Towards Circular Economy: ash management for CHP plants in the UK

Étienne Joseph Marcel BASTIEN

Master of Science Thesis KTH School of Industrial and Management Energy Technology: TRITA-ITM-EX 2020:559 Division of Heat & Power Technology SE-100 44 STOCKHOLM

Master of Science Thesis in Energy Technology

TRITA-ITM-EX 2020:559

Towards Circular Economy: management for biomass CHP plants in the UK

Étienne Joseph Marcel Bastien

Approved Examiner Supervisor at KTH

2020-10-27 Miroslav Petrov - KTH/ITM/EGI Miroslav Petrov

Commissioner Supervisor at Estover Energy Ltd. Florian Gerbier

Abstract

In response to climate change, the has committed to reach net zero greenhouse gas emissions by 2050. To reach the set target, the Committee on Climate Change is planning a notable increase of bioenergy up to 15% in the national energy mix and thus an increase of the amount of ash produced. Nowadays, ash is rarely recycled or reused but mostly landfilled, which is both costly for the industry and unsustainable. In parallel, agricultural land and suffer both from acidification and nutrient depravation due to the intensive land utilization practices.

This study reviews the potential of a circular economy of wood ash for application on land and forestry through a case study based on the ash generated by two biomass-fired CHP plants in the UK. It summarizes the general ash characteristics, the factors influencing its quality, and the optimal composition sought for application to land and forestry. The study also aims at defining the main concerns with regards to potential contamination of the environment and dangers to human health. Eventually, the process to evaluate and establish recycling of the ash for both plants was analysed in light of the above.

The study found that ash is a remarkably variable co-product, depending mainly on the fuel input and combustion conditions. Ash is a good replacement for agricultural lime and conserves a notable part of the biomass fuel nutrients. It was estimated that ash could cover a significant part of the liming and fertilising demand in the UK by 2050, and deliver a substantial financial value to a power plant. However, contamination concerns are present, especially heavy metals, which could lead to damage on the environment and reduction of the growth rate.

The study found that ash recycling in the UK is currently a challenging process, that should not be overlooked when designing a biomass project. When they exist, procedures are demanding with regards to testing, which can financially deter power plants to engage in ash recycling processes.

SAMMANFATTNING Som ett svar på klimatförändringarna har Storbrittanien åtagit sig målet att nå nollutsläpp av växthusgaser till år 2050. För att nå det uppsatta målet planerar utskottet en anmärkningsvärd ökning av bioenergi upp till 15% i den nationella energimixen och därmed en ökning av mängden producerad aska. Numera återvinns eller återanvänds aska sällan men deponeras mest, vilket är både kostsamt för industrin och ohållbart. Parallellt lider jordbruksmark och skogsmark både av markförsurning och näringsämnesförlust på grund av det mycket intensiva lantbruket.

Denna studie granskar potentialen för en cirkulär ekonomi av träaska för applicering på jordbruks- samt skogsbruksmark genom en fallstudie baserad på askan som genereras av två vedfliseldade kraftvärmeverk i Storbritannien. Den sammanfattar de allmänna askegenskaperna, de faktorer som påverkar dess kvalitet och den optimala sammansättningen som eftersträvas för applicering på mark och skog. Studien syftar också till att definiera de viktigaste problemen med avseende på potentiell miljöförorening och faror för människors hälsa. Slutligen analyseras processen för att utvärdera och etablera återvinning av askan för båda kraftverken med tanke på de ovanstående utmaningarna.

Studien visade att aska är en anmärkningsvärt variabel biprodukt, beroende främst på bränsletypen och förbränningsförhållandena. Askan är en bra ersättning för jordbrukskalk och innehåller en anmärkningsvärd del av de ursprungliga biomassans näringsämnen. Uppskattningsvis skulle askan kunna täcka en betydande del av efterfrågan på kalkning och gödning i Storbritannien till år 2050 och samtidigt leverera ett betydande ekonomiskt värde till kraftverken. Det finns emellertid kontamineringsproblem, särskilt p.g.a. tungmetaller, som kan leda till skador på miljön och minskning av tillväxttakten.

Studien bevisar att askåtervinning i Storbritannien för närvarande är en utmanande process, som inte bör förbises när man utformar ett biobränsleprojekt. När de existerar är procedurerna mycket krävande när det gäller testning, vilket kan avskräcka kraftverken ekonomiskt för att delta i askåtervinningsprocesser.

Contents 1 ASH RECYCLING: A GREAT POTENTIAL ...... 1 1.1 Motivation ...... 1 1.2 Objectives ...... 1 1.3 Methodology ...... 1 1.3.1 Literature Survey and current practices ...... 1 1.3.2 Case study and practical application ...... 1 1.3.3 Delimitations ...... 2 1.4 Background ...... 2 1.4.1 Energy Planning and Implications ...... 2 1.4.2 Nutrient Depletion & Soil Acidification Problem ...... 2 1.4.3 Ash Disposal Problem ...... 4 1.4.4 A Potential for Circular Economy ...... 5 2 THE CASE STUDY ...... 7 2.1 The Combined Heat and Power Plants ...... 7 2.1.1 CHP Plant A ...... 7 2.1.2 CHP Plant B ...... 12 2.2 The Ash Situation ...... 14 3 ASH SPREADING ON LAND: TECHNICAL REVIEW OF OPPORTUNITIES ...... 15 3.1 Characterisation of the Ash ...... 15 3.1.1 General characterization of wood-ash ...... 15 3.1.2 Factors influencing ash quality ...... 17 3.2 Properties for Ash Application to Soil ...... 19 3.2.1 On agricultural land...... 19 3.2.2 On forestry ...... 21 3.3 Evolution of the framework ...... 23 4 ASH STRATEGY: SOLVING THE ASH OFFTAKE ISSUE ...... 24 4.1 Plant A ...... 24 4.1.1 SEPA Waste Exemption Process ...... 25 4.1.2 Current Situation ...... 27 4.2 Plant B ...... 27 4.2.1 Logistics and Transport Costs ...... 28 4.2.2 European Waste Classification ...... 30 4.2.3 Desulphurization and lime injection ...... 32

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4.2.4 Ash testing and re-classification ...... 33 4.2.5 Plant B ash characteristics ...... 34 4.2.6 Spreading to land ...... 35 5 DISCUSSION & FURTHER WORK ...... 37 5.1 Plant A ...... 37 5.2 Plant B ...... 38 5.2.1 Ash reclassification ...... 38 5.2.2 Fuel Intake Evolution ...... 39 6 CONCLUSIONS ...... 40 REFERENCES ...... 41 APPENDIX 1 ...... 44

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List of Figures Fig.1: Soil weathering influence factors (Diagram: the author) ______3 Fig.2: Evolution of UK landfill tax lower and higher rates since creation in 1996 (Chart: the author) ______5 Fig.3: Plant A (Photo: the author) ______7 Fig.4: Small roundwood being weighed at the weighbridge at Plant A (Photo: the author) ______8 Fig.5: Timber Grades with indicative values of mass percentage and typical use (Chart: the author) ______9 Fig.6: Brash in forestry and on site after chipping at Plant A (Photo: the author) ______9 Fig.7: Fuel and Energy Flowchart of Plant A (Company report document) ______10 Figure 8: The log yard, the chipper, and the forwarder at Plant A (Photo: the author) ______10 Fig. 9: Plant A boiler and flue gas exhaust sketch and ash flows (Chart: the author) ______11 Fig.10: Plant A's grate fired boiler (in maintenance on the left, operating on the right) (Photo: the author) ___ 11 Fig.11: Grade C recycled wood ( visible) (Probio Energy, s.d.) ______13 Fig.12: CHP Plant B steam cycle and streams (Internal operational document of Plant B) ______13 Fig.13: A blended-ash sample from Plant A (Photo: the author) ______16 Fig.14: The effect of soil pH on nutrient availability from (Roques, et al., 2013) ______19 Fig.15: Map of basic soil types in the UK (Roques, et al., 2013) ______20 Fig.16: Estimate of ash content coverage of fertiliser nutrients and lime application need 2019 (kT) (Chart: the author) ______21 Fig.17: Estimate of ash content coverage of fertiliser nutrients and lime application need 2050 (kT) (Chart: the author) ______21 Fig.18: Blended ash at Plant A (Photo: the author) ______24 Fig.19: Summarized soil analysis results from one of the farms to be supplied with Plant A's blended ash (Soil Analysis report of selected farmland) ______26 Fig. 20: Ash Roadmap for Plant B's ash (Schematic: the author) ______28 Fig.21: Plant B's Bottom-ash (Photo: the author) ______29 Fig.22: Ash silo and container truck filling (Photo: the author) ______29 Fig.23: Fly-ash container truck loading sketch using only the middle trapdoor (Schematic: the author) ______30 Fig.24: Extract from the list of waste relevant to Plant B's ash classification (EA; SEPA; NIEA; NRW, 2018) ______32 Fig.25: Most sensitive elements levels for virgin fuel and IEDx as a percentage of the untreated ISO norm maximum limits ______35 Fig. 26: A crusher bucket mounted on a mobile plant (left) (Worsley Plant, s.d.), an on-line nibbler (right) (Gericke Ltd, s.d.) ______36 Fig. 27: A fly ash conditioner (MBE EWB, s.d.) ______36 Fig. 28: Project Status on the Ash Roadmap (Diagram: the author) ______38 Fig. 29: Detailed description of the 4 grades of waste wood ______44

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List of Tables Table 1: Wood ash "typical" composition (Risse & Gaskin, 2013)(Ash sample test results from plant A and B) ...... 6 Table 2: Analysis of various ashes in main oxides (Vamvuka & Kakaras, 2011) ...... 16

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Abbreviations and Nomenclature

APCr: Air Pollution Control residue

CHP: Combined Heat and Power

NRW: Natural Resources Wales

EA: Environmental Agency

EWC: European Waste Catalogue

IED: Industrial Emissions Directive

IEDx: Industrial Emissions Directive exempt material ("clean" recycled wood)

LOI: Loss On Ignition

MDF: Medium Density Fibreboard

MWe: Electrical Mega-Watt

MWth: Thermal Mega-Watt

NIEA: Northern Ireland Environment Agency

NOx: oxides

NPK(S): Nitrogen, , (and Sulphur), as soil nutrients

ODF: Oriented Strand Fibreboard

PK: Phosphorus and Potassium, as soil nutrients

SEPA: Scottish Environment Protection Agency

SNCR: Selective Non-Catalytic Reduction (for NOx control)

SOx: Sulphur oxides

SOH: Stem-only harvesting

TME: Trace Metals Elements

WTH: Whole-tree harvesting

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ACKNOWLEDGMENTS

I would like to thank my industrial supervisor Mr. FG for his guidance, the valuable input and support throughout the project.

I would also like to thank my colleagues for their availability, patience, and support. I am grateful for all the effort they made to make this internship an enjoyable and valuable experience: Mr. AF, Mr. MS, Mr. GDH, Mr. HP, M. VM and Mr MA. My heartfelt thanks to all my desk neighbours for their availability and efforts to integrate me in the team. A special thank to M. KH for the delicious cakes.

My thanks go to Alex, Florian, Ludovic, Nicolas, Paul and Tony who gathered as the Solteam and brought essential support during the lockdown.

Eventually I would like to thank my academic supervisor Mr. Miroslav Petrov for his precious advice on the report writing and presentation work.

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1 ASH RECYCLING: A GREAT POTENTIAL

1.1 Motivation The commissioning company is responsible for the management of two young biomass-fired CHP plants. The ashes generated were landfilled at the start of the thesis work and the projects suffered from the associated costs. The idea of recycling ash came from the necessity to reduce the costs associated to ash disposal and improve the sustainability of the process through application to soil. The author’s mission at the company was thus to evaluate and establish recycling of the ash.

1.2 Objectives The aim of this study is to examine the possibility of recycling wood-ash from biomass power plants in the UK, by spreading to land, spreading to forestry, or using as a fertiliser and liming1 agent. This topic will be explored through a case study: solving the ash offtake issue for the two power plants managed by the commissioning company. The study will define the potential of ash recycling and its technical and legal challenges. A focus will be made on wood-ash characteristics and on the main parameters affecting its chemical composition. The necessary and useful properties for application to soil will be investigated as well as the current practices and legal contexts in several countries. Based on preliminary elements of this study on ash spreading, an ash strategy plan will be defined for the biomass projects. The different steps of the established plan will be examined with regards to the environmental and economic viability of the ash offtake for both projects.

1.3 Methodology

1.3.1 Literature Survey and current practices First the current state of the art in terms of ash spreading will be investigated through a literature study. Information will be gathered from scientific publication sources but also from different collaborators on the projects and previous experiences in the company. This information will help with properly gauging the stakes during the alternative ash offtake project.

1.3.2 Case study and practical application The biomass projects specificities will be detailed with regards to the technology and fuel supply. The current ash situation of the projects will need to be defined. Information on ash and on the different factors potentially affecting the ash quality will be identified. It will then be possible to determine relevant actions to address the current ash output.

1 Liming is the application to soil of Calcium- and Magnesium-rich materials, which react as a base on acid and neutralise soil acidity. -1-

A multi-stage strategy will be developed including reasonable short, medium, and long-term objectives for the projects. This strategy plan will help reduce the environmental and economic impact of current landfilling of the ashes.

1.3.3 Delimitations The study will focus on the use of wood ash on soil as a fertiliser and liming agent in a context of circular economy although we will also widen the scope to other biomass fuels and other applications. With regards to the case study, the topic will be treated from the company's perspective focusing on economical and practical challenges.

1.4 Background

1.4.1 Energy Planning and Implications In 2019, the UK became the first major economy to sign a legally binding target to reach net zero greenhouse gas emissions by 2050. In order to reach those goals, the Committee on Climate Change proposed a contribution of up to 15% from bioenergy to the energy mix, which represents around up to 27 million dry tonnes of biomass – 49 million green tonnes2 per annum. If all these feedstocks were to be used in combustion this would represent a total ash output of around 1 million tonnes per year and a significant nutrient removal from agricultural and forest soils both in the UK and abroad. Today, woody biomass harvest from UK forests represents about 12 million green tonnes per year of which 3 million green tonnes are used for bioenergy. This produces around 52,000 tonnes of wood ash. Hence, the extraction rate from forestry and the amount of ash will increase more than tenfold 2050.

1.4.2 Nutrient Depletion & Soil Acidification Problem Intense biomass harvesting is responsible for nutrient depletion in forests. This effect is highly dependent on the harvesting style. Stem-Only Harvesting (SOH) is not responsible for as much nutrient removal as Whole-Tree Harvesting (WTH), which includes the stem and the residues: branches, foliage and sometimes stumps and roots. Indeed, the biggest share of nutrient is located in logging residues i.e. branches, foliage (Palviainen & Finér, 2011). If WTH allows to recover up to 2 times more biomass, it can lead to removing up to 6-7 times more of the principal nutrients than SOH from the forest soil (Hansen, et al., 2011). In order to evaluate the amount of nutrients available, many factors must be considered. A common approach is to use nutrient budgets. This approach evaluates the different fluxes to and from the forest ecosystem and allows to determine if any imbalance is present. This tool can enable a forester to determine whether compensation treatments are necessary.

2 Wet mass of biomass, moisture included -2-

Fig.1: Soil weathering influence factors (Diagram: the author)

With biomass energy development, extraction of forest residues has increased in recent decades. With WTH being more and more performed, the export of nutrients and acid buffering elements from forests has increased and is fated to increase even more. For the foresters, this generates a long-term need for compensation fertilising and soil amendment to maintain ideal conditions for the growth of trees primarily. Nevertheless, it is necessary to maintain the quality of surrounding waters and soil, in particular with regards to microbial activity, nutrient availability and toxicity. This will ensure that the soil quality remains suitable for plants growth. During the last few decades, an increase in global soil acidity has been observed in Europe and (Blake, 2005). There are two main reasons for the acidification of soil: acid rains and intensive crops or biomass extraction. Even though natural rain is slightly acidic, acid rains with concentration of protons 10 to 100 times higher than usual are a result of human activities. Through processes such as coal burning and sulphide ore smelting3, strong acids are released in the atmosphere in the form of particles or gases. As a contrast, fossil fuel combustion releases around 70 Tg(S) in the form of sulphur oxides whereas volcanic activity rejects around 8 Tg(S) (Berresheim, et al., 1995). Human activities are also responsible for significant emission levels of nitrous oxides, mainly through animal livestock activities, which are also strong acidic gases. In atmospheric gases, sulphur oxides and nitrous oxides are converted to sulphuric and nitric acid which are then deposited on soil, either through a wet process (acid rain) or through dry deposition4. Intensive agricultural practices are also responsible for the acidification of soils. Through soil amendment such as sulphur-based fertilisers, human agricultural activities can severely affect the soil pH. This is also true for nitrogen-based , in particular ammonia-based fertilisers and to a lesser extent urea, , and compost. On the other hand, soil acidification is accelerated

3 Smelting is one of the main processes of the extraction of metal from sulfide and oxide minerals. It separates the ore in two liquid layers: metals and liquid slag. 4 Dry deposition is another form of acid deposition. It refers to when acid compound are deposited in a particulate or gaseous form. -3- by extraction of farm and forestry produce. Indeed, most plants are slightly alkaline, and their removal generates acidification of the soil over time. Soil acidification causes several issues with regards to agriculture or forestry. One major concern is toxicity. Indeed, as pH decreases, the aluminium solubility of the soil increases, drastically under pH 4.5, until it reaches doses that are toxic to non-tolerant species, affecting their root growth and thus their global development (Andreas Aronsson & Ekelund, 2004) (Gazey, s.d.). The same mechanism can happen with manganese (Slattery, et al., 1999). Another major concern is with the nutrient availability for plants decreasing in acidic soils. Plant processes and growth are favored by soils whose pH ranges from 5.5 to 8 (Gazey, s.d.). Acidic soils can also reduce the microbial activity. Indeed, several beneficial symbiosis processes happening at slightly acidic soil levels are jeopardized by low pH soils. Coming back to the forestry industry, it is common practice to let the forestry residues rot on site to fertilise the soil for the next tree generation and maintain a stable pH level in the soil. However, with the development of biomass power plants and to a lower extent aggregated and stratified wood board producers (MDF, OSB), foresters have started to get benefit out of those former residues. Furthermore, biomass combustion energy presents multiple advantages (amongst others):  It is a renewable energy, that needs to be used at its full potential to achieve the sustainable objectives set for the UK.  Amongst renewable energies, harnessing biomass energy creates or stimulates a local economy (creation of jobs and stimulation of rural development).  It makes a more efficient use of the forestry produce, creating more financial value from the forestry residues.

1.4.3 Ash Disposal Problem

1.4.3.1 Landfilling As presented in 1.4.1, wood ash quantities could represent around 0.5 million tons a year. On a European level (EU27), this would represent around 15.5 million tons a year (Agrela, et al., 2019). This causes an obvious concern with regards to volume as landfill locations are already saturated. As landfill has been the conventional method of waste disposal for decades, landfill locations have come close to saturation at the beginning of the 21st century (Eyre-Walker, 2018). On the other hand, ash disposal is a concern for a lot of power plants operators. Indeed, the government has applied a policy that “encourages efforts to minimise the amount of material produced and the use of non-landfill waste management options, which may include recycling, composting and recovery” (GOV.UK, 2020). It is thanks to the introduction of this landfill tax and other efforts that landfill has seen a decrease in recent years. Wastes that are disposed at a landfill location are taxed at two different rates based on their hazard potential. In fact, are charged under the lower tax bracket materials that can be proven to be inert or inactive with a few exemptions. All other materials are taxed under the high tax bracket. Ash usually has a high pH (around 11-12) and can potentially contain contaminants, due mainly to fuel contamination, the burning process and the flue gas treatment systems. For those reasons, ash can easily fall under the higher tax bracket. Practically, the higher rate has been multiplied by

-4- approximately 13 since 1996. Brought in perspective with the fuel intake, the tax on ash represents approximately an augmentation of 5% of the price of biomass fuel (see Appendix).

Fig.2: Evolution of UK landfill tax lower and higher rates since creation in 1996 (Chart: the author)

This high price generates a high financial motivation for companies to move to alternative solutions and recycling or try to make their waste inert or inactive. However, there is no standardised method that will allow to avoid landfill every time. Indeed, ash is a very variable waste, whose properties depend on a multitude of factors. Therefore, ash recycling procedures are unique.

1.4.3.2 Transport On top of the landfilling costs, wood ash requires the implementation of costly national routes to remote landfill locations. Indeed, ash incurs huge transport costs for power plants. On a national scale, ash transport currently represents a cost of around £4 million to the bioenergy industry. Those costs depend heavily on the ash properties. Costs will be significantly higher for fly ash compared to bottom ash for instance, as fly ash is powdery and must be handled with more care both when loading at the plant and unloading at the landfill location. It will be transported in container trucks which are costlier and have less capacity than classic container trucks used for bottom-ash.

1.4.4 A Potential for Circular Economy Intensive biomass harvesting is responsible for the depletion in soil nutrients and a global acidification of the soil, which reduces tree growth over time. This invites to compensation measures as fertilisation and soil amendment. Fortunately, ash can be used for soil amendment due to its high pH and can be a great as most of the nutrients present in the biomass fuel end up in the ash.

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Currently, wood-ash is deemed a waste. This implies the implementation of national disposal routes to landfill locations, which incurs high haulage and landfill costs for the industry. This results in a burden of around £8 million to the bioenergy industry, which could increase more than tenfold by 2050. In order to resolve these issues and close the circle, some countries are already looking at ash spreading in forestry. Several case studies, mostly in Scandinavia and North America, are aimed at demonstrating potential benefits of ash spreading on soil properties - growth rates, nutrients availability, microbial activity, soil acidity (Joensuu & Berge, 2017; Moilanen, et al., 2015; Vance, 1996). There are also several countries looking at ways to implement the relevant legal framework and making progress towards ash recycling (EU, 2020). Wood ash would present the necessary qualities to palliate the nutrient depletion problem in some forestry and farmland sites. As observable in Table 1, ash presents a high pH which makes it a good liming agent. It is rich in macro-elements sought for fertilising such as Phosphorus and Potassium (PK) in particular. Table 1: Wood ash "typical" composition (Risse & Gaskin, 2013)(Ash sample test results from plant A and B)

Min "Typical" Wood Ash Max pH 9 10.5 13.5 Macro-elements (oxides) Calcium [%] 2.50% 15% 33% Potassium [%] 0.10% 2.60% 13% Magnesium [%] 0.10% 1% 2.50% Iron [%] 0.20% 0.84% 2.10% Phosphorus [%] 0.10% 0.53% 1.40% Ultimate Analysis Carbon [%] 3% 8% 15% Hydrogen [%] 0.32% 0.40% 0.59% Nitrogen [%] 0.02% 0.15% 0.77% Oxygen [%] 0% 2% 4% Sulphur [%] 0.01% 0.70% 1.30% Chlorine [%] 0.10% 0.50% 1% There is a great potential for circular economy and a creation of an ash market. It is vital to reframe ideas about wood ash and secure its acceptance as a co-product5 in order to set the right environment for a new circular market to thrive. Ending the wasteful and costly implications of ash disposal to landfill will reduce costs and improve bioenergy supply chains to help the UK meet its 2050 net zero GHG emissions target and progress towards a more circular economy as well as rebalancing the UK forests nutrient budgets.

5 Secondary product derived from a production process (also referred to as by-product) -6-

2 THE CASE STUDY

2.1 The Combined Heat and Power Plants This case study is based on two biomass-fired Combined Heat and Power (CHP) plants operated and managed by one specific company in the United Kingdom. The plants are: a) A 14.2 MWe plant in Scotland (hereby denoted as “Plant A”) and; b) A 27.7 MWe plant in the North of (hereby denoted as “Plant B”). Both projects were initially developed together from 2009 to 2016, with a third project under construction in the South of England, similar to Plant B.

2.1.1 CHP Plant A

Fig.3: Plant A (Photo: the author)

The £74m CHP Plant A project started operating in 2015 in Scotland. It was designed to supply 11.1 MWe to the grid and 9 MWth to a nearby distillery, replacing 90% of its former gas supply for the distillation process. Plant A is designed to take in a fuel mix of small round wood, and up to 25% of brash in the fuel mix. The plant also burns chips from sawmill rejects, mainly bent or oversize saw logs and residues.

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Fig.4: Small roundwood being weighed at the weighbridge at Plant A (Photo: the author)

There are different grades of timber in a tree and it is usually divided in several parts as presented in Fig.5. Saw logs, with a diameter from 50cm to 16cm, are the most valuable parts of tree and are handled in large sawmills. They are used to produce the construction wood, furniture, and other high-end products. Then comes the pallet logs between 16cm and 14cm in diameter that are mainly used to produce pallets. Small round wood corresponds to the upper part of the main stem, usually between 14cm and 7cm in diameter. This part of the stem is too small to go to sawmills. However, it can be used to produce panel boards of the type of Medium Density Fibreboard (MDF) and (ODF). Brash, also called Forest Residue is the part of the main stem at the top, typically below 7cm in diameter together with any side twigs and branches (Fig.6). It can be left on the ground for the nutrients it contains or be used as fuelwood or . In most cases, brash constitutes the main source of fuel for biomass-fired plants, delivered there in the form of , while the main product of the forestry operations is the more valuable from the main tree stem that goes for the production of various timber products and different types of fibreboard.

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Fig.5: Timber Grades with indicative values of mass percentage and typical use (Chart: the author)

Fig.6: Brash in forestry and on site after chipping at Plant A (Photo: the author)

The three types of fuel are processed on site and stored in a fuel hall in the form of wood chips.

The chips are then conveyed into a 46 MWth grate-fired boiler, which powers a steam cycle. After combustion, the bottom ash falls of the grate into a waterbed. On the other hand, an electrostatic precipitator removes the particles and dust from the flue gases, which forms the fly ash. Both streams are then mechanically mixed and exported in the form of blended ash. Plant A currently produces around 2000 tonnes of blended ash per year.

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Fig.7: Fuel and Energy Flowchart of Plant A (Company report document) Fig.7 gives a global overview of the different flows at Plant A. It illustrates the fuel intakes, the fuel processing, the steam cycle, the air intake, the flue gas output, and the ash output. Information about any materials entering the plant is obtained by weighing incoming and leaving trucks at the weighbridge (Fig.4). Logs can be stored in the log yard whereas brash or sawmill chips needs to be put directly in the fuel hall. A forwarder can bring logs to the chipper from the log yard.

Figure 8: The log yard, the chipper, and the forwarder at Plant A (Photo: the author)

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Boilers of both Plants A&B are vertical grate fired boiler making a U-turn at the top gas duct with an economizer section. After combustion, the bottom ash falls from the grate into a waterbed and is blended together with other solid residues collected at the second U-turn of the gas duct just before the economizer.

Fig. 9: Plant A boiler and flue gas exhaust sketch and ash flows (Chart: the author)

Fig.10: Plant A's grate fired boiler (in maintenance on the left, operating on the right) (Photo: the author)

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At Plant A, the fly ash is precipitated by an electrostatic precipitator into an ash conditioner where the bottom-ash and the fly-ash are blended to form a blended-ash stream. This blended ash is then conveyed by a screw conveyor and loaded into skips. As shown in Fig.10, the grate is travelling downwards with biomass chips delivered from the top. Air flows upwards through the grate to provide both oxygen and cooling to the grate.

2.1.2 CHP Plant B Plant B is quite similar to Plant A in design and operation, although on an ash-focused perspective, it differs in the fuel it combusts and the way the ash is processed. Plant B was designed to supply 26.3 MWe to the grid and 6.1 MWth to nearby industries. Plant B receives a mix of virgin wood6, and up to 11% of non-hazardous recycled wood in the mix. The virgin wood is made up of four different fuel types: roundwood logs, forestry residues, roundwood/sawmill chips and virgin biomass blend, a blend of forestry residues, virgin woodchips and arboricultural arisings i.e. trees and bushes removed for ecological reasons (e.g. from the sides of highways). The waste wood received at the plant is the most critical fuel supply, as it is the most probable source of contaminants. The plant only burns IED exempt clean recycled wood, which is the highest grade of waste wood. This is defined in the environmental permit as “wood waste with the exception of wood waste which may contain halogenated organic compounds or heavy metals as a result of treatment with wood preservatives or coating and which includes, in particular, waste originating from construction and demolition waste” (EU, 2017) and falling under the following European Waste Catalogue (EWC) codes: 15 01 03 – Clean waste wood biomass fuel from wood packaging or facilities processing untreated wood packaging operating under an appropriate exemption or 19 12 07 Untreated, chipped waste wood. This corresponds approximately to what is commonly referred as grade A waste wood. Indeed, the Environmental Agency classifies waste wood in four categories (EA, 2017):  Grade A waste wood, which as stated before must be visibly clean and non-hazardous and contains waste wood from the arboriculture sector, packaging waste, scrap pallets, packing cases, cable drums and off-cuts from the manufacture of untreated wood products.  Grade B waste wood consists of non-hazardous waste wood from the production of wood- based panels: for example, chipboard and medium density fibreboard.  Grade C consist of non-hazardous waste wood sourced mainly from construction and demolition activities, recycling centres and civic amenity sites.  Grade D waste wood can include any item of waste wood which has been treated, coated, painted, or otherwise contaminated with any hazardous substance. This may include for example heavy metals and in particular, , chrome, or arsenic (CCA), creosote, halogenated compound or metal pigment containing treatments, , coatings and preservatives.

6 As opposed to waste wood. -12-

Fig.11: Grade C recycled wood (paint visible) (Probio Energy, s.d.)

Whereas grade D material is clearly hazardous and results in a high contamination risk, grade A to C materials are labelled as non-hazardous. However, grade C and B have much higher tendency to be contaminated than grade A material, as shown in Fig.11. The fact that Plant B only combusts waste wood of the highest grade (grade A equivalent) and to a limited extent reduces consequently the contamination risk compared to other typical co- incineration plants usually burning grade C material. Although theoretically "clean", this fuel can contain minor residues of paint and coatings and stands therefore as the most critical intake for the plant with regards to the ash potential contaminants. At Plant B, the ash is separated in two ash streams: the incinerator bottom ash, which is made of the solid residues of combustion falling off the grate inside the boiler and the fly ash, which is the particle residues captured and condensed in the flue gas treatment system. The fly ash is then stored in silos before being taken off the plant by silo trailers as visible in Fig.12.

Fig.12: CHP Plant B steam cycle and streams (Internal operational document of Plant B)

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Plant B produces around 2300 tonnes of bottom ash and 900 tonnes of fly ash per year. These quantities can vary depending on the quality of the fuel, on the parameters of the combustion process and on the availability of the plant achieved throughout the year.

2.2 The Ash Situation During the time span considered in this study, the ash from both Plant A & Plant B was disposed of at a landfill. On top of being unsustainable, this was extremely costly for the company due to constantly rising landfill fees. The ash offtake represented a budget of approximately £615,000 per year for each of the plants. At Plant A, the blended ash needed to be transported to a landfill location 140 miles away for an approximate cost of £140 per tonne. At Plant B, the bottom-ash was transported 170 miles away for a cost of around £65/tonne whereas the fly ash was transported 45 miles away for a cost of around £210/tonne. These costs account for both the transport and the cost of landfilling including the payment of national landfill tax. As much as it was a money pit for the projects, an alternative ash disposal process would mean significant and durable savings for the project and would strongly increase its viability. At Plant B, three reasons were identified for the fly ash’s higher cost per ton. Firstly, it is a less dense material, that is very powdery and therefore requires special handling to mitigate risks for human health. It is taken off the plant by sealed silo trailers that have a smaller capacity than usual heavy-duty trucks. Secondly, the ash silo has a limited capacity and the plant could no longer operate if the silo were to become full. Therefore, a safety margin is applied to ensure continuous operation. Eventually, the fly ash was considered as air pollution control residue with regards to landfill tax and therefore taxed under the higher tax bracket at £94.15/tonne compared to £3/tonne for the lower tax bracket.

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3 ASH SPREADING ON LAND: TECHNICAL REVIEW OF OPPORTUNITIES To be able to properly assess the available opportunities, one first needs to characterize the ash product: identify the different properties requested for application to land but also the potentially harmful ones to the environment. It is also useful to analyse the current market situation in the UK and explore the context abroad with regards to legal frameworks.

3.1 Characterisation of the Ash Ash can designate the residue of combustion, gasification, or of a variety of inorganic and organic products such as coal, biomass, or wastes. In this , we will mainly focus on wood-ash from biomass plants and in particular bottom ash and fly ash from grate-fired boilers. Bottom-ash is a coarse and granular by-product collected at the bottom of a grate boiler. It is made of the incombustible part of the fuel that falls of the grate after the designed travel time in the boiler. Fly- ash is a co-product of combustion formed of fine granulates collected in the flue gas treatment system.

3.1.1 General characterization of wood-ash Wood ash is the organic and inorganic residue remaining after the combustion of wood. It is a highly alkaline product with a pH ranging from 9 to 13.5 (Etiégni & Campbell, 1990; Risse & Gaskin, 2013). Its neutralizing value is usually around 50% (of that of standard lime), which means that if used as a liming agent, one would need to spread twice the mass for the same liming effect. Calcium is indeed one of the most, if not the most, abundant elements in wood ash, whether in the form of Calcium carbonate (CaCO3) or Calcium oxide (CaO). This is what makes the ash similar to agricultural lime used for soil amendment (Risse & Gaskin, 2013). Ash also contains a significant amount of Silicon Nitrogen and Sulphur levels in wood bottom ash are usually very low (>0.1mg/kg), as it is almost completely given off as gasses during combustion, although it is possible to observe residues of sulphur in fly ash in the form of oxides (SO2, SO3). Some macroelements interesting for plant growth remain in the ash, it usually contains a remarkable amount of Potassium (1-10%), Magnesium (1-3%) and Phosphorus (1-2%) or Iron (0.1-1.5%). Ash can also contain to a lower extent Zinc, Boron and Copper. The combustion is never complete though it depends mostly on the size of the installation. Larger plants will be able to provide a more controlled combustion environment and achieve a more complete combustion. Therefore, an unburnt remains and is measured with the total quantity of carbon present in the ashes. This quantity varies significantly with contents ranging from 0-1% to 30%. Together with the Loss On Ignition (LOI), they are a good indicator of the quality of the combustion taking place in the boiler. Fly-ash is usually rather inorganic and most of the unburnt, if any, can be found in the bottom-ash (Agrela, et al., 2019). Furthermore, ashes can contain Trace Metals Elements (TME), which can contaminate the land if the ash is to be spread and must be closely monitored: Antimony, Arsenic, Cadmium, Chromium, Cobalt, Copper, Lead, Manganese, Mercury, Molybdenum, Nickel, Selenium, Thallium, Vanadium or Zinc. Some are naturally present in virgin biomass. Some are useful micronutrients whereas some others are just non-essential heavy metals. Ash tends to concentrate the trace metals already

-15- present in the fuel and can also contain contaminations from the plant’s systems, such as boiler coatings for instance. The biomass to ash ratio 7is ranging from 0.25% to 10% on a dry basis whereas values are rather ranging from 0.5% to 2.5% on a dry basis for industrial wood fired power plants (Boulday & Marcovecchio, 2016).

Fig.13: A blended-ash sample from Plant A (Photo: the author) However, it is difficult to give a precise characterisation of wood ash as it is a very variable product that entails many different generation processes. Even for a single selected scenario, a lot of factors may vary such as the fuel input, the combustion conditions, etc. For power plants burning virgin wood, the fuel intake varies with the seasons throughout the year, which will necessarily have an impact on the ash properties. Table 2: Analysis of various ashes in main oxides illustrates the variability of the ash characteristics for the main oxides content from different biomass types.

7 The biomass to ash ratio is the mass ratio of the biomass and the ash -16-

Table 2: Analysis of various ashes in main oxides (Vamvuka & Kakaras, 2011)

3.1.2 Factors influencing ash quality There are plenty of factors influencing the biomass ash quality at the plant's exit. As biomass includes a multitude of different fuels (straw, animal biomass/dejections, animal bedding, logs, brash, arboricultural arisings, waste wood…), the principal factor will be the fuel input type. Here are the main factors affecting wood-ash properties:  The ash type considered (bottom-ash, fly-ash, blended ash)  The fuel input which depends on: o The fuel mix (location, species, type, age) o The concentration in bark, part of the tree combusted o The presence of waste wood o The potential contaminations in the fuel supply chain  The fuel processing pre-combustion (debarking, chipping, metals sorting…)  The combustion technology (grate-fired boiler, fluidized bed boiler…) and potential contamination in the combustion process (slagging, coating scrapping…)  The size of the plant and the combustion conditions  The flue gas treatment system for the fly-ash  The collection technique of the ash  Any treatment applied to the ash (granulating, pelletizing…) There is a strong difference in composition between the bottom-ash and the fly-ash. Some macroelements can dissociate or volatilise depending on the temperature of combustion which can result in completely different composition in the fly ash and bottom ash of the same power plants. Influence of combustion temperature on the ash composition has been studied by several groups (Misra, et al., 1993; Etiégni & Campbell, 1990). More generally, for a grate fired boiler, fly ashes will have a higher concentration in interesting nutrients but also in potentially hazardous trace metals elements. The differences are extensive for plants that do not burn the same type of biomass (Table 2), but even when burning the same type of fuel, the location, the plant species and type of soil can greatly influence the ash composition (Pitman, 2006). Burning waste wood changes radically the perspective and this is automatically reflected in a different European waste code for the ashes coming from such power plants. Indeed, trace metals contamination concerns are much higher in the case of waste wood as there are several potential sources of contamination that need to be traced and monitored during the product's lifetime.

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Indeed, wood can often be treated, painted, or varnished, which results in contamination with pollutants and gets concentrated in the ashes. It is also important to ensure, that there is no mixing or contamination from other waste on the whole supply chain, which can also be a challenge for companies that handle simultaneously multiple types of waste. It is also notable that pre-combustion processes greatly influence the quality of ash. Indeed, bark usually has a higher ash content and different oxide composition (Pitman, 2006), so debarking can change the composition of the ash. Bark is also more prone to contain rocks and sand collected during logging, which increases the Al and Si content. Logs chipping can also induce metals contamination due to mechanical friction. The efficiency of the metal sorting installation, if any, also conditions the concentration of metals in the boiler fuel intake and thus of the ash. The type of installation notably affects the ash characteristics. Indeed, for a fluidized bed boiler, whether a circulating fluidized bed (CFB) or a bubbling fluidised bed (BFB), the bed ash (equivalent to the bottom ash for grate fired boilers) will be mixed with some of the spent sand bed. Size of the pieces will be different with smaller pieces for fluidized beds (Haglund, 2008). In general, fluidized beds provide a better combustion overall, thus the carbon content of the ash tends to be lower (Pitman, 2006). The size of the plant will have a significant impact on the composition of the ash as bigger installations will usually be able to provide a better controlled, more homogeneous combustion environment with often higher combustion temperature. Temperature has a consequent effect on the ash composition especially the repartition between bottom and fly-ash. Indeed, it strongly affects the volatilization and the dissociation of elements. The effects of temperature have been studied by several people. Ash has been found to lose mass with increased combustion temperature as well as having decreasing content in elements such as potassium, sulphur, boron, and copper (Misra, et al., 1993). The flue gas treatment system can have a noteworthy impact on the fly-ash composition, especially as some devices imply the injection of reactive material. Indeed, flue gases emitted by biomass power plants can contain sulphur and nitrogen oxides, mercury, particulates, and other pollutants that can affect local and regional air quality. Some equipment is necessary to respect legal emission levels. Power plants are frequently equipped with an electrostatic precipitator, a device that helps particulates and ash deposit by electrically charging them (Plant A). Control of nitrogen oxides levels is often performed with a reaction using urea or ammonia injection, such as the SNCR system at Plant B. Essentially, lime injection systems can help reduce the concentration of acidic gases remaining in the flue gases (Plant B). Eventually, the way ash gathered and collected can influence its characteristic. As it is often the case, bottom-ash can be collected in a wet basin under the grate, which modifies its chemical composition (Boulday & Marcovecchio, 2016).

The addition of water increases the proportion of slaked lime Ca(OH)2 as quicklime CaO gets hydrated to become calcium hydroxide. Practically, water is used to render the ash less powdery and cool it down. After collection, one can apply treatment to the ashes to fulfil requirements for certain applications such as self-hardening and crushing, screening, removal of metal scrap, mixing, granulating, maturing. This will be further elaborated with regards to ash spreading on land in section 3.2.2.3 below. Those multiple effects explain why biomass-ash and even wood-ash is so difficult to qualify. Variability of the ash product seriously constrains its marketing authorisation. Although we are able -18- to understand the main factors affecting the quality of an ash product, it remains difficult to know the composition of an ash waste by just examining the input variables. Even when the ash by- product has been defined by testing, it still needs to be monitored due to its dependency to fuel intake and combustion conditions. Besides, the effects of some parameters are still not understood properly and need to be further studied in order to be able to better predict the ash composition.

3.2 Properties for Ash Application to Soil

3.2.1 On agricultural land Several studies have shown that wood-ash could be a good replacement for lime in use as a liming agent or even for a use in conjunction with lime (Andreas Aronsson & Ekelund, 2004). Soil acidity is a widespread matter and there is a well-established demand for liming products. Lime must be applied regularly on agricultural land, where intensive extraction is carried out. Indeed, yearly crops means yearly offtake, which can affect the pH quickly. By increasing the pH on acidic soils, liming allows to bring back the land in the optimal range for vegetation growth, which is specific for each crop. As shown in Fig.14, regulating the soil pH at the right level allows for optimal plant nutrients availability. Therefore, a high concentration of Calcium makes the ash from biomass power plants a valuable product. Lime costs around £20/tonne, with a spreading cost around £5/tonne for a total cost of £25/tonne all included (Agriland, 2019).

Fig.14: The effect of soil pH on nutrient availability from (Roques, et al., 2013)

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With regards to nutrients input, the most sought nutrients are the usual NPK(S) (Nitrogen, Phosphorus, Potassium, Sulphur) as most vegetation require large quantities. This is also true for Calcium and Magnesium. Other trace elements are also demanded in more specific cases (Goulding, et al., 2007).

3.2.1.1 Influence of the type of soil According to (Roques, et al., 2013), micronutrients deficiency are more frequent for 'sandy and light,’ ‘chalk and limestone’ and ‘peaty’ soil types shown in Fig.15, which match almost perfectly with the soil types encountered in the north of England and Scotland. The ash product fertilising capacities are therefore useful in local areas. Fig.15 below shows the different soil types in the UK.

Fig.15: Map of basic soil types in the UK (Roques, et al., 2013) Ash contains a notable amount of quicklime (CaO), which is very corrosive and irritant. For this reason, one must wear protective equipment when spreading dry ashes. However, the ashes reactivity can be reduced by applying water to form slaked lime:

퐶푎푂 + 퐻2푂 => 퐶푎(푂퐻)2

Or carbonation with the CO2 present in the air to form limestone:

퐶푎(푂퐻)2 + 퐶푂2 => 퐶푎퐶푂3 + 퐻2푂

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The most sought nutrients both for land application and present in ash are potash and phosphate, as sulphur and nitrogen, though applied on land in large quantities, have very low levels in ashes. An estimate based on the projection yields the following potential coverage of fertiliser and liming needs only by woody biomass ashes, presented in Fig.16 and Fig.17.

Fig.16: Estimate of ash content coverage Fig.17: Estimate of ash content coverage of of fertiliser nutrients and lime application fertiliser nutrients and lime application need need for 2019 (kT) (Chart: the author) for 2050 (kT) (Chart: the author)

This modelling shows that the wood-ash volumes from bioenergy generation of 2050 could cover a significant part of the agricultural needs in potash, phosphate, and lime. There is therefore a strong economical motivation to create the right conditions for an ash recycling market to thrive in the UK.

3.2.1.2 Contamination Heavy metals traces are present in ash and as mentioned in 1.4.2, the acidity of the soil influences greatly the availability of contaminants and nutrients. When ash reacts with water, it helps raising the pH and thus reducing the solubility in most heavy metals but if all the alkalinity of the ash is consumed, the heavy metals keep a high solubility in the soil increasing their availability and potential damage (Karltun, et al., 2008). Thus, the trace metal elements present in the ash can aggravate the current situation. However, since ash usually has a strong liming effect, such detrimental effects should be rare and limited.

3.2.2 On forestry Most remarks in section 3.2.1 for agricultural land are applicable in general to forestry, though there are some specificities principally related to the soil activity and the time scales of applications in forestry. An intense research effort has been carried out in the Nordic countries for wood ash application on forests. According to (Andreas Aronsson & Ekelund, 2004), it makes no doubt that the recycling of wood ash by spreading on boreal forests will be a major industry in the future. Wood-ash spreading shows positive effects on growth due to better nutrient availability and soil

-21- acidity regulations. Effects have been found to last from 15 to 40 years (Boulday & Marcovecchio, 2016; Saarsalmi, et al., 2001; Skogsstyrelsen, 2002).

3.2.2.1 Influence of the type of soil Efficiency of ash as a fertilizer in forests depends on the availability of Nitrogen in the soil and its weatherability. It has been shown that tree growth can be increased on peatland soils rich in nitrogen in general (Andreas Aronsson & Ekelund, 2004) but also when the ash spread is enriched with Nitrogen. It has also been shown that wood ash fertilisation is useful on soils that have Ca, Mg, K or P deficiency (Boulday & Marcovecchio, 2016; Karltun, et al., 2008). A recent study has shown ash treatment to be particularly efficient for P and K deficient soils on Scots Pine with effects up to 26 years after application (Moilanen, et al., 2015). The study showed consistent volume growth with effects of ash being more significant than those with commercial PK fertiliser. However, a study showed that ash treatment brought no improvement of tree growth on mineral soils (Saarsalmi, et al., 2006).

3.2.2.2 Contamination concern Soil organisms such as bacteria, fungi, and their feeders are essential for the decomposition and mineralization of dead organic matter into plant available nutrients (Mortensen, 2019). Their role is essential to plants growth and the condition allowing them to thrive must be maintained. There is a strong contamination concern on Cadmium, as it can have damaging effect on the soil microbial activity. Indeed, studies have shown that an increased concentration of Cadmium can reduce the growth of wood rotting fungi and ectomycorrhiza, a form of symbiotic interaction that favours the growth of tree roots (Andreas Aronsson & Ekelund, 2004; Mortensen, 2019). Although always creating a perturbation on soil micro activities, it was found that ash application rates under 6 tons/ha allowed for a relatively rapid return to normal whereas higher application rates could cause permanent perturbation on the soil decomposer food web (Mortensen, 2019). There is also a concern on watercourses and aquatic life as ash could increase the pH and mobilize toxic compounds. The impact seems to be quite uncertain (Andreas Aronsson & Ekelund, 2004) and needs to be further examined. Thus, precaution measures apply. Another concern of contamination comes from organic pollutants such as dioxins and furans such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). Though usually low in virgin biomass ashes, they have been found to be correlated with heavy metals contamination. Procedures often include tests on those pollutants (EU, 2008). The dosage influences greatly the negative impact. Studies seem to see eye to eye on the fact that dosages above 5 to 10 tons per hectare per year generate a rise in detrimental effects on forest soils (Andreas Aronsson & Ekelund, 2004). This threshold is therefore the legal limit in several Nordic countries (Sweden: (Skogsstyrelsen, 2002), Denmark: (Karltun, et al., 2008))

3.2.2.3 Ash Conditioning Whereas it is of lesser concern for agricultural land where one seeks a rather immediate effect, it is necessary for forestry that the effects of ash are applied more progressively, although that can be important for agricultural land too in specific cases. Ash conditioning allows for smoother release of nutrients and thus contaminants over time. This contributes to decreasing the shock of the application on the vegetation and the microbial activity but also to guarantee a better nutrient availability on the long term.

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Ashes can be self-hardened and crushed. Water is applied on the ashes, which are then spread and left to harden for several days. Ashes are then crushed with an excavator for instance (Boulday & Marcovecchio, 2016). According to ADEME (Boulday & Marcovecchio, 2016), this process could cost around £10-20 per tonne. Another process is granulating. There are two methods: disc granulation (or pelletizing) and drum granulation. In drum granulation, the ash is fed into a big rotating cylinder after being moistened and forms ash granules. In the pelletizing process, the moistened ash is pressed through compression cylinders and then broken into pieces. Additional drying of the granules can then be applied in both methods, to avoid agglomeration and reduce weight. This is particularly useful for transport purposes. The ash does not need to be moistened much: between 10% and 15% for the bottom-ash, 40% for the fly-ash (Korpilahti, 2003). It is also notable that granulation is likely to fail on ash that contains too much carbon (unburnt), as the final product will be too porous and will not correctly aggregate (Karltun, et al., 2008). Costs are around £5/T to £10/T depending on the use of drying and capital investments. Eventually, mixing is practiced in some countries with materials such as lime or dolomite (Boulday & Marcovecchio, 2016). Spreading is a more difficult process in forestry than for agricultural land due to the terrain. There is less space to operate, the ground is usually bumpy and sloped terrain can be a significant challenge. Therefore, vehicle must be lightweight and very manoeuvrable. For that reason, some companies have started using spreading by helicopter in Scandinavia (Stockholm Exergi, 2019).

3.3 Evolution of the framework Ash application is not a new idea. As soon as the beginning of the 20th century, studies on the effect of ash spreading in forest on tree growth have been carried out in Sweden and Finland (Karltun, et al., 2008). With rising concern for acid rains and nutrient depletion in forests, ash has been seen as a promising product to restore balance in the second part of the century. Since the 1990-ies, with more care to minimize landfill waste and the implementation of landfill directives in several European countries, ash spreading has regained interest and more research effort has been attributed. Legislations have evolved, first in Scandinavia and then in other European countries such as the UK, Austria, , or France. More and more test projects are being carried out in Europe and even a technical expert group is working on the recovery rules for fertilising products at the European Union (EU, 2020).

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4 ASH STRATEGY: SOLVING THE ASH OFFTAKE ISSUE

4.1 Plant A In the initial financial model for both the plants, the ash offtake had been considered to have no financial impact on the project. Indeed, it was considered that the fertilising value of the ash waste would compensate for the handling costs. This was even considered a conservative hypothesis as the ashes could provide benefits for the projects by being sold as a fertiliser. In fact, ashes from virgin wood are known to contain high levels of potash as K2O and phosphorus as P2O5, which are demanded elements on the fertiliser market reaching values around £300-400/tonne, which would in theory help reaching financial net zero impact. Indeed, considering an ash product with typically 15% potash, and 5% phosphate and a typical application rate of 40 kg/ha for potash and 30 kg/ha (AIC, 2013), one would have enough ash to fertilise land the size of inner London with a plant this size. However, selling an ash waste as fertiliser is not possible without following a demanding and uncertain process to qualify the ash as a fertiliser product.

Fig.18: Blended ash at Plant A (Photo: the author) Some background work had already been carried out when I arrived at the managing company. The company had already worked with consultants to try and recycle the ash. Two outputs were identified: use of ash within bound industry and use of ash as an agricultural fertiliser. Both were to be reached by application to an End of Waste procedure as quality protocols already exist for these purposes. With the End of Waste criterion, the ash ceases to be a waste and obtains a status of product. It requires however 4 conditions (EU, 2008):  The substance or object is commonly used for specific purposes  There is an existing market or demand for the substance or object  The use is lawful (substance or object fulfils the technical requirements for the specific purposes and meets the existing legislation and standards applicable to products)  The use will not lead to overall adverse environmental or human health impacts. There are multiple example cases of ash spreading to land, ash used in cement raw industry, block or as an asphalt concrete filler. The market demand is present with public values of those materials. Whereas point 1 and 2 are straightforward to demonstrate, point 3 and 4 make the completion of the End of Waste procedure more challenging.

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Indeed, the demonstration of the innocuity of the product and its respect of the product's typical standards (here for example industrial fertilizer) demands an exhaustive set of tests carried out on a sufficient number of different samples (EA, 2016). The technical requirements on ash properties are very constraining and difficult to achieve. Moreover, the procedure requires the production of a thorough risk assessment report (EA, 2014). Consequently, it represents a significant risk for a rather small plant to invest in such a procedure with no guarantee of success. Besides, the financial outcomes need to be sufficient to cover the large test costs. This process would be more accessible for bigger projects or a consortium of plants with similar outputs. Though, even when considering similar ash products, the plants obviously lie in a competition situation, which renders collaboration difficult. It was first decided that the ash would be tested for the most relevant analysis parameters to be pre-emptively compared to threshold and typical values before applying for the End Of Waste criterion. A series of tests have been undertaken over a period of two years. Results were compared to reference levels for typical market products given by the Environmental Agency (EA) for liming material, gypsum, compost, peat, straw… The comparison showed that several trace metals contents were significantly higher than for industrial products, which made the perspective of direct End of Waste obtention difficult. When arriving at the company, the priority was to identify how other plants in the region manage their ashes. It was found that two plants were actually spreading ash to land and an entity providing ash recycling services. It was then identified that they actually spread their ashes through a specific procedure from the Scottish Environment Protection Agency (SEPA) and that an exemption existed and was feasible for our ashes. We were then put in relation with a farmer, who was already spreading ash on his land through this exemption and could guide our company through the application process and would be happy to spread the ashes.

4.1.1 SEPA Waste Exemption Process SEPA provides a legal framework for the application of ash to land. One must apply to an exemption of waste licensing to the Scottish Environment Protection Agency under the paragraph seven (SEPA, 2011). This exemption is only valid for ash resulting from the combustion of virgin fuel. The procedure shall guaranty that ash is of sufficient quality and is not spread on or near sensitive areas. Plant A only burns virgin timber. Besides its flue gas treatment system only includes an electromagnetic precipitator. No material is added. Therefore, its ashes fall directly under the waste code 10 01 01: “bottom ash, slag and boiler dust (excluding boiler dust mentioned in 10 01 04”, which is an absolute non-hazardous waste code. This facilitates greatly the procedures as it stands as a strong supportive evidence of the ash's innocuity and that the ash should be free from contaminants. Therefore, it was possible to apply for SEPA's waste exemption. The Paragraph 7 exemption includes several restrictions on the application to land. Per notification, one can spread maximum 1250 tons for the same farm, which can be stored, and on 50 hectares maximum. The exemption is valid for one year and must be renewed every year with reduced requirements. There is also a fee of £651 to pay to SEPA for the application review that drops to £217, when prolongating the exemption. It is also necessary to demonstrate that the spreading will not be hazardous neither for humans nor the environment. The applicant needs to supply a map of the whole property with indication of where the ash will be spread, any storage places,

-25- watercourses, springs, wells, work and live places, public rights of way, boreholes and archaeological sites close to the treatment area. It is also demanded to demonstrate the benefit to agriculture based on a soil analysis for each 10 ha on several given criteria such as nitrogen availability, pH, organic matter, water adsorption capacity. A folder detailing the benefit anticipated needs to be provided: it includes relevant information such as the application rate (in ton/ha), the current land use and vegetation present, the specification of land use and type of vegetation to be established, the incorporation depth of waste material (if the ash is going to be mechanically mixed with the soil) and information on the nutrient requirement of the vegetation and habitat. An assessment of the risk of pollution including a flood risk assessment (leakage possibility) if the field is located on a flood plain is also necessary. With regards to the blended ash, it is not in a powder form, so the application is less risky compared to plain fly ash in the presence of wind. Eventually, the applicant needs to furnish an analysis of the waste based on at least 3 sample tests, including, for an ash waste: moisture content, pH, proximate analysis, LOI, available nitrogen or ammonium (NH4-N) and nitrate nitrogen (NO3-N), total phosphorus, total potassium, total magnesium, TME (Cadmium, Copper, Chromium, Nickel, Lead, Zinc), physical contaminants and plastic contaminants. Even if a plant only burns virgin fuel, tests are still necessary. One must remember that virgin fuel already contains some trace metals elements and that contaminations could occur throughout the supply chain and combustion process. Though, with regards to spreading the ash on land or forestry, the amount of trace metals elements released is the same as what would have been released on the soil if the biomass had been left to rot. One must only ensure that the traces are sufficiently dispersed during the spreading operation as to not harm the environment. Soil analyses have been carried out on the two fields to be treated with Plant A's ash. As the soil analysis shows (Fig.19 below), the soil is in need for potash and phosphate treatment, whose concentrations in the plant's ashes are relatively high.

Fig.19: Summarized soil analysis results from one of the farms to be supplied with Plant A's blended ash (Soil Analysis report of selected farmland)

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4.1.2 Current Situation Once the application was complete, it took a few weeks for our file to be reviewed and accepted. Two permits were issued by SEPA and an ash disposal agreement was signed with the farmer. The ash is supplied to the farmer in small skips, rented by the plant and then taken over by the farmer who spreads it on two arable lands a few kilometres away from the plant. The exemption issued by SEPA includes several obligations: The plant must ensure that the ash is managed "without endangering human health and the environment". This includes "risks to water, air, soil plants or animals, causing nuisance through noise and odours and adversely affecting the countryside or places of special interest". To that purpose, it is compulsory to keep records about waste types and quantities transferred from the plant, stored, and applied for further notice to SEPA. Aside from the framework of the exemption, the plant has to comply with relevant obligations with regards to waste management in Scotland. Thanks to the new spreading process, costs have been drastically reduced. No landfill tax has to be paid anymore. The only costs concern the transport & spreading of the ash, and skip rental to the farmer. The cost of ash disposal has been divided by approximately 7. The current situation allows a local farmer to get a soil amendment product almost for free and to recycle the ash, while reducing operating costs of the plant.

4.2 Plant B The ash from Plant B necessitated a different approach. Indeed, the ash is of a different quality. The process being different, there were two main additional challenges to deal with: the flue gas treatment with the lime injection system and the presence of waste wood in the fuel mix. Besides, no significant work had been carried out before on the ash except some initial testing in 2017 just after plant commissioning and a strategy remained to be built. In order to find a solution, several discussions were engaged with our current ash services suppliers and other consultants in the form of a tender process. In fact, several different actors made proposals and plans to help us to achieve spreading to land but almost none of them was able to provide us with any similar former achievement. Indeed, we realised that, even though several parties tend to advertise it, no power plant in the UK had ever achieved End of Waste on their ashes yet. Those discussions enabled us to get a more precise idea of the different options available for the ash and to work out a plan to solve the current situation. The ash strategy was divided in three timespans: short-, medium- and long-term. The short-term objectives identified were the more precise qualification of the bottom- and fly-ash with testing to reclassify the two waste streams, as well as a reduction in logistics and transport costs. It was found that the reclassification would be the fastest way of reducing costs related to the ash by reducing the landfill tax rate on the fly ash product and simultaneously preparing the possibility to spread to land as current codes were labelled as hazardous. In parallel, a work on logistics and transport would allow to rapidly optimise the removal and allow for short term savings. It was then envisaged as medium-term objectives to spread the ash on land and thus being able to drastically reduce the costs associated with the ash offtake. Spreading to land would also be excellent practice towards reaching our long-term objectives.

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Finally, the long-term objectives were defined as reaching ash spreading to forests and the completion of the End Of Waste procedure in order to commercialise the ash as a fertiliser, thus eventually reaching financial net zero impact or positive value from the ash offtake. Spreading to forestry was identified as being harder to achieve as no specific framework currently exist. This would require adapting a framework directly with the environmental authorities, such as the creation of a research project. The strategy's articulation is detailed in Fig. 20.

Fig. 20: Ash Roadmap for Plant B's ash (Schematic: the author)

It is notable that the plant also rejects ash washings approximately once a year. Those are produced when ash installations are cleaned. As the ash washings represent around 3% in volume, 10% in cost and probably the same amount of work, they have been left aside in this strategy plan. The focus should be on the fly ash and bottom ash and redirected to the washings when there is no potential anymore.

4.2.1 Logistics and Transport Costs The ash is removed from the plant by an ash services supplier who manages the haulage to landfill locations. The processes are different for the bottom-ash and the fly ash (currently considered as APCr). Indeed, bottom ash being a bulky and granular product, it is transported in big articulated trucks with a capacity of 25 tonnes. The fly ash needs to be transported with a container truck due to its particulate aspect and handled carefully both when loading and unloading (see Fig.22). Those trucks have a smaller capacity. However, the volume rather than the mass is limiting the quantity of material transported.

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Fig.21: Plant B's Bottom-ash (Photo: the author)

Fig.22: Ash silo and container truck filling (Photo: the author)

Ash services cost represented around 70% of the total costs for ashes. The service is charged as a fixed "haulage cost" per load, a "disposal cost" per ton, and the landfill tax per ton different for the two ash types, as the removal processes are different and the loads are processed at two different landfill locations.

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Indeed, as the fly-ash/APCr is regarded as a hazardous waste, it needs to be handled at a specific site that possesses the relevant accreditation to landfill it. Two approaches of reducing the ash costs were evaluated: negotiating a better price from our ash services suppliers and gain on their current margin and ensuring we optimize the logistics of our ash removal on site weight wise and load wise. Concerning the logistics, an analysis of the truck loading was carried out. It was found that bottom- ash trucks were indeed close to truck capacity whereas fly-ash trucks were rather around 40% load. The maximum recorded was around 80%. Several reasons were identified. In fact, it is difficult to fill trucks properly as shown in Fig.23. The ash does not fill in the truck entirely. Loading it completely requires loading from several trapdoors, which requires a lot more time and resources.

Fig.23: Fly-ash container truck loading sketch using only the middle trapdoor (Schematic: the author)

Another point responsible for underloading of the trucks is the silo capacity. Indeed, it was found that it only represents around 120% of a truck loading capacity. There is currently only one ash silo on site. If the silo is full, the plant needs to be shut down and the economic losses are colossal. As a precaution measure, fly-ash trucks are booked in overabundance to avoid this situation. A second silo is built on site but is not commissioned yet. When available, it will help to significantly reduce the need for container trucks and increase the loading factor to eventually reduce overall ash costs.

4.2.2 European Waste Classification It was soon identified that the waste classification plays a significant role in the possibility whether to recycle ash. The first step to finding the relevant output for a waste is being able to define it properly. In the UK, it is the responsibility of the waste generator to qualify his own waste (EA; SEPA; NIEA; NRW, 2018). This work had been carried out in the past during plant commissioning. At that time, the plant was not operating as per design with frequent special tests, shutdowns, and start-ups, which would necessarily impact the quality of the ash. It had therefore been classified as "Bottom Ash - 19 01 11* : bottom ash and slag containing dangerous substances" and "Fly Ash – 19 01 13 : fly ash containing dangerous substances", which was then promptly switched to "Bottom ash – 10 01 14*: bottom ash, slag and boiler dust from co-incineration containing hazardous substances" and "Fly ash – 10 01 16*: fly ash from co-incineration containing hazardous substances" as a safe measure

-30- as long as the harmlessness of the ash had not been proven. The recent introduction of the clean recycled wood allowed moreover for a classification on design conditions. With such a hazardous code, recycling the ash was deemed impossible as one cannot spread dangerous waste on land or forest. Obviously, applying for spreading to land with hazardous waste codes would certainly lead to a refusal from the EA. There was therefore the necessity to demonstrate that the bottom- and fly-ash are not hazardous. In order to classify a waste, one needs to follow the Guidance on classification and assessment of waste WM3 (EA; SEPA; NIEA; NRW, 2018). The guidance explains how to classify a waste and identify its hazardous properties. The first step is to find the relevant code(s) from the European List of Waste, and if several codes can apply, whether the classification requires an assessment of the waste. The wastes should be classified with the code with the best fitting description.

There are three types of waste categories:  Absolute non-hazardous: the waste is classified as non-hazardous without any need for further testing and assessment  Mirror hazardous and mirror non-hazardous: those codes are called mirror entries. Those wastes require a complete assessment of hazardous properties to be classified.  Absolute hazardous: those wastes are automatically considered as hazardous

The waste category must first be chosen based on the industrial process producing the waste. The relevant category is 10: "Waste from thermal processes". The wastes then fall under 10 01: "wastes from power stations and other combustion wastes". Then, several codes can apply. The relevant extract is presented and discussed below in Fig.24. Two options were identified as relevant for both ash streams framed in Fig.24: the mirror entries 10 01 14*/10 01 15 for bottom-ash and 10 01 16*/10 01 17 for fly ash or the absolute non- hazardous entries 10 01 01 and 10 01 03. The relevant code for a co-incineration plant burning waste would be the mirror entries. However, Plant B is able to burn up to 10% of recycled wood but only of the highest standard: IED exempt material. This means that the plant only burns untreated wood and there is thus a strong argument for using the absolute non-hazardous code. There seem to be a kind of grey area for which the relevant code is not straightforward. Thus further work was needed to support one or the other classification. Even though the waste producer is responsible for the classification of its waste, it remains conditioned to the acceptance of other actors in the waste chain (haulier, handler, landfill owner…) and most importantly from the EA through audit processes.

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Fig.24: Extract from the list of waste relevant to Plant B's ash classification (EA; SEPA; NIEA; NRW, 2018)

4.2.3 Desulphurization and lime injection As a part of the flue gas treatment system, Plant B has the possibility to inject lime before the baghouse filter to control acidic components such as HCl, HF or NOx and SOx concentration levels (Emis, s.d.). This equipment is installed at Plant B due to the higher risk linked to the burning of waste wood. In fact, lime can be used both to limit the acidic gases emission levels and protect the baghouse filters from corrosion. Indeed: combustion of wood produces (as an example) sulfur trioxide gases (SO3) (British Lime Association, s.d.). In the flue gas system, those gases can cool down to the dew point temperature. They can then react with water steam as per reaction below to form sulfuric acid.

푆푂3 + 퐻2푂 => 퐻2푆푂4

By injecting dry hydrated lime also called slaked lime (Ca(OH)2), one can protect the filter bags and extend their lifetime. The lime reacts with the sulfuric acid following the neutralization reaction below to form calcium sulphate and water:

퐶푎(푂퐻)2 + 퐻2푆푂4 => 퐶푎푆푂4 + 2퐻2푂

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In continuous operation, a fly ash filter cake protects the filters. The problem being that this filter cake flows through when the plant stops operating. Practically, lime is only injected when restarting after an extended shutdown and when the installation is cold as there is a higher risk of Sulphur reaching its dew point temperature and damaging the installation. On the longer term, fly ash replaces the lime to form a protective filter cake and no more lime injection is needed. Firstly, lime is problematic for classification reasons. With regards to landfill tax, the lime injection supports the classification of the by-product as APCr, which is taxed under the higher tax bracket. However, fly ash benefits from an exemption and can be taxed under the lower tax bracket (Gov.uk, 2011). In addition, when aiming at reclassifying the ash, lime increases the corrosive and irritant aspects of the fly ash. Indeed, it raises the pH by providing even more active calcium oxides making the product more irritant and corrosive. As fly-ash is already a product with a very high pH, lime injection hinders the reclassification as absolute non-hazardous. Therefore, lime injection needs either to be limited or to be sorted out. Practically, this could mean to classify differently, the first few loads of fly ash after a lime injection. However, lime is used in very small quantities compared to the quantity of fly ash produced. Less than 1.5% of the residue's mass is lime. Therefore, there is a strong argument for classifying the whole as fly-ash. The presence of lime in fly-ash over time after an injection would probably need to be further studied but as a precaution measure, the first fly ash loads after an injection can be handled differently and sent to landfill. Given the high level of reactive calcium oxide in fly ash, the option of injecting fly ash during boiler start-ups had been envisaged. Indeed, the ash could be able to perform the action of lime, though bigger quantities could be needed. However, this was evaluated as a rather difficult thing to implement. It requires a collaboration with the EPC supplier of the plant and would currently rather be at a research level.

4.2.4 Ash testing and re-classification In the meantime, a consultant that already had achieved spreading of cement industry residues to agricultural land was contacted. They already hold a standard rule permit that allowed them to spread product under the waste code 10 01 01. This would require an application to the EA under the SR2010No4 permit. This is the English and Welsh equivalent to SEPA's paragraph 7 exemption and requires approximately 2 months. Spreading to land with the mirror entries would also be possible. However, this requires an application for a bespoke permit and an application for the SR2010No4. This procedure would last at least 6 months, would incur more costs (consultancy, application fees and operational) and risks as our consultant had never achieved it for ash spreading. The application would remain subject to the EA's acceptance.

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Thus, the objective was set to demonstrate that the ash is indeed similar to virgin fuel ash and can therefore be categorised as 10 01 01/03. Failing to demonstrate the absolute non-hazardous waste codes would shift the objective to obtaining the mirror non-hazardous codes through a complete hazard assessment. The outcomes of this re-classification report would enable our consultant to facilitate one of two different solutions for spreading ash to land. The re-classification would enable both to have sufficient supporting evidence to change the landfill tax rate to the lower tax bracket and move on to the application process for spreading to land.

4.2.5 Plant B ash characteristics As a first approach, a series of tests have been undertaken to determine the ash characteristics. Plant B's ash is particular due to the presence of IEDx in the fuel mix. The influence of this fuel intake needed to be properly quantified in order to support the reclassification. The ash has been tested for multiple elements concentration. A leachate analysis has also been carried out to evaluate the capacity of the ash to release such elements. With a fuel mix including mainly roundwood, the plant's ash falls well in the ash range described in part 3. The first results were very promising with trace metals level in general quite low, even lower than for the 2017 tests for some metals which could be explained by boiler coating scraping at the time. In case a complete hazard assessment would have been carried out, with a pH consistently higher than 11.5, the ash would have been classified as corrosive and possibly irritant. However, between 11.5 and 13.5 pH, in-vitro testing (on artificial skin) can be carried out to support the fact that the ash is non-hazardous. The assessment methodology works the same way for metal contents. They are always assumed to be present in the worst-case compound. If necessary, the tester can demonstrate that elements are present in non-hazardous form to a certain percentage and that the hazardous compound is below the limit. The complete hazardous properties assessment includes 16 categories (explosive, corrosive, ecotoxic, carcinogenic, infectious…) with consequent testing, which implies consequent costs. Following a discussion on the ash properties results, it was decided to do testing on the virgin and IEDx fuel. Indeed, it would make for a stronger point to demonstrate by comparison that the fuels are similar. It is especially interesting to focus on trace metal elements levels, which are the main concern for recycled waste wood and are a determining factor to assess whether the fuel can be considered as untreated. In fact, reclassifying as absolute non-hazardous in such a way is in better accordance with the classification and assessment of waste. Ash products are on the first hand defined by the fuel they result from and only if needed, hazard assessment testing on the ash helps settling what mirror code to use. A new series of samples representative of the virgin fuel were therefore sent for analysis. Results were compared to the IEDx fuel tests and to an ISO norm for a grade of untreated woodchips. The most sensitive elements’ levels are presented in Fig.25.

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Chlorine [w%] (dry) 100% Zinc [mg/kg] (dry) 90% Sulphur [w%] (dry) 80% 70% 60% 50% Nickel [mg/kg] (dry) 40% Nitrogen [w%] (dry) 30% 20% 10% 0% Mercury [mg/kg] (dry) Arsenic [mg/kg] (dry)

Lead [mg/kg] (dry) Cadmium [mg/kg] (dry)

Copper [mg/kg] (dry) Chromium [mg/kg] (dry)

Virgin Fuel average [%] IEDx average [%] Untreated Wood ISO norm limits [%]

Fig.25: Most sensitive elements’ levels for virgin fuel and IEDx as a percentage of the untreated ISO norm maximum limits

Even though elements levels are globally higher for the IEDx than for the virgin fuel, they remain under the limit for untreated wood set in the ISO norm. Some IEDx elements are actually higher than in reality as the limit of detection was used unaltered when determining the average ("<0.1" was considered as "0.1"). However, for some other elements, the level in the IEDx fuel is actually lower than for virgin fuel. This analysis enabled the redaction of a supporting report for the reclassification of the bottom- and fly-ash as absolute non-hazardous, which opened the gate to the possibility of spreading to land.

4.2.6 Spreading to land Due to plant different design, it was not possible to spread fly ash under code 10 01 01 as at Plant A or to blend materials as it is not allowed to blend wastes after production as per design to achieve another classification. Therefore, fly ash would need to be treated separately and under a bespoke permit to be spread to land. For larger granules, a concern arose with regards to the spreading on land, because the spreader could be damaged as well as vehicles on nearby roads being hit from those ash projectiles.

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Therefore, there was the need for screening or crushing to reduce the material size and avoid dangerous big particles. Two solutions envisaged are presented in Fig. 26. Power plants usually possess a mobile plant, that can be used for crushing the bottom ash to finer parts with a crusher bucket mounted on it. Then material can be directly crushed into skips for offtake. The other solution considered is a nibbler mounted directly in the plant streamline, which can be easier to operate as no changing of tool is necessary. Although crushing is not absolutely necessary, it helps improving the quality of the material and acceptance of the farmers.

Fig. 26: A crusher bucket mounted on a mobile plant (left) (Worsley Plant, s.d.), an on- line nibbler (right) (Gericke Ltd, s.d.) Besides, handling of fly ash poses a challenge due to its powdery aspect. Ash conditioners used to mix it with water exist on the market but can represent a significant cost and cause some issues for operation and maintenance. Indeed, cement form material is formed when mixing fly ash with water that necessitate fine tuning and maintenance, even with a specialised apparatus. On top of creating a need for more man power, ash conditioners can be costly devices (Korpilahti, 2003). A fly ash conditioner is shown in Fig. 27. A solution could be to mutualize ash from different power plants. Treating the material at another location could reduce stress on the operator but increases costs linked to transport and handling of the material.

Fig. 27: A fly ash conditioner (MBE EWB, s.d.)

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5 DISCUSSION & FURTHER WORK The ash alternative offtake process is not over at the time of redaction of this document. Indeed, several improvements can still be made such as reaching spreading to forestry or selling the ash as a fertiliser at Plant A and starting application to land at Plant B. However, a few remarks can be observed on the process followed yet and further work and possibilities.

5.1 Plant A The ash situation has been considerably improved at Plant A. When ash used to be landfilled for a total cost of around £280,000/yr, it is now spread to land for less than 20% of the original cost. The fields where spreading is carried out are located a few miles away from the plants, so costs and pollution from transport are now minimized. A local farmer beneficiates from free fertiliser and liming agent, having only the spreading at his own costs. Economically, this obviously pushes the ash topic to the background, although further progress could be made. With regards to sustainability, it could make more sense to spread the ash on forestry where the is sourced to reduce the risk of long-term acidification and nutrients deficiency. However, there is currently no clear framework for applying ash to forestry except for ecological benefit but not in the context of forestry harvesting activities (SEPA, 2011). The initial financial plan was to sell the ash for its fertilising value. Although this could be theoretically achieved through an End Of Waste application, it remains highly uncertain and the upfront costs are too consequent. Indeed, quality protocols tests are very demanding. It requires a great deal of tests on a multitude of samples, some of which are quite expensive. Sometimes, further testing could be required. This could bring the testing bill to a point where even commercialisation could not break-even with the invested amount. As the project is still relatively young, it is recommended to acquire more information and confidence about the ash product through land spreading before attempting any further procedures. For some reason, the word seemed to be spread amongst local foresters that ash would be detrimental for forestry due to copper contamination. Although this can in fact happen, it is less probable for ashes from a plant burning virgin biomass. There seem to be no strong demand from forest growers for ash from biomass power plants. This is probably due to little awareness about its fertilising potential but also to the fact that its benefits on growth are constrained to specific cases (peaty soil, PK deficiency). However, more and more research are published on the topic with a lot of relay in national forestry entities (Pitman, 2006). Forests around the power plant A are mostly comprised of Scots pine, Sitka spruce and birch on a peaty soil which would be, according to 3.2.2.1, ideal for ash spreading fertilising effects.

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5.2 Plant B Notable improvements have been achieved with regards to the ash at plant B as well. Indeed, the reclassification process completion allowed for a tax rate switch on fly-ash, that reduces the yearly cost of fly ash of around 40% and enabled starting the application for spreading to land. Nevertheless, the most crucial and difficult step was in the identification of the opportunities and the establishment of a strategy. It is essential to properly determine the ash product one is dealing with and the range of action available within the legal framework. Application process is currently under completion at Plant B. Challenges will lie in the sourcing of sufficient land area to spread the bottom and fly ash, obtaining a bespoke permit on fly ash by demonstrating innocuity, and solve challenges linked to fly ash handling and conditioning. The permit for bottom-ash should cause no major issue as well as management after the implementation of a crushing solution. Fig. 28 shows the status at the writing time of this document in the context of the strategy.

Fig. 28: Project Status on the Ash Roadmap (Diagram: the author)

5.2.1 Ash reclassification The ash reclassification process demanded special care as several codes were possible creating a grey area for the appropriate waste code to choose. Lifting the ambiguity and classifying the ash correctly required to understand the functioning of the classification process. This process is firstly based on the waste production process (source and type and waste testing analysis is only carry out in cases where it is judged necessary. Those waste codes are then crucial in the achievement of ash spreading. This method means that the list of waste must include all necessary details first-hand to properly judge the possibility of spreading. For instance, ash from virgin biomass plant is automatically classified as absolute non-hazardous even though it would probably be classified as irritant and corrosive under the full hazard assessment. While the List of Waste is updated regularly, it is a historic tool that has been developed since the seventies and sometimes adapt poorly to recycling.

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While exemption processes tend to give a superseding opportunity for recycling, they still rely heavily on waste codes, which can strongly limit opportunities. A different and more pragmatic approach giving more importance to waste properties while including a wider variety of non- hazardous and mirror codes, could allow for more efficient recycling of the ash.

5.2.2 Fuel Intake Evolution Plant B is on the verge of importing fuel from other European countries. If extended rapidly and massively, this could have an impact on the ash quality. The importation incurs several unknowns with regards to fuel quality and contamination as standards could be different in other countries and transport incurs more unknowns. Generally, new fuel intake always represents a risk and the new fuel will need to be more closely monitored especially with regards to pollutants both for maintaining the ash quality and protecting the plant. The plant could also burn energy crops in a near future, such as miscanthus. Miscanthus is rich in lignocellulosic biomass which makes it energetically interesting for combustion. It is also rich in Silicon, Chlorine and Potassium (Boulday & Marcovecchio, 2016). Miscanthus ash seems to be less subject to heavy metals contamination than wood-ash (Lanzerstorfer, 2019). Whereas the potash could be interesting for application to land, chlorine and silicon can be a higher concern for boiler slagging.

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6 CONCLUSIONS Notable progress was achieved for both CHP plants in this case study, both economically and with regards to sustainability. In total, annual savings of around 50% have been achieved, which represents around £300,000/yr. This means consequent savings for both projects. Further savings of circa £150,000/yr can be achieved by spreading ash from Plant B onto land. In parallel, sustainability has been increased with a more local output at Plant A, i.e. reduced transport, and benefits for a local farmer as free liming and fertiliser. At Plant B, the reclassification of the ash as non-hazardous enabled to simplify handling and reduce the stress on hazardous material landfill. Ash is a very variable product depending on the production process parameters. Its high pH makes it a good liming agent and the fact that it concentrates the nutrients present in the fuel intake makes it a good fertiliser. However, ash also concentrates potential contaminants and adequate testing is therefore necessary when attempting to spread ash on agricultural land. Besides, it appears to be common practice to condition both ash types before being spread which can represent deterring costs and a consequent challenge especially for the powdery fly ash. Due to its variability, each ash recycling process is different and present different challenges to solve. The case study shows that attempts at achieving spreading to land fall within a specific context depending on the national framework, local practices, and experience of chain actors. Even though research is showing more evidence of ash spreading benefits on agricultural land and forestry, waste remains a very regulated sector, subject to significative lobbying. Therefore, it is crucial for companies to identify the available opportunities. As ash properties and classification depend principally on the plant's design such as the fuel intake and flue gas treatment system, a special care at those parameters during design phase can ease recycling of the ash during operation. Although the European End of Waste procedure is quite often promoted, it remains a very difficult status to obtain, with no known holder in the ash waste sector yet. The process demands notable testing investment that are hardly recovered. Despite common reluctance of plant operators to share information about their ash output, there is an opportunity for power plants producing comparable ash to group to facilitate the recycling process. Waste frameworks beneficiate currently from renewed interest at the European level (Selin & VanDeVeer, 2015; EU, 2020). While new directives could change the current orientation on the ash waste, national environmental entities have an opportunity to seize in rationalizing ash recycling processes to allow for both a more sustainable biomass energy sector and satisfaction of liming and fertilising needs.

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REFERENCES

Agrela, F. et al., 2019. Biomass fly ash and biomass bottom ash. Agriland, 2019. How much does lime cost?. [Online] Available at: https://www.agriland.ie/farming-news/mustering-muscle-for-colourful-bandon- tractor-run/ AIC, 2013. Fertiliser Statistics. Andreas Aronsson, K. & Ekelund, N. G., 2004. Biological effects of wood ash application to forest and aquatic ecosystems. Anon., n.d. Berresheim, H., Wine, P. & Davies, D., 1995. Sulphur in the atmosphere. Blake, L., 2005. Acid rain and soil acidification. Boulday, D. & Marcovecchio, F., 2016. Valorisation des cendres issues de la combustion de biomasse, s.l.: s.n. British Lime Association, n.d. Flue Gas Treatment, s.l.: s.n. EA; SEPA; NIEA; NRW, 2018. Guidance on the classification and assesment of waste - Technical guidance WM3, s.l.: s.n. EA, 2014. End-of-waste and by-product hazard and risk assessment, s.l.: s.n. EA, 2016. Material comparators for end-of-waste decisions. Volume Part 6. EA, 2017. Waste wood, s.l.: s.n. Emis, n.d. Dry lime injection, s.l.: s.n. Etiégni, L. & Campbell, A., 1990. Physical and Chemical Characteristics of wood ash, s.l.: s.n. EU, 2008. End of Waste Criteria. EU, 2017. Industrial Emissions Directive - Article 3.31 (b) (v), s.l.: s.n. EU, 2020. STRUBIAS Project. Eyre-Walker, G., 2018. Groundsure-The reducing landfill capacity in the UK. [Online] Available at: https://www.groundsure.com/resources/the-reducing-landfill-capacity-in-the-uk/). Gazey, P., n.d. Soilquality.org.au. [Online]. Gericke Ltd, n.d. Gericke. [Online] Available at: https://www.gerickegroup.com/nibbler-nbs-nb Goulding, K., Jarvis, S. & Whitmore, A., 2007. Optimizing nutrient management for farm systems. Gov.uk, 2011. Landfill Tax-Group 5_Order 2011 4.2, s.l.: s.n.

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GOV.UK, 2020. GOV.UK - Landfill Tax. [Online] Available at: https://www.gov.uk/government/publications/changes-to-landfill-tax-rates-from- 1-april-2020/changes-to-landfill-tax-rates-from-1-april-2020 Haglund, N., 2008. Guideline for classification of ash from solid biofuels and peat utilised for recycling and fertilizing in forestry and agriculture, s.l.: s.n. Hansen, K., Stupak, I., Ring, E. & Raulund-rasmussen, K., 2011. The impact of forest management on soil quality. Joensuu, S. & Berge, H. t., 2017. Valerie Project. Karltun, E., Ingerslev, M., Mandre, M. & Varnagiryte-Kabasinskiene, I., 2008. Wood ash recycling - Possibilities and risks, s.l.: s.n. Korpilahti, A., 2003. Tuhkan esikäsittely metsäkäyttöä varten. Lanzerstorfer, C., 2019. Combustion of Miscanthus: composition of the ash by particle size. MBE EWB, n.d. [Online] Available at: http://www.ewb.hu/unloaders.html Misra, M. K., Ragland, K. W. & Baker, A. J., 1993. Wood ash composition as a function of furnace temperature. Moilanen, M., Hytönen, J., Hökkä, H. & Ahtikoski, A., 2015. Fertilization increased growth of Scots pine and financial performance of forest management in a drained peatland in Finland, s.l.: s.n. Mortensen, L., 2019. Implications of wood ash application on soil decomposer food webs. Nisbet, T. R., 2014. Forestry and surface water acidification. s.l.:s.n. Palviainen, M. & Finér, L., 2011. Estimation of nutrients removal in stem-only and whole-tree harvesting of Scots pine, Norway spruce, and birch stands with generalized nutrient equations. Pitman, R., 2006. Wood ash use in forestry - a review of the environmental impacts. Probio Energy, n.d. Probio Energy. [Online] Available at: https://probio-energy.com/recycled-wood/c-grade-recycled-wood/ [Accessed 2020]. Risse, M. & Gaskin, J., 2013. Best management practices for wood ash as agricultural soil amendment, s.l.: s.n. Roques, S., Kendall, S., Smith, K. & Newell Price, P., 2013. Review of the non-NPKS nutrient requirements of UK cereals and oilseed, s.l.: s.n. Saarsalmi, A., Kukkola, M., Moilanen, M. & Arola, M., 2006. Long-term effects of ash and N fertilization on stand growth, tree nutrient status and soil chemistry in a Scots pine stand. Saarsalmi, A., Mälkönen, E. & Piirainen, S., 2001. Effects of wood ash fertilization on forest soil chemical properties, s.l.: s.n. Selin, H. & VanDeVeer, S. D., 2015. EU Environmental PolicyMaking and Implementation: Changing Processes and Mixed Outcomes. s.l.:s.n. SEPA, 2011. Paragraph 7 Exemption, s.l.: s.n. Skogsstyrelsen, 2002. Recommandations for the extraction of forest fuel and compensation fertilising, s.l.: s.n.

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Slattery, W., Conyers, M. & Aitken, R., 1999. Soil analysis: an interpretation manual. Stockholm Exergi, 2019. Ny design på aska ger mer växtkraft till skogen. [Online] Available at: https://www.stockholmexergi.se/nyheter/ny-design-pa-aska-ger-mer-vaxtkraft-till- skogen/ Vamvuka, D. & Kakaras, E., 2011. Ash properties and environmental impact of various biomass and coal fuels and their blends. Fuel Processing Technology. Vance, E. D., 1996. Land Application of Wood-Fired and Combination Boiler Ashes: An Overview. Worsley Plant, n.d. [Online] Available at: https://www.worsleyplant.co.uk/products/crushing-buckets/

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APPENDIX 1

Fig. 29: Detailed description of the 4 grades of waste wood

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