Biorefining trends – Potential and challenges in the Kraft pulp mills
Marcelo Hamaguchi: Valmet, Brazil, [email protected] Joakim Autio: Sonja Enestam, Krister Sjoblom, Henrik Wallmo
Abstract The feasibility of biorefineries depends on many parameters, such as feedstock prices, biomass availability, biomass quality, process efficiency, electricity price, national incentives, plant scale and market development for the new products. For a number of years, research in the field of kraft pulp has focused on maximizing pulping efficiency, improving pulp quality and optimizing the chemical recovery cycle but today, the ability to convert biomass and residues into multiple products seems to have higher priority. Currently, traditional conversion of biomass into heat and pulp are being complemented by the development of technologies to improve the mill profitability, which range from replacing fossil with renewable fuels to the implementation of biorefineries, providing, therefore, an opportunity to mitigate climate change and meet the growing demands for energy, fuel and chemicals. This trend can also be seen in the recent literature on biorefining, where considerable effort has been invested in research related to biomass and waste conversion. Some of the novel processes entering the market are lignin extraction from black liquor and bio-oil generation through biomass pyrolysis. In the sugar platform, the main focus has shifted to the fractionation of biomass via hydrolysis to obtain value added products from e.g. glucose and xylose. This paper gives an overview of current trends in the biorefining market, challenges and future research needs.
Keywords: kraft pulping; biorefining; black liquor; biomass fractionation
Introduction The kraft process is a successful and dominant method for producing pulp today, reaching over 130 million tons of bleached and unbleached kraft pulp in the world. An important expansion phase of the kraft pulping industry can be attributed to the development of fast growing tree plantations and subsequent reduction in the costs of raw- material. It started with South-East Asia in the 1990s and has been now occurring in South American countries such as Brazil, Chile and Uruguay. These pulp mills are now in a good position to benefit significantly by implementing alternative biomass conversion technologies. The diminishing supply of resources is becoming a serious constraint on global development and welfare. Society and industry are being driven to find more eco-efficient raw materials and sources of energy, as well as to enhance the efficiency of existing processes. At the same time, bio-based markets are expected to expand, paving the way for cutting-edge technologies, products, and production systems: novel eco-efficient gaseous, liquid, and solid fuels as well as chemicals and materials are being discovered. As biomaterials are introduced into the market, there is a great potential for e.g. lignin-based products, chemicals, nanocellulose fibers, composites, and bio- plastics. It is estimated that by 2030, there will be several entirely new value chains. In this emerging business, it is vital to manage the entire life cycle of a product, from purchasing raw materials to end use and recycling. However, in spite of the potential of implementing the concept of biorefining, some challenges have to be overcome. Biorefinery players still have to prove some of their technologies, establish large scale facilities and convince customers that the green products are as good as the conventional ones and do not cost more. The aim of this paper is to discuss different routes for biomass conversion and discuss their potentials, current status as well as challenges faced by Brazilian pulp mills.
Alternative routes Different routes can be integrated into the kraft pulp mills (Figure 1). The choice will depend on several factors such as customer needs, investment cost and biomass availability. The impacts on the conventional operation of kraft pulping are also important and have to be carefully studied. This paper discuss three integration alternatives: lignin removal from black liquor, biomass fractionation through hydrolysis and fast pyrolysis of ______7th International Colloquium on Eucalyptus Pulp, May 26-29, 2015. Vitória, Espirito Santo, Brazil. biomass to produce bio-oil.
Wood Woodhandling Fiber Processing line Pulp biomass drying extracted chips tall oil esterification, black liquor biodiesel white pre-hydrolysis, hydrogenation liquefaction liquor hemicellulose extraction Evaporation Recovery Recausticizing pyrolysis Boiler lignin removal conversion lime cycle SE, torrefaction heat, Steam Lime Kiln bio-oil chemicals, lignin power turbine pelletizing biofuels gasification green liquor Auxiliary heat, gasification fossil biofuel power boilers fuels
upgrading synthesis / catalytic upgrading traditional pulp mill heat, alternative technologies transport power DME,CH4,H2, fuels ethanol,FT-fuel
Figure 1. Alternative biorefining technologies in kraft pulp mills
Biomass fractionation The first challenge is to reduce the recalcitrance of the biomass, which means that a pre-hydrolysis of the feedstock is required. The main goal of this first step, also called pretreatment, is to hydrolyze the hemicellulose and disrupt the crystalline structure of cellulose, so that the acids or enzymes can easily access and further hydrolyze the cellulose. This step can be the most expensive process in biomass fractionation but it has great potential for improvements in efficiency and lowering of costs through further R&D. Different requirements have to be fulfilled in order to design the most suitable pretreatment. According to Kumar et al. (2009), the process has to avoid the degradation of carbohydrates, improve the formation of sugars, minimize the formation of inhibitors to the subsequent processes, and be cost-effective. Although several technology players have been putting efforts to fulfil these requirements, the ideal process will probably never exist. Suitable process conditions will depend primarily on the feedstock type and quality. The majority of pretreatment processes being applied today are catalyzed by acid. In this method, most of the lignin stay in the solid phase, contrary to alkaline hydrolysis or organosolv (Figure 2), where delignification also takes place. Performing biomass auto-hydrolysis is also possible, which means that the process is catalyzed by the acetic acid formed from the liberated acetyl groups in the hemicelluloses. The auto-hydrolysis method can be alternatively followed by steam explosion. By adding a small amount of mineral acid, e.g. sulfuric acid, the hydrolysis reactions can be speeded up and is typically applied when softwood (usually less acetylated) is used as a raw material. Steam explosion is a common method to pretreat lignocellulosic materials. It typically initiates at a temperature of 180-210ºC (15-20 bar) for several seconds to a few minutes before the material is exposed to atmospheric pressure. The process is held for a period of time to promote hemicellulose hydrolysis and it is terminated by an explosive decompression (Kumar 2009). Figure 3 shows the results of a rapid pre-hydrolysis test using softwood fines. The biomass was subjected to treatment at 15 bar (from 30 to 210s) in Valmet’s pilot plant. The biomass is disintegrated into fine particles and hemicellulose extraction yield achieved > 85%. It is important to point out that softwood is typically more difficult to hydrolyze than hardwood, which can be attributed to e.g. differences in lignin structure and amount of carried acetyl groups.
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The main component in hardwood hemicellulose is glucuronoxylan, in which bonds between xylose units can be easily hydrolyzed. Therefore, the hemicellulose sugars can be separated from the solid phase after the pre-hydrolysis. This also avoid further degradation of xylose. On the other hand, liberation and dissolution of glucose from the cellulose require more severe conditions in chemical hydrolysis or the application of enzymatic hydrolysis. Irrespective of the different hydrolysis methods, lignin is obtained as a solid stream at the end. The lignin can be incinerated for producing steam or possibly refined for more valuable applications. The latter seems to be an interesting option to attract positive returns on investment. One example of lignin route being studied today is the catalytic hydrogenolysis (e.g. MOGHI technology) for the production of chemical and fuels.
pre- Biomass separation hydrolysis Glucose treatment
Ethanol/water, Pre-treatment catalyst liquor hemicellulose solvent hydrolysis, sugars recovery separation Lignin
Figure 2. Sugar platform based on organossolv process
An interesting source for extracting sugars for further refining is the pre-hydrolysis in the production of dissolving pulp. Currently, in the dissolving pulp mills, the hemicellulose sugars end up in the recovery boiler. If extracted before the cooking step (Figure 4), it brings the opportunity to use the hemicellulose sugars for different applications. One option is to convert them into glycols, primarily mono ethylene glycol (MEG), which in turn can be used in the production of polyethylene terephthalate (PET). The pentose sugars in the hydrolyzate can be also converted to lactic acid through a chemical process and further polymerized to poly lactic acid (PLA). PLA can be used for instance in food packaging including rigid containers, shrink wrap as well as mulch films and rubbish bags.
Figure 3. Fractionation of softwood fines through steam explosion
As stated earlier, xylose is usually the most abundant monomeric sugar present in the hydrolyzate from hardwood (e.g. eucalyptus). The other comparatively small amounts of monomeric sugars include arabinose, glucose, mannose and galactose. Compounds such as acetic acid, furan furfural (further degraded to formic acid), HMF (further degraded to levulinic acid) are inevitably generated during the e.g. acid hydrolysis process. One drawback is that these by-products result in the reduction of fermentable sugars yields. Methods for detoxification have been presented by Jönsson et al. (2013), who have also stated that the generation of by-products is strongly dependent on the feedstock and pretreatment method.
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Fiber raw Pre- Extraction Cooking Dissolving material hydrolysis Separation Bleaching pulp
Hemicellulose sugars Chemical recovery
On-site Conversion refining
Glycols Bioplastics, polymers, PET
Lactic acid PLA
Figure 4. Opportunities of C5 extraction in dissolving pulp mills
Sugars extraction: potential and challenges Several technology players are directing their attention to biomass fractionation for further production of chemicals, fuels and biomaterials. Although front-end equipment has achieved commercial applications (e.g. Valmet pretreatment for 1200 t/d of dry biomass), complete conversion processes (from feedstock to final product) have been performed today mostly at pilot and demonstration scales. Therefore, in spite of the market potential for sugars based products from biomass, the desired technology maturity with reasonable CAPEX has not been fully achieved. Performing feasibility studies is also not an easy task, since many potential processes under development are well protected. This makes the biomass fractionation technologies difficult to be assessed. For the traditional pulp mills that produce kraft pulp (excluding dissolving pulp), there are alternatives that could be possibly implemented to extract hemicellulose sugars:
Hydrolysis of wood chips: additional wood is required for a dedicated sugar platform, leading to an increase in the operational and logistic costs. On the hand, it can become viable if the whole biomass is efficiently utilized, with the further generation of high value products. Pulp wood has several advantages that includes well-developed logistics, low ash content, availability the year around and foreseeable cost. Examples of bio-based products from glucose include sorbitol (used in the food industry) and adipic acid for the production of nylon.
Hydrolysis of biomass waste: part of the waste stream can be hydrolyzed for hemicelluloses extraction prior to combustion. One of the drawbacks is that such technique would lower the power production in the mill, either by reducing the calorific value or increasing the moisture content of the extracted solid fuel. In spite of that, Lima et al. (2013) showed that xylose content in eucalypts bark can reach 15% wt (dry) after hot water treatment used to remove extractives and soluble sugars. By applying catalyst such as mineral acid, it was possible to extract most of the xylose from the feedstock.
Partial hydrolysis of chips prior to pulping: It is a challenging option since it brings impacts to the pulping process and pulp properties. There are a lot of investigations published on this topic but the risks are reasonably high considering that the core product in the mill can be affected.
Xylose from black liquor: In this process (XiviaTM by Dupont), xylose already in a hydrolyzed form can be removed from the black liquor. The main purpose is to covert the sugar into xylitol, a product that can be used as a naturally occurring sweetener. The process involves hydrogenation of xylose followed by separation and evaporation to yield crystallized xylitol. Today this xylitol in crystals can be found for 8 USD/kg.
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Case: hemicellulose-lean biomass to the boiler The objective here is to present a magnitude of impact on the power generation when a 12000 t/a xylose plant is integrated into one eucalyptus kraft pulp mill. In order to minimize the negative effects, it is assumed that part of the biomass waste, composed of bark, is pre-treated using hot water catalyzed by acid. The existing boiler has the capacity of burning 500 t/d of dry biomass waste, contributing to the total mill power generation of 142 MWe. The conditions for the integration are based on results by Lima et al. (2013) and also on internal reports.
Water/acid Cel: 45% Cel: 54.5% Hem: 25% Hem: 8.5% Lig: 27% Biomass Pretreatment and pretreated Lig: 35.5% Others: 3% waste Separation biomass Others: 1.5% Dry flow: 250 t/d Dry flow: 182 t/d Moisture: 45% Moisture: 60% HV: 17 MJ/kg (dry) HV: 18.1 MJ/kg (dry)
Steam from boiler: 104 t/h Xylose (olig + mon): 35 t/d Steam from boiler: 85 t/h Mill power generation: 142 MW Mill power generation: 137 MW
Figure 5. Example of hemicellulose extraction and impacts on the power generation
The value of 35 t/d of xylose should compensate, at least, for the plant operational costs and for the electricity that will no longer be sold to the grid. Taking this into account, and assuming an electricity sales price of 70 USD/MWh, the value of the extracted pentose sugars is expected to be ≥ USD 350 per ton. The feasibility of the xylose plant will depend, therefore, on the integrated manufacturing of high added value products. Opportunities being explored today, in addition to what were already mentioned, include the production of farnesene (e.g. blend in jet fuel), isoprene, xylitol and levulinic acid.
Lignin extraction from black liquor A portion of the total black liquor stream in the pulp mills can be withdrawn and treated using e.g. the LignoBoost process. As a result, kraft lignin is produced and lignin lean black liquor is sent back to the cycle for burning in the recovery boiler. Since the performance of the recovery boiler is often limited by the heat load, the black liquor throughput cannot be increased. Lignin accounts for about 35% of the total dry solids of a black liquor, and it is also the main contributor to the heating value. Consequently, when lignin is removed from black liquor prior to combustion, the organic dry solids flow and the heat load to the recovery boiler decreases. This means that the total dry solids flow to the recovery boiler can be increased correspondingly. There are opportunities for profits among the numerous potential applications of high quality lignin. These fall into three main categories: energy, materials, and chemicals. The high heating value of lignin (~26 MJ/kgDS) makes it suitable for use as a fuel. It can be used internally in a bark boiler or in lime kilns; or it can be sold in the form of pellets or powder. Lignin is easily dewatered to 65% dry solids content in the process, and the amount of energy required to dry lignin to >95% DS is less than 20% of the amount required to dry forest fuel. LignoBoost lignin extraction process involves a stand-alone plant that can produce highly purified lignin (<1 wt% ash content) while using a relatively small amount of wash water. The plant is built as a separate unit and can be integrated with the mill during a planned shutdown, having little, or even no, impact on the pulping process while under construction. The process begins with the precipitation step, in which black liquor taken during the evaporation step at about 40% DS is acidified using carbon dioxide gas. As the gas is absorbed by the black liquor, the solubility of lignin decreases when the pH drops. Precipitation is followed by the separation step, in which the lignin is dewatered to about 65% DS. The solid-liquid separation is performed using a pressure filter. Washing the impurities from the lignin at this stage would lead to significant filter plugging, low through flow of the wash liquor, high residual sodium content, and high water consumption. To overcome these obstacles, the unwashed lignin cake is dissolved in a re-suspension step with spent wash water from the downstream washing step. The resulting slurry is acidified with sulfuric acid to lower the pH to the same level as that in the washing step. A large proportion
______7th International Colloquium on Eucalyptus Pulp, May 26-29, 2015. Vitória, Espirito Santo, Brazil. of the remaining sodium (about 60%) is transferred from the lignin to the water phase. The re-slurried lignin is once again dewatered using a pressure filter and finally washed with low pH wash water. The filtrate is sent back for evaporation, while the spent wash water is circulated back to the re-slurrying step.
Lignin valorization: Potential and challenges The next horizon for Valmet is the open issue of further refining the lignin product to enhance its value. The availability of specialized lignin samples might support efforts to increase the market for lignin and its potential applications. Furthermore, producing high purity lignin (≥99 wt% lignin content) and specific fractions could the key to expanding the market for lignin as a precursor to green chemicals and a replacement for a wide range of products currently made from crude oil. To point out the potential of kraft lignin market, Figure 6 shows the increasing burning capacity of Brazilian black liquor along the years. Removal of only 5% of the lignin from the total black liquor represents more than half a million tons of lignin that can be delivered to the market every year.
Figure 6. Potential of lignin extraction in Brazil based on total black liquor dry solids
Lignin has been of great interest to the chemical industry due to its phenolic molecular structure. Therefore, the number of R&D projects has increased rapidly in recent years. Many companies are seeking renewable raw materials for the production of chemicals, including benzene, toluene, and xylene (BTX). Lignin could also be used as a green precursor for the synthesis of bio-plastics and vanillin, and there have already been successful trials of the production of carbon fibers from lignin (Figure 7). There are also several ongoing projects whose goal is the breaking of the lignin molecule through hydrotreating processes to produce motor fuels. From this perspective, initiatives such as exporting lignin as fuel or selling it for further refining into green products provide an excellent opportunity to begin transforming the conventional pulp mill into a multiproduct biorefinery. Lignin is an amorphous and heterogeneous aromatic polymer that is present in large amounts in plant cells and wood tissues. The lower reactivity and high steric hindrance effects, caused by its branched structure, limits the direct use of lignin for the replacement of petroleum-based chemicals and materials (Huang, 2014). Chemical modifications or conversions of lignin are then needed. In lignin depolymerization, the main goal is to convert the complex lignin compound into small and more reactive molecules for further applications. The process is very challenging, given the high stability of lignin bonds. Pyrolysis, catalytic hydrogenolysis, chemical oxidation, hydrolysis under supercritical conditions and biological route are examples of depolymerization methods being studied. It is important to point out that the oxygen content in lignin needs to be lowered in order to obtain more valuable products such as motor fuels or building blocks for the chemical industry. The feasibility of deoxygenation process, however, is still a challenge being faced today by the biorefining industry. Currently, lignosulfonates from sulfite pulping is the mostly used type of lignin to produce a variety of value- added products such as dispersing, detergents, stabilizers, binders and surfactants. The lignosulfonate is highly cross-linked and have sulfonate and phenolic hydroxyl group due to the extended lignin chains formed in the process of the sulfite delignification (Doherty et al., 2011), which makes the lignin from this process water-soluble. Kraft lignin has different chemical properties. There are no sulfonate groups present, so kraft lignin is more soluble in alkaline solution. Therefore, it can be precipitated from black liquor by lowering the pH, which is the operational ______7th International Colloquium on Eucalyptus Pulp, May 26-29, 2015. Vitória, Espirito Santo, Brazil. principle of e.g. LignoBoost process. The lignin can be also produced through biomass fractionation. However, in many applications, the lignin is recovered only after the fermentation step (e.g. in the production of ethanol 2G). Post-fermentation recovery of lignin is currently used for producing heat and power because of the presence of deconstruction enzymes and fermentation components that would require additional purification for higher value uses (Ragauskas et al., 2014).
More challenging sugars fractionated lignin kraft lignin lignosulphonates Depolymerization thermal Syringaldehyde Fractionation biological Phenolic resins e.g. organosolv thermochemical Vanilin etanol 2G
Kraft pulping with Carbonization and Activated Biomass lignin extraction activation carbon
Spinning Sulphite pulping Stabilization Carbon Carbonization fibers Surface treatment
Figure 7. Examples of valorization routes for the lignin
Vanillin and syringaldehyde: Sugar free lignosulfonates from softwoods serve as starting material to make e.g. vanillin. Today, approximately 99% (20000 t/a) of vanilla sensation comes from man-made version of vanillin (Bomgardner, 2014). The vanillin is mainly produced from a petrochemical raw material called guaiacol, which is prepared by methylation of catechol and then converted to vanillin. Solvay is one example of company that produces vanillin via catechol-guaiacol route. Hardwood lignin also yields vanillin, but it is contaminated with a similar compound called syringaldehyde which contains an additional methoxyl group (Harkin, 1969). Because they are difficult to separate economically, hardwood lignosulfonates are hardly used as a source of vanillin. On the other hand, Pinto et al. (2012) stated that the production of syringaldehyde (with antioxidant and antimicrobial properties that can be used e.g. in pharmaceutical applications) and vanillin from eucalyptus lignin is possible through oxidation in alkaline medium. In spite of the possibility of producing vanillin from lignin, the low yield can make the process not attractive for kraft lignin at short-term. In addition, the concentration of vanillin needed for flavoring is so small that market potential might be limited.
Phenols: Many methods have been developed for the production of phenols due to their commercial applicability. The dominant current route involves the partial oxidation of cumene, which is an organic compound based on an aromatic hydrocarbon with an aliphatic substitution. It is a constituent of crude oil and refined fuels. The major uses of phenol, consuming around two thirds of its production, involve its conversion to precursors to plastics. Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many herbicides and pharmaceutical drugs. The applicability depends, however, on the type of lignin. For example, the most prevalent degradation reactions occurring during kraft pulping include the cleavage of aryl ether bonds which can result in higher phenol content in lignin.
Carbon Fiber (CF): The US Department of Energy suggests that a 10% reduction in the vehicle’s weight can improve fuel economy by 6-8%. Today more than 90% of the carbon fibers in the market are made from oil derived materials, e.g. polyacrylnitrile (PAN), which is a polymerized form of acrylonitrile (ACN). The potential of lignin is large because it is possible to use it as a starting feedstock to produce bio-ACN. However, in order to enable the economic use of carbon fiber composites in vehicles, fiber production will have to increase and fiber price decrease to <7USD/kg (Christopher, 2013). Challenges for low cost CF from lignin include e.g. impurities and lignin molecular weight polydispersity. According to Ragauskas et al. (2014), lignin-based carbon fibers currently have poor mechanical properties compared with PAN- based fibers. In spite of the potential, they state that new chemical modifications of lignin and/or innovative ______7th International Colloquium on Eucalyptus Pulp, May 26-29, 2015. Vitória, Espirito Santo, Brazil.
biosynthesis strategies are needed to produce linear-fiber-forming lignin, with controlled monomer ratios and chemical architectures. These challenges show that we are some years away from making lignin based CF commercially available.
Activated Carbon (AC): There are basically two routes for the production of activated carbon: physical and chemical activation. The former consists of two steps (carbonization followed by activation), while in the latter, the steps take place simultaneously. The AC contain pores in different ranges (micropores, mesopores and macropores) and the proportions vary according to the raw material and activation method applied. For example, a coconut shell have predominance of pores in the micropores range, which can be ideal for the applications involving low contaminant concentrations. Hayashi et al. (2000) have demonstrated that pores structures of AC from lignin vary depending on the activating reagent (e.g. ZnCl2, H3PO4 and KOH) and carbonization temperature. This means that different applications can be explored through chemical activation. Physical routes usually require higher temperature for activating the material but they seem to be more environmentally friendly considering that no chemicals are used.
Case: lignin-lean black liquor to the recovery boiler The objective is to present a magnitude of impacts when a 25000 t/a lignin plant is integrated into a eucalyptus kraft pulp mill. The pulp production is assumed to remain unchanged. Since approximately 5% of the total lignin is being extracted from the black liquor, the impacts on the existing mill are not expected to be high.
Table 1. Changes after the installation of a 25000 t/a LignoBoost plant Unit Current mill data With LignoBoost Recovery boiler steam generation kg/s 188.0 181.1 Mill power generation MW 141.7 135.6 Black liquor heating value MJ/kg 14.0 13.8 Black liquor dry solids to boiler t/d 4315 4240 Estimated CO2 consumption t/d - 17.0 Estimated H2SO4 consumption t/d - 15.5
The price of lignin should at least compensate for the operational costs (e.g. chemicals consumption) and for the electricity that will no longer be exported. This means that prices are expected to be ≥ 400 USD per ton of lignin in order to make the plant more attractive. It is important to point out that the feasibility of lignin extraction plant is strongly influenced by the electricity sales price, which is assumed as 70 USD/MWh in this case.
Bio-oil via integrated pyrolysis and combustion Fast pyrolysis bio-oil can be used today as a substitute for fuel oil in firing applications. However, further development is also underway. In a fast pyrolysis process, dried wood chips are rapidly heated to about 500°C in a once-through fluidized bed reactor in an oxygen-free atmosphere. In the reactor, the wood is decomposed into organic vapors, gases, and char. In the Valmet integrated process, the heat for pyrolysis is obtained by taking 800°C sand from the adjacent fluidized bed boiler; the cooled sand and char are separated from the gas stream and returned to the boiler. The gas and vapors from the reactor are fed into a scrubber, where the vapors and some of the gases are condensed into bio-oil. The non-condensable gases are burned in the boiler or used as fluidizing media in the reactor. The non-condensable gases and char are used in the fluidized bed boiler for high pressure steam production. The concept of the integrated bio-oil production plant was developed within a consortium comprising Valmet, Fortum, UPM and VTT. Since the early 1990s, VTT has conducted extensive research using their laboratory-scale process development unit. As well, Valmet built a seven-ton per day integrated pyrolysis pilot in the Tampere R&D Centre. After this pilot phase, Fortum placed an order to build a 30 MW oil throughput plant in Joensuu, corresponding to 50 000 tons of annual bio-oil production, to be integrated into an existing bubbling fluidized bed boiler. This demonstration plant was completed in late 2013, and until now several thousands of tons of bio-oil have been produced.
Biomass to bio-oil: Future and challenges The operation of the pyrolysis process in Joensuu has been consistent. However, some equipment has required improvements, as is typical for the first version of a larger scale unit. The plant has reached its nominal ______7th International Colloquium on Eucalyptus Pulp, May 26-29, 2015. Vitória, Espirito Santo, Brazil. capacity, despite some scale-up issues. Attempts to achieve continuous production at nominal capacity are ongoing, and test runs are being carried out to optimize operation and study the effects of varying the process parameters and feedstock properties. One probable challenge for pulp mills is the restricted use of bark to produce bio-oil. The residue contains higher amount of extractives and alkali metal salts when compared to bark-free wood. Since mineral salts are known to catalyze thermal decomposition reactions, their presence can result in accelerated conversion rates, lower temperature at which pyrolytic decomposition proceeds, decrease in the bio-oil molecular weight, and in negative impacts on the product yields. Therefore, bark-free wood or residues such as wood fines/debris are preferred raw-materials. On the other hand, the steam production in the mill would not be reduced after the integration of the pyrolysis plant into the biomass boiler. Ongoing work for Valmet involves process optimization and assessing process features in continuous industrial-scale operation. The combustion of the bio-oil will continue to be investigated, while methods for upgrading it are pursued. Together with its partners, Valmet is involved in developing new technology routes through catalytic upgrading of pyrolysis oil. The goal is to achieve a quality of oil that is compatible for further refining to transportation fuels or intermediate products. Bio-oils usually have higher density, corrosiveness and viscosity compared to fuel-oil, while the sulfur and nitrogen content is usually lower. The high oxygen content of bio-oils can explain the lower heating value, immiscibility with conventional fuels, chemical instability and higher viscosity. In order to increase the H/C atomic ratio, the bio-oil needs to be upgraded, which can be carried out mainly through two different routes. The first one is the catalytic hydrotreating, usually known as hydrodeoxygenation (HDO). The catalysts for the reaction are traditional hydrodesulphurization (HDS) catalysts, such as Co–MoS2 /Al2O3, or metal catalysts (e.g. Pd/C). The second route is catalytic upgrading, where e.g. zeolites, like HZSM-5, are used as catalysts for the deoxygenation reaction (Mortensen et al., 2011). This latter process is carried out without external hydrogen sources, and therefore the resulting oil has lower heating value and H/C than conventional fuels. Therefore, in spite of the more challenging process conditions, HDO appears to have a great potential to obtain high grade oils which are compatible with the already available infrastructure for fossil fuels. Some issues that still have to be fully addressed regarding HDO-oil production include better understanding of the kinetics, a sustainable source of hydrogen and the carbon forming mechanisms.
Discussion Technology players have been making extensive efforts to increase the availability and efficiencies of biorefining equipment. At the same time, pulp mills are looking for the best way to create more value from the wood raw material. One essential question is how to find the most suitable business model considering that pulp mills do not have experience in producing or commercializing fuel or chemicals. In the same way, managing high volumes of biomass feedstock is not so simple for the chemical industry. As a consequence, strategic partnerships usually come into discussion to further arise the concept of biorefinery. It is important to point out that establishing a joint project is not straightforward and requires a reasonable amount of legal work that includes e.g. cost sharing, intellectual property and risk responsibilities. Furthermore, some of these cooperation agreements end up with the involvement of a third-part, usually responsible for developing the technology concept and machinery. One clear target for biorefineries is to gradually replace fossil based products, which should be achieved by introducing greener options into the market. It is also known that the integration of innovative technologies at industrial scale involve all kind of risks, which also explains why incentives from governments to mills, farmers or technology providers are seen as essential. Support can be through e.g. R&D direct funding, oil taxation, loan guarantees or development of current and new infrastructure throughout the country. It is important to emphasize here that it is not only legislation but also people’s choice and attitude towards more sustainable economy that will lead to an increasing demand for bio-based products in the near future. Companies are investigating products carrying the best value for money, taking into account that bio-based chemicals, fuels and materials can compete for the same feedstock. Today, many partnerships are being established but still without a clear promise that a revolutionary green product will be introduced to the market at short-term. On the other hand, once such product is found, and the market further developed, higher volumes can be produced at lower prices. This motivates technology players to keep exploring different biorefining routes and applications using different types of feedstock. It is also an opportunity for biorefineries to become more and more attractive and cost-competitive.
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Conclusions Pulp mills are in a very good position to explore the concept of biorefining. Pulp wood has several advantages that includes well-developed logistics, low ash content, availability the year around and foreseeable cost. Since minimizing possible impacts on the existing operation is of great importance, the use of biomass waste also brings several advantages. At the same time, biorefining processes have been reaching good levels of maturity. Examples include lignin extraction from black liquor, pretreatment of biomass and fast pyrolysis for the production of bio-oil. However, full scale conversion processes, from feedstock to valuable products, are still underway to become more reliably attractive. For that, extensive investments in R&D have been made by pulp mills, technology suppliers, research centers and universities. It is well known that valorization routes for lignin, sugars and bio-oil are indispensable steps for biorefineries to achieve payback more rapidly. Nevertheless, it is important to point out that the feasibility of these new technologies will depend on other factors such as electricity sales price, biomass availability, plant scale and process efficiency. There is also the need for consistent government policies and people’s attitude towards a more sustainable economy.
References
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Acknowledgements Contribution from the Valmet Biotech team is greatly appreciated.
______7th International Colloquium on Eucalyptus Pulp, May 26-29, 2015. Vitória, Espirito Santo, Brazil.