Solid Energy's Beneficial Use of Waste Streams
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WASTE OR RESOURCE: SOLID ENERGY’S BENEFICIAL USE OF WASTE STREAMS Paul Weber1, Joe Wildy1, Fiona Crombie1, William Olds1, Phil Rossiter1, Mark Pizey1, Nathan Thompson2, Paul Comeskey2, Dave Stone2, Andrew Simcock3, Andy Matheson1, Karen Adair4, Mark Christison5, Hayden Mason6, Mark Milke7 1-Solid Energy New Zealand Ltd; 2-Stockton Alliance New Zealand Ltd; 3-Biodiesel New Zealand Ltd; 4-Bio-Protection Research Centre, Lincoln University; 5-Christchurch City Council; 6-Holcim New Zealand Ltd; 7-Natural and Civil Engineering School, University of Canterbury Correspondence to: [email protected] Abstract In the last decade Solid Energy New Zealand Ltd - in collaboration with its research, applied technology and business partners - has made considerable progress in identifying waste resource streams which can be beneficially reused to support the state owned enterprise’s business objectives. Within the company’s coal business, Christchurch City’s high-quality biosolids are being used as a topsoil supplement in mine site rehabilitation. Coal ash from boilers is being used to create a capping material which reduces the formation of acid mine drainage. Cement kiln dust, a by-product of cement-manufacturing, is being used for waste rock capping to reduce acid mine drainage, and as a binding agent to fill and make safe underground voids prior to mining. Waste mussel shell is being used to create sulphate- reducing bioreactors to lower acidity and metals loads in mine water. Within Solid Energy’s renewable energy companies (Nature’s Flame and Biodiesel New Zealand) - approximately 150,000 tonnes of wood pellet fuel has been manufactured from untreated plantation-grown pinewood offcuts, shavings, and sawdust; 6,000 tonnes of used cooking oil has been collected for conversion into biodiesel. Used engine oils from mining fleet operations and other suppliers are recycled and sold for boiler fuel. Biodiesel glycerol, a by-product of biodiesel production, is used to boost the creation of methane by anaerobic digestion at Christchurch City Council’s wastewater treatment plant. The gas is then converted into electricity and used for commercial heating purposes. Finally, municipal biosolids, in part dried by methane derived from anaerobic digestion boosted by biodiesel glycerol, has been successfully trialled as added-value fertiliser for biofuel crops. 1 Introduction Solid Energy extracts, processes, markets and distributes coking, thermal and specialist coal from underground and opencast mines at Huntly in Waikato, Greymouth, and Westport on the West Coast, and in Southland. More than half of annual output is sold for export to major international steel industry customers. In New Zealand, Solid Energy’s customers include New Zealand Steel’s Glenbrook Mill and Huntly Power Station and industries such as dairy and meat processing, cement making, and the timber industry. Solid Energy businesses Nature’s Flame and Biodiesel New Zealand (BDNZ) have been developing bioenergy opportunities. BDNZ produces its high-quality Biogold™ fuel from used vegetable oil and oilseed rape grown locally as a break crop. Nature’s Flame is New Zealand’s leading producer of clean-burning wood pellet fuel. In the last decade Solid Energy New Zealand -- in collaboration with its research, applied technology and business partners -- has made considerable progress in identifying waste resource streams which can be beneficially reused to support the state owned enterprise’s business objectives, including the objective of having a net positive effect on the New Zealand environment. One component of this is the diversion for beneficial use at mine sites of bulk waste streams that might otherwise go to landfill. This results in a string of benefits. Scarce and increasingly expensive landfill space is preserved, extending the life of these facilities; the waste resource often substitutes for a raw, primary resource; the waste generator reduces their disposal costs; and the system reduces Solid Energy’s environmental impact. This paper discusses these projects. Biosolids for mine site rehabilitation Field trials using biosolids were begun in collaboration with local authorities at Stockton Mine (Christchurch City Council) and Rotowaro Mine (Watercare, Hamilton City Council) with the aim of developing successful, cost-effective systems for rehabilitating disturbed sites. From 2007 - 2011 field trials were undertaken to evaluate one-off dose rates of biosolids incorporated into the soil matrix prior to traditional rehabilitation (where the final landform is shaped, overlain with a topsoil medium, and then planted). The outcome demonstrated that a one-off dose was the most suitable approach at Stockton mine site and Solid Energy 2 and Christchurch City Council (CCC) are now actively engaged in a project which, when fully operational, will result in the beneficial reuse of up to 6,000 tonnes of dry biosolids each year. This involved significant collaboration between the parties and CCC has commissioned a $30M thermal drying system for the biosolids, which reduces transport rates and produces a more manageable product for Stockton. At Rotowaro trials proved that full-scale application was unsuitable due to operational issues. From 2009 to 2011 field trials were undertaken at Rotowaro mine to evaluate low-dose rate repeat surface applications of biosolids at application rates equivalent to the (textbook) agronomic requirement for nitrogen (200 kg N/ha.yr) and at application rates equivalent to twice the agronomic requirement for nitrogen (400 kg N/ha.yr). In August 2012 Solid Energy applied for resource consents for operational use of biosolids at Rotowaro mine and plan to start applying biosolids operationally from summer 2012/2013. Solid Energy has worked from a premise that any use of biosolids (at Rotowaro and Stockton) should not compromise future land use options and thus the soil contaminant ceiling limits for metals and organic wastewater contaminants are below agricultural limits set by the Waikato District Council and have been taken in part from the accepted NZWWA Guidelines (2003); Canadian Environmental Quality Guidelines (CCME); USEPA Region 9 PRGs (Residential soils); and the Petroleum Guidelines (1999). Nitrogen leaching is a significant issue in Waikato and thus biosolids application rates at Rotowaro are based on nitrogen loading and subsequent nitrogen loss (per Ha) rather than potential contaminants. Results from the 2009 - 2010 trials (200 kg N/ha.yr) demonstrated that the rate of nitrogen loss per unit area amounted to 5.6 kg N/ha.yr. Results from the 2010 - 2011 trials (400 kg N/ha.yr) demonstrated that the rate of nitrogen loss per unit area amounted to 5.8 kg N/ha.yr. These rates are well below the 30 kg N/ha.yr ceiling level applied to the site (this ceiling limit being typical of a dry stock farming unit in the area). Figure 1 shows nitrogen loss from Waipuna area at Rotowaro mine site into Te Whā stream and shows that immediately following the application periods there were minor spikes in the nitrogen discharge, although not significantly above the background levels. 3 Figure 1. Nitrate nitrogen concentrations in site discharge to Te Wha Stream 2009 to 2012. Pink and red lines indicate 200 kg N/ha.yr application rates; purple and blue lines indicate 400 kg N/ha.yr. Figure 2: Stockton Mine biosolids plots (month 10). Right ~300 dt/ha of biosolids. Left is the control plot which received a standard rate of inorganic fertiliser but no biosolids. To date ~2,000 dry tonnes of biosolids have been used for rehabilitation at Stockton and 2,600 wet tonnes have been used at Rotowaro for rehabilitation. Significant beneficial outcomes for all parties involved in this project are expected over the next few years. Coal ash for preventing acid mine drainage Coal combustion produces ash (clinker and fly ash) is derived from minerals present in the coal such as clay and alumina-silicates, and un-burnt coal. Typically coal ash is 5-8 wt% of the coal burnt and this material is often disposed to landfill. Solid Energy has resource consents to beneficially reuse coal ash at Stockton Mine to manage acid mine drainage 4 (AMD). AMD is generated by the oxidation of pyrite and the subsequent release of acidity and metals (Equation 1). Further details are available (e.g., Elder et al., 2011; Weber et al., 2008). Essentially, however, if oxygen is excluded, the production of AMD stops. 7 15 FeS2 + /2H2O + /4O2 Fe(OH)3 + 2H2SO4 (1) Management, monitoring, and regulatory compliance standards for the coal ash resource consent were based on the Pennsylvania Department of Environmental Protection, Residual Waste Management Regulations (25 PA Code Chapter 287) 1992, and the West Virginia Department of Environmental Protection (DEP) guidelines1. Typically, compacted fly ash has low permeabilities (10-7 m/s or lower), which means that it is difficult for water and oxygen to penetrate, thus making it an ideal covering material for potentially acid-forming material. Research has demonstrated that oxygen can be excluded by placing fly ash in 300mm layers over acid-forming rock. An additional benefit of using ash is that it has a moderate acid neutralisation capacity (ANC) ranging from 24 - 350 kg H2SO4/t (Table 1), which can also neutralise acid generated in the underlying rock. This is also reflected in the alkaline paste pH of the material (Table 1). Table 1. Paste pH and ANC results for different products used on site for AMD control. Sample Paste pH ANC (kg H2SO4/t) Various aglime products (<2mm) 8.4 - 10.8 747 - 966 CKD 11 - 13.9 479 - 788 Coal ash 5.96 - 11.96 24.5 – 354.2 Acid forming sandstone control 3.3 <1 The standard method for capping potentially acid-forming (PAF) waste rock overburden areas at Stockton to exclude oxygen and water is to use 400mm layers of compacted granite to achieve permeability of ~10-6 m/s. This material is then covered by 400mm of topsoil.