A short critical review for a Local Authority on the applicability of ‘advanced thermal technologies’ for treating MSW-derived plastic waste streams.
This review is intended to act as an overview for Local Authorities (LAs) wishing to explore the potential for ‘Advanced Thermal Technologies’ (ATT) to deal with the plastic fraction of a municipal solid waste (MSW) stream. It will briefly outline the technologies available; the environmental impact of ATT when utilising plastic as a fuel; the regulatory and emission controls issues to be dealt with; the regulatory and legislative issues to bear in mind and the cost of each option.
Advanced Thermal Technologies.
Whereas incineration is an exothermic chemical reaction based on full oxidation of the fuel materials, the suite of treatment options known as ATT are endothermic reactions – i.e., they require heat from an external source (Arena 2011), and they rely on heavily controlled oxygen conditions in order to produce various by-products. The three most common technologies are: gasification; pyrolysis and plasma gasification, although there is increasing interest in the production of fuel and fuel oils through the thermal depolymerisation of plastics. Each of these is explained below:
Gasification:
The gasification of waste material occurs at temperatures between 500 and 1400oC (Morrin, et al, 2010) and pressures up to 40 bars. The supply of oxygen is managed, in so far as it is controlled to allow a partial combustion of fuel, but not full combustion. If one considers the fire triangle (Fig 1), which stipulates that each of the three elements of the triangle must be present to produce a fire, then a reduction of any of the elements will stifle it.
Figure 1, The Fire Triangle.
Following this rule, the oxygen allowed into the combustion area is managed so that the waste cannot create a fire, despite its flammability and despite it being subjected to such high temperatures. Instead, the waste decomposes due to the heat within the reaction chamber and releases a synthetic gas which is produced from the constituent elements of the waste material (syngas). This syngas, consisting mostly of CO, CO2, H2 and N2 (Ricaud 2011) has a calorific value in the region of 5 to 10 MJ/Nm3 (Defra 2007) and can then be collected and subsequently combusted in the presence of oxygen, thus providing heat energy. The other major product of this process is an ash, which generally retains any heavy metals present in the fuel and persistent organic compounds. First exhibited in the UK by Energos on the Isle of Wight through Defra’s New Technology Demonstrator Programme (Defra 2003), there are numerous demonstrator project in the UK, and now 2 major modular gasification plants reaching the commissioning stage in Plymouth (O-Gen) and Avonmouth (New Earth Solutions).
Figure 2. 1MW gasification reactor (ITI, Sheffield).
Pyrolysis:
Pyrolysis works in a similar manner to gasification, but operates in a total absence of oxygen, rather than a managed amount, and can operate at much lower pressures and temperatures of 1.5 bar and 300-800oC. It will produce a syngas but of a higher density, and consequently it’s CV is generally double that of a gasified syngas. In addition, it produces a carbon rich ‘char’ (residue) and a tar-like oil.
Figure 3. Typical pyrolysis feedstocks and outputs
The respective levels of these 3 outputs vary according to the speed of the process and good management of the pyrolyser can lead to either high gas, high char or high oil. It therefore depends upon the desired output as to how the pyrolyser is run, as an oil to be refined into a fuel product is produced from the same waste input material, but in a very different manner to gas for usage in driving a steam cycle. Harper Adams University College in Shropshire has, in conjunction with Aston University, recently developed a 1MW pyrolyser whose early tests show great promise at utilising a number of plastic polymer fuels, specifically polypropylene (PP) and polyethylene (PE).
Figure 4. Harper Adams’ pyrolysis reactor.
Plasma Gasification:
Plasma is a stream of superheated ions (similar to a lightning bolt) which can be used to gasify waste in temperatures of over 15000oC. At these temperatures, a high amount of syngas is released, along with a metal product containing the melted metallic atoms and a ‘slag’ product containing any other remnant materials. The advantage of this process is that the gaseous product is extremely clean due to the high temperatures involved in the gasification, which is high enough to destroy hazardous elements such as dioxins, pesticides and PCBs (European IPPC Bureau, 2006). Unfortunately, this comes at a high price, as the energy overhead required for the production of the plasma stream can mean a high parasitic load (i.e., it uses much of the energy it creates).
Environmental Impact.
The environmental impact of diverting waste from landfill comes primarily from the minimisation of carbon release due to decomposition of waste in anaerobic landfill conditions. However, if that carbon is then released through the emission stack, it could be argued that there’s little, if any, carbon benefit from using the waste as fuel. Indeed, a WRAP (2008) study suggested that from 3 scenarios studied, only an (unachievable) 100% efficiency CHP pyrolysis plant, displacing a coal fired alternative would have any environmental benefit. Indeed, considering that plastic waste streams are derived from oil based, and therefore fossil carbon based sources, it is easily argued that the plastic waste stream is environmentally better diverted back into the ground (i.e., landfilled) rather than being released into the atmosphere. This way, the fossil carbon is retained within the earth’s crust rather than being atmospherically released. It is argued that with plastic waste streams, recycling is always the better environmental option then fuel usage. Some plastic waste streams (LDPE film, for example) are notoriously difficult to recycle, due to contamination and lack of source segregation, making recycling a hard to realise proposition.
Emissions and Emission Control.
Whenever waste materials are used as a fuel product, the issue of emissions is inevitably raised, not least by the anti-incineration lobby. The heavy metals and persistent organic compounds which are said to be produced and released are a great risk to public health and are allowed to be released in an uncontrolled manner. In fact, nothing could be further from the truth, as any process which utilised waste materials as fuel must comply with the Waste Incineration Directive (2000). These stringent rules are designed to prevent or limit any negative effects on the environment (WRAP, 2012). In particular, it looks to achieve significant levels of environmental and health protection through the administration of emission limit values which far exceed those imposed on traditional coal fired power stations. Although there are exceptions to WID applicability (e.g., vegetable waste, ‘untreated’ wood waste, cork waste), the imposition of the limit values when utilising waste materials should ensure a safe level of emissions from any ATT plant operating within the Directive.
Regulatory and Legislative issues.
In addition to the WID, there are other regulatory issues to bear in mind when attempting to set up and run an ATT plant, not least of which is the requirement for an Environmental Permit (EPR, 2010). Environmental permits are the overarching mechanism for regulating energy from waste facilities and fall into 2 categories: Environment Agency (EA) permits and Local Authority (LA) permits. Any facility proposing to use more than one tonne per hour of fuel will require an EA permit, those using less can instead work with the (in theory simpler) LA permit. The permit will have conditions attached which must be followed to prevent any processing activity from damaging the environment or human health. The permit itself will dictate what fuel feedstocks are allowed, the calorific value of fuel, how the material is to be stored, the chemical composition of the fuel, emissions to the atmosphere (and how they comply with the WID) and certain process controls. Strict monitoring systems are set out, and any breach of the allowances will see the permit revoked and a subsequent shut down of the plant.
Cost.
Costs of ATT plant are always difficult to ascertain, partly due to commercial sensitivities and partly due to a constantly evolving market place. Primary research by the author has highlighted 4 gasification plant systems which claim to be able to fully utilise the MSW plastic waste stream, ranging in capital expenditure (capex) from £1m (40,000 tpa) to £17m (80,000 tpa). Taking only the £1m example, if an LA with a plastic waste stream of 100,000 tpa was hoping to utilise this technology to manage its plastics entirely, it would need to invest £2.5m on the plant alone. Added to this would be land cost, building cost, planning, due diligence and consultancy costs, before ever considering collection, transport and operating expenditure. With an expected life time of 10 years, this equates to a capex cost of £250,000 per year, and possibly a similar amount in opex. With a landfill cost at the current rate of £72 landfill tax, £20 gate fee and maybe £10 transport (around £100 per tonne), 100,000 tpa would cost £1m per year in disposal to landfill. This would represent a reduction to 50% of previous costs for management of the plastic waste stream alone. Any such business plan would require, of course, a much more detailed breakdown than offered here.
Summary
The technologies generally described as Advanced Thermal Treatments have had a difficult evolution over the past few decades, with many examples failing to live up to their early promise. Many European states have attempted to introduce them as part of the national waste strategies, but have walked away following disappointing results. Japan, on the other hand, has embraced the idea of gasification, especially, and now has 40 plant up and running. Recent technological developments however, have helped to overcome some of the perceived barriers associated with ATT and the UK is now seeing a number of new plant in development, due in part to the government’s incentive programme offering greater incentives for renewable power generation. With an apparent reduction in cost for LAs to utilise this waste stream as opposed to disposing of it, it can be argued that ATT to manage plastic waste is economically viable. However, environmentally it could still be argued that plastic waste is better off in landfill. Maybe a reduction in landfill tax for unrecyclable plastics would avoid the fossil carbon based within it becoming inevitably released into the atmosphere, and ATT could then focus their efforts more on renewable biomass materials instead.
References
Advanced Plasma Power (2011) Advanced Plasma Power – The energy from waste solution: Technology Overview, available from www.advancedplsmapower.com/technology.aspx
Arena, U. (2011). Gasification: An alternative solution for waste treatment with energy recovery. Waste Management, 31(3), 405-406
DEFRA (2003) New Technologies Demonstrator Programme – Summary and Key Findings, available from http://archive.defra.gov.uk/environment/waste/residual/newtech/demo/documents/TAC-Summary.pdf
DEFRA (2007) Advanced Thermal Treatment of Municipal Solid Waste
Environment Agency (2002) Solid Residues from Municipal Waste Incinerators in England and Wales, available from http://www.environment-agency.gov.uk/business/sectors/133342.aspx
European IPPC Bureau (2006) Reference Document on the Best Available Techniques for Waste Incineration. European Comission.
Mininni, G., De Stafanis, P., Barni, E, Chirone, R, and Urciuolo, M (2008) New Technologies for MSW Thermal Treatment: The state of the art, available from http://www.acs.enea.it/documentazione/editoria/11.pdf
Morrin, S., Lettieri, P, Mazzei, L and Chapman, C (2010) Assessment of Fluid Bed + Plasma gasification for energy conversion from solid waste. In: CISA, Environmental Sanitary Engineering Centre. Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste. Venice, Italy.
Ricaud, Anne-Lise (2011) Practical and Economic Viability of Small-scale Energy from Waste. Imperial College London, London.
Williams, P.T., (1994) Pollutants from Incineration: An Overview IN Hester R.E. and Harrison R.M, Issues in Environmental Science and Technology – Waste Incineration and the Environment. The Royal Society of Chemistry, Letchworth.
WRAP (2008) LCA of CHP incineration for disposal of mixed waste. Available from www.wrap.org.uk/mixed_plastics
WRAP (2012) Energy from Waste Development Guidance. WRAP. Available from www.wrap.org.uk/efw
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