Pyrolysis & Gasification
There are a wide variety of thermal treatment systems incorporating 'advanced' or 'emerging' technologies for the treatment of municipal wastes. The most prevalent being Pyrolysis and Gasification processes.
Pyrolysis, often incorporating gasification, is a thermal process where organic materials in the waste are broken down under pressure and in the absence of oxygen. The process works best when the input waste is carbon-rich, preferably sorted or pre-sorted. Best results are obtained from single stream wastes such as sewage sludge, plastics, wood, tyres, or agricultural wastes. Where MSW is to be used it should be pre-sorted to remove the majority of the non-organics and processed to homogenise the feedstock. The Pyrolysis process produces a liquid residue and gaseous output which may be combusted to generate electricity. A solid slag is also produced which may require disposal or additional processing.
Schematic of Inputs and Outputs of a typical Pyrolysis process
Gasification usually operates at a higher temperature range to pyrolysis, with the addition of an oxidant (either air or oxygen) and the output from a pyrolysis plant may be fed into this process. Gasification of organic derived wastes will produce a gas which can be combusted to generate electricity and a char which usually requires disposal if no markets are available.
Schematic of Inputs and Outputs of a typical Gasification process
Theses processes are still at the development stage, with no full scale plants operating commercially in the UK. European experience is limited and mixed.
Typical capacity: Typically 20,000 tpa – 100,000 tpa (tend to be modular, so larger capacities may be achieved through multiples of facilities on one or more sites). One plant is however 225,000 tpa in Karlsruhe, Germany.
Land requirements: Typically 0.5 – 1.75Ha for a 50,000 – 60,000tpa sized plant. 6Ha for 225,000t facility in Germany
Capital costs: £(See full report) for 100,000tpa plant (process dependent).
Operating costs: Limited data available. Example from one system shows gate fee of (See full report))/ tonne for RDF and processed MSW for a 60Ktpa plant, if supplemented by higher gate fee materials such as clinical waste.
Staffing requirements: Staffing levels, including technical competence, management and administrative resources will required and will vary depending on the size of the facility. Typically 30 – 40 employees for plants of 60,000 – 200,000tpa
Strengths and Weaknesses of Pyrolysis & Gasification Processes
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Strengths
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Weaknesses
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Not Incineration!
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May suffer from the same negative perception as incineration, some evidence of this overseas, yet to be tested in the UK
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Qualifies for the Renewables Obligation for a substantial proportion of the feedstock processed
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Requires extensive pre-treatment to be able to handle MSW
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Efficient electricity generation through combustion of gas through engines
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Many processes will still have residues to be disposed of, some of which (from flue gas treatment) will be hazardous in nature
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Potential to recycle a large proportion of residues depending on the process
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Unproven on a commercial scale on MSW in the UK, patchy experience overseas
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High temperatures may make the system more flexible for other waste streams such as clinical
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More sensitive system than moving grate incineration technology
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Smaller units more acceptable and part of an integrated system
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More expensive (in terms of gate fee) than Energy from Waste
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Capable of being integrated with other processes such as the output from MBT / Refuse Derived Fuel (RDF) production
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Other Issues
Key issues with regards to these 'advanced' thermal treatment technologies are:
- their relatively unproven performance on MSW;
- whether they will be perceived as 'incineration by another name'; and issues of cost and commercial application of the systems in the UK waste management market.
There is considerable potential for these systems, if proven reliable, to act as components in integrated municipal waste management. They may be complementary in processing specific 'problematic' waste streams or to link in with other pre-processing MSW facilities such as MBT, RDF manufacture or steam reformation processes.
Their smaller scale implementation as envisaged to date, makes them compliant with the proximity principle and should help in public perception and in gaining planning permission over larger facilities. The efficient energy production aspects of the processes and the eligibility for Renewables Obligations Certificates are also positive characteristics of these facilities.
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