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160 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Plasma Gasification Plasma gasification is the gasification of matter in an oxygen-free environment to decompose waste material into its basic molecular structure. It does not rely on incineration but converts organic waste into a fuel gas and inorganic waste into an inert vitrified glass. In Japan the biggest industrial unit of its kind operates at the Eco-Valley site at Utashinai, Hokkaido plasma facility, built by Hitachi Metals. It began processing MSW in 2003 with a capacity of approximately 200-280 tons per day. 293 It uses 8 Marc-3a plasma systems with torches that are rated for 300 KW, operating at 2 gasification islands, each with 4 torches. The system delivers 1.5MW of net electricity output to the grid, 294 although the facility wasn’t designed with maximum power output in mind. If it had been, the predicted output would be 12MW. 295 A second plasma gasification system operates in Japan, the Mihama-Mikata WTE Facility. The system gasifies 24 tpd of MSW and 4 tpd of waste-water sludge through 2 Marc-3a plasma systems. 294 The heat output is used within the waste water treatment facility. There are two plasma gasification plants under construction in India, at Pune and Nagpur, both intended to take 72 tons per day of hazardous waste and convert it to energy. A plasma gasifier has planning permission in the USA, Florida where it is intended that 3,000 tons per day of MSW will be converted to 120MW of electricity and is expected to be operational in 2010. 296 10.2 Pyrolysis SRF Waste wood Clean wood Pyrolysis Gas Pyrolysis oils Char Process Description Pyrolysis is normally defined as the thermal degradation of a substance in the absence of any oxidising agent (other than that which forms part of the substance itself) to produce char and one or both of gas and liquid. The process converts feedstock waste into char, vapours of complex carbohydrates and permanent gases, primarily CO, H2, CH4 and CO2. This is done by the action of heat in oxygen free conditions at high temperatures between 400°C and 900°C. The proportions of the three product classes depend upon CHP 293 Eco-Valley Waste-to-Energy Facility, http://www.alternrg.ca/project_development/commercial_projects/utashinai 294 Alter Nrg and Westinghouse Plasma Corp: The Plasma Gasification Process, February, 2008, http://www.calgaryregion.ca/crp/media/32152/the_alter_nrg_plasma_gasification_process_-_turner_valley_presentation.ppt.pdf 295 Westinghouse Plasma Corporation, Waste Processing, http://www.westinghouse-plasma.com/markets_applications/waste_processing.php 296 Westinghouse Plasma Corporation, www.westinghouse-plasma.com/common/pdfs/WPC.pdf Slurry Thermochemical Fuel Heat Power
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 the temperature of the reactor and critically the rate at which heat is applied to the biomass fractions. Lower temperatures and fast heating rates favour vapours, higher temperatures and high heating rates gas, low temperature and slow heating rates favour char formation. There are two main categories of pyrolysis process, but many practical variants within them: Fast or flash pyrolysis – typically finely divided biomass waste is injected into a fluidised bed of inert material operating at 500°C. The size of the fuel and the excellent heat transfer characteristics of the fluid bed ensure a very fast heating rate which maximises the production of vapour. The vapour is subsequently condensed as a liquid that contains approximately 70% of the energy value of the waste feedstock. The by-product char and gas is used in part to provide heat to drive the process. The liquid fuel has been successfully used to fire boilers and kilns. Trials have been undertaken in reciprocating engines and gas turbines. Excess char can be sold as a product for activated carbon manufacture or reducing agent in metal production. The char can also be used as fuel either on its own or as a slurry with the pyrolysis liquids. The main use for fast pyrolysis processes at present is the manufacture of speciality chemicals and food additives although this is expected to change to energy use when the current demonstration plants in Canada come fully on stream. Slow Pyrolysis – finely diced waste is pyrolysed in either in a screw conveyor or reactor vessel that is indirectly heated. The slower heating rate favours char and liquid production over gas. Carbonisation – large pieces of waste or wood are heated in a retort. The heat is provided by burning a proportion of the vapour and gas product. A variation of slow pyrolysis using a heated screw has recently been proposed as a step in the production of thermo chemical fuels in the form of a slurry of charcoal and pyrolysis liquid product. The concept is a series of distributed fuel preparation processes on a local and regional scale that feed supply a very large gasification plant at national scale. The advantages are reduced transport costs and having the fuel in a form that it can easily be pumped into the high pressure (60 bar) processes that are used in this type of facility. Feedstocks for Pyrolysis Currently fast pyrolysis processes are being designed for both clean wood and wood extracted from the waste stream. In all cases the wood will need to be ground to less that 3mm particle size before use. Slow pyrolysis can use a wider variety of solid shredded material including SRF. Table 82 Risks Associated with Pyrolysis Level of Risk Comment Technical High Operating commercial plant in existence for oil production little experience in energy generation. Social & Planning High / Low High where process is used to produce electricity at a regional scale where it may be perceived as an untried and unsafe incinerator. Low for syn gas production or thermo chemical fuel production where it wuill be perceived as another industrial process. Financial High Few suppliers and uneven track record. Products have no established energy market and would rely on dedicated customer. Regulatory Medium Receives enhanced subsidy via the Renewables Obligation. Status of products is unclear. 161
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