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150 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 of the synthesis package and so will be described below in that section. The requirements for the more demanding end uses have been summarised by the Karlsruhe Institute of Technology. 278 . Cleaning processes, especially those for the tars, have proved exceptionally difficult to design and implement in practice and are the principal cause of project failure where engines and gas turbines are used. Options that have shown success are: For tar removal • Catalytic cracking or steam reforming on limestone or nickel catalysts. (VTT, Condens) • Filtration on ceramic filter with limestone precoat. (Gussing, Biomass Engineering) • Liquid scrubbing with vegetable oil or biodiesel. (Gussing, Xylowatt) • Wet electrostatic precipitators (REFgas) • Plasma reaction chamber. (Europlasma) For dust removal • Ceramic, or sintered metal filters (Guessing, Biomass Engineering) • Cyclones • Liquid scrubbers with vegetable oil or biodiesel. (Gussing, Xylowatt) • Plasma reaction chamber. (Europlasma) Table 78 Syngas trace contaminants and target levels in ppm 278 mg/Nm 3 Biomass gasification Gas motor Gas turbine MeOH FT (Sasol) Particles 10 4 – 10 5 < 50 < 1 0.2 n.s. Tar 0 – 20,000 < 100 < 5 < 1 n.s. Alkali 0.5 – 5 n.s. < 0.2 < 0.2 < 0.01 NH3, HCN 200 – 2000 < 55 n.s. < 0.1 < 0.02 H2S, COS 50 – 100 < 1150 < 1 < 0.1 < 0.01 Halogens 0 – 300 n.s. < 1 < 0.1 < 0.01 Heavy Metals 0.005 - 10 n.s. n.s. n.s. < 0.001 n.s = not specified Gas conversion The clean gas has many uses. It can be burned directly, to provide heat, raise steam, or drive electrical generators. It can also be used as a feedstock for further processes to synthesise methane, diesel fuel components and other chemicals and fuels. Without doubt the most successful application to date has been the direct combustion of gas in industrial processes and boilers. The most quoted example of process use is the use of fluidised bed gasifiers producing low calorific gas for heating lime kilns in the paper and pulp industry. Typically the gas is burned through a burner nozzle designed for the low calorific value directly into the combustion chamber of the process unit. One unit in Varo Sweden has been operating since 1987. 279 Gas can also be used to raise steam in a boiler which can in turn be used for power generation. This is a robust concept that carries much less technical risk than the use of engines or gas turbines. Prime examples of this are those processes based on an externally heated reactor with all the heat release in a 278 IEA Bioenergy Agreement Task 33 Spring 2009 Workshop, Karlsruhe, www.gastechnology.org/iea German country update 279 IEA Bioenergy Agreement Task 33 Spring 2009 Workshop, Karlsruhe, www.gastechnology.org/iea Swedish country update.
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 high temperature combustion chamber followed by a boiler (Ethos Energy, Thermoselect, Mitsui MES R21). These have all proved reliable in commercial service and delivered substantial emissions benefits. A special case of this is the use of gas for firing district heating plant. Through the 1980’s a Finnish company, Bioneer, installed a series of updraft gasifiers linked directly to hot water heating boilers in Finland and Sweden. These were very successful and reliable but development stopped after the company was sold to a major engineering concern. There are plans to implement a project in east London (ELSEF) that will use a fluidised bed gasifier, fuelled by SRF from a neighbouring waste management facility. Here the gas would be cleaned of acid gas components which allows the use a much higher pressure boiler and turbine than would be possible with a combustion plant and hence potential for higher electrical efficiency in power and CHP cycles. A further related application that is enjoying considerable success is the use of a circulating fluidised bed gasifier close-coupled to a coal fired boiler. Three installations have been built: Lahti in Finland, Amer in Netherlands and Electrobel Ruien in Belgium. These units take a wide range of feedstocks from SRF to waste woods and plastics. The first installation of this type, Lahti in Finland was commissioned in 1998 and has operated reliably since. Details of all of these installations can be found on the IEA Bioenergy Agreement Task 32 database of cofiring installations 280 . The inherent simplicity of this approach and it’s ability to make use of the high efficiency steam cycle of the fossil fuel plant is attractive and further installations are planned in Finland and Sweden. 279 A class of installations that is enjoying some success in the UK is the close coupled gasifier and boiler, where waste is burned directly after production. This does not allow for acid gas removal but ensures that the gas is burned in a well developed and controlled flame that reduces other emissions to a low level. One of the most developed version is the Energos gasifier which is demonstrating a unit on the Isle of Wight as part of the Defra WIP. 281 Further replications are planned in the UK. The controlled nature of the gasification and combustion reactions mean that this type of unit can be designed to burn sorted MSW and SRF in smaller heat and CHP installations than grate and fluidised bed combustors which means they can be used to power industrial heat and high load chp installations that have high GHG abatement potential. A further example of this type of technology is the Bioflame unit which is currently being marketed to produce CHP using waste wood chip as a fuel. 282 Using the gas to generate electricity from combustion in a reciprocating engine or gas turbine has proved problematical and there are very few examples of a fully commercial, reliable system operating today. The obstacle is cleaning the gas of the tar and dust contamination that can damage the engine. This apparently simple task has defeated many engineers over several decades. There has however been some progress over the last five years; • At Gussing in Austria an indirect gasifier has been operating for over 20k hours fuelling a 2.5 MWe reciprocating engine. The key to success here appears to be the combination of a ceramic filter with a liquid scrubber containing biodiesel. • Biomass Engineering Ltd in the UK have installed three downdraft/ engine systems in Germany that are generating reliably as CHP units supplying district heating. The key here seems to be close control over the feedstock sizing and quality combined with the use of a ceramic filter. • At Harboore in Denmark a fixed bed gasifier supplies three reciprocating engines in a local district heating system. Tar cleaned from the fuel gas is stored and subsequently fired in a boiler to even out peaks in heating demand. 280 http://www.ieabcc.nl/database/cofiring.php 281 Lets recycle Tuesday 18 September 2007. 282 http://www.bioflame.co.uk/ 151
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