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BIO-SNG FEASIBILITY STUDY – ESTABLISHMENT OF A REGIONAL PROJECTFigure 5.3Choren Entrained Flow Gasification systemIt will be readily appreciated that the pyrolysis and entrained flow gasification steps combine to create arelatively complex plant, albeit one that is technically demonstrated at a significant scale by Choren. Atthis point it should be appreciated that the principal aim of entrained flow gasifier concepts has been toproduce a good quality synthesis gas – a mixture of carbon monoxide (CO) and hydrogen (H 2 ) – and topresent these in a CO / H2 molar ratio that subsequently allows the efficient synthesis of more complexmolecules from these basic building blocks of organic chemistry. An efficient entrained flow gasifier willproduce very low levels of methane in the synthesis gas; methane production requires therefore theconversion of synthesis gas via a methanation step. This consumes energy and is seen by some SNGtechnology developers as a reason to pursue alternative gasification processes that provide a syngasoutput that maximises the methane content of the syngas as it is produced from the gasifier.Fluidised bed gasifiers. Fluidised bed gasifiers exhibit a number of variants – bubbling beds, circulatingbeds, indirect and direct heating, pressurised and un-pressurised, air blown or oxygen blown. Thecommon feature of fluidised bed gasifiers is that they provide a hot aerated bed of granular solid materialinto which the granulated fuel is injected. The fluidised bed provides a ”thermal flywheel” whereby heattransfer from the hot bed material is sufficient to dissociate the fuel into volatile components (syngas) andash. The gases are evolved from the top and the ash from the bottom. Conventionally, the heat of thebed is maintained by burning part of the fuel in the bed itself and the products of combustion (water andsome carbon dioxide) are evolved with the syngas.35