The use of biomass, which is considered to produce no net CO2 emissions in its life cycle, can reduce the effective CO, emissions of a coal-fired power generation system, when co-fired with the coal, but may also reduce system efficiency. The technical and environmental analysis of fluidised bed technologies, using the ECLIPSE suite of process simulation software, is the subject of this study. System efficiencies for generating electricity are evaluated and compared for the different technologies and system scales. Several technologies could be applied to the co-combustion of biomass or waste and coal. The assessment studies here examine the potential for co-combustion of (a) a 600 MWe pulverised fuel (PF) power plant (as a reference system), (i) co-firing coal with straw and sewage sludge and (ii) using straw derived fuel gas as return fuel; (b) a 350 MWe pressurised fluidised bed combustion (PFBC) system cofiring coal with sewage sludge; (c) 250 MWe and 125 MWe circulating fluidised bed combustion (CFBC) plants co-firing coal with straw and sewage sludge; (d) 25 MWe CFBC systems co-firing low and high sulphur content coal with straw, wood and woody matter pressed from olive stones (WPOS); (e) 12 MWe CFBC co-firing low and high sulphur content coal with straw or wood; and (f) 12 MWe bubbling A fluidised bed combustion (BFBC), also co-firing low and high sulphur content coal with straw or wood. In the large systems the use of both straw and sewage sludge resulted in a small reduction in efficiency (compared with systems using only coal as fuel). In the small-scale systems the high moisture content of the wood chips chosen caused a significant efficiency reduction. Net CO2 emissions are reduced when biomass is used, and these are compared for the different types and scales of fluidised bed technologies. NOx emissions were affected by a number of factors, such as bed temperature, amount of sorbent used for SO2 capture and HCl emitted. (c) 2007 Elsevier Ltd. All rights reserved.