CO-FIRING OF COAL AND PRESSED SUGAR BEET PULP IN A PILOT-SCALE FLUIDISED BED COMBUSTOR

摘要

Relatively cheap, poor quality, unprepared biomass materials can be difficult to burn efficiently on a large commercial scale because of their variable composition, relatively low calorific values and high moisture contents. Consequently it is often necessary to co-fire these materials with a hydrocarbon support fuel to ensure stable and efficient combustion. Fluidised bed combustion (FBC) is a promising method for burning mixtures of fuels with widely differing individual characteristics although there is a need for further information on the “optimum” conditions for efficient operation as well as on the proportions of support fuel which should be used in particular applications. This paper is therefore concerned with cofiring of coal with pressed sugar beet pulp, (a solid biomass with an average moisture content of 71%), on a pilot scale (<25 kW net thermal input) fluidised bed combustor. The project was undertaken in collaboration with British Sugar plc who operate a large coal-fired fluidised bed, with a nominal thermal rating of 40 MW, to generate hot combustion gases for use in subsequent drying applications. The combustion characteristics of different coal and pressed pulp mixtures were investigated over a wide range of operating conditions. For stable combustion the maximum proportion of pulp by mass in the blended fuel was limited to 50%. However under these co-firing conditions a fixed bed temperature can be achieved with 20% lower fluidising air (when compared with coal alone) since evaporation of the moisture in the pressed pulp provides additional cooling of the bed.