Co-firing of pressed sugar beet pulp with coal in a laboratory-scale fluidised bed combustor

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 co-firing of coal with pressed sugar beet pulp, (a solid biomass with an average moisture content of 71%), in a lab 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. This reduction in excess air will be beneficial for the output of the full scale plant at British Sugar since at present the flow rate of the fluidising air and hence the amount of coal which can be burnt is limited by high pressure drops in the bed air distributor system. The pressed pulp has relatively low nitrogen levels and hence a further benefit of co-firing is that NOx emissions are reduced by about 25%. Agglomeration of the bed can be a problem when co-firing biomass because of the formation of “sticky” low melting point alkali metal silicate eutectics which result in subsequent adhesion of the ash and sand particles. Consequently, longer term co-firing tests were undertaken with a 50/50 blended fuel by mass. Problems of bed agglomeration were not observed over the duration of these tests and moreover, scanning electron microscopy (SEM) studies indicated that the levels of alkali metals in the ash were relatively low.