It can be difficult to burn relatively cheap, poor quality, unprepared biomass materials in industrial heating processes because of their variable composition, relatively low calorific values and high moisture contents. Consequently the stability and efficiency of the combustion process can be adversely affected unless they are co-fired with a hydrocarbon support fuel. There is a lack of information on the “optimum” conditions for co-firing of coal and high moisture biomass as well as on the proportions of support fuel which should be used. This paper is therefore concerned with the pilot scale (<25 kW thermal input) fluidised bed combustion of blends of coal with pressed sugar beet pulp, a solid biomass with an average moisture content of 71%. The experimental work was undertaken in collaboration with British Sugar plc who operate a coal-fired 40 MW thermal capacity fluidised bed producing hot combustion gases for subsequent drying applications. The project studied the combustion characteristics of different coal and pressed pulp blends over a wide range of operating conditions. It was found that stable combustion could only be maintained if the proportion of pulp by mass in the blended fuel was no greater than 50%. However evaporation of the moisture in the pressed pulp cools the bed so that the excess air which is necessary to maintain a specified bed temperature at a fixed thermal input can be reduced as the proportion of biomass in the blended fuel is increased. Therefore, with a 50/50 blend the bed can be operated with 20% less fluidising air and this will be beneficial for the output of the full scale plant since at present the flow rate of the air and hence the amount of coal which can be burnt is restricted by supply system pressure drop limitations. A further benefit of co-firing pressed pulp 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 with a 50/50 blended fuel by mass were undertaken. Problems of bed agglomeration were not observed under these conditions with relatively low levels of alkali metals in the ash.
A liquid crystal technique has been applied to the problem of convective heat transfer downstream of a circular to square abrupt expansion. This configuration is similar to that found with a burner firing into a furnace or boiler. There is little data available in the literature for these expansions as most previous investigations have concentrated on a simple circular to circular geometry.
Liquid crystals were selected for the tests because of their ability to provide a full surface temperature map with a high spatial resolution. With the progress of image capture and processing technology a transient test method was preferred as it makes the construction of models very simple by eliminating the need to heat the surface with the inherent problem of uniformity and the difficulty in viewing the surface.
The application of liquid crystals produced results which were found to be accurate and repeatable, when compared with results obtained from other investigations in the area. It gave quantitative data, allowing spanwise and axial distributions of heat transfer to be calculated in these geometries.
The hue capturing technique thus provides quantitative, accurate and repeatable temperature measurements, and when applied to heat transfer problems is a powerful experimental tool.
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