Abstract
Differences in the densities of bed material and—especially biogenic—solid fuels prevent an ideal mixture within bubbling fluidised bed (BFB) combustors. So, the presence of fuel particles is usually observed mainly near the surface of the fluidised bed. During their thermal conversion, this leads to a release of unburnt pyrolysis products to the freeboard of the combustion chamber. Within the further oxidation, these species will not transfer their heat-of-reaction to the inert bed material in the way of a convective heat transfer, but rather increase the gas phase temperature providing probably some additional radiative heat transfer to the dense bed. In this case, the so-called heat release efficiency to the fluidised bed, being the ratio of transferred heat to the fuel input, will be reduced. This paper presents a methodology to quantify this heat release efficiency with lab-scale experiments and the observed effects of common operating parameters like bed temperature, fluidisation ratio and fuel-to-air ratio. Experimental results show that the air-to-fuel ratio dominates the heat release efficiency, while bed temperature and fluidisation ratio have minor influences.
Highlights
The principle of fluidised beds for thermal conversion of solid fuels is well-known since Winkler’s patent in 1923 with its focus on coal gasification
For a transfer of the achieved data to other fluidised bed combustion chambers, this has to be considered in a critical analysis
The heat transfer conditions in the gas phase will differ between each combustion chamber and have a slight influence on the heat release efficiency
Summary
The principle of fluidised beds for thermal conversion of solid fuels is well-known since Winkler’s patent in 1923 with its focus on coal gasification. In the 1970s, the efforts for the development of bubbling fluidised bed (BFB) coal combustion raised rapidly [1]. Power-plants like in Rivesville (USA) were still based on a bubbling fluidised bed combustion principle [2]. The circulating fluidised bed (CFB) combustion aiming at higher power capacities started to succeed in research and industry [3]. With the rise of renewable energy sources, fluidised bed combustion was found to be highly interesting for the thermal conversion of biomass. In contrast, offers an isothermal temperature distribution within the bed to face these problems, and with a beneficial emission behaviour concerning nitrogen-oxides and the possibility of an in-situ desulphurisation [3,6]. Ash-related issues occur for fluidised beds in this case, by showing a tendency for bed material agglomeration [9,10]
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