Biomas co-firing in a CFB boiler

Paper in proceeding, 2006

The aim of present paper is to investigate the effects on gas distribution in a CFB furnace from burning varied fractions of wood fuel with coal as a base fuel. The work is based on in-situ measurements of gas concentrations at different locations in the furnace of the Chalmers 12 MWth CFB boiler. The boiler was operated at two different pressure drops over the furnace, corresponding to different amounts of bed material. The latter was done since a reduced pressure drop over the furnace improves the overall boiler efficiency as long as the combustion performance can be maintained. The gas-concentration measurements in the furnace are compared to results from experiments in a cold laboratory scale fluidized bed unit in which the rates of mixing of solids (“fuel particles”) are determined at varied bed heights and gas velocities by frame-by-frame digital image analysis of video recordings. Thus, the gas distribution in the furnace is assumed to be related to the mixing of fuel particles. The cold bed experiments show a pronounced reduction in solids-mixing rate when lowering the bottom bed height. Since an increased fraction of wood in the fuel mixture leads to a larger amount of volatile gases emitted in the bottom region, a combination of a low bed height and a high biomass fraction in the fuel could result in significant maldistribution of combustible gases over the furnace cross section. The conclusion from the present boiler measurements is that the performance of an existing CFB boiler, operated satisfactorily with a low furnace pressure drop when firing coal, may deteriorate when adding wood to the fuel mixture due to lateral maldistribution of combustible gases. The results show that this effect can, to some extent be counteracted by increasing the amount of bed material in the furnace, at the cost of higher pressure-drop losses.