Particulate and PCDD/F emissions from coal co-firing with solid biofuels in a bubbling fluidised bed reactor

Abstract

In the scope of the COPOWER project (SES6-CT-2004) that aimed at investigating potential synergies of co-combustion of different biofuels with coal, the study of emissions of particulate matter and PCDD/F was carried out. The biofuels tested were meat and bone meal (MBM), sewage sludge biopellets (BP), straw pellets (SP), olive bagasse (OB) and wood pellets (WP). The tests performed include co-firing of 5%, 15% and 25% by weight of biofuels with coals of different origin. Both monocombustion and co-firing were carried out to compare the results. Combustion tests were performed on a pilot fluidised bed, equipped with cyclones and air staging was used in order to achieve almost complete combustion of fuels with high volatile contents and to control gaseous emissions. Particulate matter emissions were isokinetically sampled in the stack and their particle size analysis was performed with a cascade impactor (Mark III). The results showed that most particles emitted were below 10 μm (PM 10) for all the tests, however, with the increasing share of biofuels and also during combustion of pure biofuels, especially for olive bagasse, straw and MBM, it was observed the presence of very fine particles, below about 1 μm. With the exception of sewage sludge, greater amounts of biofuels appeared to give rise to the decrease in particulate mean diameters and increase in PM percentages below 1 μm. One factor that influenced the total amount of PM emitted, as well as the amount of coarser PM, was the formation of less unburned matter with the increased share of biofuels. However, the most important factor that could lead to the formation of very fine particles could be related with the presence of aerosol forming elements such as K, Na (in the case of MBM) and Cl in biofuels, which even resulted in higher PM emissions when the ash content of fuels decreased, as was the case of straw and olive bagasse. For some fuel mixtures with selected sulphur and chlorine contents, dioxin and furan emissions were also determined. It was verified a correlation between the increase of PCDD/F with the decrease of PM mean diameter. Among other factors, this correlation may be due to higher specific surface area and greater Cu concentration in the fly ashes.