Submicron particle formation in biomass formation

Resumen

Particulate emissions from biomass combustion have been experimentally characterized in a laboratory facility, for a variety of fuels and combustion conditions. Two particle modes were found in all the cases; while coarse particles essentially retained the original fuel ash composition, those in the submicron mode were composed only of alkali (mainly potassium) sulphates and chlorides in different proportions. A new probe (AQPS), based on aerodynamic-quenching, and a TEM allowed for the detailed study of the fine particle formation mechanisms. Evidence has been found for the existence of "formation steps": K2SO4 nucleates first in the flue gas cooling process, whereas KCl condenses on these nuclei at lower temperatures. This is in good agreement with simple thermodynamic equilibrium calculations and with theoretical models by other authors. Experimental evidence is shown, also, for the kinetic limitation of SO2 oxidation in the route to sulphate formation. The final chloride/sulphate ratio in fine particles is greatly affected by variations in the SO2 - O2 concentrations in post-combustion gases. Moderate values of extra SO2 gas for usual O2 concentrations, as those attainable in co-firing systems, result in the total disappearance of chlorine from these particles, a fact that has been experimentally confirmed in co-combustion tests in this study. This gives more basic support to field results on deposit characteristics and boiler corrosion previously reported, and opens a way for minimizing the chlorine-associated boiler operating problems in biomass combustion. The effectiveness of three different particle sampling methods was comparatively studied in post-combustion flows containing significant amounts of inorganic condensable vapours. While the use of the AQPS probe resulted in all the cases tested in the formation of ultrafine particles by rapid vapour nucleation, the artifacts caused by the probes based on N2-dilution and thermophoresis presented great vari