Effects of torrefaction on ash-related issues during biomass combustion and co-combustion with coal. Part 2: Ash fouling behavior

Abstract

Torrefaction is a promising pretreatment technology that allows the utilization of biomass on large scales. This work provides new knowledge regarding the effects of torrefaction on ash fouling during biomass combustion and co-combustion by correlating them to element repartitioning due to torrefaction, which has little been evaluated in the literature. Two herbaceous biomasses (rice husk and corn stalk) and one woody biomass (sawdust) were torrefied on a fixed bed at 270℃ in N2. The raw and torrefied biomass were fired and co-fired with two coals on a drop-tube furnace at 1300℃ in air. The samples were characterized by various techniques such as X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, and ion chromatography. The results show that the fuel interactions during co-combustion inhibit the formation of inside fouling while the incorporation of alkalis in the aluminosilicates promotes the growth of outside deposits. Torrefaction reduces the formation of chlorides and sulfates during combustion directly and consequently inhibits the inside deposition due to the fractional removal of Cl, S, and alkalis in biomass. Besides, the less condensation of chlorides/sulfates on the surface of the tube and the coarse particles tend to reduce the formation and growth of the outside deposits. However, the elemental repartitioning during torrefaction leads to more retention of alkalis which can aggravate the formation of mixed aluminosilicates during combustion and favors the outside deposition. Both co-combustion and biomass torrefaction have the potential to decrease the ash fouling tendency during biomass combustion considering that they reduce the inside fouling deposits which are problematic in practice. Based on the ash composition of the fuels, a modified fouling index [soluble (Na2O + MgO + K2O + CaO)] * [SiO2/(SiO2 + Al2O3)] is proposed which shows a superior linear correlation with the inside fouling rates of the various fuels investigated, indicating its feasibility for prediction of the ash fouling tendency.