Effects of torrefaction on ash-related issues during biomass combustion and co-combustion with coal. Part 3: Ash slagging behavior

Torrefaction is a competitive technology for biomass upgrading. Knowing that the effects of torrefaction on ash-related issues during biomass combustion and co-combustion with coal have been less investigated, Parts Ⅰ and Ⅱ of this work investigated the particulate matter emission and ash fouling behavior. To be a continuation, in the present paper, the effect of biomass torrefaction on ash slagging deposition which occurs in the high-temperature areas of the furnace was focused on. Two herbaceous biomass, one woody biomass, and their torrefied chars were fired and co-fired with two coals respectively on a drop-tube furnace at 1300℃. The slags were collected from mullite tubes placed in the furnace for quantification and characterization. Computer-controlled scanning electron microscope (CCSEM) was employed to obtain the distribution of particle size and inorganic species in the bulk ash. The results indicate that co-firing the biomass with high-Ca coal significantly reduces the ash slagging rates due to the substitution of alkalis by Ca from the silicates/aluminosilicates which inhibits the agglomeration and sintering of the ash particles, and finally reduces the inertial impaction and capture of the particles on the tube. Co-firing corn stalk with the high-Si/Al coal promotes the formation of alkaline aluminosilicates which are sticky at a high temperature. Therefore, the slagging tendency is exacerbated. Torrefaction reduces the ash slagging rate of sawdust resulting from the effective removal of S and Cl and the transformation of AAEMs (alkaline and alkaline earth metals) into less-reactive forms during torrefaction which reduces the vaporization and transformation of AAEMs into coarse particles as aluminosilicates by surface reactions. However, the ash slagging rates are increased by torrefaction for other fuels due to more retention of AAEMs after torrefaction resulting in more mixed silicates or aluminosilicates in bulk ash contributing to the growth of slagging deposits during combustion. Measures are suggested to be taken, such as optimizing the operation conditions, co-firing with high-Ca coals, using additives, etc. to alleviate the ash slagging during torrefied herbaceous biomass combustion.