Effects of torrefaction on ash-related issues during biomass combustion and co-combustion with coal. Part 1: Elemental partitioning and particulate matter emission

Abstract

Torrefaction is a promising technology to scale up the utilization of biomass for power purposes. The ash-related issues during biomass combustion and co-combustion are essential to be addressed, however, on which the effects of torrefaction are seldom concerned and need in-depth and comprehensive investigation. To provide useful information related, the authors conducted a series of studies. In the present work, the particulate matter (PM) emission during combustion is focused on. Three biomasses were torrefied at a fixed-bed reactor, and the elemental release and transformation during torrefaction were characterized through chemical fractionation analysis. The combustion tests of the raw and torrefied biomass and their blends with two typical coals were conducted on a drop-tube furnace at 1300℃ in air. The PM10 (particulate matter with an aerodynamic particle size below 10 μm) collected by a Dekati low-pressure impactor and PM10+ (particulate matter with an aerodynamic particle size above 10 μm) collected by a Dekati cyclone were characterized by a scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS). The results indicate that torrefaction effectively reduces S and Cl in biomass while slightly reducing the alkalis. The contents of refractory elements (Mg, Ca, and Fe) are hardly influenced. Torrefaction promotes the transformation of AAEMs (alkaline and alkaline earth metals) into less-reactive forms (HCl-soluble and insoluble). The vaporization of alkalis, S, and Cl during biomass combustion is reduced and the retention of AAEMs into aluminosilicates is promoted after torrefaction. This reduces the emission of PM1 (particulate matter with an aerodynamic particle size below 1 μm) from biomass combustion. During co-combustion, the PM emission largely depends on the combined effects of elemental vaporization, competing reactions (alkalis against Mg, Ca, and Fe), retention of metals into aluminosilicates, and sulfation of chlorides. The inhibited effect of torrefaction on mineral vaporization reduces the formation of PM1 by condensation. The emission of PM1-10 (particulate matter with an aerodynamic particle size between 1 and 10 μm) either increases due to the promoted retention of the metals in coarse particles or decreases by agglomerating into PM10+. Torrefaction shows a preferable performance concerning its effects on PM emission during biomass combustion and co-combustion with coal.