Effects of various occurrence modes of inorganic components on the emissions of PM10 during torrefied biomass combustion under air and oxy-fuel conditions

Abstract

In order to control PM10 (of an aerodynamic diameter of 10 μm or less) emissions from biochar combustion, the effects of various occurrence modes of inorganic components on PM10 formation are investigated. Two typical biochars (torrefied willow and wheat) were processed through sequential extraction procedures followed by combustion experiments in a high-temperature drop-tube furnace at 1400 °C. The generated particulate matter was characterised. The PM1 emissions during air combustion were linearly related to the content of K/Cl/S in water-soluble/ion-exchangeable forms, which are easily vaporised. The nucleation and coagulation of KCl/K2SO4 in gaseous species forming most of the PM1 resulted in the above-mentioned strong correlation. The PM1-10 emissions had a not very good linear relationship with the various forms of inorganic elements, and they were mainly related to the acid-soluble components. The catalysed sintering of CaO and coalescence/fragmentation of Ca-containing minerals formed most of the PM1-10 (torrefied willow). The Ca in torrefied willow mainly exists in an acid-soluble form, which interfered with the effect of other elements. The coalescence/fragmentation of Si-containing minerals in torrefied wheat generated most of the PM1-10. After various washing procedures, the different forms of Ca/K were removed and the silicate decreased during combustion, leading to a weakness in coalescence. This led to the above-mentioned relationship. When the combustion atmosphere was switched to oxy-fuel mode, the great contribution of acid-soluble Ca to PM1-10 (torrefied willow)/water-soluble K to PM1 (torrefied wheat) still exists, indicating that they were less affected by the combustion conditions.