Interactions among Inherent Minerals during Coal Combustion and Their Impacts on the Emission of PM_(10). 1. Emission of Micrometer-Sized Particles

摘要

Four pulverized bituminous coals, possessing nearly identical organic properties, were burnt in a laboratory-scale drop tube furnace to investigate the formation of PM_(10) (particulate matter less than 10 μm in diameter) and the influence of coal mineralogical properties on its emission. Coal combustion was conducted at 1450℃ in air. A residence time of about 3 s was adopted. During combustion, PM_(10) was collected by the combination of a cyclone and a low-pressure impactor, and divided into two fractions: micrometer particulates ≥ 1 μm (PM_(1+)) and submicrometer ones (PM_1). These two fractions have been discussed in the present paper and the next one, respectively. Regarding the formation of PM_(1+), it varies with the coal mineralogical property greatly. The total amounts of four refractory elements, Al, Si, Ca, and Fe, account for more than 90 wt % in PM_(1+). Accordingly, PM_(1+) mainly consists of quartz, Al-silicate, and Ca/Fe Al-silicates. Two of the coals tested in this study, lean in Ca and Fe, release about 40 mg/g-coal of PM_(1+), which is about twice those emitted from the other two coals rich in Ca and Fe. This is due to the interactions between included minerals (mainly Al-silicates) and excluded ones including calcite and pyrite. In the case of coals lean in Ca and Fe, less the inherent Si and Al (regardless of their association with the carbonaceous matrix) coalesce, and hence, they transfer into PM_(1+) directly. Correspondingly, the amount of PM_(1+) formed is similar to that of inherent minerals smaller than 10 μm in raw coals. On the contrary, in the case of coals rich in Ca and Fe (mainly existing as excluded particles as found in this study), the inherent calcite and pyrite initially decompose to form finer particles, which then collide with Al-silicates released from the coal char to form the low-melting compounds and sequentially promote the coalescence of Al-silicates. As a result, less PM_(1+) is formed. This is further evidenced during the combustion of coal density fractions.