Effect of Coal Drying on the Behavior of Inorganic Species during Victorian Brown Coal Pyrolysis and Combustion

Abstract

The purpose of this work was to determine whether coal pre-drying will affect the behavior of the low concentrations of inorganic species present in coal during combustion. A drop tube furnace was used to generate chars and ashes from both wet and dry coal. Raw coal, char, and ash were then analyzed for the presence of inorganic species (i.e., Na, Mg, Ca, K, S, Si, Al, Fe, and Ca) using X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDX). There were no major differences observed between the ash formation during combustion of dry and wet coal within the temperature ranges investigated. However, there may be other implications, such as higher furnace temperature and heat flux and slagging in the furnace. This may change the ashing behavior, which warrants further investigation. Chars contained Na, Mg, and Cl bound to the char matrix and an array of excluded minerals present in low concentrations. At 800 °C, more metals were lost from wet coal than dry coal during pyrolysis. This was possibly due to the inability of the metals to migrate back into the char matrix during rapid drying prior to pyrolysis. Char formed at 1000 °C seemed to be unaffected by the coal moisture content, likely because the rate of decomposition reactions at that temperature become significant, leading to the loss of inorganics from the char matrix as well as the coal moisture. An increase in the ratio of C–Cl to inorganic chlorine was observed with an increasing temperature. This may mean that the propensity for the formation of polychlorinated dibenzo-para-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) is increased in the dry coal case, given that that the particle temperature during coal pyrolysis is increased with a decreased moisture content. The pyrolysis rate was found to be similar for both dry and wet coal, meaning that the assumptions used in computational fluid dynamic (CFD) modeling for predicting the outcomes of changes to operating conditions in industrial boilers will still stand with pre-dried coal as the input.