科研之友
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摘要
Combustion of three coal samples of different types and origins has been conducted in a lab-scale drop-tube furnace (DTF) to examine the speciation and emission of As during air and oxy-fuel combustion. The synchrotron XANES has been used to quantify the contents of toxic As3+ and less toxic As5+ in char and fly ash samples. Irrespective of coal type and the original mode of occurrence of As, the substitution of N-2 by CO2 for oxy-fuel combustion caused little changes to the emission and speciation of As at the initial step of coal combustion, pyrolysis. The reduction of original As5+-O and/or oxidation of the As(g) derived from the decomposition of As-S association favoured the retention of approximately 20% of As as As3+ in char, which remained unchanged with the progress of the bituminous coal char oxidation, but was further vaporised during the combustion of lignites. For the oxy-fuel mode with no less than 27% oxygen to match air in terms of adiabatic flame temperature, the fraction of toxic As3+-O in the final fly ash of the bituminous coal and high-ash lignite is much lower than in air, due to the higher oxygen partial pressure in the system. However, for the As-doped low-ash brown coal, its high CO2-gasification reactivity slightly promoted the fraction of As3+-O in oxy-fuel mode that decreased with increasing oxygen content. In addition, the amount of inherent Ca and/or Fe in coal has been proven insignificant in affecting the stabilisation of As vapours, instead relying more on the availability of Ca and/or Fe active sites for As capture. For the addition of external free lime (CaO), its capture capability is also lower in oxy-fuel mode than in air, probably due to the preferential carbonation caused by vast CO2 in the reactor.
- 出版日期2013
