Emission of species of environmental and process concern during simulated oxy-fuel gasification

Abstract

The release of species of environmental concern during simulated oxy-fuel gasification in a laboratory scale fluidised bed have been investigated. Fluidising gases containing a high partial pressure of CO2 and in some tests, steam, were used. The species considered are tars, H2S and NH3 and the aim has been to gain an understanding of the impact of the gasifer operating conditions on their release. This is part of a programme of work on the potential of oxy-fuel gasification as a means of enabling the use of coal to continue as a source of energy, whilst capturing the CO2 for sequestration to minimise the impact on climate change.It has been shown that the tars released during pyrolysis are efficiently destroyed during their passage through the hot, char containing bed. The measured emission of tar was very low, particularly when compared to a conventional fixed bed gasifier. The extent of S release as H2S depends on the fuel-S and ash-Ca contents, and on the operating conditions. Increasing the partial pressure of CO2, steam and raising the temperature increased the proportion of S released. Adding dolomite, to retain S, decreased the emission level at atmospheric pressure, provided the temperature was above 950 °C. The use of this method to reduce S emissions at elevated pressure would require careful consideration, as the high partial pressure of CO2 in the fluidising gas can prevent the calcination of the dolomite and therefore reduce its effectiveness. Steam was found to increase the proportion of fuel-N converted to NH3 and the concentration also depended on the bed temperature. A peak in the concentration was noted at 850 °C, due to the opposing impacts on increased release as the temperature was raised and increasing equilibration of NH3 to N2 and H2.