Sulphur self–retention and sulphur dioxide capture with active calcium minerals in mineral–rich coals

Abstract

In order to provide information for the development of clean coal technology, the sulphur self–retention and sulphur dioxide capturing properties of minerals present in low grade coals was investigated. This study consisted of detailed mineral analyses of coal and ash samples using results obtained from QEMSCAN and separate retention (coal) and capture (ash) experiments with laboratory scale reactors. Typical South African coal samples were used in this study. The ash content varied between 37.0 wt % and 47.9 wt % with active calcium oxide (from calcite and dolomite) present between 1.22 wt % and 4.92 wt %. The total sulphur content ranged between 0.60 wt % and 1.90 wt % and was distributed between sulphate minerals, pyrite and organically associated minerals in the coal macerals. The calcium to sulphur ratio based on the active calcium ranged from 0.64 to 3.20. Sulphur self–retention experiments (using powders of particle sizes ranging from 212 um to 300 um) were carried out in a Packed Bed Balance Reactor at 900 oC at atmospheric pressure with dry air for a period of 12 h. Transformation of a large fraction of the calcium bearing minerals to sulphates was evident with total sulphur self–retention between 22.9% and 66.9% and the formation of calcium containing non–crystalline phases. Thermogravimetric Analyzer experiments using 1 mm diameter coal particles were carried out with the ash prepared (in situ with air) prior to the actual sulphation determinations. After attaining a stable ash mass, the gas stream was changed to the sulphur dioxide containing mixture (3000 ppm SO2, 8.0% CO2, 8.0% O2 and 83.7% N2) and the increase in mass as a result of the reaction of sulphur dioxide (and oxygen) with calcium oxide (calcite and dolomite) was recorded. Conversion of active calcium bearing phases in the presence of sulphur dioxide containing gas mixture, similar to the gas released during fluidised bed combustion, was employed to evaluate the desulphurisation potential of the coal ashes, and it was found that nearly 40% of the active calcium oxide was converted after 90 min reaction time at 900 ºC and that no blocking of pores occurred as a result of solid phase changes. A mathematical model based on the shrinking core model with diffusion through the product layer as the determining mechanism was found to describe the overall reaction rate.