Trace element partitioning and emission control during coal gasification

Coal contains most elements of the Periodic Table in trace quantities. During gasification and combustion, many trace elements are released from the fuel in vapour form and/or in association with particulates released by the process. In addition to causing corrosion damage to process plant, many of these trace elements (e.g As, Hg, etc.) constitute potential health hazards. It is therefore necessary to evaluate trace element contents in coals and discharges likely to arise from coal utilisation. However, there are no established protocols for trace element analyses of solid fuels and their derived products. In this study 'Inductively Coupled Plasma-Mass Spectrometry' (ICP-MS) has been used as the primary tool for evaluating trace element contents of the samples. The technique is accurate for determining trace element contents, down to ppm levels, once the original samples have been digested in acidic solutions. In our research, the performance of two acid digestion methods used to prepare sample solutions for ICP-MS analysis have been compared: an 'open vessel' acid digestion and acid leaching in microwave bombs. Up to seventeen elements in coal and ash standard reference materials have been determined (As, Ba, Be, Cd, Co, Cr, Cu, Ga, Mn, Mo, Ni, Pb, Sb, Se, Sn, V and Zn). The reduction of the sample size to 10 mg was shown not to affect the accuracy and reproducibility of the analyses that have been carried out. Once validated, these methods were used to determine the distribution of trace elements in a pilot-scale gasification plant. The results showed that, whilst a large part of the trace elements released by the coal actually condensed on solid by-products retrieved from the system (primarily ash and char), discharges of As, Hg, Pb and Se into the product gas stream were identified. Potentially hazardous Sb and Cd were found to be mainly concentrated in candle filter fines. The second stage of the study focused on the removal of corrosive and hazardous trace elements released during coal gasification (mainly As, Cd, Hg, Pb Sb and Se), by contacting with suitable sorbents. A laboratory-scale reactor was designed to simulate the capture of volatile elements in the gas clean-up stage of gasification plants. The effect of temperature on the efficiency of trace element retention by a fixed-bed of sorbent was studied in some detail. An attempt was made to distinguish between chemical capture by, and condensation on, sorbents. Sorbents such as kaolin and activated carbon were used to trace the reaction leading to the retention of trace elements. The results indicated that the nature and efficiency of the capture process between 300 deg. C and 600 deg. C was temperature dependent. Based on the overall capacity for trace element capture and water solubility of the product formed, activated carbon was found to be more reactive towards As and Hg, and kaolin towards As and Sb, at moderate temperatures (300-450 deg. C range); kaolin was more efficient at the highest temperatures studied for the Cd and Pb vapours (up to 600 deg. C). Unaffected by kaolin, Se vapours were shown to react readily with activated carbon at all temperatures tested to the extent that all Se was captured on the sorbent. Although this research confirms the potential of sorbents for limiting the emission of trace elements as part of the Hat Gas Clean-Up concept, it also indicates that a single sorbent is unlikely to be successful at retaining all the toxic polluting vapours released. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN034081 / BLDSC - British Library Document Supply CentreGBUnited Kingdo