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Chemical properties of coal which impact on gasification performance relate to those processes which do effect a change in chemical constitution, these in turn may lead to changes in physical properties such as particle size distribution and surface area of the coal. Turn-out samples obtained from a commercial-scale Sasol-Lurgi fixed-bed dry bottom (FBDB) gasifier were characterized to understand and interpret the internal chemical property behaviour and are discussed in relation to the residual C, H, N, S and O distribution profiles obtained. Thermodynamic equilibrium simulation of the organic and inorganic speciation behaviour occurring within a fixed-bed gasifier was modelled using the Fact-Sage simulation package, and used to support the measured ultimate analysis profile data obtained. The measured gasifier ultimate analysis profiles provided good insight into understanding the development of aromaticity of the char, expressed by the carbon:hydrogen ratio calculated on a mass basis. Equilibrium compositional profiles calculated for C, H, N, S and O provided discernment regarding the speciation and partitioning behaviour occurring within the fixed-bed-reactor. Fact-Sage thermodynamic equilibrium modeling of the gasifier related to the ultimate analysis results, was found to be useful in identifying an oxygen scavenging effect created by the mineral transformation behaviour occurring during reduction. It was found that oxygen-containing species such as Mg 2 Al 4 Si 5 O 18 (corderite) and Fe 2 Al 4 Si 5 O 18 (ferro-corderite) form within the reduction zone. It would appear that mineral composition is a more fundamental property than merely ash content in the gasification process, when viewed on an oxygen consumption basis.

Coal-fired power and heat production are the largest single source of Hg in the atmosphere, and in March 2005, the US-EPA ruled regarding Hg reduction from coal utilization in the USA. Appropriate Hg pollution control of technology, as well as reductions in the uses of Hg and coal-containing Hg can readily reduce the releases of Hg from coal utilities. Integrated multi-pollutant (SOx, NOx, particulate matter and Hg) control technologies may be a cost-effective approach. Prior to considering mitigation technologies, it is necessary to understand the quantity of mercury in the feed coal, its mode of occurrence (i.e. mineral or organic associations), its partitioning behaviour during the process, and the volume and species in which it is being emitted via stacks. These factors have all been investigated up to the point of release for the Sasol gasification and steam-raising plants, including other trace elements. The focus of this paper is to discuss the more recent environmental research developments by Sasol, where trace element simulation and validation of model predictions have been undertaken for the Sasol–Lurgi gasification process operating on lump coal. Fact-Sage thermodynamic equilibrium modeling was used to simulate the trace elements: Hg, As, Se, Cd and Pb gas phase and ash phase partitioning and speciation behaviour occurring in a fixed-bed pressurised gasifier. A Sasol–Lurgi Mark IV (MK IV) fixed-bed dry bottom (FBDB) gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, findings being used to validate the modeled results. This paper will focus on the behaviour of the volatile Class I trace elements: Hg, As, Se, Cd and Pb within the Sasol–Lurgi MK IV FBDB gasifier as function of coal quality. This study excludes the downstream gas cleaning partitioning and speciation behaviour of these elements, which will form the basis of a future paper. Good agreement between model-predictions and measurements have been attained in this study, with the exception of As. Hg, Cd, Pb, As and Se were all found to be highly volatile, partitioning into the gas phase. Hg was found to be the most volatile element during fixed-bed gasification and is present in the gas phase in the form of elemental Hg (g). As, Se, Cd and Pb have lower volatilities when compared to Hg, and they vary in an order: Hg > Se > Cd > Pb > As. Speciation predictions showed that: Hg, AsH 3 , H 2 Se, PbSe, Cd, CdS, and PbS/Pb/PbCl, species could potentially exist in the raw gas phase.

Gasification behaviour is particle dependent, whilst gasifier (reactor) behaviour is an averaging process of individual responses of each particle. It was hypothesized, that if it were possible to extract and analyze particles from different reaction zones within a gasifier, it may be likely to enhance the understanding of the contribution that these particles make towards gasification. This better understanding of the particle-type compositional responses could act as an enabler to further manipulate and improve gasifier performance. The primary focus of this study was to evaluate a sequential (axial) sampling “turn-out” methodology of a quenched fixed-bed commercial-scale Sasol–Lurgi gasifier, in order to present samples that accurately describe operational aspects occurring in the reaction zones within the reactor. Characterization of the chemical properties of the sample increments were expected to deliver distinct profiles of the drying, pyrolysis, reduction and combustion (ash-bed) zones, which could be used to advance the kinetic modeling capability of the process. In order to interpret the coal property transformational behaviour occurring within the commercial-scale gasifier, the proximate, Fischer tar, ultimate, and coal char CO2 reactivity analysis were conducted. The pyrolysis zone was found to be the largest reaction zone situated below the drying zone within the gasifier, followed by the reduction zone, and combustion (ash-bed) zones. Whilst the boundaries of the pyrolysis zone were very clearly defined by the residual volatile matter distribution profile, distinctive regional overlap with a “slow pyrolysis with gasification” region was observed in the bottom half of the pyrolysis zone, above which a “rapid de-volatilization” region existed. The reduction zone was found to also exhibit an overlap in zonal fronts, i.e. a gasification region occurred below the pyrolysis zone and co-existed in equal proportions, with an oxidation frontal region occurring above the combustion zone. The combustion zone was found to be very shallow, below which the ash-bed region existed. The findings clearly suggest that text book pictures showing axially-depicted reaction zones occurring within the fixed-bed gasifier, i.e. drying, pyrolysis, gasification and combustion, inadequately describe the “real” situation and in practice, overlap of reaction regions within zones indeed also transpire.