An understanding of the behaviour of a number of element phases impacting on a commercial-scale Sasol-Lurgi FBDB gasifier

Abstract

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 Mg2Al4Si5O18 (corderite) and Fe2Al4Si5O18 (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