Liquid-metal based coal gasification

Abstract

Significant environmental benefits are secured in the gasification of coal, petroleum coke, biomass and similar carbonaceous materials, if the ash content is rendered non-leachable by the production of a vitreous slag. By combining float glass and generic melt circulation technologies, a new liquid-metal based process for gasification is proposed. The first case considered focuses on low sulphur feeds for which the proposed basic flowsheet involves a pressurised horizontal liquid-metal transport gasifier, to which solid charge materials are simply introduced onto the moving liquid metal surface via a barometric leg entrained in liquid metal. The solids are transported in a straight line through an air and steam top-blown zone to gasify the solids and produce a liquid sheet of molten slag, analogous to float glass manufacture, except that the liquid-metal is force circulated through the gasifier and external heat and mass transfer functions, involving direct countercurrent contacting in irrigated packed or moving beds to remove solid particulates and gaseous pollutants at high temperature. In the context of advanced power generation, the fuel gas is cooled to around 240°C to permit straightforward filtration and then is re-heated again by direct liquid-metal contacting to at least 730°C for transmission to the combustors of a gas turbine, which yields enhanced combined cycle efficiencies. More complex flow-sheets are then explored in which metallic zinc is added non-intrusively in a separate gas circuit independent of the fuel gas circuit to effect desulphurisation and to yield a valuable by-product, zinc sulphide. It is shown that not only can a slag be produced in a single stage air-blown cycle without the need for air separation and oxygen production, but also toxic components in air,in general, can be removed; in the particular case of mercury, this can be recovered directly in elemental form.