December 2011 - SIMA

Syngas options include a coal gasifier, coke ovengas, or BOF gas. The big advantage of coalgasification is that lower grade, inexpensive domesticcoals can be used to produce a high quality reducinggas for the MIDREX Shaft Furnace.Coal GasificationThere are three general types of coal gasifiers: fixedbed, entrained flow, and fluidized bed. All threetechnologies are based on partial oxidation(gasification) of a carbonaceous (carbon containing)feed material.The general partial oxidation reaction is:2 CH + O ———> 2 CO + n H2n 2In addition to the desired CO and H , the syngas2exiting a gasifier also contains CO , H O, CH , H S,2 2 4 2NH , and particulates. If a fixed bed gasification3technology is utilized, the syngas will also containaromatic organic compounds.While each of the gasifier types can make anacceptable reducing gas for a MIDREX DR Plant,the fixed bed and fluidized bed technologies will bethe preferred choices for many locations becausethey can accommodate the high ash domestic coals.Countries of interest include India, China, and theCIS. The leading fixed bed process is the LurgiGasification process; it is well-proven, with over 102gasifiers in commercial operation worldwide, theearliest of these built in 1955. Figure 2 shows Lurgigasifiers at the Sasol Plant in Secunda, South Africa.There are a number of fluidized bed processes,including the KBR Transport Gasifier, known asTRIG, and the U-Gas Process, which is licensedby Synthesis Energy Systems. These processes arein the early stages of commercialization, but showgood promise.The Coal Gasification ProcessFigure 3 shows a simplified MXCOL processflowsheet. In the gasification processes, coal isgasified at elevated pressures by reacting with highpressure steam and high purity oxygen to produce asyngas suitable for the production of fuels andchemicals, power generation or the reduction of ironore. The fixed bed and fluidized bed gasifiers operateFigure 2Lurgi Gasifiers in Secunda, South Africaat a temperature below the ash melting point so thecoal ash is discharged from the gasifier as a solid.Because of this low operating temperature, thesetechnologies require significantly lower quantities ofoxygen than the entrained flow gasification processeswhich melt the ash.The syngas exiting the gasifier is hot, dirty, andcontains a significant amount of non reducing gascomponents. Downstream of the gasifier, the syngasis cleaned and conditioned to remove most of theundesired components and produce saleablecommodities such as sulfur and petrochemical plantfeedstocks.Figure 3MXCOL FlowsheetDECEMBER-2011/8