GB1393635A - Three stage gasification of coal - Google Patents

Abstract

1393635 Three stage gasification of coal BITUMINOUS COAL RESEARCH Inc 20 March 1972 [23 March 1972] 13290/73 Heading C5E A three-stage gasification of coal comprises, in sequence, (i) feeding a recycle stream of partially gasified char through the first stage of a threestage gasification apparatus; (ii) reacting the partially gasified char in the first stage with oxygen and superheated steam, thereby to produce a first stage synthesis gas comprising hydrogen and oxides of carbon; the reaction being performed at a pressure of at least 50 atmospheres and at a temperature such that the slag produced as a by-product of the reaction is in the molten state, e.g. at least 2500‹ F.; (iii) passing the first stage synthesis gas and slag through a second stage of the apparatus; (iv) reacting the first stage synthesis gas and slag in the second stage with feed coal and superheated steam, thereby to produce a second stage product gas comprising hydrogen, methane and oxides of carbon and having entrained therein a partially gasified char, the reaction being performed at a pressure of at least 50 atmospheres and at a temperature above the plasticity range of the feed coal, e.g. at least 1600‹ F.; (v) separating the second stage product gas and the partially gasified char from the slag; (vi) establishing a fluidized bed of the partially gasified char in a third stage of the apparatus; (vii) contacting the partially gasified char in the bed with a fluidizing stream of second stage product gas but at a temperature lower than that which prevails in the second stage, e.g. 1500‹ to 1600‹ F., thereby to produce a third stage product gas comprising hydrogen, methane and oxides of carbon and having entrained therein partially gasified char from the bed; (viii) separating the partially gasified char from the third stage product gas and recycling the separated char in admixture with steam to the first stage; and (ix) purifying and methanating the separated third stage product gas to produce a fuel gas comprising at least 70% by volume methane. The pressure in the first and second stages may be at least 70 atmospheres. Instead of steam and oxygen in the first stage, the recycle char may be reacted with preheated air, and the pressures in the first, second and third stages are 5 to 60 atmospheres. The first stage solids residence time is less than 2 seconds, the second stage coal residence time is less than 4 seconds, and the third stage partially gasified char residence time is at least 20 minutes and is at least 5 times the time taken for the fluidizing gas to pass through the stage. A constrictioa accelerates the stream of synthesis gas and slag as the stream passes from the first to the second stage. The reactants flow downwardly through the first and second stages and upwardly through the third stage. First stage slag solidifies upon passage through the second stage, and after removal for disposal at the end of the second stage reaction, the slag may be elutriated with steam to remove entrained partially gasified char, the elutriation steam and extracted char being thereafter returned to the process stream and the elutriated slag quenched prior to disposal. The second and third stage reactions are performed in a vessel having an internal wall defining a second stage reaction zone along the axis of the vessel and an annular third stage reaction zone surrounding but separated from the second stage zone by the wall, the two reaction zones being in open communication one with the other at the bottom of the vessel. Alternatively, the second and third stage reactions may be conducted in different reaction vessels, and the partially gasified char recovered from the second stage is separated into a coarse and a fine fraction, the coarse fraction thereafter being fed to the third stage for further reaction therein with second stage product gas, and the fine fraction being recycled in admixture with steam to the first stage without further reaction with the second stage product gas; and the third stage product gas and entrained char are quenched with water, treated to remove the char therefrom as a fine char stream and a coarse char stream, the fine char stream being admixed with steam and recycled to the first stage and the coarse char being admixed with steam and recycled to the third stage. The fluidizing gas used in the third stage may be the second stage product gas, purified and enriched with respect to hydrogen.