Patent GB-1393636-A (GB)
Title: Two-stage gasification of coal
IPC: C07C67/00
Filed: 1973-03-20
Abstract: 1393636 Coal gasification BITUMINOUS COAL RESEARCH Inc 20 March 1973 [23 March 1972] 13291/73 Heading C5E A two-stage gasification of coal comprises, in sequence, (i) passing a stream of partially gasified recycle char upwardly through the first stage of a two-stage gasification reactor vessel; (ii) reacting the partially gasified recycle char therein with an upwardly flowing stream of superheated steam and oxygen to produce a stream of synthesis gas comprising hydrogen and oxides of carbon; the reaction being performed at 2200-3800‹ F. and at 500-2000 p.s.i.g. pressure, and with a gas velocity of from 2-12 feet per second and a char residence time of from 0À5 to 5 seconds; (iii) discharging a slag from the bottom of the first stage; (iv) accelerating the synthesis gas from the first stage upwardly through a throat into a second stage of the two-stage gasification reactor vessel; (v) injecting feed coal and a further quantity of superheated steam into the synthesis gas while passing through the throat, the velocity of the gases through the throat and the direction of introduction of the feed coal and superheated steam being such as to prevent substantially any counter-current flow of coal or gases from the second stage to the first stage; (vi) passing the mixture of synthesis gas, feed coal and superheated steam upwardly through the second stage and reacting them therein to produce a partially gasified char entrained in a product gas comprising hydrogen, methane and oxides of carbon; the second stage reaction being performed at 1500-2000‹ F. and at 500-2000 p.s.i.g. pressure, and with a gas velocity of from 2-12 feet per second and solids residence time of from 5 to 15 seconds; (vii) separating the partially gasified char from the product gas in the second stage and recycling the separated char to the first stage; and (viii) treating the product gas to remove oxides of carbon, hydrogen sulphide and other impurities, and methanating the purified product gas to provide a fuel gas containing at least 70% by volume of methane. The synthesis gas is accelerated by passage through a throat having a cross-sectional area less than 20% of the maximum cross-sectional area of the first stage of the two-stage gasification reactor vessel. In step (ii), the first stage reaction is conducted at 2400-3200‹ F., at 750-1500 p.s.i.g. pressure, and with a gas velocity of 5-10 feet per second and a char residence time of from 1-3 seconds. In step (vi), the second stage reaction is performed at 1600-1900‹ F. at 750-1500 p.s.i.g. pressure, and with a gas velocity of from 4-8 feet per second. In step (v), the coal and superheated steam are injected into the stream of synthesis gas at respective velocities equal to at least twice the velocity of the synthesis gas through the throat, e.g. at 30 to 200 feet per second. The coal and/or superheated steam is or are projected as two or more streams differing in velocity. In step (v) the superheated steam may be injected upwardly into the stream of synthesis gas and as a plurality of separate streams spaced around the throat. In step (iii), molten slag is discharged from the first stage of the two-stage reactor under gravity through a conical slag discharge zone which constitutes a bottom section of the first stage reaction zone, the molten slag being retained therein for predetermined length of time to permit reaction of char entrained therein with the superheated steam and oxygen fed as reactants to the first stage of the gasification reactor vessel. The conical discharge section may have a minimum crosssectional area less than 4% of the maximum cross-sectional area of the first stage of the twostage gasification reactor vessel. In step (vii), the separated char is scrubbed with steam in a fluidized bed formed under the fluidizing action of the scrubbing steam, prior to recycle to the first stage, the gaseous effluent from the scrubbing step being mixed with the product gas stream prior to purification and methanation in step (viii). The recycle char may be fed to the first stage in admixture with superheated steam using a steam ejector driven by superheated steam at a pressure at least 1À3 times the pressure in the first stage.