Patent Publication Number: US-4925644-A

Title: Partial oxidation of sulfur-containing solid carbonaceous fuel

Description:
This is a continuation-in-part of copending application Ser. No. 07/062,018, filed June 15, 1987, now U.S. Pat. No. 4,801,438. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to the simultaneous partial oxidation and desulfurization of a sulfur and silicate-containing solid carbonaceous fuel. More particularly, it pertains to a process for the partial oxidation of a sulfur and silicate-containing solid carbonaceous fuel for the production of gaseous mixtures comprising H 2  and CO and containing less than about 0.05 volume % of H 2  S and COS. 
     It would be highly desirable to use comparatively low-cost readily available sulfur and silicate-containing solid carbonaceous fuels such as coal, lignite, petroleum coke and mixtures thereof for the production of gaseous mixtures comprising H 2  +CO. These gaseous mixtures are commonly known as synthesis gas, reducing gas, and fuel gas depending on the actual gaseous composition. For example synthesis gas comprises a specific H 2  /CO mole ratio, reducing gas has a high reducing ratio e.g. H 2  +CO/H 2  O+CO 2 , and fuel gas contains supplemental CH 4  and has a relatively high heat content. 
     By conventional partial oxidation gasification processes, sulfur containing gases e.g. H 2  S and COS in the amount of about 0.5 to 2.0 mole % are produced along with the H 2  +CO. These sulfur-containing gaseous impurities are undesirable. They corrode piping and equipment upon contact; and, they deactivate catalysts. Accordingly, raw gas streams from the reaction zone may require additional downstream gas purification in order to remove the sulfur-containing gases. 
     The need for means to remove a major portion of the sulfur in synthesis gas as generated by the partial oxidation process is widely recognized. In particular, removal of sulfur from synthesis gas at high temperature is of great interest. The combined-cycle thermal efficiency is improved by eliminating the need to cool the product gases prior to low temperature acid-gas wet scrubbing. Gaseous impurities, including H 2  S, COS and CO 2  are removed from raw synthesis gas by low temperature condensation and by solvent absorption in a separate gas purification operation located downstream from the gas generator in coassigned U.S. Pat. No. 4,052,176. Further, in this process, it is necessary to cool the hot raw gas stream from a temperature of 2700° F. to -70° F. before the sulfur-containing gas are separated. 
     Reference may be made to U.S. Pat. No. 4,599,955 which describes the use of a horizontal cyclone type furnace operating at a temperature of at least 2192° F. with a sulfur fixing agent to provide molten slag with the necessary fluidity. Unfortunately molten slag will attack the refractory lining of the furnace and shorten its life. Further, any iron present in the coal is reduced to elemental form. In contrast, advantageously the subject invention described herein employs a vertical free-flow unobstructed refractory lined generator operating in the fly-ash mode at a lower temperature. No supplemental calcium is added to react with the silicates and no molten slag is produced. The cost to supplemental calcium to react with the silicates is avoided. Further, the life of the refractory lining in the gas generator is increased. Also, iron is tied up in the fly-ash as an iron silicate; and substantially no elemental iron is produced. 
     SUMMARY OF THE INVENTION 
     This invention relates to a process for the production of gaseous mixtures comprising H 2  +CO and containing less than 0.05 volume % of H 2  S +COS by the simultaneous partial oxidation and desulfurization of a sulfur and silicate-containing comminuted solid carbonaceous fuel. In addition to carbon, the solid carbonaceous fuel comprises (basis solid fuel) about 0.2 to 6.0 wt. % sulfur and about 0.1 to 30 wt. % silicate compounds including iron silicate. A separate gas purification step downstream from the gas generator to remove sulfur containing gases is not necessary in the subject invention. 
     In the process, a sulfur and silicate-containing solid carbonaceous fuel and a calcium-containing compound are reacted by partial oxidation in the reaction zone of a free-flow unobstructed down-flowing vertical refractory lined gas generator with a controlled amount of free-oxygen containing gas in the fly-ash mode, and with or without a temperature moderator so that an equilibrium oxygen concentration is produced in the gas phase in the reaction zone having a partial pressure which is less than about 10 -12  atmosphere. Preferably, the solid fuel in admixture with the calcium-containing additive is suspended in a gaseous medium in the reaction zone. A burner, for example, may be used to introduce a mixed stream of ground solid carbonaceous fuel and calcium-containing additive entrained in a gaseous medium downwardly into the unobstructed free-flow reaction zone of the partial oxidation gas generator along with a separate stream of free-oxygen containing gas and with or without a temperature moderator. The gaseous medium for entraining the solid fuel particulate in admixture with calcium-containing additive is selected from the group consisting of steam, nitrogen, CO 2 , recycle synthesis gas, free-oxygen containing gas, and mixtures thereof. 
     In the reaction zone, the O/C atomic ratio is in the range of about 0.3 to 1.1., and the H 2  O/solid carbonaceous fuel weight ratio is in the range of about 0.03 to .70. Further, the total moles of calcium in the reaction zone is at least equal to about 1.0, such as about 1.2, times the moles of sulfur in the solid carbonaceous fuel. The partial oxidation and desulfurization reactions take place simultaneously at a temperature which is at least about 400° F. below the softening temperature of the ash in the solid carbonaceous fuel at an increased thermal efficiency. The reaction temperature is preferably in the range of about 1600° F. to about 2,175° F. The pressure in the reaction zone is in the range of about 2 to 250 atmosphere. Substantially no elemental iron is produced; there is substantially no formation of additional calcium silicates; and about 70 to 98 wt. % of the carbon in the original solid carbonaceous fuel is converted into carbon oxides e.g. CO+CO 2 . At least about 90 wt. % of the sulfur in the solid carbonaceous fuel in the reaction zone is converted into calcium sulfide particulate matter which leaves the reaction zone along with the fly-ash entrained in the hot raw effluent gas stream comprising H 2 , CO, CO 2 , at least one gas selected from the group consisting of H 2  O, N 2 , NH 3 , CH 4  and A; less than 0.05 volume % of H 2  S and COS, and entrained fly-ash in admixture with calcium sulfide and particulate carbon. The mole ratio in the reaction zone for H 2  O/H 2  is in the range of abut 0.1 to 0.7. The hot raw effluent gas stream is cooled and cleaned without contact with water. 
    
    
     DISCLOSURE OF THE INVENTION 
     This invention relates to the partial oxidation of sulfur and silicate-containing solid carbonaceous fuels such as coal and petroleum coke, or mixtures thereof to produce synthesis gas, reducing gas, or fuel gas which is substantially free from sulfur. More particularly, it pertains to a partial oxidation process employing an additive system for in situ capturing the sulfur from the fuel at high temperature, and to a special gasifier operating mode to maximize the effectiveness of the additive. In the case of petroleum coke feedstocks, the additive system will also help with the removal of vanadium and nickel contaminants from the gasifier. 
     From the point of view of thermodynamics and cost, the addition of supplemental calcium-containing compounds to the partial oxidation gasifier has been found to be highly effective to capture the sulfur contained in the sulfur and silicate-containing solid carbonaceous fuels. Thermodynamics favor calcium, especially at low gasification temperatures, e.g. about 2175° F. and below. However, the synthesis gas produced must have a sufficiently low oxygen potential for calcium to capture the sulfur. To produce a synthesis gas with sufficiently low oxygen potential, the following methods of feeding may be used: (1) gas-entrained feeding of solid carbonaceous fuel particulates; (2) H 2  O or CO 2  slurry feeding with skimming of the carrier liquid; and (3) feeding slurries of solid fuel particulates in a liquid hydrocarbonaceous carrier. It has been found that by the subject process, a smaller amount of calcium needs to be added than in a high temperature slagging process. In the latter approach, the required large amounts of calcium adversely affect the slag viscosity, especially if metallic iron is also present. In addition the lower steam to carbonaceous fuel ratio in the present process will increase the gasification efficiency. 
     The sulfur and silicate-containing solid carbonaceous fuel feed materials in the subject process include by definition coal e.g. anthracite, bituminous, subbituminous, lignite; coke made from coal; petroleum coke; oil shale; tar sands; pitch; and mixtures thereof. In addition to carbon, solid carbonaceous fuel comprises (basis solid fuel) about 0.2 to 6.0 wt. % of sulfur and about 0.1 to 30 wt. % of silicate compounds including iron silicate. The solid carbonaceous fuels are preferably ground to a particle size, so that 100% of the material passes through an ASTM E 11-70 Sieve Designation Standard 1.4 mm (Alternative No. 14) and at least 80% passes through an ASTM E 11-70 Sieve Designation Standard 425 μm (Alternative No. 40). The moisture content of the solid carbonaceous fuel particles is in the range of about 0 to 10 wt. %, such as 2 to 5 wt. %. The feedstock may be dried when necessary. 
     The ground solid carbonaceous fuel is mixed with a calcium-containing material selected from the group consisting of calcium oxide, calcium hydroxide, calcium carbonate, calcium acetate, calcium formate, calcium hydride, calcium nitrate, calcium chloride and mixtures thereof. Alternatively, the calcium-containing material may be mixed with the solid carbonaceous fuel before, during or after the solid carbonaceous fuel is ground. Lime e.g. calcium oxide is the preferred calcium-containing material. Sufficient supplemental calcium containing compound is introduced so that the total amount of calcium in the reaction zone comprising the moles of calcium in solid fuel and the moles of calcium in the supplemental calcium containing compound is at least equal to about 1.0, such as about 1.2, times the moles of sulfur in the solid carbonaceous fuel. 
     The solid carbonaceous fuel in admixture with supplemental calcium-containing compound is introduced into the reaction zone of a vertical refractory-lined, unobstructed, free-flow down-flowing noncatalytic partial oxidation gas generator by way of a burner. Simultaneously, a stream of free-oxygen containing gas, and optionally a temperature moderator are introduced into the gas generator. When used, the temperature moderator is selected from the group consisting of steam, nitrogen, CO 2 , cooled recycle synthesis gas, and mixtures thereof. The temperature moderator may be a separate stream passing through a separate passage in the burner, or it may be in admixture with the free-oxygen containing gas. Single and double annulus type burners for simultaneously introducing the feedstreams into the partial oxidation gas generator are shown in coassigned U.S. Pat. Nos. 3,528,930 and 3,847,564, which are incorporated herein by reference. Other suitable burners and gas generators for use in the subject process are shown and described in coassigned U.S. Pat. Nos, 4,197,281; 4,218,222; and 4,328,006, which are incorporated herein by reference. Typical partial oxidation gas generating processes employing gaseous and liquid CO 2  -solid carbonaceous fuel feeds are shown in coassigned U.S. Pat. Nos. 3,976,442, and 3,976,443 respectively, and are incorporated herein by reference. 
     The mixture of solid carbonaceous fuel and calcium-containing material is preferably introduced into the partial oxidation reaction zone as solid particles entrained in a gaseous medium e.g. steam, nitrogen, CO 2 , recycle synthesis gas, free-oxygen containing gas, and mixtures thereof. For example, a mixed stream of ground solid carbonaceous fuel and calcium-containing additive entrained in a gaseous medium is introduced into the reaction zone of the partial oxidation gas generator along with a separate stream of free-oxygen containing gas and with or without a temperature moderator. 
     Alternatively, pumpable slurries of said mixture of solid carbonaceous fuel and calcium-containing material in a liquid carrier comprising a liquid hydrocarbonaceous fuel or liquid CO 2  may be introduced into the gas generator. For example, the liquid slurry may comprise 50-70 wt. % of solid carbonaceous fuel and calcium-containing material and the remainder is liquid CO 2 . CO 2  is skimmed from the CO 2  -solid fuel mixture prior to the mixture being introduced into the burner at a temperature in the range of about -67° F. to 100° F. depending on the pressure. After skimming, the mixture of solid carbonaceous fuel and CO 2  contains 10 to 30 wt. % of CO 2  (basis weight of feed). In another embodiment, the feedstream comprises a slurry of liquid hydrocarbonaceous material and solid carbonaceous fuel. A small amount of H 2  O in liquid phase may be mixed with the liquid hydrocarbonaceous carrier, for example as an emulsion. A portion of the H 2  O i.e., about 0 to 25 weight % of the total amount of H 2  O present may be introduced as steam in admixture with free-oxygen containing gas. In one embodiment, the fuel feed to the gas generator comprises pumpable aqueous slurry comprising 50 to 70 wt. % solid particulates. In such case, steam skimming is used to reduce the amount of H 2  O entering the gas generator to abut 5 to 10 wt. % (basis weight of feed) prior to introducing the solid fuel into the reaction zone. 
     The term liquid hydrocarbonaceous material as used herein to describe suitable liquid carriers is intended to include various materials, such a liquified petroleum gas, petroleum distillates and residues, gasoline, naphtha, kerosene, crude petroleum, asphalt, gas oil, residual oil, tar sand oil, shale oil, coal derived oil, aromatic hydrocarbon (such as benzene, toluene, xylene fractions), coal tar, cycle gas oil from fluid-catalytic-cracking operation, furfural extract of coker gas oil, methanol, ethanol and other alcohols, by product oxygen containing liquid hydrocarbons from oxo or oxyl synthesis, and mixtures thereof. 
     The partial oxidation reaction takes place in the reaction zone of the gas generator at a temperature which is at least about 400° F. below the softening temperature of the ash in the solid carbonaceous fuel. Preferably, the temperature in the reaction zone is in the range of about 1600° F. to about 2,175° F. This will prevent slag from sticking to the walls of the gasifier. Usually the autothermal reaction temperature of the reaction zone is below about 2,175° F. The pressure in the reaction zone is in the range of about 2 to 250 atmospheres, say 10 to 100 atmospheres. The atoms of free-oxygen plus atoms of organically combined oxygen in the solid carbonaceous fuel per atom of carbon in the solid carbonaceous fuel (O/C atomic ratio) may be in the range of 0.5 to 1.5. With free-oxygen containing gas in the reaction zone the broad range of said O/C atomic ratio may be about 0.3 to 1.4, such as about 0.7 to 1.1. The free-oxygen is intended to include air, oxygen enriched air, i.e., greater than 21 mole % oxygen, and substantially pure oxygen, i.e. greater than 95 mole % oxygen, (the remainder comprising N 2  and rare gases). The free-oxygen containing gas and temperature moderator is provided in a controlled amount so than an equilibrium oxygen concentration is provided in the gas phase in the reaction zone having a partial pressure which is less than about 10 -12  atmosphere. It was unexpectedly found that this limitation is necessary for the calcium in the reaction zone to capture the sulfur. In conventional partial oxidation processes, such as coassigned U.S. Pat. No, 4,007,018 Slater et al., the equilibrium oxygen concentration in the gas phase in the reaction zone has a partial pressure which is greater than 10 -8  atmospheres. Under such conditions in Slater et al. there would be substantially no sulfur-capture in the fly-ash mode such as provided in the subject process. Further, the amount of oxygen as specified in the subject process will prevent the existence of elemental iron. The mole ratio H 2  O/H 2  in the reaction zone is in the range of about 0.1 to 0.7 such as about 0.2 to 0.4. The weight ratio H 2  O/solid carbonaceous fuel in the reaction zone is in the range of about 0.03 to 0.7. 
     The raw product gas leaving the reaction zone may have the following composition in mole percent: H 2  8 to 60, CO 8  to 70, CO 2  1 to 20, H 2  O 1 to 40, CH 4  nil to 30, H 2  S +COS nil to 0.05, N 2  nil to 85 and A nil to 2.0. 
     Substantially no elemental iron is produced in the reaction zone; there is substantially no formation of additional calcium silicates; and about 90 to 98 wt. % of the carbon in the original solid carbonaceous fuel is converted into carbon oxides e.g. CO+CO 2 . About 70 to 90 volume % of the carbon oxides comprises CO. At least about 90 wt. % of the sulfur in the solid carbonaceous fuel in the reaction is converted into calcium sulfide particulate matter which leaves the reaction zone along with the fly-ash entrained in the hot raw effluent gas stream. The calcium sulfide particulate matter leaves the reaction zone entrained in the hot raw product gas mixture along with the fly-ash by way of an outlet in the bottom of the reaction zone. Preferably, the bottom outlet in the reaction zone is coaxial with the central vertical axis of the unobstructed reaction zone in the partial oxidation synthesis gas generator. 
     By definition, fly-ash is the unmolten remnant of combusted particles of the solid carbonaceous fuel. On a dry basis, the fly-ash may comprise in wt. % SiO 2  10 to 50, Al 2  O 3  10 to 50, iron oxides and sulfides 0 to 40, and others. The fly-ash particles in the hot raw gas stream have a particle size in the range of about 0.01 to 1000 microns. The fly-ash has not reached the melting temperature of the mineral matter which is originally part of the solid carbonaceous fuel. Molten slag is substantially molten ash and represents the fused mineral matter of the solid carbonaceous fuel. 
     Advantageously, in the subject process there is substantially no reaction between the supplemental calcium-containing materials and the silicates in the solid carbonaceous fuel feed. Accordingly in contrast with a molten slag producing process operating at a higher temperature no supplemental calcium-containing material is introduced into the reaction to provide for this slagging reaction. Since the supplemental calcium-containing material reacts only with the sulfur in the solid carbonaceous fuel feed to produce calcium sulfide, a smaller amount of calcium-containing material is required in the instant process at a great cost savings. 
     The mixture of calcium sulfide, fly-ash and particulate carbon entrained in the hot raw effluent stream of synthesis gas from the reaction zone is separated from the synthesis gas. In a preferred embodiment, the hot raw effluent gas stream from the partial oxidation reaction may be cooled and cleaned by contact with a liquid hydrocarbonaceous fuel cooling and scrubbing agent in a conventional gas quench tank and/or in gas scrubbing equipment. For example reference is made to coassigned U.S. Pat. No. 4,007,018, which is incorporated herein by reference. A slurry of calcium sulfide, fly-ash and particulate carbon in liquid hydrocarbonaceous fuel is produced. After being steam stripped and after separating H 2  S, the sulfur-depleted slurry may be recycled to the front end of the process. There it is mixed with fresh sulfur and silicate containing solid carbonaceous fuel and make-up calcium-containing material in the preparation of fresh feed mixture to the partial oxidation gas generator. The mixture of steam and H 2  S is then sent to a sulfur-recovery facility. Water is avoided as the cooling and scrubbing agent for the hot raw effluent gas stream from the gas generation in order to prevent the calcium sulfide entrained in the hot raw effluent gas stream from reacting with the water to produce unwanted H 2  S in the product gas. 
     Alternatively, the hot raw effluent gas stream may be cooled by indirect heat exchange with a coolant before the gas scrubbing step. For example, the hot raw effluent ga stream may be passed through a waste heat boiler to produce steam and then scrubbed with a liquid hydrocarbonaceous fuel. 
     Various modifications of the invention as herein before set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be made as are indicated in the appended claims.