Patent Publication Number: US-5250083-A

Title: Process for production desulfurized of synthesis gas

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a process for the partial oxidation of a sulfur- and silicate-containing carbonaceous fuel to produce a synthesis gas with reduced sulfur content. More particularly, this invention relates to a process for the partial oxidation of a sulfur- and silicate-containing carbonaceous fuel in the presence of a temperature moderator, an oxygen-containing gas, and a sulfur capture additive which comprises a calcium-containing compound portion, a sodium-containing compound portion, and a fluoride-containing compound portion to produce a synthesis gas with reduced sulfur content and a molten slag which comprises (1) a sulfur-containing sodium-calcium-fluoride silicate phase and (2) a sodium-calcium sulfide phase. 
     It is well known by those skilled in the art that synthesis gas comprising primarily H 2  and CO, together with various amounts of other gases, may be produced via the partial oxidation gasification of a carbonaceous fuel at elevated temperatures. References disclosing such a process include co-assigned U.S. Pat. Nos. 2,818,326 (Eastman et al.), 2,896,927 (Nagle et al.), 3,998,609 (Crouch et al.), and 4,218,423 (Robin et al.), all incorporated herein by reference. Such carbonaceous fuels include crude residue from petroleum distillation and cracking processes, petroleum distillates, reduced crudes, whole crudes, asphalts, washed and unwashed coals, coal tars, coal derived oils, petroleum cokes, shale oils, tar sand oils, sludge and mixtures thereof. The production of synthesis gas from such fuels is described by, for example, co-assigned U.S. Pat. Nos. 3,544,291  (Schlinger et al.), 3,976,442 (Paull et al.) and 3,996,026 (Cole), all incorporated herein by reference. 
     It would be highly desirable to use comparatively low cost and readily available sulfur- and silicate-containing solid carbonaceous fuels for the production of synthesis gas comprising H 2  and CO. However, in conventional partial oxidation gasification processes, sulfur-containing gases (e.g. H 2  S and COS) in the amount of about 0.1 to 2.0 mole percent are produced along with the H2 and CO. These sulfur-containing gaseous impurities are undesirable, as they are pollutants, corrode piping and equipment upon contact, and deactivate downstream catalysts. Accordingly, raw gas streams from the reaction zone may require additional downstream gas purification in order to remove the sulfur-containing gases prior to the use of the synthesis gas in chemicals production, power generation, and the like. 
     Conventional processes employed in the downstream removal of sulfur and sulfur compounds from synthesis gas (often called acid gas removal processes) are described, for example, in R.F. Probstein and R.E. Hicks, Synthetic Fuels (1982) at pp. 210-21, and include both liquid absorption and solid absorption techniques. Such techniques generally require that the synthesis gas first be cooled from its production temperature of 1500-3000° F., say 1700-2200° F. to a lower temperature prior to removal of sulfur and sulfur-containing compounds. It would thus be advantageous to remove or reduce the concentration of sulfur and sulfur-containing compounds in the synthesis gas during or immediately after the production of the synthesis gas, while it is at high temperatures (i.e. in-situ sulfur removal). This would improve the thermal efficiency of the synthesis gas production process and in addition reduce costs associated with gas cooling and purification equipment and maintenance. References describing high temperature gas desulfurization are as follows: 
     Co-assigned U.S. Pat. No. 4,778,484 (Suggitt et al.), and incorporated herein by reference discloses a process for the production of desulfurized synthesis gas from a sulfur-containing carbonaceous fuel, the process comprising: (a) reacting a first portion of the fuel with an oxygen-containing gas and a temperature moderator to produce a synthesis gas; and (b) passing a second portion of the fuel in admixture with a portion of the synthesis gas and an iron-containing additive to a second reactor and thereby reacting to produce additional H 2  and carbon oxides and particulate matter comprising iron oxysulfide derived from the interaction of the iron-containing additive and the sulfur-containing gases produced by partial oxidation of the fuel. In one embodiment of this invention, an alkali metal or alkali earth metal catalyst selected from Group IA or IIA of the Periodic Table of Elements is introduced into the second reactor in admixture with the fuel and iron-containing additive. 
     Co-assigned U.S. Pat. No. 4,776,860 (Najjar et al.) and incorporated herein by reference discloses a process for the production of desulfurized synthesis gas from a sulfur-containing carbonaceous fuel, the process comprising: (a) reacting a first portion of the fuel with an oxygen-containing gas and a temperature moderator to produce a synthesis gas; and (b) reacting a devolatized second portion of fuel and carbon from the unreacted first portion of fuel in a second reactor in the presence of a calcium-containing additive to produce additional H 2  and carbon oxides and to achieve in-situ conversion of sulfur-containing gases (e.g. H 2  S, COS) into calcium sulfide. In one embodiment of this invention, an alkali metal or alkali earth metal catalyst selected from Group IA or IIA of the periodic table is introduced into the second reactor in admixture with the fuel and iron-containing additive. 
     Co-assigned U.S. Pat No. 4,801,438 (Najjar et al.) discloses a process for the simultaneous partial oxidation and desulfurization of an ash-containing solid carbonaceous fuel to produce a synthesis gas low in sulfur. The process comprises reacting the fuel and a calcium-containing material in the presence of an oxygen-containing gas and a temperature moderator to produce synthesis gas and entrained molten slag in admixture with calcium sulfide and the silicates of calcium. 
     However, the aforesaid prior art does not teach or suggest the subject process which employs a novel sulfur capture additive which comprises a calcium compound portion, a sodium compound portion, and a fluoride compound portion that produces synthesis gas with a reduced sulfur content, and a novel molten slag with increased solubility for sulfur. 
     SUMMARY OF THE INVENTION 
     The instant invention is a process for the partial oxidation of a sulfur- and silicate-containing carbonaceous fuel, preferably coal, to produce a synthesis gas with a reduced sulfur content. The process comprises partially oxidizing the fuel at a temperature in the range of 1900°-2600° F., preferably 2000°-2400° F. in the presence of a temperature moderator, an oxygen-containing gas, and a sulfur capture additive which comprises a calcium-containing compound portion present in a concentration of 0.5-10.0, preferably 3.0-7.0 weight percent (based on the weight of the carbonaceous fuel), a sodium-containing compound portion present in a concentration of 0.1-4.0, preferably 1.0-2.5 weight percent (based on the weight of the carbonaceous fuel) and a fluoride containing compound portion present in a concentration of 0.05 to 3.0 weight percent, preferably 0.3 to 1.5 weight percent (based on the weight of the carbonaceous fuel). 
     The partial oxidation of the fuel in the presence of the sulfur capture additive produces a synthesis gas comprising H 2  and CO with a reduced sulfur content and a molten slag. The process of the instant invention is advantageous in that it produces a synthesis gas with reduced sulfur content, thereby eliminating or reducing the need for cooling or further downstream treatment of the synthesis gas for sulfur removal prior to use. In addition it produces a molten slag comprising (1) a sulfur-containing sodium-calcium-fluoride silicate phase; and (2) a sodium-calcium sulfide phase. Further the slag freely flows from the partial oxidation reaction, thereby improving reactor operation and facilitating slag disposal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is the object of this invention to provide a process for the partial oxidation of a sulfur- and silicate-containing carbonaceous fuel to produce a synthesis gas comprising H 2  and CO and having a reduced sulfur content for downstream use and a molten slag which flows easily from the partial oxidation reactor, thereby improving reactor operation and facilitating disposal of the slag. 
     It is one feature of the process of this invention that a sulfur- and silicate-containing carbonaceous fuel is partially oxidized at a temperature of 1900°-2600° F. in the presence of a temperature moderator, an oxygen-containing gas, and a sulfur capture additive which comprises a calcium-containing compound portion, a sodium-containing compound portion, and a fluoride-containing compound portion to produce a synthesis gas with reduced sulfur content and a molten slag which comprises (1) a sulfur-containing sodium-calcium-fluoride silicate phase and (2) a sodium-calcium sulfide phase. 
     The sulfur- and silicate-containing carbonaceous fuel which may be employed in the process of this invention is typically solid at ambient temperatures and contains ash. The fuel may be selected from the group consisting of unwashed or washed coal (including anthracite, bituminous, sub-bituminous, and lignite), crude residue from petroleum distillate, reduced crude, whole crude, asphalt coal, coal tar, coal derived oil, petroleum coke, shale oil, tar sand oil, sludge and mixtures thereof. Such fuels ordinarily contain varying amounts of silicate, sulfur and sulfur compounds, as well as metal and metal compounds. The metals include vanadium, nickel, and iron. Typically, these fuels may contain ash in amounts as little as 0.1 wt. % -0.5 wt. % or as much as 20 wt. % -40 wt. %. They may also contain water in amounts as low as 0 wt. % -10 wt. % or as much as 30 wt. % -40 wt. % or more. Although the fuel may be used without reducing the moisture content, it is preferred, to facilitate grinding and slurrying in the case of those fuels containing large amounts of water, to pre-dry the fuel to a moisture content to 2 wt. % -20 wt. % depending on the nature of the fuel. The fuel may be ground to a particle size so that preferably 100 wt. % passes through a 14 mesh sieve and greater than 50 wt. % has a particle size within the range of 14-325 mesh sieve. 
     In the case of some pitches, asphalts, and tar sand, it may be possible to feed them as liquids by heating them to just below their decomposition temperature. A ground solid fuel alone may be employed as charge; but preferably the ground solid fuel is slurried in a liquid vaporizable hydrocarbon or water, or it is entrained in a gaseous medium. The fuel may be slurried with water, a liquid hydrocarbon fuel, liquid C0 2  or mixtures thereof. The preferred slurrying agent is water, and it is preferably present in the charge to gasification in amount of 30-120 parts per 100 parts of solid coal or petroleum coke. In typical operations, the fuel is slurried with the slurrying agent in a slurry preparation tank, where the slurry is prepared to the desired concentration, and thereafter pumped to the partial oxidation reactor by means of a slurry feed pump. Alternatively, the fuel may be entrained in a gas such as steam, carbon dioxide, nitrogen, recycle synthesis gas, air, etc. When the fuel is a liquid or a gas, no slurrying liquid is admitted with the charge fuel. 
     Typical liquid hydrocarbon charge fuels which may be employed include various oils derived from petroleum including distillates and residues such as crude petroleum, reduced crude, gas oil, cycle gas oil, coker gas oil, furfural extract of coker gas oil, etc; oil derived from coal, tar sands, lignite, etc. Such liquids may be employed in the form of a slurry which includes 100 parts of solid coal or coke with 40-150 parts, preferably 50-100 parts, say 55-60 parts of liquid. Gaseous hydrocarbon charge fuels may also be employed. 
     The slurrying agent or entraining gas employed may also act as a temperature moderator for the partial oxidation reaction. If desired, there can also be charged a supplemental temperature moderator to moderate the temperature in the reaction zone. Moderators may be necessary when the charge includes liquid vaporizable hydrocarbons in order to simultaneously achieve desired conversion level (optimum efficiency) and temperature (fixed by materials of construction). When employed, they may be admitted with any of the charge streams or separately. Typical temperature moderators may include superheated steam, saturated steam, carbon dioxide-rich gas, cooled exhaust from downstream turbines, nitrogen-in-air, by-product nitrogen from a conventional air separation unit, etc. 
     The charge to the partial oxidation reactor also includes an oxygen-containing gas. Typical of such gases which contain at least about 21 wt.% oxygen include air, oxygen-enriched air (containing more that 21 wt. % oxygen), substantially pure (e.g. greater than 95 wt. %) oxygen, etc. Commonly, the oxygen-containing gas contains oxygen plus other gases derived from the air from which the oxygen was prepared. The atomic ratio of oxygen (in the oxygen-containing gas) to carbon (in the fuel) may be 0.6-1.2. When the oxygen-containing gas is substantially pure oxygen, the ratio may be 0.7-1.2, preferably 0.9-1.0. When it is air, the ratio may be 0.8-1.2, say 1.2. When water is employed as the temperature moderator, the weight ratio of water to carbon in the fuel may be 0-2.0, preferably 0.2-0.6, say 0.5. In all of the above cases, the atomic ratio of oxygen to carbon is such that less than 60% of the oxygen stoichiometrically required for coal combustion is supplied in the process of the instant invention. 
     The charge to the partial oxidation reactor additionally comprises a sulfur capture additive which comprises calcium-containing compound portion, a sodium-containing compound portion, and a fluoride-containing compound portion. The calcium-containing compound portion is preferably selected from the group consisting of calcium oxides, calcium carbonates, calcium hydroxide, calcium acetate and mixtures thereof. 
     The sodium-containing compound portion of the sulfur capture additive is preferably selected from the group consisting of sodium oxides, sodium carbonates, organic sodium-containing compounds, sodium silicates, sodium aluminum silicates and mixtures thereof. Sodium carbonates are particularly preferred for use as the sodium-containing compound portion of the sulfur capture additive. 
     The fluoride-containing compound portion of the sulfur capture additive is selected from the group consisting of sodium fluoride, potassium fluoride, calcium fluoride and mixtures thereof. 
     In the process of the instant invention, the sulfur- and silicate-containing carbonaceous fuel is partially oxidized in the presence of a temperature moderator, an oxygen-containing gas, and the above described sulfur capture additive. In a preferred embodiment of the instant invention, the sulfur capture additive is mixed with the fuel prior to the fuel being charged into the partial oxidation reactor. If the fuel is slurried with water as previously described, the sulfur capture additive may first be ground together or separately with the fuel to a particle size of 0.1-2000 microns and then admixed with the fuel-water slurry prior to introduction to the partial oxidation reactor. The sulfur capture additive comprises 0.5-10.0, preferably 3.0-7.0 wt. % of calcium-containing compound portion and 0.1-4.0, preferably 1.0-2.5 wt. % sodium-containing compound portion, and 0.05-3.0, preferably 0.3-1.5 wt. % fluoride-containing compound portion. All of the above described weight percents are based on the weight of the solid carbonaceous fuel. 
     The sulfur- and silicate-containing carbonaceous fuel in admixture with the above described sulfur capture additive is introduced, together with oxygen-containing gas and temperature moderator (which may be the slurrying medium for the fuel, e.g. water) into a partial oxidation synthesis gas generator, typified by that set forth in co-assigned U.S. Pat. No. 2,818,216 (Eastman et al.). This generator includes an annulus-type burner (such as is typified by that set forth in co-assigned U.S. Pat. No. 2,928,460, (Eastman et al.), U.S. Pat. No. 4,328,006 (Muenger et al.) or U.S. Pat. No. 4,328,008 (Muenger et al.) in a vertical cylindrical steel pressure vessel lined with a thermal refractory material. 
     The partial oxidation reaction takes place in the reaction zone of the gas generator at a temperature in the range of 1900°-2600° F., preferably 2000°-2400° F. and a pressure of 2-250 atmospheres, say 10-100 atmospheres. The oxygen-containing gas and temperature moderator are provided in a controlled amount so that an equilibrium oxygen concentration is provided in the gas phase in the reaction zone having a partial pressure which is less than about 10 -13  atmospheres. 
     The partial oxidation of the sulfur- and silicate-containing carbonaceous fuel in the presence of a temperature moderator, an oxygen-containing gas and the above described sulfur capture additive produces a synthesis gas with the following approximate concentrations: 
     
         ______________________________________                                    
Component             Mole %                                              
______________________________________                                    
H.sub.2               8-60                                                
CO                    8-70                                                
CO.sub.2              1-20                                                
H.sub.2 O             1-40                                                
CH.sub.4              0-30                                                
H.sub.2 S + COS       &lt;0.5                                                
N.sub.2               0-85                                                
NH.sub.3              0-2.0                                               
______________________________________                                    
 
    
     The gas may be cooled and otherwise further treated via conventional means prior to use for power generation, chemicals production, and the like. 
     In addition, a molten slag which comprises (1) a sulfur-containing sodium-calcium-fluoride silicate phase and (2) a sodium-calcium sulfide phase is produced by the process of the instant invention. By definition, molten slag is the molten remnant of particles of the solid carbonaceous fuel which have been subjected to partial oxidation in the process of the instant invention. The sodium-calcium-fluoride silicate slag phase comprises about 30-70 wt. % of the reaction products of the sodium-containing compound component of the sulfur capture additive, about 90-100 wt. % of the fluoride-containing compound component of the sulfur-capture additive, 98-100 wt. % of the silicate material of the original carbonaceous fuel, about 5-45 wt % of the calcium-containing compound component of the sulfur capture additive, and about 0.5-20 wt. % of the captured sulfur compounds. The sodium-calcium sulfide slag phase comprises the reaction products of the remaining calcium-containing compound and remaining sodium-containing compound components of the sulfur capture additive and captured sulfur compounds. This phase is essentially a solution of calcium sulfide and sodium sulfide. Increased sulfur concentration in each phase represents enhanced sulfur capture from the synthesis gas during partial oxidation, with attendant advantages as previously described. 
     Molten slag typically accumulates on the vertical walls of the partial oxidation reactor, and ideally flows freely out of the reactor via the outlet located at the bottom of the reaction zone. However, incomplete melting of the ash often causes the viscosity of the slag to increase, resulting in the accumulation of slag, together with its metal constituents, on the walls and refractory of the gasification reactor. This accumulation of slag often leads to reactor operability problems as well as potential damage to the reactor refractory. 
     The process of the instant invention is advantageous over other processes which employ only a calcium-containing sulfur capture agent in that the presence of the sodium-containing component of the sulfur capture additive is necessary for the formation of the above described sodium-calcium sulfide and sodium-calcium-fluoride silicate slag phases. The solubility of sulfur in the sodium-calcium sulfide slag phase produced by the process of the instant invention is enhanced as compared with a typical calcium sulfide slag phase which is produced by processes which employ a calcium-containing sulfur capture additive without the presence of a sodium-containing component. 
     In the subject process, unexpectedly the fluoride component of the sulfur capture additive goes only into the silicate phase and not into the sulfide phase. Further, there is an increased solubility of the sulfur in the new and unobvious Na-Ca-Fl phase. 
     The process of the instant invention is additionally advantageous in terms of sulfur capture over other processes which employ only a calcium-containing sulfur capture agent in that the sodium-calcium-fluoride silicate phase produced is fluid at a lower temperature than a corresponding calcium silicate phase. This greater fluidity allows the partial oxidation reactor to be operated at a lower temperature without attendant slag plugging problems. Concurrently, the operation at lower temperatures facilitates greater capture of sulfur based upon thermodynamics. It is estimated that the presence of sodium in the silicate phase due to the sodium-containing sulfur capture additive component produces a total increase in sulfur capture in the silicate phase of about 5-10% greater than sulfur capture achieved in a calcium silicate phase where no sodium is present. In addition, the presence of fluoride-containing additive further increases the sulfur capture in the silicate phase by about 1-5%. 
     It is a critical feature of the process of the instant invention that the partial oxidation reactor be operated at temperatures of 1900°-2600° F., preferably 2000°-2400° F. to achieve optimum sulfur capture from the synthesis gas to the sodium-calcium-fluoride silicate and sodium-calcium sulfide phases. At temperatures above 2600° F. sulfur capture is substantially diminished. At temperatures below 1900° F., slag fluidity is diminished and plugging and other operational problems will arise. 
     It will be evident that the terms and expressions employed herein are used as terms of description and not of limitation. There is no intention, in the use of these descriptive terms and expressions, of excluding equivalents of the features described and it is recognized that various modifications are possible within the scope of the invention claims.