Patent Publication Number: US-4321242-A

Title: Low sulfur content hot reducing gas production using calcium oxide desulfurization with water recycle

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process and apparatus for producing low sulfur content hot reducing gas and especially that gas formed by the combustion and gasification of sulfur-bearing carbonaceous fuel. The gasification of solid carbonaceous fuel, such as by reaction with a limited quantity of oxygen to produce carbon monoxide, is well known. Either pure oxygen or air, with or without steam, may be utilized in the reaction. The products of combustion are reducing gases including carbon monoxide, hydrogen, carbon dioxide, water vapor and nitrogen. Hydrogen is produced from the hydrocarbons in the fuel, and also by reaction of injected steam with carbon, while nitrogen may be brought in by air and may also be contained in the fuel. Carbon dioxide and water vapor may also be used to react with the carbonaceous fuel and the reducing gas produced therefrom to vary the final composition of the product reducing gas. 
     One of the serious problems with gasification of carbonaceous fuels is that many commercially available carbonaceous fuels contain sulfur. Sulfur-containing reducing gases, usually predominantly hydrogen sulfide, are produced when these fuels are reacted with air or oxygen in a gasification process. These sulfur-containing gases in the reducing gas are objectionable for a number of reasons. One reason is that when the sulfur finally ends up in the atmosphere, it results in serious pollution problems. Additionally, this sulfur should be removed from the product gas before its use in many applications, such as metallurgical reducing gas where it will contaminate the metal produced, synthesis gas where it will poison the catalyst in the reaction system, or feed stock for pipeline gas where it may promote corrosion and other detrimental effects. Also, if the product gas is burned to raise steam or generate electricity, it is advantageous to remove the H 2  S before combustion rather than having to remove SO 2  from the larger volume of combusted gas. In at least two of these applications, i.e. as a reducing gas for direct reduction of iron ore or fuel for gas-turbine engines, it is desirable to remove the H 2  S while the product gas is still hot so that gas can be used directly without loss of heat values. 
     To offset the cost of desulfurizing the hot reducing gases, the byproducts of sulfur removal should be marketable: (i) recovery of sulfur from the spent absorbent, (ii) regeneration of absorbent for recycle, or (iii) marketing the treated spent absorbent, after the recovery of sulfur, for other applications. The absorbent used for desulfurization of hot reducing gases should have the capability of lowering the sulfur content of the treated gas to below 100 ppm without much changing the reducing capacity or fuel value of the gas. 
     Attempts have been made to remove the sulfur during the gasification reaction itself. U.S. Pat. No. 3,533,730, incorporated herein by reference, is an example of such a process whereby the carbonaceous fuel is reacted with a controlled quantity of oxygen beneath the surface of a molten iron bath and whereby lime on the surface of the molten iron bath is used to desorb sulfur from the iron bath. Sulfur is then recovered from the coal ash-lime-sulfur molten slag byproduct. There are serious questions concerning the practical operability of this process. The rate of coal gasification depends upon the rate of coal dissolution for a given melt size, which are relatively slow compared with volumetric gasification rates for other processes. Furthermore, the sulfur in the slag byproduct is recovered only by costly additional steps. The gasification product generally contains fly ash which also requires an extra step for removal. 
     The use of calcined dolomite has been suggested for a regenerative cycle process of desulfurization of hot reducing gases. See U.S. Pat. Nos. 3,276,203; 3,296,775; 3,307,350; 3,402,998; and 3,853,538, each incorporated herein by reference. While dolomite is an effective gas-desulfurizing agent, the most commonly proposed method of regenerating dolomite, reacting with CO 2  and H 2  O under slightly reducing conditions at pressures greater than about 50 psig and temperatures preferably about 1000°-1200° F. to liberate H 2  S, does not achieve complete regeneration of the dolomite. One of the problems is that calcium carbonate formed in the regeneration coats the regenerated dolomite thereby reducing its effectiveness. Furthermore, because the spent dolomite contains appreciable nonregenerated calcium sulfide, it must undergo expensive and complete treatment to bring it to a state suitable for disposal without causing pollution of the air and groundwater. When dolomite is calcined after having been regenerated by the above suggested process, some of the residual sulfur in the dolomite can be released, which requires difficult treatment to bring the stack gas to a condition suitable for venting to the atmosphere. 
     Copending and commonly assigned application Ser. No. 154,731, filed May 29, 1980, by E. T. Turkdogan and entitled &#34;Low Sulfur Content Hot Reducing Gas Production Using Calcium Oxide Desulfurization&#34;, incorporated herein by reference, teaches a process for removing sulfur from a hot reducing gas stream by contacting the gas stream with a fixed bed of particulate calcium oxide desulfurizing agent, such as calcined dolomite. The desulfurizing agent is used one time and is then contacted with boiling water or wet steam, preferably under pressure, to remove the sulfur from the calcium sulfide composition produced in the gas desulfurizing step. A basic problem with this process is that when the reducing gas stream initially contains significant fly ash then the fixed bed rapidly becomes plugged with fly ash, thus resulting in plant shut downs and wasted desulfurizing agent. The invention described in copending and commonly assigned application Ser. No. 158,190, filed June 11, 1980, by J. Feinman and J. E. McGreal, Jr. and entitled, &#34;Low Sulfur Content, Fly Ash Free Hot Reducing Gas Production Using Calcium Oxide Desulfurization&#34;, incorporated herein by reference, teaches a solution to the fly ash problem by using a moving bed of desulfurizing agent so that the fly ash is continually removed with the spent desulfurizing agent. After removal of the sulfur from the spent desulfurizing agent, the mixture of fly ash and desulfurizing agent is preferably disposed of and fresh desulfurizing agent is preferably used as input to the moving bed. 
     One of the basic problems that remains with the calcium oxide desulfurizing agent system is how to dispose of the sulfur containing water produced in the process of desulfurizing the calcium sulfide of the spent desulfurizing agent composition. Water desulfurizing methods are expensive. Adding such sulfur containing waste waters to streams or the like is generally unacceptable. 
     Another problem with the calcium oxide desulfurizing agent system is how to achieve sufficiently rapid desulfurization of the spent dolomite without excessive use of fresh water. 
     SUMMARY OF THE INVENTION AND BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention relates to an improved process and apparatus for producing a low-sulfur content hot reducing gas stream by (a) contacting a sulfur bearing hot reducing gas stream with a desulfurizing agent comprising a bed of solid particles comprising calcium oxide, such as dolomite, to thereby produce a low-sulfur content hot reducing gas stream and (b) contacting the calcium sulfide composition with hot liquid water at a temperature and corresponding pressure sufficient to maintain steam in the system to thereby convert the sulfide of the composition to calcium hydroxide and hydrogen sulfide and to produce a sulfur containing water, and then (c) recycling for reuse step (b) at least a major portion of the sulfur containing water produced in step (b) in combination with fresh water and condensate removed from the H 2  S stream leaving the system. Preferably the fresh water is added in an amount at least sufficient to replace the water consumed by reaction in step (b). More preferably, the fresh water added in step (c) is first used to wash the calcium hydroxide composition produced in step (b) prior to combining the fresh water with the sulfur-containing water. This assists in producing an essentially sulfur free calcium hydroxide composition without contaminating any water that will not subsequently be used in the process. The condensate, which is also relatively sulfur free, is preferably added to the last stage of contact between the solids and the boiling water to provide enhanced driving force for increasing the rate and extent of desulfurization of the spent dolomite. The weight ratio of the hot liquid water to the calcium sulfide composition immediately prior to contacting is preferably between about 1 to 1 and about 20 to 1. 
     Applicants&#39; process has the advantage of achieving effective sulfur removal from the spent desulfurizing agent while at the same time eliminating any disposal problems with the sulfur containing water stream produced in the process. Moreover, in the preferred embodiment of this invention wherein the fresh water is used to wash the calcium hydroxide produced in the process a calcium hydroxide product can be produced which has very low sulfur content and which is, therefore, readily marketable as a desulfurizing agent for stack gases. This latter result is achieved without producing additional sulfur containing water having a difficult disposal problem. While it is possible to recycle the calcium hydroxide for use in desulfurizing hot reducing gases, it is generally not practical because it would have to be agglomerated to be effective in such a process. An additional advantage of a simple one-time use for the dolomite is that significant capital expenditures are eliminated that would be required in a process involving recycle of the spent dolomite. 
    
    
     FIG. 1 is a schematic diagram of one embodiment of the desulfurization process and apparatus of this invention. 
     FIGS. 2-4 are graphs showing some results of examples. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The desulfurizing agent comprising a bed of solid particles comprise calcium oxide preferably in the form of calcined dolomite or lime. The particle size of this bed is preferably between about 1/8 inch and about 1 inch, and more preferably between about 1/4 inch and about 1/2 inch. If the particle size is below about 1/8 inch, there is a serious dust problem, and if the particle size is above about 1 inch, the efficiency of the desulfurization decreases markedly. 
     The calcium sulfide composition produced as a result of the desulfurizing step of this invention is composed of calcium sulfide, and generally fly ash. When dolomite is utilized as the desulfurizing agent, the calcium sulfide composition will also contain magnesium oxide. 
     The desulfurization step is preferably conducted at temperatures between about 600° C. and about 1200° C. and more preferably between about 800° C. and about 1000° C. 
     The hot liquid water used to react with the calcium sulfide composition is preferably in the form of boiling water, although wet steam is also acceptable. The temperature of this reaction is preferably between about 100° C. and about 394° C., and more preferably between about 140° C. and about 250° C. and the pressure is preferably between about 1 and about 218 atmospheres, and more preferably at elevated pressures such as between about 3 and about 40 atmospheres. 
     The term &#34;fresh water&#34; used herein refers to water which is added to the process from an external source and, preferably, is water having a sulfur content lower than that of the recycle sulfur containing water produced as a by-product of the process of this invention. More preferably, the fresh water will be substantially sulfur free when initially added to the process. Part of the recycle water may come from condensing the steam produced in the process. Generally, fresh water will be added in sufficient quantities to replace the water consumed in the process by reaction in step (b), that leaving with the dry hydrogen sulfide, and the water entrained in the calcium hydroxide composition after washing with fresh water to remove the sulfur from this composition. 
     The calcium sulfide composition produced in the desulfurization is preferably reduced in particle size to less than about 6 mesh and preferably less than about 30 mesh immediately prior to or during the process of contacting this composition with the hot liquid water. In one embodiment the size reduction is accomplished under water after the hot gas desulfurization step and subsequent to removing the sulfur from the calcium sulfide composition by contacting with hot water. 
     In FIG. 1 a sulfur-bearing composition such as a hot reducing gas usually containing fly ash 1 is transferred through line 2 to fixed or moving bed desulfurization means 3 where it is contacted with a particulate calcium oxide containing desulfurizing agent transported from line 4. The desulfurizing agent is preferably introduced in the uncalcined state as raw dolomite or limestone in which case calcination will occur in the upper part of bed 3 without significant loss of temperature by the hot gas or increase of CO 2  content of the hot gas. The reducing gas passes through the desulfurization means 3 to exit line 5. A calcium sulfide composition passes from the desulfurization means 3 through line 6 into sulfur recovery means 7 where it is contacted with hot liquid water at a temperature and corresponding pressure sufficient to maintain steam in the system. Fresh water, and optionally condensate from the steam produced in the contacting step, enters the sulfur recovery means 7 through line 8. Hydrogen sulfide and steam pass through exit line 9 from the sulfur recovery means 7. The steam may be condensed and reused as recycle water. A calcium hydroxide containing composition passes through exit line 10 from sulfur recovery means 7. Sulfur containing water passes through exit line 11 where it is combined with fresh water which is added to the process by means of inlet line 12. 
     In the step of contacting the calcium sulfide composition with hot liquid water, the weight ratio of hot liquid water to calcium sulfide composition immediately prior to contacting is between about 1:1 and about 20:1, and more preferably between about 6:1 and about 10:1. The hot liquid water may be in the form of steam or boiling water. 
     Preferably two or more tanks are used sequentially for contacting the sulfur containing composition with the hot liquid water. 
     Preferably the water vapor produced in the process is condensed and returned to the process, such as by adding to the final stage of the step of contacting liquid water with the calcium sulfide composition. 
     EXAMPLES 
     The process and apparatus of FIG. 1 is used to carry out the following examples. The step of contacting the calcium sulfide composition with hot liquid water is carried out by means of two separate vessels containing such water. 
     Sulfided dolomite from pilot-plant studies where the dolomite was used as a desulfurizing agent was crushed to provide feed for the experiments ranging in size from minus 10 mesh to minus 20 mesh. Following is a chemical analysis of this material. 
     
         ______________________________________                                    
Constituent   Weight Percent                                              
______________________________________                                    
CaO           2.36                                                        
CaS           60.07                                                       
MgO           35.62                                                       
Al.sub.2 O.sub.3                                                          
              0.13                                                        
Fe.sub.2 O.sub.3                                                          
              0.60                                                        
M.sub.m O     0.05                                                        
SiO.sub.2     0.72                                                        
              99.55                                                       
S Total       26.70                                                       
______________________________________                                    
 
    
     EFFECT OF WATER (H 2  O) SULFIDED CALCINED DOLOMITE (SCD) WEIGHT RATIO 
     The effect of H 2  O/SCD weight ratio was studied over the range 6 to 20 with minus 10 mesh material at atmospheric pressure and at 55 psig. The results are shown in FIG. 2 as percent sulfur removal versus time with H 2  O/SCD weight ratio and pressure as parameters. Increasing the H 2  O/SCD weight ratio above 10 did not result in a significant improvement in sulfur removal rate over the range of operating pressures studied. It is interesting to note that there is a marked increase in sulfur removal rate with increasing pressure at all H 2  O/SCD weight ratios. 
     EFFECT OF PRESSURE (SATURATION TEMPERATURE) 
     The effect of pressure (saturation temperature) on sulfur removal rate was studied with minus 10 mesh sulfided calcined dolomite and H 2  O/SCD weight ratio of 10. The results are presented in FIG. 3 as percent sulfur removed versus time with pressure as a parameter. Similar results were obtained with minus 100 mesh dolomite. The sulfur removal rate increases with increasing pressure (saturation temperature), the increase being particularly marked when the pressure increases from one to two atmospheres. 
     EFFECT OF RECYCLE H 2  O 
     Because the reactions in the treatment of sulfided calcined dolomite with boiling water result in a net water consumption, the desired H 2  O/SCD weight ratio is obtained by recycling water separated from the hydrated product. The fresh water for reaction is introduced as a wash stream to clear the hydrated solids of &#34;spent&#34; recycle water and provide the cleanest possible solids for disposal or use. Tests were, therefore, conducted to determine the effect of recycle water on the degree and rate of sulfur removal. The water separated from the slurry at the end of each batch test was used to make up the starting bath for the succeeding test. The bath for each test was made up with 80 percent recycle water and 20 percent fresh water to simulate conditions in an actual flowsheet. The results are shown in FIG. 4 as percent sulfur removed versus time with pass number as a parameter (with repeated use, the water will tend to approach a steady state sulfur content so that the retarding effect of this variable should also stabilize at some steady-state level). After two passes with recycle the sulfur removal versus time curves do reach a steady-state (superimposed). 
     EFFECT OF WASHING ON FINAL SULFUR CONTENT 
     The retarding effect of recycle water on the rate of sulfur removal is the direct result of a buildup in sulfur and &#34;sulfur ion&#34; concentration in the slurry water. All of the prior solid analyses were affected because the solids were dried before analysis without washing, with the result that sulfur contained in the residual water remained with the sample. This effect is illustrated quantitatively in the following Table 1 which presents a comparison of final percent sulfur removals for unwashed and washed samples for most of the later experiments. 
     
                       TABLE 1                                                     
______________________________________                                    
COMPARISON OF PERCENT SULFUR REMOVED                                      
IN FINAL SOLIDS - UNWASHED AND WASHED                                     
                             Percent S                                    
Dura-    Pres-               Removed                                      
Test tion    sure    H.sub.2 O/                                           
                           Size  Un-                                      
No.  Hours   PSIG    SCD                                                  
Mesh washed  Washed  Δ                                              
______________________________________                                    
34   5       0       10    100   92.9  98.4   5.5                         
35   5       15      10    100   94.1  98.9   4.8                         
36   5       30      10    100   92.3  98.6   6.3                         
37   5       44      10    100   92.1  98.6   6.5                         
38   5       55      10    100   93.1  98.8   5.7                         
40   3       55      15    10    96.4  97.7   1.3                         
41   3       0       20    10    95.5  98.3   2.8                         
42   3       55      20    10    95.5  98.2   2.7                         
43   3       0       15    100   94.9  98.9   4.0                         
44   3       55      15    100   94.6  98.8   4.2                         
45   3       0       20    100   95.8  99.2   3.4                         
46   3       55      20    100   96.2  99.0   2.8                         
47   4       0       10    200   92.0  97.9   5.9                         
48   4       55      10    200   92.2  98.4   6.2                         
49   3       55      6     10    88.8  97.5   8.7                         
50   3       30      6     10    87.2  97.2   10.0                        
51   3       0       6     10    88.7  96.6   7.9                         
52   3       55      6     10    88.6  96.5   7.9                         
53*  3       55      6     10    87.0  95.7   8.7                         
54*  4       55      6     10    84.5  95.5   11.0                        
55*  4       55      6     10    87.4  96.9   9.5                         
56*  4       55      6     10    86.3  95.4   9.1                         
______________________________________                                    
 *Runs 53, 54, 55, 56 were run with 80% recycle water 1,2,3 &amp; 4 passes    
 
    
     The data presented in this Table 1 clearly show the effects of washing on percent sulfur removal and of using higher H 2  O/SCD weight ratios when the treatment is done without recycle water. The effect of recycle water is similar to using a lower H 2  O/SCD weight ratio. The average of the Δ&#39;s between unwashed and washed samples is 4.4 percentage points for H 2  O/SCD of 10 compared with 9.1 percentage points for H 2  O/SCD of 6. 
     CONTINUOUS TREATMENT RESULTS 
     The results of some continuous boiling-water-leach tests are summarized in Tables 2 and 3. Table 2 shows the effect of residence time on the steady-state percent sulfur removal with constant H 2  O/SCD weight ratio of 6, and FIG. 3 shows the effect of H 2  O/SCD weight ratio on the steady-state percent sulfur removal with constant residence time of 12 hours. These results indicate that a residence time of at least 10 hours with H 2  O/SCD weight ratio of at least 6 will be needed to provide the desired percent sulfur removal and that certain special arrangements will be needed to overcome the adverse effects of recycle H 2  O. 
     
                       TABLE 2                                                     
______________________________________                                    
EFFECT OF RESIDENCE TIME ON STEADY                                        
STATE PERCENT SULFUR REMOVAL, H.sub.2 O/SCD = 6                           
Residence Time, Hours                                                     
                Percent Sulfur Removed                                    
______________________________________                                    
12              93.5                                                      
8               91.0                                                      
4               82.5                                                      
4 with Recycle H.sub.2 O                                                  
                75.0                                                      
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
EFFECT OF H.sub.2 O/SCD WEIGHT RATIO ON STEADY                            
STATE PERCENT SULFUR REMOVAL, RESIDENCE                                   
TIME = 12 HOURS                                                           
H.sub.2 O/SCD Weight Ratio                                                
                Percent Sulfur Removed                                    
______________________________________                                    
10               94.0                                                     
6                93.5                                                     
4                88.0                                                     
2                78.0                                                     
______________________________________