Abstract:
An alternative process whereby the calcium sulphide formed in an integrated gasification combined cycle (IGCC) system from calcium oxide and/or calcium carbonate can be safely and more or less quantitatively converted by reaction with carbon dioxide to calcium carbonate and/or calcium oxide together with sulphur dioxide. The calcium oxide and/or calcium carbonate can be reused in the integrated gasification combined cycle (IGCC) system, and the sulphur dioxide can be converted to a useful product, such as sulphuric acid. One result of this process is that since the furnace ashes do not contain a significant level of calcium sulphide, they can be safely disposed of in a land fill site.

Description:
[0001]     This is a Continuation-in-part of U.S. Ser. No. 10/600,345 filed Jun. 23, 2003. 
     
    
     BACKGROUND  
       [0002]     In the past, coal has commonly been used as a fuel in electrical power generation. Although the details have changed with time, in a coal fired power station the coal is generally burnt under oxidising conditions in a boiler unit to generate steam, which is then used to operate a turbine driven generator. Although this technology is reasonably well understood, it is still not without its drawbacks. If the coal is burnt under conditions which optimise coal consumption, and also if the coal contains significant amounts of sulphur, oxides of both sulphur and nitrogen are formed, which result in ecological damage.  
         [0003]     Techniques for removing acid oxides from flue gases, particularly sulphur oxides, are known. The commonly used one is to add a particulate calcium compound, such as calcium carbonate, or calcium oxide(lime) to the coal so as to trap the sulphur oxides as calcium sulphite and/or calcium sulphate. However, the utilisation of the calcium compound in the furnace is relatively inefficient.  
         [0004]     Alternatives to full sized power stations have been proposed. One of these is the so-called “Integrated Gasification Combined Cycle” (hereafter IGCC) technique. The IGCC technique is attractive for producing electricity from coal because of its low emissions level and its significantly improved fuel efficiency in comparison with a conventional coal fired power station. In a power generation unit using IGCC, the coal is gasified under reducing conditions, the resulting gas is burnt with air as the oxygen source and the resulting hot gas is used to power a gas turbine. The gas turbine drives the generating equipment, which can be mounted directly onto the same shaft as the gas turbine itself.  
         [0005]     However, the IGCC technique is not without its own disadvantages, one of which is that the sulphur is still present in the coal, and thus can be present in the hot gas being burnt to power the gas turbine. Due to the different conditions in the two systems, the presence of sulphur in the fuel poses a quite different problem in an IGCC system to that found in a conventional steam generating furnace.  
         [0006]     IGCC systems involve a coal gasification step which is carried out in a gasifier under reducing conditions. Due to the different chemical conditions involved in the gasifier, instead of producing sulphur oxides in the hot gas, the sulphur is present chiefly as hydrogen sulphide, H 2 S. The hydrogen sulphide must be largely removed, first due to the limit on the amount of sulphur that can be accepted in the gases going forward to the turbine stage, and second due to the toxicity of hydrogen sulphide.  
         [0007]     The step normally taken to capture the hydrogen sulphide is to react it with a calcium compound, by adding typically calcium oxide(lime) or limestone to the IGCC reactor. In the reactor, the powdered limestone reacts to produce mainly calcium sulphide, according to essentially the following reaction: 
 
CaCO 3 +H 2 S--&gt;CaS+H 2 O+CO 2    (1)
 
         [0008]     This ash product cannot be sent to a landfill site, because reaction of ground water with the calcium sulphide produces poisonous hydrogen sulphide. At a practical level, almost quantitative destruction of the calcium sulphide is required before the ashes can be disposed of safely in a landfill site.  
         [0009]     To destroy the calcium sulphide and to enhance process efficiency, it has been proposed to burn the ash product remaining from the coal gasification process, which will include calcium sulphide, calcium oxide, ash materials (from the coal or other carbonaceous feed material), and unburnt char, with air in a so-called topping cycle combustor, which is typically a pressurised fluidized bed combustor (hereafter PFBC) or preferably a circulating fluidized bed combustor thereafter CFBC). In the PFBC or CFBC topping cycle, in theory the calcium sulphide should be oxidised to calcium sulphate, more or less as proposed by Weathery in U.S. Pat. No. 5,228,399 and by Moss, in U.S. Pat. No. 4,435,148. According to both of these patents, when calcium sulphide is burnt under the correct conditions of oxygen partial pressure and temperature, reaction (2) takes place. 
 
CaS+2O 2 --&gt;CaSO 4    (2) 
 
         [0010]     It has been shown that reaction (2) does not go to completion as proposed in these two patents: Qiu et al., in Ind. Eng. Chem. Res. 37, 923-928 (1998) showed that as the calcium sulphide is oxidised the calcium sulphate is formed as a relatively hard crust of calcium sulphate on the surface of the calcium sulphide particles. Qiu et al. showed that once this hard crust has formed, the rate of oxidation of the calcium sulphide inside the calcium sulphate crust is controlled by the rate at which oxygen can be transported through the calcium sulphate crust into the calcium sulphide core of the particle. Qiu et al. showed that the oxygen transfer rate is far too slow for the process to be of any commercial usefulness.  
         [0011]     Proposals have been made to overcome this difficulty, for example by Wheelock in U.S. Pat. No. 4,102,989; in U.S. Pat. No. 5,653,955 and in U.S. Pat. No. 6,083,862, and by Turkdogan in U.S. Pat. No. 4,370,161. In these patents either very carefully controlled conditions are used (eg &#39;955) or at least one additional reagent is added to the gas (eg &#39;862 and &#39;161).  
         [0012]     This invention seeks to overcome these difficulties, and to provide an alternative process whereby the calcium sulphide formed in a gasification process where calcium carbonate or calcium oxide is used to remove sulphur can be safely and more or less quantitatively converted to other sulphur compounds which can be trapped and used for other useful purposes.  
         [0013]     In the process according to this invention several potentially competing reactions can occur; these are: 
 
CaCO 3 +H 2 S--&gt;CaS+H 2 O+CO 2    (1) 
 
CaCO 3 --&gt;CaO+CO 2    (3) 
 
CaO+H 2 S--&gt;CaS+H 2 O   (4) 
 
CaS+3CO 2 --&gt;CaO+SO 2 +3CO   (5) 
 
2CaS+3O 2 +2CO 2 --&gt;2CaCO 3 +2SO 2    (6) 
 
         [0014]     Since the reaction conditions are chosen so that the calcium sulphide produced in these reactions is not oxidised to calcium sulphate, the creation of a tightly adhering calcium sulphate crust on the particles of calcium sulphide is avoided, thus allowing the reaction producing sulphur dioxide to go more or less to completion. The resulting ash product can be disposed of safely in a land fill site.  
       SUMMARY OF THE INVENTION  
       [0015]     Thus in the broadest embodiment, this invention seeks to provide a process for removing sulphide compounds from waste calcium sulphide particles produced by a gasification process where calcium carbonate or calcium oxide is used to remove sulphur and where carbon dioxide is generated, which process comprises: (a) recovering the waste calcium sulphide particles from the gasification process; (b) recovering the carbon dioxide from the gasification process and injecting the carbon dioxide into a reactor furnace at a predetermined ratio; (c) pretreating the waste calcium sulphide particles of (a) to bring the particles to an optimal size range, if necessary; (d) reacting the waste calcium sulphide particles of (a) or (c) in the furnace with sufficient carbon dioxide at a pre-determined and controlled pressure, temperature and flowrate optimized to convert the waste calcium sulphide particles to calcium carbonate or calcium oxide and to provide a gas flow containing sulphur dioxide; (e) recovering the sulphur dioxide of step (d); and (f) recovering a substantially calcium sulphide free product that is sufficiently chemically active to be reused to remove sulphur in said gasification process.  
         [0016]     Preferably, the gasification process is the integrated gasification combined cycle technique(IGCC).  
         [0017]     Preferably, the furnace is selected from the group consisting of a pressurised fluidised bed combustor(PFBC) and a circulating fluidised bed combustor(CFBC).  
         [0018]     Preferably, the optimum temperature T is determined by the equation logp e   CO2 =−8308/T+7.079, wherein p e   CO2  is the partial pressure of CO 2 , and the furnace is operated at a temperature of about 850° C. to about 980° C.  
         [0019]     Preferably, in step (d) at least 90% of the calcium sulphide present in the waste calcium sulphide product is converted to calcium carbonate and/or calcium oxide.  
         [0020]     Preferably, in step (d) a mixture of carbon dioxide and nitrogen is used to obtain the desired carbon dioxide partial pressure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The invention will now be described in more detail with reference to the attached Figures in which:  
         [0022]      FIG. 1  shows schematically an experimental furnace used to investigate the reaction of calcium sulphide with carbon dioxide;  
         [0023]      FIG. 2  shows data for the oxidation of calcium sulphide by carbon dioxide;  
         [0024]      FIG. 3  shows data for the oxidation of calcium sulphide by carbon dioxide in the presence of water;  
         [0025]      FIG. 4  shows data for the reaction of calcium sulphide with water in the presence of nitrogen;  
         [0026]      FIG. 5  shows data for the oxidation of calcium sulphide by carbon dioxide over a long time period.  
         [0027]      FIG. 6  shows the chemical equilibrium phase diagram of the Ca—S—O—C system at a temperature of 1100° C. under weak reducing conditions. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     The invention will be described for the purposes of illustration only in connection with certain embodiments; however, it is to be understood that other objects and advantages of the present invention will be made apparent by the following description of the drawings according to the present invention. While a preferred embodiment is disclosed, this is not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present invention and it is to be further understood that numerous changes may be made without straying from the scope of the present invention.  
         [0029]     In one embodiment this invention seeks to provide a process for removing sulphide compounds from waste calcium sulphide particles produced by a gasification process, preferably an IGCC system, where calcium carbonate or calcium oxide is used to remove sulphur and where carbon dioxide is generated.  
         [0030]     In the process of the present invention waste calcium sulphide particles and carbon dioxide are recovered from the gasification process. If necessary the waste calcium sulphide particles are pretreated to bring the particles to an optimal size range of from about 0.40 mm to about 4 mm. The carbon dioxide generated by the gasification process is recovered from the gasification process and injected into a furnace at a carbon dioxide/calcium sulphide molar ratio range from about 2.7 to about 6.5.  
         [0031]     Carbon dioxide may be additionally injected into the furnace from an external source at the same predetermined ratio.  
         [0032]     The process will then react the waste calcium sulphide particles in the furnace with sufficient carbon dioxide at a pre-determined and controlled pressure, temperature and flowrate. The controlled pressure, temperature and flowrate are optimized to convert the waste calcium sulphide particles to calcium carbonate and/or calcium oxide and to provide a gas flow containing sulphur dioxide.  
         [0033]     The optimum temperature T is determined by the equation logp e   CO2 =−8308/T+7.079, wherein p e   CO2  is the partial pressure of CO 2 . The furnace is operated at an optimum temperature range of about 850° C. to about 980° C. Alternatively, heat released during the waste calcium sulphide and carbon dioxide reaction is used to maintain the optimum temperature range or, alternatively a predetermined small amount of a coal gas is injected into the furnace to maintain the optimum temperature range.  
         [0034]     By controlling the flowrate it is possible to use the heat released during the waste calcium sulphide and carbon dioxide reaction to maintain the optimum temperature range.  
         [0035]     The optimum partial pressure range of the carbon dioxide is maintained from about 0.1 MPa to about 0.25 MPa, and the optimal partial pressure range of the sulphur dioxide is maintained from about 0.007 MPa to about 0.01 MPa.  
         [0036]     The process will then recover the sulphur dioxide and a substantially calcium sulphide free product that is sufficiently chemically active to be reused to remove sulphur in said gasification process.  
         [0037]     Referring first to  FIG. 1 , the experimental furnace  10  comprises a tubular electric furnace  11  which heats the midportion  12  of a quartz tube  13 . The sample of calcium sulphide  22  was placed in a small ceramic boat  14 , which was then located at more or less the center of the furnace  11 . At the input end  13 A of the quartz tube nitrogen or carbon dioxide was fed in to the quartz tube  13  through line  15 , to which was attached an evaporator  21  fed with a controlled flow of water through line  16 . At the output end  13 B of the quartz tube the exiting gases in line  17  were first passed through a condenser  18  cooled by an ice bath  19  and then passed through line  20  to the analytical equipment(not shown).  
         [0038]     In operation, a sample of calcium sulphide having a particle size of less than about 45 μm was placed in the ceramic boat and the system flushed for about 20 minutes with carbon dioxide or carbon monoxide. The carbon dioxide used had total impurities of less than 100 ppm and therefore contained negligible amounts of oxygen. The carbon dioxide flow rate was generally maintained at about 0.6 dm 3 /min. During some of the tests the effect of water vapour was also investigated. The carbon dioxide flow rate was increased to 1 dm 3 /min. and the water flow rate was controlled by a syringe pump at 0.1 dm 3 /min.  
         [0039]     The tube furnace used was capable of reaching 850° C. in about 30 minutes. For this furnace construction the temperature differential between the calcium sulphide sample and the quartz tube is negligible. The gas in line  20  was fed to a carbon monoxide and sulphur dioxide NDIR analyser.  
         [0040]      FIGS. 2, 3 ,  4 , and  5  show the data from test runs in which a calcium sulphide sample was oxidised under varying conditions. The calcium sulphide used was obtained from Aldrich Chemicals, and on analysis was found to contain 97.1% by weight CaS. The conditions for these test runs is shown in Table 1.  
                                     TABLE 1                       FIGURE No.   Sample size, g.   Gas composition and flow rate.                                2   0.3462    0.1 dm 3 /min, CO 2         3   0.3235    0.1 dm 3 /min H 2 O + 0.1 dm 3 /min CO 2         4   0.209   0.05 dm 3 /min H 2 O + 1 dm 3 /min N 2         5   0.2069     1 dm 3 /min CO 2                      
         [0041]     The test run shown in  FIG. 2  was carried out in two passes. In the first pass the furnace was raised from room temperature to about 550° C. and then allowed to cool to near room temperature. When cool, the second pass was made with the furnace heated from near room temperature to about 900° C. This test was run according to reaction (5) above. It then follows that all of the reactant gases, carbon dioxide, carbon monoxide and sulphur dioxide, can be tracked by the analysis system.  
         [0042]     In the first pass, a small sulphur dioxide peak occurred in the temperature window of 400° C.-550° C.; this peak did not appear in the second pass. This result indicates that the first sulphur dioxide peak is caused by impurities in the calcium sulphide sample. It is also of interest that the ratio of sulphur dioxide to carbon monoxide at a value of from about 1:2.5 to about 1:2 within the temperature window of from about 800° C. to about 850° C. is larger than the stoichiometry of the reactions given above would indicate. This suggests that these simple reactions do not adequately describe the oxidation of calcium sulphide by carbon dioxide.  
         [0043]     The test run shown in  FIG. 3  was carried out to investigate the effect of water on the reaction. The carbon monoxide and sulphur dioxide profiles are very similar to those for the reaction of calcium sulphide with carbon dioxide alone. However, the ratio of carbon monoxide to sulphur dioxide is slightly lower; this is probably due to the influence of the water gas shift reaction.  
         [0044]     For the test runs shown in  FIGS. 4 and 5  the calcium sulphide samples were held for 22 hours at a temperature of 850° C. in streams of water ( FIG. 4 ) and carbon dioxide ( FIG. 5 ). The results of the analysis of the samples by quantitative X-ray diffraction (QXRD)at the end of this period is shown in Table 2.  
                                             TABLE 2                               Wt % on oxidation   Wt % on oxidation       Phase Identity   Formula   by CO 2     by H 2 O                                Calcite   CaCO 3     92.3   not present       Oldhamite   CaS   6.7   73.2       Lime   CaO   not present   7.2       Portlandite   Ca(OH) 2     not present   19.4       Crystallinity       99   99.8       Amorphous       1   0.2       content                  
 
         [0045]     It is clear from these two much longer runs that oxidation by carbon dioxide is far more effective than oxidation with water over the same temperature range. Additionally it is also noteworthy that no calcium sulphate is formed.  
         [0046]     The results shown in  FIGS. 4 and 5  and in Table 2 are very similar to those shown in the preceding figures, and the presence of multiple peaks is evident throughout the run. It is also noted that the formation of calcium oxide or calcium carbonate is not limited on thermodynamic grounds at a high carbon dioxide concentration and at a high reaction temperature; this possibility is evident from  FIG. 6 . However, at very high sulphur dioxide and carbon dioxide concentrations or partial pressures calcium sulphate can be formed. In practise, the chemical equilibrium diagram shown in  FIG. 6  is used to choose appropriate operating conditions.  
         [0047]     These results demonstrate that carbon dioxide oxidation can be used to destroy calcium sulphide more or less completely. There are different strategies whereby this reaction can be used. One strategy is to operate at a temperature above 900° C. at which the calcium carbonate is not stable to ensure that the pores in the calcium sulphide particles remain open, when converting the waste calcium sulphide particles to calcium oxide is desired. Another strategy is to operate at a temperature within the stability range of the calcium carbonate, when converting the waste calcium sulphide particles to calcium carbonate is desired. Another strategy is to operate at lower temperatures with mixtures of carbon dioxide and nitrogen such that calcium carbonate is not stable at the operating temperature of interest.