Abstract:
A process for reducing sulfur emissions and increasing particulate removal in the combustion of pulverized coal is presented. Pulverized sulfur-containing coal is injected along with combustion air into a combustion chamber and fired to create a combustion zone within the chamber, thereby creating gaseous sulfur and particulate emission products. The emission products are directed to an exhaust means where the particulate emission products are removed. A fuel having negligible sulfur content and a heating value higher than coal is injected in substantially the same direction as the coal and combustion air stream into a region of the combustion zone through which a majority of the of the gaseous sulfur emissions must pass to reach the exhaust means, thus increasing the temperature of the region in an amount sufficient to increase the sulfur content of the particulate emission products.

Description:
This is a continuation-in-part of co-pending application Ser. No. 07/627,642 filed on Dec. 14, 1990, abandoned. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to emission control for combustion processes, and more particularly, to reducing the gaseous sulfur-containing compounds and particulate matter which result from firing pulverized coal in a combustion device. 
     2. Description of the Prior Art 
     The use of sulfur-bearing coals containing mineral matter in various combustion installations (steam boilers, furnaces, and the like) results in the production of sulfur-containing compounds, such as sulfur dioxide, sulfur trioxide, and fly ash particulates. These are pollutants and must be removed to a certain extent from the exhaust gases prior to release into the atmosphere. Various methods are known to attempt to deal with this problem. 
     The sulfur-containing compounds are usually absorbed by different sorbents injected either into the firebox or into a scrubber installed downstream of the combustion device. This causes increased mineral matter in the flue gas discharged by the system, and thus requires additional use of energy (heat and electrical power) for the particulate removal process. 
     The fly ash particulates are removed from the exhaust gases by various means, such as filtration of the flue gas, or use of an electrostatic precipitator. The efficiency of an electrostatic precipitator depends on many factors, including the electrical conductivity of the particulate matter. Increased conductivity results in improved precipitation performance. The electrical conductivity in turn depends in part on the sulfur concentration in the particulate matter. In general, conductivity increases with increasing sulfur concentration in the particulate matter. Because of this, in order to improve the electrostatic precipitator performance when low sulfur coal is burned, sulfur is often introduced with the coal, or sulfur trioxide is injected into the flue gas. 
     The problem of achieving a reduction in sulfur-containing compounds and an increase in efficiency of particulate removal, while eliminating the use of additives and additional scrubbing processes or gas particulate cleaning processes, has not been adequately resolved. 
     The concept of cofiring natural gas with coal to improve emission performance is not new. Compared to coal, natural gas contains negligible sulfur and no mineral matter. It is known that partial replacement of coal with natural gas will result in a reduction in sulfur compounds and particulates proportional to the reduction in sulfur and mineral matter input. It is also known that cofiring of natural gas and oil yields a plume which is almost invisible. 
     The most significant development work on cofiring technology was conducted as a result of the Gas Research Institute&#39;s Cheswick Station project. It was discovered that cofiring natural gas with coal could result in a percentage reduction in sulfur dioxide emissions which was greater than the percentage reduction in sulfur and mineral matter input. Further, if a small amount of gas is fired simultaneously with coal, carbon burnout is improved, resulting in improved electrostatic precipitator performance and reduced plume opacity. 
     SUMMARY OF THE INVENTION 
     A process for reducing gaseous sulfur dioxide and increasing particulate matter removal in the emission products from the combustion of pulverized coal is presented. Pulverized sulfur-containing coal is injected along with combustion air into a combustion chamber and fired to create a combustion zone within the chamber, thereby creating emission products, including gaseous sulfur compounds and particulate matter. The emission products are directed to an exhaust means where the particulate matter is removed. By injecting a fuel containing negligible sulfur and having a heating value higher than coal into a region of the combustion zone through which a majority of the of the gaseous sulfur compounds must pass to reach the exhaust means, and by injecting the fuel substantially in parallel to the coal and combustion air, the temperature of the region is increased in an amount sufficient to increase the sulfur content of the particulate matter, thereby reducing the amount of gaseous sulfur dioxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a simplified combustion system. 
     FIG. 2a is a side plan view of a portion of the combustion system of FIG. 1 illustrating sulfur distribution for a conventional pulverized coal flame. 
     FIG. 2b is a plan view illustrating the operation of a conventional electrostatic precipitator. 
     FIG. 3 is a side plan view of a portion of the combustion system of FIG. 1 illustrating sulfur distribution when a pulverized coal flame is cofired with natural gas according to the present invention. 
     FIG. 4 is a perspective view of one of the four corners of a conventional tangential-fired boiler illustrating the burner arrangement. 
     FIG. 5a is a side plan view of a tangentially-fired boiler configured according to the present invention. 
     FIG. 5b is a top plan view of the boiler of FIG. 5a taken across section 5b-5b. 
     FIG. 6 is a graphical representation of the temperature of flue gas as a function of boiler elevation when pulverized coal is cofired with natural gas according to the present invention. 
     FIG. 7 illustrates a sulfur balance for a tangentially-fired boiler fired on 100% coal. 
     FIG. 8 illustrates a sulfur balance for a tangentially-fired boiler fired on 89% coal and 11% natural gas. 
     FIG. 9a illsutrates a side plan view of a wall-fired boiler configured according to the present invention. 
     FIG. 9b illustrates a top plan view of the boiler of FIG. 9a taken across section 9b-9b. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Generally 
     A process for reducing gaseous sulfur dioxide in the combustion of pulverized coal is presented. Natural gas is injected substantially in parallel with the coal and combustion air stream into an upper region of the coal combustion zone, thereby increasing the temperature of the upper region. The temperature increase promotes increased conversion of sulfur dioxide to sulfur trioxide. Since sulfur trioxide is more reactive with the particulate matter which is produced by coal combustion than sulfur dioxide, the amount of sulfur in the particulate matter is increased, and the amount of gaseous sulfur dioxide is reduced. Since sulfur increases the conductivity of the particulate matter, the ability to electrically precipitate the particulate matter is enhanced, resulting in increased particulate removal from the exhaust plume. 
     Referring now to FIG. 1, a simplified block diagram of a typical combustion unit 10 for burning sulfur-bearing pulverized coal depicts a main combustion chamber 12 comprised of a plurality of burner levels 14A, 14B, 14C, and 14D. The combustion chamber 12 is pre-warmed by firing a natural gas flame through warm-up gun 15. The combustion chamber 12 is fired on pulverized coal by injecting the coal and combustion air into the combustion chamber 12 at burners 16A, 16B, 16C, and 16D. Alternatively, the combustion chamber could be operated with other fuels, such as natural gas or oil, by firing the fuel through burners 30A, 30B, and 30C. The burners 30C, 30B and 30A are located below the burners 16D, 16C and 16B, respectively. All of the burners 30C, 30B and 30A are located below burner 16D. It should be recognized that the figure is illustrative only, and a typical configuration includes burner arrays on more than one wall, for example, a tangentially-fired boiler has burner arrays located in each of four corners and a wall-fired boiler in one of the possible configurations has burner arrays located on opposing walls 
     It should also be recognized that the burners are completely conventional in design, that is, the burners mix fuel with excess air in order to achieve complete combustion of the fuel. 
     The initial combustion of coal is started by natural gas flames provided through ignitors 18A, 18B, 18C, and 18D. In a typical boiler, ignitors 18A, 18B, 18C, and 18D are located on the wall adjacent to burners 16A, 16B, 16C, 16D and 30A, 30B, 30C. A coal flame is thus developed in the entire combustion chamber 12, and is generally referred to as the combustion zone 45. 
     Referring now to FIG. 2a, the combustion of sulfur-bearing pulverized coal yields four sulfur-containing compounds: gaseous SO 2  (sulfur dioxide) and SO 3  (sulfur trioxide), fly ash particulates, and bottom ash particulates. Most of the sulfur in the coal oxidizes to form gaseous SO 2  in the exhaust stream. A small amount of the already-formed SO 2 , oxidizes to form SO 3 . The SO 3  has a higher affinity for the remaining mineral particulate matter (e.g., calcium, magnesium, alumina) resulting from combustion and thus readily forms sulfates which make up in part the fly ash and bottom ash. The bottom ash simply falls to the bottom of the combustion chamber 12, as indicated by arrow 11, where it may be collected and safely disposed of. The fly ash and gaseous sulfur compounds, however, rise into the exhaust duct 20 of the combustion unit 10, as indicated by arrow 13. 
     Referring back to FIG. 1, the fly ash and gaseous sulfur compounds are then processed through an electrostatic precipitator 22, where additional particulate matter is removed, as indicated by arrow 23, prior to venting the exhaust into the atmosphere, as indicated by arrow 25. The effectiveness of the electrostatic precipitator 22 increases as the conductivity of the exhaust stream increases. Where the conductivity of the exhaust stream is low (resistivity high), then the Corona Effect is observed and less precipitation occurs. See FIG. 2b. 
     According to the present invention, a second fuel, preferably natural gas, is injected into the upper region of the combustion zone 45 through either burner 30B or 30C. This second fuel must have a higher heating value than coal and negligible sulfur content to optimize the process. It is preferable that the injected fuel intimately mix with the coal flame, thereby causing no change in appearance of the combustion zone 45 relative to that for operation on 100% coal in terms of size and shape, which optimizes the effectiveness of the present invention. This is accomplished by firing the second fuel into the combustion zone 45 in substantially the same direction as the coal/air stream. The higher heating value of the second fuel causes this fuel to be consumed in a short residence time. Thus, the higher heating value of natural gas promotes quick heat release in a short residence time, which raises the temperature of the upper region of combustion zone 45 and delays the quenching of the flue gas temperature, thus promoting faster burn-out of the coal particles in the upper region of combustion zone 45. In this manner, the flue gas from the lower regions of the coal combustion zone is processed through a higher temperature zone than would be possible with coal combustion alone. 
     As noted above, the sulfur contained in the coal oxidizes mostly to gaseous SO 2  during combustion. However, when the coal flame is processed through the higher temperature zone created by the cofiring of natural gas and coal in the upper region of the combustion zone, more of the already-formed SO 2  is oxidized to SO 3 . The SO 3  thus formed in the flame reacts with the mineral matter (calcium, magnesium alumina) more readily than the SO 2  in the coal ash. Thus, oxidizing the SO 2  to SO 3  increases the sulfate content in the fly ash and reduces the SO 2  content in the flue gas. This is illustrated in FIG. 3. The increased sulfate content in the fly ash increases its electrical conductivity. This increased conductivity in turn improves the collection efficiency of the electrostatic precipitator, consequently reducing the flue gas particulate content. Thus, the opacity of the flue gas emitted into the atmosphere is reduced. 
     The maximum conversion of SO 2  to SO 3  occurs when the second fuel is injected into an upper region of the combustion chamber such that already-formed SO 2  is processed through the higher temperature flame. The only limitation for the injection site is that the exhaust gas temperature of the combustion unit 10 must be maintained within certain preset limits in order to avoid fly ash slagging and fouling on the heat exchanging surfaces, which is a function of the coal ash properties. 
     In order to demonstrate certain aspects and embodiments of the present invention, the following experimental samples are provided. It should be understood that the examples are merely illustrative and not limiting. 
     B. Experimental Results 
     Tests were conducted on a Combustion Engineering tangential-fired, multiple-fuel-capable boiler rated at 450 mW and delivering 3,200,000 pounds of steam per hour at 3614 psi and 1005° F. (four burner levels operating). The unit was also supplied with overfire air for NO x  control, a cold precipitator for particulates, and a stack. The boiler was optimized for 100% coal operation. Relevant criteria for optimization included minimum slagging and fouling, maintenance of desired bulk steam temperature, no upward excursion in stack emissions, and minimum feasible heat rate. Optimum boiler performance at full load on 100% coal firing was achieved with 4.5% to 5.0% excess O 2 , 65% overfire air, windbox/furnace pressure differential of 7.0 to 8.0 inches H 2  O, coal burner levels B-C-D-E in service with no mill bias, level A out of service, and burner tilts on automatic. 
     FIG. 4 illustrates one corner of a tangentially-fired multi-fuel capability boiler with five coal burners 40a, 42a, 44a, 46a, and 48a. A coal flame is started by firing natural gas through ignitors 60a, 62a, 64a, and 66a. It is of course recognized that the boiler has three additional corners configured identically to the one illustrated. Thus, a coal flame is developed in the combustion zone 45, as illustrated in FIGS. 5a and 5b. The boiler also has the capability of being fired on 100% gaseous fuel by delivery through gas burners 52a, 54a, 56a, and 58a. Burner 52a is located between burners 40a and 42a, gas burner 54a is located between burners 42a and 44a, gas burner 56a is located between burners 44a and 46a, and gas burner 58a is located between burners 46a and 48a. 
     According to the present invention, 11% natural gas (based upon fuel Btu input) was cofired into the combustion chamber through gas burners 56 substantially in parallel with the coal and combustion air stream with the boiler operating at full-load. The resulting temperature profile of the flue gas as a function of boiler elevation is illustrated in FIG. 6. 
     Samples of the fly ash, bottom ash, and input coal were taken for firings of 100% coal, and for firings of 89% coal and 11% natural gas. Sulfur balances were then performed, and the results are illustrated in FIGS. 7 and 8. The sulfur balances showed a significant increase in sulfur retention in the fly ash. In fact, as shown in Table I, the sulfate level in the fly ash increased nearly four times, and the sulfate level in the bottom ash nearly doubled as a result of the 11% gas cofiring. 
     
                       TABLE I______________________________________   100% Coal   89% Coal, 11% GasSample    Sulfur  Sulfate   Sulfur Sulfate______________________________________Coal      0.51    0.01      0.51   0.01Fly       0.56    0.11      0.65   0.42AshBottom    0.27    0.06      0.27   0.11Ash______________________________________ 
    
     Samples of the flue gas were also taken, and the average SO 2  level for 100% coal firing was 1.05 lb/MBtu, whereas the average SO 2  level for 89% coal and 11% gas was 0.87 lb/MBtu, a 17.1% reduction in average SO 2  level. This compared well with sulfur balance results, which showed a 13.6% reduction in average SO 2  levels. Testing the flue gas for opacity showed average levels of 9.2% opacity for 100% coal firing, and 5.9% opacity for 89% coal and 11% gas firing, a 35.9% reduction in average opacity of the flue gas. 
     It should be recognized by those skilled in the art that there are additional embodiments which would be within the scope of the present invention. For example, a wall-fired boiler is illustrated in FIGS. 9a and 9b. Coal is fired through an array of coal burners mounted on either one wall or on opposing walls of the boiler. Thus, FIG. 9a illustrates coal burner rows 70a, 70b, 72a, 72b, 74a, and 74b. Natural gas is cofired into the upper region of the boiler through gas burner rows 76a and 76b. FIG. 9b illustrates the coal burners 74a 1 , 74a 2 , 74a 3 , 74a 4 , 74b 1 , 74b 2 , 74b 3 , and 74b 4  in burner rows 74a and 74b. The scope of the invention is defined by the accompanying claims.