Abstract:
A method to reduce mercury in gas emissions from the combustion of low rank coal in a combustion system including: combusting coal having a low chlorine content in the combustion system, wherein elemental mercury (Hg 0 ) is released in the flue gas produced by the combustion of the low rank coal; releasing chlorine into the flue gas by combusting a coal having a high chlorine in the combustion system; reacting the elemental mercury and released chlorine in the flue gas to oxidize the mercury; adsorbing at least a portion of the oxidized mercury generated by the combustion of the coal with an adsorbent in the flue gas, and collecting the adsorbent with the oxidized mercury in a combustion waste treatment system.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the combustion of coal and in particular to the reduction of mercury (Hg) in flue gases generated during coal combustion. 
     Mercury is a constituent part of coal mineral matter. The emission of mercury from coal-fired power plants is one of major sources of environmental mercury. The U.S. Environmental Protection Agency (EPA) has identified mercury emissions from power plants as a significant hazard to public health. The EPA is considering regulations that would require utility companies operating coal-fired power plants to minimize mercury emissions. Accordingly, there is a need for effective and inexpensive technologies to control mercury emissions from power plants. 
     Mercury is generally discharged in waste products from coal combustion. Mercury volatizes during coal combustion as elemental mercury (Hg 0 ). A portion of this mercury is oxidized as flue gas cools. It is believed that most oxidized mercury (Hg +2 ) in flue gas is present as mercury chloride HgCl 2 . Although elemental and oxidized mercury both can be adsorbed on fly ash, oxidized mercury is adsorbed more easily. Since oxidized mercury is water soluble, it can be removed by wet scrubbers that are used to control sulfur dioxide (SO 2 ) emissions. Ash borne mercury may be removed by a particulate collection system in the power plant. However, mercury that remains as elemental mercury tends to remain in the flue gas and is particularly difficult to remove by conventional combustion emission control devices. 
     Eastern bituminous coals have a high amount of chlorine, e.g., more than 800 parts per million (ppm). The chlorine released during the combustion of bituminous coals assists in oxidizing to HgCl 2  the elemental mercury released during combustion of the coals. The HgCl 2  is readily captured by fly ash, sorbents, wet scrubbers and other types of emission control technologies. 
     There is a particular need to control mercury emissions from power plants burning Powder River Basin (PRB) and lignite coals. PRB coals are mined from the Powder River Basin in Wyoming and Montana of the United States. These coals represent a significant portion of the available coal for power utilities. PRB and lignite coals are desirable, in part, because they have a low sulfur content. Flue gases from these coals tend to have desirably low sulfur dioxide (SO 2 ) emissions. The mercury in PRB and lignite coals (collectively “low rank coals”) tends to remain as elemental mercury in the flue gas, and does not readily oxidize or convert to other forms of mercury. Low rank coals tend to have a low chlorine (Cl) content, e.g., typically less than 100 parts-per-million (ppm). In the combustion of low rank coals, mercury oxidation in the flue gas is suppressed due to the low chlorine (Cl) content and the presence of other constituents in low rank coals. 
     Controlling the emission of mercury in combustion flue gas is complicated because mercury may take different forms. These forms of mercury change during the combustion process and as the flue gas cools and flows through combustion gas control systems. The effectiveness of a control technology for treating mercury emissions depends on the form or speciation of the mercury present as the flue gas passes through the controls. If the form or speciation of the mercury is not accurately known or changes during emission control treatment, then selecting an effective mercury emission control technology becomes increasingly difficult. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention is a method to reduce mercury in gas emissions from the combustion of low rank coal in a combustion system, said method including: combusting coal having a low chlorine content in the combustion system, wherein elemental mercury (Hg 0 ) is released in the flue gas produced by the combustion of the low rank coal; releasing chlorine into the flue gas by combusting a coal having a high chlorine in the combustion system; reacting the elemental mercury and released chlorine in the flue gas to oxidize the mercury; adsorbing at least a portion of the oxidized mercury generated by the combustion of the coal with a solid adsorbent in the flue gas, and collecting the adsorbent with the oxidized mercury in a combustion waste treatment system. Alternatively, oxidized mercury can be removed by wet scrubber. 
     In another embodiment, the invention is a method to reduce mercury in gas emissions from the combustion of low rank coal in a combustion system, said method comprising: combusting a Powder River Basin coal (PRB) having a low chlorine content in the combustion system, wherein elemental mercury (Hg 0 ) is released in the flue gas produced by the combustion; releasing chlorine into the flue gas by combusting a bituminous coal having a high chlorine in the combustion system; reacting the elemental mercury and released chlorine in the flue gas to oxidize the mercury and generate mercury chloride (HgCl 2 ); adsorbing at least a portion of the the HgCl 2  generated by the combustion of the coal with an adsorbent, and collecting the adsorbent with the HgCl 2  in a combustion waste treatment system. Alternatively, oxidized mercury can be removed by wet scrubber. 
     In a further embodiment, the invention is a system to treat mercury in flue gas emissions from a coal fired boiler comprising: a coal injector adapted to receive a low chlorine coal and a high chlorine coal; a combustor have a combustion chamber receiving coal from the coal injector and having a flue gas output; a combustion treatment waste system coupled to the flue gas output and a discharge for captured particulate waste, and wherein said combustor burns the low chlorine coal and the high chlorine coal such that elemental mercury (Hg 0 ) released in the flue gas produced by the combustion of the low chlorine coal is oxidized by chlorine released during combustion of the high chlorine coal, and the oxidized mercury is adsorbed by particulates in the flue gas. Alternatively, oxidized mercury can be removed by wet scrubber. 
     In another embodiment, the invention is a method to reduce mercury in gas emissions from the combustion of low rank coal in a combustion system, said method comprising: combusting coal having a low chlorine content in the combustion system, wherein elemental mercury (Hg 0 ) is released in the flue gas produced by the combustion of the low rank coal; releasing chlorine into the flue gas by combusting a coal having a high chlorine in the combustion system; reacting the elemental mercury and released chlorine in the flue gas to oxidize the mercury, and removing at least a portion of the oxidized mercury generated by the combustion of the coal with a water scrubbing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a coal fired power plant having coal blending and sorbent injection; 
         FIG. 2  is a schematic diagram of coal fired power plant having coal blending and a modified combustion system. 
         FIG. 3  is a schematic diagram of coal fired power plant having a bituminous coal injection separately of low rank coal injection. 
         FIG. 4  illustrates a boiler simulation facility. 
         FIG. 5  is a chart of the effects of mercury removal with respect to loss on ignition (LOI) of the fly ash produced by combustion of PRB, Eastern Bituminous, and coal blend. 
         FIG. 6  is a chart of the effects of mercury removal with respect to loss on ignition (LOI) of the fly ash produced by combustion of PRB, Western Bituminous, and coal blend. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a coal-fired power plant  10  comprising a supply of various types of coal  12 , a coal mixing bin  14 , a coal-fired boiler  16 , and a combustion waste treatment system  18 . The boiler includes a coal fuel injection system  20  and air injectors  22 . The combustion waste treatment system includes a sorbent injection system  24 , a particulate control device (PCD)  26  with an ash discharge  28 , wet scrubber  19 , and a stack  30  for flue gas discharge. The PCD captures fly ash and sorbent in the flue gas. The wet scrubber removes SO 2  from flue gas. 
     The coal supply  12  and mixing bin includes low rank coals  32  and another type of coal  34  having a relatively high chlorine (Cl) content, such as a bituminous coal having an average chlorine content of between 100 to 2000 parts per million (ppm). In contrast Low rank coal  32  typically has a low chlorine content, such as below 100 ppm. Low rank coals are blended with a high chlorine content coal, e.g., a chlorine content above 100 ppm. 
     The low rank and high chlorine content coals are blended in the mixing bin  14 . The ratio of low rank coal to bituminous coal is selected such that the amount of chlorine in the coal injected into the combustion zone  36  of the boiler  16  produces sufficient chlorine in the flue gases to assist in the treatment of mercury emissions. For example, the blended coal may be 30% high chlorine content coal and 70% low rank coal. The blended coal is fired in the combustion zone  36  of the boiler  16 . Alternatively, low  32  and high  34  chlorine coals can be fired separately into combustion zone  36 . 
     The chlorine in a high chlorine content coal assists in oxidation of the mercury vaporized during combustion of the low-rank coal. Oxidized mercury is relatively easily adsorbed by solid particles in the flue gas, such as sorbent and fly ash. Bituminous coal tends to generate relatively high LOI in fly ash, which can be used to adsorb mercury in the flue gas. Oxidized mercury is also relatively easily removed from flue gas by gas desulfurization system (FGD) such as wet scrubber  19 . 
     Increasing the amount of chlorine in the flue gases improves the efficiency of mercury reduction techniques that rely on the oxidation of mercury. The presence of chlorine converts the mercury in the flue gas to mercury chloride (HgCl 2 ), which may be captured using various conventional emission treatment systems. Such treatment systems  24  include injecting activated carbon (AC) upstream of a PCD. In addition, selective catalytic reduction (SCR) may be used to enhance mercury oxidation and wet flue gas desulfurization (FGD) may be used to enhance capture of HgCl 2 . Other techniques for removal of oxidized mercury in flue gases may be employed. 
     Activated carbon or other sorbent material (collectively “sorbent”) may be injected  24  in the flue gases from the boiler. The sorbent is injected downstream of the boiler and upstream of the waste treatment system  18 . 
       FIG. 2  depicts a power plant  40  similar to the plant  10  shown in  FIG. 1 . The same reference numbers have been used to label the components of the power plant  40  shown in  FIG. 2  that are the same as the components of the power plant  10  shown in  FIG. 1 . The power plant  40  has a modified combustion section  42  of the boiler  16 . The combustion section  42  is modified so that amount of carbon in fly ash generated during coal combustion is greater than would be expected in a conventional, efficient combustor of a coal-fired boiler. 
     Coal combustion in coal-fired boilers is usually not complete and generates fly ash with some carbon content. Combustion of bituminous coal tends to generate more carbon in fly ash than does the combustion of low rank coals. Adding bituminous coals to low rank coals tends to increase the amount of carbon in fly ash generated during combustion which increases reactivity of fly ash towards mercury. 
     Moreover, active high carbon fly ash is often generated during the conventional Low Oxides of Nitrogen (NO x ) Burn (LNB) process, in overfire air (OFA) injection zones  44 , and in connection with other conventional low NO x  combustion technologies. The combustion zone  42  of the boiler is configured such that active fly ash is formed in fuel-rich zones of the gas stream in the boiler. For example, an OFA injection zone  44  may be configured so that it does not completely burnout the carbon in the fly ash as the flue gases pass through the OFA injection zone. Accordingly, the amount of carbon in fly ash flowing through the boiler and downstream of the combustion zone is greater than would otherwise be expected in an efficient coal-fired boiler. 
     As the high carbon fly ash flows down stream from the boiler, the flue gas and ash cool. In a temperature range of 140° F. to 400° F., the high carbon fly ash is suitable for a baghouse or electrostatic precipitator (ESP). As the fly ash cools, the active carbon in the fly ash adsorbs mercury from flue gas. The ash adsorbs the mercury upstream and inside of the PCD  26 . The ash with adsorbed mercury is collected in the PCD  26  and discharged as ash waste discharge  28 . The power plant  40  generating reactive fly may be used with or without the sorbent injector  24  shown in the plant  10  shown in  FIG. 2 . 
       FIG. 3  depicts a power plant  50  similar to the plant  10  shown in  FIG. 1 . The same reference numbers have been used to label the components of the power plant  50  shown in  FIG. 3  that are the same as the components of the power plant  10  shown in  FIG. 1 . The power plant  50  includes a low rank coal injection system  52  that feeds low rank coal into the combustion section  42  of the boiler. The low rank coal  32  has a low chlorine content and is fired in the main combustion zone  42  of the boiler without first being blended with a high chlorine content bituminous coal. 
     Bituminous coal  54  or other high-chlorine coal is burned in a reburn zone  56  of the boiler to form high carbon fly ash and to provide the chlorine needed to oxidize the mercury released during coal combustion. The chlorine released by burning bituminous coal in the reburning zone oxidizes the mercury released by burning the low rank and bituminous coals. 
     Moreover, the combustion section  42  and the reburning zone  54  may be configured to generate high carbon fly ash from the coal combustion. As high carbon fly ash cools, the active carbon in the fly ash adsorbs mercury from the flue gas. The PCD  26  collects the ash with carbon and adsorbed Hg. 
     The benefits and effectiveness of burning low rank coal with high chlorine coal, and of generating high carbon fly ash are evident from the following test. Tests were performed in a 1.0 MMBTU/hr Boiler Simulator Facility (BSF)  60  to determine the effect of coal composition on Hg removal. The BSF facility is shown schematically in  FIG. 4 . The BSF provides sub-scale simulation of the flue gas temperatures and compositions found in a full-scale boiler. 
     As shown in  FIG. 4 , the BSF  60  includes a burner  62 , a vertically down-fired radiant furnace  64 , a cooling section  66 , a horizontal convective pass  68  extending from the furnace, an ESP  68  and a stack with flue gas sampling instruments  70  in communication with the convective pass. The burner  62  is a variable swirl diffusion burner with an axial fuel injector, and is used to simulate the approximate temperature and gas composition of a commercial burner in a full-scale, coal-fired boiler. Primary air is injected axially into the combustion zone of the boiler. Secondary air is injected radially through swirl vanes (not shown) to provide controlled fuel/air mixing in the combustion zone. The swirl number can be controlled by adjusting the angle of the swirl vanes. Numerous access ports located along the axis of the facility allow access for supplementary equipment such as reburn injectors, additive injectors, overfire air injectors, and sampling probes. 
     The radiant furnace  64  has eight modular refractory lined sections with an inside diameter of 22 inches and a total height of 20 feet. The convective pass  68  is also refractory lined, and contains air cooled tube bundles to simulate the superheater and reheater sections of a utility boiler. Heat extraction in radiant furnace and convective pass can be controlled such that the residence time-temperature profile matches that of a typical full-scale boiler. A suction pyrometer (not shown) measures furnace gas temperatures. 
     The ESP  70  for the BSF is a single-field unit consisting of 12 tubes with axial corona electrodes. Mercury concentration was measured at ESP outlet using an online Hg analyzer. The analyzer is capable of measuring both elemental (Hg 0 ) and total mercury in flue gas. Tests were conducted with Powder River Basin (PRB) coal, bituminous, and blends of PRB and bituminous coal to determine efficiency of mercury removal by fly ash across the ESP. The average temperature across the ESP was 350° F. 
       FIG. 5  is a chart  80  that presents results of the BSF tests regarding mercury emissions from the combustion of a low-rank coal (s), an Eastern bituminous only coal (s), and a blend of low-rank and Eastern bituminous coal. High carbon fly ash was formed using air staging during each test. Stoichiometric ratio (SR 1 ) in the main combustion zone was in the range of 0.5-1.0, and final stoichiometric ratio (SR 2 ) was 1.16 which corresponded to about 3% excess oxygen in flue gas. Overfire air (OFA) was injected at flue gas temperatures in the range of 1800-2500° F. Variation in OFA injection temperature was achieved by changing location of the OFA injection port. 
     Test results with PRB coal demonstrated that SR 1  and OFA injection temperature had a small effect on carbon in ash content (as indicated by levels of the LOI—loss on ignition) and Hg removal in PRB coals. Variation of these test parameters resulted in LOI in the range of 0.2% to 0.8%. Mercury removal by fly ash in PRB coals was in the range of 6% to 35%, as is shown in  FIG. 5  in the area  82  indicated by dashed lines. These tests  82  results are consistent with the mercury remaining in elemental form (Hg 0 ) in the flue gas and is not substantially oxidized. 
     Test results  84  with bituminous coal demonstrated that changes in combustion conditions had significant effect on the LOI level of the fly ash. Mercury adsorption increased as LOI increased, and the level of Hg adsorption reached about 60% at an LOI of 5%. These tests  84  are consistent with the mercury in the flue gas being oxidized and converted to HgCl 2  by the chlorine released as the bituminous coal burns. 
     Test results  86  were conducted with a PRB/bituminous coal blend (70/30) having composition of 70% PRB and 30% Eastern bituminous coals on a weight basis. These tests  86  demonstrated that a change in combustion conditions had a significant effect on LOI and Hg adsorption by fly ash. The mercury adsorption in the 70/30 coal blend was about the same as in bituminous coal only, even though the bituminous coal comprised only 30% of the coal blend. 
       FIG. 6  is a chart  90  that presents results of the BSF tests regarding mercury emissions from the combustion of a PRB coal, an Western Bituminous only coal, and a 70/30 blend of PRB and Western bituminous coals. High carbon fly ash was formed using air staging during each test. OFA air was injected at 1800° F. SR 1  and SR 2  were 1.05 and 1.16, respectively.  FIG. 6  demonstrate that mercury removal efficiency for the coal blend was 55% and was higher than that for PRB and bituminous coal only. The pilot-scale experiments results show that efficiency of mercury removal by fly ash for low-rank coal can be improved by blending them with bituminous coal. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.