Abstract:
A method of decreasing a concentration of nitrogen oxides in a combustion gas flowing through a vessel including: generating a flue gas in a combustion zone of the vessel, the flue gas containing nitrogen oxides and carbon monoxide; providing overfire air into a burnout zone of the vessel from a first injector of overfire air to oxidize at least some of the carbon monoxide in the flue gas; injecting a selective reducing agent concurrent with overfire air at a level in the burnout zone downstream of the first injector of overfire air and downstream of the oxidization of the carbon monoxide, and reacting the selective reducing agent with the flue gas to reduce the nitrogen oxides.

Description:
CROSS RELATED APPLICATION 
     This application is a divisional of application Ser. No. 10/454,597, filed Jun. 5, 2003, which application is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to reducing emission of nitrogen oxides from combustion systems, such as boilers, furnaces and incinerators. 
     A group of air pollutants produced by combustion in boilers and furnaces include oxides of nitrogen, mainly NO and NO 2 . Nitrogen oxides (NO X ) are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes. Reduction of nitrogen oxides has been the focus of many technology development efforts. 
     In modern boilers and furnaces and other such combustion vessels, emissions of nitrogen oxides (NO X ) have been greatly reduced by the use of overfire air (“OFA”) technology. In this technology, most of the combustion air goes into the combustion chamber together with the fuel, but addition of a portion of the combustion air is delayed to yield oxygen lean conditions initially and then to facilitate combustion of CO and any residual fuel. 
     Selective Non-Catalytic Reduction (“SNCR”) technologies reduce NO X  in combustion gas by injecting a nitrogenous reducing agent (“N-agent”), such as ammonia or urea, into the gas. The N-agent is injected at high temperature and under conditions such that a non-catalytic reaction selectively reduces NO X  to molecular nitrogen. Reduction of NO X  is selective because the molecular nitrogen in the combustion gas is not reduced, while the NO X  is reduced by the N-agent. 
     The N-agent is typically released into flue gas that is within an optimum temperature range or window, such as between 1700 degrees to 2200 degrees Fahrenheit (930 to 1200 degree Celsius). The flue gas often has moderate to high carbon monoxide (CO) concentrations (0.2-1.0 percent). In some SNCR applications, the CO in flue gas chemically competes with the active species in the N-agent needed for NO X  reduction. This competition reduces the effectiveness of the SNCR process and NO X  reduction, and/or moves the optimum temperature window to lower temperatures. 
     Earlier SNCR techniques circumvented the CO problem by spraying large N-agent droplets into overfire air injected into the flue gas. As the OFA and flue gas steams mix, CO is oxidized and water in the droplets evaporates as the droplets are carried to cooler regions of the boiler. This process delays the release of the N-agent until the gas temperature has reached the optimal temperature window. 
     Large droplet N-agent systems have difficulties that can reduce their effectiveness such as: long droplet residence times in the flue gas, a tortuous flow path with obstructions for the droplets, and a narrow N-agent release temperature window. If the droplets are too small, they release the N-agent upstream of the optimal temperature window where the flue gas is still too hot and render the N-agent ineffective. Under these conditions, the N-agent can generate (rather than reduce) NO X . On the other hand, if the droplets are too large, a portion of the N-agent is released after the combustion gas has cooled below the optimal temperature window causing high ammonia concentrations (ammonia slip) in the flue gas outlet stream. Finally, there is a need for better SNCR techniques to address the problems raised by high CO concentrations in the flue gas near the droplet injection location. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention is a method of decreasing the concentration of nitrogen oxides in a combustion gas flowing downstream through a vessel, comprising: generating a flue gas in a combustion zone of the vessel, the flue gas partly composed of nitrogen oxides and carbon monoxide; injecting overfire air into a burnout zone of the vessel from a first source of overfire air to oxidize at least some of the carbon monoxide in the flue gas; spraying a selective reducing agent concurrently with overfire air into a burnout zone downstream of the first source of overfire air and downstream of the oxidization of the carbon monoxide; and reacting the selective reducing agent with the flue gas to reduce the nitrogen oxides. 
     In a second embodiment, the invention is a combustion vessel having a combustion zone; a burnout zone downstream of the combustion zone; an overfire air compartment adjacent the burnout zone, wherein the overfire air compartment has an upstream air injector and a downstream air injector, and at least one agent injector for injecting a selective reducing agent into the burnout zone wherein the agent injector is placed in the air injector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a coal-fired combustion vessel. 
         FIG. 2  is a schematic diagram of a multi-compartment overfire chamber for the vessel shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  a schematic representation of a combustion system  10  such as that used in a coal-fired boiler or furnace. The combustion system  10  includes a combustion vessel  11  having a combustion zone  12 , a burnout zone  14  and an optional reburning zone  16 . The combustion zone  12  includes one or more main burners  18  mounted on at least one of the walls  20  of combustion vessel  11 . The walls form a vertical chamber for the combustion zone  12 , reburning zone  16 , burnout zone  14  and other components in the flue gas stream of the system  10 . 
     The main burners are supplied with a main fuel, such as coal, directly or through a fuel manifold  22  and with air directly or through an air box  24 . The air box may be mounted on the outside of the walls  20  opposite to the combustion zone  12  inside the vessel. The air box is a manifold that distributes air to each of the burners. 
     Combustion of the fuel injected by the main burners  18  and air from the air box  24  occurs in the combustion zone  12  of the vessel. The flue gas  26  produced by the combustion flows in a downstream direction that is upward from the combustion zone  12  to the burnout zone  14  in the vessel  11 . The main burners supply the heat energy input into the vessel. Additional heat may be released into the vessel  11  at the reburning zone  16  where a reburn fuel, such as natural gas, is combusted. The reburn fuel enters the vessel  11  through a reburn fuel injector  28 . 
     Downstream of reburning zone  16  is the burnout zone  14  where overfire air enters the vessel  11  through an overfire air injector  30 . Downstream of the burnout zone in the vessel  11 , the flue gas  26  optionally passes through a series of heat exchangers  32 . Solid particles remaining in the flue gas may be removed by a particulate control device  33 , such as an electrostatic precipitator (“ESP”) or baghouse. 
     A selective reducing agent (N-agent) is sprayed into the burnout zone  14  with the overfire air. An N-agent injector (nozzle and lance) is placed in the overfire air chamber  30  and injects the selective reducing agent into the burnout zone  14  along with overfire air. As used herein, the terms “selective reducing agent” and “N-agent” are used interchangeably to refer to any of a variety of chemical species capable of selectively reducing NO X  in the presence of oxygen in a combustion system. In general, suitable selective reducing agents include urea, ammonia, cyanuric acid, hydrazine, thanolamine, biuret, triuret, ammelide, ammonium salts of organic acids, ammonium salts of inorganic acids, and the like. Specific examples of ammonium salt reducing agents include, ammonium sulfate, ammonium bisulfate, ammonium bisulfite, ammonium formate, ammonium carbonate, ammonium bicarbonate, ammonium nitrate, and the like. Mixtures of these selective reducing agents can also be used. The selective reducing agent is provided in a solution, preferably an aqueous solution, or in the form of a powder. One selective reducing agent is urea in aqueous solution. 
     As shown in  FIG. 2 , the overfire air input chamber  30  includes a plurality of OFA injectors  34 ,  36 . These injectors are in regions of the chamber  30  from which overfire air flows through the wall  20  and into the burnout zone  14  of the vessel  11 . The overfire chamber  30  is attached to the wall  20  of the vessel. 
     The OFA injectors of the chamber  30  are arranged vertically one over the other on the wall  20  of the vessel. A lower OFA injector  34  (upstream injector in flue gas) of the chamber  30  is a conduit that provides air, e.g., at a high flow rate, into the burnout zone  14 . An upper OFA injector  36  (downstream injector in flue gas) of the chamber  30  also provides air to the burnout zone. The overfire air supplied by the downstream injector may be at a reduced flow rate than the air flowing through the upstream injector. Each of the OFA injectors may have walls that define an air conduit through which air flows to the wall  20  of the vessel, through penetrations in the wall and into the burnout zone  14  of the vessel. 
     A separator plate  46  in the chamber  30  may provide a wall separating the upper and lower OFA injectors. However, a separator plate may not be needed if the OFA injectors are not contained in one air input chamber  30 , but are separated from one another with some of the vessel wall  20  between the OFA injectors. There may be more than two OFA air injectors, but the injector furthest downstream will generally include the N-agent injector. For example, two or more upstream OFA injectors may supply air to the burnout zone  14  and a final downstream OFA injector with an N-agent injector may supply both overfire air and the N-agent to the burnout zone  14 . 
     The air from the upstream injector reduces the CO concentration in the burnout zone  14 , before the N-agent is released. Air from the downstream injector  36  flows into the burnout zone  14  with the droplets containing the N-agent. The air mass flow through the upstream OFA injector(s) may be substantially greater than the mass flow through the downstream OFA injector. The flow rates of air through each of the injectors may be controlled to regulate the amount of overfire air flowing into the vessel. Adjustable dampers  44  in each of the injectors  34 ,  36  may be used to regulate the amount of air flowing through each injector. Similarly, fans may be positioned in the overfire chamber  30  upstream of the injector and used to move air into the overfire chamber at control flow rates. 
     N-agent nozzles  38  spray the N-agent into the burnout zone. Each N-agent nozzle  38  is placed at the end of a lance  48  that extends through the downstream overfire air injector in the overfire chamber  30 . There may be a plurality e.g., three or four, of the agent injectors and lances arranged in the wall  20  and through the downstream OFA injector  36 . N-agent is introduced into the burnout zone  14  through the N-agent nozzle  38  concurrently with the air flowing through the downstream OFA injector  36 . The N-agent flows downstream as the OFA mixes with the combustion gas  26 . Once released, the N-agent chemically reacts with combustion gas to reduce the NO X  emissions. 
     Flue gas  26 , with moderate to high CO concentrations, flows upward from the combustion zone into the burnout zone  14  where they initially mix with the overfire air from the lower compartment  34  and subsequently mix with the N-agent and overfire air from the upper compartment  36 . The carbon monoxide (CO) in the flue gas flowing from the combustion and reburning zones  12 ,  16  is oxidized in the burnout zone  14  by the air flowing from the lower compartment  34  of the overfire chamber  30 . Oxygen (O 2 ) in the air reacts with the CO to form carbon dioxide. The oxidation of the CO occurs in the burnout zone  14  upstream (below) the level where the N-agent is injected. 
     By injecting air into the vessel through the upstream injector  34  that is below the N-agent injector  38 , a substantial portion of the carbon monoxide in the flue gas  26  is oxidized before the gas comes into contact with the N-agent. The oxidization of the CO upstream of the N-agent injection location may allow the N-agent to be sprayed into the flue gas with smaller droplets sizes reducing droplet residence times in the flue gas. 
     Airflow rates in the upper and lower injectors  34 ,  36  are adjusted to shield the N-agent from the flue gas until a sufficient amount of the flue gas CO is oxidized by the air from the lower compartment  34 . This usually requires that more air flow through the upstream injector  34  than the downstream injector  36 . For example, the air mass flowing through the upstream injector  34  may be four to ten times the air mass flowing through the downstream injector  36 . The low CO concentration in the flue gas that contacts the N-agent improves N-agent effectiveness by reducing the competition between CO and NO X  for active species critical to SNCR NO X  reduction chemistry. 
     The N-agent injector  38  may be a nozzle at the end of a lance  48  that extends through the downstream injector  36 . An input end of the lance, opposite to the nozzle  38 , is coupled to a source of the N-agent. There may be multiple agent nozzles and lances for N-agent injectors arranged in the upper chamber and along the wall  20  of the vessel  11 . The N-agent injector may be positioned at a level of the vessel  11  corresponding to a desired temperature of the flue gas in the burnout zone. For example, the agent injector  38  may be at a level where the temperature of the flue gas is in a range of 1,700 to 2,500 degrees Fahrenheit. The N-agent nozzle  38  may inject small droplets or gas of N-agent into the burnout zone. The small droplets release the N-agent to the combustion gas quicker than do larger droplets. 
     Pilot-scale field tests have demonstrated the negative effect that CO in combustion gas has on SNCR NO X  reduction chemistry. The presence of 2000 parts-per-million (ppm) of CO in the combustion gas has been shown to effectively eliminate the NO X  reduction achieved with N-Agent injection. For example, pilot-scale field tests conducted on a 300 kW (kiloWatt) cylindrical coal-fired furnace indicate that the N-agent reduces NO X  in combustion gas by 6 to 25 percent when CO is not present in the flue gas. However, the NO X  reduction due to the N-agent becomes negligible when CO at 2000 ppm is present in the flue gas. Accordingly, reduction of CO in the combustion gas is a factor that improves NO X  reduction when using SNCR technology. 
     Computational Fluid Dynamic (CFD) computer simulations of a typical boiler furnace demonstrated that to reduce NO X  by injecting an N-agent in overfire air, the temperature of the combustion gas entering the burnout zone should be in the temperature range from 1700 degrees to 2500 degrees Fahrenheit. The N-agent should be injected as small droplets into the gas and a split flow overfire air chamber  30  should provide substantially greater air mass flow through a lower compartment  34  than through the upper compartment  36 . The split in the air mass flow between the upstream and downstream compartments in the overfire chamber may be as great as 10 to 1, where this ratio means that ten times as much air mass flows through the lower compartment as flows in the upper compartment. The CFD results showed that NO X  was reduced by 21 percent when the air mass split was 4 to 1, and NO X  was reduced by 35 percent when the air mass split was 10 to 1. The relative adjustment of the air flow rate may be performed by moving dampers  44  in the upper and lower injectors, or adjusting the speed of fans driving air into the upper and lower injectors. 
     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.