Abstract:
An improved method for removing nitrogen oxides from concentrated waste gas streams, in which nitrogen oxides are ignited with a carbonaceous material in the presence of substoichiometric quantities of a primary oxidant, such as air. Additionally, reductants may be ignited along with the nitrogen oxides, carbonaceous material and primary oxidant to achieve greater reduction of nitrogen oxides. A scrubber and regeneration system may also be included to generate a concentrated stream of nitrogen oxides from flue gases for reduction using this method.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention pursuant to the employee/employer relationship of the inventors to the United States Department of Energy at the Pittsburgh Energy Technology Center. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a new method for destroying high concentrations of nitric oxides in gas streams. These streams may be recycle streams from regenerable NO x  scrubbers for flue gas applications, or they may be industrial waste streams such as those from nitrification processes in the chemical industry. 
     Flue gases resulting from the combustion of carbonaceous material typically contain substantial quantities of pollutants, including nitrogen oxides. These pollutants combine with other substances found in the atmosphere to produce serious environmental hazards, such as acid rain and smog. It is therefore desirable to remove these pollutants from flue gases before they are dispersed into the atmosphere. It is also desirable to decompose these pollutants into other substances not having the deleterious environmental consequences possessed by nitrogen oxides. 
     There are two types of methods by which the quantities of NO x  dispersed from combustion systems may be reduced. One type, known in the art as combustion modification, requires control over the combustion reaction producing the pollutant. These techniques have generally achieved fifty to sixty percent reductions in NO x  emissions from conventional combustion systems. 
     A specific type of combustion modification, known as reburning, has achieved reductions approaching seventy percent. Using this technique, a secondary fuel source as introduced downstream of the primary combustion zone in the combustor to achieve reductions of NO x . This technique, however, is disadvantageous in that it requires large amounts of secondary fuel to accomplish the reburning of NO x , and additionally requires downstream injection ports in the combustor to effectively control NO x  levels in the effluent stream. This method is not effective when more than seventy percent removal of effluent NO x  is required. 
     The second type of NO x  removal methods is known as post-combustion cleanup, whereby the pollutant is removed downstream of its formation. These techniques are more complex and expensive than combustion modification techniques, but are useful when NO x  reduction levels higher than seventy percent are necessary. In systems employing this technique, a dry scrubbing sorbent or an aqueous sorbent, such as an active metal chelate, are typically used to remove NO x  and other pollutants. Certain systems, such as that proposed by Walker, U.S. Pat. No. 4,615,780, additionally incorporate the step of regenerating the sorbent and producing a concentrated stream of NO x  which can be recycled as part of the combustion air. This concentrated NO x   is destroyed in the combustor and elemental nitrogen and oxygen are produced. This system produces maximum reductions of 60 to 70 percent of the NO x  produced by combustion, substantially less as compared to the present invention. 
     Another scrubber system, proposed by Harkness, et al, U.S. Pat. No. 4,612,175, uses active Fe(II)EDTA as the chelate to remove NO from flue gases simultaneously with the removal of SO 2 . This method, however, produces as an end product aqueous hydroxylamine disulfonate and other sulfonates in a sludge with the aqueous sorbent. These products in themselves create environmental hazards which render the process described by Harkness disadvantageous. 
     It should be noted that the source of the high concentration NO x  stream may be, as described above, a recycle stream from a regenerable scrubber or an industrial waste gas stream, such as that from nitrification processes as used in the chemical industry, high temperature smelting plants, nitric acid manufacturing plants, and any other process where a high concentration of NO x  is in the waste stream. The present invention relates to removal and destruction of NO x  from such high NO x  concentration waste gas streams. 
     SUMMARY OF THE INVENTION 
     The method described herein provides a technique for removing introgen oxides from gas streams, discharged from the combustion of carbonaceous materials, in greater quantities than previously known. 
     It is also an object of the invention to provide a method for the removal of NO x  resulting in the production of effluent streams of elemental nitrogen and oxygen. 
     It is a further object of the invention to provide a method for NO x  removal in which a reductant gas is used to further lower the concentration of NO x  in the effluent stream. 
     Another object of the invention is to provide a method wherein unconverted NO x  is removed from the discharge stream by a regenerable scrubber and recycled to the combustion device for conversion into elemental nitrogen and oxygen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of one embodiment of a system using the method described and claimed herein, incorporating a scrubber to produce the concentrated NO x  stream. 
     FIG. 2 is a cut-away view of a burner which may be used in the combustor, in accord with the method described and claimed herein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described with reference to the drawings. A system, such as that illustrated in FIG. 1, is provided, including a combustor 25. The combustor 25 is suitable for burning carbonaceous materials, such as coal. In the preferred embodiment of the invention, powdered or pulverized coal is mixed with substoichiometric quantities of air (ten to thirty percent of that required for full combustion) from a primary air source 20. The coal-air mixture is provided to combustor 25 through a primary air line 30 into one, or preferably more, burners 35. As illustrated more completely in FIG. 2, the coal-air mixture is provided to each burner through primary air line 30. Secondary air is also introduced into burners 35 via secondary air lines 40, having an outlet 43 downstream of primary air-carbonaceous material injection location (hereinafter &#34;injection location&#34;) 45. NO x  from outside sources, or from the recycle stream 50 (shown in FIG. 1) is injected into burners 35 through one of three gas lines: with the coat-air mixture, through primary air line 30; with the secondary air through line 40; or through auxiliary gas line 55. It is preferred to inject the NO x  through auxiliary gas line 55 having outlet 60 upstream of injection location 45. The burner 35 also includes pilot gas line 70 and pilot gas igniter 75 to cause combustion. If more than one burner 35 is used in the combustor 25, as in FIG. 1 (using four burners), it is preferable to inject NO x  through less than all of the burners 35 to allow the NO x  to contact hydrocarbons or other reductants in the system, thereby increasing NO x  reduction efficiency. 
     Referring back to FIG. 1, combustion gases are then passed out of combustor 25 via effluent gas line 80 into scrubber 85. Gases not absorbed in scrubber 85 pass through discharge line 90 to stack 95. These gases will typically include carbon dioxide, elemental nitrogen, elemental oxygen, and trace amounts of sulfur dioxide and nitrogen oxides. 
     In the preferred embodiment of the present invention, reductant gases 82, such as alkane gases, are combined with NO x  stream 50 prior to injection into burners 35 with a large volume of air. The reductant gas is mixed with NO x  stream 50 at a temperature below the combustion temperature of the gases. In the preferred embodiment of the invention, the molar ratio of reductant gas to NO x  is greater than or equal to three, and less than fifteen, to permit sufficient reduction. The injection of reductant into the NO x  stream 50 creates an oxygen deficient zone in the injection location 45 (shown in FIG. 2) in burner 35, to achieve maximum reduction of NO x . In the preferred embodiment of the invention, methane is used as the reductant gas. Other alkane gases up to butane may be used, and higher alkanes may also be used, but the higher alkanes will require vaporization before combination with NO x  stream 50. Other reductant gases which will be less useful include elemental hydrogen, carbon monoxide and other hydrocarbon gases. 
     The present invention may also be used in conjunction with another burner, known as to low NO x  burner. Such a burner (not illustrated) would consist of a primary air stream including carbonaceous material, a secondary air stream, and a tertiary air stream. As above, NO x  would be injected into the burner with the primary air or through the auxiliary gas line, and reductant gases injected with the NO x . 
     This technique for NO x  destruction can be used, for example, in conjunction with a regenerable scrubber, as shown in FIG. 1. In such an application, a scrubber 85 is provided for absorbing NO x  in effluent stream 80. The outflow 105 of scrubber 85, containing sorbent and absorbed NO x , is directed to sorbent regeneration means 110, which removes NO x  from the sorbent and returns the cleansed sorbent to scrubber 85 through sorbent line 115. Such a scrubber and regeneration means are described by Walker, U.S. Pat. No. 4,615,780, but other scrubber and regeneration means known in the art will function equally effectively. A concentrated stream 50 of NO x  is also output from regeneration means 110, and is recycled to provide NO x  input into the burners 35 at one or more of gas lines 30, 40 or 55 (as shown in FIG. 2). 
     The use of the system described above provides higher efficiency of NO x  removal then previously known in the art. This is demonstrated by the specific examples set forth below. 
     EXAMPLE I 
     This example was performed in a 227 kg/h coal combustor 25 having four burners 35 (as shown in FIG. 1) set vertically within the combustor. The lowest two burners 35 were injected with NO x  (to simulate an industrial waste gas stream or recycled NO x ) provided through the auxiliary gas line 55. No reductant gas was added to the system in this example. One and ninety-two hundredths moles NO x  were injected per mole NO x  produced in the initial combustion. A reduction of 93.9 percent of injected No x  was observed. 
     EXAMPLE II 
     In this example, the lowest two of four vertically set burners 35 in a 227 kg/h coal combustor 25 were injected with NO x  (to simulate an industrial waste gas stream of recycled NO x ), provided through the auxiliary gas line 55 of the burner. Two and five hundredths moles NO x  were injected per mole of NO x  produced in the initial combustion. Reductant gas, in the form of methane was introduced with the NO x  through auxiliary gas line 55, in the amount of 3.9 moles CH 4  per mole NO x  reduced. A reduction of 100 percent of the injected NO x  was observed. 
     EXAMPLE III 
     The lowest two of four vertically set burners 35 in a 227 kg/h coal combustor 25 were again injected with NO x  (to simulate an industrial waste gas stream or recycled NO x ) along with the coal-air mixture through inlet 30. Two and five hundredths moles NO x  were injected per mole of NO x  produced in the initial combustion. Reductant gas, in the form of methane was introduced with the NO x  through inlet 30, in the amount of 3.9 moles CH 4  per mole NO x  reduced. A reduction of 100 percent of the injected NO x  was observed. 
     Although the present invention is described in terms of specific materials, embodiments and process steps, it will be clear to one skilled in the art that various modifications can be made within the scope of the invention as described in the following claims.