Patent Publication Number: US-2015064083-A1

Title: Injector grid for high and low dust environment selective catalytic reduction systems

Description:
FIELD OF THE INVENTION 
     The present invention relates to an arrangement for supplying and mixing a reducing agent into a flue gas flowing through a duct and into a selective catalytic reduction (SCR) reactor arranged downstream of said arrangement. The subject arrangement is useful in both high and low dust environments to mitigate ash and like particulate accumulation on reducing agent nozzles and to provide uniform reducing agent flow distribution upstream of the SCR reactor. 
     BACKGROUND OF THE INVENTION 
     Combustion of a fuel such as coal, oil, natural gas, peat, waste, or the like, in a combustion plant such as a power plant or a waste incineration plant, generates a process gas. Separating nitrogen oxides denoted herein as “NOx”, from such a process gas or “flue gas”, frequently is accomplished using a reducing agent such as ammonia or urea. The ammonia or urea reducing agent is mixed with the flue gas, with the mixture then passed through a catalyst for a selective reaction of the reducing agent with the flue gas NOx to form nitrogen gas and water vapor. Usually the catalyst is installed in what is commonly called a selective catalytic reduction (SCR) reactor. The mixing of the reducing agent and the flue gas is accomplished in a gas duct upstream of the SCR reactor. 
     Reducing agent is supplied to the gas duct by a plurality of lances and nozzles arranged within the gas duct. To facilitate an even concentration distribution of NOx and reducing agent across a particular cross section of the gas duct, and thus also over a particular cross section of the SCR reactor, mixing devices are arranged in the gas duct downstream of the reducing agent supply to cause turbulent flow and mixing of the flue gas and reducing agent. 
     However, a problem in many systems is that the concentration of NOx and reducing agent is not evenly distributed in the flue gas across a particular cross section of the SCR reactor. This is a problem because the stoichiometric ratio between the NOx and the reducing agent is essential for achieving efficient reduction of the NOx content within the flue gas and a low slip of reducing agent from the SCR reactor. 
     DE 3723618 C1 discloses a device for mixing together two gases in a gas duct for a purpose such as that noted above. One of the gases is supplied by a number of nozzles arranged in rows on parallel nozzle lances. Along with the nozzle lances, a flow element in the shape of a baffle is provided arranged in such a way that a further flow channel is formed in each case on a side of the flow baffle facing away from an assigned nozzle. 
     SUMMARY OF THE INVENTION 
     An object of the present disclosure is to provide a robust injector arrangement that provides reducing agent and flue gas intermixing with reduced ash accumulation on reducing agent injection lances and nozzles over that of the described prior art device. As such, reduced ash accumulation on reducing agent injection lances and nozzles promotes improved reducing agent flow and more uniform reducing agent concentration distribution over a particular cross section of a gas duct. Additionally, installation and use of the subject injector arrangement within a gas duct results in a minimum increase in pressure drop upstream of a SCR reactor as is greatly desired. 
     The above stated object is achieved by the subject reducing agent injector arrangement useful for supplying a reducing agent in gaseous or liquid form into a flue gas flowing through a gas duct fluidly communicating with a catalyst in a selective catalytic reduction (SCR) reactor arranged downstream of the subject injector arrangement. The subject injector arrangement comprises a plurality of nozzles staggered on one or more, or two to eight, injector grid elliptical branch lances arranged in a gas duct perpendicular to the direction of flue gas flow through the gas duct. Each of the one or more, or two to eight, injector grid elliptical branch lances equipped with a plurality of nozzles is controlled by preferably one flow-adjusting control valve, although more valves could be used, for reducing agent and flue gas intermixing. As such, the plurality of nozzles are arranged to supply reducing agent within the gas duct for intermixing and consistent concentration distribution with said flue gas flowing through the gas duct. 
     The subject injector arrangement provides a relatively efficient and uniform concentration intermixing of the supplied reducing agent throughout the flue gas over a particular or given cross section of the gas duct downstream of the injector arrangement. Furthermore, the subject injector arrangement is robust with respect to variations in power plant operating conditions such as in either high or low dust environments. Supplying reducing agent using the subject injector arrangement mitigates dust, boiler ash, or like particulate accumulation on reducing agent injection lances and nozzles providing the advantage of improved reducing agent flow and uniform concentration distribution prior to entry into a downstream SCR reactor. As such, the subject injector arrangement supplies reducing agent into the passing stream of flue gas in a very evenly distributed manner regardless of environment, and minimizes pressure drops within the gas duct as desired. 
     The subject injector arrangement is thus useful for supplying and mixing a reducing agent into a flue gas flowing in a gas duct communicating with a catalyst in a selective catalytic reduction reactor downstream of the arrangement. For this purpose, the injector arrangement comprises a reducing agent supply for a supply of reducing agent for flow through fluidly connected elliptical main supply lance, elliptical branch lances, staggered injection pipes and nozzles for injection of the reducing agent from the nozzles into the flue gas flowing through the gas duct. 
     According to one embodiment, the elliptical main supply lance is fluidly connected to one or more elliptical branch lances. 
     According to another embodiment, the elliptical main supply lance is fluidly connected to two to eight elliptical branch lances. 
     According to another embodiment, the elliptical branch lances are fluidly connected to approximately 5 to approximately 20 staggered injection pipes. 
     According to another embodiment, the staggered injection pipes are equipped with removably fixed cap members each with an opening forming a nozzle. 
     According to another embodiment, the nozzles may be cleaned by removing removably fixed cap members from staggered injection pipes and replacement with new or cleaned cap members. 
     According to another embodiment, the elliptical shape of the elliptical main supply lance and the elliptical branch lances reduce pressure drop in the gas duct over that of round piping. 
     According to another embodiment, the elliptical shape of the elliptical main supply lance and the elliptical branch lances reduce particulate accumulation on nozzles improving reducing agent flow and more uniform reducing agent concentration distribution within the gas duct over that of round piping. 
     A method of using the subject injector arrangement to supply and mix a reducing agent into a flue gas flowing in a gas duct communicating with a catalyst in a selective catalytic reduction reactor downstream of said arrangement, comprises providing a supply of reducing agent for flow through fluidly connected elliptical main supply lance, elliptical branch lances, staggered injection pipes and nozzles for injection of the supply of reducing agent from the nozzles into the flue gas flowing through the gas duct. 
     According to one method, the elliptical main supply lance is fluidly connected to one or more elliptical branch lances. 
     According to another method, the elliptical branch lances are fluidly connected to approximately 5 to approximately 20 staggered injection pipes. 
     According to another method, the staggered injection pipes are equipped with removably fixed cap members each with an opening forming a nozzle. 
     According to another method, the nozzles may be cleaned by removing removably fixed cap members from staggered injection pipes and replacement with new or cleaned cap members. 
     According to another method, the elliptical shape of the elliptical main supply lance and the elliptical branch lances reduce pressure drop in the gas duct over that of round piping. 
     According to another method, the elliptical shape of the elliptical main supply lance and the elliptical branch lances reduce particulate accumulation on nozzles improving reducing agent flow and more uniform reducing agent concentration distribution within the gas duct over that of round piping. 
     Further objects and features of the subject injector arrangement and method of using the subject injector arrangement will be apparent from the following detailed description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject injector arrangement will now be described in more detail with reference to the appended drawings described below. 
         FIG. 1  is a schematic side view of a plant with a reducing agent injection grid according to the present invention. 
         FIG. 2  is an enlarged schematic side perspective view of the reducing agent injection grid of  FIG. 1 . 
         FIG. 3  is a schematic end cross sectional view taken at line  3 - 3  of the reducing agent injection grid of  FIG. 2 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Power plants are typically powered using coal, oil, natural gas, peat, waste, or like fuel fired boilers. According to the present power plant system  10  illustrated in  FIG. 1 , fuel is combusted in a boiler  12  in the presence of air A, thereby generating a flow of process gas in the form of a flue gas, FG, that flows out from the boiler  12  via a fluidly connected gas duct  14 . Through gas duct  14 , flue gas FG flows to an inlet  16  of a selective catalytic reduction (SCR) reactor  18 .  FIG. 1  illustrates an injector arrangement  19  in the form of a reducing agent injection grid  20  arranged across gas duct  14  perpendicular to the flow of flue gas FG through gas duct  14 , and upstream with regard to flue gas FG flow to SCR reactor  18 . A reducing agent supply system  22  is operative for supplying a reducing agent such as ammonia or urea, but preferably ammonia in a gas form, more preferably in a diluted gas form, and most preferably in a diluted gas form diluted with air, from a reducing agent supply  24  through a fluidly connected reducing agent pipe  26  to the reducing agent injection grid  20 . One or more flow-adjusting control valves  72  are used to control flow of reducing agent through one or more reducing agent pipes  26  to the reducing agent injection grid  20 . As such, the reducing agent injection grid  20  supplies diluted or undiluted gaseous ammonia, NH 3 , to the flue gas FG flowing through gas duct  14  prior to its flow into SCR reactor  18 . The SCR reactor  18  comprises one or more consecutive layers  28  of SCR catalyst  30  arranged inside the SCR reactor  18 . The SCR catalyst  30  can by way of example comprise a catalytically active component, such as a vanadium pentoxide (V 2 O 5 ) and titanium dioxide (TiO 2 ) substrate with other chemical additives such as wolfram trioxide (WO 3 ) and molybdenum trioxide (MoO 3 ), applied to a ceramic honeycomb carrier material or parallel plate structures (not shown). In the SCR reactor  18  the nitrogen oxides, NOx, in the flue gas FG react with the ammonia supplied through the reducing agent injection grid  20  to form nitrogen gas, N 2 , and water vapor. Following this reaction in the SCR reactor  18 , “cleaned” flue gas CG flows out from the SCR reactor  18  via a fluidly connected exit duct  32  for emission into the atmosphere via a fluidly connected stack  34 . It will be appreciated that the power plant system  10  may comprise further gas cleaning devices, such as dry and/or wet scrubbers, and particulate removers, such as electrostatic precipitators and fabric filters, not illustrated in the figures provided herewith for purposes of clarity. 
     As best illustrated in  FIG. 2 , the reducing agent injection grid  20  comprises a plurality of staggered injection pipes  36  each with a nozzle  42  fluidly connected to an elliptical main supply lance  38  via fluidly connected elliptical branch lances  40  therebetween. From reducing agent supply  24  through fluidly connected reducing agent pipe  26 , elliptical main supply lance  38 , elliptical branch lances  40  and staggered injection pipes  36 , reducing agent flows for release within gas duct  14  for intermixing with flue gas FG flowing therethrough. 
     As best illustrated in  FIG. 3 , elliptical branch lances  40  are elliptical in form. As such, each elliptical branch lance  40  is formed by opposed side walls  46   a  and  46   b.  Opposed side walls  46   a  and  46   b  form opposed exterior surface  44  and interior surface  48  of elliptical branch lance  40 . Opposed side walls  46   a  and  46   b  join at upstream apex  52  and downstream apex  54 . Flue gas FG flowing through gas duct  14  first contacts upstream apex  52  before flowing past downstream apex  54 . This elliptical form of both the elliptical main supply lance  38  and the elliptical branch lances  40 , mitigates ash and like particulate accumulation on nozzles  42  by reducing flue gas FG recirculation, or eddies, that typically occur as flue gas flows around round injection piping. Round injection piping of the prior art creates flue gas recirculation or eddies due to a relatively large flue gas initial contact area. This relatively large flue gas initial contact area blocks and redirects flue gas flow thereby causing an increased pressure drop and increased flue gas recirculation or eddie formation. As illustrated in  FIG. 3 , the elliptical form of both the elliptical main supply lance  38  and the elliptical branch lances  40  feature a relatively small flue gas FG initial contact area CA. This relatively small flue gas FG initial contact area CA provides minimal blockage and redirection of flue gas FG flow, thereby minimizing pressure drop and flue gas FG recirculation or eddie formation. Minimizing flue gas FG recirculation is highly desirable to prevent nozzle  42  plugging and resultant poor reducing agent injection and concentration distribution with flue gas FG flowing within gas duct  14 . 
     The elliptical main supply lance  38  is preferably equipped with one or more, or two to eight elliptical branch lances  40 . Interior surface  48  defines interior area  50  of elliptical branch lance  40  through which reducing agent flows to fluidly connected staggered injection pipes  36 . Each elliptical branch lance  40  ranges in length from approximately 1 meter (m) to approximately 4 m in length and may be equipped with a total of approximately 5 to approximately 20 staggered injection pipes  36 . As illustrated, staggered injection pipes  36  protrude approximately 8 centimeters (cm) to approximately 20 cm from exterior surface  44  of opposed side walls  46   a  and  46   b  of elliptical branch lances  40 . Staggered injection pipes  36  are staggered in that staggered injection pipes  36  protruding from side wall  46   a  are arranged so as to be between staggered injection pipes  36  protruding from side wall  46   b,  and vice versa. This staggered arrangement of staggered injection pipes  36  allows for more uniform distribution and flow of reducing agent within gas duct  14 . 
     It is to be understood that the number of staggered injection pipes  36  and their positioning relatively near downstream apex  54  of elliptical branch lances  40  may be varied. The number of staggered injection pipes  36  should be adapted to parameters such as the quality of the flue gas, the dimensions of the elliptical branch lance  40  and gas duct  14 , and the quantity of reducing agent and dilution air required for the SCR reactor  18 . 
     Staggered injection pipes  36  as best illustrated in  FIG. 3  protrude from exterior surface  44  of opposed side walls  46   a  and  46   b  of elliptical branch lances  40 . The staggered injection pipes  36  protrude from exterior surface  44  relatively near downstream apex  54 , as compared to upstream apex  52 , and at an angle A of approximately 45 degrees to approximately 50 degrees toward downstream apex  54  measuring from longitudinal axis L of staggered injection pipes  36  to plane P-P perpendicular to the flow of flue gas FG through gas duct  14 . Opposite from staggered injection pipe  36  connection with exterior surface  44  is staggered injection pipe  36  free end  58 . On exterior surface  60  of staggered injection pipe  36  at free end  58  is threading  62 . Threading  62  on staggered injection pipe  36  is compatible for male-female interlocking with threading  64  on interior surface  66  of cap member  68  arranged over free end  58  of staggered injection pipe  36 . Although threading  62 ,  64  is described herein for removably fixing cap member  68  to staggered injection pipe  36 , other means of removably fixing cap member  68  to staggered injection pipe  36  known to those skilled in the art would likewise be acceptable. Opening  56  through free end  70  of cap member  68  forms nozzle  42 . Threading  62 ,  64  provides for ready adjustment of nozzles  42  through use of differing cap members  68  with openings  56  of varying size. Likewise cleaning of nozzles  42  may be achieved with relative ease through removal of dirty nozzles  42  and replacement thereof with new or cleaned nozzles  42 . Each nozzle  42  is preferably operated to provide a continuous flow of reducing agent from the reducing agent supply  24 , through fluidly connected reducing agent pipe  26 , through fluidly connected elliptical main supply lance  38 , through fluidly connected elliptical branch lances  40 , and through staggered injection pipes  36  into gas duct  14 . 
     The reducing agent supply system  22  provides a ready supply of reducing agent to gas duct  14 . Reducing agent supply  24  can be in the form of a tank used in combination with a vaporization skid and flow control skid, or another suitable storage arrangement known to those skilled in the art. As nonlimiting examples, the reducing agent can be ammonia or urea. In case of ammonia, it can either be delivered to the power plant  10  in gaseous form, or be delivered in liquid form for later vaporization and dilution before injection into gas duct  14 . Maintaining ammonia and dilution air in superheated gaseous form, avoids problems associated with deposit formation due to droplets or condensation interacting with flue gas FG particulates. 
     The reducing agent supply system  22  is disclosed thus far with a single unitary reducing agent injection grid  20  comprising a plurality of staggered injection pipes  36  each with a nozzle  42  fluidly connected to an elliptical main supply lance  38  via fluidly connected elliptical branch lances  40  therebetween. However, it is to be understood that the reducing agent supply system  22  could be expanded to include one or more different reducing agent injection grids  20  positioned in gas duct  14  to be provided with different amounts of reducing agent or with different degrees of pressurization. The latter can be useful if it has been detected by measurements made downstream of the SCR reactor  18  that there is a non-uniform NOx distribution profile. 
     Additionally, reducing agent supply system  22  may be connected to a control system  74  to regulate a supply of reducing agent to gas duct  14  based on an amount of NOx measured by one or more sensors  76   b  in the flue gas FG downstream of the SCR reactor  18 . Such control system  74  may directly or by electronic signal flow-adjusting control valve  72  to control or regulate reducing agent flow through nozzles  42 . 
     As further illustrated in  FIG. 1 , a first NOx sensor  76   a  is operative for measuring the amount of NOx in the flue gas of gas duct  14  after the boiler  12  and upstream of the SCR reactor  18 . A second NOx sensor  76   b  is operative for measuring the amount of NOx in the flue gas of exit duct  32  downstream of the SCR reactor  18 . The control system  74  receives data input from the first NOx analyzer  76   a  and the second NOx sensor  76   b.  Based on that data input, the control system  74  calculates a present NOx removal efficiency. The calculated present NOx removal efficiency is compared to a NOx removal set point. Based on the result of the comparison, the amount of reducing agent supplied to the flue gas FG is adjusted for optimal efficiency. 
     It is to be understood that when a control system  74  is used, the particular embodiment described herein is only one possible solution. Control system  74  may be varied to control NO X  reduction efficiency of the SCR reactor  18 , depending upon the required outlet NO X  emission level to be achieved. 
     It is also to be understood that a load sensor (not shown) operative for sensing the load on the boiler  12  may be used. Such load could be expressed in terms of, for example, the amount of fuel, such as ton/hour of coal transported to the boiler  12 . The data signal from such load sensor is useful to further control the amount of reducing agent supplied to gas duct  14  via nozzles  42 . According to one embodiment, flue gas NOx profile data is generated on a regular basis, based on NOx measurements performed upstream and/or downstream of the SCR reactor  18 . An advantage of this embodiment is that changes in the NOx profile, such changes being caused by, for example, a change in the load on the boiler, a change in the fuel quality, a change in the status of the burners of the boiler, and the like, can be accounted for through control of the amount of reducing agent supplied to gas duct  14 , such that efficient NOx removal can be ensured at all times. 
     It is also to be understood that the NOx profile data could be obtained by making manual measurements, to determine a suitable amount of reducing agent is supplied by nozzles  42  to the flue gas FG in gas duct  14 . 
     It has been described hereinbefore, that the present invention can be utilized for reducing NO X  emissions from a process flue gas FG generated in a coal fired boiler  12 . It will be appreciated that the invention is useful also for other types of reagent injection processes, e.g., liquid sorbent injection systems, and other types of process gases, including process gases generated in gas and oil fired boilers, incineration plants, including waste incineration plants, cement kilns, blast furnaces, combined cycle plants and other metallurgical plants including sinter belts, and the like. 
     Likewise, it is to be understood that the gas duct  14  can be provided with one or more mixing plates  78  of any geometry, downstream or upstream of the reducing agent injection grid  20  to increase the turbulence and intermixing of reducing agent with the flue gas FG. 
     To summarize, the present disclosure provides an injector arrangement  19  for supplying and mixing a reducing agent RA into a flue gas FG flowing in a gas duct  14  communicating with a catalyst  30  in a selective catalytic reduction reactor  18  downstream of the injector arrangement  19 . The injector arrangement  19  comprises a reducing agent supply  24  for a supply of reducing agent RA for flow through fluidly connected elliptical main supply lance  38 , elliptical branch lances  40 , staggered injection pipes  36  and nozzles  42  for injection of the reducing agent RA from the nozzles  42  into the flue gas FG flowing through the gas duct  14 . 
     According to one embodiment, the elliptical main supply lance  38  is fluidly connected to one or more elliptical branch lances  40 . 
     According to another embodiment, the elliptical main supply lance  38  is fluidly connected to two to eight elliptical branch lances  40 . 
     According to another embodiment, the elliptical branch lances  40  are fluidly connected to approximately 5 to approximately 20 staggered injection pipes  36 . 
     According to another embodiment, the staggered injection pipes  36  are equipped with removably fixed cap members  68  each with an opening  56  forming a nozzle  42 . 
     According to another embodiment, the nozzles  42  may be cleaned by removing removably fixed cap members  68  from staggered injection pipes  36  and replacement with new or cleaned cap members  68 . 
     According to another embodiment, the elliptical shape of the elliptical main supply lance  38  and the elliptical branch lances  40  reduce pressure drop in the gas duct  14  over that of round piping. 
     According to another embodiment, the elliptical shape of the elliptical main supply lance  38  and the elliptical branch lances  40  reduce particulate accumulation on nozzles  42  improving reducing agent RA flow and providing more uniform reducing agent RA concentration distribution within the gas duct  14  over that of round piping. 
     A method of using the subject injector arrangement  19  to supply and mix a reducing agent RA into a flue gas FG flowing in a gas duct  14  communicating with a catalyst  30  in a selective catalytic reduction reactor  18  downstream of said arrangement  19 , comprises providing a supply of reducing agent RA for flow through fluidly connected elliptical main supply lance  38 , elliptical branch lances  40 , staggered injection pipes  36  and nozzles  42  for injection of the supply of reducing agent RA from the nozzles  42  into the flue gas FG flowing through the gas duct  14 . 
     According to one method, the elliptical main supply lance  38  is fluidly connected to one or more elliptical branch lances  40 . 
     According to another method, the elliptical branch lances  38  are fluidly connected to approximately 5 to approximately 20 staggered injection pipes  36 . 
     According to another method, the staggered injection pipes  36  are equipped with removably fixed cap members  68  each with an opening  56  forming a nozzle  42 . 
     According to another method, the nozzles  42  may be cleaned by removing removably fixed cap members  68  from staggered injection pipes  36  and replacement with new or cleaned cap members  68 . 
     According to another method, the elliptical shape of the elliptical main supply lance  38  and the elliptical branch lances  40  reduce pressure drop in the gas duct  14  over that of round piping. 
     According to another method, the elliptical shape of the elliptical main supply lance  38  and the elliptical branch lances  40  reduce particulate accumulation on nozzles  42  improving reducing agent RA flow and more uniform reducing agent RA concentration distribution within the gas duct  14  over that of round piping. 
     It will be appreciated that numerous variants of the above described embodiments of the present invention are possible within the scope of the appended claims.