Economizer bypass with ammonia injection

Abstract

A NOx removal system for a furnace or boiler with an economizer has two separate ammonia injection grids (AIG) upstream of an SCR. A primary AIG is positioned adjacent the flue gas outlet of the economizer and upstream of an economizer bypass outlet. A secondary AIG is located within an economizer flue gas bypass. The separate AIGs permit a reduction in the overall length of the flue between the economizer and SCR, thereby reducing the NOx removal system footprint, cost and quantity of materials.

Description

FIELD SAND BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to the field of utility boilers and furnaces, and in particular to a new and useful Selective Catalytic Reduction (SCR) installation in which separate ammonia injection grids (AIGs) are located in each of the economizer outlet flue and economizer flue gas bypass of a boiler for control of NOx emissions.

[0002] NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other species generated during combustion of fossil fuel in boilers and furnaces. Conversion of molecular and fuel nitrogen into NOx is promoted by high temperatures and high volumetric heat release rates found in boilers.

[0003] SCR systems are used to clean impurities from the exhaust gases of boilers and furnaces, and in particular, to reduce NOx emissions. Ammonia is injected into the boiler exhaust gas stream. Ammonia is typically introduced using sparger tubes to spray ammonia into the exhaust gas stream. The sparger tubes form an ammonia injection grid (AIG). A chemical reaction occurs with the exhaust gases in the presence of an SCR catalyst, which removes a large portion of NOx from the exhaust gases and converts it to water and elemental nitrogen.

[0004] Additional details of SCR systems for NOx removal are provided in Chapter 34 of Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright® 1992, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.

[0005] It is desirable to maintain the temperature of flue gases leaving the last heat trap of a boiler entering an SCR at or above the NOx reduction catalyst's minimum operating temperature. The minimum temperature must be maintained even as the boiler load is reduced.

[0006] Presently, a preferred method for maintaining the flue gas temperature above the SCR operating minimum is to install an economizer bypass duct system. The economizer bypass duct transports flue gases from upstream of the last heat trap, or economizer, to just prior to entry into the NOx reduction device—the SCR. The diverted flue gas flowing the through the bypass has a higher temperature since it did not pass over the economizer heat exchange surfaces. Thus, the diverted flue gas raises the temperature of the flue gases entering the SCR above the minimum operating temperature.

[0007] The flow of flue gases through the bypass is controlled using dampers in the economizer outlet and in the bypass. By opening or closing the dampers, the percentage flow through the bypass is easily adjusted.

[0008] However, it has been found that in certain new boiler sites, or where existing boilers are being retrofitted with SCR systems, that there is insufficient space to install an economizer bypass duct in a cost-effective manner. In such cases, either there would be no control over the temperature of the flue gases entering the SCR, or the installation of a bypass would be so costly as to be prohibitive of making the modification.

[0009] As the catalysts used in SCR systems are carefully engineered and expensive, it is beneficial to be able to control the stoichiometry and temperature of the exhaust gas/ammonia/catalyst reaction. Insufficient temperature of the flue gases in the SCR where the ammonia is injected will result in salt formation of fouling ammonia salts in the flues.

[0010] An ammonia injection grid (AIG) design presently in use has multiple levels of sparger tubes having spray openings. The AIG is located within a single designated space, positioned downstream of the economizer and bypass outlets. The spray openings are oriented parallel with the exhaust gas flow, toward the catalyst. The position of the AIG is selected to avoid spraying ammonia into portions of the flue that will have too low a temperature during bypass mode operation, so as to prevent formation of ammonia salts in the flue. Groupings of the tubes are supplied by independently controlled supply headers to create zones within the grid.

[0011] While the grid design permits greater control over the dispersion of ammonia into the exhaust gas stream, it also results in blockage of a large area of the downstream exhaust gas flow path. A large number of tubes and pipes are needed to provide ammonia to each zone of the grid, which creates the blockage. The blockage in turn results in a pressure drop between the furnace side and exhaust side. The pressure drop is not desirable as it adversely affects the overall efficiency of the boiler or furnace system.

[0012] And, to ensure proper flue gas mixing and velocity, the flue length must be a minimum length both up and downstream of the AIG. Thus, locating the AIG downstream of the bypass outlet results in a necessary extension of the flue length, further resulting in increased footprint, cost and materials.

[0013] Current consensus with respect to the operation of SCR's is to provide the highest level of flow with temperature and chemical component uniformity. Techniques such as zonal AIG injection, static mixing and judicious use of flow correcting devices have all been used in known SCR designs to attempt to provide the most uniform flow and blend composition as practical. However, these techniques still do not provide a perfectly uniform flow or blend, so that improvement is possible.

SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a NOx removal system for furnaces and boilers which is more compact than current systems, while preventing the formation of fouling ammonia salts.

[0015] It is a further objection of the invention to provide an SCR installation for furnaces and boilers which is less costly than current systems while providing the same or better NOx removal from flue gases.

[0016] Accordingly, an SCR installation of the invention for a furnace or boiler having an economizer and economizer bypass has two ammonia injection grids. A primary AIG is located between the economizer outlet and the economizer bypass outlet, while a secondary AIG is positioned within the bypass duct. Placing the AIGs in these locations permits the use of the fluework for flue gas velocity distribution and mixing of flue gases and ammonia in a shorter overall flue length. The bypass and economizer are each provided with dampers for controlling the flow of flue gases through the respective ducts. The ammonia injection grids can be controlled to provide ammonia in proportion to the flow of gases through the economizer and bypass as well.

[0017] The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The sole FIGURE is a side elevation schematic view of an economizer and SCR reactor of a furnace or boiler in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Referring now to the drawing, a NOx removal system 10 of the invention is shown used in an economizer section 50 of a furnace or boiler. The economizer section 50 includes economizer 52 and economizer bypass duct 54. An SCR reactor 60 is located at the outlet end of the flue gas duct 58. Two separate ammonia injection grids (AIG) 20, 30 are provided in the economizer section 50.

[0020] Primary AIG 20 is positioned in economizer flue 56 between economizer flue gas outlet 55 and bypass duct outlet 57. Secondary AIG 30 is located near the upper end of economizer bypass duct 54. Each of the primary and secondary AIGs 20, 30 is positioned downstream of control dampers 40, 42. Bypass control dampers 40 are located adjacent the economizer bypass duct inlet 53. Economizer dampers 42 are arranged in the economizer flue 56 just upstream of the primary AIG 20. The primary and secondary AIGs 20, 30 are connected to ammonia supply and control 80, which is operated to control the delivery of ammonia through each AIG 20, 30.

[0021] As in known NOx removal systems, the dampers 40, 42 are adjusted between opened and closed to control the flow of flue gases through the economizer bypass duct 54 versus the economizer 52. That is, the percentage of the flow of flue gases through the economizer bypass duct 54 can be varied between 0-100% inclusive.

[0022] During non-bypass mode, when the flue gases all pass through the economizer 52, ammonia is only provided to the primary AIG 20. As flue gases are directed through the economizer bypass duct 54, some of the ammonia is directed to the secondary AIG 30, so that the flow to primary AIG 20 is reduced. Alternatively, when flue gases are directed primarily through the economizer bypass duct 54, the AIGs 20, 30 can be operated to only provide ammonia to the economizer bypass duct 54. The formation of fouling ammonia salts due to low temperature in the economizer 52 is prevented by directing the appropriate portion of the ammonia from the supply and control 80 to the secondary AIG 30.

[0023] Flue gas velocity distribution and ammonia/flue gas mixing during the bypass mode of operation is not as critical compared to non-bypass mode due to increased flue gas retention time in the SCR 60 at low heat input/flue gas flow. Therefore, the number of ammonia injection points in the secondary AIG 30 and the length of the economizer bypass duct 54 upstream and downstream of the secondary AIG 30 are not as critical compared with the same parameters in the primary AIG 20. Thus, the secondary AIG 30 may be constituted by a single set of injection pipes, a set of injection pipes with multiple openings for ammonia injection or even a single pipe with one or more injection holes. The pipes may be arranged in an array of parallel pipes with the injection openings oriented directly upstream, or at an angle to the travel of flue gases.

[0024] During bypass mode operation when flue gas is passed through both the economizer 52 and economizer bypass duct 54, injection of a portion of the ammonia at each of the primary AIG 20 and secondary AIG 30 will result in a better ammonia to NOx distribution at the SCR reactor 60 entrance. The ammonia has sufficient time, even in a shortened flue gas duct 58 between the economizer 52 and SCR reactor 60, to mix and distribute within the flue gases. Further, introduction of ammonia to the flue gases in the economizer bypass duct 54 prevents introduction of large amounts of ammonia-less flue gases to the flue gas stream at the bypass duct outlet 57.

[0025] As a result, the length of the flue gas duct 58 can be reduced by up to about 50% of the length of conventional NOx removal systems. The duct length reduction translates to significant cost savings and smaller footprint furnaces and boilers. The invention permits these savings without sacrificing the efficiency or quality of NOx removal from flue gases.

[0026] While not shown in the drawing, static mixing devices positioned downstream of the secondary ammonia injection grid 30 can be used to improve the mixing and ammonia distribution efficiency within the economizer bypass duct 54.

[0027] While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles.

Claims

1. A NOx removal system for a furnace or boiler having an economizer with a flue gas bypass duct for bypassing flue gas around the economizer, the bypass duct extending between a bypass duct inlet located upstream of the economizer and a bypass duct outlet downstream of an economizer flue gas outlet, the bypass duct outlet being upstream of a selective catalytic reduction reactor, the system comprising:

a primary ammonia injection grid positioned between the economizer flue gas outlet and the bypass duct outlet; and

a secondary ammonia injection grid positioned within the flue gas bypass duct.

2. A system according to claim 1, further comprising ammonia control means for controlling the amount of an ammonia supply injected through each of the primary and secondary ammonia injection grids.

3. A system according to claim 2, further comprising first dampers adjacent the economizer outlet and second dampers adjacent the bypass duct inlet.

4. A system according to claim 3, wherein the primary ammonia injection grid is positioned downstream of the first dampers.

5. A system according to claim 3, wherein the secondary ammonia injection grid is positioned downstream of the second dampers.

6. A system according to claim 3, wherein the first and second dampers are adjustable to direct from 0-100% of the flue gases through the bypass duct.

7. A system according to claim 6, wherein the ammonia control means is adjustable to direct from 0-100% of the ammonia through the secondary ammonia injection grid.

8. A system according to claim 1, wherein the secondary ammonia injection grid comprises at least one injection pipe.

9. A system according to claim 1, wherein the secondary ammonia injection grid comprises a plurality of injection pipes each having a plurality of injection holes.

10. An ammonia injection system for introducing ammonia to a flue gas stream of a furnace or boiler having an economizer and an economizer flue gas bypass upstream of a selective catalytic reduction reactor, the system comprising:

a primary ammonia injection grid positioned between a flue gas outlet of the economizer and a bypass outlet;

a secondary ammonia injection grid located in the economizer flue gas bypass; and

control means for controlling a percentage of an ammonia supply delivered to each of the primary and secondary ammonia injection grids.

11. A system according to claim 10, wherein the control means is adjustable to deliver from 0-100% of the ammonia supply through the secondary ammonia injection grid.

12. A system according to claim 10, wherein the secondary ammonia injection grid comprises at least one injection pipe.

13. A system according to claim 10, wherein the secondary ammonia injection grid comprises a plurality of injection pipes each having a plurality of injection holes.