GB2271517A - Flue gas NOx reduction in a fluidized bed reactor - Google Patents

Abstract

In a bubbling bed fluidized bed incinerator, ammonia or urea is injected through nozzles 42 and mixed with the waste gases in an upper freeboard region 19 above the bed 24. This reduces the nitrogen oxide content of the waste gases. <IMAGE>

Description

FLUE GAS NOX REDUCTION IN A FLUIDIZED BED REACTOR FIELD OF THE INVENTION The invention is directed to the reduction of nitrogen oxide levels in the flue gas from fluidized bed combustion units, particularly fluidized bed combustors of the bubbling bed type.

BACKGROUND OF THE INVENTION Fluidized bed reactors are well-known means for generating heat and, in various forms, can carry out the processes of drying, roasting, calcining, incineration and heat treatment of solids in the chemical, metallurgical and other material processing fields. They are also used for the generation of hot gases, including steam, for use in driving electric power generation equipment, for process heat, for space heating, or for other purposes.

Fluidized bed reactors typically comprise a vessel having a substantially horizontal air distributor or constriction plate, which supports a bed of particulate solids in the reaction chamber and separates the reaction chamber from a windbox below the air distributor. Combustion air is introduced into the windbox and passes through the air distributor in sufficient volume to achieve a gas velocity that expands or fluidizes the solids bed, suspending the particulate solids of the bed in the flowing air stream and imparting to the individual particles a continuous random motion. A fluidized bed in appearance and properties resembles a boiling liquid.

Conducting a combustion reaction in a fluidized bed has important advantages which include attainment of a substantially uniform bed temperature, combustion at relatively low temperatures, say 1400 -1700 F, and a high heat transfer rate.

Nitrogen oxides are generated when fuel is burned, both from thermal fixation of nitrogen in the air and from conversion of nitrogen present in the fuel. The former reaction is favored at high temperatures (above 1800OF) while the latter occurs at all temperatures. There is much concern with minimizing NOB emissions in combustion systems due to their poisonous effect at low exposure levels and to their involvement in the formation of photochemical fog.

The problem of NOx reduction in flue gas has been addressed by injection of ammonia with and without catalysts. U.S. Patent No.

3,900,554 suggests non-catalytic removal of NOx from flue gases exiting a conventional furnace by injecting ammonia into a gas stream at a temperature in the range 1600 -2000 F. Reduction of NOx in flue gases using ammonia with various catalysts is suggested in U.S. Patent Nos. 3,887,683; 4,056,600; 4,010,238; 4,049,777and 4,070,440.

The injection of ammonia or an ammonia-producing precursor directly into the fluidized bed combustion region of a furnace is suggested in U.S. Patent No. 4,181,705. U.S. Patent No. 4,756,890 suggests injection of ammonia or an ammonia precursor into the stream within a high temperature cyclone separator at a location where there is a strong vortex region to obtain an efficient mixing of the NO, reducing agent and the combustion product flue gas. This arrangement is particularly suitable for those fluidized bed reactors of the circulating bed type where large volumes of gas and solids are returned to the combustion reactor through a cyclone. It is not suitable for fluidized bed reactors of the bubbling bed type serving as incinerators because the incinerators do not employ cyclone separators.

Gases from incinerators have very high moisture contents and can contain other gaseous emissions which make NOx reduction by ammonia/urea non catalytic system more difficult. Some endo- thermic fluid bed applications have similar problems. Reducing gases can exist and combustion can continue at the level of secondary air introduction into an incinerator. Selective catalytic reduction by NH3 or urea is adversely affected by reducing gases and combustion.

SUMMARY OF THE INVENTION It is necessary in effecting NOx reduction in the flue gases of an incinerator of the bubbling bed fluidized bed reactor type to obtain a good mixing of the injected ammonia and the combustion gases and to effect a retention time before cooling as determined by the process conditions.

In locating the optimum region or volume for injection of the ammonia, the upper freeboard region within the reactor presents a substantial volume in which combustion has essentially been completed. At the same time, this is a region or volume in which mixing currents are present. It is into this region, then, that the ammonia is injected in a manner calculated to enhance the mixing currents already present. In this region reduction of NOx is initiated, and these reactions continue as the flue gas moves out of this region into and through the ductwork toward the heat exchanger or other unit which alters gas temperature.

The ductwork alone will not provide sufficient retention time for completion of the NOx reduction reaction and it is for this reason, too, that injection of ammonia is provided through the roof of the reactor into the upper freeboard region of the reaction chamber, thereby assuring the necessary retention time to approach equilibria for the NOx reduction reactions. With the combined volume of the upper freeboard and the exhaust ductwork, a retention time of 0.75 to 3 seconds is achieved and that is sufficient at the operating temperatures to approach equilibria of the desired reactions.

DESCRIPTION OF THE DRAWINGS Figure 1 is an elevational view, partially in section, of a fluidized bed reactor incorporating the NOx reduction structure of the invention, and Figure 2 is a plan view of the roof of the fluidized bed reactor of Figure 1 showing the NOx reduction apparatus in place thereon.

DETAILED DESCRIPTION OF THE INVENTION In Figure 1 a fluidized bed reactor 10 is shown having a vessel wall 11 which comprises a steel shell 13 and a refractory ceramic lining 14. The reactor 10 is supported by grillage beams 36 which rest on a concrete pad or foundation 38. The reaction chamber 16 in the main portion of the fluid bed reactor 10 is separated from the windbox 17 in the lower portion of the fluid bed reactor by a perforated steel constriction plate or the perforated ceramic dome 21 as shown. It will be understood that the steel constriction plate is suitable for cold windbox operation while the perforated ceramic dome is used when the incoming air is heated; that is, "hot windbox" operation. The upper freeboard region of the reaction chamber 16 has special importance in this invention and is designated by the reference character 19. The ceramic dome 21 is provided with a number of tuyeres 22 for providing communication between the windbox 17 and the reaction chamber 16. A conduit 27 is provided for introducing feed stock into the reaction chamber 16. A fluidized bed 24 is illustrated in the reaction chamber 16 and a conduit 26 is provided for draining the bed material or removal of solid particulate products if required. A conduit 29 is provided for supplying air to the windbox 17. A duct 34 is provided for the off-gases emanating from the reaction chamber 16. A conduit 28 may be provided for introducing secondary air into the reaction chamber 16.A ring bustle 41 is installed on the roof 31 of the reactor 10 and there are a plurality of nozzles 42 extending from the ring bustle to penetrate the roof shell 11' and the roof insulation 13' and to extend a short distance into the region 19 of the reaction chamber 16. In determining the reaction volume of the system, the region 19 has a volume ranging from one-fifth to one-quarter the volume of the reaction chamber 16 and to this must be added the volume of the duct leading from region 19 to the heat exchanger or alternate unit.

In Figure 2, which is a plan view of the fluidized bed reactor 10, particularly showing the roof 31 with the ring bustle 41 and its associated nozzles 42, the spray pattern of the nozzles is indicated by the circular overlapping dotted line patterns 50. It will be seen that the nozzles 42, equidistantly spaced along ring bustle 41, provide rather complete spray coverage of the region under the roof 31 and so assures satisfactory mixture of the NH3 or urea sprayed with the flue gases in the region to promote the desired reactions.

The desired reactions which take place in region 19 and the duct 34 leading therefrom are: 1500 F-1700 F 4NH3 + 4NO + O2 ----- > 4N2 + 6H2O and/or 1500 F-1700 F 4N113 + 2N02 + 02 ------ > 3N2 + 6H2O In operation, the fluidized bed reactor 10 has within the reaction chamber 16 a body of particulate material 24 which is supported by the perforated dome or constriction element 21. Air supplied by a blower (not shown) through conduit 29 to the windbox 17 moves upward through the tuyeres 22 of the constriction element 21 into the bed material 24 and expands that bed to a substantial height within the combustion chamber 16. Above the expanded bed 24 is the freeboard region of the combustion chamber 16 into which combustible gas and fine particles are ejected from the bed 24.

Secondary air is often introduced into the freeboard region just above the expanded bed 24. Combustion takes place primarily in the expanded bed 24 at temperatures of 1350OF to 1600OF and at the level of secondary air introduction and thereabove in reaction chamber 16 where gases and suspended fine solid particles are burned at temperatures of 1350OF to 17000F. The upper freeboard region 19 of reaction chamber 16 is isothermal with only minimum combustion occurring and temperature is in the range of 1500OF to 17000F. It is into this region that the ammonia/urea is injected. This upper freeboard region, together with the ductwork, provides the retention time required for the desired reactions. The ammonia/urea is furnished through supply hose 44 to the ring bustle 41.The nozzles 42, of which there may be from 4 to 8 or more, all connected to the ring bustle 41, direct the ammonia/urea into the region 19 in a manner to assure thorough mixing with the combustion products in the region. The amount of injected compounds used is somewhat in excess of that required to complete the desired reactions, but with a minimum of excess ammonia/urea remaining to escape into the atmosphere.

EXAMPLE A fluidized bed reactor about 40 feet high and having a maximum diameter at the reaction chamber of about 27 feet is provided with a ceramic combustion dome separating the reaction chamber from the windbox below.

A bed comprising inert material (i.e. silica sand) is charged into the reaction chamber. After preheating, it is fluidized in the combustion zone of the reaction chamber by introducing a flow of air from the windbox sufficient to establish an air flow of from 1.5 ftlsec to 4 ft/sec. This air flow expands the charge in the reaction chamber and forms a fluidized bed of the bubbling bed type. Waste material containing organics is fed into the reaction chamber and oxidized.

Combustion of the organic material in the charge takes place at a temperature of from 1350OF to 17000F.

Above the bubbling bed, secondary air is admitted into the reaction chamber to complete combustion of the gases rising from the bubbling bed and the fine solids suspended in the gas. The secondary combustion occurring at the level of introduction of secondary air is minor and usually does not increase the gas temperature.

The gas leaving the secondary combustion zone is cooling as it rises and it contains a substantial quantity of NOx generated in the combustion zones below. Ammonia is injected into this gas through the roof nozzles slightly in excess of. the stoichiometric amount required to react with the NOx.The retention time of the mixed gases in the freeboard region and exhaust duckwork is approximately 2 seconds and the temperature is about 16500F. In this case 60-80% reduction of NO, concentration is effected.

There has thus been disclosed a simple and effective method for reducing the nitrogen oxide levels produced in the combustion of fuels in bubbling bed fluidized bed reactors.

Claims (8)

1. A process for reducing the nitrogen oxide content of the waste gases of a bubbling bed fluidized bed incinerator comprising the step of mixing ammonia or urea with the waste gases in the upper freeboard region and exhaust duct of the incinerator by injection of ammonia or urea through the roof of said fluidized bed incinerator.

2. The process of claim 1 wherein the gases in said upper freeboard region and exhaust duct are at a temperature in the range of 1500OF to 1700OF.

3. The process of claim 2 wherein the combined volume of said upper freeboard region and exhaust duct is selected to provide sufficient time for said waste gases and the injected ammonia to intimately mix and to attain substantially reaction equilibria.

4. The process of claim 3 wherein said selected combined volume provides a retention time of from 0.75 to 3 seconds.

5. The process of claim 4 wherein the temperature of the gases in said upper freeboard region and exhaust duct is about 1650OF.

6. The process of claim 5 wherein said combined volume of said upper freeboard region and exhaust duct is selected to provide a retention time of approximately 2 seconds.

7. A process for reducing the nitrogen oxide content of waste gases of a bubbling bed fluidized bed incinerator, substantially as described with reference to the drawings.

8. A process for reducing the nitrogen oxide content of the waste gases of a bubbling bed fluidized bed incinerator, substantially as set forth in the foregoing Example.