PROCESS FOR REMOVING NITROUS OXIDES FROM A GAS

Abstract: A gas containing nitrous oxides is introduced by pipe (28) to the air chamber (16) of a fluidised bed (20) of particulate material. Carbonaceous fuel is added to the bed (20) by pipe (24) and combustion of the fuel takes place in the bed (20). Additional air is introduced by pipe (26) above the bed (20). The carbonaceous material scavenges the oxygen in the nitrous oxygen.

Description: This invention relates to a process for removing nitrous oxides from a gas, and more particularly, to such a method in which the gas is passed through a fluidized bed reactor operating in a manner to remove the nitrous oxides from the gas. Combustion systems utilizing fluidized beds as the primary source of heat generation are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulphur generated by the combustion of the coal, to fluidize the bed and promote the combustion of the fuel at relatively low temperatures. An added advantage is that the fluidized bed combustion process has the ability to produce lower emissions of nitrous oxides (NOx) than conventional combustion systems. U.S. Patent 4,308,810 discloses an apparatus and method in which the reduction of NOx emissions from a fluidized combustion system is reduced even further from the reduction achieved by the standard fluidized bed. The process comprises the steps of: (a) providing a non-catalytic fluidized bed of particulate material including a nitrogen-containing fuel without catalyst and an adsorbent for the capture of sulfur oxides (SOx), wherein the temperature in said non-catalytic fluidized bed is within the range of 816° to 899°C; (b) introducing air into said fuel to fluidize said bed and promote the combustion of said fuel to form a primary combustion stage; (c) introducing additional fuel to said bed to replenish the combusted fuel; (d) controlling the fuel feed rate or controlling the air feed rate to achieve combustion in said primary combustion stage under continuous substoichiometric conditions, and (e) providing a second combustion zone which is not a fluidized bed by introducing air into the combustion off-gas above said fluidized bed to produce combustion under stoichiometric conditions, wherein the temperature in said second combustion zone is within the range of 704° to 899°C; whereby said combustion process has a reduced NOx emissions level while maintaining efficient capture of sulfur oxides (SOx). In this technique, the amount of air added to the bed is less than that required for complete combustion resulting in the production of extremely low NOx. To reduce carbon monoxide and methane to acceptable levels, overfire air is added above the upper surface of the fluidized bed which results in a large reduction of the carbon monoxide with little or no increase in the already low NOx emissions. The bed material includes an adsorbent for the sulphur generated by the combustion of the fuel, and the above technique does not compromise the sulphur capture performance. U.S. Patent No. 4,218,427 discloses a non-catalytic dry-contact reduction method for reducing the NOx content of stack gases produced by stationary combustion sources, e.g., coal-fired boilers which comprises: (a) introducing an effective amount of coal having a volatile content of at least about 15 weight percent and the gaseous mixture into a fluidized bed reduction zone; (b) maintaining the fluidized bed reduction zone at a temperature within the range from about 400° to 700°C, thereby producing NOx reducing agents comprising fluid phase coal devolatilisation/gasification products, substantially reducing the nitrogen oxides in the gaseous mixture, and forming a treated gaseous mixture containing oxidizable products and a solids phase comprising coal char and ash; (c) separating the solids phase from the treated gaseous mixture containing oxidizable products; (d) adding sufficient oxygen-containing gas to the treated gaseous mixture in a second zone to convert substantially all of the oxidizable products to carbon dioxide and water without reformation of nitrogen oxides, and (e) venting the resulting gas. However, these prior art techniques have not been applied to the reduction of nitrous oxides from external gases such as the gases received from an oil refinery or the like, which are laden with NOx and with sulphur. According to the present invention there is provided a process for removing nitrous oxides from a gas from an external source, comprising: providing a bed of particulate material including a particulate carbonaceous fuel material; passing a controlled amount of air through the bed to fluidize the particulate material and to promote combustion of the particulate carbonaceous fuel material in which the amount of fluidizing air passed through the bed is less than that required for complete combustion of the particulate carbonaceous fuel; adding additional particulate carbonaceous fuel material to the bed; adding a controlled amount of additional air above the area of introduction of the fluidizing air in which the amount of additional air added is sufficient to complete the combustion of the particulate carbonaceous fuel material; and passing the externally produced gas through the bed so that the particulate carbonaceous fuel material scavenges the oxygen in the nitrous oxides. The present invention provides a process for the removal of nitrous oxides from a gas from an external source in which the gas to be treated is passed, with a controlled amount of fluidizing air, through a bed of carbonaceous material to fluidize the bed while a controlled amount of additional air is added above the area of introduction of the air passing through the bed, to remove the nitrous oxides from the gas. The amount of air passed through the bed is less than that required for complete combustion of the carbonaceous material and the amount of additional air added is sufficient to complete the combustion. The carbonaceous material scavenges the oxygen in the nitrous oxides. The invention will now be described with reference to the accompanying drawing which is a schematic view depicting a fluidized bed reactor which is utilized in the process of the present invention. Referring specifically to the drawing, the reference numeral 10 refers in general to a fluidized bed reactor which includes a vessel 12 having a perforated plate, or grate 14, extending across the lower portion thereof to define an air inlet chamber 16 and a combustion chamber 18. A bed of particulate material, shown in general by the reference numeral 20, is supported on the plate 14 and includes a particulate carbonaceous fuel material, such as coal and a particulate adsorbent, such as limestone, for adsorbing the sulphur generated by the combustion of the coal. A primary air inlet pipe 22 extends through the rear wall of the vessel 12 in communication with the air chamber 16, and a particulate carbonaceous fuel inlet pipe 24 extends through the front wall of the vessel 12 in communication with the chamber 18 at an area just above the upper level of the bed 20. It is understood that the pipes 22 and 24 are connected to sources (not shown) of pressurized air and particulate fuel, respectively, for introducing the air and fuel into the air chamber 16 and the combustion chamber 18, respectively. An air flow control device 25, which may be in the form of a damper, or valve, is disposed in the air and fuel inlet pipes 22 and 24, respectively, for controlling the flow of air and fuel into the vessel 12 for reasons to be described. The particulate adsorbent material may be introduced onto the bed from an inlet pipe (not shown) extending into the chamber 18 in the same manner as the fuel inlet pipe 24. A secondary air inlet pipe 26 extends through the front wall of the vessel 12 above the location of the fuel inlet pipe 24, as viewed in the drawing, and in communication with the chamber 18. The pipe 26 is connected to a source of pressurized air (not shown) and functions to introduce this air into the chamber 28 above the bed 20 and above the area of introduction of the fuel from the fuel inlet pipe 24, for reasons to be explained. An inlet pipe 28 is provided through the front wall of the vessel 12 in communication with the air chamber 16 and is connected to a source (not shown) of process gas from an external source, such as an oil refinery, or the like, and functions to introduce this gas into the chamber 16 for treatment, as will be described. A solids discharge pipe 30 extends from the bed 20, through the front wall of the vessel 12, and to an area externally of the vessel for discharging spent and unspent particulate material from the bed, for reasons that will be described. An outlet pipe 32 extends through the rear wall of the vessel 12 in communication with the upper portion of the chamber 18 for receiving a mixture of flue gases and the gases to be treated, including entrained solid particulate material, from the bed 20 and for passing same to a cyclone separator 34. The separator 34 functions in a conventional manner to separate the gases from the entrained solid particulate material by centrifugal forces, with the former being discharged, via an outlet pipe 36 to external equipment (not shown) such as heat recovery unit, and the latter falling into a hopper 37 disposed on the lower portion of the separator. A pipe 38 is connected to an outlet formed in the lower portion of the hopper 37 and extends through the rear wall of the vessel 12 and into the bed 20. The pipe 38 thus functions to reinfect the particulate material from the cyclone separator 34 back into the bed 20, for reason that will be described. It is understood that a plurality of water tubes, one of which is shown by the reference numeral 40 can be disposed in the combustion chamber 18 above the bed 20. The respective end portions 40a and 40b of the tubes 40 extend through the rear wall of the vessel 12 and are connected to a water circuit (not shown) for passing water through the tubes to control the temperature inside the vessel 12. In operation, pressurized air from an external source along with the gases to be treated from another process or source are passed, via the inlets 22 and 28, respectively, into the chamber 16, from which they pass through the perforated plate 14 and into the bed 20 of particulate carbonaceous fuel material and adsorbent. The velocity of the air is sufficient to fluidize the bed 20 and promote the combustion of the particulate fuel and adsorbent material. The gases not combusting in the bed combine with the gaseous products of combustion and with the air introduced via the conduit 26 to form a flue gas that passes through the length of the vessel 12 before exiting from the outlet pipe 32. During this passage, the flue gas entrains the relatively fine particulate material in the bed and the resulting mixture is passed into the separator 34. The volume of air introduced into the chamber 16 is carefully controlled so that is insufficient to completely combust the fuel in the bed 20 but sufficient to generate a combustible off-gas which combines with the gases from the inlet 28. Additional air is introduced, via the pipe 26 into the vessel 12 above the bed 20 in sufficient quantities to complete the combustion. The combustibles in said gas are incinerated by the heat from the bed 20 and the sulphur in the gases are captured by the adsorbent particulate material in the bed 20. Also, the nitrous oxides are removed from the gases due to the presence of the carbonaceous material in the bed and the addition of the additional air, via the pipe 26, above the bed. The gases in the separator 34 are separated from their entrained particulate material and the latter is reinjected, via the conduit 38, back into the bed 20. The relatively clean gases from the separator 34 pass, via the conduit 36, to external equipment for further treatment and/or use. It is understood that the fluidized bed boiler 10 can be of a "bubbling" type or a "circulating" type. In the bubbling type, the air passing through the plate 14 causes the material in the bed 20 to expand and take on a suspended, or fluidized, state. The gas velocity is typically two to three times that needed to develop a pressure drop which will support the bed weight (e.g., minimum fluidization velocity), causing the formation of bubbles that rise up through the bed and give it the appearance of a boiling liquid. The bed 20 exhibits a well-defined upper surface as shown in Figure 1, and the density is typically 30% volume of solids. In a circulating bed the mean gas velocity, as a fraction of the minimum fluidizing velocity, is increased above that for the bubbling bed, so that the bed surface becomes more diffused and the solids entrainment for the bed is increased requiring a relatively high solids recycle. According to this process the density of the circulating bed is relatively low, such as between 5 and 20% volume of solids due to the small particle size and the high solids throughout. The velocity range of the circulating bed is between the solids terminal, or free fall, velocity and a velocity beyond which the bed would be converted into a pneumatic transport line. Also the high solids circulation required by the circulating bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the vessel, and therefore further decreasing the nitrogen oxides formation. Also, the high solids loading improves the efficiency of the separator used to separate the gas from the solids for solids recycle. The resulting increase in sulphur adsorbance and fuel residence times reduces the adsorbent and fuel consumption.

Claims: 1. A process for removing nitrous oxides from a gas from an external source, comprising: providing a bed of particulate material including a particulate carbonaceous fuel material; passing a controlled amount of air through the bed to fluidize the particulate material and to promote combustion of the particulate carbonaceous fuel material in which the amount of fluidizing air passed through the bed is less than that required for complete combustion of the particulate carbonaceous fuel; adding additional particulate carbonaceous fuel material to the bed; adding a controlled amount of additional air above the area of introduction of the fluidizing air in which the amount of additional air added is sufficient to complete the combustion of the particulate carbonaceous fuel material; and passing the externally produced gas through the bed so that the particulate carbonaceous fuel material scavenges the oxygen in the nitrous oxides. 2. A process as claimed in Claim 1 in which the fluidizing air is passed from the lower portion of the bed upwardly through the bed. 3. A process as claimed in Claim 1 or Claim 2 in which the additional air is added to an area above the bed. 4. A process as claimed in any one of Claims 1 to 3 further comprising the step of discharging a portion of the particulate material from the bed in quantities to eliminate the build up of particulate material in the bed. 5. A process as claimed in any preceding Claim further comprising the steps of discharging flue gases containing entrained particulate material from the bed, separating the entrained particulate material from the flue gases and reinjecting the separated particulate material back into the bed. 6. A process as claimed in any preceding Claim in which the particulate material includes an adsorbent for the sulphur in said gas and the sulphur generated by the combustion of the particulate carbonaceous fuel material. 7. A process as claimed in any preceding Claim in which the combustibles in said gas are incinerated by the heat from the fluidized bed. 8. A process as claimed in any preceding Claim in which the quantities of particulate material and air passed through and added to the bed are regulated so that the bed operates as a circulating bed. 9. A process as claimed in any preceding Claim further comprising the step of passing a cooling fluid in a heat exchange relation to the bed to control the temperature of the bed.

Inventors: GARCIA-MALLOL, JUAN ANTONIO
Applicants: FOSTER WHEELER ENERGY CORPORATION
Application year: 1988
Main classification: F23G7
IPC codes: F23C99, F23G5, B01D53, F23G7, F23C10