Method for reducing gaseous emission of halogen compounds in a fluidized bed reactor

Abstract: A method for reducing gaseous emission of halogen compounds in a fluidized bed reactor (12) in which the fine particles entrained in flue gases are used to form a temporary layer of particles on the baghouse filter (38) to absorb halogen gases.

Description: This invention relates to a method to reduce the emission of halogen compounds in gaseous products resulting from the combustion of halogen containing fuels in fluidized bed reactors. Substantial efforts have been made to reduce emission of halogen compounds in gaseous products resulting from the combustion of halogen containing fuels, such as certain coals, industrial and municipal wastes, to comply with environmental regulations. In general, there are three prior art methods to reduce halogen emissions in flue gases: wet scrubbing, spray drying and dry-solids contact. In both the wet scrubbing and spray drying processes, a reaction vessel provides a region in which an interaction between a mixture of water and an alkaline sorbent-material, such as lime, and the flue gases can take place. The mixture of water and sorbent material forms an alkaline solution which is highly conducive to the absorption of halogen compounds, such as hydrogen halide. Unfortunately, both the wet scrubbing and spray drying processes suffer from major problems with scaling and corrosion resulting from the presence of an aqueous solution phase. The dry-solids contact process, while avoiding the problems associated with the aqueous solution phase, suffers from a relatively low halogen removal efficiency due to relatively slow solid-gas reaction kinetics. The dry-solids contact process typically involves the injection of a dry, alkaline sorbent-material, such as limestone, into the combustion vessel of a fluidized bed reactor. Unfortunately, only the most reactive halogen, fluorine, is retained in the sorbent material while only a small portion of the most abundant halogen, chlorine is retained due to the elevated temperatures disposed within the combustion vessel. In other known dry-solids contact processes, a dry, alkaline, sorbent material, such as lime, is introduced into the flue gases upstream from a baghouse and the sorbent material is distributed over the input side of a baghouse filter. The filter thus provides a region in which interaction between the sorbent material and the flue gases can take place. This latter process of dry scrubbing is generally considered too expensive for use in many industrial fluidized bed reactors because it incurs a significant cost disadvantage by using lime instead of limestone since the cost of lime is as much as ten times the cost of the limestone. Accordingly, there remains a need in the art for a dry-solids contact process to remove halogen compounds from flue gases without incurring the additional cost of using lime. WO-A-88/10291 discloses a fast fluidized bed reactor in which a reactant, such as an oxide or hydroxide which is converted to an oxide during combustion, is added to the fuel to react with the low melting point alkaline compositions in the fuel to produce alkali metal compounds having a high melting point. This allows the operation of the reactor at a high temperature whilst preventing agglomeration of the bed material, sintering of the fuel and enhanced formation of deposits on operable components of the reactor. According to the present invention, a method of combustion in a fluidized bed for reducing gaseous emission of halogen compounds comprising the steps of: forming a bed of solids particles, including a fuel material and a sorbent material, in a fluidized bed reactor; introducing air to the bed to fluidize the particles to promote the combustion of the fuel particles which generate sulfur oxides, the flue gases from the combustion entraining a portion of the particles; and, separating a portion of the entrained particles from the flue gases, is characterized by the further steps of: passing the flue gases with the remaining portion of the entrained particles to a baghouse; establishing temporary layer of the remaining portion of entrained particles in the baghouse; and, controlling the temperature of the gases and remaining portion of entrained particles entering the baghouse within a range to permit absorption of halogen compounds in the flue gases by the remaining portion of entrained particles in said temporary layer, said remaining portion of the entrained particles including the fuel material and the sorbent material and wherein the sorbent material absorbs the halogen compounds. With the present invention, it is possible to reduce the emission of halogen compounds in the gaseous products resulting from the combustion of halogen containing fuels, and carrying this out in an economical manner. The present invention preferably provides a method which provides the required residency time and temperature for the gaseous products to effect proper scrubbing of the halogen compounds. The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred but nonetheless illustrative embodiment in accordance with the present invention when taken in conjunction with the drawing which illustrates a schematic view of the system of the present invention. The method of the present invention will be described in connection with a fluidized bed reactor forming a portion of a natural water circulation steam generator, shown in general by the reference numeral 10 in the drawing. The steam generator 10 includes a fluidized bed reactor 12 having four walls. It is understood that each wall is formed by a plurality of vertically-disposed tubes interconnected by vertically elongated bars or fins to form a substantially rectangular, contiguous and air-tight structure. Since this type of structure is conventional, it is not shown in the drawings nor will it be described in any further detail. A plenum chamber 14 is disposed at the lower portion of the reactor 12 into which pressurized air from a suitable source (not shown) is introduced by conventional means, such as a forced-draft blower, or the like. A perforated air distribution plate 16 is suitably supported at the lower end of the combustion chamber of the reactor 12, and above the plenum chamber 14. The air introduced through the plenum chamber 14 passes in an upwardly direction through the air distribution plate 16 and may be preheated by air preheaters (not shown) and appropriately regulated by air control dampers as needed. The air distribution plate 16 is adapted to support a bed 18 of particulate material consisting in general, of crushed coal, as well as limestone, and/or dolomite, for absorbing a portion of the sulfur oxides (SOx) formed during the combustion of the coal. A fuel distributor 20 extends through the front wall of the reactor 12 for introducing particulate fuel into the bed 18, it being understood that other distributors can be associated with the walls of the reactor 12 for distributing particulate sorbent material and/or additional particulate fuel material into the bed 18, as needed. A multiplicity of air ports 21 are provided through a side wall of the reactor 12 at a predetermined elevation from the bed 18 to introduce secondary air into the reactor 12 for reasons to be described. It is understood that additional air ports at one or more elevation can be provided through the sidewalls of the reactor 12 as needed. An opening 12a formed in the upper portion of the rear wall of the reactor 12 by bending back some of the tubes (not shown) forming the latter wall and connecting the reactor 12 with a cyclone separator 22 of conventional construction. Gases thus enter the separator 22 from the reactor 12, and swirl around in an annular chamber 22a defined in the separator to separate a portion of the entrained relatively-fine particles therefrom by centrifugal forces, before the gases leave the separator 22. The separator 22 includes a hopper portion 22b into which the separated fine particles fall before being passed back into the reactor 12 by a recycle conduit 24. A duct 26 is disposed above, and connected to, the cyclone separator 22 and operates to pass the separated flue gases which contain entrained relatively-fine particulate material that was not separated out in the separator 22 to a heat recovery enclosure 28 that is formed adjacent the duct 26. An opening 28a is formed in the upper wall portion of the heat recovery enclosure 28 to receive the relatively-clean hot flue gases from the duct 26. The heat recovery enclosure 28 is of conventional construction and operates to transfer heat from the hot flue gases to a cooling medium such as water which is in fluid flow relationship with flow conduits, and the like, of the steam generator 10. A gas flow duct 30 is formed adjacent the heat recovery enclosure 28 for receiving the relatively-clean flue gases from the enclosure 28 and divides into two branch ducts 30a and 30b. An upper economizer 32 is disposed in branch duct 30a and operates to transfer heat from the flue gases to water flowing through conventional water flow circuitry of the economizer. A damper 34 is disposed in branch duct 30b and operates to control the flow of flue gases through branch duct 30a for purposes that will be described later. A gas flow duct 36 is provided below the branch ducts 30a and 30b for connecting a baghouse 38 in gas flow communication with the ducts 30a and 30b. A halogen monitoring device 40 and a temperature monitoring device 42 are connected to the duct 36 and monitor the halogen content and temperature, respectively, of the flue gases entering the baghouse 38. The temperature monitoring device 42 is electrically connected to the damper 34 and sends the damper 34 control signals to regulate the flow of the flue gases through the duct 30b and, consequently, the temperature of the flue gases to the baghouse 38. The baghouse 38 is of a conventional design and contains, for example, fabric filters in the path of the gases as they pass through the baghouse. An outlet duct 44 extends from the baghouse 38 for discharging gases from the baghouse to an external stack, or the like. A second halogen monitoring device 46 is connected to the duct 44 for monitoring the halogen content of the flue gases exiting the baghouse 38. The halogen monitoring devices 40 and 46 are electrically connected to a control device 48 which operates to produce control signals on a control line shown in part by the reference numeral 50. The control line 50 is used to control the baghouse cycle rate and/or the limestone feed rate as necessary to control the emission of halogen compounds. In operation of the steam generator 10, a quantity of start-up coal with limestone for absorbing a portion of the sulfur oxides generated as a result of the combustion of the coal, is introduced to, and spread over the upper surface of, the particulate material in the bed 18. Air is introduced into the plenum chamber 14 and passes through the coal and limestone within the bed 18 and the start-up coal and limestone is ignited by burners (not shown) positioned within the bed. As the combustion of the coal progresses, additional air is introduced into the plenum chamber 14 at a relatively high pressure and velocity. Alternately, the bed 18 can be warmed up by a burner located in the plenum chamber 14. The primary air introduced through the plenum chamber 14 comprises a fraction of the total air required for complete combustion of the coal so that the combustion in the lower section of the reactor 12 is incomplete. The latter section thus operates under reducing conditions and the remaining air required for complete combustion of the coal is supplied by the air ports 21. When operating at maximum capacity, the range of air supplied through the plenum 14 can be from 40% to 90% of that required for complete combustion, with this amount varying according to the desired bed temperature, while the remaining air (60% to 10%) is supplied through the ports 21 to complete the combustion. The high-pressure, high-velocity, combustion-supporting air introduced by the air distribution plate 16 from the plenum chamber 14 causes the particles of the relative-fine particulate material, including particles of coal ash and limestone, to become entrained within, and to thus be pneumatically transported by, hot flue gases consisting of air and the gaseous products of combustion. This mixture of entrained particles and flue gases rises upwardly within the reactor 12 to form a gas column containing the entrained particles. The relatively coarse particles accumulate in the lower portion of the reactor 12 along with a portion of the relatively fine particles while the remaining portion of the relatively fine particles pass upwardly through the gas column. The mixture of the hot flue gases and a portion of the relatively fine particles travel the length of the gas column and exit from the reactor 12 through the opening 12a. A portion of the relatively fine particles are separated from the hot flue gases within the separator 22 and are recycled back to the fluidized bed 18 through the recycle conduit 24, while the remaining portion of the relatively fine particles remain entrained in the flue gases. Particulate fuel material is supplied, in addition to the recycled portion of fine particles, at a sufficient rate to saturate the gas column formed above the bed 18 in the reactor 12, i.e., maximum entrainment of the relatively fine particles by the flue gases is obtained. The mixture of hot flue gases and fine particles pass through the heat recovery enclosure 28 in a heat exchange relation with water passing through conventional water flow circuitry (not shown), to transfer heat from the mixture prior to the mixture entering the duct 30 including the branch ducts 30a and 30b. The damper 34 receives control signals from the temperature monitoring device 42 and operates to control the temperature of the mixture entering the baghouse 38 by regulating the flow of the mixture through the duct 30b, and therefore through the duct 30a to regulate the transfer of heat from the mixture flowing through the latter duct to the economizer 32. Thus, the mixture enters the baghouse 38 at a controlled temperature range which preferably is between 525°F and 615°F (270°C and 325°C respectively). A portion of the fine particles in the mixture entering the baghouse 38 are particles of limestone which are both unsulfated and have undergone chemical conversion to calcined limestone as a result of the high temperature in the reactor 12. According to a feature of this invention, the mixture of flue gases and entrained fine particles enter the baghouse 38 and the particles accumulate on the baghouse filter so as to form a temporary layer of sufficient thickness for the flue gases in the mixture to take between 0.1 and 1.0 seconds to traverse the layer. The above-mentioned controlled temperature range is conducive to the absorption of halogen compounds in the flue gases by the calcined limestone particles which accumulated on the filter, and the baghouse cycle rate and/or the limestone feed rate are regulated by the control device 48 to maximize the absorption of halogen gases as indicated by the halogen monitoring devices 40 and 46 whose outputs can be used to control the baghouse cycle rate and/or the limestone feed rate, as described above. It is thus seen that the method of the present invention utilizes the limestone in the entrained fine particles contained in the flue gases for the absorption of halogen compounds resulting from the combustion of fuels containing halogen. The use of the limestone particles in this manner results in significant cost savings in that it avoids the recurring costs associated with the procurement of halogen sorbing compounds, such as lime, in addition to the non-recurring cost associated with the equipment required for the injection of halogen sorbing compounds. Although not specifically illustrated in the drawing, it is understood that additional necessary equipment and structural components will be provided, and that these and all of the components described above are arranged and supported in any appropriate fashion to form a complete and operative system. It is also understood that variations may be made in the method of the present invention without departing from the scope of the invention. For example, the fluidized bed reactor need not be of the "circulating" type but could be any other type of fluidized bed in which halogen containing fuels undergo combustion in the presence of sulfur-oxide sorbing-materials, such as limestone. Further, the absorption of halogen by the limestone can be augmented by injection of other alkaline sorbent material, such as lime, limestone or other halogen sorbing-materials.

Claims: A method of combustion in a fluidized bed for reducing gaseous emission of halogen compounds comprising the steps of: forming a bed of solids particles, including a fuel material and a sorbent material, in a fluidized bed reactor; introducing air to the bed to fluidize the particles to promote the combustion of the fuel particles which generate sulfur oxides, the flue gases from the combustion entraining a portion of the particles; and, separating a portion of the entrained particles from the flue gases, is characterized by the further steps of: passing the flue gases with the remaining portion of the entrained particles to a baghouse; establishing temporary layer of the remaining portion of entrained particles in the baghouse; and, controlling the temperature of the gases and remaining portion of entrained particles entering the baghouse within a range to permit absorption of halogen compounds in the flue gases by the remaining portion of entrained particles in said temporary layer, said remaining portion of the entrained particles including the fuel material and the sorbent material and wherein the sorbent material absorbs the halogen compounds. The method of Claim 1 further comprising the step of recycling said separated solids to said reactor. The method of Claim 1 or 2, wherein said temperature control step comprises the step of selectively extracting heat from at least a portion of said gases and entrained particles. The method of Claim 3 wherein temperature control step further comprises the step of monitoring the temperature of said flue gases entering and/or exiting the baghouse and controlling said step of extracting in response to said temperature. The method of Claim 3 wherein said step of extracting comprises the steps of dividing said flue gases into two streams, said heat being extracted from one of said streams, and regulating the relative flows of said streams. The method of any preceding claim wherein the temperature of said flue gases entering the baghouse is controlled to between 525° and 615° Fahrenheit (270° and 325° Centigrade). The method of Claim 1 further comprising the step of monitoring the halogen content of said flue gases entering said baghouse and controlling the baghouse cycle rate in response to said halogen content. The method of Claim 1 wherein said step of forming comprises the step of introducing said sorbent material into said reactor and further comprising the step of monitoring the halogen content of said flue gases entering said baghouse and controlling the rate of said latter introduction in response to said halogen content.

Inventors: GARCIA-MALLOL JUAN ANTONIO, GARCIA-MALLOL, JUAN ANTONIO
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1995
Main classification: F23J15
IPC codes: F23G5, B01J20, B01J8, B01D53, F23C10, F23J15