The invention relates to a process and plant for the purification of gases, particularly for the desulphurization of and NO.sub.x -removal from flue gases by multistage adsorption and catalytic reduction in gravity-flow moving beds of granular, carbon-bearing materials contacted by a transverse stream of the gas, a minimum of two such moving beds being arranged in series with reference to the gas route so that NO.sub.x -elimination takes place in the second or any downstream moving bed.
Referring to the state of the art, it is known from West German Patent DE-OS 2911712 to separate gaseous and solid pollutants from gas streams by adsorption and/or adhesion in gravity-flow moving beds of granular, carbon-bearing material, especially activated coke or activated carbon, in that a transverse flow of the gas stream to be purified passes through these moving beds. Referring to West German Patent DE-OS 3232546, it is also known to arrange several moving beds in series with reference to the direction of flow of the gas to be purified and to provide for an additional feature in that the various moving beds may be interconnected with the gas and solids routes to reflect a quasi counter-current principle. The state of the art further teaches that purification steps may be allocated in a substance-related pattern to various moving beds arranged in series. Elimination of nitrogen oxides is performed in a known way with ammonia serving as a reducing agent. This method depends on diluted or undiluted gaseous ammonia or an aqueous ammonia solution being injected into the inlet duct of the first moving bed or into the outlet duct of the first moving bed or the inlet duct of the second moving bed or even directly into the moving beds, for example see West German Patents DE-OS 3232544, DE-0S 3039477, and DE-OS 2911712.
Chemical properties governing the NO.sub.x -elimination step prevent selective reaction with NO.sub.x of the ammonia injected into the flue gas stream to be purified. Secondary reactions take place with the other flue gas constituents, for example SO.sub.2, hydrochloric acid, hydrofluoric acid, and oxygen, in addition to the desired reaction. Therefore, referring to the state of the art, according to West German Patent DE-OS 2911712, the more-recent processes and plants use the first moving bed for removing from the gas stream the major portion of sulphur dioxide, for example, while the elimination of nitrogen oxides is performed in a second moving bed after injection of ammonia. According to the state of the art, this method cuts ammonia requirements at comparable NO.sub.x -elimination rates. In addition, the formation of ammonium salt deposits on the granular, carbon-bearing material of the moving bed is lowered.
Despite said improvements, the processes and plants incorporating the state of the art still have a serious disadvantage. Large volumes of gas, such as flue gas emissions from industrial furnaces, require the fabrication, installation and operation of large items of equipment. At the same time, consideration must be given to the economical use of energy and raw materials since the process requires large volumes of adsorption agent to be moved. It is common practice to allow the loading of the adsorption agent to a point where the risk of a breakthrough of sulphur dioxide exists over a wide range in the lower part of the first moving bed. Consequently, concentrations of sulphur dioxide are bound to increase from top to bottom on the outlet side of the first moving bed. The stream in the outlet duct of the first moving bed is subject to the formation of gas streaks with comparatively high concentrations of sulphur dioxide. A significant portion of the ammonia injected for the reaction of NO.sub.x -elimination downstream of the outlet duct of the first moving bed will be consumed by side reactions in the gas streaks of high SO.sub.2 concentration and is, therefore, lost for the reaction of NO.sub.x -elimination. In the upper part of the second moving bed, the catalytic reaction of NO.sub.x -elimination takes place in line with design conditions while such reaction can be reduced to zero by side reactions in the lower part. Therefore, excess ammonia must be added to achieve a certain mean degree of NO.sub.x -elimination. However, the addition of excess ammonia must be limited because excess ammonia might break through in the upper part of the second moving bed and itself pollute the purified flue gas.
Referring to the suggestions outlined in West German Patent DE-OS 3232543, the plant may operate with a high excess of ammonia and a corresponding ammonia breakthrough if provision is made for a third moving bed arranged, with reference to the gas flow, downstream of the second moving bed to eliminate ammonia from the flue gas by neutralization yielding ammonium sulphate and ammonium bisulphate. According to the suggestions outlined in said publication, this is achieved by admitting to the third moving bed at least part of the granular, carbon-bearing material laden with sulphur dioxide and sulphuric acid, respectively. However, this measure entails a dramatic rise in the overall pressure drop across the plant and, consequently, a dramatic rise in energy requirements while a substantial portion of the ammonia intended for the reaction of NO.sub.x -elimination is lost owing to side reactions.
Referring to West German Patent DE-OS 3014934 Al, it is suggested that the predesulphurized flue gas be admixed with a higher quantity of ammonia in the lower part than in the upper part of the second moving bed. This method may substantially avoid a local admission of excess ammonia, but the formation of significant amounts of ammonium sulphate in the lower part of the second moving bed remains a characteristic feature. In addition, a locally variable admixture of ammonia cannot be achieved at reasonable expense because the advent and the local extent of a sulphur dioxide breakthrough as well as the SO.sub.2 concentrations involved are not steady-state phenomena but are subject to continuous variation depending on variations of the initial sulphur dioxide concentration and of the firing rate of the furnace.
For reasons of flow kinetics, to avoid, for example, the entrainment of particulate matter from the moving bed on the discharge side, the gas flow velocity across the discharge surface must not exceed a certain definite limit. This limit depends, for example, on the kind of the granular, carbon-bearing material of the moving bed, on the angular position of the proposed louver-type sheets which form the gas-permeable wall on the inlet and outlet sides of the moving bed housing, etc. Therefore, the size of the inlet surface is invariable. The only variable is the bed depth that is likely to prevent an SO.sub.2 breakthrough. This, in turn, means a higher pressure drop across the plant as the bed depth is increased.
Without increasing the depth of the first moving bed, i.e. without additional pressure drop across the plant, a breakthrough of sulphur dioxide in the lower part of the first moving bed could also be avoided by increasing the traveling speed of the first moving bed. However, this will entail a markedly higher mechanical loss of granular, carbon-bearing adsorption agent which must be manufactured by a process that is expensive from the standpoint of both energy and raw materials.
Moreover, streaks of sulphur dioxide with the inevitable formation of ammonium salt deposits on the granular, carbon-bearing material of the second moving bed, i.e. the NO.sub.x -elimination stage, may adversely affect the known processes by creating flow disturbances in a specific part of the second moving bed. These disturbances are due to the presence of ammonium sulphate which causes the formation of conglomerations of adsorption agent granules and/or of incrustations on the housing wall. This is of major importance for operational reliability because, in areas of poor flow, it affects significantly the dynamic heat balance between the heat composed of heat of adsorption, dilution and reaction in the moving bed on the one hand and the quantity of heat that is dissipated as sensible heat by the temperature rise of the effluent gas (directly proportional to the quantity of gas) on the other hand. The result may be what is called a hot spot which might impose a shut-down of the entire plant.
For the elimination of nitrogen oxides from low-oxygen waste gas, West German Patent DE-OS 2635652 describes a process which uses, at temperatures ranging from 400 to 600 degrees centigrade, a special granular, carbon-bearing material with metal oxide ingredients to serve as adsorption agent and catalyst. This process does not permit desulphurization of and NO.sub.x -elimination from low-oxygen flue gas at temperatures ranging from 50.degree. to 200.degree. C.
In actual operation, the processes described in said publications as the state of the art evidence at least one, but generally several of the disadvantages outlined before.