METHOD FOR CLEANING PROCESS GASES

Abstract: A method for cleaning process gases, such as flue gases formed during combustion in a boiler, especially during combustion of waste, in which method a sorbent, such as an absorbent and/or an adsorbent, is caused to react with gaseous pollutants present in the process gases, such as acidifying components and/or heavy metals, and/or organic compounds, thereby forming particulate pollutants, which are separated in a subsequent dust separator, such as a bag filter (6). In the method, the moisture content and temperature of the process gases as well as the content of at least one of the gaseous pollutants are measured. The relative humidity of the process gases is increased from a substantially constant basic level (BL) to an increased level (BI), this increase (I) being initiated by said content exceeding an upper limit value (UL). The amount of sorbent supplied to the process gases is then increased. The relative humidity of the process gases is decreased after a limited first period (t51), and/or this decrease is initiated by said content being below a lower limit value (LL), thereby restoring the relative humidity to said basic level (BL).

Description: METHOD FOR CLEANING PROCESS GASES FIELD OF THE INVENTION The present invention relates to a method for cleaning process gases, such as flue gases formed during combustion in a boiler, especially during combustion of waste, in which method a sorbent, such as an absorbent and/or an adsorbent, is supplied to the process gases for reaction with gaseous pollutants that are present therein, such as acidifying components and/or heavy metals and/or organic compounds, whereby particulate pollutants are formed, which are separated in a subsequent dust separator. BACKGROUND OF THE INVENTION For sorption of gaseous pollutants, two different methods are usually applied, the so-called dry method and the so-called wet-dry method. As a rule, the dry method is applied in the combustion of waste and is distinguish¬ ed in that a sorbent in the form of a dry, finely divided powder is supplied to the flue gases in a contact reactor. The sorbent can be, for instance, an absorbent, such as lime, which thus absorbs the acidifying components that are present in the flue gases, whereby particulate pollutants are formed, which together with the remaining so-called residual products are separated in a subsequent dust separator, such as a bag filter. To obtain an operating temperature that is suitable for the absorption, the flue gases are usually cooled by injection of water which is evaporated therein in a cool¬ ing tower arranged before the contact reactor. The wet-dry method is usually applied in the combustion of waste or coal and is distinguished in that a sorbent solution or sorbent suspension is injected into the flue gases in a contact reactor. Also in this case, the separation of the particulate pollutants formed takes place in e.g. a bag filter. An operating temperature that is suitable for the sorption is obtained by the water in the solution or suspension being evaporated in the ilue gases. Both methods result in a completely dry residual product. It is known that the sorption capacity and, thus, the degree of separation depends to a great extent on the temperature and moisture content at which the sorption takes place. The ratio of the temperature of the flue gases to the moisture content thereof can be expressed in their relative humidity or so-called condition. It may be mentioned that relative humidity is, by definition, the ratio of the current partial pressure ofthe water vapour to the corresponding saturation pressure at the same temperature. The moisture content ofthe flue gases derives on the one hand from the inlet moisture content after combustion and, on the other hand, from the amount of water supplied for cooling the flue gases. With an increased moisture content, the sorption capacity increases, i.e. the tendency ofthe sorbent to react with the gaseous pollutants of the flue gases, which, for a satisfactory sorbent utilisation, makes it necessary to decrease the temperature as much as possible. There is, how¬ ever, an upper limit for the moisture content in respect of the separated residual pro- ducts. At too high a moisture content, the residual products will be moist and, thus, sticky, which results in clogging ofthe filter bags ofthe bag filter. The residual pro¬ ducts will also be practically impossible to handle. The hygroscopic calcium chloride formed by the reaction between lime as absorbent and the hydrogen chloride of the flue gases is very important to the moisture content ofthe separated residual products. On the other hand, too low a moisture content reduces the sorption capacity, which makes the sorbent consumption increase for a given degree of separation. Thus, it is difficult to control the condition ofthe flue gases to a level which is optimum for the process concerned. The temperature and moisture content ofthe flue gases as well as the amount of acidi¬ fying components often fluctuate widely, among other things owing to great varia¬ tions in the composition ofthe fuel that is being burnt, especially in the combustion of waste. These fluctuations may also be very rapid and thus result in emission peaks, i.e. temporarily high outlet contents ofthe gaseous pollutants ofthe flue gases. The controlling of the condition of the flue gases to a level that is optimum for the process concerned thus is rendered still more difficult. According to conventional technique in, for instance, the dry method, the temperature in the dust separator is set at a constant level by the required amount of water being controlled and injected into the flue gases, the temperature of the flue gases being continuously measured, preferably directly after the cooling tower. Since the moisture content of the flue gases fluctuates as described above, the condition thereof cannot be controlled by this technique. To avoid the sticky residual products, the average value of the moisture content must therefore be selected by a great margin in relation to the defmed saturation curve ofthe calcium chloride when lime is used as absorbent, which thus implies that a relatively high constant temperature must be selected. Consequently, this results in a lower degree of separation and/or a highei consumption of sorbent. The lime is injected into the flue gases as usual in a contact reactor arranged between the cooling tower and the dust separator. The amount of lime is supplied with respect to the content of acidifying components. Since a control ofthe emission of acidifying components is only connected to the supply of lime and not integrated with the control ofthe water supply or the cleansing, it is thus not pos¬ sible to achieve optimum conditions for the separating of acidifying components. Owing to inertia in the control system and the lime in itself, this has a slow response in respect ofthe absoφtion of acidifying components, especially in case of emission peaks. It thus takes a relatively long time before a sufficient amount of lime has been supplied to the flue gases to overcome the content of acidifying components, which increases in case of an emission peak. That stated above will appear from the curves A, B and CO in Fig. 1 , the sum ofthe times tl and t2 being the total response time of the lime. It is thus also difficult to satisfy the greater and greater requirements that are plac¬ ed on emission guarantees with permissible maximum average values over short times, such as the new EU requirements which are connected to 24-hour and 30-minute average values. The 24-hour average values for hydrogen chloride and sulphur dioxide according to these new rules will be, for instance, 10 and 50 mg/Nπr , respectively, and the corresponding 30-minute average values will be 60 and 200 mg/Nnr , respectively. It is also difficult to comply with customers' requirements, which do not allow emission to exceed a given maximum momenta- neous limit value. EP 596 229 discloses a method concerning the dry and the wet-dry method for clean¬ ing Hue gases formed in the combustion of waste. Here, use is made of a calcium- containing absorbent for separating the acidifying components ofthe flue gases. To achieve a favourable condition and at the same time prevent the separated residual products from being overmoistened owing to hygroscopic calcium chloride, the inlet temperature and moisture content ofthe flue gases to the contact reactor as well as inlet gas flow are measured. A quantity of water that is necessary for this puφose can be determined in consideration of the corresponding measured values of the outlet temperature and the possible moisture content ofthe flue gases leaving the contact reactor. The quantity of water is controlled in this manner and injected into the flue gases for cooling to a predetermined operating temperature and moisture content, i.e. condition, which thus lies at a predetermined constant basic level relative to the satu¬ ration curve defined for the calcium chloride. As a result, the problem with the form- ing of overmoistened, sticky residual products is in fact eliminated. However, the relatively great distance between the above-mentioned constant level and the satura¬ tion curve of the calcium chloride does not result in an optimum absoφtion capacity, which leads to a lower degree of separation and, consequently, a higher consumption of absorbent in order not to exceed the set limit values of the emission, especially when the emission temporarily has high contents of acidifying components. The con¬ trol system for the condition of the flue gases will also be relatively complicated and expensive to accomplish since it requires extensive measuring equipment for measur¬ ing the above-mentioned parameters. Besides, the consumption of absorbent will be higher since the monitoring of the emission and the controlling of the cleansing are not integrated with this control system. A higher consumption of absorbent implies an increased amount of unused absorbent in the residual products, which results in increased costs for the provision of absorbent as well as the deposition of these resi¬ dual products. OBJECT OF THE INVENTION The object of the present invention thus is to provide, in view of that stated above, such a method for cleaning process gases, such as flue gases, preferably by means of a rapid, simple and inexpensive control system, that the set limit values for the emis- sion of the gaseous pollutants of the process gases are not exceeded, especially when there are high contents of these pollutants, and to ensure the handling of the residual products while the consumption of sorbent for separating said pollutants is kept at a minimum. SOLUTION This object is achieved by a method, which is of the type mentioned by way of intro¬ duction, the moisture content and temperature of the process gases being measured, and which according to the present invention is characterised in that the content of at least one of the gaseous pollutants in the process gases is measured. The relative humidity of the process gases, which is determined on the basis of said moisture content and temperature, is increased from a substantially constant basic level to an increased level, said increase being initiated by said content exceeding an upper limit value. The amount of sorbent supplied to the process gases is increased. The relative humidity is decreased after a limited first period, and/or this decrease is initiated by said content being below a lower limit value, thereby restoring the relative humidity to said basic level. The basic level of the relative humidity is preferably in the range of about 2-40%. The basic level is determined, based on experience of each individual process, by the para¬ meters affecting the relative humidity, such as temperature, moisture content, handl¬ ing of residual products, corrosion, and is the optimum condition that can be allowed for continuous operation ofthe process concerned, without any problems with the above-mentioned residual products. In, for instance, the combustion of waste accord¬ ing to the dry method, the relative humidity can be in the range of about 4-6%, corre¬ sponding to a temperature range of about 110-150°C, whereas the relative humidity in, for instance, the combustion of coal can be in the range of about 30-40%, corre- sponding to a temperature range of about 60-80°C. The relative humidity is increased by the above-mentioned increase which preferably is in the range of about 0-50%, and is then decreased during said first period which preferably is in the range of 0-6 h, especially about 1-20 min. The increase ofthe rela- tive humidity and said first period are determined on the basis of each individual pro¬ cess. It should be noted that the increase and also the decrease of the relative humidity takes place very rapidly, preferably during a period in the order of about 20-40 s. The relative humidity is preferably increased before increasing the supply of sorbent, the sorbent being present in the form of a dry powder, but can advantageously also be wholly or partly increased by an increased supply of a sorbent solution or sorbent sus¬ pension. The increased amount of added sorbent, sorbent solution or sorbent suspen¬ sion, which according to the invention is integrated with the controlling of the relative humidity, is determined on the basis of each individual process in respect ofthe con- tent of gaseous pollutants ofthe process gases. Preferably, the inlet and/or outlet content of gaseous pollutant(s) is measured, such as hydrogen chloride and/or sulphur dioxide and/or mercury. By inlet content is meant the raw gas content, while by outlet content is meant the content of gaseous pollutant in the cleaned process gases which are emitted into the atmosphere. The absorbent preferably consists of lime, such as limestone, burnt lime or calcium hydroxide, of sodium bicarbonate or of sodium carbonate, and the adsorbent prefer¬ ably consists of carbon, such as activated carbon or coke. The relative humidity is preferably increased by supplying a fluid, especially water, which is evaporated in the process gases, and/or by decreasing the temperature prefer- ably by means of an energy recovery step, especially a heat exchanger, such as an eco- nomiser connected to the boiler. The cleansing ofthe dust separator can be carried out after a limited second period 5 after the last cleansing operation, which preferably is in the range of about 0-24 h, and is preferably carried out when the relative humidity is kept on said basic level or below said basic level, the relative humidity being decreased so as to be increased again to said basic level after the cleansing operation. The amount of sorbent supplied to the process gases is preferably increased when the relative humidity is below said 0 basic level in order to compensate for the non-optimum condition of the process gases. By being, during cleansing, on or below said basic level as stated above, the handling of the residual products is ensured. According to the present invention, a very favourable condition of the process gases 5 is quickly attained owing to the rapid increase of the relative humidity. This means a very rapid response in respect of the reaction between the sorbent present in the pro¬ cess and the gaseous pollutants until the increased amount of supplied sorbent gives an effect. It has been found that the response time in respect ofthe soφtion for the relative humidity is very much shorter than the corresponding response time for the 0 sorbent. Since the relative humidity ofthe process gases is above the basic level which is already optimal for the continuous operation ofthe process involved, the sorbent gets a very good soφtion capacity owing to the very favourable condition. This rapid increase of the relative humidity in combination with the slower supply of 5 sorbent results in an increased degree of separation and, thus, a lower level of emis¬ sion while a minimum consumption of sorbent for separating the gaseous pollutants present in the process gases is achieved. The greater and greater requirements that arc placed on emission guarantees with permissible maximum average values over very short times and also momentaneous limit values can thus be complied with. The 30 increase of the relative humidity in fact results in an undesired increased moisture of the separated residual products. Since said increase takes place for a short time only, the resulting moist residual products, however, cause no problem, but they are diluted in the considerably greater amount of dry residual products. i;ι DESCRIPTION OF THE PROPOSED EMBODIMENT The invention will now be described in more detail with reference to the accompany¬ ing Figures. Fig. 1 illustrates schematically an example ofthe dynamics ofthe control system according to the invention. Fig. 2 illustrates schematically an example of an installation, in which the inventive method can be utilised. Fig. 3 illustrates an example ofthe outlet content of hydrogen chloride according to the present invention and the conventional technique, in which the 24-hour average value in both cases does not exceed a given maximum level of emission. Fig. 1 thus schematically illustrates an example ofthe dynamics ofthe control system for the relative humidity in respect ofthe content of an acidifying component, which here consists of hydrogen chloride, in flue gases which are formed in the combustion of waste, as well as the effect ofthe invention in relation to the conventional tech- nique (see also Fig. 3). Curve A in Fig. 1 represents the fluctuation of the inlet content, i.e. the raw gas con¬ tent, ofthe hydrogen chloride ofthe flue gases, and curve B represents the change, corresponding to the fluctuation ofthe inlet hydrogen chloride content, of the sorbent supplied to the flue gases for reaction with the acidifying components thereof, such as hydrogen chloride and sulphur dioxide, and the formation of particulate. separable pollutants. The sorbent consists of an absorbent in the form of finely divided lime powder. The content of inlet hydrogen chloride varies according to curve A typically in the range of 200-1000 mg/Nm " , and the content of added lime varies according to curve B typically in the range of 200-3000 mg/Nm 3 . The fluctuation of the inlet content of hydrogen chloride according to curve A results, according to conventional technique, in undesired peaks of emission, i.e. a tempora¬ rily high content of outlet hydrogen chloride, which is represented by curve CO. The content of outlet hydrogen chloride varies typically in the range of 5-30 mg Nm " . These emission peaks depend on the fact that the lime supply which is increased according to curve B is not sufficiently rapid. On the one hand, the actual inertia in the control system causes a time delay tl from the giving of the control signal for an increased supply of lime until the amount of lime supplied to the flue gases begins to increase and, on the other hand, it takes a time t2 before a sufficiently increased amount of lime has been supplied to overcome the increased content of hydrogen chloride. The times tl and t2, respectively, are in the order of 1 -5 min and are evident from Fig. 1. The sum ofthe times tl and t2 constitutes the total response time ofthe lime, whereupon the lime gives full effect. Merely an increased supply of lime thus is not sufficiently rapid to manage these high emission peaks, and therefore the relative humidity ofthe flue gases is increased for a short time according to the present invention, which appears from curve D, represent¬ ing the control ofthe relative humidity. This takes place based on an optimum, con¬ stant basic level BL of the relative humidity when the measured content of outlet hydrogen chloride exceeds a predetermined upper limit value UL. The basic level BL of the relative humidity is accomplished by controlling the operating temperature in respect ofthe outlet moisture content ofthe flue gases according to the process con¬ cerned. The relative humidity of the flue gases is increased by increasing the amount of supplied water. The supply of water takes place practically immediately without any considerable time delay, which is indicated by the very short period p3 which thus constitutes the total response time ofthe relative humidity. This means that the flue gases quickly get a very favourable condition for the reaction of their acidifying com¬ ponents and the lime present in the process, the outlet content of hydrogen chloride rapidly decreasing, which appears from curve Cl representing the outlet content of hydrogen chloride in the method according to the invention. The increase I of the relative humidity to the increased level BI takes the time t4, whereupon the relative humidity gives full effect. After the time delay tl , the amount of lime supplied to the flue gases begins to increase, the relative humidity being decreased after a period t51 limited according to the process involved when a sufficient amount of lime has been added, so as to return to the optimum constant basic level BL ofthe relative humidity'. The above-described method according to the invention is repeated once more when the content of hydrogen chloride again exceeds the upper limit value UL. Although the outlet content of hydrogen chloride, thanks to the increase I of the rela¬ tive humidity, rapidly decreases, the supply of lime is increased, as stated above. The supplied amount of lime thus reacts with the acidifying components o the flue gases partly during and essentially after the above-mentioned increased level BI. Since the increase 1 of the relative humidity has a much quicker response to the separation of acidifying components compared with an increase ofthe supply of lime only, emis¬ sion peaks can in this manner be kept down on a level according to set limit values until the lime itself again takes over the emission control. This is apparent from the content of outlet hydrogen chloride according to curve Cl . The increase I ofthe relative humidity creates a very favourable condition ofthe flue gases, which results in a high utilisation of the lime and, thus, a low consumption of sorbent. As the increased level BI ofthe relative humidity is of short duration only, this condition can be permitted, which is above the optimum level, possible for con- tinuous operation, i.e. the basic level BL. The moist calcium chloride for ed, which results in the separated residual products becoming sticky, thus constitutes no prob¬ lem thanks to the short duration ofthe increased level BI of the relative humidity since the separated moist amount of residual products is diluted in a considerably greater separated dry amount of residual products, which thus serves as a buffer. The period t52, which appears from curve D, between two successive cleansing operations is determined according to the process involved, the relative humidity being tempora¬ rily decreased below the basic level BL so as to be increased again to the basic level BL after cleansing. In the cleansing operation, the amount of lime is increased to com¬ pensate for the non-optimum condition (not shown). Alternatively, the cleansing ope- ration can be carried out on the basic level BL. As a result, the handling of the resi¬ dual products is ensured. It should be noted that the curves according to Fig. 1 are not to scale, but only serve to illustrate the principle according to the present invention. Fig. 2 is a schematic view of an installation regarding the dry method for flue gas cleaning, in which the method according to the present invention can be used to con¬ trol in an optimum fashion the emission of gaseous pollutants by means of a control system, especially in the case of emission peaks, and to ensure the handling of the residual products. The installation comprises a cooling tower 2, a contact reactor 4 and a fabric filter in the form of a bag filter 6 for separating residual products from flue gases formed in the combustion of waste. Fuel consisting of waste, such as household waste and industrial waste, is supplied to a boiler 1. The flue gases formed during combustion in the boiler 1 contain substan¬ tially dust and acidifying components, such as sulphur dioxide and hydrogen chloride. and have an outlet temperature from the boiler 1 in the range of about 200-300°C. The temperature and moisture content of the flue gases formed var)- rapidly and strongly above all owing to the inhomogeneous composition of the waste that is being burnt. The flue gases first pass through a cooling tower 2, in which they are cooled by injec¬ tion of an amount of water which is controlled with regard to the outlet temperature and moisture content o the flue gases leaving the bag filter, such that an optimum, constant basic level BL of the relative humidity is obtained. Water is injected in con¬ ventional manner with the aid of a nozzle assembly 3. The cooled flue gases are now passed into the contact reactor 4 and are supplied with lime in the form of finely divided calcium hydroxide. After cooling in the cooling tower 2, the flue gases thus have a condition which is very favourable for the reaction between the acidifying components thereof and the calcium hydroxide. Subsequently, the flue gases are passed into a bag filter 6, in which the formed residual products consisting of dust, reaction products and unutilised calcium hydroxide are separated. The cleansing of the filter bags of the bag filter 6 can take place at predetermined regular intervals, the residual products being removed, as indicated by arrow 7, through a conduit for further handling and deposition. The outlet content of hydrogen chloride in the cleaned flue gases leaving the bag filter 6, as well as the outlet temperature and moisture content of the flue gases are measured continuously, and these parameters are indicated in Fig. 2 by HCl, Temp and H2O, respectively. The cleaned flue gases are finally conducted via a fan 8 to a chimney 9 for emission into the atmosphere. When the outlet content of hydrogen chloride now exceeds the predetermined maxi¬ mum 24-hour average value UL of e.g. 10 mg HCl/Nm3, the relative humidity is briefly increased by an increased injection of water into the flue gases in the cooling tower 2 from the optimum, constant basic level BL, which is established by experience of the process involved and amounts to e.g. 5.6% RH, to the increased level BI of e.g. 7.3% RH, i.e. an increase I of about 30%. The supply of calcium hydroxide is increased in parallel to meet the increased content of hydrogen chloride in the flue gases by an amount of e.g. 1000 mg/Nm3. The control signals of the above-mentioned parameters as measured are processed in a control unit 10 consisting of a control system. As a result, the amount of water and calcium hydroxide supplied to the flue gases is controlled, which is indicated by the dashed lines W and S, respectively. The relative humidity is once more decreased from the increased level BI after the period t.51, which is predetermined according to the process concerned and amounts to e.g. 5 min, to the above-mentioned basic level BL when a sufficient amount of lime has managed to be supplied to the process. Moreover, two successive cleansing operations of the filter bags of the bag filter 6 will be carried out after the period t52 of, for instance, 1 h when the relative humidity is kept on the above-mentioned basic level BL, i.e. when temporarily high contents of acidifying components are not present. Alternatively, the relative humidity is temporarily decreased to a predetermined level below the basic level BL, the amount of calcium hydroxide being increased to com- pensate for the non-optimum condition. The controlhng of the relative humidity, car¬ ried out by the control unit 10, in respect of the cleansing of the bag filter 6 is indicat¬ ed by the dashed line C. In this manner, the control system performs a complete con- trol of the emission with a very low consumption of calcium hydroxide, while ensur¬ ing the handling ofthe residual products. Since only temperature, moisture content and content of hydrogen chloride need to be measured, the quick control system will be simple and inexpensive. Fig. 3 shows in a very simple manner an example of the outlet content of hydrogen chloride according to the invention, curve El, and according to conventional tech¬ nique, curve EO, whose average values both comply with the permissible maximum emission level ME, which is set at 10 mg HCl Nm 3 and indicates the 24-hour average value. To ensure according to conventional technique that this average value ofthe emission ofthe outlet content of hydrogen chloride is not exceeded, an amount of calcium hydroxide is supplied continuously, corresponding to an emission level of 5 mg HCl/Nm owing to the emission peaks. During the periods between these emis¬ sion peaks, there is supplied and, consequently, consumed more calcium hydroxide than is required so as not to exceed the above-mentioned 24-hour average value. The amount of calcium hydroxide which, however, needs to be supplied according to the invention corresponds to an emission level according to curve El, which is about 9 mg HCl Nm 3 , i.e. immediately below the emission level ME. Considerably smaller margins can thus be permitted, which results in a considerably lower consumption of calcium hydroxide. In tests using calcium hydroxide as absorbent having the above- mentioned permissible maximum emission level ME, it has been found that a very high degree of utilisation of the absorbent is achieved. By analysing the unutilised amount of calcium hydroxide in the residual products separated from the bag filter 6, the following results were obtained. The unutilised amount of calcium hydroxide according to the invention was found to be about 5% by weight o he total amount of residual products. This result should be compared with the corresponding unutilised amount of calcium hydroxide of about 20% by weight of the residual products accord¬ ing to conventional technique. In these tests, a fourfold saving of the consumption of calcium hydroxide could thus be established. Of course, the invention is not restricted to the above-described method or application and can be modified in various ways within the scope of the appended claims. For instance, the sorbent may consist of an adsorbent, such as activated carbon, or a combination of an absorbent and an adsorbent instead of an absorbent only, such as lime. For example, the relative humidity can be controlled in respect of the outlet content of sulphur dioxide, mercury or dioxins, instead of the outlet content of hydrogen chloride. For instance, the relative humidity can be controlled in respect of the inlet content of hydrogen chloride, sulphur dioxide, mercury or dioxins, instead of the outlet content of hydrogen chloride. For instance, the relative humidity can be controlled by controlling the temperature of the flue gases discharged from the boiler 1 by means of a heat exchanger or an econo- miser connected to the boiler 1 , instead of by supplying water to the flue gases in the cooling tower 3. For instance, the method according to the invention can be applied to the cleaning of gaseous pollutants that are present in the flue gases and formed in the combustion of coal, instead of in the combustion of waste. For instance, the method according to the invention can be applied to the wet-dry method, a solution or a suspension of the sorbent being supplied to the flue gases, after optional cooling thereof, in the contact reactor 4, instead of to the dry method, the sorbent being supplied to the flue gases in the form of finely divided dry powder. In the wet-dry method the relative humidity is wholly or partly controlled by the sup¬ ply of the solution or suspension.

Claims: CLAIMS 1. A method for cleaning process gases, such as flue gases formed during combustion in a boiler, especially during combustion of waste, in which method a sorbent, such as an absorbent and/or an adsorbent, is supplied to the process gases for reaction with gaseous pollutants that are present therein, such as acidifying components, especially hydrogen chloride and sulphur dioxide, and/or heavy metals, especially mercury, and/ or organic compounds, especially dioxins, thereby forming particulate pollutants, which are separated in a subsequent dust separator (6), such as a fabric filter, especial¬ ly a bag filter, and in which method the moisture content and temperature ofthe pro¬ cess gases are measured, c h a r a c t e r i s e d in that the content of at least one of the gaseous pollu¬ tants is measured, that the relative humidity ofthe process gases is increased from a substantially constant basic level (BL) to an increased level (BI), that this increase (I) is initiated by said content exceeding an upper limit value (UL), that the amount of sorbent supplied to the process gases is increased, that the relative humidity is decreased after a limited first period (t51), and/or that this decrease is initiated by said content being below a lower limit value (LL), thereby restoring the relative humidity to said basic level (BL). 2. The method as claimed in claim 1, c h a r a c t e r i s e d in that the basic level (BL) ofthe relative humidity is in the range of about 2-40%. 3. The method as claimed in claim 1 or 2, c h a r a c t c r i s e d in that the relative humidity is increased by said increase (I) which is in the range of about 0-50%. 4. The method as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the relative humidity is increased and then decreased during said first period (t51) which is in the range of about 0-6 h, preferably 1-20 min. 5. The method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the relative humidity is increased before increasing the supply of sorbent, the sorbent being present in the form of a dry powder. 6. The method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the relative humidity is increased wholly or partly by an increased supply ofa sorbent solution or a sorbent suspension. 7. The method as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that the inlet and/or outlet content of gaseous pollutant(s), such as hydrogen chloride and/or sulphur dioxide and/or mercury, is measured. 8. The method as claimed in any one of claims 1-7, c h a r a c t e r i s e d in that the absorbent consists of lime, such as limestone, burnt lime or calcium hydroxide, of sodium bicarbonate or of sodium carbonate. 9. The method as claimed in any one of claims 1-8, c h a r a c t e r i s e d in that the adsorbent consists of carbon, such as activated carbon or coke. 10. The method as claimed in any one of claims 1-9, c h a r a c t e r i s e d in that the relative humidity is increased by supplying a fluid, preferably water, and/or by decreasing the temperature by means of an energy recovery step, preferably a heat exchanger, such as an economiser connected to the boiler (1). 11. The method as claimed in any one of claims 1-10, c h a r a c t e r i s e d in that cleansing ofthe dust separator (6) is carried out after a limited second period (t52) after a preceding cleansing operation, said period being in the range of about 0-24 h. 12. The method as claimed in any one of claims 1-11, c h a r a c t e r i s e d in that cleansing ofthe dust separator (6) is carried out when the relative humidity is kept on said basic level (BL) or below said basic level (BL), the relative humidity being decreased so as to be increased again to said basic level (BL) after the cleansing operation. 13. The method as claimed in any one of claims 1-12, c h a r a c t e r i s e d in that the amount of sorbent supplied to the process gases is increased during cleansing when the relative humidity is below said basic level (BL).

Inventors: ODENMO PETER, ODENMO, PETER
Applicants: FLAEKT AB, ODENMO PETER, ABB FLAEKT AKTIEBOLAG, ODENMO, PETER
Application year: 1996
Main classification: B01D53
IPC codes: B01D46, B01D53, B01J20