METHOD AND PROCESS FOR CLEANING A PRODUCTGAS OF A GASIFICATION REACTOR

Abstract: The object of the invention is a method for purifying a productgas of a gasification reactor of tars and other organic compounds condensing out of the gas while it cools in a fluidized bed reactor acting as cracker. The invention is characterized in that the purification is accomplished in a spouted-bed reactor (1, 15), where the productgas (2) is fed into the reactor (1, 15) from at least one inlet point, beneath the inlet point of the productgas (2) is arranged a bubbling fluidized bed (5) that is maintained by an oxidizing gas (6) fed into the reactor (1, 15) and which forms a first zone, the productgas (2) forms above its inlet point a second or spouted zone, where fluidized particles (9) move along with the spout (8) and where purification is mainly accomplished, and above the spouted zone is arranged a third or equalizing zone,where the fluidized particles (9) are separated from the purified gas (12). An inventive object is even an equipment for carrying out the method.

Description: Method and Process for Cleaning a Productgas of a Gasification Reactor The invention relates to a method for cleaning a productgas of a gasification reactor of tars and other organic compounds that condense out of the gas while it cools in a fluidized bed reactor acting as cracker. The invention relates also to equipment for carrying out the method. In many applications of gasification techniques a crucial problem is the formation oi tar-like impurities. Among other things, tars cause clogging of heat exchangers and of gas filters and preclude the use of this gas in motors. A catalyzed technique for purifying gas has been an object of research for many years. According to familiar techniques prevention of the formation of tar and efficient decomposition of tars may be accomplished in three alternate ways: 1. The formation of tars is minimized in the gasification reactor itself with the aid of a catalytically active bed material, high gasification temperature and air distribution of the gasification reactor. If, in addition, a sufficiently efficient decomposition of the tars is attained, the final purification may be realized by washing with water. 2. Tar-like compounds and ammonia are decomposed in a separate catalyzer with the aid of a nickel-based cell catalyst. 3. The tars are decomposed in a separate catalyzer with the aid of a lime-based catalyst. The first alternative requires, however, in practice gasification temperatures of more than from 900°C to 950°C this being not feasible with each and every fuel. Also, it has been noted that in the gasification reactor itself it is difficult to reach tar contents of less than 2-4 g/m 3 n because the contact between the gas and the catalytically active material is not good enough. Thus, in many applications a separate secondary cracking is required. According to research done by the applicant a complete decomposition of tarry compounds may be accomplished in principle with both nickel-based and calcium-based catalysts. However, a drawback of the nickel-based catalysts is the high prize of the material, its toxicity as well as difficulties relating to realizing equipment run-ups and run-downs. Furthermore the lifetime of such catalysts is so far not known. The use of lime-based catalysts, that is the use of limestone and/or dolomite, requires calcination of calcium carbonate, which may be accomplished easily in gasification pressures that come in to question in motor applications and boiler applications (1-5 bar) when the temperature is maintained at over 780-850°C. In experiments conducted with applicant's small test equipment full conversion of tars has been reached in a fixed bed reactor already with a very short residence time (less than 0.5 s) and a moderate temperature of 850-900°C. In a fixed bed reactor the contact between the tar-containing gas and the bed material is ideal, but the reactor is, however, extremely vulnerable to clogging caused by dust and finely divided limestone and is therefore not applicable to industrial use. In Patent Publication SE 8,703,816 a solution is put forward that applies a separate moving bed catalysis reactor with which reasonably good results have been obtained. However, a moving bed reactor is large and expensive and calcium - containing bed materials are ground there very fast due to the high flow rate (typic ally about 5 m/s). In practice this leads to high consumption of limestone. Then again, in US Patent 4,865,625 there is proposed a method that is based on a nickel catalyst where the productgas from the gasifier is introduced to a traditional fluidized bed cracker containing nickel-containing bed material and operated at a rather low temperature of 550-570°C in order for the catalyst not to sinter. According to the applicant's own experiments operating at such a low temperature easily leads to formation of charcoal in the catalyst bed that with time clogs the catalyst. Therefore, in practice, the catalyst has to be regenerated. In the traditional fluidized bed cracker used in the above US Publication the productgas is introduced to the reactor through the distributor plate supporting the catalyst bed. In order to avoid clogging of the distributor plate the productgas leaving the gasifier has to be filtered at first. This, again, may be difficult because the tars contained in the productgas may clog the filter. Problems associated with known techniques are thus with nickel-containing catalysts a high prize, toxicity as well as difficulties related to realizing equipment run-ups and run-downs, and with limestone-based catalysts formation of dust as well as high consumption of the catalyst. In addition to the common fluidized bed reactors and moving bed reactors also spouted-bed reactors are known. However, a problem limiting their use is controlling fluidization of the bed material, while this is successfully accomplished in traditional reactors only at very high gas inlet rates (of the order of 50 m/s in general) and by applying certain relations between critical dimensions (for example particle size of the bed material, conical angle, diameter of the inlet tube and inner diameter of the reactor). In practice, this limits the use of the spouted-bed reactor and leads to a high pressure loss in the inlet tube. For example at gas inlet rates that are too low the bed material plunges into the inlet tube clogging the inlet flow o f gas. This happens also when the diameter of the gas inlet tube in relation to the size of the bed material used reaches too large a value. Thus an objective of the invention is to develop a method for purifying the productgas of a gasification reactor of tars and of other organic compounds condensing out of the gas while it cools in a cracker reactor, which method solves the problems mentioned above and meets the following demands: a) the contact between the gas and the catalyst is efficient, b) the method is not sensitive to clogging by dust and c) the method can be realized on industrial scale with costs low enough. These demands have now been met as in presented in accompanying claims. The inventive method is characterized in that the cleaning is accomplished in a spouted-bed reactor, where - the productgas is fed into the reactor from at least one inlet point, - beneath the inlet point of the productgas is arranged a bubbling fluidized bed that is maintained by an oxidizing gas fed into the reactor and which forms a first zone, - the productgas forms above its inlet point a second or spouted zone, where fluidized particles move along with the spout and where purification is mainly accomplished, and - above the spouted zone is arranged a third or equalizing zone, where the fluidized particles are separated from the purified gas. Thus, in the inventive method the productgas leaving the gasifier is introduced into a secondary cracker reactor, the bottom part of which functions by spouted bed principle and the upper part of which functions as on equalizing and separation space. A spouted bed reactor is a technique known to one skilled in the art per se from other applications. According to a preferred embodiment of the invention between the spouted zone and the equalizing zone of the cracker reactor there is arranged a forth or fluidized bed zone that functions as a traditional fluidized bed. The operation temperature needed for cracking of tars - 700-1200°C, preferably 800-1000°C - is reached by regulating the inlet of the fluidization gas of the bubbling fluidized bed. The bed material in the first and/or the forth zone may comprise limestone, dolomite, sand or a mixture of at least two of these depending on the residual tar level aimed at. In addition it is possible to use some other inert and non-grindable material. Even with inert materials a better cracking result is reached than with an empty reactor, because all bed material surfaces are to some extent active especially at high temperatures (over 900°C). According to one embodiment of the inventive method the productgas to be purified is fed into the reactor by a rate that is 15-50 m/s, preferably 20-45 m/s. In the equalizing zone the flow rate of the gas flow is yet again 0.1-1.2 m/s, preferably 0.3-1.0 m/s. Further, the fluidized bed of the first zone is bubbled with the aid of an oxidizing gas which gas is fed into the reactor at a rate that is 0.4-2.0 m/s, preferably 0.5-1.5 m/s. Said oxidizing gas may be air, oxygen, water vapor or a mixture of at least two of these. According to a preferred embodiment of the invention said oxidizing gas is fed into the reactor through a distributor plate. The productgas to be cleansed by the inventive method may be for example a gasification product of charcoal, peat, solid biomass or waste-based recirculated fuel. A further object of the invention is an equipment for carrying out the inventive method, which equipment is characterized in that it comprises - a conical part, in which a spouted-bed can be arranged, - beneath the conical part a first cylindrical part, - a distributor plate arranged in the first cylindrical part and at least one inlet opening for oxidizing gas arranged in the distributor plate, - at least one inlet tube for productgas that is arranged essentially in the middle of the first cylindrical part, parallel with the axis of the reactor, between which walls of said inlet tube and cylindrical part a fluidized bed maintained by oxidizing gas can be arranged. According to one preferred embodiment of the invention the reactor comprises additionally a second cylindrical part that is arranged above the conical part. Thus in the equipment according to the invention a productgas to be purified is introduced to the reactor through an inlet tube situated in the bottom part of the reactor which inlet tube is placed inside a larger outer tube. The oxidizing gas needed for cracking and used for partial combustion is introduced through the distributor plate in the space between the inlet tube and the outer tube. The tubes are dimensioned such that in this intervening space used for introduction of oxidizing gas reins a flow rate that is beneficial from the point of view of functioning of the bubbling fluidized bed (typically 0.5-1.5 m/s). The surface height in the reactor may be adjusted according to the tar removal degree desired on a higher or lower level. As its lowest the reactor comprises only the spouted zone and above that an equalizing zone from which the particles swept away with the gas are returned back below. Then again, the upper part of the reactor is dimensioned such that it functions as a separator for the bed particles swept away with the spout (flow rate is typically about 0.3-1 m/s). When more bed material is used a traditional fluidized bed J described forms in the reactor above the spouted zone which furthers cracking of tars and allows for a longer residence time. According to one embodiment of the invention the equipment may be used even in large cracking reactors whereby there are more inlet tubes and they are essentially parallel to each other. This kind of application is described more in detail below. By the inventive method and equipment the following advantages are attained compared to known reactor solutions: particles contained in the productgas don't cause problems such as in traditional fluidized bed crackers equipped with a distributor plate and the fluidization produced by an oxidizing gas stabilizes the spouted bed function such that an even functioning is attained with widely varying inlet flow rates of the productgas (even below 20 m/s) and with bed materials of varying particle sizes. Further, fluidization with an oxidizing gas carries likewise part of the pressure drop caused by fluidization of bed material, thanks to which the pressure of the incoming productgas is lower, which is desirable in order to prevent leaks of feed equipment and other openings. In addition the incoming oxidizing gas is very efficiently mixed with the productgas because bed material particles thrown upwards by the gas spout return back downwards along the conical walls directly into the incoming air flow to be drawn from it again back into the upwardly mobile spout. Further advantages of the inventive method and equipment comprise suitability of the space situated beneath the conical upper part and fluidized with the oxidizing gas even to removal of used bed material when it is necessary to change bed material for example due to poisoning due to reaction between HC1 and limestone or for example due to agglomeration. The inventive equipment is presented in the following more in detail with the aid oi drawings where: Fig. 1 presents an equipment according to the first embodiment of the invention in side-view, Fig. 2a presents a part of an equipment according to the second embodiment of the invention in side-view, and Fig. 2b presents a cross-sectional view AA of the equipment presented in Fig. 2a. Fig. 1 presents an equipment according to the first embodiment of the invention in side-view. The equipment 1 comprises an inlet tube 3 for a productgas 2, a bubbling fluidized bed 5 arranged beneath an inlet opening 4. The fluidized bed 5 is bubbled by introducing air 6 through a distributor plate 7. Bed material particles 9 thrown upwards by gas spout 8 formed by productgas 2 are returned back downwards along the conical walls 10 directly into the incoming flow of air to be drawn from it again back into the upwardly mobile spout 8. Above the gas spout 8 there is still a fluidized bed zone 11 that furthers cracking of tars and allows for a longer residence time. In addition, the drawing presents an outlet channel 13 for purified gas 12 as well as an outlet channel 14 for the bed material of fluidized bed 5. In Fig. 2a there is presented a part of an equipment according to the second embodiment of the invention in side-view. The equipment 15 is larger than the one presented in Fig. 1, and it comprises an inlet channel 3 for productgas 2 that is divided to multiple feed channels of which the side-view presented here comprises three, 3a, 3b and 3c. Additionally, the drawing presents introduction of air 6 through a distributor plate 7 as well as a bubbling fluidized bed 5. The solution illustrated in Fig. 2a allows for an efficient mixing and a controlled fluidization even in a large sized cracker in which good results are not obtained by using only one large inlet tube. When the introduction of gas is divided to several smaller tubes that are placed inside the same space fluidized by air even in large crackers as efficient a mixing is obtained as in a small cracker in addition to which the plunging of bed material into the inlet tube typical of spouted-bed reactors is avoided. In Fig. 2b there is presented a cross-sectional view AA of the equipment featured in Fig. 2a. In the drawing there are presented inlet tubes for gas 3a-3i and the holes 16 in distributor plate 7 are illustrated. Evidently, there may be some other number of said inlet tubes 3 and holes 16 than the one presented in the drawing, and the holes 16 may be situated according to desires. The inventive equipment has also been tested in a bench-scale sized cracker, the diameter of the upper part of which was about 100 mm and that was connected to a fluidized bed gasifier. With the inventive cracker a series of tests was carried out where the changing parameters were the bed material of the gasifier and of the cracker, the amount of the bed material in the cracker plus the amount of fluidization air as well as the temperature of the cracker. Results of the experiments are presented in Table 1. In all experiments presented in Table 1 the air coefficient of gasification and the amount of bed material in the cracker and the amount of fluidization air were kept constant. In comparative experiment A there was no bed in the cracker and no air was introduced to it whereby the temperature in the cracker was lower than in other experiments. In experiments B-E the tar content of the gas reaching the cracker was of the same order of magnitude (6 g/m 3 n ) as in experiment A. On the contrary in experiment F the tar content of the gas reaching the cracker was lower because the limestone bed in the gasifier lowered the tar content already in the gasifier. From the results in Table 1 (experiment B) it may be seen that the oxidization occurring merely in the cracker lowers the tar content of the gas approximately to a half. When the bed material of the cracker was changed to catalytically active limestone (experiment D), the tar content was lowered to the level of 1 g/m 3 n . The effect of the operation temperature of the catalyst may be seen in results of experiments C-E. An increase in the operation temperature of the cracker by 100°C lowered the tar content from the level of 2.3 g/m 3 n to a third. When the tar content of the gas was lowered already in the gasifier (experiment F, limestone bed) the tar content in the cracker was lowered to the level of 0.3 g/m 3 n . Table 1. Purification of gas by the inventive cracker H = sand, K = limestone Thus, with the inventive cracker a very good tar conversion of 85-90% may !v attained. When the tar content of the productgas of the gasifier was lowered by changing a limestone bed to the gasifier the tar content reached was quite close to the level attainable with the known nickel-monolith catalyst. Obviously the applications of the inventive cracker are not limited to the purification of productgas of fluidized bed gasification presented in the example but rather the method may be applied, for example, even to treatment of gases formed in fixed bed gasifiers or in other processes.

Claims: Claims 1. A method for cleaning a productgas of a gasification reactor of tars and othei organic compounds that condense out of the gas while it cools in a fluidized bed reactor acting as cracker, characterized in that the cleaning takes place in a spouted-bed reactor (1, 15), in which - the productgas (2) is fed into the reactor (1, 15) from at least one inlet point, - beneath the inlet point of the productgas (2) is arranged a bubbling fluidized bed (5) that is maintained by an oxidizing gas (6) fed into the reactor (1, 15) and which forms a first zone, - the productgas (2) forms above its inlet point a second or spouted zone, where fluidized particles (9) move along with the spout (8) and where purification is mainly accomplished, and - above the spouted zone is arranged a third or equalizing zone, where the fluidized particles (9) are separated from the purified gas (12). 2. A method according to claim 1, characterized in that between the spouted zone and the equalizing zone of the reactor (1, 15) there is arranged a forth or fluidized bed zone. 3. A method according to claim 1 or claim 2, characterized in that the productgas (2) to be cleansed is introduced into the reactor (1, 15) at a rate that is 15-50 m/s, preferably 20-45 m/s. 4. A method according to one of the preceding claims, characterized in that in order to bubble the fluidized bed (5) of the first zone an oxidizing gas (6) is introduced to the reactor (1, 15) at a rate that is 0.4-2.0 m/s, preferably 0.5-1.5 m/s. 5. A method according to one of the preceding claims, characterized in that said oxidizing gas (6) is air, oxygen, water vapor or a mixture of at least two of these. 6. A method according to one of the preceding claims, characterized in that said oxidizing gas (6) is introduced into the reactor (1, 15) through a distributor plate (7). 7. A method according to one of the preceding claims, characterized in that fluidized particles (9) are of limestone, dolomite, sand or a mixture of at least two of these. 8. A method according to one of the preceding claims, characterized in that the temperature of the reactor (1, 15) is 700-1200°C, preferably 800-1000°C. 9. A method according to one of the preceding claims, characterized in that the flow rate of the gas flow in the equalizing zone is 0.1-1.2 m/s, preferably 0.3- 1.0 m/s. 10. A method according to one of the preceding claims, characterized in that the productgas (2) to be cleansed is a gasification product of charcoal, peat, solid biomass or waste-based recirculated fuel. 11. An equipment for cleaning a productgas of a gasification reactor of tars and other organic compounds condensing out of the gas while it cools, which equipment comprises a fluidized bed reactor acting as cracker and which comprises inlet openings for the productgas as well as an outlet channel for the purified gas, characterized in that the reactor (1, 15) comprises - a conical part, in which a spouted-bed can be arranged, - beneath the conical part a first cylindrical part, - a distributor plate (7) arranged in the first cylindrical part and at least one inlet opening for oxidizing gas (6) arranged in the distributor plate (7), - at least one inlet tube (3) for productgas (2) that is arranged essentially in the middle of the first cylindrical part, parallel with the axis of the reactor (1, 15), between which walls of said inlet tube and cylindrical part a fluidized bed (5) maintained by oxidizing gas (6) can be arranged, and - a second cylindrical part that is arranged above the conical part. 12. An equipment according to claim 11, characterized in that of said inlet tubes (3) there are more than one and that they are essentially parallel to each other.

Inventors: KURKELA ESA, SIMELL PEKKA, STAAHLBERG PEKKA, KURKELA, ESA, SIMELL, PEKKA, STAAHLBERG, PEKKA
Applicants: KURKELA ESA, SIMELL PEKKA, STAAHLBERG PEKKA, VALTION TEKNILLINEN, KURKELA, ESA, SIMELL, PEKKA, STAAHLBERG, PEKKA, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 2000
Main classification: C10K1
IPC codes: C10K1