WO2003029389A1

WO2003029389A1 - Method and apparatus for the gasification of fuel on a fluidised bed reactor

Info

Publication number: WO2003029389A1
Application number: PCT/FI2002/000775
Authority: WO
Inventors: Esa Kurkela; Matti Nieminen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2001-10-02
Filing date: 2002-10-01
Publication date: 2003-04-10
Also published as: PT1432779E; FI120770B; JP4445261B2; FI20011925A0; DE60221549D1; ATE368719T1; JP2005504167A; EP1432779B1; DK1432779T3; DE60221549T2; ES2292803T3; FI20011925A; EP1432779A1

Abstract

The invention relates to a method and apparatus for gasifying fuel in a fluidised bed reactor (1) containing fluidised solid material particles. The fuel (8) is introduced into the reactor bottom part, and product gas (10) formed in gasification is led from the reactor top to a separator, such as a cyclone (11), which separates solid particles from the gas for recirculation to the reactor. In accordance with the invention, a bubbling fluidised bed (2) containing coarser particles, and above this, a circulating bed containing finer particles are maintained in the reactor by recirculating the particles separated from the product gas to the reactor, to the top of the bubbling fluidised bed or above this. The reactor may comprise a lower part (2) for the bubbling fluidised bed and an upper part (4) larger in cross-section for the circulating fluidised bed, the speed of the ascending gas flow in the circulating bed being equal to or lower than that of the bubbling bed. The separating limit in the cyclone may be adjusted such that the discharged product gas flow entrains solid particles, which have a binding effect on tacky ash particles. The product gas is cooled in two successive heat exchangers (19, 20) before filtration of the gas. The method is suitable for gasification of biomasses and recycled fuels forming tars and/or containing chlorine, thus providing a solution to the problems of fouled and clogged gas ducts.

Description

Method and apparatus for the gasification of fuel on a fluidised bed reactor

This invention relates to a method for gasifying fuel in an ascending gas flow in a fluidised bed reactor containing solid fluidised material particles, comprising supply of fuel to the reactor bottom part and leading the product gas produced from the reactor top part to a separator, by means of which solid particles are separated from the gas and are returned to the reactor. In addition, the invention relates to a gasification apparatus for implementing the method.

Fuels suitable for gasification comprise finely divided biofuels and waste such as saw cuttings, municipal waste, packaging materials and plastic waste. The product gas obtained can be utilised at power plants by substituting it to the plant fuel, such as charcoal, oil or natural gas.

There are two main types of fluidised bed reactors, one of which being based primarily on a stationary bubbling fluidised bed. The bubbling fluidised bed consisting of relatively coarsely divided fluidised particles remains in position supported by an ascending air flow blown into the reactor space. The speed of the air flow is typi- cally of the order 1 m/s. The solid matter concentration is low in a gas flow above a clearly limited bubbling fluidised bed. The temperature of the reactor space above the fluidised bed in a bubbling fluidised bed reactor can be raised by additional air supply or dropped by injecting cooling water into the gas flow. To increase charcoal conversion, dust particles present in the gas flow can be separated with a distinct cyclone, in which the particles are returned to the bottom of the reactor space. "Winkler gasifiers" of this type are described in DE patent specifications 195 48 324 and 27 51 911.

The second main type of fluidised bed reactors is a circulating fluidised bed, in which solid fluidised particles raise along with the air flow blown into the reactor. The air flow speed, which typically is of the order of 5 m/s, is higher and the size of the fluidised particles is smaller than those of a fluidised bed reactor. The fluidised particles are entrained by the product gas into the cyclone, where the particles and the carbonisation residue derived from the fuel are separated and returned to the bottom of the reactor space. To obtain the retention time required for the gasification reaction, circulating fluidised bed reactors have been given a height substantially higher than that of bubbling fluidised bed reactors. Other properties typical of circulating fluidised beds comprise even temperature and relatively even viscosity of the solid matter suspension in the reactor space, without a clearly limited fluidised bed characteristic of bubbling fluidised beds. A typical fuel gasification process based on circulating fluidised bed reactors is disclosed in FI lay-out print 62554.

Gasification of finely divided biomasses and plastic-containing waste performed with current methods and equipment involves the problem of the formation of tarlike compounds in abundance. Before combustion at the power plant, the product gas is filtered in order to remove ashes and heavy metals with filters having an operating temperature in the range from 200 to 450 °C. To this end, the gas derived from the reactor at a temperature in the range from 800 to 100 °C needs to be cooled with a heat exchanger, and then tars are condensed on the surfaces of the gas ducts, the heat exchanger and the filter, which become clogged. A second source of problems in current gasification processes is the chlorine contained in the fuel, such chlorine being abundantly present especially in plastic waste or similar waste fuels. Chlorine reacts with calcium used as fluidised particles or entrained by the fuel, forming compounds, which also adhere to the gas ducts and heat exchangers and cause clogging of these. This process has been observed to reach a maximum when the cooling product gas is in the temperature range of approx. 720 to 780 °C.

The problems above have been noted in gasification tests conducted by the applicant by means of a bubbling fluidised bed reactor regarding waste containing wood residue, such as saw cuttings, and plastic in abundance. With both the types of fuel, there was abundant tar formation regardless of the fact that the fuel was fed into a bubbling fluidised bed. Part of the finely divided fuel rose without being gasified along with the air flow into the top part of the reactor space, which was almost free of fluidised particles. With plastic-containing fuel, the pyrrolysis resulted in great amounts of heavy hydrocarbons, which are not given enough time to disintegrate in the relatively low fluidised bed. These compounds rise from the solid matter in the reactor into the almost empty top part, where the lack of catalysing particle surfaces prevents them from disintegrating. For this reason, the expansion of the flow cross- section of the gasifier top carried out in many known fluidised bed gasifiers and the great reactor height do not bring any solution to the tar problem. In tests conducted by the applicant, the decrease in tar concentration in the fluidised particles in the empty top of the reactor space at 900 °C and with a 10 second-delay was less than 5%, and the product gas ducts were clogged due to both condensed tars and to calcium/chlorine compounds present in the finely divided dust. The applicant has examined the gasification of the same fuels with conventional circulating fluidised bed reactors as well. In these tests, the problem of tar in saw cuttings was relieved by using limestone or dolomite as fluidised particles, which catalyse tar disintegration after they have been calcinated into calcium oxide. How- ever, there are still problems caused by lime being ground due to the high flow rate and the resulting high lime consumption and increased dust amount. With chlorine- containing fuels, lime is not of any notable assistance, presumably due to the reaction of the released HC1 and lime particles, which deactivates these fuels. Likewise, there still remained the problem of clogged gas ducts due to tacky calcium/chlorine compounds. In a test conducted with waste fuel, the product gas duct was almost completely clogged in a 30 hour-run. The deposit was noted to contain calcium and chlorine in abundance. Nor is the problem due to the fluidised material used, because waste fuel as such contains sufficiently lime for the gas ducts to be clogged. In US patent specification 5,658, 359, the problem has been solved by feeding sepa- rately coarse sweeping material, such as sand, into the product gas, yet this involves the inconvenience of costs incurred by the additional equipment and the abrasive action of coarse sand on the gas ducts and the heat exchanger.

The applicant's previous FI Patent Application 981817 describes a gasification method using a circulating fluidised bed reactor, in which the bed material consists of a mixture of hard and coarse material and readily ground and porous material. The objective of this is to achieve a situation, where the tacky alkali metals in the ashes are bound to the finely divided calcium particles and rapidly pass through the circulation cyclone out of the gasifier. In the examples of the application, the as- cending gas flow has a speed of 5 m/s, which is usual for a circulating fluidised bed gasifier, and the dust separated by the effective circulation cyclone is recirculated into the coarser sand bed at the bed bottom. In this manner, rapid grinding of the circulated lime is enhanced and good carbon conversion is achieved even at a relatively low temperature, since the carbonaceous solid matter being circulated is re- turned to the oxidative zone at the reactor bottom. The method described is suitable for materials rich in alkalis, in which chlorine is bound to the sodium or potassium in the fuel. By contrast, the tests conducted by the applicant have shown that materials containing a surplus of chlorine relative to sodium and potassium, as is often the case of waste fuels, there is strong formation of a calcium and chlorine contain- ing deposit in the gas pipe following the cyclone.

The purpose of the invention is to provide a solution to the problems explained above, which allows the problems of fouled and clogged gas ducts and heat ex- changers due to tars and/or calcium/chlorine compounds to be prevented or substantially relieved while the consumption of solid fluidised material remains moderate during the process. The method of the invention is characterised by the fact that a bubbling fluidised bed containing coarser fluidised material particles is maintained in the reactor by means of a gas flow, and above this a circulating bed is maintained, which contains finer fluidised material particles, and that circulated particles separated from the product gas are returned to the top of the bubbling fluidised bed or above this in the reactor. Consequently, the central idea of the invention is to maintain a bubbling fluidised bed and a circulating bed in parallel in the process, with these beds operating substantially separately without intermixing. This achieves an optimal process, which combines the benefits of the two basic solutions: bubbling fluidised bed gasification and circulating fluidised bed gasification.

The bubbling fluidised bed at the reactor bottom of the invention requires a rela- tively low flow speed, and then an adequate retention time is achieved in a reactor that is substantially lower than current circulating fluidised bed reactors. Fuel is gasified in the bubbling fluidised bed, from where the gas passes to the upper circulating bed, where circulating finely divided fluidised particles have a catalysing particle surface for tar degradation. For finely divided circulating particles to be fluid- ised, a low flow speed of the bubbling bed is sufficient, providing adequate retention time and decreasing disintegration and dust formation caused by particle collision. The recirculation of circulated particles to the level of the upper edge of the bubbling fluidised bed or above this yields the special benefit of the particles not having to compensate the high pressure loss of the order of 50 to 200 mbar of the bubbling bed, which is the case of conventional bubbling fluidised bed reactors, where recirculation takes place to the bottom of the bubbling bed or to the bottom of the reactor space.

Waste containing plastic and a number of biofuels are typically rich in evaporating substances and/or the carbonisation residue formed by these is extremely reactive. This is why fluidised bed gasification readily exceeds the required 95% carbon conversion, and hence it is not as crucial to separate cyclone dust with maximum precision and to return it to the fluidised bed as it is in the conventional Winkler gasifiers mentioned above. This allows the separation limit of the separator to be adjusted in such a manner that smaller solid particles in the product gas are not returned to form part of the circulating bed in the reactor, but are left in the product gas flow to be cooled, where they serve to substantially alleviate the clogging problem caused by the calcium/chlorine compounds. With the separation limit of solid particles in the separator being in the range of about 50 to 70 μm, solid particles of a size of 10-70 μm are obtained in the product gas flow, which decrease the proportion of tacky adhering components in the solid matter of the gas, bound these and prevent tacky material from adhering to the gas ducts, the heat exchanger and the gas filter. The bub- bling fluidised bed may have a particle size e.g. in the range from 0.3 to 1.5 mm, and with the separation limit of the separator mentioned above, a fraction can be returned to the fluidised bed whose particle size is in the range from 50 to 300 μm. Particles of the latter order of magnitude ascend in the gas flow, which has equal or lower speed than that of a bubbling bed, preferably e.g. in the range from 1 to 1.5 m/s. The reactor space top containing the circulating bed can be given a larger width than that of the lower part containing the bubbling bed in such a way that the expansion will compensate the gas flow increase generated by gasification and prevent flow acceleration.

The fluidised material supplied to the reactor is a particulate inorganic substance, which may be inert or reactive, yet does not form combustive product gas as does fuel.

In accordance with the invention, the reactor may be supplied with two different fluidised particle sizes so that coarser particles form a bubbling fluidised bed in the reactor and more finely divided particles form a circulating bed. The fluidised material particles to be added, which may consist of e.g. sand, will then preferably get into the variation range mentioned above between the bubbling bed and the circulating bed. Optionally, the feed material may consist exclusively of coarsely divided solid particles e.g. of the order of 0.3 to 1.5 mm, whose material is friable, so that the material, after it has been ground in the bubbling bed, joins the ascending air flow and thus forms fluidised material in the circulating bed. In a bubbling bed and a circulating bed, a continuing solid matter fraction is further obtained from the finer fraction formed, passing from the separator to the product gas flow, and this fraction serves to prevent clogging of the gas ducts and the heat exchanger caused by tacky ashes. Lime is a particularly suitable powderised fluidised material, however, magnesium oxide and kaolin are also usable.

In order to minimise clogging of gas ducts, it is further advantageous to provide product gas cooling in two subsequent steps with two separate heat exchangers, the first one cooling the gas to the temperature range 600 to 720 °C and the second one further to a temperature of 450 °C or less. The first heat exchanger passes the gas by the temperature range 720 to 780 °C, which is critical in terms of clogging, so that ash components containing calcium and chlorine loose their adhesiveness, thus eliminating the risk of clogging in the second heat exchanger operating at a lower temperature and the gas filter. The first heat exchanger, in turn, is preferably given wide dimensions so that its gas flow rate will be low, and the gas pipes may be disposed vertically, the risk of adhesion being thus minimised.

An additional measure for further avoiding clogging is to feed a sorbent, such as a suitable sodium, potassium or calcium compound into the product gas flow in or before the heat exchanger in order to bind ash particles.

The gasification apparatus of the invention implementing the method described above comprises as elements known per se a fluidised bed reactor, fluidised material consisting of solid particles, a feed inlet for producing an ascending gas flow in the reactor, a fuel feed inlet, a feed inlet for introducing fluidised material into the reactor, a gas exhaust duct starting from the reactor top, a separator for separating solid matter particles from product gas that has left the reactor, and a return line for returning separated particles to the reactor. In accordance with the invention, the apparatus is characterised by the fact that a bubbling fluidised bed containing coarser fluidised material particles can be provided in the reactor and above this, a circulating bed containing finer fluidised material particles can be provided, so that the return line for particles returning from the separator to the reactor ends at the top level of the bubbling fluidised bed or above this.

The reactor space preferably comprises a lower part for the bubbling fluidised bed and an upper part wider in cross-section for the fluidised bed. The ratio of the diameter of the upper part to that of the lower part is most appropriately in the range from 1.15 to 1.5. The parts may be spaced by a conically expanding portion, whose conical angle to the vertical axis of the reactor space is obtuse, preferably less than 10 ° and most preferably less than 7 °. This expansion prevents the gas flow rate in a circulating bed from exceeding that of a bubbling bed.

The separator for separating solid particles returning to the reactor circulating bed from the finer particles remaining in the gas flow consists of a cyclone. A wide cyclone dimensioned for a low flow speed preferably below 15 m/s can be used, caus- ing low loss of pressure, preferably less than 15 mbar. This facilitates the return of particles from the circulating bed to the reactor, and the product gas will retain solid particles that are able to sweep dust off the gas duct walls and to bind tacky adhesive ash components. The invention is explained more in detail below with reference to the accompanying drawing, which exemplifies a gasification apparatus of the invention comprising a bubbling fluidised bed and an upper circulating bed.

The gasification apparatus illustrated in the drawing comprises a fluidised bed reactor 1, which consists of a cylindrical lower part 2, an upper conically expanding part 3 and a cylindrical top part 4, the top part 4 having larger cross-sectional area than the lower part 2. In the reactor 1, in the vicinity of its bottom, there is a grill 5, un- der which gasification air or vapour is fed from the feed inlet 6. In said lower reactor part 2, a stationary bubbling fluidised bed 7 is maintained supported by the ascending air flow, the fluidised bed consisting of solid, finely divided fluidised particles having a size above 0.2 mm, preferably in the range from about 0.3 to 1.5 mm. The finely divided fuel to be gasified, which may consist of biomass, such as saw cuttings or municipal or industrial waste, such as plastic waste, is fed through the inlet 8 to the reactor bottom part 2, into the fluidised bed 7. The fuel is gasified in the bubbling fluidised bed 7, so that the formed gas passes into the ascending gas flow in the reactor space while ashes 9 are removed from the reactor bottom.

A circulating bed is maintained with the gas flow in the conically expanding part 3 of the fluidised bed reactor 1 and in the upper cylindrical part 4, the fluidised solid particles in this circulating bed being smaller than those of the lower bubbling fluidised bed 7, having preferably a size in the range of 50 to 300 μm. The conical expansion 3 of the reactor space is preferably dimensioned with the increase of the cross-sectional surface of the space corresponding to the increase in gas amount caused by the fuel gasification, so that the speed of the ascending gas flow in the bubbling bed in the narrower lower part 2 of the reactor is the same as in the circulating fluidised bed in the upper part 4 of the reactor, of the order of 1 to 1.5 m/s. These operating parameters can be adjusted by means of the gasification air and the fuel supply, and it is even conceivable that the gas flow speed in the circulating bed is lower than that of the bubbling bed.

The product gas flow is conducted from the top of the reactor space 1 through the duct 10 to the separating cyclone 11, which separates the fluidised particles from the circulating flow to be taken through the duct 12 back to the fluidised bed reactor 1. The return duct 12 joins the reactor space at the level of the upper edge of the bubbling fluidised bed 7 as illustrated in the drawing. The bubbling bed and the circulating bed will thus be separated from each other without intermixing of fluidised particles between the beds. However, in the practice, the height of the bubbling bed 7 may vary to some extent, in other words, the upper edge of the bed may momentarily rise slightly above the end of the return duct 12, or accordingly, drop slightly below this. However, it is essential that the circulating particles returning to the re- actor 1 compensate with their weight in duct 12 only the loss of pressure occurring in the circulating bed and the cyclone, but not the notably greater loss of pressure occurring in the bubbling bed, which would be the case, should the circulating particles be fed into or below the bubbling bed.

The drawing shows two feed inlets 13, 14 for fluidised particles to be introduced in the process, so that coarser particles of the bubbling bed 7 can be introduced in the fuel supply 8 and finer particles of the circulating bed can be introduced in the return inlet 12 of the circulating flow. The particles to be introduced may be e.g. sand fractions, whose particle size is different in the bubbling bend and the circulating bed. On the other hand, friable fluidised material such as lime can also be used, which, when comminuted in the bubbling bed 7, produces the more finely divided fluidised particles in the circulating bed. In that case, it may be enough if fluidised material is fed only to the bubbling bed 7 through the inlet 13. The circulating bed can optionally be supplied with secondary air raising its temperature through inlet 15 or cooling water or vapour dropping its temperature through inlet 16, in the vicinity of the top part of the reactor space.

The cyclone 11 has the task of separating the fluidised particles of the circulating bed returning to the reactor 1 from the product gas flow, which is conducted through ducts 17, 18 to be cooled in two successive heat exchangers 19, 20. The gas flow removed from the cyclone 11 to the duct 17 typically has a temperature of the order of 800 to 1000 °C, and it is cooled to the temperature range 600 to 720 °C with the first heat exchanger 19 in the flow direction. Cooling below the temperature range of about 720 to 780 °C, which is problematic in terms of the adhesion of ash particles and clogging caused by this, will thus occur in the piping of the heat exchanger 19, which is suitably vertically positioned and has been given sufficiently wide dimensions, and in which the gas flow speed is low, e.g. approx 4 m/s. With the particle separating limit in the cyclone 11 being about 50 to 70 μm, solid particles of the size of 10-70 μm will remain in the product gas flow, and they will have en essential impact in binding ashes and preventing clogging. In addition, a sorbent, such as calcium, sodium or potassium compounds reacting with chlorine can be fed to the heat exchanger 19 through the inlet 21. If necessary, cooling water or vapour can also be fed in the gas flow of the heat exchanger 19. The heat exchanger 19 acts simultaneously as a separator for removing fly ashes to the outlet 22.

In the heat exchanger 20 that is second in the flow direction, the gas flow is cooled to a temperature of 450 °C or less. The cooled product gas continues to the filter 23, whose operating temperature is in the range from 200 to 450 °C and from which the final, purified product gas is exhausted to the duct 24 and the separated finest dust fraction is removed to the discharge outlet 25.

It is obvious to those skilled in the art that the applications of the invention are not confined to the implementation given as an example above, but may vary within the scope of the accompanying claims. Thus, for instance, it is conceivable that the return duct 12 for circulating flow joins the reactor space at a higher point than that shown in the drawing, with the circulating particles being fed into the conically ex- panding part 3 of the reactor, or even into the reactor top part 4. Then the bubbling bed 2 and the circulating bed will operate completely independently of each other, without the particles being even momentarily intermixed at the bed interface.

The operability of the process of the invention was confirmed with an approx. 600 kW pilot equipment, which performed totally 460 hours of gasification tests on recycled fuel prepared from household refuse and on mixtures of recycled fuel and wood. The pilot apparatus comprised all the essential components of the invention. A solid fluidised bed consisting of coarse bed material was maintained at the bottom of the fluidised bed gasifier, and above this a circulating bed consisting of finer sand, lime and ash particles was maintained. The product gas was conducted through a widely dimensioned cyclone, in which the gas in the inlet pipe had a flow rate less than 15 m/s, and the bed material separated by the cyclone returned through the return inlet the pipe located above the fluidised bed of the gasifier. The pressure difference between the cyclone and the circulation line was low, less than 5 mbar, and the circulation acted reliably during the tests runs, and during the gasification tests, there were not even once any problems caused by clogging of the return duct for cyclone ashes, which is typical of conventional bubbling fluidised beds.

After the cyclone, the product gas and the particles having typically a size of 0 to 70 μm entrained by the gas through the cyclone passed first through the first heat exchanger. This heat exchanger contained vertical heat exchanger pipes, and the gas flowed from the top towards the bottom. The gas was cooled to a temperature of approx. 550 °C in this heat exchanger, which had been correctly designed in terms of clogging prevention. Owing to the heat exchanger design, the suitable particle load of the product gas and the proper temperature level (below 600 °C), clogging of the gas line was completely avoided, which had caused problems in the tests the applicant had previously performed with conventional bubbling fluidised bed and circulating fluidised bed gasifiers. After the first heat exchanger, the gas was cooled to the filter operating temperature in a conventional heat exchanger unit, which comprised horizontal heat exchanger piping. In some periods, finely divided calcium hydroxide was injected into the product gas before the filter in order to enhance chlorine retention that occurred with the aid of bed lime and alkali metals contained in the fuel. The filter comprised fibre filters reinforced with ceramic, which resisted a temperature of about 500 °C. The filter was used at a temperature of about 400 °C.

The recycled fuel of examples 1 to 2 represents recycled fuel substance of poor quality, which is derived exclusively from household refuse. In the practice, wood and packaging waste from stores and businesses are usually also available at gasification plants, this waste having better composition quality than household waste and being thus suitable for an advanced gasification process. It is obvious to those skilled in the art that the implementation of the invention is not restricted to the ex- emplified fuels, but instead, the invention may utilise various waste and biofuels, which have the common features of a large amount of evaporating substances and the presence of a substance generating chlorine or other tacky deposits.

Example 1

The pilot equipment was used for gasifying chopped recycled fuel prepared from household refuse, having a humidity of 26% and the following elemental composition of dry matter: C: 50.2%, H: 6.8%, N: 1.1%, S:0.2%, Cl: 0.70%, 0:26.2% and ashes 14.8%. The fuel was disintegrated to a particle size under 20 mm. The bed material in the gasifier consisted of a mixture of sand and limestone. No chlorine removing sorbent was injected into the product gas duct before the filter, the chlorine retention being based on the alkali metals of the fuel proper and bed calcium. The particle size of sand was selected such that part of the sand remained in the lower bed and part was captured into the circulating flow above the bed. Limestone was fed along with sand into the bed, where is was calcinated and ground, getting into the circulating flow and eventually reaching the gas duct along with the product gas in a finely powdered state. The gasification conditions and the results are shown in the following table. The carbon conversion of the gasification was above 98% and 96.3% of the chlorine in the fuel was mainly retained in fly dust. All of the heavy metals, except for mercury, were retained with a separation degree above 99% in the ash flows of the gasi- fier.

Example 2

Example 2 was conducted in the same way as example 1, except that the average humidity of the recycled fuel made from household refuse was 23% and the elementary composition of dry matters was as follows: C: 49.0%, H: 6.6%, N: 1.1%, S: 0.2%, Cl: 0.61%, O: 26.6% and ashes 15.9%. The bed material in the gasifier consisted of the same mixture as in example 1. To enhance chlorine retention, chlorine removal sorbent was injected in the product gas duct before the filter. This gasifica- tion was also successfully conducted with good results, which appear from the following table. Carbon conversion exceeded 98%, the chlorine retention degree was 96.9% and the efficiency of separation of heavy metals exceeded 99%. The pressure difference between the heat exchanger and the gas duct remained constant.

The gasification in examples 1 and 2 was performed as part of a 200-hour extended test run, which showed that the process operated reliably and efficiently and that no deposits were formed on the product gas line or in the heat exchanger. The pressure difference between the heat exchanger and the gas duct remained constant throughout the test, at a pressure below 2 mbar.

Example 3

In example 3, the gasifier was run over an overall period of about 80 hours. In this example, the fuel consisted of a mixture of dry wood and recycled fuel prepared from household refuse, in which the proportion of wood was about 30% by weight and the proportion of recycled fuel was 70% by weight, the average humidity of the mixture was 19% and the elementary composition of dry matter was as follows: C: 49.4%, H: 6.5%, N: 0.8%, S: 0.1%, Cl: 0.57%, O: 31.9% and ashes 10.7%. The bed material in the gasifier consisted of a mixture of sand and lime. No chlorine remov- ing sorbent was injected into the product gas duct. Nor were any measurements of chlorine or heavy metals made, but instead, the test focussed on the determination of the performance of the gasifier with a mixture of wood and recycled fuel. The operation of the apparatus was good with this fuel mixture as well, and despite the somewhat lower reactivity of wood compared to recycled fuel, carbon conversion was still 97.5%. No deposits were formed in the heat exchanger or the gas ducts, and the pressure loss remained constant. The conditions and results are indicated in the following table.

Table

nd = not determined

Claims

1. A method for gasifying a fuel in an ascending gas flow in a fluidised bed reactor (1) containing solid fluidised material particles, comprising the supply of fuel (8) into the reactor bottom part and leading the product gas (10) formed from the reactor top part to a separator (1), by means of which solid particles are separated from the gas and are returned to the reactor (12), characterised in that the gas flow is used to maintain in the reactor (1) a bubbling fluidised bed (2, 7) containing coarser fluidised material particles and above this a circulating bed (4) containing more finely divided fluidised material particles, and in that the particles that have been separated from the product gas for circulation are returned to the top part of the bubbling fluidised bed of the reactor or above this.

2. A method as defined in claim 1, characterised in that the bubbling fluidised bed (1) contains fluidised material particles having a size above 0.2 mm and preferably in the range from 0.3 to 1.5 mm.

3. A method as defined in claim 1 or 2, characterised in that the circulating bed contains circulating fluidised material particles having a size in the range from 50 to 300 μm.

4. A method as defined in any of the preceding claims, characterised in that the product gas (17) removed from the separator (11) contains solid particles having a size in the range from 10 to 70 μm.

5. A method as defined in any of the preceding claims, characterised in that the gas flow rate in the circulating bed is equal to or lower than that of the bubbling bed.

6. A method as defined in claim 5, characterised in that the gas flow rate in the cir- culating bed is in the range from 1 to 1.5 m/s.

7. A method as defined in any of the preceding claims, characterised in that the reactor is supplied with coarser fluidised material particles (13), which end in the bubbling fluidised bed and more finely divided fluidised material particles (14), which end in the circulating fluidised bed.

8. A method as defined in any of claims 1-6, characterised the reactor (1) is supplied with fluidised material particles having a size in the range from 0.3 to 1.5 mm and a friable material, so that disintegration of the particles results in the particles passing from the bubbling fluidised bed to the circulating fluidised bed.

9. A method as defined in claim 8, characterised in that the particles to be added are lime particles.

10. A method as defined in any of the preceding claims, characterised in that the fuel (8) is fed into the bubbling fluidised bed (7).

11. A method as defined in any of the preceding claims, characterised in that the separator is a cyclone (11), in which the separating line between solid particles is approx. in the range from 50 to 70 μm.

12. A method as defined in any of the preceding claims, characterised in that the product gas (17) removed from the separator (11) is led to a heat exchanger (19), where the gas is cooled to the temperature range 600 to 720 °C, and subsequently to another heat exchanger (20), where the gas is further cooled to a temperature of 450 °C or less, followed by filtering (23) of the product gas.

13. A method as defined in claim 12, characterised in that a sorbent (21) for binding ash particles is added to the product gas in the first heat exchanger (19).

14. A method as defined in any of the preceding claims, characterised in that the fuel (8) to be gasified is a finely divided biomass, such as wood pulp.

15. A method as defined in any of claims 1-13, characterised in that the fuel (8) to be gasified contains plastic.

16. A method as defined in any of claims 1-13, characterised in that the fuel (8) to be gasified contains chlorine.

17. A gasification apparatus suitable for the method defined in any of the preceding claims, comprising a fluidised bed reactor (1), fluidised material consisting of solid particles, a feed inlet (6) for generating an ascending gas flow in the reactor, a fuel feed inlet (8), a feed inlet (13, 14) for adding fluidised material to the reactor, a gas exhaust duct (10) starting from the reactor top, a separator (11) for separating solid particles from product gas removed from the reactor, and a return line (12) for returning the separated particles to the reactor, characterised in that a bubbling fluid- ised bed (7) containing coarser fluidised material particles can be provided in the reactor (1) and above this a circulating bed containing more finely divided fluidised material particles, the return line (12) for particles returning from the separator to the reactor ending at the level of the top of the bubbling fluidised bed or above this.

18. An apparatus as defined in claim 17, characterised in that the reactor comprises a lower part (2) for the bubbling fluidised bed (7) and an upper part (4) larger in cross-section for the circulating fluidised bed.

19. An apparatus as defined in claim 17 or 18, characterised in that the separator is a cyclone (11).

20. An apparatus as defined in any of claims 17 to 19, characterised in comprising two heat exchangers (19, 20) in succession in the flow direction for gradual cooling of product gas led from the separator (11), and a filter (23) for filtering the cooled product gas.