Prior art apparatus for gasifying fuel in fixed layer gasifiers exist in two basic types, that is cocurrent gasifiers and countercurrent gasifiers.

In cocurrent gasifiers the fuel is fed to upper parts of the gasifying reactor, from which it is flows due to gravity through drying, pyrolysis, oxidizing and reducing zones. In general, the gasifying air is introduced directly to the hottest combustion zone, through which even the products of pyrolysis travel. Also, the diameter of the combustion zone is often smaller than in other parts of the reactor. With this kind of gasifier rather pure product gas is obtained and the tars disintegrate rather efficiently in the process. After further purification the product gas is applicable as motor fuel.

The biggest problems with cocurrent gasifiers lie in their applicability for certain fuels only, melting of the fuel ashes, scaling of the gasifier as well as a low efficiency. A cocurrent gasifier is applicable only to good quality fuels in piece form that flow without interference through the gasifyig reactor, such as chopped wood, briquets, wood charcoal in piece form and good quality chips. When using other biofuels the operation of the gasifying reactor is easily disturbed due to channeling of the bed and arching of the fuel. On the other hand, the temperature in the combustion zone rises often to such high values that the ashes of many biofuels melt. In practice, a nearly ash-free pure wood fuel or a fuel the ashes of which don't melt easily are used in cocurrent gasifiers. Furthermore, the scaling of the gasifier of the cocurrent gasifier is difficult, because while the gasifying reactor and its narrowing, choke, diameters get larger, it is very difficult to accomplish an evenly hot oxidizing zone and due to this, the tars are not totally disintegrated. Furthermore, a total conversion of the remaining coal is not accomplished in the gasifying reactor whereby the efficiency remains low.

In known countercurrent gasifiers the fuel also flows due to gravity downwards from the above. However, in countercurrent gasifiers the gasifying air is introduced upwards from down below in a direction opposite to the flow of fuel. The fuel is thus dried and undergoes pyrolysis in upper parts of the gasifying reactor, whereby the pyrolysis products enter almost as such into the product gas, that is, the tars do not disintegrate as in cocurrent gasifiers. On the other hand, the remaining charcoal from pyrolysis travels eventually through the oxidizing zone, where in practice all combustible material is reacted, whereby the conversion of charcoal is more or less complete. The thermal efficiency of the countercurrent gasifier is thus higher than that of the cocurrent gasifier. In addition, the problems of melting ashes encountered with the cocurrent gasifiers can be avoided by regulating the temperature of the oxidizing zone by means of added steam. Furthermore the gasifying reactor can be designed as a simple chute oven where choke structures are not needed. Even the variety of possible fuels is larger than with cocurrent gasifiers. However, the fuel must be mainly in piece form and it must flow due to gravity of its own accord in the reactor. Moreover the content of particulates in the product gas is low, and almost all of the ashes may be removed in oxidized form from the bottom of the gasifying reactor.

Problems encountered with the countercurrent gasifier are the high content of tar and low temperature of the product gas as well as pressure drop across the high fuel bed used. Due to the high content of tar and low temperature of the product gas it has to be combusted without delay in a boiler, dryer or some other such combustion apparatus situated near the gasifying reactor. The gas is not applicable to motor use contrary to the product gas of a cocurrent gasifier. Furthermore, these properties of the product gas cause from time to time clogging of the gas pipeline. A clogging gas pipeline causes the pressure to drop, like the height of the fuel bed mentioned above. This is all the more marked, when the fuel encompasses finely divided material such as sawdust. Pressure drops, in turn, can cause drainage problems of the feeding apparatus. Moreover, when the humidity of the fuel exceeds 45%, an unstable combustion is often the problem.

There have been attempts to avoid the problems mentioned above. e. g. with the method described in US Patent 4 498 909, where artificial ash-material is fed to the countercurrent gasifier in addition to the light biofuel with low ash content. Hereby the flow of the fuel in the gasifying reactor is somewhat improved, whereby the variety of usable fuels is somewhat increased. However, in order to achieve a

crucial improvement large amounts of inert material should be used, whereby the heat economy of the gasifier would decrease.

Thus an objective of the invention is to free the countercurrent gasifier of the problems mentioned above and at the same time, produce product gas, the tar content of which is at the level of the cocurrent gasifier. In addition, an objective is to produce gas with a low tar content of various types of biofuels that are not applicable to traditional fixed layer gasifiers. This has been accomplished as represented in accompanying claims.

An objective of the invention is thus a method for gasifying carbon-containing fuel in a fixed layer gasifier according to the countercurrent principle, where fuel is fed to the gasifying reactor to at least one point, which is, when seen from the reactor bottom up, at a height that is 20-70% of total height of the reactor, whereby below the feed point the fuel bed forms a primary zone, where pyrolysis, reductive and oxidative reactions occur, and whereby primary air is fed from under the fuel bed and from the reactor, from above of the fuel bed, product gas is removed. The invention is characterized in that to the gasifying reactor (7), to the secondary zone above the feed point of the fuel (1) and above the fuel bed, secondary air is fed at least to the immediate vicinity of the fuel bed.

According to a preferred embodiment of the invention fuel is fed to the gasifying reactor to a point, the height of which is, when seen from the reactor bottom up, 30- 50% of total reactor height. According to a particularly preferred embodiment of the invention fuel is fed to the gasifying reactor essentially at its imaginary vertical axis.

On the other hand, clearly the fuel may be fed to the gasifying reactor also near the reactor wall, for example. Moreover, it is preferred that the fuel is fed with the aid of a feed pipeline extending to the point of feed, whereby it is especially preferred that the feed pipeline is vertical.

In addition in the inventive method it is preferred that the fuel is fed to the gasifying reactor in a compulsory manner. This kind of compulsory feeding of fuel to the gasifying reactor can be accomplished with a screw feeder, for example, that feeds new fuel against the bed in the gasifying reactor. A compulsory feeding of fuel is preferably accomplished with the aid of a vertical feed pipeline when wishing to feed the fuel to the imaginary vertical axis of the gasifying reactor.

Those skilled in the art recognize that the number of feed points depends on the size of the gasifying reactor, that is, for example, the bigger the reactor, the more feed

pipelines are preferably used. It is even possible to arrange the feed pipelines in such a manner that they are arranged to different points of the reactor cross-section.

It is preferred that the feed points are in the same plane when viewed vertically.

By compulsory feed of the fuel the drainage and arching problems typical of the fixed layer gasifiers are avoided. In addition, more fuel types can be used than in traditional methods, where fuel must flow undisturbed through the reactor. The fuel may be, for example, coal, peat, solid biomass such as sawdust or wood chips, or waste-based recycled fuel.

Secondary air is fed to the process most preferably in several stages, of which the first feed takes place to essentially immediate vicinity of the pyrolysis layer. By feeding secondary air over the primary layer the temperature of the secondary zone can be raised to a level of 400-1000°C, preferably 650-800°C. In these temperatures pyrolysis products formed in gasifying, that is tars, exist in gaseous form and partially decompose. Thanks to the increased temperature and partial decomposition of tars clogging of the product gas pipeline, a problem that is typical of the countercurrent gasifiers, is avoided. On the other hand, the primary zone temperature is downgraded enough by feeding water vapor in order for the fuel ashes not to melt in the gasifier.

In addition, on the bottom of the gasifying reactor is preferably arranged a movable grate according to some known technique per se, for example a rotating cone- shaped grate. By means of the compulsory feeding and movable grate a situation is created, where the flow of the fuel in the gasifying reactor is not solely dependent on the fuel's own weight. Thus the arching and channeling problems are all the more diminished. Furthermore it is evident, that while the reactor size increases, it is preferred to use more than one grate.

The primary air is fed to the reactor preferably through the grate. The primary air can consist also of some other oxidizing gas than air, for example oxygen or a mixture of oxygen and water vapor. It is even possible to add water vapor to the air.

In order to avoid drainage problems of countercurrent gasifiers, according to the inventive method carrier air can be fed into the fuel in at least one point of the fuel feed line. This carrier air forms at the same time part of the secondary air needed.

Moreover, according to the inventive method to the gasifying reactor is arranged a cracking organ for the product gas which constitutes a tertiary zone. The cracking organ may be a thermal or catalytic cracking organ according to some technique

known per se, which may in addition be based on tertiary air feed. Thus the easily decomposed and unstable organic compounds contained in the product gas of the countercurrent gasifier are decomposed in two or more stages, of which the first is the decomposition in the secondary zone described above, and other decompositions taking place in the cracking organ of the tertiary zone. It is especially preferred that the temperature of the tertiary zone is higher than the temperature in the secondary zone, for example 800-1100°C. With the combination described above the formation of heavy polyaromatic compounds that are difficult to decompose and of soot can be minimized, these being typical to traditional cracking of product gas from a traditional countercurrent gasifier.

Furthermore an object of the invention is an apparatus for application of the method described above, which apparatus comprises a fixed layer gasifier, where the opening for fuel of one or more feed inlets is situated in the gasifying reactor at a height that is 20-70% of the total height of the gasifying reactor, and which has a feed inlet for primary air as well as an outlet channel for product gas. The apparatus is characterized in that the inlet opening for fuel of a feed pipeline is situated in the gasifying reactor at a height that is 20-70% of the total height of the gasifying reactor and that in the gasifying reactor, above the opening of a feed inlet is arranged at least one secondary air feed inlet.

In addition, the secondary air feed inlets of the apparatus are preferably arranged in annular and staged manner in the vertical direction of the gasifying reactor in one or more planes.

As an example may be stated that when the height of the gasifying reactor is 3-4 m, the height of the primary zone of the gasifying reactor is about 0,7-1,5 m. With the inventive apparatus it is thus possible to use a low fuel bed, whereby the pressure drop remains low. This increases further the efficiency of the apparatus.

The invention is illustrated more in detail with reference to the accompanying Figure 1, that presents schematically the principle of one inventive mode of apparatus.

In Figure 1 is presented feeding of fuel 1 through a feeding channel 2 to a purge container 3. In the feeding channel 2 is arranged a feed pipeline inlet 4 for carrier air of the fuel 1 and on both sides thereof, block feeders 5. At the bottom of the purge container 3 there is a resolve unit 6 that disperses the fuel 1 to an even mass. From the bottom of the purge container 3 fuel 1 is transported to a gasifying reactor 7 with

the aid of screw conveyors 10,11 that are mounted in feed pipelines 8 and 9. In addition, more feed pipelines for carrier gas can be arranged in feed pipelines 8,9.

The inlet opening of feed pipeline 9 is situated in the middle of gasifying reactor 7 to which the screw conveyor 11 is compulsorily feeding fuel 1. Evidently the pretreatment of fuel of the inventive apparatus before it is fed to the gasifying reactor may be realized also by some other means known per se.

Further, in Figure 1 is presented secondary air feed pipeline inlets 12,13 that are arranged in the gasifying reactor 7, above the inlet opening of the feed pipeline 9. It is preferred that secondary air inlet is accomplished through several annularly mounted air nozzles that are situated in more than one plane vertically. On the bottom of the gasifying reactor 7 there is also a movable grate 14 according to some technique known per se to those skilled in the art. For example, the grate 14 may be a three-dimensional cone-shaped, periodically and slowly rotating grate shown in the picture. Through the grate 14 primary air and steam 15 are fed into the gasifying reactor through a feed inlet 16. It is preferred that the grate 14 is designed such that primary air and steam 15 are fed into the gasifying reactor 7 in more than one plane.

Also, at the bottom of the gasifying reactor 7 there is arranged an outlet opening 17 for removal of bottom ashes from the gasifying reactor 7. The outlet opening 17 is according to some technique known per se, for example a bottom valve.

In Figure 1 is also presented an outlet channel 18 for product gas, the opening of which in situated in upper parts of the gasifying reactor 7. According to a preferred embodiment of the invention, product gas is introduced to a cracker 19, which in the apparatus shown is of a two-piece design. The cracker 19 that is shown comprises in the first place a thermal cracker 20 into which tertiary air 21 is introduced, and thereafter a catalytic cracker 22. The thermal and catalytic crackers are according to some technique known per se. It is also evident that the product gas may be purified by some other means known to those skilled in the art. The purified product gas 23 is obtained from the apparatus through outlet channel 24.

The feasibility of the inventive method has been shown in the experiments according to the following examples, where forest waste, willow chips ans sawdust are gasified with an apparatus of about 300 kW. None of the experimental fuels would be applicable as fuel of a cocurrent gasifier, and problems related to fuction would be likely to occur even in traditional countercurrent gasifiers.

Example 1 As a fuel for the gasifier forest waste (forest waste 1) was used, the volumetric weight of which was 263 kg/m3 and the thermal value LVH (in dry matter) of which was 19.6 MJ/kg. The fuel's dry matter composition and sieve analysis are given in Table 1. The gasifier according to the invention operated smoothly, and feeding of fuel functioned automatically and without problems. The content of non- combustibles determined from bottom ashes was less than 1%.

In this experiment, the temperature in the secondary zone of the gasifying reactor was 660°C and the temperature of the gas exiting from a thermal cracker connected to the rear of the gasifying reactor was 710°C. The composition of the product gas exiting from the gasifying reactor (in volume % in dry gas) was CO = 25.1%, C02 = 9.8%, H2 = 9.3%, CH4 = 3.6% the rest being mainly nitrogen. Likewise, the concentrations of the components of a thermally cracked product gas were: CO = 19.6%, C02 = 11.6%, H2 = 7.6%, CH4 = 2.8%. The dust content of the product gas was when measured after cracking 0.1-0.5 g/m3n and the tar contents before and after cracking were 2.8-3.5 g/m3n and 1.1-2.2 g/m3n.

Example 2 As a fuel for the gasifier forest waste (forest waste 2) was used, the volumetric weight of which was 245 kg/m3 and the thermal value LVH (in dry matter) of which was 19.6 MJ/kg. The fuel's dry matter composition and sieve analysis are given in Table 1. The gasifier according to the invention operated smoothly, and feeding of fuel functioned automatically and without problems. The content of non- combustibles determined from bottom ashes was less than 1%.

In this experiment, the temperature in the secondary zone of the gasifying reactor was 750°C and the temperature of the gas exiting from a thermal cracker connected to the rear of the gasifying reactor was 820°C. The composition of the product gas exiting from the gasifying reactor (in volume % in dry gas) was CO = 25.3%, C02 = 8.3%, H2 = 9.1%, CH4 = 3.3% the rest being mainly nitrogen. Likewise, the concentrations of the components of a thermally cracked product gas were: CO = 23.1%, C02 = 9.5%, H2 = 9. 1%, CH4 = 3.4%. The dust content of the product gas was when measured after cracking 0.1 g/m3n and the tar contents before and after cracking were 2.2-2.9 g/m3n and 0.7 g/m3n.

Example 3 As a fuel for the gasifier willow chips were used, the volumetric weight of which was 178 kg/m3 and the thermal value LVH (in dry matter) of which was 18.6 MJ/kg.

The fuel's dry matter composition and sieve analysis are given in Table 1. The gasifier according to the invention operated smoothly, and feeding of fuel functioned automatically and without problems. The content of non-combustibles determined from bottom ashes was less than 1%.

In this experiment, the temperature in the secondary zone of the gasifying reactor was 750°C and the temperature of the gas exiting from a thermal cracker connected to the rear of the gasifying reactor was 870°C. The composition of the product gas exiting from the gasifying reactor (in volume % in dry gas) was CO = 25.1%, C02 = 9.8%, Ho = 9.3%, CH4 = 3.6% the rest being mainly nitrogen. Likewise, the concentrations of the components of a thermally cracked product gas were: CO = 19.6%, C02 = 11.7%, H2 = 8.4%, CH4 = 2.8%. The dust content of the product gas was when measured after cracking 0.25 g/m3n and the tar contents before and after cracking were 2.7-2.9 g/m3n and 0.3-0.4 g/m3n.

Example 4 As a fuel for the gasifier sawdust was used, the volumetric weight of which was 190 kg/m3 and the thermal value LVH (in dry matter) of which was 19.0 MJ/kg. The fuel's dry matter composition and sieve analysis are given in Table 1. The gasifier according to the invention operated smoothly, and feeding of fuel functioned automatically and without problems. The content of non-combustibles determined from bottom ashes was less than 1%.

In this experiment, the temperature in the secondary zone of the gasifying reactor was 760°C and the temperature of the gas exiting from a thermal cracker connected to the rear of the gasifying reactor was 870°C. The composition of the product gas exiting from the gasifying reactor (in volume % in dry gas) was CO = 25.6%, C02 = 11.6%, H2 = 11.1%, CH4 = 5.7% the rest being mainly nitrogen. Likewise, the concentrations of the components of a thermally cracked product gas were: CO = 22.5%, C02 = 11.1%, H2 = 10.0%, CH4 = 4.3%. The dust content of the product gas was when measured after cracking 0.5-0.6 g/m3n and the tar contents before and after cracking were 2.1-3.7 g/m3n and 0.6 g/m3n.

Table 1. The Fuels Used in the Examples 234Example1 wasteFuelForest 1 Forestwaste 2 chips Sawdust Volumetric 263 245 178 190 weight (kg/m3) LVH (MJ/kg) 19.6 19. 6 18. 6 19.0 Composition of dry matter (w%) C 51.0 51.8 50.2 50.9 H 6.1 5.8 5.6 6.2 N 0.7 0.5 0.4 0.1 S 0.015 0.04 0.04 0.01 O (as difference) 40.05 39.76 41.36 45.59 Ashes 2. 1 2. 1 2. 4 0. 2 Sieve analysis (w%) > 31. 5 mm-2. 4 2.5 8.0-16.0 mm 0. 1 23.4 19.9 3.15-8.0 mm 38.7 25.9 39.1 2.0-3.15 mm 17.5 12.6 19.0 19.5 1.0-2.0 mm 24.4 16.3 10.5 34.4 < 1.0 mm 19.3 7.4 9.0 46.1

In all examples the tar contents of the product gas after the secondary zone of the gasifying reactor were about 2-4 g/m3n. In traditional countercurrent gasifiers the tar content of the prodct gas is of the order of 50 g/m3n. The tar content of product gas achieved by the inventive method was even lower than what has been determined with applicant's own fluidized bed gasifiers, and is very close to the level (about 1,5 g/m3n) that was realized when gasifying ideal chopped wood with commercial Martezon Cocurrent Gasifier.

The purification of the product gases of the examples was investigated also by introducing the product gases to both a thermal and a catalytic cracker to which tertiary air was fed. In examples 2 and 3, where the temperature of the cracker was 820-870°C, the tar content of product gases was after the thermal cracker 300-700

mg/m3n, and when the product gases were additionally driven through a nickel catalysis cracker, a tar content of 10-150 mg/m3n was achieved. At the same end temperature traditional thermal cracking of the product gas of a traditional countercurrent gasifier produces a tar content of 1500-4000 mg/m3n, that is, by the inventive method considerably lower tar contents are realized.