Biofuel is found abundantly in form of e. g. wood chips. The fuel has a relatively large volume in relation to its energy content. It is therefore not very economic to utilise biofuel in large central systems that require that the fuel has to be transported over relatively great distances.

Instead, the biofuel can advantageously be utilised in smaller, locally placed thermal gasification installations having smaller gasifiers for producing gas for a gas engine which e. g. can power an electric generator for electricity production. For this purpose, the produced gas must however be rather pure, that is the gas must not have any significant content of particles and tar.

Such small gasifiers are normally of the fixed bed type. By means of counter-current gasification, a good heat recovery is obtained but the produced gas is on the other hand very impure. By means of co-current gasification, a far purer gas is obtained but the resulting disadvantage is that a separate heat recovery system is required in order for the installation to be able to obtain a high utilization ratio of the energy content of the biofuel.

Most commercial and semi-commercial gas purification systems are of the liquid purification type. Typically a scrubber is

thus used to transfer impurities in form of tar from the gaseous phase to the liquid phase. Often the scrubber will also remove the dust which has not already been removed in e. g. a dust separator placed before the scrubber system. The scrubber furthermore cools the gas which becomes saturated with moisture.

The heat content of the gas can be recovered but only up to relatively low temperatures. The heat can thus only be utilised at low temperatures as e. g. is the case in district heating installations.

Even if excellent results are obtained by means of the above scrubber system, the application of these liquid purification systems however always encounters the very serious problem of having to get rid of the highly polluted and very toxic discharge water which is an inevitable by-product of the liquid purification process. There is no general good solution to this problem.

Systems using gas-liquid or gas-gas heat exchangers have the problem of the tar and dust content of the gas being inclined to be deposited as encrustations on the heating surfaces of the heat exchangers. Thereby, the heat transfer will in time be reduced in a given heat exchanger so much that it will be necessary to clean the heat exchanger in order to restore its capacity. This task is difficult and laborious, and problems can furthermore arise in getting the removed encrustations deposited.

The consequence of the above disadvantages have so far been no appreciable application of a process in a thermal gasification installation in which the heated gas from the gasifier is led through a heat exchanger and during this cooled by means of cooling gas or air. The considerable advantages that otherwise

could be obtained by means of such a process have therefore never been implemented.

The object of the invention is to provide a thermal gasification installation of the kind mentioned in the opening paragraph for by means of dry purification of the gas from the gasifier producing a producer gas of such a high quality that the gas can be used to power a gas engine without any problems.

The novel and unique features according to the invention, whereby this is achieved, is the fact that the installation is arranged to periodically make the gas flow and airflow change heat exchanger sides.

The gas from the gasifier normally has a not insignificant content of tar which is inclined to be deposited as encrustations on the heating surfaces in the cold end of the heat exchanger. Thereby, the output of the heat exchanger is reduced.

By letting the gas flow and airflow change heat exchanger sides, it is obtained that air is now flowing on the heat exchanger side which was previously flown through by gas, and that the former cold end of the heat exchanger now becomes its heated end.

The heated air in the now heated end of the heat exchanger affects the tar encrustations on the encrusted heating surface with such a high temperature that the encrustation crack, disintegrate and are volatilized and in this state are led out of the heat exchanger together with the air. Thereby, the previously encrusted heating surfaces is cleaned so that the output capacity of the heat exchanger is restored.

The heated outgoing air from the heat exchanger can advantageously be utilised for the gasification process in the gasifier whereby the content of combustible gasses from the tar encrustations brought along by the air at the same time are exploited. Thereby the thermal gasification installation obtains an optimum good energy economy.

If more air is required for cooling the gas in the heat exchanger than required in the gasification process in the gasifier, the heat exchanger can furthermore be adapted for the flowing through of another airflow which during this also is heated by the passing heated gas. This heated second airflow can be utilised for successively drying the biomass and/or as heat source in a district heating installation.

The thermal gasification installation can furthermore have a dust separator for freeing the gas of particulate matter before the gas reaches the heat exchanger.

The invention will be explained in greater detail below, describing only exemplary embodiments with reference to the drawing, in which Fig. 1 is a diagrammatic view of a thermal gasification installation according to the invention, Fig. 2 is a diagrammatic view of a heat exchanger for the thermal gasification installation in fig. 1 in one stage of operation, and Fig. 3 is the heat exchanger in fig. 2 in a second stage of operation.

The thermal gasification installation in fig. 1 is generally designated by the reference numeral 1. The installation is utilised for gasifying a biofuel 2 supplied from a storage

container 3. The biofuel is led by means of a conveyor 4 in the direction indicated by the arrow to a gate 5 which successively leads the biofuel down into a gasifier 6.

The gasifier is of a kind known per se and will therefore not be described in detail in this application. For the installation shown is used a co-current gasifier to ensure that the gas obtains a high discharge temperature.

The gasifier has an ash outlet 7 and is via a discharge channel 8 connected to a dust separator 9 for removing particulate matter from the gas. From the dust separator the gas flows via a gas channel 10 into a heat exchanger 11, in which the gas is cooled down and purified of tar. The now purified gas is led via a second gas channel 12 to a gas engine 13 that powers an electric generator 14 for generating electricity.

In the heat exchanger the gas is flowing counter-currently to a first and a second airflow from a first blower 15 and a second blower 16, respectively.

The first airflow is led via a first air channel 17 into the gasifier at the top for being used in the gasification process that takes place in the gasifier. As mentioned earlier, the first airflow is flowing co-currently to the biofuel in the gasifier in order to thereby give the gas a high discharge temperature.

The second airflow is led via a second air channel 18 to, in the case shown, a drying device 19 for drying the biofuel 2.

Figs. 2 and 3 are very diagrammatic views of how the heat exchanger 11 in fig. 1 is arranged and how it operates. The heat exchanger is divided into a first and second flow-through section 20' ; 20". Each of these sections is by a first and second

heat transferring wall 21', 22' ; 21", 22"respectively, again divided into a first, second and third heat exchanger side 23', 24', 25' ; 23", 24", 25"respectively.

The first heat exchanger side 23'in the first flow-through section 20'is via a channel 26 connected to the second heat exchanger side 24"in the second flow-through section 20", whereas the second heat exchanger side 24'in the first flow- through section 20'via a second heat exchanger channel 27 is connected to the first heat exchanger side 23"in the second flow-through section 20". The third heat exchanger sides 25' ; 25" in the first and second flow-through section 20' ; 20" respectively are not connected to each other.

It is to be noted that the two flow-through sections 20' ; 20" alternatively can be directly connected to each other, that is without the heat exchanger channels 26 and 27.

As shown in fig. 2, the gas x D D D 1S flowing through the heat exchanger from right to left via the first heat exchanger side 23"of the second section, the second heat exchanger channel 27 and the second heat exchanger side 24'of the first section. At the same time the first airflow y ils flowing in the opposite direction, that is from left to right via the first heat exchanger side 23'of the first section, the first heat exchanger channel 26 and the second heat exchanger side 24"of the second section, whereas the second airflow z -=== is flowing in the same direction as the first airflow but only through the third heat exchanger side 25'of the first section. The gas x is thus flowing through the heat exchanger counter-currently to the two airflow y and z. The first section 20'therefore becomes the cold end of the heat exchanger.

The heat exchanger 11 is set up in such a way in dependence of the three mass flow and the temperature of these flow at the entrance to the heat exchanger that the tar content of the gas cannot condense in the heated second section 20"but only in the cold first section 20', that is on its heat transferring walls 21'and 22'. Thereby encrustations are deposited on these walls, said encrustations will successively reduce the performance of the heat exchanger so that the thermal gasification installation no longer will be able to function effectively.

In order to avoid the development of such a situation, the thermal gasification installation is arranged in such a way (not shown) that the gas flow x and the first airflow y periodically can change heat exchanger side and change flow direction, whereas the second airflow z simultaneously changes both position and flow direction.

This situation is shown in fig. 3. As can be seen, the previous cold first section 20'has now become the heated section of the heat exchanger in which section the first airflow intensely heated by the heated gas flow x sweeps the two walls 21'and 22'on which encrustations to some extent now have been deposited. The heated first airflow y affects these encrustations with heat so that the encrustations crack, disintegrate and are volatilized and are in this state absorbed by the first airflow y. Thereby the heat exchanger is cleaned of encrustations.

While the first heat exchanger section 20', 20"is cleaned of encrustations in this way, encrustations are however at the same time deposited on the heating surfaces of the second section. Therefore, there is periodically changed back and forth between the operating condition in fig. 2 and the one in

fig. 3 so that encrustation are never deposited on the heating surfaces of the heat exchanger to an unacceptable extent.

As mentioned earlier, the first airflow y is utilised in the gasification process that takes place in the gasifier. Thereby its heat content is fully exploited. Furthermore the combustible products absorbed by the airflow y during the removal of the encrustation on the heating surfaces of the heat exchanger are turned to account. As also mentioned earlier, the heat content of the second airflow z is at the same time utilised for drying the biofuel.

The heat exchanger is furthermore arranged in such a way that the gas is cooled to about 10-20°C above the dew-point temperature, that is to between 70 and 80°C before it without any further purification now can be utilised as fuel for the gas engine.

The thermal gasification installation according to the invention can thus always function with an optimum good energy economy and produce a pure producer gas which is suitable as fuel for a gas engine. The gas is purified dryly whereby the disadvantages connected to the conventional liquid purification methods are avoided.

The heat exchanger is described above and shown in figs. 2 and 3 entirely in principle; and it can of course be arranged in any expedient way within the scope of the invention.