The present invention relates to a method and an installation for thermal gasification of solid fuel, such as biomass fuel, wherein the gasification process comprises pyrolysis of the fuel, an at least a partial chemical reaction, such as an oxidation, of volatile pyrolysis products, and a gasification of char formed by the pyrolysis.

Installations for performing such gasification process and having a separate reactor or zone for each of said process steps are known. These known installations may comprise mechanical means, such as a conveyor screw, for transferring char from the prolysis reactor to a fixed char bed. Furthermore, combustion installations in which the fuel is exposed to a pyrolysis, and in which a whirling motion is imparted to the pyrolysis products by a substantially tangential flow of oxygeneous gas, are known, for example from JP 11248123 A and from WO 00/01987.

The present invention provides a method of the above type, which may be performed in an installation having only a single rector chamber, and which is much more simple than those required for the known methods. Thus, the present invention provides a method of thermal gasification of solid fuel, said method comprising providing a layer of solid fuel forming a fixed bed, heating the fuel in said bed so as to form volatile fuel components by pyrolysis, causing at least part of said volatile components to move upwardly into a reactor chamber defined above the layer of solid fuel, supplying at least one reactant into said reactor chamber, contacting said at least one reactant with the volatile components in the reactor chamber so as to cause said components to at least partly react chemically with the at least one reactant, and discharging gaseous products from the reactor chamber through the layer of solid fuel.

The reactant or reactants supplied into the reactor chamber may be of any type, which may chemically react with at least some of the volatile fuel components or pyrolysis products so as to obtain a desired net result. As an example, the reactant or reactants may combine chemically with the fuel components in an exothermic process, so that the gaseous products discharged through the fuel bed are hot and may heat the solid fuel therein. The volatile fuel components usually contain tar components or tarry substances, which to a substantial degree may be decomposed when heated due to the exothermic chemical reaction and/or by at least partial chemical reactions with at least some of the

reactants supplied to the reactor chamber. Preferably, such chemical reactions are exothermic and may be obtained by choosing a reactant, which comprises oxygen, such as air, so that the chemical reaction in the reactor chamber comprises at least partial oxidation of the volatile fuel components formed by the pyrolysis of the solid fuel. It is important, however, that the solid fuel or char in the fuel bed is not oxidized to any substantial degree in order to avoid formation of undesired passages through the bed.

Therefore, the oxygen is preferably supplied into the reactor chamber in such an amount and/or in such a manner, that the gaseous products discharged through the layer of solid fuel contains substantially no free oxygen. This may, for example, be obtained by supplying an amount of oxygen, which is sufficient to oxidize the volatile fuel components or pyrolysis products in the reactor chamber only partly and/or by supplying the oxygen into the upper part of the reactor chamber and/or by directing the gas flow or flows containing the oxygen in directions away from the fuel bed, so that substantially all of the free oxygen is consumed by oxidation of the volatile fuel components, before the resulting gaseous products are discharged through the fuel bed.

The solid fuel in the fixed fuel bed may be heated to a pyrolysis temperature in any suitable manner, for example by an external heat source, such as by combustion heat generated by burning the only partly oxidized gaseous products discharged from the reactor chamber through the fuel bed. In the preferred embodiment, however, the heating of the fuel in the fuel bed is at least partly obtained by the hot gaseous products discharged there through. Such heating of the fuel causes not only that in a pyrolysis process the solid fuel is decomposed into volatile fuel components and char, but also that the char thus formed is gasified. Thus, by using the method according to the invention the solids of the fuel is oxidized only to a very limited extent, while the volatile or gaseous components may be oxidized sufficiently to decompose tarry substances therein and to produce enough heat to sustain the pyrolysis and gasification processes in the fuel bed by heat transfer when the gaseous products from the reactor chamber is discharged through fuel layer or bed. From the fuel bed these combustible gaseous products may be passed to any suitable combustion or heating device or to a gas storage.

As indicated above, other reactants than oxygen may be used. Thus, said at least one reactant may comprise water vapour and/or carbon dioxide in addition to or as

alternatives to oxygen. The reactant or reactants supplied into the reactor chamber may be mixed with inert gaseous media, such as nitrogen.

The solid fuel in the fixed bed may, for example be heated to a temperature of 200° C- 1300° C, preferably 1000° C-1100° C, so as to cause said pyrolysis of the solid fuel.

The volatile fuel components or pyrolysis products should preferably continuously be moved upwards from the fuel bed into the reactor chamber where they should be mixed with and brought into intimate contact with the reactant or reactants. This is preferably obtained by passing said at least one reactant, which is gaseous and possibly combined with other gaseous media, into the reactor chamber at such positions and in such a manner that the volatile components are whirled up from the fuel bed and into the reactor chamber. The whirling motion may, alternatively, be generated or promoted by other means, such as gas flows not containing a reactant and/or by mechanical means. As another possibility, one or more gas flows may be directed towards the upper surface of the fuel bed so as to cause turbulence within the reactor chamber.

In the presently preferred embodiment the volatile components are moved upwardly from the fuel bed and into the reactor chamber by forming in the reactor chamber one or more whirls each having an axis intersecting said fuel bed, preferably substantially at right angles to the fuel bed. Such whirl or whirls may be generated by gas flows and/or by mechanical means arranged within the reactor chamber, such as wall formations promoting a tangential or circumferential gas flow within the reactor chamber. The volatile fuel components may then be moved upwardly along the axis or axes of the whirl or whirls thus formed and may be mixed with the reactant or reactants in the upper part of the reactor chamber.

In principle, said at least one reactant may be intermittently introduced into the reactor chamber. It is preferred, however, to feed the reactant or reactants into the reactor chamber continuously, and the gaseous products are preferably also continuously discharged from the reactor chamber through the layer of solid fuel.

The solid fuel may be fed to the fuel bed batch wise via a fuel inlet formed in the upper part of the reactor chamber, but the fuel is preferably fed continuously.

The present invention further provides an installation for thermal gasification of solid fuel, said installation comprising a reactor chamber, means for supporting a layer of solid fuel forming a fixed bed forming the bottom part of the reactor chamber, means for supplying solid fuel to said fuel bed, means for heating the fuel in said bed so as to form volatile fuel components by pyrolysis, means for causing a least part of said volatile components to move upwardly into the reactor chamber defined above the layer of solid fuel, means for supplying at least one reactant into said reactor chamber, whereby said at least one reactant is brought into contact with the volatile components in the reactor chamber so as to cause said components to at least partly react chemically with the at least one reactant, and means for discharging gaseous products from the reactor chamber through the layer of solid fuel.

The said means for causing a least part of said volatile components to move upwardly into the reactor chamber may comprise any suitable means, such as gas flows and/or mechanical means arranged or formed within the reactor chamber. In the preferred embodiment, however, they comprise said means for supplying at least one reactant into said reactor chamber, which supply means comprise one or more gas inlet openings or nozzles formed in the reactor chamber wall above said fuel bed, said nozzles being arranged and shaped so as to cause a whirling movement of the gaseous product in the reactor chamber, when gas including the at least one reactant is passed there through and into the reactor chamber. The inlet openings or nozzles may be arranged and shaped so that the gas flowing there trough and into the reactor chamber above the fuel bed in addition to a possible radial velocity component also obtains a peripheral or tangential and/or an axial velocity component.

The various gaseous media, some of which contain the reactant or reactants, separately or mixed, may be passed into the reactor chamber through the various inlet openings or nozzles as flows of pressurized gas. The flow rates and pressures of the various gas flows may be the same or different for the various inlet openings or nozzles. The nozzle or nozzles is/are preferably positioned and shaped so that gas flow (s) passed there through cause (s) in the reactor chamber the formation of one or more gas whirls each having an axis intersecting said fuel bed and preferably extending at right angles thereto.

In a preferred embodiment a single such whirl is generated within the reactor chamber.

This may be obtained by arranging the gas inlet openings or nozzles in such a manner

and by blowing flows of gas there through in such a way that the gases obtain a resulting angular velocity in a plane at right angles to the longitudinal axis of the reactor chamber, which is also the axis of the whirl. Thus, the tangential velocity of the gas introduced through the nozzles causes rotation of the gas above the fuel bed, whereby volatile components or pyrolysis products from the upper part of the fuel bed is whirled upwardly along the centre axis of the whirl. The gas inlet openings or nozzles may also be arranged and shaped such that the gas flow also obtains a velocity component, which is parallel with the axis of the whirl, whereby an ejector effect is obtained in this direction. In addition to the gas flows at right angles to the longitudinal axis of the reactor chamber gas flows being directed substantially at right angles or having velocity components at right angles to the plane of the fuel bed may be provided. Thereby the desired flow of volatile pyrolysis products from the fuel bed and upwardly into the rector chamber for mixing with the reactants may be promoted.

The supply means for reactants may feed the reactant or reactants into the reactor chamber batch wise or in a pulsating manner. However, preferably the said supply means are adapted to feed said at least one reactant continuously into the reactor chamber, and the discharge means for gaseous products may similarly be adapted to discharge the gaseous products continuously from the reactor chamber through the layer of solid fuel.

Such discharge means may comprise gas ventilators or pumps or other gas transporting means in case a forced gas flow is required.

The means for supplying fuel to the fuel bed are preferably adapted to supply the fuel into the reactor chamber continuously or batch wise via a fuel inlet formed in the upper part of the reactor chamber and may be in the form op any conventional mechanical transporting or conveyor means, such as a conveyor belt or a conveyor, or an airlock for feeding the fuel batch wise.

The invention will now be further described with reference to the drawings, wherein Fig. 1 is a diagrammatic axial sectional view of an installation or gasifyer according to the invention, Fig. 2 is a diagrammatic cross-sectional view of the gasifyer shown in Fig. 1, Fig. 3 is a diagrammatic axial sectional view as shown in Fig. 1, where the gas flow pattern has been indicated,

Fig. 4 is a diagrammatic cross-sectional view showing the arrangement of gas nozzles, and Fig. 5 is a longitudinal sectional view of a gas inlet or nozzle shown in an enlarged scale.

Figs. 1-4 diagrammatically illustrate a gasifyer or reactor comprising a preferably cylindrical housing 10 having a grate 11 arranged in the lower part thereof. An inlet 12 for solid fuel is arranged at the upper part of the housing 10 for batch wise or continuous supply of fuel so as to form a fixed fuel bed 13 on top of the grate 11. One or more inlets or nozzles 14 for supply of gas containing or entraining one or more reactants, such as oxygen, water vapour and carbon dioxide, is/are arranged almost tangentially to the peripheral housing wall as best shown in Fig. 4. Each of the gas inlets or nozzles 14 opens into a reactor chamber 15, which is defined in the housing 10 above the upper surface of the fuel bed 13. Gaseous products from the reactor chamber 15 is discharged via the fuel bed 13 through a gas outlet 16, which is communicating with an outlet chamber 17 defined in the housing 10 below the grate 11.

Fig. 5 shows an example of a nozzle 14, which is provided with a plurality of differently directed nozzle openings 18, and Fig. 4 illustrates have such nozzles may be arranged in the housing 10.

The gasifyer illustrated in the drawings may be operated as follows : The solid fuel in the fuel bed 13 fed into the housing 10 through the fuel inlet 12 is exposed to a pyrolysis, whereby tarry, volatile pyrolysis products are formed in the upper layer 19 of the fuel bed 13. Gas flows from the nozzles 14 containing oxygen and possible other reactants create a whirling movement of the gas in the reactor chamber as illustrated by arrows 20 in Figs. 2-4. The whirl thus generated has an axis substantially at right angles to the upper surface of the fuel bed 13. The whirl creates secondary flows 21 (Fig. 3) in the upper layer 19 of the bed 13, whereby the volatile pyrolysis products are moved upwardly along the centre axis of the whirl as illustrated by an arrow 22 in Fig. 1 and is thoroughly mixed with the oxygen and possible other reactants introduced through the nozzles 14. Thereby the volatile pyrolysis products are partly oxidized and the tarry substances therein are decomposed while the gases in the reactor chamber 15 are heated by the exothermic chemical reaction. In the upper part of the reactor chamber 15 the upwardly directed gas flow 22 is reversed, and the hot gaseous products, which contain almost no free oxygen, are passed downwardly through the fuel bed 13 and the

grate 11 as indicated by arrows 23 in Figs 1 and 3. Finally, the combustible gas is passed through the gas outlet 16 to a combustion device or to a gas storage, not shown.

The fuel in the bed 13 is heated by the hot gas, so that the pyrolysis process in the upper layer of the bed with freshly supplied fuel may be maintained, and also the lower layers of the bed containing char are heated and exposed to water vapour and other oxidation products, whereby the char is gasifyed.

It should be understood that various modifications and changes of the embodiment shown in the drawing could be made within the scope of the present invention. As an example, the reactor chamber need not necessarily have a circular cross-sectional shape. Thus the cross-section could be elliptical or polygonal. Furthermore, the peripheral wall of the housing 10 could have formations for promoting the desired whirling movement of the gas in the reactor chamber 15.