The present invention applies to a procedure for burning wood fuel and other similar non-slagging biofuels like peat etc. which are contained in a fuel container. These fuels are characterized also by a low ash content and considerable production of pyrolysis gas during heating with limited supply of air already at comparatively low temperatures. The residues which are formed during the pyrolysis are also quite reactive in the final prpcesses of gasification and combustion.

This category of fuels is potentially very attractive from an environmental point of view because of, among other things, a frequently very low sulphur content. On the contrary these fuels contain organic nitrogen which gives nitrogen oxides also at rather low temperatures of combustion when these fuels are subjected to burning.

The thermoche ical roperties which have been indicated above have, however, so far constituted a complication. The large production of pyrolysis gas already at low temperatures constitutes a risk for uncontrolled emissions of cancerogenic polyaromatic compounds as well as precipitation of so-called creosote or tar in the chimney. Chimney fires in houses with wood burners have caused considerable damage.

During the last years the attention has been focussed

on the emissions of cancerogenic substances in the com¬ bustion of fossil fuels as well as biofuels as wood and peat. The risk for such emissions because of incomplete combustion is probably not negligible in combustion on a small scale with large pieces of fuel, e.g. logs, in furnaces which are primitive from a combustion tech¬ nical point of view.

A primary purpose with the present invention is to supply a procedure which permits complete and environ- mentally acceptable combustion particularly of wood, peat and lignite also on a small scale for residential heat etc. with small emissions of nitrogen oxides, cancerogenic substances and particulate matter at a small excess of air.

A second purpose is to supply fast control of the heat¬ ing power within wide limits to suit the actual demand.

A third purpose is to produce a high efficiency and a high final temperature of combustion at good fuel economy and efficient utilization of the fuel gas heat.

fourth purpose is to achieve the above advantages and properties also in the combustion of fuel in the form of larger fuels like boiler wood or briquettes.

The invention will also produce a number of other advan¬ tages which will be evident from the following description.

The procedure according to the invention has been deve¬ loped particularly for biofuels like wood and peat but can also be used with other fuels like lignite, etc.

The procedure is also particularly suited for burning of boiler wood and logs but may also be used with wood chips and other fuels in a finely divided shape. It can be dimensioned and designed in detail for the fuel preferred.

The invention is using a new combustion technical com¬ bination. The new principle permits a new combustion process which is on one hand adapted to the special pro¬ perties of the fuels which has been mentioned above and which are preferred and on the other hand satisfies the requirements indicated above with respect to complete and efficient combustion.

Many attempts have been made to design furnaces for efficient and complete combustion of wood fuels, fre- quently in connection with container burning. Natural measures have been pre-heating of the combustion air, supply of secondary air in a special flame space or a design which forces developed gases through the hearth.

The Swedish patent 6251 thus describes a furnace with a fuel container and a space surrounding this container where the combustion air is pre-heated. The wood is sinking down in the upright fuel container towards a horizontal grid with an ash box below the grid.

The Swedish patent 7454 describes supply of primary air to the hearth which is arranged on a vertical grid with final combustion with supply of secondary air.

Other means and procedures which are in principle similar are described in other earlier patents like the Swedish patents 11515, 14383, 29004, 93704 etc.

The Swedish patent 99156 describes combustion with an inverted container. Combustion is taking place in a horizontal hearth in the upper part of the fuel charge .

The Swedish patent 99532 aims at complete combustion with secondary air by means of a furnace chamber which has a comparatively small width in relation to its length which gives the flame space the shape of a slot. The Swedish patent 148925 shows also how pre-heated air is supplied for final combustion in a special chamber prior to ^" this ^" flame space. The Swedish patent 118540 describes a gas flame outlet which is disposed in the middle part of a watercooled plane grid.

None of these principles satisfy the above requirements as well as the present invention which refers to proce- dure for combustion of non-slagging solid fuels like wood, peat and lignite which contain a large amount of volatile matter in a furnace with a fuel container for container firing with a grate and a grate space and a connecting flame space for combustion of fuel gas which is leaving the grate space characterized in that hot recirculated flue gas is brought in contact with the fuel in a narrow gasification zone in the grate space for production of a fuel gas which fuel gas is burnt in the flame space .

The invention shall now be described by means of the Figures 1, 2 and 3.

Figure 1 shov/s quite schematically the principle for the new procedure .

Figure 2 shov/s a useful furnace for carrying out the procedure according to the invention leaving out details

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of construction.

Figure 3 shows in principle other embodiments of furnaces for the procedure according to the invention.

Figure 1 shows a fuel container (1) with a grate (2) and the solid fuel (3) . Hot flue gas with a temperature of more than 200° (4) from the flame space (5) is supp¬ lied to a narrow gasification zone (6) in and within the grate (2) in the grate space (7) . The fuel gas (8) which is formed during the gasification of the fuel flowing against the slot (9) at the upper edge of the grate (10) where it is mixed with combustion air (11) for combustion in the flame space (5) to the flue gas (12) whereby the flow (4) is recirculated whereas the remaining flue gas (13) is utilized in heat exchangers etc. prior to leaving to the chimney.

Figure 2 shows the principle for a furnace according to the invention in a preferred embodiment.

The fuel container (1) , which may have a length of about 1 meter and a square section of about 0,2 x 0,2 meter for a small house, is ending downwards with the grid (2) which is inclined towards the horizontal plane.

The fuel container (1) contains the fuel charge (3) , e.g. logs, which are standing on the grid (2) .

It is frequently useful to arrange another preferably horizontally disposed plane grid (14) for the purpose to catch not completely burnt out fuel which may fall through the grid (2) . The ash is collected in the ash box (15) .

The flow of flue gas which is recirculated to the gasi-

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fication zone (6) is discharged from the upper part of the flame space with the channel (16) containing the fan (17) and the heat exchanger (18) for reheat of the flue gas. The flue gas is then fed to the inlet (19) by means of a pipe system in the hottest part of the fur¬ nace, (this pipe system is not shown in the drawing) .

The fuel gas thus formed is flowing up along the grid (2) towards the slot or the constriction (9) . The slot connects the grid space (7) with the flame space (5) .

The hot ^" flue gas contains nitrogen, carbon dioxide, steam and a small quantity of oxygen. The oxygen con¬ tent is controlled by the air/fuel gas ratio in the flame space and by eventual extra supply of recirculated flue gas by the conduit (20) with the damper (21) and the fan (22) . (Cf Sv. pat. appl. 8001801-3) A very fast pyrolysis and gasification of the fuel'is taking place in the thin zone of gasification. The pyro¬ lysis and gasification residue is oxidized by the oxy¬ gen in the reaction gas. Organic nitrogen compounds are decomposed down to nitrogen. The solid wood fuel is thus converted to an environmentally acceptable fuel gas.

The fuel gas is burnt in the flame space (5) by addition of air in or next -to the slot where intense mixing is taking place. The area of the flame space is then in- creasing upwards which causes separation of particulate matter. The flame space may also contain buffles (23) so as to produce a circular gas movement. Additional air for final combustion and dilution can be added in inlets which are tangentially disposed (24) to produce a cyclone effect.

It is surprising that the contact between fuel and

recirculated hot flue gas will generate a fuel gas which can be combusted completely and in an environmentally acceptable way in the flame space. It is not possible to get a simple explanation to this feature.

A detailed analysis of differences between the object of the invention and other designs,e.g. according to the Swedish patents cited above,will however give some clues. None of these known designs give the same rapid and intense gasification process in a small zone com¬ bined with the feature that all gases formed rapidly go to the flame space.

These conditions which can be described as flash pyro- lysis/gasification according to the last findings,, in this area will give a fuel gas with a higher heating value and improved combustion properties depending on, among other things, higher content of methane. The pyrolysis is also more complete with a smaller amount of. pyrolysis residues and furthermore is more reactive in gasification and final combustion.

These suitable conditions are not obtained in earlier furnaces which are characterized by a slower pyrolysis process and a longer residence time for the produced gas with thermochemical conversion of primary pyrolysis products towards equilibrium composition.

The technical effect of the invention, the complete combustion, is of course governed by the combustion properties of the fuel gas. The properties of the fuel gas can not be described completely by means of analysis of fuel gas samples. The gas has a different composi¬ tion immediately after the pyrolysis and gasification reactions and may contain reactive intermediate products

of different kind like ions, radicals, etc. The flash pyrolysis and the short residence time, which is in th ^' e range from about 0.1 second or below to a couple of seconds, is of great importance for the properties of the fuel gas.

The reaction pattern at the procedure according to the invention with a reaction gas for pyrolysis and gasi¬ fication, consisting of hot recirculated flue gas, with a content of reactive intermediate products, differs completely from e.g. partial oxidation with preheated air. The temperature in the recirculated flue gas should be above 200°C and may preferably amount to 550 - 750 C. Another important circumstance is the low oxygen content in the recirculated flue gas which should not exceed about 10% and preferably be below

2-3%. Eventual addition of air should be adjusted to give this level.

A very small part of the fuel charge is in each moment taking part in the gasification process. The process may be described so that the fuel charge is burnt like a cigar with its glo.w towards the grate. This is caused by the remarkable feature that only the lowest part of the fuel charge is taking part in the process. Thanks to this the heating power can be rapidly changed from spare power to f ll power and vice versa.

The combustion process is of course controlled by con¬ trolled supply of the reaction gas e.g. air and the re¬ circulated combustion gas. In general it is desired to work with as low excess of air as possible. It is pos- sible to operate near a stoichiometric air supply by careful adjustment of process conditions.

At simpler embodiments with natural draft and manual con-

trol of dampers under constant operating conditions ob¬ servation windows into the grid and ^* the flame ^" space respectively help here for adjustment of the supply of air. A flow gas thermometer and a look on the com- bustion gas leaving the chimney gives additional in¬ formation at manual operation.

The geometrical arrangement with the inclined grate, the thin slot and the connecting flame space consti¬ tutes a very useful furnace design for carrying out the procedure according to the invention. The invention is therefore . not limited to these geometrical requirements. The narrow gasification zone and the short and intense contact between hot recirculated flue gas and fuel can also be obtained with other embodiments of the furnace as shown in Figure 3. Figure 3a thus shows a horizontal gasification zone (6) whereby the grate (2) has an oblong shape so that the residence time of the fuel gas in the gasification zone is short. Figure 3b shows a vertical gasification zone (6) and Figure 3c a fuel con- tainer ending downwards with a double grate so as to produce two opposite gasification zones. The symbols in Figure 3a, b and c are the same as in Figure 1 and 2.

The technical effects of the invention can be illustra¬ ted by the following example. The methane content of the fuel gas is a good indicator of the properties of the gas . Two-stage combustion can take place in a furnace according to Figure 2 - which by the way is subject to a simultaneous patent application (Sw. pat. appl. 8001801-3) - either with addition of preheated air to the gasification zone which is described in the said simultaneously filed patent application or with addition of hot recirculated flue gas. The recirculated flue gas has a temperature around 750°C and an oxygen content of 2% . Air for the partial combustion in the

control experiment is preheated to 150 C. The methane

3 contained in the fuel gas is 0,02 standard m /kilogram fuel in the control test and more than twice as high,

3 00,,0055 ssttaamndard m /kilogram fuel with recirculated hot flue gas.

The invention can be used with many different fuels by modification of existing furnaces or by new design so as to adapt the furnace for conditions for the new pro¬ cedure. It should be no difficulty for the artisan knowing the spirit of this invention and the state of art in combustion technology to design, manufacture and use furnaces for the new procedure so as to be able to obtain the great advantages with the invention in parti¬ cular complete combustion of solid fuels in an environ- mentally acceptable manner. , .