The invention concerns a method as defined in the introduction of claim 1 and a furnace as defined in the introduction of claim 7 for the incineration of solid fuel.

The method and furnace according to the invention are suited to many applications, such as for use as a normal heating furnace and incinerator, which is heated with wood or other conventional solid fuels. However, the method and furnace according to the invention are especially well suited to the incineration of all types of waste such as solid municipal waste. This is enabled by the structure of the furnace and by the high temperature achieved with the method, where no polluting flue gases are released to the environment.

Municipal waste is conventionally taken for example to landfills, but as landfills are becoming full and as the volume of poorly disintegrating plastic-based waste is increasing, it has been necessary to develop new methods for disposing of waste. One way to dispose of waste is incineration. In prior art solutions, the methods used for this have comprised fixed bed combustion or incineration in a fluidized bed boiler.

However, the major problem in fixed bed combustion is that the combustion residues released from the process create adverse environmental impacts. The incineration process results in, inter alia, much ash, which is only partially burned waste and contains considerable amounts of for example heavy metals and very toxic organic compounds, such as dioxins and poly aromatic hydrocarbons. Incineration of waste in a fluidized bed boiler is more environmentally friendly than fixed bed combustion, but the problem is that only about 50 to 70% of waste is suited to incineration in a fluidized bed boiler. This is why non-combustible waste must be separated from combustible waste and disposed of in some other way.

Waste may be burned either on its own or as a mixed fuel with for example coal, peat or wood. The latter case is referred to as co-incineration. However, the use of waste as a fuel imposes specific requirements on incineration as compared to the use of most other solid fuels. As an example, the composition of waste fuel may vary considerably, because the fuel may come from many different sources. Such an inhomogeneous composition requires a high incineration temperature and a high volume of combustion air in order to achieve complete combustion.

As an example, publication FR 532880 presents one known furnace for the incineration of solids, where the furnace comprises at least a chamber provided with a grate, a core provided with a jacket, and a gas space for cycling the flue gases in the furnace.

The purpose of the present invention is to eliminate the above-mentioned drawbacks and to accomplish a low-cost, reliable and energy-saving method for the incineration of solid fuel, and a furnace which is also suitable for the incineration of municipal waste. Another objective is to accomplish a method and furnace which does not create harmful emissions to the environment irrespective of which fuel is used in the furnace. The characteristics of the method according to the invention are presented in the characterizing part of claim 1. Correspondingly, the characteristics of the furnace according to the invention are presented in the characterizing part of claim 7. The characteristics of the other embodiments of the invention are presented in the other claims.

The advantage of the method and furnace according to the invention - in what follows, these are referred to jointly as the invention - is the simple and inexpensive structure of the furnace, which enables a very high temperature even in excess of 1,000 to 2,000°C in the incineration stage of flue gases. Due to the high incineration temperature and closed cycle of flue gases, the furnace can also be used for incinerating municipal waste without the furnace producing harmful emissions to the environment. Another advantage is that as a result of the high temperature reached, the furnace is very energy-efficient, and it can achieve savings of up to 80% in heating costs.

In what follows, the invention is described in more detail by means of application examples by making reference to the below simplified and schematic drawings, where: Figure 1 presents a schematic and simplified front view of one furnace according to the invention,

Figure 2 presents a schematic and simplified front view of a furnace depicted in

Figure 1, as a section along line II-II in Figure 3,

Figure 3 presents a schematic and simplified top view of a furnace depicted in

Figure 1, as a section mainly along line III-III in Figure 2,

Figure 4 presents a schematic and simplified top view of another core structure of the furnace according to the invention, as a section along line III-III in Figure 2. Figure 1 presents a schematic and simplified front view of one furnace 1 according to the invention, with the furnace 1 connected to tanks 2, which collect combustion residues, by means of connecting channels 3. Moreover, the furnace 1 has a steam exhaust channel 4, along which the water heated in the furnace is led as steam to the heat use equipment or heat recovery equipment. Furthermore, the furnace 1 comprises a door 5, through which the fuel is fed to the grate of the furnace, and a fuel feed track 5a, along which the fuel is supplied to be fed into the furnace. In order to obtain supplementary air, for example in the gasification stage, the furnace has supplementary air channels 7 which provide supplementary air for example to the space above the grate. The collection tanks 2 have a door 6 or corresponding hatch for the removal of the acidic water solution, which has accumulated on the bottom of the collection tanks 2 as a combustion product of the incineration process, and for the further recovery of various substances from the water.

Figure 2 presents a front view of a furnace depicted in Figure 1, as a section along line II-II in Figure 3. Correspondingly, Figure 3 presents a schematic and simplified top view of a furnace depicted in Figure 1, as a section mainly along line III-III in Figure 2. The upper right corner of the core 9 describes the second uppermost intact brick layer 14, while the left part describes the brick surface as a section along the line III-III. Figures 2 and 3 have been simplified, and they are not necessarily on scale.

The furnace 1 has a core 9, which is surrounded by a thermally-insulated jacket 8 so that there is a gas space 8a between the core 9 and the inner surface of the jacket 8 for the cycling of flue gases. The gas space 8a contains a group of parallel steam generation tubes 18, which contain water that vaporizes by the heat of the flue gases and the furnace, and the resulting hot steam is carried along channel 4 to the heat use equipment or heat recovery equipment, which is not depicted in the figures. The furnace 1 also comprises pumping equipment and a return channel for leading the steam, which cools in the heat use equipment or heat recovery equipment and transforms back to water, as water back to the steam generation tubes 18. The pumping equipment and return channel are not presented in the figures.

Completely or at least partly inside the core 9 is the chamber 10 of the furnace. At the bottom part of the chamber 10, there is the grate 1 1, onto which the fuel is fed. The core 9 of the furnace is built of ceramic bricks or other corresponding material, which withstands very high temperatures. The core 9 of the furnace has essentially blind side walls 12, but the top end of the core 9 is partly open. The core 9 consists mainly of a cell structure 9a, which contains a group of channels 15 in the depth direction of the chamber 10 of the furnace, channels 16 which are in cross direction to the depth direction of the chamber 10, and channels 17 which are in an upward direction in the chamber 10. The channels 15-17 have been accomplished by laying the bricks forming the core material for example so that the brick row 13 in every second layer is in cross direction to the depth direction of the chamber 10, and the brick row 14 in every second layer is in the depth direction of the chamber 10. Moreover, the brick rows 13 in the same layer are at the horizontal distance away from each other, and the brick rows 14 in the same layer are also at the horizontal distance away from each other. In this way, the core 9 contains multiple upward flow channels 17 and essentially horizontal connecting channels 15 and 16. At the same time, the quantity of the channels 15-17 allows the core 9 to have a large material surface area, past which the flue gases flow and which is therefore heated by the flue gases.

Figure 4 presents a schematic and simplified top view of another cell structure 9a of the core 9 of the furnace according to the invention, as a section at the same point along line III-III in Figure 2 as the core structure presented in Figure 3. In the core structure presented in Figure 4, the bricks in the core 9 have been laid in a way different from the structure in Figures 2 and 3. The bricks do not form uniform horizontal rows, but the bricks form uniform vertical brick columns so that in the longitudinal direction of the bricks, there is a gap between the consecutive brick columns, and the gap forms the vertical channel 17 of the core, with the vertical channel 17 extending from the chamber 10 all the way to the top edge of the core 9. The adjacent column rows are placed attached to each other and overlapping each other so that the vertical channels in one column row are closed on both sides of the row by means of the bricks in the adjacent column row, and at the edges by means of the blind side walls 12.

The top end of the core 9 is partly open and has a cell structure so that it contains a group of channels 17 which run upward from the chamber 10. At the same time, the quantity of the channels 17 allows the core 9 to have a large material surface area, which is heated by the flue gases as they flow past the material surface area.

The furnace 1 according to figures 1-3 and 4 heats up quickly and an extremely high temperature above the chamber 10 is achieved due to the large material surface area in the channels 15-17 running through the core 9 and the cell structure of the core 9 and also by the supplementary air channels 7. The structure of the furnace enables achieving a temperature in excess of +2000°C easily. These temperatures are high enough to combust all the combustible material placed on the grate 11 so that combustion is as complete as possible and so that the combustion residues are mainly acidic water 20, which is led to tanks 2 for treatment and processing. The furnace 1 has a closed flue gas circulation, which is why no flue gases are released to the atmosphere. The operating principle of the core structure presented in Figure 3 is essentially similar to the core structure presented in Figure 4. In order to achieve complete combustion, the flue gases are circulated inside the furnace as indicated by the arrows in Figure 2. From the upward flow channels 17, the hot flue gases are led to the top part of the gas space 8a and from there further downwards to the parts of the gas space 8a located on the sides of the core 9. From these parts of the gas space 8a, some of the flue gases are led into the tanks 2 and some via the channels or openings 19 located beneath the grate 11 back into the chamber 10 of the furnace for re-combustion. At this stage, it is possible to achieve a very high temperature when new oxygen-rich combustion air is supplied at the same time into the combustion chamber via the supplementary air channels 7.

It is obvious for a professional in the field that the various embodiments of the invention are not only limited to the above examples, but the embodiments can vary within the framework of the below claims. This is why the laying of the bricks in the core can differ from that presented above. It is also obvious that the grate solution and fuel feed of the furnace can be different from those presented above.

Furthermore, it is obvious that the combustion residues can be led into the collection tanks at a different point and in a manner different from that presented above.