Combustion device and method of operating the same

Abstract: The invention relates to a method for cleaning products created during a combustion process, by admixing air to the combustion products and removing by after-burning remnants of harmful substances still present in the combustion products. The air can herein be heated prior to admixing thereof to the combustion products. The air can also be admixed in counterflow to the combustion products. The invention further relates to a combustion device (1) provided with a combustion chamber (4), an opening (3) connected to the combustion chamber (4) for drawing in ambient air (P1) and an opening (5) connected to the combustion chamber (4) for discharging combustion products (E) to the environment. The device further has an installation for cleaning the combustion products (E) connected thereto, which cleaning installation has means arranged between the combustion chamber (4) and the discharge opening (5) for admixing air (S6) to the combustion products for the purpose of after-burning these products. Finally, the invention relates to a cleaning installation for use in such a combustion device.

Description: The invention relates to a combustion device including an installation for cleaning products created during a combustion process as described in the preamble of claim 1, and to a method for operating this device as described in the preamble of claim 10. Such a combustion device and method of operation are known from US-A-5 263 471. It is meanwhile generally known that the gases released in a combustion process are for a significant part harmful to people and the environment. In combustion processes wherein solid fuels such as wood are used, the gaseous combustion products consist particularly of carbon dioxide (CO 2 ), carbon monoxide (CO), unburnt hydrocarbons (C x H y ) and nitrogen oxides (NO x ). In addition, solid combustion products are formed such as ash and residues which are carried along with the flue gas flow into the atmosphere. Of the stated products, attention is currently focussed on the unburnt hydrocarbons, particularly the polycyclic aromatic hydrocarbons (PAH). Different methods have been tested to reduce the emission of harmful combustion products. A possible solution is to increase the combustion temperature, whereby a more complete combustion is achieved. The efficiency of the combustion process is however adversely affected hereby as a result of greater losses in the discharge. Another applied cleaning method makes use of filtering of the combustion gases before they are blown into the atmosphere. This however results in an increase in the resistance in the discharge, which has an adverse effect on the combustion. In the above-identified prior art document US-A-5 263 471 an installation is disclosed whereby the emission of harmful gases by combustion devices, including wood-burning stoves, is further limited in comparison to the above methods. This prior art cleaning installation can be applied in a combustion device provided with at least one combustion chamber, at least one opening connected to the combustion chamber for drawing in ambient air and at least one opening connected to the combustion chamber for discharging combustion products to the environment. It has means arranged between the combustion chamber and the discharge opening for admixing air to the combustion products for the purpose of after-burning the products. By this arrangement secondary air may be guided along the rear wall of the combustion chamber and preheated. This preheated secondary air may then be introduced into the combustion chamber from above thus after-burning remnants of harmful substances still present in the combustion products. The secondary air and the combustion products are thus in counterflow. Because in this installation the harmful gases are burnt and converted into harmless compositions, very high combustion temperatures or costly after-treatments of the combustion gases, such as filtering thereof, can be dispensed with. The combustion process is moreover enhanced by after-burning as opposed to filtering, so that the efficiency of the process increases. Because the combustion temperature is not increased no great discharge losses occur. Furthermore, from US-A-4 665 889 a wood burning stove is known having several features for reducing emissions output and improving energy efficiency. This stove has an air inlet from where the air is channeled through preheat ducts. A small amount of incoming air is channeled directly into a combustion zone through a startup duct. As the temperature in the combustion zone increases most of the incoming air is channeled naturally to the preheat ducts. Part of the air flowing through these preheat ducts is then channeled into the combustion zone to provide fresh oxygen. The air flow in this prior art combustion device is thus determined by the temperature only, without any control means being present. The present invention now has for its object to provide a cleaning installation of the type described above having improved controllability and a greater effectiveness. According to the invention this is achieved by controllable means for closing the air feed opening, said closing means comprising a valve actuated by a drive under control of a controller for opening only at the beginning and the end of the combustion process. Preferred embodiments of the combustion device according to the invention form the subject-matter of the dependent claims 2-8. The invention also relates to a method of operating the combustion device described above for cleaning products created during a combustion process. In accordance with the invention such a method, which comprises the steps of admixing air to the combustion products and removing by after-burning remnants of harmful substances still present in the combustion products as disclosed in US-A-5 263 471, is characterized in that the combustion process, after start-up thereof, is sustained substantially only with after-burning air. Preferably applied variants of the method according to the invention are described in the dependent claims 11-13. The invention is now elucidated with reference to two embodiments, wherein reference is made to the annexed drawing, in which: Fig. 1 shows a partly cut away perspective view of a combustion device not forming part of the invention; and Fig. 2 shows a partly cut away perspective view of a combustion device in accordance with the invention. The combustion process of a solid fuel such as wood comprises four phases. At a relatively low temperature of about 100°C the wood begins to dry through evaporation of the moisture still present therein. At higher temperatures of about 150 to 350°C degassing or pyrolysis of the wood then occurs. The chemical structure of the wood is herein broken down with formation of volatile compounds such as CO, H 2 O, CH 4 and lower hydrocarbons as well as tar-like components which are volatile at decomposition temperature but which condense at lower temperature. When the temperature is increased still further to about 550°C and oxygen is added the volatile compounds resulting from the pyrolysis burn. The remaining solid component consists of almost pure carbon and burns at a temperature of roughly 800°C when oxygen is added. A combustion device 1 (fig. 1) comprises a housing 2 in which is defined a combustion chamber 4. The latter is joined to the environment by an opening 3 for feed of ambient air P1 and an opening 5 for discharge of the combustion products E into the environment. Formed between the air feed opening 3 and the combustion chamber 4 is an air feed duct through which the ambient air or primary air for combustion can reach the combustion chamber 4 as according to arrows P1, P2 and P3 through openings 16 in a removable ash-pan 25 arranged under combustion chamber 4 and through a pipe heat exchanger 13 and a grid 17 arranged above the ash-pan 25. Fuel can further be added to the combustion chamber 4, for instance in the form of logs. It is of course also possible for another fuel, for instance gas, to be fed to the combustion chamber 4. On its side facing the room area for heating the combustion chamber 4 is closed off by one or more window panels 6. Heating of the room area therein takes place by means of heat radiation through the window panel(s) 6. In order to increase the heat transfer to the area for heating the housing 2 of the combustion device 1 takes a double-walled form, wherein between an outer wall 7 and a middle wall 8 of housing 2 a duct 9 is defined through which so-called secondary air can flow as according to arrows S1, S2 and S3. This secondary air is drawn in through an aperture 10, wherein suction can take place in natural manner or can be supported by a fan 11. The sucked in air is guided round through the duct 9 and along the top of the combustion chamber 4 and the outlet duct 5 and thereby heated, and flows finally through a number of outflow openings 12 into the room area for heating, where the secondary air functions as source of convection heating. The generated heat can be adjusted by controlling the feed of primary air to the combustion process. For this purpose the through-flow openings 15 in the ash-pan 25 are provided with slidable valves 16. The main feed opening 3 is also provided with a closing valve 26 which is opened by the air flow and closed by the force of gravity. In order to clean the gases created in chamber 4 during combustion a cleaning installation is connected to the combustion device 1. This cleaning installation comprises a feed duct 18 which is formed by the rear wall 19 of the combustion chamber 4 and the middle wall 8 of housing 2 and which runs out into a part 20 which is arranged overhanging the combustion chamber 4 and which is provided with an outflow grid 21. The feed duct 18 is connected by means of a number of tubes 14 of the heat exchanger 13 to the suction opening 10 for secondary air. A portion of the secondary air drawn in by the fan 11 is therefore guided as according to arrow S4 through the lower tubes 14 of heat exchanger 13 in the direction of the front part of combustion device 1 and then returns as according to arrow S5 through the upper tubes 14 of heat exchanger 13 to the feed duct 18 in order finally to be admixed from the top as according to arrow S6 in counterflow to the combustion products which under the influence of the heat in the combustion chamber 4 rise in the direction of discharge opening 5. The admixed air brings about after-burning of the products present in combustion chamber 4, whereby remnants of harmful substances which may be present therein are burnt and converted into less harmful compounds. The thus cleaned combustion gases finally leave combustion chamber 4 along a path defined by a cover plate 22 and flow into the environment through discharge duct 5. As a result of the after-burning of the combustion products these latter are not only cleaned, but the energy efficiency of the combustion device also increases in comparison with the situation where no after-burning takes place. This is because fuel particles burnt during the normal combustion process are also converted into heat during the after-burning. The heater with after-burner can therefore be operated with less fuel than conventional heaters, whereby the emission of harmful substances is reduced still further. In order to prevent the window panel(s) 6 of combustion device 1 being excessively dirtied by soot particles released during combustion, the combustion device is further provided with air feed channels 23, 24 arranged respectively on the upper side and underside of the window panel(s) 6. A part of the drawn-in secondary air is hereby guided along parallel to the window panel(s) 6 as according to arrows S7 respectively S8, whereby as it were a "curtain" of clean air is formed which prevents deposits on the window panel(s) 6, as described in the Netherlands publication 9000658. In the combustion device 1 according to the invention (fig. 2) the main feed opening 3 is provided with a valve 26' which can be opened and closed by means of a drive 27 under control of a controller 34. The main feed opening 3 is further connected over a duct 35 to the space 36 under the actual burning location 37. In duct 35 is further arranged a fan 38, the operation of which is likewise controlled by the controller 34. The feed of primary combustion air to combustion chamber 4 can thus be adapted to the progress of the combustion process. This is detected by sensors 28 arranged in combustion chamber 4. The valve 26' is only opened by the controller when a fresh quantity of fuel is placed in combustion chamber 4, in order to initiate the combustion process and to compensate the oxygen deficit which occurs as a result of a rapid degassing in the initial phase of the combustion process. Herein the fan 38 is then also switched on as required. At the end of the combustion process extra oxygen is again required for good combustion of the remaining solid carbon. During the combustion process, however, valve 26' is closed and fan 38 switched off. The combustion process is then sustained by the secondary air which flows into the combustion chamber as according to arrows S6 and S9. The secondary air is not guided through beneath the burning location through a tube heat exchanger but carried round the burning location 37 via a bypass duct 39 as indicated by arrows S4 and S5, which burning location consists of a number of solid rods. In order to nevertheless optimally preheat this secondary air an additional heat exchanger 29 is arranged in the bypass duct 18 running behind the combustion chamber 4. This heat exchanger is formed by a number of parallel heating ribs 30 protruding into duct 18 and connected to a plate 31 let into the rear wall 19 of combustion chamber 4. Plate 31 and ribs 30 are herein preferably formed integrally, for example as casting of aluminium or another metal which conducts well. In the second embodiment the part 20' of the cleaning installation protruding above combustion chamber 4 extends further than in the first embodiment. The combustion gases must hereby cover a longer path from the combustion chamber 4 to the discharge opening 5, whereby a better mixing and after-burning is obtained. The outflow apertures 21' are moreover further removed from the rear wall 19 than in the first embodiment. Thus achieved is that the secondary air flow S6 is directed practically perpendicularly of the flow of combustion gases, which enhances after-burning. This is further enhanced in that the angle between the protruding part 20 and the rear wall 19 is almost a right angle (in the embodiment shown 95°). Additionally arranged in the front wall 41 of the protruding part 20' are extra outflow apertures 42 through which the secondary air flow S9 likewise exits in perpendicular direction of the flue gas flow. In order to prolong the time the combustion gases stay in the device 1 and thereby improve efficiency, a further C-shaped baffle 32 is arranged in front of the discharge opening 5 above the protruding part 20 of the cleaning installation. This causes additional whirls in the outflowing combustion gases. In order to improve the heat transfer from the combustion gases to the secondary air S3 flowing to the space for heating, a wall 33 between the secondary air duct 8' and the upper part of the combustion chamber 4 takes a corrugated form. This also prolongs the time the combustion gases stay at the top of the combustion chamber 4. The intensity of the combustion process can be controlled not only by regulating the feed of primary and secondary air but also by regulating the discharge of the combustion gases, or the "draught". Arranged for this purpose in the discharge duct 5 is a displaceable closing slide 43 which can be operated via a control rod 44 protruding at the front from the combustion device 1. The controller 34 can also be connected to a control panel 45 arranged on the outside of combustion device 1 for entering a control program or manual entering of operating commands.

Claims: Combustion device (1) provided with at least one combustion chamber (4), at least one opening (3) connected to the combustion chamber (4) for drawing in ambient air (P1), at least one opening (5) connected to the combustion chamber (4) for discharging combustion products (E) to the environment, and at least one installation for cleaning the combustion products (E) connected to the combustion device (1), which cleaning installation has means arranged between the combustion chamber (4) and the discharge opening (5) for admixing air (S6) to the combustion products for the purpose of after-burning these products, characterized by controllable means (26') for closing the ambient air feed opening (3), said closing means comprising a valve (26') actuated by a drive (27) under control of a controller (34) for opening only at the beginning and the end of the combustion process. Combustion device (1) as claimed in claim 1, characterized in that the cleaning installation has means for heating the air for admixing to the combustion products. Combustion device (1) as claimed in claim 2, characterized in that the heating means comprise at least one heat exchanger (13;29) connected for heat-transfer to the combustion chamber (4). Combustion device (1) as claimed in any of the preceding claims, characterized in that the mixing means are adapted for admixing the air in counterflow to the combustion products. Combustion device (1) as claimed in claim 4, characterized in that the mixing means comprise at least one air feed duct (18) debouching into the upper part of the combustion chamber (4). Combustion device (1) as claimed in claim 5, characterized in that the cleaning installation comprises means (11) for urging the air for admixing through the feed duct (18) to the combustion chamber (4). Combustion device (1) as claimed in any of the preceding claims, characterized by means arranged in the combustion chamber (4) in the vicinity of the discharge opening (5) for prolonging the time combustion gases stay therein. Combustion device (1) as claimed in claim 9, characterized in that the means for prolonging the time of stay comprise at least one substantially C-shaped baffle (32) partially covering the discharge opening (5). Method of operating the combustion device as claimed in claim 1 for cleaning products created during a combustion process comprising admixing air to 0 the combustion products and removing by after-burning remnants of harmful substances still present in the combustion products, characterized in that the combustion process, after start-up thereof, is sustained only with after-burning air. Method as claimed in claim 9, characterized in that the air is heated prior to admixing thereof to the combustion products. Method as claimed in claim 10, characterized in that the air is heated by heat released 0 during the combustion process. Method as claimed in any of claims 9-11, characterized in that the air is admixed in counterflow to the combustion products.

Inventors: VAN GORP CORNELIS FRANCISCUS J, VAN GORP, CORNELIS FRANCISCUS JOSEPHUS MARIA
Applicants: GORP CORNELIS FRANCISCUS JOSEP, VAN GORP, CORNELIS FRANCISCUS JOSEPHUS MARIA
Application year: 1995
Main classification: F23G7
IPC codes: F23G7, F23L9