A method for combusting multifarious waste material, an oven to be used thereby, as well as an universal waste combustion system with a number of such ovens

Abstract: The waste to be combusted is subjected to a self maintaining pyrolisis in a long, horizontal chamber oven (1) under oxygen depleted conditions, and is subsequently completely combusted under supply of adequate air. The oven is provided with a first row (22) of closable air inlet apertures at an upper level and a second row (20) of closable air inlet apertures at a lower level. A larger air inlet aperture (21) is provided in each one of the side walls in the vicinity of the oven throat (6) and air inlet apertures (19) are also provided in the off-gas conduit (15).

Description: This invention relates to a method for combusting multifarious waste material, in which the waste to be combusted is subjected to a pyrolysis in a horizontal chamber oven under oxygen depleted conditions,and is subsequently completely combusted under supply of adequate air and to an oven to be used thereby. Such a method as well as an oven for it are known from EP-A-173 628. Waste removal is one of the most serious problems of the present time. In order to be able to stand up to the continuously increasing quantities of waste materials, waste is more and more removed by means of combustion. In practice solid and liquid waste materials are mainly combusted in ovens with a sliding grid or ovens with a rotating drum. In ovens with a sliding grid waste materials are continuously supplied to a moving grid and air is blown through the burning mass by way of apertures in the grid. The temperature may increase thereby locally to over 1000°C, while elsewhere the temperature may remain below 800°C. Under such conditions much fly-ash is formed, while in the areas, where the temperature is too low, unpleasantly smelling substances will remain or be formed by incomplete combustion, which by their unpleasant smell alone will already create a burden for the environment. In an oven with a rotating drum waste materials are kept in motion by the slow rotation of the cylindrically shaped oven, under which conditions the advantage is obtained that the areas of too low a temperature, such as in an oven with a grid, may be avoided. However, the temperature for an oven with a rotating drum should not rise much above 1000°C in order to prevent, the fluid slag may deposit against the wall. Downstream of an oven with a rotating drum a chamber for after burning may be positioned, wherein the temperature is increased to for example 1150°C by additional combustion of fuel. The residence times of gases in hot areas (temperature of more than 800°C) of the ovens amount in general to 1 to 3 sec. The incomplete combustion, the relatively low temperature, the short residence times of the compounds in the hot areas, provide the conditions for the formation of many poisonous and unpleasantly smelling compounds, such as dioxins and benzofuranes. To avoid such problems and to arrive to a more complete and efficient waste combustion, it is proposed according to EP-A-173 628 to perform the combustion in two stages. In the first stage combustion takes place at low temperatures with a small quantity of air. In this so-called pyrolysis stage the waste materials are partly decomposed by thermal pyrolysis reactions. The gases and fumes developed pass through an off-gas conduit, the so-called "gaine de fumées" to a post combustion chamber where post combustion of the gases takes place. After the pyrolysis there is admitted further air in the oven chamber and the temperature is increased for complete combustion. The oven system used according to EP-A-173 628 comprises a horizontal, tube-like oven with refractory oven walls, resistent to high temperatures, which oven is provided at the front side with a charging opening, closable with an oven door and at the rear side with an oven throat, emerging into a horizontal off-gas conduit, the diameter of which is small in comparison to that of the oven, whereby an air inlet aperture is provided in each one of the side walls in the vicinity of the oven throat, and whereby air inlet apertures are also provided in the off-gas conduit. The combustion chamber of said oven includes a burner, necessary to ignite and sustain the pyrolysis combustion. Because the pyrolysis at such low temperatures is rather incomplete, lots of undecomposed gases will be present in the exhaust gases from said combustion chamber. Therefore the exhaust gases are passed through the horizontal off-gas conduit into a post combustion chamber where a necessary post combustion will take place. Said post combustion chamber is therefore provided with air inlet channels and a second burner. The method and system according to EP-A-173 628 has the disadvantage that no complete pyrolysis can be obtained. Because of the low temperatures used there will be lots of contaminating gases and also solids (fly ash) present in the exhaust fumes from the combustion chamber, whereas the countercurrent of air introduced through the inclined air inlet channels in the off-gas conduit, at least in the pyrolysis stage of the combustion, is not sufficient for stopping all such solid matter. Anyhow, it cannot be avoided that both by fumes and solid particles contamination of the surroundings is completely avoided. Further all thermal energy of the combustion in the post combustion chamber is lost and not used for the pyrolysis process. It is now the object of the present invention to provide an improved method for two stage combusting of multifarious waste in which the disadvantages of EP-A-173 628 are eliminated, and which makes it possible to work in an economical way without unnecessary loss of useful thermal energy. Thereto the present invention provides a method as defined in the preamble, characterized in that in an elongate horizontal chamber oven and startingfrom its rear end a self-sustaining pyrolysis is performed at temperatures of 1100-1450°C, that the emerging hot pyrolysis gases are combusted at the rear end of the chamber oven and in an off-gas conduit connected therewith that optionally catalytically active substances are added to the hot pyrolysis gas flow in the off-gas conduit, and that the flue gas emerging from the off-gas conduit is rapidly cooled and purified by contacting with a gas washing liquid. According to the invention the pyrolysis is performed at temperatures of 1100-1450°C under which conditions an efficient and complete pyrolytic decomposition occurs whereby organic materials are decomposed into carbon and simple gases such as carbon monoxides and hydrogen, while inorganic materials are decomposed into simple oxides. According to the invention the pyrolysis is in an efficient way made self-sustaining because the hot combusting pyrolysis gases at the rear end (the throat) of the chamber oven and in the adjacent part of the off-gas conduit provide the heat for sustaining the pyrolysis in the remaining part of the elongate combustion chamber oven. The invention further provides a chamber oven to be used in the two stage pyrolysis combustion. Such chamber oven comprising a horizontal, tube-like oven with refractory oven walls, resistent to high temperatures, which oven is provided at the front side with a charging opening, closable with an oven door and at the rear side with an oven throat, emerging into a horizontal off-gas conduit, the diameter of which is small in comparison to that of the oven, whereby an air inlet aperture is provided in each one of the side walls in the vicinity of the oven throat, and whereby air inlet apertures are also provided in the off-gas conduit, is characterized in that the long side walls of the very elongate oven are each one provided with an upper row of closable air inlet apertures at the upper side, divided over the wall length, and a lower row of closable air inlet apertures at the lower side, all apertures divided over the wall length, that the off-gas conduit near the oven throat turns off square from the longitudinal direction of the oven, and an injection aperture is provided into the wall of the off-gas conduit at this turn-off, directed along the longitudinal axis of the turned off off-gas conduit, and that at least the upper wall of the oven has a concavely domed shape in order to reflect the heat radiation of the pyrolysis process in focus. By the provision of the rows of air inlet apertures in the side walls of the oven an efficient oven control is possible and the pyrolysis process can be adjusted and controlled in a desired manner. Efficiently the processing may be such thereby that the off-gas conduit near the oven throat turns off square from the longitudinal direction of the oven, and an injection aperture is provided into the wall of the off-gas conduit at this turn-off, directed along the longitudinal axis of the turned off off-gas conduit. By means of a suitable spray device catalyzing liquids, gases and air may be injected throught this injection aperture into the off-gas conduit. In addition a pilot-burner and/or support burner may be mounted there, in order to be able to adjust the combustion of the off-gas in a desired manner. According to a particularly advantageous embodiment at least the upper wall of the oven has a concavely domed shape, in order to reflect emanated heat of the pyrolysis process in focus. In practice it is customary, that the oven, once filled with the waste to be pyrolysed, is ignited from the rear side, that is near the oven throat. To that end care has been taken, that specifically in the rear section of the oven a properly combustible charge, for example paper, celluloid, etc. is present. The emanated heat, caused by this intense combustion, occurring under heat supply, reflects by way of the domed wall towards the inside of the oven, and heats the material present there. By adjusting the air inlets in the oven it may be arranged that hereby the complete oven is gradually heated to pyrolysis temperature, whereby the various air inlets during the pyrolysis are blocked, in order to maintain an oxygen depleted atmosphere. In order to achieve heating of the oven as efficiently as possible, the upper wall of the oven is preferably completely or partially covered with heat-isolating layer of for example clay at the exterior. In addition the oven may further be provided with a concrete cover plate, comprising a weakening for the eventuality of gas explosions. Such gas explosions might occur if for example especially in the starting period of the oven, there is still too much air in the waste material positioned in the oven, whereby locally an sudden fierce combustion might occur. For the chamber oven according to the invention, a closable vent hole is provided in the oven door, through which the waste to be treated will be charged. This vent hole is blocked during pyrolysis, but is opened during the subsequent combustion of the pyrolised material, in order to achieve an additional air draught theretrough. Finally the invention provides a universal waste combustion system, consisting of one or a number of pyrolysis combustion units, each one consisting of three chamber ovens as described above, a central flue gas chamber, with which the off-gas conduits of the chamber ovens are connected, and a gas washing reactor connected with the flue gas chamber, for primary flue gas purification, and a central gas washing column with a number of superimposed washing steps, the gas washing reactors of the pyrolysis combustion units being connected in combination with said central gas washing column. For practical reasons the central flue chambers are thereby preferably each one provided with an emergency chimney. Such a system is efficiently adjusted to the fact, that there are factually three phases in each oven cycle, that is pyrolysis, ash combustion and annealing and cooling down. Each one of these phases has a duration of one or a plurality of days, so that efficiently in the oven unit the first oven ccan be pyrolysed, while in the second oven, where pyrolisation has already taken place, ash combustion occurs, while the third oven is in its annealing phase. In this manner a substantially continuous operation will be possible for such a unit. By using a plurality, for example two, of such units, waste types of varying qualifications may moreover be treated simultaneously. The off-gas, emerging from the flue gas chamber, is subsequently purified in the central gas washing column in the manner as described in the earlier mentioned Dutch Patent Application No.8902490. The invention will be further elucidated in the following with reference to the drawings. In the drawings: Fig.1 shows an embodiment of a chamber oven for pyrolysis combustion according to the invention in horizontal cross-section; Fig.2 a vertical cross-section of the oven of Fig.1, and Fig.3 a diagrammatical view of a universal waste combustion system according to the invention, whereby a number of such ovens are used, as well as a central gas washing system. An embodiment of a chamber oven according to the invention is shown in Figs.1 and 2 in respectively horizontal and vertical cross-section. The horizontal oblong oven has a long oven chamber 1 wirth side walls 2, a bottom 3, a roof or upper wall 4 and a rear wall 5, which consist of high quality refractory material, resistent to high temperatures of over 1450°C. The oven roof 4 is made concavely domed while the side walls 2 at the inner side are also slightly concavely rounded out. This concave shape is intended to reflect heat emanated during the pyrolysis, toward the interior of the oven. The rear wall 5 and the oven throat 6 als show a concave vaulting. The side walls 2 of the oven are supported by steel beams 7, which are held together by tension bars 8. The oven roof 4 is made thinner than the oven side walls 2 and is covered with a layer of clay 11, leaving the centre part uncovered. Above the clay and the oven roof is a concrete cover plate 9, having in the centre a conical, removable weakening part 10. This so-called gas roof provides a protection for the eventuality of explosions. The oven is fitted into a concrete bedding 12, whereby clay 13 is also applied between the bottom of the oven and the bedding. The layers of clay 11 and 13 both act as heat-isolation, in order to avoid heat of the oven from getting lost to the exterior. At the front the oven has a charging opening, closed off by an oven door 14. The waste to be combusted is charged into the oven through this charging opening. At the rear of the oven near the throat 6 is a off-gas conduit 15 at the right side above, that in the embodiment shown comprises two square turn-offs, the first one of which near the oven throat. At this section the wall 16 of the conduit is square to the walls 17 of the conduit and forms as it were a type of "bottom" of the off-gas conduit 15. In this "bottom" 16 an injection opening 18 is present in the centre, which serves for injecting catalysing liquids, air, and passage of the pilot flame and pilot burner (not shown). In the walls 16 and 17 of the conduit regularly distributed air inlet apertures 19 are present. The oven itself is also provided with air inlet openings. Thus a row of air inlet openings 20, regularly distributed over the length of the oven, is present in each one of the long side walls 2 downwards near the bottom 3. A larger air inlet 21 is provided in each wall 2 adjacent ot the oven throat 6. At the upper side of both side walls are rows of small air inlets 22, which act in particular for controlling the pyrolysis process. In the oven door 14 is finally also an air inlet 23, which acts as vent hole during the combustion after the pyrolysis and as injection opening for liquids to be combusted. Now the functioning of this oven will be described. Into the very large oven, for example with a length of 16 meters and a height of 4 meters, a charge of the waste to be combusted is introduced at the front. An oven charge may consist of a mixture of numerous materials, both combustible as well as non-combustible, in more or less finely divided form, as well as coarse, such as for example barrels, may further comprise liquids, slurries, shredder, soil, etc. A condition is, that there should be an average energy-content of at least 7 MJ/kg, in order to function in a profitable manner. On charging, care is taken, that at the rear side near the oven throat 6 sufficient properly combustible material is present, for example photographis film, paper, waste wood and the like. Prior to igniting the oven, the off-gas conduit is first heated. This is done by injecting combustible gas or liquid by way of the injection opening and to ignite this by means of the support burner or pilot burner.Owing to the many air inlets a proper combustion occurs in the off-gas conduit, whereby after a short time a sufficiently high combustion temperature in the off-gas conduit is achieved. Subsequently the combustible material present at the rear side of the oven is ignitedd by means of a fuse or plug by way of one of the air inlets 19. As a consequence of the ample air supply by way of the air inlets 19 and the proper vent conditions in the heated off-gas conduit a fast, complete combustion occurs here, the radiation of which is reflected against the domed oven walls, inwards and forwards in the oven. Hereby a gradual heating of further advanced and lower located material occurs. During this process the greater part of the air inlet apertures, as well as the vent hole in the oven door, are closed down, for example with plugs of glass-wool, so that little oxygen may enter into the oven proper. Hereby no normal; combustion will occur in the oven, but a pyrolysis, which is maintained by the combustion at the rear side of the oven. Adjustment of the required heat and the speed of thepyrolysis process occurs by opening or closing of the small air inlet apertures 22 at the upper side of the side walls. In particular at the start of the process, whereby some air is present in the charged waste, explosive reactions may occur, which in general however may be adjusted. If explosive conditions would occur, the protection of the light gas roofing warrants in a customary manner, that the explosion may be deviated through the roof of the oven. It is essentiel for the oven according to the invention that the off-gas conduit, through which the off-gases of the pyrolysis are passed and combusted, has a relatively small diameter in comparison with the diameter of the oven chamber. It is hereby achieved, that the hot gas will stay for a long period of time in the oven and will contribute to the maintenance of the pyrolysis. The considerable advantage of the pyrolysis treatment is, that thereby a substantially complete decomposition will occur of the waste to be processed, whereby organic compounds will be decomposed substantially into carbon , carbon monoxide and hydrogen, while inorganic materials are converted into oxides, which however in contrast to normal combustion will give rise to little slag formation. The slagstill formed can to be removed easily by distributing a layer of sand covered with with a thin layer of paper cuttings, on the refractory bottom 3 of the oven at the start of the process. During the pyrolysis process this paper layer will carbonize and the possible slag will deposit onto this carbon layer, and may be removed later on with ease. The chamber oven according to the invention operates particularly efficient. The large space in the chamber oven and the very low gas velocities in this area cause a long residence time of hte gases to 30 sec. Thus a high temperature of 1250°C may be achieved and maintained in this chamber oven, and the influx of oxygen at numerous places through the inlets, causing a temporary excess of oxygen on leaving the chamber oven will cause a complete decomposition of larger organic molecules. A complete combustion is ultimately achieved in the narrow off-gas conduit, where by means of the spacious injection opening 18, as well as the air inlets in the continuation of the off-gas conduit is seen to adequate oxygen for complete combustion. It is thereby of importance that in particular at the start of the off-gas conduit a very high temperature is maintained. In addition, as a consequence of the absence of turbulences, very little fly ash is formed in the pyrolising waste, a matter causing a considerable problem in the normal combustion by means of grid- and rotation ovens. Once the pyrolysis reactions have been completed, carbon rich matter remains in the oven. Now the next phase is started, the combustion, to which end the air inlets 21 at the lower side of the side walls of the oven are opened and air is gradually introduced. From this moment onwards liquids may also be injected for combustion by way of the injection opening 23 in the oven door. The air supply initially takes place by normal supply from the atmosphere; for an intensive after burning one may ultimately resort to supply of pressurized air by means of a source for pressurized air 24. Under these conditions a complete combustion or roasting of the residual substances is achieved. During this latter phase the vent hole-injection opening 23 in the oven door 14 is also fully opened. Finally the oven extinguishes, whererafter the front side of the oven is opened, and after the ash has been moisted with water, it is removed together with possible slags. The total process requires three to eight days. During the pyrolytic phase it may be necessary to add auxiliary materials, which act catalytically, by means of injection opening 18 during the combustion of the off-gases in the off-gas conduit. If for example an excess of nitrogen oxides is present in the off-gas, a solution of ammonia may for example be injected, whereby ammonium nitrate is formed, which decomposes into nitrogen, water and oxygen. In the case of a too high CO-content solutions comprising chromiun and copper ions for example, may be injected. These enter into the gas flow and will be converted into copper oxide and chromium oxide, which are efficient catalysts for the conversion of CO with abundantly present oxygen into carbon dioxide. Otherwise the flue gas formed in the invention will in general not be discharged directly, but will be supplied to a suitable gas washing installation for further purification. In an oven as described above, having dimensions of 16 meters long and 4 meters high, the oven volume is such, that it may comprise a charge from 10 to 60 tons. This charge may consist of combustible = energy rich and incombustible = energy poor material. The total should provide so much energy on complete combustion, that the required temperature of well above 1000°C is achieved and maintained over a number of days. To that end it is required that the oven charge has an average energy content of at least 7 MJ/kg. This latter value determines the ratio between combistible and incombustible waste material in the oven charge. In the following an example is given of an oven charge suitable thereby: The estimated average energy content of this charge is ± 12 MJ/kg. During the combustion phase ± 10 tons of aqueous liquid, for example residual liquid of developer, may be injected into the oven and be combusted. If these 10 tons of waste are added to the total oven charge, the average energy content over the 56 tons of waste amounts to ± 10 MJ/kg. In Fig.3 a universal waste combustion system according to the invention is shown diagrammatically, whereby efficiently a number of the above described ovens are applied. This system comprises two pyrolysis combustion units, each one of three pyrolysis combustion ovens according to the invention, designated with A. These ovens, each of 50 tons, are in each unit connected with a common flue gas chamber B, provided with an emergency chimney. The flue-gas chamber B of each combustion unit is connected with a primary gas washing reactor C, and all primary gas washing reactors C are in turn connected with a common gas washing column D, wherein a number of gas washing steps are positioned superimposed to each other Thereby the system, consisting of the primary gas washing reactors C and the central gas washing column D, corresponds with the system described in the earlier mentioned Dutch Patent Application No.8902490, aimed at the purification of flue gases. Just like in that case a residual liquid from the photographic industry or something similar is used as gas washing liquid. The substantial advantage of the pyrolysis combustion units, consisting of three parallel positioned chamber ovens A resides in that the action of the pyrolysis combustion according to the invention has three phases, to wit: A) the pyrolysis phase, which takes 1 to 3 days, B) the ash combustion phase, which equally takes 1 to 3 days, and C) the annealing and cooling down phase, which takes 1 to 2 days. By three parallel positioned ovens of such a pyrolysis unit initially the first oven is charged and ignited. After three days the pyrolysis in this first oven is completed and there a start is made with the ash combustion phase, while in the second oven the pyrolysis is started. After 5 or 6 days the third oven is ignited for the pyrolysis, while the first oven is then in the cooling down phase and the second oven starts with the ash combustion phase. This cycle can be continued, so that waste may be processed uninterruptedly. The substantial advantage of such a way of processing is that on the one hand continued working may be performed on a semi-continuous basis, while on the other hand also various types of waste may be processed in charges, whereby also the destination and further treatment or recycling of the ash may be chosen. The treatment of the gases by means of the central gas washing column may proceed continuously. In the above the invention has been elucidated with reference to more or less specific examples of a pyrolysis oven and a universal combustion system with two combustion units, each one with three such pyrolysis ovens. The ovens may be modified with respect to shape and with respect to positioning. For a universal combustion system more combustion units than the two according to the example, may be used in addition. Thus for example four of such systems may be centrally connected with a gas washing system. An efficient positioning is for example one, whereby four combustion units, each one with a primary gas washing reactor, are connected with a central washing tower with a number of gas washing stages.

Claims: A method for combusting multifarious waste material, in which the waste to be combusted is subjected to a pyrolysis in a horizontal chamber oven under oxygen depleted conditions, and is subsequently completely combusted under supply of adequate air, characterized in that the pyrolysis is performed at temperatures of 1100-1450°C, that the emerging hot pyrolysis gases are combusted at the rear end of the chamber oven and in an off-gas conduit connected therewith under supply of adequate air or oxygen, that catalytically active substances are added to the hot pyrolysis gas flow in the off-gas conduit, and that the flue gas emerging from the off-gas conduit is rapidly cooled and purified by contacting with a gas washing liquid. A method according to claim 1, characterized in that the gas washing liquid is chosen from residual liquids giving a high chemical oxygen consumption, comprising complexing agents, compounds of heavy metals, sulphur and nitrogen compounds, originating from fixing baths and the like from the photographic, photochemical and galvanic industries. A method according to claim 1 or 2, characterized in that the chamber furnace is charged with waste, sorted in such a manner that the average energy content of the oven amounts to at least 7 MJ/kg. A chamber oven for use in the method according to claims 1 - 3, comprising a horizontal, tube-like oven with refractory oven walls, resistent to high temperatures, which oven is provided at the front side with a charging opening, closable with an oven door and at the rear side with an oven throat, emerging into a horizontal off-gas conduit, the diameter of which is small in comparison to that of the oven, whereby an air inlet aperture is provided in each one of the side walls in the vicinity of the oven throat, and whereby air inlet apertures are also provided in the off-gas conduit, characterized in that the long side walls of the very elongate oven are each one provided with an upper row of closable air inlet apertures at the upper side, and a lower row of closable air inlet apertures at the lower side, all apertures divided over the wall length, that the off-gas conduit near the oven throat turns off square from the longitudinal direction of the oven, and an injection aperture is provided into the wall of the off-gas conduit at this turn-off, directed along the longitudinal axis of the turned off off-gas conduit, and that at least the upper wall of the oven has a concavely domed shape in order to reflect the heat radiation of the pyrolysis process in focus. A chamber oven according to claim 4, characterized in that the bottom of the oven is concavely domed. A chamber oven according to any of the claims 4 - 5, characterized in that the upper wall of the oven is completely or partially covered with a heat isolating layer of clay at the exterior, and that the oven is covered with a concrete cover plate, comprising a weakening for the eventuality of gas explosions. A chamber oven according to one of the claims 4 - 6, characterized in that a heat isolating layer of clay is present between the bottom of the oven and the bedding of the oven. A chamber oven according to any one of the claims 4 - 7, characterized in that the oven door is provided with a closable vent hole injection opening. A universal waste combustion system, characterized in that this system comprises one or a number of pyrolysis combustion units, each one consisting of three chamber ovens according to claims 4 - 8, a central flue gas chamber, with which the off-gas conduits of the chamber ovens are connected, and a gas washing reactor connected with the flue gas chamber, for primary flue gas purification, and a central gas washing column with a number of superimposed washing steps, the gas washing reactors of the pyrolysis combustion units being connected in combination with said central gas washing column.

Inventors: NEVELS LEONARDUS MATHIJS MARIE, NEVELS, LEONARDUS MATHIJS MARIE
Applicants: NEVELS LEONARDUS MATHIJS M, NEVELS, LEONARDUS MATHIJS MARIE
Application year: 1990
Main classification: F23G5
IPC codes: F23J15, F23G5, F23M5