METHOD AND APPARATUS FOR HEATING A ROTARY KILN DESIGNED FOR GASIFICATION AND PYROLYSIS OF ORGANIC MATERIAL

Abstract: Method for heating a rotary kiln (1) designed for gasification and pyrolysis of organic material. The energy for heating the rotary kiln (1) is supplied by means of a radiation heat exchanger (2), positioned inside and longitudinally in the rotary kiln (1), and the energy supply to the radiation heat exchanger (2) is provided in a controlled manner over the length thereof, in order to achieve a temperature distribution inside the rotary kiln (1), which is optimised for the desired gasification and pyrolysis. The energy supply to the radiation heat exchanger (2) is provided by controlled combustion of a combustible gas, such as the pyrolytic gas inside the radiation heat exchanger (2). A surprising advantage is achieved by controlled partial combustion of the pyrolytic gas (8) inside the radiation heat exchanger (2), the most condensable part of the pyrolytic gas (8) hereby being combusted so that the tendency to condensation of pyrolytic gas in the succeeding pipelines is reduced.

Description: METHOD AND APPARATUS FOR HEATING A ROTARY KILN DESIGNED FOR GASIFICATION AND PYROLYSIS OF ORGANIC MATERIAL TECHNICAL FIELD The present invention relates to a method and an apparatus for heating a rotary kiln for gasification and pyrolysis of organic material. BACKGROUND ART When pyrolysing organic material like coal, biomass, and different types of waste, a heating of the material is usually performed by means of a heating medium being caused to transfer heat to the material without direct contact, whereby the organic material is decomposed into pyrolytic gas and coke, subsequently usable for different purposes. When using a rotary kiln for gasification and pyrolysis, the cold organic material is supplied at one end and is heated during its flow through the rotary kiln, ultimately leaving the rotary kiln at the opposite end in the form of hot pyrolytic gas and coke. The heating of the rotary kiln is usually performed by supplying a heating medium like e.g. hot flue gas to a jacket surrounding the rotary kiln or to a bundle of tubes, positioned longitudinally in the rotary kiln, through which the heating medium is circulated. The heating medium possessing a high temperature when introduced into the system is gradually cooled, the heat being transferred through the wall of the rotary kiln or tubes, whereby the organic material is heated. The heating medium is supplied with energy by burning a secondary fuel, or in certain circumstances by burning the pyrolytic gas, in an external combustion process. In order to achieve a high conversion velocity it is necessary to maintain a controlled optimal temperature over the whole length of the rotary kiln. However, this is not possible with the plants described above, the heating medium being cooled concurrently with the heating of the supplied organic material. Alternatively this can under certain circumstances be compensated by raising the inlet temperature of the heating medium, this however often being limited by the fact that the organic material does not tolerate heating to high temperatures, which may induce unwanted reactions and accordingly, a certain maximum inlet temperature for the heating medium is imposed, which means that the organic material temperature during its flow through the rotary kiln will decrease to a level, at which the speed of pyrolysis is relatively low. This means that such rotary kilns will be substantially bigger to achieve the capacity which would be achieved with an optimal temperature over the whole length of the kiln. Usually the pyrolytic gas generated by pyrolysis contains major amounts of condensable material which means that parts of the gas will condensate, if the temperature decreases during the removal of the gas from the plant. Often it is not sufficient to lead the gas away in isolated tubes and accordingly, it has been suggested to heat these tubes by means of a heating jacket, which however has appeared to lead to carbonization of parts of the pyrolytic gas, whereby the tubes are clogged by such carbonized material. From EP-704 658 a rotary kiln for gasifying waste material is known in which oxidizing agent for combustion of the gases developed by the process is supplied via a lance positioned longitudinally and openly in the rotary kiln and in which the supply of oxidizing agent can be controlled over the length of the rotary kiln in order to control the temperature distribution. However, this construction has a number of disadvantages. The oxidizing agent can inadvertently come into direct contact with the waste material in such places, where sufficient gas production and temperature to maintain a flame are not present, e.g. at the infeed end for waste material. This means that oxidizing conditions will be present around the waste material with consequent risk of formation of toxic components and risk of gas explosions. The flames radiate directly onto the waste material and may hit the waste material resulting in the risk of local overheating and consequently unwanted reactions. The liberation of gas from the waste material will be unevenly distributed over the length of the rotary kiln and with varying calorific value and composition in such a way that it will hardly be possible to control the combustion and thereby the heat production via the separate nozzles. DISCLOSURE OF THE INVENTION Based on this Prior Art, it is the object of the invention to provide a method and a plant of the above mentioned kind, whereby an optimal temperature can be maintained over the whole length of the rotary kiln and the above mentioned disadvantages are avoided. According to the invention, this is achieved by a method of the above mentioned kind, which is characterised by the features stated in the characterising part of claim 1 , and a plant of the above mentioned kind, which is characterised by the features set out in claim 6. By using a radiation heat exchanger positioned inside the rotary kiln and controlled supply of energy to this radiation heat exchanger, a controlled temperature distribution in the rotary kiln, which can be adapted to optimum operation conditions for the desired gasification and pyrolysis, can be achieved. The energy supply to the radiation heat exchanger is preferably provided by combustion of a combustible gas inside the radiation heat exchanger, .this combustible gas preferably being the pyrolytic gas provided by gasification and pyrolysis of the organic material, preferably being led through the inside of the radiation heat exchanger in opposite flow direction of the flow direction of the organic material inside the rotary kiln, the pyrolytic gas preferably being supplied with a controlled amount of combustion air, preferably being controlled with respect to both amount and position for the supply of the combustion air, in such a way that the energy supply can be controlled over the length of the radiation heat exchanger. The combustion air is preferably supplied to the pyrolytic gas via an air lance comprising suitable air nozzles over the length of the lance in such a way that a suitable distribution of the energy supply over the length of the radiation heat exchanger is achieved. The air lance can be mounted axially movable inside the radiation heat exchanger in order to control the position for supply of energy to the radiation heat exchanger. Preferably the plant is provided with a preheating arrangement for starting up the plant. In such embodiments of the invention, in which the total amount of produced pyrolytic gas passes through the radiation heat exchanger and is partly combusted inside the radiation heat exchanger a heating and thereby overheating of the pyrolytic gas is achieved concurrently, whereby subsequent condensation of the gas in pipelines is avoided. This is surprisingly further augmented by the fact that the more condensable parts of the pyrolytic gas has a higher thermal reactivity than the non-condensable parts of the gas, whereby the condensable parts are combusted first thereby reducing the problems with clogging of the subsequent gas pipelines. Moreover, it has appeared that the most reactive parts of the pyrolytic gas at the temperatures normally used (above approximately 450°C) will ignite spontaneously when adding air so that no ignition apparatus is needed to start up the combustion of pyrolytic gas inside the radiation heat exchanger. BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be explained in more detail by means of a preferred embodiment referring to the drawing, in which Fig. 1 shows a plant in accordance with the invention which is suitable to carry out the method in accordance with the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT The plant shown in Fig. 1 comprises a rotary kiln 1 which is isolated and lined, in order to maintain constant temperatures in the rotary kiln during the operation thereof. At the outlet end, the rotary kiln 1 is connected to an isolated and lined reversing chamber 5. The organic material to be gasified and pyrolised inside the rotary kiln 1 is supplied at the inlet end of the rotary kiln 1 by means of a feeding system 4, the further advancement of the organic material being provided by means of the rotation of the rotary kiln 1. Inside the rotary kiln a radiation heat exchanger 2 is positioned directly connected to the gas discharge tube 9. An air lance 3 is mounted axially movable inside the radiation heat exchanger 2. Air 10 is supplied via one end of the air lance 3 and is blown out through air nozzles 7 which are positioned in the outer wall of the air lance and distributed over the length of the air lance. In order to control the air supply, the air lance 3 may be constructed with separate channels connected to separate air nozzles 7, debouching into different zones inside the radiation heat exchanger 2. As an alternative, the control of the position of supply of combustion air inside the radiation heat exchanger 2 may be performed by the axial displacement of the air lance 3 and by controlling the amount of supplied air 10, the distribution of the air nozzles 7 over the air lance may in advance be adapted to the optimal distribution of the energy supplied to the radiation heat exchanger 2. The embodiment of the plant in accordance with the invention shown in Fig. 1 comprises an oil or gas burner 6 positioned in the reversing chamber in order to start up the gasification and pyrolysis process. When starting up the plant, this preheating system 6 is started and the heat therefrom is sucked into the radiation heat exchanger 2, thereby being heated and liberating its heat to the rotary kiln 1. When the desired operation temperature has been reached, organic material is supplied to the rotary kiln 1. The organic material is then heated, partly by direct radiation from the radiation heat exchanger 2, partly by contacting the hot lining of the rotary kiln 1 which is continuously heated by the radiation heat exchanger 2. When the organic material is heated, the volatile constituents are liberated as pyrolytic gas 8. The pyrolitic gas 8 is sucked out of the rotary kiln 1 and in through the radiation heat exchanger 2 and onwards to the outlet 9 for pyrolytic gas. By adding air 10 through the air nozzles 7 of an air lance 3, a partial combustion of the pyrolytic gas 8 will be induced. As mentioned above, the pyrolytic gas may ignite spontaneously by adding air and depending on the amount and position of the air supply the radiation heat exchanger 2 will be heated to a certain extent. When the combustion of the pyrolytic gas has been started, the preheating system 6 may be turned off and the control of the temperature of the rotary kiln 1 can then be provided alone by partial combustion of a greater or lesser part of the produced pyrolytic gas 8. By controlling the air supply 10 through the air lance 3 and the position of the air lance 3, it will be possible to control the temperature over the whole length of the rotary kiln 1 with high precision and the heating can be achieved alone by partial combustion of the produced pyrolytic gas 8 and without the use of external combustion systems. Although the invention has been explained above in connection with a preferred embodiment, it will be evident for a man skilled in the art that the basic principle can be achieved in other different ways. Thus, in principle the radiation heat exchanger 2 may be supplied with energy in another way than by combustion of part of the pyrolytic gas, although this is preferred, and as mentioned above gives the further advantages that the pyrolytic gas is heated to avoid condensation and the more condensable parts of the pyrolytic gas being burned first, whereby the tendency of condensation of parts of the pyrolytic gas is further reduced.

Claims: CLAIMS 1. Method for heating a rotary kiln (1) designed for gasification and pyrolysis of organic material c h a r a c t e r i s e d by the energy for heating the rotary kiln (1) being supplied by means of a radiation heat exchanger (2), positioned inside and longitudinally in the rotary kiln (1), and that the energy supply to the radiation heat exchanger (2) is provided controlled over the length thereof, in order to achieve a temperature distribution inside the rotary kiln (1), which is optimized for the desired gasification and pyrolysis. 2. Method in accordance with claim 1 , c h a r a c t e r i z e d by the energy supply to the radiation heat exchanger (2) being provided by controlled combustion of a combustible gas inside the radiation heat exchanger (2). 3. Method in accordance with claim 2, c h a r a c t e r i z e d by the pyrolytic gas produced by gasification and pyrolysis of the organic material being used as the combustible gas for the energy supply to the radiation heat exchanger (2). 4. Method in accordance with claim 3, c h a r a c t e r i z e d by the total amount of produced pyrolytic gas (8) being fed through the inside of the radiation heat exchanger (2) and part of this pyrolytic gas (8) being combusted inside the radiation heat exchanger (2) for the energy supply thereto, the control being performed by adding combustion air to the pyrolytic gas (8), controlled and distributed over the length of the radiation heat exchanger (2). 5. Method in accordance with claim 4, c h a r a c t e r i z e d by the control of partial combustion of the pyrolytic gas (8) being performed by control of both amount of combustion air as well as position for introduction of combustion air. 6. Plant for gasification and pyrolysis of organic material comprising a rotary kiln (1), means (4) for supply of organic material to the rotary kiln (1), heating means (2) for heating the organic material inside the rotary kiln (1) and means (9) for removing the produced pyrolytic gas (8), c h a r a c t e r i z e d by the heating means (2) being constructed as a radiation heat exchanger (2), extending generally over the whole length of the rotary kiln (1), and that the energy supply to the radiation heat exchanger (2) is controlled over the length thereof. 7. Plant in accordance with claim 6, cha racte rized in that the end of the radiation heat exchanger (2), which is positioned closest to the means (4) for supply of organic material into the rotary kiln (1), is connected to the means (9) for discharging the produced pyrolytic gas, and that the opposite end of the radiation heat exchanger (2) is in open connection with a reversing chamber (5) provided by the outlet end of the rotary kiln (1), whereby the produced pyrolytic gas (8) is fed through the inside of the radiation heat exchanger (2), and the plant further comprising an air lance (3) for controlled supply of combustion air (10) for the pyrolytic gas (8) fed through the radiation heat exchanger (2). 8. Plant in accordance with claim 7, c h a r a c t e r i z e d by the air lance (3) being mounted axially movable inside the radiation heat exchanger (2). 9. Plant in accordance with claim 7 or 8, characterized by the air lance (3) being provided with air nozzles (7) distributed over the length the air lance (3) for supply of combustion air for combustion of part of the pyrolytic gas (8). 10. Plant in accordance with any of the claims 6 - 9, c h a ra cte ri ze d by further comprising a preheating system (6) for heating the rotary kiln (1) when starting up the plant.

Inventors: KRISTENSEN OLE, KRISTENSEN, OLE
Applicants: ANSALDO VOLUND AS, KRISTENSEN OLE, ANSALDO VOELUND A/S, KRISTENSEN, OLE
Application year: 1998
Main classification: C10B53
IPC codes: C10B1, C10B47, F23G5