METHOD OF BURNING A PARTICULATE FUEL AND USE OF THE METHOD FOR BURNING SLUDGE

Abstract: Method for the production of hot drying gas by the burning of flowable, biological refuse in an incinerator (1) which comprises a vertical cyclone furnace (2), into which there is an injection of fuel (6) tangentially together with the primary combustion air, secondary combustion air (7) and tertiary combustion air (8) which is injected into a throat (5). In the bottom of the furnace there is provided a cooled, rotating ash scraper (11). The waste gas is conducted via a throat (5) to a secondary combustion chamber (3) in which an incineration of residuals takes place, and from which the drying gas (4) is removed. Combustion-retarding air (9) is injected into the hottest area of the cyclone furnace (2), so that sintering and the formation of slag is avoided.

Description: METHOD OF BURNING A PARTICULATE FUEL AND USE OF THE METHOD FOR BURNING SLUDGE BACKGROUND OF THE INVENTION The invention relates to a method of the kind disclosed i the preamble to claim 1. Such a method is known, for example from USA patent no. 4,398,477, where the two combustion chambers consist of tw cyclone furnaces arranged one above the other and connecte via an opening with a reduced clearance, a so-calle throat. The fuel, which consists of rice hulls, is blow together with the primary air into the lower vertical cyc lone furnace, and the waste gas is then burned in the uppe cyclone furnace during the introduction of additional com bustion air through tangential nozzles. There is hereb achieved an optimal incineration of the fuel, and the re sidual product in the form of ash can be removed from th bottom lower cyclone furnace by means of a cooled, rotatin ash scraper. In order to achieve optimal incineration of the fuel, the temperature in the lower furnace is in the order of 1200"C. Such a high temperature is unfavourable, in that during the combustion of biological fuels at this temperature there are formed relatively large amounts of nitrogen oxides, so- called NOx'es, which are poisonous. From the applicant's own previously-submitted international application, published under no. 090/05272, there is known a sludge drying apparatus in which, for example, sewage sludge is dried down to less than 10% water content in a rotating dryer, after which the dried sludge is used as fuel in a furnace which delivers the thermal energy neces¬ sary for the rotating dryer. However, it has proven to be almost impossible to incinerate the dried sludge in a no mal cyclone furnace, the reason being that the dried slud and similar types of fuel vitrify to form a kind of sl filled with porous pores which have an insulating effec while at the same time it is highly viscous, thus renderin the removal of the slag impossible. Therefore, use is mad in practice of other types of furnaces, e.g. fluid-be ovens, for the incineration of fuels which are aqueous o of low energy content, such as e.g. dried biologica sludge. Furnaces of such a type are suitable only for larg amounts of fuel and require a long start-up time, and thu furnaces of this type are not suitable if they cannot b used in continuous operation. Moreover, this type of fur nace demands a comprehensive process regulation with spe cially-trained personnel. Therefore, when it is required to dispose of refuse fro smaller towns or urban areas, it is necessary either to us other methods for the disposal of the biological sludge o to transport this to larger communal plants for incinera tion. ADVANTAGES OF THE INVENTION By proceeding as disclosed and characterized in claim 1, i is possible to use a cyclone furnace for the incineratio of dried, flowable biological refuse of the kind which can¬ not otherwise be burned in a cyclone furnace. The cyclone furnace has the great advantage that it is relatively cheap to produce, that it is compact and results in an intensive combustion and, what is very important, the cyclone furnace is quick and easy to start up. Consequently, a cyclone fur¬ nace for the incineration of biological refuse does not need to operate continuously. By arranging and controlling a furnace as disclosed and characterized in claim 1, it is possible to burn bio-fuel of low calorific value without the fuel sintering, whic gives rise to the formation of slag and sintering in the combustion zone. The combustion zone usually lies slightl below the middle of the furnace, the reason being that the fuel will not ignite until it has reached a good distance down towards the bottom and has achieved the ignition tem¬ perature. The introduction of combustion-retarding gas to the ash separation area, e.g. oxygen-deficient air in the form of wet flue gas, will retard combustion so that this is less intense, and the sintering formation is avoided. At the same time, there is hereby achieved a reduction in the formation of NOx because the surplus air is decreased and, providing that the temperature is around 850°C, the C0- formation can be held at an acceptably low level. The waste gas formed by the controlled and retarded combus¬ tion is burned after the throat in a secondary combustion chamber which is merely a large, brick-lined chamber in which the post-combustion takes place. In order to burn out the flue gas from the retarded combustion, it is necessary for the secondary combustion chamber to be of sufficient size for a reduction of the CO-content in the waste gas, and to give the waste gas an adequate period of time in the chamber, i.e. in the order of 0.5 - 2 seconds. By proceeding as disclosed and characterized in claim 2, sintering and the formation of slag at the throat is avoid¬ ed, even during the use of dried biological sludge with low ash content. Moreover, a particularly good ash separation is achieved if the diameter of the throat is small in rela¬ tion to the diameter of the cyclone combustion furnace, e.g. a diameter which is less than half of that of the cyclone furnace, and if the air velocity is around 60 - 100 m/sec. By proceeding as disclosed and characterized in claim 3, can be ensured that no sintering of the fuel with slag fo mation can occur at any place within the combustion area the cyclone furnace. All ash/slag will fall to the bott in the conical area of the furnace, where by means of cooled, rotating ash scraper it can be removed from t furnace in the normal manner, e.g. by means of an a sluice. By proceeding as disclosed and characterized in claim the operational reliability of the furnace is increased that a uniform and complete incineration of the fuel achieved. The fuel is measured and screened so that it h the desired particle distribution. The smallest particl are ignited quickly and ensure the combustion, while t large particles are held by the centrifugal force in t periphery of the primary chamber until combustion has tak place. If a poor fuel is used, i.e. fuel with a high ash conte or high water content, a subsidiary-firing must be estab lished as disclosed and characterized in more detail i claim 5. The subsidiary-firing plant can also be used i connection with the start-up of the combustion furnace However, by proceeding as disclosed in claims 1-4, when fuel with a calorific value of around 1700 kcal/kg or high er is used, it is possible to maintain a constant combus tion of the fuel without subsidiary-firing. By proceeding as disclosed and characterized in claim 6, complete combustion of the waste gas is achieved, so tha the CO-content is burned to C0 2 , and a suitably low C0- content is achieved without any significant formation o NOx. By proceeding as disclosed and characterized in claim 7, i is ensured that sintering of the fuel can not take place a any point in the furnace during combustion, and at no poin in the furnace is there any formation of viscous slag. Th combustion throughout the whole of the primary combustio chamber is a so-called dry (non-slagging) combustion, th only waste products of which are ash and flue gas, an where the ash is of such a consistency that it can be re moved without problems by means of a commonly-known, rotat ing ash scraper. Experiments have shown that in connection with the burnin of fuel which consists solely of biological refuse in th form of dried sludge, the best incineration of the fuel i achieved by proceeding as disclosed and characterized i claim 8. The method according to the invention has been develope mainly for use in connection with refuse incineratio plants as disclosed in more detail in claims 9 and 10, bu can naturally also be used in connection with the burnin of other forms of biological fuel. THE DRAWING The method according to the invention will now be described in more detail with reference to the drawing, which shows the principle of an incinerator comprising a vertical cyc¬ lone furnace which is connected to a secondary combustion chamber via a throat. DESCRIPTION OF EMBODIMENT EXAMPLES In the drawing is seen an incinerator 1 for bio-fuels, e.g. dried sludge, and comprising a primary combustion chamber in the form of a vertical cyclone furnace 2, a throat 5 and a secondary combustion chamber 3 for subsequent incinera- tion of the waste gas from the cyclone furnace. In the bottom of the conical part 12 of the cyclone furnac there is provided a rotating ash scraper 11 which is air cooled in the normal manner, and which scrapes the ash 1 out through a not-shown ash sluice 10 or an ash conveyo with product lock. The top of the secondary chamber 3 is arranged for the re moval of the hot waste gas 4, which for example can be use directly in a rotary drier as described in more detail i International Patent Application No. PCT/DK89/0024 (W090/05272), and to which reference is made to all exten in connection with the use of the hot drying gas 4. The primary air 6 together with the fuel is blown i through tangential injection nozzles. The fuel is bio-fuel, e.g. dried sludge, as explained in more detail in th above-mentioned international application. The dried bio fuel in the form of sludge is dried down to less than 15%, preferably 10%, water content, pulverised in a mill an screened, e.g. through a 5 mm sieve. The main part of th fuel, i.e. at least 75%, has a particle size of less than 1 mm, and the maxiumum particle size due to the sieve is 5 mm. At the same level, or possibly slightly higher up in the cyclone furnace than that at which the primary air is in¬ jected, the secondary air 7 is injected through a series of tangential nozzles, and tertiary air 8 is blown into the throat 5 itself, similarly through a number of tangential nozzles. A modest amount of combustion air is also injected through the cooled ash scraper 11, in that cooling-air is introduced into the combustion chamber through openings in the ash scraper 11. At some distance down in the cyclone furnace, preferably a around the mid-point or immediately below, the injecte fuel 6 will be ignited and will burn. In order to control and dampen the intensity of the combustion, so that the fuel does not sinter and give rise to the formation of slag in the combustion zone, combustion-retarding air 9 is in¬ jected directly into the combustion zone via tangential nozzles in the direction of rotation for the combustion. The combustion-retarding air is air with reduced oxygen content and/or with high moisture content, so that the oxygen content of the air is reduced approx. 30-50% in re¬ lation to normal atmospheric air, and the air has a temper¬ ature in the order of 100-200 β C, preferably 150°C. The air, for example, is recirculated drying air with a temperature of approx. 150 β C from the rotary dryer in the above- mentioned international application. The amount of com¬ bustion-retarding air 9 can be set once and for all, de¬ pending on the capacity of the furnace. Primary air, sec- ondary air and tertiary air is also set once and for all, similarly depending on the capacity of the furnace. The temperature of the furnace is controlled at approx. 850*C. If the temperature falls, the amount of injected fuel is increased. If the temperature rises, the amount of injected fuel is reduced. There is hereby achieved a very simple and reliable form of control, which at the same time ensures that the temperature does not exceed 950-1000°C at any point in the primary chamber. With an incinerator of the kind described, and controlled as explained above, a cyclone combustion 13 is achieved whereby with the use of gravitation and the special form of injection for the combustion air, the combustion takes place in a downwardly-directed spiral movement as shown in the drawing, and where the waste gas, similarly sketched in the drawing, is transferred via the throat 5 to the post- combustion chamber 3 for incineration. The post-combust chamber 3 is at least of the same size as the cyclone f nace, but will normally have a volume which ensures t the period of time for which the waste gases are in chamber is at least 0.5 sees. The following table shows a series of different values incinerators controlled according to the invention and u in connection with recirculated waste gas (drying air) biological fuel from a rotary dryer as disclosed in above-mentioned international application. Prerequisites: 60 g solids per person equivalent per hours; the dried sludge has 20% solids, which 40% is ash. Operational time per we is 100 h. *) This drying air, which for example is used in a rotar dryer as disclosed in the above-mentioned internationa application, has a temperature of approx. 850°C and NOx content of less than 100 ppm. **) The air has a temperature of 100-150"C, an oxygen con tent of 10 - 12% and a moisture content of 0.4 kg wate per kg dry air. At the start-up of the incinerator, oil or gas, e.g. N-gas is introduced in the secondary air 7 by means of not-show nozzles. These nozzles are also used for subsidiary firin if the fuel has a calorific value of less than 170 kcal/kg.

Claims: C L A I M S 1. Method for the production of hot drying gas by the i cineration of fuel consisting of flowable biological refu in an incinerator which comprises a primary combusti chamber in the form of a vertical cyclone furnace (2), into which the fuel (6) is injected tangentially in t upper half of the furnace together with the primary co bustion air, into which secondary combustion air (7) is injected ta gentially in the same plane as that at which the prima air is injected or higher in the primary combustion cha ber, where the ash is removed from the bottom area (12) of t furnace by means of a rotating, cooled ash scraper (11 and from which the waste gas is transferred via an apertu (5) in the top of the furnace to a secondary combusti chamber (3), c h a r a c t e r i z e d in that in addition, combustion retarding gas (9) is injected tangentially into the as separation area of the cyclone furnace. 2. Method according to claim 1, c h a r a c t e r i z e in that in addition, tertiary combustion air is injecte tangentially and directly into the reduced aperture (5) and that the secondary combustion air is injected immedi ately below the aperture, and that said injections ar effected with relatively high air velocity. 3. Method according to claim 1 or 2, c h a r a c t e r i z e d in that the amount of combustion-retarding air in jected constitutes at least 10% of the total amount of ai injected into the cyclone furnace, and is in the order o half of the amount of primary combustion air. 4. Method according to claim 3, c h a r a c t e r i z e in that a flowable fuel is used, and which is pulverise and screened so that at least 75% of the fuel has a par ticle size of less than 1 mm, and such that the maximu particle size is 5 mm. 5. Method according to claim 4, c h a r a c t e r i z e in that subsidiary firing with oil or gas injection int the secondary air is employed if the calorific value of th fuel is less than 1700 kcal/kg. 6. Method according to claim 1, c h a r a c t e r i z e in that the secondary combustion chamber is of a volum which is at least sufficient for the waste gases to exis herein for at least 0.5 sees. 7. Method according to any of the claims 1-6, c h a r a c t e r i z e d in that the injection of combustion air an combustion-retarding air is effected in such a manner tha the temperature does not exceed 950-1000°C at any point i the cyclone furnace. 8. Method according to claims 1 and 2, c h a r a c t e r i z e d in that the amount of injected combustion-retard- ing air is in the order of approx. half of the amount o primary air, and that the amount of secondary air is of th same order as the amount of primary air. 9. Use of the method according to any of the claims 1-8 for the incineration in a cyclone combustion furnace of bio¬ logical sludge with a water content of less than 25%, and where the hot drying gas is used for a preceding drying the biological sludge in a drying plant, and where mo drying air from the drying plant is recirculated to incinerator and is used as combustion-retarding air in cyclone furnace. 10. Use of the method according to any of the claims 1-8 a cyclone combustion furnace in a drying plant for aqueo masses, e.g. biological sludge, and as disclosed in mo detail in International Patent Application N PCT/DK89/00246 (W090/05272).

Inventors: CHRISTENSEN JOERGEN STEEN, CHRISTENSEN, JOERGEN, STEEN
Applicants: ATLAS IND AS, ATLAS INDUSTRIES A/S
Application year: 1992
Main classification: F23G5
IPC codes: F23C9, F23G5, F23G7, F26B21