METHOD AND PLANT FOR COMPOSTING AND DRYING ORGANIC MATERIAL

Abstract: The invention relates to a method and tunnel composting plant for drying organic material, in particular for manufacturing fuel from organic material, in particular from sludges and possible support substances, by exploiting mechanical composting. In the method, the organic materials are divided into the batches required by mechanical compost processing operating on the batch principle, each batch being composted separately and dried as a separate phase by means of thermal energy external to the batch, preferably produced by composting other batches.

Description: METHOD AND PLANT FOR COMPOSTING AND DRYING ORGANIC MATERIAL The present invention relates to a method and tunnel composting plant for drying organic material, particularly for producing fuel from organic material, in particular from sludges and possible auxiliary materials, by utilizing mechanical composting. The organic materials are first brought to a total solids content that will permit composting, generally of about 30 %. The materials that have been mixed and pre-dried in this way are divided into the batches required for mechanical composting operating on the batch principle, the materials of each batch being dried to the desired solids content. The materials are dried using the heat produced during composting. In this case, the organic materials are pulp and paper industry fibre sludges, bio-sludges, paste sludges, and/or debarking plant sludges. However, bark material from the timber processing industry and other organic materials can also be considered. The sludges created in the pulp and paper industry can be divided mainly into the following sludges: fibre sludges, principally so-called noil (wood fibre) , bio-sludges, sludges from biological waste treatment plants, paste sludges, kaolin-content sludges from the coating kitchen, sludges from debarking plants. Though fibre sludge alone can be mechanically dried to a total solids content of up to 40 %, it is preferable to mix the sludges together to form mixed sludge, because especially bio- sludges (mostly bacterial material) cannot be drier to more than 12 - 16 %. Mixed sludge can be dried, for example, in a steam-screw press, to 30 % . At present, mixed sludge is burned in bark-fired boilers, due to the lack and difficulty of exploiting it in other ways. The problem is the low solids content of the sludge, which results in losses in boiler capacity and increased emissions and costs. Thermal drying is also expensive (high drying temperature) , while detrimental odours also arise during the drying process. The water in organic material is so tightly bound to the cell structure that an unreasonable amount of energy is required to separate it by thermal drying or by pressing. Finnish patent 100191 discloses a method for manufacturing fuel from organic sludge or bio-waste. According to the method, the sludge is dried by composting, the nitrogen released in composting being collected in a scrubber. Composting provides several advantages. The quality of the material is homogenized and the detrimental microbes in it die. On the other hand, part of the combustion value of the material is lost in composting. Usually, the heat generated by composting is wasted. Swedish patent publication 447827 discloses a method of for stack-composting a material with a high water content, especially bark material, on top of an air-conditioned base, to dry it to make fuel. Exhaust air extractors are located inside the stack, the heat of the exhaust air being transferred to the replacement air by means of a heat exchanger. Thus, the heat obtained from composting is used to dry the material. However, the method disclosed is quite difficult. Initially, the arrangement is beneficial, but the biological process, and thus the production of heat, slows down and finally ceases as drying progresses. The present invention is intended to improve the process according to the state of the art and especially to increase the yield of fuel and its thermal value. The characteristic fea- tures of the invention are disclosed in the accompanying Claims. To a great extent, the invention is based on the obser- vation that the cell structure of the organic material that is most difficult to dry is rapidly broken down by means of composting, after which thermal drying becomes easier and mechanical composting can be discontinued. The composting of mixed sludge starts rapidly precisely with the aid of bio-sludge, which is the most difficult type of sludge to process. For example, the wood fibres in fibre sludge are preserved for longer and the combustion value of the material is retained. By means of the method according to the invention, the material can be dried easily and rapidly to a moisture content of less than 50 %, for instance, to 30 %. Other advantages and embodiments of the invention will become apparent in connection with the following examples. The invention differs from the method of the Swedish publication referred to above in that the effective drying phase becomes a separate process only after the material has decomposed sufficiently. Though the Swedish publication discloses the principle of using the heat from composting to dry the material, the system proposed is not efficient. Apparently the inventor in question has not understood the basic idea itself that the cell structure of the organic material should be broken by thorough composting, before thermal drying. Thermal drying in the start-up phase is partly wasted, because the capillary and colloidal water nevertheless does not separate from the unbroken network of cells. One the other hand, during the final phase, composting does not produce a great deal of excess heat. In the following, the invention is described with reference to the accompanying drawings. Figure 1 shows various alternatives of the drying process. Figure 2 shows the development of the air flow and temperature of the composting and drying process. Figure 3 shows one type of composting plant specially for drying . According to Figure 1, composting 11, which would normally last for 10 - 14 days, is interrupted after only 6 - 7 days. According to the simplest alternative I, the partially composted material, which has been separately dried for, e.g., two days, is moved outdoors, preferably into a stack formed under a canopy, where it continues to compost slightly for 2 - 3 weeks. In any event, the storage evens the moisture content through the entire batch, so that the fuel is of even quality. Mechanical drying reduces both the duration of the stack drying and its dependence on weather conditions. Drying continues with the aid of the heat produced by even the least degree of composting and of the dry outdoor air. The internal temperature of the stack continues to rise considerably over several weeks. The outdoor air only cools the thin surface layer. In tests made outdoors in summer and without a canopy, the stack dried to the target moisture content within a few weeks, despite periods of rain. Only in October - November has the weather proven to be so unfavourable that drying does not take place. According to alternative II, drying is completed with the aid of a longer period of mechanical drying than that described above (3 - 4 days 50 % target moisture content) . This entirely avoids dependence on weather conditions. Mechanical drying is also essential, if the target moisture content of the fuel is much below 50 %. Some power plants are prepared to accept fuel with a moisture content of 30 %, when it has quite a good combustion value. The process mainly comprises a normal composting process and a drying phase, in which a large quantity of air is used. If necessary, the mixed sludge to be dried is mixed with a stabilizing substance such as bark or woodchips, and is then placed in a composting tunnel and is composted for 6 - 7 days, depending on the moisture content. When composting mixed sludge, a stabilizing substance may not be needed at all. One such batch of mixed sludge contained 8500 kg total solids of fibre sludge, 6500 kg of bio-sludge, and 2000 kg of paste sludge. In another trial batch, 0,4 m 3 of bark/tonne of mixed sludge was used. Generally, the preferable mixed sludge contains 35 - 70 % total solids of fibre sludge, 15 - 45 % total solids of bio-sludge from a biological waste treatment plant, and possibly other sludges. In this case, the mixed sludge can be composted essentially as it is, with the proportion of the auxiliary substance being less than 10 % total solids. When decomposition is optimized in the composting of municipal bio-waste, the optimization takes place by regulating the heat in the process phase to about 50°C. This takes place by regulating the ratio of circulation air to heated fresh air. The composting is started, however, by raising the temperature of the compost material considerably (to 60 - 70°C) for a short time, thus killing the detrimental bacteria. After the composting phase, the material is dried, preferably in the same composting tunnel, in which the thermal energy produced by the composting of other batches is exploited. After the composting phase, the moisture content of the compost material is 55 - 65 % (generally 68 - 50 %) , the moisture content being reduced as desired during post-drying, i.e. usually to 45 - 50 %, but sometimes to as little as 30 %. After the drying phase, (3 - 4 days) , the material is either taken directly to be burned, or else to an intermediate stockpile. According to Figure 2, a large quantity of circulation air is used in the starting phase and a large quantity of fresh air in the drying phase. In the method, composting has the following tasks: to produce the free energy required for drying to carry out pre-drying to 55 - 65 % to render the sludge odourless - to render the sludge hygienic to break down the cell structure of the organic material (water in the cells) , especially of bio-sludge, when thermal after-drying at a low temperature will become possible to reduce the amount of detrimental nitrogen in combustion. After-drying has the following advantages: the free energy produced by composting is exploited, so that the duration of the composting can be shortened and the size of the plant reduced the desired moisture content of the fuel is ensured under all conditions. The method can be used to manufacture fuel efficiently and economically from odorous sludges and other organic materials. The overall economy is greater than, for example, digesting, purely thermal drying, and mechanical drying, as the free energy of the process itself is used for drying. The process is simple and manageable. The end product can be utilized in existing boilers without any special arrangements, in which case it can be used to replace conventional fuels. In addition, environmental detriments can be effectively controlled. The accompanying Figure 3 shows one possible plant assembly, together with flow diagrams. Here, the tunnel composting plant itself is otherwise conventional, except for a more comprehen- sive air treatment system than usual. The principal components of the tunnel composting plant in Figure 3 are composting tunnels 13 equipped with an air distribution base 13.1, in addition to a scrubber 15 and a biological air cleaner 16 for cleaning the exhaust gases. In addition to these, the composting plant 1 includes a comprehensive air- conditioning system. Fresh intake air brought from outside is heated in heat exchanger 17 with the heat of the scrubber 15 and led to distribution duct 10. From here, the heated, oxygen- rich, and dry fresh air indirectly with circulation air through 3-way dampers 11 and propelled by fans 12 to the air-distribu- tion bases 13.1 of the composting tunnels 13. These feed the air evenly to the material being processed, the air being exhausted through other 3-way dampers, either to the cold exhaust line 18', or to the exhaust line 18 leading to the gas scrubber 15, from which circulation air is also taken into circulation air duct 19. From scrubber 15, the air is led to biological cleaner 16, which removes odour-producing compounds from the exhaust air. The water circulation of scrubber 15 forms a heat recovery apparatus together with heat exchanger 17. It is particularly essential that the air feed and removal from each composting tunnel 13 can be selected individually and be implemented for the process phase desired at the time, with the aid of 3-way dampers 11 and 14, as well as with the aid of fans 12. The air exhausted from the composting process (start-up and process phases) has a temperature of 40 - 60°C and a relative humidity of nearly 100 %, so that the circulation air cannot be used for drying. The exhaust air in the drying phase is so cold that it is not worth recovering heat from it. One the other hand, it also does not require scrubbing, so that it can be led directly to the biological cleaner 16. Some of the warm air flow of exhaust line 18 is recirculated through circulation air duct 19 to the composting tunnels 13 that are selected at the time. Each of the three composting tunnels 13 of the plant in Figure 3 is shown as operating in a different phase. Usually, at least two of the composting tunnels are simultaneously in the process phase. There are preferably at least four composting tunnels, to be able to ensure an even and efficient production of heat. 100 % circulation air is fed under high pressure to the com- posting tunnel in the start-up phase in Figure 3, so that the material heats up and composting starts rapidly. The air is exhausted through exhaust line 18. At the very beginning of the start-up phase, when exhaust air is quite cold, exhaust air can be fed to the cold exhaust air line 18'. A day later, composting will have started effectively, moving next to the process phase . Maximum production of heat is maintained during the process phase, when the temperature of the batch is set at about 50°C (usually in the range 45 - 55°C) . Generally, the quantity of air if only half of that in the previous phase and is mainly regulated by adjusting the amounts of fresh air and circulation air to achieve the target. In the drying phase, a large quantity of air is once again used, which is only heated and thus dry fresh air and can thus bind a large amount of moisture. Even though the temperature of the batch drops, the drying is efficient. The cold exhaust air is led past the scrubber, as described above. In another plant embodiment, the final drying takes place using a separate dryer, through which the material is moved after composting. The dryer used can be a dryer intended for drying chicken manure, that is as such known, for example the 'Jansen drying system' manufactured by the Dutch firm Jansen Machinefabrik & Konstruktiebedrij f B.V., Barneveld. In it, the material travels on belt conveyors, with drying air led transversely through the conveyor.

Claims: Claims 1. A method for drying organic material, in particular for manufacturing fuel from organic material, in particular from sludges and possible support substances, by utilizing mechanical composting, in which method, the organic materials are first brought to a total solids content permitting composting, generally of about 30 %, the mixed and thus pre-dried materials being divided into the batches required by mechanical compost- ing operating on the batch principle, the materials of each batch being dried to the desired total solids content, and in which the heat produced by composting is used to dry the materials, characterized in that, the processing of each batch is divided into two principal phases, in the first of which the cell structure of the organic material is broken by composting to permit thermal drying and the moisture content is reduced to the range 68 - 50 % , preferably in the range 65 - 55 %, after which, in the second principal phase, the materials of the batch are dried by means of thermal energy external to the batch, preferably produced by composting other batches. 2. A method according to Claim 1, characterized in that the batches are processed in a tunnel composting plant, in which there are several composting tunnels (13) with an air-condi- tioning system, as well as fresh air heating with the aid of the exhaust air, and that both of the principal phases of the processing of each batch, i.e. composting and separate drying, are carried out in the same composting tunnel (13) , by directing either fresh air or circulation air or a mixture of both in an optional ratio into the composting tunnel according to the process being carried out at the time. 3. A method according to Claim 2, characterized in the fresh air is heated using the heat available from the scrubber (15) . 4. A method according to Claim 2 or Claim 3, characterized in that the first principal phase, i.e. mechanical composting, is divided into a short start-up phase and a process phase, with the batch temperature in the start-up phase being raised for a 5 short time to the range 55 - 75°C, most preferably in the range 60 - 70°C, by feeding the batch with essentially only the plant's warm circulation air, and the batch being fed in the process phase with both fresh air and circulation air, to maintain an essentially level temperature in the range 45 - 10 55°C, most preferably at about 50°C. 5. A method according to one of Claims 2 - 4, characterized in that the second principal phase, i.e. mechanical drying, takes place by leading only heated fresh air to the batch. 15 6. A method according to one of Claims 2 - 5 in the composting of mixed sludge, which mixed sludge contains 35 - 70 % total solids of fibre sludge, 15 - 45 % total solids of bio-sludge from a biological treatment plant, and possibly other sludges, 20 characterized in that the mixed sludge is composted essentially as such, the amount of support material being less than 10 % total solids. 7. A method according to Claim 1, characterized in that the 25 thermal drying in the second phase is carried out mainly by using a separate air dryer. 8. A tunnel composting plant for manufacturing fuel from organic materials and possible support substances by exploiting 30 mechanical composting, which plant includes several composting tunnels (13) with an air treatment system, a fresh air feed system with heating devices (17), an exhaust air system with gas scrubbing devices (15), and a circulation air system (14, 18, 19) to lead the exhaust air back to the composting tunnels 35 (13), characterized in that the air treatment system of each composting tunnel (13) includes duct damper devices (11) that can be independently operated on the feed side, so that fresh air and circulation air in an optional ratio can be connected to the air feed of each composting tunnel (13) . 9. A tunnel composting plant according to Claim 8, characterized in that the circulation air duct (19) is connected to the main exhaust duct (18) leading to the gas scrubber (15) and that the plant includes an auxiliary exhaust duct (18') bypassing the gas scrubber (15) and duct dampers (14) for connecting the air exhaust of each composting tunnel to the main and auxiliary exhaust duct in an optional ratio.

Inventors: MUTKA KARI, MUTKA, KARI
Applicants: MUTKA KARI, VAPO OY, MUTKA, KARI
Application year: 1999
Main classification: C10L5
IPC codes: C02F11, C05F17, F26B1, F26B3, F26B23