FIELD OF THE INVENTION

The invention is related to a method for treating biomass and especially such a method for treating biomass in which the biomass is dried in a process in which an essential part of the dried biomass is recycled back to be mixed to the biomass coming into the process. The invention is also related to biological utilization of different kinds of biomasses.

BACKGROUND OF THE INVENTION Many biomasses, like peat, sludge and different kinds of vegetable wastes, are utilized biologically in growth substrates, fertilizers, for soil improvement in general or for producing biogas in bioreactors. A significant problem in that is that these biomasses, as well as many other ones, include a lot of fibrous material, i.e. long chain carbon compounds, like cellulose, hemicellulose etc. which are very difficult for micro-organisms to be exploited. Degradation of cellulose to shorter chain compounds, e.g. sugars, intensifies biological processes. A remarkable part of the waste masses of the paper industries consists of biomasses, mostly cellulose fibres, and breaking down cellulose may be an advantageous solution also for handling these masses.

In composting there are micro-organisms, mainly in the thermophilic area, which break down also cellulose. Efficient thermophilic composting, which requires that temperature is held at about 60 °C, is possible only in a properly controlled composting reactor. The time ,,required for composting is in every case several days, and a composted mass must further be aftertreated, e.g. dried or matured and mixed with various improvement materials for obtaining a usable product. Composting also consumes carbon from a biomass and produces carbon dioxide. Moreover, composting large amounts of biomasses in a reactor is an expensive solution.

It is known to use enzymes for intensifying composting. In conventional composting it is, however, problematic to get an enzyme distributed efficiently in a mass to be composted. Further to that, a temperature prevailing in composting is seldom favourable for action of an enzyme, i.e. preferably more than 60 °C. E.g. the use of cellulase improves to some extent degradation of cellulose in composting but does not reduce significantly the disadvantages of composting as a biomass handling method.

JP 10120482 A presents a method for accelerating composting by adding to a compost mass xylanase breaking down especially lignin and cellulase breaking down cellulose. JP 52107970 A presents a similar quite widely defined method for handling vegetable matter.

In the applicants international patent application WO 00/58229, the solution for treating and utilizing sludge is presented in which sludge is dried to hydrofobized granules and an essential part of the dried granules are circulated back for making sludge to attach to the surface of hydrofobized granules. The treating process may include composting whereby circulation of the granules several times through composting in thermophilic area breaks relatively well also fibrous matter, e.g. cellulose. The solution then has also the above stated disadvantages of composting solution. hi another solution presented in WO 00/58229, sludge is first treated by only drying and circulating granules. This method, which is a basis for the solution presented here, is very fast and economically advantageous but fibrous matter is not broken down in the treatment, and the final product is accordingly not the best possible in view of biological utilization.

JP 2001025799 A presents a method of treating sludge in which suitable enzymes, mainly cellulase, is mixed to so called concentrated sludge (usual dry matter content 3 to 5 %) coming from settling phase. The purpose is to break down cell wall and membrane structures in sludge so that further drying would be easier. Enzymes are added in a basin which may be provided with additional mixing and heating for improving the influence of enzymes. After enzyme treatment, sludge is further dried mechanically by e.g. centrifuging or belt filter pressing after which the dry matter content of the sludge has been 26 %, according to the publication. Not until after that, the sludge is dried, probably in a thermal drier. In our opinion, adding enzymes to compressed sludge seems not quite reasonable because the sludge of which 95 % is water must be heated to 60 to 70 °C for making the enzymes to affect properly, which requires a lot of energy. As to the influence of enzymes, adding them to as wet sludge as that is not advantageous, either.

It is also known to add cellulase and other enzymes to either a gasifying or acidifying reactor when producing gas from a biomass. The affect of enzymes is then reduced by the fact that circumstances in reactors are usually not favourable for the action of enzymes. Digesting processes normally work in the mesophilic area, and the temperature is therefore less than 40 ⁰ C which is too low for enzymes. The water content of sludge normally fed to the reactors is 80 to 95 %, and the enzymes are diluted too much.

SUMMARY OF THE INVENTION

An object of the invention is firstly to provide an efficient and economically advantageous method for utilizing biomasses.

An object of the invention is especially to provide a method with which possibilities to utilize different kinds of biomasses, e.g. sludge, collected biowaste or peat, are significantly improved.

A further object of the invention is to provide a treating method which makes sure hygienization of biomass and meets the requirements of environmental legislation becoming more stringent.

A further object of the invention is to provide a method with which treated biomass is safe and easy to handle and use and may be applied to many utilization purposes.

To achieve these objects, a method for treating biomass, in which method the biomass is dried in a process in which an essential part of the dried biomass is recycled back to be mixed to the biomass coming into the process, is characterised in that what is defined in the characterising part of claim 1. Other claims define various embodiments of the invention. The method of the invention is very advantageous because the same circumstances suit well for both drying and efficient treatment with enzymes, e.g. cellulase. Also the recycling of the biomass has an advantageous influence in both drying and enzyme treatment. Wet biomass added in the mixing phase is deposited on dry mass granules or particles which makes drying easier. An enzyme or mixture of enzymes which is added each time as new biomass is added affects most efficiently the added mass. Because biomass is circulated in the process, it goes through several enzyme treatments which of course improves the effect of the enzymes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention together with some of its embodiments is described in further detail in the following with reference to the accompanying drawings, in which:

Fig. 1 presents schematically a process for applying the method according to the invention;

Fig. 2 presents schematically another process for applying the method according to the invention; and

Figs. 3 to 5 present some embodiments of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Better enzyme products for breaking down cellulose are developed continuously. The same concerns hemicellulases breaking down hemicellulose. A cellulase breaks cellulose down to glucoses and other shorter chain carbon compounds. Enzyme products are often developed for certain purposes, for treating certain biomasses, and they include therefore often several different enzymes. With different cellulases, optimal circumstances for the affect vary. For the solutions presented in this application, the most suitable are enzymes which affect most efficiently in the temperature range of 60 to 70 ⁰ C and when the dry

matter content of a biomass is 60 to 70 %. For celMase enzymes in general, a favourable pH range is 3,5 to 5,5 which is characteristic for peat and vegetable waste. Cellulase enzymes are produced also for pH range 6 to 8 which is typical for e.g. many sludges. The mark % means in every context here below weight percents. In the example of Fig. 1, the basis for the solution is a process provided for direct granulation. The process corresponds to the method presented in Fig. 3.

The final product of the process are grains dried to a dry matter content of e.g. about 95 %. Drying and granulation occur in a drum 6 from the end of which dry grains are taken out to a screen 7 and divided so that a desired fraction is taken out from the process as a final product and stored in a dry grain silo 9, and an essential part of the grains is circulated via a crusher 10, circulation conveyor and container 3 provided for circulated grains back to the beginning of the process. Grains are dried efficiently also during the circulation. Ventilation of cool dry air is arranged for the screen, crusher and circulation conveyor, and dry air is blown by a fan 12 into the circulation grain silo 3. By crushing it is secured that the new grains formed on the recycled grains are small enough for hygienization, for example. So called dried sludge, the dry matter content of which is 20 to 30 %, is brought into the process through a sludge silo 1. With this sludge, so large amount of recycled grains are mixed that the dry matter content of the mixture is e.g. 50 to 55 %. Together with the mixing, an enzyme or mixture of enzymes is added into the mass, as indicated by the reference sign 4. The mixed mass, in which the new wet sludge is deposited on the surface of dry grains, may be fed directly to the drum 6. It may also be pressed by means of an extruder 5 to pieces which are fed into the drum. Based on our experiences, pressing into pieces quickens drying, and it seems also to help mixing the enzyme efficiently and evenly with the mass. The pieces are e.g. pieces with round section having a diameter of 5 to 10 centimeters. In the continuously rotating drying drum, blades are stirring the mass which is conveyed slowly from the inlet end towards the other end at which heated dry air is brought into the drum through a heater unit 11 of a heat exchanger. A suction apparatus 13 creates at the inlet end a region of low pressure which makes the warm air to flow efficiently through the mass to be dried towards the inlet end of the drum. Exhaust air is cleaned in a cyclone 14 and led into an exhaust air pipe 15. The exhaust air may be led into the pipe also through a heat exchanger or odor removing equipment, for example.

The operation of the drum may be continuous so that a certain amount of new sludge and a corresponding amount of recycled grains are taken into the mixer e.g. every 2 hours, and at the same time a certain amount of an enzyme or mixture of enzymes is added. The cellulase product to be added may be e.g. a product of AB Enzymes GmbH named Econase CE, the estimated need of which is 1 kilogram for each dry matter ton of biomass. Because

the biomass is circulated in the process and the enzyme thereby affects it several times, a suitable amount of the enzyme to be added each time may be e.g. 0.5 kilograms per each dry matter ton of new sludge taken into the process. Before mixing the enzyme with the biomass, it may be diluted with water at a ratio of 1:2 to 1:5, for example. By increasing the amount of the enzyme with dilution, it is easier to get it mixed evenly with the biomass.

In the method of Fig. 3, mixing of incoming biomass, the dry matter content of which is 20 to 30 %, with recycled grains occurs in phase 31 in which at the same time a certain amount, e.g. the above mentioned amount, of an enzyme or mixture of enzymes is added into the mass. The mixed mass, the dry matter content of which is 50 to 55 %, may be pressed into pieces in phase 32 or that phase may be skipped by advancing directly to phase 33 in which the mass is dried and granulated. In the phase 33 the mass is first heated to a temperature which is favourable for the action of the selected enzymes. E.g. the enzyme product Econase CE affects best within the temperature range of 60 to 70 °C. The mass is then heated first to this temperature range in which it is held for 1 hour, which is a long enough affecting time for this enzyme. The preferred dry matter content for the action of the enzyme is 60 to 65 %. As the mass is treated in the drum provided with stirring and drying air circulation, also its dry matter content is brought at least close to the best possible treatment conditions.

For securing the hygienization of the mass, the temperature may after that be raised to e.g. 75 to 80 ⁰ C for at least 1 hour. After that drying is continued further in a temperature above 70 ⁰ C, for example, until the dry matter content of the grains is 70 to 95 %, e.g. about 80 % as mentioned above. The grains taken out from the drum are screened in phase 34 from which a desired fraction and amount is taken into the dry grain store and a necessary suitable amount is circulated after crushing phase 35 back to the beginning of the process. Cellulase enzymes suitable for the above described purpose work best in slightly acidic circumstances in which the pH is preferably less than 6. Therefore sludge or some other biomass, the pH of which is normally a little higher, may be treated together with enzyme treating with diluted acid, e.g. acetic acid or strongly diluted hydrochloric acid, lowering the pH. The temperature of the mass treated in a drum drier or granulator is held at at least 75

°C for at least 1 hour, and each time after addition of new mass the above mentioned hygienization treatment is repeated. If the process is operating continuously, it is secured that the flow of the mass through the drum includes a period securing hygienization. In our opinion, it is not reasonable to raise the temperature of the mass in any phase above, at least not remarkably above, the range of 70 to 80 °C. Firstly, it could involve the risk of fire because e.g. the risk of dust inflammation is increasing as the temperature is

raised and because the biomass may include easily inflammable gasifying matters, like e.g. peat includes. On the other hand, also the drying process is economically most advantageous and easiest to realize and control as the temperatures are held within the mentioned ranges. Besides the action of the enzymes, also the drying of the mass is partly biological process in which the heat generated by biological reactions contributes, and in view of biological actions the most suitable temperatures are preferably below than over 70 ⁰ C. The temperature of the air fed into the drying or granulation drum is of course a little higher than the aimed temperature of the mass.

Drying of the biomass is continuing during the circulation in the screen, crusher, circulation conveyor and circulation mass container by means of the ventilation of cool dry air and exhaust of moist air arranged in connection with them. A suitable temperature for drying air is e.g. 15 to 20 ⁰ C. Warm grains coming from the drum are drying efficiently when brought to dry cool air due to vapor pressure difference, and at the same time the grains are shrinking and become more compact. The final product are compact granules the dry matter content of which is 80 to 95 %.

Besides the above mentioned heat treatment, hygienization is secured also by crushing the circulated grains small enough so that the new grains formed on them are small enough and the temperature is raised high enough throughout the grains. Because the mass is held close to the hygienization temperature already before hygienization phase, the raising of the temperature high enough is in practice secured. It is to be noted also that if pressing the mass into pieces before starting the drying and heat treatment is included in the method, the most of the pieces are normally broken down completely during the heat treatment before hygienization phase. If bigger pieces remain in the treatment, the holding time in the hygienization temperature is increased respectively. The method of Fig. 4 comprises essentially the same phases as the method of Fig. 3, and only the granulation and drying phase 33' differs from the corresponding phase of Fig. 3 in that the enzymes are added at the beginning of the phase 33' and not in the mixing phase 31 and that the biomass is heated to 60 to 70 ⁰ C at the same time stirring it.

Fig. 2 presents another example of the process in which the method according to the invention is applied. An example of the corresponding method is presented in Fig. 5. The enzyme treatment occurs in a reactor 22 which is provided with a heating device 26, 27. The biomass is added into the reactor from up, and the bottom 23 of the reactor is constructed so that the mass may pass through it to a conveyor 24 under the bottom, and, on the other hand, air is let into the reactor, as indicated by arrows A, the ventilation being intensified by a suction apparatus 28. The mass is after enzyme treatment discharged by means of the conveyor into a dryer 25 by which it is dried to dry matter content of 70 to 95 %.

Granulation may be combined with the drying, and it may be carried out in the same way as described above in relation to the process of Fig. 1. A part of the dried mass is brought into a store 28 and a part is circulated back via a screen 7 and crusher 8 back to the beginning of the process and there into a dry matter silo 19 into which also some other dry biomass, e.g. mass including cellulose, may be brought to be used as a solid additive in the process. The biomass to be added to the process for treatment is brought into a silo 16 and from it through a crusher 17 into a mixer 20 in which circulated dry biomass and possibly some other solid additive from the silo 19 is mixed with it, so that the dry matter content of the mixture is e.g. 50 to 55 %. In the mixer a certain amount of an enzyme or mixture of enzymes in relation to the amount of dry matter of the biomass brought into the process is added to the mass in the same way as described above in relation to the process of Fig. 1 and method of Fig. 3. The mixed and enzymated biomass may be fed into the reactor directly or also in this case after pressing it into pieces by means of an extruder 21.

A possible heat treatment in an enzymation reactor is described with reference to Fig. 5. Crushing, mixing, addition of enzymes and possibly pressing into pieces in phases 36, 37 and 38 may be similar to the corresponding phases described above. In phase 39, the mass is heated in the reactor first to e.g. 60 to 70 °C in which it is held for 1 hour which is a long enough time for action of the enzymes. After that, the mass is heated to 75 °C for 1 hour for securing the hygienization. In phase 40, the mass discharged from the reactor is dried to dry matter content of 70 to 95 % after which it is screened in phase 41. At the same time a part of it is taken as a final product into a store and the remaining part is circulated through a crushing phase 42 back to be mixed in phase 37 with the biomass coming into the process.

If the biomass treated with cellulose, e.g. the hydrolysis residue of a digester or final products of separately collected biowaste, produce odor, the odor problem may be removed e.g. by means of a treatment with hydrogen peroxide. For example 40 percent hydrogen peroxide may then be used and dilute it at a ratio of 1:2 to 1:5. Hydrogen peroxide oxidizes odor gases, like hydrogen sulphide. At the same time hydrogen peroxide hygienizes e.g. sludge and biowaste, i.e. kills possible plant and other pathoghens in the masses. Among others, hydrogen peroxide kills Clostridium bacteria which reduces sulphides. Also peat has absorbing properties and includes antiseptic and odor removing components. Adding peat as a dry matter into a biomass to be treated (e.g. 10 to 15 percent peat) reduces harmful odors in the final product.

A biomass to be treated with the method according to the invention may be e.g. sewage sludge, as it is in the process of Fig. 1. Also the hydrolysis residue of a digester may be treated with the method. A biomass to be treated may include also biomasses from agriculture or animal husbandry which include a lot of cellulose and hemicellulose, or

biomass waste produced by food industries. A biomass may include also milled peat or crushed sod peat. Also peat itself may be treated advantageously with the method according to the invention. Besides that, a biomass may include matter from trees or other plants, like bark, bark waste, sawdust, wood chips or straw, or waste paper a lot of which is often included in separately collected biowaste. A mass to be treated may also be waste mass from which only a part is biomass, e.g. fibreclay, the waste produced by paper industries, the typical contents of which are 25 % biomass (fibres), 25 % clay and 50 % water.

A biomass treated with the method of the invention is dry hygienized matter (in many cases granulated) in which a significant part of cellulose and possibly hemicellulose have been broken down with enzyme treatment in connection with drying the mass. Therefore the final product is suitable to be used as a soil improving material or as a basic material in making fertilizer. In biogas production, treating a biomass with the method of the invention before feeding it into a bioreactor increases significantly gas production because there are significantly more short chained carbon compounds available for microbes than the mass originally had. A sludge, for example, may be treated with the method of the invention to grains which are fed directly to a digesting reactor. A granulated mass is also suitable for realizing different kinds of biofilters, e.g. in cleaning waste water.

The technical values presented above, e.g. temperatures, treating times, dry matter contents and added amounts of enzymes, are only examples. As enzyme products are developed, for example, some other values may become more advantageous in realizing the method of the invention. The above described exemplary processes include a phase securing hygienization. There may also be applications in which it is not necessary, and the temperature of a mass may be held a little lower during the whole process.

The dry matter content of a mass coming to the drying phase may vary within the range of 40 to 65 %, for example.

Also the realizations of drying drums, reactors or equivalent devices used for applying the method may vary in many ways. E.g. the process described with reference to Fig. 2 may be realized also in a horizontal reactor which may by also a rotating drum, whereby the reactor and the drier may be united. The invention may vary within the scope of the accompanying claims.