FIELD OF THE INVENTION

The invention is related to a method for production of peat-based fuel.

BACKGROUND OF THE INVENTION

Generally known peat-based fuels include milled peat, peat pellets and sod peat. Milled peat is normally produced in the following way. E.g. a 20 millimeter thick layer of peat is cut by milling from the surface of a bog, where the moisture content is normally 70 to 80 percent, for drying. The milled layer of peat is turned 1 to 3 times in the course of drying. Because solar energy is utilized in the drying process, peat must be produced in summertime and the milling must be carried out at dry weather. The drying takes normally 2 to 3 days. Temperature, air humidity, wind and peat quality are the evaporative factors affecting the time needed for drying. The aim in the drying is to reduce the moisture content to about 40 percent.

Finally, the dried layer of peat is collected into stacks from which milled peat is then transported to incineration plants as needed.

In an average summer there are 40 to 50 days suitable for production, and so it is possible to produce 15 to 20 production lots. Milled peat varies in composition. It is partly granular and may include slightly decomposed parts of plants but a remarkable part of it is always fine dusty matter. As milled peat is used as a fuel mainly in large incineration plants, its moisture content must be approximately of the order of said 40 percent so that danger of explosion is avoided. Peat pellets are made of fine milled peat which is dried and pressed into pellets in an essentially same way as wood pellets are made. The diameter of peat pellets is typically within 8 or 12 millimeters. The moisture content varies normally within 8 to 15 percent. Similarly to wood pellets, peat pellets break down turn into dust quite easily. In small or relatively small incineration plants where pellets are normally burnt, fuel supply systems, however, are such that dusting is not a severe problem. In sod peat production, a machine cuts peat from the surface of a bog 30 to 50 centimeter deep by means of a lifting disc or screw provided with cutters. The moisture content of the peat is normally more than 80 percent. At the same time, the machine works the mass and extrudes it through nozzles on the field for drying. The extrusion produces typically either a bar with a diameter of 40 to 70 millimeters or strip with wave-like section which are broken into pieces. In the latter case, so called wave-like sod peat is concerned. It is, of course, possible to press pieces into some other form. Normally, the pieces are let to dry 2 to 4 weeks, during which period they are turned 1 to 2 times. The aim is to reduce the moisture content of the pieces to 35 to 40 percent.

Finally, the dried pieces are collected and stored in piles from which sod peat is then transported to incineration plants as needed. For making sod peat production more efficient, a method is presented in which peat is after extrusion transferred for drying to an asphalt field provided with heating pipes. Heat is produced e.g. by solar panels. The drying of the pieces is intensified remarkably because asphalt absorbs efficiently heat from solar radiation and because it is additionally heated from inside. The efficiency of the production is, however, weather dependent also in this method.

Sod peat is dried efficiently on the side which is towards the sun but at the same time the dried layer of peat forms an insulation at the surface preventing heat to flow inside the piece. The same concerns to some extent also the influence of heat coming from underneath when drying pieces on an asphalt field. Therefore also sod peat is only partly tight and highly consistent and partly easily crumbling and dusting. Danger of dust explosion is reduced with the use of sod peat as sod peat is burnt in grate boilers. Dust problem is as a whole remarkably smaller than with milled peat but it must be taken into account also as sod peat is treated.

The dusting problem of the known peat fuels concerns not only treating and use but also production and environmental influences. Peat dust is spreading from peatlands to the environment and is washed into water systems.

The moisture content of the order of 40 percent in the peat fuels used in large scale: milled peat and sod peat, naturally reduces remarkably the amount of energy produced in combustion but it has also other adverse effects. It increases corrosion in combustion equipment and formation of nitrogen oxides in combustion process. The energy efficiency is reduced also by the fact that large amounts of water in addition to the fuel itself are transferred from one place to another in transport.

A big problem of the present production methods of the peat fuels is the very great dependency of the production on weather conditions. As mentioned above, there are in average 40 to 50 days favorable for production per year. In rainy summer, production may be even less than one half of a normal production.

SUMMARY OF THE INVENTION

An object of the invention is to present such a method for production of peat-based fuel by which the above mentioned problems are remarkably reduced. To achieve this object, a method according to the invention for production of peat- based fuel, in which method peat is lifted from a bog for supplying it into a production process, is characterized in that which is defined in the characterizing part of the independent claim 1. Other claims define various embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is disclosed in further detail in the following with reference to the accompanying drawings, in which:

Figs. 1 and 2 present schematically examples of the realization of the production of peat-based fuel by means of the method according to the invention;

Fig. 3 is a flow chart presenting generally the method according to the invention; Fig. 4 is a flow chart presenting an advantageous embodiment of the method according to the invention;

Fig. 5 is a flow chart presenting another advantageous embodiment of the method according to the invention;

Figs. 6 and 7 present schematically a possible realization of biological drying in the method according to the invention; and

Fig. 8 illustrates drying and shrinking of a piece of peat in the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 presents an example in which the method according to the invention is realized totally in connection with a large peat production area. The peat is lifted from the bog by means of e.g. an excavator (not presented) and supplied then through a feeding hopper 9 provided with a trash rack to a conveyor 10. By means of the trash rack bigger stumps and equivalent matter are removed from the peat so that they can not damage the conveyor. The conveyor 10 feeds the peat into a silo 11. The dry matter content of peat lifted from a bog is typically 10 to 25 percent.

Peat may be lifted from a bog also at a larger distance from the conveyor and then transfer it by means of a dumper or tractor towed trailer into the feeding hopper. The conveyor is advantageously constructed so that it is easily moved. Peat may also be transported from farther away directly into the receiving silo 11 by means of suitable vehicles.

A silo 13 contains dry crushed biomass, preferably crushed hydrophobic peat having the dry matter content of more than 90 percent. Dry crushed matter is mixed in a mixer 12 with peat so much that the dry matter content of the mixture obtained is high enough for aerobic microorganisms to function. A high enough dry matter content is typically about 40 percent and it is achieved by mixing about 0.5 to 0.6 cubic meters hydrophobic crushed matter with one cubic meter wet peat. Peat is advantageously well decomposed sedge peat with which the best properties are obtained for the peat-based fuel produced by the method according to the invention.

Next, the mixture is extruded into pieces in phase 14. This is made by means of e.g. a press provided with a suitable pipe-like nozzle from which 5 to 10 centimeter thick bar is brought which is typically broken into 15 to 30 centimeter long pieces. In the following phase, the pieces are dried biologically. This is made e.g. by collecting the pieces into a pile or stack of a suitable size. This is illustrated in Figs. 6 and 7. In connection with the press 21 there may be a belt conveyor 22 movable within a certain range and lifting height for feeding the pieces P into the pile S. Under the pile there is advantageously a grate-like or other type of platform G provided with holes above the ground level L, but the pile may be made also on dry soil surface or a suitable cover placed on the ground. The pile is covered by a tight cover 23 with wide fitting, i.e. so that there is a wide enough space R between the cover 23 and the pile S in view of ventilation and removal of moisture. The hems 24A and 24B of the cover are e.g. foldable and may be set so that the distance from them to the ground may be adjusted. In that way the air flows A are adjusted suitable for the drying process under e.g. different wind conditions. The moisture evaporated from the pile as it is warming up rises with the warm air up and out through a discharge uptake or equivalent device 25 provided with an adjusting plate or equivalent device 26 for adjustment of the flow V.

A suitable cover is e.g. a tight tarpaulin used on building sites, in which tarpaulin a firm base fabric is coated with thermoplastic material on both sides. The coating arrangement may be made also in the same way as the tents used widely on building sites. The frame of a tent is made e.g. of steel or aluminum and is formed by e.g. 5 to 10 meter long modules which may be moved easily. The best ventilation is provided in the way presented in Fig. 7 by arranging adjustable intake openings in the bottom of the sides and adjustable discharge channels at the top, wherein the discharging air at the top causes negative pressure and draws at the bottom in cool dry air capable to receive the moisture evaporated continuously from the pieces. The realization of the ventilation may, of course, vary in many ways.

The purpose is to make aerobic microorganisms in the pile to start working so that the temperature of the pieces and the pile rises relatively quickly to the thermophilic range close to 60 ⁰ C. According to experiments, the temperature rises to this range normally within 10 to 15 hours. It is essential in the method according to the invention that each piece is drying like a separate unit itself. During extrusion some moisture pressed out from the mass remains on the surface of a piece so that the piece is dryer in the inside than at the surface. Therefore, the biological reaction is started in the piece from the inside and the warming inside drices moisture outwards towards the surface layer and out from the piece. Fig. 8 illustrates the progress of drying in a piece. A quickly rising temperature T is achieved in the core of the piece P and the moisture starts to be pushed towards the surface and out from the piece as indicated by arrows V. At the same time the piece starts to shrink which is indicated by arrows F illustrating the shrinking force. The drying going on as indicated by arrow D, the reaction is moving closer to the surface and the piece shrinks step by step evenly at each side. In our opinion, the shrinking forces affect so that as if moisture is pressed out from the piece and it is drying more than which is possible to reach by means of the heat produced by the microbiological reaction only.

The method differs from the present drying methods also in that the drying is realized as a continuous efficient process in which the temperature of the pieces is kept continuously high, optimally close to 60 °C, and drives moisture out from the pieces. For example, in the conventional drying of sod peat the pieces are typically warmed up in the daytime and cooled down again in the night evaporating moisture as long as their temperature is higher than the air temperature. Sometimes rain and damp air turn drying backwards. And as mentioned above, the drying is uneven also otherwise for which reason the pieces dried quickly and efficiently by the method of the invention have incomparable properties in comparison with conventional sod peat.

The biological reaction is stopped gradually as the dry matter content of the pieces approaches 60 percent. Thereafter, the pieces are allowed to cool down because the drying and shrinking still continue during the cooling due to the influence of vapor pressure difference.

Under good conditions the pieces are dried to the dry matter content of 60 to 70 percent within 3 to 4 days. In rainy weather the drying takes a little longer time, about one week. The production period may be lengthened by placing the biological drying in a hall like in the example of Fig. 2.

The pieces dried biologically by the method according to the invention are approximately equivalent to sod peat as a fuel but are more homogeneous in consistency, tighter and less dusting than pieces dried by means of solar energy. It is, however, possible to continue the drying and shrinking of the pieces further until they are hydrophobic, i.e. to the dry matter content of more than 90 percent. A preferable drying method of the second phase is thermal, efficiently shrinking drying at low temperatures below 100 ⁰ C. In the preferred method a drum is used in which the temperature of the pieces is raised to 70 to 90 °C. For at least two reasons, it is important to keep the temperature clearly below 100 ⁰ C. When raising the temperature, useful compounds are gasified and vaporized from the pieces and their fuel value is reduced. Additionally, gases cause danger of fire in drying equipment. Driving water out from the pieces by boiling causes porosity in the pieces and prevents shrinking. It is also advantageous to maintain a little overpressure, e.g. of the order of 0.3 bar, in the drying drum. The moisture is then stored in the drying air in the drum up to the saturation point until an overpressure valve opens a exhaust pipe. In this way the energy efficiency of the drying process is improved. Additionally, a portion of the energy of the exhaust air may be recovered by means of a heat exchanger.

It is necessary to keep the pieces in the drum only a time long enough so that their temperature or in fact the temperature of the water included in them is raised to e.g. 80 °C. After that, the pieces are taken out from the drum for final drying and shrinking which is made by allowing the pieces dry in a dry and cool air flow. This phase may be realized by means of e.g. lifting the pieces by means of a suitable conveyor into a continuously operating drying tower which is filled from above and discharged from below and in which a dry and cool air flow directed from below upwards is arranged by means of blow or suction. In the same way, flushing with dry and cool air may be arranged in connection with the conveyor whereby the pieces are drying and shrinking first on the conveyor and then in the tower. This kind of shrinking final drying is important for achieving as good as possible properties for the produced fuel. In view of energy economy, the final drying is very advantageous especially under conditions in which dry and cool air is easily available for the final phase of drying.

The drying drum may be realized so that the air blown into the drum is heated by a burner functioning by means of crushed fuel produced by the method according to the invention, for example.

The drying of the second phase may be made also by other methods, e.g. by radiating heat in a continuously operating drying oven or in a microwave dryer of the type of present bakery ovens. The pieces may also be crushed before the second phase drying but or opinion is that shrinking is achieved better for entire pieces than crushed ones.

The pieces or crushed fuel are dried until they are hydrophobic, i.e. to the dry matter content of more than 90 percent and preferably at least 95 percent.

In the solution of Fig. 1, both the biological drying 16 and the thermal drying 17 and the final drying and shrinking 18 are placed in a hall 15 in the production area. The most of the pieces dried into hydrophobic stage are transported into a store located in the production area or some other place. A necessary portion of the dried pieces are brought through crushing 19 into a store 20 of crushed fuel from which it is supplied through the silo 13 into the drying process.

In the example of Fig. 2, the silos 11 and 13 for peat and dry matter, respectively, mixing 12, extrusion 14 and the biological drying 16 have been realized at the production field Kl. The pieces dried biologically are transported from there e.g. to a center K2 serving several bogs or production fields, in which center the thermal drying 17 and the final drying and shrinking 18 are placed in a hall 15 A. The most of the hydrophobic pieces are brought into a fuel store 27 and a necessary portion through a crusher 19 into a store 20 for crushed fuel from which it is transferred as return transport into the crashed matter silo 13 at the production field.

The method according to the invention is applied preferably in the way described above so that only peat lifted from a bog is treated and utilized. When starting a production, as no dry crashed peat produced by the method itself is available, material crushed and possibly further dried from sod peat produced in that or some other production area may be used instead.

The dry biomass mixed with peat may include also wood or cutter chips, sawdust, straw, chipped energy willow, reed canary grass and so on.

The flow chart of Fig. 3 presents generally the method according to the invention. In phase 1, peat having typically the dry matter content of 10 to 25 percent is lifted from a bog. In phase 2, it is mixed with that much dry biomass, which may be dried hydrophobic peat or even to a great extent other dry biomass, that the dry matter content of the mixture is high enough, normally al least 40 percent, for aerobic microorganisms to work. In phase 3 the mixture is extruded into pieces. In phase 4, the pieces are collected in a pile with wide fitting so that ventilation and preservation of heat are suitable for aerobic microorganisms to work for raising the temperature of the pieces preferably to 50 to 60 ⁰ C and achieving biological drying in that way and for removing the moisture evaporated from the pieces. In phase 5, biological drying is maintained and continued as long as the moisture content is high enough for the functioning of the microorganisms keeping up the process. The dry matter content to which the functioning of the microorganisms continues is of the order of 60 percent. Finally, the biological drying dying down, the pieces are allowed to cool in phase 6 because then they shrink in the best way and the drying is also continued further until the temperature descends to the level of the temperature of the environment and the vapor pressure difference between the pieces and the environment which is drying the pieces ceases from affecting. Thereafter, the dried pieces may be transferred first into a store in which the drying in favorable circumstances, air humidity being low and ventilation being good, may continue. The pieces may be used as such or after crushing e.g. as fuel or crush them for use as basic material for soil improvement materials or fertilizers. The pieces may also be transferred for after-treatment including further drying and shrinking of the pieces or in some cases possibly crushed matter made from the pieces with methods using low temperatures below 100 ⁰ C until the produced fuel mass is essentially hydrophobic having the dry matter content of more than 90 percent and preferably more than 95 percent. Fig. 4 presents an alternative of the method according to the invention in which the fuel is made of peat, exclusively. This alternative differs from that of Fig. 3 only in that in phase 2A corresponding to phase 2 only matter crushed from the produced pieces of peat is mixed with peat. After the biological drying and the following cooling and shrinking, the pieces are transferred in phase 7A to the final drying in which their temperature is raised first to a thermally suitable temperature below 100 °C, e.g. to 80 °C, and then they are allowed to dry and shrink in dry and cool air until their dry matter content is more than 90 percent, i.e. they are essentially hydrophobic.

Thereafter, the most of the produced pieces are utilized as fuel and the necessary portion of the pieces are crushed in phase 7B and stored in phase 7C to be recirculated into phase 2A as indicated by reference sign 8.

The method may be applied also so that additionally other biomass to be dried is mixed with peat, e.g. wood chips, for which as such it is not possible to apply corresponding biological drying because there is not enough easily available carbon necessary for aerobic microorganisms to function. Peat functions then as an intermediary substance in the drying because the moisture content between it and the other biomass which remains moister is leveled and also the other biomass is finally dried into essentially the same moisture content than the peat. In this way also other biomasses may be utilized for producing fuel with remarkably better properties.

This kind of embodiment of the method according to the invention is presented in Fig. 5 the process of which is otherwise similar to the process of Fig. 3 but phase 2 is replaced by phase 2B in which peat is mixed with other biomass to be dried and additionally with that much dry biomass that the dry matter content of the mixture is high enough, normally at least 40 percent, for aerobic microorganisms to work.

By drying and shrinking the produced pieces are made tight and highly consistent. For this, at first, the pieces or grains break down or dust extremely little in comparison with the corresponding fuels produced by conventional methods. The best properties are achieved when the drying is continued until the material is hydrophobic. Hydrophobization achieves for the pieces or grains also special bioelectric properties affecting so that they attract and bind small particles and dust. For the same bioelectric properties, the pieces and grains also reject moisture and water and do not absorb them like e.g. wood or peat pellets which break down quickly under influence of humidity. To pieces or grains dried and shrunk well by means of the method according to the invention it is peculiar that they maintain the hydrophobicity exceptionally well.

The fuel produced by the method according to the invention may be used as such like sod peat in e.g. large heat generation plants provided with grate boilers. The pieces may include also other biomass than peat as described above. A portion of the produced sod peat is in every case crushed and used as dry matter in the process itself and possibly as a fuel in thermal drying. It is also possible to make crushed material of the size 8 to 12 millimeters or 6 to 10 millimeters from the pieces to be utilized like wood or peat pellets. Fine material produced in crushing and other treatments may be used e.g. in the production of bioethanol or biodiesel.

The method according to the invention and the peat-based fuel produced by the method have several advantages:

- A great advantage in comparison with the peat-based fuels produced by conventional methods, with which the danger of dust explosion must be prevented e.g. by keeping the moisture content of the fuel high enough to reduce the danger, is that breakup and dusting of the fuel are minimal.

- Due to high dry matter content the fuel value is remarkably better than with other corresponding fuels.

- A small portion of water in the fuel reduces significantly formation of nitrogen oxides in combustion or in the utilization of the produced material for manufacturing bioethanol or biodiesel.

- Combustion temperature may be raised from the conventional temperature of about 850 0C even to 1500 to 1700 ⁰ C which increases energy efficiency and decreases remarkably the amount of ash produced in combustion. - The compactness achieved by the shrinking drying raises significantly the volume weight of the fuel and improves the energy efficiency in its use. The amounts and costs in transport are reduced remarkably in the production and utilization of the fuel.

- With the method of the invention peat-based fuel may be produced remarkably regularly in comparison with the conventional methods and production is much less weather dependent. If the biological drying is made in a hall, it is possible to lengthen the production period significantly in relation to present one. Although a part of the energy of peat is lost in the drying process in the method of the invention, the advantages obtained replace many times this loss.

The ecological benefits of fuel production are increased remarkably also in peat production areas because much less peat dust spreading to the environment and peat sludge drifted into the water systems are produced in comparison with the conventional methods.

The invention is not restricted to the embodiments described above but may vary within the scope of the accompanying claims.