The present invention relates to a method for producing fuel peat, in which - the peat is detached from the bog from the lifting area, the detached peat is transferred as a high-consistency mass, with a solids content of 8 - 30 %, preferably 15 - 25 %, from the bog to a drying field, which is a special base designed for drying, on which the transferred peat is spread as a thin layer and dried mainly by utilizing direct solar energy, in which case the peat dries through the effect of solar radiation and wind, and - after drying, the peat is transferred from the drying location to further use or storage.

The invention also relates to a drying field created to apply the method.

An increasing number of bogs in production are reaching the stage in which their surface areas are decreasing and production is becoming more difficult as the peat layer thins. On the other hand, the requirements of environment permits are being tightened with respect to emissions to waterways, dust, and noise. Production possibilities are also limited in the vicinity of housing. It is becoming increasingly difficult to obtain permits for new bogs, particularly those in a natural state, making it necessary to exploit old bogs and their edge area, as well as small forest-ditched bogs, even more thoroughly than previously.

A so-called hydro-peat method, for example GB 166,576, is known, in which a water jet is used to detach peat from a bog and pump it as a very thin sludge to a drying field, which is usually an area of bog taken out of production. On the field, the mass is spread as a thin sludge and allowed to dry for several weeks, after which cuts, preferably as a briquette-sod pattern, are made at least in the longitudinal direction in the drying paste-like mat, when separate briquettes form as the mass dries. Using this technique, it may be that only a single harvest is obtained in a summer, so that the total production output remains modest.

Finnish patent FI-93855 discloses a peat-production method, in which wet peat is detached from a bog and collected in a stockpile at a selected place on the peat field, the dried side surface of which is turned at regular intervals to the other side. Even using this technique the production output remains modest .

The basic problem when drying peat appears to be that drying takes place on a wet surface, which hinders the drying process. Often, the drying that takes place during the day is lost as a result of dew forming at night. Finnish patent publication 56853 discloses a method, in which the peat is lifted in winter and placed as small sods on plastic to dry, in which case these sods dry sometimes even before the bog has had time to thaw. According to the prior art, peat cannot be effectively dried on a bog area, because the peat field itself does not dry, due to the peat's water-retaining properties. In the traditional method, material is removed from the surface of the bog, exposing new wet peat. At the same time, the drying conditions become poorer as the surface of the bog approaches the level of the watertable. On some production fields (e.g., SU1314071), widely-spaced subsurface drains have been used. However, on a freshly-exposed surface, water is not easily absorbed, nor does the water travel into these subsurface drains. The water remains standing for a long time in hollows.

The applicant's publication EP 1524310 discloses a method according to the preamble, in which peat is transported some distance away from the production area to a special field to dry. Very good results in terms of production efficiency have been obtained using an asphalt field, but such a special field construction is very expensive. Conventional milled-peat pro- duction produces 500 MWh/ha. Using such a field method, and particularly using an asphalt field, drying outputs of 5000 - 10 000 MWh can be achieved. In terms of logistics, trailer transport is disadvantageous, in that the mass must be loaded a second time onto the spreading trailer. For this method, high-consistency-mass pumped transfer has been developed, though this too has the disadvantage of a second loading, unless complex automatic spreading machines are used.

The present invention is intended to create a simpler field method for peat production, the investment costs of which would be much lower than in the prior art, but which would otherwise exploit the advantages of the field method. The invention is characterized by what is stated in the characterizing portion of Claim 1. The characteristics of the drying field according to the invention are stated in Claim 11.

It is surprising that even a bog surface can act as a drying base, if it is dried effectively and aged in order to alter its surface properties favourably. Creating a drying field on a bog area requires efficient subsurface and perimeter drainage. At least the surface part of the bog must be compacted and dried, which typically takes 2 - 3 years, but at least one year. As a result of solar UV radiation and oxygen-rich conditions, the surface layer dries and hardens to a depth of a few centi- metres, while over time cell changes too take place. In particular, the capillary properties of the field become favourable through drying. Once the decomposed peat has compacted and its surface has become very dry (typically to a 20 - 30-% moisture content) , it finally becomes water repellent and does not easily become wet. This does not happen on a normal production field, as the peat is harvested at a moisture content of about 40 % and a new fresh peat layer is always exposed once the previous layer has been milled off. In the method according to the invention, the surface layer becomes wet during heavy rain, but the slope of the field and the subsurface drains dry it rapidly. Under the thin dry surface layer, the moisture content is still quite high, typically 75 %, down to the subsurface- drainage depth. However, the fuel peat on top of it is dried to become considerably drier (e.g., 40-% moisture content) . In between is a thin dry surface layer, which rapidly dries if it becomes wet.

Closely-spaced subsurface drainage is essential, as water moves very slowly in peat. Though it appears to be technically possible to accelerate the aging of the bog surface by a suitable acid treatment, this is not to be recommended, due to environmental risks.

In the present method, the field surface is not degraded, so that its structure remains good for drying material. The few conditioning measures that may be necessary to maintain the profile of the field will create at most momentary drawbacks.

Using the field solution according to the invention, it is easily possible to achieve peat production of 2500 MWh/ha, which is less that the yield of an asphalt field, but many times that of conventional production. On the other hand, the creation of such a field costs only a fraction of the creation of an asphalt field. In addition, such a field can be made right next to the lifting area, thus reducing transportation distances. It is often possible to lift the peat mass directly onto the spreading trailer, which simplifies production logistics .

Using the method according to the invention, 20 - 25 harvests of granular peat can be obtained annually, i.e. about 100

MWh/ha/harvest and a total of as much as 2000 - 2500 MWh/ha in a production season. During periods of good evaporation, i.e. when moisture evaporates at a rate of 6 - 7 iran/d, the drying of granular peat takes only about one day. At a normal evaporation rate, less than 5 mm/d, the drying of granular peat takes 2 - 4 days. An advantage of the short drying period is that production can be timed optimally on the basis of weather forecasts. In normal peat production, if rain wets a nearly dry peat layer, a long wait is necessary for the field itself and the milled peat layer to dry. The method according to the invention can exploit short dry periods of a few days between periods of rain better than the prior art. A second advantage is that the conditioned peat forms denser granules or sods than milled peat, which withstand rain and mechanical handling well.

In one embodiment, the peat is spread on the drying field as small sods, i.e. lumps. Spreading can be done using certain types of manure spreader. As drying progresses, the lumps are collected, when they disintegrate to form granular peat. In this case, the advantage of granular peat is its greater gran- ule size compared to milled peat. Thanks to its larger granule size, granular peat's dust loss is considerably less than that of milled peat and it withstands rain better. The properties of the fuel-peat product obtained correspond mainly to those of rough milled peat, but there are fewer fines in it and its thermal value and density are greater than those of milled peat .

In sod-peat production, the shape of the sod being spread is, as such, not decisive. However, it is preferable if the contact surface with the base is minimized, as with a corrugated sod.

In the following, the invention is described with reference to the accompanying figures, which show some applications of the invention. Figure 1 shows the overall arrangement of the peat work site, as a layout diagram, Figure 2a shows a cross-section of a drying field with subsurface drainage, Figure 2b shows the moisture profile of a drying field without subsurface drainage, and Figure 3 shows a top view of the drainage arrangement of the drying field of Figure 2.

In fuel-peat production, the stages of the method are as follows :

1) subsurface drains are laid and feeder ditches are dug in the drying field while a perimeter ditch is dug around the entire area,

2) the drying field is allowed to age and dry for at least one year, preferably for at least 2 - 3 years,

3) peat is detached from the bog in the lifting area, which is close (generally less than 1 km) from the drying field,

4) the detached peat is moved from the lifting area to the drying area, next to the drying field,

5) the peat is spread on the drying field to dry by a spreader device, so that the peat becomes lightly conditioned lumps,

6) as drying progresses, the lumpy peat is collected in a ridge, when the lumps disintegrate to form granules,

7) the dried peat granules are collected and trans- ferred from the drying location for further use or storage,

8) each new harvest is collected according to stages 3 - 7.

Instead of lightly conditioned lumpy peat, sod peat can be produced, in which more strongly conditioned peat is pressed through a nozzle to form sods that are substantially larger and denser than granules, which also have a compact surface. These remain in shape better than lumps when they are gathered into a ridge. Machines known from milled and sod-peat production can be used in ridging.

In the method according to the invention, the drying field 12 is preferably located in the immediate vicinity of the lifting area 10 (Figure 1) . Thus an excavator or other work machine can be used to lift the peat directly from the bog to the spreading trailer, which is used to transport the peat to the drying field 12 and spread it there. Depending on the site, a separate transfer can be used before loading the spreading trailer, if the distance is more than 500 m.

Typically, the drying field 12 is built on a field that has been taken out of production, on an old stockpile base, on new areas, preferably on a low edge area, in which there is a thin, 0, 3 - 1, 5-m, preferably 0, 5 - 1-m peat layer on top of the subsoil 14, according to Figure 2a. This realizes the use of fields, which have been lowered, for sod-peat production and a more thorough exploitation of peat resources. A perimeter ditch is dug around an area of the selected size of the peat layer 16 and feeder ditches are dug at 20 - 40-m intervals. The open ditches 18 extend down to the subsoil 14. After that, subsurface drains are laid in each strip, preferably in two layers, i.e. an upper layer at a depth of 0, 3 m (0,25 - 0, 5 m) and a lower layer at a depth of 0, 7 m (0, 4 - 0, 9 m) . The upper subsurface-drain layer is about halfway down the peat layer 16, or slightly higher, while the lower layer is closer to the bottom of the ditch. Thus, the perimeter and feeder ditches will be lower than the subsurface drains, so that water coming from the subsurface drains will flow into the feeder ditches and from there into the perimeter ditch. The subsurface drains prevent the groundwater from rising by capillary action through the peat layer. The subsurface drains are preferably pipe subsurface drains, because their operating life is not short. So-called mole drains can also be used.

Subsurface drains laid on two levels are advantageous, as the deep subsurface drains prevent water rising by capillary action and the subsurface drains on the surface dry the surface layer rapidly after rain. Thus, in all conditions, water is removed optimally. Without subsurface drainage, the groundwater level will extend nearly to the peat surface in the ridge between the feeder ditches, according to Figure 2b, when the moisture profile 22 of the ridge will typically be curved. In this connection, effective subsurface drainage refers to the ground- water level being reduced with the aid of subsurface drainage to a depth of at least 0,6 m, and also that the groundwater level will at no point rise close to the surface, but will remain everywhere at a considerably low level.

According to Figures 2 and 3, the subsurface drains are at intervals of 2,5 m (generally 2 - 10 m, preferably 2,5 - 5 m) . Every second subsurface drain is preferably deeper. Thus, subsurface drainage is preferably made to two levels 0,3 - 0,7 m apart from each other. The subsurface drainage can also comprise transverse subsurface drains. If the subsurface drains are installed too widely apart in the peat layer, they will not be able to prevent sufficiently the water from rising by capillary action. The width of a strip is generally 20 - 40 m. The length of a strip can be in the order of 100 m.

In the method according to the invention, the drying field is sloping, so that most of the surface water can flow into the feeder ditches. The fall of the drying field is 1:1000 - 5:100, preferably 1:100. The fall can be made in the drying field, for example, with the aid of milling, before the field is aged.

The drying field is allowed to age for at least one year and preferably 2 - 3 years before starting production, when the thin surface of the peat layer 16, at a depth of a few centimetres, which is here marked as the layer 17', dries and its cellular structure disintegrates at least to some extent. The surface of the peat layer then compacts and hardens and becomes a durable drying base, which allows water to percolate through it poorly. However, the thin surface layer dries to become very dry and does not conduct water upwards by capillary action. Otherwise, the upper part 17 of the peat layer 16 dries to a moisture content of 75 % down to the subsurface drain level.

The limiting factor in lifting peat is the solids content of the raw peat, which should be more than 15 %. The drier the raw peat, the greater the drying result that will be achieved. In sod-peat production, the maximum solids content of the raw peat is less than 20 %, as a spreading trailer cannot form a sod from peat as dry as that. In this connection, references to sod peat mean peat pieces, which have a granule size of more than 15 mm, with a small deviation. In milled-peat production, there is no upper limit to the solids content, but a solids content of more than 30 % seldom appears. A sufficiently dry layer on new bogs is generally 0,5 - 1,0 m and in old areas even as much as 2, 0 m (1, 5 - 2, 5 m) .

The peat lifted from the lifting area is transferred as a high- consistency mass with a solids content of 8 - 30 %, preferably

15 - 25 %, from the bog to the drying field, where it is spread with the aid of a spreading device. Using the method according to the invention, the peat is spread on the drying field as thin layers, preferably 2 - 5 cm (in other words, 16 - 40 kg/m ² ) , which dry rapidly through the effect of sunlight and wind, in about 1 - 4 days, depending on the spreading thickness and the evaporation. Drying to the selected final moisture content takes place in a ridge, in which the peat is collected as granules or sods. The dried fuel-peat product is transferred to a store, generally with a moisture content of at most 55 %, preferably 35 - 50 %. In granular-peat test production, a Jeantil® manure spreader has been used successfully to spread the peat. The manure- spreader trailer divides the mass into small pieces and the mass is conditioned by being slightly compacted, so that lumps without fines arise. The manure-spreader trailer includes a bottom conveyor (chain conveyor) as well as vertical screws, which throw the lumps onto the drying field, when the compacted lumps spread as somewhat flat sods on the surface of the drying field. For this purpose, it is also possible to use some other spreading device possessing a mechanically similar conditioning and spreading power. Of the total number of the lumps, at least 90 % are 20 - 200 g in weight and about 25 cm ² (generally 15 - 100 cm ² ) in size. The drying of the lumps is facilitated by their size and shape, which give them a large surface area. In addition, the lumps' contact surface area with the base is small, so that they also dry slightly from underneath.

One product obtained by using the method has properties that correspond to, or are somewhat better than rough milled peat. The following values are used and achieved in spreading and production (the preferred target values appear in square brackets) :

- loading 20.5 kg/m ² (wet 74,8 weight-%) [16 - 40 kg/m ² ]

- solids 5,2 kg _dm /m ² [4 - 7 kg _dm /m ² ] (dm = dry matter) - cubic weight 406 kg/m ³ (normal milled peat 378 kg/m ³ )

- moisture content 44,9 m-% (normal milled peat 48,1 m-%)

- effective thermal value 1,21 MWh/m ³ (normal milled peat 0,95 MWh/m ³ )

- production per harvest about 280 MWh/ha - in 2008 3 harvests were obtained, totalling 840 MWh/ha. For reasons of comparison it should be noted that normal milled- peat production in the area in question was 400 m ³ /ha under the same production conditions, i.e. 360 MWh/ha. These are the results for one summer in one area and are intended as an example. The quality of the peat is very good. The spreading output of the spreading machine (Jeantil®) was about 70 mVhour/trailer and the production output 43 MWh/machine hour. The logistical advantage that was immediately gained was that the spreading trailer could be used to trans- port the peat mass from the lifting location to the drying field (distance less than 500 m) . In sod-peat production, the production target for a single trailer unit is 14 000 m ³ per year and requires a surface area of about 20 ha. The spreading capacity is 40 - 50 t/h with a distance of 300 - 500 m.

In terms of the drying of the peat, the important factors used in the method are the aged and sloping peat surface of the drying field, which reduces the absorption of rainwater into the drying base. Thanks to the slope, most of the rainwater flows rapidly into the feeder ditches while, as a result of aging, the compacted surface does not absorb as much rainwater as a normal bog base.

As drying progresses, the partly or already completely dried lumps can be collected in a ridge, when the lumps disintegrate into granules being, however, substantially coarser and denser than milled peat. The granular peat is allowed to dry to the target dryness, after which it is transferred to further use or storage. By means of the method according to the invention, a granular-peat harvest can be gathered as many as 25 times in a season, i.e. the production cycle is rapid. The granule size of the finished, dried fuel peat is 3 - 8 mm, preferably 4 - 6 mm, so that it is coarser that normal milled peat.

According to one embodiment, the surface of the drying field can be bound with a selected binder, particularly to reduce dust formation.

The moisture and solids percentages above and in the Claims refer to the portion of the mass.