Manufacture of lump peat for energy purposes

PRESENT STATE OF THE TECHNOLOGY

Modern lump peat machines generally consist of a harvesting and a discharge unit. The harvesting unit consists of a rotating, vertical circular disc equipped with a driver, a screw conveyor, or a chain feed. In all cases the peat is collected from the section between the surface of the peat layer and down to a depth of approximately 20 - 50 cm.

The harvesting unit can also, when using the field-press method, consist of an excavator bucket or it can also consist of a bucket elevator. In all cases, the function of the harvesting unit is to move the peat from the natural peat deposit into the machine.

After the harvesting unit has moved the peat into the machine, it is handled by the discharge unit, which usually consists of a conveyor screw enclosed in a tubular casing. The task of the screw conveyor is to press the peat out through one or more nozzles placed at the end of the conveyor. In the nozzles, the peat is formed into 'sausages'/pieces that fall out onto/are placed on the surface of the deposit or on another surface to dry in the air.

The lump peat machine is often drawn by a tractor. The harvesting unit and discharge unit are driven by the tractor engine or by an engine unit independent of the tractor.

The raw peat's quality is usually measured by determining the degree of humification on the von Post scale, where H 1 designates an insignificantly humified (decomposed) plant structure and H 10 designates a completely humified (decomposed) plant structure. For the quality of the finished peat, there is a direct relationship with the humification, which is that a high degree of humification provides good quality.

One way of imparting a higher quality to the peat than that corresponding to the humification of the raw peat is to knead the peat, which implies that the peat is processed mechanically on its way through the lump peat machine.

The advantages of kneading have long been known within the branch. But the significance has been neglected in most cases in the design of the lump peat machines available on the market at present. What has principally been given priority is the machines' volumetric capacity. It is true that a certain, limited kneading is achieved in the machines, but this is more to be regarded as a secondary effect of the harvesting and discharge device.

If the kneading is neglected, the humification of the raw peat should not generally be less than H 5 for an acceptable quality to be obtained. As a peat deposit normally contains raw peat with humification varying between H 3 and H 8, a final product of very irregular quality is obtained. In many cases, the surfaces that contain low humification peat, H 3 and H 4, must be completely excluded from production.

When manufacturing lump peat it is therefore important that the peat be kneaded optimally. This applies particularly to low and medium-humified peat as the kneading affects the raw peat's structure and properties so that the finished lump peat's specific weight, density and dry-matter content increase. Even the strength of the individual pieces of peat increases. A low-humification peat, e.g. H 3, can by optimal kneading give lump peat of the same quality as a normally kneaded H 7.

Optimal kneading gives the greatest quality enhancement with low-humification peat. The enhancement decreases successively with increasing humification and may be said to be negligible at H 8.

Swedish Patent No. 467 497 contains a description of an invention designed to be applied to existing lump peat machines and which increases the kneading in these. The invention consists of an kneading unit located in the lump-peat machine's discharge device. The kneading unit consists (see Fig. 1 ) of a widening, a chamber (3), that encloses the machine's existing screw conveyor. Inside the chamber and parallel to the screw conveyor there are one or more rotors (4) consisting of a rotating axle with a radially affixed cutting/processing tool (5).

The peat is fed into the chamber by the screw conveyor ( 1 ). As the screw capacity is considerably greater than the amount of peat supplied via the machine's harvesting unit, the screw in the front part of the chamber is not completely filled. In the rear of the chamber a pressure is generated that compresses the peat due to the resistance that occurs when the peat is to be pressed out through the nozzles (2) located at the end of the screw. The resulting pressure is both axial and radial. The radial pressure presses some of the flow of peat out towards the rotor (4) where the peat is kneaded at the same time as the rotor moves back towards the chamber's front part where the kneaded peat is returned to the screw.

The length of the section within which the axial force is exerted, i.e. from the nozzles and back to the chamber, varies due to factors that include the inner structure of the peat (friction properties). Low-humification peat whose friction is high therefore provides a shorter section where axial pressure is exerted than high-humification peat. If the section in which axial pressure is exerted is less than the distance A (see Fig. 1 ) no radial pressure is obtained in the chamber that can press the peat against the rotor. If the section in which axial pressure is exerted is considerably longer than the distance A, the opposite effect is obtained, i.e. the radial force inside the chamber becomes too great, resulting in overloading of the rotor.

The shortcoming of the kneading unit described in Swedish Patent No. 467 497 is that the distance A is constant, which means that the kneading ability decreases with lower humification and increases with higher humification, which is completely contrary to what is desirable. In the extreme cases, this results in the kneading unit being underloaded and overloaded.

DESCRIPTION OF THE INVENTION

The size and axial extent of the pressure obtained by the screw conveyor within and after the chamber and which is caused by the screw movement towards the nozzles is mainly dependent on two factors, namely the peat's structure (friction properties) and the contact (clearance) of the screw with the surrounding pipe. The relationship between these factors and the pressure obtained can generally be described by saying that peat with a coarse structure, i.e. peat with low humification, provides an axial force over a short distance, while peat with a fine structure, i.e. peat with high humification, provides an axial force over a long distance.

If the axial extent of the pressure is greater than the distance A, i.e. larger than the distance between the chamber and the nozzles, a radial pressure is obtained in the chamber that presses the peat out towards the rotor. If the axial extent of the pressure is less than A no radial pressure is obtained in the chamber and, thus, no peat is pressed out against the rotor.

As the structure of the raw peat in a peat deposit varies both vertically and horizontally, if the distance A is constant the rotor will be unevenly loaded. The load can, depending on the peat structure, vary from overloading to no load at all.

The invention is a complement to Swedish Patent 467 497 and implies basically that the distance A is made variable by means of a regulator (6) that reduces or increases the flow of peat from the screw conveyor ( 1 ) to the rotor (4) so that the rotor (4) is always subjected to the optimal load in relation to the engine power available and independent of the peat structure encountered.

The movement (C) of the regulator (6) is obtained from a linear actuator(9) controlled by a sensor that senses the load on the rotor (4) . The regulator (6) closes and opens when the rotor (4) tends to be overloaded and underloaded respectively.

The kneading chamber (3) including rotor (4) is extended so that the kneading chamber casing (8) encloses the screw conveyor's casing tube (7) over the section B. On the screw conveyor's casing tube (7) and within the extended kneading chamber (B) a regulator (6) is located with a throughput opening (D/E) located in line between the screw conveyor ( 1 ) and the rotor (4) . The peat which via the regulator (6) subjects the rotor (4) to load on the extended section (B) will be moved out to the kneading chamber (3) partly by the radial pressure from the screw conveyor ( 1 ) and partly by the rotor's processing tool (5).

The invention is also shown in Figures 1 to 3, where

Fig. 1 shows the principle of the design of the kneading device and the location of the regulator.

Fig. 2 shows the regulator in a section perpendicular to the screw conveyor.

Fig. 3 shows the regulator in a section parallel to the screw conveyor.

CORRECTED

ADVANTAGES OF THE INVENTION

With the location of the regulator between the screw conveyor and the kneading chamber rotor, the flow of peat towards the rotor can be regulated continually so as to achieve the optimal load and thus the optimal kneading can be obtained irrespective of the peat structure encountered. As the movement of the regulator is controlled by the rotor load, the flow of peat towards the rotor can be optimized in relation to the instantaneously available engine power.

Without the regulator, the distance A must be adjusted to a pre-assessed mean value of peat structures to be encountered. This results in the rotor being overloaded or underloaded when deviating structures are encountered. Optimal kneading cannot thereby be obtained.

With optimal kneading the quality of lump peat increases, which, in addition to increasing the value of the product, also provides production advantages in the form of a shorter production cycle and less loss.