A heating product and a method for preparing thereof.

The present invention relates to a method for preparing a heating product as depicted in the introduction of claim 1. This heating product has the character of agglomerate, i. e. a unit pressed together comprising particles, such as granules, pellets and briquettes. In the following the term "pellets" will be used.

Background

Sewage sludge is a problem material for most sewerage systems. Due to the content of harmful materials, such as heavy metals, a problem has arisen in connection with use of sewage sludge as fertilizing substance or soil improvement means or in open disposal sites. This has further lead to the need for other ways to utilize the sewage sludge and the value in it. Many attempts have been done to find practical uses of sewage sludge as energy source.

From US patent 4,225,457 (Schulz) it is known to use sewage sludge as a cementing agent when pressing briquettes of torn solid waste.

From US patent 5,797,972 (Schulz) it is known to prepare mechanical stable heating briquettes with sewage sludge as the main ingredient, with the addition of coal and calcium as cementing agent.

From US patent 4,395, 265 it is known that dried, ground sewage sludge is added to peat particles to bind the peat particles together to a pellet for heating. It is not known that this has lead to a useful product, even though it is open for pulverizing of the final result and blowing it into a combustion chamber.

From US patent 5,389,114 it is (Forder 1995) it is known to mix 77% sewage sludge, 12,5% peat and 12,5% sawdust as a raw material for heating pellets. The production process comprises several phases with mechanical and chemical treatment, and disinfection with microwaves. This method is complicated and requires use of chemical additives.

From US patent 5,972,058 (Turkkeli, 1999) it is known to use granulated peat as additive in a process where particles are prepared with basis in sewage sludge. This process requires that the sewage sludge is extruded in such a way, together with the peat particles, that the cell structure in

the sewage sludge opens and the peat particles absorb the sludge. It is further required that the particles are coated with a suitable coating and further are cured to give sufficient stability. From the Korean patent application 200302906 (Ook 2003) it is known to press peat particles, sewage sludge and zeolite in the ratio 65:25:10. Hence this method requires an extra additive.

From the Swedish patent 524283 (Lindahl, 2002) it is known to press briquettes or the like of peat particles and sewage sludge, followed by an impregnation of the briquettes with paraffin wax or similar treatment agent containing oil.

None of the known methods have been satisfactory with regard to the most important parameters, production cost and product quality. With regard to production cost, the known methods have either required complicated equipment and/or a long lasting process. The requirements to product quality are satisfied by use of expensive additives and particular steps in the production process.

Purpose

The main purpose of the invention is to reach a production method which makes it possible to convert sewerage sludge to agglomerate, that is granules, pellets or briquettes, with satisfying characteristics regarding storing and transport, and which allows an effective and environmental satisfying combustion in different stoves.

There is a wish to create pellets or other compositions consisting of particles that are pressed together, which compositions give sufficient strength for transport and storing. This should be achieved without the use of additives.

It is also a purpose to create a combustion product which allows that eventual rests of heavy metals in the sewage sludge may be gathered and treated in a satisfactory way.

There is also a wish to provide a combustion product with good heat values and complete combustion, with acceptable emissions of so called greenhouse gases.

The Invention

The invention is described in the claims. The method is identified by a) The sewage sludge is dried until the dry substance constitutes 60 - 90 % and is then ground,

b) A peat with long fibres is utilized, which is dried to a dry substance value of 60 - 90 % and is ground to a particle size in the area 0,2 - 10 mm, c) A particle mixture is made with approximately 20 - 60% by weight of peat and 40 - 80 % by weight of sewage sludge particles, and d) This mixture of particles is extruded for pressing the particles together.

It has turned out that this method requires simple equipment and is simple to implement. A good binding together of the briquettes is achieved, and firmness, and satisfying characteristics for transport and storing. Moreover, no addition of expensive impregnating or strength increasing additives is required.

The claims 2 - 6 states particularly advantageous features of the invention.

In claim 7 a new combustion product is described, with basis in the present invention. This has shown to have good characteristics regarding handling and transport and satisfying firmness and combustion qualities.

The invention makes it possible to prepare a heating product which is cheaper and have lower costs than comparable known heating products.

Example

Below an example of a method and a heating product is described, where the method is illustrated in the drawings, where

Fig. 1 shows a flow chart for an embodiment of a production process according to the present . invention, while

Fig. 2 shows schematically an installation for accomplishment of the method according to the present invention.

In Fig. 1 each step in one embodiment of the process according to the invention is shown schematically in a flow chart.

Peat was extracted, in particular sod with long fibres, when ploughing with known equipment. The peat was thereafter put in cones for draining off before it was pressed and thereafter ground in a grinding mill. During the grinding it may be alternatively be added other organic materials, such as

sawdust, bark or farming waste containing fibres with a certain content of dry materials, possibly also manure.

The sewage sludge was received after draining off. It was then centrifuged until approximately 20% contents of dry materials, and was transported to a closed chamber for storing without gas released to the air.

The sewage sludge was thereafter ground in a mill and together with peat particles it was dried in a heated drum. The exit temperature was 60 - 70 degrees Celsius. The mixed mass of particles was then transferred to hammering, with an exit temperature of 30 - 40 degrees Celsius. Before the pelletizing machine, a silo was used in order to have an intermediate storing of the mass, and to provide the correct amount to a pelletizing machine. Therefore it is provided for adding steam to the mass before pellets are prepare, so that one has optimum temperature 30 - 40 degrees Celsius and 60 - 90 % dry substances, preferably 80 - 90 % dry substance.

In Fig. 2 it is shown the elements in a production plant based on reception of peat blocks and sewage sludge in bulk.

The starting point for the process and product according to the invention is sewage sludge. In the example it was used sewage sludge extracted from an official cleaning plant which receives sewer from houses. This sewerage sludge was in advance dried off to increase the contents of dry substances.

The installation comprised a mechanical press 11 driven by an engine 12 and with an inlet opening 13 for blocks of peat. This was not vitally necessary to use if the drying was sufficient.

From the press 11 the peat mass was transferred through a screw transporter 14 with a motor 15 which can further remove water from the peat mass.

The peat mass is then transported to a grinding mill 16 driven by a motor 17 and which tears up the peat mass in particles.

The sewage sludge which is brought into the plant is deposited in a closed chamber 18, to avoid gas evaporation, infective agents and smell from the sewage sludge. The gas which develops in the

chamber 18 is unpleasant. It may be pumped out with a pump 19, and be burned in the stove of the drying unit.

The invention requires a certain consistency and water content in the particles which is to be compressed. Therefore the sewage sludge is centrifuged in advance. The Figure shows a mechanic press 20 to be used when necessary.

In the press 20 the contents of dry materials has increased to 20 - 30 %.

To prepare the sewage sludge for mixing with the peat particles, it is transported from the press 20 to a grinding mill 22 driven by a motor 23. The grinding mill 22 can for example be adapted to be driven by a tractor, for grinding of bio mass.

The peat particles from the grinding mill 16 and the sludge particles from the grinding mill 22 are transferred to a drum drying unit 24, for example with not shown screw transporters or declining gutters.

The drum drying unit 24 is driven by a motor 25 and it heatens the particles to a temperature suitable for the further treatment as it will be described below. A drum stove which is fuelled by its own combustibles is used.

As a pre-processing for pelletizing, the mixture of particles is hammered in a hammer mill 26 driven by a motor 27. In the hammer mill the particles are crushed and have a temperature of 30 - 40 degrees Celsius before they are brought to a pelletizing machine 28 driven by a motor 29. From the pelletizing machine 28 the ready pellets 30 sent over a shaking table 31 to remove dust.

Thereafter they are stored in a container 32. From there the pellets mass may be transferred in bulk or by pipe transport to a combustion installation.

To adapt the temperature and humidity in the mass that is brought to pelletizing, it is an advantage if it by the inlet of the pelletizing machine 28, which for example may be a screw pelletizer, is supplied water vapour from a boiler 33 or another steam generator. It may be used steam without overheating.

All motors and driving devices in this installation are included in a control system which allows control of the process so that it can be accomplished continuously, with adaptation to varying mass qualities, temperatures and other conditions which requires adaptation of the process.

In the pelletizing equipment the particles melts together, with basis in the content of lignin in the peat particles. This both binds together and cures the surface. Pellets made in accordance with the method according to the invention get a consistency and firmness which allow blowing and suction through pipes and transport in bulk and besides storing under controlled humidity conditions.

It has appeared that heavy metals that may be present in the in the sewage sludge remains in the ash rest after combustion. This makes the depositing simpler and prevents the spreading of heavy metals in the nature, which has occurred when using sewage sludge as natural fertilizer or soil improvement agent.

A particularly advantageous composition of components is 30 - 50 % by weight peat and 50 - 70 % by weight of sewage sludge, in both cases with basis in dried mass, as it described earlier. But this composition may vary in the area of 20 to 60% by weight peat and 40 to 80 % by weight sewage sludge. This balance will depend on the contents of fat in the sewage sludge and the contents of lignin in the peat mass. Besides it is controlled by the desired burning characteristics of the finished burning product.

Pellets that are made of approximately 30% by weight peat and approximately 70% sewage sludge was measured in standardised "Ligno-test" equipment. Samples showed that approximately 98, 7 % of the pellets were not disturbed by the test influence. From the existing results, it is assumed that this composition is very favourable.

The pellet mass had a bulk density of approximately 0, 85 kg/litre. This has appeared to be favourable for the combustion, since it gave complete combustion, without any unburned rests in the ash.

The result of the method was depending on adaptation of the grade of grinding, humidity and temperature of the actual raw materials, but it brougth the possibility of production of a heating product, which was easy to pelletize and had good storing characteristics.