GB2086763A - A process for reducing waste solids material - Google Patents

Abstract

A process for reducing and pelletising waste solid materials wherein the raw material is initially passed through a drier and then into a grinding machine wherein the material is mechanically worked and fine ground material is caused to pass through a perforate screen by means of a gas stream taken from the drier at a temperature and relative humidity greater than that of the ambient air. The ground material is fed through a ripener in which steam is added, and then to a press to produce a pelletised product. <IMAGE>

Description

SPECIFICATION A process for reducing waste solids mate- rial THIS INVENTION relates to a process for reducing waste solids material, and preferably for conditioning such material prior to forming pellets thereof in a pelleting press.

In a convenient process of this kind refuse or biomass is first conditioned in a drier to reduce its moisture content from around 55% to less than 20% so that it can be readily pulverised in a grinding machine. The socalled low grade heat energy issuing from the drier is usually discharged to atmosphere at a rate of something like 11 32 cubic meters per minute and at a temperature of approximately 120"C.

In the grinding machine, a stream of gas, usually ambient air, is passed through the machine where the material is mechanically worked so that fine ground material passes through a perforate screen assisted by the stream of gas. The ground material leaves the machine thoroughly cooled by virtue of the gas stream passing through the machine, and its moisture content has been further reduced.

In many processes it is preferable prior to supplying the ground material to the pelleting press, to pass it first through a ripener which includes driven stirrers to keep the material moving, and at the entrance to which steam is fed to raise the temperature and humidity of the material passing to the press. Also, additives may be introduced at this stage. Thus a considerable quantity of steam is needed to return the material to its level of temperature and humidity before it issued from the drier.

With some materials, the steam is not readily absorbed by the fibrous material, so that considerable energy must be expended in the pelleting press to produce pellets with the necessary degree of hardness and other desired characteristics.

An object of the present invention is to provide a process for reducing waste solid materials, which ensures a saving in the energy needed to perform the process, and which yields a product which can more readily be worked in a subsequent process step.

According to the present invention, there is provided a process for reducing waste solids material in a grinding machine wherein the material is mechanically worked and fine ground material is caused to pass through a perforate screen by means of a stream of gas directed through the screen, characterised in that the temperature and relative humidity of the gas stream are maintained at a level greater than that of the ambient air.

An example of a process carried out in accordance with the invention will now be described with reference to the accompanying flow diagrams in which: Figure 1 represents a process carried out according to conventional steps; and Figure 2 represents a process carried out in accordance with the invention.

Referring now to Fig. 1, there is provided a drier into which waste solids material is placed, in this example, at a rate of 978 tonnes per day. The conditions of the material when it enters the drier are that the temperature is 20"C, and there is a 55% moisture content. Heating energy equivalent to 14,890 kilowatts is supplied to the drier together with a stream of treatment air at a rate of 11 32 cubic meters per minute such that the temperature of air leaving the drier is approximately 120"C and moisture equivalent to 46 tonnes per day is removed. The material leaves the drier at a rate of 510 tonnes per day and at approximately 60"C with a moisture content of 13.7%.

The material enters a pulveriser or grinding machine at approximately 50,C. 900 kilowatts of driving energy are supplied to the pulveriser, and ambient air is directed therethrough at a rate of 850 cubic meters per minute and at 20"C with a relative humidity of 70%. in the pulveriser, approximately 8 tonnes per day of moisture are removed.

The material leaves the pulveriser at approximately 25 C and at a rate of 502 tonnes per day, with a moisture content of 12.3%, and enters a ripener at approximately 22"C. In the ripener it is necessary to raise the humidity of the material to a level at which it can be supplied to a pelleting press, and preferably the temperature must be raised also. Thus, a considerable amount of steam is injected into the material entering the ripener since the moisture is not readily absorbed into the dried fibrous material. If required additives may be introduced into the ripener together with the steam.

Thus the material leaves the ripener at a rate of approximately 527 tonnes per day and at 77"C, with a moisture content of 16.6%.

In the pelleting press 1100 kilowatts of power are supplied to drive the material through the pelleting die.

Referring now to Fig. 2, there is illustrated a process in accordance with the invention wherein material having the same parameters as that supplied in the process of Fig. 1, is supplied to the drier. In this case, the heat energy supplied to the drier is 14,730 kilowatts, i.e. 160 kilowatts less than was supplied in the process of Fig. 1. In this case, only 451 tonnes per day of moisture is removed in the drier and the material leaves the latter at a higher temperature i.e.

80"C and at a higher moisture content i.e.

16.6%.

The material enters the pulveriser at 70"C, and, of the 11 32 cubic meters per minute of air stream leaving the drier, a proportion equivalent to 850 cubic meters per minute is introduced into the pulveriser as the process gas stream, and this air is at 120"C and has a relative humidity of 80%. The power supplied to the pulveriser in this process is at a level of 1100 kilowatts which is greater than that required in the pulveriser of Fig. 1 since material is supplied thereto at a higher rate, i.e. 527 tonnes per day as opposed to 510 tonnes per day. Moisture is removed in the pulveriser at a rate of 3 tonnes per day.

The material leaves the pulveriser at a rate of 524 tonnes per day and at 80"C with a moisture content of 16%. It enters the ripener at 75"C, and steam at a rate of 3 tonnes per day, as opposed to 25 tonnes per day in the case of Fig. 1, is introduced. The material leaves the ripener at a rate of 527 tonnes per day with the moisture content thereof increased by only 0.6% in the ripener.

Thus, the material enters the pelleting press at a temperature greater than that in the process of Fig. 1 but with the same moisture content. The power required to drive the press under these improved conditions is approximately half that required in the process of Fig.

1 and is therefore in the region of 550 kilowatts.

It is clear that by introducing so-called low grade heating energy from the drier into the pulveriser, the power input to the drier can be significantly reduced, and the necessity to remoisturise and re-heat the material in the ripener is substantially removed. This represents a considerable saving in the overall power consumption of the process of Fig. 2 when compared with that of Fig. 1. in the example, the heat energy saving in the drier is 1 60 kilowatts. There is an overall saving in electrical power supply of 350 kilowatts calculated from a reduction in power to the press against a slight increase in power to the pulveriser. There is a considerable saving in the amount of steam which must be generated for supply to the ripener, and this represents approximately 850 kilowatts.The total energy saving according to this example is in the region of 1 360 kilowatts.

A further saving is realised in that the physical size of the ripener and thus the power needed to drive it, can be reduced when compared with the conventional process in which a longer residence time was needed for the material to re-absorb sufficient moisture.

Whilst the invention has been illustrated by reference to an example of a process for densification of fibrous material, the principal feature of novelty resides in the concept of using so-called low grade heat energy in a pulveriser or grinding machine such that the ground material issuing therefrom is well conditioned for further treatment. It will be appreciated that, in some cases, the material entering the process is sufficiently dry to be supplied directly on the pulveriser, and in this case, the drier can be located downstream of the pulveriser in the plant, so that the problem of retaining heat in the material as it travels from the drier to the press is overcome by the simple expedient of reducing the physical distance between the two machines.

The transfer of hot air from the drier to the pulveriser can be direct or, where the introduction of combustion gases into the pulveriser is undesirable, via a heat exchanger.

Claims (5)

1. A process for reducing waste solid materials in a grinding machine wherein the material is mechanically worked and fine ground material is caused to pass through a perforate screen by means of a stream of gas directed through the screen, characterised in that the temperature and relative humidity of the gas stream are maintained at a level greater than that of the ambient air.

2. A process according to claim 1, wherein the material is initially fed to a drier wherein a stream of heated gas contacts said material to reduce its moisture content, the gas stream from said drier being utilised as said gas stream in the grinding machine to which the dried material is fed.

3. A process according to claim 1, or claim 2, wherein said ground material is subsequently worked into pellets.

4. A process according to claim 3, wherein steam is injected into the ground material prior to the pelleting step.

5. A process for reducing waste solid materials, substantially as hereinbefore described, with reference to Fig. 2 of the accompanying drawings.