GB2082628A - Waste material treatment - Google Patents

Abstract

A method of producing a generally homogeneous fuel having a generally consistent particle size and calorific content from a heterogeneous waste material having components of diverse moisture contents, calorific values and micro-organism contents comprises: passing the waste material through a rotating drum; controlling the volume of waste material in the drum, the speed of rotation of the drum and the rate of flow of the material through the drum so as to obtain therein a residence time and a component collision frequency and intensity sufficient to obtain: (a) transfer of moisture between the components via mutual component contact, (b) breakdown of said components into small particles, and (c) intimate admixture of the small particles. The residence time is controlled to be sufficiently short so as to minimise micro-organism growth and activity and substantially to prevent any appreciable temperature rise. The fuel is obtained from the admixture of particles by separating those particles less than a predetermined size.

Description

SPECIFICATION Waste material treatment This invention relates to the treatment of waste material, in particular domestic refuse.

In general waste material is used as landfiil and/or burnt to reduce its bulk. This latter process involves the construction of special furnaces with little or no useful energy being recovered. This is because domestic refuse contains a heterogeneous mixture of a plurality of components of varying calorific values and moisture contents which in an 'as collected' form are difficult to utilise for energy production due to the differing physical characteristics and calorific values of each element. The following table is a typical breakdown of the constituents of domestic refuse.

Constituent Percent Calorific Value on Moisture by Weight an 'as received' Content basis - BTUllb Finedust & inder 19 4,150 19% Paper 30 6,250 5.5% Vegetable matter 21 2,900 76.5% Metals 9 Glass 9 Textiles 3 6,950 14% Plastic 3 16,100 -- Unclassified 6 7,500 2.9% It is an object of the present invention to provide a method of producing a waste or refuse derived fuel with minimal processing and which fuel will be of generally consistent particle size and calorific value and can be handled by conventional solid fuel handling equipment.

According to the present invention there is provided a method of producing a generally homogeneous fuel having a generally consistent particle size and calorific content from a heterogeneous waste material having components of diverse moisture contents, calorific values and micro-organism contents, said method being characterised by the combination comprising (A) passing the waste material through a rotating drum; (B) controlling the volume of waste material in the drum, the speed of rotation of the drum and the rate of flow of said material through the drum so as to obtain therein a residence time and a component collision frequency and intensity sufficient to obtain: (a) transfer of moisture between said components via mutual component contact, (b) break down of said components into small particles, and (c) intimate admixture of the small particles; said residence time being sufficiently short so as to minimise micro-organism growth and activity and to substantially prevent any appreciable temperature rise associated therewith whereby release of water vapour is substantially prevented and the aggregate calorific value of the waste material fed to the drum is substantially retained in the admixture of particles; and (c) separating from said admixture of particles those particles less than a predetermined size to provide said fuel.

The fuel obtained with the method of the invention is a relatively light material having a small particle size and generally uniform moisture content and which is readily combustible, especially in combination with conventional fuels such as coal, in suitable furnace installations such as for example a chain grats type boiler, and thus constitutes a useful fuel i.e. one from which useful amounts of energy can be recovered.

A significant advantage over other previously known processes of deriving fuel from waste material is that the present invention does not require the use of any additives or any mechanical apparatus for breaking down the waste material to reduce its particle size (this being achieved simply by the waste particles colliding with each other), nor indeed any other special treatment processes in obtaining the waste derived fuel. This naturally results in the avoidance of expenditure on such items and helps to minimise the capital and running costs of the necessary plant. Thus preferably the method of the invention is carried out in the absence of the addition of any extraneous material to the waste material and advantageously in the absence of any external mechanical means for breaking down the waste material to reduce its particle size.

The waste material derived fuel provided by the present invention as well as being combustible itself assists in the maintenance of good firebed conditions when dispersed in a bed of coal since the tendency is for the refuse derived fuel to burn more quickly than the coal so that air passages are created in the firebed which lead to better comustion of the coal. Tests have indicated that the refuse derived fuel produced by the method of the invention can be efficiently burned as a mix with coal in proportions exceeding 20% by volume. The refuse derived fuel has a lower sulphur and chlorine content than coal and, therefore, dilutes these elements when it is mixed with coal and this factor coupled with the reduced amount of air required to be passed through the firebed results in reduced flue-gas corrosivity.

The refuse derived fuel may, of desired, be burned by itself or it can be mixed with other fuels or further processed by air classification for pelletising.

In a further aspect the present invention provides a fuel produced by the method of the invention as well as a method of treatment of waste material comprising the steps of producing a fuel by a method of the present invention, burning said fuel, and recovering heat energy therefrom. Thus an important advantage provided by the present invention is that it permits the recovery of useful amounts of energy from waste material whilst at the same time disposing thereof in a convenient and economic manner.

In the method of the invention the preferred residence time for the waste material in the rotating drum is not more than 12 hours, and is desirably from 4 to 12 hours. The speed of rotation of the drum is generally from 1 to 5 r.p.m., for example, 321 revolutions per minute whilst the quantity of material in the drum is desirably maintained at from 40 to 60, for example 50, per cent by volume of the internal volume of the drum in order to maximise the particle size reduction resulting from collisions between different pieces of waste material, and to ensure intermixing ofthe particles.

The operation of the drum is peferably controlled so that the particle size of at least any vegetable and paper components, but preferably also any other combustible components, is reduced to 50 mm or less. In practice, it has been found possible by means of the invention to readily produce a substantially homogeneous material having a particle size of the order of 5 mm or less which is of low density (i.e contains a substantial proportion of air which assists combustion) and is easily combustible. With drum rotation speeds as described above a convenient size of drum providing a residence time within the preferred range is of the order of 15 m. in length and 4 m. in diameter.

Of course waste material, especially domestic refuse, varies considerably from one area to another and from one day to the next. It is therefore desirable to have the ability to control the treatment conditions to some extent according to requirements. This may conveniently be effected by varying the speed of rotation of the drum.

If the refuse is retained in the rotating drum for more than 12 hours, it is likely that the micro-organisms which are already present in the refuse will multiply by utilising the carbon in the refuse which is consumed in the oxidation of carbon to carbon dioxide which results in the liberation of energy in the form of heat causing in turn a rise in temperature.

This is undesirable and is avoided by the present invention since this effect would reduce the carbon content and thus the calorific value of the refuse since carbon is the major contributor of energy units and result in loss of weight of the material. Any water vapour thus released as a result of a temperature increase is likely to condense on the sides of the rotating drum and on the cooler fresh refuse entering the drum with the possibility of being re-absorbed and increasing the percentage of moisture retained relative to the net weight of the material.

Tests have indicated that where temperatures are allowed to increase due to long retention time and micro-organism activity, the resultant product is likely to have a moisture content in the order of 35% thus reducing its value as a fuel.

The particle admixture obtained in the rotary drum at its downstream end is subjected to a separation process on a particle size basis so as to separate the fuel from any unwanted and non-combustible fractions.

This process is conveniently achieved by a screen. The resulting fuel will have a composition and properties dependent on the components of the refuse from which it has been produced but in general is likely to have characteristics of the order of: Particle Size Moisture Content Calorific Value Density Up to 50 mm. 28% 4,500 BTU/lb. 9.5 cwts/cu.yd.

Since there is generally a proportion of refuse or waste material more suitable to be 'tipped' or sent for use as 'landfill', the waste material is desirably also initially screened to remove any fine dust component of the wast material which can be used as cover material for non-combustible residues of the process of the invention in landfill operations. This initial screening step also helps to reduce the final ash content of the waste material derived fuel.

Further preferred features of the invention will appear from the following description given by way of example of two preferred embodiments illustrated with reference to the accompanying drawings in which: Figure 1 is a schematic elevation of a first apparatus for use in the method of the invention; and Figure 2 is a corresponding view of a second apparatus.

Figure 1 shows an apparatus for producing fuel derived from waste material and comprises a feeding means 1 for feeding waste material into a rotary drum 2 which is mounted with its longitudinal rotational axis substantially horizontal. The drum is provided with a drive means 3 for continuous rotation thereof for the purpose of tumbling the material contained therein in the manner hereinbefore described.

At its outlet end the drum is provided with a screen section 4. Two streams of material issue from said outlet end: a first fraction 5 and a second fraction 6, the former passing radially outwardly through the screen section 4 and the latter axially outwardly from the drum 2.

The first fraction 5 is capable of being used as a fuel without further treatment but it will consist of particles of varying density all with a particle size less than the mesh size of the screen section. If it desired to grade the fuel further, it may be passed to a classifier 7 which includes an air blower 8 arranged to blow a stream of air through the falling particles to separate those of lower density 9 from those of higher density 10. The lower density material 9 may then be pelletised, if required, to increase its density for better economy of transport. Such pelletised refuse derived fuel will have a relatively high calorific value since the heavier low calorific value particles will have been substantially removed in the classification process.

Figure 2 shows an apparatus similar to that described in Figure 1 but has an additional fine screen section T1 at the upstream end of the rotary drum 2 for screening out the dust component of the refuse. This dust component can subsequently be used to coverthe second fraction 6 in landfill operations. Removal of the fine dust particles will also help to reduce the ash content of the refuse derived fuel 5.

Claims (9)

1. A method of producing a generally homogeneous fuel having a generally consistent particle size and calorific content from a heterogeneous waste material having compsonents of diverse moisture contents, calorific values and micro-organism contents, said method comprising (A) passing the waste material through a rotating drum; (B) controlling the volume of waste material in the drum, the speed of rotation of the drum and the rate of flow of said material through the drum so as to obtain therein a residence time and a component collision frequency and intensity sufficient to obtain:: (a) transfer of moisture between said components via mutual component contact, (b) breakdown of said components into small particles, and (c) intimate admixture of the small particlses; said residence time being sufficiently short so as to minimise micro-organism growth and activity and to substantially prevent any appreciable temperature rise associated therewith whereby release of water vapour is substantially prevented and the aggregate calorific value of the waste material fed to the drum is substantially retained in the admixture of particles; and (C) separating from said admixture of particles those particles less than a predetermined size to provide said fuel.

2. The method as claimed in claim 1, wherein said residence time is less than 12 hours.

3. The method as claimed in claim 1, or claim 2, wherein said residence time is greater than 4 hours.

4. The method as claimed in any preceding claim, wherein said drum rotation speed is in the range 1 to 5 revolutions per minute.

5. The method as claimed in any preceding claim, wherein the volume of waste material in the drum is maintained within the range 40-60% by volume of the internal volume of the drum.

6. The method as claimed in any preceding claim, wherein said predetermined particle size is 50 mm.

7. The method as claimed in any preceding claim, wherein at the drum inlet the waste material is subjected to a preliminary screening process to remove fine dust particles therefrom.

8. The method as claimed in any preceding claim, wherein the separated particles providing the fuel as subjected to a grading process on the basis of particle density.

9. The method as claimed in claim 1 and substantially as hereinbefore described with reference to either --of the embodiments illustrated in the accompanying drawing.