GB2297608A - Fluidised bed incinerator - Google Patents

Abstract

A fluidised bed incinerator for the destruction of wastes, wherein waste is fluidised by addition to a fluidised bed of inert material maintained by an upwardly-flowing hot air stream in said incinerator and it is incinerated in this form, the fluidised bed being maintained at a temperature of 800{C maximum and the waste being fed in at a rate of from 10-100g/sec. The incinerator internal pressure is preferably kept at below atmospheric. The incinerator is useful for most types of liquid and solid wastes, and is particularly useful for organic hazardous wastes which are reduced to environmentally-acceptable ash and emissions.

Description

IMPROVEMENTS IN OR RELATING TO ORGANIC COMPOUNDS The invention relates to incinerators for the incineration of waste materials, particularly those of a hazardous nature.

The disposal of increasing volumes of waste materials, much of it of a very hazardous nature, is one of the great problems facing industry today. One method of disposal which has been widely advocated is incineration, in which the waste is burnt at a temperature which is sufficiently high to ensure the conversion of the waste to an environmentally-acceptable gaseous emission and a harmless ash which can then be disposed of, either as land fill or in some practical use such as filler in concrete.

One of the problems of this method is that the wide variety of wastes sometimes require very different incineration techniques and conditions, and what will work satisfactorily with one type will not necessarily satisfactorily work with another. For example, some incinerators are useful only for liquid wastes, others only for solids. A further problem is that the incineration of many wastes requires a high energy input, usually derived from non-renewable fossil fuels, even though much waste is potentially combustible.

The nearest approach to a universal incinerator for all wastes is the fluidised bed type.

In this type, the waste is added to a bed of hot particulate inert material such as graded ash or sand which is "fluidised" in the incinerator by an upwardly-flowing hot air stream and is incinerated in this bed. The combustion process is efficient and when the waste is organic containing no objectionable metals such as mercury, the waste gases often need minimal treatment before they can be safely vented to atmosphere. The concept is described in, for example, "Chemical Engineering" of May 9, 1988 at page 30.

It has now been found that an especially high-perfomling fluidised bed incinerator may be provided by adhering to certain critical design parameters. There is therefore provided a fluidised bed incinerator for the destruction of wastes, wherein waste is fluidised by addition to a fluidised bed of inert material maintained by an upwardly-flowing hot air steam in said incinerator and it is incinerated in this form, the fluidised bed being maintained at a temperature of 800"C maximum and the waste being fed in at a rate of from 10-lOOg/sec.

The invention further provides a method of disposing of waste material by fluidising it in a fluidised bed of inert material and incinerating it therein, the bed being maintained at a temperature below 800"C and the waste material being fed into the bed at a rate of from 10-lOOg/sec.

Fluidised bed incinerators are commercially available and any of the known types may be modified to give an incinerator according to the invention.

It has been found that the combination of the bed temperature and the waste feed rate give not only excellent results for nearly all wastes (a fine ash with often a total absence of clinker) but also it allows the burning of the waste itself (when it is combustible) to help sustain the process and thereby reduce the energy costs. Preferably, the bed temperature should be from 600 -700 C and the waste feed rate from 40-80g/sec.

The fluidising hot air flow should be such that a stable fluidised bed is created. The flow rate will naturally depend on the nature of the waste, but it has been found that, for the purposes of the invention, a rate of from 1.5-2.5m/sec is necessary. The production of such a flow rate will depend on many variables such as air nozzle size, number and distribution, incinerator volume and so on, but the skilled person, given the need for such a flow rate, can readily produce a suitable arrangement to give this flow rate in any given case.

In a particularly preferred embodiment of the invention, the interior pressure of the incinerator is kept at a pressure which is lower than atmospheric. This not only prevents a leakage of material to the outside, but also makes the process work better. The pressure should be maintained at a level between atmospheric and lOmm water gauge below atmospheric. Preferably the pressure is from 4-6mm water gauge below atmospheric.

The residence time of the waste in the incinerator needed for complete combustion will vary according to the nature of the waste, but it will typically be around 2 seconds, although it can vary between about 1 and 5 seconds.

It is a feature of this invention that, when the waste is organic and contains no objectionable metals, the final products will be a non-toxic ash and combustion gases which need only nominal scrubbing before venting to atmosphere - often, a simple cyclone separator and a venturi scrubber (to remove entrained very fine particulate matter and, for example, when organic chlorides are present, to quench rapidly the gases from high temperature to low temperature) are all that are necessary. When the waste contains heavy metals (which will concentrate in the ash) or materials such as mercury (which will vapourise and could therefore be emitted into the atmosphere), other artrecognised treatments and/or particular disposal will be needed.However, it is a feature of this invention that even in these cases of highly hazardous materials, the present invention will give ash and gaseous emissions which are relatively easy to deal with.

The invention is further described with reference to the drawing which is a schematic representation of a possible embodiment.

An incinerator 1 made of firebricks has a main combustion chamber 2 equipped with oil burners 3 and 4 and a waste feed port 5. The incinerator also has post-combustion chambers 6 and 7. At the base of main combustion chamber 2 are rows of fluidising air injection nozzles 8 which provide the upwardly-moving air stream for the fluidised bed.

The nozzles are supplied with air by means of a forced draught fan 9 and a conduit 10.

Part of the air supply to the nozzles is passed through a heat exchanger 12 which derives its heat from the hot exhaust gases from the incinerator. These gases pass through two heat exchangers 11 and 12, then to a cyclone separator 13 and along conduit 14 to a venturi scrubber 15,17 which is fitted with a recycling pump 16. Any fine particulate material is trapped and collected in a sump 18, while the gas passes through an alkali scrubber 19 (with recycling pump 20) and then through a moisture separator 21. The gases are then drawn by an induced draught fan 22 into a chimney 23 and vented to atmosphere.

In operation, a solid waste is mixed with calcium hydroxide lime and fed into the incinerator through the feed port 5. (In the case of a liquid waste, addition may be through the oil burner 3). A fluidised bed of hot sand is already present, having been preheated by the oil burner 3 (and once the process is running, partially by the heat exchanger 12). Additional heat can be supplied by oil bumer 4. The bed is run at a temperature of between 600 -700 C. The combustion process is completed in postcombustion chambers 6 and 7 and the exhaust gases then pass out of the incinerator through heat exchanger 11 (whose heat may be used, for example, to generate low pressure steam or hot water) and follows the route described hereinabove to chimney 23 and the atmosphere. The ash which is the solid residue of the incineration falls to the bottom of the combustion chambers 2,6, and 7 and may be removed through a series of ports 24.

In such an apparatus, the vast majority of organic wastes, no matter what their composition, will be reduced to non-toxic ash and flue gas which easily passes the strictest environmental standards.

The skilled person will readily realise that there are possible many obvious variations of the above-described process which will still fall within the scope of this invention.

The above-mentioned apparatus will deal with a large variety of wastes, but, for example, if only a few wastes are dealt with, it may be possible to optimise the process and thereby achieve greater economies. For example, it might be possible to dispense with the scrubbing stage 19.

Claims (10)

CLAIMS:

1. A fluidised bed, waste disposal incinerator, wherein waste is fluidised in by addition to a fluidised bed of inert material maintained by an upwardly-flowing hot air steam in said incinerator and it is incinerated in this form, the fluidised bed being maintained at a temperature of 800C maximum and the waste being fed in at a rate of from 10-1 oOglsec.

2. A fluidised bed, waste disposal incinerator according to claim 1, wherein the temperature of the bed is from 600'-700"C.

3. A fluidised bed, waste disposal incinerator according to claim 1 or claim 2, wherein the feed rate is from 40-80glsec.

4. A fluidised bed, waste disposal incinerator according to any one of claims 1-3, wherein the interior pressure of the incinerator is maintained at a pressure which is below atmospheric pressure.

5. A fluidised bed, waste disposal incinerator according to any one of claims 14, wherein the pressure in the incinerator is maintained at a level between atmospheric and 10 mm water gauge below atmospheric.

6. A fluidised bed, waste disposal incinerator substantially as described with reference to the drawing.

7. A method of disposing of waste material by fluidising it in a fluidised bed of inert material and incinerating it therein, the bed being maintained at a temperature below 800"C and the waste material being fed into the bed at a rate of from 10-lOOg/sec.

8. A method according to claim 7, wherein the temperature of the bed is from 600"- 700or.

9. A method according to claim 7 or claim 8, wherein the feed rate is from 40 80g/sec.

10. A method substantially as described with reference to the drawing.