GB2286542A

GB2286542A - Treating waste gas

Info

Publication number: GB2286542A
Application number: GB9402012A
Authority: GB
Inventors: James Robert Smith; Robert David Chapman
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1994-02-02
Filing date: 1994-02-02
Publication date: 1995-08-23
Also published as: GB9402012D0

Abstract

An apparatus and method of treating exhaust gas containing one or more undesirable components. The apparatus includes a recirculating fluidized bed of heated chemical reagents through which the exhaust gas is passed for treatment prior to it being vented to atmosphere. A fan blows in fluidising air or waste gas, with gas also being added through the inlet above it. Other matter can be added via ports 36 and 37 and reagent through chute 24. Material leaving the bed is separated in cyclone 16 and treated gas removed through duct 14a and solid matter returned to the bed. <IMAGE>

Description

EXHAUST GAS TREATMENT The present invention relates to a method of treating exhaust gas and substances and relates particularly to the conversion of certain hazardous exhausts in gaseous, liquid or solid form by chemical reactions into safe solid or gaseous products.

The exhaust gases, liquids or solids (herein after referred to collectively as exhaust products) may be the product of a combustion process such as for example, the incineration of municipal waste, or may comprise a mixture of exhaust products generated or used during a manufacturing process, such as for example the etching of semi-conductor devices.

Table A attached hereto lists a number of pollutants and the various control technology options available for their treatment. Currently, there is no single technology able to satisfactorily cater with all the listed pollutants.

An example of cleaning apparatus for use in the conditioning of dry exhaust gases is described in British Patent Application Number 8813270.9. The apparatus comprises a cylinder having a heating furnace positioned therein and a removable cartridge positionable within the cylinder and having an exhaust gas inlet and outlet. The interior of the cartridge is provided with an absorbent/reagent such as for example silicon and/or lime and/or copper oxide which acts to absorb or convert elements of the exhaust gas as is passed there through. The heater acts to heat the absorbent/reagent to its optimum working temperature. After a predetermined operating time cycle, the cartridge is replaced and the old cartridge can be disposed of without special treatment.

Due to the relatively high pack density, the comparatively small quantity of adsorbent/reagent used, the above apparatus does not lend itself to use in the cleaning of high flowrates of exhaust gas.

Large density packed beds of reagent are known for use in exhaust gas treatment. However, in such arrangements it is difficult to heat the reagent to its optimum operating temperature.

It is an object of the present invention to provide a method and apparatus for the treatment of exhaust products which reduces and possibly eliminates the problems associated with the above mentioned methods.

Accordingly, the present invention provides a method of treating exhaust gas containing one or more undesirable components, the method including the steps of firstly fluidising a bed of chemical reagents and secondly passing said exhaust gases through said fluidised bed so as to facilitate a reaction between at least one of said undesirable components and the reagents thereby to remove at least one of said undesirable components from said exhaust gas.

Advantageously the method includes the further step of heating the reagent to a temperature of between 4500C and 12000C and preferably between 5000C and 800at thereby to facilitate the optimisation of reaction or incineration of undesirable components.

Conveniently, the exhaust gas may be passed through the fluidised bed in a pre-heated state, thereby to heat the reagent.

In a particularly advantageous arrangement, the exhaust gas is passed to the fluidised bed from a combustion process in which one or more of the undesirable components are produced. In such an arrangement, it will be appreciated that the heat created during combustion can be used to heat the reagent, thereby eliminating the requirement for any external heating apparatus.

In a further advantageous arrangement, the bed may be fluidised in a fluidised incinerator, thereby to facilitate incineration and gas cleaning in one step.

For best results the reagent is removed from the fluidised bed after a predetermined residence time.

The reagent may be removed and replaced by fresh reagent in a continuous or substantially continuous process.

The reagent may be recirculated in its fluidised state so as to cyclically expose it to any incoming exhaust gas and to facilitate the transportation of said reagent with said exhaust gas thereby to increase the contact time there between.

For convenience, the above mentioned method maybe carried out after the exhaust gas has been passed through at least one stage of a boiler forming part of a waste incinerator thereby enabling the temperature of the exhaust gas to be lowered to a temperature at which the reagent is most efficiently operated.

Alternatively, there may be provided an apparatus for treating exhaust gas containing one or more undesirable components, the apparatus comprising a bed of chemical reagents, fluidising means for fluidising said bed and directing means for directing said exhaust gas through said bed of chemical reagents when in said fluidised state thereby to facilitate a reaction between at least one of said undesirable components and the reagents thereby to remove at least one of said undesirable components from said exhaust gas.

Conveniently, the fluidising means may comprise a fan for driving or drawing air and/or exhaust gases through the reagent.

Improved results may be achieved when the apparatus comprises a fluidised incinerator having a fluidising zone in which said bed is fluidised, a rising zone through which the reagent and exhaust gas is passed in a fluidised state and in which said reaction takes place, a separator for separating cleaned exhaust gas from the fluidised reagent and recirculating means for recirculating at least part of the reagent to the fluidising zone.

Means my be provided for removing spent reagent and supplementing any remaining reagent with fresh reagent.

Advantageously the apparatus further includes a cyclone separator for separating cleaned exhaust gas from the fluidised reagent.

An injection means may be provided for the direct injection of waste product in liquid or solid form into the interior of the incinerator for destruction by a combustion or reaction process.

The reagent may comprise silicon, or a silicon rich alloy or substance.

The silicon substance may include or may comprise ferro silicon alloy, manganese silicon alloy or commercially pure silicon.

The reagent may further comprise or include magnesium oxide or calcium oxide, magnesium carbonate or calcium carbonate, magnesium hydroxide or calcium hydroxide, copper oxide or a combination thereof.

An oxygen or air injecting means may be provided for injecting oxygen or air into the fluidised reagent so as to enable the reaction to take place in the presence of oxygen.

The present invention will now be more particularly described by way of example only with reference to the following drawings in which: Figure 1 is a schematic cross-sectioned representation of a fluidising incinerator incorporating features of the present invention.

Referring to figure 1, a container 10 advantageously in the form of a fluidised incinerator acts to house a bed of reagent 12 through which exhaust gas 14 is passed for cleaning. The incinerator 10 includes a fluidising zone 10a, a mid portion or rising zone lOb through which the reagent and exhaust gas is passed in a fluidised state, a separator 16 for separating cleaned exhaust gas 1 4a from the fluidised reagent and recirculating means 18 for recirculating at least part of the reagent to the fluidising zone 10a. The separator 16 may, for example, comprise a cyclone separator, as shown in figure 1, in which cleaned gas is drawn from the centre portion of the cyclone whilst reagent is allowed to drop down towards the recirculating means 18 which may comprise a simple pipe section connecting the cyclone 16 to the fluidising zone 10a. A fan, shown schematically at 20, may be provided for fluidising the bed 12 and/or for driving/drawing air and/or exhaust gases through the bed of reagent 12. A chute 22 or similar device is provided for directing fresh reagent 12 into the container 10 as and when desired. Flow control means 24 in the form of, for example, gate 26 and actuator 28 may be provided for controlling the flow of reagent into the container. A simple trap door 32, or other such device, is provided together with actuator 34 to facilitate the removal of spent reagent 12 as and when required. An additional inlet 36 may be provided for injecting liquid or solid waste products into the incinerator 10 for destruction therein. An oxygen injector 37 is also provided. Additional burners 13 may be provided to assist the main burners during the start up procedure.

The reagent itself may comprise any one of a number of materials known for their ability to react with the particular undesirable elements of the gases being processed and convert them into safer solid or gaseous products. Preferably, the chemical reagents used are solids thereby reducing the risk of carry-over contamination from any gas conditioning equipment. Such solid chemical reagents are easier and safer to handle than liquid reagents and also lend themselves to fluidisation in the method to be described below.

Chemical exhaust gas conditioning according to the present invention may be carried out in a series of reagent beds, each comprising a different reagent, or may be carried out in one step in a reagent bed comprising a combination of reagents, each selected for their own particular reaction properties. For convenience, the latter alternative has been selected for description herein.

Examples of reagents include silicon or silicon rich alloys or substances, specifically but not exclusively, manganese silicon alloys, ferro silicon alloys and commercially pure silicon at an elevated temperature. The effect of the passage of exhaust gases including, for example, C-C1 or CFC's or organo-chlorine compounds (both aromatic and aliphatic) through the bed of hot silicon is that the gases react with the silicon to form silicon tetra-halide(s). The resultant mixture of gases being more reactive and more readily removed by subsequent chemical reaction stages. The resultant gas mixture may also be reacted with magnesium oxide or calcium oxide as will be described later herein. Hot silicon also causes complete or partial break-down of other exhaust gases including silane, diborane, borane, phosphine, arsine and ammonia into hydrogen (which either passes through the reactor for reaction in a later stage, or is reacted by another reactant present in this stage) and other elements which will be wholly or partially retained by the silicon.

The silicon used is most economically a metallurgical grade of crystalline or poly-crystalline material with a purity of approximately 98.5%. Higher purity silicon can be used effectively. It is also possible to use silicon-iron alloys known as "ferrosilicon" in place of or mixed with silicon: this increases the ability of the system to retain boron, phosphorous or arsenic, but gas transport of slightly volatile iron halides from the silicon section can cause blocking of gas pipes.

The silicon used should be in the form of granules or lumps, graded in size depending on the expected gas flow through the system. Granules capable of passing through a mesh sieve of between 0.1 and 1.5mm have been found to be particularly effective. Powdered silicon is undesirable as it may catch fire in an oxygen rich exhaust stream. Ferro silicon and other alloys are preferred.

The silicon should be heated in the temperature range 200-7000C with the preferred temperature of 350-5500C. The silicon can be contained in any suitable container, eg. stainless steel, mild steel, graphite, ceramic or quartz, and heat supplied through the walls or alternatively by induction heating by an internally placed heat source or by the heat present in the exhaust gases being treated.

Additional reagent materials include lime in, for example, granulated form. this converts silicon tetrahalides into a calcium silicate and a calcium dihalide in an exothermic reaction. It also converts boron trihalides into calcium borate and calcium dihalides and decomposes tetraethylorthosilicate vapour into calcium silicate and diethyl ether, and residual silane or borane are converted into a mixture of hydrogen and calcium silicante and silicon or calcium borate ad boron.

The lime needs to be soft permeable structure strong enough to support the weight of the column of reagent without crushing to powder when the bed is not fluidised. The lime must be kept free of water vapour during storage and may be of a similar size to that described in the silicon section above.

The lime needs to be maintained at an elevated temperature. A temperature in the region of 100-6000C would be suitable with a preferred range of 250-5500C. A further reagent in the form of copper oxide silica and lime may be provided.

This reagent will be required when the exhaust gases/vapours present are residual phosphine arsine, elemental phosphorous and arsenic. These substances react with the copper oxide rich reagent present to form copper phosphate or arsenate and water vapour. In addition carbon monoxide is converted to carbon dioxide and hydrogen will reduce the copper oxide to copper and water.

The reagent (a mixture of copper oxide silica and lime) must first be prepared in a suitable form to be loaded into the reaction vessel. The requirement is to provide a granule that is porous and of a size to allow the free passage of gas.

In an ideal operation, a reagent containing copper oxide is maintained at an elevated temperature of within the range 1 50-6000C and preferably within a range of between 200-400"C.

As will be known soda lime is normally calcium hydroxide with sodium hydroxide. Reference in this specification to soda lime should also include a mixture of calcium hydroxide and potassium hydroxide.

Additionally, there may be provided a quantity of magnesium oxide or calcium oxide or magnesium carbonate or calcium carbonate or magnesium hydroxide or calcium hydroxide or a material containing these materials in the bed. The magnesium oxide or calcium oxide material is heated to a temperature of 3500C to 8000C. This magnesium oxide or calcium oxide material reacts with many acids and acts as a filter for contaminants to reach and poison the silicon surface.

A further advantage of the magnesium oxide or calcium oxide bed held at elevated temperature is the disassociation of some gas species on hot base materials. Hydrogen selinide, silane and dichlorosilane will decompose on base materials held at elevated temperatures while other hydrides such as arsine, phosphine and diborane thermally decompose on the same surface.

In our experiments we show that when air containing chlorobenzine at a significant level is passed through a bed of silicon at elevated temperature (up to 6000C) the chlorobenzine reacted with silicon and the toxic products were removed by a subsequent stage. The silicon surface did not passivate; no detectable carbon, carbide film was detected. Where it is required to pass carbon-halide compounds in the absence of air over a similar bed we have found that silicon carbide is formed. This carbide is also effective in reacting with the incoming vapours, but blinding will eventually reduce the effectiveness of the silicon surface. The use of ferro-silicon alloys overcomes this problem in part by the iron reacting with carbon on the surface of the ferro silicon, reducing the rate of blinding of the surface. The presence of oxygen also removed carbon deposits while the oxides of silicon and iron retain adequately reactivity at these temperatures. a It will also be possible to include a de Nox catalyst in the bed so as to reduce nitrogen oxide to nitrogen.

The present apparatus may also be used in the disposal of dioxins, furans and other volatile organic compounds. The compounds could be injected into the fluidised bed once it is at its elevated operating temperature.

TABLE A : GENERIC CONTROL TECHNOLOGIES FOR WASTE COMBUSTION Pollutant Control Technology Options Particulate Matter Centrifugal Separation Electrostatic Precipitation Fabric Filtration Wet Scrubbing Hydrogen Chloride (HCl) ) Wet Scrubbing Hydrogen Fluoride (HF) } Semi-dry Scrubbing Sulphur Dioxide (SO: ) Dry Scrubbing Carbon Monoxider CO) Catalytic Oxidation Nitrogen Oxides (NOX) Selectlve Catalytic Reducrion Selective Non-Catalytic Reduction Oxidation/Adsorption Reduction Processes Mercury (Hg), Cadmium (Cd), Lead (Pb) As for Particulate Matter, Scrubbing also partially effective Other Heavy Metals As t'or Particulate Matter. Scrubbing also partially effective Polvchlorinate biphenois (PCBs) Semi-dry and Dry Scrubbing claimed to be effective Dioxins and Furans PCDDs. PCDFs) As for Particulate Matter, Semi-dry aria Drv Scrubbing aiso claimed to be effective

Claims

CLAIMS 1. A method of treating exhaust gas containing one or more undesirable components, the method including the steps of firstly fluidising a bed of chemical reagents and secondly passing said exhaust gases through said fluidised bed so as to facilitate a reaction between at least one of said undesirable components and the reagents thereby to remove at least one of said undesirable components from said exhaust gas.
2. A method as claimed in claim 1 including the further step of heating said reagents to a temperature between 450 C and 12000C and preferably between 5000C and 6000C.
3. A method as claimed in claim 1 or claim 2 in which the exhaust gas are passed through the fluidised bed in a pre-heated state, thereby to heat the reagent.
4. A method as claimed in anyone of claims 1 to 3 in which the exhaust gas is passed to said fluidised bed from a combustion process in which said one or more undesirable components are produced.
5. A method as claimed in any one of claims 1 to 4 in which the bed is fluidised in a fluidised incinerator.
6. A method as claimed in any one of claims 1 to 5 including the further steps of separating spent reagents from the fluidised bed and removing it therefrom and replacing it with fresh reagent as required.
7. A method as claimed in claim 6 in which said spent reagent is removed from said fluidised bed after a predetermined residence time.
8. A method as claimed in claim 6 or claim 7 in which removal of spent reagent and replacement by fresh reagent is a continuous or substantially continuous process.
9. A method as claimed in any one of claims 1 to 8 in which said reagent is recirculated in its fluidised state so as to cyclically expose said reagent to any incoming exhaust gas and to facilitate the transportation of said reagent with said exhaust gas thereby to increase the contact time there between.
10. A method as claimed in any one of the previous claims when carried out after said exhaust gas has passed through at least one stage of a boiler forming part of a waste incinerator.
11. A method substantially as described herein with reference to and as illustrated in the accompanying drawings.
12. An apparatus for treating exhaust gas containing one or more undesirable components, the apparatus comprising a bed of chemical reagents, fluidising means for fluidising said bed and directing means for directing said exhaust gas through said bed of chemical reagents when in said fluidised state thereby to facilitate a reaction between at least one of said undesirable components and the reagents thereby to remove at least one of said undesirable components from said exhaust gas.
13. An apparatus as claimed in claim 12 in which the fluidising means comprises a fan for driving or drawing air and/or exhaust gases through the bed of reagents.
14. An apparatus as claimed in claim 12 or claim 13 in which said apparatus comprises a fluidised incinerator having a fluidising zone in which said bed is fluidised, a rising zone through which the reagent and exhaust gas is passed in a fluidised state and in which said reaction takes place, a separator for separating cleaned exhaust gas from the fluidised reagent and recirculating means for recirculating at least part of the reagent to the fluidising zone.
15. An apparatus as claimed in any one of claims 12 to 14 including removing and supplementing means for removing spent reagent and supplementing the remaining reagent with fresh reagent.
16. An apparatus as claimed in claim 14 or claim 15 including a cyclone separator for separating cleaned exhaust gas from the fluidised reagent.
17. An apparatus as claimed in any one of claims 14 to 16 including injection means for the direct injection of liquid or solid waste products into the incinerator for destruction by a combustion or reaction process.
18. An apparatus as claimed in any one of claims 12 to 17 in which the reagent comprises silicon, or a silicon rich alloy or substance.
19. An apparatus as claimed in claim 18 wherein the silicon substance includes or comprises ferro silicon alloy, manganese silicon alloy or commercially pure silicon.
20. An apparatus as claimed in claim 18 or 19 wherein the reagent comprises or includes magnesium oxide or calcium oxide, magnesium carbonate or calcium carbonate, magnesium hydroxide or calcium hydroxide or copper oxide or a combination thereof.
21. An apparatus as claimed in any one of claims 12 to 20 including oxygen or air injecting means for injecting oxygen or air into the fluidised reagent so as to enable reaction to take place in the presence of oxygen.
22. An apparatus substantially as described herein with reference to and as illustrated in Figure 1 of the attached drawings.