WO1992019365A1

WO1992019365A1 - Stack gas cleaning process

Info

Publication number: WO1992019365A1
Application number: PCT/GB1992/000723
Authority: WO
Inventors: Bernard John Crewdson; Brian Mills
Original assignee: Imperial Chemical Industries Plc
Priority date: 1991-04-24
Filing date: 1992-04-21
Publication date: 1992-11-12
Also published as: AU1551992A; ZA922913B; GB9108754D0

Abstract

A gas cleaning process for treating a gas stream which contains dioxin forming components including hydrogen chloride. The process comprising selectively removing the hydrogen chloride, by using for example sodium β aluminate, at a temperature above that at which chlorinated dioxins are formed thereby preventing their formation. The conventional problem of the disposal of dioxin contaminated material is thus reduced.

Description

Stack Gas Cleaning Process

This invention relates to a process for cleaning a gas, for example a stack gas or a refinery off-gas, and in particular to the removal of hydrogen chloride from such gas. It is known that stack gas formed during the combustion of a material such as coal, refuse or chemical waste, contains a variety of components including hydrogen chloride, water, oxygen and carbonaceous residues. Similarly, the regeneration of a refinery catalyst, which requires the removal by burning of carbonaceous deposits formed on the refinery catalyst, produces a refinery off-gas which also contains such components. The hydrogen chloride in the refinery off-gas being derived from the catalyst modifiers commonly used with the refinery catalyst. The aforementioned components have a propensity to react, at temperatures below those at which they have been formed during the respective combustion or regeneration process, to produce members of classes of compounds known as dioxins, and furans. Dioxins are generally highly toxic, the most toxic of those found in stack gas and refinery off-gas being those which contain chlorine, e.g. 2,3,7,8-tetrachloro dibenzo(b.e)(1,4) dioxin, (TCDD).

In a number of countries the level at which dioxins are emitted to the atmosphere is regulated by legislation. Usually, the permitted limits for discharge are expressed in terms of the equivalent amount of TCDD. Thus, conventionally, in order to be allowed to discharge stack gas or refinery off-gas to the atmosphere the dioxins, in particular TCDD, present in the gas have to be removed to a satisfactory level.

Methods of removing dioxins from stack gas before being vented to atmosphere include the use of absorbents, such as linear or crosslinked polymers as disclosed in EP-A-0343674 or activated carbon as disclosed in EP-A-0208490, onto which they are absorbed. When spent, the absorbent is disposed of or regenerated. In either event the presence of the dioxins requires that both the spent absorbent and any subsequent waste product derived therefrom are required to be handled with extreme care.

Additionally, stack gas often contains dust particles which are also required to be removed before the stack gas is vented to the atmosphere. Unfortunately, such dust particles are usually filtered out at low temperatures, i.e. at or below those temperature at which dioxins can form, and thus may also be contaminated with dioxins thereby requiring decontamination of the collected dust before final disposal thereof.

We have found that a gas mixture which contains dioxin forming components, such as those found in a stack gas or a refinery off-gas, can be treated so as to reduce the formation of certain dioxins, in particular TCDD, to the extent that further treatment to remove other dioxins is not necessary thereby allowing the treated gas to be vented to atmosphere. Accordingly we provide a process for cleaning a gas comprising a mixture of hydrogen chloride, water, oxygen and carbonaceous residues, in which process at least some of the hydrogen chloride is removed from the gas at a temperature which is between a first temperature at which the gas is formed and a second temperature at which dioxins and/or precursors thereto are formed, before the gas is cooled to the second temperature, such that the amount of chlorine containing dioxins and/or precursors thereto capable of being formed at the second temperature is reduced. Stack gas suitable for treatment by the present invention may be that formed during the combustion of coal in coal fired power stations, or the incineration of waste such as refuse, including industrial, domestic and hospital refuse, sewage sludge and chemical wastes. Refinery off-gas that may be treated includes that formed by the removal by burning of carbonaceous deposits formed on the refinery catalysts such as those used in catalytic reformers.

Typically, stack gas is formed at a first temperature at or in excess of 850°C, for example in the range 850 and 1100°C, although temperatures as high as 1500°C are known in respect of the combustion of coal. Typical refinery off-gas is formed at a first temperature in excess of 500°C, e.g. in the range 500 to 850°C.

Precursors to dioxins are usually formed on cooling the gas to a temperature of about 550°C, with actual formation of dioxins occurring on further cooling to a temperature below 400°C, for example in the range 250 to 400°C.

Thus, in order to substantially prevent the formation of chlorine containing dioxins, the removal of hydrogen chloride from the gas is preferably accomplished at a temperature in the range from 250 and 1500°C, in particular from 400 to 850°C„ and especially from 400 to 550°C.

The hydrogen chloride is preferably removed by contacting the gas with a suitable particulate absorbent or adsorbent. Suitable particulate absorbents comprise non-volatile oxidic materials that react with hydrogen chloride at elevated temperatures to form relatively non-volatile inorganic chlorides. A particularly suitable absorbent is sodium β aluminate which may be formed by soaking alumina in a solution of sodium hydroxide, and thereafter heating the soaked alumina to form the desired sodium β aluminate. An example of such a material is "Katalco 810" as sold by ICI Katalco.

The absorbent is typically disposed in at least one bed during contact by the gas. The bed may be static, fluidised, or continuously movable. In a preferred form the absorbent is disposed in at least two beds thereby allowing for the possibility of continued treatment of the gas through one bed whilst the absorbent in an other bed is being renewed. Additionally, where two or more beds are used, one bed may be of a static type and an other bed may be fluidised or otherwise continuously movable. After being used for hydrogen chloride removal for a period of time the effectiveness of the absorbent will diminish, i.e. the absorbent will be spent. When sodium β aluminate is used as the absorbent, the absorbate may be released from the spent absorbent by washing with water to produce an aqueous solution of

SUBSTITUTESHEET sodium chloride. The washed absorbent may then be used in the preparation of fresh absorbent, or as a raw material in the production of alumina. Alternatively, the spent absorbent may be discarded with little or no contamination by dioxins. The level of hydrogen chloride initially present in the gas will depend on the specific nature of the gas forming process. Commonly, hydrogen chloride concentrations up to 10000 mg.m.-³, and more particularly in the range 500 to 1000 mg.m^-3 (expressed in terms of the volume of gas as if measured at 0°C and 1.013 bar absolute) are found in stack gas and refinery off-gas.

In order to reduce the formation of chlorine containing dioxins to an acceptable level the residual concentration of hydrogen chloride in the gas is preferably below 100 mg.m^-3, particularly in the range 0 to 10 mg.m^-3 and especially in the range 0 to 1 mg.m^-3.

The process of the present invention may be used in conjunction with other gas cleaning processes such as the removal of dust particles and other gaseous contaminants.

The present invention is further illustrated with reference to the accompanying figures, wherein

Figure 1 is a diagrammatic representation of a typical stack gas cleaning process; and

Figure 2 is a diagrammatic representation of a typical refinery off-gas production and cleaning process. In Figure 1, a hot gas stream of incinerator combustion products (1), which contains hydrogen chloride and other dioxin forming compounds, is passed through a high temperature filter (2) to produce fly ash (3) and a fly-ash free hot gas stream (4). The fly-ash free stream (4) then contacts a hydrogen chloride absorbent (5) to produce a hydrogen chloride free stream (6). The hydrogen chloride free stream (6) may then pass to a N0_X (oxides of nitrogen) removal stage (7) to which may be added ammonia (8). The resulting gas (9) then passes to a heat recovery system (10) wherein steam is raised and a cooled gas stream (11) produced. The cooled gas stream (11) is then subjected to a mercury removal

SUB stage (12), with the optional addition of sodium sulphide (13). The mercury free gas (14) then passes through a final filtration stage (15) and vented to atmosphere (16).

In Figure 2, an oxygen containing stream (1) is mixed with recycle gas (2) to form a mixed gas stream (3). The mixed gas stream (3) is heated by passage through a heater (4) to form a hot gas stream (5). The hot gas stream (5) passes through the refinery catalyst (6) thereby regenerating the catalyst and forming a refinery off-gas (7) which contains hydrogen chloride and other dioxin forming compounds. The refinery off-gas then passes through a hydrogen chloride absorbent (8) to produce a hydrogen chloride free stream (9) which is then cooled in cooler (10) and the then cooled gas (11) is optionally dried in drier (12). The dried gas (13) may then be split between a purge (14) which is vented and the remainder (15) which is recompressed in compressor (16). The high pressure gas (17) is then mixed with an inert carrier gas stream (18) to form the recycle stream (2).

SUBSTITUTE SHEE^°

Claims

1. A process for cleaning a gas comprising a mixture of hydrogen chloride, water, oxygen and carbonaceous residues, in which process at least some of the hydrogen chloride is removed from the gas at a temperature which is between a first temperature at which the gas is formed and a second temperature at which dioxins and/or precursors thereto are formed, before the gas is cooled to the second temperature, such that the amount of chlorine containing dioxins and/or precursors thereto capable of being formed at the second temperature is reduced.

2. A process as claimed in claim 1 wherein the amount of hydrogen chloride in the gas at the first temperature is in the range 500 to 1000 mg.m^-3.

3. A process as claimed in either claim 1 or claim 2 wherein the amount of hydrogen chloride remaining in the gas at the second temperature is below 100 mg.m^-3.

4. A process as claimed in any one of claims 1 to 3 wherein the gas is a stack gas.

5. A process as claimed in claim 4 wherein the first temperature is at least 850°C.

6. A process as claimed in claim 5 wherein the first temperature is in the range 850 to 1100°C.

7. A process as claimed in any one of claims 1 to 3 wherein the gas is a refinery off-gas.

8. A process as claimed in claim 7 wherein the first temperature is in the range 500 to 850°C.

9. A process as claimed in any one of claims 1 to 8 wherein the second temperature is below 400°C.

10. A process as claimed in any one of claims 1 to 9 wherein the temperature at which the hydrogen chloride is removed from the gas is within the. range 400 to 850°C.

11. A process as claimed in any one of claims 1 to 10 wherein the hydrogen chloride is removed by contacting the gas with a particulate absorbent or adsorbent.

12. A process as claimed in claim 11 wherein the hydrogen chloride is removed by contacting the gas with a particulate absorbent comprising a non-volatile oxidic material that reacts with hydrogen chloride to form a relatively non-volatile inorganic chloride.

13. A process as claimed in either claim 11 or claim 12 wherein the absorbent is sodium β aluminate.

14. A process as claimed in any one of claims 11 to 13 wherein the absorbent is disposed in at least one bed during contact by the gas.

SUBSTITUTESHEET