FLUE GAS TREATMENT WITH MIXTURE OF ALKALI METAL BICARBONATE AND ALKALI METAL CARBAMATE

Abstract: A method of treating a flue gas to reduce the content of acidic gases therein comprises treating the flue gas with an effect amount of a treatment agent which comprises an alkali metal bicarbonate and an alkali metal carbamate.

Description: FLUE GAS TREATMENT ITH MIXTURE OF ALKALI METAL BICARBONATE AND ALKALI METAL CARBAMATE The present invention relates to the treatment of flue gases. The term "flue gas" as used herein relates to gases produced in industrial combustion processes. Thus, for example, flue gases are produced in the combustion processes employed in power generation plant, waste incinerators, steam raising boilers, glass production furnaces and metal refining furnaces. Flue gases generally contain acidic gases which, if not removed prior to discharge ofthe flue gas into the atmosphere, may cause an unacceptable deterioration of air quality. Examples of such acidic gases include sulphur oxides (SO nitrogen oxides (NO*) and hydrogen halides (HX). Various processes are known and used for the removal of acidic gases from flue gases. Thus, for example, it is known to treat flue gases by injection of dry powder sorbents, e.g. CaO, Ca(OH) 2 , Na 2 C0 3 , NaHCO 3 , MgO, sodium sesquicarbonate, or mixtures thereof. These materials are generally more effective for removing SO x and HX gases than NO x gases. Techniques for reducing NO x gases include injection of ammonia gas into the flue gas or, more conveniently, injection of urea as a solution or dry powder sorbent into the flue gas. When it is desired to reduce both SO x (and HX) and NO x gases it is generally necessary to deploy a SOJHX reduction technique simultaneously or sequentially with a NO x reduction technique. This does however require two different sorbents with the associated costs of purchasing and using each such separate sorbent via separate installations. It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantages. According to a first aspect of the present invention there is provided a method of treating a flue gas to reduce the content of acidic gases therein the method comprising treating the flue gas with an effective amount of a treatment agent which comprises a combination of an alkali metal bicarbonate and an alkali metal carbamate. The method of the invention may be used for treating a flue gas containing SO x> NO x or HX gases or any combination thereof. The alkali metal bicarbonate is primarily effective for reducing SO x and HX gases whereas the alkali metal carbamate provides a source of ammonia effective for reducing NO x gases. Thus a single treatment agent may used for the reducing SO x , HX and NO x gases in a flue gas. Ifthe flue gas contains SO x and NO x gases, reaction of the SO x gases with the bicarbonate gives the alkali metal sulphate which catalyses reduction of NO x gases. Generally the amount of water in the treatment agent will not exceed 0.5% by weight thereof. It is highly preferred that the water content does not exceed 0.2% by weight, even more preferably does not exceed 0.1% by weight, and most preferably does not exceed 0.05%. Preferably the treatment agent comprises at least 3%, preferably 3% - 15% by weight ofthe alkali metal carbamate, the balanc being substantially comprised ofthe alkali metal bicarbonate and water (if present). Preferably the alkali metal is sodium. The treatment agent preferably has a maximum median particle size of 80 microns and is free flowing. It is also preferred that the treatment agent is comprised of alkali metal bicarbonate which has been co-produced with the alkali metal carbamate. The co- production proceeds initially with crystallisation of the carbamate which provides nuclei on which the bicarbonate crystals grow. The resulting product is comprised of particles of alkali metal bicarbonate which at least partially enclose the alkali metal carbamate. The treatment agent may be prepared by reaction of an ammoniacal solution of an alkali metal salt (preferably the halide, most preferably the chloride) with carbon dioxide. Preferably the treatment agent is produced from a saturated solution of the alkali metal salt which is then substantially saturated with respect to ammonia prior to carbonation. The reaction of an ammoniacal solution of an alkali metal salt and carbon dioxide produces a precipitate comprising particles of alkali metal bicarbonate which at least partially enclose the alkali metal carbamate. Most conveniently, the treatment agent is crude sodium bicarbonate as produced during the Ammonia Soda process (also known as the Solvay process). This process involves reaction of carbon dioxide with ammoniacal brine (sodium chloride solution) which initially produces a crude sodium bicarbonate containing sodium carbamate. Normally this crude product is either converted to sodium carbonate or refined to produce 'pure' sodium bicarbonate. However the crude product (possibly after further treatment as described below) is eminently useful as a treatment agent for use in accordance with the invention. Thus, for example, the product may be dried at a temperature which reduces the moisture content of the product (preferably to less than 0.5%) without causing significant decomposition of the alkali metal carbamate and the bicarbonate. If desired, the effectiveness of the product may be further increased by addition of other reducible nitrogen species, e.g. ammonium carbamate, urea, ammonium bicarbonate etc. The treatment agent may be used in a number of ways. Thus, for example, the agent may be injected into the flue gas using conventional injection equipment. Generally a filter will be provided in the flue gas stream to remove the solid product resulting from reaction of the treatment agent and acidic gases. If the flue gas to be treated contain only SO x and/or HX gases, then it is convenient that injection of the treatment agent takes place at a position close to the filter where the temperature of the gas will be at least 150°C (e.g. in the range of 160°-300°C). This gives stoichiometric reduction of SO x and HX gases. It is however more preferred to introduce the treatment agent into or close to the combustion zone where the temperature is sufficiently high to cause possible decrepitation (i.e. auto-disintegration of the agent into smaller particles) thus increasing the surface area of the treatment agent and rendering it more effective for removal of acid gases. To achieve decrepitation, the treatment agent should be introduced into the flue gas at a region where the temperature thereof is 500 to 1000°C, more preferably 750 to 900°C. Such temperamres also cause reduction of NO x gases. Certain of the treatment agents described herein are novel compositions and therefore according to a second aspect ofthe present invention we provide a treatment agent for use in reducing the content of acidic gases in a flue gas, the agent comprising a combination of an alkali metal bicarbonate and an alkali metal carbamate, the agent optionally containing water in an amount no more than 0.5% by weight. Preferred features of the treatment agent described in relation to the first aspect of the invention are also applicable to the second aspect.

Claims: CLAIMS 1. A method of treating a flue gas to reduce the content of acidic gases therein the method comprising treating the flue gas with an effective amount of a treatment agent which comprises a combination of an alkali metal bicarbonate and an alkali metal carbamate. 2. A method as claimed in claim 1 wherein the agent optionally contains water in an amount of up to 0.5% by weight. 3. A method as claimed in claim 2 wherein the agent optionally contains water in an amount up to 0.2% by weight. 4. A method as claimed in claim 3 wherein the agent optionally contains water in an amount up to 0.1% by weight. 5. A method as claimed in claim 4 wherein the agent contains water in an amount up to 0.05% by weight of water. 6. A method as claimed in any one of claims 1 to 5 wherein the agent contains at least 3% ofthe alkali metal carbamate. 7. A method as claimed in claim 6 wherein the agent contains 3 to 15% by weight ofthe alkali metal carbamate. 8. A method as claimed in any one of claims 1 to 7 wherein the alkali metal is sodium. 9. A method as claimed in any one of claims 1 to 8 wherem the treatment agent is comprised of particles of alkali metal bicarbonate which at least partially enclose the alkali metal carbamate. 10. A method as claimed in any one of claims to 1 to 9 wherein the treatment agent has been prepared by a reaction of an ammonical solution of an alkali metal salt with carbon dioxide. 11. A method as claimed in any one of claims 1 to 10 wherein the treatment agent is introduced into the flue gas at a region where the temperature thereof is in the range of l60°C to 300°C. 12. A method as claimed in any one of claims 1 to 10 wherein the treatment agent is introduced into the flue gas at a region where the temperature thereof is in the range of500 to lOOO°C. 13. A method as claimed in any one of claims 1 to 12 wherein the treatment agent is introduced into the flue gas at a region where the temperature thereof is in the range of 750 to 900°C. 14 A method as claimed in any one of claims 1-13 wherein the treatment agent is injected into the flue gas. 15. A treatment agent for use in reducing the content of acidic gases in a flue gas, the agent comprising a combination of an alkali metal bicarbonate and an alkali metal carbamate, the agent optionally containing water in an amount no more than 0.5% by weight. 16. An agent as claimed in claim 15 wherein the agent optionally contains water in an amount up to 0.2% by weight. 17. An agent as claimed in claim 16 wherein the agent optionally contains water in an amount up to 0.1 % by weight. 18. An agent as claimed in claim 17 wherein the agent contains water in an amount up to 0.05% by weight of water. 19. An agent as claimed in any one of claims 15 to 18 wherein the agent contains at least 3% ofthe alkali metal carbamate. 20. An agent as claimed in claim 19 wherein the agent contains 3 to 15% by weight ofthe alkali metal carbamate. 21. An agent as claimed in any one of claims 15 to 20 wherein the alkali metal is sodium.

Inventors: THORPE MALCOLM JOHN, THORPE, MALCOLM, JOHN
Applicants: BRUNNER MOND & CO LTD, THORPE MALCOLM JOHN, BRUNNER MOND & COMPANY LIMITED, THORPE, MALCOLM, JOHN
Application year: 1996
Main classification: B01D53
IPC codes: B01D53