GB1578150A - Removal of the oxides of sulphur and/or nitrogen from the flue gases of power generation apparatus - Google Patents

Description

(54) REMOVAL OF THE OXIDES OF SULFUR AND/OR NITROGEN FROM THE FLUE GASES OF POWER GENERATION APPARATUS (71) We, THE RALPH M.

PARSONS COMPANY, a corporation organised and existing under the laws of the State of Nevada, United States of America, of 100 West Walnut Street, Pasadena, California 91124, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- For a few years and in the interest of the environment, low sulfur fossil fuels were used in the generation of energy by the combustion of low sulfur coal and similar low sulfur carbonaceous materials.

Depleting fuel reserves, however, have dictated the necessity of combusting fossil fuels of high sulfur content.

With this, considerable interest has developed in the ability to combust high sulfur fuels and still emit a flue gas to the atmosphere which is sufficiently low in the oxides of sulfur, so that a problem will not be presented from an ecological standpoint.

Many processes have been proposed for the removal of the oxides of sulfur from the stack gases emitting from the boiler sections of power generation systems.

Most are complicated and involve additional operating and maintenance expense in addition to high initial capital cost for new installations. They are also cumbersome and costly to adapt to existing installations.

Some involve injection scrubbing operations, which entail additional raw materials and material handling cost, add nothing to fuel efficiency, rather decrease it, and result in slurry disposal problem.

In another process, sulfur dioxide is scrubbed from the gas and regenerated as sulfur dioxide. Operating costs are high and the oxides of nitrogen introduce complications to sulfur dioxide removal.

Further, sulfur dioxide is not a desirable byproduct and must be converted to sulfuric acid or to sulfur at a considerable additional expense.

According to the present invention, there is provided a method to improve the economy of fuel burning, power generators such as power plant boilers and the like, while at the same time minimising objectionable emission of the oxides of sulfur and nitrogen to the atmosphere.

In accordance with an embodiment of the present invention, the fuel to air ratio is adjusted so that the products of the combustion zone of the boiler, while containing oxides of sulfur and nitrogen, will still contain sufficient hydrogen and carbon monoxide to reduce the oxides of sulfur to hydrogen sulfide and carbonyl sulfide and the oxides of nitrogen to inert nitrogen and/or ammonia. A portion of reduction occurs during heat transfer in the boiler and the balance catalytically at a temperature from 3000 to 8000F in the presence of a catalyst capable of converting the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen, ammonia or mixtures thereof. The catalyst may also hydrolyze formed carbonyl sulphide to hydrogen sulfide. Following reduction of the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and ammonia, the flue gas stream is passed through the extraction zone where the formed hydrogen sulfide is extracted prior to venting the flue gas to the atmosphere.

In carrying out the process of the invention, it is preferred that the amount of hydrogen and carbon monoxide formed during the combustion of the carbonaceous fuel be 30 to 60% in excess of the stoichiometric amount required to reduce the sulfur present as sulfur dioxide to hydrogen sulfide and carbonyl sulfide. This ensures complete consumption of the oxygen present in the air fed to the combustion zone and provides the driving force for both the non-catalytic and catalytic reduction of the oxides of sulfur and nitrogen to hydrogen sulfide and inert nitrogen and ammonia.

In carrying out the process of this invention, the flue gas stream ultimately discharged to the atmosphere will contain minimal quantities of the oxides of nitrogen and oxides of sulfur below a 10 ppm level to meet or exceed the most stringent regulations for emissions of the oxides of sulfur to the atmosphere.

In addition to permitting utilisation of conventional high sulfur fuels for power generation a particular advantage of the process of this invention is a generation of energy from low BTU gaseous hydrocarbon fuels such as those obtained by the gasification of coal.

The present invention is a method of removing at least the oxides of sulphur and/or nitrogen from the flue gas formed in the combustion zone of the boiler of power generating apparatus, comprising combusting a sulfur bearing carbonaceous fuel in the combustion zone of a boiler of a power generator in a deficiency of air to form a reducing flue gas stream at a temperature from about 2000 to about 3000OF comprising carbon dioxide, carbon monoxide, hydrogen, sulfur dioxide, the oxides- of nitrogen and water, the hydrogen and carbon monoxide content of the formed high temperature reducing flue gas stream being in excess of the stoichiometric amount required to reduce the formed sulfur dioxide, cooling the high temperature reducing flue gas stream in said boiler to a temperature of from 300 to 8000F, to extract heat values therefrom and thermally reduce a portion of the oxides of nitrogen to nitrogen, ammonia or a mixture thereof, and a portion of the sulfur dioxide to a mixture of hydrogen sulfide and carbonyl sulfide, catalytically converting by reduction residual sulfur dioxide to hydrogen sulfide, the oxides of nitrogen to nitrogen, ammonia or a mixture thereof and hydrolyzing at least part of the carbonyl sulfide to hydrogen sulfide by passing the cooled reducing flue gas stream through a catalytic conversion zone in the presence of a catalyst capable of converting the sulfur dioxide to hydrogen sulfide, the oxides of nitrogen to nitrogen and ammonia and hydrolyzing carbonyl sulfide to hydrogen sulfide, and extracting the formed hydrogen sulfide from the flue gas stream and venting the flue gas stream to the atmosphere.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which illustrates one scheme for modifying a power generator in accordance with the practice of the invention.

With reference to the drawing in a power generator i0, the boiler 12 is supplied with a sulfur bearing carbonaceous fuel such as pulverises sulfur bearing coal or sulfur bearing hydrocarbon liquid in line 14 which enters along with preheated air from duct 16 in line 18 to combustion section 20.

Combustion occurs in a deficiency of oxygen to generate a mixture of carbon monoxide and hydrogen in a stoichiometric excess over that required for reduction of SO2. Preferably, the stoichiometric excess of carbon monoxide and hydrogen is from 30 to 60%.

This ensures that the effluent from the combustion zone will be oxygen free and contain sufficient excess hydrogen and carbon monoxide for the reduction of the oxides of sulfur, predominantly sulfur dioxide in accordance with the following reactions.

By creating reducing conditions in the combustion zone a portion of the SOx as SO2 is reduced to H2S and COS by reactions (1) and (2) above during the production of useful energy in the heat transfer sections of the boiler. The oxides of nitrogen are also reduced to inert nitrogen and ammonia.

In addition to combustion zone 20, boiler 12 normally contains a radiant boiler section, a convection boiler section, and a high temperature economiser and may be followed by electrostatic precipitator 22 to remove fly ash. Other means to remove ash can also be employed. For instance, cyclones, bag filters and the like may be employed, as effluent from these systems is normally sufficiently fine to pass through the catalyst section employed and can be removed in the liquid H2S absorption systems used in this invention. The air required for combustion is blown into air preheater 26, and passes by duct 16 through high temperature economiser 24, where it enters the combustion zone, through line 16 normally at temperature from 500 to 6000F.

The combustion products in transferring their heat by convection and radiation to boiler feed water are coiled from their adiabatic combustion temperature to 20000F to 30000F in the upper portion of the combustion zone of boiler 12.

As gas temperature in boiler 12 reduces, the conditions which favour the reduction of the oxides of sulfur, sulfur dioxide to hydrogen sulfide and carbonyl- sulfide occur. As the temperature drops below about 2000OF, for instance, equilibrium begins to favour their formation, with carbonyl sulfide formation being maximised at a temperature of about 1200OF. In addition to the reduction of the oxides of sulfur some of the oxides of nitrogen as well as any hydrogen cyanide present will also be reduced. Because rates of reaction decrease with temperature the flue gases leaving boiler 12 will still contain residual quantities of the oxides of sulfur and nitrogen as well as other sulfur species.

To effectively eliminate them the gas stream now at a temperature from 3000F to 800OF, preferably 500 to 8000 F, is passed through added catalyst zone 30. Catalyst zone 30 contains one or more metals, which may be present as their sulfides typically supported on alumina, silica or aluminasilica which are capable, under reducing conditions, of converting the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and/or ammonia by respective reactions with hydrogen and water. Typical of the metals which may be employed are the Group VIII metals such as cobalt, nickel, rhodium, palladium, iridium and platinum, as well as the lower sulfides and oxides of molybdenum and chromium, promoted aluminum oxides and the like.

After conversion of the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and/or ammonia, the flue gas stream is passed through a low temperature air preheater 26 and to a hydrogen sulfide extraction unit 32.

Because SO,, SO2 and NOX are virtually eliminated from the flue gas, gas temperature can be reduced to 120 to 150"F in the air preheater 26 without causing corrosive dilute acids such as sulfuric, polythionic sulfurous and nitric acids to condense in the duct work or contaminate the chemicals used in hydrogen sulfide extraction unit 32.

Any number of methods are feasible for hydrogen sulfide removal with absorption methods being preferred. For instance, the cooled tail gas may be passed through alkaline absorption solutions which are continuously regenerated by oxidation to produce elemental sulfur using catalysts such as sodium vanadate, sodium anthraquinone disulfonate, sodium arsenate, sodium ferrocyanide, iron oxide and iodine.

A convenient alternative is to use absorption solutions containing amines, sulfonates and potassium carbonates which can be continuously regenerated by steam stripping to produce hydrogen sulfide.

The preferred hydrogen sulfide extraction system is one which involves the alkaline absorption of hydrogen . sulfide and oxidation to produce sulfur. The preferred system is known as the "Stretford Process", which employs a solution containing sodium carbonate, sodium vanadate and sodium anthraquinone disulfonic acid as the absorbent used in the absorber. The absorbed hydrogen sulfide is oxidised by sodium vanadate to form sulfur in the absorber and retention tank (not shown), and the absorbing solution is then regenerated by oxidation with air in an oxidiser (not shown). The sulfur is recovered from the solution by conventional means such as flotation, filtration, centrifuging, melting, decantation under pressure and the like.

The Stretford Process for stripping hydrogen sulfide from the tail gas is particularly preferred because the flue gas contains carbon dioxide as this component is not extracted. Accordingly, chemical and/or utility requirements are substantially reduced.

After hydrogen sulfide is extracted, the residual flue gas is vented to the atmosphere by stack 34.

Example Pulverised coal containing 3.6% sulfur is burned at the rate of 150 tons per hour. The amount of air used for the combustion is equivalent to 96.5% of the theoretical air required for complete combustion representing an oxygen deficiency of 3.5%.

The heat of combustion is extracted by the boiler and generated as steam, The gas stream leaves the boiler at a temperature of 600 to 100OF with some of the heat passing to the air entering the boiler. The gas stream is then passed through a high efficiency electrostatic precipitator to reduce solid particulate content to 0.02 grains per standard cubic foot, then passes through a fixed bed of a cobalt molybdenum catalyst where residual sulfur dioxide is converted to hydrogen sulfide and oxides of nitrogen to a mixture of inert nitrogen and ammonia. The amount of hydrogen and carbon monoxide in the flue gas leaving the catalyst zone is 0.5 /,, by volume, with temperature rise across the catalyst bed being between 10 to 20OF.

The gas stream after being used to supply heat to the incoming air in the preheater is passed to a Stretford unit where the contained hydrogen sulfide is removed prior to venting the gas stream to the atmosphere.

Concentration of sulfur dioxide in the gas stream is less than 100 ppm.

WHAT WE CLAIM IS: 1. A method of removing at least the oxides of sulphur and/or nitrogen from flue gas formed in the combustion zone of the boiler of power generating apparatus comprising combusting a sulfur bearing carbonaceous fuel in the combustion zone of a boiler of a power generator in a deficiency of air to form a reducing flue gas stream at a temperature from about 2000

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. addition to the reduction of the oxides of sulfur some of the oxides of nitrogen as well as any hydrogen cyanide present will also be reduced. Because rates of reaction decrease with temperature the flue gases leaving boiler 12 will still contain residual quantities of the oxides of sulfur and nitrogen as well as other sulfur species. To effectively eliminate them the gas stream now at a temperature from 3000F to 800OF, preferably 500 to 8000 F, is passed through added catalyst zone 30. Catalyst zone 30 contains one or more metals, which may be present as their sulfides typically supported on alumina, silica or aluminasilica which are capable, under reducing conditions, of converting the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and/or ammonia by respective reactions with hydrogen and water. Typical of the metals which may be employed are the Group VIII metals such as cobalt, nickel, rhodium, palladium, iridium and platinum, as well as the lower sulfides and oxides of molybdenum and chromium, promoted aluminum oxides and the like. After conversion of the oxides of sulfur to hydrogen sulfide and the oxides of nitrogen to inert nitrogen and/or ammonia, the flue gas stream is passed through a low temperature air preheater 26 and to a hydrogen sulfide extraction unit 32. Because SO,, SO2 and NOX are virtually eliminated from the flue gas, gas temperature can be reduced to 120 to 150"F in the air preheater 26 without causing corrosive dilute acids such as sulfuric, polythionic sulfurous and nitric acids to condense in the duct work or contaminate the chemicals used in hydrogen sulfide extraction unit 32. Any number of methods are feasible for hydrogen sulfide removal with absorption methods being preferred. For instance, the cooled tail gas may be passed through alkaline absorption solutions which are continuously regenerated by oxidation to produce elemental sulfur using catalysts such as sodium vanadate, sodium anthraquinone disulfonate, sodium arsenate, sodium ferrocyanide, iron oxide and iodine. A convenient alternative is to use absorption solutions containing amines, sulfonates and potassium carbonates which can be continuously regenerated by steam stripping to produce hydrogen sulfide. The preferred hydrogen sulfide extraction system is one which involves the alkaline absorption of hydrogen . sulfide and oxidation to produce sulfur. The preferred system is known as the "Stretford Process", which employs a solution containing sodium carbonate, sodium vanadate and sodium anthraquinone disulfonic acid as the absorbent used in the absorber. The absorbed hydrogen sulfide is oxidised by sodium vanadate to form sulfur in the absorber and retention tank (not shown), and the absorbing solution is then regenerated by oxidation with air in an oxidiser (not shown). The sulfur is recovered from the solution by conventional means such as flotation, filtration, centrifuging, melting, decantation under pressure and the like. The Stretford Process for stripping hydrogen sulfide from the tail gas is particularly preferred because the flue gas contains carbon dioxide as this component is not extracted. Accordingly, chemical and/or utility requirements are substantially reduced. After hydrogen sulfide is extracted, the residual flue gas is vented to the atmosphere by stack 34. Example Pulverised coal containing 3.6% sulfur is burned at the rate of 150 tons per hour. The amount of air used for the combustion is equivalent to 96.5% of the theoretical air required for complete combustion representing an oxygen deficiency of 3.5%. The heat of combustion is extracted by the boiler and generated as steam, The gas stream leaves the boiler at a temperature of 600 to 100OF with some of the heat passing to the air entering the boiler. The gas stream is then passed through a high efficiency electrostatic precipitator to reduce solid particulate content to 0.02 grains per standard cubic foot, then passes through a fixed bed of a cobalt molybdenum catalyst where residual sulfur dioxide is converted to hydrogen sulfide and oxides of nitrogen to a mixture of inert nitrogen and ammonia. The amount of hydrogen and carbon monoxide in the flue gas leaving the catalyst zone is 0.5 /,, by volume, with temperature rise across the catalyst bed being between 10 to 20OF. The gas stream after being used to supply heat to the incoming air in the preheater is passed to a Stretford unit where the contained hydrogen sulfide is removed prior to venting the gas stream to the atmosphere. Concentration of sulfur dioxide in the gas stream is less than 100 ppm. WHAT WE CLAIM IS:

1. A method of removing at least the oxides of sulphur and/or nitrogen from flue gas formed in the combustion zone of the boiler of power generating apparatus comprising combusting a sulfur bearing carbonaceous fuel in the combustion zone of a boiler of a power generator in a deficiency of air to form a reducing flue gas stream at a temperature from about 2000

to about 3000OF comprising carbon dioxide, carbon monoxide, hydrogen, sulfur dioxide, the oxides of nitrogen and water, the hydrogen and carbon monoxide content of the formed high temperature reducing flue gas stream being in excess of the stoichiometric amount required to reduce the formed sulfur dioxide, cooling the high temperature reducing flue gas stream in said boiler to a temperature of from 300 to 800OF, to extract heat values therefrom and thermally reduce a portion of the oxides of nitrogen to nitrogen, ammonia or a mixture thereof, and a portion of the sulfur dioxide to a mixture of hydrogen sulfide and carbonyl sulfide, catalytically converting.by reduction residual sulfur dioxide to hydrogen sulfide, the oxides of nitrogen to nitrogen, ammonia or a mixture thereof and hydrolyzing at least part of the carbonyl sulfide to hydrogen sulfide by passing the cooled reducing flue gas stream through a catalytic conversion zone in the presence of a catalyst capable of converting the sulfur dioxide to hydrogen sulfide, the oxides of nitrogen to nitrogen and ammonia and hydrolyzing carbonyl sulfide to hydrogen sulfide, and extracting the formed hydrogen sulfide from the flue gas stream and venting the flue gas stream to the atmosphere.

2. A method as claimed in claim 1 in which the cooled reducing flue gas stream is fed to the catalytic conversion zone at a temperature from 5000 to 8000F.

3. A method as claimed in claim I in which the catalyst contains a metal selected from cobalt, nickel, rhodium, palladium, iridium, platinum, molybdenum, chromium and mixtures thereof contained on a support selected from alumina, silica, alumina-silica and mixtures thereof.

4. A method as claimed in claim I in which the former hydrogen sulfide is extracted from the flue gas by contacting the flue gas with a hydrogen sulfide absorption solution.

5. A method as claimed in claim 4 in which the absorbed hydrogen sulfide is oxidised to elemental sulfur using a catalyst selected from sodium vanadate, sodium anthraquinone disulfonate, sodium arsenate, sodium ferrocyanide, iron oxide and iodine.

6. A method as claimed in claim 4 in which the flue gas stream is cooled to a temperature of from 1200 to 150OF prior to contact with the absorption solution.

7. A method as claimed in claim I and substantially as hereinbefore described with reference to the accompanying drawing.