CA1095692A - Power generation - Google Patents

Abstract

Abstract of the Invention A sulfur bearing carbonaceous fuel, particularly a low BTU gaseous fuel, is burned in a deficiency of air in the combustion sons of a boiler to form a reducing gas containing H2 and CO in an amount in excess of that required to reduce the generated SOx to H2S and COS. The oxides of sulfur and nitrogen are, in part, converted to H2S, N2 and/or NH3 during heat transfer in the boiler.
The balance is passed through a catalyst chamber where sulfur species are converted to H2S and oxides of nitrogen to N2 and/or NH3. The formed H2S is extracted from the flue gas prior to venting to the atmosphere and recovered as free sulfur. Catalytic conversion is carried out at a temperature from about 300°F to 800°F.

Description

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IrG 1 POWER GENER~TION
., . ......................................................... .. . .-Background of the In~ention ~or a ~ew years and in the lnterest of the ecology, low sulfur fossil fuels were used in the generation of : energy by the combustion of low sulfur coal and similar low sul~ur carbonaceous materials.
Depleting uel~reserves, however, have dictated the necessity of combusting fossil ~uels o~ high sulfur i: ;!S content.
W~th this, considerable interest has developed in the abillty to combus:t hi~h suIur uels and still eml~ ~ flue gas to the atmosphcrc which i3 suficien~1y low in the oxidcs o~ sulfur tha~c a problem wi11 not be presentcd rom 3 an ~ecology standpoint.
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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 addit~nal 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 rejection 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 desir-able by-product and must be converted to sulfuric acid or to sulfur at a considerable additional expense.

Summary of the Invention .
; In accordance with the present invention there is provided an improved process for the generation of power through the extraation of heat generated from the combustion of sulfur bearing ; carbonaceous fuels which comprises:
a) combusting the sulfur bearing carbonaceous fuels in the combustion zone of the boiler of a power generator in a deficiency of air, the~e being provided sufficient air to maintain stable combustion and above the amount at which signi-ficant amounts of free carbon will be formed, but less than that theoretically required to achieve complete oxidation of the carbon, hydrogen and sulfur content of the fuel, thus forming a high " lQ95~9Z
temperature reducing flue gas stream 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 to hydrogen sulfide;
b) cooling the high temperature reducing flue gas stream in said boiler to a temperature of from about 300 to 800F, to extract heat values therefrom and reduce a portion of the sulfur dioxide to a mixture of hydrogen sulfide and carbonyl sulfide and the oxides of nitrogen to a nitrogen compound selected from the group consisting of nitrogen, ammonia and mixtures thereof;
c) catalytically converting residual sulfur dioxide to hydrogen sulfide and the oxides of nitrogen to a nitrogen compound selected from the group consisting of nitrogen ammonia and mixtures thereof 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 and the oxides of nitrogen to nitrogen and ammonia;
d) extracting the formed hydrogen sulfide from the flue gas stream and venting the flue gas stream to the atmosphere.
There i9 therefore provided a process for the elimination of sulfur emissions to the atmosphere in the operation of power generators based on the combustion of the sulfur bearing carbonaceous fuels, such as power plant boilers and the like while conserving or increasing plant efficiency.
The fuel to air ratio is adjusted so that the products of the combustion zone of a boiler, while containing oxides of sulfur and nitrogen, will contain sufficient hydrogen and carbon monoxide .

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to reduce the oxides of sulfu~ 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 about 300 to about 800F 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 carbon-sulfur compounds 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 an 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 is preferably about 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 insures 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 particular, in the practice of the process of the invention the sulfur bearing carbonaceous fuel is combusted in a deficiency of air in a combustion zone of a boiler of power generation apparatus to form a high temperature flue gas stream which comprises the oxides of carbon, including 1 carbon monoxide; oxides of sulfur; oxides o~ nitro~en;
hydro~en and water. The gas stream is then passed through the hc~t extrac~ion sectlons o the boiler where energy is generated in the form of useful steam ~nd the like, while temperature reduced. At least the oxides of sulfur react with hydrogen and the carbon monoxide present to form hydrogen sulfide and carbonyl sulfide. When the gas stream ~s cooled to a temperature of rom about 350 to about 800.F9 pre~erably from about 500 to about 800F, the gas stream i8 passed through a catalytic conversion zone where residual oxides o sulfur are converted to hydrogen sulf~de and the oxides of nitrogen to inert n~trogen and/or ammonia. As water is present or formed, carbonyl sulfide present will also be catalytically hydrolyzed to hydrogen sulfide. The formed hydrogen sulfide is.then extracted from the flue gas stream prior to venting the flue gas stream to the atmosphere.
In carxying out the process of this invention, the 1ue gas stream ultimately discharged to the atmosphere will contain minimal quantities of ~he oxides o~ nitrogen and oxides o sulfur below a 100 ppm level to meet or exceed the most stringent regulations for emissions o~ the oxides o sulfur ~o the atmosphere.
In addition to permitting utilization of conventional ~5 high sulur fuel~ for power generation a particular advantage o the process of this invention is a generation o~ energy from low BTU gaseous hydrocarbon uels such as thosQ
ob~ained by the gasification of coal.
Further, bcc~use the resultant ~inal volume o~ 1ue gas 3 generated, when me~surcd as standard conditlons, per unlt of 1~9 56~92 l ¦ ~wer ~eneratccl wlll normally be 15 to 20Z less than ~mploycd in the convcntional practice wllere fuel is com~usted in the prescnce of 15% or more excess air. This permits a s~gni~icant reduction in equipment size and, therefore, capital cost o equipmen~ used for power generation.

e Drawin~
The drawing illustrates one 6cheme for carrying ou~
the process of this invention.

De_ ri~tion According to the present invention, there is provided an improved process for the generation of power from sulfur bearing carbonaceous fuels while at the same time eliminating the oxides of sulfur and nitrogen from the resultant stack or ~lue gases.
In it~ more salient aspect, process o~ this invention ; comprises increasing the overall ratio of fuel to the air

2 supplied to the combustion æone of a boiler to ~he extent that reducing conditions will prevail in thè flue gas at the dischar~e o~ the combustion zone. The reducing conditions re~ul~ ~rom the use o~ a deficiency o~ air. ln particular, the amount of air ~ed to the combustion zone is sufficiently low such that the effluent fxom the combustion zone will contain an excess hydrogen and carbon monoxide over that required for the reduction of thc ~ormed oxides of sul~ur to hydro~cn sulid~ and carbonyl sul~id~, precrably rom about 40 to 60% in excess of the stoichiometric requircment.

3 ~1i5 insures tha~ thc ~fluent from the combu~tion zonc will Il 109 569 2 1 I ~ oxy~en frcc nnd con~in su~ici~nt exc~ss hydro~cn and ¦ carbon monoxidc for thc rcduction of the oxides of sulfur, prcdominantly sul~ur dioxidc in accordance wlth the ~ollowin~ reactions.
. S2 t 3H2~H2S ~ 2H20 (1) S2 ~ 3C0~ \ COS ~ 2C02 In ~eneral, to provide a hydrogen and carbon monoxide in an amount of 50~/~ in excess over that required to l achieve the desired reactions will require the deficiency of air below the theoretical a~r required for complete oxidation of the carbon, hydrogen and sulur constituents of about equal to ~he sulfur content o~ the fuel. For example, if the fuel contains about 3% sulfur, the amount of air supplied would be about 97% of theoretical air.
Wh~le the process is applicable to a wide variety of fuels, lt is particularly adapted ~o the conversion of suLfur bearing ~uels of low BTU content, such as those as formed ¦ in the gasification of coal. Fuels of this nature generally ¦ have a BTU content o~ about 200-500 BTU per standard cubic foot.
¦ By creating reducing conditions in the combustion zone ¦ a portion o~ the Sx as SO2 is reduced to H~S and COS by ¦ reactions tl) and (2) above during the production of useful energy in the he~t transfer sect~ons o~ the boiler. The zsl oxides of nitrogen are also reduced to inert nitrogen and ammonia. ~ major portion o the reduction is carried out I catalytically at temperatures ~rom about 300 to about 800F, ¦ preferably ~rom about 500 to abou~ 800F. Pre~erably, the l c~talyst is al80 capable of convertin~ COS and CS2 ~o H2S as weLl as thc ox~dcs o~ nl~ro~cn to lneFt nltro~en l . 1 09569Z

1 an~.,or ammonia`. Following this, the ormcd H2S is separa~ed ~rom t~le ~luc ~as stream usin~ ~bsorption .
processes such as the Stre~ford process wlich converts , the H2S to sul~ur.
With reference now to the Figure in power generator 10, b~iler 12 is supplied with a sulfur bearing carbonaceous fuel in line 14 which enters along with prehested air from duc~ 16 in line 18 to combustion section 20.
.. The ~ulfur bearing carbonaceous fuel ed to combustion ~ec~ion 20 may be derived from any source such as for instance, a pulveriæed coal as well 8S hydrocarbon 1uid which may be nonmally liquid or gaseous in nature. .
Particularly use~ul uels are low BTU gaseous caxbonaceous fuels, such as those obtained by the gasification of coal.
Fuels of ~his nature no~inàlly have a heating value of sbout 200 to about 500 BTU per standard cubic feet.
To achieve essentially complete conversion of ~he sulfur bearing hydrocarbon contained to yield a reducing gas stream, there is provided sufficient air to maintain stable combustion and above the amount at which significant amounts of free carbon will be fo~med, but less than that theoretically required to achieve complete oxidation of the carbon, hydrogen and sulfur content of the fuel.
To provide a sufficient driving force to achieve ~5 reduction o~ sulfur dioxide to hydrogen sulfide and carbonyl sul~ide the amount of hydrogen and carbon monoxide ; generated is greater than that stoichiometrically required to reduce all o the su1ur in the fuel taken as ~ulfur .. dioxide to hydrogen sulfide and carbonyl sulide. Preferably 3 the ~mount of ~ir supplied providcs ~n oxygen deic~cncy to l , 10~569Z

1 ~he extent tllat the total ~mount of hydro~cn and carbon monoxide formed dur~ng combustion will be from about 30 to about 60% in excess of that required to reduce all of the ~ul~ur values tsken 8S sulfur dloxlde to hydrogen sulfide and carbonyl sulide by the reactions.
In combustion zone 20, there occurs the oxidation of hydrocarbon to form the oxides of carbon and water and the sulfur to the oxides o~ sulfur normally present as . sulfur dioxide. Although some sulfur tr~ox~de will also be formed, the amount be~ng minimal, however, ~5 due to the reduced amount of oxygen a~ailable.
. Conversion will take place at the tempe~ratbuFes present, B normal b between about 2000 and 3000F, ~ ~between 2400 to 2600F. The prevailing combustion temperature favors the formation of sulur dioxide as well as carbon monoxide and hydrogen, to exclusion of hydrogen sulfide and carbonyl sulide formation despite `the prevailing reducing conditions.
As a result, the flue gas exiting combustion zone 20 contain sulfur dioxide.
Following combustion zone 20, the 1ue gas is transported through a radiant boiler section, a convection boiler section, and a high temperature economizer and may be followed by electrostatic precipitator 22 to remove ~ly ash and any carbon ~ormed. Other means to remove ash and carbon can Z5 also be employed. For instance, cyclone, bag 11ters and the like may also be employed as effluent from these sys~ems i8 normally su~ficlently f~ne to pass throu~h the catalyst 8ection employcd and can b2 removed in thc l~quid H2S
ab~orption systems used in thi3 invention.

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1 The a~r rcquircd for combus~ion is blown into air prcheater 26~ and passes by duct 16 throu~h hi~h temperature economizer 24, where it enters the combustion zone throu~h line 18 normally at tempera~ures from S00 to 600F.
The combustion products in transferring their heat by convection and radiation to boiler feed water for ~team are cooled in boiler 12 from their adiabatic combustion to a temperature from about 300F to about 800F, preferably from about 500F to about 800F.
As gas temperature in boiler 12 reduces, the conditions which favor the reduction of the oxides of ~ulfur, sulur dioxide to hydrogen sulide and carbonyl sulfide occur. As the temperature drops below about 2000~F, or instance, equilibrium ~egins to ~avor their formation, with carbonyl sulfide formation being maximized at a : . temperature of about 1200~F. In addi~ion to the reduc~ion of ~he oxides of sulfur some o the oxides of nitrogen as well as any hydrogen cyanide present will ~lso be reduced.
Because rates of reaction decrease with temperature the ~lue gases leaving boiler 12 will still contain residual quantities of the oxides o~ sulfur and nitrogen as well as other sulf~r species.
To effectively el~minate them, the gas stream now at ~5 a temperature from about 300F to about 800F is passed through an added catalyst zone 28. Catalyst zone 28 con~ains one or more metals or their sulfides typically supported on an ~lum~na, silica or alumina-silica which are capable, under reducin~ conditions, of conve~tin~ the ox~des of sulfur to hydrogen sulfide and the oxldes of nitro~en to , ' \~ .
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~ 09 S6 ~ 2 1 ~ncrt nitro~cn and/or ammonia by respectlv~ reaetions with hydro~cn and/or watcr. Typic~l o~ thc metals which ¦ may be employed are ~he Group VIII metals such as cobalt, ¦ nicklel, rhodium, palladium, iridium and platinum, as S ¦ well as the lower sulfides and oxides of molybdcnum and ¦ chromium, promoted aluminum oxides and the like.
¦ Besides hydrogenation of the oxides of sulfur to ¦ hydrogen sulide with conversion of the oxides of nitrogen ¦ to inert nitrogen andlor ammonia, the water presen~ in the 10¦ gas stream will simultaneously cause the carbon sulfur compounds such as carbonyl sulfide and carbon disulide to hydroly~e to hydrogen sul~ide. The extent of total conversion of sulfur compounds to hydrogen sulfide in both bo~ler 12 and catalyst ~one 28 is such that the 1ue gas stream a~ter hydrogen su~ ide removal will contain less than about 100 ~pm sulur calcul~ted~
A~ter conversion o ~he residual noxious sulfur species to hydrogen sulfide and the oxides of nitrogen to inert nitrogen snd/or ammonia, the flue gas stream is passed 2 through a low temperature air preheater 26 and to a hydrogen . sulfide extraction unit 30.
Because Sx and NOX are virtually el~mina~ed ~rom the : flue gas, gas temperature can be saely reduced to a temperature of from about 120 to about 150F in the air prehatex 26 with-Z out causing corrosive dilute ac~ds such as sùlfuric, poly-thionic, sulfurous and nitric acids to condense in the duct work or contaminate the chemicals used in hydrogen sulfide extrac~ion unit 30.
Any number o methods are feasible for hydrogcn sulfide 3 removal wi~h absorption methods being pre~errcd. For instanco, thc coolcd tail ~as may be pnssed throu~h ~lknlinc ~09 569 2 1¦ nDsorp~ion solu~lons wh~ch are continuously regenera~ed by oxidation to produce elenlentnl sul~ur usin~ catalysts I such as sodium vanadate, sodium anthraquinone ~isulfonate, ¦ sodil~ arsenate, sodium ferrocyanide, iron oxide, iodine 51 and like catalysts.
A convenient alternative is to use absorption solutions containing amines, sulfonates, potassium carbonates and like absorben~s for hydrogen sul~ide which can be continuously regenerated by steam stripping to produce hydrogen sulfide.
The preferred hydrogen suifide extraction system is one which involves the alkaline absorption of hydrogen sulide and o~idation to produce sulfur. The preferred 6ystem is known as the "Stxetford Process", which employs a solution containing sodium carbona~e, sodium vanadate and sodium anthraquinone d~sulfonic acid as the absorbent used in the absorber. The absorbed hydrogen sulfide is ox~dized by sodium vanadate to ~orm sulfur in the absorber and retention tank (not shown), and the absorbing solution is 2 then regenerated by oxidation with air in an oxidizer (not sho~l). The sulfur is recovered ~rom the æolution by conventional means such as 10tation, filtration, centrifuging, meltin~, decantation under pressure and the like.
The Stret~ord Process for stripping hydrogen sulfide from the tail gas is particularly prefexred ~ecause the flue g~s contains carbon dioxide as this component is not extracted.
Accordin~ly, chemicnl ;md/or utili~y requi~cments are substantially reduced.

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1 ~ftcr hydro~en sulf~de is ex~rac~cd, ~hc residual flue ~as is vcnted to ~he atmospllere by stack 32.
In carryil~ out the process of tl~e invention onc o~ thc most material ad~antages is e~fec~ of reduction of sulfur J 5 emiss~ons to the atmosphere. Sulfur dioxide emiss~ons can be readily reduced to exceptionally low levels and certainly below 100 ppm wl~ich meet or exceeds present regulations for the combustion of sul~ur bearing fuels in power generators. This will permit the use o~ more economical high sul~ur uels for power generation without creating a pollution hazard and while maximizing convers~on energy to useful power.
Process of this invention also eliminatas the oxides of nitrogen emitted to the atmosphere to exceptionally low and acceptable levels and far lower than any current method for obtaining energy by the combustion of carbonaceous uels in power generators.
In addition to permitting recovery o~ the sul~ur con~ained in the fuel as ~ree sulfur, the sulfur recovery is accomplished without creating a corrosive waste water or solid disposal problem which would only create one environmental problem to replace another.
Another important consideration is that the resultant final ~olume of ~lue gas vented to the atmosphere per unit ~5 of power generated will`be in the order of 10 to 20Z~ less than with conventional combustion practices. This permits considerable reduction in both equipment size and capital c~st.

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1 ¦ EX~MrLE
¦ Pulveri~ed coal containing 3.6Z sulur ~ burned at I the ra~c of 150 tons per hour. The amount of air used ~or ¦ the combustion is equivalent to 96~5% of the theoretical 5 ¦ air required for complete combustion representing an oxygen ¦ de~iciency of 3.5%. ~he heat of combus~ion is extracted ¦ by the boiler and generated as steam. The gas stream ¦ leaves the boiler at a temperature of 600 to 700F with ¦ some o the hea~ passing to the air entering the boiler.
10 ¦ The gas s~ream is then passed through a high eficiency ¦ electrostatic precipitat~r to reduce solid particulate ¦ content to 0.02 grains per standard cubic feet, then passed ¦ through a fixed bed of 8 cobalt molybdenum catalyst where l residual sulfur dioxide is converted to hydrogen sulfide 15¦ and oxides of nitrogen to ~ mixture of iner~ nitrogen and ¦ ammonia. The amount of hydrogen and carbon mono~ide in the flue gas leaving the catalyst zone is 0.5% by volume, l with temperature rise across the catalyst bed being between ¦ 10 to 20F. , 201 The gas stream after bei~g used to supply heat to the incomin~ 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 25 ¦ sul r dioxidc in thc gas strea= i6 les- than 100 pp~.

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Claims (10)

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. Improved process for the generation of power through the extraction of heat generated from the combustion of sulfur bearing carbonaceous fuels which comprises:
(a) combusting the sulfur bearing carbonaceous fuels in the combustion zone of the boiler of a power generator in a deficiency of air, there being provided sufficient air to maintain stable combustion and above the amount at which signi-ficant amounts of free carbon will be formed, but less than that theoretically required to achieve complete oxidation of the carbon, hydrogen and sulfur content of the fuel, thus forming a high temperature reducing flue gas stream 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 to hydrogen sulfide;
(b) cooling the high temperature reducing flue gas stream in said boiler to a temperature of from about 300 to 800°F, to extract heat values therefrom and reduce a portion of the sulfur dioxide to a mixture of hydrogen sulfide and carbonyl sulfide and the oxides of nitrogen to a nitrogen compound selected from the group consisting of nitrogen, ammonia and mixtures thereof;
(c) catalytically converting residual sulfur dioxide to hydrogen sulfide and the oxides of nitrogen to a nitrogen compound selected from the group consisting of nitrogen ammonia and mixtures thereof 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 and the oxides of nitrogen to nitrogen and ammonia;
(d) extracting the formed hydrogen sulfide from the flue gas stream and venting the flue gas stream to the atmosphere.

2. A process as claimed in claim 1 in which the formed hydrogen and carbon monoxide content of the high temperature reducing flue gas stream is at least from about 30 to about 60% in excess of that stoichiometrically required to reduce the contained sulfur dioxide.

3. A process as claimed in claim 1 in which the combustion zone is maintained at a temperature of from about 2000 to about 3000°F.

4. A process as claimed in claim 1 in which the formed carbonyl sulfide is catalytically hydrolyzed to hydrogen sulfide in the catalytic conversion zone.

5. A process as claimed in claim 1 in which the cooled reducing flue gas stream is fed to the catalytic conversion zone at a temperature from about 500 to about 800°F.

6. A process as claimed in claim 1 in which the catalyst contains a metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, molybdenum, chromium and mixtures thereof contained on a support selected from the group consisting of alumina, silica, alumina-silica and mixtures thereof.

7. A process as claimed in claim 2 in which the catalyst contains a metal selected from the group consisting of cobalt, nickel, rhodium, palladium, iridium, platinum, molybdenum, chromium and mixtures thereof contained on a support selected from the group consisting of alumina, silica, alumina-silica and mixtures thereof.

8. A process as claimed in claim 1 in which the formed hydrogen sulfide is extracted from the flue gas by contacting the flue gas with a hydrogen sulfide absorption solution.

9. A process as claimed in claim 8 in which the absorbed hydrogen sulfide is oxidized to elemental sulfur using a catalyst selected from the group consisting of sodium vanadate, sodium anthraquinone disulfonate, sodium arsenate, sodium ferrocyanide, iron oxide ant iodine.

10. A process as claimed in claim 8 in which the flue gas stream is cooled to a temperature of from about 120°F
to about 150°F prior to contact with the absorption solution.