Cogeneration System With Low NO.sub.x Combustion of Liquid Fuel

This invention relates to the cogeneration of electricity and carbon dioxide (CO.sub.2) with low NO.sub.x combustion of vaporizable liquid fuels.

Cogeneration systems have received considerable attention in recent years. Many processes have been proposed for the cogeneration of electricity and saleable refrigeration or heating steam. A few processes have been suggested for the cogeneration of electricity and CO.sub.2. By and large, cogeneration has been based on fuel combustion associated with a gas turbine. However, the operation of gas turbines has two disadvantages: combustion is carried out with a large amount of excess air which reduces thermal efficiency, and the products of combustion or flue gas has a high content of nitrogen oxides (NO.sub.x) and other atmospheric pollutants.

For example, U.S. Pat. No. 4,528,811 to Stahl shows several cogeneration systems involving gas turbines. Stahl points out that gas turbines are normally operated with about 300% excess air over the stoichiometric requirement; he proposes to curtail the use of excess air by recycling the combustion products to his two-portion combustor. However, such recycling is at the expense of compressing the expanded combustion products back to the pressure in the combustor. Moreover, such recyling does not curtail the formtion of NO.sub.x and other pollutants. To the contrary, Stahl's divided combustor which admittedly generates high temperatures may well increase the formation of NO.sub.x, carbon monoxide (CO) and unburned hydrocarbons, particularly inasmuch as Stahl uses only 10% excess air in the combustion zone of his combustor.

U.S. Pat. No. 4,797,141 to Mercader et al describes a process for recovering heat contained in the combustion gases from an engine or turbine that drives an electric generator as well as heat given off by the engine or turbine. The recovered heat is used in the absorption separation of CO.sub.2 from the combustion gases. However, the combustion gases from internal combustion engines are no less polluted than those from gas turbines.

In short, there is an urgent need to overcome the handicaps of cogeneration systems involving fuel combustion and gas turbines, namely, the use of a large amount of excess air and the emission of high levels of atmospheric pollutants.

Accordingly, a principal object of this invention is to avoid the use of gas turbines in cogeneration systems by substituting steam turbines therefor.

A related principal object is to generate steam for the steam turbines by burning a liquid fuel in a manner that minimizes the emission of NO.sub.x and other gaseous pollutants.

A further object is to limit the amount of excess air used in the combustion to yield a flue gas of increased CO.sub.2 content.

Another important object is to recover CO.sub.2 from the flue gas by absorption and desorption effected with some of the generated steam.

Still another important object is to liquefy the recovered CO.sub.2 with refrigeration produced with energy derived from the generated steam.

These and other features and advantages of the invention will be evident from the description that follows.

In accordance with this invention, the cogeneration of electricity and CO.sub.2 through low NO.sub.x combustion of a vaporizable liquid fuel comprises preheating combustion air in an amount not exceeding about 25% in excess of the stoichiometric requirement of the liquid fuel, injecting the liquid fuel into the preheated air to form a completely gaseous mixture of the fuel and air, effecting flameless combustion of the gaseous mixture on the outer surface of a porous fiber burner to produce high-pressure steam and yield hot flue gas with a low content of NO.sub.x and other gaseous pollutants, and passing the hot flue gas in heat exchange with the combustion air to effect the aforesaid preheating thereof.

The high-pressure steam is expanded with the performance of work which is used to drive an electric generator.

The flue gas after heat exchange with combustion air is treated in an amine absorption system for the separation and recovery of CO.sub.2 wherein heat for stripping absorbed CO.sub.2 is provided by a portion of the high-pressure steam after being partially expanded with the performance of work.

Part of the energy of the high-pressure steam may be utilized to compress the refrigerant vapor of a refrigeration system, particularly when liquid rather than gaseous CO.sub.2 is the desired cogeneration product together with electricity. In such case, the gaseous CO.sub.2 recovered by the amine absorption system is compressed, cooled and dehydrated, and finally liquefied with refrigeration produced with energy from the high-pressure steam.

Inasmuch as the production of CO.sub.2 is a basic purpose of the cogeneration of this invention, the selection of a vaporizable liquid fuel, preferably alcohol, leads to an advantage over the use of natural gas and normally gaseous hydrocarbons. Thus, while the methane molecule requires 2 oxygen molecules to form CO.sub.2 (CH.sub.4 +20.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O), methanol requires only 1.5 oxygen molecules (CH.sub.3 OH+1.50.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O). Inasmuch as air has roughly 4 molecules of nitrogen for every molecule of oxygen, the CO.sub.2 molecule formed from methane will be mixed with 8 nitrogen molecules while CO.sub.2 formed from methanol will be mixed with 6 nitrogen molecules. Clearly, production of CO.sub.2 by combustion of methane results in a flue gas wherein CO.sub.2 is diluted by roughly 33% more nitrogen than when CO.sub.2 is produced from methanol. This difference in CO.sub.2 dilution by nitrogen diminishes when the liquid fuel is a hydrocarbon but not significantly. For example, a C.sub.10 -hydrocarbon will yield each CO.sub.2 molecule admixed with 6.2 molecules of nitrogen compared to 8 nitrogen molecules when methane is the fuel. Hydrocarbon fractions containing aromatic and olefinic compounds are very desirable fuels for this invention. Thus, benzene yields CO.sub.2 admixed with only 5 volumes of nitrogen which is obviously better than CO.sub.2 in 6 volumes of nitrogen formed from methanol. In short, as the ratio of hydrogen to carbon of the fuel decreases the ratio of CO.sub.2 to nitrogen or concentration of CO.sub.2 in the flue gas increases.

Alcohols, particularly methanol which is the cheapest, and petroleum fractions containing aromatic and olefinic compounds are preferred fuels. As pointed out in U.S. Pat. No. 4,865,543 to Garbo, the liquid fuel used with the porous fiber burner must be completely vaporizable and may be any hydrocarbon fraction having an end boiling point not exceeding about 650.degree. F. Preferably, the liquid fuel will have an end boiling point not exceeding about 500.degree. F.; naptha and kerosene fractions are economic fuels within this preferred boiling range. Other sources of liquid hydrocarbon fractions that can be utilized by the invention are tar sands, shale oil, coal liquefaction and synthetic fuels. Alcohols derived from agricultural products may in some places be economic fuels for this invention. The liquid fuel must be free of any ash residue to prevent clogging of the porous fiber burner.

An advantage of porous fiber burners is that all of the combustion air is mixed with the fuel and rarely is more than 25%, but often is not more than about 15%, in excess of the stoichiometric requirement. This limitation of combustion air obviously helps in obtaining a flue gas wherein CO.sub.2 is not excessively diluted.

For a fuller understanding of the invention, the further description will refer to the appended drawing which is a flow diagram of the cogeneration system of the invention in a preferred integrated embodiment.

The description of the flow diagram will include data of a specific example of the invention.

Combustion air is drawn in by blower 10 driven by electric motor 11 at inlet 12 and impelled through line 13, heat exchanger 14 and line 15 into tee connection 16 into which methanol is injected via line 17. Methanol from a supply (not shown) flows through line 18 to pump 19 which feeds the methanol to line 17 at the rate of 32,160 pounds per hour. The methanol injected into tee connector 16 is dispersed in combustion air, preheated to a temperature of 280.degree. F. in exchanger 14, whereupon the methanol is completely vaporized. The amount of preheated combustion air which is admixed with the methanol is limited to 10% in excess of the stoichiometric requirement. The mixture of air and vaporized methanol passes from tee 16 via line 20 into porous fiber burner 21 in steam boiler 22.

Burner 21 has a porous fiber layer on its convex side. The production of porous fiber burners is well known. Basic teachings of such burners are found in U.S. Pat. No. 3,275,497 and U.S. Pat. No. 3,383,159 which adds aluminum powder in the porous fiber layer to suppress the formation of CO. While burner 21 is simply represented as having a large convex-lens shape, other shapes, such as tubular, may be used. In fact, the shape of burner 21 will usually be selected depending on the size and configuration of boiler 22. Inasmuch as the porous fiber layer of burner 21 becomes a glowing surface during combustion that radiates infrared energy, burner 21 is disposed in boiler 22 so that the glowing surface faces the steam-generating tubes therein. Also, while a single burner 21 is shown in boiler 22 to simplify the diagram, it will be appreciated that generally a multiplicity of burners 21 will be used in any large boiler 22.

Hot flue gas, containing only 15 ppm NO.sub.x, 24 ppm CO and 2 ppm unburned hydrocarbons, leaves boiler 22 at a temperature of 350.degree. F. and passes through line 23 and heat exchanger 14 from which it exits at a temperature of 160.degree. F. via line 24. Steam generated in boiler 22 passes at a pressure of 600 psia via line 25 into steam turbine 26. Fully expanded steam leaves turbine 26 through line 27 and passes through water-cooled condenser 28. The pressure at the exit port of steam turbine 26 is about 2.5 inches of mercury column. The condensate formed in condenser 28 drains via line 29 into pump 30 which returns it via line 31 to boiler 22. The work performed by steam turbine 26 is utilized to drive electric generator 32 which is connected thereto. Generator 32 produces 17.9 megawatts (MW) of electricity.

Flue gas flowing in line 24 enters blower 33 driven by electric motor 34 and is pressurized enough to flow via line 35 into and up through absorption column 36 in scrubbing relation with counter-flowing amine solution. Regenerated amine solution enters column 36 via line 37 and CO.sub.2 -laden solution leaves column 36 through line 38. Heat of absorption is dissipated by circulating the amine solution of column 36 through line 39, pump 40, line 41, water-cooled exchanger 42 and line 43. The discard gas, predominantly nitrogen with about 10% by volume of water vapor, leaves column 36 through line 44.

Pump 45 passes the CO.sub.2 -enriched amine solution from line 38 through line 46, heat exchanger 47 and line 48 into stripping column 49. Stripping heat is supplied to column 49 by circulating amine solution through lines 50,51, reboiler 52 and line 53. Reboiler heat is obtained by drawing steam from an intermediate stage of steam turbine 26 through line 54 connected to reboiler 52. The steam entering reboiler 52 is at a pressure of 70 psia and temperature of 320.degree. F. The condensate formed in reboiler 52 flows through line 55 to pump 30 which returns the total condensate of lines 29, 55 to boiler 22 via line 31.

Stripped amine solution from line 50 is returned by pump 56 through line 57, heat exchanger 47, line 58, water-cooled exchanger 59 and line 37 to absorption column 36. Part of the heat in the stripped amine solution is transferred in exchanger 47 to the CO.sub.2 -laden amine solution flowing into stripping column 49.

CO.sub.2 gas released from the amine solution rises in column 49 and together with water vapor passes via line 60 through water-cooled condenser 61 to discharge into separator 62. Water condensate returns from separator 62 via line 63 as reflux in column 49. Water is discarded from separator 62 through drain line 64 at the same rate at which water vapor enters column 36 with the flue gas from line 35.

CO.sub.2 leaves separator 62 via line 65 at slightly above atmospheric pressure and a temperature of about 100.degree. F. The gas is saturated with water vapor but otherwise is CO.sub.2 containing a very small fractional percentage by volume of other gases such as nitrogen and oxygen. At this point, the CO.sub.2 gas could be utilized to enhance the recovery of crude oil from an underground formation as taught in U.S. Pat. No. 3,443,332 to Keith or to provide an inert atmosphere as required in some chemical or metallurgical operations. When necessary, the CO.sub.2 would be dehydrated before use.

Liquid CO.sub.2 is a more valuable form of the product that is in great demand, e.g., by manufacturers of carbonated beverages and frozen foods. To effect liquefaction, CO.sub.2 gas from line 65 is compressed by centrifugal compressor 66, driven by electric motor 67, to a pressure of 220 psia and flows via line 68 through water-cooled condenser 69 wherein the gas temperature is reduced to about 100.degree. F. with the result that moisture in the gas is condensed. The cooled steam flows through line 70 into separator 71 wherein water condensate is knocked down and discharged via drain line 72.

CO.sub.2 gas saturated with moisture exits separator 71 via line 73 and passes through dryer 74 wherein residual moisture is removed from the compressed CO.sub.2 gas. Dryer 74 may be filled with a molecular sieve that adsorbs moisture but any of various known dryers may be used to dehydrate the CO.sub.2 gas. Dry CO.sub.2 gas passes through line 75 and heat exchanger 76 wherein it is liquefied. Liquid CO.sub.2 discharges via line 77 into storage tank 78 at a temperature of -20.degree. F. and pressure of 215 psia. Liquid CO.sub.2 is recovered at the rate of 41,700 pounds per hour.

The refrigeration required for the liquefaction of CO.sub.2 in exchanger 76 is provided by compressing a refrigerant vapor, such as DuPont Freon R-12, cooling the compressed refrigerant to liquefy it, subcooling the liquefied refrigerant, and expanding the subcooled liquid to yield refrigeration with the evaporation of the liquid refrigerant. Specifically, the energy for compressing the refrigerant vapor is derived from steam turbine 26 by having centrifugal compressor 80 driven by steam turbine 26. Compressor 80 raises the pressure of Freon R-12 to 135 psia and the temperature to about 200.degree. F.

The hot compressed refrigerant vapor exiting compressor 80 via line 81 passes through water-cooled condenser 82 and at a temperature of 100.degree. F. continues to flow as liquid in line 83 to subcooler exchanger 84. The subcooled refrigerant leaves subcooler 84 at a temperature of 20.degree. F. through line 85 and passes through pressure-reducing valve 86. The liquid refrigerant discharges from valve 86 at a pressure of 12 psia and temperature of -30.degree. F. via line 87 into heat exchanger 76 wherein it is vaporized by heat abstracted from CO.sub.2 undergoing liquefaction therein. Refrigerant vapor passes from exchanger 76 through line 88, sub-cooler 84 and line 89 to return to compressor 80 at a temperature of 80.degree. F. and pressure of 10 psia. The refrigeration cycle delivers 510 tons of refrigeration to exchanger 76 at a temperature level of -30.degree. F. However, the temperature level of the delivered refrigeration can be varied, e.g., over the range of about 20.degree. F. down to -60.degree. F., if the refrigeration is to be utilized for other purposes such as freezing fish and meat.

Electric motors 11, 34, 67 and those (not shown) for pumps 19,30,40,45,56 consume a total of about 2.4 MW of electric power. Pumps to circulate cooling water to exchangers 28,42,59,61,69,82 and fans of a water-cooling tower are estimated to consume approximately 0.9 MW. Hence, of the 17.9 MW of power produced by generator 32, 14.6 MW are available for sale after satisfying the power requirements of the cogeneration system.

Recapping the example described in relation to the flow-sheet, the hourly combustion of 32,160 pounds of methanol with only 10% in excess of the stoichiometric requirement of air with a porous fiber burner yields a flue gas containing a remarkably low content of pollutants, thus achieving an important advance in environmental protection. Besides producing a flue gas in which CO.sub.2 is not excessively diluted by nitrogen, the combustion produces enough high-pressure steam to drive an electric generator delivering 17.9 MW of power and to drive a refrigerant compressor delivering 510 tons of refrigeration. Moreover, steam is available for operating the reboiler of a CO.sub.2 stripping column in an amine absorption system used to recover CO.sub.2 from the flue gas. The recovered CO.sub.2 gas with a purity of about 99.9% by volume is liquefied by applying the refrigeration generated within the system. Thus, the cogeneration system has a production capacity of 500 tons per day of liquid CO.sub.2.

While the cogeneration example produces electricity and liquid CO.sub.2, when only gaseous CO.sub.2 is desired as a product of cogeneration, 360,000 SCFH of CO.sub.2 will be delivered by line 65 and 510 tons of refrigeration at a temperature of -30.degree. F. will be available at heat exchanger 76 as a third saleable product of cogeneration. In short, the cogeneration system can simultaneously deliver three products: refrigeration, electric power and substantially pure CO.sub.2 gas, while discharging into the atmosphere a vent gas, predominantly nitrogen, with an unusually low content of pollutants: NO.sub.x, CO and unburned hydrocarbons.

On the other hand, the cogeneration system may be desired for the production of only electricity and gaseous CO.sub.2. In such a system, all of the elements with reference numerals 66 through 89 will be eliminated; not only will the plant cost be reduced but also the output of generator 32 will be increased to 19 MW in the absence of refrigerant compressor 80. The elimination of motor 67 and of water-cooled exchangers 69, 82 will obviously reduce the portion of the generated electricity which is consumed within the system.

In some cases, the liquid fuel, e.g., kerosene, would not be completely vaporized by injecting it into the combustion air preheated solely by heat exchange with hot flue gas from the steam boiler. In such circumstances, supplemental heat must be provided to ensure complete vaporization of the liquid fuel. Referring to the flowsheet, supplemental heat is added to preheated air in line 15 by diverting a portion or all of it to branch line 15A connected to coil 91 in heater 90. While heater 90 may take any known form, it is shown as having burner 92 supplied with fuel by line 93.

Limiting the amount of combustion air to not more than about 25% in excess of the stoichiometric requirement, preferably not more than about 15% in excess, leads to high thermal efficiency. Such a limited amount of combustion air is made practical by the use of a porous fiber burner which permits surface flameless combustion of the fuel yielding a flue gas with a remarkably low content of pollutants which on a volume basis rarely exceed about 20 ppm NO.sub.x, 50 ppm CO and 5 ppm unburned hydrocarbons. The suppresion of CO emission is enhanced by a small amount of fine aluminum powder uniformly distributed in the porous fiber layer of the burner as taught by aforesaid U.S. Pat. No. 3,383,159.

The term amine used herein is a shortened word for a class of chemical absorbents known as alkanolamines. The two amines which have proved to be of principal commercial interest are monoethanolamine and diethanolamine. While the specific example of the invention is based on monoethanolamine, other amines are available for use in the invention. A fairly broad discussion, entitled Alkanolamines for Hydrogen Sulfide and Carbon Dioxide Removal, by Kohl and Riesenfeld in their textbook Gas Purification (pages 29-109, 4th Edition, 1985, Gulf Publishing Company), presents the technology of separating CO.sub.2 from other gases and is made a part of this disclosure.

Variations and modifications of the invention will be apparent to those skilled in the art without departing from the spirit and scope of the invention. For example, if it is desired to reduce the gas temperature of the flue gas before it enters absorption column 36, a water-cooled exchanger may be inserted in line 35 or, more simply, water may be injected into line 35 to drop the gas temperature. A single steam turbine 26 with line 54 for drawing steam from an intermediate pressure stage of turbine 26 is shown in the drawing; however, two steam turbines connected in tandem may replace turbine 26 and line 54 can then draw steam from the connection between the two turbines. Claim language such as: the steam turbine has a port at an intermediate pressure stage, is intended as a simple expression for two steam turbines in tandem. Branch line 65A is provided to permit the delivery of some of the recovered CO.sub.2 in gaseous form for one customer while the remainder is liquefied for another customer. The liquid fuel may also be preheated to facilitate its complete vaporization in the preheated combustion air. Repeated mention of a porous fiber burner in the specification and claims is clearly a simple expression for a multiplicity of such burners supplied in parallel with the mixture of vaporized liquid fuel and combustion air. Accordingly, only such limitations should be imposed on the invention as are set forth in the appended claims.