INTEGRATED OXY-FUEL COMBUSTION AND NOX CONTROL

Abstract

The efficiency of selective non-catalytic reduction reactions for converting NOx in a flue gas to molecular nitrogen is improved by producing the flue gas from combustion of fuel with oxidant having an oxygen concentration above that of air.

Description

This application claims priority from U.S. Provisional Application Ser. No. 60/875,940, filed Dec. 20, 2006, the content of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to improvements in the combustion of hydrocarbon fuels in apparatus such as heaters, boilers, kilns, and furnaces.

BACKGROUND OF THE INVENTION

Many industrial processes, such as heaters, boilers, kilns, furnaces, and the like, rely on the combustion of hydrocarbon fuels to generate heat that is used in the process, for instance, to heat a chemical process stream, to heat water into superheated steam for power generation, to heat and/or melt and/or fuse feed material, and the like. The flue gas that is produced by the combustion generally contains, among other products, nitrogen oxides (referred to herein as “NOx”, by which is meant any oxide of nitrogen regardless of the atomic ratio of the oxygen to the nitrogen, and mixtures thereof). Since NOx is considered to be a pollutant, operators of these processes must control or reduce the amount of NOx in the flue gas stream emitted from the combustion.

One technique for controlling the amount of NOx emitted from combustion processes is known as selective non-catalytic reduction (also referred to as “SNCR”). In this technique, flue gas is treated with a reducing reagent (typically ammonia or urea) to convert nitrogen oxides in the flue gas to molecular nitrogen, N₂. However, this technique is not as adaptable to combustion processes as might be desired, because of the characteristics of the flue gas that is produced by the combustion.

Thus, there remains a need for methodology to improve the efficiency and adaptability of selective non-catalytic reduction technique in combustion processes.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is its adaptation to combustion processes, and specifically to a method of modifying a combustion process, comprising

(a) providing a combustion apparatus in which fuel and gaseous oxidant, preferably air, are combusted, wherein the combustion produces flue gas containing NOx and containing carbon monoxide in a concentration higher than 200 ppm and having a temperature at which adding the nitrogen-containing reducing reagent added in step(d) to the flue gas would oxidize the reagent to form NOx,

(b) increasing the oxygen concentration of the gaseous oxidant fed to said combustion apparatus to a level at which combustion of said fuel with said gaseous oxidant decreases the carbon monoxide concentration of said flue gas to less than 200 ppm, and decreases the temperature of said flue gas to a temperature within the range within which the reaction of the reducing reagent added in step

(d) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas, and

(c) combusting said fuel with said gaseous oxidant having said increased oxygen concentration to form flue gas that contains carbon monoxide at a concentration higher than 200 ppm, contains NOx, and has a temperature in a said range, and

(d) feeding into said flue gas produced in step (c) a nitrogen-containing reducing reagent and reacting it with NOx in said flue gas to form N₂ at a temperature within said range within which the reaction of the reducing reagent added in step (d) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas.

Yet another aspect of the present invention is a method of operating a combustion process, comprising

(a) combusting fuel and gaseous oxidant having an oxygen concentration higher than that of air in a combustion apparatus to produce flue gas containing NOx and containing carbon monoxide under conditions under which, if the gaseous oxidant were air, the carbon monoxide concentration of the flue gas would be greater than 200 ppm, except that the oxygen concentration of said gaseous oxidant is at a level at which the flue gas produced by said combustion of said fuel with said gaseous oxidant contains carbon monoxide at a concentration below 200 ppm, and the temperature of said flue gas is in a range within which the reaction of the reducing reagent added in step (b) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas, and

(b) feeding into said flue gas produced in step (a) a nitrogen-containing reducing reagent which and reacting it with NOx in said flue gas to form N₂ wherein the temperature of said flue gas is within said range.

A further aspect of the present invention is a method of improving the efficiency of a process that lowers the amount of NOx in a gaseous stream of flue gas by reacting a nitrogen-containing reducing reagent with said NOx to convert said NOx to N₂, comprising

(a) combusting fuel and gaseous oxidant that has an oxygen concentration higher than that of air in a combustion apparatus to produce flue gas containing NOx and carbon monoxide, while maintaining the oxygen concentration of said gaseous oxidant at a level at which (i) the flue gas produced by said combustion of said fuel with said gaseous oxidant contains carbon monoxide at a concentration below 200 ppm, and (ii) the temperature of said flue gas is in a range within which the reaction of the reducing reagent added in step (b) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas, and

(b) combining flue gas produced in step (a) with nitrogen-containing reducing reagent and reacting it with NOx in said flue gas at a temperature within the range set forth in step (a)(ii) to form N₂ while controlling the concentration of carbon monoxide in said flue gas and the temperature of said flue gas by controlling the concentration of oxygen in the gaseous oxidant that is combusted in step (a).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowsheet of one embodiment in which the present invention may be carried out.

FIG. 2 is a flowsheet of an alternate embodiment in which the present invention may be carried out.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is useful in any industrial process in which combustion of hydrocarbon fuel is carried out to generate heat which is used for a useful purpose. Many examples of such processes exist. Among them are process heaters, wherein combustion is carried out in a combustion chamber and the heat of combustion is conducted through the walls of the combustion chamber to gaseous, liquid or solid material in heat-conductive relation to the combustion chamber (such as through tubes in the combustion chamber walls), wherein the heat is employed to raise the temperature of the material, to bring about a phase change such as evaporation, and/or to promote a chemical reaction such as steam-methane reforming. Other examples include boilers, where the heat of combustion is conveyed to feed water in tubes surrounding the combustion chamber, in order to heat the water and convert it to superheated steam which can then be fed to turbines for generation of electric power. Other examples include furnaces, such as glass melting furnaces and incinerators, wherein material is melted and/or combusted. Preferred examples include calciners and kilns such as cement kilns, wherein solid feed material is heated so that it undergoes a chemical transformation.

Combustion apparatus with which the present invention can be practiced includes a combustion chamber, at least one burner, apparatus for providing fuel and gaseous oxidant to the burner (or to each burner) to be combusted, and an outlet for flue gas that is produced by combustion of the fuel and oxidant at the burner within the combustion chamber.

Referring to FIG. 1, combustion apparatus is shown which includes combustion chamber 1 and burner 2 which can be located in a side, end, floor or roof of combustion chamber 1. Combustion apparatus that is used in many commercial operations includes more than one such burner, but FIG. 1 illustrates one burner for ease of description.

Feed lines 3 and 4 respectively represent the feeding to burner 2 of fuel and of gaseous oxidant for combustion with the fuel at burner 2 within the combustion chamber. The fuel is preferably fed to the burner 2 in one stream and it can be fed out of burner 2 in one stream or multiple streams. The oxidant can be fed to the burner in one stream or in multiple streams and it can be fed from the burner from one location or from multiple locations. A well-known example is burners that feed oxidant from an opening or group of openings one distance from the fuel stream and that feed oxidant from an opening or group of openings that are further from the fuel stream than said one distance.

The combustion within combustion chamber 1 produces stream 5 of flue gas which comprises the gaseous products of the combustion of the fuel and oxidant. There can be one such stream, or multiple streams from the combustion chamber 1.

In the embodiment of the present invention in which material is passed through the combustion chamber 1 to be processed (i.e. to be heated and thereby to be melted, fused, and/or chemically transformed, as the case may be), the material is preferably fed and withdrawn from points at or near opposite ends of the combustion chamber. More preferably, the material being processed flows countercurrent to the flow of fuel and oxidant into the combustion chamber and the flow of flue gas out of the combustion chamber. Thus, referring to FIG. 1, material that would be processed within combustion chamber 1 is preferably fed as stream 11 passing into combustion chamber 1 near the end of combustion chamber 1 from which flue gas stream 5 passes, and the processed material 12 passes out of combustion chamber 1 at or near the point at which fuel and oxidant are fed into combustion chamber 1 to be combusted at burner 2.

Any of a number of different fuels and types of fuels can be combusted. That is, the fuel can be gaseous, liquid, solid (preferably in finely divided solid form), or a mixture thereof. Suitable gaseous fuels include natural gas, methane, propane, petroleum gas, and any other gaseous combustible matter, and mixtures thereof. Suitable liquid fuels include fuel oil and any other combustible liquid hydrocarbon and mixtures thereof. Suitable solid fuels include coal, coke, solid waste, any other solid combustible material, and mixtures thereof.

In heretofore known, conventional operation of apparatus such as that illustrated in FIG. 1, the oxidant fed to burner 2 within combustion chamber 1 is air. The fuel and air are fed to the burner and combusted within combustion chamber 1 in amounts relative to each other to provide the desired combustion of the fuel. Typically, and particularly when all of the gaseous oxidant for combustion is fed with the fuel at the burner, the flue gas exiting combustion chamber 1 contains carbon monoxide in amounts in excess of 200 ppm. Flue gas characterized by such high temperatures and such high concentrations of carbon monoxide cannot be treated effectively by selective non-catalytic reaction techniques for removal of NOx from the flue gas before it is emitted to the atmosphere. Also, typically, when combustion is carried out using air as the only oxidant and hydrocarbon fuel such as natural gas, oil or coal as the fuel, the temperature of the flue gas exiting combustion chamber 1 is so high (typically, over 2200° F. or even over 2000° F.) that nitrogen-containing reducing reagent added to the flue gas is oxidized to NOx rather than contributing to converting NOx to N₂ as desired. The method of the present invention provides ways to adapt industrial combustion processes of the type described herein, so that the selective non-catalytic reaction techniques can be applied more efficiently and more effectively to the flue gas produced by the combustion.

That is, in accordance with the present invention, the oxygen concentration of all of the oxidant stream or streams fed to combustion chamber 1 for combustion with the fuel is raised to a level so that the aggregate oxygen concentration of the oxidant fed to the combustion chamber (that is, the total amount of oxygen fed to the combustion chamber relative to the total amount of all gaseous streams fed to the combustion chamber including any gas such as air that is used to transport the fuel through the burner) is greater than the oxygen concentration of dry air, i.e. greater than 20.9 vol. %. This increase in the oxygen concentration of the oxidant can be achieved by premixing a gaseous stream having a high oxygen concentration, preferably at least 90 vol. % oxygen, with air so as to create a gaseous oxidant having the desired oxygen concentration, before the resulting mixture is fed to burner 2. Alternatively, a gaseous stream of oxygen, preferably containing at least 90 vol. % oxygen, is fed directly to the burner where it mixes with air and/or other oxidant streams to be combusted at the burner. In a preferred embodiment, the stream of air is replaced by a stream of high-purity oxygen, preferably containing at least 90 vol. % oxygen.

Preferably, the aggregate oxygen concentration of the gaseous oxidant fed to the combustion apparatus is at least 30 vol. %, and more preferably at least 90 vol. %.

The aggregate oxygen concentration of the gaseous oxidant fed to combustion chamber 1 for combustion with the fuel is increased to a level at which the flue gas produced by combustion of the fuel with the resulting oxidant satisfies conditions as to carbon monoxide concentration and temperature.

One condition is that the flue gas produced by the combustion has a carbon monoxide concentration that is below 200 ppm. The higher oxygen concentration fed to the combustion chamber achieves this condition by promoting increased conversion of the fuel to carbon dioxide. Satisfying this condition lessens the ability of carbon monoxide to inhibit the conversion of NOx to molecular nitrogen.

A second condition is that the increased concentration of the oxygen fed to the combustion chamber for combustion also reduces the temperature of the flue gas emerging from combustion chamber 1 into an effective range within which the efficiency of the selective non-catalytic reduction reaction is increased. The upper end of this range is characterized as a temperature at which (and above which) the reducing reagent is converted to additional NOx, rather than converting NOx to molecular nitrogen. The lower end of this effective temperature range is characterized as a temperature at which (and below which) the reaction of NOx and reducing reagent proceeds to less than 90% of stoichiometric completion under ideal mixing conditions, by which is meant that with all reagents completely mixed less than 90% of the reagent that is not in stoichoimetric excess (or less than 90% of all reagents if all are present in stoichiometrically equal amounts) is consumed in the reaction by which NOx is converted to molecular nitrogen. The effective range for the temperature of the flue gas emerging from any particular combustion chamber 1 can be determined experimentally. Typical upper ends of the range defined in this way are on the order of 2000° F. to 2200° F., and typical lower ends of the range defined in this way are on the order of 1700° F. to 1800° F., but the ranges may vary with particular apparatus and combustion conditions.

The appropriate aggregate oxygen concentration of the gaseous oxidant to feed to the combustion chamber that satisfies these conditions can readily be determined, by comparing measurements of the oxygen concentration of oxidant fed into the combustion chamber, with measurements of the temperature, the carbon monoxide concentration, and the NOx content, of the flue gas emerging from the combustion chamber, preferably while holding other inputs constant.

The flue gas stream 5 preferably contains unreacted oxygen, in an amount up to about 5% in excess of stoichiometric.

Carrying out combustion in chamber 1 as described herein with oxidant present in a concentration higher than that of air also improves the processing of material that passes through the combustion chamber. Processing is improved in the rate of throughput of material that is achieved.

Returning to FIG. 1, stream 7 which comprises reducing reagent is fed into flue gas stream 5 and the reducing reagent reacts with NOx to convert the NOx into N₂. Preferred reducing reagents include urea, ammonia, and mixtures thereof. The stream comprising reducing reagent can be fed into the flue gas in, for example, a duct carrying the flue gas, a vessel, or another unit such as a heat exchanger through which the flue gas stream passes. The reducing reagent is preferably fed as an aqueous stream thereof which is sprayed or sparged into the flue gas stream. The temperature of the flue gas stream should still be within the aforementioned effective range. Preferably, the temperature of the flue gas is not raised or lowered out of that effective range between the combustion chamber 1 and the point at which reducing reagent is fed into the flue gas stream. The flue gas containing reaction products of the reduction reaction, principally water and molecular nitrogen, comprise stream 8 which can be vented to the atmosphere, fed to a scrubber, or fed to another chemical processing unit.

The invention can be practiced in other embodiments too. FIG. 2 illustrates one such type of embodiment. Flue gas stream 5 passes into preheater or preprocessor unit 10, wherein fuel fed as stream 13 and gaseous oxidant fed as stream 14 are combusted to produce heat. The combustion in unit 10 produces flue gas stream 15 that contains carbon monoxide and NOx among the combustion products. Reducing reagent 7 is then fed into the flue gas stream 15 to convert NOx in the stream 15 to molecular nitrogen. Preferably, stream 16 of material to be processed is fed into and through unit 10 where it is heated by direct or indirect heat exchange with the gaseous stream in unit 10. The heated material passes from unit 10 as stream 11 which, as described herein, is fed to combustion chamber 1.

In an alternative embodiment, the incoming stream 16 of material to be processed is heated by the gaseous stream before the reducing reagent is fed into the gaseous stream, following which the material passes through unit 10, and then into combustion chamber 1.

The present invention is illustrated further in the following simulated example. Hypothetical process data for a cement kiln, as if operated using only air as the source of oxygen for combustion, and using with and without oxygen injection into the combustion air fed to the burner, were calculated using a kiln balance program that accounts for energy and mass balances in the kiln. The oxygen combustion system is designed and controlled in a manner to minimize CO emissions from the kiln. The example is intended to illustrate the beneficial effects of the added oxygen on key kiln operating parameters.

			10%
		Air	Oxygen
	Units	Baseline	Addition
Clinker production	ton/hr	77	85
Kiln firing rate	MBtu/hr	250	263
Stoichiometric oxygen requirement	Scfh	512000	538000
Total air feed	Scfh	2692000	2560000
Oxygen to kiln	Scfh	0	54000
Flue gas from kiln	Scfh	3452000	3402000
Solids temperature from kiln	° F.	2300	2300
Preheated air temperature	° F.	1200	1200
Flue gas leaving kiln	° F.	2000	1950

The addition of pure (99.9 vol. %) oxygen results in approximately 10% increased production from the kiln. Although the overall firing rate is increased to generate more production, the amount of air fed to the kiln is reduced, because of the addition of the oxygen. This also results in a reduction of the flue gas amount.

In this example the temperature of the solids leaving the process, and the temperature of the air fed to the kiln, remain unchanged by the addition of the oxygen. However, the addition of oxygen results in a significant reduction of the flue gas temperature that is leaving the kiln. This effect can be beneficial for the operation of the selective non-catalytic reaction system downstream of the kiln.

A preferred aspect of the method of the present invention is that the aggregate oxygen concentration of the gaseous oxidant fed to the combustion chamber can be varied as necessary to maintain the carbon monoxide concentration of the flue gas at a level that remains within a predetermined range of values below 200 ppm. Similarly, the aggregate oxygen concentration of the gaseous oxidant fed to the combustion chamber can be varied as necessary to maintain the temperature of the flue gas emerging from combustion chamber 1 at a level that remains within a predetermined range.

Controlling the oxygen concentration of the gaseous oxidant fed to the combustion chamber thus provides the ability to maintain steady conditions for the flue gas which becomes the feed stream into the selective non-catalytic reduction reaction, despite fluctuations in the combustion step such as fluctuations in the specific energy content of the fuel (that is, the amount of energy that can be derived by complete combustion of the fuel per unit mass of the fuel), fluctuations in the feed rate of the fuel to the combustion process, and/or fluctuations in the moisture content of the fuel that is fed to combustion chamber 1.

Claims

1. A method of modifying a combustion process, comprising

(a) providing a combustion apparatus in which fuel and gaseous oxidant, preferably air, are combusted, wherein the combustion produces flue gas containing NOx and containing carbon monoxide in a concentration higher than 200 ppm and having a temperature at which adding the nitrogen-containing reducing reagent added in step(d) to the flue gas would oxidize the reagent to form NOx,

(b) increasing the aggregate oxygen concentration of the gaseous oxidant fed to said combustion apparatus to a level at which combustion of said fuel with said gaseous oxidant decreases the carbon monoxide concentration of said flue gas to less than 200 ppm, and decreases the temperature of said flue gas to a temperature within the range within which the reaction of the reducing reagent added in step (d) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas, and

(c) combusting said fuel with said gaseous oxidant having said increased oxygen concentration to form flue gas that contains carbon monoxide at a concentration higher than 200 ppm, contains NOx, and has a temperature in a said range, and

(d) feeding into said flue gas produced in step (c) a nitrogen-containing reducing reagent and reacting it with NOx in said flue gas to form N2 at a temperature within said range within which the reaction of the reducing reagent added in step (d) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas.

2. A method according to claim 1 wherein the fuel that is combusted in said combustion apparatus is gaseous.

3. A method according to claim 1 wherein the fuel that is combusted in said combustion apparatus is liquid.

4. A method according to claim 1 wherein the fuel that is combusted in said combustion apparatus is solid.

5. A method according to claim 1 wherein the aggregate oxygen concentration of the gaseous oxidant is increased in step (b) to at least 30 vol. %.

6. A method according to claim 1 wherein the aggregate oxygen concentration of the gaseous oxidant is increased in step (b) to at least 90 vol. %.

7. A method according to claim 1 wherein the combustion apparatus is a kiln.

8. A method according to claim 7 wherein the fuel that is combusted in said combustion apparatus is gaseous.

9. A method according to claim 7 wherein the fuel that is combusted in said combustion apparatus is liquid.

10. A method according to claim 7 wherein the fuel that is combusted in said combustion apparatus is solid.

11. A method according to claim 7 wherein the aggregate oxygen concentration of the gaseous oxidant is increased in step (b) to at least 30 vol. %.

12. A method according to claim 7 wherein the aggregate oxygen concentration of the gaseous oxidant is increased in step (b) to at least 90 vol. %.

13. A method of operating a combustion process, comprising

(a) combusting fuel and gaseous oxidant having an oxygen concentration higher than that of air in a combustion apparatus to produce flue gas containing NOx and containing carbon monoxide under conditions under which, if the gaseous oxidant were air, the carbon monoxide concentration of the flue gas would be greater than 200 ppm, except that the oxygen concentration of said gaseous oxidant is at a level at which the flue gas produced by said combustion of said fuel with said gaseous oxidant contains carbon monoxide at a concentration below 200 ppm, and the temperature of said flue gas is in a range within which the reaction of the reducing reagent added in step (b) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas, and

(b) feeding into said flue gas produced in step (a) a nitrogen-containing reducing reagent which and reacting it with NOx in said flue gas to form N2 wherein the temperature of said flue gas is within said range.

14. A method according to claim 13 wherein the fuel that is combusted in said combustion apparatus is gaseous.

15. A method according to claim 13 wherein the fuel that is combusted in said combustion apparatus is liquid.

16. A method according to claim 13 wherein the fuel that is combusted in said combustion apparatus is solid.

17. A method according to claim 13 wherein the aggregate oxygen concentration of the gaseous oxidant combusted in step (a) is at least 30 vol. %.

18. A method according to claim 13 wherein the aggregate oxygen concentration of the gaseous oxidant combusted in step (a) is at least 90 vol. %.

19. A method according to claim 13 wherein the combustion apparatus is a kiln.

20. A method according to claim 19 wherein the fuel that is combusted in said combustion apparatus is gaseous.

21. A method according to claim 19 wherein the fuel that is combusted in said combustion apparatus is liquid.

22. A method according to claim 19 wherein the fuel that is combusted in said combustion apparatus is solid.

23. A method according to claim 19 wherein the aggregate oxygen concentration of the gaseous oxidant combusted in step (a) is at least 30 vol. %.

24. A method according to claim 19 wherein the aggregate oxygen concentration of the gaseous oxidant combusted in step (a) is at least 90 vol. %.

25. A method of improving the efficiency of a process that lowers the amount of NOx in a gaseous stream of flue gas by reacting a nitrogen-containing reducing reagent with said NOx to convert said NOx to N2, comprising

(a) combusting fuel and gaseous oxidant that has an oxygen concentration higher than that of air in a combustion apparatus to produce flue gas containing NOx and carbon monoxide, while maintaining the oxygen concentration of said gaseous oxidant at a level at which (i) the flue gas produced by said combustion of said fuel with said gaseous oxidant contains carbon monoxide at a concentration below 200 ppm, and (ii) the temperature of said flue gas is in a range within which the reaction of the reducing reagent added in step (b) with NOx proceeds to at least 90% of stoichiometric completion under ideal mixing conditions and said reducing reagent is not oxidized to form NOx in the flue gas, and

(b) combining flue gas produced in step (a) with nitrogen-containing reducing reagent and reacting it with NOx in said flue gas at a temperature within the range set forth in step (a)(ii) to form N2 while controlling the concentration of carbon monoxide in said flue gas and the temperature of said flue gas by controlling the concentration of oxygen in the gaseous oxidant that is combusted in step (a).

26. A method according to claim 25 wherein the fuel that is combusted in said combustion apparatus is gaseous.

27. A method according to claim 25 wherein the fuel that is combusted in said combustion apparatus is liquid.

28. A method according to claim 25 wherein the fuel that is combusted in said combustion apparatus is solid.

29. A method according to claim 25 wherein the aggregate oxygen concentration of the gaseous oxidant combusted in step (a) is at least 30 vol. %.

30. A method according to claim 25 wherein the aggregate oxygen concentration of the gaseous oxidant is combusted in step (a) is at least 90 vol. %.

31. A method according to claim 25 wherein the combustion apparatus is a kiln.

32. A method according to claim 31 wherein the fuel that is combusted in said combustion apparatus is gaseous.

33. A method according to claim 31 wherein the fuel that is combusted in said combustion apparatus is liquid.

34. A method according to claim 31 wherein the fuel that is combusted in said combustion apparatus is solid.

35. A method according to claim 31 wherein the aggregate oxygen concentration of the gaseous oxidant is combusted in step (a) is at least 30 vol. %.

36. A method according to claim 31 wherein the aggregate oxygen concentration of the gaseous oxidant is combusted in step (a) is at least 90 vol. %.