CA1091570A - Multi-stage process for combusting fuels containing fixed-nitrogen chemical species - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Fuels containing fixed-nitrogen chemical species are combusted in a multi-stage process. The process which converts substantially all of the fixed-nitrogen into molecular nitrogen (and thus avoids the formation of significant amounts of nitrogen oxides from the fixed-nitrogen) consists of four steps: (a) mixing said fuel with at least one first oxidizing agent in amounts such that the equivalence ratio of said fuel to said oxidizing agent is at least about 1.2; (b) partially combusting the mixture resulting from step (a) in at least one first stage at a first temperature of about 1950 to about 2400°K., with a residence time of at least 0.01 second; (c) mixing the combustion products resulting from step (b) with at least one second oxidizing agent in an amount such that the equivalence ratio of combus-tion products to the total amount of oxidizing agents in the mixture will be about 1.0 to less, such mixing taking place under conditions such that the temperature of the mixture will not exceed about 1750°K.;
and (d) completely combusting the mixture resulting from step (c) in at least one second stage at a second temperature of less than about 1750°K.

Description

~saC~7~3 :~ 1 SUM~IARY OF THE INVh'NTION

2 The present invention relates to a multi-stage pro-

3 cess for combusting a fuel containing ~ixed-nitrogen chemical

4 species which comprises the steps of: (a) mixing said fuel with at least one first oxidizing agent in amounts such that 6 the equivalence ratio of said fuel to said oxidizing agent is 7 at least about 1.2; (b) partially combustin~ the mixture re-` 8 sulting from step (a) in at least one first stage at a first 9 temperature of about 1950 to about 2400K.,`with a residence time of at least 0.01 second; (c) mixing the combustion pro-11 ducts resulting from step (b) with at least one second oxi-12 dizing agent in an amount such that the equivalence ratio of 13 combustion products to the total amount of oxidizing agents s~ 14 in the mixture will be a~out 1.0 or less, such mixing taking - 15 place under conditions such that the temperature of the mix-16 ture will not exceed about 1750K.; and (d) completely com-17 busting the mixture resulting from step (c) in at least one 18 second stage at a second temperature of less than about 19 1750K.
It is well known that common fuels such as coal, ~ 21 coal liquids, diesel oils, bunker oils, crude oils, shale - 22 oils, natural gas, etc. contain varying amounts of fixed-` 23 nitrogen chemical species. It is also well known that com-24 bustion of such fuPls will produce varying amounts of nitro-gen oxides (e.g. 150-1500 ppm.), depending on the ~ype and 26 quantity of fixed-nitrogen chemical species as well as the 27 furnace and burner arrangements.
. 28 It is axiomatic that it would be desirable to mini-29 mize the formation of nitrogen oxides without any significant impairment of the combustion efficiency. This desirable re-.. ~, , .

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1 suit has been achieved by means of the instant multi-stage 2 combustion process, since this process results in the conver-3 sion of substantially all of the fixed-nitrogen chemical 4 species contained in ~he fuel into innocuous molecular nitro-gen (rather than nitrogen oxides) without any significant con-6 comitant impairment of combust:ion efficiency.

8 U.S. Patent 3,048,13:L teaches a two-stage method 9 for combusting nitrogen-containing fuels in order to minimize the production of N0X species in the combustion products.
11 However, the results achieved by the ins~ant process surpass 12 those of this patent. Moreover, this patent contains no - 13 teaching whatsoever of the four critical steps (outlined above) - 14 of ~his process.
The M.S. thesis by Howard W. Chou entitled "Fate 16 of Ammonia In Fuel Rich Flames" (deposited in the library of 17 the Massachusetts Institute of Technology on October 25, 18 1976) indicates the desirability of combusting fuels under 19 fuel-rich conditions (i.e. high equivalence ratios) and at elevated temperatures. However, the Chou thesis does not in-21 dicate the necessary residence times for the first stage com-` 22 bustion. Moreover, Chou did his wor~ at flame temperatures 23 corresponding to adiabatic or less and at equivalence ratios 24 greater than unity. In contrast thereto, this process in-volves three inter-related parameters in the first stage:
26 high temperatures (e.g. 1950-2400K.), high equivalence 27 ratios (e.g. at least 1.2) and minimum residence times ~e.g.
28 at least 0.01 second).
29 Other relevant prior art processes are summarized - 30 in the paper entitled "Mechanisms and Kinetics/NOx Formationl' 31 by A. F. Sarofim et al. which was presented at the 69th an-32 nual meeting of the American Institute of Chemical Engineers -~ 33 on November 30, 1976.

This combustion process is multi-stage in nature, 36 i.e. it involves one or more first stages and one or more 37 second stages. The combustion process may be practiced with 38 any desired type of combustion chamber/burner, so long as the .~

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rj7 1 chamber/burner is capable of being utilized in accordance 2 wlth the four critical steps outlined above. Further, the 3 same combus~ion chamber(s) employed in the second stage(s) 4 may be the same as or different from that employed in the first stage(s~.
6 The first step of this process involves mixing a 7 fuel with a first oxidizing agent. The fuel may be a solid, 8 a liquid, a gas or a mixture thereof ~uch as the common fuels 9 previously mentioned. The quantity and type of ~ixed-nitro-gen chemical species contained in the fuel is relatively un 11 important; however, most common fuels contain less than about 12 5 wt. % of such chemlcal species.
13 Typically, the first oxidizing agent is air; how-14 ever oxygen, o~ygen mixed with an inert gas such as helium, etc. may also be employed instead of air. If desired, the 16 air may be enriched with oxygen, e.g. 6-15 wt. % of oxygen 17 may be added to the air, based on the weight of the addition-18 al oxygen plus air. Further, it may also be useful to pre-19 heat the air to a Pemperature in the range of 450 to 700K~
prior to its admixture with th~ fuel. If desired, the fuel 21 may also be preheated to similar temperatures prior to admix-22 ture with the air.
23 The amount of oxidizing agent mixed with the fuel 24 is such that an equivalence ratio of at least about 1.2 is 25 obtained; preferably, the equivalence ratio is in the range :
26 of 1.2 to 1.9, most preferably 1.4 to 1.7. The equivalence 27 ratio (usually referred to as ~)is defined as:
28 actual fuel '!. ' 29 equivalence ratio (0) - actua ~ a ~nt StOlC ometrlc ue 31 stoichi~ t~r~Rl~Rr-- g agent 32 For complete combustion (e.g., oxidation of carbon monoxide 33 to carbon dioxide), 0 should be equal to or less than 1Ø
34 Where 0 has a value equal to or greater than 1.2, carbon will be oxidized to carbon monoxide plus carbon dioxide. It 36 should also be noted that while a condition of 0 ~ 1.0 is de-37 sirable from a complete ~ mbustion point o~ view, such condi-38 ~tion favors conversion of fixed-nitrogen chemical species in-39 to nitrogen oxides. Thus by combusting in at least two ,.... .
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1 stages in whi~h the first stage(s) 0 ~ 1.2 and ~he second 2 stage(s) 0 c 1.0, both minimiZcltion of the ormation of ni-3 trogen oxides and maximization of complete combustion are ob-4 tained.
The mixture of fuel alnd first oxidizing agent may 6 be formed externally to, or wit:hin, a suitable combustion 7 chamber. In the second step of this process, the mixture is 8 partially combusted (i.e. carbon is oxidized to carbon monox-9 ide plu3 carbon dioxide) in at least one ~irst stage. The combustion ~emperature of this first stage is maintained in 11 the range of about 1950 to about 2400K., preferably 1975 to 12 2100K. Fur~her, the residence time of the fuel and oxidiz-13 ing agent during the combustion reaction is maintained a~ a 14 level of at least about 0.01 second, preferably 0.03 second, e.g. 0.5 second.
16 The combustion products resulting from the second 17 step (i.e. the first stage combustion) are then mixed with a 18 second oxidizing agent (which may be the same as or different 19 from the firs t oxidizing agent employed in the first step).
Typically, the second oxidizing agent is also air, but it may 21 be any o the other choices enumerated above for the first 22 oxidizing agent. The amount of oxidizing agent employed in 23 the third step is such that the equivalence ratio of com-24 bustion products to the total amount of oxidizing agents (i.e.
any remaining first oxidizing agent plus the added second ox-26 idizing agent) is equal to or less than 1.0, e.g. 0.95-0.99.
27 Since an equivalence ratio of 1.0 or less favors 28 the formation of NOX species at elevated temperatures, it is 29 necessary that the mixing in the third step take place under conditions such that the temperature of the combustion pro~
31 ducts - second oxidizing agent mixture is maintained at a 32 level not in excess of about 1750K., e.g. 1200-1700K. This 33 may be readily accomplished by several techniques, e.g. cool-34 ing of the combustion chamber, transfer of the combustion products to a different "cold" combustlon chamber, coollng of 36 tha combustion products (e.g. by suitable heat exchangers) to 37 a temperature of less than 1300K. Furthermore, cooling may 38 not necessarily be required, e.g. the temperature of the com-~, .

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- 5 -1 bustion products in relation to the temperature, requisite 2 amount, and rate of mixing, of the second oxidizing agent may 3 be such that the temperature will at all times be below about 4 1750~.
S The mixing of combustion products and second oxidiz-

6 ing agent may~ as in the case of the first step, take place

7 external to, or within the same or different combustion cham-

8 ber as was employed in the first stage combustion (i.e. the

9 second step). Further, wherë the second oxidizing agent is chosen to be air, the air may be enriched with the same ll levels of additlonal oxygen as mentioned above with respect 12 to the first oxidizing agent (provided that an equivalence 13 ratio of ~ 1.0 is maintained for the mlxture).
14 The second oxidizing agent(e.g. air) may be pre-heated and/or enriched with oxygen as was the case with re-16 spect to the first oxidizing agent. Further, the second oxi-17 dizing agent may be diluted with combustion products andtor 18 inert gases prior to and/or during admixture wlth the com-19 bustion products resulting from the first stage. These al-ternatives, however, are subject to the proviso that the 21 equivalence ratios and maximum temperatures outlined above .
22 must nevertheless be maintained. ~ ~
23 In the fourth step, the mixture of combustion pro- ~ -~4 ducts and second oxidizing agent is completely combusted in at least one second stage (in the same or different combustion 26 chamber as that employed in the first stage combustion~. The 27 term "completely" combusted is used herein ~o denote that par-28 tially oxidi2ed combustion products ~e.g. carbon monoxide) re- `
29 sulting from the first stage combustion are further oxidized to their highest oxidation state (e.g. carbon dioxide). The 31 fourth step is carried out at a temperature of less than about 32 1750K., preferably 1200 to 1700K. The residence times for 33 the second stage are not critical, i.e. they need be only long 34 enough to oxiclize substantially all of the carbon monoxide 35 (from the first stage combustion) into carbon dioxide. Typi-36 cally, residence times of 0.1 to O.S second will be sufficient 37 for the second stage combustion.

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2 In this Example and in Example 2 below, attention 3 was focused on the first stage of the combustion process.
4 Once the conversion of fixed nitrogen chemical species to molecular nitrogen has been maximized by ~he first stage pro-6 cess conditions of this invention, completing the combustion 7 in the second stage (at equivalence ratios of 1.0 or less) 8 and at lower temperatures (about 1750K. or less) presen~s 9 no problem vis-a-vis minimization of NOX formation.
The apparatus employed in Examples 1 and 2 con-11 sisted of an electrically-heated vertical muffle-tube furnace;
12 the furnace was constructed of zirconia and was 5.40 cm. I.D.
13 x 60.64 cm. long (the heated zone was 35.56 cm. long). The 14 reactants (i.e. pre-mixed fuel and air) were fed to the bot-tome of the muffle-tube through a porous plug flat flame 16 burner of 2.54 cm. diam~ter (~he face of the burner was 17 located 4.92 cm. below the heated zone). ~as sa~ples for 18 analysis of 29 NO and NOX were withdrawn through a water-19 cooled stainless steel probe that was axially located 41.91 cm. above the burner face. ~as samples for analysis of HCN
21 and N~3 were withtrawn from the cool burner exhaust duct, 22 approximately 99 cm. above the burner face.
23 In Example 1, methane was doped with appro~imately 24 5,000 ppm NO and mixed with air so as to result in a mixture having an equivalence ratio o 1.7. The results in terms of 26 the output of the sum of NO, NH3 and HCN as a mole percent of 27 the input NO versus various adiabatic flame temperatures are 28 shown in Table I below.

Output, Mole % Flame Tem~.~ K
31 30.7 1762 32 17.3 1824 33 10.5 1884 34 8.04 1940 6.57 1994 36 6.32 2045 , . . _ 38 Example 1 was repeated under the same conditions, :,, -..

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1 except that the methane was doped with 4376 ppm NH3 (instead 2 of the 5000 ppm NO). Table II set forth below indicates the 3 wall temperature o~ the combus~ion chamber, the adiabatic 4 flame temperature, the equivalence ratio and the output, mole fraction of input NH3 appearing as the sum of NH3, NO plus 6 HCN in the combustion products.

8 Wall Adiabatic Output, 9 Temp., FlameEquivalence Mole

10 K. Tem~.,K.Ratio Fraction

11 1658 2232 1.01 0.703

12 1658 2191 1.13 0.584

13 1658 2121 1.22 0.522

14 1658 2052 1.31 0.411 1658 1981 1.40 0.266 16 1658 1915 1.49 0.253 17 165~ 1849 1.58 0.233 18 1658 1705 1.78 0.409 19 1658 1570 1.99 0.433 1658 L450 2.19 0.408 :
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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A multi-stage process for combusting a fuel containing fixed-nitrogen chemical species which comprises the steps of:
(a) mixing said fuel with at least one first oxi-dizing agent in amounts such that the equivalence ratio of said fuel to said oxidizing agent is at least about 1.2;
(b) partially combusting the mixture resulting from step (a) in at least one first stage at a first temperature of about 1950 to about 2400°K., with a residence time of at least 0.01 second;
(c) mixing the combustion products resulting from step (b) with at least one second oxidizing agent in an amount such that the equivalence ratio of combustion products to the total amount of oxidizing agents in the mixture will be about 1.0 or less, such mixing taking place under condi-tions such that the temperature of the mixture will not ex-ceed about 1750°K.,; and (d) completely combusting the mixture resulting from step (c) in at least one second stage at a second temper-ature of less than about 1750°K.

2. The process according to claim 1, wherein the first temperature is in the range of 1975 to 2100°K.

3. The process according to claim 1, wherein the residence time is at least 0.03 second.

4. The process according to claim 1, wherein the equivalence ratio of the mixture resulting from step (a) is in the range of about 1.2 to about 1.9.

5. The process according to claim 41 wherein the equivalence ratio is in the range of 1.4 to 1.7.

6. The process according to claim 1, wherein the first oxidizing agent is air.

7. The process according to claim 6, wherein the air is preheated to less than about 800°K. prior to its ad-mixture with said fuel.

8. The process according to claim 1, wherein the fuel is a solid, liquid, gas, or a mixture thereof.

9. The process according to claim 1, wherein the second oxidizing agent is air.

10. The process according to claim 1, wherein the second temperature is in the range of 1200 to 1700°K.