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Control through Combustion Modifications D. W. PERSHING and E. E. BERKAU U. S. Environmental Protection Agency, Office of Research and Monitoring, National Environmental Research Center, Research Triangle Park, N.C. 27711
The chemistry of NO formation during atmospheric pressure, hydrocarbon combustion is reviewed. During natural gas combustion, NO forms via a thermal fixation mechanism. During heavy oil and coal combustion, NO is formed via the fixation mechanism and through the conversion of bound nitrogen in the fuel to NO. The fixation mechanism depends strongly on both the local gas temperature and the local stoichiometry; therefore, temperature-reduction techniques such as flue gas recirculation and water injection and reductions in oxygen availability through low excess air firing or staging are effective in practical NO control. The conversion of fuel nitrogen seems to depend on the amount of nitrogen present in the fuel and oxygen availability. x
x
x
x
T*he Air Quality Act of 1967 (Public Law 90-148 as amended) and the Clean Air Amendment of 1970 (Public Law 91-604) assigned to the Environmental Protection Agency (EPA) the responsibility of (a) developing emission standards for existing sources to be enforced by the States, and (b) setting performance standards for new sources to be enforced by EPA. Under these laws, EPA pubhshed in the Apr. 30, 1972, Federal Register the "National Primary and Secondary Ambient Air Quality Standards," which set the maximum acceptable ambient nitrogen oxides (NO*) concentration at 55 parts per billion (annual arithmetic mean measured as nitrogen dioxide). In an effort to guide the States in meeting this ambient level, EPA recently pubhshed the following as "Standards of Performance for New Stationary Sources" (Federal Register, vol. 36, No. 247, Dec. 23, 1971): 218 Jimeson and Spindt; Pollution Control and Energy Needs Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
19.
PERSHING AND BERKAU
New sources
Chemistry of Nitrogen Oxides
219
Fuel Burning Emissions,
Ibs/MMBtu
Gas
Oil
Coal
0.2
0.3
0.7
Nitrogen oxides—primarily nitric oxide (NO) and nitrogen dioxide (N0 )—are formed during the combustion of fossil fuels with air. Table I shows source estimates developed for EPA. These estimates of pollutant emission rates are based on emission factors developed by past stacksampling data, material balances, and engineering appraisals of other sources similar to the listed sources. As Table I indicates, almost half of the total national NO* emissions result from stationary fuel combustion. In principle, NO* emissions from fossil fuel-combustion systems can be reduced by three methods: fuel cleaning (removal of the fuel nitrogen), combustion modification, and flue gas treatment. Combustion modification appears to be by far the easiest and most economical of the three. Therefore, EPA has assigned to the Control Systems Divisions, Combustion Research Section (CRS) the responsibility for assisting industry in developing technology to re2
Table I. Estimates of N O * Emissions in the U . S. by Source, 1968 (1
3
Source Mobile fuel combustion Motor vehicles Gasoline Diesel Aircraft Railroad Vessels Non-highway users Stationary fuel combustion Coal Fuel oil Natural gas Wood Solid waste Open burning Conical incinerators Municipal incinerators On-site incinerators Coal waste banks Forest burning Agricultural burning Structural fires Industrial processes Total
NO
x
Emissions, tons/yr
Per Cent Total 40
6,600,000 600,000 40,000 400,000 300,000 300,000 48 4,000,000 1,110,000 4,640,000 230,000 11 450,000 18,000 19,000 69,000 190,000 1,200,000 280,000 23,000 200,000
1
20,669,000
100
Jimeson and Spindt; Pollution Control and Energy Needs Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
220
POLLUTION CONTROL AND ENERGY NEEDS
duce stationary-source emissions through changes in the combustion system. The purpose of this paper is to review the chemistry of NO* formation in flames, to explain what CRS is presently doing to advance the understanding of this chemistry, and to relate the chemistry to existing control technology. Chemistry of NO Formation from Thermal Fixation x
Nitrogen oxides are formed during the combustion of coal, oil, natural gas, etc., by two mechanisms. The first mechanism is high temperature thermal fixation of molecular oxygen and nitrogen present in the combustion air; the second mechanism is the reaction of atmospheric oxygen with nitrogen-containing compounds introduced in the fuel. Both mechanisms result primarily in NO because the residence time in most stationary combustion processes is too short for the oxidation of NO to N 0 , even though N 0 is thermodynamically favored at lower temperatures (3). NO, however, does oxidize in the atmosphere to N 0 , which is a primary participant in photochemical smog. For many years it was generally assumed that the thermalfixationof NO occurred according to the mechanism suggested by Zeldovich et al. 2
2
2
(4). 0 + N *=± NO + N
(1)
N + 0 -
A
100
A
A A
50
A A A A
0.50
1.00 STOICHIOMETRIC RATIO
1.50
Figure 6. Emissions from propane-air combustion in a jetstirred reactor Detailed finite rate calculations were used from C O and H on as before: poor agreement was still observed. Poor agreement may be a result of the lack of a detailed hydrocarbon combustion mechanism or of other reactions with nitrogenous intermediates. EPA is presently in the process of extending this work (21) by conducting detailed flame probing on premixed and diffusion flames in an axisymetric laboratory furnace burning C O , H , methane, and propane. The premixed burner and jet-stirred combustor data will be used in conjunction with detailed, finite rate plug flow and well stirred analyses to determine the chemistry of thermal NO* formation during gaseous combustion. 2
2
Chemistry of NO
x
Formation from Fuel Nitrogen Conversion
For many years it was assumed that NO was formed only by high temperature fixation of atmospheric nitrogen and oxygen; recent experimental studies have indicated that the conversion of bound nitrogen in
Jimeson and Spindt; Pollution Control and Energy Needs Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
230
POLLUTION CONTROL AND ENERGY NEEDS
the fuel to N O may be of equal or greater importance in the formation of NO-p during coal and residual oil combustion. In an early EPA (NAPCA) evaluation of fuel additives in a small experimental furnace, Martin et al. (27) noted that certain nitrogen-containing additives, notably various amines and nitrates, increased NO* emissions through approximately 50% conversion of the nitrogen in the additive to NO. In flat flame laboratory experiments, Shaw and Thomas (28) have shown that addition of fuel nitrogen compounds such as pyridines, amines, and cyanides to C O flames increases the NO* emissions. Values to 1300 ppm NO* were measured with and without molecular nitrogen during low temperature C O combustion (860-1145°K). Both CO-0 -argon and C 0 - 0 - N mixtures were studied with maximum conversions of about 40-50% of the fuel nitrogen to N O . Martin and Berkau (29) investigated the amount of N O produced in an experimental furnace from burning a standard distillate oil (less than 0.01% nitrogen) doped with various nitrogen compounds (pyridine, quinoline, and piperidine). The fraction of fuel nitrogen converted to N O increased with stoichiometric ratio and decreased with fuel-nitrogen concentration, although the absolute levels increased with both. The percentage of fuel nitrogen converted to NO generally ranged from 20 to 70%. Elshout and van Duuren (30) have interpreted the data of Smith (31) on large source emissions to show that the fuel nitrogen concentration was related to the NO* emissions from boilers of a given size. Similar data are reported in the EPA (NAPCA)-sponsored Esso NO* Systems Study (32). Bartok et al. (12) examined the addition of a number of nitrogen compounds (including NO, N 0 , N H , ( C N ) , and C H N H ) during methane-air combustion in a jet-stirred reactor. Under excess air conditions, essentially complete conversion (or retention) was observed; however, the fraction of the additive which oxidized to NO* decreased sharply on the fuel-rich side. Jonke (33) studied the N O emissions from a coal-fired, fluid-bed combustor operating at a bed temperature below 1300°K and observed 580 ppm NO emissions with both N - 0 and argon-0 . This corresponds to about 25% conversion of the fuel nitrogen. Turner et al. (34) experimented with doped distillate fuels and residual oils in a package boiler and discovered that 40-80% of the fuel nitrogen, depending on concentration, is converted to N O during combustion under normal operating conditions. This conversion is further substantiated by the data from a West Coast utility shown in Figure 7. Thus, based on the results of laboratory studies, it appears that fuel nitrogen is potentially important in the overall formation of NO^. Unfor2
2
2
x
2
3
2
2
2
3
2
2
Jimeson and Spindt; Pollution Control and Energy Needs Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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PERSHING AND BERKAU
231
Chemistry of Nitrogen Oxides
700 0.5% N 100% CONVERSION 600
500
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400
300
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A
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0.25% N 100% CONVERSION n
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