Ind. Eng. Chem. Res. 1989,28, 1004-1010
1004
Effect of Fuel Sulfur on Nitrogen Oxide Formation in a Thermally Stabilized Plug-Flow Burner Lisa D. Pfefferle* Chemical Engineering Department, Yale University, P.O. Box 2159 Y S , New Haven, Connecticut 06520
Stuart W. Churchill Department of Chemical Engineering, The University o f Pennsylvania, 220 South 33rd Street, Philadelphia, Pennsylcania 19104
Premixed ethane/air flames in a thermally stabilized plug-flow burner were doped with fuel-N (ammonia) and fuel-S (hydrogen sulfide) t o investigate the effect of sulfur on nitrogen oxide formation. Sulfur was found t o reduce both thermal NO, and fuel-NO, emissions from the thermally stabilized burner a t equivalence ratios from 0.8 to 1.6. Reductions in thermal NO, between 5% and 10% were noted for sulfur (H2S)dopings of from 0.03 t o 0.06 wt %. T h e observed reduction in fuel-NO, varied from 10% to 20%. These experimental results are in contrast to those of previous investigators using lower temperature, backmixed burners, which show enhancement of fuel-NO, in the presence of sulfur additives for most stoichiometries and for all fuels with the exception of CO. A model is proposed which correctly predicts the qualitative effect of sulfur additives on fuel-NO, emissions from both the plug-flow and conventional backmixed burners, supporting both sets of experimental results. Since many fuels contain appreciable amounts of both fuel-bound sulfur (fuel-S) and fuel-bound nitrogen (fuel-N), understanding the interaction of sulfur compounds with the mechanisms for the formation of nitrogen oxides is of primary importance in designing a burner that will minimize NO, emissions. Previous investigations have shown that fuelsulfur can affect the net formation of NO, in combustion systems. Many investigators have found that sulfur in the fuel reduces the production of nitrogen oxides from atmospheric nitrogen (thermal NO,) (Wendt and Ekmann, 1975; Tang et al., 1981; Pfefferle, 1984). The effect on the production of nitrogen oxides from fuel-bound nitrogen (fuel-NO,) appears to be much more difficult to explain due to the complexity of the mechanism and multiplicity of intermediate species. Tseregounis and Smith (1983) found sulfur consistently enhanced fuel-NO, emissions from rich premixed acetylene or hydrogen flames. They observed the degree of enhancement to increase with the equivalence ratio (4; the equivalence ratio is here defined as the ratio of the actual fuel/oxidizer divided by the ratio of fuel/ oxidizer required for complete conversion of the fuel to C 0 2 and water) for both atmospheric and low-pressure flames (Tseregounis and Smith, 1983,1984). The overall increase in NO, output was found to be caused by a decrease in the rates of destruction of NO, in the postflame zone. Wendt et al. (1979),using a premixed flame stabilized on a porous metal disk, found the enhancement or reduction of fuelNO, by fuel-S to depend upon the equivalence ratio and the distance of the measurement from the primary flame zone. They found the effect of sulfur on fuel-NO, enhancement to be greatest at the richest equivalence ratio studied (4> 2 ) (Wendt and Corley, 1984). Other investigators, using jet-stirred reactors, found fuel-NO, enhancement by fuel-sulfur a t fuel-rich equivalence ratios shifting to slight reduction or no change for fuel-lean conditions (Chen et nl., 1984; Malte et al., 1982). In the thermally stabilized burner (TSB) used by Tang et al. (1981) and by us, sulfur was found to reduce the NO, produced from fuel-nitrogen at all equivalence ratios (from 0.8 to 1.6) and postflame locations studied. The objective of the current studv was to determine the effect of fuel-S .-_I__._.
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* To whom all correspondence should be addressed.
on the NO, formed in the TSB and to try to reconcile the differences in behavior observed by prior investigators with different burner geometries. Proposed Mechanisms The mechanisms for oxidation of fuel-S in combustion systems have been studied previously. Under fuel-lean adiabatic operating conditions, sulfur oxidizes almost exclusively to SO2,with the exception of 0.5’70 or more, which further oxidizes to SO3. As the fuel fraction of the mixture increases, however, concentrations of SO can rise to significant levels, and a t equivalence ratios of 1.3 or more, the concentrations of SH, H2S, S2,COS, and CS2can also attain appreciable superequilibrium levels in the postflame zone. Muller et al. (1979) studied sulfur chemistry in stoichiometric and fuel-rich Hz/O2/N2 flames doped with hydrogen sulfide. Using quantitative laser-fluorescence measurements to determine OH, SH, Sz,SO, and SOz, they postulated a set of reactions that describes the oxidation of sulfur in such a flame. Sulfur additions were found to accelerate significantly the recombination of H radicals and subsequently that of OH radicals and 0 atoms. This provides an explanation for the reduced production of thermal NO, in the presence of fuel-sulfur. The lower concentration of oxidizing radicals then depresses the rate of the reactions N + OH = N O + H (R1)
Nz + 0 = NO + N (R2) in the Zeldovich mechanism, resulting in lower overall NO, production. In the case of fuel-nitrogen, however, the reduction in the formation of NO, through the extended Zeldovich mechanism as a result of the decreased concentration of oxidizing radicals does not appear to be the dominant mechanism of interaction with sulfur. Other possible mechanisms by which sulfur can enhance or reduce NO, include the following: (1)The first mechanism is the direct interaction between S and N species such as N + SO - + N O + S (R3) (Wendt and Ekmann, 1975).
0SS8-~~8S~/89/2628-1004$01.~0/0 \C 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 1005 NH3
1+
A MIXING CHAMBER
97“
191mm--
0 DISTRIBUIION CHAMBER
OH, H
-
C COMBUSTION CHAMBER
Law T e m p
High Temp
NHz
+NOJ NNH
+
OH
+ NO1 N z + HNO
+MI
H+NO
1
\OH
I
-
0
NH (+HzO)
1\
N
FUEL
A
CAST CERAMIC
-
BED1 AIR
OH, 02 THERMOCOUPLE
HNO
[ ? IPI / - 10 ‘ I s R h
CROSS-SECTION
OF C
~+OH,H /+M
NO
NO+H+M
\
Figure 1. Schematic diagram of the mechanism for fuel-NO, formation. First and third paths are from Miller et al. (1981).
-STEEL
CASE
AOIABAIIC FLAME TEMPERATURE ,
