United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/S7-88/007 July 1988
v°/EPA Project Summary
Effect of Fuel Sulfur on Nitrogen
Oxide Formation in Combustion
Processes
J. 0. L. Wendt.T. L. Corley, and J. T. Morcomb
Because chemically bound
nitrogen in fossil fuels has been
shown to be an important contributor
to nitrogen oxide (NOX) emissions,
much research has been focused on
mechanisms governing the oxidation
of fuel nitrogen to fuel NO. This has
been done, hoping that the insight
and understanding gained thereby
will lead to development of new
combustion modifications for NOX
control. However, many fuels
containing chemically bound
nitrogen also contain sulfur in
various amounts. Indeed, insofar as
pulverized coal is concerned, a wide
variation in sulfur content is
possible, even from coals with
similar nitrogen contents. The
question can, therefore, be posed, is
the sulfur content of a fuel likely to
have a major influence on the
resulting NOX emissions?
Specifically, will the presence of fuel
sulfur cause major changes in
mechanisms of fuel NO formation?
Furthermore, since there exist a wide
variety of possible combustion
conditions and combustion
modifications, it is to be expected
that the potential importance of
SOx/NOx interactions depends not
only on the fuel quality, but also on
the conditions under which the fuel
is burned.
This Project Summary was
developed by EPA's Air and Energy
Engineering Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that Is fully
documented in a separate report of
the same title (see Project Report
ordering Information at back).
Introduction
This report describes results of a far
ranging study to determine: (1) if sulfur
has a measurable effect on fuel nitrogen
conversion in practical combustion
configurations, and (2) the mechanisms
through which this effect, if it exists, is
implemented. Results from the practical
turbulent diffusion flame studies did in
fact indicate that sulfur may have a first
order influence on fuel NO emissions
when the fuel is burned under hot, poorly
mixed combustion conditions in which
fuel nitrogen processing occurs in long-
lived, hot rich zones. These results.
which are of practical significance.
prompted further studies on fuel nitrogen
processing in hot, rich, premixed, laminar
flame environments.
The project was divided into four
phases:
1. Fuel sulfur effects on NOX formation
in turbulent diffusion flames.
2. Interactions of fuel sulfur and fuel
nitrogen in fuel-rich premixed
laminar flames.
3. Post-flame behavior of nitrogenous
species in the presence of fuel sulfur
(rich, moist CO/Ar/Oa flames)
4. Post-flame behavior of nitrogenous
species in the presence of fuel sulfur
(rich, moist CH4/He/02 flames)
Each phase logically followed the
previous one and originated from the
need to obtain more fundamental data to
understand and generalize the effects
observed, Phase 1 was concerned only
with exhaust NO measurements; Phase 2
-------
with NO profiles; Phase 3 with NO, HCN,
NH3, and N2 profiles in CO flames and
their kinetic modeling; and Phase 4 with
profiles of the same species in CH4
flames.
Phase 1: Turbulent Diffusion
Flames
Interactions between certain fuel sulfur
compounds and nitric oxide (NO) in
turbulent gaseous and distillate oil
diffusion flames were experimentally
investigated in a 75,000 Btu/hr (22 kW)
laboratory combustor. Aerodynamcis, air
preheated conditions, and overall excess
air conditions were varied to determine
their role in any such interaction.
Results indicated that adding sulfur
dioxide (SOa) to natural gas flames could
enhance or inhibit NO emissions. Local
flame stoichiometry and temperature,
which were influenced by fuel injector
type, determined which effect was
observed and the extent to which it
occurred. Thiophene (C4H4S) and
pyridine (CsHsN) were added to No. 2
diesel oil to determine effects of fuel
sulfur on conversion of chemically bound
fuel nitrogen to NO. No discernible effect
was observed at "zero" air preheat
conditions. No emissions were enhanced
at high air preheat conditions. Adding
802 to natural gas flames doped with
ammonia (NH3) significantly increased
the conversion of NH3 to NO at high air
preheat conditions.
Inhibition effects were explained in
terms of homogeneous catalysis of
recombination reactions by SO2
Hydrogen abstraction reactions involving
reduced form were considered to explain
the enhancement effect.
Phase 2 : No Profiles in
Premixed Laminar Flames
Significant fuel sulfur and fuel nitrogen
interactions were observed in the post-
reaction zone of premixed flat flames.
Methane/oxygen/argon flames of
stoichiometric ratios between 0.40 (4> = 2
.5) and 1.2 ( =0.833) were doped with
cyanogen and with sulfur additives (S02
and H2S). The resulting profiles of NO in
fuel-rich studies exhibited an
enhancement of peak NO formation due
to sulfur species. These peak NO values
decayed rapidly in the later portions of
the flame, indicating the continuation of
low temperature (below 1600 K)
reactions. In some instances, the
presence of sulfur species cause the
decay or reduction of NO to proceed
below the base line values at very long
(greater than 0.1 sec) residence times.
Phase 3: Nitrogenous Species
Profiles in CO Flames
Experimental measurements of NO,
Nj, and other nitrogenous species, in the
post-flame gases of rich (<{> = 2.17)
premixed laminar CO/Ar/02 (trace H2)
flames, with fuel nitrogen as NO, €2^,
and NHa, and fuel sulfur as S02, allowed
the nitrogen balance to be closed to
within 7%. In the absence of
hydrocarbons and with only a trace of
hydrogen, NO decayed homogeneously
to form N2 at high temperatures, and the
fate of the fuel nitrogen was independent
of the type of fuel nitrogen species. The
effect of fuel sulfur was to decrease
post-flame No levels and increase N2
more rapidly. The observed decay in NO
and formation of N2 were consistent with
detailed kinetic calculations employing
only the reverse Zeldovich mechanisms
to form N and N2- There was no
.evidence of other N2 formation
mechanisms being important for these
hydrogen-poor flames, at either high or
low temperatures. Calculations also
showed that the most plausible effect of
S02 in the mixture was to increase the
steady state N atom concentration
through direct interactions between N,
NO, S. and SO.
Phase 4: Nitrogenous Species
Profiles in CH4 Flames
Profiles of NH3, HCN, NO, and N2 in
premixed CH4/He/O2/C2N2 flames
allowed the nitrogen balance to be
closed to with 3% in the far post-flame
region of rich flames at stoichiometric
ratios greater than 0.58 ($<1.72). In the
flame front and the near post-flame, a
(decaying) discrepancy in the nitrogen
balance was attributed to N2> which was
being destroyed to form N2- For
exceedingly rich sooting flames
(SR = 0.46,* = 2.1 7), closure was
impossible due to nitrogenous species
adsorbed on solid particles. In all cases
the effect of sulfur was to increase the
sum of NO + HCN + NH3 and to
decrease N2- This is consistent with
previous data on hydrocarbon flames.
Results showed that sulfur accelerates
hydrocarbon decay rates, thereby
inhibiting hydrocarbon breakthrough,
which apparently influences the XN
species profiles in the post-flame.
Certainly, hydrocarbons and possibly
hydrogen play a critical role in the
enhancement effect that fuel sulfur has
on fuel nitrogen conversion.
Conclusions
Results from this project, taken as a
whole, indicate that, under hot rich
conditions often considered optimum for
NO abatement by staged combustion fuel
sulfur tends to increase the conversion of
fuel nitrogen in hydrocarbon fuels.
-------
-------
J. O. L Wendt, T. L. Corley, and J. 7. Morcomb are with the University of
Arizona, Tucson, AZ 85721.
W. Steven Lanier is the EPA Protect Officer (see below).
The complete report, entitled "Effect of Fuel Sulfur on Nitrogen Oxide Formation
in Combustion Processes," (Order No. PB 88-208 178/AS; Cost: $25.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-88/007
0000329 PS
U S EMVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S OiARSORK STREET
CHICAGO It 60«04
------- |