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NOx Combustion Modification Abstracts
NOx Control Technology Requirements under the United States' 1990 Clean Air Act Amendments Compared to Those in Selected Pacific Rim Countries
C. Andrew Miller Robert E. Hall Richard D. Stern Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Combustion Research Branch (MD-65) Research Triangle Park, NC 27711
Abstract
The 1990 Clean Air Act Amendments (CAAAs) require reduction of nitrogen
oxide (NOx) emissions under two provisions: Title I requires
control of NOx from all source types for the purpose of
attaining ambient air quality standards for NOx and ozone;
and Title IV requires control of NOx from coal-fired utility
boilers for the reduction of acid rain precursors. Title IV is the
more straightforward of the two, and sets national emission standards
for dry-bottom wall-fired and tangentially fired boilers based on
low NOx burner technology (LNBT), defined by EPA to include
separated overfire air (OFA). Emission standards for other boiler
types are to be promulgated by 1997. NOx controls under
Title I are more complex, and are based on reductions necessary to
reduce local and regional ambient levels of NOx and ozone.
Control technology requirements under Title I are based on Reasonably
Available Control Technology (RACT) as defined by EPA's Office of
Air Quality Planning and Standards; however, emission levels are set
by the states according to local conditions. Technologies defined
as RACT include low NOx burner technology, selective non-catalytic
reduction (SNCR), and selective catalytic reduction (SCR). These and
other combustion modifications and flue gas treatment technologies
are described. NOx emission regulations and technology
requirements in the U.S. are compared to those in selected Pacific
Rim countries.
"On-Line Measurement
of Nitrous Oxide from Combustion Sources by Automated Gas Chromatography,"
J.V. Ryan and W.P. Linak, 5th International Workshop on Nitrous Oxide
Emissions, Tsukuba, Japan, July 1-3, 1992.
On-Line Measurement of Nitrous Oxide from Combustion Sources by Automated Gas Chromatography
Jeffrey V. Ryan Acurex Environmental Corporation Environmental Systems Division P.O. Box 13109 Research Triangle Park, NC 27709 William P. Linak U.S. Environmental Protection Agency Air and Energy Engineering Research Laboratory Combustion Research Branch, MD-65 Research Triangle Park, NC 27711
Abstract
The combustion of fossil fuels is suspected to contribute to the measured
increases in the ambient concentrations of nitrous oxide (N2O).
Characterization of N2O emissions from fossil fuel combustion
and associated pollution control systems has been hindered by a sampling
artifact whereby N2O may be generated from nitrogen oxides,
sulfur dioxide, and moisture present in the sample vessel while these
samples await analysis. To truly assess the N2O emissions from
fossil fuel combustion, a real-time or near real-time measurement technique
is required. To accomplish this, a gas chromatograph equipped with an
electron capture detector was configured and automated. This system is
capable of detection levels below ambient concentrations and a practical
quantifying range of 0.1 to 200 ppm. A pre-column backflushing system
negates the effects of interferants present in fossil fuel combustion
emissions. The automated system is capable of one on-line measurement
every 8 minutes and has been used to evaluate N2O emissions
from a variety of combustion sources, fuels, and post-combustion pollution
control techniques.
"Nitrous Oxide Emissions from
Fossil Fuel Combustion," W.P. Linak, J.A. McSorley, R.E. Hall, J.V.
Ryan, R.K. Srivastava, J.O.L. Wendt, and J.B. Mereb, Journal of Geophysical
Research, V. 95, No. D6, pp. 7533-7541, 1990.
Nitrous Oxide Emissions from Fossil Fuel Combustion
William P. Linak, Joseph A. McSorley, and Robert E. Hall Combustion Research Branch, MD-65 Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 Jeffrey V. Ryan and Ravi K. Srivastava Acurex Environmental Corporation P.O. Box 13109 Research Triangle Park, NC 27709 Jost O. L. Wendt and Jamal B. Mereb Department of Chemical Engineering University of Arizona Tucson, AZ 85721
Abstract
The role of coal combustion as a significant global source of nitrous
oxide (N2O) emissions was re-examined through on-line emission
measurements from six pulverized-coal-fired utility boilers and from
laboratory and pilot-scale combustors. The full-scale utility boilers
yielded direct N2O emission levels of less than 5 ppm.
The sub-scale combustor test data were consistent with full-scale
data, and also showed N2O emission levels not exceeding
5 ppm, although these levels increased slightly when various combustion
modifications to lower NO emissions were employed. These on-line emission
measurements are very different from previously published data. The
discrepancy is shown to be due to a sampling artifact by which significant
quantities of N2O can be produced in sample containers
which have been used in establishing the previously employed N2O
data base. Consequently, we conclude that N2O emissions
bear no direct relationship to NO emissions from these combustion
sources, and that this direct source of N2O is negligible.
Other indirect routes for the conversion of NO into N2O
outside the combustor and other combustion sources not examined by
this study, however, cannot be ruled out.
"Nitrous Oxide Behavior
in the Atmosphere, and in Combustion and Industrial Systems,"
J.C. Kramlich and W.P. Linak, Progress in Energy and Combustion Science,
V. 20, pp. 149-202, 1994.
Nitrous Oxide Behavior in the Atmosphere, and in Combustion and Industrial Systems
John C. Kramlich Department of Mechanical Engineering, FU-10 University of Washington Seattle, Washington 98195 USA William P. Linak Combustion Research Branch, MD-65 Air and Energy Engineering Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711 USA
Abstract
Tropospheric measurements show that nitrous oxide (N2O) concentrations
are increasing over time. This demonstrates the existence of one or more
significant anthropogenic sources, a fact that has generated considerable
research interest over the last several years. The debate has principally
focused on (1) the identity of the sources, and (2) the consequences of
increased N2O concentrations. Both questions remain open, to
at least some degree.
The environmental concerns stem from the suggestion that diffusion of
additional N2O into the stratosphere can result in increased
ozone (O3) depletion. Within the stratosphere, N2O undergoes
photolysis and reacts with oxygen atoms to yield some nitric oxide (NO).
This enters into the well known O3 destruction cycle. N2O is
also a potent absorber of infrared radiation and can contribute to global
warming through the greenhouse effect.
A major difficulty in research on N2O is measurement. Both
electron capture gas chromatography and continuous infrared methods have
seen considerable development, and both can be used reliably if their
limitations are understood and appropriate precautions are taken. In particular,
the ease with which N2O is formed from NO in stored combustion
products must be recognized; this can occur even in the lines of continuous
sampling systems.
In combustion, the homogeneous reactions leading to N2O are
principally NCO + NO -> N2O + CO and NH + NO -> N2O
+ H, with the first reaction being the most important in practical combustion
systems. Recent measurements have resulted in a revised rate for this
reaction, and the suggestion that only a portion of the products may branch
into N2O + CO. Alternately, recent measurements also suggest
a reduced rate for the N2O + OH destruction reaction. Most
modeling has been based on the earlier kinetic information, and the conclusions
derived from these studies need to be revisited.
In high-temperature combustion, N2O forms early in the flame
if fuel-nitrogen is available. The high temperatures, however, ensure
that little of this escapes, and emissions from most conventional combustion
systems are quite low. The exception is combustion under moderate temperature
conditions, where the N2O is formed from fuel-nitrogen, but
fails to be destroyed. The two principal examples are combustion fluidized
beds, and the downstream injection of nitrogen-containing agents for nitrogen
oxide (NOx) control (e.g., selective non-catalytic reduction
with urea).
There remains considerable debate on the degree to which homogeneous vs.
heterogeneous reactions contribute to N2O formation in fluidized
bed combustion. What is clear is that the N2O yield is inversely
correlated with bed temperature, and conversion of fuel-nitrogen to N2O
is favored for higher-rank fuels. Fixed-bed studies on highly devolatilized
coal char do not indicate a significant role for heterogeneous reactions
involving N2O destruction. The reduction of NO at a coal char
surface appears to yield significant N2O only if oxygen (O2)
is also present. Some studies show that the degree of char devolatilization
has a profound influence on both the yield of N2O during char
oxidation, and on the apparent mechanism. Since the char present in combustion
fluidized beds will likely span a range of degrees of devolatilization,
it becomes difficult to conclusively sort purely homogeneous behavior
from potential heterogeneous contributions in practical systems.
Formation of N2O during NOx control processes has
primarily been confined to selective noncatalytic reduction. Specifically,
when the nitrogen-containing agents urea and cyanuric acid are injected,
a significant portion (typically > 10%) of the NO that is reduced is
converted into N2O. The use of promoters to reduce the optimum
injection temperature appears to increase the fraction of NO converted
into N2O. Other operations, such as air staging and reburning,
do not appear to be significant N2O producers. In selective
catalytic reduction the yield of N2O depends on both catalyst
type and operating condition, although most systems are not large emitters.
Other systems considered include mobile sources, waste incineration, and
industrial sources. In waste incineration, the combustion of sewage sludge
yields very high N2O emissions. This appears to be due to the
very high nitrogen content of the fuel and the low combustion temperatures.
Many industrial systems are largely uncharacterized with respect to N2O
emissions. Adipic acid manufacture is known to produce large amounts of
N2O as a byproduct, and abatement procedures are under development
within the industry.
"NOx Abatement by Fuel-Lean
Reburning: Laboratory Combustor and Pilot-Scale Package Boiler Results,"
C.A. Miller, A.D. Touati, J. Becker, and J.O.L. Wendt, Twenty-Seventh
Symposium (International) on Combustion/The Combustion Institute, 1998,
pp. 3189–3195
NOx Abatement by Fuel-Lean Reburning: Laboratory Combustor and Pilot-Scale Package Boiler Results
C. Andrew Miller, Air Pollution Technology Branch, U.S. Environmental Protection Agency Research Triangle Park, NC 27711 A. Dahman Touati, ARCADIS Geraghty & Miller, P.O. Box 13109, Research Triangle Park, NC 27709 J. Becker and J.O.L. Wendt, Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721
Abstract
Although nitrogen oxides (NOx) abatement by reburning or
secondary fuel addition has been demonstrated in the field, its application
has heretofore been reserved primarily for overall fuel-rich or reducing
conditions in the reburning zone. Contrary to predictions by detailed
kinetic modeling of premixed systems, the unmixed environments often
present in large full-scale units allow substantial NOx
reduction under overall fuel-lean conditions in the reburning zone.
To explore this further, two experimental studies were conducted.
First, systematic tests in a 17-kW down-flow laboratory combustor,
in which nitric oxide (NO) in the oxidant was destroyed in long, axial,
methane/air, diffusion flames, showed that substantial reduction of
NO was possible under overall fuel-lean conditions. Variations of
burner Reynolds number showed that this was a first-order process
with respect to primary NO concentration under laminar flow conditions,
but approximately second order with respect to primary NO under turbulent
diffusion flame conditions. These results were then corroborated by
tests in a pilot-scale, 0.9-MW, package boiler simulator, in which
reburning natural gas was introduced, in an axial coflowing mode,
into a flue gas containing almost 6% O2 and NO concentrations
ranging from 600 to 2600 ppm. These larger-scale, turbulent flow tests
also showed that fuel-lean reburning could achieve 40%–50% NO destruction
with carbon monoxide emissions of less than 100 ppm, and that the
effectiveness of fuel-lean reburning depended only weakly on reburning
zone temperature, with a maximum effectiveness at a modestly cool
flue gas temperature of 1000 K. These results, which differ markedly
from those for conventional fuel-rich reburning, are interpreted in
light of pertinent mechanisms.
"Evaluation of Tire-Derived
Fuel for Use in NO Reductions by Reburning," C.A. Miller, P.M. Lemieux,
and A. Touati, Journal of the Air & Waste Management Association,
Vol. 48, pp. 729-735, 1998
Evaluation of Tire-Derived Fuel for Use in NO Reductions by Reburning
C. Andrew Miller, Paul M. Lemieux, U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Research Triangle Park, NC 27711 A. Touati, Acurex Environmental Corporation, P.O. Box 13109, Research Triangle Park, NC 27709
Abstract
Tire-derived fuel (TDF) was tested in a small scale (44 kW or 150,000
Btu/hr) combustor to determine its feasibility as a fuel for use in reburning
for control of nitrogen oxide (NO). TDF was gravity fed into upward flowing
combustion gases from a primary natural gas flame doped with ammonia (NH3)
to simulate a high NO combustion process. Emissions of NO, oxygen (O2),
carbon dioxide (CO2), carbon monoxide (CO), and particulate
matter (PM) were measured. The tests varied the nominal primary NO level
from 600 to 1200 ppm and the primary stoichiometry from 1.1 to 1.2, and
used both natural gas and TDF as reburn fuels. The reburn injection rate
was varied to achieve 8 to 20% of the total heat input from the reburn
fuel. NO emissions reductions ranged between 20 and 63% when using TDF,
depending upon the rate of TDF injection, primary NO, and primary stoichiometry.
NO emission reductions when using natural gas as the reburn fuel were
consistently higher than those when using TDF. While additional work remains
to optimize the process and evaluate costs, TDF has been shown to have
the potential to be a technically viable reburning fuel.