United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-83-025 June 1983
Project Summary
Bench-Scale Evaluation of
Non-U.S. Coals for NOx
Formation Under Excess Air and
Staged Combustion Conditions
S. Chen, D. Pershing, and M. P. Heap
This report summarizes results of
bench-scale fuel screening experiments.
Twenty non-U.S. coals (including lig-
nite, subbituminous, and bituminous)
were tested in a 21-kWt refractory-
lined tunnel furnace. NOX emissions
were measured as a function of coal
composition and initial fuel/air con-
tacting rate under excess air condi-
tions. In addition, inflame measure-
ments were made to quantify the in-
fluence of stoichiometry and tempera-
tu re on the fate of fuel nitrogen species
under staged combustion conditions.
The results show that NOX emissions
are generally correlated to fuel nitro-
gen content; however, such factors as
hydrocarbon volatile content and the
partition of fuel nitrogen between char
and volatile fractions are also impor-
tant Moreover, coals with high volatile
contents give high NOX emissions
under well mixed or excess air con-
ditions, but give low NOX emissions
under staged conditions or with long
axial flames. Both increasing flame
temperature and decreasing particle
size decrease NOX emissions in staged
combustion but increase it in well
mixed combustion. Increasing the rate
of heat extraction from staged com-
bustion, however, generally reduces
NOX emissions via a complex mech-
anism.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Background - NOX Formation
During the combustion of coal, nitrogen
oxides (NOX) are formed by at least two
fundamentally different chemical mech-
anisms: high temperature fixation of at-
mospheric nitrogen (leading to thermal
NOX), and the oxidation of nitrogen chem-
ically bound in the fuel (leading to fuel
NOJ. The formation of thermal NOX in gas
systems has been studied extensively and
is usually described using a modified
Zeldovich mechanism.
At least 70% of the NOX formed during
the combustion of pulverized coal are due
to the oxidation of coal-bound nitrogen.
Therefore, it is of major importance in the
development of low emission systems to
understand the phenomena which control
the fate of this fuel-bound nitrogen. The
current hypothesis concerning the fate of
coal nitrogen during pulverized coal com-
bustion can be simplified to include the
following three processes:
• Thermal Decomposition - As the
coal particle is heated, it decomposes.
Volatiles are evolved, and the nitro-
gen is spilt between the volatile and
solid fractions. The nitrogen remain-
ing in the solid, char nitrogen, is a
function of the temperature attained
by the particle and the time at that
temperature.
• Gas-Phase Reactions - The gas-
phase nitrogen produced from the
volatile coal fractions can react to
form NO, NH3, HCN, or N2. The
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formation of NO is favored under
fuel-lean conditions, and the forma-
tion of N2 is favored under fuel-rich
conditions.
• Char Burnout - The char nitrogen
associated with the solid, either as a
pyrolysis product of tars or as the
original coal char, can also be oxi-
dized to form NO during char burn-
out Char nitrogen conversion effici-
ency is usually assumed to be low.
This simplified hypothesis provides a
basis to understand the potential fuel
effects involved in NO formation during
pulverized coal combustion as well as to
recognize the steps involved in the optimi-
zation of a staged combustion system.
Control Technology
Most NOX control technology is based
on the use of combustion modification
techniques because they have proven cost-
effective compared to stack gas scrubbing.
The most successful combustion modifi-
cation technique for coal has been staged
combustion: through external staging
(biased firing, over-fire air ports, off-stoi-
chiometric combustion), burner modifi-
cations (staging via delayed mixing), or
compartmentalization (primary combus-
tion furnace). In all of the concepts a
portion of the combustion air is removed
from the normal burner air and added at
some distance downstream. Field testing
and pilot-scale studies have demonstrated
the potential of external staged combustion.
During the past 10 years, the U.S. EPA
has devoted considerable effort to under-
standing the mechanisms involved in the
formation of NOX during the combustion
of pulverized coal. This understanding has
led to the development of a low-NOx
pulverized-coal burner (the Distributed
Mixing Burner, DMB) which is suitable for
retrofit to coal-fired boilers. EPA's DMB
uses a combination of proper fuel injector
and distributed air addition to create con-
ditions which minimize fuel NOX produc-
tion and yet provide a combined heat-
release zone which is compatible with
current boiler combustion chambers.
Purpose and Scope
A problem associated with applying the
DMB to coal-fired boilers is the impact of
the fuel type on burner performance. In
this instance, performance refers not only
to pollutant emissions but also to ignition
stability, combustion efficiency, and the
necessity to reoptimize burnout to ac-
count for differences in coal properties.
The purpose of this program is to assess
the application of low NOX coal combus-
tion technology to non-U.S. coals. The
overall objectives are to define the level of
NOX control likely to be achieved with non-
U.S. coals and to determine the perform-
ance of the DMB on non-U.S. coals. The
overall effort consists of bench-scale fuel
screening studies and pilot-scale burner
evaluation. Goals of the bench-scale in-
vestigation are to determine the NOX for-
mation characteristics of non-U.S. coals
under both excess air and staged com-
bustion conditions, and to provide a basis
for comparison with U.S. coals and for the
selection of fuels to be tested in the pilot-
scale investigations.
Results
Fuel NO formed during the combustion
of pulverized coal can be isolated by re-
placing the combustion air with an arti-
ficial oxidant mixture containing C02, Ar,
and 02. It appears that under conditions
typical of commercial practice, oxidation of
nitrogen chemically bound in the coal is
the major source of NOX emissions Tests
with 20 non-U.S. coals under excess air
conditions indicated that, although the
nitrogen content of these coals ranged
from 0.8 to 2.5% (DAF), the emissions
correlated generally with those obtained
from 28 U.S. coals with a smaller range of
nitrogen content Fuel NO emissions were
reduced as the rate of fuel/air mixing was
reduced. In general, fuel NO emissions
increased with increasing fuel nitrogen
content; however, fuel properties such as
hydrocarbon volatile content and the par-
tition of nitrogen between the char and
volatile fractions were also found to be
important In general, coals which evolve
large amounts of reactive volatile nitrogen
under inert-pyrolysis conditions give high
exhaust emissions with rapidly mixing
burners and relatively low exhaust emis-
sions with long axial flames. Conversely,
those coals which tend to retain a large
fraction of their nitrogen in the solid-phase
until the char burnout regime, tend to
produce relatively low NO under well-
mixed conditions. It was found that the
exhaust emissions could be correlated in
terms of total fuel-nitrogen content, reac-
tive volatile nitrogen content as determined
by inert-pyrolysis, and nitrogen content of
the ASTM char.
Increasing flame temperature and de-
creasing particle size both increased NOX
emissions with rapidly mixed flames be-
cause both changes enhanced the evolu-
tion of fuel-nitrogen from the coal particle.
Conversely, with axial diffusion flames,
increasing temperature or decreasing par-
ticle size was found to be beneficial be-
cause both changes promote the evolution
of fuel nitrogen species within a fuel-rich
flame core.
Detailed measurements of first-stage
and exhaust species concentrations sug-
gest that a staged combustion system
must be optimized with respect to first-
stage stoichiometry and residence time,
fuel properties, and heat extraction rate.
As first-stage stoichiometry is decreased,
the NO formed in the fuel-rich zone de-
creases, but other oxidizable gaseous ni-
trogen species increase as does nitrogen
retention in the solid-phase material exit-
ing the first stage. Total fixed-nitrogen
(TFN = NO+NH3+HCN) generally in-
creases with increasing fuel nitrogen and
correlates well with fuel-nitrogen. Increas-
ing the residence time in the fuel-rich
stage allows TFN species to decay toward
low equilibrium values and thus reduces
NO emissions.
The distribution of the TFN species
leaving the first-stage is strongly depen-
dent on the coal composition. Of the nine
coals tested in detail, none produced the
high HCN concentrations previously ob-
served with the Utah and Texas lignite
coals from the U.S. The medium volatile
Line Creek bituminous coal formed es-
sentially no NH3, and very little HCN, even
under extremely fuel-rich conditions. This
behavior was directly analogous to that
observed previously with a low-volatile
Pennsylvania anthracite and with coal
chars. In general, the first-stage NO per-
centage decreased significantly with de-
creasing coal rank from bituminous to
lignite. Conversely, the relative importance
of NH3 grew with decreasing rank. In
general, HCN was greater than NH3 with
bituminous coals, but less than NH3 with
all of the subbituminous and lignite coals.
Second-stage TFN conversion decreases
as the TFN distribution shifts in favor of
HCN and NH3, and as the hydrocarbon
content of the second-stage reactants in-
creases. The percentage conversion of
char nitrogen to NO in the second stage is
low (less than 20%) and appears to be
inversely proportional to the first-stage
stoichiometric ratio. Exhaust emissions can
be correlated in terms of the gas-phase
TFN, and the char nitrogen entering the
second stage.
Increasing the rate of heat extraction
from a staged combustion system generally
reduces exhaust NO emissions via a com-
plex mechanism. Reduced second-stage
flame temperatures have little effect on
solid-phase nitrogen conversion, but they
dramatically decrease gas-phase TFN con-
version due to a shift in controlling flame
chemistry. If the reactants are cooled suf-
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ficiently to bring the bulk gas temperature
at the first-stage exit to approximately
1200 K, it is possible to obtain selective
NO reduction (by NH3) in the second-
stage flame zone and, therefore, reduce
the exhaust NO emissions. However, the
effectiveness of this concept is strongly
dependent on the combustor design (first-
stage temperature profile and residence
time) and fuel chemistry. Increasing heat
extraction appears to be most favorable
with low-rank coals, because they produce
large amounts of NH3 and relatively less
first-stage NO. Conversely, high-rank coals
(e.g., Russia) may be less influenced by
the cooling in the first stage because they
produce relatively little NHs. Additionally,
first-stage cooling may decrease the rate
of TFN decay m the fuel-rich zone and,
hence, increase both the TFN and char
nitrogen carry-through into the second
stage. Finally, because of the extremely
low temperatures required for the selec-
tive NO+ NH3 reaction, carbon burnout in
the second stage may be a significant
problem.
S. Chen, D. Pershing. and M. P. Heap are with Energy and Environmental
Research Corp., Irvine. CA 92714.
Dennis C. Drehmel is the EPA Project Officer (see below).
The complete report, entitled "Bench-Scale Evaluation ofNon-U.S. Coals for NO*
Formation Under Excess Air and Staged Combustion Conditions," (Order No. PB
83-196 014; Cost: $14.50, 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:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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