United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S7-87/011 May 1987
Project Summary
Reburning Application to Firetube
Package Boilers
J. A. Mulholland, E. E. Stephenson, C. Pendergraph, and J. V. Ryan
A pilot-scale experimental research
program has been conducted to examine
the physical and chemical phenomena
associated with the NOX control tech-
nology of reburning applied to gas- and
liquid-fired firetube package boilers.
Reburning (staged fuel combustion)
diverts some of the fuel and combustion
air from the main burner(s) for injection
into the post-flame gases, resulting in
three distinct zones in the boiler radiant
section: a fuel-lean primary zone, a
fuel-rich reburning zone, and a fuel-lean
burnout zone. NOX is reduced both by
reduction of NOX formation and by
destruction of primary flame NOX by
secondary fuel radicals.
Several hypotheses were evaluated.
Results indicate that the overall NO,
reduction via reburning has a reaction
rate order of about 1.5 with respect to
primary NOX. Secondary fuel-bound
nitrogen is found to reduce reburning
effectiveness. An overall gas-phase re-
action time constant of 50 ms is ob-
served for the reburn zone nitrogenous
species; the characteristic time constant
is greater for liquid fuel reburning due
to droplet vaporization requirements.
Increased temperature increases N2
formation in the fuel-rich zone, but
decreases N2 formation in the burnout
zone. Secondary fuel injector design
does not significantly influence reburn-
ing effectiveness in the firetube package
boiler class because of large-scale
turbulent structures in the reacting flow
that dominate secondary fuel jet pa-
rameters. Nitrogen-free fuel oil yields
slightly greater NO, reductions than
natural gas in reburning application.
These results were obtained on a 1.0
MW (3.5 x 10' Btu/hr) research boiler
simulator and verified on a commercial
Scotch package boiler (2.5 x 106
Btu/hr).
It is shown that, with minimal facility
modification, NOX emissions from a
firetube package boiler can be reduced
by 50% or more from an initial level of
200 ppm (measured dry, at zero % O2)-
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 docu-
mented In a separate report of the same
title (see Project Report ordering In-
formation at back).
Introduction
Experimental research has been con-
ducted to study the physical and chemical
phenomena associated with gaseous and
liquid fuel reburning applied to two fire-
tube package boilers. The test results
show that reburning is an effective way
to control the emission of oxides of
nitrogen (NOX), even at low initial NOX
levels. For initial NOX levels of >100 ppm,
50-80% NOX reductions have been ob-
tained. Reburning can be coupled with
other NOX control technologies to yield
NOX emissions of<100 ppm.
Reburning (staged fuel combustion) is
an in-f urnace NOX control technology that
diverts some of the fuel and combustion
air flows from the main burner(s) for
injection into the post-flame gases. With
this combustion modification, three
stoichiometrically distinct zones are es-
tablished in the boiler radiant section: a
fuel-lean primary zone, a fuel-rich re-
burning zone, and a fuel-lean burnout
zone. NOX reduction via reburning occurs
both by the molecular destruction of NOX
formed in the primary combustion zone
through reactions with secondary flame
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radicals and by an overall reduction of
NOX formed due to distributed mixing of
fuel and air. Figure 1 is a cartoon depicting
this process.
Bench-Scale and Other Tests
Bench-scale tests performed elsewhere
identify the important reburning NOX
reduction mechanisms. It has been shown
that the methylidyne radical (CH") result-
ing from pyrolysis of the staged fuel reacts
with NO (formed in the primary combus-
tion zone) to form hydrogen cyanide (HCN).
In the reburning zone, HCN reacts to
form amines which can react with nitric
oxide (NO) to form molecular nitrogen.
Alternatively, fixed nitrogen species can
oxidize in the reburning and burnout
zones to form NOX.
Full-scale field tests in Japan in the
late 1970s and early 1980s showed that
50% NOX reduction is achievable with
reburning over a wide range of initial NOX
conditions burning gas, oil, or coal in the
reburning zone. These promising results
raised fundamental questions leading to
Reburning /V0« Destruction Mechanism:
CH
/VO«
HCN
NH,
NH3
NO
Burnout
Air
Reburn
Fuel
Main
Burners
Figure 1. Reburning.
further research and development in the
U.S. Environmental Protection Agency
(EPA), the Department of Energy (DOE),
the Electric Power Research Institute
(EPRI), and the Gas Research Institute
(GRI) have sponsored various parametric
reburning studies.
EPA Tests
To bridge the gap between fundamental
study of reburning mechanisms and full-
scale reburning application, the U.S. EPA
conducted in-house reburning tests from
May 1983 through May 1985 at its
Environmental Research Center in Re-
search Triangle Park, NC. Two pilot-scale
firetube package boiler test facilities (a
research simulator and a commercial
unit) were modified for reburning applica-
tion, with these key characteristics:
• Nominal firing rate — 0.6 - 0.9 MW
(2-3x106Btu/hr).
• Fuels — natural gas and light and
heavy fuel oils.
• Low primary NOX levels — 50 - 250
ppm (all NOX concentrations reported
dry, at zero % 02).
• Low reburning zone temperatures
— 1300 - 1600 K (1900 - 2400°F).
The test facilities simulate the thermal
environment and turbulent diffusion flame
aerodynamics of small firetube package
boilers. The simple furnace geometry,
with a single centerline burner mounted
to a horizontal, cylindrical boiler, results
in a nearly two-dimensional system,
allowing for complete spatial (and tempo-
ral) characterization of the combustion
gas temperature, velocity, and composi-
tion. Natural gas and light and heavy fue
oils were fired in the primary and reburn-
ing burners, with ammonia and pyridine
dopants used as fuel nitrogen surrogates
A schematic of the research package
boiler simulator is shown in Figure 2.
Baseline conditions were establishec
to focus study on low primary NOX level:
for two reasons: (1) to evaluate ar
apparent discrepancy in results reportec
by various investigators, and (2) to targe
most likely candidates for reburninj
application. First, laboratory and field dat;
reported by Japanese investigators in
dicate that NOX reduction by reburning i;
independent of initial NOX level; however
data from bench-scale tests conducted ii
the U.S. indicate that NOX reduction b'
reburning decreases as initial NOX level:
decrease. Second, application of reburnini
is likely to be coupled with primary flami
NOX control (e.g., low NOX burner). Cos
effective primary combustion modifica
tions exist to reduce NO, levels to betweei
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cxhaus<
Primary
Air
Burnout
Air
Low NO* \
Drecombustion
Chamber
Burner
Primary^* Reburning Burnout
Zone Zone Zone
Primary
Fuel
Figure 2. Package boiler simulator schematic
Reburnmg
Fuel
200 and 400 ppm; reburning can then be
applied to reduce NOX emission by an
additional 50%
Reburning parametric tests included
both input/output and detailed sampling
of combustion gas temperature, velocity,
and speciation. The test matrix included
variation of the following parameters to
evaluate several hypotheses:
able reburning fuel nozzles were used to
adjust secondary fuel mixing rates. The
temperature in the boiler was changed
by insulating portions of the radiant zone.
EPA Findings
Several findings from the research
boiler simulator reburning experiments
were significant. Figure 3 shows the ef-
Primary Zone
• Initial NOX concentration
• Stoichiometry
• Fuel type
• Flame shape
Reburning Zone
Stoichiometry
Fuel nitrogen content
Residence time
Temperature
Mixing rate
Fuel type
Fuel staging location
Burnout Zone
• Stoichiometry
• Temperature
• Mixing rate
These parameters were varied indepen-
dently to as great a degree as possible.
Primary NOX concentration was controlled
by varying the level of primary fuel
dopant; primary zone Stoichiometry, by
primary combustion air flow rate; and
primary flame shape, by primary axial
and radial air split. Reburning zone
Stoichiometry was varied by varying the
amount of secondary fuel addition (with
primary load held constant) Reburning
zone residence time was varied by varying
the location of burnout air addition. Vari-
fects of initial NOX concentration, reburn
zone Stoichiometry, and fuel nitrogen
content. The overall reburning reaction
rate order with respect to primary NOX is
about 1.5, as NOX reductions decrease
with decreasing levels of primary NOX.
There is an optimum reburn zone Sto-
ichiometry for NOX reduction, dependent
on initial NOX level and reburning fuel
nitrogen content. The optimum Stoichio-
metry for primary NOX destruction is
about 0.9; however, dilution effects from
staged combustion typically result in an
optimum reburn zone Stoichiometry for
overall NOX reduction in the range of 0.7
- 0.9. Secondary fuel nitrogen content is
shown to have a large impact on re-
burning effectiveness, significantly re-
ducing NOX reductions for primary NOX
levels less than 250 ppm. However, at
high initial NOX levels (i.e., greater than
500 ppm), reburning fuel nitrogen can
actually enhance NOX reduction.
Figure 4 shows reburning effectiveness
as a function of reburn zone residence
time for reburning with natural gas,
distillate fuel oil, and a distillate/residual
fuel oil mixture. A minimum reburn zone
gas-phase reaction rate time was found
to be 50 msec, which translates to a
reburn zone length of 0.61 m (2 ft) and
reburn zone bulk residence time of 200
msec in the firetube package boilers
tested. Fuel oil reburning required longer
residence times to allow for droplet
vaporization.
Other important findings included the
lack of significant overall effects of tem-
perature and mixing rates in these tests.
Over the limited range tested, tempera-
ture was not found to have a major
influence on overall reburning effective-
ness. While increased temperature
increased N2 formation in the:reburnmg
zone, N2 formation in the burnout zone
was significant under low temperature
reburning conditions. Reburning fuel
nozzle design did not significantly in-
fluence reburning effectiveness in these
tests because of large-scale} turbulent
eddy structures in the reacting flow that
dominated reburning fuel mixing. Thus,
secondary fuel jet parameters did not
control reburning zone mixing rates in
these tests. Detailed probing in the
furnace of temperature, velocity, and
speciation provided a deeper understand-
ing of the reburning process. The time-
resolved nitrogen species profiles shown
in Figure 5 demonstrate the kinetic and
mixing limitations of reburning.
Conclusions
These results, characterizing the
dependence of reburning effectiveness
on major parameters, were found to be
consistent for natural gas and fuel oil
firing in the commercial boiler. Light fuel
oil was found to yield slightly greater NOX
reductions that natural gas in reburning
application. With minimal facility modifi-
cation, Figure 6 shows that NOX emissions
can be .educed by 50 - 60% from an
initial level of 200 ppm with either natural
gas or fuel oil reburning. Reburning is a
way to control NOX emissions with a
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minimum fuel-rich zone length. However,
for reburning to be effective, secondary
fuel must be staged downstream of the
primary combustion zone, which requires
sufficient boiler length.
NOX formed during combustion by the
reduction and oxidation of molecular
nitrogen and nitrogen contained in the
fuel contribute to the degradation of air
quality as well as to acid deposition and
forest damage. Reburning is one of many
NOX control technologies presently avail-
able. While NOX control strategies exist
that can meet current New Source Per-
formance Standards (NSPS) for NOX
emissions, few technologies exist that
can control NOX emissions to < 100 ppm.
Natural gas reburning can be coupled
with other NOX control technologies to
achieve this low emission level. Of course,
in selecting a NOX control strategy for a
particular application, cost analysis must
be considered along with technical
feasibility. Reburning, while more ex-
pensive and/or difficult to implement than
some other in-furnace NOX control tech-
nologies (such as air staging or low NOX
burners), is still much less expensive
than pre-combustion fuel cleaning and
post-combustion catalytic reduction.
700
30
0.7
Figure 3. Natural gas reburning effectiveness.
4
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700
90
80
100
Reburn Zone Bulk Residence Time, msec
200 300 400
I
Q)
Q.
I
i
70
60-
50
40
Distillate/Residual Mixture Fuel Oil
Natural Gas
/VOP,,= 185 ppm
SRR=0.89
\
0.25 0.50 0.75 1.00
Reburn Zone Length, m
1.25
1.50
Figure 4. Reburning zone residence time effect.
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200
775
750
725
s
&
c,- 700
•3>
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S«R = O.S4
Primary Fuel: Distillate/Residual Fuel Oil Mixture
Uncontrolled NO, = 265 ppm
46%
Primary Fuel: Distillate Fuel Oil:
Uncontrolled NO, - 125 ppm
§
g 50
I
i 25
I
Gas Distillate Distillate/Residual
Fuel Oil Fuel Oil
61%
51%
Natural Distillate Distillate/Residua/
Gas Fuel Oil Fuel Oil
Figure 6. North American boiler reburning application test results
J. Mulholland. E. Stephenson, C. Pendergraph, and J. Ryan are with Acurex
Corporation. Research Triangle Park, NC 27709.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Reburning Application to Firetube Package
Boilers." (Order No. PB 87-177 515/AS; Cost: $36.95, subject to change}
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
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
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United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S7-87/011
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