United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-94/208 March 1995
EPA Project Summary
NOX Control Technologies
Applicable to Municipal Waste
Combustion
D. M. White, K. L. Nebel, M. Gundappa, and K. R. Ferry
Several technologies are available for
reducing nitrogen oxide (NOX) emis-
sions from municipal waste combus-
tors (MWCs), including combustion
controls, natural gas injection (NGI),
selective non-catalytic reduction
(SNCR), and selective catalytic reduc-
tion (SCR). The full report documents
the key design and operating param-
eters, commercial status, demonstrated
performance, and cost of NGI, SNCR,
and SCR, and identifies technology re-
search and development needs associ-
ated with them.
Two NGI processes have been devel-
oped: (1) Methane de-NOXSIM uses gas
injection to inhibit NOX formation and
appears capable of reducing NOX emis-
sions from MWCs by approximately
60% and (2) reburning uses gas injec-
tion to create reducing conditions that
convert NOX formed in the primary com-
bustion zone to molecular nitrogen.
Because of the relatively high tempera-
tures required for these NOx-reduction
reactions, it may be difficult to suc-
cessfully apply reburning to modern
mass-burn waterwall MWCs. Long-term
emission reductions are 45-65% for
SNCR and 80-90% for SCR. Operation
of SNCR processes near the upper end
of their performance range can result
in unwanted emissions of ammonia or
other by-product gases. An advanced
version of SNCR using furnace pyrom-
etry and additional process controls
appears capable of achieving high NOX
reductions with less reagent than is
needed for conventional SNCR. The
combination of NGI and SNCR (ad-
vanced NGI) may be able to achieve
overall NOX reductions of 80-85%.
Comparing costs, SCR is the most
capital intensive, followed by advanced
SNCR and advanced NGI. Capital costs
of NGI and conventional SNCR are com-
parable. In terms of tipping fee impact
and cost effectiveness, conventional
SNCR generally has the lowest costs
of the evaluated technologies. For NGI,
these costs depend on whether waste
is diverted and tipping fee revenues
are lost when applying this technol-
ogy, along with the price of natural
gas. Depending on the selected NGI
scenario, the resulting tipping fee im-
pacts and cost effectiveness values can
be the highest of the evaluated tech-
nologies. After specific NGI scenarios,
the next highest tipping fee impacts
and cost effectiveness values are for
SCR. These high costs result from high
capital costs, as well as the cost of
catalyst replacement and disposal.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Nitrogen oxides (NOX) are of environ-
mental significance because of their role
as a criteria pollutant, acid gas, and ozone
precursor. The current New Source Per-
formance Standards (NSPS) for municipal
waste combustors (MWCs) (40 CFR Part
-------
60, Subpart Ea) limit NOX emissions to a
daily average of 180 parts per million (ppm)
at 7% oxygen (O2), dry basis.* By com-
parison, typical NOX emissions from mod-
ern mass-burn waterwall (MB/WW) MWCs
range from 220 to 320 ppm. To comply
with the NSPS, most recently built MWCs
have used a combination of combustion
controls to limit NOX formation and selec-
tive non-catalytic reduction (SNCR) to con-
vert NOX to molecular nitrogen (N2). Be-
cause of pressure to achieve even lower
emission levels, questions have been
raised regarding the potential for advance-
ments in NOX control technologies. To re-
spond to these questions, the U.S. Envi-
ronmental Protection Agency's (EPA's) Air
and Energy Engineering Research Labo-
ratory and the Department of Energy's
National Renewable Energy Laboratory ini-
tiated this assessment of three alternative
NOX control technologies: natural gas in-
jection (NGI), SNCR, and selective cata-
lytic reduction (SCR). The objectives of
the assessment were to (1) document the
key design and operating parameters,
commercial status, demonstrated perfor-
mance, and cost of each technology and
(2) identify technology research and de-
velopment needs.
The assessment of achievable NOX
emissions presented by the report is based
on the average NOX reduction potential of
these technologies applied to "typical"
MWCs. The assessment does not exam-
ine the potential severity or length of short-
duration excursions in performance that
can affect continuously achievable NOX
emission rates associated with short aver-
aging periods. The assessment also does
not consider potential limitations on tech-
nology performance that may result from
combustor-specific design or operating re-
strictions.
NOX Formation
The chemistry of NOX formation is di-
rectly tied to reactions between nitrogen
and O2. To understand NOX formation in
an MWC, a basic understanding of com-
bustor design and operation is useful.
Combustion air systems in MB/WW MWCs
include both undergrate (also called pri-
mary) air and overgrate (also called sec-
ondary or overfire) air. Undergrate air, sup-
plied through plenums located under the
firing grate, is forced through the grate to
sequentially dry (evolve water), devolatilize
(evolve volatile hydrocarbons), and burn
out (oxidize nonvolatile hydrocarbons) the
waste bed. The quantity of undergrate air
* Unless otherwise noted, all NOx concentrations used in
this summary are corrected to 7% O2 and are on a dry
basis.
is adjusted to minimize excess air during
initial combustion of the waste while maxi-
mizing burnout of carbonaceous materials
in the waste bed. Overgrate air, injected
through air ports located above the grate,
is used to provide turbulent mixing and
destruction of hydrocarbons evolved from
the waste bed. Overall excess air levels
for a typical MB/WW MWC are approxi-
mately 80% (180% of stoichiometric [i.e.,
theoretical] air requirements), with
undergrate air accounting for 60-70% of
the total air. In addition to destruction of
organics, one of the objectives of this
"staged" combustion approach is to mini-
mize NOX formation.
NOX is formed during combustion
through two primary mechanisms: fuel NOX
formation and thermal NOX formation. Fuel
NOX results from oxidation of organically
bound N2 present in the municipal solid
waste (MSW) stream. Thermal NOX re-
sults from oxidation of atmospheric N2.
Fuel NOX is formed within the flame
zone through reaction of organically bound
N2 in MSW materials and O2. Key vari-
ables determining the rate of fuel NOX
formation are the availability of O2 within
the flame zone, the amount of fuel-bound
N2, and the chemical structure of the N2-
containing material. Fuel NOX reactions
can occur at relatively low temperatures
[<1,100°C (<2,000°F)]. Depending on the
availability of O2 in the flame, the N2 com-
pounds will react to form either N2 or NOX.
When the availability of O2 is low, N2 is
the predominant reaction product. If sub-
stantial O2 is available, an increased frac-
tion of the fuel-bound N2 is converted to
NOX. Testing conducted in the 1970s and
1980s using coal showed that in O2-rich,
highly mixed systems approximately 50%
of the fuel-bound N2 can convert to NOX;
in O2-starved staged-combustion systems,
however, the rate of conversion decreases
to near 5%. Other testing has shown that
N2 associated with volatile compounds is
more readily converted to fuel NOX than
N2 associated with nonvolatile materials.
Still other research involving coal and oil
combustion indicates that the extent of
conversion is related to the amount of N2
available, with the degree of conversion
decreasing as the amount of fuel-bound
N2 increases.
Thermal NOX is formed in high-tempera-
ture flame zones through reactions be-
tween N2 and O2 radicals. The key vari-
ables determining the rate of thermal NOx
formation are temperature, the availability
of O2 and N2, and residence time. The key
reactions resulting in thermal NOX forma-
tion are
followed by
N + O,
NO + O
(2)
N2 + O
NO + N
(1)
N + OH ^± NO + H (3)
Because of the high activation energy re-
quired for Reaction (1), thermal NOx for-
mation does not become significant until
flame temperatures reach 1,100°C
(2,000°F). Kinetic calculations (assuming
30% excess air, average MSW proper-
ties, and residence time of 0.5 second)
predict thermal NOX concentrations of <10
ppm in MWCs. However, local flame tem-
peratures may exceed 1,100°C and ther-
mal NOX concentrations may be greater
than these calculated model results.
Examination of MWC operating condi-
tions suggests that most of the NOX emit-
ted from MWCs (>80%) is attributable to
fuel-bound N2. Based on typical MSW N2
contents of 0.3-0.7%, the expected NOX
emissions—assuming all of the fuel-bound
N2 is converted to NOX—would be 1,000-
2,500 ppm at 7% O2. As noted earlier, how-
ever, actual emissions are generally be-
tween 220 and 320 ppm at 7% O2, indicat-
ing that perhaps 10-30% of the fuel N2 is
converted to NOX, with most of the remain-
der forming N2.
A number of evaluations of MWC NOX
emissions data have attempted to define
the role of N2-containing materials in MSW
(e.g., grass, leaves, wood, and food
wastes) on NOX emissions. The first of
these evaluations was presented in 1987
and suggested that fluctuations in mea-
sured NOX levels were attributable to sea-
sonal fluctuations in MSW composition.
This evaluation was based on NOX com-
pliance test data obtained from a number
of MWCs in the U.S. and overseas at
different times of the year, and concluded
that the seasonal variations in measured
NOX concentrations might be the result of
variations in the amount of yard waste in
the MSW at different times of the year. A
similar comparison of NOX emission con-
centrations versus time of year for a num-
ber of U.S. MWCs compiled by EPA in
1989 to support the MWC NSPS did not
show any significant relationship between
NOX concentration and time of year.
A more recent evaluation of NOX con-
tinuous emission monitor data from 11
MWCs located in the northeast U.S. found
no seasonal variations in NOX concentra-
tions. This evaluation compared monthly
average NOX data from the individual
MWCs (covering 12-36 consecutive
months of operation for each unit) with
the estimated fraction of yard waste found
in northeastern MSW during each of the
four seasons. Monthly average NOX con-
centrations from these units varied from
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140 to 310 ppm but did not show any
consistent relationship between NOX con-
centrations and the estimated percentage
of yard waste.
Yet another evaluation compared NOX
concentrations measured during nine test
runs conducted at the MB/WW MWC in
Burnaby, British Columbia, to the amount
of high-N2 organics (grass, leaves, brush,
stumps, wood, food waste, textiles, rub-
ber, and footwear) in the MSW fired dur-
ing each run. During these tests, high-N2
organics accounted for 25-47% and yard
waste accounted for 4-30% of the total
waste stream. The estimated average N2
content of the entire stream during each
run ranged from 0.34 to 0.66%. NOX con-
centrations in the flue gas during the runs
varied from 261 to 304 ppm. Statistical
analysis of the MSW and flue gas data
from each run showed no relationship (at
a screening 80% confidence level) be-
tween NOX concentrations and MSW char-
acteristics. The data suggest that, because
of the staged-combustion design of the
Burnaby MWC and other modern MB/WW
MWCs, variations in NOX emissions appear
to be attributable to differences in combus-
tor design and operation, rather than waste
composition.
NOX Control Technologies
NOX control technologies can be divided
into two subgroups: combustion controls
and post-combustion controls. Combus-
tion controls limit the formation of NOX
during the combustion process by reduc-
ing the availability of O2 within the flame
and lowering combustion zone tempera-
tures. These technologies include staged
combustion, low excess air, flue gas recir-
culation (FGR), and NGI. Staged combus-
tion and low excess air reduce the flow of
undergrate air in order to reduce O2 avail-
ability in the combustion zone. Another
option is FGR in which a portion of the
combustor exhaust is returned to the com-
bustion air supply to both lower combus-
tion zone O2 and suppress flame tem-
peratures by reducing the ratio of O2 to
inerts [N2 and carbon dioxide (CO2)] in the
combustion air system. One or more of
these approaches are used by most mod-
ern MWCs. Test data for these techniques
indicate that they can reduce NOX con-
centrations by 10-30% compared to
baseline levels from the same units. For
NGI, two processes have been developed:
(1) Methane de-NOxSM uses gas injection
to inhibit NOx formation and (2) reburning
uses gas injection to create reducing con-
ditions that convert NOX formed in the
primary combustion zone to N
The most used post-combustion NOX
controls for MWCs include SNCR and
SCR. SNCR reduces NOX to N2 without
the use of catalysts. With SNCR, one or
more reducing agents are injected into
the upper furnace of the MWC to react
with NOX and form N2. SNCR processes
include Thermal DeNOx™, which is based
on ammonia (NH3) injection, NOXOUT™,
which uses urea injection, and the addi-
tion of urea followed by methanol. Ther-
mal DeNOx and NOXOUT have been used
predominantly to date.
SCR is an add-on control technology
that catalytically promotes the reaction
between NH3 and NOX. SCR systems can
use aqueous or anhydrous NH3, with the
primary differences being the size of the
NH3 vaporization system and the safety
requirements.
Technical Approach
This assessment was conducted using
information obtained from published lit-
erature and contacts with technology ven-
dor and MWC industry personnel. For each
technology, information was collected on
the key process variables; commercial ap-
plications in Europe, Japan, and the
U.S.; recent research and development
activities; and costs. In addition to the
current versions of NGI and SNCR, ad-
vanced concepts for the two technologies
were examined, including advanced NGI
(which combines conventional NGI and
SNCR) and advanced SNCR (which em-
ploys additional process control equipment
to enhance conventional SNCR perfor-
mance). This information was then used
to develop a series of computer-based
spreadsheets designed to maintain basic
material and energy balances for the tech-
nology and to calculate technology costs.
The output from these spreadsheets is
presented in two formats. The first format
is a table presenting capital costs, tipping
fee impacts, and cost effectiveness lev-
els** for each NOX control technology as
a function of MWC size and a second key
technology variable. An example of this
"Capital costs include purchased equipment costs,
installation, engineering and home office expenses,
and process and project contingencies. Tipping fee
impact is calculated by dividing the technology's total
annualized cost by the annual tonnage of MSW pro-
cessed. Tipping fee impact is an incremental cost that
indicates the potential cost of the technology on the
MSW generator. However, it does not necessarily
reflect the amount by which the plant's tip fee will
increase as a result of applying the control technology.
Cost effectiveness is calculated by dividing the
technology's total annualized cost by the tonnage of
reduced NOX emissions and indicates the cost of the
control relative to its environmental benefit.
output, based on conventional SNCR, is
shown in Table 1. For example, the esti-
mated capital cost for SNCR operating at
60% NOX reduction on a 400 ton per day
(tpd) MWC is $1,980/tpd of capacity. For
the same NOX reduction level and MWC
size, the tipping fee impact is estimated at
approximately $1.50 per ton of MSW pro-
cessed, and the cost effectiveness is ap-
proximately $1,270 per ton of NOX re-
moved from the flue gas.
The second output format is a graph
showing the sensitivity of tipping fee im-
pact and cost effectiveness to key pro-
cess variables. An example of this output,
also based on conventional SNCR, is pre-
sented in Figure 1. For example, the pro-
cess variable having the greatest effect
on cost is MWC size. Compared to the
400 tpd "reference" MWC, reducing plant
size to 100 tpd increases the tipping fee
impact from roughly $1.50 per ton of MSW
to almost $4.00 per ton (shown on the left
Y-axis), and increases the cost effective-
ness value from $1,270 per ton of NOX
removed to $3,380 per ton (shown on the
right Y-axis).
Technical Status of Evaluated
Control Technologies
The technical status of each evaluated
control technology is summarized in Table
2. As noted, two NGI processes have been
developed. Methane de-NOx uses gas in-
jection to inhibit NOX formation and ap-
pears capable of reducing NOX emissions
from MWCs by approximately 60%. The
second approach, reburning, uses gas in-
jection to create reducing conditions that
convert NOX formed in the primary com-
bustion zone to N2. Because of the rela-
tively high temperatures required for these
NOx-reduction reactions, it may be difficult
to successfully apply reburning to modern
MB/WW MWCs. Short-duration tests of
these processes have been conducted at
MWCs in the U.S. and Europe. However,
neither process has been adequately de-
veloped and demonstrated to be consid-
ered ready for commercial applications.
Both SNCR processes (Thermal DeNOx
and NOXOUT) and SCR are considered to
be commercially available. Long-term NOX
reductions are 45-65% for SNCR and 80-
90% for SCR. Operation of these pro-
cesses near the upper end of their perfor-
mance range can result in unwanted emis-
sions of unreacted NH3 or other by-prod-
uct gases. An advanced version of SNCR
using furnace pyrometry and additional
process controls has been tested on at
least two MWCs and appears capable of
achieving high NOX reductions with less
reagent than is needed for conventional
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Table 1. Model Plant Cost Estimates for Conventional SNCR a
Total Capital Cost
($1000/TPD Capacity)
NOX Reduction (%)
100 TPD Mass Burn MWC
400 TPD Mass Burn MWC
750 TPD Mass Burn MWC
45
5.05
1.97
1.49
60
5.06
1.98
1.50
65
5.07
2.00
1.52
Tipping Fee Impact
($/ton MSW)
45
3.74
1.30
0.92
60
3.91
1.47
1.09
65
4.06
1.62
1.24
Cost Effectiveness
($/ton NOx)
45
4,308
1,496
1,058
60
3,378
1,268
940
65
3,235
1,289
986
*$/ton can be converted to $/Mg by multiplying by 1.1.
3,455
1
Unit Size (tpd) 100
Reagent Cost ($/ton) 210
Annualized Capital ($1000/yr) 52
Reference MWC Parameters
Uncontrolled NOX = 250 ppm
NOX Reduction = 60%
Unit Size
Reagent Cost -•• Annualized Capital I
Figure 1. Effect of unit size, reagent cost, and annualized capital on tipping fee impact and cost effectiveness for conventional SNCR.
SNCR. Combining NGI and SNCR to
achieve an overall NOX reduction of 80 -
85% may be feasible but will require test-
ing to evaluate the interactions between
the temperature and residence time re-
quirements of each technology.
Comparative Costs of
Evaluated Control
Technologies
As part of this study, cost evaluations
were conducted for several variations of
NGI, SNCR, and SCR. For the evaluation
of NGI, two scenarios were examined.
The first scenario, referred to as NGI-100,
assumes the MWC is firing MSW at 100%
of its design heat input capacity. There-
fore, under this scenario, the rate of waste
fired must be reduced by an amount com-
parable to the heat input from natural gas.
This results in a reduction of tipping fee
revenues. The second scenario, referred
to as NGI-85, assumes that the MWC is
firing MSW at 85% of its design heat input
capacity because of insufficient MSW flow
or because the unit was designed with
excess heat input capacity. In this case,
natural gas can be used to reduce NOX
emissions without displacing any waste
and, therefore, without a loss of tipping
fee revenues. The average NOX reduction
assumed in both scenarios is 60%.
Four variations of SNCR technology
were examined: (1) conventional SNCR,
with an average NOX reduction of 60%,
(2) advanced SNCR, with an average NOX
reduction of 60% with lower reagent use
compared to conventional SNCR, (3) a
second advanced SNCR option with a NOX
reduction of 70% at a higher reagent feed
rate, and (4) advanced NGI, which com-
bines conventional NGI with conventional
SNCR to achieve a NOX reduction of 80%.
The SCR evaluation focused on a cold-
side system with a NOX reduction level of
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Table 2. Technical Overview of Evaluated NOX Control Technologies
Technology
Commercial Status
NOV Control Performance
Technical Issues
NGI:
Methane
deNOxSM
NGI:
Reburning
SNCR:
Thermal
DeNO™
Tested on MWC in Olmsted
County, Minnesota.
Applied to fossil fuel boilers;
use on MWCs limited to test
program in Malmo, Sweden.
Applied to six MWCs in U.S.
plus others overseas.
Testing achieved up to 60%
NOX reduction without
increasing CO emissions.
MWC testing encountered high
CO levels when NOX reductions
exceeded 30 - 40%.
Achieved short-term reductions
of 45 - 75%, depending on NH3
injection rate. Plume visible at
higher reduction levels.
Scaleup of technology to larger
furnaces (> 100 tpd).
Reburn zone temperatures in
MWCs may be too low for NOX
reduction reactions.
Impact of furnace temperature
swings on NOX and NH3
emissions. Control of NH3 slip
and visible plume.
SNCR:
NOXOUT™
Advanced
SNCR
(improved
process
control)
Advanced NGI
(conventional
NGI plus
SNCR)
SCR
Applied to two MWCs in U.S.
plus others overseas.
Tested at MWCs in Lancaster,
Pennsylvania, and Munich,
Germany.
Concept only.
Installed at 20 MWC plants in
Europe and Japan.
Comparable to NH3 injection.
Achieved 60 - 75% NOX
reduction with less reagent than
is needed with conventional
SNCR. Plume visible at higher
reduction levels.
Potential for 80% reduction.
80 - 90% A/CL reduction.
Similar to NH3 injection. N2O
emissions also of concern.
Demonstration of long-term
performance capability.
Additional process controls
may benefit other combustor
operations.
Interaction of temperature and
residence time needs for each
technology.
Catalyst life in hot-side systems.
80%. A limited evaluation of a hot-side
SCR system was also conducted.
Costs for the different control technolo-
gies are compared in Figures 2, 3, and 4.
Figure 2 presents the capital costs for
each of the technologies. The advanced
SNCR system with the 70% NOX reduc-
tion and the hot-side SCR system are not
represented, since only a limited analysis
of these scenarios was conducted. As
shown by the figure, the capital costs for
each of the technologies increase with
increasing unit size. SCR is the most capi-
tal intensive of the technologies, costing 4
to 5 times more than the next highest
technology. Advanced SNCR and ad-
vanced NGI have the next highest capital
costs, with both technologies estimated to
cost $1 to 2 million per combustor. The
capital costs associated with NGI and con-
ventional SNCR are comparable, at less
than $1 million per combustor.
The tipping fee impacts and cost effec-
tiveness values presented in Figures 3 and
4, respectively, include both annualized
capital costs and operating and mainte-
nance costs. Conventional SNCR gener-
ally has the lowest costs of the technolo-
gies. NGI-100 has the highest tipping fee
impacts for all but the 100 tpd MWC, for
which SCR has slightly higher costs. Cost
effectiveness values for NGI-100 are the
highest for all combustor sizes. Both of
the NGI-100 and NGI-85 scenarios result
in an incremental cost increase; however,
the revenue loss associated with diverting
MSW when firing natural gas under NGI-
100 results in much higher costs for this
control technique. These revenues are not
lost with the NGI-85 scenario, plus rev-
enues are received under this scenario
from sale of additional electrical produc-
tion. The other key variable affecting both
NGI scenarios is the price of natural gas.
The average gas price used in these fig-
ures is$3.50/106Btu.
The high tipping fee impacts and cost
effectiveness values with SCR result from
the high capital costs for this technology,
as well as from the cost of catalyst re-
placement and disposal. Advanced SNCR
and advanced NGI have higher costs than
NGI-85 applied to the small combustor
size but are fairly similar when applied to
the medium and large combustor sizes.
-------
72-
70-
8-
§
o| e-
o
4-
2-
0 ~r
^n^
^
^
^
p
700 400 750
Unit Size (tpd)
\ \ NGI- 10060% \ \ NGI-85 60 %
ea
t-
{
I \ Conventional SNCR 60%
d] Advanced SNCR 60% Q Advanced NGI 80% \ \ SCR 80%
Figure 2. Comparison of capital cost.
25-
20-
42 75-
Tipping Fee Impa
3
5-
0-
^
t
tm
^
100
^s
fSS.
^^
Til
400
-------
18-
16-
14 -
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-------
D. M. White, K. L. Nebel, M. Gundappa, and K. R. Ferry are with Radian Corp.,
Research Triangle Park, NC 27709.
James D. Kilgroe is the EPA Project Officer (see below).
The complete report, entitled "NOx Control Technologies Applicable to Municipal
Waste Combustion," (Order No. PB95-144358; Cost: $27.00, 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
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use $300
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
EPA/600/SR-94/208
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