United States Air and Energy Engineering

Environmental Protection Research Laboratory

Agency Research Triangle Park, NC 27711

Research and Development EPA/600/SR-94/208 March 1995

E AP

NOX Control Technologies

Applicable to Municipal Waste

Combustion

Several technologies are available for

reducing nitrogen oxide (NOX) emissions from municipal waste combustors (MWCs), including combustion

controls, natural gas injection (NGI),

selective non-catalytic reduction

(SNCR), and selective catalytic reduction (SCR). The full report documents

the key design and operating parameters, commercial status, demonstrated

performance, and cost of NGI, SNCR,

and SCR, and identifies technology research and development needs associated with them.

Two NGI processes have been developed: (1) Methane de-NOXSM uses gas

injection to inhibit NOX formation and

appears capable of reducing NOX emissions from MWCs by approximately

60% and (2) reburning uses gas injection to create reducing conditions that

convert NOX formed in the primary combustion zone to molecular nitrogen.

Because of the relatively high temperatures required for these NOX-reduction

reactions, it may be difficult to successfully 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 pyrometry 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 (adProject Summary

D. M. White, K. L. Nebel, M. Gundappa, and K. R. Ferry

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 comparable. 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 technology, along with the price of natural

gas. Depending on the selected NGI

scenario, the resulting tipping fee impacts and cost effectiveness values can

be the highest of the evaluated technologies. 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 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

Nitrogen oxides (NOX) are of environmental significance because of their role

as a criteria pollutant, acid gas, and ozone

precursor. The current New Source Performance Standards (NSPS) for municipal

waste combustors (MWCs) (40 CFR Part
2

60, Subpart Ea) limit NOX emissions to a

daily average of 180 parts per million (ppm)

at 7% oxygen (O2

), dry basis.* By comparison, typical NOX emissions from modern 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 selective non-catalytic reduction (SNCR) to convert NOX to molecular nitrogen (N2

). Because of pressure to achieve even lower

emission levels, questions have been

raised regarding the potential for advancements in NOX control technologies. To respond to these questions, the U.S. Environmental Protection Agency's (EPA's) Air

and Energy Engineering Research Laboratory and the Department of Energy's

National Renewable Energy Laboratory initiated this assessment of three alternative

NOX control technologies: natural gas injection (NGI), SNCR, and selective catalytic reduction (SCR). The objectives of

the assessment were to (1) document the

key design and operating parameters,

commercial status, demonstrated performance, and cost of each technology and

(2) identify technology research and development 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 examine the potential severity or length of shortduration excursions in performance that

can affect continuously achievable NOX

emission rates associated with short averaging periods. The assessment also does

not consider potential limitations on technology performance that may result from

combustor-specific design or operating restrictions.

NOX Formation

The chemistry of NOX formation is directly tied to reactions between nitrogen

and O2. To understand NOX formation in

an MWC, a basic understanding of combustor design and operation is useful.

Combustion air systems in MB/WW MWCs

include both undergrate (also called primary) air and overgrate (also called secondary or overfire) air. Undergrate air, supplied 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

is adjusted to minimize excess air during

initial combustion of the waste while maximizing 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 approximately 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 minimize 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 results 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 variables 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 compounds will react to form either N2 or NOX.

When the availability of O2

 is low, N2 is

the predominant reaction product. If substantial O2

 is available, an increased fraction 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-temperature flame zones through reactions between N2 and O2

 radicals. The key variables 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 formation are

N2 + O →← NO + N (1)

followed by

N + O2 →← NO + O (2)

N + OH →← NO + H (3)

Because of the high activation energy required for Reaction (1), thermal NOx

 formation does not become significant until

flame temperatures reach 1,100°C

(2,000°F). Kinetic calculations (assuming

30% excess air, average MSW properties, and residence time of 0.5 second)

predict thermal NOX concentrations of <10

ppm in MWCs. However, local flame temperatures may exceed 1,100°C and thermal NOX concentrations may be greater

than these calculated model results.

Examination of MWC operating conditions suggests that most of the NOX emitted 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, however, actual emissions are generally between 220 and 320 ppm at 7% O2, indicating that perhaps 10-30% of the fuel N2 is

converted to NOX, with most of the remainder 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 measured NOX

 levels were attributable to seasonal fluctuations in MSW composition.

This evaluation was based on NOX compliance 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 concentrations versus time of year for a number 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 continuous emission monitor data from 11

MWCs located in the northeast U.S. found

no seasonal variations in NOX concentrations. 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 concentrations from these units varied from

* Unless otherwise noted, all NOx concentrations used in

this summary are corrected to 7% O2

 and are on a dry

basis.
3

140 to 310 ppm but did not show any

consistent relationship between NOX concentrations 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, rubber, and footwear) in the MSW fired during 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 concentrations 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) between NOX concentrations and MSW characteristics. 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 combustor 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. Combustion controls limit the formation of NOX

during the combustion process by reducing the availability of O2

 within the flame

and lowering combustion zone temperatures. These technologies include staged

combustion, low excess air, flue gas recirculation (FGR), and NGI. Staged combustion and low excess air reduce the flow of

undergrate air in order to reduce O2 availability in the combustion zone. Another

option is FGR in which a portion of the

combustor exhaust is returned to the combustion air supply to both lower combustion zone O2

 and suppress flame temperatures 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 modern MWCs. Test data for these techniques

indicate that they can reduce NOX concentrations by 10-30% compared to

baseline levels from the same units. For

NGI, two processes have been developed:

(1) Methane de-NOX

SM uses gas injection

to inhibit NOx

 formation and (2) reburning

uses gas injection to create reducing conditions that convert NOX formed in the

primary combustion zone to N2

.

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

TM, which is based

on ammonia (NH3

) injection, NOXOUTTM,

which uses urea injection, and the addition of urea followed by methanol. Thermal 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 literature and contacts with technology vendor and MWC industry personnel. For each

technology, information was collected on

the key process variables; commercial applications in Europe, Japan, and the

U.S.; recent research and development

activities; and costs. In addition to the

current versions of NGI and SNCR, advanced concepts for the two technologies

were examined, including advanced NGI

(which combines conventional NGI and

SNCR) and advanced SNCR (which employs additional process control equipment

to enhance conventional SNCR performance). This information was then used

to develop a series of computer-based

spreadsheets designed to maintain basic

material and energy balances for the technology 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 levels** for each NOX control technology as

a function of MWC size and a second key

technology variable. An example of this

output, based on conventional SNCR, is

shown in Table 1. For example, the estimated 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 processed, and the cost effectiveness is approximately $1,270 per ton of NOX removed from the flue gas.

The second output format is a graph

showing the sensitivity of tipping fee impact and cost effectiveness to key process variables. An example of this output,

also based on conventional SNCR, is presented in Figure 1. For example, the process 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 effectiveness 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 injection to inhibit NOX formation and appears capable of reducing NOX emissions

from MWCs by approximately 60%. The

second approach, reburning, uses gas injection to create reducing conditions that

convert NOX formed in the primary combustion zone to N2. Because of the relatively 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 developed and demonstrated to be considered 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 processes near the upper end of their performance range can result in unwanted emissions of unreacted NH3

 or other by-product 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

**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 processed. 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.
4

Table 1. Model Plant Cost Estimates for Conventional SNCR a

Total Capital Cost Tipping Fee Impact Cost Effectiveness

($1000/TPD Capacity) ($/ton MSW) ($/ton NOx)

NOx Reduction (%) 45 60 65 45 60 65 45 60 65

100 TPD Mass Burn MWC 5.05 5.06 5.07 3.74 3.91 4.06 4,308 3,378 3,235

400 TPD Mass Burn MWC 1.97 1.98 2.00 1.30 1.47 1.62 1,496 1,268 1,289

750 TPD Mass Burn MWC 1.49 1.50 1.52 0.92 1.09 1.24 1,058 940 986

a$/ton can be converted to $/Mg by multiplying by 1.1.

SNCR. Combining NGI and SNCR to

achieve an overall NOX reduction of 80 -

85% may be feasible but will require testing to evaluate the interactions between

the temperature and residence time requirements 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. Therefore, under this scenario, the rate of waste

fired must be reduced by an amount comparable 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 combines conventional NGI with conventional

SNCR to achieve a NOX reduction of 80%.

The SCR evaluation focused on a coldside system with a NOX reduction level of

Figure 1. Effect of unit size, reagent cost, and annualized capital on tipping fee impact and cost effectiveness for conventional SNCR.

4

3.5

3

2.5

2

1.5

1

Reference MWC Parameters

Uncontrolled NOx = 250 ppm

NOx Reduction = 60%

3,455

3,023

2,591

2,160

1,728

1,296

864

Cost Effectiveness ($/ton of NOx)

Tipping Fee Impact ($/ton of MSW)

Unit Size (tpd) 100 200 300 400 500 600 700

Reagent Cost ($/ton) 210 240 270 300 330 360 390

Annualized Capital ($1000/yr) 52 60 67 75 82 90 97

Unit Size Reagent Cost Annualized Capital
5

Table 2. Technical Overview of Evaluated NOX Control Technologies

Technology Commercial Status NOX Control Performance Technical Issues

NGI: Tested on MWC in Olmsted Testing achieved up to 60% Scaleup of technology to larger

Methane County, Minnesota. NOx reduction without furnaces (> 100 tpd).

deNOX

SM increasing CO emissions.

NGI: Applied to fossil fuel boilers; MWC testing encountered high Reburn zone temperatures in

Reburning use on MWCs limited to test CO levels when NOX reductions MWCs may be too low for NOX

program in Malmo, Sweden. exceeded 30 - 40%. reduction reactions.

SNCR: Applied to six MWCs in U.S. Achieved short-term reductions Impact of furnace temperature

Thermal plus others overseas. of 45 - 75%, depending on NH3 swings on NOX and NH3

DeNOX

TM injection rate. Plume visible at emissions. Control of NH3

 slip

higher reduction levels. and visible plume.

SNCR: Applied to two MWCs in U.S. Comparable to NH3 injection. Similar to NH3 injection. N2O

NOXOUTTM plus others overseas. emissions also of concern.

Advanced Tested at MWCs in Lancaster, Achieved 60 - 75% NOX Demonstration of long-term

SNCR Pennsylvania, and Munich, reduction with less reagent than performance capability.

(improved Germany. is needed with conventional Additional process controls

process SNCR. Plume visible at higher may benefit other combustor

control) reduction levels. operations.

Advanced NGI Concept only. Potential for 80% reduction. Interaction of temperature and

(conventional residence time needs for each

NGI plus technology.

SNCR)

SCR Installed at 20 MWC plants in 80 - 90% NOX reduction. Catalyst life in hot-side systems.

Europe and Japan.

80%. A limited evaluation of a hot-side

SCR system was also conducted.

Costs for the different control technologies 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 reduction 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 capital intensive of the technologies, costing 4

to 5 times more than the next highest

technology. Advanced SNCR and advanced 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 conventional SNCR are comparable, at less

than $1 million per combustor.

The tipping fee impacts and cost effectiveness values presented in Figures 3 and

4, respectively, include both annualized

capital costs and operating and maintenance costs. Conventional SNCR generally has the lowest costs of the technologies. 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 NGI100 results in much higher costs for this

control technique. These revenues are not

lost with the NGI-85 scenario, plus revenues are received under this scenario

from sale of additional electrical production. The other key variable affecting both

NGI scenarios is the price of natural gas.

The average gas price used in these figures 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 replacement 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.
6

Figure 3. Comparison of tipping fee impact.

12

10

8

6

4

0

2

100 400 750

Unit Size (tpd)

Capital Cost ($)

(Millions)

NGI-100 60%

Advanced SNCR 60% Advanced NGI 80%

NGI-85 60% Conventional SNCR 60%

SCR 80%

Figure 2. Comparison of capital cost.

25

20

15

10

5

0

100 400 750

Tipping Fee Impact ($/ton MSW)

Unit Size (tpd)

NGI-100 60% NGI-85 60% Conventional SNCR 60%

Advanced SNCR 60% Advanced NGI 80% SCR 80%
7

Figure 4. Comparison of cost effectiveness. Cost Effectiveness ($/ton NOx)

Conventional SNCR 60%

18

0

100 400 750

Unit Size (tpd)

NGI-85 60%

Advanced SNCR 60% Advanced NGI 80% SCR 80%

NGI-100 60%

(Thousands)

16

14

12

10

8

6

2

4
8

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

EPA/600/SR-94/208

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT NO. G-35