v-xEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
-Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-030 Aug. 1981
Project Summary
i
*
Selective Catalytic
Reduction and NO,
Control in Japan
Gary D. Jones
A four-member study team traveled
in Japan during March 1980 to assess
NO, flue gas treatment technology
and related areas. The overall goat of
the study was to obtain new informa-
tion on current issues concerning
application of this technology and
update information previously pub-
lished. A total of 28 equipment vendors,
process operators, governmental
agencies, and industry groups were
contacted. There has been substantial
progress recently with regard to com-
mercial applications of selective cata-
lytic reduction (SCR) technology to
gas- and oil-fired boilers. In several
applications, SCR systems are oper-
ated continuously and are successfully
removing 80 percent of NOx from the
flue gas stream. Current development
and demonstration efforts are aimed
at applying SCR technology to coal-
fired boilers since that fraction of
Japan's total electric power genera-
tion is expected to increase to 12.5
percent in 1995 and since most of the
new, coal-fired boilers will use flue gas
treatment technology for NOX control.
Four SCR systems on coal-fired boilers
are scheduled to start up in 1980 and
81. Thus, the Japanese activity in the
NO* control field should provide valu-
able information to interested parties
in the U.S. in the next 4 years.
This Project Summary was devel-
oped by EPA's Industrial Environmen-
tal 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
The Environmental Protection Agency
is investigating methods of controlling
nitrogen oxides (NO*) emissions from
fossil-fuel-fired utility and industrial
boilers since these sources comprise a
major source of NOx emissions. Coal-
fired boilers are of particular interest
since they emit the highest concentra-
tions of NOx. Currently, NOX emissions
are reduced by the use of special "Low
NOx" burners and other combustion
controls. Another technique, developed
primarily in Japan, treats the flue gas to
achieve 80-90 percent NOx removal.
This technique is usually termed selec-
tive catalytic reduction (SCR) or catalytic
de-NOx since it involves injecting am-
monia (NH3) into the flue gas prior to a
catalytic reactor which reduces NOx to
N2 according to the following reactions:
4NO + 4NH3 + 02 - 4N2 + 6H20
2N02 + 4NH3 + 02 - 3N2 + 6H2O
SCR has been widely applied com-
mercially in Japan to several oil- and
gas-fired boilers. Coal-fired applications
will soon be on-line. However, because
this technology has not been applied in
the U.S., there are several unanswered
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questions regarding the application of
SCR technology to coal-fired boilers. A
study team consisting of J.David Mobley
of EPA/IERL-RTP, Jumpei Andoof Chuo
University, Gary D. Jones of Radian
Corporation, and J. Douglas Maxwell of
TVA, travelled in Japan during March
1980 to develop answers to these
questions. Information was gathered
from a wide variety of sources. Equip-
ment vendors included those of SCR
processes, SCR catalysts, other N0»
control processes, air preheaters, and
instruments. The variety of installations
inspected included utility and industrial
boilers firing gas, oil, or coal. Govern-
mental groups on both the national and
local level were contacted as well as
industry groups.
The results of the study can be broken
down into four primary subject areas:
SCR process design, application of SCR,
SCR installations, and additional aspects
of SCR.
SCR Process Design
Catalyst
The catalyst is the "heart" of an SCR
system and, consequently, is the subject
of many design considerations. Spe-
cially shaped catalysts that avoid being
plugged by fly ash have been developed
for use with oil- and coal-fired boilers.
The shapes are typically classified as
honeycomb, plate, or pipe. For gas-fired
applications, the conventional pellet
shape is preferred.
The same basic catalyst material is
used by all SCR vendors and consists of
TiOz and VzO5 as major components.
Each vendor also includes some addi-
tional proprietary compounds to affect
special properties, such as the SOz
oxidation potential. The catalyst ele-
ments can be supplied in three forms:
composed solely of catalyst material
(homogeneous), or composed of either a
ceramic or metallic substrate that is
coated with catalytic material. There is
no agreement on which type is best;
each offers unique advantages. Of the
systems currently under construction
on coal-fired boilers, two are using a
homogeneous catalyst and two are
using a coated catalyst.
One aspect of using V205 as an SCR
catalyst is that it will also catalyze the
oxidation of S02 to S03; with a V205-rich
catalyst, the conversion can be as high
as 5 percent. The performance of one
formulation can be shown as a function
of temperature; however, note that
other factors also affect the rate of S02
oxidation. High SO3 concentrations can
cause problems due to ammonium
bisulfate (NH450°C, the catalyst particles will
undergo sintering and the catalyst
activity will be reduced. Low tempera-
tures, <300°C, can allow NH4HS04 to
form on the catalyst if sufficient SO3 is
present. Temperatures <100°C can
allow water deposition which can per-
manently reduce the catalyst activity.
The desired operating temperature
range is 350 to 400°C: for some applica-
tions a temperature control system is
recommended by the vendor to maintain
a minimum temperature.
/VOX Removal
NOx removals of 90 percent and
higher are potentially obtainable with
SCR systems, although most systems
installed in Japan usually operate at 80
percent removal. The vendors indicate
that 90 percent removal can be achieved,
but will cost more since more catalyst
and a higher NH3:NOx mole ratio will be
necessary.
NH3 Emissions
Increasing MOx removal also has the
effect of increasing NH3 emissions.
Keeping the NH3 emissions low at 90
percent NO. removal levels will require
either additional catalyst or some other
process for NH3 removal. To go from 80
to 90 percent NOx removal without
increasing NH3 emissions is estimated
to require about 30 percent additional
catalyst. The process vendors indicate
that at either 80 or 90 percent NOx
removal, slip or unreacted NH3 can be
limited to <10 ppm.
Costs
The costs quoted for SCR systems by
all of the vendors are similar. Investment
costs for a typical 80 percent control
system applied to an oil-fired boiler are
about 3000 to 4000 yen/kW ($12 to
$16/kW). A similar system applied to a
coal-fired boiler would be about ¥=6000
to 7000 yen/kW ($24 to $28/kW).
These figures are for new units. The
cost of SCR systems installed on existing
boilers can be substantially higher. No
general figures are available for retrofit
systems due to the high variability
between individual installations.
One vendor indicated that investment
costs for a 90 percent NOx removal
system are 20 to 25 percent higher than
those for an 80 percent system applied
to an oil-fired boiler if increased NH3
emissions are allowable. For 90 percent
NOX control with low NH3 emissions, the
costs will be 40 to 50 percent greater,
rather than 20 to 25 percent.
Application of SCR
Flue Gas Processing
Considerations
When installing a boiler which in-
cludes an SCR system (or retrofitting an
SCR system to an existing unit), it is
necessary to consider the flue gas
processing system as a whole since the
location of equipment relative to other
pieces can have significant downstream
impacts. The temperature requirements
of the NOx reduction reactions are 370
±~50°C, and economics limit the loca-
tion of the reactor to upstream of the air
preheater. Temperature regulation may
be necessary to limit temperature ex-
cursions outside this range. Where
required, temperature control can be
obtained by either load regulation or
bypassing hot floe gas around the
economizer and mixing it with the main
gas stream.
In Japan, the particulate control
devices are almost always electrostatic
precipitators (ESP), installed either
upstream of the SCR system or down-
stream of the air preheater. This option
results in the two basic arrangements of
flue gas processing equipment, shown
in Figure 1.
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Air
Boiler SCR „ , ,
Preheater
Arrangement A. Cold-Side Paniculate Removal — Example: Chugoku Electric Shimonoseki Power-station
Stack
Boiler
Air
Preheater
FGD
Stack
Arrangement B. Hot-Side Paniculate Removal — Example: Electric Power Development Company Takehara
Power Station
Figure 1. Basic equipment layout options for processing flue gas from a coal-fired boiler.
In Japan, hot-side ESP's will be used
with most of the SCR systems currently
under construction on coal-fired boilers.
There are several reasons for this: one is
that, when these systems were designed
several years ago, SCR catalysts had not
been adequately proven with high ash
loadings. Other equally important rea-
sons are that cold-side ESP's are not
effective with the low sulfur coal some-
times received in Japan, and the concern
that NH3 compounds in the collected fly
ash will affect by-product utilization.
However, it is anticipated that many
SCR systems currently being designed
will utilize cold-side ESP's.
Downstream Impacts and
Countermeasures
The SCR system can potentially im-
pact the air preheater, ESP, and FGD
system. The most important of these is
the air preheater. Several plants have
reported problems with air preheater
plugging and corrosion by NhUHSCU,
product of a condensation reaction
between NH3, SOs, and HjO: tn many
cases, more rigorous soot blowing
procedures have controWed the plugging
to the extent that water washing the air
preheater is only required once per year
during a scheduled boiler outage. In
other cases, however, water washing is
necessary every few months, even with
NHa and SOa concentrations of <4 ppm
and a temperature of 140°C. The plug-
ging occurs at the interface between the
intermediate- and cold-temperature
elements where the temperature is cool
and soot blowing is least effective.
Where more severe problems are
anticipated, such as after a hot-side
ESP, a modified air preheater design
will be used. This design consists of
combining the intermediate and cold
sections into a single element which
permits more effective soot blowing
which, for the new design, is done from
both the hot and cold sides. Also, the
large element is made of corrosion
resistant, coated material similar to
typical cotd-end elements.
The NH3 emitted by SCR systems is
not expected to adversely impact the
operation of a cold-side ESP; on oil-fired
boilers in Japan, NH3 is often injected
upstream of these units for ash condi-
tioning. However, the NH3 can affect the
fly ash utilization in by-products by
depositing on the surface of the fly ash.
The impact of SCR systems on baghouse
efficiency is not known in Japan where
it is being investigated at two pilot units.
Systems in which a limestone FGD
unit exists downstream of an SCR unit
will be operational in the near future in
Japan. There are no adverse effects of
SCR systems anticipated on the perfor-
mance of-downstream limestone/gyp-
sum FGD systems. The FGD system will
absorb NH3; however*, reliability and
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SOa removal should not be affected.
Environmental Impacts
SCR systems on gas- and oil-fired
boilers in Japan emit NH3; however,
since the concentrations are 3-10 ppm,
it is not considered to be a serious air
quality impact. Ammonia emissions
from full-scale, coal-fired applications
have not been quantified; however,
downstream equipment will probably
mitigate any air impacts.
Visible plumes of ammonium sulfite
have been known tooccurduring certain
atmospheric conditions with NH3-based
FGD systems. Based on Japanese ex-
perience with situations in which 10
ppm of NH3 enters the FGD system,
Japanese researchers do not feel that
SCR systems will cause visible plume
formation.
SCR systems can also have water
impacts resulting from: NHs in the
leachate from ash that has been used as
landfill, NH4HS04 in the air preheater
wash water, NH3 in FGD blowdown, and
ammonia compounds in the water used
to wash gypsum produced by an FGD
system. Where the concentrations are
high enough to cause water pollution
problems, an activated sludge water
treatment system may be necessary.
Solid waste from SCR systems consists
of spent catalyst material. A definite
disposal method has not yet been estab-
lished primarily because none of the
commercial SCR units have required a
catalyst change. This area requires
further investigation for U.S. applica-
tions.
SCR Installations
Extent of Application
SCR systems have been installed on
many gas- and oil-fired boilers and are
operating successfully. One application
to a coal-fired boiler and four others are
scheduled to startup within a year.
With all of the coal-fired applications,
the catalyst and reactor are designed in
a vertical downflow arrangement to
minimize the potential for plugging by
fly ash. Soot blowers are installed on
some units above the beds to control
ash buildup. These are installed as a
conservative design measure in case
plugging develops; however, they are
not expected to be necessary. There are
up to four beds within the reactor
consisting of one or two layers of
catalyst modules, each of which corf-
tains many catalyst elements.
In some cases a dummy layer of
catalyst modules is installed at the
reactor inlet to prevent abrasion of the
catalyst surface.
Operation and Maintenance
The labor requirements of the opera-
ting full-scale systems are small. No
additional operating personnel are
required and maintenance labor consists
primarily of NH3 and catalyst loading,
and cleaning the air preheater during
the annual outage. Since there have
been no catalyst changes to date, the
labor estimates for this work vary widely.
Operators indicate that the SCR proc-
esses themselves are very reliable,
essentially 100 percent. However, in
some cases, a boiler shutdown has been
necessary where air preheater plugging
has occurred. In most cases, steps have
been taken to reduce the plugging rate
to the extent that cleaning is only
required during normal boiler outages.,
Additional Aspects of SCR
Government Agencies
Environmental regulation in Japan is
similar to that in the US in that a federal
agency establishes the minimum re-
quirements for the country as a whole.
Local governments have the option of
adopti ng the federa I sta nda rds or esta b-
lishing their own, more stringent stan-
dards. The Japan Environment Agency
recently modified standards for ambient
air quality and established three con-
centration ranges based on a daily
average sample. Above 0.06 ppm NC>2,
countermeasures must be undertaken
to reduce the level to 0.06. Within the
range 0.04 to 0.06 ppm NO2, no signifi-
cant change is allowed; areas below
0.04 ppm NC>2 are allowed to deteriorate
to 0.04 ppm. The Environment Agency
also sets emission standards for point
sources. Emission limits for new sources
are shown in Table 1.
Table 1. National Emission Limits for
The local governments, city and pre-
fectural, have required many of the SCR
applications. This occurs primarily when
the plant owners negotiate with the
local governments over a plant siting or
expansion. Citizen groups appear to
have a significant voice in these deliber-
ations. The cooperative spirit which
exists between the local citizens, the
plant personnel, and the government
provides the driving force for the plant's
compliance with the negotiated emis-
sion limits and the successful imple-
mentation of new control technology.
Further, the local governments utilize
sophisticated air quality monitoring.
systems to ensure that the public is
protected and that regulations are
complied with.
Industry Groups
The two industry groups contacted
are looking to the future when coal will
be a much more significant energy
source. Coal-fired boiler capacity is
expected to increase from 4410 MW in
1978 to about 34,500 MW by 1995 and
NOX control will probably be required on
all of these. They feel that combustion
modifications and SCR have been suc-
cessfully demonstrated on gas- and oil-
fired boilers and that these techniques
will also be useful on coal-fired units.
Instrument Design
The application of SCR systems in
Japan has necessitated the development
of instruments capable of measuring
NOx and NH3. Coal-fired applications
present the most difficult situation for
the instruments since the flue gas
contains SO2, SO3, and fly ash. Chemi-
luminescent monitors, most commonly
used for NO« measurement, apparently
operate satisfactorily with coal-fired
flue gas.
Usually the NH3 monitor is a chemi-
luminescent NO monitor with an up-
New Sources
Fuel
Gas
Oil
Coal
Unit Size
WOO Nnf/hr
>500
40 - 500
10-40
<10
>500
10 - 500
<10
All Sizes
Oz Concentration
%
5
5
5
5
4
4
4
6
/Vox
Emission Limit
ppm
60
100
130
150
130
150
180
400
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stream catalytic converter which con-
verts NH3 to NO. Sulfur oxides apparently
interfere with the monitoring equipment
and there have been additional prob-
lems with the converters giving incom-
plete NO conversions. An alternative
NH3 emissions monitor for coal-fired
flue gas has not been perfected; how-
ever, several techniques are being
developed.
Conclusions
SCR has been applied to many gas-
and oil-fired boilers, both utility and
industrial, and these are operating
successfully. Full-scale tests on coal-
fired utility boilers are just beginning;
several units will be on-line in the next
year or two. These full-scale units will
be incorporating some new design
features developed specifically to deal
with coal-related problems such as air
preheater plugging and catalyst erosion.
Gary D. Jones is with Radian Corporation, 8501 Mo-Pac Blvd., A ustin, TX 78759.
J. David Mobley is the EPA Project Officer (see below).
The complete report, entitled "Selective Catalytic Reduction and NO > Control in
Japan," (Order No. PB 81-191 116; Cost: $20.00, 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
ir U S GOVERNMENT PRINTING OFFICE, 1981 - 757-012/7256
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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