United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-93/188 December 1993
EPA Project Summary
Evaluation of Simultaneous
SO2/NOX Control Technology
Kevin R. Bruce and Walter F. Hansen
The Clean Air Act Amendments of
1990 (CAAA) have led to accelerated
research into novel sulfur dioxide (SO2)
and nitrogen oxide (NO) control tech-
nologies for coal-fired industrial boil-
ers. One of these technologies com-
bines sorbent injection and selective
non-catalytic reduction for simultaneous
SO2/NOx removal. The work presented
herein concentrated on characterizing
three process operational parameters
of this technology: injection tempera-
ture, sorbent type, and reductant/pol-
lutant stoichiometric ratio. A slurry com-
posed of a urea-based solution
(NOxOUT A or NOxOUT A+) and various
calcium-(Ca-) based sorbents was in-
jected at a range of temperatures and
reactant/pollutant stoichiometries in a
natural-gas-fired, pilot-scale reactor with
doped pollutants.. Up to 80% reduction
of SO2 and NOx at reactant/pollutant
stoichiometric ratios of 2 and 1.5, re-
spectively, was achieved. SO2 emission
reductions from slurry injection were
enhanced moderately when compared
with dry sorbent injection methods, pos-
sibly caused by sorbent fracturing to
smaller, more reactive particles. Emis-
sions from ammonia (NH3) slip
(unreacted nitrogen-based reducing
agent) and nitrous oxide (N2O) forma-
tion were reduced in comparison with
other published results, while similar
NOx reductions were obtained. In-
creased carbon monoxide (CO) emis-
sions, caused by the decomposition of
urea, were moderate. Emissions of CO,
NH3, and NO for the enhanced urea
solution (NoxOUT A+) were substan-
tially less than the levels observed dur-
ing urea (NOXOUT A) injection. The in-
jection of the urea-based solution en-
hanced SO removal, probably because
of the formation of (NH^CAfSOJ, H2O.
The results of this pilot-scale study
have shown high reduction of both SO,
and NO.
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
The project work reported here was ini-
tiated through a Cooperative Research and
Development Agreement (CRADA) be-
tween EPA and Nalco Fuel Tech, a com-
mercial licensor of a urea-based reducing
agent injection technology for NOx reduc-
tion.
Experimental testing of Nalco Fuel
Tech's urea-based NOxOUT A and
NOXOUT A+ reducing agents for NOx con-
trol, in combination with Ca-based sorbent
injection for SO2 control, was conducted
from June to November 1991. Testing was
performed at EPA's Environmental Re-
search Center in Research Triangle Park,
NC, in a pollutant-doped, natural-gas-fired
50,000 Btu/h (15 kW) furnace.
The project scope of work included test-
ing furnace sorbent injection of several
Ca-based sorbents to remove SO2 from
flue gas. The tested sorbents came from
a single source of commercially prepared
slaked lime [Ca(OH) ], limestone (CaCO3),
and quicklime (CaO). A comparison of
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SO2 removal efficiency was made between
dry and slurry injection. The effect of
CaCO3 sorbent particle size was also stud-
ied.
Slurry sorbent injection was found to be
superior to dry injection for SO2 removal.
Dry injection of Ca(OH)2 achieved a maxi-
mum of 60% SO removal (at a Ca/S ratio
= 2), while the Ca(OH)2 slurry removed
72%. Removal efficiency with Ca(OH) was
superior to that with CaCO3 in both dry
(43%) and slurry (58%) testing. CaO was
tested in slurry form by slaking to form
Ca(OH)2 slurry, and compared to the com-
mercially prepared Ca(OH)2. The slaked
CaO proved identical in its SO2 removal
performance to the commercially prepared
Ca(OH)2.
Both NOXOUT A and NO OUT A+
achieved maximum NOx removal when in-
jected at a temperature of about 1,100 °C.
Almost no difference in the two reducing
agents existed at the optimum tempera-
ture; approximately 80% NOx removal was
observed for both reducing agents. At tem-
peratures higher than the optimum, NOx
removal efficiency dropped quickly. At
about 1,170°C, the reducing agents be-
gan producing NOx, caused probably by
high temperature oxidation of the NH3 pro-
duced by urea decomposition. At tempera-
tures lower than the optimum, NOx re-
moval efficiency gradually decreased!!
Varying the molar ratb of reducing agent
(urea) to baseline NOx, or N/NOx|( showed
that increasing N/NO^ to a value of near 2
produced significant improvement in NOx
removal. Further N/NOx| increases had little
or no effect on removal efficiency.
The work mentioned above also en-
tailed characterizing NH3, N2O, and CO
emissions produced by injecting the re-
ducing agent over a range of tempera-
tures and N/NOxl. Each reducing agent
produced maximum NH3 slip (unreacted
nitrogen-based reducing agent) at the
lower injection temperatures; around
821 °C, the amount of slip was about 140
ppm for both NOXOUT A and NOXOUT A+.
As injection temperature increased, NH3
slip for the NOXOUT A decreased quickly,
while slip from NOXOUT A+ dropped off
almost completely at around 875 °C. At
900-1,000 °C, slip generated by NOxOUT
A gradually decreased to a level of about
60 ppm. Throughout this temperature re-
K. Bruce and W. Hansen are with Acurex Environmental Corp., Research Triangle
Park, NC 27709.
Brian K. Gullett is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Simultaneous SO2/NOx Control Tech-
nology," (Order No. PB94-114741/AS; 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
gime, NOXOUT A+ produced negligible NH3
slip (<10 ppm).
CO emissions produced by NOxOUT A
rose gradually from about 20 ppm at
800 °C to a maximum of 25 ppm at
1,100 °C. NOXOUT A+ produced low CO
at 800-1,000 °C (<10 ppm), with a maxi-
mum of 50 ppm around 1,100 °C.
NO production by NOxOUT A was neg-
ligible at lower injection temperatures (ap-
proximately 25 ppm), but increased with
injection temperature to a maximum of
200 ppm at approximately 1,150 °C, about
42% of the NOx reduced. NOXOUT A+
produced only moderate levelsx of N2O
(typically <40 ppm, less than 20% of the
NOx reduced) over the entire temperature
range. A maximum of about 30 ppm was
observed at around 1,150 °C.
Aqueous ammonia solution was injected
to ensure that these results would be re-
producible on other facilities. Available data
for NOx removal using aqueous ammonia
injection showed comparable results to
others' work. These data validated improve-
ments shown by this work with both
NOXOUT solutions and suggested the ap-
plicability of these results to other facilities.
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