Energy and Power En gi neering, 2011, 3, 317-324
doi:10.4236/epe.2011.33039 Published Online July 2011 (http://www.SciRP.org/journal/epe)
Control of an Industrial SCR Catalyst Using
Ceramic NOx Sensors
Joshua Schmitt, Daniel B. Olsen
Engines and Energy Conversion Laboratory, Colorado State University, Fort Collins, USA
Received June 15, 2011; revised July 8, 2011; accepted July 13, 2011
Abstract
Selective Catalytic Reduction (SCR) catalysts respond slowly to transient inputs, which is troublesome when
designing ammonia feed controllers. An experimental SCR test apparatus installed on a slipstream of a Coo-
per-Bessemer GMV-4, 2-stroke cycle natural gas engine is utilized. Ammonia (NH3) feed rate control algo-
rithm development is carried out. Two control algorithms are evaluated: a feed forward control algorithm,
using a pre ammonia injection ceramic NOx sensor and a feed forward plus feedback control algorithm, using
a pre ammonia injection ceramic NOx sensor and post catalyst ceramic NOx sensor to generate feedback sig-
nals. The feed forward algorithm controls to constant user input NH3/NOx molar ratio. The data show the
lack of pressure compensation on the ceramic NOx sensors cause errors in feed forward NOx readings, re-
sulting in sub optimal ammonia feed. The feedback system minimizes the post catalyst ceramic NOx sensor
signal by adjusting the NH3/NOx molar ratio. The NOx sensors respond to ammonia + NOx; therefore, the
feed forward plus feedback algorithm minimizes the sum of NOx emissions and ammonia slip. Successful
application of the feedback control minimization technique is demonstrated with feedback periods of 15 and
5 minutes with molar ratio step sizes of 5% and 2.5%, respectively.
Keywords: SCR, Selective Catalytic Reduction, Ammonia Injection, NOx Emissions, NOx Sensor,
Minimization Algorithm
1. Introduction
Selective Catalytic Reduction (SCR) is an aftertreatment
technique for reduction of oxides of nitrogen (NOx) from
the exhaust from combustion devices. SCR requires a
reagent be blended with exhaust upstream of the SCR
catalyst, which then reacts with NOx across the SCR
catalyst. The reagent is typically ammonia (NH3) or a
chemical that breaks down to form ammonia, such as
urea. The reagent feed rate must be precisely controlled
to achieve high efficiency NOx reduction, while limiting
ammonia slip [1]. Reagent feed rate control techniques
have been studied to improve SCR performance. In the
case of mobile applications, the high level of transients
requires fast feedback response. In the case of stationary
engine applications operating condition changes are
slower. Control techniques for slow, stationary applica-
tions are developed in this work. Feedforward algorithms
are used to follow basic system transitions. Feedback
algorithms are used to compensate for feedforward errors,
such as sensor drift and ammonia injector nozzle clog-
ging.
Schär et al. [2] tested feedback and feedforward algo-
rithms. In that work four feedforward techniques were
implemented. The algorithms were tested in a manner
that required much faster response than tests described in
this paper. Schär et al. [2] used a feedback signal gener-
ated with a ceramic NOx sensor. Ammonia interferes
with ceramic NOx sensors. Ceramic NOx sensors respond
approximately as is shown in Equation (1) [2],
x
CNS NONH
RC 0.65C
3
(1)
where RCNS is the ceramic NOx sensor reading and x
NO
and 3
NH are concentrations of NOx and ammonia, re-
spectively. This is important in SCR feedback applica-
tions because both ammonia and NOx are present post
catalyst. As a result, it is difficult to determine whether
the sensor is responding to NOx or ammonia.
C
C
In this work we experimentally explore feedforward
and feedback SCR control algorithms on an SCR system
for industrial reciprocating natural gas engines. The
feedforward approach utilizes a pre catalyst NOx sensor
measurement to set the ammonia flowrate, given a con-

J. SCHMITT ET AL. 318
stant NH3/NOx set point. In the feedback algorithm a
second NOx sensor is used post catalyst. The feedback
approach used is a new approach to SCR control. The
post catalyst NOx sensor signal is minimized to deter-
mine the optimal NH3/NOx ratio, which is then used to
set the ammonia feed rate. A catalyst slipstream is used
for the experimental evaluation, integrated with a Coo-
per-Bessemer GMV-4 large bore natural gas engine.
2. Experimental Setup
Figure 1 shows the exhaust flow schematic. Shown is
each component in the slipstream that will be discussed
in order of its respective position on the slipstream. The
gas flow in Figure 1 starts at the engine, flows as indi-
cated by the arrows, and ends where exhaust is emitted
into the atmosphere. The SCR slipstream removes a
small portion of exhaust from each of the four exhaust
elbows, conditions it, directs it through the SCR catalyst,
then reconnects with the main exhaust. Exhaust condi-
tioning is done through temperature, exhaust flow, and
reagent concentration control. Conditioned exhaust flows
into the catalyst material where NOx and ammonia are
catalytically reduced. After passing through the catalyst
and through a flow measurement orifice, the slipstream
gas is recombined with the main exhaust stream.
2.1. Engine
The test engine is a Cooper Bessemer GMV-4TF,
four-cylinder, two stroke, lean-burn, natural gas, internal
combustion engine, rated at 440 bhp (330 kW). Engine
torque is controlled by a water brake dynamometer. Igni-
tion is performed using pre-combustion chambers. Intake
and exhaust pressure are controlled, which allows intake
boost to be adjusted from 3.5"Hg (11.8 kPa), to 23"Hg
(77.9 kPa) gauge. Exhaust backpressure was always set
at 2.5"Hg (8.46 kPa) less than intake pressure, and con-
trolled by a butterfly valve in the main exhaust stream.
Engine out NOx was controlled by varying boost at con-
stant load and speed, which varies trapped equivalence
ratio. Further description of the test engine is in [4] and
[5].
2.2. Slipstream
Figure 2 is a photo of the SCR slipstream. The slip-
stream was designed to receive exhaust gas from the four
exhaust elbows, each of which corresponds to one of the
engine cylinders. Each elbow connected the exhaust port
of the cylinder to the main exhaust manifold. A heat ex-
changer controlled temperature of the exhaust gas and
the operating temperature of the catalyst. The heat ex-
changer was a cross flow type, in which compressed air
flowed across a finned tube bank. Temperature of down-
stream exhaust gas was controlled by varying flow rate
of compressed air through the heat exchanger. The heat
exchanger was able to control catalyst temperature be-
tween 450 and 600˚F (505 to 589 K). A butterfly valve
controlled exhaust flow rate through the slipstream. The
butterfly valve was located inside the slipstream pipe and
positioned by a Belimo AF24-SR actuator.
The aqueous ammonia injector was an air assisted type,
supplied by CPI International. The design used two con-
centric stainless steel tubes, one 1/8" (3.2 mm) diameter,
and the other 1/16" (1.6 mm) diameter. The smaller tube
had a calibrated crimp on its end. Compressed air flowed
through the smaller tube, and aqueous ammonia flowed
in the annulus. Aqueous ammonia was atomized by high
velocity air exiting the calibrated crimp. The aqueous
Figure 1. Exhaust flow schematic.

J. SCHMITT ET AL. 319
Figure 2. Picture of SCR slipstream.
y=1.1008x‐15.532
y=1.8303x‐25.564
0
100
200
300
400
500
600
700
800
900
1000
0100 200 300 400 500 600 700 800 9001000
CEMSNOx
[pp
m
]
SensorNOx[ppm]
PreammoniaInjection
PostammoniaInjection
Figure 3. Ceramic NOx sensor signal vs CEMS NOx in the
presence of ammonia and the absence of ammonia.
ammonia air assisted atomizer was mounted to an elbow
in the flow stream so that the atomizer could spray in the
same direction as the exhaust flow, without modifying or
bending the atomizer. A vane mixer was used to ensure
gaseous homogeneity. The mixer was placed between the
ammonia injector and the catalyst. Experimental and
CFD analyses were done by Ivaturi [6] to quantify re-
agent mixing.
A commercial company provided the catalyst modules.
The cylindrical modules were 9" (22.9 cm) diameter by
5" (12.7 cm) long. The cylinders had 1/16" (1.6 mm)
square cross hatching, creating channels, or monoliths. A
vanadia-titania mixture coated the surface of the catalyst,
which catalyzed the chemical reactions between NOx and
ammonia. The exact composition of the washcoat on the
catalyst modules was unknown.
To measure slipstream exhaust flow, a 1.75" (4.45 cm)
diameter orifice, with a pressure measurement before and
after, was used. Differential pressure across the orifice,
static pressure at the orifice, and temperature at the ori-
fice were measured to calculate exhaust flow.
2.3. Emissions Measurement
Exhaust was sampled with averaging probes and flowed
through a heated sample line. The heated sample line,
temperature controlled to 230˚F (383 K), directed the
sample into a Rosemount Continuous Emissions Meas-
urement System (CEMS) and a Nicolet Magna Fourier
Transform Infra-Red (FTIR) spectrometer. Carbon diox-
ide (CO2), CO, oxygen (O2), total hydrocarbons (THC),
and NOx, were measured using five dedicated measure-
ment modules in the CEMS. The CEMS incorporates a
chiller that condensed water out of the sample, so all
measurements made by the CEMS analyzer were dry. An
FTIR spectrometer was used to measure ammonia, water,
and hydrogen cyanide. The FTIR spectrometer sampled
wet exhaust gas. For more details on the emissions
measurement equipment, see [7].
2.4. Data Acquisition and Control
Measurements were made using National Instruments
data acquisition systems and LabVIEW software. The
National Instruments hardware consisted of a compact
field point, cFP 2100 unit with: TC 120, AI 110, AO 200,
and DIO 550 input/output modules. A program written in
LabVIEW controlled basic functionality of the slip-
stream system, including catalyst temperature, sample
line temperature, space velocity, and NH3/NOx molar
ratio. The LabVIEW program also read and recorded
basic system parameters.
During catalyst testing, ECM ceramic NOx sensors,
part number 06-01, were used to create feedforward and
feedback loops. The sensors were mounted to an O2 bung,
which was welded directly to the side of the slip stream
pipe. NOx sensors were connected to an ECM CANopen
NOx/O2 Module, which communicated via ModBus to an
ECM NOx 5210 module. The 5210 module communi-
cated with two NOx sensors at a time, and relayed the
signal, via 0 - 5 V analog, to a National Instruments
compact field point unit. The sensors detected NOx, O2,
and air/fuel ratio.
Figure 3 shows trends of ceramic NOx sensor readings
plotted against CEMS NOx readings, measured with a
Chemi-Luminescence Detector (CLD), of the same ex
haust gas. The first series plots the readings absent of
ammonia, while the second series plot is in the presence
of 0.85 NH3/NOx molar ratio. The ceramic NOx sensors
have a positive reaction to ammonia. Ceramic NOx sen-
sors have cross sensitivity to ammonia, and when tested,
sensitivity was 0.65 that of NOx [2]. This means that for
every 100 ppm of ammonia, the ceramic NOx sensor
returned a 65 ppm higher NOx concentration. In applica-
tion, sensitivity to ammonia does not affect feedforward
control, but is troublesome in feedback control. While
the feedforward ceramic NOx sensor can be placed
up-stream of ammonia injection, the feedback sensor is
always immersed in both ammonia and NOx. Therefore,

J. SCHMITT ET AL. 320
neither post catalyst NOx concentration nor post catalyst
ammonia concentration can be measured accurately us-
ing a ceramic NOx sensor.
Filtering the NOx sensor signal is necessary because
the sensor noise band was often greater than the slip-
stream NOx concentration, especially post-catalyst. The
noise band was typically 30 ppm, and post catalyst NOx
concentrations approached 5 ppm. The filter imple-
mented a least squares linear fit to the previous one min-
ute of data.
Calibration of the NOx sensor was performed using
exhaust gas and the CLD. Ammonia was first purged
from the slipstream. For one calibration point the engine
was operated at 100% load and low boost (large trapped
equivalence ratio), which yields higher NOx emissions.
For the other calibration point the engine was operated at
100% load and high boost (lower NOx level). Five min-
ute averaged points were used. The upper and lower span
concentrations of the pre-catalyst sensor were 314 ppm
and 52.8 ppm. These span values corresponded to 3.15 V
and 1.85 V, respectively. The post-catalyst NOx sensor
was spanned between 11.6 ppm and 52.8 ppm, corre-
sponding to 1.85 V and 2.83 V, respectively.
Post-catalyst NOx sensor 0-5V analog communication
to National Instruments equipment was set up to include
negative NOx concentrations. This was done because
when 0 V corresponds to 0 ppm NOx and the actual NOx
concentration is 5 ppm, noise fluctuations cause much
data to be lost through truncation of the 0 - 5 V analog
signal. The analog signal cannot communicate negative
voltage, so any part of the NOx sensor noise that is less
than zero results in a zero reading, which is incorrect.
Instead, 0 V was set to correspond to −50 ppm, so no
data was lost in analog communication at low NOx con-
centrations.
3. Results
The feedforward algorithm used a constant molar ratio
calculation. The ammonia injection rate is computed
from the exhaust flowrate, NOx concentration, and de-
sired NH3/NOx molar ratio. Figure 4 shows the feedback
control algorithm loop. The feedback algorithm used
feedforward calculations to create ammonia flow rate.
The feedback algorithm provided an updated NH3/NOx
molar ratio to the feedforward algorithm. The parameter
space velocity is used in this study. It is proportional to
exhaust flowrate and inverse residence time. Space ve-
locity is computed by dividing the standard volumetric
flowrate by catalyst envelop volume.
3.1. Feedforward Control Testing
To test the feedforward control algorithm, a set of tran-
Ex haust Flow
Rate
NOx 
Concentration
Ammoniato
NOxMolar
Ratio
Ammoni a
InjectionRate
Catalyst,
Re duc e s NOx
andAmmonia
CeramicNOx Sens o r,
Respondstothesumof
NOxandammonia
Fee dba c k
Sign a l
Signal
Minimization
Technique
Figure 4. Flow diagram of the feedba ck algorithm.
sient flow conditions was imposed. There were three
transitions: 1) at 1.0 hour is a step transition in which
space velocity, pre catalyst NOx, and catalyst tempera-
ture were increased from 7000 1/hr, 50 ppm, and 500˚F
[533 K] to 10,000 1/hr, 150 ppm, and 525˚F [547 K],
respectively; 2) at 1.5 hours, a step transition of space
velocity, pre-catalyst NOx, and catalyst temperature from
10,000 1/hr, 150 ppm, and 525˚F [547 K], to 13,000 1/hr,
200 ppm, and 550˚F [561 K] , respectively; and 3) a slow
transition, starting at hour two, in which space velocity,
pre-catalyst NOx, and catalyst temperature were reduced
from 13,0001/hr, 200 ppm, and 550˚F [561 K], to 7000
1/hr, 50 ppm, and 500˚F [533 K], linearly over the dura-
tion of two hours. This test map was designed to repre-
sent loading and unloading of an industrial, natural gas
engine. Figure 5 shows actual space velocity, tempera-
ture, and pre-catalyst NOx variables throughout the point.
Space velocity followed the two step inputs and the ramp
down closely throughout the point. This was because
space velocity was controlled by the slipstream, inde-
pendent of engine exhaust flow. Catalyst temperature did
not reach the objective due to slow heat exchanger re-
sponse and varying engine exhaust temperature from
NOx control adjustments. Temperature oscillated on the
ramp down, and did not stabilize at 500˚F [533 K] at the
end of the data point. NOx varied significantly from the
objective. NOx was adjusted manually by changing en-
gine boost, which changed trapped air/fuel ratio. The
transitions in Figure 5 are good representations of
in-field catalyst operation and provide a good test for the
feedforward algorithm.
Figure 6 shows the results of the feedforward control
test for the transients shown in Figure 5. When ammonia
feed was turned on, NOx reduction approached 60%, and
ammonia slip approached 2 - 3 ppm. This is because
ammonia feed rate was too low. Low ammonia feed rate
is an error that can be explained by ceramic NOx sensor
pressure compensation. Ceramic NOx sensors are sensi-
tive to pressure changes, but the sensors used in this ap-
plication were not pressure compensated. The sensors
were calibrated at 10,000 1/hr space velocity, and initial

J. SCHMITT ET AL.321
0
2000
4000
6000
8000
10000
12000
14000
012345
Time[hr]
SpaceVelocity[1/hr], PreCatalystNOx
[ppm x10]
500
510
520
530
540
550
560
570
580
590
600
CatalystTemperature[K]
CatalystTemperature
SpaceVelocity
PreCatalystNOx
Figure 5. Experimental feedforward parameters.
0
20
40
60
80
100
120
012345
Time
[hr]
NOxReduction[%],AmmoniaSlip[ppm]
NOxReduction
AmmoniaSlip
Figure 6. Result of feedforward test map.
startup was 7000 1/hr space velocity. Since the exhaust
flow control valve was upstream of the feedforward NOx
sensor, reduced space velocity caused reduced pressure
at the sensor location, resulting in reduced feedforward
ceramic NOx sensor readings. This caused a lean condi-
tion, in which not enough ammonia was injected. NOx
reduction was less than optimal, and ammonia slip was
low.
At hour one, space velocity, temperature, and NOx
were stepped up. NOx reduction increased to 80% after
an upward, then downward NOx reduction peak. The
downward peak was caused by slow ammonia injector
response, in which the ammonia to NOx ratio decreased,
because of slow ammonia injection response. Low am-
monia slip and 80% NOx reduction is representative of a
slightly lean condition.
When the second transition was made at 1.5 hr, space
velocity, temperature, and NOx inlet concentration in-
creased. After this transition, NOx reduction increased to
around 97%, followed by an ammonia slip spike about
30 min later. This ammonia slip spike is due to ceramic
NOx sensor pressure compensation. When space velocity
was increased to 13,000 1/hr, exhaust flow and pressure
were higher than that at which the sensor was calibrated,
causing a high NOx reading, and ammonia overfeed.
Thirty minutes later, an ammonia surge occurred. This is
because ammonia had been overfed for a half hour, dur-
ing which the catalyst became oversaturated with ammo-
nia. Subsequently excess adsorbed ammonia released and
ammonia slip remained high for about one hour before
slowly decreasing.
Space velocity, temperature, and NOx decreased
slowly and linearly during the third transition. At the
start of the downward ramp, ammonia was just starting
to spike from the ammonia overfeed, so NOx reduction
was high throughout the ramp. Ammonia slip slowly
decreased from the overfeed incident, and at about 3 hr
and 50 min the catalyst approached a lean condition.
NOx reduction and ammonia slip decreased, approaching
NOx reduction and ammonia slip of 80% and 2 - 3 ppm,
respectively.
Purely open loop, feedforward control is poor if ce-
ramic NOx sensors are used without pressure compensa-
tion. When using feedforward control, catalyst perform-
ance is only as good as the accuracy of the feedforward
sensors. In this case, without pressure compensation, the
NOx sensor is accurate within about 40%, and the ce-
ramic NOx sensor is the limiting factor in emissions re-
duction.
Adsorbed ammonia can build up and, when released,
can cause high ammonia slip for an hour or more. Am-
monia adsorption is extensive at these temperatures. The
catalyst adsorbs ammonia in the form of a wave propa-
gating from the front of the catalyst material, ending at
the back of the catalyst material. Because of this, ammo-
nia slip does not increase until the entire catalyst is satu-
rated. Once excessive ammonia begins to slip, ammonia
continues to slip until the catalyst is no longer saturated.
Ammonia desorption propagates through the catalyst
front to back, and the ammonia desorption wave must
propagate through the entire catalyst before ammonia
slip stabilizes.
When adequate ammonia is in the catalyst, the catalyst
does not transmit high frequency inputs. As stated by
Schär et al. [2], the SCR catalyst can act like a low pass
filter when proper ammonia is adsorbed in the catalyst.
Inadequate ammonia flow is indicated by high frequency
NOx concentration variation (peaks and valleys), and low
ammonia slip, which can be seen in the first hour of
catalyst operation, in Figure 6. NOx reduction increases
and decreases rapidly during the first two hours of testing
and, when the catalyst had adsorbed sufficient ammonia,
NOx reduction stabilized and high frequency peaks and
valleys disappeared.

J. SCHMITT ET AL. 322
3.2. Feedback Control Testing
Feedback algorithms, or closed loop control techniques,
are effective at compensating for long term calibration
errors. In this case, long term error can be caused by in-
accurate initial NOx sensor calibration or sensor drift.
The ceramic NOx sensor signal feedback algorithm
(Figure 4) was designed to correct these calibration er-
rors. Fast transient effects caused by engine load transi-
tions, space velocity transitions, NOx concentration
variation, and temperature changes, are handled by the
feedforward system. Most long term errors progress
slowly over hours or days, so the stabilization timeframe
of the feedback system should be able to compensate for
these errors over a few hours. Feedback testing was done
at steady state, and stabilization time was the focus.
If the ceramic NOx sensor responds proportionally to
the sum of ammonia and NOx, minimizing this signal
would minimize the sum of ammonia and post-catalyst
NOx. To initiate the process, a small transition in
NH3/NOx molar ratio is made. In response, catalytic re-
duction either improves or diminishes, and the ceramic
NOx sensor signal either increases or decreases.
There are four possibilities:
1) The system is operating lean (too little ammonia)
and the feedback system steps ammonia down;
2) The system is lean and the feedback system steps
ammonia up;
3) The system is rich (too much ammonia) and the
feedback system steps ammonia up;
4) The system is rich and the feedback system steps
ammonia down.
The second and fourth operations improve SCR per-
formance, while the first and third operations reduce
catalytic performance. If the transition decreased the
signal, another step is taken in the same direction. If the
transition increased the signal, the next step is taken in
the opposite direction. Eventually, the algorithm will
cross the feedback ceramic NOx sensor minimum, and
reverse direction, oscillating back and forth across the
optimum NH3/NOx molar feed ratio. Through this
method, the ceramic NOx sensor signal is minimized.
The first test was performed with a 15 min decision
time and 5% step increment, and the second test was
done with a 5 min decision time and 2.5% step increment.
A step increment is a step in NH3/NOx molar ratio. The
size of the step increment is relative to stoichiometric
molar ratio. Decision time is the time between steps. The
15 min test was started at 0.5 NH3/NOx molar ratio, and
the 5 min test was started at 0.8 NH3/NOx molar ratio.
The test was done to see if the algorithm approached an
appropriate molar ratio, and how long the algorithm took
to stabilize.
Figure 7 shows the result of the first feedback control
test. Ammonia was turned on at time zero. Molar ratio
was the controlled parameter in the feedback system.
NOx reduction increased to about 50%, which is ex-
pected since NH3/NOx molar ratio was around 0.5. At
about 15 min, when NOx reduction dropped off momen-
tarily, the ammonia feed pump malfunctioned. After this,
the algorithm increased the molar ratio appropriately. At
about 1 hr and 45 min, when NOx reduction dropped off
again, there was another pump malfunction. At this point,
the algorithm made one incorrect step, but corrected, and
the system took about four hours to stabilize.
Figure 8 shows NH3/NOx molar ratio and post catalyst
ceramic NOx sensor signal. In the figure, molar ratio be-
gins low, and the signal is resultantly high. As the feed-
back loop increases the molar ratio, the catalyst ap-
proaches stoichiometric operation, ammonia and NOx
slip decrease, and the ceramic NOx sensor signal de-
creases. At 1 hr and 30 min, the feedback algorithm
made an incorrect decision and decreased NH3/NOx feed
ratio. At this point, NOx increased, increasing the ce-
0
20
40
60
80
100
120
140
01234
Time[hr]
NOxReduction[%],AmmoniaSlip[ppmx10]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
AmmoniatoNOxMolarRatio
NoxReduction
AmmoniaSlip
MolarRatio
Figure 7. Feedback control with 15 min decision time.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
00.511.522.533.54
Time [hr]
Post Catalyst Ceramic NOx Sensor Signal [v]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Ammonia to NOx Molar Ratio
CeramicNOxSensorSignal
MolarRatio
Figure 8. Post catalyst NOx sensor signal and NH3/NOx
molar ratio for feedback control during 15 min decision
time test.

J. SCHMITT ET AL.323
ramic NOx sensor signal. The algorithm reversed its di-
rection, and continued to an appropriate molar ratio.
The minimization algorithm proved very effective and
robust with a 15 minute decision time and a 5% incre-
ment. The system approached an appropriate molar ratio,
despite equipment malfunctions. The equipment mal-
functions, although unplanned, displayed control algo-
rithm robustness.
Figure 9 shows the result of the second feedback con-
trol test. In Figure 9, ammonia was turned on and NOx
reduction increased to about 90%. The algorithm, at this
point, made incorrect decisions, decreasing molar ratio to
0.7, until NOx reduction decreased to 85%, and the con-
troller began making correct decisions. Over the course
of the next hour and a half, the system increased molar
ratio to somewhere between 0.8 and 0.85, and stabilized.
Figure 10 shows the inputs and outputs of the feed-
back algorithm during the test. In the beginning, the post
catalyst ceramic NOx sensor detected a surge. This is
because NOx reduction was low at the beginning of this
0
20
40
60
80
100
120
140
01
Time[hr]
NOxReduction[% ],AmmoniaSlip[p pmx10]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Ammonia/NOxMolarRatio
2
MolarRatio
NOxReduction
AmmoniaSlip
Figure 9. Feedback control with 5 min decision time.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
00.25 0.5 0.7511.25 1.5 1.75
Time [hr]
Post Catalyst Ceramic NOx Sensor Signal [v]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Ammonia to NOx Molar Ratio CeramicNOxSensorSignal
MolarRatio
Figure 10. Post catalyst ceramic NOx sensor signal and
NH3/NOx molar ratio during five minute decision time test.
data point. As ammonia feed was turned on, NOx reduc-
tion dropped quickly. As the algorithm initially made
incorrect decisions, the post catalyst ceramic NOx signal
increased. Around 0.5 hours, the algorithm began mak-
ing correct decisions. The post catalyst ceramic NOx
sensor signal began decreasing. At about 1.25 hours, the
system stabilized. The post catalyst ceramic NOx signal
leveled, and the molar ratio control signal oscillated
above and below the optimum.
With a 5 min decision time and 2.5% step size, the
system made incorrect decisions, but stabilized much
faster than the 15 min decision time algorithm. When the
system was turned on, NOx reduction increased, de-
creasing the feedback signal from the ceramic NOx sen-
sor. The algorithm reduced molar ratio for several steps,
which was incorrect. Although the 5 min decision time is
significantly faster than the 15 min decision time feed-
back system, the 15 min system is fast enough to correct
for sensor drift, and more robust than the 5 min system.
The 15 min decision time system made very few incor-
rect decisions during stabilization, whereas the 5 min
decision time system made many incorrect decisions.
The feedback system should ensure that long term sensor
drift does not significantly affect engine emissions. Since
sensor drift occurs in the timeframe of hours and days,
both the 5 min and 15 min systems should be sufficiently
fast. NOx reduction was around 98% on both systems at
the stabilization point, while maintaining ammonia slip
under 5 ppm. This shows that the control technique is
very effective at ensuring the catalyst is operating prop-
erly.
These tests showed the algorithm response given con-
stant space velocity, temperature, and NOx concentration.
The tests did not test the feedback algorithm sensitivity
to varying inputs. If NOx, were to increase rapidly, caus-
ing rapid ammonia slip or NOx reduction transition, the
feedback algorithm might respond to the varying input,
as if the transition was initiated by a feedback step. More
research is needed to evaluate the feedback algorithm
with variable inputs. Incorporation of pressure compen-
sated NOx sensors may be necessary to achieve accept-
able performance with variable inputs.
4. Conclusions
Control systems were developed for SCR systems to
control ammonia injection flow rate. Two algorithms
were experimentally evaluated. The first was a feedfor-
ward control algorithm that used a ceramic NOx sensor to
detect pre catalyst NOx. The second was a feedforward
plus feedback algorithm which used a pre and post cata-
lyst ceramic NOx sensor to generate feedforward and
feedback signals, respectively.

J. SCHMITT ET AL. 324
The feedforward control algorithm was inaccurate
following space velocity transients, because the ceramic
NOx sensor was not pressure compensated. This lead to
overfeeding of ammonia at high space velocities and
underfeeding of ammonia at low space velocities.
The feedforward plus feedback algorithm used an al-
gorithm that minimized the post catalyst ceramic NOx
sensor signal. This feedback technique controlled the
molar ratio set point. Minimization of the post catalyst
ceramic NOx sensor signal is a new approach for utiliz-
ing ceramic NOx sensors that is independent of sensor
calibration. This approach capitalizes on NOx sensor
ammonia interference to improve SCR control. Two de-
cision times were tested, a 15 min decision time and a 5
min decision time. The 15 min decision time algorithm
was able to approach appropriate ammonia feed, a 40%
correction, in about 4 hours at steady state feedforward
conditions. The 15 min decision time algorithm was ro-
bust and operated fast enough to account for sensor drift
in stationary engine applications. The 5 min decision
time algorithm stabilized much faster, in about 1.5 hours,
but was less robust.
5. Acknowledgements
This work was funded by the Pipeline Research Council
International.
6. References
[1] M. Chen and S. Williams, “Modeling and Optimization
of SCR-Exhaust Aftertreatment Systems,” SAE Interna-
tional, 2005, Article No. 2005-01-0969.
[2] C. M. Schär, C. H. Onder and H. P. Geering, “Control of
an SCR Catalytic Converter System for a Mobile
Heavy-Duty Application,” IEEE Transactions on Control
Systems Technology, Vol. 14, No. 4, 2006, pp. 641-653.
doi:10.1109/TCST.2006.876634
[3] C. M. Schär, “Control of a Selective Catalytic Reduction
Process,” IMRT Press c/o Institut für Messund Regeltech-
nik, ETH Zentrum, 2003.
[4] D. Simpson, “Precombustion Chamber Design for Emis-
sions Reduction from Industrial Natural Gas Engines,”
Master’s Thesis, Colorado State University, Fort Collins,
2009.
[5] J. Lisowski, “Diagnostic Techniques for Precombustion
Chambers in Large Bore Lean Burn Natural Gas En-
gines,” Master’s Thesis, Colorado State University, Fort
Collins, 2007.
[6] K. Ivaturi, “Experimental and CFD Investigation of
Re-agent Mixing in an SCR System,” Master’s Thesis,
Colorado State University, Fort Collins, 2007.
[7] T. Moosman, “FTIR Spectroscopy for 2-Stroke, Lean
Burn Gas Engines Emphasizing Low-Level Detection of
HAPs,” Master’s Thesis, Colorado State University, Fort
Collins, 2005.