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B.16 NONSELECTIVE CATALYTIC REDUCTION28,29,30

B.16.1 Background

Nonselective catalytic reduction (NSCR) is an add-on NOx control technology for

exhaust streams with low O2 content. Nonselective catalytic reduction uses a catalyst reaction to

simultaneously reduce NOx, CO, and hydrocarbon (HC) to water, carbon dioxide, and nitrogen. 

The catalyst is usually a noble metal. The conversion occurs in two sequential steps, as shown in

the following equations:

Step 1 Reactions: 2CO + O2 ÿ 2CO2

2H2 + O2 ÿ 2H2O

HC + O2 ÿ CO2 + H2O

Step 2 Reactions: NOx + CO ÿ CO2 + N2

NOx + H2 ÿ H2O + N2

NOx + HC ÿ CO2 + H2O + N2

The step 1 reactions remove excess O2 from the exhaust gas because CO and HC will more

readily react with O2 than with NOx. The O2 content of the stream must be kept below

approximately 0.5 percent to ensure NOx reduction.

One type of NSCR system injects a reducing agent into the exhaust gas stream prior to

the catalyst reactor to reduce the NOx. Another type of NSCR system has an afterburner and two

catalytic reactors (one reduction catalyst and one oxidation catalyst). In this system, natural gas

is injected into the afterburner to combust unburned HC (at a minimum temperature of 1700°F). 

The gas stream is cooled prior to entering the first catalytic reactor where CO and NOx are

reduced. A second heat exchanger cools the gas stream (to reduce any NOx reformation) before

the second catalytic reactor where remaining CO is converted to CO2.

The control efficiency achieved for NOx ranges from 80 to 90 percent. The NOx

reduction efficiency is controlled by similar factors as for SCR, including the catalyst material

and condition, the space velocity, and the catalyst bed operating temperature. Other factors

include the air-to-fuel (A/F) ratio, the exhaust gas temperature, and the presence of masking or

poisoning agents. The discussions in section B.15 for SCR relating to catalyst issues and space

velocity also apply to NSCR. The operating temperatures for NSCR system range from

approximately 700° to 1500°F, depending on the catalyst. For NOx reductions of 90 percent, the

temperature must be between 800° to 1200°F. One source indicates that the O2 concentration

for NSCR must be less than 4 percent, and another source indicates that the O2 concentration

must be at or below approximately 0.5 percent.

B.16.2 Indicators of NSCR Performance
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The key indicators for NSCR are the same performance indicators for SCR with a few

exceptions (minus the NH3/NOx ratio, outlet NH3 concentration, and sulfur content of the fuel). 

Outlet NOx concentration, catalyst bed inlet temperature, catalyst activity, pressure differential

across the catalyst bed, catalyst bed outlet temperature, inlet gas flow rate, and outlet O2

concentration were discussed above. Table B-16 lists these indicators and illustrates potential

monitoring approaches for NSCR.

B.16.3 Illustrations

The following illustration presents an example of compliance assurance monitoring for

NSCR:

16a: Monitoring catalyst bed inlet temperature, catalyst bed outlet temperature, and

catalyst activity.

B.16.4 Bibliography
CAM

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TABLE B-16. SUMMARY OF PERFORMANCE INDICATORS FOR NSCRs Parameters Performance indication Approach No. 

Illustration No. 

Example CAM Submittals Comment Primary Indicators of Performance Outlet NOX concentration Direct measure of outlet concentration. Most direct single indicator of NSCR performance. 

Catalyst bed inlet temperature Indicator that bed inlet is of sufficient temperature to initiate reduction. Also an indicator that bed inlet temperature is not too high for catalyst longevity. 

Catalyst activity Periodic check of catalyst activity gives an indication of catalyst fouling or masking. Must periodically clean and/or replace catalyst to ensure reduction is occurring. 

Other Performance Indicators Catalyst bed outlet temperature Indicator of level of reduction occurring in the catalyst bed and that temperature does not exceed design limits of catalyst. Higher temperatures improve reduction of CO and VOC but encourage generation of NOX. 

Pressure differential across catalyst bed Indicator of bed fouling or plugging. Increase in pressure differential indicates that bed is becoming fouled or plugged. Changes in pressure differential are likely to be gradual. 

Outlet O2 concentration Indicator of dilution rate. Generally monitored in combination with NOX, VOC, or CO concentration to allow for correcting concentration to a specified percent O2. 

Comments: None.
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CAM ILLUSTRATION

No. 16a. NONSELECTIVE CATALYTIC REDUCTION FOR NOX CONTROL

1. APPLICABILITY

1.1 Control Technology: Nonselective catalytic reduction [065]

1.2 Pollutants

Primary: Nitrogen oxides (NOX)

Other: Nitric acid (HNO3), ammonia (NH3)

1.3 Process/Emission units: Internal combustion engines (ICE)

2. MONITORING APPROACH DESCRIPTION

2.1 Indicators Monitored: Catalyst bed inlet temperature, catalyst bed outlet temperature,

and catalyst activity.

2.2 Rationale for Monitoring Approach

• Catalyst bed inlet temperature: Indication that the gas stream is at sufficiency

temperature to initiate reduction on the catalyst bed; too high a bed temperature may

generate NOX rather than reduce NOX.

• Catalyst bed outlet temperature: Indication that the reaction is occurring in the

catalyst bed; too high a bed temperature may generate NOX rather than reduce NOX.

• Catalyst activity: Indication of the catalyst’s ability to promote the reduction of

NOX.

2.3 Monitoring Location

• Catalyst bed inlet temperature: Inlet to the catalyst bed.

• Catalyst bed outlet temperature: Outlet to the catalyst bed.

• Catalyst activity: Removal of a small portion of catalyst for testing.

2.4 Analytical Devices Required: Thermocouples or other temperature instrumentation.

2.5 Data Acquisition and Measurement System Operation

• Frequency of measurement:

– Catalyst bed inlet temperature: Measure continuously.

– Catalyst bed outlet temperature: Measure continuously.

– Catalyst activity: Measure periodically.

• Reporting units:

– Catalyst bed inlet temperature: Degrees Celsius or Fahrenheit (C or F).

– Catalyst bed outlet temperature: Degrees Celsius or Fahrenheit (C or F).

– Catalyst activity: Percent active compared with new catalyst (or other as

appropriate).

• Recording process:

– Catalyst bed inlet temperature: Recorded automatically on strip chart or data

acquisition system.

– Catalyst bed outlet temperature: Recorded automatically on strip chart or data

acquisition system.

– Catalyst activity: Manually recorded in NSCR maintenance log.
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2.6 Data Requirements

• Baseline catalyst bed inlet temperature, catalyst bed outlet temperature, and catalyst

activity measurements concurrent with emission test.

• Historical plant records of catalyst bed inlet temperature, catalyst bed outlet

temperature, and catalyst activity measurements.

2.7 Specific QA/QC Procedures

• Calibrate, maintain, and operate instrumentation using procedures that take into

account manufacturer’s recommendations.

2.8 References: ______

3. COMMENTS

None.