Development and Implementation of an Alternative Emissions Monitoring Program for Carbon Monoxide Compliance for Fluidized Catalytic Cracking Unit CO Boilers

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Courtesy of Mostardi Platt Environmental

This paper presents the results of an alternative emissions monitoring test program for the Fluidized Catalytic Cracking Unit (FCCU) CO boilers at major oil refinery. Alternative emissions monitoring systems can be used in lieu of other compliance monitoring techniques (e.g., periodic emissions testing, continuous emissions monitoring) to relate monitored operating conditions which might indicate potential problems in emission control performance. Alternative emissions monitoring can enable a source to obtain information in a timely manner to avoid, or reduce, operational problems that could result in excess emissions.

The flue gas from the FCCU regenerator contains approximately 10% carbon monoxide (CO) by volume, which is commonly referred to as “CO gas.” To control the emissions of CO, the CO gas stream is passed through two boilers (East and West) designed with sufficient residence time, turbulence, and temperature to fully combust the CO contained in the FCCU Regenerator flue gas. The FCCU is also equipped with piping which enables the FCCU regenerator CO gas to bypass the boilers and be directed to the common stack shared by the East and West boilers. This bypass piping is equipped with a butterfly valve that is normally operated in the fully closed position. To ensure against leakage, the valve is routinely packed with ceramic fiber insulation.

One of the main objectives of the alternative monitoring test program was to determine what effect the operating load of the CO boilers had on CO emissions at the boiler outlets leading to the common stack. This evaluation was performed to ascertain if variation of operating loads for the CO boilers would cause changes in CO emissions. The other main objective was to determine what effect leakage of the bypass valve had on temperatures downstream of the valve and CO emissions in the common stack. To perform this evaluation, emissions and operating data were collected during baseline and test (bypass leakage) conditions. Some of the test conditions evaluated resulted in CO emission levels in the common stack above the 200 ppm (corrected to 50% excess air) emission limit.

The test results demonstrated that the operating load of the East and West boilers had no measurable effect on CO levels in the common stack. However, by forcing leakage of the bypass valve, there were measurable temperature increases downstream of the valve that were directly related to the magnitude of leakage of CO gas across the bypass valve. As valve leakage increased, so did the CO levels in the common stack.

As a result of this test program, the facility has implemented an alternative emissions monitoring system by installing a permanent temperature sensor downstream of the bypass valve, and connecting the temperature signal to the plant process computer. The process computer has been programmed to alert the operators of temperature increases downstream of the bypass valve. As a corrective action for significant temperature increases above normal baseline levels, the bypass valve is repacked in accordance with facility maintenance procedures. By monitoring these temperatures and taking the appropriate corrective action, the facility has a very high degree of compliance assurance that CO emissions in the common stack remain below the applicable limit.

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