Industrial Electronics

Ammonia injection control for stack NOx scrubbers relies on precision ST100 mass flow meters

18 October 2021

The engineering team at a coal-fired electric power plant utilizing ammonia (NH3) for nitrous oxide (NOx) removal by its pollution control system installed the ST100 series thermal flow meter from Fluid Components International (FCI) based on its superior repeatability in measuring NH3 under harsh operating conditions.

The ST100 series flow meter allowed the plant team to optimize its selective catalytic reduction system. This step improved performance while substantially reducing NH3 consumption and costs. The meter, with its rugged thermal dispersion sensor head, provides a highly repeatable measurement solution to control vaporized NH3 in such applications.

Power plant engineers were experiencing problems with inconsistent measurement of the vaporized NH3 injected into their flue gas as a means to reduce NOx emissions. The original system utilized a volumetric Source: FCISource: FCIflow sensor, which wasn’t well suited for controlling the NH3 gas injection system. Switching to the mass flow sensing ST100 series meter, as opposed to taking a volumetric measurement, solved the false readings issue.

Given the meter’s standard wide flow turndown ratio of 100:1 (up to 1000:1 based on application), and the ability to measure mass flow instead of volumetric flow, the engineers decided to conduct a pilot test. Insertion style ST100A thermal flow meters were installed on the primary vaporized ammonia feed lines to their nozzle grid for the test.

With the resulting improved measurement reliability, engineers then installed in-line style ST100AL thermal flow meters at each of the nozzle locations. After proving out the benefits in one operating unit, ammonia injection systems were instrumented for all of the units within the facility.

The ST100 meter can be factory calibrated to measure virtually any popular process gas as well as mixed gases. The technology is suitable for use in wet and dirty gas applications, having no small ports prone to fouling. The basic insertion style air/gas meter features a thermal flow sensing element that measures flow from 0.25 up to 1000 SFPS (0.07 NMPS to 305 NMPS) with accuracy of ±0.75% of reading, ±0.5% of full scale. The basic in-line style meter is available for 2 inch and smaller line sizes.

The meter’s transmitter is unsurpassed in meeting both a plant’s current and future need for outputs, process information and communications. Whether the output required is traditional 4-20 mA analog, frequency/pulse or advanced digital bus communications such as HART, Modbus, PROFIBUS or FOUNDATION Fieldbus, the ST100 Series will meet instrumentation integration needs. Its advanced bus communications are all third party certified and registered.

Developed with a graphical, multivariable backlit LCD display, the ST100 series meter brings new meaning to the term “process information.” Its sophisticated readout continuously displays all process measurements and instrument status for easy on-site viewing by technicians, and it has the ability to query for service diagnostics via an integral, optical human-machine interface that does not require declassification of hazardous areas.

The meter is designed for extreme industrial process and plant conditions and can be used in service up to 850º F (454º C) and is available with both integral and remote (up to 1,000 feet (300 m)) electronics versions. The ST100 meter is agency approved for hazardous environments, including the entire instrument, the transmitter and the rugged, NEMA 4X/IP67 rated enclosure.

Global approvals include: ATEX, CPA, CRN, EAC/TRU CU, FM, FMc, FDT, GOST, IECEx, Inmetro and NEPSI. Third party failure rate data per IEC 61508 is available that demonstrates suitability of the hardware architecture for SIL 1 (HFT=0) applications.

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