General Electric Energy and Environmental Research Corporation (GE) has conducted a pilot-scale catalyst test program for the PRCI to evaluate commercial ammonia selective catalytic reduction (SCR) technology for small and medium-size gas turbines used in the pipeline industry. The pilot-scale catalyst evaluation was conducted at the GE Test Site in Santa Ana, California using the GE Catalyst Evaluation Facility (CEF).
The primary objective of the test program was to evaluate byproduct emissions at steady state and transient operating conditions for two commercially available SCR catalysts used in power generation gas turbine applications. Both NOX removal efficiency and ammonia slip behavior were also examined to validate expected catalyst trends and activity. Even thought the study replicated expected field catalyst process conditions as well as possible (e.g., flue gas temperatures, space velocities, and inlet species concentrations), the data and results are from pilot-scale testing only, and consequently may differ from actual gas turbine field tests. However, the system configuration as operated did appear to capture field conditions well.
The results of the steady-state catalyst testing followed the expected trends for NOx reduction and ammonia slip with regard to changes in ammonia & NOx stoichiometric ratios (NSR), temperature, and space velocity through the catalyst (residence time). The catalysts performed very well even at non-optimal conditions with an NSR of 0.5 and temperatures of 625 °F or 850 °F. A temperature of 725 °F and NSR of 1.0 also appear to be the near optimal conditions for NOx reduction and ammonia slip.
NOx and ammonia responses to step changes in NSR, temperature, and flow were characterized, but were limited by the natural temperature response of the test rig, which was on the order of one hour. Transient NOx and NH3 response time to step changes in NSR were on the order of 100 to 700 seconds, depending on catalyst type and final NSR. No significant byproduct emissions were generated by either catalyst for both steady-state and transient tests. Some byproduct emissions were introduced to the system as a result of injecting propane upstream of the catalyst blocks. However, subsequent tests showed that in the absence of propane injection the initial byproduct emissions fell below the minimum detection limits of the FTIR emissions analyzer.
When propane was injected as total hydrocarbon, formaldehyde was present at the catalyst inlet in concentrations up to 1.4 ppm, depending on the temperature. Acrolein concentrations were up to 0.4 ppm, also depending on temperature. The extruded catalyst reduced a substantial portion of the formaldehyde independent of NSR, whereas the metallic catalyst was able to consistently reduce some of the formaldehyde only at NSR levels above 1. Neither catalyst had any effect on acrolein emissions. No measurable amounts of hydrogen cyanide and acetaldehyde were found during any of the tests.
The overall results of this work indicate that the use of ammonia-based SCR in variable load applications does not appear to generate any unforeseen toxic emissions, such as cyanides. This is noteworthy because the physical conditions under which cyanides are manufactured commercially have some overlaps with that conditions found in gas turbine exhaust, and the excess, reactive nitrogen species in ammonia-SCR exhaust carry the potential to create particularly troublesome byproduct emissions.
The report is available to PRCI members in PRIME by clicking here.