BP’s Cherry Point refinery in Blaine, Wash., recently completed a 50-day stability trial of Emerson's Rosemount CT5400 hybrid quantum cascade laser (QCL)/tunable diode laser (TDL) analyzer to characterize the flue gas from its calciner hearth for environmental reporting. The analyzer is installed after the chiller in the stack's vacuum eductor, and uses chemiluminescence, infrared, ultraviolet (UV) and paramagnetic technologies to analyze O2, CO2, CO, SO2 and NOx.
"All of our heaters have operating permits issued by EPA and state regulators, which specify emission limits, and require continuous emissions monitoring system (CEMS) testing," said Ryan Holgate, analyzer engineer, maintenance, BP Cherry Point. "This testing includes daily validation of analyzers with check gas, quarterly stability testing against check gases, and yearly relative accuracy test audit (RATA) of stack gases compared to a third-party reference.
"In our daily validation testing over 50 days, all operations were well within limits, and we found QCL to be very stable,” Holgate said. He and Dave McMillen, business development manager in North America for Emerson's Rosemount QCL technology division, presented "RATA using hybrid QCL/TDL spectrometer for emissions monitoring" this week at the 2017 Emerson Global Users Exchange in Minneapolis.
Laser learnings
McMillen explained that recent quests for more stable laser-based analyzers arise from the challenges of using other, less stable technologies. "The non-dispersive infrared (NDIR) and chemiluminescence methods developed in the 1940s were good in their day, and are still used now, but they rely on broad-spectrum light sources and filter wheels with moving parts," said McMillen. In recent years, he added, laser spectroscopy technologies are yielding even more stable analysis and more precise emissions measurements that allow easier regulatory compliance.
"Lasers are more stable because they only look at the frequencies we want to absorb," added McMillen. "They're also reliable, fast, very repeatable, don't drift even over 10-15 years, don't need calibration, and give us good absorption peaks for multiple analyte measurements, which are calculated from thousands of scans. Bringing together QCL and TDL for CEMS lets us measure several gases at once without a fuss, and gives us higher analyzer availability. Performing multiple different measurements is done by 'chirping' six lasers over 1 millisecond by applying power to heat them, and then rapidly cooling them with a Peltier cooler in 500 nanoseconds.
"BP has experience with TDL analyzers, and found them to be reliable, fast and repeatable. Hybrid QCL/TDL analyzers add further capabilities for CEMS, including multiple measurements, very low drift and high analyzer availability."
RATA arrives
Holgate added that BP Cherry Point's annual RATA compares CEMS data to information collected from an independent EPA test method, and passing a RATA means matching the test method, though it's not necessarily a measure of accuracy.
"A RATA usually includes nine runs at 21 minutes per run, and data is recorded at one-minute averages," said Holgate. "The tester performs analyzer bias checks between each run, and traverses the stack diameter to eliminate any stratification concerns by moving the probe every seven minutes.
"We needed to match the performance of the EPA's existing testing method before we could adopt QCL technology. We did 10 runs, and we passed for all gases—O2 , CO2, CO, SO2 and NOx—within the EPA's specs," added Holgate. "During the 50-day trial, QCL showed it was stable, met the EPA's performance requirements, and had lower maintenance requirements. We anticipate that QCL/TDL will give us greater data availability with less downtime, and QCL has already been approved for our next two heater applications."