Current and pending EPA rules and regulations are requiring power plant, cement plant, incinerator, refinery and boiler operators to improve their abilities to continuously measure a variety of emissions, including HCl, Hg, SO2, NOx, O2, CO2, total hydrocarbons and particulate materials—at lower levels and with higher proven accuracy than ever before.
One existing and one emerging technology promise to help them do so more simply, more reliably and with less maintenance than they might imagine.
“Measurements of total, elemental and ionic Hg must be made at very low and varying concentrations,” said Steve Gibbons, business development manager, ABB Measurement & Analytics, at his presentation on Measurement Technology Trends to Meet EPA Emissions Regulations at ABB Automation & Power World today in Houston. “HCl is soluble and sticky and must be measured at levels of less than 1 ppm in the power industry and 3 ppm in cement plants.”
Conventional continuous emission monitoring system (CEMS) analyzer technologies are challenged by these and other requirements, resulting in unreliable results, complex sample handling systems, multiple analyzers and high maintenance costs.
In general, stack gas emission analyzers work by measuring light absorption at specific wavelengths. Their ability to discriminate among emissions of interest depends on their ability to separate distinct spectrum peaks, and their sensitivity to low concentrations is improved by increasing the length of the light beam path through the gases.
As a baseline, consider a Fourier-transform infrared (FTIR) analyzer, using incoherent infrared light.
“Water and CO2 peaks interfere, so you can introduce different kinds of light sources to get a full spectrum,” Gibbons said. “You can use optical filters to remove unnecessary frequencies, or take them out with software. Or you can use coherent light to look only at specific wavelengths.”
Tunable diode laser adsorption spectroscopy (TDLAS) is useful for HCl analysis.
“You can run the laser across the stack for a path length of the stack diameter, or use a mirror to double the path length to 20-40 ft.” But to measure very low levels, you need a longer light path. Cavity enhanced adsorption spectroscopy (CEAS) brings the gases of interest into a small cell with mirrors and uses many passes to obtain paths as long as 12 miles, with the added complexity of a heated sampling system.
CEMAS FTIR technology is in its fourth generation at ABB. Introduced in 2013, it’s built on 20 years of experience and now is used in 1,600 installations around the world.
“Getting a real sample to the analyzer is the challenge,” Gibbons said, “Not the analyzer technology. NASA uses our FTIR to measure CO2 in the Earth’s atmosphere.”
The ACF5000 FTIR CEMS uses a thermoelectrically cooled detector. Separating the gas path from the electronics contributes to a 10-15 year life.
“But you can only measure what you can see,” Gibbons said, “So the layout of the system is critical.” The sample path must be kept above 356 °F at all times; connections must be minimized and distances as short as possible. The heart of the system is the “ASP block,” which minimizes tubing and connections and eases maintaining the minimum temperature.
An optional validation unit uses films or gas cells for each type of gas to verify calibration by rotating them into the light path. Verification can be manual or automatic.
"We’ve submitted it to EPA for acceptance in PS-18 as an alternative to test gases,” Gibbons said.
The emerging technology is coming from Los Gatos Research, which ABB purchased in 2013. Los Gatos invented CEMAS and has many patents on the technology. The new version is called Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) and uses an optical cavity with a path length of 20 miles or longer to produce fast readings at concentrations as low as parts per trillion (ppt).
“It can quantify multiple gases in complex mixtures, it’s very robust, and exact alignment, gas pressure and temperature are not critical,” Gibbons said. “The mirrors are easy to remove and can be cleaned by anyone, anytime, in 30 minutes or less, with practice.”
The analyzer can use near infrared (NIR), ultraviolet (UV), “or anything in between,” Gibbons added.
“You can use more than one laser at the same time for multi-component measurements…HCl, H2O, NH3 in one analyzer, with dilutions up to 150:1. And for most applications, calibrations are unnecessary. We just plug it in and it runs.”
Visualization of the spectra in real time gives confidence that the measurement is working. Field trials are currently running at seven U.S. cement plants.