For each type of CEMS the EPA has published performance specifications used for “evaluating the acceptability of the CEMS at the time of or soon after installation and whenever specified in the regulations”. The EPA has also published quality assurance “used to evaluate the effectiveness of quality control (QC) and quality assurance (QA) procedures and the quality of data produced by any CEMS”. Those performance specifications and QC QA procedures are used to determine if a plant is in compliance with their emission standards.
NESHAP compliance has put plants on a steep learning curve with CEMS and although there’s still time for them to get up to speed, they are learning about the operation of these systems through the school of hard knocks.
Particulate matter (PM) is the term the EPA uses for “a mixture of solid particles and liquid droplets found in the air”. Some PM is filterable (it can be either a solid or liquid material at exhaust stack temperatures), while some PM is condensable (it can be either a vapour or a gas at exhaust stack temperatures but quickly coalesces after leaving the stack, leaving a visible plume). PM is also classified by size. PM2.5 refers to combustion particles and other materials with a diameter less than 2.5 µm. PM10 includes dust and other materials less than 10 µm in diameter.
Plants using PM CEMS with EPA Performance Specification 11 (PS 11) cannot find enough certified facilities that use the method, so there are still big questions regarding both the precision and bias of PM CEMS. It is an extensive certification process (anywhere from three to five days to complete) and it is also expensive – correlating responses relative to emission concentrations determined by the reference method requires either spiking or baghouse bypass. The certification process is complex; the correlation may require the Agency to allow PM emissions to exceed limits during the certification process. That seems backward, but maybe an analogy will help. Suppose one sheet of paper had to be weighed using a truck scale. How would one prove that the truck scale worked? If 100 000 sheets of paper were placed on the scale, the total weight could be found and divided to determine the weight of just one sheet. Although the truck scale would not find the weight of the one sheet, the user would at least be assured that the scale worked.
The process is also a frustrating one. The correlation test is difficult to pass at low PM emission levels and there are operational limitations, including equipment uptime on wet stacks. Furthermore, the method is affected by changes in particle size, colour, density and shape, all of which skew the measurement of CEMS that use light scattering techniques. Some plants have expressed concerns that the units using light scattering techniques cannot be used after a wet scrubber and others have worries regarding the fact that aerosols may be picked up in the back half portion.
The end game for effective PM CEMS requires more accurate flow rate monitors and more precise clinker production data; however, even with these changes, some states will still require opacity monitoring.
There are alternative methods for PM measurement but these possess their own unique problems. Method 5, for example, is not representative of long-term conditions and triboelectric variants have not demonstrated an ability to pass the PS 11 correlation test. Hot/wet extractive tests are prohibitively expensive and probably require installation downstream of wet scrubbers.
Mercury in the cement industry can be found in raw materials or in the coal used for combustion. Mercury CEMS require compliance with one of five EPA performance specifications: total vapour phase Hg CEMs (PS 12A); total vapour phase Hg using a sorbent trap monitoring system (PS 12B); total vapour phase Hg using carbon sorbent traps (PS 12B); total vapour phase Hg using an instrument analyser (PS 30A); total vapour phase mercury using sorbent trap sampling and an extractive or thermal analytical technique (PS 30B).
Plants using cold vapour atomic fluorescence CEMS (PS 12A) still lack a reliable NIST (National Institute for Science and Technology) traceable protocol standard for a calibrated reference. These same plants have also found that high operating probe temperatures significantly increased probe wear. If the exhaust gas temperature is too hot, or if the exhaust gas flow is too high, there are problems in evaluating how much of the mercury is oxidised and how much is elemental. The breakdown between oxidised and elemental mercury is not an issue for EPA, but it could be a potential concern for a plant trying to control oxidation to improve the effectiveness of their mercury abatement.
Instrument analysers are costly, complex and difficult to maintain. They take a high degree of skill to operate, maintain and troubleshoot. Part of the reason for the complexity is the requirement for dedicated air, water and power connections. Another reason is that PS 30A requires either argon or chlorine gas cylinders and nearly a full day to purge the system with argon prior to sample preparation. System reliability issues can impact the time required and generate long delays. Additional problems reported by plants with PS 30B include cracking of sample tubes if the sampling point has a very large pressure deviation from ambient pressure.
Regardless of the CEMS used for mercury, plants are having a difficult time passing the relative accuracy test procedure (RATA). And just as with PM CEMs, there are not enough certified facilities using the method so the same issue regarding precision and bias arises.
Total hydrocarbons are the sum of all hydrocarbon emissions as measured using a flame ionisation detector. The key assumption behind this detector is that it responds to all hydrocarbons the same way it responds to propane; therefore the detector is calibrated using propane. THC CEMS must comply with EPA Performance Specifications 8 (VOC CEMs), 8A (THC CEMs) and EPA Method 25 (total gaseous non-methane organic compounds). Plants are realising that non-dispersive infrared (NDIR) units are not accurate enough at low THC concentrations. Plants that have flame ionisation detectors are experiencing problems as well: reliability at low temperatures; maintaining consistent flow, pressure and temperature; sensitivity from contamination of the air supply; sensitivity to pressure; blockages in fine tubing and difficulty in maintaining the flame. FID units also tend to have relatively longer downtimes in comparison to other CEMS units, plus the storage, handling and recordkeeping requirements for H2 cylinders and carbon free air cylinders are rigorous. THC CEMS measuring raw mill stacks require additional oxygen and water monitors to ensure these units can report THC concentrations dry at 19% oxygen. With such low emission figures, there is also a very real concern regarding cross contamination by ambient THC. (Note that ambient air typically contains approximately 2 ppm of THC, although levels of 3 – 5 ppm have been observed.)
PS 8A is an alternative method for THC CEMS that extracts a gas sample through a heated sample line and heated filter to a flame ionisation detector; but PS 8A has its own problems. Plants are discovering maintenance and reliability issues, along with frequent blockages in both the filters and the sampling units. Other problems include maintaining proper sample temperatures, maintaining flame in the oven and even flameout of FID units.
HCl or hydrochloric acid can be formed during the combustion process of some fuels. EPA Performance Specification 15 (Extractive FTIR CEMs) and EPA Method 320 (Vapor Phase Organic and Inorganic Emissions by Extractive FTIR) both address HCl emissions. These are complex units that require the use of highly reactive gas. Concerns have arisen about achieving continuous reliable measurements and meeting the requirements of PS 15. Measurements below 3 ppm HCl require longer path lengths and that means larger cells and longer response times. Plants are worried that there are no NIST traceable protocol standards for a calibration reference and no proven units that can reliably measure HCl continuously at levels below 3 ppm. Adding to the complexity is the availability and cost of HCl standard bottles. HCl, at a level of 3 ppm, is at a point where particulate buildup in filters may actually scrub out HCl emissions within the gas stream. That scrubbing may, in effect, make the testing non-repeatable.
Alternative HCl measurements include EPA Method 320, tunable diode laser (TDL) and infrared (IR) gas correlation. The problem with these alternatives is that Method 320 is not representative of long-term conditions, TDL has no EPA approved compliance procedure and IR gas correlation has a limited history of use. IR gas correlation also has its own set of problems with calibration gas, reliability, scrubbing issues with the sample condition and system issues.
Written by Rick Bohan, Portland Cement Association, USA.
This is an abridged version of the full article, which appeared in World Cement’s IEEE-IAS/PCA Conference Supplement 2014. Subscribers can view the full article by logging in.
Read the article online at: https://www.worldcement.com/the-americas/21042014/beyond_analysis_part_2/