Continuous emissions monitoring systems (CEMS) are installed in a wide range of chemical and combustion processes to measure pollution from smoke stacks. These analytical devices require calibration with specialty gas mixtures to comply with environmental legislation.
Few acronyms are as well known in the environmental industry as CEMS, Continuous Emissions Monitoring System. The purpose of the system is clear: stack gas measurement with precision and permanent online availability.
To achieve 100% continuous uptime is challenging and legislation in states across Australia allows for only short periods of outage for calibration or other maintenance activities. With that target in sight, the best CEMS systems will achieve more than 97% uptime on an annual basis. For this uptime to be valid, the analytical instrument must be both functional and properly calibrated.
Calibration of CEMS instruments is achieved with high precision specialty gas calibration mixtures. Compliance with legislation can best be achieved with the use of mixtures which are traceable to national reference materials and have suitable NATA accreditation to back up the traceability claim. The typical choice would be to use ISO 17025 accredited calibration standards, some of the most sophisticated products in the Coregas specialty gases range.
Industries requiring CEMS calibration gas mixtures and typical calibration gases
Natural gas combustion for power generation and other applications such as glass melting
Most natural gas sources contain a high proportion of light hydrocarbons such as methane and ethane. The combustion is relatively clean and produces a narrow emissions spectrum. Oxides of carbon (CO and CO2) and nitrogen (NO and NO2, collectively known as NOx) are inevitable. Selective reduction DeNOx units with, or without, catalysts (SCR or SNCR) are often installed to reduce the NOx emissions and these units require the addition of ammonia, either via direct injection or through the addition of urea which decomposes to form ammonia in the high temperatures of the combustion chamber exhaust gases. NDIR and chemiluminescence analysers are regarded as some of the best direct-read online instruments for CEMS applications to measure these pollutants. An FID might be used to measure uncombusted methane or total hydrocarbons in the exhaust.
As laser measurement technology is becoming more commercially available and sample handling techniques are evolving, ammonia measurement to control the 'slip' of ammonia through the SCR or SNCR unit is also becoming common. Oxygen measurement with a zirconia analyser is often used for combustion process control to adjust the air-fuel injection ratio or will be required to adjust the FTIR measurement readings, if FTIR technology is employed.
A common suite of calibration gas mixtures might be:
6% Oxygen, balance Nitrogen
1000 ppm CH4, balance Nitrogen
100 ppm CO, balance Nitrogen
100 ppm NOx, balance Nitrogen
10 ppm Ammonia, balance Nitrogen
Associated instrumentation gases are likely to be:
Natural gas-fired power station in Port Adelaide, Torrens Island, South Australia
Coal combustion for power generation
In comparison to natural gas, coal is a dirty fuel. The burning of coal is associated with sulphur emissions in addition to oxides of nitrogen and carbon. In some cases, mercury measurement in the emissions gases is also undertaken. The instrumentation selection would often be as for natural gas because the SO2 can also be detected using an NDIR or a UV-pulsed UV-fluorescence detector.
A common suite of calibration gas mixtures and instrumentation gases might be as above for the natural gas combustion, plus:
100 ppm SO2, balance nitrogen calibration gas mixture
500 ppm NOx, balance Nitrogen calibration gas mixture
In some states of Australia, such as Victoria, the NOx CEMS emissions limits from coal-fired power generation are higher than from natural gas, hence the higher NOx concentration in the calibration gas mixture above.
Loy Yang - brown coal-fired power station in the Latrobe Valley, Victoria
Liquid hydrocarbon fuels combustion for power or steam generation
The combustion of highly refined liquid fuels such as automotive petrol results in an emissions footprint to natural gas combustion. During the refining process, most of the sulphur in the fuel is removed so that the emissions are relatively clean. However, the liquid fuels commonly used in power or steam generation will typically be heavier sour fuels, similar to the 'bunker fuel' that has been common in the shipping industry in past decades. These fuels often contain high levels of sulphur and therefore result in SO2emissions from the combustion furnace. Typically the SO2 is scrubbed out from the exhaust gases using an alkaline medium of lime and water. The suite of typical CEMS calibration gases will therefore mirror that required for coal combustion. Instrumentation for liquid fuels combustion CEMS would be similar to the cases above.
Co-combustion of waste materials with natural gas or liquid fuels
Waste incineration for environmental management or the co-combustion of waste products with fuels to produce heat is generally the most challenging of CEMS applications. It is extremely difficult to control the input to the combustion furnace because the waste often comes from uncontrolled sources. In some cases, such as the combustion of rubber tyres, there is a good understanding of the input fuel material, but in many other cases this is not so. The production of comparatively low value products with a high thermal input, such as cement kilns, often relies on co-combustion of waste materials for a low cost fuel source.
The emissions spectrum from waste incineration and co-combustion is very wide. It will include all of the species mentioned above for natural gas, coal and heavy liquid fuels plus many additional exotic pollutants. With such a range of pollutant species to investigate, it would be common to use an FTIR instrument. This analytical technique also allows for speciation within the oxides of nitrogen.
A common suite of calibration gas mixtures and instrumentation gases might be as above for the natural gas and coal combustion, plus: