Though considered progressive at the time, the US Clean Air Act, passed in 1969, has proven insufficient. Amended in 1990, sweeping revisions have been implemented in an attempt to curb the three major threats to the nation's environment and to the health of millions of Americans: acid rain, urban air pollution and toxic air emissions. Bill Worthington, ABB Analytical, outlines some new developments in continuous emission monitoring systems (CEMS).
CEMS can broadly be broken into three types: extractive, in situ and parameter. Extractive methods can be further broken into two techniques: direct source level and dilution. In situ methods can be either path or point types. Parameter methods rely on physical measurements such as temperature, pressure, or other process measurements to predict emissions and thus do not incorporate on-line analytical techniques.
In the acid rain programme, dilution extractive techniques have predominated due to the desirability of a wet basis measurement. These systems have been successful in this, but have severe limitations as pollutant concentrations reach lower levels. Variables such as stack temperature, stack pressure and molecular weight have an unfavourable impact on the dilution technique. A dilution sample system is not appropriate where an analyser with the requisite sensitivity is not available.
In general, reactive and condensable gases such as HCl, NH3 and formaldehyde present the greatest measurement challenges. Such gases may react with other components within the stack gas stream: they may condense or be absorbed by liquid condensate within a cold extractive sampling system; they may adsorb onto surfaces; or they may polymerise before reaching the analyser. Thus, depending upon the components making up the flue gas stream, special sampling equipment may be needed and special operation and maintenance procedures may be required to achieve reliable results.
Both extractive methods rely on similar analytical techniques. A current summary of which includes: non-dispersive infra-red gas analysers; ultraviolet (UV) gas analysers; chemiluminescent gas analysers; paramagnetic oxygen; flame ionisation detection gas analysers; gas chromatography; mass spectrometry; and Fourier transform infra-red analysers.
A brief overview of these analytical techniques and advances in their technologies follows.
Non-dispersive infra-red gas analysers (NDIR) use several different detection techniques. Opto-pneumatic detectors, commonly known as Luft detectors (from their inventor, Karl Luft), interference filter photometers (IFC) and gas filter correlation (GFC) are the more predominant types.
The theory of operation of these infra-red methods is similar and is based upon absorption of infra-red energy in the 2-11 micron wavelength range. Simple molecules with less than five or six atoms have infra-red absorption spectra with fine structures. Gases fitting this description are ideal for Luft and GFC types of analysers, which correlate the spectra of the sample gas with the spectra of the pure component of interest. Interference filter correlation is capable of measuring gases with either fine structure in the spectra or broadband absorption.
Recent developments in NDIR analysers include the ability to have multiple optical benches in one analyser and stacked detectors that result in as many as four gases being measured in a single instrument. Microprocessors have added many capabilities to modern NDIR analysers, which include a "platform" approach where a single system controller provides the user interface for as many as three analytical modules.
Improvements have also been made in the calibration of NDIR gas analysers. By using calibration cells - small optical cells with actual test gases encapsulated within - analyser calibration can be checked without bottles of expensive test gases.
Ultraviolet analysers are, in general, more sensitive than NDIR analysers, with the added advantage of water being transparent to UV. SO2 and NOx are frequently measured with this technology. Other possibilities are NO2, H2S, CS2, COS, Cl2, NH3 and other gases. The state-of-the-art UV analyser uses a four-beam optical approach such that a double quotient can be formed, and removes variables such as dirty windows and ageing of components so that the result is a more robust and stable analyser.
Paramagnetic analysers are commonly used to measure oxygen. Paramagnetic analysers are normally used in CEMS to measure oxygen as the diluent gas. Newer analysers of this type are more resistant to corrosion and are smaller than their predecessors, which results in a faster, more stable response.
There are several types of paramagnetic analysers. The more widespread is the "dumb bell" type, or more properly, magnetodynamic, which uses a small dumb bell-shaped optical component to physically determine oxygen concentration. These have the limitation of having moving parts and optical components exposed to the sample gas, which may be corrosive. Another type of paramagnetic is the "Magnetic wind" or thermomagnetic analyser, where there are no moving parts and the construction is of highly corrosion resistant materials, resulting in an extremely robust analyser with a long service life.
Microprocessor technology has also advanced paramagnetic analysers by allowing for surrogate calibration mixtures for difficult or impossible to prepare gas mixtures or very toxic mixtures.
Chemiluminescence is a technique that has been used to measure NO. Widely used for CEMS with low NOx levels, chemiluminescent analysers provide good sensitivity and selectivity to NO.
Flame ionisation detection (FID) techniques are widely used to measure hydrocarbons. During the combustion of organic substances in a hydrogen flame electrically charged particles are produced. The resulting current of these ions is proportional to the organic carbon content. FIDs have very large dynamic ranges, from 10-100,000mg of organic carbon/ m3.
The state-of-the-art hydrocarbon analyser uses a heated system from the point of the sample interface throughout the entire analytical system to avoid cold spots where heavier hydrocarbons may adsorb and cause erroneous results. Eductor systems eliminate the troublesome problem of heated sample pumps. Modern FID analysers feature self-monitoring, automatic fault recognition and logging functions. Some FID analysers also include options for a sparger, or water stripper to measure volatile organics in water.
Gas chromatography is a very versatile analytical technique that separates the sample stream into its individual components for measurement. Process or on-line gas chromatography differs dramatically from its laboratory counterpart, with the only common area being the separation concepts. Current designs integrate a basic analyser (chromatograph), controller and microprocessor package. These systems can perform as stand alone units or be interfaced with multiple chromatographs, distributed control systems, or a host computer.
Significant changes have been made in valves, columns, column systems and detectors. Tough, polyamide-coated fused silica columns with stabilised stationary phases have reached a high level of reliability and are routinely used in process chromatographs. The most popular detectors for on-line chromatographs are still the thermal conductivity and flame ionisation detectors, but other component selective detectors are being used, including electron capture, flame photometry, photo-ionisation and chemiluminescence.
Process chromatographs are used in CEMS when the pollutant of interest is more exotic than the criteria pollutants. In the USA, for example, the CAAA lists 188 hazardous air pollutants, most of which can be reliably measured by on-line chromatography.
Mass spectrometers have been a basic laboratory tool for many years. The mass spectra of a compound provides a positive "fingerprint" identification of that compound. Mass spectrometers also have inherently broad chemical applicability because the ionisation process is fairly uniform for all compounds. One instrument can measure many compounds providing a cost-effective means of monitoring. They are inherently fast, sensitive and capable of wide dynamic range(ppb).
The mass spectrometer is quantitative and linear, since its output is directly proportional to the concentration of the species in the sample. It is a reliable, stable and low maintenance instrument because it is electronically based rather than chemically based as with many single sensors. Finally, the mass spectrometer offers maximum flexibility because its broad capabilities can be selectively used under programmable microprocessor control.
Recent advances Recent advances in technology now provide the elements necessary for continuous on-line applications such as CEMS and air monitoring, including: the turbo molecular vacuum pump, allowing operation in minutes, high reliability and low maintenance; and the quadrupole filter, providing measurement of any mass from 0-400amu, low cost computing power and packaging for continuous operation environments.
The mass spectrometer is used often to measure hazardous air contaminants which require fast analysis. Where many sample points are required, speedy analysis allows a multi-stream sampling concept to be used. Such flexibility makes it possible to monitor many different streams using several different methods for several different gases.
Fourier transform infra-red (FTIR) analysers are another technology that has been taken from the laboratory and used on-line for CEMS following the advent of inexpensive and powerful microprocessors - one advantage being that several gases can be monitored at one time. Heated versions are available that make it possible to monitor hard to handle gases like HCl and NH3 down to 10ppm. These units find applications in the monitoring of combustion sources, toxic waste incinerators and industrial processes as well as ambient monitoring applications for hazardous air pollutants. Basically, the FTIR is able to produce a very detailed infra-red spectra over a range of typically 2.5-25 microns wavelength. Through the power of modern microprocessors, the spectra is calculated from an interference pattern and matched mathematically with the sample data held in memory. A variety of algorithms have been developed to obtain the concentration values from the wealth of data provided by this instrument.
An interesting feature offered by this technique is the storing of digital spectra, not just the calculated values. At some later time, the stored spectra can be processed to determine concentrations of other gas components that were deemed unimportant at the time. For example, if a process upset caused the release of gases not normally present, the spectra could be evaluated to determine if the release actually occurred and to what extent.
The demand created by the environmental regulations of the 1990s, together with the advent of economical and powerful microprocessors, has moved CEMS further along the development curve by focusing on features that save time and money. The promise of increased productivity through microprocessor technology is realised in CEMS through extended online time and remote maintenance capabilities.
Sometimes the best predictor of the future is the past. History indicates CEMS and other types of environmental monitoring can not only satisfy regulatory requirements, but can assist with production cost reduction and efficiency in many cases. Technology and products are available to make most measurements. The formula to the best and most successful monitoring project is to find the most cost-effective approach, that has the best workable solution, and that is in the hands of the end user.