The Federal Clean Air Act Amendments of 1990 require the EPA and various State Regulatory Agencies to monitor, reduce, and control NOx (Nitrous Oxide), NH3 (Ammonia), and SO2 (Sulfur Dioxide) emissions. In the United States, regulatory controls are being imposed on power, cement, paper, and pulp industries. These regulatory controls encourage the installation of such pollution control devices as scrubbers, using NH3 to reduce the emission levels of NOx, and flue gas desulfurization to reduce emission levels of SO2.
Scrubbers are extremely efficient; they can often reduce levels of NOx and NH3 below 10 ppm (parts per million). The difficulty with these applications is obtaining a representative sample from the process in order to measure and verify the NOx and NH3 levels. For that, an emissions analyzer that can measure very low levels (below 10 ppm) accurately and reliably is needed.
Traditional and dilution extractive techniques do not work well in low level applications of NOx and NH3 due to their highly polar (attractive) nature in the presence of moisture. These extractive systems require the transport of the process gases from the point of sample acquisition to the monitor's location. Transporting the process gases requires costly, maintenance-intensive sample handling/conditioning equipment, including heated sample pumps and line, conditioners, air dryers, and air clean-up systems. It is virtually impossible to obtain representative data utilizing such sample transport and measurement techniques.
Where the application permits, the preferred measurement technique is almost always in situ. Traditional in situ measurement requires no sample acquisition; a reliable and accurate measurement can be made under normal process conditions without the formation of ammonia salts, sulfates, H2SO4, or moisture, as often occurs using the extractive measurement techniques.
The newest in situ measurement technique, integrated with a state-of-the-art UV process diode array monitor, offers a very discriminating qualitative as well as quantitative measurement for NOx, NH3, and SO2. This technique can monitor low level NOx and NH3 concentrations of 1-5 ppm. In California, this technology is applied on the control of three scrubbers at these levels. The only maintenance required for these in situ instruments has been the replacement of UV lamps about every one and a half years since their installation, four years ago.
In situ technology works well in high-temperature applications exceeding 1,000(F. It can also be used in high-dust and high-moisture applications and saturated streams with moisture content as high as 50%. Designed for minimal maintenance and unattended operation, these systems require no calibration adjustment for a period of one year.
Clearly, the in situ UV process diode array technology is the preferred method for process and emissions measurements of NOx, NH3, and SO2. Verifiable proof of this comes from TUV, an independent European testing body. TUV test results document a side-by-side comparison of the extractive and the in situ emission measurement techniques at a hazardous waste incinerator. Over a six-month period of operation the results indicate that the in situ measurement technique was the preferred one for measuring NOx, NH3, and SO2. For example, data gathered for the NH3 in situ measurement revealed that monitor uptime was greater than 96%. Standard deviation of each monitor was less than one. The accuracy of the in situ monitor vs. three calibration standards was less than or equal to 0.2% of the instrument range (0-30 ppm).
