The direct measurement of emissions, performed by Queensland and NSW Laboratory , can be undertaken by either:
- Continuous emission monitoring systems (CEMS) or
- Discrete sampling tests (“Stack test”)
Continuous emission monitoring systems (CEMS) provide a continuous record of emissions over an extended and uninterrupted period of time. It allows for the continuous determination of a gaseous or particulate matter concentration or emission rate using pollutant analyzer measurements. Once the concentration and flow rate is known, emission quantities are calculated by multiplying the pollutant concentration by the volumetric flow rate over a period of time. The volumetric flow rates are either measured continuously or derived from other measurements such as fuel rates, fuel and combustion characteristics.
Advantages CEMS provide an effective means to verify pollutant emissions compliance from combustion devices. They are considered the most accurate method for measuring emissions over time. CEMS have several advantages including real-time results, availability of operational data for system optimization and information concerning the temporal variations of emissions. Limitations CEMS may not be very accurate for pollutants having very low concentrations and are also expensive. Retrofits of CEMS into existing facilities can be very expensive. CEMS also requires regular maintenance and calibration of analysing equipment. Initial manual testing is required to evaluate the CEMS performance. In addition periodic testing may be required to provide a check on the results produced from continuous monitoring. There may be missing data when daily zero and span checks and maintenance tests are carried out.
Applicability CEMS are well suited for monitoring of criteria pollutants such as SO2, NOX, and opacity. Particulate matter concentration can be estimated from correlation tests between opacity and particulate concentration. In NSW, coal-fired power stations are required to monitor SO2, NOX and opacity continuously. The US’s Standards of Performance for Electric Utility Steam Generating Units specifies that all existing coal-fired units having a generator greater than 25 megawatts and all new coal units must use CEMS for monitoring SO2 , NOX , flow, and opacity. Facilities that are regulated under the Acid Rain Program are required to install a diluent gas (O2 or CO2 ) monitor in addition to the above The requirements for CEMS have been extended to cover a wider range of pollutants such as volatile organic compounds (VOC), H2S and CO .
Potential applications of CEMS for monitoring of hazardous air pollutants are discussed in a handbook prepared by the USEPA. There are basically two methods that are used to monitor gas concentrations continuously: extractive and in-situ. In extractive systems, gas is withdrawn from the stack, conditioned then analyzed, whereas in in-situ systems gas is not extracted but analyzed directly in the stack by the analyzer. The choice of a CEM depends on both the regulatory requirements and the types of pollutants monitored. All CEM systems are able to sample, analyze, and record data at least every 15 minutes.
All emissions and flow data can be reduced to 1-hour averages. Total emission over a year can be calculated by adding the 15 minute or hourly average emissions during the period. This can be very time consuming. A longer averaging time may become necessary for example a 30-day rolling average emission value. Performance specificationsPerformance Specifications that are used to determine whether a particular CEM is acceptable at the time of installation are included in Appendix B Part 60 of CFR 40. The performance specifications are the minimum procedures that are required to determine if a CEMContinuous Emission Monitoring is capable of providing reliable measurements. The NSW EPA has also prescribed continuous emission monitoring methods by adopting the USEPAUnited States Environmental Protection Agency’s performance specifications.
In general, short-term stack tests will give the most accurate data for any given point in time. Typically stack tests provide better long term (annual) emission estimates if the test is undertaken under conditions which are representative of the normal operating conditions. Unlike CEMS which are applicable to a restricted number of substances, stack tests can be used to obtain emission estimates for all of the pollutants of relevance to coal fired combustion - from the criteria pollutants to hazardous air pollutants.
Limitations Stack testing provides a “snapshot” of emissions during the period of the test. It is applicable only to the conditions at the time of the testing or monitoring. Differences from run to run may occur even under identical operating, sampling and analytical conditions. As a result, duplicate or triplicate sampling may be required in some cases. Considerable labour resources are required in undertaking stack tests and hence this method can be expensive if used on a routine basis. In addition, analytical costs for substances like dioxins can be very high.
For many substances emitted in low concentrations, reporting based on stack tests may show significant variability due to measurement and analytical variability and uncertainty. In such cases other EETs may be more appropriate. Difficulties in obtaining representative samples limit the application of this method to power stations. Issues such as very large duct cross-sections, complex flow patterns and fluctuations in velocity, particulate loading and temperatures can influence the accuracy of the test results.
Applicability: A discrete sampling test is often used for compliance testing and as a reference or calibration test for CEMS. It can also be used to determine pollution control efficiency or to optimize operation and improve energy efficiency. The compliance test is normally performed at process conditions affecting the maximum pollutant emissions eg at maximum production rates. Therefore it may not accurately reflect the emissions that would result under normal operating conditions.
When using stack sampling test data to estimate emissions, tests should be conducted under “normal”operating conditions Base load coal power stations lend themselves to discrete sampling tests. They are normally operated in a regular and consistent way. Their emissions do not vary much with time, therefore tests at periodic intervals can be used to determine annual average emissions.