Compliance monitoring of a stationary source can be accomplished on either a continuous or an intermittent basis. Some stationary sources use a continuous emission monitoring (CEM) system which is permanently installed on a stack or process ductwork to measure the emissions of one or more pollutants. The emissions data is recorded, averaged and stored by a computer data acquisition system (DAS). Instead of, or in addition to, the CEM system measurements, a source may conduct a one-time or periodic compliance emissions test to measure the magnitude of one or more of its emissions. A test typically consists of three discrete measurement runs, each run lasting one or more hours depending on the test method and pollutant concentration(s).
A CEM system can be used to demonstrate continuous compliance with an applicable emissions standard, to calculate total emissions over a particular period of time, to show the effectiveness of pollution control equipment, or as a means of controlling a process or to monitor operations. Typically, the CEM system consists of a sampling probe, a sample conditioning system to remove moisture and/or particulate, a sampling pump, and one or more gas concentration analyzers for measurement of the pollutant concentrations. In some cases, a volumetric flow monitoring system may also be installed to provide a stack gas velocity measurement. The DAS collects the continuous stream of concentration measurements from the gas concentration analyzer(s) and averages and stores the data. The DAS can calculate from the pollutant concentration and stack gas velocity a pollutant mass flow rate in pounds per hour (lb/hr) which can be averaged and stored. In some cases, the pollutant emissions are expressed as a ratio of pollutants emitted to the heat input (i.e. fuel combusted by the device or process), typically in the units of pounds of pollutant per million British thermal units (lb/MMBtu). If the CEM system is required by a state or federal rule or program, the CEM system is required to be calibrated daily, audited quarterly and recertified at least once per year. The CEM system must meet the accuracy standards of these quality assurance procedures for the data to be considered "valid".
An advantage to using a CEM system is that it provides emissions data under all source operating conditions, including varying loads and operating scenarios, and during malfunctions, startups and shutdowns. Typical pollutants that can be monitored by CEM systems include the products of combustion (oxides of nitrogen, carbon dioxide and sulfur dioxide), diluent gases (carbon dioxide and oxygen), and less common gases and pollutants including, but not limited to mercury, total reduced sulfur, volatile organic compounds, and particulate matter.
Continuous opacity monitors (COMs) are another kind of CEM system which monitors the opacity of a smoke plume. Opacity is a measure of the degree to which the smoke blocks visible light and, although it is not necessarily directly proportional to the amount of particulate matter emissions, it is an indicator of overall combustion efficiency or control of particulate emissions.
Sources which are required to comply with a pollutant cap- and trade program or that choose to participate in an emissions trading program will usually install and operate a CEM system to measure the actual amounts of pollutants emitted and the number of credits (typically equivalent to one ton of a pollutant) that it needs to purchase or that it can trade, bank or sell.
Compliance Emissions Tests
A compliance emissions test, also commonly known as a stack test (although not limited to only stack locations), is conducted to measure the amount of air pollution that is emitted from an emissions point at a stationary source. The emissions test is performed following specific procedures developed by the US Environmental Protection Agency (EPA) or, depending on the pollutant and industry involved, by a method developed by one of the other Federal or state regulatory agencies. The New Hampshire Department of Environmental Services (DES) requires a stationary source to conduct compliance emissions testing when DES has determined that it is necessary for the source to demonstrate its compliance status with applicable emission limits. Approximately 50 compliance emissions tests are performed in New Hampshire each year, all of which are coordinated with and witnessed by a DES representative.
A typical compliance emissions test consists of three measurement runs, each run lasting one or more hours depending on the test method and pollutant concentration. The testing should occur during conditions of plant operation that are representative of normal operation, but also during which maximum emissions are expected. In some cases, testing is done at multiple loads. Typically, the test collects concentration data for one or more compounds or pollutants of interest, the percent oxygen and/or carbon dioxide, the percent moisture content, as well as volumetric stack flow. In the case of pollution control devices that may be required to meet a minimum destruction or removal efficiency, compliance emissions testing must be done at both the inlet and outlet of the device. Generally, each compliance emissions test requires: (1) the submittal to DES of a pretest protocol at least 30 days prior to the test date which describes the emissions stack testing program description; and (2) a pretest meeting to be held approximately 2 weeks prior to the test date in order for all parties involved to comprehensively plan for the test. DES is involved in all aspects of the testing and technically reviews and approves the final test report that shows the emission results.
The equipment used to collect and analyze the stack gas being sampled during a compliance emissions test is basically the same as described above for a CEM system. However, instead of being permanently installed, the sampling equipment and analyzers are transported to the source specifically for the test and then removed. Similar to the Continuous Opacity Monitoring system discussed in the section above, there is a specific test method (US EPA Method 9) which has been developed to determine the opacity of emissions from a stack or process. This method is conducted by making visual observations during the test period and requires no analytical equipment.
A compliance emissions test is required if any one of the following conditions applies:
- A stationary source is subject to a Federal air pollution emissions or control standard which requires compliance emissions testing;
- A stationary source is subject to a state rule that requires compliance emissions testing;
- DES has determined that the emission rate estimated for the stationary source using available technical data is close to the threshold above which a violation of the ambient air limit or emission standard might occur;
- A stationary source wishes to opt out of a Federal permitting program, and DES must verify the source’s emissions are below the applicable threshold(s) of the program;
- A source has installed new pollution control equipment to comply with an emission limit or program, or previously installed pollution control equipment is believed to have degraded in its effectiveness to control pollution since the completion of the last compliance emission test or performance verification of the device; or
- There is some other technical concern that a source may be exceeding an emission limit.
Depending on the situation or applicable regulations, the compliance emissions test may be required only once, or it may be required at regular intervals (e.g., once every three to five years) depending on the applicable rules or devices.
For sources required to perform compliance emissions testing as part of the conditions of a Temporary Permit, a charge is imposed to cover expenses incurred by the engineer’s time to travel to the site and witness the test, and to review the final compliance emissions test report.