Emissions trading—the ability for firms to buy, sell and trade emissions allowances—has encouraged many organizations to re-think using their continuous emissions monitoring system (CEMS) simply for regulatory compliance. Innovative facilities are finding ways to use information traditionally obtained for regulatory compliance to modify and improve process performance and lower operating costs.

Under the U.S. Clean Air Amendments of 1990, companies may trade SO₂ (sulfur dioxide) allowances in an open market where a dollar value is placed on emissions allowances. Typically, facilities gain emissions credits when they voluntarily reduce emissions below the level required by law, or emit less than allowed by a current allocation or permit. Allowances (credits) are commonly measured in pounds, tons, pounds per day, or pounds per season. For example, one ton of SO₂ equals one allowance. Companies frequently monitor scrubber SO₂ discharge and, by maintaining levels below requirements, can manage their emission allowances to enhance their trading position and profit.

Facilities using lime-fed scrubbers for emissions management can integrate the CEMS SO₂ measurement to the scrubbers' lime feed control strategies to produce savings in several ways:

• Reduce the total amount of lime required;
• Reduce the amount of power required to operate the scrubber;
• Reduce the amount of lime waste and the associated disposal fees; and
• Utilize blends of lower-priced fuels.
• More allowance trading

In 1994, the South Coast Air Quality Management District (SCAQMD) of Southern California established the RECLAIM (regional clean air incentives market) trading program for oxides of nitrogen (NO_X ). The goal of RECLAIM is to reduce NO_X emission levels in Southern California by 75%. Similar NO_X programs may be established in the Eastern United States in the near future.

Controlling excess combustion air is a common method to control NO_X emissions. Using in situ oxygen (O₂) analyzers and/or in situ carbon monoxide (CO) analyzers, the "quality" of combustion can be determined by these measurements. Integrating these measurements to trim combustion airflow, optimum combustion levels can be maintained and lower NO_X emissions achieved.

Lower mandated NO_X requirements are leading to the use of Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) technologies. For example, CEMS can ensure proper ammonia (NH₃) feed to the SCR/SNCR by measuring NO_X levels before and after ammonia injection. Pre-ammonia injection measurements can be used for feedforward control, and post-ammonia injection measurements can be used to trim the feedback control to minimize the excess ammonia (ammonia slip) while maintaining emission levels of NO_X and/or NH₃ below regulatory limits.

For nearly three decades, driven by ever tightening emissions regulations worldwide, facilities have invested in analyzers, shelters, sample conditioning systems, sample lines, probes, and other hardware and software simply to prove that their emissions levels meet regulatory requirements. Data gathered by emissions monitoring systems is frequently saved in a DAS (data acquisition system). Today, this same data can be provided to control systems and integrated into control strategies.

Two key factors have spurred the rising use of CEMS as a process control tool. First, with current regulatory requirements mandating at least 95% CEMS data reliability, manufacturers have developed products reliable enough to meet regulatory requirements and tough enough to meet industrial installation and maintenance requirements.

Secondly, improvements in analyzer designs, the increased usage of microprocessor electronics, and standardized data communication protocols make connecting a CEMS unit to a control system easier. Today's CEMS analyzers include advanced diagnostic and operational information that can be digitally communicated to the control system where it can be used to manage emissions, minimize system downtime, and maximize profits.

In addition to regulating the levels of SO₂ and NO_X in stack emissions, CEMS can be used for other process control applications. For example, measurements of opacity, or particulate in the stack emissions, can be used to maximize the blending of different priced fuels to save money, without exceeding emission levels.

CEMS can also be used for electrostatic precipitator control, offering the ability to control collection plate rapping frequency and precipitator efficiency. Electrostatic precipitators use a series of parallel vertical plates to collect and remove emission particulates. Electrodes centered between the plates provide an electric field, which charges passing stack particulates, causing them to migrate to the collecting plates where they form a layer of ash. Typically, this ash layer is removed by periodically "rapping" or striking the collection plates, dislodging the ash in sheets. Using CEMS measurements, users can precisely determine when rapping is necessary, saving precipitator operation and maintenance costs.

An investment in a CEMS should not be viewed as only an expense to comply with pollution agency mandates; rather, by using the latest CEMS developments, valuable contributions to emission and process control can be obtained, leading to enterprise wide cost savings.

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