January 2007 ISSUE

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The Value of an Accurate Air Emissions Inventory

The onus on Alberta industry to accurately inventory its emissions is greater than ever before. Which system will you use and how will you make sure it performs up to standard?

BY PARESH THANAWALA, P.ENG., QEP
PEGG Contributor

It is of utmost importance for engineers and all others involved in developing and implementing an air quality management plan to ensure the accuracy of estimated air emissions. The consequences of using poor estimates are risky — potential downstream environmental and financial liabilities associated with decisions made on inaccurate data.

Third-party verification of emissions is almost a necessity where emission trading and fees are involved in implementing air quality management plans. Developing an accurate emission estimate is ultimately the operator’s responsibility and should be treated far more seriously than using a traditional “number crunching” process would demonstrate.

The federal government has proposed the new Clean Air Act to protect the health of Canadians, due to concerns about the worsening of the air quality and increasing emissions of greenhouse gases, which are believed to contribute to climate change. The Clean Air Act proposes to regulate and enforce emission reduction targets, especially during the industry growth periods.

It is proposed to reduce emissions of greenhouse gases by 40 to 60 per cent from the 2003 level by the year 2050. Achieving this goal will also require increased use of emerging and innovative technologies. Canadian industries will also have to rely on emission offsets and carbon trading to achieve this goal. An accurate estimation of emissions and verification are important in this process and for any air quality management or risk assessment process.

Both industry and regulators have recognized limitations with respect to data quality of several emission inventories compiled so far. Also, the United States Environmental Protection Agency has formed a working group to deal with uncertainties associated with emission factors and emission inventories.

Improving the quality of emissions estimates is a challenging task for all of us. As such, there are some key steps needed to improve air emission inventories. We need to develop quality improvement programs on an ongoing basis, conduct critical review of estimation methods, and enhance efforts on monitoring of emission sources where estimates are known to be highly uncertain. We also need to conduct research to improve existing tools and develop new tools for emission estimates.

The accuracy of an emission estimate relies heavily on a good knowledge of all influencing variables and their accuracy, as well as the quality of assumptions and algorithms used. In most cases, estimating emissions involves use of sound engineering practice and requires a fundamental knowledge of physics or chemistry.

Emission inventories and estimates are used in several ways such as

• air dispersion modeling/risk analyses leading to ambient air quality and/or health impact assessments to facilitate urban planning or development of emergency response plans

• defining control levels for pollution control technology

• designing heights of stacks and flares

• assessing compliance levels with permits, approvals

• developing regulations, guidelines and strategy to control air pollution on a regional or a national level

• developing emission trading systems

• defining emission fees as an incentive to reduce pollution.

• complying with national or international reporting obligations such as the National Pollutant Release Inventory and the Kyoto Protocol.

Understanding Errors
A simple error analysis can tell us how errors associated with individual variables (or parameters) can compound the error in the end result. As an example, if the volumetric flow rate and the pollutant concentration in the exhaust gases from a stack had an inherent error of 10 per cent each, it is easy to deduce that the error in the emission rate of that pollutant can be up to 20 per cent.

If we stretch this concept to a detailed air quality modeling/risk analysis, it is quite easy to visualize that errors can be compounded at each input of the assessment step (such as accuracy of meteorological and emissions data, risk factors, acceptable health criteria and so on). Errors in individual input parameters can potentially add a significant error in the end results upon which we rely to make important and costly decisions. This example just stresses the value of an accurate emission estimate.

Comparing Methods
Described in Figure 1 are some commonly used methods for estimating emissions. The figure illustrates relative accuracies, costs and risks associated with each method based on an analysis presented by the U.S. Environmental Protection Agency and author’s judgment, experience and knowledge. It is recognized that other estimation methods exist but are not included in this article.
Also, variations in accuracies may occur from those shown in Figure 1, depending on the type of process or equipment.

Continuous Emission Monitors
Continuous emission monitors are generally installed on point sources such as stacks. They provide a highly reliable measure of short-term as well as long-term emission estimate — as long as they are maintained and operated properly.

In Alberta operators are required to follow Air Monitoring Directive, which stipulates quality assurance and quality control requirements of instruments. Continuous emission monitors should demonstrate a minimum uptime of 90 per cent each month and comply with relative accuracy audit requirements of the CEM Code in Alberta.

Source Emission Testing
Stack surveys are useful in assessing short-term emission rates and verify compliance with approved emission limits. Stack survey data also serve to verify CEM data accuracy (and vice-versa).

Operators with point sources emissions are usually required to conduct stack surveys in accordance with their approvals under the Alberta Environmental Protection and Enhancement Act. When stack surveys are conducted according to the Stack Survey Code and by trained and certified personnel, they provide a highly accurate measure of emissions.

Source emission testing can also be used to test non-point sources for emission type and quantity (fugitive or low-level emission sources such as tank/vent emissions, waste water ponds, etc.). Again, it is extremely important to ensure quality of sampling and analysis methods, through quality assurance and control systems, and by following provincial air monitoring directives, codes and guidelines. 

Mass Balance
This method follows the principle that mass of a substance entering a process must equal to the mass of that substance leaving the process, plus that accumulated in the process and lost in products and wastes. Emission estimates are generally of a high quality, except in processes where only a very low amount of the substance entering the process is emitted in the atmosphere.

The method is simple, relatively less expensive and generally well suited for estimating longer-term emissions (daily, monthly or annually), especially if process throughputs are variable. This method is quite suitable, for example, for estimating sulphur dioxide emissions from power plant stacks and volatile organic compounds from furniture-making industry.

The method will not work well, however, for estimating emissions from large area sources, such as tar sands tailings ponds.

Manufacturer’s Data
Manufacturers of pollution control equipment have developed their own emission factors based on conducting a series of tests on air emissions from these equipments. Data accuracy is reasonably good; however, it is highly recommended that the user validates accuracy of these factors by conducting simple stack tests.

Care should be exercised to ensure emission data is used for the correct rating of the equipment (usually heat input, power output rating or inlet gas flow), equipment design (such as number of strokes and lean or rich burn design for an engine), and operating parameters (such as engine speed) for which the factors are applicable.

Emission Factors
The U.S. Environmental Protection Agency provides Air Program, Document 42 emission factors for estimating emissions from several industries, point and area sources. An AP 42 emission factor is a representative value, one that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant.

AP 42 factors are rated for their accuracies as A for excellent; B for above average, C for average, D for below average and E for poor.

Factor ratings do not imply statistical error bounds or confidence intervals about each emission factor. Table 1 presents a frequency count of AP 42 factors in various ratings. These data suggest that only about nine per cent of emission factors posted by the agency in the Factor Information Retrieval System are rated A or B.

Table 1
Rating and Percentage Count of AP 42 Emission Factors

AP 42 emission factors should be used with a lot of care and good judgment. Risks of using emission factors should be evaluated carefully for uncertainty carried through in decision making and their implications to costs for protection of the environment and human health.

In general, factors should not be used to define a control technology capability (such as maximum achievable or best control technology), set approval/permit limits, check compliance, and define emission reductions for a control technology. Factors of a higher quality rating can be used where critical decisions on air quality management employ air dispersion modeling and risk analysis methods.
Industry-specific emission factors have generally been arrived at by conducting several source emission tests for a type of industry. In general, industry-specific emission factors are more reliable compared to AP 42 factors.

Professional Role
A sound engineering judgment or assessment of emission methods is almost inevitable for engineers involved in estimating emissions, using them for air quality management plans, defining carbon credits, and designing emission control technology.

Providing leadership in estimating air emissions in most cases is more of an engineering function and responsibility.

Paresh Thanawala, P.Eng., QEP, is president of 2020 Environmental Solutions, which offers services in air quality management, waste management and environmental management systems. Reach him by phone at 403-255-3171 or by e-mail at perry58@telus.net.