Accurate VOC, HAP Measurement Critical For Permit Compliance

Understanding and correctly measuring and reporting ethanol facility volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions are critical for a facility to avoid unexpected, unwanted and costly compliance mistakes. VOC and HAP emissions are generated at various steps in ethanol production. In accordance with each ethanol facility’s air emission permit, these emissions must be measured and reported for certain sources at the facility on a periodic basis.

VOCs and HAPs are regulated by the U.S. EPA. The EPA’s broad definition of a VOC is “any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions,” which encompasses a very long list of compounds. Fortunately that list can be narrowed down considerably for a dry mill ethanol facility.

The EPA’s definition for HAP is “those pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or adverse environmental effects.” This definition is much more limited in scope and, in fact, the original list of HAPs published by the EPA contained only 189 individual compounds or elements. Many of these 189 HAPs are also VOCs, however, only four are generally present and tested for in the air emission streams of a dry mill ethanol facility.

In general, the EPA has delegated the oversight of air emission regulations to the states. The states, however, also apply the EPA definitions and the federal limits in the process of permitting and regulating dry mill ethanol facilities. Each state has some degree of flexibility in how it implements and oversees these programs within its respective region. This results in some variations in air emission permit language and enforcement personnel preferences in each state.

To avoid compliance mistakes, it is important to know what VOC and HAP compounds are typically present at ethanol production facilities and what differentiates a HAP from a VOC. Grasping common terminology regarding the description and reporting of these compounds and the various common methods of measurement for VOCs and HAPs is important as well. Ethanol production facilities that avoid compliance mistakes are aware of the advantages, disadvantages and challenges associated with each of the VOC and HAP measurement methods. Knowing these points is key to maximizing plant ethanol production while simultaneously maintaining constant compliance with air emission regulations.

VOCs Explained
The manufacture of ethanol from corn starch in the dry mill ethanol industry involves the conversion of starch to sugar with enzymes and sugar to ethanol with yeast. A number of VOCs are generated in the fermentation process. Some are controlled by wet scrubbers on the fermentation vents. Additional VOCs are driven off in the handling and drying of the distillers dried grains with solubles and typically controlled by a regenerative thermal oxidizer (RTO). The fermentation scrubbers and dryers RTOs tend to be two of the larger sources of VOC emissions in an ethanol facility.

The VOCs generated are the result of the natural actions of the enzymes and yeast on the various constituents of the corn that have been milled and added to the fermenters. An excellent resource for understanding these processes and the resulting VOCs is “The Alcohol Textbook,” edited by Jacques, Lyons and Kelsall. While no single master list of VOCs from ethanol production has emerged, some common VOCs tested for include acetaldehyde, acetic acid, acetoin, acrolein, 2,3-butanediol, 2,3-butanedione, ethanol, ethyl acetate, formaldehyde, 2-furaldehyde, isoamyl alcohol and methanol. Not all of these VOCs are emitted from all sources, so selectivity is required in choosing which ones to evaluate for a given source.

Some of these VOCs are dependent on the conditions of the fermentation, and ratios of these compounds can vary depending on which specific enzymes and strains of yeast are used. Obviously, ethanol is the product, so it will always be present and in most cases is the most prevalent VOC. Some of the VOCs are not always present and may reflect undesired conditions in the fermentation.

The list of VOCs is subdivided into special compounds of interest known as HAPs. Some of the VOCs listed above are also HAPs, including acetaldehyde, acrolein, formaldehyde and methanol. HAPs are subject to more stringent emission requirements than VOCs in general. For some facilities very stringent limits have been placed on HAP emissions. Acrolein limits, in particular, can require very low mass emission rates.

Testing Methods
The EPA provides several methods for testing and reporting the VOCs and HAPs generated in the production of ethanol. The most commonly used methods include Method 18, Method 25A and Method 320.

In the configuration used in testing ethanol facilities, Method 18 is typically composed of a series of water impingers followed by silica gel tubes. The samples are collected in the field and returned to a laboratory for analysis, typically using gas chromatography with flame ionization detection for the compounds listed above. They can be analyzed on-site if equipment can be provided. Other analytical techniques can also be applied, if needed, for other compounds or requirements. Concentrations and mass rates of individual compounds are measured and reported. Detection limits of individual compounds are variable, but the final detection limit can be controlled by varying sampling rates, times and volumes, and employing sample concentration techniques to achieve very low detection limits.

Method 25A is a field instrumental method (using a heated flame ionization detector) that measures the concentration of VOCs against a standard gas, typically methane or propane. Method 25A does not measure individual compounds, only the total response of the gas stream to the standard, so it is not applicable for determining HAPs nor specific compound emission rates. The detection limit for Method 25A is based on the instrument and calibration gases used, but generally one part per million is typical (as methane or propane).

Method 320 is another field instrumental method that measures VOC and HAP concentrations based on Fourier transform infrared (FTIR) spectroscopy. The method is capable of measuring individual and HAP concentrations. Detection levels are variable depending upon instrument characteristics and the presence and concentration of interfering compounds such as water and carbon dioxide. The availability of necessary reference spectra for all targeted compounds can present challenges.

Another EPA method that is frequently cited is Method 207, designed to aid in determining and confirming the list of compounds that any given specific source may be emitting. For a source that has not been previously screened for appropriate VOC and HAP compound lists, or if the source has undergone an operational change that could affect the VOC and HAP emission profiles, many agencies recommend or require that this method be employed in advance of VOC or HAP compliance testing.

Keeping in Compliance
The air emission permit for each facility will give the various emission levels that each source to be tested must achieve to maintain compliance. There are usually separate limits for VOC and HAP emissions (although the HAP compounds do contribute to the total VOC measurement). Some permits will also include the specific methods to be applied for the emission testing program. Many permits, however, allow sufficient flexibility for the ethanol facility environmental manager and the testing company to present appropriate test methodologies in a written test plan.

Careful review of individual permit requirements is necessary to prepare an efficient and effective request-for-quote for obtaining proposals that will meet a facility’s compliance requirements. Determination of concentration levels and the associated mass rates for each source to be tested is important information in selecting the most appropriate test methodologies to be employed.

Thorough communication of permitting requirements and close consultation with testing firms facilitate an effective and successful emission compliance testing program. Understanding the test methods used and how they relate to VOC and HAP measurement is useful for plant environmental personnel in evaluating air emission permits and compliance test reports.

In summary, VOCs and HAPs are a natural result of the process of producing ethanol in a corn dry milling ethanol facility. Most facilities already have stringent control systems in place such as wet scrubbers and RTOs for the removal of these compounds. Understanding the differences between these compounds and how they are measured and reported is important for the successful operation of the plant and adequate compliance with a facility’s air emission permit.
   
Author: Dan Despen
President, Interpoll Laboratories
763-786-6020
[email protected]