CEP November 2003 www.cepmagazine.org 51
Environmental Protection
must comply with a wide vari...
Environmental Protection
52 www.cepmagazine.org November 2003 CEP
consider your specific process conditions. It also aver-...
CEP November 2003 www.cepmagazine.org 53
stack tests have their inaccuracies, based on normal
error expected with equipmen...
Environmental Protection
54 www.cepmagazine.org November 2003 CEP
ment of an emissions inventory is the formation of an ap...
CEP November 2003 www.cepmagazine.org 55
that in many cases, data needed in an engineering equa-
tion or material balance ...
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Basics of Preparing an Air Emissions Inventory


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Basic approaches and techniques to estimate air emissions from a wide variety of sources, using approved strategies

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Basics of Preparing an Air Emissions Inventory

  1. 1. CEP November 2003 www.cepmagazine.org 51 Environmental Protection ODAY, A MANUFACTURING PLANT must comply with a wide variety of new air quality regulations, such as the national emission standards for hazardous air pollutants (NESHAPs). Because compliance with such rules is tied, for the most part, to rates of air emissions, it is also tied to operations. Even the highly publicized changes in New Source Review (NSR) regulations, while generally more favorable for industry, still re- quire plants to develop a complete facility-wide emissions profile. In addition, many government agencies in charge of enforcing environmental regulations have under- gone budget cutbacks, resulting in requirements for facilities to self-enforce rules by declaring their compliance with applicable standards and reporting any deviations in periodic reports. This raises the stakes in terms of keeping track of emissions. False- ly claiming compliance can result in serious enforce- ment penalties. How does a facility keep track of its emissions and compliance status efficiently and in a changing work environment? By performing a thorough, technical emissions inventory — that is, by developing pro- cess-specific factors to determine air emissions from all of your processes, preferably linked to easy-to-ob- tain production data. There are several techniques that can be used to estimate emissions from your processes. You may need to employ several of these at your facility. Emission factors The U.S. Environmental Protection Agency (EPA) maintains a compendium of emission factors for many different processes called AP-42 (1), which can be found at www.epa.gov/ttnchie1/ap42. The emis- sion factors are based on information obtained by the EPA over many years, including measurements per- formed on actual operating equipment. Most of these factors are normalized in terms of pounds of pollu- tant emitted per usage (for example, pounds per thou- sand gallons of fuel oil combusted). AP-42 generally offers emission factors in both metric and English units. In addition to the EPA-published factors, some states, industry associations and manufacturers pub- lish their own emission factors based on data they have gathered. An advantage of using AP-42 or other published emission factors is the simplicity of the method. En- gineering calculations and testing are not required, nor is it necessary to hire an engineering consultant. Just look up the appropriate emission factors and multiply them by the usage in the period of time of interest to estimate emissions. Because most facili- ties keep usage records, such as the quantity of fuel combusted, this is simple and verifiable. However, there are several major disadvantages to using AP-42 emission factors. One is the non-specific nature of many of these factors. AP-42 is a compila- tion of emissions data of potentially many types of similar equipment and process conditions; it does not To keep track of its emissions and its regulatory compliance status, a plant must perform a thorough, process-by-process emissions inventory. Preparing an Air Emissions Inventory TT Marc Karell Malcolm Pirnie, Inc.* * The author is now with Environmental Resources Management’s (ERM’s) New York, NY, office, and can be reached at Marc.Karell@erm.com, Phone (212) 447-1900, Fax (212) 447-1904. Used with permission from Chemical Engineering Progress, November 2003. Copyright © American Institute of Chemical Engineers 2003. All rights reserved.
  2. 2. Environmental Protection 52 www.cepmagazine.org November 2003 CEP consider your specific process conditions. It also aver- ages data obtained over a long time period. Although the EPA updates many AP-42 emission factors periodically, any published factor may include emissions information from “older” units that were not necessarily manufac- tured to minimize emissions. Therefore, AP-42 emission factors are considered by most to be conservative, over- estimating actual emissions. Thus, another disadvantage of using AP-42 emission factors is running the risk of overstating your emissions to such an extent that your fa- cility may, on paper, exceed an applicability threshold and thus unnecessarily subjecting your facility to a regu- latory program. Determining emissions using other more site-specific estimation techniques may demonstrate that emissions are considerably lower than calculated using AP-42 factors and that your facility does not belong in a particular regulatory program. If this is not important, then using AP-42 or other EPA-published emission fac- tors can be a quick and acceptable way to estimate emis- sions at your plant — at least as a first step. Overall, emission factors are most advantageous when they are specific to the equipment your plant is using. For example, many manufacturers of combustion equip- ment provide emission factors for different pollutants for the specific or related models of equipment for sale. Be- cause the factors are based on testing of that specific model, there is a good chance that emissions of that unit in your plant will be similar. Material balance Another common technique for estimating emissions is a material balance, where the fate of each compound is quantified throughout its lifecycle in a plant. If a plant is able to estimate the quantity of compound entering the plant (purchased or used in its processes), the quantity con- sumed and the quantity disposed of in solid waste or in its wastewater and lost in any spills, then the difference can be a reasonable estimate of losses by other means, which would mainly be evaporation (air emissions). Many of these quantities can be estimated using stan- dard operating procedures (SOPs) and purchase, batch and waste disposal records. In this case, a material bal- ance could represent a relatively inexpensive method to estimate emissions. However, using material balances has several poten- tial disadvantages. Because the fraction of material not accounted for and, therefore, considered emitted is gen- erally very small, any error in a measurement or calcula- tion of any parameters will have a major percentage im- pact on the emissions estimate. For example, consider a plant that uses 100,000 lb/mo of a solvent to facilitate a chemical reaction. It estimates 98,000 lb of the solvent is disposed of in waste based on measuring the contents of selected waste drums and wastewater samples. There- fore, by applying a material balance, we find that the plant emits into the air 1 ton/mo of that solvent. Howev- er, if the error in measuring the contents of the various waste drums and wastewater was only about 2%, then the total quantity of the solvent emitted could have been closer to 4,000 lb, twice that of the original estimate. For a complex material balance with many fates of the com- pound in question, even larger calculation errors would be common. More important, material balances may pos- sibly underestimate emissions, potentially resulting in compliance issues for the plant. For this reason, in the background document for the Miscellaneous Organic NESHAP (MON) maximum achievable control technolo- gy (MACT) standards, the EPA states that facilities should not use material balances to estimate emissions from batch processes. Therefore, it is generally recommended that material balances only be used to estimate emissions for process- es whose chemicals have a known, simple fate. For ex- ample, material balances could be useful in estimating emissions from coating operations. The solvent that car- ries the pigment or resin to the substrate is fully emitted into the atmosphere. Solvent emissions can, thus, be sim- ply and accurately estimated as equal to the solvent frac- tion of the quantity of coating used. Direct measurement Direct measurement of emissions, or “stack testing,” to develop emission rates represents the “true” emission estimation technique, since a part of the actual exhaust is sampled during operation and analyzed. Typically, a probe is inserted in the exhaust to pull out a representa- tive sample, which is transported to a laboratory for anal- ysis or for immediate analysis in a continuous emissions monitor (CEM). The EPA and some states have pub- lished techniques for sampling and analyzing that must be adhered to. Many states require approval of a formal protocol and final report for the findings to be accepted for permitting or compliance purposes. An advantage of stack testing is its acceptance. If per- formed according to protocol, the results essentially are indisputable and considered an accurate representation of emissions from that process under those conditions. A major disadvantage is the cost. Generally, a spe- cialty firm with experienced testers, the right equipment, and a laboratory is hired to perform stack testing. Be- cause triplicate sampling is necessary, even basic stack testing of one point from one process can cost at least several thousand dollars. For testing of several pollu- tants and several process conditions and/or stacks, the cost can be significantly higher. In addition, stack test- ing represents a “snapshot” of emissions under those specific process conditions during the time of the test. For a complex process or for many processes, the emis- sions measured during the stack test may not be repre- sentative of the entire process or facility. Finally, even
  3. 3. CEP November 2003 www.cepmagazine.org 53 stack tests have their inaccuracies, based on normal error expected with equipment and sample handling dur- ing field sampling and lab analysis. Stack tests are most useful when determining emis- sions from a small number of specific sources and steps. Engineering equations Emissions may also be estimated based on equations that are themselves based on the fate of the compounds during the physical actions that they undergo during pro- cess steps. The driving force of the physical action and the chemical properties of the components, mainly the volatilities, influence the emission rate. The EPA has published several documents or rules containing such en- gineering equations to estimate emissions, such as Refs. 2–3. While the equations are not provided here, many are derived from the ideal gas law. Engineering equations that may be used to estimate emissions are available for the following common industrial process steps: Equipment filling. When a volume of material is added to a vessel, such as a reactor or a tank, an equal volume of vapor is displaced and emitted from the ves- sel, laden with volatiles from existing compounds and any being added. The emission rate may be calculated based upon the pollutants’ volatilities and the rate at which the vapor is displaced. The equations compute the vapor mole fractions and emissions of various com- pounds in a multi-component system. Gas sweep. When equipment (such as containers or ves- sels partially filled with liquids) is purged with an inert gas (such as nitrogen), volatile compounds are swept into the purge gas and emitted. The emission rate is determined based upon the rate of the sweep, the pressure of the airspace in the vessel, and the vapor pressures of the pollutants. Evacuation. The emission rate for the contents of a vessel emitted after it has been evacuated is calculated based upon the free space in the vessel, time of evacua- tion, differential system pressures, and vapor pressures of volatile components. Heating. When the contents of a reactor or tank are heated, thermal expansion causes a volume of vapor to be displaced at a relatively high temperature. Emissions are calculated based upon the change in temperature of the components, the exit temperature of the vessel, the system pressure, the headspace volume, and the vapor pressures of the volatile components. Gas evolution. New compounds may be formed and volatilized during a reaction. The rate of evolution of the gas and its molecular weight are needed to determine the vapor mole fraction, from which volatile emission rates may be calculated. Vacuum distillation. Emissions from distillation may be estimated based upon the components’ volatilities. The equations consider the condensation of the exhaust stream to recover solvent. The EPA equation for emis- sions is based upon a driving force (air leaking into the system) and the relative volatilities of the components. Equations have also been published for vacuum dry- ing, evaporation and other operations. Plant personnel can plug actual operating parameters into the appropriate equations to estimate emissions from each batch step of a process. The EPA has fully accepted engineering equations as a valid method to estimate emissions in many applica- tions. For example, emission models (i.e., the use of pro- cess-specific equations) is the preferred method for esti- mating volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions from (4): • mixing operations (material loading, heat-up losses and surface evaporation) • product filling • vessel cleaning • wastewater treatment operations • material storage • spills. In its summary of public comments on one proposed NESHAP (5), the EPA agreed with a commenter’s re- quest to allow a facility to estimate emissions based on engineering equations so it can be less dependent on stack testing. However, control equipment that has an input HAP rate of ≥10 tons/yr must perform stack tests under worst-case operating conditions to determine con- trol efficiency. This also applies to sources affected by other NESHAP-affected facilities, including the pharma- ceutical and MON MACT standards. Using engineering equations as an emissions invento- ry technique has a number of advantages. While many of the equations are based on theoretical relationships, this approach may be superior in many applications to emis- sion factors and material balance, because it is based on actual process conditions and, in many cases, will be more accurate. Another advantage of the engineering equations method is its efficiency, as the same equations can be used repetitively and consistently for dozens or hundreds of operations. Commercially available software (e.g., PirnieAIR, PlantWare, and Emission Master) can auto- mate the process and save time. Therefore, using engi- neering equations, even for many processes and steps, should be significantly less expensive than conducting multiple stack tests. For many processes, engineering equations represent a good compromise in terms of ef- fort, cost and potential error compared to the other tech- niques discussed here. How to perform an emissions inventory Preparation. Compiling a thorough, plant-based emis- sions inventory is a highly technical exercise that com- bines inputs from the production, management and envi- ronmental staffs. Therefore, the first step in the develop-
  4. 4. Environmental Protection 54 www.cepmagazine.org November 2003 CEP ment of an emissions inventory is the formation of an ap- propriate task force with representatives from these dif- ferent disciplines. The task force should be committed to the common goal of determining air emissions, while si- multaneously looking at cost-effective opportunities to reduce the risks of accidental discharges, optimize pro- cess operations, and minimize production costs. While the effort could be aided with experienced professionals from outside the company whose role would be to supply technical expertise and an independent perspective, the tools are available for you to perform the emissions in- ventory totally internally. Assessing emission sources. The task force should begin by assessing all processes that could potentially re- sult in air emissions and categorizing them. Typically, there may be a combustion category consisting of all boilers and engines. Other categories may include wastewater treatment, surface coating, solvent storage, tank cleaning and solvent recovery. Similar products should be in the same group if they contain similar com- pounds and/or are produced in a similar fashion. Determining how to estimate emissions. For each cat- egory, decide which technique previously discussed is most appropriate to estimate emissions. A particular technique may not be ideal for all categories. For exam- ple, depending upon the information available, you may choose emission factors for combustion sources, material balance for surface coating and wastewater, engineering equations for all manufacturing processes, and stack test- ing for a small number of key emission sources. The task force must decide how to apply each technique and what basic data must be collected. Write a plan. At this point, the task force should com- pose a written plan consisting of the goals of the effort, the listed categories and components, the techniques se- lected for each category, the specific approach for each technique, the data needed to achieve the goals, and the specific responsibilities of team members. This plan can save considerable time in keeping the diverse task force focused on the ultimate goals. Also, because this emis- sions inventory may become the basis of re-permitting or of new regulatory requirements, it may be valuable to re- view the plan with people outside the group, such as the corporate environmental department or the appropriate environmental regulatory agency. You do not want to ex- pend all that energy to prepare the inventory only to learn later that the agency does not agree with some of the technical choices, such as a technique chosen to esti- mate emissions of a category or the accuracy of the data to be gathered. Data gathering. To develop an accurate emissions in- ventory for batch processes using engineering equations, the task force must review process information, such as the SOP or batch data sheets, and select the steps that will result in air emissions for categorization (filling, heating, etc.). Relevant information needed to use the equations, such as the charging rate and chemicals pre- sent, must be gathered for every emitting step. In addi- tion to reviewing the plant’s SOPs, permits, flow dia- grams, site maps and process equipment layouts, the task force should discuss operations with knowledgeable plant operators. While this task could potentially result in a large volume of data, it can represent an easy-to-ac- cess “encyclopedia” that may have many other uses in the future. As discussed earlier, commercially available software can efficiently store information, as well as quickly and consistently compute emissions. Be aware Emissions inventory leads to major cost savings This example illustrates how a plant got a direct monetary benefit from a thorough emissions inventory. A paint manufacturing firm had used the AP-42 emission factor of 1% of total solvent usage in order to determine emissions from its paint manufacturing operations. This simple factor enabled the plants to quantify emissions easi- ly and cheaply. A number of its plants obtained Title V oper- ating permits based on this and, for the most part, operated no air-pollution control equipment for solvent emissions. However, anticipating that the MON MACT rule might af- fect the company’s facilities and require a huge capital in- vestment to install and operate stringent VOC controls, the firm used an emission estimation software program to per- form a thorough process-oriented evaluation of the emis- sions at many of its plants in several states. Some assess- ments were performed only by internal personnel, while other plants, short of manpower, contracted with an out- side engineer. The plants determined categories of paint products and selected a single complex product with a high solvent concentration to represent emissions of all products in each category. Using the software, these plants determined that the AP-42 emission factor had overstated emissions significantly. The newly calculated emission rates developed using engineering equations and process-specific information demonstrated that VOC and HAP emissions were below the applicability thresh- olds, exempting these facilities from Title V and MON MACT requirements. Several regulatory agencies reviewed the emissions inventories and equations for estimating emissions, concurred with the new information, and reper- mitted the plants as non-major sources. Without this effort, each plant would have had to spend hundreds of thousands of dollars on capital costs and considerable annual operating and maintenance costs for air-pollution control equipment that would have con- trolled much lower levels of emissions than they would have been designed for.
  5. 5. CEP November 2003 www.cepmagazine.org 55 that in many cases, data needed in an engineering equa- tion or material balance may not be readily available and may take more time than expected to uncover. Data collection should be based upon reasonable worst-case conditions and should anticipate, where possible, fore- seeable changes in plant conditions. Develop emissions and prepare inventory. Once the data have been collected, run the calculations to develop the emission rates, whether via engineering equations, material balance or emission factors. Where possible, the emissions calculated should be normalized to a con- venient unit so that the same basis can be applied to other processes in the category and future processes. For example, natural gas combustion emissions should be computed in pounds of pollutant per million cubic feet of gas so that annual emissions can be easily computed based on natural gas usage. Similarly, for many pharma- ceutical and chemical manufacturing processes, emis- sions should be computed in pounds of pollutant emitted per kilogram, thousand pounds, or thousand gallons of product manufactured. It is very important to perform a quality check on the calculations. With the large amount of data involved, it is inevitable that even simple errors will occur, such as recording incorrect information. The task force should perform, at a bare minimum, a reality check to ensure that inappropriate emissions have not been calculated (e.g., an insignificant step in a process that results in very high emissions, a material balance calculation that shows negative emissions). In addition, emissions from sources that contribute significantly to total plant emis- sions and those that are relatively critical to compliance should be thoroughly reviewed. The task force should plan for some time to recalculate key quantities. The final emissions inventory should be recorded both electronically and in paper form. One or several sub- folders and binders may be necessary to record the infor- mation. While the final emissions inventory should con- tain a summary section so that managers quickly see the bottom line, it should also contain as much process data as possible in case questions arise in the future or pro- cess changes are implemented. While the members of the task force are ensconced in data and assumptions as they are performing the emissions inventory, it is very likely that small, but critical, details will be forgotten over time. Therefore, keep thorough records of data and as- sumptions, even if they seem elementary. The emissions inventory should be a “living” docu- ment. If changes in operations or an expansion are pro- jected, then the inventory should be revisited to deter- mine if emissions will change. A thorough emissions inventory should be easy to edit, particularly if software is used. The plant should perform a minor inventory re- view on a routine basis, typically every two or three years. The value of an emissions inventory While there is no doubt that a thorough, process-based emissions inventory represents a significant investment of time, the value more than makes up for it, both in the short term and the long term. A thorough emissions in- ventory informs the plant of which air quality regulations apply, and which ones do not, in a definitive manner. If the inventory demonstrates that air-pollution control equipment is necessary to comply with a particular re- quirement, then accurate technical information will be available to assist in the design of the equipment, which can result in a cost savings. The cooperation of process and environmental staff and the use of the emissions in- ventory to anticipate change are additional benefits. Fi- nallly, many facilities that have done this have learned that the emissions inventory based on actual process con- ditions makes an excellent teaching tool for new process and environmental engineers. CEP MARC KARELL, P.E.*, is a senior project engineer at Malcolm Pirnie, Inc. (104 Corporate Park Dr., Box 751, White Plains, NY 10602; Phone: (914) 641-2653; Fax: (914) 641-2645; E-mail: mkarell@pirnie.com). He has 18 years of experience in air-quality permitting, emissions inventories, air pollution control, and monitoring for a variety of chemical process industries, and has worked in industry, consulting and for government. He has a BS in biochemistry from New York Univ., an MS in biochemistry from the Univ. of Wisconsin, and an MS in chemical engineering from Columbia Univ. He is a licensed professional engineer in New York, is a member of AIChE, and has published many articles on industrial air- pollution control. Literature Cited 1. U.S. Environmental Protection Agency, “Compilation of Air Pol- lutant Emission Factors, AP-42, Fifth Edition, Volume 1: Stationary Point and Area Sources,” U.S. EPA, Office of Air Quality Planning and Standards, Research Triangle Park, NC, available at www.epa.gov/ttnchie1/ap42 (chapters updated on an ongoing basis). 2. U.S. Environmental Protection Agency, “National Emission Stan- dards for Pharmaceutical Production,” 40 CFR, Part 63.1257. 3. U.S. Environmental Protection Agency, “Control of Volatile Or- ganic Compound Emissions from Batch Processes,” U.S. EPA, Of- fice of Air Quality Planning and Standards, Research Triangle Park, NC (1994). 4. Emission Inventory Improvement Program (EIIP), “Preferred and Alternative Methods for Estimating Air Emissions from Paint and Ink Manufacturing Facilities,” published jointly by the State and Territorial Air Pollution Program Administrators (STAPPA), the Association of Local Air Pollution Control Officials (ALAPCO), and U.S. Environmental Protection Agency (EPA), Volume II: Chapter 8, Table 8.3-1 (Aug. 2000). 5. U.S. Environmental Protection Agency, “The NESHAP for Polyether Polyols Manufacturing Industry: Summary of Public Comments and Results,” Publication No. EPA-453/R-99-002b, Sec- tion 1.2.10 (May 1999). * The author is now with Environmental Resources Management’s (ERM’s) New York, NY, office, and can be reached at Marc.Karell@erm.com, Phone (212) 447-1900, Fax (212) 447-1904.