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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

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

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