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Environmental Protection

ODAY, A MANUFACTURING PLANTmust comply with a wide variety of new airquality regulations, such as the national

emission standards for hazardous air pollutants (NESHAPs). Because compliance with such rules istied, for the most part, to rates of air emissions, it isalso tied to operations. Even the highly publicizedchanges in New Source Review (NSR) regulations,while generally more favorable for industry, still re-quire plants to develop a complete facility-wideemissions profile.

In addition, many government agencies in chargeof enforcing environmental regulations have under-gone budget cutbacks, resulting in requirements forfacilities to self-enforce rules by declaring theircompliance with applicable standards and reportingany deviations in periodic reports. This raises thestakes 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 andcompliance status efficiently and in a changing workenvironment? By performing a thorough, technicalemissions inventory — that is, by developing pro-cess-specific factors to determine air emissions fromall of your processes, preferably linked to easy-to-ob-tain production data.

There are several techniques that can be used toestimate emissions from your processes. You mayneed to employ several of these at your facility.

Emission factorsThe U.S. Environmental Protection Agency (EPA)

maintains a compendium of emission factors formany different processes called AP-42 (1), which canbe found at www.epa.gov/ttnchie1/ap42. The emis-sion factors are based on information obtained by theEPA over many years, including measurements per-formed on actual operating equipment. Most of thesefactors 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 generallyoffers emission factors in both metric and Englishunits. In addition to the EPA-published factors, somestates, industry associations and manufacturers pub-lish their own emission factors based on data theyhave gathered.

An advantage of using AP-42 or other publishedemission 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 andmultiply them by the usage in the period of time ofinterest to estimate emissions. Because most facili-ties keep usage records, such as the quantity of fuelcombusted, this is simple and verifiable.

However, there are several major disadvantages tousing AP-42 emission factors. One is the non-specificnature of many of these factors. AP-42 is a compila-tion of emissions data of potentially many types ofsimilar 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 KarellMalcolm 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 ChemicalEngineers 2003. All rights reserved.

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consider your specific process conditions. It also aver-ages data obtained over a long time period. Although theEPA updates many AP-42 emission factors periodically,any published factor may include emissions informationfrom “older” units that were not necessarily manufac-tured to minimize emissions. Therefore, AP-42 emissionfactors are considered by most to be conservative, over-estimating actual emissions. Thus, another disadvantageof using AP-42 emission factors is running the risk ofoverstating your emissions to such an extent that your fa-cility may, on paper, exceed an applicability thresholdand thus unnecessarily subjecting your facility to a regu-latory program. Determining emissions using other moresite-specific estimation techniques may demonstrate thatemissions are considerably lower than calculated usingAP-42 factors and that your facility does not belong in aparticular 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 whenthey are specific to the equipment your plant is using.For example, many manufacturers of combustion equip-ment provide emission factors for different pollutants forthe specific or related models of equipment for sale. Be-cause the factors are based on testing of that specificmodel, there is a good chance that emissions of that unitin your plant will be similar.

Material balanceAnother common technique for estimating emissions is

a material balance, where the fate of each compound isquantified throughout its lifecycle in a plant. If a plant isable to estimate the quantity of compound entering theplant (purchased or used in its processes), the quantity con-sumed and the quantity disposed of in solid waste or in itswastewater and lost in any spills, then the difference can bea reasonable estimate of losses by other means, whichwould mainly be evaporation (air emissions).

Many of these quantities can be estimated using stan-dard operating procedures (SOPs) and purchase, batchand waste disposal records. In this case, a material bal-ance could represent a relatively inexpensive method toestimate emissions.

However, using material balances has several poten-tial disadvantages. Because the fraction of material notaccounted 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 aplant that uses 100,000 lb/mo of a solvent to facilitate achemical reaction. It estimates 98,000 lb of the solvent isdisposed of in waste based on measuring the contents ofselected 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 variouswaste drums and wastewater was only about 2%, then thetotal quantity of the solvent emitted could have beencloser to 4,000 lb, twice that of the original estimate. Fora complex material balance with many fates of the com-pound in question, even larger calculation errors wouldbe common. More important, material balances may pos-sibly underestimate emissions, potentially resulting incompliance issues for the plant. For this reason, in thebackground document for the Miscellaneous OrganicNESHAP (MON) maximum achievable control technolo-gy (MACT) standards, the EPA states that facilitiesshould not use material balances to estimate emissionsfrom batch processes.

Therefore, it is generally recommended that materialbalances 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 estimatingemissions from coating operations. The solvent that car-ries the pigment or resin to the substrate is fully emittedinto 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 measurementDirect measurement of emissions, or “stack testing,”

to develop emission rates represents the “true” emissionestimation technique, since a part of the actual exhaust issampled during operation and analyzed. Typically, aprobe 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 emissionsmonitor (CEM). The EPA and some states have pub-lished techniques for sampling and analyzing that mustbe adhered to. Many states require approval of a formalprotocol and final report for the findings to be acceptedfor permitting or compliance purposes.

An advantage of stack testing is its acceptance. If per-formed according to protocol, the results essentially areindisputable and considered an accurate representation ofemissions 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 stacktesting of one point from one process can cost at leastseveral thousand dollars. For testing of several pollu-tants and several process conditions and/or stacks, thecost can be significantly higher. In addition, stack test-ing represents a “snapshot” of emissions under thosespecific 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

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stack tests have their inaccuracies, based on normalerror 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 equationsEmissions may also be estimated based on equations

that are themselves based on the fate of the compoundsduring the physical actions that they undergo during pro-cess steps. The driving force of the physical action andthe chemical properties of the components, mainly thevolatilities, influence the emission rate. The EPA haspublished 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 arederived from the ideal gas law. Engineering equationsthat may be used to estimate emissions are available forthe following common industrial process steps:

Equipment filling. When a volume of material isadded to a vessel, such as a reactor or a tank, an equalvolume of vapor is displaced and emitted from the ves-sel, laden with volatiles from existing compounds andany being added. The emission rate may be calculatedbased upon the pollutants’ volatilities and the rate atwhich the vapor is displaced. The equations compute thevapor 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 thepurge gas and emitted. The emission rate is determined basedupon the rate of the sweep, the pressure of the airspace in thevessel, and the vapor pressures of the pollutants.

Evacuation. The emission rate for the contents of avessel emitted after it has been evacuated is calculatedbased upon the free space in the vessel, time of evacua-tion, differential system pressures, and vapor pressuresof volatile components.

Heating. When the contents of a reactor or tank areheated, thermal expansion causes a volume of vapor tobe displaced at a relatively high temperature. Emissionsare calculated based upon the change in temperature ofthe components, the exit temperature of the vessel, thesystem pressure, the headspace volume, and the vaporpressures of the volatile components.

Gas evolution. New compounds may be formed andvolatilized during a reaction. The rate of evolution of thegas and its molecular weight are needed to determine thevapor mole fraction, from which volatile emission ratesmay be calculated.

Vacuum distillation. Emissions from distillation maybe estimated based upon the components’ volatilities.The equations consider the condensation of the exhauststream to recover solvent. The EPA equation for emis-

sions is based upon a driving force (air leaking into thesystem) 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 parametersinto the appropriate equations to estimate emissions fromeach batch step of a process.

The EPA has fully accepted engineering equations asa 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 hazardousair pollutant (HAP) emissions from (4):

• mixing operations (material loading, heat-up lossesand 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 onengineering equations so it can be less dependent onstack testing. However, control equipment that has aninput HAP rate of ≥10 tons/yr must perform stack testsunder worst-case operating conditions to determine con-trol efficiency. This also applies to sources affected byother 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 ofthe equations are based on theoretical relationships, thisapproach may be superior in many applications to emis-sion factors and material balance, because it is based onactual process conditions and, in many cases, will bemore accurate.

Another advantage of the engineering equationsmethod is its efficiency, as the same equations can beused repetitively and consistently for dozens or hundredsof 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 conductingmultiple stack tests. For many processes, engineeringequations 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 inventoryPreparation. 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-

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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 tothe common goal of determining air emissions, while si-multaneously looking at cost-effective opportunities toreduce the risks of accidental discharges, optimize pro-cess operations, and minimize production costs. Whilethe effort could be aided with experienced professionalsfrom outside the company whose role would be to supplytechnical expertise and an independent perspective, thetools are available for you to perform the emissions in-ventory totally internally.

Assessing emission sources. The task force shouldbegin by assessing all processes that could potentially re-sult in air emissions and categorizing them. Typically,there may be a combustion category consisting of allboilers and engines. Other categories may includewastewater treatment, surface coating, solvent storage,tank cleaning and solvent recovery. Similar productsshould 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 ismost appropriate to estimate emissions. A particulartechnique may not be ideal for all categories. For exam-ple, depending upon the information available, you maychoose emission factors for combustion sources, materialbalance for surface coating and wastewater, engineeringequations for all manufacturing processes, and stack test-ing for a small number of key emission sources. The taskforce must decide how to apply each technique and whatbasic 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 eachtechnique, the data needed to achieve the goals, and thespecific responsibilities of team members. This plan cansave considerable time in keeping the diverse task forcefocused on the ultimate goals. Also, because this emis-sions inventory may become the basis of re-permitting orof new regulatory requirements, it may be valuable to re-view the plan with people outside the group, such as thecorporate environmental department or the appropriateenvironmental regulatory agency. You do not want to ex-pend all that energy to prepare the inventory only tolearn later that the agency does not agree with some ofthe technical choices, such as a technique chosen to esti-mate emissions of a category or the accuracy of the datato 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 asthe SOP or batch data sheets, and select the steps thatwill result in air emissions for categorization (filling,

heating, etc.). Relevant information needed to use theequations, 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, thetask force should discuss operations with knowledgeableplant operators. While this task could potentially resultin a large volume of data, it can represent an easy-to-ac-cess “encyclopedia” that may have many other uses inthe future. As discussed earlier, commercially availablesoftware can efficiently store information, as well asquickly and consistently compute emissions. Be aware

Emissions inventory leads to major cost savings

This example illustrates how a plant got a direct monetarybenefit from a thorough emissions inventory.

A paint manufacturing firm had used the AP-42 emissionfactor of 1% of total solvent usage in order to determineemissions from its paint manufacturing operations. Thissimple 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, operatedno 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, thefirm 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, whileother plants, short of manpower, contracted with an out-side engineer. The plants determined categories of paintproducts and selected a single complex product with ahigh solvent concentration to represent emissions of allproducts in each category. Using the software, theseplants determined that the AP-42 emission factor hadoverstated emissions significantly. The newly calculatedemission rates developed using engineering equationsand process-specific information demonstrated that VOCand HAP emissions were below the applicability thresh-olds, exempting these facilities from Title V and MONMACT requirements. Several regulatory agencies reviewedthe emissions inventories and equations for estimatingemissions, concurred with the new information, and reper-mitted the plants as non-major sources.

Without this effort, each plant would have had to spendhundreds of thousands of dollars on capital costs andconsiderable annual operating and maintenance costs forair-pollution control equipment that would have con-trolled much lower levels of emissions than they wouldhave been designed for.

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that in many cases, data needed in an engineering equa-tion or material balance may not be readily availableand may take more time than expected to uncover. Datacollection should be based upon reasonable worst-caseconditions and should anticipate, where possible, fore-seeable changes in plant conditions.

Develop emissions and prepare inventory. Once thedata have been collected, run the calculations to developthe 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 toother processes in the category and future processes. Forexample, natural gas combustion emissions should becomputed in pounds of pollutant per million cubic feetof gas so that annual emissions can be easily computedbased on natural gas usage. Similarly, for many pharma-ceutical and chemical manufacturing processes, emis-sions should be computed in pounds of pollutant emittedper kilogram, thousand pounds, or thousand gallons ofproduct manufactured.

It is very important to perform a quality check on thecalculations. With the large amount of data involved, itis inevitable that even simple errors will occur, such asrecording incorrect information. The task force shouldperform, at a bare minimum, a reality check to ensurethat inappropriate emissions have not been calculated(e.g., an insignificant step in a process that results invery high emissions, a material balance calculation thatshows negative emissions). In addition, emissions fromsources that contribute significantly to total plant emis-sions and those that are relatively critical to complianceshould be thoroughly reviewed. The task force shouldplan for some time to recalculate key quantities.

The final emissions inventory should be recorded bothelectronically 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 thebottom line, it should also contain as much process dataas possible in case questions arise in the future or pro-cess changes are implemented. While the members of thetask force are ensconced in data and assumptions as theyare performing the emissions inventory, it is very likelythat small, but critical, details will be forgotten overtime. 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 emissionsinventory should be easy to edit, particularly if softwareis used. The plant should perform a minor inventory re-view on a routine basis, typically every two or threeyears.

The value of an emissions inventoryWhile there is no doubt that a thorough, process-based

emissions inventory represents a significant investmentof time, the value more than makes up for it, both in theshort term and the long term. A thorough emissions in-ventory informs the plant of which air quality regulationsapply, and which ones do not, in a definitive manner. Ifthe inventory demonstrates that air-pollution controlequipment is necessary to comply with a particular re-quirement, then accurate technical information will beavailable to assist in the design of the equipment, whichcan result in a cost savings. The cooperation of processand 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 learnedthat the emissions inventory based on actual process con-ditions makes an excellent teaching tool for new processand 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 18years of experience in air-quality permitting, emissions inventories, airpollution control, and monitoring for a variety of chemical processindustries, and has worked in industry, consulting and for government.He has a BS in biochemistry from New York Univ., an MS in biochemistryfrom the Univ. of Wisconsin, and an MS in chemical engineering fromColumbia Univ. He is a licensed professional engineer in New York, is amember of AIChE, and has published many articles on industrial air-pollution control.

Literature Cited1. U.S. Environmental Protection Agency, “Compilation of Air Pol-

lutant Emission Factors, AP-42, Fifth Edition, Volume 1: StationaryPoint and Area Sources,” U.S. EPA, Office of Air Quality Planningand Standards, Research Triangle Park, NC, available atwww.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), “Preferredand Alternative Methods for Estimating Air Emissions from Paintand Ink Manufacturing Facilities,” published jointly by the Stateand Territorial Air Pollution Program Administrators (STAPPA), theAssociation 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 forPolyether Polyols Manufacturing Industry: Summary of PublicComments 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.