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Article # 0067

Basics of Surface Coating and Abrasive Blasting Emission Calculations

This article will examine the current calculation procedures approved by the Texas Commission on Environmental Quality (TCEQ) for estimating air emissions from surface coating and abrasive blasting operations. The procedures will be illustrated by giving an example of a process for manufacturing steel storage tanks. “Tank Manufacturers, Inc.” will be the fictional name of the example company.

Steel Tank Manufacturing Process

Abrasive Blasting

Tank Manufacturers, Inc. will manufacture steel tanks from sheets of steel. The steel will be cut and shaped into the appropriate size with hand-held or manually operated machines and then welded. Then, the interior and/or exterior surfaces will be cleaned by dry abrasive blasting using black beauty or steel shot as blast media.

Particulate emissions will be vented to a dust collector that will achieve at least 99.9% filtration efficiency.

Surface Coating

After blasting, the cleaned tank bodies are coated on the interior and/or exterior surfaces using high volume, low pressure (HVLP) spray application equipment. In general, the interior of tanks may be painted with a chemical-resistant lining coating. The exterior of tanks may be coated with a primer and then a topcoat. Low-VOC coatings are used for the surface coating operations whenever they meet the technical specifications of the customer. Particulate (PM) emissions will be controlled with a filter that will achieve at least 99% filtration efficiency.

Maximum Emissions Data and Calculation Procedure

Abrasive Blasting

Steel shot or coal slag will be used as the blast media. The TCEQ particulate emission factors will be utilized to estimate emissions. The dust collector filtration system used in the blast booth will provide a minimum capture efficiency of 99.9% of PM and emissions will be vented through EPN-B.

The TCEQ currently applies the emission factors for coal slag to all blasting media. These factors are:

0.0023 lb/lb usage for PM (particulate size 30 microns in diameter and less)

0.0006 lb/lb usage for PM₁₀ and PM_2.5 (particulate size 10 and 2.5 microns and less)

The usage rate of blast media will be 1500 lb/hr/tank. Two tanks per hour may be blasted, and up to 3000 tons/yr of blast media will be used. The filtration system will reduce emissions by 99.9%. The control factor will be (1 – 0.999 = 0.001).

The resulting emissions are:

Hourly:

(1500 lb media/hr/tank) * (2 tanks/hr) * (0.0023 lb PM emissions/lb media) * (0.001) =

0.0069 lb/hr PM

(1500 lb media/hr/tank) * (2 tanks/hr) * (0. 0006 lb PM₁₀/PM_2.5 emissions/lb media) * (0.001) =

0.0018 lb/hr PM₁₀/PM_2.5

Annual:

(4000 tons media/yr) * (0.0023 ton PM emissions/ton media) * (0.001) =

0.0092 tons/yr PM

(4000 tons media/yr) * (0.006 ton PM₁₀/PM_2.5 emissions/ton media) * (0.001) =

0.0024 tons/yr PM

Surface Coating

The physical properties of three example coatings (one of each type: interior lining, exterior primer, exterior topcoat) are given in the calculations below. The minimum information needed to complete emission calculations for the TCEQ are: (1) density of the material, (2) total volatile and particulate content of the material, (3) maximum hourly and annual usage rate of the material, (4) 100% speciation of the chemicals comprising the coatings, (5) overspray percentage and (6) particulate control efficiency.

Item 4 is needed to evaluate the emissions to determine whether predicted concentrations of each chemical will be detrimental to the area surrounding the plant. Individual chemical species must add up to at least 100% by weight. Often, manufacturers will provide a range for the weight percent of each chemical to avoid disclosing confidential composition information. In this case, the weight percent of the upper end of the range should be used in hourly calculations. The total of the emissions from individual chemical species will often exceed the actual emissions calculated using the overall volatile and non-volatile content of the material.

In this example, all three of the given coatings will be sprayed on the tanks. The particulate control efficiency of the filters will be 99% and the overspray efficiency will be 20% (the value generally accepted for HVLP spray guns.) The TCEQ also generally allows the assumption that 90% of the coating overspray particulate will fall out inside the work area prior to reaching the filters and not be emitted into the air through the exhaust stack. It is assumed that 100% of any volatile emissions are emitted.

Hourly Emissions from Surface Coating

It is conservatively assumed that each type of coating process can take a minimum of one hour (i.e. 1 tank coated per hour). Up to two tanks at a time may be coated with one of the interior, exterior prime or exterior topcoat materials. A maximum hourly emission rate of overall VOC, overall PM and each speciated component is calculated for each coating on a per tank basis.

For the maximum hourly overall PM and VOC emissions, the coatings with the highest VOC and the highest PM content are used as the worst case scenario to be permitted on the maximum allowable emission rate table (MAERT). For example, if the highest-VOC coating had a maximum hourly emission rate of VOC estimated at 10 lb/hr, the permitted VOC emission rate would be 20 lb/hr from EPN-2 (i.e. assuming the highest-VOC coating is used in on two tanks at the same time at the maximum usage rate).

For speciated emissions, a composite theoretical coating will be evaluated using the maximum hourly emission rate of each species from each possible coating. The coating with the highest estimated emission rate of a particular species on an hourly basis will be used as the worst case. For example if coating A has emissions of 2 lb/hr xylene and 4 lb/hr toluene (on a per tank basis), while coating B has emissions of 3 lb/hr xylene and 3 lb/hr toluene (on a per tank basis), the theoretical “worst case” coating would have emissions of 3 lb/hr xylene and 4 lb/hr toluene (on a per tank basis). The permitted emissions would be 6 lb/hr xylene and 8 lb/hr toluene (assuming two tanks coated at the same time.) This procedure will simplify coating calculations while providing a conservative estimate of maximum emissions.

Format of Emission Calculations:

(Usage gal/hr) * (wt fraction volatile) * (density of coating lb/gal) = lb/hr volatile material

(Usage gal/hr) * (wt. fraction PM) * (density lb/gal) * (0.1 fallout factor) * (0.01 control factor) * (0.2 overspray fraction) = lb/hr particulate material

Annual Emissions from Surface Coating

To calculate the annual emissions, an example is given that shows the result when 500 tanks are coated with all three coatings (interior, exterior prime and exterior topcoat). These tons/yr emission rates represent a conservatively high estimate of annual emissions from this process. Please note that all tanks do not receive all three coatings. Many receive only an interior lining coating. Therefore, the number of tanks produced is a minimum number and not an accurate compliance measure for compliance with the annual emission rates.

Emissions Tables

The following tables show the emission calculations for each of the three types of coatings in the example:

Worst Case Hourly Composite Coating Result

The following table shows the composite hourly emission rates for the three coatings. The emission rates in this table are used for purposes of evaluating the maximum possible emission rate of each chemical. In the case of ethyl benzene and xylene, the maximum hourly emission rates of two different coating are evaluated to give the maximum possible hourly emission rate of these chemicals.

Overall Hourly and Annual Emissions Result

The overall annual VOC and PM emission rates are obtained by adding the overall VOC and PM emission results from all three coatings. The overall hourly VOC and PM emission rates are obtained by finding the coating with the highest VOC hourly emission rate for two tanks and the coating with the highest PM hourly emission rate for two tanks. There is no size distribution for particulate emissions and so the PM/PM₁₀/PM_2.5 emissions are assumed to be equivalent.

These results are as follows:

VOC emissions = 74.2 lb/hr and 27.1 tons/yr

PM/PM₁₀/PM_2.5 = 0.06 lb/hr and 0.017 tons/yr

Biography

Karen M. Bullard, P.E. is an Engineering Partner and the President of Bullard Environmental Consulting, Inc. She has over 22 years experience in environmental engineering, compliance and permitting. She worked for the Texas Commission on Environmental Quality (TCEQ) for four years as an Air Permit Specialist in the Coatings and Combustion Section, where she developed a thorough understanding of the governmental procedures and policies in Texas. Karen has a Bachelors of Science Degree in Chemical Engineering from the University of Texas at Austin.

Karen M. Bullard, P.E. No. 88449

Final edition completed June 27, 2015 from previously composed material.

Article # 0067