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Article # 0045



by Jack L. Bullard 




This article describes various considerations when renewing an environmental air quality permit in the State of Texas.  A permit renewal or “renewing a permit” means that an “applicant” has already prepared and submitted an application to construct and operate the facility and has been issued such permit, but after a specified period of time (typically between 5 and 15 years of the initial issuance date) is undergoing a review of the “permitted” facilities current operation.  The purpose of the renewal process is for the regulating or permitting agency to have an opportunity to reevaluate the facilities operation after a period of time to see if the facility continues to operate in compliance with current State and Federal rules and regulations and to give the facility a chance to consolidate other permits that may have been issued for the same physical location.   

This article reviews the operation of a previously permitted electric generation facility at a water utility plant.  The water utility plant utilizes purchased electricity for normal operation of it’s facility processes, but as a precautionary measure maintains a small diesel-fired generator to supply electric power in case of an interruption of its main outside source of electricity.  The diesel engine that propels the generator is the main source of potential air pollutants at the facility, but an evaluation  of current operation and equipment inventory determined that a small diesel fuel tank at the facility - that stores fuel for the engine’s operation - is also a source of volatile organic compound (VOC) emissions and its permit is to be consolidated into the primary permit as part of the permit renewal process.  The fictional city is called “Anycity” and the site is assumed to be located in Texas. 


Process Description


Anycity Water Utilities is renewing their air quality permit to continue operation of a standby diesel engine at its Pump Station in Texas. 

The diesel engine’s main purpose is to provide emergency power in case of outage or reduction in the primary electric power. The engine is linked to a 1.0 kilowatt electric generator and is currently limited by permit condition to 50 hours of operation annually.  Anycity plans to retain this level of operation and does not propose any modification that would increase existing emissions potential from the engine. 

In addition to the engine, a 2000 gallon fixed-roof storage tank is located at the site to provide diesel fuel.  The storage tank meets permit-by-rule (PBR) 30 TAC §106.472.   The PBR that covers the storage tank’s operation is to be consolidated into the primary air quality permit that is being renewed. Low-sulfur diesel fuel (or No. 2 Fuel Oil) is supplied to the engine at an average flow rate of 1 gallon per min.  The diesel engine operates at approximately 1000 hp at maximum load conditions to drive the electric generator when necessary. The exhaust can contain the combustion products volatile organic compounds (VOC), nitrogen oxides (NOx), sulfur dioxides (SO2), particulate matter (PM or PM10), and carbon monoxide (CO).


Emissions Data And Calculations


There has been no change in the operation of, or physical modification to, the diesel engine and storage tank that would increase their emission potential above the levels first permitted.  Emission calculations are being updated for the purposes of this renewal using the best source of emission factors available at the writing of this report.  Vendor emission factors were obtained for most air pollutants. The change in emission rates between this application and the original permit are due to different emission estimation techniques only.  The engine emission calculation methodology is based on the TCEQ guidance document entitled, Reciprocating Engines and Stationary Gas Turbines, and common industry practice.


NOx (as NO2), hydrocarbon, and PM (as PM10) emission factors are based on vendor estimates for this model of engine and the load condition that results in the highest hourly emission rate.   An engine load analysis was performed and indicates the maximum NOx and PM/PM10 emission rates occur at 100% engine load and maximum hydrocarbon emissions at 75% load.  No vendor emission factor estimates were available for CO, so a CO emission factor from Environmental Protection Agency’s (EPA) Compilation of Air Pollutant Emission Factors (AP-42), Section 3.3, Page 3.3-6, Table 3.3-1 was utilized assuming maximum load conditions.


The majority of particulate matter from diesel exhaust is less than 10 microns and as a conservative measure PM10 is assumed equal to total PM for the purposes of this emission estimate. 


A hydrocarbons (HC) emission factor is provided by the vendor.  Hydrocarbon is a total organic compound (TOC) measurement that includes both volatile and non-volatile organic compounds as well as methane and ethane.  The common organic compound category for environmental permitting is non-methane, non-ethane volatile organic compounds or NMVOC.  As a conservative measure and for purposes of these calculations it is assumed that NMVOC is equal to HC. 


SO2 emissions are based on the fuel flow rate of 1 gpm at maximum load/brake horsepower and a maximum sulfur content in liquid fuel of 0.5 weight percent (wt%).  Note that the existing permit has a special condition citing 0.5 wt% sulfur in liquid fuel limit.  The SO2 emission calculations assume complete conversion of the sulfur into SO2.


The only other source of emissions is a small 2000 gallon storage tank and associated piping and minor process equipment (valves, connectors).  This equipment operates under permit-by-rule 30 TAC §106.472.  Emissions for the tank were calculated per TCEQ Guidance and the latest version of EPA’s TANKS computer program which incorporates EPA’s Air Pollutant Emission Factors (AP-42) and American Petroleum Institute (API) air emission methodology for storage tanks as applicable.  A copy of the TANKS program is available for download and public use from the U.S. Environmental Protection Agencies (EPA) website.


The latest AP-42 emission factors for fugitives from minor process equipment (e.g., valves, pump seals, connector leaks, and drains) along with TCEQ guidance were utilized to estimate emissions from these fugitive sources.  Copies of the emission factors and guidance are available from the EPA and Texas Commission on Environmental Quality (TCEQ) websites.


Federal Air Quality Regulatory Requirements


Two main federal air regulatory programs are evaluated for the purposes of this article, the New Source Performance Standards (NSPS) and the National Emission Standards for Hazardous Air Pollutants (NESHAPS).


The Pump Station has two main emission sources: the diesel engine that is the subject of this renewal and a 2000 gallon capacity diesel storage tank (and associated piping) operating under permit-by-rule (30 TAC §106.472).  As of the date of this renewal, no NSPS has been promulgated for small diesel engines that drive electric generators.  


The storage tank was built after July 23, 1984 which makes NSPS Subpart Kb (Volatile organic liquid storage vessels for which construction, reconstruction or modification commenced after July 23, 1984) potentially applicable.  But since the tank’s capacity is less than 10,600 gallons (or 40 m3), this subpart is not applicable to the storage tank per 40 Code of Federal Regulations (CFR) Part 60.110b(a). Therefore, no NSPS apply to this permit renewal project.


No NESHAPS have been promulgated for diesel engines that drive electric generators or for small above-ground storage tanks storing diesel fuel.  Therefore, NESHAPS codified in 40 CFR 61 and 40 CFR 63 are not applicable to these facilities or this project.


State Regulatory Requirements


Each state has regulatory requirements specific to that state.  All states must meet federal environmental regulatory requirements, but can create additional requirements or expand requirements as long as they are more stringent then the federal requirements and do not contradict or interfere with federal requirements.  When evaluating the construction of a new emission source or operation of an existing source the permit engineer should become familiar with all local, state, and federal permitting regulatory requirements.  The section that follows list Texas state regulatory requirements in effect as of the date of this article as they pertain only to the diesel storage tank that is being consolidated into the permit.  The tank meets section (1) of  Permit-by-Rule 30 TAC §106.472 listed below: 


Permit-by-Rule §106.472 for 2000-gallon Diesel Storage Tank


Organic and Inorganic Liquid Loading and Unloading. Liquid loading or unloading equipment for railcars, tank trucks, or drums; storage containers, reservoirs, tanks; and change of service of material loaded, unloaded, or stored is permitted by rule, provided that no visible emissions result and the chemicals loaded, unloaded, or stored are limited to:


(1) the following list: asphalt, resins, soaps, lube oils, fuel oils, waxes, polymers, detergents, lube oil additives, kerosene, wax emulsions, vegetable oils, greases, animal fats, and diesel fuels;

(2) water or wastewater;

(3) aqueous salt solutions;

(4) aqueous caustic solutions, except ammonia solutions;

(5) inorganic acids except oleum, hydrofluoric, and hydrochloric acids;

(6) aqueous ammonia solutions if vented through a water scrubber;

(7) hydrochloric acid if vented through a water scrubber;

(8) acetic acid if vented through a water scrubber;

(9) organic liquids having an initial boiling point of 300 degrees Fahrenheit or greater.


Facilities loading, unloading, or storing butyric acid, isobutyric acid, methacrylic acid, mercaptans, croton oil, 2-methyl styrene, or any other compound with an initial boiling point of 300 degrees Fahrenheit or greater listed in 40 Code of Federal Regulations 261, Appendix VIII shall be located at least 500 feet from any recreational area or residence or other structure not occupied or used solely by the owner of the facility or the owner of the property upon which the facility is located.


Best Available Control Technology (BACT)


There has been no change in the operation of, or physical modification to, the diesel engine and storage tank that would increase their emission potential above the levels first permitted.  The diesel engine’s maximum annual operation will remain at 50 hours and the maximum short-term fuel flow rate to the engine will remain at 1 gpm.   At this level of operation each individual pollutant has an emission potential of less than 1 ton/yr.  The station limits sulfur content in the fuel oil to 0.5 wt% by permit special condition.  The size of these units, limits on hour of operations, and emission specifications and parameters meet current state and federal BACT


Conclusions and Considerations


This article discussed some of the considerations involved when preparing a renewal of an existing air quality environmental permit application.  These considerations include:



In conclusion, it is very important for the permit engineer to prepare a history of facility operation, an inventory of its current equipment that may be a source of air pollutants and the current operation of that equipment, and evaluate proposed operation against current state and federal regulatory requirements to assess compliance and propose appropriate changes to demonstrate compliance as is necessary.




Jack L. Bullard, P.E. is an Engineering Partner of Bullard Environmental Consulting, Inc. He has over 19 years experience in environmental engineering, compliance, and permitting. Jack has a Bachelors of Science Degree in Chemical Engineering from the University of Texas at Austin.


Jack L. Bullard, P.E. No. 83547


Final Edition Completed December 27, 2008 using Previously Composed Material.


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