1 The Road to Single Digit NOx for Oilfield Once-Through Steam Generators John Nowakowski, Tom Robertson Fives North American Combustion, Inc., Cleveland, OH Beverly K. Coleman, Stein Storslett, Nicholas Brancaccio Chevron U.S.A., Inc., Richmond, CA, San Ramon, CA and Bakersfield, CA Conference Paper for AFRC 2013 Executive Summary This paper will walk you through how the San Joaquin Valley (SJV), where heavy oil extraction is enhanced by steam injection, came to have some of the strictest limits on Nitrogen Oxides (NOx) emissions in the world and what technological advances have been necessary to meet these limits while maintaining acceptable steam generator performance. This also paper explains the motivation for a collaborative project amongst the University of Utah, Chevron USA Inc. (Chevron), and Fives North American Combustion, Inc. (Fives North American) to examine NOx formation mechanisms in modern steam generators. The University of Utah will present the technical results of our collaboration in a separate paper titled Evaluating the NOx Performance of a Steam Generator for Heavy Oil Production: Validation/Uncertainty Quantification in the Field. The SJV has been designated as a "non-attainment area" for Federal and California ambient ozone and PM2.5 standards for many years. The SJV Air Pollution Control District (APCD) has promulgated regulations to control direct and/or precursor emissions of PM2.5 and ozone. NOx is a precursor for both pollutants. This has led SJV to have some of the strictest NOx limits on stationary sources in the world. These NOx limits have been a driving force for steam generator burner technology changes in SJV. Steam generators producing 60-80% quality steam are used in thermally enhanced oil recovery (TEOR) processes. In the SJV there are over 400 such generators, of which over 150 are operated by Chevron North America Exploration and Production Company (a division of Chevron U.S.A., Inc.). Over the past 40 years in SJV, NOx regulations have led to reducing steam generator NOx emissions over 95%, from over 300 ppm while burning recovered heavy oil in the mid-1960s and 1970s to limits today of 5 to 15 ppm (depending on when the steam generator was installed and compliance was achieved) with natural gas-burning, low-NOx combustion systems. If NOx reductions beyond the current lowest limits are required, further combustion technology innovation will be required. Meeting the ongoing challenge of ultra-low steam generator emissions requires commitment by both oil producers and combustion system providers. Partnerships, such as the one Chevron and Fives North American have entered with the University of Utah to explore combustion dynamics in steam generators, will also be important during the next 50 years of thermally enhanced oil recovery for heavy oil producers. 2 Steam Generation for Enhanced Oil Recovery Thermally Enhanced Oil Recovery (TEOR) Thermally Enhanced Oil Recovery describes a process whereby a hydrocarbon-containing reservoir formation is heated in order to increase the oil production capability as well as the amount of recoverable reserves that may be extracted from the reservoir. This recovery process is generally utilized for hydrocarbons classified as Heavy Oil (API gravity less than 22.3 °API), which are difficult to extract from the reservoir due to their high viscosity. The viscosity of the oil is inversely influenced by the temperature, typically in a logarithmic relationship, making it possible to increase the oil mobility (i.e., reducing the viscosity) by elevating its temperature. The predominant source of the heat energy for TEOR operations comes from the injection of steam into the reservoir. Once the steam is in the reservoir, the temperature of the reservoir rock and in-situ fluids increases due to the delivery of heat energy by the steam as it condenses. The steam for this process is produced in a special type of boiler, known as the Oilfield Once-through Steam Generator (OTSG). In the State of California, the successful commercial application of the TEOR process began in the 1960's with various oilfield operators throughout SJV.1 Figure 1: Oilfield Once-Through Steam Generator Oilfield Once-Through Steam Generator (OTSG) As shown in Figure 1, the oilfield OTSG has been specifically developed for thermal recovery applications and is based on a forced circulation, once-through concept, where the pressure is contained in relatively small tubes. The OTSG units are generally designed to produce a wet saturated steam, with the steam quality ranging from 60% up to 80% steam, in a single serpentine flow path, and at operating pressures 3 typically between 450 to 2500 psig. Wet saturated steam of this quality may be injected directly into the reservoir formation. For some special TEOR applications, the approximately 20% to 40% residual liquid water may be separated in an external separator vessel and disposed of, leaving steam vapor of essentially 100% quality for injection. The injection of the residual liquid water - which is analogous to blowdown water found in traditional drum boiler systems - depends on reservoir and well completion characteristics; typical practice is to inject the liquid water with the steam vapor, hence the term "wet steam." Steam generators for thermal flooding applications differ from conventional steam boilers in that feedwater containing a relatively high percentage of dissolved solids may be used provided that the solids have been converted to a soluble form and will remain in solution while in the generator tubes. In general, control systems are relatively simple and the operator attention required is less than for a conventional boiler. The source of this feedwater is often the water that is recovered from the reservoir with the Heavy Oil. In mature TEOR operations, this produced water is generally at elevated temperatures and thus represents an opportunity for heat conservation and reduces the amount of fuel required to produce the steam for injection. Today, these steam generators are typically gas-fired and are installed in steam plants which may consist of a number of steam generators with discharge into a common steam distribution system. Standard oilfield steam generators are rated for a gross heat release (HHV) of 62.5 million Btu/hr (MMBtu/hr) or 27.5 MMBtu/hr, though recent developments at local manufacturers have produced a steam generator rated for a heat release of 85 MMBtu/hr. The standard steam generators are often referred to as either 50's or 22's, respectively for 50 MMBtu/hr or 22 MMBtu/hr, respectively. The term ‘50 MMBtu/hr.' corresponds to the net heat added to the working fluid assuming a legacy 80% thermal efficiency. Since the actual performance varies due to many different factors, the convention is to define the generator rating in terms of the gross heat release rating. The 62.5 MMBtu/hr steam generators are the most commonly used units in continuous steam injection applications. Steam Generator Emissions Steam generator emissions typically consist of the following: • Major Products of Combustion (POC): o Carbon Dioxide o Water Vapor • Trace Pollutant Species: o NOX (Thermal, Prompt, Fuel) o Unburned Fuel (CO / Unburned Hydrocarbons) o Particulates • Inert Reactants (Excess Air) 4 NOX Formation and Control "NOX" refers to the sum of nitrogen oxide (NO) and nitrogen dioxide (NO2); NOX permit limits are one of the main drivers for steam generator technology upgrades in the SJV for reasons described later in the paper. The vast majority of NOX formed during combustion in steam generators is in the form of NO which is converted to NO2 in the atmosphere. In steam generators, NO can be formed by three distinct mechanisms: thermal, prompt, and fuel. "Thermal NOx" is formed when air is heated to a high enough temperature to dissociate the N2 and O2 molecules, and the resulting atomic N and O react to form NO as shown by the extended Zeldovich mechanism (as shown below). 2,3 Thermal NO formation is an exponential function of flame temperature and a square root function of oxygen concentration (as shown below). Extended Zeldovich Mechanism O + N2 ↔ NO + N N + O2 ↔ NO + O N + OH ↔ NO + H Thermal NO Formation d[NO]/dt = A[N2][ O2]1/2 exp(-E/RT) "Prompt NOx" is formed when atmospheric nitrogen reacts with hydrocarbon radicals early in the combustion process to form NOx precursors as suggested by the Fenimore mechanism. HCN and CN rapidly react to form NOx.4 Fenimore Mechanism CH + N2 ↔ HCN + N C + N2 ↔ CN + N Bound nitrogen in the fuel can contribute to "Fuel NOx." Examples of bound nitrogen include amines and mercaptans (both of which might be in the gas phase or bound to long-chain hydrocarbons in liquids like crude oil) and ammonia. Fuel NOX is generally a greater issue for liquid and solid fuels than for gaseous fuels, unless ammonia is present in the gas. Nitrogen gas (N2) in the fuel (or in the air) is not bound nitrogen (but could contribute to thermal or prompt NOx). The mechanism by which fuel NOX is formed starts with the pyrolysis of the parent molecule to form HCN and CN and proceeds with same HCN-CN chemistry as the prompt NOx formation mechanism.5,6 For fuels without an appreciable amount of bound nitrogen, thermal NOX is the dominant formation mechanism, though the proportion of prompt NOX becomes noticeable in units operating below 10 ppm total NOX. All industrial combustion installations experience some degree of non-homogeneous combustion, occurring in localized areas within the flame containing varying air/fuel stoichiometry. While near-stoichiometric areas have the highest possible flame temperature, off-stoichiometric (generally fuel-lean) areas have lower flame temperatures; these variations in turn produce respectively high and low NOX in those regions. The conditions that minimize NOX are peak flame temperature lower than 2800 °F, which reduces thermal NOX, and highly uniform fuel/air mixtures which reduce/avoid additional prompt NOX, and low residence times. 5 However, flame temperature can only be reduced so much before the balance between burner stability, low NOX, and completeness of combustion becomes unfavorable. Therefore, the most efficient way to reduce NOX is to optimize the flame region by operating in a relatively narrow temperature range with good mixing conditions. Air Quality Drivers in the San Joaquin Valley Ambient Air Quality Standards The Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to set health-based standards, which are called National Ambient Air Quality Standards (NAAQS), for six criteria pollutants, including ozone (O3), particulate matter (PM), and nitrogen dioxide (NO2). Similarly, California sets ambient air quality standards that are at least as strict as the federal standards. Ozone is a constituent of smog.7 Ozone is created by chemical reactions of NOX and VOCs in the presence of sunlight.8,9 NOx can also contribute to PM2.5 formation; PM2.5 is particulate matter with an aerodynamic diameter less than 2.5 micrometers. Therefore, NOx is a criteria pollutant on its own, but also a potential precursor for ozone and PM2.5. There are several harmful effects associated with ozone, PM2.5, and NOX emissions including irritation of the respiratory system and aggravation of asthma conditions for susceptible individuals. Ambient NO2 standards for the United States (US) and California (CA) over the last five decades are shown in Figure 2. California introduced the first ambient air quality standard for NO2 in 1962 of 250 ppb for a 1-hour maximum value. The California 1-hour NO2 standard remained at 250 ppb from 1962 to 2007, when it was lowered to 180 ppb, and an annual standard of 30 ppb was introduced.10 The United States annual NO2 ambient standard has not changed from 53 ppb since first promulgated in 1971, but a 1-hour standard of 100 ppb was introduced in 2010.11 Figure 2: Ambient Air Quality Standards for NOx 6 Ambient NO2 standards have not changed drastically over the last 40 to 50 years, although new standards have been introduced in the last decade. However, ambient ozone standards have significantly decreased and have been the driver for further NOX reductions in many regions. Ambient ozone standards for the United States (US) and California (CA) for the last six decades are shown in Figure 3. The first ambient ozone standard was set by California in 1955 at 500 ppb of oxidant (as defined by the test methodology) for a 1-hour averaging period. The pollutant of concern was updated from ‘oxidant' to ozone in 1975. The current California ozone ambient air quality standards are 0.090 ppm for a 1-hour averaging period and 0.070 ppm for an 8-hour averaging period, which are stricter than the federal standards.12 The US EPA first promulgated an ambient oxidant standard in 1971, and then an ozone standard in 1979 of 0.120 ppm for a 1-hour averaging period. In 1997, EPA adopted an 8-hour standard of 0.08 ppm, and revised that standard to 0.075 ppm in 2008. In 2005, EPA revoked the original 1-hour ozone standard for areas that had attained the standard, but areas that have not demonstrated attainment are still required to show compliance. 13 Figure 3: Ambient Air Quality Standards for Ozone* *Explanation for the increase in the US 1-hr standard in 1979: The standard changed from ‘total photochemical oxidants' to ‘ozone' only, and ‘not to be exceeded more than one hour per year' became ‘Attainment is defined when the expected number of days per calendar year, with maximum hourly average concentration greater than 0.12 ppm, is equal to or less than 1.'13 NOx can also be a precursor for PM2.5 formation. In 1997, EPA set the PM2.5 NAAQS to 15 μg/m3 on an annual basis and 65 μg/m3 on a 24-hour basis. In 2006, the 24-hour standard was lowered from 65 to 35 μg/m3. In 2012, separate primary and secondary NAAQS were established for annual PM2.5 of 12 and 15 μg/m3, respectively.14 (The secondary standard for 24-hour PM2.5 is the same as the primary standard of 35 μg/m3.) Primary standards set limits to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protection against visibility impairment, damage to animals, crops, vegetation, and buildings.15 7 Designations Once a NAAQS is set for a pollutant, EPA designates each area as in attainment or non-attainment for that pollutant. Ozone non-attainment areas can be classified as marginal, moderate, serious, severe, or extreme non-attainment. If an area is designated non-attainment, the local air district and state must make an emission reduction plan to reach the standard by the deadline set by EPA and submit a State Implementation Plan (SIP) to EPA. Once the SIP is approved it is federally enforceable, meaning it may be enforced by EPA or the public through lawsuits. Penalties or fees may be levied for not meeting the attainment deadline. The San Joaquin Valley is in attainment for NO2, but designated as non-attainment for PM2.5 and extreme non-attainment for ozone. As alluded to above, ozone standards and now PM2.5 standards - not NOX standards - drive NOX regulations in SJV. Ozone and NOx in SJV The SJV has unique and challenging air pollution circumstances because the geography and meteorology of the Valley exacerbate air pollution issues. The Valley has high temperatures, little precipitation, and relatively stagnant wind conditions, all of which are conducive to ozone formation. Pollutant transport from neighboring air basins can also contribute to ozone issues.16 The SJV is also expected to be one of the fastest growing areas of California, and population increases are expected to result in increases of NOX and VOCs emissions, the precursors of ozone. The California Department of Finance projects the population increase in the Valley to be twice as high as the California average increase during 2010 to 2020 (i.e., 18% increase for SJV as compared with 9% for California on average).16 As mentioned before, SJV is designated attainment for NO2, but classified as non-attainment for ozone and PM2.5. Ozone and PM2.5 standards, not NO2 standards, have driven NOX reductions in SJV. From 1997-1999, the SJV had 80 days over the US 1-hour ozone standard, ranking it amongst the worst ozone regions in the nation.17 SJV was initially designated as serious non-attainment. In 2001, SJV was reclassified ("bumped up") from serious to severe for the US 1-hour standard.18 In 2004, the San Joaquin Valley Air Pollution Control District (ACPD) petitioned EPA and was granted approval to be reclassified from severe to extreme for the 1-hour federal standard.18 For the US 8-hour standard, SJV was reclassified to extreme in 2010.18 It should be noted that air quality in San Joaquin continues to generally improve as can be seen in Figure 4, but the area remains in non-attainment because ozone levels are not decreasing quickly enough to meet federal standards. 8 Figure 4: County Days over Federal 1-hr Ozone Standard16 After reclassification to extreme non-attainment, the APCD submitted an attainment plan for the federal 1-hour ozone standard in 2004. The 1-hour standard was revoked in 2005. However, litigation ultimately resulted in reinstatement of some portions of the requirements of the 1-hour standard and areas that had not attained the 1-hour standard were still required to show compliance and pay penalties for non-compliance. After a lengthy process, EPA approved the 2004 plan in 2010, but due to litigation, ultimately withdrew its approval, and SJV withdrew the plan in 2012. As of June 2013, SJV is proposing a new plan to meet the revoked 1-hour standard. The hearing is slated for September 2013.19 In addition to the plan to meet the US 1-hour ozone standard, the APCD has also developed an attainment plan for the 1997 US 8-hour standard (0.08 ppm). The plan was due in 2007 and was approved by EPA in 2012. The final attainment deadline is 2024. An attainment plan for the revised US 8-hour standard that was adopted in 2008 (0.075 ppm) is due in 2015, and the final attainment deadline is 2032.19 Progress towards the federal standards is progress towards the stricter California standards. PM2.5 and NOx in the SJV While efforts to reduce ozone have driven NOx reductions in SJV for the last 40 years, efforts to reduce PM2.5 have also started to drive NOx reductions in the past few years. SJV has a 2008 plan to comply with the 1997 PM2.5 NAAQS,20 and a 2012 plan to comply with the 2006 PM2.5 NAAQS by 2019.21 Both plans include NOx reductions. Air Emissions and Regulations in SJV As a result of ozone and PM2.5 exceedances, the APCD is required to make air pollution regulations to reduce the ambient ozone and PM2.5 concentrations. It should be noted that the APQD has the authority to control emissions from stationary and area sources, but does not have authority to control motor vehicle tailpipe emissions, which are regulated by EPA and the California Air Resources Board (CARB). According to CARB Emission Inventory data,22 mobile sources are estimated to account for 80% of NOX emissions in SJV (2010 projections based on 2008 data are the latest data available, as shown in Figure 5). NOX from Oil & Gas Production, a subset of Stationary Sources, but shown as a separate category in 9 Figure 5, accounts for 0.06% of NOX currently emitted in the SJV. Steam generator NOX emissions would be included in this category. Figure 5: Sources of NOX in SJV (2010 projections based on 2008 data)22 In an effort to improve the air quality in the SJV, the APCD has adopted numerous air emission control rules for stationary and area sources that include NOX emission limits. As shown in Figure 6, these rules have been designed to reduce NOX emissions by an estimated 95 tons per day.23 The total estimated NOX emissions from stationary and area sources was approximately 97 tons per day in 2010.22 The rules that apply to steam generators are 4305, 4306, and 4320 (the red stripes in Figure 6). Figure 6: SJV APCD Planned NOX Reductions for Area and Stationary Sources by Regulation23 History of NOx Reductions in SJV Steam Generation As Heavy Oil resources were being developed in the mid 1960's, hundreds of steam generators began to be commissioned. Originally, the fuel for oilfield steam generators was the same crude oil they recovered - locally variable, but typically containing nitrogen in a range of 0.9% by weight, which 10 contributes to fuel NOX.24 Running ‘uncontrolled' in regards to NOX emissions, steam generators operated at levels up to 350 ppmvd25 (parts per million on a dry volumetric basis, corrected to 3% dry O2). In 1980, the first NOX permit limit in SJV for steam generators was promulgated, and it limited NOx from oil-fired steam generators to 150 ppmvd. Modifications to the combustion system were made to achieve the desired level of NOX emissions reductions, and consisted of staging a portion of combustion air about halfway down the radiant section of the steam generator. This created a locally fuel-rich condition for most of the flame, enabling atomic nitrogen to recombine due to the unavailability of oxygen, thus avoiding the formation of a great deal of fuel NOX.26 In the 1990's, the combination of available and relatively inexpensive natural gas, the increasing value of the produced heavy oil, and evolving environmental regulations led to a series of retrofits which converted the steam generators from oil-fired to gas-fired, updated the control systems to improve performance capabilities, and included modifications that were developed to achieve reductions in the NOX emissions. With the conversion from oil-firing to gas-firing, and the substantial elimination of bound nitrogen from the fuel, generator NOX emissions were reduced to less than 95 ppmvd. The next major effort in the mid-to-late 1990's involved the addition of flue gas recirculation (FGR) to gas-fired steam generators to reduce the NOX emissions to less than 30 ppmvd. FGR is a process which recycles some of the products of combustion (i.e., stack exhaust) back to the intake of the forced draft blower and dilutes the amount of oxygen in the combustion air stream supplied to the burner. This serves to both slow down the combustion process and reduce the peak flame temperatures; thus achieving a reduction in the amount of NOX emissions produced.27 Although FGR reduces NOX, the increased mass flow through the blower - and hence reduced residence time in the generator - carry electrical, thermal efficiency, and maintenance penalties. The next modification to the combustion process consisted of replacing the steam generators' diffusion style burner with an ultra-low NOx burner, which in the case of Fives North American is a lean premix with staged fuel combustion system. The majority of the fuel is combusted in the lean premix core, which results in consistent, low flame temperatures and ultra-low thermal NOX. The staged fuel is diluted by and combusts within the products of combustion (POC) from the lean premix core, which reduces both remaining localized peak flame temperatures and oxygen availability, hence offering low thermal NOX from combustion of the balance of the fuel. This combustion system upgrade required a significant upgrade to the steam generator control systems as well. This modification was successfully able to achieve a further reduction in the NOX emissions levels to a level less than 15 ppmvd without the need for FGR. Used in combination, ultra-low NOx burners with FGR can be used to achieve even lower NOX levels: • 9 ppm limit - Ultra-low NOx burner achievable with ≤ 10% FGR rate • 5 ppm limit - Ultra-low NOx burner achievable with ≤ 30% FGR rate 11 In order to be in compliance with NOx permits limits, operators typically run a unit such that NOx emissions are at least 20% below the permitted limit. For example, with a limit of 15 ppm NOx, the target for normal operation would be roughly 12 ppm NOx. Attempting to meet a NOx limit of 5 ppm means consistently operating the steam generator at 3-4 ppm NOx. With a 5 ppm limit, even 1 ppm is significant. The history of NOX emissions and control technologies are summarized in Table 1. Table 1: NOX Emission Compliance History in SJV Time Fuel Emission Control Technology NOX Emissions, ppmvd (3% O2)* Pre-1988 Heavy oil None Up to 350 ppm 1988 - 1994 Heavy oil Staged combustion air 150 ppm limit (SJV Rule 4405) 1994 - 1997 Natural Gas None 95 ppm limit (SJV Rule 4351) 1997 - 2006 Natural Gas Flue gas recirculation (FGR) 30 ppm limit (SJV Rule 4305) 2006 - 2010 Natural Gas Ultra-low NOx burner 15 ppm limit (SJV Rule 4306) 2010+ Natural Gas Ultra-low NOx burner plus FGR 5-15 ppm limits depending on compliance date and option (SJV Rule 4320) *Note that SJV air quality regulations for steam generators and similar fired equipment typically have several compliance options, and only the option that was most commonly adopted by Chevron is identified here. There are other NOx emissions reduction technologies such as ammonia injection, either in the presence (selective catalytic reduction - SCR) or absence (selective non-catalytic reduction - SNCR) of a catalyst. These have been evaluated28, but not widely implemented for oilfield steam generators. Due to a significant number of operational, maintenance, and performance limitations, this technology is not practical for use with OTSGs. Modifications to the combustion system or the operating conditions are preferable to installing separate NOx control units such as SCRs, so long as combustion adjustments are safe, cost-effective and do not unacceptably decrease performance or increase emissions of pollutants other than NOX. The ultimate goal is to find robust, pollutant-optimized, energy efficient and cost-optimized control technologies. 12 Conclusions and Future Work Oilfield steam generator usage for thermally enhanced oil recovery has now surpassed the half-century mark in SJV. Utilizing combustion technology innovation, emissions from steam generators have decreased as NOX limits have declined radically during this period, from historical levels of over 300 ppm to today's limits of 5 to 15 ppm, as shown in Figure 7. Figure 7: Typical NOx limits over time for steam generators in the San Joaquin Valley Coupling this trend with the expected California population increase and ongoing demand for recovered oil, it is reasonable to believe that SJV steam generator NOX limits will remain at an ultra-low level and possibly even decrease further; however, at 5 ppm (and below) everything matters, including air/fuel mixing, air/FGR mixing, accommodation of fuel variation, burner flow distribution, ambient air temperature variation, and FGR proportioning. Meeting the ongoing challenge of ultra-low steam generator emissions requires commitment by both oil producers and combustion system providers. Partnerships, such as the one Chevron and Fives North American Combustion have entered with the University of Utah to explore combustion dynamics in steam generators, will also be important during the next 50 years of TEOR for Heavy Oil producers. Acknowledgements The authors thank Philip Smith, Jennifer Spinti, Michal Hradisky, and Jeremy Thornock at the Institute for Clean and Secure Energy at the University of Utah for their modeling expertise in our collaboration to understand an minimize NOx production in oilfield once-through steam generators. We also thank Karen Graul and Dan Vanderzanden in Chevron Energy Technology Company (ETC) for their guidance of this research project and their thoughtful feedback on this conference paper and presentation. Thanks also to Chris Rabideau, also in ETC, for his significant input on the NAAQS section of this paper. 13 References 1 Rintoul, Drilling Through Time, California Department of Conservation, Sacramento California, 1990. 2 Zeldovich, Oxidation of Nitrogen in Combustion and Explosions, Acta Physicochem. USSR: 21:577-625, 1946. 3 Lavoie, Heywood, and Keck, Experimental and Theoretical Investigation of Nitric Oxide Formation in Internal Combustion Engines, M.I.T. Fluid Mech. Lab. Publ. 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Engineering and Science, 14 (3). pp. 7-13. 8 Haagen-Smit, Chemistry and physiology of Los Angeles smog, Industrial Engineering Chemistry, 44:6, 1342-1346, June 1952. 9 US EPA Basic Information about Ozone: http://www.epa.gov/air/ozonepollution/ 10 California NOx Ambient Standard History: http://www.arb.ca.gov/research/aaqs/caaqs/no2-1/no2-1.htm 11 US EPA NOx NAAQS History: http://www.epa.gov/ttn/naaqs/standards/nox/s_nox_history.html 12 California Ozone Ambient Standard History: http://www.arb.ca.gov/research/aaqs/caaqs/ozone/o-hist/o-hist.htm 13 US EPA Ozone NAAQS History: http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_history.html 14 US EPA PM2.5 NAAQS History: http://www.epa.gov/ttn/naaqs/standards/pm/s_pm_history.html. 15 US EPA National Ambient Air Quality Standards (NAAQS): http://www.epa.gov/ttn/naaqs/ 16 San Joaquin Valley Air Pollution Control District 2012-2013 Report to the Community: http://www.valleyair.org/2012-13AnnualReport.pdf 17 US EPA Region 9, Reclassification of the San Joaquin Valley Ozone Non-attainment Area to Severe, October 23, 2001: http://www.epa.gov/region9/air/sjvalley/pdf/fact1001.pdf 14 18 US EPA Region 9, San Joaquin Valley Ozone Reclassification website: http://www.epa.gov/region9/air/sjvalley/ 19 SJV APCD Ozone Plans: http://www.valleyair.org/Air_Quality_Plans/Ozone_Plans.htm 20 San Joaquin Valley 2008 PM2.5 Attainment Plan (for the 1997 NAAQS): http://www.valleyair.org/Air_Quality_Plans/AQ_Proposed_PM25_2008.htm. 21 San Joaquin Valley 2012 PM2.5 Attainment Plan (for the 2006 NAAQS): http://www.valleyair.org/Air_Quality_Plans/PM25Plans2012.htm. 22 CARB Emission Inventory Data (select Annual Average, NOx, All Sources, 2010 and SJV APCD): http://www.arb.ca.gov/app/emsinv/fcemssumcat2009.php 23 Providence Engineering Report to San Joaquin Valley Air Pollution District on the Analysis of PAMS Data 1994-2007, Contract No 08-10-07, February 2010: http://www.providenceeng.com/P/Files/othertechnicalinfo/455-002-001ER%20Final%20Report%20Narrative.pdf 24 McDanial & Aha, Baseline Repeat Tests: NOx Research and Development Program, Supplemental Report 1, KVB3 42000-1637, 1981. 25 Taback, Determination of Air Pollutant Emission Factors for Thermal and Tertiary Oil Recovery Operations in California, KVB 5807-84242000-1637, 1980. 26 Barnhart & Diehl, Control of Nitrogen Oxides in Boiler Flue Gases by Two-Stage Combustion, Journal of the Air Pollution Control Association, 10:5, 397-406, 1960. 27 Bacon, The Petroleum Engineer, 11:13, 51-56, 1940. 28 Aha, Ammonia Injection Tests: NOx Research and Development Program, KVB3 42000-1192, 1981.