7.6 Power Plants: Gas/Oil Turbines
Categories 297 and 1595
7.6.1 Introduction
Categories 297 and 1595 account for criteria pollutant emissions (particulate, organic, NOx, SOx, and CO) from oil and gas fired turbines at the electric power plants in the San Francisco Bay Area.
Modern, highly efficient power plants use the gas turbine Combined Cycle (CC). A combined-cycle gas turbine power plant consists of one or more gas turbines equipped with heat recovery steam generators to capture heat from the gas turbine exhaust. Steam produced in the heat recovery steam generators powers a steam turbine to produce additional electric power. Use of heat from the main turbine exhaust gas results in high thermal efficiency compared to other combustion-based technologies. Combined-cycle plants currently entering service can convert about 50 percent of the chemical energy of natural gas into electricity. Additional efficiency can be gained in combined heat and power (CHP) applications (co-generation), by bleeding steam from the steam generator, steam turbine or turbine exhaust to serve direct thermal loads, such as food and chemical processing.
7.6.2 Methodology
Point Sources are typically operations that emit air pollution into the atmosphere at a fixed location within a facility, for which the Air District has specific operational information. For many of these point sources the Air District issues a permit to operate, e.g., refinery cooling towers. These could also be a collection of similar equipment / sources located across multiple facilities, e.g., reciprocating engines.
During the permit to operate (PTO) issuance process, the BAAQMD collects information from the operating facility and/or determines from published literature, e.g., EPA’s AP-42 document300 characteristics of a source including maximum throughput, emission factors for emitted pollutants, and control factors associated with downstream abatement devices. These characteristics are then stored for future use in the BAAQMD’s internal database. Facilities that hold a permit to operate are required to renew this permit periodically (this period varies based on facility and source type). Upon renewal, the facilities are requested to provide any updates to source characteristics as well as the source throughput for the last 12 months. This throughput, in combination with the emission factors and controls factors stored in the internal database, are used to estimate annual emissions at the source level. These source level emissions are then sorted and aggregated into categories.
Further speciation and quality assurance of emissions are performed as a part of the inventory process. The BAAQMD staff also perform a systematic crosswalk between the California Emissions Projection Analysis Model’s (CEPAM)301 source category classification (Emission Inventory Code - EICs) and the District’s source category classification (category identification number - cat_ids), which ensures consistency in the annual emissions reporting process (CEIDARS) to California Air Resources Board (CARB). The last part of the inventory development process includes forecasting and back casting, and aggregation into sub-sectors and sectors for documentation purposes. For those years where no data is available, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using either interpolation or another mathematical approach (see Trends section). Finally, emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated for anomalies that are then investigated and addressed.
Categories 297 and 1595 are point source categories and follow the above methodology for emissions estimates.
PM speciation: The PM2.5/PM and the PM10/PM ratios applied to these categories are consistent with size fractions of speciation profiles developed by the California Air Resources Board (CARB) and published on their emissions inventory webpage302.
For category 297, CARB PM speciation profile number is 114; PM2.5 constitutes 97.0% and PM10 constitutes 97.5% of total PM. For category 1595, CARB PM speciation profile number is 123; PM2.5 constitutes 99.2% and PM10 constitutes 99.4% of total PM.
The ROG/TOG ratios applied to this category, or this group of related categories are based on an Air District internal speciation profile. Multiple data sources have been used for developing speciation profiles, such as Air District-approved source tests, TOG speciation ratios used by other regional air quality agencies, and relevant literature including latest speciation profiles developed by CARB303 and the US Environmental Protection Agency304.
For categories 297 and 1595, ROG to TOG ratios are 0.88 and 0.09, respectively. Further assessment and improvement of ROG/ TOG speciation profiles has been planned in future inventory updates.
7.6.3 Changes in Methodology
No changes to methodology were made in this version of the base year emissions inventory.
7.6.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
7.6.5 Trends
(a) Historical Emissions / History
Historical emissions for point sources are derived from source-specific data provided by the facility on throughputs, compiled or reported emission factors, and regulation-based control factors. This information is archived in the BAAQMD’s internal database which is queried to retrieve the data for historical and current years. Interpolation techniques to account for missing data are used when necessary, this is the case for years 1991-1992.
The fuel oil usage at the power plants has been decreasing over the years. Since 1996, electric utility power plants have been prohibited from burning fuel oil by the District Regulation 9, Rule 11, except for emergency situations. Category 297 emissions prior to 1987 were derived from Base Year 1983 trend values. Historical emissions have varied with fuel usage activity. Emission projections for base years prior to 1996 were developed from natural gas and fuel oil usage estimates provided to the District by PG&E.
(b) Future Projections / Growth
Forecasting of point source emissions is done based on calculations as shown in the equation below using recently updated growth profiles and a base year of 2020. The growth profiles for the current base year inventory have been verified and updated to represent the most likely surrogate for growing emissions for a given category up to year 2040. Forecasting for point source emissions includes impact of in-place regulations but does not include estimation of controls that will theoretically be implemented as part of future policy emission targets or proposed regulation and legislation.
\[ \text{PE} = \text{Gr} * \text{Ci} * \text{Ei} \]
\(PE\) = projected emissions of pollutant i in a future year
\(Gr\) = growth rate by economic profile of industry or population
\(Ci\) = control factor of pollutant i based on adopted rules and regulations
\(Ei\) = base year emissions of pollutant i
For Categories 297 and 1595, it is assumed that annual emissions, over the years, would tend to follow the manufacturing, wholesale, and transportation sector jobs growth in the Bay Area Air District.
The employment and population data used were obtained from, the Association of Bay Area Government’s (ABAG’s) 2017 “Projections” reports305.
7.6.6 Uncertainties
A step-increase in TOG emissions for category 1595 may be seen for the period 2009-2010. This is due to a sustained Air District effort to gradually include and update methane emissions factors for various source types over time. For years 1990-2008, high uncertainty in the TOG emissions estimates is expected; further refinement in backcasting of historical TOG emissions is planned in future inventory updates.
7.6.7 Contact
Author: Sukarn Claire
Reviewer: Ariana Husain
Last Update: November 06, 2023
7.6.8 References & Footnotes
USEPA. AP-42. Compilation of Air Pollutant Emission Factors. https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors↩︎
California Emissions Projection Analysis Model (CEPAM). https://ww2.arb.ca.gov/applications/cepam2019v103-standard-emission-tool↩︎
PMSIZE. CARB. 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
ORGPROF. CARB. 2022. https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
SPECIATE. USEPA. 2022. https://www.epa.gov/air-emissions-modeling/speciate↩︎
The Association of Bay Area Governments (ABAG). https://abag.ca.gov/↩︎