7.5 Power Plants
Categories 293 and 294
7.5.1 Introduction
Categories 293 and 294 estimate criteria pollutant emissions (particulate, organic, NOx, SOx, and CO) and greenhouse gas emissions (CO2, CH4, and N2O) from gas and oil fired boilers at the steam turbine type power plants in the San Francisco Bay Area. All of the large steam turbine/boiler type electric power plants are located in San Francisco and Contra Costa counties.
A power generation plant which converts heat energy of fuel combustion into electrical energy by utilizing a steam turbine and alternator, is known as a steam turbine power plant. Steam is produced in the boiler by combusting fuel. The steam is then expanded in the turbine to spin the steam turbine. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy.
7.5.2 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 document294 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)295 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 293 and 294 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 web-page296.
For category 293, CARB’s PM speciation profile number is 120; PM2.5 and PM10 constitutes 100% of total PM. For category 294, CARB’s PM speciation profile number is 130; PM2.5 constitutes 92.7% of total PM and PM10 constitutes 99.7% 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 CARB297 and the US Environmental Protection Agency298.
For categories 293 and 294, ROG to TOG ratios are 0.42 and 0.83, respectively. Further assessment and improvement of ROG/ TOG speciation profiles has been planned in future inventory updates.
7.5.3 Changes in Methodology
No changes to methodology were made in this version of the base year emissions inventory.
7.5.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
7.5.5 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.
For these categories, NOx emissions from 1980 through 2002 were based on actual fuel usage data provided by power plants operating in the Bay Area. Emissions over the years have fluctuated by a factor of about 2.5 based on external factors, such as amount of rainfall and the availability of low-cost hydroelectric power during that year. The NOx emissions have varied from over 30 tons/day to as low as 10 tons/day. After 2002, there was a large decrease in throughput at two of the region’s larger power plants which caused a drop in emissions from 2003 and years going forward.
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.
The Bay Area boiler/steam turbine type power plant NOx emissions have decreased from historical (e.g., 1994) levels of over 30 ton/day to about 1 ton/day in 2007. This was due to shutting down of the older inefficient boilers and the stringent Best Available Retrofit Control (BARCT) requirements of Regulation 9, Rule 11, for existing utility power plants (0.057 Lb/MMBTU or 48 ppm in 2003, 0.018 lb/MMBTU or 15 ppm in 2005) and the BACT requirements of Regulation 2, Rule 2 (Adopted December 19, 2012) for new power plants (0.0089 lb/MMBTU or 7.5 ppm). The NOx emission concentrations are given in ppm by volume, dry basis at 15% O2.
In general, historical emission trends have varied with fuel usage activity. Prior to Base Year 1987, these two categories were grouped as one category.
(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 293 and 294, it was 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” reports299.
7.5.6 Uncertainties
A step-increase in TOG emissions for these categories may be seen for the periods of 2000-2001 and 2007-2008. This is due to a sustained Air District effort to gradually include and update methane emissions factors for various source types over these two periods of 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.
7.5.7 Contact
Author: Sukarn Claire
Reviewer: Ariana Husain
Last Update: November 06, 2023
7.5.8 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/↩︎