7.10 Turbines
Categories 305 and 306
7.10.1 Introduction
Categories 305 and 306 account for criteria pollutant emissions (particulate, organic, NOx, SOx, and CO) emissions from fuel combustion in turbine engines in the San Francisco Bay Area. Category 305 accounts for emissions from gas fired turbines and Category 306 accounts for emissions from oil fired turbines.
A combustion turbine is a rotary engine that extracts energy from a flow of combustion gases. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine’s blades, spinning the turbine which powers the compressor and drives their mechanical output.
7.10.2 Methodology
Point Sources are operations that emit air pollution into the atmosphere at a fixed location within a facility, for which the Air District has issued 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, 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.
Categories 305 and 306 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 webpage314.
For category 305, the CARB PM speciation profile number is 120; PM2.5 and PM10 constitutes 100.0% of total PM. For category 306, the CARB PM speciation profile number is 114; PM2.5 constitutes 96.7% of total PM and PM10 constitutes 97.6% 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 CARB315 and the US Environmental Protection Agency316.
For categories 305 and 306, ROG to TOG ratios are 0.70 and 0.84, respectively. Further assessment and improvement of ROG/ TOG speciation profiles has been planned in future inventory updates.
The Air District Regulation 9, Rule 9 controls NOx and SOx emissions from fuel combustion in stationary turbines.
7.10.3 Changes in Methodology
No changes to methodology were made in this version of the base year emissions inventory.
7.10.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
7.10.5 Trends
(a) Historical Emissions / History
Historical emissions for point source emissions are derived from source-specific throughputs provided by the permitted facility, compiled/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.
(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.
In addition to the uncertainty due to methane emission factors, all pollutant emissions for category 305 are heavily dictated by sources at a single facility. Therefore, future and current trends in this category are highly sensitive to production associated with this facility.
\[ \text{PE} = \text{Gr} * \text{Ci} * \text{Ei} \]
\(PE\) = 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
The emissions growth profiles for categories 305 and 306 are developed based on future industrial activity in accordance with the Association of Bay Area Government’s (ABAG) 2017 projection reports.
7.10.6 Uncertainties
A step-increase in TOG emissions for category 305 may be seen for the period 2007-2009. 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.
7.10.7 Contact
Author: Sukarn Claire
Reviewer: Ariana Husain
Last Update: November 06, 2023
7.10.8 References & Footnotes
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↩︎