JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION
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Previous research has investigated emission factors for boilers that combust a mixture of coal and biofuels (Cao et al. 2008; Cheng et al. 2018; Jetter and Kariher 2009; Jetter et al. 2012), but there is a lack of empirical data on emission factors for boilers that burn unconven- tional biomass-derived fuels. Such an information gap could pose risks to industry owners (specifically biore- fineries) and operators, as they may encounter delays in obtaining the required permits prior to construction and operation due to inappropriate emission estimates in their permit applications. The goal of this study is to carry out a comprehensive survey to identify available information on empirical emission factors for boilers that burn one or a mixture of biomass-derived fuels. We use several existing sources, including little-known/underutilized sources such as per- mits/permit applications, stack testing, and industry- sponsored collection efforts. The information collected and analyzed can be useful to a broad audience, including federal and state air quality planners, permit writers, industrial facilities utilizing biomass-derived waste fuels, and biofuel industries and investors, who may have a keen interest in utilizing biomass-derived fuels as a source of process energy. Our research aims to leverage previous research that is less known to stakeholders, who could benefit from accessing such information. Although our review does not focus on each individual factor men- tioned, the reviewed data implicitly incorporates the variability introduced by these factors.
(c) Material balance analysis (e.g., amount of sulfur or nitrogen in the feed and product streams) (d) Emission factors (e.g., EPA’s AP-42 compilation of emission factors (EPA 1995). In the absence of emission measurements and data from performance tests, emissions from boilers are often esti- mated based on emission factors (approach d). An emis- sion factor for a boiler is typically expressed as pounds (lbs) of pollutant per million British thermal units (MMBtu) of the boiler heat input capacity (Eberle et al. 2017; Komariah et al. 2013). EPA has compiled emission factors for approximately 200 emission source categories, which are available in the Compilation of Air Pollutant Emissions Factors (AP-42) database. AP-42 emission fac- tors are developed and compiled using data from multiple source tests, material balance studies, and engineering estimates. The most relevant emission factors for burning biomass fuels in a boiler are those in AP-42 Chapter 1.6: “Wood Residue Combustion in Boilers.” The database provides data quality ratings ranging from A to E (with A being the best) for emission factors to indicate the quality and robustness of the data. A rating of D is assigned when the emission factors are developed based on a small number of facilities, and thus are not necessa- rily representative of the industry, or when there is evi- dence of variability within the source population. In the case of wood residue combustion, the emission factors were last updated in 2003, and the ratings for the pollu- tants range from A to D, depending on the type of feed- stocks in the industry population. Ratings for several pollutants are poor (C or D), suggesting relatively high uncertainties in the estimates. Over the last two decades, continued efficiency improvements in industrial boilers have likely led to decreased emission factors (Bowman et al. 2009). The efficiency improvement could be due to several efforts, including boiler design that takes fuel parameters (moist- ure, density, size) into consideration to maximize combus- tion efficiency as well as boiler optimization and tuning (through monitoring of temperature, automatic adjust- ment of air-to-fuel ratios, and fuel feed rate adjustment) (ACHR News 2016; Bases 2011; Industrial Controls 2021). Emission factor data sources We focus on emission factors for six criteria air pollu- tants (NO x , SO 2 , CO, VOC, Pb, and PM 2.5 /PM 10 ) as well as selected hazardous air pollutants (HAPs; including acetaldehyde, formaldehyde, and two acid gases – hydrochloric acid [HCl] and hydrogen fluoride [HF]), which are expected to be the major contributors to total
Methodology Approaches for emission estimation
Emissions inventories are important for air quality man- agement and are utilized by local, state, and federal agen- cies as well as industries and consultants for projecting air quality impacts. Developing an emissions inventory is essential for supporting the development of air permit applications, determining the permitting requirements for a new source or modification of an existing source, project- ing future air quality impacts, characterizing pollution transport, and developing emission control strategies (Bhatt et al. 2022; Komariah et al. 2013). Emissions (typi- cally expressed in tons per year) can be calculated or estimated by one or a combination of the several different approaches (no particular order of importance), including: (a)Emission measurement based on continuous emission monitoring systems (b) Emission data from performance tests (e.g., by boiler manufacturers, chemical kinetic data from combustion labs, industries using boilers)
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