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A. BHATT ET AL.
HAP emissions. Because the emission factors from Chapter 1.6 of the AP-42 database are mainly represen- tative of various wood residues burned in the form of hogged wood, bark, sawdust, shavings, chips, mill rejects, sanderdust, or wood trim, they may not accu- rately represent emissions from boilers burning uncon- ventional biomass fuels. So, our review starts by analyzing unconventional literature sources, such as technical reports, industry-sponsored surveys, permits and permit applications, and stack testing, that do not appear in typical online search engines and biblio- graphic databases. Where appropriate, we use a material balance approach to estimate emission factors for certain pollutants. We then compare the emission factors collected from these data sources with those for wood residue combustion in boilers in AP-42. We made the following efforts to identify emission data from boilers that burn unconventional fuels. Ɣ We identified specific operational boilers that burn unconventional biomass-derived fuel(s) such as sludge, lignin, biogas, off-gases from the plant, etc. and contacted the facilities and the respective state regulatory agencies for air permits and permit applications. Air permits contain detailed informa- tion on the emission factors, boiler fuel, and boiler characteristics, which makes them a preferred source of data when information is not available publicly. Ɣ We also contacted regulatory agencies for stack test data tied to boilers that burn unconventional bio- mass fuels. Stack test data is one of the most pre- ferred data sources for estimating emission factors; however, detailed information on control efficien- cies, stream characteristics, and operating capacity of the boiler at the time of testing is required for estimating the uncontrolled emission factors, which is not always available in the stack testing reports. Ɣ We reviewed publications on emissions from wood combusting boilers, including data collected by the National Council for Air and Stream Improvement (NCASI), an independent research organization established by a consortium of pulp and paper industry members in North America. The data on emission factors from NCASI includes detailed information on boiler feed and composition along with boiler type, which makes it a useful source for characterizing data. Ɣ We analyzed the Compliance and Emissions Data Reporting Interface (CEDRI), a U.S. EPA program designed to manage and collect environmental data
submitted to the agency to meet various reporting requirements, for relevant emission factors. Ɣ We connected with Consortium for Research on Renewable Industry Materials (CORRIM), a research organization that focuses on developing life cycle assessment-related data specific to the wood industry. CORRIM conducted a survey and collected data from 16 different wood mills located in the northwest and southeast regions of the United States, of which data for 12 mills contained emissions of aerosol and gas- eous pollutants we report in this paper. Results and discussion We analyzed the emission data from each source obtained, which includes boilers burning either a single biomass fuel or a mixture of solid and gaseous biomass fuels. The statistics show a varying range of values depending on the fuel mixture, process technology uti- lized, variance in time when the data was reported, and other process parameters. Table 1 summarizes the data obtained from our analysis as compared to EPA’s AP-42 emission factor data. A total of five data sources for emission factors are reported. We also include compli- ance or permit limits specified in permit applications or stack test data when available. To ensure consistency, we report uncontrolled emis- sion factors for all data sources. In cases where the original data sources do not specify the emission reduc- tion efficiencies, thereby preventing us from back calcu- lating the uncontrolled emission factors, we use the average emission reduction efficiencies for the specific control technologies reported in EPA’s factsheet to esti- mate the uncontrolled emission factors. Also, we only report certain air toxics (HAPs) because the emission factors for other hazardous pollutants were several orders of magnitude lower and thus, through engineer- ing judgment, considered to be insignificant. We exclude emission data that (1) utilizes fuels containing a large portion of conventional fuels, such as coal or natural gas; and/or (2) does not provide sufficient infor- mation on emission controls or boiler heat input capa- city to allow us to estimate emission factors. Details on other data sources and air toxics not included in the main paper are in the Supplementary Information (SI). The following subsections describe the detailed results for all the sources considered. Stack test data We obtained stack test data on several air pollutants from boilers burning a mixture of solid biomass and gaseous fuel at two facilities: (a) POET Biorefining
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