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Table 2. Summary of operating information from stack test data sources. Data Source Boiler Capacity (MMBtu/hr) Boiler Feed
Control Technology
Year of Stack Test 2016, 2017, 2019
POET Biorefining
267
Biomass, biogas, digester sludge, filter cake, syrup, biomass fines, and net wrap
Selective non-catalytic reduction (SNCR), flue gas desulfurization, baghouse
125 500
2015 2019
We Energies
Logging residue, wood, bark, wood waste, forest residues, wastewater sludge (NG at startup)
SNCR, baghouse
another for a maximum worst-case scenario, based on varying blended fuel composition depending on the plant’s steam demand. A nominal typical fuel blend con- sists of 41.6% lignin-rich stillage, 27.2% ethanol thin stil- lage syrup, 24.0% corn stover, 6.8% biogas, and 0.40% wastewater sludge, based on dry ton per day feed rates. The emissions of PM and NO x from the boiler were reduced by using baghouse and SNCR, respectively. A scrubber was used to reduce emissions of SO 2 , VOC, and sulfuric acid. ABBK does not have any stack test data reported in the public domain. We Energies Biomass Cogeneration Facility As stated in “Stack test data” Section, the We Energies Biomass Cogeneration Facility operates an 800-MMBtu/ hr circulating fluidized bed boiler. The permit applica- tion (Wisconsin Department of Natural Resources 2019b) from 2019 includes controlled emission factors, which we use, along with average destruction efficiencies for emission controls from EPA’s factsheets, to calculate the uncontrolled emission factors (EPA 2016). Summary of emission factors from air permits Table 3 summarizes the operating information on type of feed, operating capacity, control technologies, and year of air permit (refer to SI for detailed informa- tion). Figure 2 shows the emission factors reported in ABBK’s and We Energies’ air permit applications. As shown in Figure 2, the emission factors estimated for ABBK are higher than those for We Energies Cogeneration facility for all the pollutants except HF. The pretreatment chemicals used in ABBK that break down cellulose and hemicellulose in the biomass add nitrogen and sulfur downstream of the process (e.g., in the boiler feed), likely increasing the emission rate of NO x and SO 2 . In ABBK’s permit applications, the emis- sion factors for the maximum worst-case fuel blend include a 20% safety buffer (on a mass basis) applied to the nominal residual components (ethanol lignin-rich stillage and ethanol thin stillage syrup) used as the boiler fuel. This increase in residual components changes the blended biomass fuel composition, leading to a decrease in the overall ash content in the blended biomass and thus lower PM emission factors than the worst-case
(utilizing several chemicals to break cell walls so that basic sugar components can be extracted for the downstream fermentation process), it is likely that many unwanted components (e.g., chemicals, enzyme residues) were added to the fuel streams fed to the boiler, increasing emissions. For POET, the emission factors reported for PM, NO x , and CO vary year to year demonstrating wide ranges (note that emission factors for some years are not available from the data source), whereas emission factors for SO 2 and VOC are more consistent, ranging from 0.48–0.6 lb/MMBtu and 0.013–0.019 lb/MMBtu, respec- tively. One possibility is that the variation in fuel compo- sitions could lead to changes in combustion performance and conditions across different years. Air permits We obtained air permit applications and construction permits for two facilities burning biomass fuels in their boilers: Abengoa Bioenergy Biomass of Kansas (ABBK) and We Energies Biomass Cogeneration Facility. Given that the information on emission controls, boiler feed, and boiler heat input capacity is described in detail in the air permits and application, we use the uncontrolled emission factors directly when explicitly reported. Abengoa Bioenergy Biomass of Kansas (ABBK) ABBK (not operational as of 2020) was intended to con- vert biomass to ethanol with a 22-MW cogeneration process that included a biomass boiler rated at 500 MMBtu/hr burning a mixture of lignin, hydrolysis resi- dual stillage syrup, biomass, biogas, and sludge. The application for an air permit to construct was submitted in 2011 (WLA Consulting, Inc. 2011) and was amended in 2014 for the construction of four emergency engines as a prevention of significant deterioration (PSD) 1 permit (Burns & McDonnell 2014). The air permit application includes two sets of emission factors for the biomass-fired stoker boiler, one for a nominal typical scenario and 1 Prevention of significant deterioration (PSD) applies to new major sources or major modifications at existing sources of pollutants where the area the source is located is in attainment or unclassifiable with the National Ambient Air Quality Standards (NAAQS). See https://www.epa.gov/nsr/ prevention-significant-deterioration-basic-information for details.
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