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Policy Analysis
Potential air pollutant emissions and permitting classifications for two biorefinery process designs in the United States Annika Eberle, Arpit Bhatt, Yimin Zhang, and Garvin A. Heath Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017
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Environmental Science & Technology
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Potential air pollutant emissions and permitting classifications for two
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biorefinery process designs in the United States
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Annika Eberle, Arpit Bhatt, Yimin Zhang,* and Garvin Heath
4
National Renewable Energy Laboratory, Golden, CO, 80401 USA
5
Abstract
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Advanced biofuel production facilities (biorefineries), such as those envisioned by the United States
7
(U.S.) Renewable Fuel Standard and U.S. Department of Energy’s research and development programs,
8
often lack historical air pollutant emissions data, which can pose challenges for obtaining air emission
9
permits that are required for construction and operation. To help fill this knowledge gap, we perform a
10
thorough regulatory analysis and use engineering process designs to assess the applicability of federal
11
air regulations and quantify air pollutant emissions for two feasibility-level biorefinery designs. We find
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that without additional emission-control technologies both biorefineries would likely be required to
13
obtain major source permits under the Clean Air Act’s New Source Review program. The permitting
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classification (so-called “major” or “minor”) has implications for the time and effort required for
15
permitting and therefore affects the cost of capital and the fuel selling price. Consequently, we explore
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additional technically feasible emission-control technologies and process modifications that have the
17
potential to reduce emissions to achieve a minor source permitting classification. Our analysis of air
18
pollutant emissions and controls can assist biorefinery developers with the air permitting process and
19
inform regulatory agencies about potential permitting pathways for novel biorefinery designs.
20 *
Contact information for corresponding author: National Renewable Energy Laboratory, Strategic Energy Analysis Center, 15013 Denver W Pkwy, Golden, CO, 80401, USA. Email:
[email protected].
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1 Introduction
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The U.S. Department of Energy’s Billion-Ton Studies estimate that by 2030 the United States could,
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without interfering with other vital farm and forest products, sustainably produce enough biomass
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feedstock to displace 30% of U.S. 2005 petroleum consumption using biofuels.1–3 To take advantage of
25
the potential benefits associated with biofuels4,5, the United States developed the Renewable Fuel
26
Standard6 to increase the use of renewable fuels. However, because cellulosic biofuel conversion
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technologies are still being developed, their non-GHG air pollutant emissions are not well quantified.
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Although prior research has explored the technical and economic feasibility of cellulosic biorefineries7–10
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and the regulatory and legal impediments to commercialization,11–17 none of these studies has
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developed detailed estimates of air pollutant emissions for multiple biofuel conversion pathways. As a
31
result, biorefinery developers are often uncertain about the type and quantity of pollutants their
32
facilities may emit, and air emission permitting officials are challenged to determine regulatory
33
applicability and compliance.
34
Despite these challenges, the Clean Air Act (CAA)18 requires new stationary sources that expect to emit
35
regulated pollutants greater than specific thresholds to quantify their potential to emit (PTE) and obtain
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air permits before construction. The PTE is defined as the greatest amount of regulated air pollutant
37
emissions that a facility could release to the atmosphere under its physical and operational design.19 It
38
must be evaluated for several types of pollutants, including criteria air pollutants—those with National
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Ambient Air Quality Standards—and their precursors (e.g., sulfur dioxide [SO2], carbon monoxide [CO],
40
particulate matter [PM], nitrogen oxides [NOx], ozone regulated through its precursors such as NOx and
41
volatile organic compounds [VOCs], and lead [Pb]); pollutants outlined in the New Source Performance
42
Standards (NSPSs); and hazardous air pollutants (HAPs).20 Uncertainties in determining the applicability
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of certain regulations and estimating the PTE could lead to a lengthy permit review process, which could
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cause construction delays or design modifications and thereby impact production costs. PTE 2 ACS Paragon Plus Environment
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uncertainties could also mean that permitted emission limits are not met, which could cause
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environmental harm and result in compliance violations.
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To help mitigate the risks associated with biorefinery air permitting, we estimate the PTE for two
48
feasibility-level process designs that produce cellulosic biofuels and perform a thorough regulatory
49
analysis to evaluate the applicability of federal regulations. We show that if built according to their
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design specifications these two biorefineries would likely be classified as major sources under the
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federal New Source Review (NSR) program. However, with additional, technically feasible emission
52
control options or design modifications, both biorefineries could achieve minor source permitting
53
classifications.
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There are both positive and negative implications of analyzing feasibility-level process designs. Since the
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final process design, location, and size of each facility may differ from the designs explored here, our
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estimated PTE is considered preliminary. However, by analyzing these biorefinery designs near the
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beginning of their development, this work can help biorefinery developers formulate permitting
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strategies and mitigate permitting risks. Our work may also inform regulatory agencies about PTE
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uncertainties and control options that could lead to more streamlined permitting processes.
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2 Methods
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We performed regulatory analyses, estimated the PTE, and assessed the potential permitting
62
classifications for two feasibility-level biorefinery process designs. We focused solely on federal air
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regulations because state air regulations depend on a variety of factors such as the local air quality
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status and state implementation plans. We performed this analysis for permits required to construct
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and, when applicable, to operate the biorefinery.
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Our analysis is based on two process designs capable of processing 2000 dry megagrams (Mg) of
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biomass feedstock per day into hydrocarbon biofuel products: (1) a biochemical technology pathway
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focused on biological conversion of cellulosic sugars (hereafter referred to as sugars-to-hydrocarbons)21
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and (2) a thermochemical pathway focused on fast pyrolysis of whole biomass.22 Several baseline
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emission control technologies were included in these designs, including baghouses, flue gas
71
desulfurization (FGD), and selective non-catalytic reduction and over-fire air systems to reduce PM, SO2,
72
and NOx emissions, respectively (see Supporting Information [SI] for details).
73
To help identify mitigation strategies and to understand whether a given pollutant is emitted from
74
operations key to biofuel conversion or from supporting operations, we disaggregated each design into
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four emission-release areas: core operations (e.g., pre-processing, conversion), supporting operations,
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truck traffic, and equipment leaks (Figure 1). Additional details about our methods and results are
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documented in Zhang et al.17 and Bhatt et al.23
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Figure 1. Overarching emission release areas and potential air pollutants emitted from two process designs:
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sugars-to-hydrocarbons and fast pyrolysis. Although the conversion processes differ for these two
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biorefineries, their sub-processes can be categorized into four overarching emission release areas: core
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operations, supporting operations, truck traffic, and equipment leaks.
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To identify the types of pollutants likely to be emitted from each area, we analyzed each unit
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operation,21,22 consulted the process design engineers, utilized the U.S. Environmental Protection
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Agency’s (EPA’s) Compilation of Air Pollution Emission Factors (AP-42) report,24 and examined air
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permits for facilities employing similar processes. We then reviewed the Code of Federal Regulations
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(CFR) for all NSPSs (Title 40, Part 60)25 and NESHAPs (Title 40, Parts 61 and 63)26,27 that may apply to the
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entire biorefinery and/or to specific equipment and operations. We also used the EPA’s Applicability
90
Determination Index database28 to assess applicability or monitoring requirements under the NSPS and
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NESHAP programs. If the biorefinery or a specific portion of the facility met the applicability criteria of
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an NSPS or NESHAP, we determined the federal regulation to be applicable.
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The Standard Industrial Classification (SIC) of the facility determines which NSPSs and NESHAPs apply to
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the entire facility. Since permits for similar cellulosic biorefineries indicate that these facilities will fall
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under SIC 28 (chemicals and allied products), we conducted our analysis under the assumption that both
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biorefineries would fall under this SIC (see SI for details).29,30 The results presented here are for two
97
specific biorefinery designs; biorefinery developers and owners should consider all NSPSs and NESHAPs
98
when planning compliance measures for their facility.
99
2.1 Estimating the Potential to Emit
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According to the EPA31, there are four standard methods that can be used to calculate PTE: (1)
101
test data, (2) material balances, (3) source-specific models, and (4) emission factors. We used
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three of these approaches (i.e., material balances, emission factors, and source-specific models),
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along with a variety of data sources to estimate the PTE for each process and unit operation
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(Table 1; Tables S2-S4 in the SI).
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Table 1. Methods and data sources used to estimate the PTE for both the sugars-to-hydrocarbons and fast
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pyrolysis designs. (Additional details, including pollutant-specific emission factors, are provided in Tables S2-
108
S4 in the Supporting Information.)
Emission Source for Each Biofuel Pathway Emission Release
PTE Estimation Data Sources
Sugars-toArea
Approach
Fast Pyrolysis Hydrocarbons
Core operations
[1]
Feedstock handling
Fast pyrolysis system ;
and pretreatment;
Hydrotreating and
1. Air permits for analogous equipment/operations Enzymatic hydrolysis;
hydrocracking;
Emission factor
[3]
[2] 24
2. EPA’s AP-42 report Product recovery and
Product separation;
upgrading
Hydrogen production
Enzyme production
Fast pyrolysis system
3. Engineering judgment
1. Process models [1]
Material balance 2. Engineering judgment
Supporting
1. Air permits for analogous
operations
equipment/operations
[3]
24
2. EPA’s AP-42 report
3. EPA’s Emission Estimation Loading operations;
Loading operations;
Protocol for Petroleum Emission factor
Boiler; Utilities
[2] 32
Utilities
Refineries
4. Emission limits in NSPS Subpart IIII
33
5. SCAQMD’s Guidelines for 34
Cooling Towers
Source-specific Storage
model
Wastewater
1. EPA’s TANKS 4.09D
Storage
Wastewater treatment
Material balance
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software
1. Process models
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treatment;
2. Engineering judgment
Boiler Equipment leaks
[4]
Leaks from process
Leaks from process
vents, process
vents, process
equipment, storage
equipment, storage
1. EPA’s Protocol for Leak Emission Estimates Emission factor
[2]
Truck traffic
[4]
2. Air permits for analogous equipment/operations
tanks, and wastewater tanks, and wastewater treatment
36
treatment
[3]
3. Engineering judgment
On-site transportation On-site transportation of of feedstock, raw
feedstock, raw materials, Emission factor
[2]
24
1. EPA’s AP-42 report
materials, and finished and finished products products
109
Abbreviations include EPA for Environmental Protection Agency, NSPS for New Source Performance Standards, and SCAQMD
110
for South Coast Air Quality Management District.
111
[1]
112
handling, and the fast pyrolysis reactor. As a result, we use multiple PTE estimation approaches to estimate emissions from this
113
system (see SI for details).
114
[2]
115
activity factors (AFs)—the amount of activity associated with a unit operation or piece of equipment (e.g., heat input capacity of
116
the boiler). AFs were derived from the process designs, process models (e.g., Aspen Plus), and engineering judgment.
117
[3]
118
enzymatic hydrolysis in the sugars-to-hydrocarbons design while Cool Planet LLC air permit is relevant for the ash and sand
119
handling in fast pyrolysis). When appropriate, we scaled the emission factor based on the facility size or production rate for the
120
corresponding operation.
121
[4]
The fast pyrolysis system is a combination of several areas, including the on-site char combustor (sand heater), sand
The emission factor approach multiplies emission factors (EFs)—the amount of pollutant emitted per rate of activity—by
37
Relevant air permits may vary by unit process (e.g., the air permit for Abengoa Bioenergy Biomass of Kansas is relevant for 38
Emissions from equipment leaks and truck traffic are considered to be fugitive emissions (emissions not feasibly collected).
122 123 124
After identifying the emission calculation methods in Table 1, we estimated the PTE by adhering to the
125
following guidelines prescribed by federal regulations:19,39
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1)
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Assume the maximum capacity under the biorefinery’s physical and operational design (i.e., continuous operation at design capacity with the highest possible throughput);
2)
Use the worst-case emission factor (e.g., if multiple fuel sources are possible for a
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combustion unit, assume year-round usage of the fuel that emits the most of each
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pollutant); and
131 132
3)
Account for federally enforceable limitations (e.g., regulatory provisions and practically enforceable, EPA-approved permit limits).
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As required by regulations for chemical processing plants, we also included fugitive emissions (emissions
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not feasibly collected) in the form of equipment leaks and truck traffic.
135
PTE estimates for biorefineries are often uncertain because air permitting processes are
136
location-specific and emissions data for novel processes are often unavailable (see SI for details).
137
However, the regulatory definition of PTE does not allow for the inclusion of uncertainty. PTE is
138
intended to provide a conservative estimate for the upper bound of potential emissions. It
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should always be larger—and often much larger—than the facility’s actual emissions. As a result,
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we present only a single value for the PTE of each regulated pollutant, which represents the
141
maximum amount of emissions the biorefinery may emit.
142
To evaluate how much the PTE is impacted by federally enforceable limitations not imposed by physical
143
and operational design (item 3 above), we report two estimates of potential emissions: the uncontrolled
144
PTE (includes items 1 and 2) and the PTE (includes items 1-3). The PTE is always less than or equal to the
145
uncontrolled PTE because PTE takes into account additional controls to achieve emission limits required
146
by applicable federal rules (NSPSs, NESHAPs) or included in an approved air permit. The PTE value, not
147
the uncontrolled PTE, determines the source’s permitting classification.
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2.2 Determining the Permitting Classification
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Before constructing a new chemical process plant like a biorefinery, a developer must apply for a
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construction permit under the NSR program.40 In general, if a source exceeds an applicable major source
151
threshold for at least one regulated pollutant under the NSR program, it is classified as a major source;
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otherwise it is classified as a minor source. If the facility is classified as a major source under NSR, or if
153
the PTE of HAPs exceeds the major source threshold, then the developer must also apply for an
154
operating permit under the Clean Air Act’s Title V program.41
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We made a preliminary determination of the permitting classification for each biorefinery by comparing
156
the facility’s PTE to the corresponding major source thresholds. These thresholds vary by pollutant,
157
permitting program, industrial classification, and attainment status; lower thresholds may apply in non-
158
attainment areas but only for the non-attainment pollutant(s) (see Table S8 in the SI and Zhang et al.17).
159
For this analysis we assume these facilities will be located in attainment areas where the default major
160
source thresholds for pollutants emitted from a chemical process plant are 100 short tons per year (TPY)
161
(90.7 Mg per year) for criteria air pollutants under the NSR program and 10 TPY for a single HAP or 25
162
TPY for any combination of HAPs under the Title V program.
163
2.3 Evaluating the Ability to Obtain Minor Source Classification
164
Major sources are subject to more stringent permitting requirements such as the implementation of
165
additional emission controls and increased monitoring and reporting (see SI for details). For example,
166
under the NSR permitting program, a new major source is required to install the Best Available Control
167
Technologies (BACT), conduct an air quality analysis, perform additional impact analysis (e.g., on soils,
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vegetation, visibility), and involve the public in the review process. These additional permitting
169
requirements could delay the construction of a proposed project and increase the project’s capital and
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operating costs.
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To mitigate risks and costs, biorefinery developers would prefer their facility to be classified as a minor
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source. Therefore, if our estimated PTE exceeded the major source thresholds, we examined additional
173
emission controls and/or process modifications that may allow each biorefinery to be classified as a
174
minor source. We used the EPA’s Reasonably Available Control Technology (RACT)/BACT/Lowest
175
Achievable Emission Rate (LAER) Clearinghouse database,42 analogous air permit applications (e.g.,
176
Abengoa’s Bioenergy Biomass of Kansas air permit application43), and other EPA documentation44 to
177
identify potential emission control technologies. We then consulted with the process design engineers
178
and used engineering judgment to identify which of these potential control options or process
179
modifications could be feasibly implemented.
180
For the sugars-to-hydrocarbons design, we incorporated the additional control technologies and any
181
required process modifications into the process model and executed process simulations to verify the
182
design’s performance. We then used published emission reduction efficiencies of the control devices,
183
along with changes in the design parameters (e.g., heat input capacity of the boiler), to update the PTE
184
estimates and determine whether a minor source permitting classification could be achieved. For the
185
fast pyrolysis pathway, we more simply identified the feasible emission control technologies and
186
modified the PTE estimates using the control efficiencies of these technologies.
187
3 Results
188
Both biofuel conversion pathways will likely emit similar types of air pollutants. The supporting
189
operations and core operations areas are expected to emit PM, VOCs, NOx, CO, SO2, and HAPs, along
190
with other regulated NSR pollutants. For fugitive emissions, truck traffic will likely emit PM and
191
equipment leaks are expected to emit VOCs and HAPs (Figure 1).
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As shown in Table 2 (see Tables S5-S7 in the SI for details), both biorefineries will likely be subject to
193
several federal air regulatory requirements under the NSPSs25 and the NESHAPs27, including
194
•
NSPSs for
195
o
Industrial-Commercial Institutional Steam Generating Units (40 CFR 60, Subpart Db);
196
o
Stationary Compression Ignition Internal Combustion Engines (40 CFR 60, Subpart IIII)
197
o
Volatile Organic Liquid Storage Vessels (40 CFR 60, Subpart Kb)
198
•
NESHAPs for
199
o
Chemical Manufacturing Area Sources (40 CFR 63, Subpart VVVVVV)
200
o
Major Sources: Industrial, Commercial, and Institutional Boilers and Process Heaters (40
201
CFR 63, Subpart DDDDD)
202
o
Miscellaneous Organic Chemical Manufacturing (40 CFR 63, Subpart FFFF)
203
o
Stationary Reciprocating Internal Combustion Engines (40 CFR 63, Subpart ZZZZ).
204 205
Table 2. Federal regulations potentially applicable to the sugars-to-hydrocarbons and fast pyrolysis
206
biorefinery designs examined here.
Emission
Sugars-to-Hydrocarbons
Fast Pyrolysis
Release Area
Affected Equipment
Affected Equipment
Potential Federal Regulation
Potential Federal Regulation
If facility is a major source of
If facility is a major source of
HAPs: NESHAP Subpart FFFF
HAPs: NESHAP Subpart FFFF
Bioreactor process Core operations
or
Dryer vent
or
If facility is an area source of
(continuous process)
If facility is an area source of
vent (batch process)
Supporting
Boiler
HAPs: NESHAP Subpart
HAPs: NESHAP Subpart
VVVVVV
VVVVVV
NSPS Subpart Db
Gasoline storage
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tank
operations
If facility is a major source of HAPs: NESHAP Subpart FFFF or If facility is an area source of HAPs: NESHAP Subpart VVVVVV
If facility is a major source of
If facility is a major source of
HAPs: NESHAP Subpart DDDDD Process heaters
HAPs: NESHAP Subpart DDDDD
or
(fired reboiler and
or
If facility is an area source of
methane reformer)
If facility is an area source of
HAPs: NESHAP Subpart JJJJJJ Emergency fire
NSPS Subpart IIII
pump and generator
HAPs: No regulation applies
Same as sugars-to-hydrocarbons NESHAP Subpart ZZZZ
If facility is a major source of HAPs: NESHAP Subpart FFFF or
Not applicable because the cooling water flow rate is
If facility is an area source of
below 8,000 gal/min
Cooling tower
HAPs: NESHAP Subpart VVVVVV If facility is a major source of HAPs: NESHAP Subpart FFFF or
Equipment Equipment leaks [1]
Same as sugars-to-hydrocarbons If facility is an area source of
leaks
HAPs: NESHAP Subpart VVVVVV [1]
Truck traffic
Not applicable
Not applicable
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25
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Abbreviations include NSPS for New Source Performance Standard in Title 40, Part 60 of the Code of Federal Regulations
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(CFR) and NESHAP for National Emission Standard for Hazardous Air Pollutants in Title 40, Parts 61 and 63 of the CFR.
26
27
209 210
[1]
Emissions from equipment leaks and truck traffic are considered to be fugitive emissions (emissions not feasibly collected).
211 212 213
3.1 Estimated Potential to Emit and Preliminary Permitting Classification
214
As described in Section 2.1, we report the uncontrolled PTE and the PTE. The uncontrolled PTE accounts
215
for the maximum capacity to emit taking into account physical and operational limitations. For the
216
sugars-to-hydrocarbons design, there were no physical and operational design constraints, and the two
217
constraints that exist in the fast pyrolysis design did not have a substantial impact on the PTE results
218
(see SI for details). The PTE—the value considered under air permitting requirements—adds federally
219
enforceable emission limitations to the uncontrolled PTE. For example, in the fast pyrolysis biorefinery,
220
we assume the facility’s permit application will incorporate additional federally enforceable limitations
221
to reduce HAP emissions from the dryer and gasoline loading operations by 95% and 60%, respectively,
222
because these limitations would help the biorefinery avoid more stringent requirements by reducing the
223
HAP emissions to below the major source thresholds (see SI for details).
224
The impact of federally enforceable limitations on PTE varies by pollutant (top panel of Figure 2) and can
225
either be substantial (e.g., VOC emissions are reduced by ~93% in the sugars-to-hydrocarbons
226
biorefinery when federally enforceable limitations are included) or trivial (e.g., the uncontrolled PTE and
227
PTE values for CO are similar for both biorefineries). However, even after incorporating federally
228
enforceable limitations, the PTE for both biorefineries likely still exceeds the major source thresholds for
229
specific pollutants (e.g., 100 TPY for criteria air pollutants; 25 TPY for total HAPs). As a result, both
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biorefineries would likely be classified as major sources under the NSR program. In addition, the sugars-
231
to-hydrocarbons biorefinery would likely be a major source of HAPs under the Title V program.
232 233
Figure 2. Preliminary estimates for the uncontrolled potential to emit (PTE), the PTE in tons per year (TPY),
234
and the relative contribution of each emission release area (core operations, supporting operations,
235
equipment leaks, and truck traffic) to the PTE for each biofuel design. The uncontrolled PTE only
236
incorporates physical and operational design constraints, while the PTE includes these constraints plus
237
federally enforceable limitations. Note that the y-axes in the top panel are plotted on a log-scale.
238
Abbreviations include particulate matter (PM), volatile organic compounds (VOCs), nitrogen oxides (NOx),
239
carbon monoxide (CO), sulfur dioxide (SO2), criteria air pollutants (CAPs) and hazardous air pollutants
240
(HAPs), which represent total HAPs.
241 242
The contribution of the emission release areas to the PTE for each pollutant differs for each design
243
(bottom panel of Figure 2). For sugars-to-hydrocarbons, supporting operations (specifically the boiler) 15 ACS Paragon Plus Environment
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contribute most to the emissions of air pollutants that exceed the major source thresholds (i.e., VOCs,
245
NOx, CO, SO2, and HAPs). For fast pyrolysis, core operations, specifically the methane reformer for
246
hydrogen production and char combustor and dryer for the fast pyrolysis system, contribute most to air
247
pollutant emissions that exceed the major source thresholds (i.e., PM, NOx, and SO2).
248 249
3.2 Additional Emission Controls to Achieve Minor Source Classification
250
Since major source permits are more stringent, complex, and therefore more time- and cost-intensive
251
than minor source permits, we explored feasible, state-of-the-art emission control technologies and
252
process modifications that may allow each biofuel conversion pathway to be classified as a minor source
253
(see Methods). We only evaluated additional control options for the unit operations that contribute
254
most to the air pollutant emissions that exceed the major source thresholds (Table 3). These processes
255
include the boiler system in the sugars-to-hydrocarbons biorefinery and the methane reformer and
256
dryer in the fast pyrolysis biorefinery.
257
Table 3. Selected, technically feasible emission control options
258
PTE to below major source thresholds.
Biofuel
Emission
pathway
source
Sugars-to-
Boiler
Hydrocarbons
Control option
[2]
[1]
that could be implemented to reduce the
Targeted pollutant(s) and method for emission reduction (as compared to published design)
1. Install a catalytic oxidizer control
CO, VOCs, HAPs: New control technology reduces
technology
emissions of CO by 90%, VOCs by 70%, and HAPs (formaldehyde and acetaldehyde) by 90%
2. Install a selective catalytic
NOx: New control technology has higher emission
reduction system in place of a
reduction efficiency (88%)
selective non-catalytic reduction control technology
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3a. Reduce the amount of lignin fed
CO, HAPs, NOx, VOCs, SO2: Process modification lowers
into boiler and therefore reduce the
the heat input capacity of the boiler, which decreases
size of the boiler
pollutant emissions CO, HAPs, NOx, VOCs: Process modification uses
3b. Change the fuel type being fed to
different fuel, which decreases the worst case emission
the boiler by re-routing all lignin to
factor
produce coproducts (e.g., chemicals) SO2: Process modification decreases the sulfur content of the fuel, which lowers the SO2 combustion emissions Fast Pyrolysis
[3]
Dryer
1. Install flue gas desulfurization
SO2: New control technology reduces emissions by 92%
2. Install a baghouse
PM: New control technology reduces emissions by 99%
Methane
3. Install a selective non-catalytic
NOx: New control technology reduces emissions by 45%
reformer
reduction control technology
259
Abbreviations include carbon monoxide (CO), volatile organic compounds (VOCs), hazardous air pollutants (HAPs), NOx (nitrogen
260
oxides), SO2 (sulfur dioxide), and particulate matter (PM).
261
[1]
262
the primary pollutant contributors in each process design: the boiler in the sugars-to-hydrocarbons design and the methane
263
reformer and dryer in fast pyrolysis design.
264
[2]
265
design, options 1 and 2 are implemented with either 3a) or 3b).
266
[3]
267
feedstock and then released to the atmosphere (see SI for details). No exhaust (or flue gas) from the char combustor is directly
268
discharged to the atmosphere.
We only considered additional emission control options (i.e., emission control technologies and process design modifications) for
We assume that all of the control options are implemented cumulatively for both process designs. In the sugars-to-hydrocarbon
Emissions from the dryer include hot flue gas from the on-site char combustor, which is used to dry the incoming biomass
269 270
While the implementation of these emission control options will change the PTE and therefore impact
271
the applicability of federal regulations, we do not account for any such changes to federally enforceable
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limitations. Instead, the PTE estimates in this section build directly upon the PTE described in Section 3.1
273
and only account for changes to emissions that result from each control option. Final PTE values will
274
need to be re-evaluated once a final decision is made regarding the control options.
275
3.2.1
276
For the sugars-to-hydrocarbons design, the pollutants that exceed the NSR major source thresholds are
277
CO, SO2, NOx, VOCs, and HAPs, with the boiler contributing the most to these emissions. To reduce these
278
emissions, we considered the cumulative effect of two control technologies: (1) a catalytic oxidizer
279
control technology to reduce CO by 90%, VOCs by 70%, and HAPs (formaldehyde and acetaldehyde, in
280
particular) by 90%;45 followed by (2) a selective catalytic reduction (SCR) system in place of a selective
281
non-catalytic reduction (SNCR) system included in the design to reduce NOx by 88%.46 Despite these
282
additional control technologies, the PTE of several pollutants remained above the major source
283
thresholds; only NOx and VOCs were reduced below their 100 TPY thresholds (see Figure S1 in SI).
284
Since these two control technologies did not reduce the PTE of CO, SO2 and HAPs below their respective
285
major source thresholds, we considered process modifications that could further reduce emissions from
286
the boiler. In the current design, the boiler burns three fuels: (1) lignin from enzymatic hydrolysis, (2)
287
sludge from wastewater treatment, and (3) biogas from anaerobic digestion. Although substituting a
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natural gas boiler for a biomass boiler could be one feasible strategy to reduce these emissions, there
289
are tradeoffs associated with this option. For example, the sludge has valuable heat content (about 15%
290
of the total heat input to the boiler) and a biomass boiler allows the facility to use this fuel to generate
291
steam rather than disposing of it off-site. Furthermore, the biorefinery will likely want their products to
292
qualify for cellulosic biofuel renewable identification numbers (RINs), which provide an additional
293
economic incentive for selling biofuels that have a GHG reduction relative to petroleum. Replacing the
294
biomass boiler with a natural gas boiler could prevent the produced biofuels from qualifying for RINs
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because CO2 emissions from biomass (including lignin, biogas, sludge) are currently classified as biogenic
296
and therefore do not contribute to the biofuels’ life cycle GHG emissions. As a result, instead of
297
implementing a natural gas boiler, we considered changing the composition of the biomass boiler’s fuel
298
stream to help reduce emissions.
299
For SO2 emissions, we estimated the PTE via the material balance approach, which involved using the
300
process model to determine the sulfur content present in the fuel stream. For all other pollutants, we
301
estimated changes in the boiler’s PTE using the emission factor approach (Activity Factor [AF] x Emission
302
Factor [EF]). We followed the federal guidelines for boilers that combust multiple fuels and assumed
303
year-round usage of the fuel that emits the most of each pollutant (the worst-case fuel). When the lignin
304
fraction is greater than zero, the value of the EF remains unchanged (i.e., equal to the EF for the worst-
305
case fuel (lignin)). However, removing fractions of lignin from the boiler reduces the required heat input
306
capacity of the boiler, which decreases the AF because less fuel is combusted (electricity is instead
307
imported to satisfy the facility’s power requirements). If lignin is completely removed from the boiler’s
308
fuel stream then sludge is the worst-case fuel for HAPs and CO emissions, which changes the EF.
309
Decreasing the lignin content of the boiler’s fuel stream has a substantial impact on the facility’s PTE for
310
CO, HAPs, and SO2 (Figure 3). Furthermore, because the HAP and CO emission factors are much lower
311
for sludge than for lignin, completely diverting lignin considerably decreases these emissions and
312
reduces the PTE below the major source thresholds for CO, SO2 and HAPs.
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Figure 3. The emissions of three major pollutants—carbon monoxide (CO), total hazardous air pollutants
314
(HAPs), and sulfur dioxide (SO2)—vary depending on the fraction of lignin in the boiler’s fuel (colored lines;
315
left axis). To better visualize the emission reductions that occurs when lignin is completely diverted from
316
the boiler’s fuel stream (0% lignin fraction), we only show results for lignin fractions less than or equal to
317
fifty percent of the amount assumed in the design case. The large decrease in PTE for 0% lignin fraction
318
occurs because the worst-case emission factor changes from lignin to sludge. If all of the lignin is diverted
319
from the boiler’s fuel stream, the facility-wide PTE values for CO, total HAPs, and SO2 are estimated to be
320
below the major source thresholds: 100 TPY for criteria pollutants under the NSR program (dashed grey
321
line) and 25 TPY for total HAPs under the Title V program (dotted grey line). Although the direct emissions
322
from the facility decrease with lower lignin content, the amount of electricity imported (right axis, black
323
line) increases. Note that these emissions were calculated after implementing two additional controls on
324
the boiler: a catalytic oxidizer and higher efficiency flue gas desulfurization system.
325 326
One additional benefit of this strategy is that the lignin diverted away from the boiler could be used to
327
generate value-added co-products (e.g., chemicals)47,48 or pelleted and sold for off-site use. However,
328
the lower heat content of the non-lignin fuel streams would impact the biorefinery’s energy balance.
329
While the original sugars-to-hydrocarbons design is able to generate enough on-site steam and
330
electricity to meet the biorefinery’s energy demands, diverting lignin would require the facility to import 20 ACS Paragon Plus Environment
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electricity from the grid to satisfy its power requirements (it would not need to make any changes to
332
satisfy the facility’s steam demand). This tradeoff could affect the life cycle GHG emissions of the
333
biofuels produced.
334
3.2.2
335
Our preliminary estimate of the PTE for the fast pyrolysis design indicates that NOx, PM, and SO2 will
336
likely exceed the NSR major source thresholds. As a result, we explored three additional, pollutant-
337
specific emission control technologies. We did not explore any process modifications for the fast
338
pyrolysis design because we found that emission control technologies could reduce the PTE below major
339
source thresholds.
340
In the fast pyrolysis design, the dryer is a significant source of SO2 and PM emissions and the methane
341
reformer is a major contributor of NOx emissions. As a result, we considered three additional emission
342
controls: 1) an FGD control system upstream of the dryer exhaust to reduce its SO2 emissions by 92%,49
343
2) a baghouse on the dryer to capture 99%50 of the PM from the vent stream, and 3) an SNCR control
344
technology on the methane reformer to reduce its NOx by 45%.51 The installation of these controls
345
technologies could reduce the PTE of SO2, PM, and NOx below their respective major source thresholds
346
(see Figure S2 in SI) and allow the fast pyrolysis design to achieve minor source classification.
347
4 Discussion
348
Since our results are specific to two feasibility-level process designs and further iterations of these
349
designs may result in process modifications, our results should be considered preliminary. There are
350
significant uncertainties inherent in our analysis due to a lack of specific design parameters and the
351
subsequent use of general procedures and best-available data sources for estimating emissions. To
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more accurately assess the final PTE, future work will need to examine the final process designs and
353
make use of operational data from pilot-scale or larger facilities, when available.
354
Although it may be feasible for biorefineries using these two designs to achieve minor source
355
classifications, these determinations might not be practically achievable or cost effective. For instance,
356
while catalytic oxidizers are currently used to reduce CO emissions from gas-fired boilers, their use on
357
biomass-fired boilers has not been proven in a commercial setting. Furthermore, while our prior work
358
has shown that the cost is insignificant of implementing emission controls to achieve compliance with
359
applicable NSPS and NESHAP regulations in the sugars-to-hydrocarbons design,10 installing BACT
360
emission controls could have a significant impact on the economics of the biorefinery (see Bhatt et al.10
361
for cost estimates). A detailed evaluation of how the costs of incorporating the minor-source-
362
classification strategies explored here compare to the costs of being classified as a major source is
363
beyond the scope of this analysis. However, the additional control options analyzed here could be cost
364
prohibitive and should be explored in future research.
365
In addition, the size of a biorefinery can have a significant impact on the cost, emissions, and regulatory
366
requirements. We evaluated these two biorefineries according to the size specified in the process
367
designs, and we do not explore the implications of biorefinery sizing in this work. However, because
368
major source thresholds are independent of facility size, it is possible that a minor source classification
369
could be achieved without the need for additional emission controls, merely by designing the facility at a
370
smaller scale. Such a strategy would have other cost implications, e.g., capital costs, financing options,
371
and economies of scale, which should be considered in a complete cost-benefit analysis.
372
One of the challenges we encountered while estimating the PTE for these two process designs was the
373
lack of data available to estimate the emission factors for some novel processes and non-traditional
374
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EPA’s databases, engineering publications, performance test data from the same or a similar emission
376
unit, and manufacturing’s performance tests to develop emission factors. For example, the worst-case
377
emission factors for non-fossil-powered boilers are not well documented so we used EPA’s AP-42 report
378
to identify the most representative emission factors. In addition, no estimates of VOC emissions exist
379
publicly for some specific biofuel conversion processes, e.g., aerobic respiration of cellulosic and
380
hemicellulosic sugars to long-chain fatty acids using enzymatic hydrolysis, so we estimated these
381
emissions using values from publically available air permits with similar processes.
382
As more biorefineries become operational, many will be required to perform stack testing to ensure that
383
the emissions from their facility do not exceed their permit limitations, and these data should eventually
384
make their way into EPA’s emission factor databases. Thus, as more projects become operational and
385
the amount of stack testing data increases, the uncertainty in the PTE estimates for these new processes
386
should be reduced. However, if regulatory agencies were able to decrease the uncertainty associated
387
with the future determination of emission factors for novel processes and non-traditional fuels, either
388
through further research studies or clearer regulatory guidelines, developers could more accurately
389
estimate PTE values for biorefineries and other emerging industries. Such work may help mitigate the
390
uncertainty associated with the air permitting regulations and encourage the development of new
391
biorefineries.
392
The duration of the air permitting process could impact biorefinery developers’ investment risks and
393
potentially the cost of produced fuels. However, the challenges associated with this process are often
394
overlooked. Our work may help regulators and biorefinery developers understand the source and type
395
of emissions, along with which federal regulations might be applicable to various unit operations. Our
396
analysis may also assist biorefinery developers with assessing their PTE, permitting classification, and
397
additional control options to mitigate permitting risks. This information is particularly important for the
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398
deployment of new technologies, which typically incur longer permitting processes as developers work
399
to assure regulators that air quality regulations and permitting conditions can be met.
400
5 Acknowledgements
401
This work was funded by the U.S. Department of Energy’s Bioenergy Technologies Office (agreement
402
number 22588). The authors would like to thank Bob Sidner, Mae Thomas, and Jason Renzaglia of
403
Eastern Research Group for technical support and Daniel Inman and Ryan Davis of the National
404
Renewable Energy Laboratory for contributions and comments. The U.S. Government retains and the
405
publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a
406
nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this
407
work, or allow others to do so, for U.S. Government purposes.
408
6 Supporting Information
409 410 411
SI−S.1: Process Design Summaries; SI−S.2: Supplementary Methods; SI−S.3: Supplementary Results; S1S.4: Supplementary Discussion
412
7 Author Contributions
413
A.E. and A.B. contributed equally to this work; A.E. wrote the manuscript and assisted with the analysis;
414
A.B. performed the chemical process modeling and assisted with writing the manuscript; Y.Z. and G.H.
415
supervised the analysis and assisted with writing the manuscript.
416
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