General Conformity Training Modules: Appendix A Sample Emissions Calculations
1. External Combustion Sources
External combustion units such as boilers and space heaters provide heat for building HVAC systems, heating water, and generating steam. These units use natural gas, diesel, propane, coal, or other petroleum-based fuel.
Source test data pertain to specific equipment. Thus it is best to use these data to obtain the most accurate emissions. However, if source test data are not available, emission factors can be used to calculate annual emissions. AP-42, Compilation of Air Pollutant Emission Sources, presents emission factors for external combustion sources based on the fuel type and size of the unit:
Section 1.1 Bituminous and Subbituminous Coal;
Section 1.2 Anthracite Coal;
Section 1.3 Fuel Oil;
Section 1.4 Natural Gas; and
Section 1.5 Liquefied Petroleum Gas.
Annual emissions from external combustion sources can be calculated using the following equation:
AEi = EFi x Q x (1- CEi/100)
Where,
AEi = Annual emissions of chemical i (lb i / yr)
EFi = Chemical i emission factor for fuel and unit size used (lb i/MMscf fuel)
Q = Max potential or actual amount of fuel used (MMscf fuel /yr)
CEi = Chemical i emission control efficiency (percent)
100 = Factor for converting percent to a fraction
For example, actual annual carbon monoxide (CO) emissions from a 2.5 million British Thermal Units per hour (MMBtu/hr) natural gas-fired boiler which consumes 3.5 million standard cubic feet (MMscf) in one year is calculated as follows:
AECO = (84 lbs CO/MMscf nat gas) x (3.5 MMscf nat gas) x (1 - 0/100)
= 294 lb CO/yr [assuming no controls, CEi = 0]
where emission factors for natural gas-fired boilers with a heat rating between 0.3 MMBtu/hr and 100 MMBtu/hr are obtained from Table AP-42, Section 1.4, Table 1.4-1.
2. Internal Combustion Sources
There are two methods for calculating annual emissions from internal combustion units. If the unit’s brake horsepower (bhp) and annual hours of operation are available, the following equation can be used:
AEi = EFi x bhp x t
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor (lb i/bhp-hr)
t = Total annual number of hours of operation (hr/yr)
bhp = Unit brake horse power (bhp)
The rated bhp can either be obtained from the manufacturer or the engine literature that came with the engine. Emission factors are also available from AP-42 Section 3.3 for gasoline and diesel-fired internal combustion engines. If criteria pollutant emission factors in lb/bhp-hr are available from the manufacturer, these should be used before applying general engine emission factors from AP-42, Compilation of Air Pollutant Emission Sources.
For example, NOx emissions of a diesel- powered 250 bhp internal combustion engine was run for 2,080 hours in a year and the emission factor are calculated as follows:
AENOx = (0.031 lb NOx/bhp-hr) x 250 bhp x 2,080 hr/y
= 16,120 lb NOx/yr
An alternative method is used if fuel consumption is provided. First, the annual amount of heat input (MMBtu) using the following equation:
Q = Umax x Hv x ρ x t/(efficiency x 106)
Where,
Q = Annual heat input (MMBtu)
Umax = Maximum potential fuel usage of the engine (gal fuel/hr)
Hv = Heating value of the fuel (BTU/lb fuel)
ρ = Fuel density (lb fuel/gal fuel)
t = Annual actual hours of operation (hr/yr)
CEi = Chemical i emission control efficiency (percent (%))
Annual emissions can then be calculated by using the following equation:
AEi = EFi x Q
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor for fuel and unit size used (lb i/MMBtu)
Q = Annual heat inputfrom equation above (MMBtu)
For example, a diesel internal combustion engine with a maximum fuel usage of 18 gal/hr was run for 2,080 hr/yr and an estimated efficiency of 80 percent. The actual annual NOx emissions are calculated by first finding the maximum heat value:
Q = (18 galfuel/hr) + (19,300 Btu/lb) x (7.5 lb fuel/gal fuel) x (8,760 hr/yr) x (1 MMBtu/106) x (1/80 % eff.)
= 28,530 MMBtu
AENOx = 4.41 lb NOx x 28,530 MMBtu
= 125,817 lb NOx/yr
3. Construction
Particulate emissions from building and road construction may substantially affect local air quality for a temporary period. Construction activities include land clearing, drilling and blasting, ground excavation, cut and fill operations (i.e., earth moving), and construction of a given facility. The amount of particulate emissions is proportional to the area of land being worked on and the level of construction activity. Equipment traffic is a major contributor of emissions. Particulate emission factors for construction activity operations are:
E = 2.69 megagrams (Mg)/hectare/month of activity
E = 1.2 tons/acre/month of activity
It is strongly recommended that when estimating emissions for a particular site, the construction process be broken down into component operations where each component involves traffic and material movements, and emission factors from other AP-42, Compilation of Air Pollutant Emission Sources, sections are used to generate estimates. AP-42 Table 13.2.3-1 lists the dust sources involved with construction, along with the recommended emission factors.
In addition to the on-site activities given in Table 13.2.3-1, substantial emissions are possible because of material tracked out from the site and deposited on adjacent paved streets. A secondary source of emissions occurs from traffic passing the site (i.e., not just that associated with the construction) can resuspend the deposited material and may be a far more important source than all dust activity within in the construction site.
The annual emissions of a chemical from construction activities can be calculated using the following equation:
AEi = EFi x [PR x (LF/100) x U x H x D] x 0.002205
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor (g i/hp-hr)
PR = Power rating (hp)
LF = Load factor (percent (%))
100 = Factor for converting percent to a fraction
U = Number of units
H = Hours of operation per day (hr/day)
D = Number of day of operation (day)
0.002205 = Conversion Factor (lb i/g i)
For example, a diesel-powered forklift with a power rating of 94 horsepower (hp) is used for construction activity six hours a day, and 31 work days, its actual PM 10 emissions are:
AEPM10 = (.06 g PM10/hp-hr) x (94 hp x 48/100) x (6 hr x 31 days) x .002205 lb/g PM10
= 1.11 lb PM10
4. Fuel Storage
VOC emissions from fuel storage are calculated based on the methodologies presented in EPA’s AP-42, Compilation of Air Pollutant Emission Sources, Section 7. The EPA-established emission estimation equations for calculating emissions from fuel storage tanks were developed by the American Petroleum Institute (API). They are quite complex and are specific to the type of storage tank used. In general, the equations calculate the working and standing losses from storage tanks which are summed to provide the total emissions associated with a specific storage tank. Working losses refer to the emissions from receiving fuel. Standing losses are primarily due to temperature changes and refer to losses from the evaporation of the fuel from the storage tank. These emissions are generally released through vents or other mechanisms.
5. Fuel Transfer
Fuel transfer operations involve the loading of fuel into fuel storage tanks, tanker trucks, aircraft, and vehicles and/or equipment. As liquid fuel is loaded into a source (e.g., into the main storage tanks, a tanker truck cargo tank, an aircraft tank, a vehicle/equipment tank, or a bowser), vapors are displaced and emitted into the atmosphere. The amount of emissions released is dependent on several factors such as the type of fuel being transferred, temperature, and the loading method as described below. Working losses will occur from fuel transfer during the loading of fuel storage tanks, tanker trucks, vehicles, equipment, and aircraft refueling.
6. On-Road Vehicles
On-road vehicles can be classified into eight vehicle categories according to vehicle type, gross vehicle weight (GVW), and fuel type. Table 1 lists the eight vehicle categories and a brief description of each.
Table 1. Vehicle Categories for On-road Vehicles
|
Vehicle Type Category |
Description |
|---|---|
|
LDGV |
Light-duty gasoline-fueled vehicles (i.e., gasoline passenger cars) |
|
LDGT1 |
Light-duty gasoline-fueled trucks, type 1 (includes gasoline pickup trucks, sport utility vehicles, and vans with a GVW of 6,000 pounds or less) |
|
LDGT2 |
Light-duty gasoline-fueled trucks, type 2 (includes gasoline pickup trucks, sport utility vehicles, and vans with a GVW from 6,001 pounds to 8,500 pounds) |
|
HDGV |
Heavy-duty gasoline-fueled vehicles (includes all gasoline vehicles with a GVW exceeding 8,500 pounds) |
|
LDDV |
Light-duty diesel-powered vehicles (i.e., diesel passenger cars) |
|
LDDT |
Light-duty gasoline-fueled trucks (includes diesel pickup trucks, sport utility vehicles, and vans with a GVW of 8,500 pounds or less) |
|
HDDV |
Heavy-duty diesel-powered vehicles (includes diesel trucks and buses with a GVW exceeding 8,500 pounds) |
|
MC |
Motorcycles |
In addition to vehicle category, emission factors for motor vehicles are dependent on several other variables such as model year, mileage, speed, temperature, altitude, fuel properties (e.g., additives, vapor pressure, sulfur content, etc.), possible tampering, possible inspection/maintenance programs, operating mode (i.e., percent operation in cold start, stabilized, and hot start modes), emission control system, etc. Emissions from on-road vehicles are usually calculated using “typical” (or “average) emission factors. Typical emission factors for CO, NO, and non-methane hydrocarbons are found in Appendix H of AP-42, Volume II Mobile Sources. The annual emissions from on-road vehicles can be calculated using the following formula:
AE i = VMT ´ EF i ´ 0.002205
Where,
AEi = Annual emissions of chemical i (lb i/yr)
VMT = number of vehicle miles traveled per year (miles/yr)
EFi = Chemical i emission factor (g i/mile)
0.002205 = Factor to convert grams to pounds (lb i/g i)
For example, in the category of light duty gasoline vehicles (LDGV), the total annual miles driven onsite is 12,972 miles; therefore, actual CO emissions are:
AECO = 12,972 miles/yr x 10.2 g CO/mile x 0.0022 lbs/g
AE CO = 291.09 lb CO/yr
In order to calculate total emissions from all vehicles, emissions from each vehicle category are summed. If more specific emission calculations are required (e.g., emissions calculated based on different speeds, temperatures, operating modes, etc.), they can be performed using EPA Mobile Source computer programs. These programs can be obtained from the following internet address: https://19january2021snapshot.epa.gov/moves. The most current EPA program for calculating CO, NO, and VOC emissions from on-road vehicles is Motor Vehicle Emission Simulator (MOVES) which superscedes MOBILE6.2.
The total direct and indirect emission for a project includes the emissions from vehicles at the federal facilities as well as emissions from vehicles servicing the facilities and employees commuting to the facilities. The on facilities VMT can be estimated based upon traffic data, or surveys. The off facility VMT for service vehicles can be estimated based upon the average distances to serviced centers in the nonattainment or maintenance area. The VMT for commuters should be based upon the average commuting distance within the nonattainment or maintenance area for employees at the facility.
7. Off-Road Vehicles and Equipment
Emissions from off-road vehicles and equipment used at an installation can be estimated based upon the type of off-road vehicles and equipment used. Engine rating, fuel use, and total operating time for off-road vehicles and equipment can be obtained from maintenance records or interviews with operating personnel. Emission factors for off-road vehicles and equipment can be obtained from EPA’s NONROAD2008 model. For natural gas engines, emission factors for criteria pollutants and precursors can be obtained from AP-42, Compilation of Air Pollutant Emission Sources, Section 3.2 Table 3.2-3 Gaseous Emission Factors for 4-Stroke Rich Burn Natural Gas Engines.
Method 1
When fuel usage data are not available, the emissions can be calculated using the estimated hours of operation. Actual emissions of criteria pollutants for the specific equipment type can be estimated by multiplying the estimated annual number of operational hours by their engine rating, equipment load factor, and respective emission factors. The load factor is the portion of available power at which the type of engine typically operates. The equation is as follows:
AEi= EFi x [(PO x (LF/100) x OT)] ´ 0.002205
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor (g i/mile)
PO = Rated power output of the vehicle/equipment engine (hp)
LF = Loading Factor (% of Maximum Power)
100 = Factor for converting percent to a fraction
0.002205 = Conversion Factor (lb i/gi)
For example, for the diesel-fired tractor tow support units with seven units, each with an engine ratings of 77 hp, operated at 56% of their maximum power, for a total of 1,297 hours in a given year; their actual annual VOC emissions are:
AEVOC = (0.451 g VOC/mile) x 77 hp x (56/100) x (1,297 hrs/yr) x (.002205 lb/g VOC)
= 55.62 lb VOC/yr
Method 2
When the fuel usage data are not available the emissions can be calculated by converting the fuel usage into a power output (i.e., horsepower-hours). Actual emissions of criteria pollutants for the specific equipment type can be calculated by multiplying the annual power output by the applicable emission factors.
AEi = EFi x [(FC x FD)/BSFC] x 0.002205
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor (g i/hp-hr)
FC = Fuel consumption (gal/yr)
FD = Fuel density (lb fuel/gal fuel) [note default values include: 6.09 for gasoline, 7.11 for diesel, and 6.8 for JP-8 jet fuel]
BSFC = Typical brake-specific fuel consumption for the vehicle/equipment (lb fuel/hp-hr)
0.002205 = Conversion Factor (lb i/g i)
For example, for the diesel-fired mower with a BSFC of 0.4 lb/hp-hr, which consumed 416 gallons of diesel fuel in a given year; its actual annual CO emissions are:
AECO = (5.01 g CO/hp-hr) x (416 gal fuel/yr) x (7.11 lbs/gal fuel) x hp-hr/0.4 lbs x 0.002205 lbs/g CO
= 81.67 lb CO/yr
Method 3
When the fuel usage data and the emission factors are available based on mass of pollutant per volume of fuel consumed, the following equation can be used:
AEi = EFi x FC x 0.002205
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor (g i/hp-hr)
FC = Fuel consumption (gal fuel/yr)
0.002205 = Conversion Factor (lb i/g i)
For example, the annual NOx emissions of a 4,000 pound diesel-powered forklift using 21 gallons of diesel fuel in a particular year are:
AENOx = (4.502 gNOx/hp-hr) x (21 gal fuel/yr) x 0.002205 lb/g NOx
= 0.21 lb NOx/yr
8. Aircraft Operations
Flying operations include landing and takeoff (LTO), touch and go (TGO), and low approach (LA). An LTO cycle includes taxiing between the hangar and the runway, taking off and climbing out, approach and descent from the local pattern, followed by a touch down and taxiing in. TGOs include only a takeoff, climbout, and an approach. LAs include only approach and climbout. Each of these activities has an actual duration and associated emission factors for criteria pollutants and precursors that are based on the engine power settings. Annual emissions from flight operations can be estimated using the FAA’s Emission and dispersion Modeling System (EDMS). EDMS provides data on flight emissions and ground support equipment emission. In flight emissions can also be calculated using the following equation:
AEi = EFi x (FFR/1000) x (TIM/60) x NE x 0.002205
Where,
AEi = Annual emissions of chemical i (lb i/yr)
EFi = Chemical i emission factor for the aircraft engine at a particular
setting (lb i/1000 lb fuel)
FFR = Fuel flow rate per engine (lb fuel/yr)
1000 = Factor for converting “lb/hr” to “1000 lb/hr”
TIM = Time in Mode (min/cycle)
60 = Factor for converting minutes to hours (min/hr)
NE = Number of engines on the aircraft
In-flight emissions only include emissions in the nonattainment or maintenance area. Emissions above the mixing height for the area or beyond the boundaries of the area are not included. Three thousand feet above ground level is used as a default mixing height when more specific data is not available.
As an example, CO emissions during the taxiing out mode for the C-130H aircraft with four T56-A-15 engines and 2,368 LTOs are calculated as follows:
AECO = (3.84 lb CO/1000 lb fuel) x 1,200 lb fuel/hr x (0.58 hr/LTO) x (2,368 LTOs/yr) x 4 engines/aircraft
= 25,315 lbs CO/yr
Emissions from all five operating modes of a specific engine in an LTO cycle are then added to obtain total annual emissions from LTO operations.
9. Paved Roads
Vehicle travel over a paved surface such as a road or parking lot produces particulate emissions. Direct particulate emissions include vehicle exhaust, brake wear, and tirewear. Resuspended loose materials from the road surface also contribute to particulate emissions. AP-42, Compilation of Air Pollutant Emission Sources, Section 13.2.1 provides a paved road emission factor that only addresses particulate emissions from resuspended material. EPA’s MOBILE6.2 or MOVES2010 model are used to estimate particulate emissions from vehicle exhaust, brake wear, and tire wear. The following equation can be used to calculate size-specific emission factors for vehicle travel over a paved surface:
EFi = k (sL/2)0.98(W/3)0.53(S/30)0.16
Where,
EFi = Size-specific emission factor (units matching the units of k)
k = Particle size multiplier for particle size range and units of interest (see below)
sL = Road surface silt loading (g/m 2)
W = Average weight of the vehicles traveling the road (tons)
S = Average vehicle speed of the vehicles traveling the road (mph)
The emission factor shown above only represents particulate emissions from resuspended surface material from vehicles traveling paved roads. To obtain an emission factor representing total particulate emissions, the emission factors for the exhaust, brake wear and tire wear obtained from either EPA’s MOBILE6.2 or MOVES2010 model should be added to the emissions factor calculated from the empirical equation. Total particulate emissions can then be estimated with this aggregate emission factor.
In addition, when calculating the emission factor from the above equation, the average weight and speed for all vehicles traveling the road should be used. For example, if 99 percent of traffic on the road is 2-ton cars and trucks while the remaining 1 percent consists of 20-ton trucks, then the mean weight “W” is 2.2 tons.
10. Unpaved Roads
Particulate emissions result from vehicle travel over unpaved surfaces. Direct particulate emissions include vehicle exhaust, brake wear, and tire wear. Resuspended loose materials from the road surface also contribute to particulate emissions. AP-42, Compilation of Air Pollutant Emission Sources, Section 13.2.2 provides an unpaved road emission factor that only addresses particulate emissions from resuspended material. EPA’s MOBILE6.2 or MOVES2010 model are used to estimate particulate emissions from vehicle exhaust, brake wear, and tire wear. For vehicles traveling on unpaved surfaces at industrial sites, emissions are estimated from the following equation:
E = k (s/12)a(W/3)b
And, for vehicles traveling on publicly accessible roads, dominated by light duty vehicles, emissions can be estimated using the following equation:
E = [k (s/12)a(S/30)d - C] / [(M/0.5)c]
Where k, a, b, c, and d are empirical constants (AP-42, Section 13.2.2, Table 13.2.2-2) and
E = Size-specific emission factor (lb/VMT)
s = Surface material silt content (%)
W = Mean vehicle weight (tons)
M = Surface material moisture content (%)
S = Mean vehicle speed (mph)
C = Emission factor for 1980s vehicle fleet exhaust, brake wear and tire
Wear (lb/VMT).
Constants k, a, b, c, and d can be found in AP-42, Section 13.2.2, Table 13.2.2-2. Table 13.2.2-3 contains the range of values for surface material silt content, mean vehicle weight, surface material moisture content, and mean vehicle speed. Table 13.2.2-4 presents the emission factor for 1980s vehicle fleet exhaust, brake wear and tire wear.
The emission factor shown above only represents particulate emissions from resuspended surface material from vehicles traveling unpaved roads. To obtain an emission factor representing total particulate emissions, the emission factors for the exhaust, brake wear and tire wear obtained from either EPA’s MOBILE6.2 or MOVES2010 model should be added to the emissions factor calculated from the empirical equation. Total particulate emissions can then be estimated with this aggregate emission factor.
In estimating the particulate emission factor for resuspended material from vehicles traveling on unpaved surfaces, the average weight, speed, and number of wheels for all vehicles traveling the road is to be used. For example, if 98 percent of traffic on the road are 2-ton cars and trucks while the remaining 2 percent consists of 20-ton trucks, then the mean weight is 2.4 tons.
11. Storage Piles
Outdoor storage piles of minerals in aggregate form are usually left uncovered, partially because of the need for frequent material transfer into or out of storage. Particulate emissions arise from material loading onto the pile, wind, and loadout from the pile. Truck and equipment traffic also form dust emissions. Particulate emissions may be estimated by using one of the following equations:
EFi = [k(0.0016)] x [(U/2.2)1.3 / (M/2)1.4] (kg/megagram [Mg])
EF i = [k(0.0032)] [(U/5)1.3 / (M/2)1.4 (lb/ton)
Where,
EFi = Emission factor
k = Particle size multiplier (dimensionless)
U = Mean wind speed, meters per second (m/s) (miles per hour [mph])
M = Material moisture content (%)
Section 13.2.4 of AP-42, Compilation of Air Pollutant Emission Sources, provides a range of values for aerodynamic particle size multiplier (k), silt content (percent), moisture content (percent), and wind speed (m/s and mph) for calculating the emission factor from the equations above.
When estimating emissions from equipment traffic (trucks, front-end loaders, dozers, etc.) between or on piles, it is recommended that the equations for vehicle traffic on unpaved surfaces be used, which are listed in Section 13.2.2 of AP-42. For vehicle travel between storage piles, the silt value(s) for the areas among the piles (which may differ from the silt values for the stored materials) should be used.
Worst-case emissions from materials-handling operations typically occur under dry, windy conditions. The methodology on treatment of dry conditions for Section 13.2.2, vehicle traffic on unpaved roads, should be used.