When a facility manager hears the word emissions, the first image is usually a smokestack. That is understandable. Stacks are visible, regulated, and monitored. But combustion sources are only one piece of the emissions puzzle. Process emissions from chemical reactions, fugitive leaks from valves and flanges, and evaporative losses from storage tanks and coating operations all contribute to a facility's total emissions profile. Many facilities discover during their first serious emissions inventory that the sources they were not tracking account for more tonnage than the ones they were.
Understanding where your emissions actually come from is the first step toward managing them efficiently. Every ton of emissions that goes unreported is a compliance risk. Every ton that gets reported but could have been reduced is money left on the table, whether in permit fees, raw material waste, or energy inefficiency. This guide breaks down the major emission source categories, explains how to identify the ones that matter most at your facility, and shows how a handful of calculations can reveal where to focus your reduction efforts.
Combustion Sources: The Emissions You Already Know About
Combustion is the most straightforward source of facility emissions. Every boiler, furnace, heater, generator, and engine that burns fuel produces carbon dioxide, nitrogen oxides, carbon monoxide, particulate matter, and sulfur dioxide. The quantities depend on fuel type, combustion efficiency, and operating load. Natural gas burns cleaner than fuel oil, which burns cleaner than coal. But even a well-tuned natural gas boiler produces roughly 117 pounds of CO2 per million BTU of heat input, along with measurable quantities of NOx and CO.
The key to accurate combustion emissions accounting is knowing your fuel consumption. If you track natural gas purchases in therms or MCF, you can calculate CO2 emissions with EPA emission factors: 53.06 kg CO2 per million BTU for natural gas, 73.16 for distillate fuel oil, and 95.27 for bituminous coal. Multiply your annual fuel consumption by the heating value and the emission factor, and you have your annual CO2 from combustion. Most facilities find that boilers and process heaters account for 60-80% of their total greenhouse gas emissions.
NOx and CO emissions depend heavily on combustion conditions. Excess air, flame temperature, burner design, and maintenance all affect how much NOx forms. A boiler running with 30% excess air produces significantly more NOx than one tuned to 10% excess air, because the extra oxygen and higher flame temperature promote thermal NOx formation. CO is the opposite indicator: high CO means incomplete combustion, which wastes fuel and can signal burner problems. The ideal combustion setup minimizes both NOx and CO simultaneously, which requires careful tuning.
Many facilities underestimate their combustion emissions because they only count the large units. A 500 HP boiler running year-round dominates the inventory, but twenty space heaters, a paint booth air makeup unit, two dock heaters, and a parts washer heater collectively add another 10-15% that often goes untracked. For Title V applicability, every BTU counts. Walk the facility with a clipboard and list every fuel-burning device, including the ones nobody thinks about.
Natural Gas: 53.06 kg (117 lbs)
Propane: 63.07 kg (139 lbs)
Distillate Fuel Oil (#2): 73.16 kg (161 lbs)
Residual Fuel Oil (#6): 75.10 kg (166 lbs)
Bituminous Coal: 93.28 kg (206 lbs)
Source: EPA 40 CFR 98 Subpart C, Table C-1
Fuel Combustion Emissions Calculator
Calculate CO2, NOx, SOx, and PM emissions from fuel combustion using EPA AP-42 emission factors. Supports natural gas, propane, diesel, fuel oil, and coal with annual emissions totals and cost-per-ton estimates.
Process Emissions: What Your Operations Release
Process emissions come from chemical or physical transformations that occur during manufacturing, not from fuel combustion. Examples include CO2 released from calcining limestone in cement production, VOCs released during coating and painting operations, CO2 from welding operations that use shielding gas, and acid fumes from metal pickling or etching. These emissions are driven by production volume, raw material composition, and process design rather than energy consumption.
VOC emissions from coating operations are among the most common process emissions in industrial facilities. When solvent-based paint dries, the solvent evaporates and becomes a VOC emission. A facility using 500 gallons per month of a coating that contains 4.5 lbs of VOC per gallon emits 2,250 lbs of VOC per month, or 13.5 tons per year. That single source can push a facility from minor source to major source status under Title V, triggering permit fees, monitoring requirements, and compliance reporting obligations that cost tens of thousands of dollars annually.
Chemical reaction emissions are harder to estimate because they require knowledge of the process chemistry. Concrete batch plants release particulate matter from aggregate handling and mixing. Metal foundries release particulate and metal fumes during melting and pouring. Plastics operations release various organic compounds during extrusion, molding, and curing. Each process has its own set of emission factors, published by the EPA in AP-42 (Compilation of Air Pollutant Emission Factors), that provide default estimates based on production rates and material types.
The gap between estimated and actual process emissions can be large. AP-42 factors are averages based on source tests conducted across many facilities, and they may overestimate or underestimate your specific operation by a factor of two or more. If a process emission source is close to a regulatory threshold, consider conducting a source test or using a more refined estimation method. The cost of a stack test ($5,000 to $15,000) can be justified if it proves your actual emissions are below a threshold that would otherwise require expensive controls or an upgraded permit.
Boiler Efficiency & Stack Loss Calculator
Calculate boiler combustion efficiency from stack temperature and flue gas analysis. See stack heat loss, excess air percentage, and annual fuel savings from tuning. Supports natural gas and oil-fired boilers.
Fugitive Emissions: The Leaks Nobody Sees
Fugitive emissions are releases that do not come from a stack, vent, or other defined point source. They escape from equipment leaks, open containers, transfer operations, and material handling. In petrochemical and refining facilities, fugitive emissions from valves, flanges, pump seals, and compressor seals can represent 2-10% of total facility VOC emissions. In a facility with thousands of valves and fittings, that adds up to significant tonnage.
Refrigerant leaks are a fugitive emission category that surprises many facility managers. Commercial and industrial HVAC systems, chillers, and refrigeration equipment use fluorinated gases with extremely high global warming potential. R-410A has a GWP of 2,088, meaning one pound of leaked R-410A is equivalent to 2,088 pounds of CO2 in terms of climate impact. A 50-ton chiller holding 30 lbs of R-410A that leaks 10% per year releases the CO2 equivalent of burning 330 gallons of gasoline, just from a slow refrigerant leak that nobody notices.
LDAR (Leak Detection and Repair) programs are required for many industrial facilities under EPA regulations and state implementation plans. These programs require periodic monitoring of valves, pumps, connectors, and other components using portable VOC analyzers (Method 21) or optical gas imaging cameras. When a leak is detected above the action level (typically 500 or 10,000 ppm depending on the regulation), the component must be repaired within a specified timeframe. Facilities with good LDAR programs routinely find that 1-3% of their components are leaking at any given time.
Even if your facility is not required to have a formal LDAR program, identifying and fixing leaks is almost always cost-effective. A leaking valve releases product that you paid for. A leaking flange fitting on a compressed air line wastes energy. A leaking refrigerant connection costs hundreds of dollars per pound to recharge. A systematic walk-through with a soap solution for compressed air, a refrigerant leak detector for HVAC, and a VOC sniffer for chemical processes will almost always find leaks that pay for the inspection time within weeks.
Building Your Emissions Inventory
An emissions inventory is a systematic accounting of every emission source at your facility, the pollutants released, and the quantities. It is the foundation of every air permit, every compliance report, and every emissions reduction strategy. Without an accurate inventory, you cannot know whether you are in compliance, whether a new project will trigger permit modifications, or where to invest in reductions for the best return.
Start by walking the facility and listing every source that releases anything to the atmosphere. Categorize each source as combustion, process, or fugitive. For each source, identify the pollutants released (CO2, NOx, SO2, PM, VOC, HAPs) and the estimation method (fuel records, material balance, emission factors, or source testing). Assign operating hours: a boiler that runs 8,760 hours per year produces very different annual emissions than an emergency generator that runs 100 hours per year.
Material balance is the most reliable estimation method for many process emissions. The principle is simple: if you bring 10 tons of solvent into the facility and none of it leaves in your product or waste stream, then 10 tons of solvent was emitted to the atmosphere. For coatings, the VOC content is listed on the Safety Data Sheet. Multiply usage by VOC content and you have your VOC emissions. For liquid chemicals, track purchases minus inventory change minus waste disposal to get annual usage, then apply the appropriate vapor pressure or emission factor.
Once the inventory is complete, rank sources by emissions. In almost every facility, a Pareto pattern emerges: three or four sources account for 70-80% of total emissions. These are the sources to focus on for permit compliance, emissions reduction, and cost savings. A 10% improvement in the efficiency of your largest boiler may save more emissions than eliminating your tenth-largest source entirely. The inventory gives you the data to make smart allocation decisions instead of guessing.
Combustion: boilers, heaters, furnaces, generators, engines, flares
Process: coating, mixing, welding, curing, chemical reactions
Fugitive: valves, flanges, pump seals, storage tanks, loading
Mobile: forklifts, yard trucks, fleet vehicles (may be excluded from stationary source permits)
Do not forget incidental sources: space heaters, dock heaters, lab hoods, solvent wipe rags.
Emergency Generator Emissions Calculator
Calculate emissions from emergency and standby diesel generators. Check RICE NESHAP compliance with runtime hour tracking, non-emergency use limits, and annual emissions totals for permit applications.
Setting Reduction Priorities
Once you know where your emissions come from, the question becomes where to spend your reduction dollars. The cost per ton of emissions reduced varies enormously across source types. Improving boiler combustion efficiency by tuning excess air costs almost nothing and can reduce NOx by 10-20% and CO2 by 2-5%. Switching from solvent-based to waterborne coatings can cut VOC emissions by 60-80% but requires process qualification and may affect product quality. Installing selective catalytic reduction on a large combustion source can achieve 90% NOx reduction but costs hundreds of thousands of dollars.
The first priority should always be no-cost and low-cost operational improvements. Tune combustion equipment annually. Fix compressed air leaks (which reduce energy use and therefore combustion emissions from power generation). Repair refrigerant leaks. Close lids on solvent tanks. Switch to lower-VOC cleanup solvents. These measures typically cost less than $1,000 per ton of emissions reduced, and some have negative cost because they save energy or materials.
The second priority is process changes that reduce emissions at the source. Reformulating coatings to lower VOC content, switching from solvent cleaning to aqueous cleaning, replacing high-GWP refrigerants with lower-GWP alternatives, and improving process efficiency to reduce waste all attack emissions at the source rather than treating them after the fact. Source reduction is almost always cheaper than add-on controls over the life of the equipment.
Add-on controls like scrubbers, thermal oxidizers, baghouses, and SCR systems are the last resort. They are expensive to install, expensive to operate, and they create their own secondary waste streams. A thermal oxidizer that destroys 98% of VOC emissions also burns natural gas to maintain its operating temperature, adding combustion emissions. The net reduction is still positive, but the cost per ton is much higher than source reduction. Reserve add-on controls for situations where permits require a specific control efficiency or where source reduction cannot achieve the required emission rate.
Combustion tuning: $50–$500/ton CO2
Leak repair (LDAR): $500–$2,000/ton VOC
Coating reformulation: $1,000–$5,000/ton VOC
Refrigerant leak repair: $200–$1,000/ton CO2e
Thermal oxidizer: $5,000–$20,000/ton VOC
SCR (NOx): $2,000–$10,000/ton NOx
Always start with the cheapest options and work up.