Gas Furnace Emissions Control: NOx, CO, and VOC Compliance in 2026
Operating a gas-fired furnace in 2026 means dealing with air emissions regulations. The regulations vary by jurisdiction, but the pollutants of concern are consistent: nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs). Each pollutant has a different formation mechanism, a different control technology, and a different cost to manage. Here is what the regulations look like and what the operators actually do to comply.
The NOx problem.
NOx forms in two ways in a gas furnace. Thermal NOx forms when nitrogen and oxygen in the combustion air react at high temperature - the reaction rate roughly doubles for every 100 degrees C above 1300 degrees C. Fuel NOx forms when nitrogen in the fuel reacts with oxygen, but natural gas has essentially no fuel-bound nitrogen, so fuel NOx is negligible for natural gas furnaces.
Thermal NOx is the main concern. A conventional atmospheric burner firing natural gas with 10 percent excess air at 1100 degrees C produces roughly 50 to 100 ppm of NOx (corrected to 3 percent O2). A high-temperature furnace (above 1200 degrees C) can produce 100 to 200 ppm.
The regulations for industrial furnace NOx vary. Typical limits are 50 to 100 ppm in most jurisdictions, 30 to 50 ppm in stricter jurisdictions (California, parts of the EU), and below 30 ppm in the strictest (some US air districts, Japan for some source categories). The trend is toward tighter limits.
The control technologies for NOx fall into two categories: combustion modifications and post-combustion treatment.
Combustion modification 1: low-NOx burner design. A low-NOx burner uses internal flue gas recirculation (FGR) or staged combustion to lower the peak flame temperature. The FGR approach mixes 10 to 30 percent of the flue gas back into the combustion air, which lowers the flame temperature by 50 to 150 degrees C and cuts NOx by 30 to 60 percent. Low-NOx burners typically produce 20 to 50 ppm of NOx. They cost 20 to 50 percent more than standard burners, but the operating cost is the same.
Combustion modification 2: excess air reduction. Operating the burner at the minimum excess air (typically 2 to 5 percent, instead of the conventional 10 to 20 percent) lowers the flame temperature and cuts NOx. The trade-off is that too little excess air causes incomplete combustion, which raises CO. Modern burners with good air-fuel ratio control can operate at low excess air without CO problems.
Combustion modification 3: furnace temperature reduction. Lowering the furnace temperature lowers the peak flame temperature and the NOx. This is sometimes an option (running the furnace at 1000 degrees C instead of 1100 degrees C) but is limited by the process requirement.
Post-combustion treatment 1: Selective Catalytic Reduction (SCR). An SCR system injects ammonia or urea into the flue gas, and the ammonia reacts with NOx on a catalyst to form nitrogen and water. SCR can remove 70 to 95 percent of the NOx, achieving emissions of 5 to 15 ppm. The system is expensive ($200,000 to $2,000,000 installed depending on the flue gas flow) and the operating cost is significant (ammonia or urea, catalyst replacement, energy for heating the flue gas to the catalyst operating temperature of 300 to 400 degrees C).
Post-combustion treatment 2: Selective Non-Catalytic Reduction (SNCR). An SNCR system injects ammonia or urea into the flue gas at a high temperature (800 to 1100 degrees C), and the ammonia reacts with NOx without a catalyst. SNCR can remove 30 to 70 percent of the NOx, achieving emissions of 20 to 50 ppm. The system is cheaper than SCR but the reagent consumption is higher and the NOx removal is less.
For most heat treatment furnaces, low-NOx burners plus excess air reduction achieve compliance with the typical 50 to 100 ppm limits. SCR is reserved for the strictest limits and the largest sources.
The CO problem.
CO forms from incomplete combustion of the natural gas. CO emissions are typically very low (under 50 ppm) in a well-tuned burner with adequate excess air. CO spikes when the burner is starved of air, when the burner is dirty, or when the furnace is cold (poor ignition).
CO is regulated less stringently than NOx. Typical limits are 100 to 500 ppm. CO is rarely the compliance driver for a gas furnace, but a high CO reading often indicates a burner problem that should be fixed.
The control technology for CO is proper burner maintenance and tuning. Annual burner service (clean nozzles, check ignition, verify air-fuel ratio) keeps CO low. Some jurisdictions require continuous CO monitoring on the stack.
The VOC problem.
VOCs from a gas furnace come from two sources. First, incomplete combustion of the natural gas can produce small amounts of VOCs (mostly methane and ethane that escape unburned). Second, VOCs from the charge (lubricants, machining oils, paint, plastic) volatilize when the charge is heated. The VOCs from the charge are often the larger source.
VOC emissions are regulated through air permits. The limits depend on the jurisdiction, but typical limits are 10 to 50 mg per cubic meter of flue gas, or 90 percent destruction efficiency for an afterburner.
The control technologies for VOCs depend on the source. For combustion VOCs, proper burner tuning is the main control. For charge VOCs, the furnace has to either burn them in the combustion chamber (with adequate temperature and residence time) or capture them with an afterburner.
An afterburner is a small combustion chamber downstream of the main furnace. The afterburner burns the VOCs at 700 to 900 degrees C with a residence time of 0.5 to 2 seconds. Afterburners are common on heat treatment furnaces that process oily or painted parts. They are also common on aluminum melting furnaces, where the organics from the charge need to be controlled.
The continuous emissions monitoring (CEMS) question.
A large gas furnace (typically above 10 to 50 MMBtu/hr input) may be required to install a CEMS - a continuous emissions monitoring system that measures NOx, CO, O2, and flow on the stack, and reports the data to the regulatory agency. The CEMS is expensive ($200,000 to $1,000,000 installed) and requires ongoing maintenance and calibration.
A CEMS is not just a regulatory burden. The data is also valuable for process control - the operator can see the NOx trend, the CO trend, and the excess air level, and adjust the burners for optimal performance. A well-maintained CEMS is a process tool as well as a compliance tool.
The compliance cost.
For a 5 to 20 MMBtu/hr heat treatment furnace, the compliance cost for NOx and CO control is typically $20,000 to $100,000 in capital (low-NOx burners) plus $5,000 to $20,000 per year in operating cost (slightly higher fuel consumption, burner maintenance, and reporting). For a furnace requiring SCR, the capital cost is $500,000 to $2,000,000 and the operating cost is $50,000 to $200,000 per year.
The compliance cost is a real operating expense. Operators should budget for it when comparing furnace options. A gas furnace with full emissions control might have a higher total cost than an electric furnace in a region with a clean grid, even if the gas is cheaper per kWh.
The future of gas furnace emissions.
The trend is toward tighter limits. Most jurisdictions are tightening NOx limits on industrial sources by 20 to 50 percent over the next 5 to 10 years. The technology to comply is available (low-NOx burners, SCR), but the cost is going up.
Author: MONTE INTELLIGENCE emissions control team. For gas furnace emissions audits and burner upgrades, contact helenxu@cnlymonte.com.
