What Are the Benefits of a Low-NOx Boiler?

Low-NOx boilers help companies comply with EPA requirements, improve efficiency

Published: Wednesday, August 18, 2021 - 12:03

Nitrogen oxides (NOx) are harmful chemical compounds produced as a byproduct of normal boiler function. The EPA regulates them due to the damage they can cause to humans and the environment.

Low-NOx boilers, outfitted with special components that reduce nitrogen oxides, are essential for organizations that want to comply with EPA requirements while also potentially improving efficiency. Factory owners should understand the benefits of low-NOx boilers, as well as how new models use industrial design to minimize the production of this dangerous compound.

The effects of NOx exposure

The excessive production of NOx can pose risks to workers’ lungs and the environment. Short-term exposure to nitrogen oxide can make people more vulnerable to asthma and respiratory infections. They may also find their sense of smell diminished. Long-term exposure can cause more serious health problems, like chronic lung disease.

Boiler operators and workers who spend much of their time near it are most vulnerable to health problems. However, everyone on-site may be exposed to nitrogen oxide and the risks associated with it.

Nitrogen oxides in the atmosphere can have a harsh environmental impact. High levels may damage vegetation and prevent new plants from growing.

The EPA regulates NOx emissions in the United States in accordance with the Clean Air Act. The agency is required to create national ambient air quality standards (NAAQS) for nitrogen oxides that determine how much NOx a facility can produce in a certain amount of time.

Currently, the primary NAAQS for NOx are a “one-hour standard at a level of 100 ppb [parts per billion] based on the three-year average of 98th percentile of the yearly distribution of one-hour daily maximum concentrations, and an annual standard at a level of 53 ppb.” All major stationary sources of NOx are expected to follow these standards.

Organizations that don’t follow these standards may be subject to legal action under the Clean Air Act. In the past, companies found in violation have been required to upgrade existing equipment to reduce emissions, pay civil penalties, and invest in projects that improve health in communities impacted by facility emissions.

How do boilers generate NOx?

The nitrogen oxides generated by a boiler are generally regarded as being in one of two categories—thermal NOx and fuel NOx. In both cases, the NOx is produced by the flame the boiler uses to heat water.

Thermal NOx is produced when atmospheric nitrogen and oxygen undergo a chemical reaction in the boiler’s flame. A boiler’s flame temperature has a significant impact on the amount of thermal NOx produced. In general, the more energy in the flame, the more nitrogen oxide will be created during normal operations.

The shape of the flame can also affect thermal NOx. The less surface area and more volume it takes up, the greater amount of nitrogen oxide will be produced.

Fuel NOx is produced by the reaction of nitrogen in the boiler fuel with atmospheric oxygen. It can account for up to half or more of a boiler’s NOx emissions if it uses an energy source rich in nitrogen, like oil or coal.

Strategies for low-NOx boiler design

Industrial designers have created boilers that produce less NOx without significantly impairing performance. The use of new techniques may help to improve efficiency in some cases, though they may make boilers more expensive to manufacture.

Low-NOx boilers employ various strategies to reduce the nitrogen oxides produced by boiler operation, including:
• Steam injection
• Selective or nonselective catalytic reduction (SCR or NSCR)
• Flue-gas recirculation
• The use of natural gas, rather than nitrogen-rich fuels like oil, to limit fuel NOx

Components that enable these strategies are sometimes built directly into boilers, while others can be installed on an existing one. This reduces NOx emissions while minimizing installation and part costs.

Different types of industrial boilers may require various NOx-minimizing strategies. For example, low-NOx firetube versions often employ specialized burners or flue-gas recirculation to reduce emissions. Steam injection, SCR, NSCR, and flue-gas recirculation all target thermal NOx production.

With steam injection, a system deposits steam into the primary combustion zone of a boiler. This reduces the peak combustion temperature of the flame and reduces potential NOx emissions.

Selective and nonselective catalytic reduction solutions work to remove NOx after it is formed. With the use of a catalyst, these systems convert NOx into diatomic nitrogen and water.

In flue-gas recirculation, gases produced by the combustion process, including nitrogen oxides, are recirculated into the main combustion chamber. This process reduces NOx emissions and improves boiler efficiency.

Boilers that use fuels with little nitrogen, like natural gas, tend to produce less NOx by default.

Upgrading existing boilers

Typically, organizations wanting to take advantage of these boiler design strategies will need to upgrade to a new model.

Low-NOx boiler retrofits are also a possibility for companies that can’t afford to fully replace their existing high-NOx boiler. Retrofitters typically install low-NOx burners rather than use a back-end solution like a selective catalytic reduction or nonselective catalytic reduction system. This is due to the relative ease and low cost of a new burner installation.

It’s comparatively simpler to install, but it’s not possible to just swap the old high-NOx burner for a new low-NOx alternative. Retrofitters must also consider the boiler’s fan capacity, burner spacing, and existing windbox when making the upgrade.

In addition to reducing the amount of NOx a boiler produces, many of these changes will help cut down on other kinds of emissions—like carbon monoxide production.

While some emissions-reducing upgrades do not provide performance improvements, many do help cut down on boiler energy and maintenance costs. In some cases, it may be possible to upgrade existing boilers in a way that reduces their NOx emissions.

Flue gas recirculation, for example, cuts down while also reducing the boiler’s particulate emissions and overall stack opacity. With this approach, a boiler is retrofitted or designed with a system that recirculates a small amount of flue gas back into a boiler’s flame, typically with the use of a digital control system.

Flue gases may be recirculated using an external system, a system of fans and piping that redirects flue gas from the stack to the burner, or an induced system, which uses the combustion air fan for the same purpose.

The lower flame temperature helps reduce both the energy consumption and emissions production of the boiler. Reduction in NOx emissions will vary depending on the type of fuel being burned, but in some boilers, it can be as high as 75 percent.

Other methods offer lower reductions in NOx emissions but may be cheaper to implement.

For example, low excess air (LEA) firing reduces the amount of air with which a boiler is fired. Boilers are typically fired with a certain amount of excess air as a safety measure. However, excess air can also significantly increase the amount of NOx a boiler produces.

Limiting the amount of firing air through oxygen controls and changes to boiler design can reduce NOx emissions by 5 to 10 percent. Optimizing the fuel/air ratio in this way can reduce energy costs, too. If there was a particular abundance of excess air to begin with, limiting air may also yield some performance improvements.

Improving boiler performance can also reduce emissions by cutting down on the fuel necessary to generate heat and the amount of flue gas produced as a result. Both recirculation and limiting excess air mean less fuel is needed for day-to-day operations, helping to reduce costs.

Smaller changes will require less capital investment but may also be less likely to provide significant efficiency improvements. Nonetheless, increasing boiler efficiency by even a few percentage points results in significant savings by reducing fuel costs.

Alternatively, the replacement of legacy equipment with new, low-NOx tools can sometimes significantly reduce maintenance costs. It’s not uncommon for newer models of low-NOx boilers to include efficiency-improving features and design elements. Investing in one of these boilers can allow a facility to cut down on both energy consumption and NOx emissions at the same time. An upgrade can also be a way to sidestep the expense of maintaining older equipment, which tends to increase over time.

Why businesses upgrade to low-NOx boilers

High-NOx boilers can cause a facility to produce NOx emissions in excess of existing EPA guidelines and may harm the health of facility workers. The environment will also pay the price.

Adopting low NOx boilers or investing in retrofits can help facility owners remain compliant with regulations, protect on-site workers, and burn fuel more efficiently during operation. The return on investment for a low NOx boiler depends on the strategies used to reduce nitrogen oxide production and whether a business opts for an entirely new machine or a retrofit.

About The Author

Emily Newton

Emily Newton is the editor-in-chief of Revolutionized, an online magazine exploring the innovations disrupting the scientific and industrial sectors.