Globally, across nearly every industry, OEMs and end users are looking for ways to improve their heat-treat equipment and processes. In addition, they are facing increasingly stringent NOX emissions regulations while focusing on delivering the process and product quality their customers expect.
In this article, I will explore the complexities faced by OEMs and end users related to heat treatment. I also will offer advice on selecting an appropriate burner and control combination for common industrial heating applications.
A Few Chemistry Basics
It is important to note that it is not possible to achieve the perfect combustion of air and natural gas, which is defined as:
CH4 + 2(O2 + 3.76N2)›CO2 + 2H2O + 7.52N2
At high temperatures, nitrogen in the combustion air reacts with oxygen to form NOX — a collective term for nitric oxide (NO) and nitrogen dioxide (NO2) — and with carbon in fuel to form carbon moNOXide (CO). All three of these byproducts are under scrutiny in combustion processes around the world.
Managing Changes in Emissions Standards
Because OEM furnace manufacturers typically sell their products in multiple regions around the world, they need to be able to build furnaces that can deliver the emissions levels needed to meet local standards. Some standards apply nationally while others are set at a regional, state or even city level. Dealing with the sheer volume, variation and constant change in these requirements is a full-time job in itself.
Typically, emissions standards are stated such that when process exhaust gas is measured, levels of NOX and CO may not be above a certain threshold, measured in parts per million (ppm). In low emissions, low temperature applications such as California’s South Coast Air Quality Management District, NOX is required to be below 30 ppm, and CO is required to be below 200 ppm, corrected to 3 percent O2. High temperature applications have more lenient requirements of 60 ppm NOX or less, corrected to 3 percent O2. One concept that has been adopted in California’s San Joaquin Valley is the use of best available control technology (BACT), which emphasizes technologies such as mass-flow control.
Across the world, emissions standards are tightening. In Canada, for example, Base-Level Industrial Emission Requirements (BLIERs) have been established as a minimum standard of environmental performance across major sectors and equipment types. As a result of BLIERs, NOX emissions limits in some areas have decreased by 50 percent — from 30 ppm to 15 ppm — within a short period of time.
A recent set of European directives also requires lower emissions over time, with the latest targets being below 100 ppm NOX corrected to 3 percent O2 in all applications. Meanwhile, in Germany, emissions requirements are part of the TA-Luft standard, which is used by authorities to define allowable emissions permits for manufacturing facilities.
China is moving toward lower emissions vehicles, and industrial processes are starting to follow suit. In recent years, cities such as Beijing have seen tighter emissions for boiler applications — from less than 50 ppm NOX to less than 30 ppm NOX. In addition, China has mandated the same stringent NOX levels as the European directives and is applying the standards across all industries — not just to boilers.
India and other high growth regions are on a similar path and will likely adopt stricter industrial process emissions requirements in the coming years.
Self-recuperative burners have a uniquely designed internal combustor that draws exhaust gases back into the flame to cool it, lowering thermal NOX. When paired with flameless mode, best in class NOX emissions can be achieved.
Choosing Suitable Thermal Equipment to Reduce NOX
Before you choose equipment to lower NOX in your process, it is helpful to understand how NOX is formed.
NOX comes from three primary sources:
- Thermal NOX.
- Fuel NOX.
- Prompt NOX.
Thermal NOX is triggered by heat from the combustion reaction and is relevant when attempting to reduce NOX. Fuel NOX and prompt NOX are inherent in fuel and nitrogen reaction properties, respectively, and cannot be changed significantly.
To reduce NOX, one must focus on reducing thermal NOX. This can be achieved through a combination of burner design, equipment selection strategies and air-fuel control schemes.
Considerations for Selecting a Burner
Selecting a burner that meets your emissions targets is imperative and requires a thorough understanding of the solutions available.
It is helpful to know that compared to high temperature applications, reduced NOX numbers can be achieved in low temperature, air-heating applications because the overall temperature is lower, which reduces thermal NOX. In general, in the past, regulations for ultra-low NOX emissions have applied to low temperature, air-heating burners. The industrial boiler burner industry, however, is quickly beginning to achieve low double-digit and single-digit NOX emissions. NOX requirements tend to be higher for high temperature applications, but as with low temperature applications, they are constantly being driven down by regulations.
One common design for lower NOX and CO in single-burner applications is a swirl burner. Swirl is achieved in a variety of ways such as nozzle or burner-body design. No matter the design, the goal is to mix air and gas well enough to ensure complete combustion. Additionally, mixing improves combustion uniformity, which reduces peak flame temperature and results in lower NOX.
It is important to note that complete combustion reduces CO production by converting it to CO2. Uniform combustion reduces NOX production by reducing peak temperatures.
For line-style burners, a common way to achieve a thorough mixing of air and fuel is through a premix design in combination with high excess air. To premix the air and fuel, mixing plates are used. Like the swirl burner, the cut outs and holes in the mixing plates cause the air and fuel to mix comprehensively, ensuring uniform combustion.
For higher temperature applications, one approach to reducing NOX is via flue-gas recirculation (FGR). Some self-recuperative burners have a uniquely designed internal combustor that draws exhaust gases back into the flame to cool it, lowering thermal NOX. It should be noted that FGR will reduce NOX, but it also can reduce overall efficiency.
Another method of reducing NOX in high temperature applications is staged combustion. Burners designed for staged combustion typically use internal baffles or switching valves to split the combustion air or gas into primary and secondary streams.
The telltale sign of a burner designed for staged combustion is an extra set of holes or slots for the secondary stream to travel through and into the combustion chamber. The idea of staged combustion is to introduce primary air or gas into the fuel stream — as is typical in burner designs — but at sub-stoichiometric conditions. The secondary stream, required to complete combustion, is introduced away from the flame, delaying combustion and lowering peak flame temperature. This reduces the creation of thermal NOX. The lowest level of NOX is achieved with this type of burner when operating at low excess air levels of 5 percent (or 1 percent O2).
Flameless combustion also is used to reduce NOX. The general principle is that once the combustion chamber reaches the autoignition temperature (approximately 1400°F [750°C]), the combustion of the fuel gas shifts from within the burner to the space outside of the burner or into the radiant tube. This spreads the combustion over a larger volume instead of concentrating it at the nozzle, so the temperature is lower. The overall temperature within the chamber still is high enough to cause combustion of the fuel gas but low enough to reduce NOX levels.
Complete combustion helps to reduce CO production by ensuring all products of combustion are converted to CO2 while uniform combustion reduces NOX production by reducing peak temperatures.
Choosing a Control Scheme
Specifying an appropriate control scheme is just as important as the burner design to control emissions.
For flameless combustion, you should choose a flame safety with a high temperature bypass. When switching from flame mode to flameless, there is no longer a flame for the sensor to detect, and a standard flame safety would detect a loss of flame inside the combustor. A flame safety with a high temperature bypass will allow the burner to continue operating after switching to flameless.
Pulse firing is a technique used in multi-burner applications to lower NOX by operating the burners in two positions: either high/low or on/off. This method can reduce NOX because when the burner is at high fire, it is operating at optimum NOX performance. When it is at the low end or off, it is making less heat, reducing fuel usage and NOX. Unique controls and valves are required because each burner has to be on and off for a certain amount of time, and this cycle is timed with the other burners in the system to deliver the required heat to the process.
One of the best ways to control emissions is to use a mass-flow control system, which combines control valves and flowmeters for gas and air along with a burner-management system. These components “talk” to each other in order to provide precise electronic control of the air and fuel flow to the burner. The flowmeters provide feedback to the burner-management system, which adjusts the control valves accordingly. This type of system can respond to and automatically compensate for changes in combustion or process conditions such as temperature, humidity and air density. This allows it to maintain the best air-fuel ratio for optimal burner performance and NOX emissions.
In conclusion, OEM furnace builders and end users who want to upgrade their heat treatment processes must make a number of decisions when it comes to meeting tightening emissions requirements. First, they must research and select the optimal burner and control combination for their control application — specifically, one that meets the NOX requirements of the country, region or city in which it will be sold. Second, they must take these critical steps while continuing to achieve the process and product quality required for commercial success.