Across nearly every industry and region of the world, OEMs and end users are looking for ways to improve their heat-treat equipment and processes. They are facing increasingly stringent NOx emissions regulations while also being challenged to be more eco-friendly by reducing fuel usage and lowering their carbon footprint.
This article explores the complexities faced by today’s OEMs and end users when facing emissions and carbon reduction challenges, and it offers practical advice on burner and control combinations to meet these challenges.
The Chemistry Basics
It is important to note that it isn’t 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
Since OEM furnace manufacturers typically sell their products in multiple markets around the globe, they need to be able to build furnaces that can deliver the emissions levels their customers need 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 or stated in parts per million (ppm), pounds/MM Btu or mg/Nm3. 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% O2. High-temperature applications used to have more lenient requirements of 60 ppm NOx or less, corrected to 3% O2, but these levels are being ratcheted down. This makes it more and more challenging for companies to stay in compliance.
Across the world, we are seeing emissions standards tightening. Canada, Europe and China are implementing reductions in permitted emissions levels, and India and other high-growth regions are on a similar path and will likely adopt stricter industrial process emissions requirements in the coming years.
Choosing Suitable Thermal Equipment to Reduce NOx but also Increase Efficiency
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-bound NOx and prompt NOx. Thermal NOx is triggered by heat from the combustion reaction and is relevant when attempting to reduce NOx. Fuel-bound and prompt NOx are inherent in fuel and nitrogen reaction properties, respectively, and cannot be changed significantly.
If you add the need or desire to burn less fuel/be more efficient/reduce your carbon footprint (be eco-friendly), then preheating the combustion in higher-temperature applications is a proven and common method to accomplish this. Increasing the combustion air temperature increases the flame temperature, however, which will increase your thermal and subsequently your total NOx emissions.
As a result, to reduce NOx, we focus on reducing thermal NOx, which can be achieved through a combination of burner design, equipment selection strategies and burner-control schemes.
Considerations for Selecting a Burner to Deliver Low NOx and Higher Efficiency
Selecting a burner that meets your emissions targets as well as efficiency is imperative and requires thorough knowledge of the application and understanding the solutions on the market.
It is helpful to know that the techniques to reduce NOx in burners used in low-temperature applications can be dramatically different than in high-temperature applications and will even vary by the type of process.
Reduced NOx numbers can be achieved in low-temperature, air-heating applications because the overall temperature is lower, which reduces thermal NOx. NOx regulations are generally higher for high-temperature applications, but – as with low-temperature applications – they are constantly being driven down by regulations.
Fig. 1. High-velocity burners
For the purposes of this discussion, we will focus on higher-temperature applications (e.g., heat-treating applications) and the advances in burners and controls used to both reduce emissions and increase efficiency.
Traditionally, high-temperature, direct-fired heat-treating furnaces will have multiple high-velocity burners to use the high jet velocity to promote uniformity, heat transfer and get good turndown. There are also low-NOx designs available.
To increase your efficiency, we look at some type of recuperation or method to preheat the combustion air, but that increase in air temperature is going to increase the NOx emissions.
In most indirect-fired applications, the burners will fire in tubes so that the products of combustion are not introduced into the furnace since some processes incorporate certain atmospheres in the furnace to aid in particular heat-treating types. Traditional tube-fired applications have and still use U or W tubes with burners that produce long, lazy flames to try and get better temperature uniformity.
Fig. 2. Tube-fired burner
To be more efficient, plug recuperators are installed in the exhaust end of the tube to preheat the combustion air to the burner. Again, we become more efficient but at the sacrifice of higher NOx with the higher combustion air temperatures.
So, what happened? This drove burner manufacturers some years ago to get creative and develop the self-recuperative, high-velocity-style burners. These burners incorporate an integral recuperator that allows the combustion air to be preheated internally in a compact, easily installed burner designed to produce lower NOx levels with the preheated air.
For direct-fired furnaces, with the addition of an eductor to pull the exhaust gas over the recuperator, this eliminates the need for an external recuperator and hot-air piping to traditional burners and greatly simplifies system installation and long-term maintenance of the combustion system.
Fig. 3. Ecomax direct-fired burner
In indirect applications, the self-recuperative burners were placed in single-ended radiant tubes in which the burner fires through an inner tube. The exhaust stream passing between the inner and outer tube along with some exhaust gas recirculation helped to reduce NOx and get excellent temperature uniformity.
Fig. 4. Single-ended radiant tube
As NOx emissions levels continue to be reduced by controlling agencies, the emissions levels in this self-recuperative technology need to be reduced with those requirements. These emissions have been reduced by varying techniques of flameless combustion.
The general principle is that once the combustion chamber reaches auto-ignition 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 chamber. 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 the NOx levels.
Below auto-ignition temperature, the burner is in a traditional type of flame mode.
Fig. 5. Flame mode
There is a relatively high reaction speed and energy density in the flame, a lower intake of exhaust gas in the flame zone and higher flame temperature. Therefore, there are higher levels of NOx.
Above auto-ignition temperature (plus a safety factor) around 1560°F (850°C), the burner is forced into a flameless combustion mode. This is accomplished by fuel staging or a combination of air and fuel staging (in some of the latest designs) without igniting the burner.
Fig. 6. Flameless combustion
This allows a high intake of exhaust gas to the combustion air and, therefore, a low reaction speed and energy density in the reaction zone, which lowers the peak temperatures in the reaction zone and minimizes the formation of NOx.
The most recent designs and innovations have resulted in some of the lowest NOx performance from this technology to date.
Power is nothing without control. Pulse firing is a technique used in multi-burner applications to reduce NOx by operating the burners in two positions: 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 and reducing fuel usage and NOx. It is a superior method to obtain furnace uniformity. Unique controls 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.
In the case of flameless combustion, an on/off pulse-firing method is used. In addition, 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 a flameless state.
Honeywell’s Approach to the NOx/Efficiency Challenge
Honeywell’s ECOMAX® LE is one example of a burner-control solution offering low emissions, process efficiency and low operating costs. Developed for OEM furnace builders and end users who want to upgrade their heat-treatment processes, it is a self-recuperative burner with a ceramic combustor and radiant tube or open-fired with an eductor assembly.
The solution features a patent-pending way to control the air and gas and switches from flame mode to flameless, with a switching temperature of 1560°F (850°C). The burner operates in flame mode until the switching temperature is reached, and then it switches to flameless mode for optimal NOx performance. In flameless mode, the flame combusts over the entire volume of the radiant tube, lowering peak flame temperature. The result is best-in-class NOx emissions.
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 and the desire or need to be eco-friendly.
First, they must research and select the optimal burner and control combination for their process application – specifically, one that meets the NOx requirements and efficiency targets of the country, region or city in which it will be installed.
Second, they must take these critical steps while continuing to achieve the process and product quality required for commercial success.
For more information: Contact Brian Kelly, applications engineering manager, Honeywell Thermal Solutions; tel: 717-821-2083; e-mail: Brian.Kelly2@Honeywell.com; web: https://thermalsolutions.honeywell.com/us/en.
All images provided by the author.