In Burners 104, I introduced the concept of the modern nozzle-mix burner. (To start at Burner History 101 or any previous part of this five-part series, use the links at the bottom of the page.) Continuous development from the 1920s through the 1950s resulted in numerous designs with excellent flame stability over a wide range of firing rates and ratios. The versatility of these burners made them suitable for a variety of applications.
Having overcome the firing rate and fuel/air ratio issues, designers moved on to the challenge of molding the flame into shapes suitable for specific applications. By controlling the velocity and direction of the air and gas flowing into burners, they were able to create a variety of flame configurations from flat, disc-shaped to long and pencil-slim (figure 1).
The 1960s saw the development of the so-called high velocity or high momentum burners, where a restricted outlet on the firing tube or block forces the rapidly expanding combustion gases to exit at very high speeds. Because of their high operating temperatures, many direct-fired furnaces and kilns can't employ circulating fans like ovens and dryers. Heat transfer was primarily by radiation, and this often led to difficulty in getting uniform temperatures and high productivity. The jets of flame and hot gases from high velocity burners (figure 2) entrain furnace gases, stirring them up like a fan would, improving temperature uniformity and heat transfer rates.
By the 1980s, most of industry's operating flexibility and heat transfer requirements were being met. Just when it looked like the pace of burner design and development would slow down, a new challenge arose -- reducing NOX emissions. This proved to be a tough one: Many of the burner design features that supported highly intense firing rates, high efficiency and low carbon monoxide and hydrocarbon emissions also contributed to higher NOX levels. Combustion air preheating had become a popular way to reduce energy consumption on high temperature applications, but it promoted high flame temperatures, one of the prime contributors to high NOX levels. The trick would be driving down NOX without sacrificing all the benefits that had been gained over the years.
Most NOX forms in that fraction of a second when the flame temperature climbs to a peak of 2,800 to 3,200oF (1,538 to 1,760oC). If that peak temperature can be forced below 2,800oF, or if the length of time above 2,800oF can be minimized (figure 3), less NOX will form. This has led to several different approaches:
Slowing down the rate of combustion, allowing radiant heat loss from the flame to release some of the heat from the burning fuel, pulling the temperature down. Staged-air and fuel burners operate on this principle. In staged-air burners, at first, only part of the combustion air is allowed to come into contact with the fuel, creating a fuel-rich, low temperature flame. This flame is allowed to release part of its energy before the balance of the combustion air is introduced to finish the combustion process. Staged-fuel burners are the other side of the coin -- the fuel stream is divided, so the burner runs lean (excess air) in the early stages. The rest of the fuel is added to the flame a little way downstream, after it has already given off some of its heat to the process and cooled a bit.
Injecting some sort of thermal ballast into the flame just before it tends to hit its highest temperature. Steam and water injection work, but cooled flue gases have been the most widely used. This technique is known as flue gas recirculation.
- Lowering the oxygen content of the combustion air to reduce flame temperatures. Called air vitiation, this technology usually involves blending flue gases with the combustion air. Typically, the air's oxygen level is diluted to 18 to 19 percent.
Some premix burners have gotten a fresh lease on life in the battle against NOX.
Lean premix burners operate close to their lean limit of stability, creating relatively cool flames with low levels of NOX. In low temperature applications like ovens, the low flame temperatures are no impediment to heat transfer or productivity.
Some types of infrared burners have exceptionally low NOX emissions even though they operate close to correct ratio and have high flame temperatures. Their flames are thin sheets of burning gases, closely hugging the burner face. Gas residence time in the flames is very short, and the flame cools quickly as it radiates energy to the process. An offshoot of infrared burners, catalytic generators operate by reacting air and gas over a platinum-catalyst treated surface. Oxidation, not a flame, takes place at 750 to 1,000oF (399 to 538oC), yielding very low NOX emissions.
What lies in the future? It will depend on what industry and environmental authorities want, but it's probably safe to say we haven't seen the end of the road yet.
Thanks for participating in this series of classes. You can pick up your diplomas at the door.
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