Oilheat Combustion Oilheat Combustion is a controlled chemical reaction. All matter has potential energy and different substances have different energy levels. Every gallon of heating oil contains about 139,000 Btus of potential heat energy. When heating oil is burned, the heat energy in the fuel is discharged as it combines with air and creates water and carbon dioxide, Figure 5. Essentially, oil’s carbon and hydrogen atoms’ high energy is converted to water and carbon dioxide using little energy. And since energy cannot be gained or lost, the energy difference is given off as heat. What Happens when Heating Oil Burns? Heating oil is 85% carbon and 15% hydrogen. When heating oil burns, the hydrogen in the fuel reacts with the oxygen in the air to produce water vapor (H2O). The carbon in the fuel combines with the oxygen to create carbon dioxide (CO2). Since oil burners operate in widely variable conditions, there is some incomplete combustion and a small amount of carbon monoxide (CO) and free carbon (smoke and soot) may also be produced. Every two hydrogen atoms in heating oil need one oxygen atom from the air and every one carbon atom needs two oxygen atoms for the conversion to take place. Only 21% of air is oxygen; the remaining 79% is nitrogen. This means a lot of air must move through the burner to provide enough oxygen for the fire. Every gallon of heating oil contains 6.3 pounds of carbon and .9 pounds of hydrogen. With perfect combustion, the fuel mixes with 1,305 cubic feet of air and produces 23 pounds of CO2, eight pounds of water and 78 pounds of nitrogen! All that nitrogen that came in with the oxygen will now absorb a great deal of heat and go up the chimney. 8 National Oilheat Research Alliance Time, Temperature, Turbulence Oil burners and heat exchangers use time, temperature and turbulence to maximize their effectiveness. Time—As the oil and air are injected into the combustion area, the mixture must stay in the area long enough to burn. However, new fuel and air are also being forced in and if the gases are moving too fast, the oil in the combustion area may be forced out before it has time to burn, creating smoke and carbon monoxide. Flame retention burners use high static pressure to create additional time for the oil to burn through recirculation. This recirculation is created by the drop in pressure in the center of the air swirl, like the eye of a tornado. The lower pressure pulls some of the hot flame gases toward the burner head. These hot gases add heat to the fuel droplets coming out of the nozzle, speeding up their vaporization and burning rates. This yields a nice, clean, stable fire close to the burner head. Although it seems contradictory, the higher air velocity and pressure blowing out of the burner causes the flame to burn closer to the burner head. Temperature—Oil will burn when the temperature in the combustion area is above the ignition point (about 600°F). Turbulence —Turbulence is critical to combustion. As oil moves into the combustion area, it must be thoroughly mixed with oxygen or it will not burn. Burners subject the oil to high pressures to break it up into small particles, then swirl it with the air in the combustion area. This turbulence ensures that each droplet of oil is surrounded by enough oxygen to guarantee combustion. In high-pressure atomizing burners, several factors control the quality of air-oil mixing.
NORA Advanced Efficiency
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