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NORA Oilheat Technicians Manual

quantity of combustion air is needed so all the fuel will burn completely. Too much air will lower efficiency, and too little air causes incomplete combustion and smoke. During combustion, new chemical substances are created from the fuel and the air. These substances are called combustion gases. Most come from chemical combinations of the fuel and oxygen, but the gases can also include chemical combinations from the air alone. When a hydrocarbon-based fuel (oil) burns, the exhaust gases include water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). Combustion gases can also include carbon monoxide (CO), oxides of nitrogen (NOx, nitrogen + oxygen) and since sulfur is present in the fuel, sulfuric oxide (sulfur + oxygen). As the fuel and air are turned into combustion gases, heat is generated. Heat is required to start combustion and is itself a 7-4 Combustion product of combustion. Once combustion gets started, we don’t have to continue to provide the heat source, because the heat produced by the combustion process will keep things going. What is air? Air is 20.9% oxygen, 78% nitrogen, and 1.1% other gases. For every one part of oxygen, we get four parts nitrogen, see Figure 7-2. Nitrogen is an inert gas and most of it goes through the combustion process unchanged. It cools the chemical reaction (burning temperature) and lowers the maximum heat content deliverable by the fuel. Therefore, it is impossible to achieve combustion efficiencies above 95% for most fuels, including natural gas, when air is used as the source of oxygen for the combustion process. With flame retention burners, some of the nitrogen combines with the oxygen to form nitrogen oxide or NOx. Figure 7-2: Combustion process in a burner Combustion Air 1,400 Cubic Feet Per Gallon 21% Oxygen, 78% Nitrogen Spark Atomization Combustion Fuel Oil 85% Carbon, 15% Hydrogen Trace of Sulfur Vaporization Chapter 7 Combustion Heat 138,690 Hot Nitrogen Carbon Dioxide CO 2 Water Vapor H O 2 Nitrogen Oxides NOx Excess Oxygen O 2 Sulfur Oxides SOx Free Carbon Smoke Carbon Monoxide CO


NORA Oilheat Technicians Manual
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