19 Combustion of Gas Combustion Characteristics of Natural Gas The

19 combustion of gas combustion characteristics of

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1.9 Combustion of Gas Combustion Characteristics of Natural Gas The stoichiometric ratio for natural gas (and most gaseous fuels) is normally indicated by vol- ume. The air to natural gas (stoichiometric) ratio by volume for complete combustion vary between 9.5:1 to 10:1 Figure 1.5 Coal Combustion
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1. Fuels and Combustion 23 Bureau of Energy Efficiency Natural gas is essentially pure methane, CH 4 . Its combustion can be represented as follows: CH 4 +2O 2 = CO 2 + 2H 2 O So for every 16 kgs of methane that are consumed, 44 kgs of carbon dioxide are produced. (Remember that the atomic weights of carbon, oxygen and hydrogen are 12, 16 and 1, respectively.) Methane burns, when mixed with the proper amount of air and heated to the combustion temperature. Figure 1.6 shows the process with the amount of air and fuel required for perfect combustion. Figure 1.6 Combustion of Natural Gas Low-And High-Pressure Gas Burners. The important thing in all gas-burning devices is a correct air-and-gas mixture at the burner tip. Low-pressure burners (figure 1.7), using gas at a pressure less than 0.15 kg/cm 2 (2 psi), are usually of the multi-jet type, in which gas from a manifold is supplied to a number of small single jets, or circular rows of small jets, centered in or discharging around the inner cir- cumference of circular air openings in a block of some heat-resisting material. The whole is encased in a rectangular cast-iron box, built into the boiler setting and having louver doors front to regulate the air supply. Draft may be natural, induced, or forced. Figure 1.7 Low Pressure Gas Burner
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1. Fuels and Combustion 24 Bureau of Energy Efficiency In a high-pressure gas mixer (figure 1.8), the energy of the gas jet draws air into the mixing chamber and delivers a correctly proportioned mixture to the burner. When the regulating valve is opened, gas flows through a small nozzle into a venturi tube (a tube with a contracted section). Entrainment of air with high-velocity gas in the narrow venturi section draws air in through large openings in the end. The gas-air mixture is piped to a burner. The gas-burner tip may be in a variety of forms. In a sealed-in tip type, the proper gas-air mixture is piped to the burner, and no additional air is drawn in around the burner tip. Size of the air openings in the venturi tube end is increased or decreased by turning a revolving shutter, which can be locked in any desired position. Excess air levels in natural gas burner is in the order of 5%. 1.10 Draft System The function of draft in a combustion system is to exhaust the products of combustion into the atmosphere. The draft can be classified into two types namely Natural and Mechanical Draft. Natural Draft It is the draft produced by a chimney alone. It is caused by the difference in weight between the column of hot gas inside the chimney and column of outside air of the same height and cross section. Being much lighter than outside air, chimney flue gas tends to rise, and the heavier out- side air flows in through the ash pit to take its place.
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  • Spring '17
  • Jerome Ramos

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