Fuels, Combustion, and Flue Gas Analysis PDF

?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis OBJECTIVE 2 Explain/define combustion, incomplete combustion, combustion products, and write balanced combustion equations. COMBUSTION Combustion is the chemical reaction between the combustible elements of a fuel and the oxygen in air at a...

?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis OBJECTIVE 2 Explain/define combustion, incomplete combustion, combustion products, and write balanced combustion equations. COMBUSTION Combustion is the chemical reaction between the combustible elements of a fuel and the oxygen in air at a rate that produces useful heat energy. Air is primarily a mixtire of oxygen and nitrogen, with small amounts of other gases. The main combustible elements in the fuel are carbon and hydrogen (often combined as hydrocarbons), and some faels also contain sulfur. During combustion, the carbon and hydrogen combine with oxygen to form carbon dioxide (€02) and water vapour (H^O). Ifsulfur is present, it will combine with oxygen to form sulfur dioxide (SOz). However, the corrosive and toxic nature of sulfur compounds makes sulfur undesirable in a fuel. The composition of dry atmospheric air is shown in Table 4. Table 4 - Composition of Dry Atmospheric Air 02 NZ Ar COz Other1 Total % vol 20.95 78.08 0.93 0.040 0.0025 100 % mass 23.20 75.47 1.28 0.062 0.0016 100 Notes: 1. Other constituents include neon, helium, krypton, hydrogen, and xenon. 2. For combustion calculations, argon (Ar), carbon dioxide (€02), and the other gases are added to nitrogen (N). This gives the following: percent by volume of 0^ 21%, N2 79%, percent by mass of 0^ 23%, N3 77%. PERFECT (STOICHIOMETRIC) COMBUSTION Perfect combustion (also called stoichiometric combustion) is a theoretical process where there is just enough 02 in the supplied air to react with all the combustibles in the fael. The process conforms exactly to the coefficients in the combustion equation. For example, the combustion of propane ^Hg) has the equation: C3Hg + SOz ^ 3C02 + 4H20. Perfect combustion would only occur if the oxygen-fuel ratio could be maintained at exactly 5:1 and the fuel was completely burned. This condition is virtually impossible to achieve in any commercial or industrial burner because it is difficult to meter the air and fuel with perfect accuracy. It is also difficult to bring the air and fuel into contact in such a way that the fuel is completely consumed without any excess air. Stoichiometric combustion is, therefore, a theoretical condition used as a reference point for actual performance. The air-fuel ratio (AF) for perfect combustion is called the stoichiometric ratio. 116 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 COMPLETE COMBUSTION Complete combustion occurs when all the combustibles in the fuel react with oxygen and the reaction uses more air than the minimum amount theoretically required. That is, excess air is supplied and used. Complete combustion is attainable if the boiler furnace is properly designed for both the fuel being burned and the boiler steam load. After complete combustion, the boiler furnace stack gases contain C02, SO^, H^O, 02, N3, and ash. There is an increase in N2 above the value calculated for perfect combustion due to the extra nitrogen supplied with the excess air. For complete combustion to occur, the following conditions are required: a) Sufficient air must be admitted to ensure all combustibles are burned. Some of this air must be admitted close to the surface of the fire. b) The temperature must be high enough to ignite the combustible gases released from the fuel. c) The air must have a turbulent flow within the furnace to ensure that oxygen contacts all the combustibles present. d) The combustion gases must remain in the hot zone for sufficient time to allow combustion to complete. Equations for Complete Combustion The following equations represent the combining of carbon, hydrogen, and sulfur (the combustible elements) with oxygen, during complete combustion. The coefficients in each equation represent the number of molecules. For combustion calculations, this number is usually assigned units of kilogram-moles (kmol). The number of kilomoles (n) multiplied by molar mass (M) gives mass (m). Equation for complete combustion of carbon: carbon + oxygen -> carbon dioxide C + 02 -> C02 1 kmol 1 kmol 1 kmol 12 kg 32 kg 44 kg Equation for complete combustion of hydrogen: hydrogen + oxygen -^ water vapour 2H2 + 02 -> 2H20 2 kmol 1 kmol 2 kmol 4 kg 32 kg 36 kg Equation for complete combustion ofsulfur: sulfur + oxygen -> sulfur dioxide 2 + 02 ^ S02 1 kmol 1 kmol 1 kmol 32 kg 32 kg 64 kg As a non-combustible element, the nitrogen in the air does not combine with oxygen, but passes through the furnace unchanged except for an increase in its temperature. 3rd Class Edition 3 • Part A2 117 ^ Chapter 3 • Fuels, Combustion, and Flue Gas Analysis INCOMPLETE COMBUSTION Incomplete combustion occurs when some of the carbon and hydrogen in the fuel do not react with oxygen and instead pass to the boiler furnace stack. In this case, the stack gas contains CO-^, S02, H^O, N2, CO (carbon monoxide), H^, possibly C (carbon as soot), and some CH4 (methane) or other hydrocarbons. The stack gas may or may not contain free 02. Of course, incomplete combustion is undesirable. Incomplete combustion reduces the efficiency of a boiler, since the maximum heat energy available in the fuel is not released in the furnace. Instead, some of the heat is lost up the stack with the unburned combustibles. In extreme cases, the products of incomplete combustion (such as soot) may also deposit onto boiler surfaces, which adversely affects heat transfer and further reduces efficiency. Equations for Incomplete Combustion If any of the requirements for complete combustion are missing, then the combustible elements do not combine completely with oxygen. The following equations represent the incomplete combining of oxygen and the combustibles. Equation for incomplete combustion of carbon: carbon + oxygen ^ carbon monoxide C + VzO^ -> CO 1 kmol + Vs kmol 1 kmol 12kg + 16kg 28kg Equation for incomplete combustion of hydrogen: hydrogen + insufficient oxygen -> water vapour + free hydrogen 2Hz + V202 ^ H20 + Hz 2 kmol V-2. kmol 1 kmol 1 kmol 4kg 16kg 18kg 2kg The formation of the free hydrogen is undesirable because it is a combustible element and, if unburned, represents a waste of fuel. Equation for incomplete combustion ofsulfur: sulfur 2S + insufficient oxygen ^ + 02 -> sulfur dioxide + free sulfur S02 + s 2kmol 1 kmol 1 kmol 1 kmol 64kg 32kg 64kg 32kg Similarly, the formation of free sulfur is undesirable as it represents a waste of fuel. Sulfur in a fuel is considered an impurity even though it is a combustible element. Free sulfur in the flue gases are also undesirable because it can combine with water (another combustion product) to form acids, which are corrosive to th^ metal surfaces in the outlet of a furnace or boiler. 118 3rd Class Edition 3 • Part A2