Fuel Oil Properties and Combustion Analysis PDF

Fuels, Combustion, and Flue Gas Analysis ' Chapter 3 OBJECTIVE 7 Describe the properties, classifications, and combustion characteristics of fuel oil. Analyze combustion equations for fuel oil. FUEL OIL Crude petroleum is sometimes burned; however, it usually contains lighter gasoline fractions w...

Fuels, Combustion, and Flue Gas Analysis ' Chapter 3 OBJECTIVE 7 Describe the properties, classifications, and combustion characteristics of fuel oil. Analyze combustion equations for fuel oil. FUEL OIL Crude petroleum is sometimes burned; however, it usually contains lighter gasoline fractions which lower the flash point and present a fire hazard. Limited fractional distillation, or topping, removes the lighter gasoline and produces a safe fuel oil. The term fuel oil covers a multitude of petroleum products, which range from crude petroleum, to a light fraction similar to kerosene, to gas oil, and to a heavy residue (after any gases, gasoline, and some of the kerosene are distilled out). As shown in Table 11, there are specifications that identify and compare several grades of fuel oil. Fuel oil (such as Grade No. 4, 5, and 6) used for steam generation is a liquid residue that remains after the more volatile petroleum constituents are removed. Grades No. 1 and 2 are distillation products (distillates) and are sometimes called light and medium domestic fuel oil. Grade No. 6, heavy industrial fuel oil or bunker C oil, is specified mainly by viscosity. Table 11 - Typical Analysis and Properties of Fuel Oils No. 4 N^ No-6fue' Grade No. 1 fuel No. 2 fuel Type Distillate (kerosene) Distillate Light Amber 40 32 21 17 12 0.8251 0.8654 0.9279 0.9529 0.9861 lb./U.S. gallon 6.870 7.206 7.727 7.935 8.212 kg/m3 (15.5°C) 823 864 926 951 984 Viscosity, SSU, 38°C 31 x10~6 35x10-6 77x10-6 232x10~6 1660x10-6 Pour point, °C Below-18 Below-18 -12 -1 18 Colour API gravity, 15.5°C (GOT) Specific gravity, 15.5/15.5°C fuel Very light Light residual residual Residual Black Black Black Temperature for pumping, °C Atmospheric ! Atmospheric -10 min 2 min 38 Temperature for atomizing, °C Atmospheric | Atmospheric -4 min 54 93 Trace i Trace 2.5 5.0 12.0 Carbon residue, % Sulfur, % 0.1 0.4-0.7 0.4-1.5 2.0 max 2.8 max Oxygen and nitrogen, % 0.2 0.2 0.48 0.70 0.92 Hydrogen,% 13.2 12.7 11.9 11.7 10.5 Carbon, % 86.5 86.4 86.10 Sediment and water, % Trace Trace 0.5 max 1.0 max 2.0 max Ash, % Trace Trace 0.02 0.05 0.08 45350 44750 43500 42850 42566 Heating value (kJ/kg) 3rd Class Edition 3 • Part A2 85.55 ; 85.70 141 ^ Chapter 3 • Fuels, Combustion, and Flue Gas Analysis The following descriptions help clarify the information provided in Table 11. Specific and API Gravity Specific gravity (SPGR) is commonly known as the ratio of the density of a solid or liquid to the density of water at 4°C and atmospheric pressure. For petroleum products, including fuel oils, the specific gravity is measured at 15.5°C; this way, all of the products can be compared at the same temperature. In Table 11, the designation is specific gravity 15.5/15.5°C. Notice in Table 11 that the specific gravity of the fuel oils increase as the grade increases. That is. No. 6 oil has a higher specific gravity than No. 4 oil. A more common reference to gravity in the petroleum industry is API gravity. The American Petroleum Institute (API) specifies an API gravity at 15.5°C for each product. The unit for API gravity is degrees API. For example. No. 1 fuel oil has an API gravity of 40 degrees API. Notice that the API gravity gets smaUer as the fuel oil gets heavier. That is, No. 6 oil has a lower API gravity than No. 4 oil. The conversion from specific gravity (SPGR) to API gravity can be made using the following formula: Degrees API gravity = (141.5/SPGR at 15.5°C) - 131.5 Heating Value The heatuig value of fuel oil, expressed as kj/kg, increases as the specific gravity of the oil decreases. This increase in the heating value is due to an increase in hydrogen content as the oil becomes lighter. The heating value ranges from 42 566 kj/kg for No. 6 oil to 45 350 kj/kg for No. 1 oil (these figures are approximate since the exact composition varies within each grade). Viscosity Viscosity is a fluids internal resistance to flow. One method to measure viscosity is with a viscosimeter, where the viscosity is expressed in units called Saybolt seconds universal (SSU). This viscosity is the time taken, in seconds, for 60 cm3 (60 millilitres) of oil to run through a standard size orifice at 38°C. The viscosity of fuel oil decreases as the temperature increases, but becomes nearly constant above 120°C. Therefore, when fuel oil is heated to reduce the viscosity and promote proper atomization, there is little to be gained by heating the oil above 120°C. Since burners operate most efficiently with oil of constant viscosity, it is desirable to operate in the viscosity range where temperature variations have the least effect. Pour Point The pour point of a fuel oil is the lowest temperature at which the oil will flow. Combustion of Oil Oil can be vaporized into its hydrocarbon gas components if the temperature is sufficiently high. This is seldom the case in the short time available in the combustion zone of the furnace. In practice, the oil is atomized into extremely smaU droplets, either mechanically or using steam or air. This atomization presents more surface area to collect heat, and thus promotes vaporization and combustion. 142 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ COMBUSTION ANALYSIS FOR FUEL OIL Fuel oil composition analysis is typically done on a percent mass basis exactly as it was described for coal in section 6.2. Summary of Fuel Oil Combustion Equations by Mass Combustion analysis for fuel oil is done using the same mass coefficients as coal, based on the fuel composition by percent carbon, hydrogen, and sulfur. Fuel oils may contain a small percentage of ash which is non-combustible, and therefore not included in the analysis. IkgC + 267 kg 02 -> 3.67 kg COz lkgH2 + 8kg02 -> 9kgH20 IkgS + 1kg 02 ^ 2kgS02 Example 16 A fuel oil analysis has the composition by mass of C 87%, H2 12%, and S 1%. Calculate the theoretical air required and the mass of the combustion products. Nitrogen can be neglected in the analysis. Take the composition of air to be 23% 0^ and 77% N2 by mass. Solution 16 It is suggested to draw up a table for the combustion analysis. Using the percent composition by mass listed in the question, calculate the mass for each constituent based on 1 kg of fuel oil. Using these masses, calculate the oxygen required for combustion and the products of combustion using the ratios summarized in section 7.2.1. I m I kg/kg fuel c 0.87 HZ 0.12 s 0.01 Total 1 kg fuel 02 kg/kg fuel C02 kg/kg fuel 0.87x2.67 = 0.87x3.67 = 2.32 3.19 HgO kg/kg fuel 0.12x8 = 0.12x9 = 0.96 1.08 SOz kg/kg fuel 0.01 x 1 = 0.01 x2 = 0.01 0.02 2.23+0.96+0.01 = 3.29 kg 02 3.19 kgC02 1.08 kg N30 0.02 kg S02 Check mass balance: Mass ofreactants excluding nitrogen = 1 +3.29 = 4.29 kg Mass of products excluding nitrogen = 3.19 + 1.08 + 0.02 = 4.29 kg The mass balance verifies the combustion analysis as tabulated. The theoretical oxygen required for combustion is 3.29 kg 02/kg fuel. Taking the composition of air to be 23% oxygen by mass, the theoretical air required is 3.29 x ~^-= 14.30 kg air/kg fuel oil. (Ans.) The masses of the various products of combustion are 3.19 kg CC>2, 1.08 kg Vi^O, and 0.02 kg S02. (Ans.) 3rd Class Edition 3 • Part A2 143 ?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Example 17 A fuel oil analysis has the composition by mass of C 85%, H2 14%, S 1%, and ash 1%. Calculate the mass of air supplied and the mass of dry flue gas produced if 50% excess air is supplied. Take the composition of air to be 23% 0^ and 77% N2 by mass. Solution 17 Using the percent composition by mass listed in the question, calculate the mass for each constituent based on 1 kg of fuel oil. Using these masses, calculate the oxygen required for combustion and the products of combustion. The ash is non-combustible, so it is not included in the calculation. m kg/kg fuel c 0.85 HZ 0.14 s 0.01 Total 1 kg fuel 02 kg/kg fuel COz kg/kg fuel 0.85x2.67 = 0.85x3.67 = 2.27 3.12 HzO kg/kg fuel 0.14x8 = 0.14x9 = 1.12 1.26 S02 kg/kg fuel 0.01 x 1 = 0.01 x2 = 0.01 0.02 2.27+1.12+0.01 = 3.40 kg Os 1.26 kg HsO 3.12 kg COs 0.02 kg S02 Check mass balance: Mass ofreactants excluding nitrogen =1+3.40= 4.40 kg Mass of products excluding nitrogen = 3.12 + 1.26 + 0.02 = 4.40 kg The mass balance verifies the combustion analysis as tabulated. The theoretical oxygen required for combustion is 3.40 kg 02/kg fuel. Taking the composition of air to be 23% oxygen by mass, the theoretical air required is 3.40 x —— = 14.78 kg air/kg fuel oil. The actual air supplied is 150% x theoretical air = 150% x 14.78 = 22.17 kg air/kg fuel oil. (Ans.) The mass offlue gas is equal to the mass of air supplied plus 1 kg of fuel =23.17 kg flue gas/kg fuel. The mass ofH^O produced is 1.26 kg H20/kg fuel. The mass of dry flue gas = 23.17 - 1.26 = 21.91 kg dry flue gas/kg fuel. (Ans.) 77 Alternatively, the amount of nitrogen in the air is 22.17 x —— = 17.07 kg N2/kg fuel. The amount of 02 in the flue gas is 50% x 3.4 = 1.7 kg 02/kg fuel. Mass of dry flue gas is 3.12 kg CO^ + 0.02 kg SOz + 17.07 kg N2+1.7 kg 02 = 21.91 kg dry flue gas/kg fuel. (Ans.) 144 3rd Class Edition 3 • Part A2