Natural Gas Combustion Analysis PDF

Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ OBJECTIVE 8 Describe the properties and combustion characteristics of natural gas. Analyze combustion equations for natural gas. NATURAL GAS Of all the fossil fuels, natural gas is the most desirable for steam generation purposes. It is free...

Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ OBJECTIVE 8 Describe the properties and combustion characteristics of natural gas. Analyze combustion equations for natural gas. NATURAL GAS Of all the fossil fuels, natural gas is the most desirable for steam generation purposes. It is free of ash and mbces readily with air, which gives complete combustion with low amounts of excess air. Raw natural gas maybe sweet gas or sour gas. Sweet gas is free of hydrogen sulfide and is sometimes used directly for boiler operation. Natural gas is usually processed before use. Natural gas, direct from a well, is a mixture of several gases, including: • Methane (€N4) • Ethane (CzHg) • Propane ^Hg) • Butane (€49:10) • Hydrogen sulfide (I-^S) • Nitrogen (N2) • Carbon dioxide (€02) • Traces of other gases Table 12 shows a typical analysis of sour natural gas; the values are percentages by volume. Table 12 - Typical Sour Natural Gas Analysis Constituent Percentage 77.73 Methane Ethane 5.56 Other hydrocarbons 4.21 Hydrogen sulfide 7.00 Carbon dioxide 5.50 After the gas is treated, only the hydrocarbons remain, since the nitrogen (N2)> carbon dioxide (C02), hydrogen sulfide (N28), and any moisture are removed. Lighter hydrocarbons, such as ethane (C2H6), propane (C3Hg), and butane (C4Hio) are often removed and sold separately. The remaining natural gas, which is used for combustion, is greater than 95% methane (CH^). Heating Value Since the constituents of natural gas may vary, the heating value of a particular gas varies according to these constituents. Expressed in terms of mass, the heating value is usually in the range of 46 420 to 55 700 kj/kg. However, since the heating value of a gas is more commonly stated in terms of volume, natural gas usually has a heating value of about 37 250 kj/m3 at a standard temperature of 15.5°C and pressure of 101.3 kPa (atmospheric pressure). Note: When combustion is proper, a natural gas flame is blue with a yellow tip. 3rd Class Edition 3 • Part A2 145 Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Advantages of Natural Gas The advantages of natural gas as a fuel include the following: a) A storage facility is not required. b) Since natural gas is clean burning, no ash is produced to leave deposits on the heating surfaces. c) Since the flue gas contains only N3, C02, and H.^0, stack emissions are relatively clean. d) Natural gas easily mixes with air, so complete combustion is possible. e) Natural gas does not require extensive handling equipment. f) Natural gas is easy to control. Disadvantages of Natural Gas The disadvantages of natural gas as a fuel include the following: a) The hydrogen content in the gas decreases the efficiency of combustion. The heat available to transfer energy to the boiler is reduced since the combustion of each kilogram of hydrogen produces nine kilograms of water. This water leaves the boiler as superheated water vapour with an approximate heat loss of 25 900 kj per kilogram of hydrogen burned. b) Natural gas is usually more expensive than solid and liquid fuels, due to the cost of processing and transmitting. c) Long, large diameter pipelines are often required to transmit the gas to and from the plant. d) Natural gas is highly explosive when mbced with air in the correct proportion, so it must be handled and burned carefully. e) Processed natural gas is colourless and odourless. Therefore, an odorant, usually a mercaptan, must be added to make gas leaks more detectable. COMBUSTION ANALYSIS BY MASS The following equations represent the combining of methane with oxygen to form carbon dioxide and water. The coefficients in each equation represent numbers of molecules. These numbers also represent kilomoles which can be converted to mass. Assuming that natural gas is 100% methane, the combustion analysis by mass for natural gas is: CH4 + 202 ^ COz + 2H20 The coefficients are equivalent to number ofkilomoles: 1 kmol + 2 krnol -> 1 kmol + 2 kmol Multiply the number ofkilomoles (kmol) by the formula mass (kg/kmol) for each constituent to get mass (kg): 1x16= 16kg 2x32= 64kg 1x44= 44kg 2x18=36 16 kg CH4 + 64 kg 02 44 kg COz 36 kg H^O lkgCH4 + 4kg02 ftk§co2 fJkgHzO Assuming that air is 23.2% oxygen by mass, the mass of air containing 4 kg of oxygen would be 4x^oo-= 17.24kg air 23.2 The mass of nitrogen in the combustion air is 17.24 - 4 = 13.24 kg N2. 146 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis ' Chapter 3 The combustion equation can be re-written with air and nitrogen included: CH4 + air -> CO^ + 2H^O + 7.52 N2 1kg + 17.24 2.75kg 2.25kg 13.24kg These calculations show that the stoichiometric ratio for methane is 17.24 kg air/kg fuel. This is the air-fuel ratio at 100% theoretical air. When excess air is used, the air-fuel ratio increases proportionately. For example, with 10% excess air, the theoretical air is 110%. In this case, the air-fuel ratio is 110% x 17.24 = 18.96 kg air/kg methane fuel. Example 18 A boiler is fired on natural gas (assumed to be 100% methane). Calculate the air-fuel ratio if the excess air is 15%. Solution 18 If the excess air is 15%, then the theoretical air is 115%. Air-fuel ratio = theoretical air x 17.24kg air/kg fuel 115% xl 7.24 = 19.83 kg air/kg fuel (Ans.) Example 19 If a boiler uses 500 kg/h of methane and has an airflow of 10 320 kg/h, what is the percent of excess air? Solution 19 The actual air-fuel ratio as supplied is: 10 320 ^__ AF supplied = ^"^"" kg air/kg = 20.64 kg air/kg fuel The theoretical air-fuel ratio is 17.24 kg air/kg fuel. The percent of excess air can be calculated in terms of air-fuel ratio as follows: supplied Percent excess air = 1 —'— — I x 100% -A-Ftheoretical (20'6147^7'24) x 100% = 19.7% (Ans.) 3rd Class Edition 3 • Part A2 147 Chapter 3 • Fuels, Combustion, and Flue Gas Analysis COMBUSTION ANALYSIS BY VOLUME Since methane is a gaseous fuel, the combustion analysis can also be done by volume. To simplify the calculations, the following combustion analysis for natural gas is done with the assumption that natural gas is 100% methane. CH4 + 202 ^ C02 + 2H20 The coefficients are equivalent to number ofkilomoles: 1 kmol + 2 kmol -^ 1kmol + 2 kmol The coefficients are also equal to volumes of gas: 1m3 + 2 m3 ^ 1m3 + 2m3 Note that there is no volumetric contraction in this reaction. For some chemical reactions, there is a decrease in volume as the reactants combine. Basing the analysis on 1 m3 of methane fuel, the stoichiometric volume of oxygen is 2 m3. This means that 1 m3 of methane has complete combustion with 2 m3 of oxygen. At this volume ratio, the theoretical air is 100% and the excess air is 0%. Taking the composition of air to be 21% 02 by volume, .3 ^^^ - ^=./210° 2 m~> oxygen x — ^3 m^ .,-. air 9.52 m3 air This volume of air contains 9.52 - 2 = 7.52 m3 of nitrogen. The combustion equation can be re-written as: CH4 + 9.52 Air ^ C02 + 2H20 + 7.52 N3 In this analysis, it is assumed that the I-^O remains as a vapour. Example 20 A boiler burns 50 m3/h of methane gas at 10% excess air. Calculate the air flow and volume of combustion products formed per hour. Solution 20 Calculate the theoretical air requirements. CH4 + 9.52 Air -^ COz + 2H20 + 7.52 N3 Multiply all the coefficients by 50 to obtain the following volumes: 50CH4 + 476 Air ^ 50 COz + 100 HzO + 376 N3 The theoretical air is 476 m3/h. The air flow supplied is 110% x 476 = 523.6 m3/h. (Ans.) In one hour, 50 m3 of 002,100 m3 ofE^O, and 376 m3 of N3 are produced. (Ans.) 148 3rd Class Edition 3 • Part A2