3A2E3 C3 Obj 6.pdf

?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis OBJECTIVE 6 Describe the properties, classifications, and combustion characteristics of coal. Analyze combustion equations for coal. COAL CLASSIFICATIONS The American Society for Testing and Materials (ASTM) classifies coal into four main gr...

?& Chapter 3 • Fuels, Combustion, and Flue Gas Analysis OBJECTIVE 6 Describe the properties, classifications, and combustion characteristics of coal. Analyze combustion equations for coal. COAL CLASSIFICATIONS The American Society for Testing and Materials (ASTM) classifies coal into four main groups with several sub classifications (see Table 9). The four main groups are: 1. Anthracite 2. Bituminous 3. Sub-bitmiinous 4. Lignite Anthracite Anthracite coal (Figure 6) is hard, dense, very britde, shiny, black, and non-friable with no layering. It has a high percentage affixed carbon and a low percentage of volatile matter, which is mostly methane (CH^). Anthracites include a variety of slow burning fuels, which range at a stage between graphite (a very pure form of carbon) and bituminous coal (less carbon than anthracites). Most anthracite coals have a lower heating value than the highest grade ofbituminous coal. Anthracite coal is expensive, has a high ignition temperature, and burns slowly. This makes it an unsuitable fuel for utiUty boilers. Semi-anthracites are dark grey and distinctly granular. They have lower percentages of fixed carbon and higher percentages of volatile matter. The lower fbced carbon content makes them burn faster and the higher volatile matter content lowers the ignition temperatire. This increases the stability of the ignition. Figure 6 - Anthracite Coal (SHTRAUS DMYTRO/Shutterstock) 134 3rd Class Edition 3 ' Part A2 I ^ Fuels, Combustion, and Flue Gas Analysis • Chapter 3 Bituminous Bituminous coal (Figure 7) is the largest group of coals. They tend to produce a sticky, cohesive mass when heated. The carbon content is less than in anthracite, but the volatile matter content is higher. The composition of the volatile matter is more complex than in anthracite and the calorific value is higher. Bituminous coals burn easily, especially when pulverized. They are not well suited for stoker firing since they bake onto the surface of the coal bed, prevent an even air supply, and cause losses through unburned fuel. Low volatile bituminous coal is greyish black and granular. High volatile bituminous coal has distinct, thin layers of shiny black coal, alternating with dull, charcoal-like layers. Medium volatile bituminous coals have the characteristics of both low and high volatile coal; some are granular, soft, and easily crumbled, while others have a faint indication of layered structure. Figure 7 - Bituminous Coal (SHTRAUS DMYTRO/Shutterstock) Sub-Bituminous Sub-bituminous coals are black in colour and have high moisture content. They disintegrate when exposed to air and are difficult to store. They burn freely and do not cake. Due to their high moisture content, they are not usually accepted for power plant use. 3rd Class Edition 3 • Part A2 135 r®- Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Lignite Lignite coals (shown in Figure 8) are dark brown, have a layered structure, and often contain remnants of woody fibres. The name comes from the Latin lignum, which means wood. Freshly mined lignite is tough, but not hard. When exposed to air, it loses moisture rapidly and crumbles. Even when it seems dry, the moisture content oflignite may be as high as 30%. Due to its high moisture content and low heating value, it is not economical to transport over long distances. Since lignite is found close to the surface, which makes strip mining relatively easy, thermal power stations are often located close to the lignite deposit. Figure 8 - Lignite Coal (Losmandarinas/Shutterstock) Table 9 provides an overview of the main classes of coal and their sub groups. Since this is an ASTM standard, the calorific values are given in United States Customary System (USCS) units. To convert BTU/lb to kj/kg, multiply BTU/lb by 2.326. For example, Bituminous A has a calorific value of 14,000 BTU/lb x 2.326 = 32 564 kj/kg. 136 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 f£ Table 9 - Classification of Coals by Rank Fixed carbon limits, Volatile matter limits, % Calorific value limits, BTU/lb (moist, (dry, mineral- (dry, mineral- mineral-matter- matter-free basis) matter-free basis) free basis) % Class and group Equal or greater than Less than Equal or greater than Less than Equal or greater than Less than I Anthratic 1. Meta-anthracite 98 2. Anthracite 92 98 2 8 3. Semi-anthracite 86 92 8 14 78 86 14 22 69 78 22 31 69 31 2 11 Bituminous 1. Low-volatile bituminous coal 2. Medium-volatile bituminous coal 3. High-volatileA bituminous coal 14000 4. High-volatile B bituminous B coal 13000 14000 5. High-volatile C bituminous coal 11 500 13000 10500 11 500 9500 10500 8300 9500 Ill Sub-bituminous 1. Sub-bituminous A coal 2. Sub-bituminous B coal 3. Sub-bituminous C coal IV Lignitic 6300 1. LigniteA 8300 6300 1. Lignite B Reprinted from ASTM Standards D 388, Classification of Coals by Rank 3rd Class Edition 3 • Part A2 137 Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Typical Coals Table 10 shows the constituent percentages of some typical coals. Table 10 - Constituent Percentages of Coals Fixed carbon Volatile Moisture Ash Head (kJ/kg) Cam rose 37.9 26.6 28.5 7.0 18700 Crows nest 57.8 24.9 2.5 14.8 29000 Drumheller 44.2 30.7 18.2 6.9 22700 Wabamun 39.5 26.5 21.8 12.2 18580 Bienfait 33.0 26.0 35.0 6.0 17130 Estevan 30.8 24.4 35.2 9.6 15580 Pennsylvania 66.5 20.6 3.4 9.5 31 610 District COMPOSITION ANALYSIS FOR COAL Composition analysis for coal is typically done on a percent mass basis. The mass of air required for the combustion of each constituent and the masses for the products of combustion can be calculated by starting with the combustion equations for carbon, hydrogen, and sulfur. Combustion of Carbon The following equation shows the combustion of carbon: carbon + oxygen -^ carbon dioxide C + 02 -> C02 1 kmol + 1 kmol -^ 1 kmol 12kg + 32kg ^ 44kg Convert these values to 1 kg of carbon by dividing each mass by 12: 32_8_^^ 44_H_ 1 + it= t= 2-67 ~> it= T= 3-67 lkgC+ 2.67 kg 02 -> 3.67 kg 002 Therefore, 1 kg of carbon requires 2.67 kg of oxygen and produces 3.67 kg CO^. Combustion of Hydrogen The following equation shows the combustion of hydrogen: hydrogen + oxygen -> water vapour 2H2 + 02 -> 2H20 2 kmol + 1 kmol ^ 2 kmol 4kg + 32kg ^ 36kg Convert these values to 1 kg of hydrogen by dividing each mass by 4: 1 32 ^=9 IkgHz + 8 kg 02 ^ 9kgH20 Therefore, 1 kg of hydrogen requires 8 kg of oxygen and produces 9 kg H^O. 138 3rd Class Edition 3 • Part A2 Fuels, Combustion, and Flue Gas Analysis • Chapter 3 ^ Combustion of Sulfur The following equation shows the combustion of sulfur: sulfur + oxygen ^ sulfur dioxide S + 02 -> S02 1 kmol + 1 kmol -> 1 kmol 32kg + 32kg ^ 62kg Convert these values to 1 kg ofsulfur by dividing each mass by 32: - +IJ=1-1=2 IkgS + 1kg 02 -> 2kgS02 Therefore, 1 kg of sulfur requires 1 kg of oxygen and produces 2 kg S02. Summary of Coal Combustion Equations by Mass Compiling the results of the analysis by mass gives the following set of mass values, which the Power Engineer should become familiar with: 1 kg C + 2.67 kg 02 ^ 3.67 kg COz lkgH2 + 8 kg 02 ^ 9kgH20 IkgS + lkg02 ^ 2kgS02 Since the nitrogen in the air is a non-combustible element, it does not combine with oxygen. Rather, the nitrogen passes through the furnace unchanged, except for an increase in temperature. Example 15 The coal supplied to a boiler contains 78% carbon, 6% hydrogen, 9% oxygen, and 7% ash by mass as fired. The air supplied is 50% in excess of that required for theoretical combustion. Calculate the actual air supplied and the mass of dry flue gas produced per kilogram of fuel. Terminology The term dryflue gas refers to all the gaseous constituents in the flue gas, except for the water (steam) produced from the combustion of hydrogen. 0 Solution 15 For 1 kg of fuel, there is 0.78 kg C, 0.06 kg H, 0.09 kg 0^ and 0.07 kg ash. Mass of combustible Mass of QZ required Carbon 0.78 kg 0.78 x 2.67 = 2.08 kg 03 Hydrogen 0.06 kg 0.06 x 8 = 0.48 kg 0-^ Total 2.08 + 0.48 = 2.56 kg 0-^ 3rd Class Edition 3 • Part A2 139 Chapter 3 • Fuels, Combustion, and Flue Gas Analysis Oxygen required for combustion = 2.56 kg 02/kg fuel Amount of oxygen in fuel = 0.09 kg 02/kg fuel 02 required from air = 2.56 - 0.09 = 2.47 kg 02/kg fuel Stoichiometric air = 2.47 x —— kg air/kg fuel = 10.65 kg air/kg fuel Actual air supplied = 10.65x1.5 = 15.98 kg air/kg fuel (Ans.) Total mass offlue gas is equal to the supplied air plus the 1 kg mass of fuel: Mass offlue gas = 15.98 + 1 = 16.98 kg fluegas/kg fuel The 0.06 kg of N2 produces: 0.06x9 = 0.54 kg HzO Therefore, the mass of dry flue gas is 16.98 - 0.54 = 16.44 kg dry flue gas/kg fuel. (Ans.) Note: The calculated value for theoretical (stoichiometric) air can be verified with the formula in the PanGlobal Academic Supplement. 100 .. |8^ . ^( ^ Oi\ . Ji , Theoretical air = -^ x | ^C + 8 ( N2 --^-) + S | kg air/kg fuel J In this equation, the chemical symbols have the following meaning: C = kg carbon per kg of fuel = percent carbon H2 = kg hydrogen per kg of fuel = percent hydrogen 02 = kg oxygen per kg of fuel = percent oxygen S = kgsulfurperkgoffuel = percent sulfur Theoretical air = 1^- x | ^ x 0.78 + 8 ( 0.06 - 0^9) + 01 kg air/kg fuel ^° x [2.08 + 8 x 0.04875 + 0] kg air/kg fuel 23 100 x (2.47) 23 = 10.74 kg air/kg fuel (Ans.) This value is very close to the value previously obtained in the question. 140 3rd Class Edition 3 • Part A2