Question 12.1: A bituminous coal is burned in a boiler furnace under the fo......
A bituminous coal is burned in a boiler furnace under the following conditions:
• Coal composition (in mass fractions): C = 0.78, H = 0.07, O = 0.04, A = 0.06, and M = 0.05
• Lower heating value of coal, LHV = 31.4 MJ/kg
• Oxyfuel stoichiometric combustion of coal in pure oxygen
• Conventional stoichiometric combustion of coal in air
Determine (a) the combustion oxygen mass and volume, (b) the flue gas volume and composition, and (c) the adiabatic combustion temperature.
Density ρ: 1.429 kg/m³ (oxygen) and 1.293 kg/m³ (air); specific heat of flue gas: c_{pg} = 1.34 kJ/m³K.
Oxyfuel Stoichiometric Combustion of Coal in Pure Oxygen
1. Mass and volume of oxygen required for stoichiometric combustion of 1 kg coal
m_{O_{2}}\,=\,2.67C+8\mathrm{H- O}=2.67\times0.78+8\times0.07-0.04=2.6\,\mathrm{kg/kg}V_{\mathrm{O_{2}}}\,=\,m_{\mathrm{O_{2}}}/{\bf r}_{\mathrm{O_{2}}}\,=\,2.6/1.429\,=\,1.82\,{\mathrm{m}}^{3}/\mathrm{ kg}_{\mathrm{}}
2. Flue gas volume and composition
V_{C O_{2}}\,=\,1.867\,\mathrm{C}\,=\,1.867\times0.78\,=\,1.46\,\mathrm{m}^{3}/\mathrm{kg}V_{\mathrm{H_{2}O}}\,=\,1\,1.11\,\mathrm{H}+1.24\,M=\,1\,1.11\times0.\mathbf{0}7+1.24\times0.05\,=\,0.84\,{\mathrm{m}}^{3}/{\mathrm{kg}}
V_{\mathrm{g}}=V_{\mathrm{CO_{2}}}+V_{\mathrm{H_{2}O}}=1.46+0.84=2.3\,\mathrm{m}^{3}/\mathrm{kg}
3. Molar fractions of flue gas components
r_{\mathrm{CO_{2}}}\,=\,V_{\mathrm{CO_{2}}}/V_{\mathrm{g}}\,=\,1.46/2.3\,=\,0.634{\mathrm{~and~}}r_{H_{2}O}\,=\,V_{\mathrm{H_{2}O}}/V_{\mathrm{g}}\,=\,0.84/2.3\,=\,0.36\mathbf{6}4. Ignoring sensible heat of oxygen and fuel, adiabatic combustion temperature (see Equation 2.50)
t_{\mathrm{ad}}=\mathrm{LH}\mathrm{V}/(V_{\mathrm{g}}\mathbf{r}{_{\mathrm{g}}}c_{\mathrm{pg}})=\mathrm{LH}\mathrm{V}/(V_{\mathrm{g}}c_{\mathrm{pg}}^{\prime})~~~^{\circ}\mathrm{C} (2.50)
t_{\mathrm{ad}}=\mathrm{LHV}{ {}}/{( \mathrm{V}_{g}\,c_{pg}})=31,400/(2.3\times1.34)=10,188^{\circ}CConventional Stoichiometric Combustion of Coal in Air
Stoichiometric volume of air requirement per unit mass of fuel
V_{\mathrm{air}}=V_{O_{2}}/0.21=1.82/0.21=8.67\,{\mathrm{m}}^{3}/{\mathrm{kg}}1. Flue gas volume and composition
V_{C O_{2}}\,=\,1.46\,{\mathrm{m}}^{3}/{\mathrm{kg}},\;V_{\mathrm{H_{2}O}}\,=\,0.84\,{\mathrm{m}}^{3}/{\mathrm{kg}},\;V_{\mathrm{N_{2}}}\,=\,0.79,V_{\mathrm{air}}\,=\,0.79\times8.67\,=\,6.85\,{\mathrm{m}}^{3}/{\mathrm{kg}}
V_{\mathrm{g}}\,=\,V_{\mathrm{CO_{2}}}\,+\,V_{\mathrm{H_{2}O}}\,+\,V_{\mathrm{N_{2}}}\,=\,1.46\,+\,0.84\,+\,6.85\,=\,9.15\,\mathrm{m^{3}/kg}
2. Molar fractions of flue gas components (in % by volume)
r_{\mathrm{CO_{2}}}\,=\,1.46/9.15=0.159,\,r_{\mathrm{H_{2}O}}\,=\,0.84/9.15=0.092,r_{N_{2}}=6.85/9.15=\mathbf{0}.749
3. Ignoring sensible heat of air and fuel, adiabatic combustion temperature
t_{\mathrm{ad}}=\mathrm{LHV}/(V_{\mathrm{g}}\,c_{\mathrm{pg}})=31,400/(9.15\times1.34)=2561^{\circ}CCalculation results for bituminous coal are summarized in Table 12.1.
Example 12.2 presents a comparison of oxyfuel combustion with conventional combustion of natural gas.
| TABLE 12.1 Flue Gas Volume and Composition and Adiabatic Combustion Temperature for Stoichiometric Oxyfuel and Conventional Combustion of Coal (Example 12.1) |
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| V_{\mathrm{{g}}}\left(\mathrm{m}^{3}{/\mathrm{Kg}}\right) | r_{\mathrm{CO}_{2}}\,(\operatorname{m}^{3}/\operatorname{m}^{3}) | r_{H_{2}O}\ ({\mathrm{m}}^{3}/{\mathrm{m}}^{3}) | r_{N_{2}}\left({\mathrm{m}}^{3}/{\mathrm{m}}^{3}\right) | t_{\mathrm{ad}}\left(^{\circ}\mathrm{C}\right) | |
| Oxyfuel combustion | 2.3 | 0.634 | 0.366 | 0 | 10,188 |
| Conventional combustion | 9.15 | 0.159 | 0.092 | 0.749 | 2561 |