Question 12.3: A fuel oil is burned (i) in pure oxygen or (ii) in air under......
A fuel oil is burned (i) in pure oxygen or (ii) in air under stoichiometric conditions.
Fuel oil composition (in mass fractions) is C = 0.87, H = 0.11, and O = 0.02; LHV of fuel = 42 MJ/kg.
Determine (a) the mass and volume of combustion oxygen or air, (b) the flue gas volume, (c) volumes and molar fractions of flue gas constituents, and (d) the adiabatic combustion temperature.
The density of oxygen is 1.429 kg/m³ and that of air is 1.293 kg/m³ (air); specific heat of flue gas is c_{pg} = 1.34 kJ/m³K.
Oxyfuel Stoichiometric Combustion of Fuel Oil in Pure Oxygen
1. Mass and volume of oxygen required for stoichiometric combustion of 1 kg coal
m_{O_{2}}\,=\,2.67\ C+8\ \mathrm{H}+{{O}}\,=\,2.67\times0.87+8\times0.11-0.02=3.18\ {{k g/k g}}V_{\mathrm{O_{2}}}\,=\,m_{\mathrm{O_{2}}}/\mathbf{r}_{\mathrm{O_{2}}}\,=\,3.18/1.429\,=\,2.23\,\mathrm{m^{3}/k g}
2. Flue gas volume and composition
V_{C O_{2}}\,=\,1.867\,\mathrm{C}=\,1.867\times0.87\,=\,1.624\,\mathrm{m}^{3}/\mathrm{kg}V_{\mathrm{H_{2}O}}=11.11\ \mathrm{H}=11.1 1\times0.1 1=\mathbf{1}.222\,m^{3}/\mathrm{kg}
V_{g}=V_{C O_{2}}+V_{H_{2}O}\,=\,1.624+1.222\,=\,2.846\,\mathrm{m}^{3}/\mathrm{kg}
3. Molar fractions of flue gas components
r_{\mathrm{CO}_{2}}\,=\,V_{\mathrm{CO}_{2}}/V_{\mathrm{g}}\,=\,1.624/2.846\,=\,0.57\mathrm{~and}r_{\mathrm{H_{2}O}}\,=\,V_{\mathrm{{H_{2}O}}}/V_{\mathrm{g}}\,=\,1.222/2.846\,=\,0.43
4. Ignoring sensible heat of oxygen and fuel, adiabatic combustion tempera-ture (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}})=42,000/(2.846\times1.34)=11,012 ^{\circ}CConventional Stoichiometric Combustion of Fuel Oil in Air
1. Volume of air requirement per unit mass of fuel oil
V_{\mathrm{air}}=V_{\mathrm{O_{2}}}/0.21=2.23/0.21=10.61\,{\mathrm{m}}^{3}/{\mathrm{kg}}2. Flue gas volume and composition
V_{\mathrm{CO_{2}}}\,=\,1.624\,\mathrm{m^{3}}/{\mathrm{kg}},\;V_{\mathrm{H_{2}O}}\,=\,1.222\,\mathrm{m^{3}}/{\mathrm{kg}},V_{\mathrm{N_{2}}}=0.79,\,V_{\mathrm{air}}=0.79\times10.6\mathbf{1}=8.38\,\mathrm{m^{3}/k g}
V_{\mathrm{g}}=V_{\mathrm{CO_{2}}}+V_{\mathrm{H_{2}O}}+V_{\mathrm{N_{2}}}=1.624+1.222+8.38=11.226\,\mathrm{m}^{3}/\mathrm{kg}
3. Molar fractions of flue gas components
r_{\mathrm{CO_{2}}}\,=\,1.624/11.226=0.145,\,r_{\mathrm{H_{2}O}}\,=\,1.222/11\,.226\,=\,0.109,r_{N_{2}}=8.38/11.226=0.746
4 Ignoring sensible heat of air and fuel, adiabatic combustion temperature
t_{\mathrm{ad}}=\mathrm{LHV}{ {}}/{( \mathrm{V}_{g}\,c_{pg}})=42,000/(11.226\times1.34)=2792 ^{\circ}CTable 12.3 contains significant data of selected oxyfuel combustion demonstration projects.
| TABLE 12.3 | ||
| Oxyfuel Combustion Demonstration Projects (Status: 2013) | ||
| Project, Country | Plant Capacity | Project Description |
| CS Energy, Australia | 30 MWe | Pulverized coal steam power plant (retrofit) |
| Total Lacq, France | 30 MWe | Gas-fired oxyfuel new boiler, 150,000 t/A CO_{2} to be stored in depleted oil/gas reservoir |
| Vattenfall Cottbus, Germany | 30 MWt | Lignite-fired new plant |