Oxyfuel computation flame temperature In Sect. 3.5.1, we calculated the adiabatic flame temperature of CH4 burned with air at 298 K, when they are premixed perfectly with an equivalence ratio of 1.0 and the combustion is complete. Now let us redo the calculation using pure oxygen instead of air.

Question

Oxyfuel computation flame temperature
In Sect. 3.5.1, we calculated the adiabatic flame temperature of [latex]CH_{4}[/latex] burned with air at 298 K, when they are premixed perfectly with an equivalence ratio of 1.0 and the combustion is complete. Now let us redo the calculation using pure oxygen instead of air. Determine the corresponding constant pressure adiabatic flame temperature.

Accepted Answer

First of all, set up stoichiometric combustion reaction equation, using the methods introduced in Sect. 3.4,
[latex]CH_{4} + 2O_{2} → CO_{2} + 2H_{2}O[/latex]

For an adiabatic constant pressure system, with the reactant temperature of [latex]T_{R} = T_{0}[/latex] = 298 K, the left-hand side (LHS) of Eq. (12.15)
[latex]\sum\limits_{R}{n_{i}}\left[\left(a_{i}(T_{R} - 298 K)+\frac{b_{i}}{2}(T^{2}_{R} - (298 K)^{2} \right)+h^{o}_{f,i} \right][/latex]
[latex]=\sum\limits_{P}{n_{i}}\left[\left(a_{i}(T_{a} - 298 K+\frac{b_{i}}{2}(T^{2}_{a} - (298 K)^{2} \right)+h^{o}_{f,i} \right][/latex] (12.15)

becomes
[latex]LHS=h^{o}_{f,CH_{4}} + 2h^{o}_{f,O_{2}}[/latex] (1)

The right-hand side (RHS) of the equation is simplified as
[latex]RHS=\left[\left(a_{CO_{2}}(T_{a} - 298 K+\frac{b_{CO_{2}}}{2}(T^{2}_{a} - (298 K)^{2} \right)+h^{o}_{f,CO_{2}} \right][/latex]
[latex]+2\left[\left(a_{H_{2}O}(T_{a} - 298 K+\frac{b_{H_{2}O}}{2}(T^{2}_{a} - (298 K)^{2} \right)+h^{o}_{f,H_{2}O} \right][/latex] (2)

Looking into Table A.4, we can get the parameters needed (Table 12.4)
Substituting these values into Eq. (1) and Eq. (2) leads to
LHS = -74,980 (J/mole)

[latex]RHS=\left[\left(44.3191(T_{a} - 298 K) +\frac{0.0073}{2}(T^{2}_{a} - (298 K)^{2} \right) - 394,088\right][/latex]
[latex]+2\left[\left(32.4766(T_{a} - 298 K) +\frac{0.00862}{2}(T^{2}_{a} - (298 K)^{2} \right) - 242,174\right][/latex]

Simplification of the equation gives
[latex]0.01227 T^{2}_{a} + 109.2723 T_{a} - 837109 = 0[/latex]

Solving this equation we can get, [latex]T_{a}[/latex] = 4930.8 K. This calculated value is much higher that air-fuel combustion: [latex]T_{a}[/latex] = 2341 K.

Table 12.4 Parameters used in Example 1.22

Species	[latex]h^{o}_{f,i}[/latex] (J/mole)	[latex]C_{p}[/latex](T) (J/mole · K)
Species	[latex]h^{o}_{f,i}[/latex] (J/mole)	[latex]a_{i}[/latex]	[latex]b_{i}[/latex]
[latex]CO_{2}[/latex]	−394,088	44.3191	0.0073
[latex]H_{2}O[/latex]	−242,174	32.4766	0.00862
[latex]O_{2}[/latex]	0	30.5041	0.00349
[latex]CH_{4}[/latex]	−74,980	44.2539	0.02273

From Table A.4, we can get

Species	[latex]h^{o}_{f,i}[/latex] (J/mole)	[latex]C_{p}[/latex](T) (J/mole · K)
Species	[latex]h^{o}_{f,i}[/latex] (J/mole)	[latex]a_{i}[/latex]	[latex]b_{i}[/latex]
[latex]CO_{2}[/latex]	−394,088	44.3191	0.0073
[latex]H_{2}O[/latex]	−242,174	32.4766	0.00862
[latex]N_{2}[/latex]	0	29.2313	0.00307
[latex]O_{2}[/latex]	0	30.5041	0.00349
[latex]CH_{4}[/latex]	−74,980	44.2539	0.02273

Although it is only estimation, the comparison does show that the oxyfuel combustion flame temperature is higher than that of conventional combustion with air. As a result, the boiler of the oxyfuel combustion process requires special materials that can survive extreme temperature.
Another concern of the oxyfuel combustion process with sulfur containing fuels is the high [latex]SO_{x}[/latex] concentration without the dilution of nitrogen, resulting in high corrosion on the ducts. Extra costs are associated with concentrated oxygen production by costly air separation units.
The benefit is a simple process for carbon capture after combustion. Without nitrogen and [latex]NO_{x}[/latex] in the flue gas, it contains mainly [latex]H_{2}O[/latex] and [latex]CO_{2}[/latex]. After the removal of soot and [latex]SO_{2}[/latex], if any, [latex]CO_{2}[/latex] can be readily separated from water vapor by condensation in a cooler. This highly concentrated [latex]CO_{2}[/latex] is ready for transportation and storage.

Question 12.2: Oxyfuel computation flame temperature In Sect. 3.5.1, we cal...

Related Answered Questions

Verified Answer:

CO2 solubility in physical IL Estimate the amount of CO2 in kg that can be absorbed into 1,000 kg of IL at 2 MPa and 300 K using the Carvalho and Coutinho [12] model. ...

Verified Answer:

How much water and CaCO3 is needed to capture 1,000 kg of CO2 from the flue gas? How much calcium bicarbonate (Ca(HCO3)2) is produced? ...

Verified Answer:

Verified Answer: