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[edit] Analysis

This section provides a combustion analysis for a few typical fuel cases (carbon, hydrogen, sulfur, coal, oil and gas) when the fuel reacts with air at stoichiometric conditions.

In the presented combustion analysis, both fuel and air are at standard inlet combustion conditions of 298 K and 1 atm of absolute pressure. Furthermore, combustion is complete and with no heat loss.

During the combustion, a large amount of reactants' chemical energy gets released in the form of thermal energy.

Enthalpy of combustion (HHV) is the difference between the reactants enthalpy value minus the combustion products enthalpy value at the standard reference temperature, which is 298 K.

When the reactants enthalpy value is equal to the combustion products enthalpy value, one can calculate the combustion products flame temperature (adiabatic temperature).

The plot in Image 1 depicts the reactants and combustion products enthalpy value change with an increase in the temperature.

Physical properties for both reactants and combustion products are very important and need to be known in order to carry out successful combustion calculations.

The plot in Image 2 depicts how the reactants and combustion products species enthalpy values change with the temperature. The physical properties provided in this plot come from the JANAF Thermochemical Data - Tables, 1970.

It is interesting to note that the enthalpy value for basic combustion elements such as carbon (C), hydrogen (H), sulfur (S), oxygen (O) and nitrogen (N) is equal to zero at the standard combustion conditions of 298 K and 1 atm.

Also, it should be mentioned that for ideal gas species, the enthalpy value is only dependent on the temperature.

In addition to knowing the reactants and combustion products physical properties, for any kind of combustion analysis and calculations, it is important to know both fuel and oxidant compositions.

For solid and liquid type fuels, the fuel compositions is given on the weight basis for a unit mass amount. For the gas type fuels, the fuel compositions is provided on the mole/volume basis for a unit volume amount. In this analysis, CH₄ is the only gas fuel considered. In order to keep the combustion analysis simple and straightforward, the CH₄ composition is provided on the weight basis. Oxidant composition is usually given on the mole/volume basis.

Table 1 provides the fuel composition.

Table 1: Fuel Composition

Fuel	C kg/kg	H kg/kg	S kg/kg	N kg/kg	O kg/kg	H2O kg/kg	CH4 kg/kg
Carbon	1.000	0.000	0.000	0.000	0.000	0.000	-
Hydrogen	0.000	1.000	0.000	0.000	0.000	0.000	-
Sulfur	0.000	0.000	1.000	0.000	0.000	0.000	-
Coal	0.780	0.050	0.030	0.040	0.080	0.020	-
Oil	0.863	0.136	0.000	0.000	0.000	0.000	-
Gas	-	-	-	-	-	-	1.000

Table 2 provides the standard air composition.

Table 2: Oxidant/Air Composition

Oxidant	N kg/kg	O kg/kg	N2 mol/mol	O2 mol/mol
Air	0.766	0.233	0.789	0.210

Again, in this combustion analysis, only the stoichiometric combustion is analyzed. Results of such analysis are provided, including a combustion products composition on weight and mole/volume basis, a flame temperature, a stoichiometric ratio and a higher heating value (HHV).

Table 3 provides the combustion products composition on the weight basis.

Table 3: Combustion Products on Weight Basis

Fuel	CO2 kg/kg	H2O kg/kg	SO2 kg/kg	N2 kg/kg	O2 kg/kg
Carbon	0.294	0.000	0.000	0.705	0.000
Hydrogen	0.000	0.254	0.000	0.745	0.000
Sulfur	0.000	0.000	0.378	0.621	0.000
Coal	0.250	0.041	0.005	0.703	0.000
Oil	0.203	0.078	0.000	0.717	0.000
Gas	0.151	0.123	0.000	0.724	0.000

Table 4 provides the combustion products composition on the volume basis.

Table 4: Combustion Products on Mole Basis

Fuel	CO2 mol/mol	H2O mol/mol	SO2 mol/mol	N2 mol/mol	O2 mol/mol
Carbon	0.210	0.000	0.000	0.789	0.000
Hydrogen	0.000	0.347	0.000	0.652	0.000
Sulfur	0.000	0.000	0.210	0.789	0.000
Coal	0.171	0.068	0.002	0.756	0.000
Oil	0.133	0.126	0.000	0.739	0.000
Gas	0.095	0.190	0.000	0.714	0.000

When considering coal, oil and gas as the fuel, coal has the largest amount of CO₂ in the combustion products on both weight and mole basis.

Table 5 provides the combustion products flame temperature, stoichiometric ratio and the fuel higher heating value.

Table 5: Other Fuel Characteristics

Fuel	Flame Temperature K	Stoichiometric Ratio (see note below)	HHV kJ/kg
Carbon	2,462	11.433	32,779.8
Hydrogen	2,527	34.300	141,866.8
Sulfur	1,970	4.287	9,261.3
Coal	2,495	10.477	32,937.9
Oil	2,485	14.530	47,630.0
Gas	2,328	17.123	50,151.2
Note: Stoichiometric ratio is the mass of air required for complete combustion of a unit mass of fuel. Thus, 1 kg of carbon fuel requires 11.433 kg of air for complete, ideal combustion.

Today, global warming is becoming more evident and it is being said that it is primarily caused by CO₂ emissions. A detailed combustion analysis, as it is provided here, can be very useful in determining different fuel and technology scenarios that would result in the reduction of current CO₂ emissions.