Calculating the Heat of Combustion for Natural Gas
Maintaining the efficiency of a heating process depends on knowing (or calculating) the heat of combustion of a fuel. In some cases, it’s sufficient to use an approximate value, such as 1,000 BTU/ft3 for natural gas. But for fuel-intensive processes, and as NOx minimization becomes more important, you may want a more accurate value. We show how to calculate it here, with details in the downloadable Excel workbook HeatCalc.xlsx.
First, you need the composition of the supplied gas. Next, you need values for the heat of reaction for oxidation of each constituent to CO2 and H2O. These are then summed to obtain the heat of combustion (∆Hcomb) of the gas.
Heats of reaction are usually (but not always!) calculated or listed for mole amounts at 25°C, then converted to ∆Hcomb on a mass or volume basis. Care must be taken when calculating or using handbook data for ∆Hcomb values because there are two different conventions used in specifying the H2O part of the combustion products. In some tabulations, ∆Hcomb is based on liquid water as the combustion product, and in others, water vapor is the product form. The heat produced by the combustion of a fuel is called its heating value, which is equal to the value of ∆Hcomb but of opposite sign. When liquid water is the product, it is called the higher (or gross) heating value (HHV). When water vapor is the product, it is called the lower (or net) heating value (LHV). The difference between the two values is ∆Hvap of water. Since combustion products are above the boiling point of water, the LHV is a better indication of a fuel’s useful heat.
Be aware that some handbooks use 288.7 K (60°F) as the STP temperature unit instead of 273.15 K (0°C, 32°F) used here. It’s best to find or calculate molar heating values and then convert to a mass or volumetric basis of your choice, as outlined in the Excel workbook available for download at www.industrialheating.com/HeatCalc.
Calculating the LHV of Methane
The ∆Hcomb of one mole of methane (CH4) at 298.15 K is the heat of reaction between CH4 and O2 to form CO2 (g) and H2O(g), according to Equation 1. Table 1 shows values of ∆H°formation of several natural gas reactants and products. Equations 2 and 3 show the calculation for ∆H°reax (i.e. ∆Hcomb ) of methane from these values.
CH4(g) + 2O2 (g)
→ CO2 (g) + 2H2 O(g); LHV = –∆ H °reax at 298.15 K (25°C, 77°F) 1)
∆Hcomb = ∆ Hrx = ∆ H°f of CO2 (g) + 2∆H°f of H2O(g) – ∆H°f of CH4(g) 2)
∆Hcomb = –393.51 + 2(–241.81) + 74.81 = –802.32 kJ/mole of CH4 3)
The LHV of methane is thus 802.3 kJ/mole at 298.15 K (25°C, 77°F). Similar arithmetic using ∆H°formation of H2 O(liq) gives HHV = 890.4 kJ/mole. The difference (88 kJ) is the heat of vaporization of 2 moles of water. Table 2 shows LHV and HHV for methane combustion in different units using methods outlined in workbook HeatCalc.xlsx. Download this workbook at www.industrialheating.com/HeatCalc.
Internet Sources of Heat of Formation and Heat of Combustion Data
Although chemistry handbooks are often used as a source of thermodynamic data, you may find it easier to use web sources. The FactSage program is probably the easiest to use, followed by the NIST Chemistry WebBook. The data in Tables 1 and 2 came from database program FREED. The differences in data between these sources is minimal. IH