Available Combustion Heat
Much of the heat produced by fuel combustion is unavailable for productive use. Figure 1 shows the flow of product and heat for a typical process. Previous columns showed that hot offgas contains much of the combustion heat, while a lesser but still significant amount is lost through the walls of the furnace.[1,2] Only the remaining heat is available for raw-material processing. A main objective of combustion management is to maximize the available heat. Here, we define the gross available heat is the heat of combustion minus the heat carried out in the offgas, while the net available heat is the gross available heat minus the heat loss. Both are expressed as a percent of the combustion heat.
This article shows how to calculate the % available heat using methane as fuel and an excess of dry normal air as the oxidant. Details are in the downloadable Excel workbook AvailableHeatCalc.xlsx, which accommodates different natural gas compositions and oxygen enrichment of the air. This workbook uses the data and calculation techniques developed in earlier Combustion Concepts workbooks.
We start by collecting data on the heat of methane combustion and the HT-H25 values of the offgases. A previous Excel workbook gives DH°comb for CH4 at 25°C = -802.3 kJ/g-mole for H2O(g) as the combustion product and -890.4 for H2O(l). Table 1 shows the heat-content-equation parameters from the FREED database for the product gases in J/g-mole for Celsius temperature.
A material balance is used to calculate the amount of reactants and products over a large range of percent excess air. On a basis of one g-mole of methane burned, AvailHeatCalc.xlsx shows that the air/NG ratio varies from 9.52-19.05 as the air varies from stoichiometric to 100% excess. For example, at 40% excess air, 13.33 g-mole of air are required, with 3.81 moles in excess of stoichiometric.
A heat balance is used to calculate the heat content of each combustion product. The first step is to select an offgas temperature and use it in the Table 1 formulae to calculate the species molar heat content. We then multiply the species amount by its molar heat content to get its heat content and repeat this for each species to get the heat content for the entire 14.33 moles of offgas. This was done for 800°, 900° and 1000°C. The gross available heat is the algebraic combination of DH°comb for CH4 at 25°C and HT-H25 for the offgas. The % available heat is the available heat value divided by the DH°comb for CH4 at 25°C (either -802.3 or -890.4). The latter value gives an artificially low value of % available heat because industrial processes never produce H2O(l) as a combustion product.
Figure 2 shows the benefit of minimizing the extent of excess air to maximize the available heat. Equation 1 gives the statistical results of a correlation between the two variables, where T is the offgas temperature in Celsius and H2O(g) is the combustion product.
% Gross Available Heat = (-0.00040T + 0.033)(%XS air) – 0.049T +104.9 
The downloadable worksheet at www.industrialheating.com/AvailHeatCalc has additional descriptive text on natural gas combustion, source of all data and complete material-balance values. Goal Seek can be used for what-if calculations, such as seeking the adiabatic flame temperature. A similar program, BurnerCalc.xlsx, includes oxidant humidity, oxidant temperature and NG temperature as process variables. IH
1. Arthur Morris, “Making a Heat Balance,” Industrial Heating, December 2012
2. Arthur Morris, “Making a System Balance (Part 1),” Industrial Heating, February 2013
3. Arthur Morris, “Calculating the Heat of Combustion for Natural Gas,” Industrial Heating, September 2012