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1. What factors influence the emissivity of fossil-fuel combustion products?

Radiant emission from flames and hot combustion products in high temperature furnaces and boilers is usually the primary mode of heat transfer. Consequently, to calculate the heat transfer in any combustion plant, it is important to know the [GLOSS]emissivity[/GLOSS] of the gases.

The radiation from flames and combustion products is primarily due to carbon dioxide and water vapour.  The total emissivity (e_g) of the combustion products is a function of the number of molecules of absorbing gases (CO₂ and H₂O) in the path of the radiant beam. Consequently, emissivity increases as both the beam length and the pressure of CO₂ and H₂O increases. Emissivity is in fact correlated to the product of [GLOSS]partial pressure[/GLOSS] and  [GLOSS]beam length[/GLOSS] (see CF95 for a simple formula), where the partial pressure (p bar) is the sum of the partial pressures of the absorbing gases (p =p_C + p_w where p_c is the partial pressure of CO₂ in bar, and p_w that of water vapour). The temperature of the gas is also important; emissivity decreases as temperature increases. 

The presence of soot or carbon particles in the gases also increases emissivity, giving rise to the luminous appearance of certain flames. Soot can increase the total emissivity of flames significantly.  The effect of soot on emissivity can be calculated using the downloadable spreadsheet calculator at the top of this page.

Data on the total emissivity of CO₂, H₂O and soot mixtures are available in the literature [1-4]. In some cases the data have been usefully correlated using a mixed grey gas model [5,6,7], which can then be applied in ZONE models for the calculation of thermal radiation in furnaces.

2. What are the partial pressures of radiating gases in natural gas combustion products?

Most natural gas fuels consist predominantly of methane, which has four hydrogen atoms for every carbon atom. Consequently, the products of combustion of natural gas tend to comprise approximately 2 volumes of water vapour (H₂O) for every volume of carbon dioxide (CO₂) i.e. the ratio of partial pressures of H₂O to CO₂ is approximately 2:1 (for pure methane it is exactly 2:1).   This differs from the combustion products of most fuel oils that have a ratio close to one.  Consequently, combustion product emissivity data sets are generally presented in the literature for two conditions:

Data for p_w/p_C = 2 (for natural gas combustion)

Data for p_w/p_c = 1 (for fossil fuel oil combustion)

The stoichiometric products of natural gas combustion contain approximately 18% H₂O and 9% CO₂ by volume; these are concentrations on a wet basis. For atmospheric pressure combustion, these concentrations are equivalent to partial pressures of 0.18 bar and 0.09 bar respectively. As air/fuel ratio is increased, the partial pressures decrease. Fig. 1 gives a simple plot of p_w and p_c for normal atmospheric pressure combustion over a range of air/fuel ratio conditions from [GLOSS]stoichiometric[/GLOSS] to 200% [GLOSS]excess air[/GLOSS].