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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 fuel oil combustion products?

Fuel oils include a wide range of liquid hydrocarbon fuels such as kerosene, gas-oil, and light, medium and heavy fuel oils. Their compositions are often complex but generally they all contain around 85% by mass of carbon and 12% by mass of hydrogen (the balance being sulphur, oxygen, nitrogen and ash).  Consequently the products of combustion of fuel oils contain approximately equal volumes of water vapour and carbon dioxide. In practice the ratio of water vapour to carbon dioxide by volume is around 1¡À0.1.  This differs from the combustion products of natural gases that have a ratio of two.  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 fuel oil combustion contain approximately 13% H₂O and 13% CO₂ by volume; these are concentrations on a wet basis. For atmospheric pressure combustion, these concentrations are equivalent to partial pressures of 0.13 bar. 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 of a typical fuel oil over a range of air/fuel ratio conditions from [GLOSS]stoichiometric[/GLOSS] to 200% [GLOSS]excess air[/GLOSS]. 

3. What is the emissivity of fuel oil combustion products?

The emissivity of fuel oil combustion products is plotted in Fig.2 for a range of values of the product pL (partial pressure x beam length, where p = p_C + p_w) and for the normal range of temperatures occurring in furnaces (600 to 2400K). If you don’t know the partial pressures, simply read off appropriate values for p_w and p_c from Fig.1 for your known excess air level.  Add these together and multiply by the mean radiating beam length in metres, then read off the emissivity from Fig. 2 for the corresponding gas temperature.

It should be noted that the emissivity values in Fig.2 approach a limiting value at high partial pressure-beam length products that is significantly less than one. This is because in combustion products that contain no carbon or soot, there are bands in the spectrum where the gases do not radiate (so-called ‘windows’). The combustion products are non-grey.