Heat transfer.

The total heat transfer occurring within a furnace can be broken down into four main areas:

– radiant heat transfer from the flame to the product;

– radiant heat transfer from the exhaust gases to the product;

– convective heat transfer from the exhaust gases to the product;

– radiant heat transfer from the furnace structure to the product.

Radiant heat transfer from the flame. It is the carbon particles in the fuel that are the source of radiation from the flame. There are fewer particles in an oxy-fuel flame due to the fact that, owing to the improved thermal efficiency, less fuel is used for a given heat input requirement compared with air-fuel. However, the total flame radiation is also a function of the temperature difference to the fourth power between the flame and the material being heated. Thus, as the adiabatic flame temperature of an oxy-fuel flame is around 2 700°C compared with 1 800°C for air-fuel, this more than compensates for the lower number of carbon particles, giving overall, higher total radiation from the flame. This accounts for the improvement in radiant heat transfer in oxy-fuel combustion.

Radiant heat transfer from the exhaust gases. Radiant heat transfer from the exhaust gases is a function of the molecular species within the exhaust gases plus the temperature and velocity of the exhaust gas stream. The exhaust gases present in air-fuel combustion are predominantly nitrogen with smaller quantities of CO₂ and water vapour. The products of oxy-fuel combustion however, are CO₂ and water vapour. These tri-atomic molecules have excellent radiation heat transfer properties, whereas nitrogen does not transfer its heat energy by radiation at all. Thus the atmosphere from oxy-fuel combustion consisting purely of hot CO₂ and water vapour transfers heat much more effectively that the mostly nitrogen containing air-fuel atmosphere.

Convective heat transfer. Heat conduction within gases is quite poor. Thus, velocity and turbulence have a significant effect upon the extent of heat transfer by convection. For this reason convection contributes significantly to the overall heat transfer in an air-fuel fired furnace due to the large volumes of exhaust gas that travel through the furnace. This is not so however in the case of oxy-fuel as the exhaust gas volumes are reduced by over 75%, and the high efficiency of radiant heat transfer by water vapour and CO₂ from the exhaust gas is by far the primary source of heat transfer.

Radiant heat transfer from the furnace sidewalls. Radiant heat transfer of a surface depends to a large extent upon the emissivity and absorptivity of the material surface. In oxy-fuel combustion efficient radiant heat transfer from both the exhaust gas and directly from the flame effectively transfers heat to the furnace sidewalls. Radiation heat transfer from the furnace sidewalls back into the furnace is then dependent to a large extent upon the emissivity and absorptivity of the surface. In addition, the relative positions of the radiating and receiving surfaces affect heat transfer rate. The quantitative measure of this ability is termed the 'arrangement or view factor’, which corresponds to the fraction of radiation that comes from the radiating surface to the receiving surface. Thus, the actual radiation heat transfer from the walls is the black body rate multiplied by the emissivity factor and the arrangement or view factor.