A fuel-fired furnace may be visualized as a gaseous heat source (the flame and furnace gases), a sink, and a refractory enclosure. Heat is transferred to the sink surface by radiation and convection from the furnace gases and radiation from refractory surfaces. Generally speaking, the dominant mode of heat transfer is radiation, but in some heating plant, for example shell boilers, convection between the combustion gases and the heat sink will be significant and need to be accurately accounted for Rhine and Tucker (1991). The roles of the heat source, sink, and refractory in the heat transfer process are described in the following sections.

A. Heat source

The heat to a fuel-fired furnace is provided primarily by the combustion reaction. The heat liberated by the combustion of a unit mass of fuel is given by the heat of reaction or calorific value of the fuel. For fuels containing hydrogen, two calorific values are reported: the gross calorific value, determined by assuming that all the water vapor produced during the combustion process is condensed and cooled to 288 K; and the net calorific value, determined assuming that the water vapor formed during combustion remains in the vapor phase. The source of oxygen for combustion is generally air. An excess of air above the stoichiometric requirement is used to ensure complete combustion of the fuel. Typical excess air requirements are 10% for gaseous fuel, 15-20% for liquid fuels, and 20% or more for pulverized solid fuels. Table 1 and Table 2 give the compositions, calorific values, and air requirements for the components of most industrial gaseous fuels, and for typical liquid and solid fuels. These days with concerns about climate change, and particularly carbon dioxide emissions, many industrial applications, such as boiler plant for power generation, will be fired on renewable fuels. Often the renewable energy sources are co-fired with conventional fossil fuels, such as pulverized coal. The burners on the power station boiler will be fed with a mix of coal and solid renewable fuel. Generally speaking it is possible to co-fire up to 15% with the renewable fuel, because of limitations in the milling processes. In smaller installations 100% firing with renewable fuels is possible. This can be carried out in fluidized bed combustors or chain-grate stokers, depending on the size of the application.