288 ENVIRONMENTXLENGIIVEEIUNG
EXAMPLE 14.5. If carbon is combusted as
C + 02 i, C02 + heat,
how much air is required per gram of carbon? One mole of oxygen is required for each
mole of carbon used. The atomic weight of carbon is 12 g/g-atom and the molecular
weight of 02 is 2 x 16 = 32 g/mole. Hence 1 g of C requires
32
14- = 2.28 g02.
Air is 23.15% 02 by weight; total amount of air required to combust 1 g of C is
2.28
0.2315= 9.87 g air.
The yield of energy from combustion is measured as the calories of heat liberated
per unit weight of material burned. This is the heat of combustion or, in engineering
terms, the heat value. Heat value is measured using a calorimeter, in which a small
sample of fuel is placed in a water-jacketed stainless steel bomb under high pressure of
pure oxygen and then fired. The heat generated is transferred to the water in the water
jacket, and the rise in water temperature is measured. Knowing the mass of the water,
the energy liberated during combustion can be calculated. In SI units the heat value
is expressed as kilojoules (kJ) per kilograms; in British units, as British thermal units
(Btu) per pound. Table 14-3 lists some heats of combustion for common hydrocarbons,
and gives some typical values for refuse and RDF. The rate at which heat goes into a
boiler is sometimes called the heat rate.
In designing any combustion operation, the system must be analyzed for thermal
as well as materials balance. Materials balance requires that the total mass of the inputs
(air and fuel) must equal the mass of the outputs (stack emissions and bottom ash).
Table 14-3. Qpical Values of Heats of CombustionHeat of combustionCarbon (to C02) 32,800 14,100
Hydrogen 142,000 61,100
Sulfur (to S02) 9300 3980
Methane 55,500 23,875
Residual oil 41,850 18,000
Raw refuse 9300 4000
RDF (air classified) 18,600 8000