Environmental Engineering FOURTH EDITION

Reuse, Recycling, and Resource Recovery 289

For thermal balance the heat input must equal the heat output plus losses. Example

14.6 shows calculations for a typical thermal balance.

EXAM- 14.6. Aprocessed refuse containing 20% moisture and 60% organic material

is fed to a boiler at a rate of 1000 kg/h. From a calorimetric analysis of the refuse, a

dry sample was determined to have a heat value of 19,000 Wkg. Calculate the thermal

balance for this system.

The heat from combustion of the RDF is

&omb = (19,OOOkJ/kg)(1000kg/h) = 19 X 106kJ/h.

The organic fraction of the RDF contains hydrogen, which is combusted to water.
Therefore, the heat of combustion value includes the latent heat of vaporization of
water, because the water formed is vaporized during the combustion. Since this heat is
absorbed by the water formed, it is a heat loss. Assuming that the organic constituents
of the RDF are 50% hydrogen (by weight), and given that the latent heat of vaporization
of water is 2,420 kJkg, the heat loss from the vaporization is

Hvsp = (1000kg/h)(0.6organic)(0.5H) (2420kJ/kg) = 0.726 x 106kJ/h.

The RDF also contains moisture that is vaporized:

Hmois = (1000 kg/h) (0.2 moisture) (2420 W/kg) = 0.484 x lo6 W/h.

There is heat loss associated with radiation, usually assumed as 5% of the heat input.
Not all of the organic material will combust. Assume that the ashes contain 10% of the
organic material, so that the heat loss is

HIad = (19 x lo6 kJ/h)(0.05) = 0.95 x lo6 kJ/h

Hnoncom = (10/60)(19 x 106kJ/h) = 3.17 x lo7 W/h.

The stack gases also contain heat, which is usually assumed to be the difference between
the heat of combustion and the other losses calculated:

Heat input = Heat output

Hcomb = Hvap + Hmois + Hrad + Hnoncom + Hstack

19 X lo6 = (0.726 + 0.484 -I- 0.95 + 3.17)106 -k &tack

Hsmck = 13.67 X 1O6kT/h.

Some of the 13.67 x lo6 W/h may be recovered by running cold water into the boiler
through the water wall tubes and producing steam. If 2000 kg/h of steam at a tempera-
ture of 300°C and a pressure of 4000 kPa is required and the temperature of the boiler
water is 80"C, calculate the heat loss in the stack gases.
Heat in the boiler water is

= (2000kg/h)(80 + 273)K (0.00418) kJLkg-K = 2951 W/h.