Page 116 - Materials Chemistry, Second Edition

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Characterization of Solid Waste 87
SOLUTION

E 0.051 [F 3.6 (CP)] 0.352 (PLR)
0.051 [24 3.6 (37)] 0.352 (7)
10.48 MJ/kg

Two heat of combustion parameters are of significance: high heating value and low heating value.
The higher heat of combustion includes the latent heat of vaporization of water molecules gener-
ated during the combustion process. The reaction for the combustion of cellulose and the conse-
quent formation of water is

(C H O ) 6nO → 6nCO 5nH O (4.5)
6 10 5 n 2 2 2
This water results solely from the combustion process, i.e., hydrogen is oxidized to form a water
molecule. Therefore, even a seemingly dry sample of MSW will generate moisture, and this free
water must be evaporated. The energy required may be substantial and may result in an inefficient
combustion process. Subtracting the latent heat of vaporization of water provides a lower heat of
combustion; this value represents the net heat available during the incineration of MSW.
The high and low heating values can be estimated from composition data of the material. The
higher heat value (HHV) is calculated using the equation

HHV (MJ/kg) 0.339 (C) 1.44 (H) – 0.139 (O) 0.105 (S) (4.6)

The lower heat value (LHV) is calculated as

LHV HHV (in MJ/kg ) – 0.0244 (W 9H) (4.7)

where W represents the mass % of water and H the wt % of H in the waste.
The as-received heat value of a waste is approximately proportional to the carbon content of the
waste. The heat values of plastics and, to a lesser extent, of paper, are among the highest because
of their high carbon content and relatively low ash and moisture contents. In contrast, yard waste
and food waste, although mostly organic, possess lower heat values because of their high moisture
contents (Liu and Liptak, 2000).
The following four factors must be considered when evaluating MSW as a potential fuel
(Pfeffer, 1992):

● Only dry organic matter yields energy.
● Ash reduces the proportion of organic fuel per pound of MSW.
● Ash retains heat when removed from the furnace, therefore wasting heat.
● Water reduces the amount of organic fuel per pound of MSW and requires a significant
amount of energy for removal (evaporation).

The heat contents for various fractions of MSW appear in Table 4.19.

4.5.4 FUSION POINT OF ASH

The fusion point of ash provides information on its physical behavior under high temperatures, i.e.,
softening and melting. The fusion point of MSW ash is the temperature at which the ash from waste
combustion forms “clinker” by fusion and agglomeration. The fusion point should, ideally, corre-
late with the potential for boiler fouling by the ash.