Page 112 - Materials Chemistry, Second Edition

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Characterization of Solid Waste 83
● Fixed carbon, the combustible residue left after volatile matter is removed; ignition at
600 to 900°C.
● Ash, the weight of residue after combustion in an open crucible.

Moisture content and ash represent the noncombustible component of the MSW. Moisture is
undesirable in MSW as it adds weight to the fuel without adding to the heating value. In addition,
moisture content will adversely affect heat release from the fuel. Ash similarly adds weight without
providing heat energy. Furthermore, ash retains heat when removed from the furnace; as a result,
potentially useful heat is lost to the environment.
The volatile matter and the fixed carbon content are the preferred indicators of the combustion
capability of MSW. The volatile matter is the portion of MSW converted into gas as the tempera-
ture increases. Gasification occurs before the onset of combustion. In many incineration systems
these carbonaceous gases are drawn away from the heating mass and carried to a secondary com-
bustion chamber where combustion of the fuel gas occurs (see Chapter 9). Heat release is rapid and
combustion is complete within a short time (Pfeffer, 1992).
Fixed carbon is the solid carbon residue that has settled on the furnace grates. Combustion
occurs in the solid state, i.e., on the surface of this “char” material. The rate of combustion is
affected by the temperature and surface area of the char. A waste fuel with a high percentage of
fixed carbon will require a longer retention time in the combustion chamber to achieve complete
combustion as compared with a fuel low in fixed carbon (Pfeffer, 1992).
The value for fixed carbon from laboratory results is calculated as follows (Liu and Liptak, 2000):

% fixed carbon 100% % moisture % ash % volatile matter (4.1)

A limitation of proximate analysis is that it does not provide an indication of possible pollutants emit-
ted during combustion. These data are determined by conducting ultimate analysis. Proximate analysis
data for the combustible components of MSW and bulk samples of MSW are presented in Table 4.18.

TABLE 4.18
Typical Proximate Analysis of MSW and MSW Components
Proximate analysis (% by wt)
Waste Type Moisture Volatiles Fixed Carbon Noncombustable (ash)
Food mixed 70.0 21 3.6 5.0
Paper mixed 10.2 76 8.4 5.4
Newspapers 6.0 81 11.5 1.4
Cardboard 5.2 77 12.3 5.0
Plastics mixed 0.2 96 2 2
Polyethylene 0.2 98 <0.1 1.2
Polystyrene 0.2 99 0.7 0.5
PVC 0.2 87 10.8 2.1
Textiles 10 66 17.5 6.5
Yard wastes 60 30 9.5 0.5
Wood mixed 20 68 11.3 0.6
Glass 2 96–99
Metals 2.5 94–99
Domestic 10–40 30–60 3–15 10–30
MSW
Source: Kiely, G., Environmental Engineering, McGraw-Hill, New York, 1997. Reproduced with kind
permission of the McGraw-Hill Companies.