Solid Waste
One of the difficulties with refuse is the variability of its chemical composition. For a study comparing various refuse incineration and gasification processes, the State Environmental Office of Nordrhein-Westfalen developed a “standard refuse” composition, which is given in Table 4-19. The lower heating value was specified as lOMJ/kg.
Loffler (1998) reports the composition of other types of refuse as specified for a circulating fluid-bed gasifier in Riidersdorf.
Care should be exercised, however, when evaluating such data, which is very locality specific. In particular, local regulations on separation and recycling of household waste including plastics can have a dramatic effect on the heating value.
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Table 4-19 “Standard Refuse” in Nordrhein-Westfalen, Germany |
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Composition |
Mass% |
Ash Composition |
kg/1000 kg |
|
C |
27.16 |
SiOz |
110 |
|
H |
3.45 |
ai2o3 |
34 |
|
о |
18.39 |
CaO |
31 |
|
N |
0.30 |
Fe |
30 |
|
s |
0.20 |
Na20 |
15.2 |
|
Cl |
0.50 |
Fe203 |
15 |
|
Moisture (H20) |
25.00 |
MgO |
4.5 |
|
Ash |
25.00 |
A1 |
4 |
|
Total |
100.00 |
k2o |
3 |
|
Zn |
1.5 |
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|
Pb |
1.0 |
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|
Cu |
0.5 |
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|
Cr |
0.2 |
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|
Ni |
0.075 |
||
|
Cd |
0.01 |
||
|
Hg |
0.005 |
||
|
As |
0.005 |
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Source: Berghoff 1998 |
Feedstock Sizing. Typically, solid wastes in the “as-received” state have a highly irregular and asymmetrical lump size. Both for reasons of transport into and in the reactor, size reduction to a fine homogeneous material is required. Drying is in most cases also a requirement to ensure a smooth-flowing pneumatic transport.
There are a number of different approaches to pre-preparation. Most of these involve mechanical shredding and metals removal using magnetic and electric devices. Much refuse cannot be ground to a sufficient degree of fineness to permit slurry or dense phase fluid transport. Thus mechanical transport or dilute fluidized transport are generally the only possibilities. The latter type of transport cannot be economically performed under pressure, for then relatively large amounts of transport gas would be required, which would render the process uneconomical.
Gorz (1998) has proposed an initial pyrolysis of the refuse simply to ensure a material that can then be ground to a size suitable for an entrained-flow reactor.
Moving bed processes such as BGL have a different requirement from fluid-bed and entrained-flow reactors. The feedstock is transported mechanically and introduced into the reactor via lock hoppers (which allows pressure operation), but the reactor bed can tolerate only a limited amount of fines without excessive pressure drop.
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Table 4-20 Various Feeds for a CFB Refuse Gasifier |
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|
Lignite Char |
Coal |
Old Wood |
Refuse |
Rubber |
|
|
C, Wt% |
9.05 |
78.00 |
43.40 |
40.00 |
64.40 |
|
H, wt% |
0.55 |
4.61 |
4.77 |
5.69 |
10.62 |
|
O, wt% |
1.56 |
4.12 |
30.88 |
29.25 |
5.63 |
|
N, wt% |
0.13 |
1.14 |
0.23 |
0.79 |
0.76 |
|
S, wt% |
0.54 |
0.45 |
0.10 |
0.22 |
1.50 |
|
Cl, wt% |
0.00 |
0.08 |
0.02 |
0.75 |
0.79 |
|
Ash, wt% |
78.17 |
9.60 |
1.50 |
17.30 |
14.10 |
|
Moisture, wt% |
10.00 |
2.00 |
19.10 |
6.00 |
2.20 |
|
Total, wt% |
100.00 |
100.00 |
100.00 |
100.00 |
100.00 |
|
LHV, MJ/kg |
3.3 |
31.4 |
15.4 |
15.9 |
34 |
|
Source: Lojfler 1998 |
Thus a lump size of 20-70 mm is desirable. Material preparation therefore includes a pelletizing step in addition to initial shredding, metals removal and drying (Hirschfelder, Buttker, and Steiner 1997).
Heating Value. As indicated in Tables 4-19 and 4-20, the heating value of waste material can vary considerably. The drying required to facilitate pneumatic transport will in many cases be sufficient to ensure satisfactory operation of the gasification process. One should recognize, however, that wastes with excessive inert material cannot be gasified without a support fuel. The lower limit is 7-8 MJ/kg (Gorz 1998).
