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Cracking of Lipids for Fuels and Chemicals 241

8.5 Cracking by In Situ Catalysts
This method is applicable for cellular biomass containing lipids, e.g.,
sewage sludge or organic residues from rendering plants. The
European Union is looking for new markets for both materials. On the
one hand, treatment of municipal and industrial wastewaters gener-
ates huge quantities of sludge, which is the unavoidable by-product
especially if biological processes are used. Management of this residue
poses an urgent problem. The residue contains about 60% of bacterial
biomass and up to 40% of inorganic materials such as alumina, sili-
cates, alkaline and alkaline earth elements, phosphates, and varying
amounts of heavy metals [56]. On the other hand, returning animal
meal (AM) or meat and bone meal (MBM) from the rendering plant into
the food cycle is forbidden by law since the BSE crisis [50, 51]. Besides
burning, low-temperature conversion (LTC) of these organic materials
offers an alternative disposal method [52–54]. LTC is a thermocat-
alytic process whereby organics react to hydrocarbons as the main
product [12].
The conversion of bacterial biomass or organic residues from render-
ing plants to oil may be formally defined by considering the starting
materials and the end products. The principal components of these sub-
strates are proteins and lipids. They make up about 60–80% of this bio-
mass. The average elemental composition of neutral lipids is C H O .
6
92
50
An empirical formula for proteins is (C H 135 N O S) . From these com-
38
x
70
18
pounds, nonpolar hydrocarbons of the general elemental composition
C H have to be produced [13, 55].
n
m
Obviously, LTC removes the heteroatoms from both principal com-
ponents. In general, it splits off functional groups from complex bio-
mass. The process operates at moderate temperatures (380–450 C),
essential atmospheric pressure, and the exclusion of oxygen. Under
these conditions, heteroatoms from organics are removed as ammo-
nia (NH ), dihydrogensulfide (H S), water (H O), and carbon dioxide
3
2
2
(CO ). This decomposition scheme may serve as a model for the for-
2
mation of coal from primarily plant sources. Carbohydrates (C H O )
6
10
5 n
are the principal components in plants. The elimination of water from
carbohydrates produces elemental carbon, according to the following
reaction:
H O ) – 5H O → C
(C 6 10 5 n 2 m
Consequently, carbohydrates of bacterial mass will be converted to
carbon, mainly in the form of graphite [56, 57]. Therefore, the forma-
tion of oil from complex biomass will always be accompanied by the for-
mation of carbon. Figure 8.18 depicts the mechanism for the production
of oil from lipids by LTC [58].