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 gener­ates 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, sili­cates, 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 thermocat­alytic process whereby organics react to hydrocarbons as the main product [12].

The conversion of bacterial biomass or organic residues from render­ing plants to oil may be formally defined by considering the starting materials and the end products. The principal components of these sub­strates are proteins and lipids. They make up about 60-80% of this bio­mass. The average elemental composition of neutral lipids is C50H92O6. An empirical formula for proteins is (C70H135N18O38S)x. From these com­pounds, nonpolar hydrocarbons of the general elemental composition CnHm have to be produced [13, 55].

Obviously, LTC removes the heteroatoms from both principal com­ponents. In general, it splits off functional groups from complex bio­mass. 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 ammo­nia (NH3), dihydrogensulfide (H2S), water (H2O), and carbon dioxide (CO2). This decomposition scheme may serve as a model for the for­mation of coal from primarily plant sources. Carbohydrates (C6H10O5)n are the principal components in plants. The elimination of water from carbohydrates produces elemental carbon, according to the following reaction:

(C6HMO5)n — 5H2O — Cm

Consequently, carbohydrates of bacterial mass will be converted to carbon, mainly in the form of graphite [56, 57]. Therefore, the forma­tion of oil from complex biomass will always be accompanied by the for­mation of carbon. Figure 8.18 depicts the mechanism for the production of oil from lipids by LTC

It is worth mentioning that the ash content (Table 8.9) includes nat­ural catalysts (e. g., alumina and silicates) that substantially influence the yield and composition of LTC products. Table 8.9 shows results of the conversion of these organic residues. Yields of oil, solid product, water, volatile salts (NH4Cl, NaHCO3), and noncondensable gases (NCG: CO2, H2, C-1-C-4 alkanes and different alkenes) are given in Fig 8-19. Digested sludge produces less oil than aerobically stabilized sludge. This correlates with the carbon content in Table 8.9. The food chain of anaerobic bacteria efficiently removes organic carbons as biogas (CH4/CO2). Thus it is no longer available for the production of oil in sub­sequent LTC. AM shows higher yields of oil due to its higher content of fat and proteins (Table 8.9). The viscosities of untreated oils at 40oC are as follows: DS, 14 mm2/s; AS, 35 mm2/s; AM, 27 mm2/s; and MBM, 21 mm2/s. In comparison, diesel from a filling station has a viscosity of

about 4 mm2/s. The solid products consist of carbon, nonvolatile salts (e. g., CaKPO4), and metal oxides or sulfides. Especially in the case of AM and MBM, the solid product is of commercial interest due to its high content of phosphate. It is free of proteins [59].

As with natural crude oils, the hydrocarbon mixtures obtained by LTC of lipids containing biomass are of a highly complex composition. For example, Fig. 8.20 shows the gas chromatogram of oil derived from

sewage sludge AS [61]. Peaks assigned by numbers correspond to the aliphatic, unbranched saturated hydrocarbons. The peak appearing before the n-alkane corresponds to the n-alkenes.

The predominant aliphatic nature of oils produced is readily ascer­tained by NMR spectroscopy. Figure 8.21 depicts the 1H-NMR spectro­gram of oil from DS with about 5% of aromatic protons.

Infrared spectroscopy (see Fig. 8.22) reveals the presence of C-H — stretching frequencies at 2850-3000 cm-1. In addition, the spectrum provides clear evidence of hydrogen bonding due to a broad absorption band of 3350 cm-1. Thus, decarboxylation of lipids in the presence of in situ catalysts is not complete. This is consistent with the higher vis­cosities in comparison to diesel. A special loop reactor for recycling catalytic activity to overcome these problems has been designed [62].

Hydrocarbons are derived from both lipids and proteins in the sewage sludge in the presence of in situ catalysts. However, oil produced from proteins under anaerobic LTC conditions is high in nitrogen and sulfur: Amines, purins, and mercaptanes are trace contaminants that are formed. Consequently, this oil smells and is a nuisance, and upgrad­ing (e. g., over H-ZSM-5 as catalyst) is essential [64]. The useful oil is

produced from lipids. When sewage sludge was spiked with triolein, representative of unsaturated triglycerides, the compound did not sur­vive the LTC [65]. As a result, sludge was extracted with toluene using a Soxleth extraction method to yield 12 wt.% lipids. Pyrolysis of sewage sludge lipids over activated alumina produced liquid hydrocarbons containing mostly alkanes [65]. Even the carboxylic acid fractions of the lipids that were separated were completely converted. This is in contrast to direct sewage sludge LTC, where long-chain carboxylic acids are detectable in the IR spectrum (see Fig. 8.22). The reason is the lower content of catalytically active in situ material. Pyrolyzed liquid products from sewage sludge lipids contain virtually no nitro­gen or sulfur (see Table 8.10). Only this liquid has a potential for use as a base for commercial fuels [65].

8.5 Conclusion

The potential offered by lipids for alternative fuel and chemicals is widely recognized. Various sources from plant seeds to animal fat are commercially available. Cracking converts polar esters into nonpolar hydrocarbons. Highly efficient conversion technology should include use of catalysts, e. g., zeolites such as H-ZSM-5 or Y-type representatives. At 380-450oC, alkanes and alkenes are predominantly found in the liquid product. With increasing temperatures up to 550OC, the product spectrum shifts to alkylbenzenes with 1,3,5-trimethylbenzene as the main product. For commercial fuel production based on lipids, assess­ment of oxidation stability and deposit formation are essential. Influences on regulated and nonregulated emissions have to be analyzed. Attention should be paid both to the NOx content of exhaust gas and to the particle size distribution with special focus on ultrafine particles. In addition, mutagenic tests for potency of particulate matter extracts are recommended. Finally, it has to be kept in mind, that the replacement of fossil fuels by biofuels may not bring the intended climate cooling due to the accompanying emissions of N2O from the use of N-fertilizers in crop production. Much more research on the sources of N2O and the nitrogen circle in connection with biofuels from lipids is needed.