The biology of an “oxidation pond” is not well understood. The algae-versus — bacterial growth needs to be controlled, and anaerobicity and temperature need to be maintained properly. The carbonaceous matter tends to ferment, and methane is produced instead of carbon dioxide. The end product, methane, can be used either as a direct fuel or through a suitably designed fuel cell. Microbial methane and hydrogen production are discussed later.

The general reaction for ethanol production during fermentation is

In this reaction, the microorganisms work as a catalyst.

Crop description. Madhuca indica—commonly known as madhuka, yappa, mahuda, mahua, mauwa, mohwa, hippe, butter tree, mahwa, mahula, or elupa—belongs to the family Sapotaceae and grows up to 21 m high. This deciduous tree is distributed mainly in India (see Fig. 4.9). The kernels are 70% of seed by weight. Seeds content includes 35% oil and 16% protein. Main fatty acids are palmitic acid (16-28.2%), stearic acid (20-25.1%), arachidic acid (3.3%), oleic acid (41-51%), and linoleic acid (8.9-13.7%) [106].

Main uses. Traditionally, it has been used as a source of natural hard fat in soap manufacture. The seed oil is used as an ointment in rheuma­tism and to prevent dry, cracked skin in winter. It is used in foods, cos­metics, and lighting. The cake presents toxic and bitter saponins that preclude its use as animal feed. However, mahua cake can be used as organic manure [106]. Several approaches to produce biodiesel can be found. Ghadge and Raheman have proposed a two-step pretreatment to reduce high FFA levels. Transesterification was carried out adding 0.25 v/v methanol and 0.7% KOH. Fuel properties were found comparable to those of diesel fuel [107]. Some authors have proposed different successful

alternatives to produce biodiesel: ethanol and sulfuric acid, and methanol and NaOH [108-110]. Puhan et al. have found better diesel engine performance for methyl esters compared to ethyl and butyl esters, while ethyl esters show lower NOx emissions compared to the rest [111]. Systematic studies on the lubrication properties of biodiesel have shown that the preferred range of blending with diesel fuel is 5-20% [51].

Hawkins et al. [53] have conducted combustion studies on methyl and ethyl esters of degummed sunflower oil, maize oil, cottonseed oil, peanut oil, soybean oil, and castor oil. Fuel properties of the esters were very sim­ilar to each other, except the esters of castor oil which were much more vis­cous. The heating values of ethyl esters were also considerably lower. Engine results indicated that the power output for esters varied from 44.4 to 45.5 kW, with diesel delivering 45.1 kW. The brake thermal efficiencies were also slightly higher than diesel. High esterification yields (around 90%) must be obtained to avoid choking of injector tips. Further, sticking of injec­tor needles after a shutdown time of 48 h has been reported.

Fort and Blumberg [54] have tested a diesel engine with a mixture of cottonseed oil and ME of this oil. Results indicate that viscosity and den­sity increased whereas the heating value and the cetane number decreased, when the percentage of the cottonseed oil was increased in the blend. The durability test with 50-50% cottonseed oil and ME was terminated after 183 h of running the engine, because the engine was noisy. After disassembly, the engine indicated severe wear scouring and

TABLE 6.2 Comparison of Biodiesel Production by Acid, Alkali, and Enzyme

Acid-catalyzed Base-catalyzed

transesterification transesterification

heavy carbon deposits. But specific emissions and visible smoke char­acteristics of diesel fuel and esterified cottonseed oil were comparable.

Ziejewski and Kaufman [55] conducted a long-term test using a 25-75% blend of alkali-refined sunflower oil and diesel fuel in a diesel engine, and compared the results with that of a baseline test on diesel fuel. Engine power output over the tested speed range was slightly higher for this blend. At 2300 rpm, the difference was 25%. At 1800 rpm, the gain in power was 6%. The smoke level increased at a higher engine speed from 1 to 2.2 and decreased at a lower engine speed. Greater exhaust temperature was caused by a higher intake air temperature. The major problems experienced were:

1. Abnormal carbon buildup in the injection nozzle tips.

2. Injector needle sticking.

3. Secondary injection.

4. Carbon buildup in the intake port and exhaust-valve stems.

5. Carbon filling of the compression ring grooves.

6. Abnormal lacquer and varnish buildup.

Tahir [56] has determined the fuel properties of sunflower oil and its ME. The properties were favorable for diesel engine operation, but the problem of high viscosity (14 times higher than diesel at 37°C) of sunflower oil might cause blockage of fuel filters, higher valve-opening pressure, and poor atomization in the combustion chamber. Transesterification of sun­flower oil to its ME has been suggested to reduce viscosity of the fuel. The viscosity of ME at 0°C was closer to that of No. 2 diesel fuel, but below 0°C, it was not possible because of the pour point of — 4°C.

Pryor et al. [57] have conducted a short-term performance test on a small, test diesel engine using crude soybean oil, crude degummed soy­bean oil, and soybean ethyl ester. The engine developed about 3% more power output with crude legume soybean oil, but the development was insignificant with soybean ethyl ester. The fuel flow of soybean oil was 13-30% higher and for the ethyl ester it was 11-15% higher, depending upon the load on the engine. The exhaust temperature throughout the test was 2-5% higher for soybean oil and 2-3% lower for ethyl ester than the diesel fuel.

Clark et al. [58] have tested methyl and ethyl esters of soybean oil as a fuel in CI engine. Esters of soybean oil with commercial diesel fuel additives revealed fuel properties comparable to diesel fuel, with the exception of gum formation which manifested itself in problems with the plugging of fuel filters. Engine performance with esters differed little from the diesel fuel performance. Emissions of nitrous oxides for the esters were similar, or slightly higher than diesel fuel. Measurement of engine wear and the fuel injection test showed no abnormal charac­teristics for any of the fuels after 200 h of testing.

Laforgia et al. [59] has prepared biodiesel from degummed vegetable oil with 99.5% methanol and an alkaline catalyst (KOH). On engine performance, pure biodiesel and blends of biodiesel combined with 10% methanol had a remarkable reduction in smoke emissions. When the injection timing was advanced, better results were obtained.

Pischinger et al. [60] have conducted engine and vehicle tests with ME of soybean oil (MESO) 75-25% gas oil—MESO blend and 68-23-9% gas oil—MESO—ethanol blend. The fuel properties of the blend indi­cated a 6% lower volumetric calorific value of the ester, a drastic reduc­tion in kinematics viscosity, and a greater ethane number than that of gas oil. The engine results indicated about 7% higher BSFC with a mar­ginal difference in power and torque in comparison with gas oil. The smoke emission was much lower with ME.

Ali et al. [61] have observed that engine performance with diesel fuel—methyl soyate blends did not differ to a great extent up to a 70-30% (v/v) from that of diesel-fueled engine performance. There was a slight increase in NOx emissions with increasing methyl soyate con­tent in the blends at higher speeds but at lower speeds there was a quadratic trend with diesel fuel content.

Carbon monoxide emissions were very similar for blends up to 70-30% (v/v) diesel fuel—methyl soyate blends at any speed. Visible smoke decreased with increasing speed and methyl soyate content. More smoke was produced with neat diesel fuel at full load.

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.

There are essentially two types of reactions in photosynthesis: a series of light-dependent reactions that are temperature independent (or light reaction) and a series of temperature-dependent reactions that are light

independent (or dark reactions). The rate of the light reaction can be increased by increasing light intensity, and the rate of the dark reaction can be increased by increasing temperature to a certain extent (see Fig. 2.2).

Ethanol can be produced by using continuous flow fermentors arranged in a series with complete sugar utilization or high ethanol concentra­tion. With two fermentors arranged in a series, the retention time can be chosen so that the sugar is only partially utilized in the first, with fermentation completed in the second. Ethanol inhibition is reduced in the first fermentor, allowing a faster throughput. The second, lower — productivity fermentor can now convert less sugar than if operated alone. For high product concentration, productivity of a two-stage system has been 2.3 times higher than that of a single stage [47, 75].

A two-stage continuous ethanol fermentation process with yeast recir­culation is used industrially by Danish Distilleries Ltd., Grena, for molasses fermentation (see Fig. 3.8). Two fermentors with 170,000-L volume produce 66 g/L ethanol in 21-h retention time [76].

Research in most of the nonedible oil crops previously mentioned has been insufficient. To determine the viability of their use as a source of biodiesel and to optimize the transesterification as well as engine per­formance, more research is needed. But, there are also other nonedible and low-cost edible oily crops and trees that could be exploited for biodiesel production. Amongst them, allanblackia, bitter almond, chaul — moogra, papaya, sal, tung, and ucuuba produce oils that hold immense potential to be used as a raw material for producing biodiesel. Most of them grow in underdeveloped and developing countries, where govern­ments may consider providing support to the activities related to col­lection of seeds, production of oil, production of biodiesel, and its utilization for cleaner environment. Hence, to facilitate its integration, a legal framework should be legislated to enforce regulations on biodiesel. Biodiesel should be seriously considered as a potential source of energy, particularly in underdeveloped and developing countries with very tight foreign exchange positions and insufficient availability of traditional fuels.

This method is the easiest but requires anhydrous ethanol, because methanol has limited solubility. A maximum of 10% diesel can be sub­stituted due to the lower solubility of methanol in diesel. No component changes; only adjustments of injection timing and fuel volume delivery are required to restore full power. Dodecanol is an effective surfactant for methanol-diesel fuel blends. Straight-run gasoline is an economical additive for ethanol-diesel blends.

Solubility of alcohols in diesel fuels is a function of (a) fuel tempera­ture, (b) alcohol content, (c) water content, (d) specific gravity of diesel, (e) wax content, and (f) hydrocarbon composition. Methanol solubility in diesel increases as the aromatic content goes up.

7.7.1 Alcohol-diesel fuel emulsions

Here, an emulsifier extends the water tolerance of alcohol-diesel blends. In general, equal volumes of alcohols and emulsifiers are required for suitable emulsions. No component changes, but injection volume and timing are adjusted for diesel fuel with alcohol then solutions, i. e., up to 35% diesel substitution is possible. Addition of ignition improvers, e. g., cyclohexanol nitrate, up to 1% helps increase the alcohol percentage up to 35% while maintaining a cetane rating at permissible levels. Cost of emulsifiers and poor low-temperature physical properties of emulsions limit the use of this technique. Stable emulsion requires the use of costly surfactants. Using higher-order alcohols improves the stability of blends at temperatures as low as —20oC.

A fuel cell power system requires the integration of many components. The fuel cell produces only dc power and utilizes only certain processed fuels. Besides the fuel cell stack, various components are incorporated in a fuel cell system. A fuel processor is required to allow operation with conventional fuels; a power conditioner is used to tie fuel cells into the ac power grid or distributed generation system; for high-temperature fuel cells, a cogeneration or bottoming cycle plant is needed to utilize rejected heat for achieving high efficiency. A schematic of a fuel cell power system with interaction among various components is shown in Fig. 9.13.