Other processes can also be used to produce high-quality diesel fuel from vegetable oil or animal fat. Less appropriately, pyrolysis and hydrogenation products, diesel-vegetable oil blends, microemulsions of alcohols and water in vegetable oils and fermentation butanol are sometimes also referred to as biodiesel; in fact, they are just alternative (renewable) diesel fuels.

For use as diesel fuels vegetable oils and animal fats with high viscosity and low volatility must be modified in order to gain sufficient engine compatibility. Viscosity values of vegetable oils typically vary between 27.2 and 53.6 mm2/s, whereas those of vegetable oil methyl esters are between 3.59 and 4.63 mm2/s. Apart from transesterification with low alcohols (MeOH, EtOH) various other processes exist to improve the fuel combustion characteristics of biomass (in particular vegetable oils and fats) by reduction of viscosity:

• blending (dilution) of vegetable oil with other fuels (diesel, alcohols);

• microemulsification with short-chain alcohols (MeOH, EtOH);

• thermal treatment (pyrolysis, gasification) producing alkanes, alkenes, carboxylic acids and aromatic compounds;

• catalytic cracking to (cyclo)alkanes;

• hydrotreating; and

• ozonation.

Among these primary technologies only dilution, microemulsification and heating do not chemically alter the raw material.

Co-solvent blending of vegetable oils with other fuels like alcohol may bring the viscosity to within specification. Microemulsions cannot generally be recommended for long-term use in diesel engines (see Section 4.3). The development of stable bio-crude oil (BCO)/diesel oil emulsions is under way [51, 52].

Pyrolysis is a thermochemical conversion process in conditions of significantly less oxygen than required for complete combustion, which offers a way to convert (solid) biomass into an easily stored and transported liquid or bio-oil for use in producing chemicals and fuels [53]. Torrefaction is a mild form of low-oxygen pyrolysis (573 K) producing biomass without moisture and volatiles. In pyrolysis, the feedstock is subjected to moderate-to-high temperatures in the absence of oxygen and degrades to a liquid (preferred product), gaseous and solid phase. The product distribution depends on biomass composition and pyrolysis conditions (T, AT, t). Table 1.6 shows

various pyrolysis conditions and main products. Pyrolysis modes are flash or ultrafast (up to 1000 K/s with rapid condensation of vapour products), fast (300 K/min) and slow (30 K/min), all usually up to 773-873 K. Processes can be adjusted to favour charcoal, pyrolytic oil, gas or methanol production with a fuel-to-feed efficiency up to 95%. Slow heating rates tend to favour production of volatile gases, organic acids and aldehydes, mixed phenols and char. High heating rates tend to minimise char formation and liquid production and maximise gas production. Pyrolysis can be used for the production of bio-oil by various processes (e. g. flash pyrolysis) [53-56]. The yield of pyrolytic liquid, also known as bio-oil, depends on pyrolysis conditions, including relatively short residence times tr (0.5-2 sec.), moderate temperatures (673-873 K), and rapid quenching at the end of the process. The multi-component liquid is highly oxygenated, and contains up to 15-20% water. The elemental composition of bio-oil resembles that of biomass (cellulose, hemicellulose and lignin) rather than that of petroleum oils. Fast pyrolysis technologies for energy application have recently entered the stage of commercialisation (25 kt/yr; DynaMotive, West Lorne, Canada). Bio-crude oil (BCO) use for power and heat production represents a main goal in the biomass sector. BCOs have already been used in modified diesel engines (see Section 4.2). Their use in internal combustion engines offers several advantages in terms of logistics (compared to bio-energy from solid biomass) as well as in terms of power-to-heat ratio and efficiency. Direct use of pyrolysis oils requires significant adaptations of technology to fuel characteristics. Other problems with bio-oil include high corrosivity. The biomass pyrolysis-to-energy chain is not yet fully developed.

The pyrolysis technology that is closest to commercialisation is the pyrolysis of high lignin-containing lignocelluloses. Ensyn (Canada)’ s Rapid Thermal Processing (RTP) can use sawmill residuals and construction and demolition wood waste as well as fast-growing wood species such as willow and poplar, liquefying 75% of the original in a fast pyrolysis process to ‘bio-oil’.

Pyrolysis and cracking of oils and fats produce compounds that are smaller than triglyceride molecules and are, therefore, more suitable as a fuel. In petroleum refining hydrocracking is widely used for converting various fractions to lighter and more valuable products, especially distillates such as jet fuels, diesel oils and heating oils. Ester-containing vegetable oils have been converted into hydrocarbons by pyrolysis at 573-973 K in the presence of a catalyst [57]. Pyrolysis or thermal catalytic cracking has been applied to various oilseed feedstocks, including soybean [58, 59], rapeseed [60, 61], tropical vegetable oils (babassu, piqui, palm, copra) [62, 63], tung oilseeds [64], soybean oil cake [65] and sunflower oil cake [66]. Pyrolysis and hydrocracking of SBO, hydrogenolysis of palm oil (PMO) and rapeseed oil (RSO), and hydrocracking of SBO and babassu oil were reviewed by Dunn [67]. Cracking of canola oil (used as a model compound for waste vegetable oils) to fuels and chemicals at 673-773 K using a variety of catalysts yields mainly Cx-C5 hydrocarbon gases, liquid aromatics and aliphatics, and coke [68]. Da Rocha Filho et al. [69] studied catalytic hydrocracking of vegetable oils obtained from Passiflora edulis (maracuja), Astrocaryum vulgare (tucuma), Mauritia flexuosa (buriti), Orbignya martiana (babassu) and soybean. In catalytic cracking of triglycerides (TGs) using a variety of catalysts, including SiO2/Al2O3 [63], NiMo/g-Al2O3 [69], Ni/SiO2 [70], Al2O3 [71], MgO [71], zeolites [72-74], a gasoline-like fuel is more likely to be formed than a diesel-like fuel. Bio-oil was obtained in tubular reactor fixed-bed hydropyrolysis experiments from extracted Euphorbia rigida at a hydrogen pressure of 15 MPa at 823 K [75]. The main shortcoming of such biofuels lies in the economics of the process with the use of expensive vegetable oils as new raw materials.

Pyrolysis of vegetable oilseeds avoids the complicated and expensive extraction step [60]. Some drawbacks of pyrolysis are:

• use of tremendous amounts of heat;

• production of a wide range of compounds due to low selectivity;

• removal of oxygen from substrate molecules (thus eliminating one of the benefits of oxygenated fuels); and

• creation of solid residues of ash and carbon, thus requiring additional separation steps.

Vegetable oils and animal fats can be hydrogenated catalytically under formation of a product that can be used as a commercial fuel (see Section 15.4). Finnish Patent No. FI 100,248 to Aalto et al. (to Neste Oy) [76] describes a two-step process for producing middle distillate from vegetable oil by hydrogenating fatty acids of vegetable oil or triglycerides, to give n-paraffins, and then branched-chain paraffins by isomerising the n-paraffins.

Neste Oy has recently commercialised a renewable diesel process based on hydrotreated vegetable oils (HVOs) and animal fats [45, 77] (see also Section 15.4.1). US Patent Appl. No. 2006/0207166 A1 to Herskowitz et al. [78] describes a one-step hydrodeoxygenating/hydroisomerising process for producing a low-sulphur diesel fuel composition (mixture of C14 to C18 (iso)paraffins) from vegetable and/or animal oil using a dual-functional Pt/ SAPO-11 catalyst at 643-683 K at 2-4 MPa. Hydrocracking was inhibited. Composition and fuel characteristics of the produced diesel may be adjusted by varying the vegetable and/or animal starting product, process conditions and catalyst used. Paraffinic diesel fuel compositions provide superior fuel properties, especially for low-temperature performance (e. g. density, viscosity, cetane number, lower heating value, cloud point and CFPP), to FAME biodiesel.

US Patent No. 6,364,917 B1 to Matsumura and Murakami [79] discloses a method of converting virgin plant oils and/or waste vegetable oils into (diesel) fuel, by heating the oil, mixing the oil with water and/or ozone and agitating the mixture of oil and water dissipating the ozone. Ozonation (aeration with ozone-oxygen mixed gas) at close to normal temperature (313-353 K) involves no other chemicals aside from ozone (i. e. electrical power) and readily allows transformation of waste oils and requires low plant construction costs. The end product has lower average molecular weight than petrodiesel as a result of formation of ozonides (1,2,4-trioxolane rings) and bond rupture in the fatty acids composing the vegetable oil (i. e. aldehyde formation) [80]. Obviously, the chemistry of the resulting fuel differs totally from that of hydrotreating, pyrolysis or transesterification technology. As ozonation hardly produces any post-refining waste matter it thus shows remarkable efficacy as a recycling system.