Even though vegetable oils are the main feedstock for the production of first generation biofuels, soon their production has troubled the public opinion due to their abated sustainability and to their association with the food vs. fuel debate. As a result the technology hasshifted towards the exploitation of both solid and liquid residual biomass. Waste Cooking Oils (WCOs) is a type of residual biomass resulting from frying with typical vegetable fryingoils (e. g. soybean-oil, corn-oil, olive-oil, sesame-oil etc). WCOs have particular problems regarding their disposal. In particular grease may result in coating of pipelines within the residential sewage system and is one of the most common causes of clogs and sewage spills. Furthermore, in the cases that sewage leaks into the environment, WCOs can cause human and environmental health problems because of the patho­gens contained. It has been estimated that by disposing 1 lit of WCO, over 1,000,000 of liters of water can be contaminated, which is estimated as the average demand of a single person for 14 years.

Catalytic hydroprocessing of WCO was studied as an alternative ap­proach of producing 2nd generation biofuels [20-24]. Initially catalytic hydrocracking was investigated over commercial hydrocracking catalysts leading not only to biodiesel but also to lighter products such as biogaso­line [20], employing a continuous-flow catalytic hydroprocessing pilot — plant with afixed-bed reactor. During this study several parameters were considered including hydrocracking temperature (350-390°C) and liquid hourly space velocity or LHSV (0.5-2.5 hr-1) under high pressure (140 bar), revealing that the conversion is favoured by high reaction temper­ature and low LHSV. Lower and medium temperatures, however, were more suitable for biodiesel production while higher temperatures offered better selectivity for biogasolineproduction. Furthermore, heteroatom re­moval (S, N and particularly O) was increased while saturation of double bonds was decreased with increasing hydrocracking temperature, indicat­ing the necessity of a pre-treatment step.

However catalytic hydrotreatment was later examined in more detail as a more promising technology particularly for paraffinic biodiesel produc­tion (Figure 1). The same team has studied the effect of temperature (330- 398°C) on the product yields and heteroatom removal[21]. The study was conducted in the same pilot plant utilizing a commercial NiMo/Al2O3hy — drotreating catalyst over lower pressure (80 bar). According to this study, the hydrotreatingtemperature is the key operating parameter which defines the catalyst effectiveness and life. In fact lower temperatures (330°C) fa­vour diesel production and selectivity. Sulfur and nitrogen removal were equally effective at all temperatures, while oxygen removal and satura — tionof double bonds were favoured by hydrotreating temperature. The same team also studied the effect of the other three operating parameters i. e. pressure, LHSV and H2/WCOratio [22]. Moreover they also studied the hydrocarbon content of the products [23] qualitatively via two-dimen­sional chromatography and quantitatively via Gas Chromatographywith Flame Ionization Detector (GC-FID), which indicated the presence of C15-C18 paraffins. Interestingly this study showed that as hydrotreating temperature increases, the contentof normal paraffins decreases while of iso-paraffins increases, revealing that isomerization reactions are favoured by temperature.

The total liquid product of WCO catalytic hydrotreatment was further investigated in terms of its percentage that contains paraffins within the diesel boiling point range (220-360°C)[24]. The properties of WCO, hy­drotreated WCO (total liquid product) and the diesel fractionof the hy­drotreated WCO are presented in Table 3. Based on this study the overall yieldof the WCO catalytic hydrotreatment technology was estimated over 92%v/v. The properties of the new 2nd generation paraffinic diesel prod­uct indicated a high-quality diesel with highheating value (49MJ/kg) and high cetane index (77) which is double of the one of fossil diesel. An ad­ditional advantage of the new biodiesel is its oxidation stability (exceeding 22hrs) and negligible acidity, rendering it as a safe biofuel, suitable for use in all engines. The properties and potential of the new biodiesel were further studied [25], for evaluating different fractions of the total liquid product and their suitability as an alternative diesel fuel.

TABLE 4: Properties of different pyrolysis oils according to literature Types of Pyrolysis Biooils Properties Test Methods [26] [27] [28] [29] [30] [31] [32] pH pHmeter 2.2 2.5 2~3 2.5 Density 15C (Kg/L) ASTM D4052 1.207 1.2 1.15-1.2 1.192 1.2 1.19 HHV (MJ/Kg) DIN51900 17.57 LHV (MJ/Kg) DIN51900 15.83 Solids Content (%wt) Insolubles in Ethanol 0.06 Ash content (%wt) ASTM D482 0.0034 <0.1 0.1 0.15 0-0.2 Pour point ASTM D97 -30 -30 Flash point ASTM D93 48 40-65 40-65 51 Viscosity (cP) @ 40C 40 40-100 40-100 43-1510 Viscosity 20°C 9mm2/s) ASTM D445 47.18 Viscosity 50°C (mm2/s) ASTM D445 9.726 Carbon (%wt) ASTM D5291 42.64 40.1 51.1 ~52 39.17 54-58 39.4-46.7 Hydrogen (wt%) ASTM D5291 5.83 7.6 7.3 ~6.4 8.04 5.5-7 7.2-7.9 Nitrogen (wt%) ASTM D5291 0.1 0.1 ~0.2 0.05 0-0.2 0.2 Sulphur (%wt) ASTM 0.01 0.032 Clorine (%wt) ASTM 0.012 AlkaliMetals (%wt) ICP <0.003 Oxygen (wt%) 52.1 41.6 ~40 52.74 35-40 45.7-52.7