Despite the bright prospect of biodiesel production, efforts to commercialize it have been very limited. One of the major obstacles has been the high price associated with refined oil feedstock, which makes up nearly 70−75% of the total production costs. Hence, in order to reduce the cost of biodiesel production, using cheaper feedstocks such as waste oil or low-quality oil has been proposed. Sewage sludge (SS), a relatively inexpensive feedstock, is a promising raw material for such a purpose. However, its use has been challenging as a result of impurities such as water and large amounts of free fatty acids (FFAs), which are common in most waste materials. Thus, economically viable biodiesel conversion technology must be developed and implemented.

...Although these catalysts [for transesterification] are relatively cheap, they are very sensitive to FFAs and the water content in oils, since the side reactions of saponification and hydrolysis, respectively, can occur. It is well-known that the conventional catalytic processes used for biodiesel production cannot efficiently handle such inexpensive lipid feedstocks without additional pretreatment steps, which add extra processing costs.

...the main objective of the present work was to develop and mechanistically validate the simplest non-catalytic transesterification process for the conversion of the lipids extracted from SS into biodiesel in a continuous system under ambient pressure. This work also proved that efficient non-catalytic biodiesel conversion could be achieved by using only activated alumina (Al₂O₃) in the presence of carbon dioxide (CO₂). Moreover, this study showed the economic feasibility of biodiesel production from SS.

—Kwon et al.

Biodiesel conversion at various temperatures (a) and various volumetric ratios of MeOH to oil at 380 °C (b). Credit: ACS, Kwon et al. Click to enlarge.

The researchers postulated that temperature could be the main driving force for the transesterification reaction for non-catalytic biodiesel conversion in a continuous flow system—i.e., the activation energy can be reached by providing thermal energy. The activation energy of the transesterification reaction (11.2 kJ mol⁻¹) is lower than those of other catalytic applications, such as methane reforming (209.2 kJ mol⁻¹), they noted.

In general, triglycerides start to evaporate at ∼400 °C, and thermal cracking is observed at ∼550 °C. Thus, they said, non-catalytic biodiesel conversion should be carried out at temperatures lower than 550 °C. In addition, the use of a porous material such as activated alumina provides a way to increase the contact time for the triglycerides and methanol.

In their experiments, they used a tubular reactor (TR) packed with activated alumina. A split-hinged furnace delivered the experimental temperature in the range of 250−500 °C; an insulation collar at the end of the furnace was used to block heat transfer during operation and to protect the quartz tubing. The lipids extracted from SS and methanol were continuously fed into the tubular reactor using a gear pump and a high-performance liquid chromatography (HPLC) pump.

The heterogeneous transesterification reaction mainly occurs in the pores, and the reaction rate is enhanced by the thermal energy. Then, the converted biodiesel and byproduct (glycerin) are eluted from the pores as a result of their relatively lower boiling points compared to that of the triglycerides. Considering the average molecular size of triglycerides (i.e., 2 nm), a porous material having meso- and macropores would be suitable for the non-catalytic transesterification reaction. In addition, this biodiesel conversion was free from saponification, since no basic catalyst such as KOH or NaOH was used. This observation allowed us to combine the esterification of FFAs and transesterification of triglycerides into a single process.

—Kwon et al.

They also found that non-catalytic biodiesel conversion can be enhanced by the presence of carbon dioxide. Utilizing CO₂ expedited the rate of the transesterification reaction without leading to the thermal cracking of the oil feedstock and provided favorable conditions for the transesterification reaction by means of impeding the reversible transesterification reaction.

In their results, they found that the C₁₆₋₁₈ range of biodiesel is most abundant. Although thermal cracking of the resultant biodiesel was observed as a result of the relatively high experimental temperatures, the amounts of cracked chemical species are negligible. Some aromatic compounds were also identified. Among them, the most abundant aromatic compound was toluene. Compared with those of other aliphatic hydrocarbons, they noted, the amount of toluene was negligible. As these aliphatic and aromatic compounds are common in petroleum-derived diesel, these chemical species should not be problematic, they suggested.

They concluded that the projected yield of oil from SS is 980 000 L ha^–1 year^–1—superior to those from microalgal and soybean oils, which are 446 and 2,200 L ha^–1 year^–1, respectively. According to their case study in South Korea, the price of the lipids extracted from SS was approximately US$0.03 L^–1—lower than those of all current biodiesel feedstocks.

The new process for converting the lipids extracted from SS shows high potential to achieve a breakthrough in minimizing the cost of biodiesel production owing to its simplicity and technical advantages, as well as environmental benefits, they concluded.

Resources

• Eilhann E. Kwon, Sungpyo Kim, Young Jae Jeon, and Haakrho Yi (2012) Biodiesel Production from Sewage Sludge: New Paradigm for Mining Energy from Municipal Hazardous Material. Environmental Science & Technology doi: 10.1021/es3019435