Biofuel Production Using Charcoal and CO2 to Transform Animal Fats

The need for renewable fuels is increasing as the fossil fuel crisis becomes more severe.  Animal fats, an inexpensive source of triglyceride, are a potential cost-effective feedstock for biodiesel production (Kwon et al. 2012).  However, animal fats, which contain up to 6 wt% of free fatty acids (FFA’s), must be pre-treated before undergoing conventional catalytic processes (Crabbe et al. 2001; Naik et al. 2008).  Without pretreatment steps, impurities in the feedstock will react with the catalysts and limit the biofuel yield by producing soap.  Kwon et al. aim to prove that an efficient non-catalytic biodiesel conversion using only charcoal and CO2is possible.  They determined the optimal conditions for this conversion, including temperature, pressure, and feeding ratio of raw materials.  Previous studies on non-catalytic conversion suggest an optimal temperature of 250 °C, a pressure of 10MPa or higher, and a methanol-to-oil molar ratio of 6:1.  In the present study, Kwon et al. determined optimal operating conditions for the conversion of animal fats to biodiesel to be at a temperature of 350500 °C under ambient pressure, and volumetric flow rates of extracted lipid and methanol (MeOH) to be 10 and 3 ml min1, respectively. Shelby Long
Kwon, E. et al., 2012. Transforming animal fats into biodiesel using charcoal and
CO2. Green Chemistry 14, 1799–1804.

                  Kwon et al. analyzed the production process of biodiesel by transforming animal fat into biodiesel using charcoal and CO2.  They obtained cooking oil from a local restaurant, beef tallow and lard from the local slaughterhouse, charcoal from the local market, and MgOCaO/Al2O3 that was generated from magnesium slag from a magnesium-smelting factory.  They determined the acid value (AV), an indicator of oil quality, with the following equation: AV = A x c x 56.11/m (A = volume of KOH solution use to titrate sample; c = concentrations of KOH solution; m = sample mass).  They first examined the non-catalytic biodiesel conversion of used cooking oil to biodiesel using a pressure reactor.  For this experiment they used MgOCaO/Al2O3 as a catalyst.  Kwon et al. carried out the experiment at a temperature of 130–250 °C and maintained pressure by filling the reactor with nitrogen (N2) and CO2.  To further examine the effect of temperature on the conversion process they replicated the previous experiment but varied the temperature from 250–500 °C.  Kwon et al. replicated the same experiment a third time, but added a virgin catalyst, activated Al2O3, in order to examine the catalytic element effect of MgO–CaO on the transesterification reaction.  For their main experiment, Kwon et al. tested the conversion of beef tallow and lard into biodiesel using charcoal.  They packed charcoal into an airtight reactor and maintained the temperature at 250–500 °C while oil feedstock, MeOH, and CO2 reaction medium were continuously added into the reactor.  The mixture was allowed to settle for 2 hours after the reaction before the contents were analyzed.
Kwon et al. achieved an approximately 98% biodiesel conversion rate of used cooking oil after 30 minutes.  This high conversion rate suggests that CO2 can enhance transesterification.  CO2 is believed to enhance the efficiency of the transesterification process by accelerating bond dissociations, also known as thermal cracking (Kwon et al. 2009).  By examining the biodiesel conversion of used cooking oil, Kwon et al. determined that the conversion rate is more responsive to changes in temperature than to pressure.  They also found that non-catalytic biodiesel conversion can be completed using porous materials. The pores, such as those in charcoal, act as small reactors, while the high temperature drives the transesterification reaction.  One of the main findings Kwon et al. observed was that under atmospheric pressure and a relatively high temperature, the conversion cost can be decreased by almost 70%, compared to standard commercial processes.
                  The researchers suggest that the mass decay of lard they observed at the comparatively low temperatures of 120–140 °C may be due to low molecular lipids and moisture in the lard.  Also, the thermal decomposition of lard was observed to be lower than that of beef tallow, which may be attributed to its lower amount of saturated fat.  In addition, they also found that the thermal degradation pattern for animal fats is similar to that of vegetable oil.  The biodiesel conversion efficiency of lard and beef tallow was almost identical at 400 °C.  There was no evidence of thermal cracking taking place in the experiment.
                  Kwon et al. achieved a conversion efficiency of beef tallow and lard into biodiesel of 98.5 (+ 0.5) % under ambient pressure and at temperatures higher than 350 °C.  They determined these to be the optimal operating conditions.  Based on their observations, the researchers assert that the production of biodiesel using charcoal and CO2 has the potential to be a highly cost-effective biofuel conversion process.
Other Sources
Crabbe, E., et al., 2001. Biodiesel production from crude palm oil and evaluation of butanol extraction and fuel properties. Process Biochemistry 37, 65–71.
Naik, M., et al., 2008. Production of biodiesel from high free fatty acid Karanja (Pongamia pinnata) oil. Biomass Bioenergy 32, 354–357.
Kwon, E., et al., 2009.  ASME Conference Proceedings, 231–236. 

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