Production of Green Diesel From Brown Grease Via Decarboxylation

Brown grease (BG) is a mixture of waste fats and oils collected from wastewater traps.  The free fatty acid (FFA) content of BG can range between 50-100%.  While BG is a potential low cost and non-food competing feedstock source for biofuels, it is a challenging feedstock due to its high FFA content. However, FFAs are potentially good candidates for decarboxylation reactions where the oxygen is removed as carbon dioxide, producing hydrocarbons referred to as “green diesel.”  This process is a low cost alternative to hydrogenation, where excess hydrogen is required along with high pressure conditions in order to remove oxygen as water to form straight chain hydrocarbons in the diesel fuel boiling range. Approximately 75% of BG consists of unsaturated FFAs with oleic acid representing the largest portion (around 50%) ¹. Studies on the decarboxylation of oleic acid have shown that the hydrocarbon selectivity is low because the unsaturated carbon-carbon double bond (C=C) in the oleic acid is saturated first under the reaction conditions in the presence of H₂, and therefore decarboxylation of oleic acid yields stearic acid (saturated FFA) as an intermediate and n-heptadecane as final product². In the absence of H_2, the key challenge is catalyst deactivation. We explored the conversion of brown grease to green diesel via decarboxylation with minimal hydrogen consumption. The purpose of this study is to develop a mechanism for the conversion of FFAs to n-paraffins, in the diesel fuel distillate range, to provide a better understanding of the processing conditions required and optimization of the reaction conditions in order to further develop the current green diesel process.  Experiments were conducted using a 100-mL stirred Hanwoul high pressure batch reactor and a 5wt.% Pd catalyst on an activated carbon support (Aldrich).  Results demonstrated that all the BG compounds can be fully converted in 9 hours to straight chain hydrocarbons using mild reaction conditions (15 bar and 300 ^oC) and an inert gas with 10 vol% H₂. However, the decarboxylation reaction rate was observed to be very slow due to the presence of unsaturated FFAs in the BG. To further investigate the effects of unsaturated FFAs in the feedstock on the reaction rate, a pre-hydrogenation step was also performed before decarboxylation as part of study.

References

1.         Kim, M.; DiMaggio, C.; Yan, S.; Wang, H.; Salley, S.O.; Ng, K.Y.S., Performance of heterogeneous ZrO2 supported metal oxide catalysts for brown grease esterification and sulfur removal. Bioresource Technology 2010, doi:.1016/j.biortech.2010.10.105

2.         Immer, J. G.; Kelly, M. J; Lamb, H. H., Catalytic reaction pathways in liquid-phase deoxygenation of C18 free fatty acids. Applied Catalysis A:General 2009, 375, 134-139