Biodiesel production from municipal waste using lipase catalysis

Abstract

Municipal solid waste (MSW) and sewage sludge are attractive feedstocks for biofuel production through hydrothermal liquefaction (HTL) due to high moisture content, availability, and low costs. HTL has the advantage of not requiring any energy-intensive dewatering and drying processes compared to some other processes. HTL-biocrude has unwanted properties, such as a high water content, high viscosity, high ash, and high oxygen content, which limit the application thereof. Enzymatic esterification and transesterification are promising methods for the upgrading of biocrude that need to be further investigated. Enzymatic esterification of biocrude oil should be studied in detail to evaluate the effect of lipase catalysis on other biocrude components, not only on fatty acids; and the effectiveness of the overall fatty acid methyl ester (FAME) conversion should be determined. The HTL experiments were conducted in a high-pressure batch type autoclave reactor with simulated municipal solid waste and sewage sludge as feedstock and solvent. The reaction temperature chosen was 300 °C with a residence time of 20 minutes. For each esterification experiment, a 50 mL pear-shaped reaction flask was used in which 0.1 g of lipase (Novozym 435) and 1 g of biocrude oil (2.63 mmol) were weighed to obtain a 10 mass % enzyme loading. A magnet for stirring and an appropriate amount of methanol was added to the reaction flask to obtain the desired methanol to oil molar ratio. Methanolysis continued for 6 hours under constant stirring at 12.5 Hz. The results of the HTL of municipal waste (MW) showed the following product yields for: biocrude (6.80 mass %), biochar (14.82 mass %), biogas (9.52 mass %), and aqueous product (57.84 mass %). The biocrude was further characterised and a higher heating value (HHV) of 31.20 MJ/kg, a moisture content of 3.98 ± 0.85 mass %, a methyl ester and fatty acid content of 1.2 ± 0.21 % and 41.5 ± 3.38 % were obtained. The enzymatic esterification resulted in the following optimum conditions: reaction time of 6 hours at 30 °C and an oil to methanol molar ratio of 1:3 with a constant loading of 10 mass % Novozym 435 (N435). At the optimum conditions, a FAME conversion of 95.83 % was achieved with a HHV of 34.95 ± 1.22 MJ/kg and a methyl ester yield of 48.66 ± 5.40 %. Various esterification conditions were investigated to observe their effect on the FAME conversion. The FAME conversion increased as the reaction time increased until the optimum reaction time was reached. Furthermore, a decrease in the FAME conversion was observed due to alcohol inhibition that was attributed to the denaturation of the enzyme. No significant difference in the FAME conversion was observed at the different temperatures.

The boiling range distribution of the biocrude oil was investigated and compared with the produced biodiesel for different reaction conditions. There was a slight decrease in the kerosene fraction and a significant increase in the diesel fraction due to the conversion of fatty acids to FAME. There was also a significant decrease in the heavy fraction that could be explained by the fact that the boiling point of the methyl ester of a fatty acid was lower than the boiling point of the fatty acid itself. This resulted in the fatty acids with boiling points in the heavy fractions boiling range to be observed in the diesel fraction boiling range. The only significant increase observed for each fraction was the esters. Used N435 catalyst, acetone-regenerated N435 catalyst, as well as - tetrahydrofuran (THF), hexane-, dimethylsulfoxide- (DMSO), tert-butanol- and acetonitrile regenerated N435 catalyst were reused, and the results demonstrated that used N435 catalyst could be reused without solvent regeneration. However, acetone-regenerated N435 catalyst obtained insignificantly higher conversions. The reusability of N435 catalyst and acetone-regenerated N435 catalyst obtained a FAME conversion of 83.60 % and 86.34 %, respectively, after four cycles. The results obtained for lipase-catalysed esterification indicated that it is a promising route for MW generated biocrude oil upgrading using mild reaction conditions compared to thermochemical routes. It was also observed that N435 catalyst was highly stable in the reaction environment and could be reused and still obtain high FAME conversions.