Experimental and computational exploration of advanced biodiesel fuels and hybridisation process evaluation of feedstocks and their chemical combinations

Abstract

To address the alarming crisis of global energy demand, environmental degradation and climate change, biomass derived diesel fuel is one of the superior renewable fuel options, considered as suitable alternative to petroleum fuel. Important fuel characteristics of biomass derived diesel fuel ranges from being recyclable available local fuel to auspicious performance in combustion emission reduction. In this study, waste oil and other indigenous tropical seed oils, which include; used sunflower oil (USO), linseed oil (LSO), marula seed oil (MSO), baobab seed oil (BSO) and Trichilia emetica kernel oil (TEKO) were investigated for biodiesel production and further scrutinised for the hybridization process for effective applications. The process of hybridization applied was a two-pathway approach via in-situ and ex-situ transesterification reactions. Biological wastes mineral-rich materials such as eggshells, banana peels and pawpaw peels were used to produce the bio-alkaline catalysts. The waste materials were washed with distilled water, dried in the oven and further subjected to high temperature of calcination in the furnace. Eggshells were calcined at 900 oC for 3 h while pawpaw and banana peel were calcined for 3 h at 700 oC respectively. The calcined ash of eggshells and banana peel, eggshells and pawpaw peels were bonded respectively via wet impregnation method and further activated at high temperatures to obtain hybridized bio-alkaline catalysts. The synthesized samples of all catalyst were characterized using Fourier transforms infrared (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The catalysts produced were applied in the production of biodiesel from waste and underutilized oils such as used sunflower oil (USO), linseed oil (LSO), marula seed oil (MSO), baobab seed oil (BSO) and Trichilia emetica kernel oil (TEKO) under an optimized transesterification reaction process. The operating parameters considered viz methanol-to-oil ratio, catalyst loading, and reaction time temperature were investigated and optimized using Response surface methodology (RSM) to obtain the best operation condition for the maximum yields. The optimized condition established from the biodiesel fuel produced was used as a standard for the transesterification reaction condition for the single and hybrid oils. The two pathways hybrid process; In-situ (co-mingling of oils prior transesterification) and Ex-situ (comingling of the single biodiesel fuels after transesterification) was used to evaluate and compare the differences between the two processes and how effective they can be deployed commercially. The four crude oils considered for the study (USO, LSO, MSO and BSO) were analysed while fractions of them were individually converted via transesterification to obtain single biodiesel fuels (SOBFs): used sunflower oil methyl ester (USOME), linseed oil methyl ester (LOME), marula oil methyl ester (MOME) and baobab oil methyl ester (BOME). Then the remaining fractions were pre-treated and co-mingled in 27 various combinations to form new oils (of bi-and poly-hybrids) called the hybridized oils (HOs). These different combinations were then trans-esterified to obtain hybridized oil methyl esters (HOMEs) - In-situ hybridization. Thereafter, the SOBFs - (USOME, LSOME, MSOME and BSOME) were hybridized in the same pattern following the same ratios to form new products termed hybridized methyl ester (HMEs) - Ex-situ hybridization. All the produced biodiesel fuels: USOME, TEKOME, LOME, MOME, BOME and HOMEs were individually blended with petrol-diesel and their chemo-physical properties were analysed and compared with the international (ASTM and EN) and South African (SANS) standards. The impact of the chemical combinations on the physico-chemical properties of all the biodiesel produced was investigated and computed using artificial neural networks (ANN). Their influence on the important thermophysical fuel properties such as cetane number and calorific values were also evaluated. The characterization results revealed that eggshell is an excellent source of natural CaO while the banana and pawpaw peels are rich in potassium compounds such as: KCl, K2SO4, K2CO3, K2O which are efficient catalyst compounds for biodiesel production. The hybridized catalysts were found to be effective and of high basicity and active in oil conversion to biodiesel. The process of in-situ and ex-situ hybridization and their blends with petro-diesel were found to be a very effective approach to be adopted in the biodiesel production process. High conversion of biodiesel yields was obtained via the process of in-situ transesterification, indicating that the transesterification process is not affected by the number of mixing ratios of oils. The two process pathways offered improved properties that are much more conformable to standards than most of the single biodiesel produced fuels. Some properties such as density, acid value, viscosity, calorific value and cetane number were found a bit lower in ex-situ than in in-situ hybrids under the same hybrid conditions. The predicted properties obtained from the two protocols by ANN show good alignment with the experimental values with high regression coefficients close to unity (1). The improved fuel properties obtained following these protocols were within the international and South African standard specifications. The general principles and model predictions of the subsequent properties of biodiesel presented in this study will serve as a database and template for effective development for the overall biofuels application