Calcium Rich Food Wastes Based Catalysts for Biodiesel Production

Transcription

1 Chemical Eng. Dept., ISEL From the SelectedWorks of João F Gomes 2017 Calcium Rich Food Wastes Based Catalysts for Biodiesel Production João F Gomes Available at:

2 DOI /s ORIGINAL PAPER Calcium Rich Food Wastes Based Catalysts for Biodiesel Production M. Catarino 1 M. Ramos 2 A. P. Soares Dias 1 M. T. Santos 2 J. F. Puna 2,3 J. F. Gomes 2,3 Received: 15 December 2016 / Accepted: 12 June 2017 Springer Science+Business Media B.V Abstract Biodiesel produced from food wastes can help to solve several environmental issues: anthropogenic carbon emissions due to fossil fuels combustion and waste management. Biodiesel was produced using waste frying oils (WFO) and calcium rich food wastes such as mollusk, shrimp, eggs shells and cuttlebone to produce calcium based heterogeneous catalysts by calcination. The characterization of chalky white calcined powders by XRD showed diffraction lines typical of lime but some samples were slightly contaminated with calcite. The powders with low crystallinity showed high hydration rate presenting XRD features ascribable to nanocrystals of calcium hydroxide. The post reaction samples presented mainly lines due to calcium diglyceroxide and methoxide. Thermograms of used catalysts showed some weight loss of these calcium compounds, confirming the presence of such phases. All prepared catalysts were effective in catalyzing the methanolysis of soybean oil. A FAME yield around 96% was obtained after 2.5 h of reaction. When using WFO, the FAME yield was only 65% with simultaneous production of soap. The use of WFO and soybean oil mixtures attenuates the loss of catalytic performances. The obtained glycerin s presented a light color characteristic of heterogeneous catalyzed processes. FTIR spectra of glycerin s showed * M. T. Santos tsantos@deq.isel.ipl.pt LAETA, IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, s/n, Lisboa, Portugal ADEQ, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro 1, Lisboa, Portugal CERENA, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, s/n, Lisboa, Portugal some features belonging to matter organic non glycerin and methanol. The catalyst reutilization without intermediate reactivation indicated that catalysts are somewhat stable. When WFO was used, the reused catalysts showed improved performance probably due to the formation of calcium diglyceroxide. Nevertheless, calcium diglyceroxide is bound to promote homogeneous catalysis and consequent deactivation. Keywords Food wastes Biodiesel WFO Calcium catalysts Deactivation Introduction In modern society, man is a huge energy consumer and waste producer, but has also an imperative need of having renewable sources of energy and solutions for waste management. Solving both issues simultaneously would have excellent profits for society. Food wastes (FW) represents a significantly fraction of municipal solid wastes [1]. One-third of the food produced for human consumption is lost along the food supply chain. Theoretically, food waste can be a useful resource for biofuels production [2]. Food wastes are characterized to have a variable chemical composition which may comprise a mixture of carbohydrates, lipids, proteins [3] and, also, inorganic elements such as calcium. Different biofuels can be produced depending on the FW composition using, for instance, several biochemical and thermochemical processes. The main processes to produce energy from FW reported in the literature are [3]: Transesterification of oils and fats to produce biodiesel. Vol.:( )

3 Fermentation of carbohydrates to produce bioethanol and biobutanol. Anaerobic digestion to produces biogas. Dark fermentation to produce hydrogen. Pyrolysis and gasification. Hydrothermal carbonization. Incineration. Biodiesel, which is a mixture of fatty-acids methyl or ethyl esters, is pointed out as a feasible renewable substitute of fossil diesel [4]. Nowadays, 95% of the produced biodiesel is derived from edible oils and there isn t enough arable land to increase biodiesel production, in order to meet world demand [5]. The use of waste frying oils and non-edible animal fats are an alternative feedstock that could avoid the problematic food or fuel? issue, making biodiesel a sustainable fuel. Biodiesel presents several advantages in its use, since is safe, non-toxic and biodegradable. In fact, the transportation sector is a major contributor to greenhouse gases emissions (GEE) and it s imperative to find a low carbon fuel for this sector. Biodiesel leads, also, to a substantial reduction of GEE s emissions, mainly CO 2. According with European Legislation (Directive 2009/28/EC), the utilization of biodiesel produced from waste frying oils (WFO) must be promoted and is considered a renewable energy source. National authorities (D.G.E.G. 2016) shows that the total production of biodiesel in Portugal, in 2014, was 340,781 tons, but only 5% was produced from WFO s and animal fat. These numbers show that only a small amount of biodiesel is manufactured from WFO s, which calls for a substantial increase on its utilization. WFO have three main sources: domestic from households, industrial when produced in the food industry and service sector from hotels, catering, canteens, and others. For example, in last years, according with Portuguese Environmental Agency (A.P.A. 2010), the amount of WFO s produced in Portugal was estimated to around 43,000 65,000 tons, with 62% collected from the domestic sector, 37% from the service sector and 1% from the industry sector. Is expected that the WFO collection increase due to the growth of collection points, according to the Portuguese legislation framework. Waste frying oils, restaurants grease and animal fats or even spent coffee beans [6, 7] are low price potential raw materials for biodiesel production. Several researchers pointed out the use of WFO as a key component in reducing biodiesel costs [8] since the greatest barrier to biodiesel use lies in the cost compared with fossil fuel. WFO contains free fatty acids, moisture, and other minor contaminants such as NaCl, which will reduce the efficiency of traditional transesterification methods [7]. Pretreatment of WFO, prior to the transesterification step, with basic catalysts, would increase the production costs [9]. Two-step processes starting from an acid catalyzed esterification followed by a traditional basic catalyzed transesterification are usually reported as an effective way to avoid the undesirable effect of free fatty acids [10, 11]. Since a larger number of reaction steps (two-steps instead of a single step) result on an increase in the production costs, it would be advantageous to process WFO using traditional reaction systems, preferably with basic heterogeneous catalysts. Lime catalysts have been extensively studied for the methanolysis of vegetable oils [12]. In a recent review on catalysts for biodiesel, Avhad and Marchetti [13] stated that, among basic catalysts, lime was found to be gaining both scientific and industrial attention, because of its high basicity, low solubility in methanol, and its easy production from natural resources. Wastes, such as egg shells, mollusks, and crustacean shells, are calcium rich materials available from food industries, like calcium carbonate, which can be used as cheap raw materials, to obtain CaO, by a calcination process [14 16]. Many researchers used egg shells to prepare CaO catalysts for biodiesel production, and mollusk shells have also been reported as raw materials for lime catalysis [17 20]. According with Portuguese statistics (I.N.E. 2016), only in Portugal, 749 tons of crustacean and 19,172 tons of mollusks were captured. Egg production, for consumption only, in last year was 106,784 tons, whereas 10% weight of eggs is shell, corresponding to approximately 10,678 tons. To improve the knowledge on the CaO/biodiesel catalytic system, several catalysts were prepared from food wastes with high content of Ca: egg shells, mollusk shells and shrimp shells. Particular emphasis was given to the post reaction catalysts characterization, in order to characterize the deactivation processes usually mentioned for such catalysts when low grade oils are used. Materials and Methods Materials and Catalysts Preparation The methanolysis tests were performed using soybean oil, alimentary grade, from a local producer (Sovena) and WFO collected from restaurants and ISEL cafeterias. The WFO samples were filtrated under vacuum filtration, to remove solids and dried by heating (110 C) during 1 h. The oils acidity was determined by titration with ethanolic KOH solution (0.1 M) using phenolphthalein indicator (0.5% W/W ethanolic solution) [21]. Calcium rich materials obtained from domestic wastes and collected from the beach were used to prepare Ca based catalysts. The eggshells and the cuttlefish bone were used as raw, while the remaining shells were cooked. All

4 the Ca food wastes were washed, dried and reduced to powder using a ceramic mortar. The calcium catalysts were obtained by powders calcination in a muffle at 800 C, for 3 h using a heating rate of 5 C/min. The calcination temperature was chosen from the thermal degradation profile of each raw material obtained by thermogravimetric analysis, under air flow (30 C/min). Then, a catalyst, named as Shellmix, was prepared mixing equal amounts of all shells with exception of the cuttlebone, as it is the less abundant Ca rich waste. The powder sample was calcined in the above reported conditions. Catalysts Characterization The catalysts, fresh and post reaction samples, were characterized by X-ray diffraction (XRD) to identify the Ca crystalline phases and the degree of crystallinity of each phase. The diffractograms were recorded with a Rigaku Geigerflex diffractometer with Cu K α radiation at 40 kv and 40 ma (2 /min). The post reaction catalysts were also characterized by thermogravimetry, under air flow, to evaluate the reaction species which remains adsorbed [21] and, thus, to infer about their stability. The thermograms were acquired (30 C/min) for mg of powder samples in alumina crucibles using a Netzsch TG-DTA-DSC thermobalance. Reaction Procedure The methanolysis reactions, at methanol reflux temperature, were carried out using 5% (W/W) of catalysts (oil basis) and a methanol/oil molar ratio of 12. For each reaction batch, the catalyst was previously contacted with methanol for 1 h (65 C) and then, the pre-heated oil (100 g, 100 C) was added to the reaction flask. After the reaction period (2.5 h) the slurry, containing the catalyst, the unreacted oil, the methyl esters (biodiesel), and the glycerin, was cooled down and the catalyst was removed under vacuum filtration. The collected liquid was transferred into a decantation funnel for gravity separation of the glycerin, from the oil phase containing the biodiesel and the unreacted oil. The FAME content of the oil phase was evaluated by thermogravimetry under air flow [22]. The post reaction catalysts were dried overnight at 105 C before characterization and eventual reutilization. More details on the reaction procedure are given elsewhere [21]. The catalysts stability was studied by re-using the same catalyst sample in consecutive reaction batches without intermediate reactivation procedure. Determination of Biodiesel Yield Since biodiesel and oil show different decomposition temperatures [23], thermogravimetry can be used to evaluate the reaction yield. The purity of the obtained biodiesel can also be inferred from thermograms since methanol, glycerin and Ca soaps have distinguishable decomposition temperatures. The biodiesel (oily) phase, containing the FAME and the unreacted oil, was characterized by thermogravimetry to evaluate the reaction extent [22]. Liquid samples ( mg) were heated from room temperature to 1100 C under an air flow of 20 C/min. The thermal degradation profiles (TG) were differentiated (DTG) to underline the different degradation processes. The mass composition was computed using the equipment software. The thermal decomposition of neutral oil, WFO and pure FAME obtained by homogeneous catalysis, using sodium methylate as catalyst, were used as a standard. As previously mentioned [23], the thermal degradation of oil and biodiesel occurs at different temperatures, with maximum temperatures separated by more than 50 C, which allows thermogravimetry to be used to quantify FAME composition in oil/biodiesel mixtures. Glycerin Phase Characterization Glycerin is the main by-product (10% W/W) of biodiesel production and its valorization can reduce the biodiesel production costs [24]. The purity of glycerin phase is a key factor for its valorization. Usually heterogeneous catalysis produces better quality glycerin than the corresponding homogeneous process. As reported by Dias et al. [25], the color of such phase can be used to perceive the occurrence of heterogeneous catalyst leaching and consequent occurrence of homogeneous catalyzed processes. The glycerin obtained in each reaction test, was characterized by ATR- FTIR using a ATR accessory with a ZnSe crystal. The IR spectrum for p.a. grade glycerin was used as standard. Spectra were acquired with a resolution of 4 cm 1 and a Kubelka Munk correction was used. Results and Discussion Catalyst Preparation and Characterization of Fresh Samples Due to the relevance of the oil acidity on the basic catalyzed methanolysis process [10, 21], this parameter was evaluated by titration with KOH ethanolic solution, according with European Standard EN 14,204 and, the average values obtained were: 0.57 mg of KOH/g and 2.18 mg of KOH/g, for soybean oil and WFO, respectively. The thermal degradation profiles of raw Ca rich materials (Fig. 1) exhibited several processes corresponding to dehydration and thermal degradation of organic materials

5 Waste Biomass Valor Fig. 1 Thermal degradation differential profiles (DTG) of the raw Ca wastes (under air; 30 C/min) Temperature ( 0C) DTG (mg/min) Intensity (a.u.) lime portlandite calcite Scallop Pink shell clam Scallop Clam Mussel Pink shell clam Cuttlefish Eggs Shrimp Crab Limpet Intensity (a.u.) -16 Ca Diglyc portlandite Ӿ lime Shellmix _WFO Pink shell clam Crab Eggs Mussell Clam Egg Crab Shrimp Mussel Scallop Limpet Cu lebone Cu lebone Clam 5 Shrimp θ(0) Fig. 2 XRD patterns of fresh catalysts prepared by calcination at 800 C (phase identification using standard JCPDS files for lime, portlandite and calcite) for temperatures lower than 500 C and, a main process above 800 C, ascribable to the CaCO3 thermochemical conversion into CaO [26]. The catalysts were prepared by calcination at 800 C to minimize sinterization phenomena, which occurs at temperatures higher than 1000 C, hence reducing the loss of surface area. However, as can be seen in Fig. 2, for some samples, such temperature was too low to perform the total conversion of C aco3 into CaO. The scallop derived catalyst presented the highest content of C aco3 showing θ (o) Fig. 3 XRD diffractograms of post reaction Ca catalysts, with refined soybean oil and WFO, after one transesterification step (calcium diglyceroxide, lime and portlandite phases identification using standard JCPDS files) intense diffraction lines corresponding to calcite. The catalyst prepared from cuttlebone was less crystalline than the others and the XRD pattern presented features belonging to portlandite [Ca(OH)2]. Such result can be related with the composition of the cuttlebone raw material being mainly aragonite, whereas all the others were calcite. Catalyst Characterization of Post reaction Samples The post reaction samples were characterized by XRD (Fig. 3) to evaluate eventual modifications leading to

6 deactivation, according to Catalysts Characterization. As a result of a high contamination with oil, the XRD characterization of the catalyst obtained from limpet shell was prevented. The diffractograms shown in this figure exhibited lime XRD features which were almost vanished, leading to XRD patterns belonging to calcium diglyceroxide and calcium methoxide [25]. Only mussel derived post reaction catalyst presented lines ascribable to nanocrystalized portlandite (wide lines). The Shellmix catalyst after reaction with pure WFO showed a similar XRD pattern with lines belonging to calcium diglyceroxide and methoxide. After the second reaction batch, this catalyst showed a XRD pattern which cannot be identified with none of the Ca phases previously reported. The XRD patterns of the post reaction catalysts are somewhat different from previously published data [25] which report the calcium hydroxide (portlandite) as the main crystalline phase. In fact, the authors only detected calcium diglyceroxide after several reutilizations of the same catalyst sample. This apparent disagreement can be due to the low calcination temperature used. The majority of authors refer higher calcination temperatures, which can be as high as 1000 C, for analogous catalysts, and they underline an improvement of the catalyst stability for higher calcination temperatures [17]. As reported by Dias et al. [10], the post reaction catalysts can be characterized by thermogravimetry, under air flow, to evaluate the reaction mixture components adsorbed by Ca powders and to confirm, also, the formation of calcium diglyceroxide and calcium methoxide species. If the post reaction catalyst contains large amounts of oily species adsorbed, since they are hydrophobic, the catalyst surface will not activate methanol species because they are hydrophilic. Such catalyst would be inactive in a consecutive reaction batch but, from the thermogram profile, a reactivation temperature can be chosen. In Fig. 4, the thermograms of post reaction catalysts are displayed. For all the analyzed samples, large amounts of oily species were thermal degraded in the range C. For lower temperatures, the thermal degradation of calcium diglyceroxide was present [10]. Figure 4 shows also, the DTG profiles for post reaction samples, using soybean oil and several Ca catalysts from food wastes. The main weight loss processes were observed at 200, 500 and 800 C. At 200 C, the thermal decomposition of calcium diglyceroxide takes place whereas, at 500 C, the decomposition of Ca(OH) 2 occurs. As described elsewhere [10, 27, 28], at 800 C, the conversion of calcium carbonate, initially presented in the post reaction catalyst or formed by carbonation in situ, into calcium oxide takes place. These results evidenced several changes suffered by the catalysts during reaction. The presence of calcium diglyceroxide in the post reaction catalysts had a detrimental effect on the catalyst stability [25]. Determination of Biodiesel Yields The biodiesel yield (FAME, % W/W) was evaluated by thermogravimetry, without purification of the oily phase collected after gravity settling of the reaction mixture and separation of glycerin phase. Typical thermograms, in Fig. 5, show different thermal decomposition processes for WFO (or oil) and biodiesel. Oil and WFO decompose at higher temperature than the corresponding methyl esters. All the oily phases analyzed presented minor weight loss for temperatures lower than 100 C which can be attributed to thermal degradation/drying of methanol/water contaminants. The FAME yields computed from thermogravimetry data are displayed in Fig. 6 and Table 1. Data in Fig. 6 shows analogous catalytic behaviors for all the catalysts prepared from mollusk shells, when soybean oil was used. For such catalysts, and in tested conditions, the FAME yields were higher than 95%, close to the range of previous published data for similar catalysts [29]. For the same conditions, the shrimp shell and cuttlebone derived catalysts showed lower catalytic performances which can be related to their lower crystallinity and/or higher hydration (XRD patterns belonging to portlandite for cuttlebone catalyst). The reuse of the scallop catalyst, in a second reaction batch, showed strong deactivation. Such low catalyst stability is ascribed to the low calcination temperature used. Some researchers mention a stability improvement for higher calcination temperatures [17]. Data in the Table 1 shows results for scallop and Shellmix catalysts using WFO and mixtures WFO + soybean oil. Co-processing equal amounts of WFO and soybean oil, the scallop catalyst was slightly more active than the Shellmix catalyst, but the WFO acidity promotes a depreciation of the scallop catalyst behavior. This loss of catalytic performances was expected, since the free fatty acids from WFO partially neutralize the basic active sites on the catalyst surface [10]. The Shellmix catalyst, with lower catalytic activity than the scallop one, showed high tolerance to the WFO acidity since the mixture 1:1 and 3:1 (WFO:oil) had almost the same FAME yield. Only pure WFO promotes a net decrease of its catalytic activity. The reuse of Shellmix catalyst in a second reaction batch with WFO, presented higher FAME yield than the first reaction batch. This apparent catalyst activation, during the first reaction batch, is due to the calcium diglyceroxide formation which promotes unwanted homogenous catalysis (faster than the heterogeneous) and consequent catalyst leaching [25].

7 Fig. 4 Thermograms of the post reaction catalysts using soybean oil (under air; 30 C/ min) 100 Temperature (ºC) Sample mass (%) Clam Shrimp Cuttlebone Pink shell clam Egg Limpet Mussell Crab Scallop Temperature (ºC) DTG Clam Shrimp Cuttlebone Pink shell clam Egg Limpet Mussell Crab Scallop Glycerin Phase Characterization The obtained glycerines, for all catalytic tests, were characterized by ATR-FTIR. The spectra in Fig. 7 shows IR features similar to those of pure glycerin (used as standard) with the most intense IR band around 1030 cm 1 (pure glycerin) but with a small displacement toward lowers wave numbers (around 1022 cm 1 ) due to methanol contamination. The absorption bands at 1580 cm 1, attributable to carboxylate species from soaps [30], are visible and, as expected, more intense when pure WFO was used. The band around 1740 cm 1 from contamination with esters and carboxylic acids is also observed for all the glycerin samples. However, the light color of all the glycerin samples seems to indicate lower contamination with the above referred matter organic non glycerin (MONG). Conclusions First generation biodiesel, a mixture of fatty acids methyl esters, was produced using low value raw materials to obtain a cheaper and sustainable fuel. Frying oils and calcium rich shells (eggs and mollusk among others) wastes from alimentary industry were used. The raw Ca rich materials, mostly calcite (CaCO 3 ) and portlandite (Ca(OH) 2 ), were calcinated at 800 C to produce lime catalysts. In standard conditions (methanol reflux temperature, 5% (w cat /w oil ) of catalyst and methanol/oil = 12 molar ratio), high FAME yields were obtained for all the tested catalysts when alimentary refined soybean oil was used. For pure WFO a decline of the catalysts activity was observed with lower FAME yield and soap formation. These drawbacks

8 Fig. 5 Thermograms (TG and DTG) of FAME obtained by homogeneous catalysis used as standard, WFO and some biodiesel phases Fig. 6 FAME yield, assessed by thermogravimetry, obtained for soybean oil using the lime catalysts from Ca rich alimentary wastes (5% w cat /w oil ; methanol/oil = 12; 2.5 h, 65 C) Scallop batch#2 Scallop Mussel Crab Clam Shrimp Cu lebone Pink shell clam Limpet Eggs FAME yield (%) are due to the WFO acidity, which was quite higher, about 2 mg KOH/g oil. These drawbacks were overcome using WFO/soybean oil mixtures. Data pointed out that WFO can be processed mixed with neutral oil without significant loss of the catalytic performances thus avoiding the use of acid catalysts which require higher reaction temperature and pressure conditions, for the same reaction time. The catalysts deactivation was investigated using the same catalyst sample in consecutive reaction batches without intermediate regeneration and only dried overnight at 105 C. As expected, the WFO promotes faster catalyst deactivation than the neutral oil. The decline in the catalyst activity can be attributed to the leaching of calcium diglyceroxide formed (homogeneous contribution) which, in presence of WFO, forms soap that leads to more difficult biodiesel purification processes and, also, a decreasing of the biofuel mass yield. Finally, it is possible to conclude that these natural food wastes catalysts, rich in Ca compounds, are very active and suitable for biodiesel production through the transesterification process, following an alkaline heterogeneous catalysis mechanism, even with WFO as raw-material, but with lower acidity or mixed with refined vegetable oils. Future work must be addressed to clarify the improvement of these food wastes catalysts activity, with the decreasing of oil acidity in WFO. Nevertheless, these heterogeneous basic food wastes catalysts showed to be high potential to catalyze the transesterification of WFO to

9 Table 1 FAME yield using WFO and WFO/Soybean mixtures assessed by thermogravimetry (under air, 30 C/min), using lime catalysts from Ca rich food wastes (5% Wcat. /Woil; methanol/oil = 12; 2,5 h; 65 C) Catalyst Raw material FAME yield (%) Egg shell Refined soybean oil 97.2 Limpet 96.4 Pink shell clam 97.1 Cuttlebone 86.4 Shrimp shell 94.2 Clam 96.8 Crab 95.6 Mussel 96.2 Scallop batch# Scallop batch# Scallop WFO + soybean oil (1:1) 90.9 Shellmix 82.6 Shellmix WFO + soybean oil (3:1) 82.0 Shellmix batch#1 WFO 62.5 Shellmix batch# Kubelka Munk Kubelka Munk Glycerin clam crab eggshell limpet mussel pink_shell_clam shrimp scallop Wave number (cm -1 ) Wave number (cm -1 ) scallop_ 50%WFO shellmix_ 50%WFO shellmix_ 75%WFO shellmix_ WFO shellmix_ WFO_batch#2 Glycerin Fig. 7 ATR-FTIR spectra of the obtained and commercial (used as standard) glycerin s produce first generation FAME biodiesel and, for that reason, they play an important role of waste solid valorization policy, through its recycling, besides WFO s valorization. Acknowledgements FCT Fundação para a Ciência e Tecnologia, Lisboa, Portugal, for funding project PTDC/EMS-ENE/4865/2014. Professor Manuel Francisco da Costa Pereira (CERENA, IST) for the availability of the thermobalance as well as the essential help in the XRD data analysis. Atlantic University for the FTIR acquisition spectra. Centro de Estudos de Engenharia Química do ISEL, for muffle utilization in the activation of food wastes catalyst fresh samples. References 1. Pham, T.P.T., Kaushik, R., Parshetti, G.K., Mahmood, R., Balasubramanian, R.: Food waste-to-energy conversion technologies: current status and future directions. Waste Manag. 38, (2015) 2. Kiran, E.U., Trzcinski, A.P., Ng, W.J., Liu, Y.: Bioconversion of food waste to energy: a review. Fuel 134, (2014) 3. Girotto, F., Alibardi, L., Cossu, R.: Food waste generation and industrial uses: a review. Waste Manag. 45, (2015) 4. Ajala, O. E., Aberuagba, F., Odetoye, T. E., Ajala, A. M.: Biodiesel: sustainable energy replacement to petroleum-based diesel fuel a review. ChemBioEng Rev. 2, (2015) 5. Leung, D.Y.C., Wu, X., Leung, M.K.H.: A review on biodiesel production using catalyzed transesterification. Appl. Energy 87, (2010) 6. Jenkins, R.W., Ellis, E.H., Lewis, E.J., Paterson, M., Le, C.D., Ting, V.P., Chuck, C.J.: Production of biodiesel from Vietnamese waste coffee beans: biofuel yield, saturation and stability are all elevated compared with conventional coffee biodiesel. Waste Biomass Valorization 8, (2017) 7. Canakci, M.: The potential of restaurant waste lipids as biodiesel feedstocks. Bioresour. Technol. 98, (2007) 8. Talebian-Kiakalaieh, A., Amin, N.A.S., Mazaheri, H.: A review on novel processes of biodiesel production from waste cooking oil. Appl. Energy 104, (2013) 9. Rathore, V., Newalkar, B.L., Badoni, R.P.: Processing of vegetable oil for biofuel production through conventional and nonconventional routes. Energy Sustain. Dev. 31, (2016) 10. Dias, A. P., Puna, J., Correia, M. J., Nogueira, I., Gomas, J., Bordado, J.: Effect of the oil acidity on the methanolysis performances of lime catalyst biodiesel from waste frying oils (WFO). Fuel Process. Technol. 116, (2013) 11. Cai, Z.-Z., Wang, Y., Teng, Y.-L., Chong, K.-M., Wang, J.-W., Zhang, J.-W., Yang, D.-P.: A two-step biodiesel production process from waste cooking oil via recycling crude glycerol esterification catalyzed by alkali catalyst. Fuel Process. Technol. 137, (2015) 12. Kouzu, M., Hidake, J.: Transesterification of vegetable oil into biodiesel catalyzed by CaO: a review. Fuel 93, 1 12 (2012) 13. Avhad, M., Marchetti, J.: A review on recent advancement in catalytic materials for biodiesel production. Renew. Sust. Energy Rev. 50, (2015) 14. Viriya-empikul, N., Krasae, P., Puttasawat, B., Yoosuk, B., Chollacoop, N., Faungnawakij, K.: Waste shells of mollusk and egg as biodiesel production catalysts. Bioresour. Technol. 101, (2010) 15. Sirisomboonchai, S., Abuduwayiti, M., Guan, G., Samart, C., Abliz, S., Hao, X., Kusakabe, K., Abuluda, A.: Biodiesel production from waste cooking oil using calcined scallop shell as catalyst. Energy Convers. Manag. 95, (2015)

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