Synthesis of char-based acidic catalyst for methanolysis of waste cooking oil: An insight into a possible valorization pathway for the solid by-product of gasification
Graphical abstract
Introduction
In the last few decades, energy demand has increased drastically worldwide, with mineral oil being the main source of energy. Depletion of fossil fuel reserves, increase of global warming and fluctuating in mineral fuel prices have gained attention worldwide [1]. Biodiesel has similar properties to fossil fuels and it can be considered as a suitable alternative to complement them [2]. Biodiesel is a mixture of fatty acid methyl esters (FAMEs) that are produced from oil in the presence of catalyst and solvent (alcohol) by the transesterification reaction [3]. Several types of oils have been used for biodiesel production around the world, in particular, the first generation oils termed as edible oils such as palm oil [4] soybean oil [5] and canola oil [6]. Other than the feedstock, process conditions such as oil to alcohol (mostly methanol is used because it is cheaper than other primary alcohols) ratio, reaction temperature and reaction time have a great impact on biodiesel properties and its production cost [7], [8]. The range of reaction conditions highly depends on the nature of oil/fat used for biodiesel production. In the case of low-quality feedstocks, higher oil to methanol ratios, longer reaction times and higher amounts of catalyst should be used. Theoretically, for the occurring of the transesterification reaction, three moles of alcohol are required for each mole of oil. Nonetheless, in reality, more than three moles are needed to proceed the reaction in the forward direction.
The extensive utilization of edible oils in the biodiesel industry resulted in a fuel versus food controversy. Second-generation oils (non-edible oils) such as Jatropha [9], neem oil [10], rubber seed oil [11], kapok seed oil [12], palm fatty acid distillate (PFAD) [13] and waste cooking oil [14] have been explored to avoid the controversy. Third generation oil, which is algae oil, has also been tested for biodiesel synthesis [1].
In comparison of fossil fuels, biodiesel is more expensive (approximately one and a half time more expensive), because of the feedstock cost, which shares around 70–95% of the total cost of biodiesel [15]. Therefore, it is stated that the use of low-cost feedstock such as waste cooking should reduce the overall production cost. Other than feedstock, a suitable heterogeneous catalyst saves a lot of costs. Acidic catalyst reaction is slow, need longer time than usual and their corrosiveness increase the maintenance cost. Other main costs are catalyst, solvent and waste treatment [16].
Currently, homogeneous (H2SO4, KOH/NaOH) catalysts are used in transesterification process [17]. The problems associated with the homogeneous catalysts are: on one side, the need of a high amount of energy for purification, separation of by-products and catalyst from produced esters and on other side, it is impossible to recover and use again [18]. The disposal of acidic wastewater produced during purification and separation process is a serious issue being faced by the industries [19]. Because of these issues, there is a growing trend in the field of heterogeneous catalysts (acidic or basic) for esters production.
Heterogeneous catalysts have several advantages over homogeneous catalysts. They can be reused many times, easily separated from the esters layers and there is no generation of wastewater during the purification stage [20]. Several kinds of heterogeneous catalysts have been reported, such as alkali metal catalysts, alkali and alkaline earth oxide, zeolites based catalysts, zirconia-alumina, amberlyst-15 [21], [22]. Unfortunately, high cost, sensitivity to water, rapid deactivation, low acidic site are the major obstacles in the application of these catalysts [18]. In recent years, there is a great interest shown in the generation of carbon-based catalysts. Nonetheless, this type of catalysts is still not fully explored. Carbon-based catalysts have several advantages over other metal-based catalysts; they have higher surface area and they are more stable under acidic and basic conditions. Carbon-based D-glucose solid acid catalyst preparation method was developed by Toda et al. They used this catalyst for biodiesel production from long chain fatty acids [23]. Chen et al. [24] used the glucose-starch mixture and prepared acid catalysts. They claimed that mixture of glucose-starch solid acid catalyst gives 96% conversion on the conditions of oil to methanol ratio 10:1, reaction temperature 80 °C and reaction time 6 h. Li et al. [24] investigated rice husk char based solid acid catalysts for the production of oleic acid for biodiesel production. They claimed that, with this catalyst, the oleic acid conversion was around 98.7%. This catalyst shows very high reusability, after seven cycles the conversion yields decrease from 98.7 to 96%. Hidayat et al. [25] applied the coconut shell char based solid acid catalyst for the esterification of palm fatty acid distillate.
Biomass gasification is a thermochemical process that converts biomass into syngas, liquid tar and solid char. Both tar and char are the unwanted byproducts. Char yield depends on the process conditions and the design of the reactor but, typically, it is around 2–5% of the used feedstock. In South Tyrol (Italy), there are around 40 small-scale biomass gasification plants working. More than 1500 tons of char are produced every year only in this region and industries need to pay the huge amount for its disposal. The char used in this study was collected from a downdraft gasifier during a monitoring campaign which details were published previously [26].
From a general perspective, this work aims at investigating a possible valorization route for char, in particular looking at the possibility of using it for synthesizing an acidic catalyst to convert WCO into biodiesel. The prepared catalyst was characterized by using FTIR, FESEM, NH3-TPD, and BET by N2 adsorption-desorption methods. Meanwhile, reaction process variables such as oil to methanol ratio, catalyst loading, reaction temperature, and reaction time for transesterification of WCO were also studied. Reusability study of the catalyst was conducted to analyze the effectiveness of the produced catalyst. Furthermore, important fuel properties have also been investigated.
Section snippets
Materials and methods
The WCO was collected from the canteen of the Free University of Bolzano, Italy. WCO free fatty acid value (FFA) was measured by following the AOCS method [27] and resulted to be 6 wt%. Methanol (99.9%), sulfuric acid (98%), n-hexane (99.5%), acetone (99.5%), methyl heptadecanoate internal standard for GC (99%) were purchased from Sigma Aldrich and Merck. Char samples used in this study were collected from a local commercial small-scale gasification plant based on downdraft reactor. The samples
Catalyst characterization
The acid density and the acid distribution were confirmed by the TPD-NH3 illustrated in Fig. 1. The spectrum illustrated that the desorption of the ammonia occurred at two different temperature points which were 420–465 °C and 890–987 °C, thus showing the presence of two types of acid sites. At lower temperature, the spectrum confirms the presence of weak bronsted acid sites whereas at higher temperature shows strong bronsted acid sites [28]. After the sulfonation of char, sulfur contents
Conclusion
The present paper proposes the use of char from small-scale biomass gasification plants to produce acid catalyst through sulfonation process. The synthesized wood-char catalyst shows high acid density, surface area, porosity and reusability. The wood-char catalyst has been used for the transesterification of waste cooking oil and showed a high catalytic activity. The 96% WCO methyl esters conversion yield was achieved under the optimized conditions of 6 wt% of wood-char catalyst loading, 9:1
Acknowledgements
The authors would like to acknowledge the Autonomous Province of Bolzano, Provincia Autonoma di Bolzano – Alto Adige for the financial support to the NEXT GENERATION project: “Novel EXTension of biomass poly-GENERATION to small scale gasification systems in South-Tyrol”, CUP B56J16000780003 and Inisiatif Putra Berkumpulan (Kod Project: GP-IPB/2016/9490400), Universiti Putra Malaysia, Malaysia.
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2022, Journal of Hazardous MaterialsCitation Excerpt :Numerous studies have attempted to convert the low-cost WCO feedstock into bioenergy using different technologies, including gasification, pyrolysis, hydrotreating, transesterification, etc. (Kim et al., 2020; Landwehr et al., 2021; Lam et al., 2016). Briefly, at high temperatures (>700 °C), without combustion, with a controlled amount of steam and/or oxygen, WCO could be rapidly converted into syngas at high conversion rates (Ahmad et al., 2018). However, the WCO gasification process suffers from high capital cost, significant input of gasification agent, and high heat input requirement (Franco et al., 2003; Watson et al., 2018).
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