Sustainable Processes Reusing Biomass Potassium-Rich Ash as a Green Catalyst for Biodiesel Production: A Mini-Review

1. Introduction

With the increase of the population, the improvement of living-level and the requirements of the transportation, the upward increase in the demand for fossil fuels has become a long-term trend. However, using these non-renewable fuels has negative impacts on the environment and the human-being health. When fossil fuels are combusted or burned, they release large amounts of air pollutants [1], including carbon dioxide, sulfur oxides, nitrogen oxides, particulates and other compounds (e.g., methane, nitrous oxide) or traces of hazardous substances (e.g., heavy metals, dioxins). Some of them belong to greenhouse gases, thus causing global warming and climate change. In this regard, one of the solutions for mitigating the climate crisis or greenhouse effect is based on the use of renewable fuels from the environment-friendly sources like edible oils (or their spent cooking oils) and lignocellulosic alcohols. Over the past decades, diesel fuel from renewable resources for the production of biodiesel in replacements of petroleum-based chemicals has drawn attention in particular. According to the data on the global biodiesel consumption [2], the worldwide biodiesel consumption in 2023 amounted to 65.86 million metric tons. This figure was very different from only 2.2 million metric tons consumed in 2004. The consumption amounts were expected to increase further over the coming years by exceeding 75 million metric tons in 2030 (estimated). Although biodiesel exist some drawbacks (e.g., high production costs and the competition with the food), it has some features such as sustainability, biodegradability, low sulfur content, and low toxicity [3].

Biodiesel, a mixture of fatty acid alkyl esters (FAAEs), is commonly obtained by the chemical reaction of edible/ non-edible oils & fats and waste/recycled cooking oils (vegetable oils) with an alcohol (usually methanol). This reaction is called as transesterification (or alcoholysis) in the presence of an acid/base catalyst at moderate temperature. Figure 1 showed the transesterification reaction for producing biodiesel and glycerol (a by-product). Herein, the term “triglyceride” is used for edible/non-edible oils that are liquid state at normal temperatures. Generally, they are water-insoluble products of naturally occurring plants like peanut, sunflower, linseed and olive. A triglyceride is an ester product obtained from one molecule of glycerol and three molecules of fatty acids like palmitic acid (a saturated fatty acid with a 16-carbon chain by noting C16:0), oleic acid (a mono-unsaturated omega-9 fatty acid by calling C18:1) and linoleic acid (a di-unsaturated fatty acid with containing 2 double bonds at the ninth and 12th carbons, thus denoting C18:2). The most commonly used catalysts are alkaline by using potassium hydroxide (KOH) and sodium hydroxide (NaOH). Therefore, the combined amount of residual potassium and sodium in the 100% biodiesel fuels was limited to 5 mg/kg by two major specifications or standards (i.e., the ASTM D6751 in the USA and the EN 14214 in Europe) [4].

To date, different catalysts have been used to produce biodiesel. They may be alkaline, acid, or enzymatic catalysts [5,6,7,8]. Although enzymatic catalysts feature the non- formation of soaps and the easy refining of crude biodiesel, they have been limited to produce biodiesel commercially due to the high operation costs and long reaction time [9,10]. On the other hand, acid catalysts involved in the transesterification reaction often require longer reaction time and higher ratios of oil to alcohol. More seriously, these catalysts are very corrosive, causing the difficulty of managing them. In this regard, the commercial biodiesel production is mostly based on employing homogeneous alkaline catalysts such as KOH and NaOH. It was summarized that the alkali-catalyzed transesterification process has several advantages over other catalyzed ones, including lower operating temperature (< 80℃) at normal pressure and high biodiesel yields within short reaction times, indicating its effective performance as compared with acid-catalyzed transesterification. Despite significant features of homogeneous alkaline catalysts, it is difficult to be recovered and reused after the transesterification reaction, causing the necessary neutralization process by water washing in the refining and thus generating a large amount of wastewater to be treated [11,12]. In addition, these alkaline salts or their solutions must be careful for safe handling during the operation and storage due to their chemical properties and health hazards.

To overcome the drawbacks of homogeneous alkaline catalysts, many review papers on biodiesel production by using heterogeneous base catalysts have been published [12,13,14,15,16,17,18,19,20,21,22]. From the economic and environmental points of view, these solid base catalysts are also expensive and non-renewable. Therefore, there were several review papers of focusing on the reuse of biomass-based ash [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38], which may be rich in silica (Si) and alkali & alkaline earth elements such as calcium (Ca). In the present work, we reviewed potassium-rich ash from herbaceous & agricultural biomass resources as a green catalyst for biodiesel production in the transesterification reaction. Their potassium-containing mineral may be the forms of oxides, carbonates, hydroxides, chloride, nitrates, sulfides, and phosphates, and their mixtures.

2. An Overview of Biodiesel Production Processes

As mentioned above, the core process for biodiesel production is based on transesterification reaction where the interchange of an R group of esters (i.e., triglycerides) with R group of alcohol (methyl group from methanol, or ethyl group from ethanol) under the catalysis by acid or base catalysts (seen in Figure 1), thus yielding a different alcohol (i.e., glycerol, as a by-product) and ester (i.e., fatty acid methyl ester, or biodiesel). Commercially, biodiesel production generally employed homogeneous alkaline catalysts (e.g., sodium hydroxide and potassium hydroxide) due to the high reaction efficiency under the mild conditions. However, the contents of free fatty acids and water in the starting feedstock should not be more than threshold values when using these alkaline catalysts. More noticeably, the neutralization process must be performed by water washing, which generates a large amount of wastewater. Figure 2 illustrated the schematic diagram of biodiesel production by using methanol as a strong nucleophile [29], which also includes several processes (except for transesterification) like pretreatment of feedstock, separation of biodiesel product mixture (e.g., decanting, or gravity settlement), water-washing of crude biodiesel, drying of wet biodiesel (for removal of residual water) by evaporation), and refining of crude glycerol (and recycling of methanol) by distillation process. In the separation process, the biodiesel mixture is first allowed to cool down at room temperature (from about 60-80℃) and settle in the settling tank. The polar fractions with heavier density at the bottom contain crude glycerol, unreacted methanol and remaining catalyst, settles, while the non-polar fractions with ester-rich phase (i.e., crude biodiesel) exist the upper layer.

Generally speaking, the first process in the biodiesel production system is to remove the free fatty acids (FFA) present in starting oils because it can render the production of biodiesel by the reaction of FFA with alkali, thus causing saponification and emulsification. These excessive soaps in the crude biodiesel products can reduce the yield of biodiesel and also make the separation processes (including glycerol separation and water washing) more complicated [39]. To avoid this undesirable problem, the pretreatment of feedstock oils is processed in two stages. The first stage is to lower the FFA contents to the desired level by the esterification with a strong acid (e.g. sulfuric acid). In the subsequent stage, the purified feedstock is reacted with a homogeneous/ heterogeneous catalyst to produce the FAME (i.e., biodiesel). Another core consideration in the biodiesel production is to adopt a proper catalyst. The most effective transesterification reaction may be via enzymatic catalysts, but this method is very expensive and slow as compared to those by acid or base catalysts. The commonly used homogeneous catalysts are based on the use of NaOH and KOH. They have led to higher FAME yields at shorter reaction time. However, homogeneous catalysts have some disadvantages, including soap formation, non-recyclability, non-renewability, non-easy for separation and equipment corrosion [40]. In this regard, the heterogeneous base catalysts produced from biomass-based ash have prevailed with their features, which include environmentally benign, catalyst reusability by easy separation, relatively low toxicity, elimination of wastewater treatment from water-washing step, and consequent reduction in production cost. These biomass-based catalysts can be derived from crop-, wood-, herb-, and animal-based residues or remainders, which have been reviewed by several researchers [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38].

3. Overview of Solid Heterogeneous Catalysts for Biodiesel Production

As stated above, solid heterogeneous catalysts are promising candidates for producing biodiesel from vegetable/edible oils or waste cooking oils. Alkaline earth metal oxides (e.g., CaO, MgO) are commonly used in the biodiesel production due to their high reaction rates in comparison with those of acid catalysts. Other alkali forms, including supported alkali metals, supported alkaline earth metal oxides, alkali & alkaline earth composites, zeolites & modified zeolites, lanthanide-based, ion exchange resins, zirconia(Zr)-based oxides, and oxides of other transition metals (e.g., zinc), were also investigated in the literature [13,14,15,16,17,18,19,20,21,22,23]. Potassium oxide (KOH) was also used for alternative of CaO. However, the latter features lower solubility, higher reactivity (basicity), lower cost and easier handling in comparison with the former. Obviously, these base catalysts are less corrosive and can be performed in various reactors, resulting in low cost and more environment-friendly processes due to the catalyst recyclability and continuous production. Moreover, wastewater treatment costs can be reduced by using heterogeneous catalysts as compared to the biodiesel production process with homogeneous catalysts. In addition, such catalyst can tolerate those oils with higher FFA contents, thus eliminating the pretreatment (pre-esterification) stage (as seen in Figure 2).

Despite the advantages of solid heterogeneous base catalysts, producing efficient, environment-friendly and low-cost catalysts from renewable resources is essential in order to highlight the sustainability in the biodiesel production. Various cost-effective heterogeneous solid-base catalysts from a wide variety of biological and agricultural residues for sustainable biodiesel production have been investigated over the past decade [24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. Most of these biomass resources enrich in metallic base minerals such as potassium (P), silicon (Si), magnesium (Mg) and calcium (Ca) [28]. For example, some plat-based ash materials contain higher contents of P. They include banana peel ash, cocoa pod husk ash, coconut husk ash and palm bunch ash. The biomass-based ash with high Si contents often referred to rice-derived residues such as rice straw and rice husk. Another notable Ca-rich biomass should be associated with the outer shell of shellfish (e.g., oyster, clam, mussel) and chicken egg. It is mainly composed of calcite (one of carbon carbonate, CaCO₃), which must be calcined at above 800℃ to produce calcium-rich catalyst (i.e., calcium oxide, CaO) [41,42].

4. Biomass Potassium-Rich Ash as a Green Catalyst for Biodiesel Production

It was reported that biomass or its ash is normally enriched in inorganic minerals by denoting oxides such as calcium oxide (CaO), potassium oxide (K₂O), magnesium oxide (MgO), sodium oxide (Na₂O), phosphorus pentoxide (P₂O₅), alumina (Al₂O₃), iron oxide (Fe₂O₃), silica (SiO₂), sulfur trioxide (SO₃), and titanium dioxide (TiO₂) in comparison with coal ash [43,44,45,46]. On the other hand, high variability in biomass (or biomass ash) characteristics could limit its industrial valorization such as a green catalyst with solid heterogeneous form for biodiesel production [47]. These inorganic components could lead to tendency of slagging, fouling, and corrosion in the industrial boilers [48,49,50]. According to the classification by Vassilev et al. [43], these inorganic compositions can be classified into three groups:

(1)

Ash elements with acidic feature and high melting points

They include Si, Al, Fe, Na and Ti. For example, melting point of silicon dioxide (SiO₂) is about 1700°C.

(2)

Ash elements with base feature and less volatiles at high temperature

They can refer to Ca and Mg. In the case of calcium oxide, its melting point is as high as over 2,500°C.

(3)

Ash elements with base feature and more volatiles at high temperature

They can involve K, P and S. For instance, melting point of potassium oxide (K₂O) is approximately 400°C.

It is well known that the mineral matters or inorganic species could be phosphates, carbonates, chlorides, silicates, hydroxides, fluorides, sulfates, nitrates, and other mineral classes [41]. Table 1 listed the data on melting temperature and boiling temperature of common potassium-containing salts, which were compiled from their safety data sheet (SDS). It showed that most of them may be decomposed, evaporated or vaporized at higher temperatures, thus decreasing the basicity and catalyst (i.e., biomass ash) performance. Li et al. studied the evaporation rate of potassium chloride in combustion of herbaceous biomass (corn stover) [51]. It was found that that most potassium-containing mineral exists as potassium chloride (its melting temperature at 772°C), which will be released at normal combustion temperatures (about 800°C) under the investigation of thermogravimetric analyzer (TGA).

Although Ca/biomass-based base catalysts have some advantages, they also involve the challenges such as high cost due to high temperature for calcination in the CaO preparation step, thus limiting its commercial biodiesel production. Under the considerations of caustic catalyst KOH and biomass-based podash (a potassium-rich ash derived from biomass like cocoa pod husk) for commercial biodiesel production, Table 2 summarized the mineral compositions of some herbaceous & agricultural biomass with higher potassium (K) content [43], including bamboo whole, bana grass, buffalo gourd grass, alfalfa straw, mint straw, almond hulls, almond shells, coffee husks, cotton husks, grape marc, olive residue, pepper residue, plum pits, soya husks, sunflower husks, walnut blows, walnut hulls & blows, and walnut shells. It also showed that herbaceous & agricultural biomass contains higher ash contents, thus having more ash-forming elements (K, Ca, Si) than most of forestry or woody biomass. Recently, biomass-derived heterogeneous potassium-rich catalysts have been a popular valorization research in the biodiesel production field due to its potentiality for cost reduction and environmental sustainability. These biomass-based potassium-rich ash precursors for biodiesel production catalysts include cocoa pod husk [52,53,54,55,56,57,58,59,60,61], kola nut pod husk [59,62], pumpkin pod husk [59], Annona squamosa L. peel [63], coconut husk [64], palm oil [65], orange peel [66], Sesamum indicum plant [67], Heteropanax fragrans (Kesseru) [68], radish (Raphanus sativus L.) leaf [69], tucumã peel (Astrocaryum aculeatum Meyer) [70], pineapple (Ananás comosus) leaf [71], Avocado peel [72], plantain fruit peel [73], Brassica nigra plant [74], Carica papaya stem [75], banana trunk [76], Musa acuminata peel [77], moringa leaf [78], palm kernel bunch [79], palm kernel shell husk [80], Musa balbisiana Colla peel [81], Karanja seed shell [82], sugarcane leaf [83] , walnut shell [84] , hazelnut shell [85] , Acai seed [86] , wheat bran [87] , ginger leaf [88], and banana peel [42,56]. The inorganic elements of these herbaceous & agricultural biomass and its ash were preliminarily determined by energy dispersive X-ray spectroscopy (EDS or EDX) or X-ray fluorescence (XRF). However, the more accurate data on these elements listed in Table 2 must be performed by inductively coupled plasma optical emission spectrometry (ICP-OES) orinductively coupled plasma with atomic emission. spectroscopy (ICP-AES).

5. Conclusions and Perspective

In conclusion, a comprehensive overview focusing on the use of potassium-rich solid catalysts from a wide range of herbaceous & agricultural biomass-based ashes is presented. The practical application of these catalysts in biodiesel synthesis have been studied to produce successfully the green fuels with high yield from various non-edible vegetable oils. We also summarized some advantages of heterogeneous catalysts in comparison with homogeneous catalysts. In addition, producing efficient, environment-friendly and low-cost solid base catalysts from renewable resources is essential in order to highlight the sustainability in the biodiesel production. Due to cocoa pod husk (CPH) that’s rich in potassium-containing minerals, it was found that its derived ash may be the most commonly used solid base catalyst in the biodiesel production. Further research should be also needed to test the transesterification reactivity of these CPH-derived catalysts after purification.

Furthermore, the future efforts will concentrate on the cost-down of potassium-rich biomass-based ash, especially for those with significant generation amounts around the world such as cocoa pod husk, palm kernel (its shell and empty bunch), coffee residues (spent coffee ground, coffee husk) and banana (its peel, leaf and trunk). Due to the high energy content of these biomass-based residues (dry basis), they are often reused as solid auxiliary fuels in the industrial boilers, thus producing large amounts of bottom ash for recycling. One of valorization routes is to improve their purities for enhancing the catalytic performance in the transesterification process. In this perspective, we are actively looking for green potassium-rich catalysts, particularly by biomass-derived husks, in the sustainable biodiesel production.