1. Introduction
According to the recent report from the World Energy Outlook 2018 [
1], 93% of the world’s carbon capacity is already in use up to 2040. Consequently, there is a very narrow space for the development of fossil fuel projects over this period without contradicting international objectives about climate change. This implies that it is becoming inevitable to push on the development of alternative and renewable energy resources for the supply of reliable and environmentally efficient energy to the growing economic activities around the world. Among such alternative sources are biofuels [
2], which are mainly preferred for their carbon neutral character, their renewability, as well as the fact that they can be produced in decentralized manners from abundant and versatile resources. Biodiesel is one of the promising biofuels to substitute conventional fossil diesel. It has a number of environmental and technical benefits over conventional fossil diesel. Environmentally, biodiesel is non-toxic, biodegradable, and its greenhouse gas (GHG) emission is very low compared to the conventional fossil diesel [
3,
4]. The technical benefits are associated with its use for fuel; for example, its possession of more oxygen to favor complete combustion and its better lubricating character to reduce engine wear [
5].
Biodiesel can be produced from different oil and fat resources, which are found everywhere. Such feedstocks include edible and non-edible plant oil, animal fat, as well as waste oils and fats. The production of fuel quality biodiesel from oil and fat feedstock mainly involves the transesterification reaction with alcohol in the presence of some kind of catalyst or without catalyst at the supercritical condition. The transesterification reaction catalyzed by homogeneous base catalysts like NaOH and KOH is the conventional way of producing biodiesel at an industrial scale [
6], which requires relatively better-quality feedstock like edible oil with very low free fatty acid (FFA) content [
7,
8]. Such high-quality feedstock is usually associated with a high price. In addition, it creates food versus energy controversies. These reasons altogether have been making biodiesel the expensive alternative fuel compared to its counterpart-fossil diesel, because the cost of feedstock can take up to 80% of the total cost of biodiesel production [
9,
10]. Comparatively, the heterogeneous alkali-catalyzed transesterification reaction has advantages of easy catalyst recovery and reusability for multiple times [
11,
12]. Unlike the homogeneous ones, the heterogeneous alkali-catalyzed transesterification can tolerate a considerable amount of FFA in the feedstock. For instance, Avhad et al. [
13] reported that using glycerol-enriched calcium oxide as a heterogeneous alkali catalyst, a 96.1% of crude
Jatropha curcas oil containing high free fatty acid could be converted into biodiesel within 7 h. In addition, most of such heterogeneous base catalyst types can be easily prepared from cheap resources, indicating the potential to reduce biodiesel production cost. For example, industrial wastes (red mud, slag, ash) and biological wastes (chicken eggshells, mollusk shells, animal bones) have huge potential towards developing a cheap catalyst for low-cost biodiesel production [
14]. Among the heterogeneous alkali catalysts developed for biodiesel production, the main ones include basic zeolites, alkaline earth metal oxides, and hydrotalcites [
15].
The conventional chemical catalyst options also include homogeneous and heterogeneous acid catalysts. In general, acid-catalyzed transesterification is very efficient in the production of biodiesel from feedstock with very high FFA content [
16,
17]. However, the problems usually associated with the use of acid catalysts are high reaction temperature, longer reaction time, and corrosion of the equipment due to the acid catalyst [
10]. There are some substantial advantages of solid acid catalysts over the homogeneous ones. This includes ease of catalyst separation from the reaction media, which lowers product contamination and ease of catalyst regeneration and reuse, as well as much-minimized equipment corrosion [
18].
The other most promising technologies for the production of biodiesel from least cost feedstock involve CaO-based catalysts, enzyme catalysts, ionic liquid (IL) catalysts, and nanoparticle catalysts. Calcium oxide, as a catalyst, has such advantages as an abundance occurrence, better catalytic property, easy separation from the product stream, reusability for multiple times, nontoxicity, and least cost character for feasible production of biodiesel from lower-quality feedstock [
19,
20]. Boey et al. [
21] did a study on the production of biodiesel from waste cooking oil using a CaO catalyst derived from waste sources like mud crab shells and cockleshells. They calcined the CaO obtained from these wastes at 900 °C for 2 h separately and mixed them in a 1:1 mass ratio to catalyze the transesterification of the oil. According to their result, a 98% conversion could be achieved within 3 h for optimum reaction conditions of 5 wt.% catalyst and methanol to oil molar ratio of 13:1 at methanol refluxing temperature [
21]. In another study, Sasiprapha et al. [
22] assessed the production of biodiesel from used oil using CaO catalyst derived from river snail shells. For optimum reaction conditions of methanol to oil ratio of 9:1, catalyst amount of 3 wt.%, and reaction temperature of 65 °C, they could achieve 92.5% conversion of oil to biodiesel within 3 h [
22].
Even though the enzyme-catalyzed approach for biodiesel production is the expensive option, primarily due to the cost of the enzyme, the technical performance of most enzyme catalysts for the production of fuel quality biodiesel is very significant. Enzyme for the catalysis of biodiesel production has such advantages over the chemical catalysts as being less energy-intensive, allowing easy recovery of glycerol from the product stream and efficient conversion of acidic oil (oil with high FFA content) to biodiesel [
23,
24]. A study done by Cervero’ et al. [
25] indicated that a 95% conversion of soybean oil to biodiesel could be reached within 24 h using Novozyme 435 enzyme catalyst at optimum reaction conditions of 5 wt.% enzyme load, 3:1 molar ratio of ethanol to oil, and a temperature of 37 °C. Ketsara et al. [
26] also studied the production of biodiesel from used palm oil using mixed enzymes in a solvent-free environment. The studied mixed enzyme contained
Pseudomonas fluorescens and
Candida rugose. According to their result, 89% conversion could be realized within 12 h for optimum reaction conditions of 3:1 ethanol to oil molar ratio, 10% enzyme dosage, 2% water content of the oil feedstock, and a 45 °C reaction temperature [
26].
The other group of promising catalysts for biodiesel production are ionic liquids, which are generally known as solvents and green catalysts in chemical processes. Several ionic liquids are being used for the catalysis of biodiesel production from various low-cost feedstock alternatives. The use of such ionic liquids for biodiesel production provides considerable advantages over most other catalyst categories. Some of such advantages are low corrosion of equipment, ease of separation, recyclability, and less wastewater production [
27]. In addition, the lower reaction time, together with the ability to produce good quality biodiesel from low-cost feedstock, could make ionic liquid catalysis a better alternative than most of the catalyst options for biodiesel production. Feng et al. [
28] studied the transesterification process to produce biodiesel from palm oil using Brønsted acidic ionic liquid as a catalyst. They found out that conversion of 98.7% of the oil to biodiesel could be achieved within 2.5 h when the optimum reaction conditions are methanol to oil molar ratio of 21:1, catalyst dosage of 3 wt.%, and reaction temperature of 120 °C [
28]. In another study, Ullah et al. [
29] investigated the production of biodiesel from waste palm cooking oil using the acidic ionic liquid as a catalyst. They used specific ionic liquid butyl-methyl imidazolium hydrogen sulfate (BMIMHSO
4) as catalyst, and the highest biodiesel yield of 95.6% could be achieved with optimum reaction conditions of 5 wt.% of BMIMHSO
4, methanol to oil molar ratio of 15:1, 1 h reaction time at 160 °C reaction temperature and agitation speed of 600 rpm [
29].
Similarly, nano-catalysts are also becoming very interesting for the production of biodiesel from low-quality feedstock as they do have higher catalytic activity due to having large pore size and large surface area. Having large pore size and large surface area means possessing a more active catalytic surface, because the active surface of a catalyst, which is its vital property, increases when the size of the catalyst is reduced [
30]. Such higher catalytic character enables the use of a smaller amount of the catalyst compared to other catalyst options, and this has considerable economic benefits for large-scale production processes. Generally, by using nano-catalysts, better conversion of oil feedstock to biodiesel can be achieved at relatively medium temperature and shorter reaction time. The study done by Bet-Moushoul et al. [
31] indicated that the oil conversion range of 90%–97% could be attained within 3 h using CaO-based gold nanoparticles as a heterogeneous catalyst for transesterification of sunflower oil with methanol. For this conversion, the optimum reaction conditions were a reaction temperature of 65 °C, methanol to oil molar ratio of 9:1, and a catalyst loading of 3 wt.% [
31].
Table 1 shows some of the recent studies done on the optimum reaction conditions required to produce biodiesel from different feedstock types using bulk CaO, ionic liquid, enzyme, and nanoparticle catalysts.
Even though these four catalyst categories are technically capable of producing fuel quality biodiesel from various feedstock options, the relative economic feasibility of each production alternative remains unclear, as there are no such considerable studies performed so far to investigate the economic competitiveness of the alternatives. Accordingly, this study aimed at evaluating the techno-economic performances of bulk CaO, enzyme, ionic liquid, and nanoparticle catalyst technologies to produce biodiesel fuel from low-quality and cheap oil feedstock. The study would compare the proposed catalyst technologies in terms of their technological efficiencies and economic feasibility. Such an approach would give a complete view of the practicability of the process routes for sustainable production of biodiesel fuel. In addition, the study could be used as a preliminary estimate of the whole set up of the projects based on which detail estimates for the actual implementation of the efficient and affordable production process could be carried out. As to our knowledge, there are no similar investigations and comparisons performed among the catalyst technologies mentioned in this study.
The study was entirely based on process simulation involving all the unit procedures required to produce fuel quality biodiesel. These process simulations were designed using two commercial software - Aspen Plus and SuperPro. The technical performance evaluation was done based on the relative amount and purity of the product biodiesel and the byproduct glycerol, as well as the relative amount of biodiesel produced per amount of oil feedstock. Whereas the economic performance assessment was performed using economic parameters, such as total investment cost, unit production cost, net present value (NPV), internal rate of return (IRR), return over investment (ROI), and gross margin. The sensitivity of the technology options towards the change in market values of oil purchasing cost, catalyst purchasing cost, and biodiesel price was also assessed using NPV as a parameter.
3. Results and Discussion
The material and energy balance of the four technology options were carried out based on determined equipment size and the optimum reaction conditions taken for each dominant reaction in the processes. Using the results from material and energy balance together with the latest prices of raw materials, utilities, labor, and equipment, the techno-economic performances of the technology options have been evaluated and presented as follows.
3.1. Technical Performances
All the process options could provide fuel quality biodiesel and pure glycerol, proving that the catalysts used together with the unit procedures involved in separation and purification of the crude biodiesel could attain high-quality products. Accordingly, the biodiesel from all technology options fulfilled the American Society for Testing and Materials (ASTM) standards for biodiesel fuel quality. However, there was a slight variation in the amount of biodiesel and glycerol produced. In terms of biodiesel product, technology option IV had the highest performance, with about 98.98 kg/h product variation from the least performing one. This was mainly owing to the high catalytic activity of the nano-CaO particles, which favors the high conversion of the oil into biodiesel within relatively short reaction time. It might also be due to the negligible occurrence of the saponification reaction when zinc doped nano-CaO catalyst was used [
48], which also minimizes the likeliness of the catalyst being used by the FFA in the process of saponification. Relatively, the least performance in terms of biodiesel product was indicated in the ionic liquid catalyst option. This was due to the lower conversion percentage achieved in the given optimum reaction conditions taken from the literature [
39].
Similarly, the higher glycerol production amount was attained in technology option IV, with a product variation of 23.43 kg/h glycerol from the least performing option. This was again due to the variation in the achievement of oil conversion percentage according to the required optimum reaction conditions. Consequently, the option I and IV did have a relatively highest performance as they provided more amount of biodiesel from the same amount of feedstock used.
Table 5 indicates the relative technical performances of the technology options studied.
The catalysts from all the technology options could be recovered and reused for a number of times. This would help to reduce a considerable amount of money, which otherwise could be spent to purchase the extra catalyst. Another advantage of the processes was that in all the process options, except technology option II, there was no waste produced. In option II, the waste stream was composed of unreacted oil and calcium soap that could be further purified for economic benefits. The unreacted oil from technology option I and option III could be recycled directly to the processes, whereas the one from technology option IV should pass through a treatment step before it is reused in order to reduce the FFA content. This is because 76% of the unreacted oil from this process was composed of FFA, which was left unreacted in the nano-catalysis process.
3.2. Economic Performances
Technology option I was the most expensive alternative, mainly due to the very high cost of the enzyme, Novozyme 435. Even though this catalyst could be repeatedly used for more than 200 times [
54], and the process could give the second higher biodiesel product, the higher total investment cost of the option could not make it economically feasible for the production of biodiesel fuel. The higher production cost in option I was also attributed to its relatively larger reactor volume required due to longer reaction time. Because the larger the equipment volume, the higher would be the costs of equipment, facilities, and utilities.
The second expensive option was the technology option III. Its total investment cost was almost half of that of the option I and 37% higher than the least cost option, which was option II. This was mainly because of the second larger volume of reactor required due to longer reaction time as well as because of additional centrifugal decanter required to separate the catalyst. The larger and the more equipment we use, the higher would be the utility cost and the other equipment-associated costs. Technology option II had the least total capital investment cost because it required smaller equipment sizes due to minimum reaction time, and the catalyst involved was the cheapest among the catalyst options studied.
Even though technology option IV was the second cheapest option, the higher cost of the nano-CaO catalyst could still make it economically infeasible at the optimum market prices of raw materials and outputs. Similarly, option III was also found to be economically infeasible at the current market prices of raw materials and outputs. However, for the optimum market values of inputs and outputs considered, option II was the most feasible option with positive NPV, lower unit production cost, higher IRR, ROI, and gross margin.
Table 6 summarizes the economic performances of the technology options. It highlights the comparative economic performances of the process options for the given market scenario. The first part indicates the total investment cost, followed by expenditures in cost categories. The calculated revenues from the product and byproduct, as well as the value of the calculated economic parameters, are also indicated in the Table.
The higher amount of biodiesel product and glycerol byproduct for the nano-CaO-catalyzed option results in a relatively higher value of total annual revenue incurred, as shown in
Table 6. The lowest total annual revenue was recorded for ionic liquid-catalyzed option with about 684,976 US
$/year lower than the revenue from the nano-catalyzed option. The enzyme-catalyzed option scored the highest unit production cost with about 0.964 US
$ increment per kilogram of biodiesel product compared to the bulk CaO-catalyzed option. Positive after-tax IRR was recorded only for bulk CaO-catalyzed option. For the enzyme-catalyzed option, the gross margin, ROI, and NPV were the lowest, followed by the ionic liquid-catalyzed option.
Except for the bulk CaO-catalyzed option, the other three process options showed negative NPV. This indicated that the investment of each project was not profitable because the present value of the net cash flow in each project, within the projects’ lifetime, was lower than the present initial cash required to establish them. This was exhibited more by the values of IRR for each process option. The after-tax IRR of the bulk CaO-catalyzed option was positive, whereas, for the other three options, the calculated amount was negative, meaning that the projects did not perform well over time. For instance, without considering the discounted cash flow, i.e., only based on the amount of return and cost of investment, the nano-CaO-catalyzed option seemed efficient as it had a higher percentage of ROI. However, its NPV and IRR were negative, implying that the investment in this process option is not viable within the given lifetime.
Concerning the relative economic performances of the technologies, divergent results might be obtained if calculations are done in a different market scenario or using market values of inputs and outputs, which are not comparable to what has been used in this study. This implies that such performances are expected to be different for countries with different market scenarios.
3.3. Sensitivity Analysis
The economic feasibility of the studied technology options was very diverse, mainly due to the cost variation among the catalysts as well as the number and size of equipment required to attain fuel quality biodiesel. Thus, it seems reasonable to test how sensitive the technology options are towards the change in market values of the inputs and outputs. Among the various economic variables, oil cost and catalyst cost comprise the higher percentage of the raw materials’ cost. Similarly, biodiesel is the main product to get the desired revenue from the projects. Therefore, in this study, the economic effect of variations of oil purchasing cost, biodiesel selling price, and catalyst purchasing cost was evaluated in terms of NPV (at 7% interest rate); and the results among the technology options were compared and presented as follows. We considered biodiesel price since biodiesel is the main product, and its price fluctuation could have a direct effect on the feasibility of the businesses. We considered oil cost because the cost of feedstock took a higher share of raw material cost. In addition, we took the cost of catalysts for sensitivity analysis in order to indicate how the respective cost of the studied catalysts affect the businesses as well as to indicate the maximum possible cost of each catalyst for the economic feasibility of the businesses.
3.3.1. Effect of Change of Oil Cost on NPV
The trend at which the technology options respond towards change in oil purchasing cost was almost similar. However, the option I was found to be economically infeasible for all ranges of the oil purchasing cost considered. For option III, the maximum cost of oil feedstock was about 0.39 US
$/kg, beyond which the option would be economically infeasible. Option II was found to be more tolerant of the market variation of oil cost. It could, still, be economically feasible up to 0.59 US
$/kg of the oil purchasing cost. In comparison, option IV was found to be the second most tolerant of market fluctuations of the oil purchasing cost. Nevertheless, it could be economically feasible for oil purchasing cost less than 0.51 US
$/kg.
Figure 5 indicates the effect of the change in oil purchasing cost on NPV of the technology options.
3.3.2. Effect of Change of Biodiesel Selling Price on NPV
The changing trend of the technology options towards the variation of the market values of biodiesel price was almost similar. Option I was unprofitable for the considered ranges of biodiesel selling prices. For this option to be economically feasible, biodiesel should be sold at a very high price (1.8 US
$/kg), which is practically impossible. On the contrary, technology option IV could be economically feasible, with almost half of this price (0.97 US
$/kg). This designates that the production of biodiesel fuel using technology option I should be subsidized to make the fuel economically competitive with fossil diesel in the market. For the biodiesel selling price range considered, option II was found to be more tolerant of the possible fluctuation of biodiesel price and could still be economically feasible at a biodiesel price as low as 0.77 US
$/kg.
Figure 6 indicates the effect of variation of the biodiesel selling price on NPV of the technology options.
3.3.3. Effect of Change of Catalyst Purchasing Cost on NPV
The entire techno-economic comparison was among these four catalyst technologies. Thus, evaluating the effect of the market values of the four catalyst types could give reasonable ground for selecting the technology option that is more tolerant of fluctuating cost of materials in the market. The trend at which the NPV changes with the catalyst cost was almost the same for all technological options. However, some get negative NPV at very low-cost values and some at relatively higher values. For instance, an option I could still be economically feasible for about 61 US
$/kg cost of an enzyme catalyst. This was mainly due to its higher reusability that could reduce the total cost of the catalyst. Nevertheless, this higher price, indicated here, is not enough to buy the very expensive enzyme catalysts, especially immobilized ones [
54]. This demands more investigation on enzyme catalysts, which could be produced with a cost as low as 60 US
$/kg while possessing the same catalytic performance as indicated here. Option III was found to be the most sensitive towards a change in the value of catalyst purchasing cost. It was economically feasible for a catalyst cost of less than 4.1 US
$/kg. Option IV got its negative NPV for a catalyst cost of more than 5 US
$/kg. The cheapest catalyst was the bulk CaO catalyst in option II. It could be prepared from wastes, such as eggshell, crab shell, and river snail shell, among others. The cheap cost and higher reusability of the CaO catalyst made option II more tolerant of the possible fluctuations of catalyst purchasing cost in the market. It got its negative NPV for a catalyst cost more than 7 US
$/kg, which seems to be far from its current market value, as indicated in
Table 2.
Figure 7 shows the effect of variation of catalyst cost on the NPV of each technology option.