Jatropha curcas is a tall bush or small tree from the Euphorbiaceae family that grows up to 5–7 m tall [166]. The origins of the Jatropha tree are unknown, but it may have originated in Mexico or Central America. It was first introduced to Africa and Asia before expanding to the rest of the world. This drought-tolerant species grows in arid and semi-arid environments, preferring well-drained soils with excellent aeration, and can adapt to peripheral soils with low nutritional value. It grows fairly quickly, starts producing seeds after 12 months, reaches peak productive capacity after five years, and can produce seeds for up to 50 years [167] is a promising, commercially viable nonedible vegetable oil. The decorticated seed of Jatropha contains 43–59% oil [168] and 30–40% [169], depending on the variety. The plant is critical for tackling climate change issues since a mature plant or tree can absorb 18 lbs of CO₂ annually [170]. Becker et al. [171] found that one hectare of J. curcas can absorb up to 25 tons of atmospheric CO₂ every year for around 20 years. The oil of J. curcas contains approximately 47.3% crude fat, 24.6% crude protein, and 5.54% moisture [172]. Like most other nonedible oils, J. curcas contains a high concentration of FFAs. Oils obtained from the Jatropha plant contain saturated fatty acids such as stearic acid (7.0%) and palmitic acid (14.2%), as well as unsaturated fatty acids such as linoleic acid (32.8%) and oleic acid (44.7%) [173].

The cost of producing biodiesel is dependent on the size and type of seeds used (Table 11) [174]. Hence, Jatropha seeds appear to be a viable option for biodiesel production, as shown in Figure 12.

Recently, many researchers have investigated the pyrolysis of available vegetable oils from nonedible plants and their cakes, including J. curcas [175]. Telfah [176] concludes that Jatropha oil has a high FFA content of around 15%. Hence, a two-step pretreatment process is used to reduce the FFA content to less than 1%. The first step is a pre-esterification step conducted in methanol in the presence of a 1.0% (w/w) sulfuric acid (H₂SO₄) catalyst at a 0.6 (w/w) methanol to oil ratio at 50 °C for 1 h. The second step transesterifies the layer of Jatropha oil obtained using a 0.24 (w/w) methanol to oil ratio in the presence of an alkaline catalyst of 1.4% (w/w) sodium hydroxide (NaOH) at 50 °C for 2 h to produce Jatropha biodiesel with a yield of around 90%. When processed using 12 wt% methanol and 1.0 wt% H₂SO₄ for 2 h at a temperature of 70 °C, the acid value of the obtained Jatropha oil can be diminished from 14 mg KOH/g oil to less than 1 mg KOH/g oil. The FFA content was reduced by 97% after reacting with 4 wt% solid acid and molar methanol to FFA ratio of 20:1 for 2 h at 90 °C. Phospholipids are eliminated during the pre-esterification process, eliminating the need for a separate degumming operation. A 98% biodiesel yield was generated by transesterification when using a 1.3% KOH catalyst and molar methanol to oil ratio of 6:1 at 64 °C for 20 min. Qudah et al. [174] describe another two-step process. A homogeneous acid-catalyzed pre-esterification reaction was performed at a constant temperature of 60 °C. A 100 mL round-bottomed flask was filled with 25 g of Jatropha oil and treated with concentrated H₂SO₄ (1.2% w/w acid to oil ratio) and dried methanol (25% w/w methanol to oil ratio); the mixture was refluxed 60 °C for 3 h. After washing the mixture to remove residual methanol and sulfuric acid, the products were dried with anhydrous Na₂SO₄. In the transesterification step, a KOH catalyst was used in a homogeneous base-catalyzed transesterification for 3 h at 60 °C. A 250 mL round-bottomed flask was filled with 40 g of pre-esterified Jatropha oil and heated to the appropriate temperature; the mixture was then treated with methanol (6:1 methanol to oil ratio) and KOH (1.2% w/w KOH to methanol ratio). The mixture was refluxed for 3 h while being continuously stirred. The products were then washed with distilled water and dried with anhydrous Na₂SO₄. This two-step process reduced the acid value of the raw Jatropha oil obtained to 0.30 mg KOH/g oil.

Qudah et al. [174] found that dolomite is a natural rock found in Jordan that contains 66% Ca, 28% Mg, and 6% Fe that can be used as a catalyst in the transesterification process. The dolomite should be crushed, finely ground, and exposed to a series of thermal treatments under flowing oxygen or ambient air before use. Two procedures that employed a dolomite catalyst were tested, and both involved treatments at 800 °C for 2 h. In the first procedure, 10 g of ground Jatropha seeds were blended with 4.0 g of dolomite catalyst, 100 mL of chloroform, and 75 mL of methanol in a 250 mL volume round-bottomed flask. The mixture was refluxed for 10 h, filtered to separate the Jatropha powder, and evaporated using a rotary evaporator to remove the solvent from the filtrate. In the second procedure, 10 g of ground Jatropha seeds were blended with nearly 100 mL chloroform in a 250 mL round-bottomed flask. After filtration, the liquid phase was transferred into a 250 mL flask and blended with 4.0 g of dolomite catalyst and 75 mL of methanol. The mixture was constantly agitated for 2 h at 60 °C and subsequently refluxed for 10 h before being evaporated with a rotary evaporator to remove the solvent under reduced pressure. To optimize the transesterification process (>96% conversion rate) of Jatropha seed oils at 60 °C and 6:1 methanol to oil ratios, the dolomite should be thermally activated at 800 °C for at least 30 min. This heightened activity is correlated with the creation of CaO but not MgO, which forms when heated to 600 °C. The minimum mass ratio of catalyst to oil required to achieve such an ideal biodiesel yield is 1:50 (2%); It is important to note that the catalyst cannot be recycled and can only accomplish a 2% transesterification rate if reused. Nevertheless, activated dolomite can be considered to be a less expensive alternative to the more commonly used KOH catalyst.

The viscosity of J. curcas oil/diesel fuel blends at ratios of 50:50 and 40:60 is similar to that of pure diesel fuel at the temperature of 328–333 K and ~318 K, respectively, while a mixture of J. curcas oil/diesel fuel at a ratio of 30:70 has a viscosity similar to diesel fuel at the temperature of 308–313 K [177].

Figure 13 shows that biogas technology has grown in popularity over the last few decades [178]. Biogas can be used as a replacement for natural gas (NG) in stoves. It can also be used to produce electricity by feeding an internal combustion engine that is directly connected to an electrical generator. The extra heat from the combustion engine can heat an entire room (space heating) as well as heat the anaerobic digester [36]. However, these applications have not yet been widely adopted in developing countries [178]. Al-hamamre et al. [31] concluded that Jordan is theoretically capable of producing approximately 706.9 MCM of biogas per year even without taking the availability of energy into account; this is equivalent to around 388.1 MCM of NG per year or 2.57 MCM of barrel oil per year. Al-hamamre et al. [37] reported that the total amount of biogas that Jordan could theoretically generate is around 428.19 MCM/year and that it can also generate around 235.5 MCM of methane (CH₄) per year. This amount would account for 29.2% of Egypt’s total NG imports in 2011 (Table 12). Al-hamamre et al. [36] also found that if Jordanian biogas was used for electricity generation, this would generate approximately 917.41 GWh_e of power per year, which would be 17% more than the total amount of power imported from Syria and Egypt (784.3 GWh_e), assuming a conversion efficiency (biogas to electricity) of 27.5%.

As a sustainable source of energy, biogas can play a key role in maintaining the balance between long-term advancement, economic strength, and the preservation of the environment in Asia [179]. Recently, the anaerobic digestion of agricultural and industrial waste has emerged as one of the most appealing sustainable energy solutions. The most popular digester designs are fixed domes (Chinese type) and floating covers (Indian type) [178]. However, Surendra et al. [180] found that these two types of digests are difficult to adopt in developing countries; they do not provide temperature control, have prohibitively expensive installation costs for farmers, and require specialized skills. The biogas produced from biological waste is mostly a natural process. Hence, a majority of the project’s costs involve the collection of waste and the construction of the biogas plant [158]. Al-hamamre et al. [37] found that the total annual power that Jordan might theoretically generate from CH₄ was around 698.1 GWh_e, which is equivalent to around 5.09% of energy consumption in 2011 and 39.65% of the power imported from Egypt and Syria. Abu Qdais et al. [181] also calculated that if all potential biomass in Jordan was used to generate energy in 2012, about 53.35% of the primary energy consumption could be replaced by NG or could replace about 66.23% of the total amount of NG imported from Egypt. They found that 28.34% of this biogas could be generated from animal waste and that 62.18% of this amount could be derived from sheep and goat waste, with poultry residues contributing the remaining 32.29%. Biogas production from dung could be used as a substitute for conventional fossil fuels in the generation of electricity and would thus contribute to the reduction of GHG emissions and other pollutants [178]. Al-hamamre et al. [36] found that the use of anaerobic digestion to produce biogas would be an alternative to traditional waste disposal methods in Jordan (such as landfilling or incineration), which emits toxic gases such as CO₂ and carbon monoxide (CO). Jafar and Awad [178] reported that the most common applications for biogas were limited to lighting and cooking in developing countries. Utilizing biogas in rural areas would cut down on firewood consumption, prevent deforestation, and boost soil productivity; hence, biogas could contribute significantly to sustainable development. Milbrandt and Overend [182] concluded that the amount of biogas produced is primarily dependent on the digester volume. The main components of biogas are CH₄ (50–70%) and CO₂ (30–50%), with the rest being trace gases (hydrogen and nitrogen) [36]. Wang Ris [183] found that a pretreatment process was necessary to enhance CH₄ production via anaerobic digestion. Al-hamamre et al. [36] concluded that when co-digesting agricultural waste with cattle manure, the yield of biogas products increased by 0.62 L/kg volatile solid (VS). About 85.62 GWh_e of electricity could be obtained if the MSW in the Akaider landfill was used, saving approximately 92.86–194.8 × 10³ t/year of CO₂. Furthermore, nearly 538.2 GWh_e of electricity can be obtained by using the MSW in the Al-Ghabawi landfill with CO₂-equivalent savings of 582–1220 × 10³ t/year. Each ton of MSW used to produce electricity can reduce CO₂ emissions by around 725–1520 kg. If the entire supply of MSW was used to obtain electricity, this number could reach 1005 × 10³–2108 × 10³ kg. Malkawi et al. [158] found that in the absence of “fuel costs”, operational expenses tended to be less than that of traditional power plants. However, due to socioeconomic and institutional barriers, the prospective growth of the Asian biogas industry is relatively poor [179]. Aggarangsi et al. [184] found that biogas digesters significantly decrease environmental pollution.

A summary of Jordanian bioenergy research and how it compares to global bioenergy technology is presented in Table 13 and Table 14.