Pakistan’s energy is mainly dependent on the imported fossil fuels as the explored fossil fuels of the country are insufficient to meet the country’s current energy needs. Meanwhile, these fossil fuels have negative environmental consequences and are too expensive to electrify remote areas. To address the country’s serious energy shortages, Pakistan’s Alternative Energy Development Board (AEDB) has suggested to introduce energy mix to meet the increasing energy demand and fuel the economy. Renewable energy endorsing unique environmentally friendly nature, constant supply, wider availability and ease of integration into existing infrastructure. Biodiesel is considered the best and most easily accessible source of energy among all renewable energy resources. However, there is still substantial room for development of renewable energies in Pakistan. This literature review examines the availability of biomass resources in Pakistan and their potential for meeting the country’s rapidly growing energy demand, boosting Country economy and creates new employments in the near future.
- renewable energy
- fossil fuels
- bifunctional heterogeneous catalysts
Energy has been a fundamental need of a human society. On the other hand, energy consumption has increased exponentially due to rapid growth in population and modernization . The population of world has grown after Second World War, from two billion to seven billion in the 21st century [2, 3]. Currently fossil fuels are the major source for the primary energy of the world (Figure 1) [4, 5].
According to the International Energy Outlook 2013 set by the U.S Energy Information Administration [6, 7], the total energy consumed in 2010 was 5.5282 × 1020 J, which is predicted to rise further to 8.6510 × 1020 J by 2040. Accordingly, the total world energy consumption will grow by 56% between 2010 and 2040; as given in Figure 2. The mismatch between the energy supply and energy demand has increased dramatically all over the world.
The limited fossil fuelsand the associated problems such as energy security environmental issueshave emphasized the need for sustainable, reliable renewable energy sources.
In the view of the current energy scenario, renewable energy sources could be fantastic choice for the world to meet the increasing energy demand and socio economic development. Renewable energy sources are gaining much attention due to their non-toxicity, biodegradability and low emissions profile as compared to petro diesel [8, 9]. According to US energy information administration, there are seven countries (Paraguay (100), Iceland (100%), Costa Rica (99%) Norway (98.5%) Austria (80%), Brazil (75%) and Denmark (69.4%) in the World to have or very near to 100 percent renewable energy sources. Resources of renewable energy are available on large scale such as hydropower, solar, biomass, wind and geothermal energy Figure 1. The fossil fuels substitution with renewable energy sources will have very positive effect on greenhouse gases emissions. It has been reported that 2% replacement of fossil fuels with renewable energy sources will result in 1.8% reduction of emissions of CO2 while replacement of 100% will lead to 90% reduction . In the current energy scenario, renewable energy sources could be a fantastic choice for the World to meet the increasing energy demand. Among them, biodiesel is considered to be the most reliable and consistent source of renewable energy supply.
2. Biodiesel as an emerging energy resource
Biodiesel may be defined as an oxygenated, non-toxic, biodegradable, eco-friendly and sulfur-free alternative diesel oil. Chemically biodiesel may be defined as a fuel that is composed of mono-alkyl esters of long chain fatty acids obtained from renewable sources such as animal fats, vegetable oilsthat comply the ASTM and European quality standards. Different natural oils are used for the production of biodiesel such as coconut, rapeseeds, soybeans and waste cooking oil (Figure 3).
3. Biodiesel production technologies
Several efforts have been made to produce derivatives of vegetable oil that can approximate the performance and properties of hydrocarbon-based diesel fuels. The problems associated with the vegetable oil to be used as diesel fuel are high viscosity, low stability against oxidation and the subsequent reactions of polymerization, low volatility due to which incomplete combustion occurs, resulting in the formation of high amount of ash . Different process can be used in order to change these properties such as direct use or blending, micro emulsion, pyrolysis (thermal cracking) and the most conventional process is the transesterification.
3.1 Direct use or blending
In beginning of 1980, there was a considerable discussion about the use of vegetable oil as a fuel. The concept of using food as a fuel was explained in 1981 by Bartholomew, demonstrating that petroleum should be the alternative fuel for combustion rather than the vegetable oil. Direct use of vegetable oils has been considered impractical and not satisfactory for both direct and indirect diesel engines. The high viscosity, free fatty acid content, acid composition and the formation of gum due to polymerization and oxidation during storage and combustion are the obvious problems.
Ma et al.  highlighted two severe problems such as incomplete combustion and oil deterioration associated with the direct use of vegetable oil as a fuels. Therefore, it will be significant to dilute the vegetable oils with some materials such as diesel fuels, ethanol or solvents to reduce the density and viscosity of vegetable oils.
Bilgin et al.  reported that 4% ethanol addition to diesel fuel increased the brake torque, brake thermal efficiency and brake power while decreasing the consumption of brake specific fuel. As the ethanol boiling point is less than the diesel fuel, ethanol could assist the process of combustion through an unburned blend spray.
3.2 Pyrolysis or thermal cracking
Generally, pyrolysis may be defined as the thermochemical decomposition of feedstock at medium (300–800℃) to high temperatures (800–1300℃) in an inert atmosphere. Pyrolysis means a chemical change that is caused by the application of thermal energy in the absence of oxygen or air or by the application of heat in the presence of catalyst that results in the bonds cleavage and formation of various small molecules . Being a type of destructive distillation, it is performed in an inert atmosphere in the temperature range of 300–1300℃. Based on the operating conditions, pyrolysis may be classified into three subclasses such as conventional pyrolysis that occur in the temperature range of 550 K–900 K, (400–500℃) fast pyrolysis occurring in 850 K–1250 K (400–650℃) and the flash pyrolysis occurs in the 1050 K–1300 K (700–1000℃) range of temperature. Pyrolysis is the process used for the synthesis of fuel from triglycerides, vegetable oil, animal fats or natural fatty acids. Fast pyrolysis is used for the bio-oil production. Vegetable oils can be cracked to improve cetane number and reduce the viscosity. The products obtained as a result of cracking include carboxylic acids, alkanes, alkadienes, alkenes and aromatics in various proportions. Rape seed oil, cotton seed oil, soybean oil and other oils with the use of appropriate catalyst were successfully cracked to get biofuel.
Micro-emulsions are isotropic, translucent or clear, thermodynamically stable dispersion of water, oil, surfactants, co-surfactants (amphiphilic molecule) for stabilization. In micro-emulsions, the droplet diameters range from 100 to 1000 Å (10 nm–100 nm). A micro-emulsion can be made of vegetable oils with an ester and dispersant (co-solvent) or vegetable oil with alcohol and surfactant with or without diesel fuels .
Alcohols such as ethanol or methanol are frequently used as a viscosity lowering additives. Whereas higher alcohols are used as surfactants. The alkyl nitrates are also used as cetane improvers. It has been reported that micro-emulsion can results in the reduction of viscosity, increase in cetane number and good spray characters in the biodiesel. However, continuous use of micro-emulsified diesel causes problems in engine such as formation of carbon deposits, injector needle sticking and incomplete combustion.
3.4 Transesterification (alcoholysis)
Transesterification is a process that involves the reaction of triglycerides such as vegetable oil, with alcohol in the presence of a catalyst to produce 3 moles of fatty acid esters and one mole of glycerol . Catalyst is used to increase the rate and yield of the reaction. The reaction is reversible. Excess alcohol is used to shift the equilibrium to the product side. Suitable alcohols such as methanol, ethanol, propanol, butanol and amyl alcohol are used for the transesterification reaction. Among these methanol and ethanol are most frequently used because of their low cost and physical and chemical advantages (polar and shortest chain alcohol). The fatty acid methyl ester (FAME) obtained by this process can be used as an alternative fuel for diesel engines . The catalyst used for transesterification may be acid or base (homogeneous or heterogeneous) and lipase enzymes. Transesterification reaction depends on various factors such as catalyst concentration, nature of the feedstock, molar ratio of alcohol-oil, agitation rate, temperature, reaction time, amount of free fatty acids and moisture content . Transesterification is a reversible reaction and proceeds by mixing the reactants under heat. In this process, 1 mole of triglyceride react with 3 moles of alcohol gives 3 mole of fatty acid alkyl ester and 1 mole of glycerol in a sequence of three reversible reactions where the triglyceride are converted to diglycerides and then to monoglycerides as shown in Figure 4. From each step, one molecule of alkyl ester is produced (Figure 5).
4. Catalysts used for biodiesel production
The catalysts used in the transesterification reaction, are extremely important to the group. The presence of a catalyst speeds up the reaction, increasing the yield of the final product. These catalysts are classified into two major categories: homogeneous catalysts and heterogeneous catalysts, each of which can further be divided into subgroups. The classification is shown in Figure 6.
4.1 Homogeneous transesterification
4.1.1 Homogeneous base-catalyzed transesterification
The base catalysts used for the process of Transesterification include KOH, NaOH, carbonates and corresponding potassium and sodium alkoxides such as sodium ethoxide, sodium methoxide, sodium butoxide and sodium propoxide. The alkaline catalyzed Transesterification reactions are 4000 times faster than acid catalyzed Transesterification reactions. As compared to acidic catalyst, the base catalyst are less corrosive to industrial equipments, hence alkaline catalysts are mostly employed in commercial. However, the base catalysedTransesterification reaction is affected significantly by the presence of free fatty acid (FFA) and moisture content in the feedstock. Therefore, the glycerides and alcohol used for Transesterification must be substantially anhydrous. It has been recommended that the FFA contents should be less than 2%, whereas the moisture content below 0.5 wt%. As the value of FFA is inversely proportional to the conversion efficacy, therefore small amount of water and high FFA contents present in animal fats and vegetable oils results in the deactivation of the catalyst and cause saponification (soap formation), which consequently decrease the biodiesel yield and renders the separation of glycerol and ester . So, low free fatty acid content in triglycerides is required for base catalyzed Transesterification. Homogeneous acid catalyst is then referred for Transesterification.
Generally, the mechanism of base-catalyzed Transesterification of animal fats or vegetable oils involves four steps [13, 21]. In the first step, the base react with the alcohol gives an alkoxide and protonated catalyst. In the second step, nucleophilic attack of the alkoxide at the carbonyl group of the triglycerides and generates a tetrahedral intermediate. In the third step, alkyl ester and corresponding anion of diglyceride is produced. The final step involves the deprotonation of the catalyst to regenerate the active species that is able to start another catalytic cycle by reacting with the second molecule of the alcohol. Same mechanism is followed by the diglycerides and monoglycerides to convert to a mixture of alkyl esters and glycerol. The mechanism is summarized in the Figure 7.
4.2 Homogeneous acid-catalyzed transesterification
Mineral acids such as H2SO4, HCl and H3PO4are widely used for the acid catalyzed transesterification reaction. Acid catalysts are recommended for the oils that have higher free fatty acid contents such as waste oil or palm oil . Such types of oils are first treated with acid catalyst (esterification) before the basic transesterification in order to convert the free fatty acids to esters. In this case, the FFA is esterified until the free fatty acid content becomes lower than 0.5%  In acid catalysis the oil is treated with acid catalyst and gives biodiesel and water but the water must be removed immediately because it will results in the soap] formation in base catalyzed transesterification.
In the acid catalyzed transesterification, the protonation of carbonyl group of the ester results in the formation of carbocation, which after a nucleophilic attack of the alcohol produces a tetrahedral intermediate. This intermediate then eliminates the glycerol to form a new ester and to regenerate the catalyst. This mechanism is related to a monoglyceride. However, this reaction can be extended to di- and triglycerides (Figure 8).
4.3 Bio-catalyst (enzyme) catalyzed Transesterification
In enzyme catalyzed Transesterification, the reaction is catalyzed by various lipases such as candida rugasa, candida Antarctica, immobilized lipase (lipozyme RMIM) pseudomonas cepacia, pseudomonas spp. Or rhizomucarmiehei. The yield of biodiesel greatly depends on the type of enzyme used . 60% biodiesel yield was achieved from transesterification of soyabean oil using commercially avalaibleimobalized lipase (Lipozyme RMIM) [26, 27]. More importantly sufficient time is required for the enzyme catalyzed Transesterification as compared to base catalyzed Transesterification. However, the various parameters such as pH, temperature, solvent, type of micro-organism that generate enzyme etcmust be optimized to achieve the industrial goals. This process is highly selective, more efficient, produces less side products or waste i.e., environmentally favorable and involves less consumption of energy because reaction can be carried out in mild conditions .
Arumugam et al.  used the sardine oil (byproduct of fish industry) as a low cost feedstock for the production of biodiesel. The FFA content of the oil was high (32mgKOH/G of oil) and the lipase enzyme immobilized on activated carbon was used for the Transesterification. Various reaction conditions were optimized such as methano/oil ratio 9:1, water content 10 v/v% and temperature 30℃. Reusability of the catalyst was studies for 5 cycles and 13% drop in FAME yield occurred.
4.4 Heterogeneous Transesterification
In heterogeneous catalysis, the phase of the catalyst is different from the phase of the reactants. Heterogeneous catalysts are very important in various fields such as industrial bulk chemical production, synthesis of selective chiral molecueles and energy . Various process problems associated with homogeneous Transesterification, such as regeneration or separation of the catalyst, soap formation, disposal of byproducts, treatment of waste effluents and corrosion in case of acid catalyst have been solved by the use of heterogeneous Transesterification. Heterogeneous catalysts they are easily recovered at the end of the reaction by decantation or filteration, reusablility, show potential activity, selectivity, longer catalyst lifetimes and cost effective green process . Interestingly heterogeneous catalysts could be used in certain harsh conditions such as high temperature and pressure. Heterogeneous catalysts may be solid base catalyst or solid acid catalyst.
Heterogeneous catalysts can be designed to bring out entrapment and grafting of the active molecules on the surface or inside the pores of the solid support such as alumina, silica or ceria. Mixed metal oxides , transition metal oxides , ion exchange resin , Alkali earth metal oxides  and alkali metal compounds supported on zeolite or alumina  have been used in different chemical reactions such as aldol condensation, isomerization, oxidation, Michael condensation, Knoevenagel condensation, and transesterification .
4.4.1 Heterogeneous base catalyst
Heterogeneous base catalysts are used to overcome the constraints such as saponification that hinders the glycerol separation from the layer of methyl ester associated with the homogeneous base catalysts. These catalysts show superior catalytic activities under mild conditions and are non-corrosive, environmentally friendly, have less disposal problems and easily separated from the reaction mixture [38, 39]. Moreover, the properties of these catalysts can be tuned accordingly to enhance activity, selectivity and longer catalyst lifetime. Various metal-based oxides such as alkali metal, alkaline earth metals and transition metal oxides can be used as a base catalyst for the biodiesel production from oils by trans-esterification process. The structure of metal oxides consists of cations (positive metal ions) that possess Lewis acid characteristics and anions (negative oxygen ions) that possess Brønstedbase characteristics. The combination of Lewis acid and Bronsted base characteristics make them potential catalyst for transesterification reaction.
184.108.40.206 Alkaline earth and alkali metal-based catalyst
Alkaline earth metal oxides such as CaO, MgO, BaO, BeO and SrOhave successfully been used as a catalysts for biodiesel production by many researchers.
Calcium oxide is favored ecofriendly material that haslonger life time because it is cheap catalyst, moderate reaction conditions and high activity. Generally, calcium hydroxide and calcium nitrate are used as precursors for the CaO production. Recently, several calcium-rich waste materials such as mollusk shell and bones, chicken eggshells have been used for CaO synthesis to minimize the biodiesel production cost, problem of waste disposal.
Demirbas  described the supercritical conditions effect on the sunflower oil catalytic Transesterification in the presence of 3 wt% of CaO with 60–120 mesh size, 40: 1 of alcohol/oil molar ratio, at pressure of 24 MPa and 252℃ The author reported 98.9% yield of methyl ester in reaction time of 26 min.
220.127.116.11 Mixed metal-based catalyst
Mixed metal oxides consist of two or more type of metal cations. Oxides may be binary, ternary and quaternary and so on with respect to the presence of the number of different metal cations . Mixed metal-based oxides are mainly used as basic catalyst depending on the mixture of the catalyst. More importantly, the basicity of these catalysts can be tuned by changing their chemical composition and procedure for synthesis. Similarly, activation energy, type of synthesis method and structure of the catalyst have a strong impact on the final basicity of the mixed metal oxides.
It has been reported that, calcining MgO with ZrO2 gives a bimetallic oxide MgO-ZrO2having high basicity character and is almost unaffected by dissolution. Similarly, MnO, CuO and CuO supported on Al2O3 have been investigated in transesterification reaction at room temperature, yielded upto 97%. Al2O3-ZnO mixed oxide and rare earth oxides were studied but require high temperature for biodiesel production from vegetable oils. Calcium bimetallic oxides such as CaCeO3, CaZrO3, CaMnO3, CaTiO3 and Ca2Fe2O5 have also been investigated for the transesterification at 60℃, which displayed good activity and reusability [42, 43].
Xie et al.  used the Zinc aluminate catalyst (ZnAl2O4) in a batch processing for the biodiesel production from waste cooking oil. More than 95% ester yield was obtained at temperature greater than 150 C, alcohol to oil molar ratio 40:1, stirrer speed of 700 rpm, reaction time of 2 h and varying the catalyst amount in the range of 1–10 wt%. The catalyst was reused for 3 cycles and the yield reduced after the 3 run. The authors reported that the decrease may be due to the carbon deposition on the surface catalyst or loss of tiny particles of the catalyst during the process of recovery.
Basic catalyst may have several problems during the process of transesterification because they are sensitive to free fatty acid content. If the free fatty acid content is higher than 2 wt %, soap formation occurs resulting in decrease in the yield of biodiesel. The downstream purification process raises problems such as producing a large amount of wastewater .
4.4.2 Heterogeneous acid catalysts
18.104.22.168 Metal oxides/mixed metal oxides
Metal oxides such as FeTiO, ZrFeO, ZrFeTiO and Cesium-doped heteropolyacid have been used successfully as solid acid catalysts for the Transesterification of oil using ethanol and methanol as a solvent. Acid catalysts are insensitive to water content and free fatty acid (FFAs) present in the feedstock and is a are preferable method for cheaper feedstock .
Alhassan et al.  developed Ferric-manganese-based solid catalyst by impregnating the support material of sulfated zirconia with Fe2O3-MnO. The catalyst wascalcined for 3 h at 600℃. The synthesized catalyst was then used for the waste cooking oil Transesterification. The author found 96.5% yield of biodiesel under optimum reaction conditions of oil to alcohol molar ratio of 1:20, at 180℃ temperature and catalyst loading of 3 wt%. The yield of the catalyst remained the same (96.5%) for 6 runs but decreased upto 87% upon the seven run. They reported that the decrease may be due to blockage of the energetic centers as a result of the accumulation of triglycerides in the pores of the catalyst.
22.214.171.124 Heteropoly acid derivatives
Heteropolyacids and their salts are also used as solid acid catalysts for the biodiesel production. HPAs withKeggin structure can be prepared very easily as compared to other HPAs. They possess high thermal stability and are preferably used for production of biodiesel from different feedstocks. Keggin-type HPA has a low specific surface area, which can be overcome using appropriate supportive material. Similarly, HPAs supported on the carriers are used in biodiesel production because of their structural mobility and superacidity.
Sakthivel et al.  used the tungstophosphoric acid (HPW) and MCM-48-supported HPW catalysts for the esterification of long chain fatty acids and alcohol in supercritical CO2 (sc-CO2) medium. High yield was obtained in the supercritical CO2 medium due to the rapid diffusion of reactants and products in the MCM-48 channels and high contact of the reactants with the catalyst.
Acidic catalyst may have several problems such as very slow reaction rate, corrosive to reactors and pipelines. Normally, high reaction temperature, high oil to methanol molar ratio and long reaction time are required .
4.4.3 Heterogeneous bifunctional (acid: base) catalysts
As the alkali catalyzed transesterification of the feedstock with higher FFA contents can produce low yield of biodiesel, because the FFA reacts with the alkali catalyst and produce the foam that results in separation and emulsification problems . To solve this problem, a two steps catalytic process for the biodiesel production is recommended. In the first step, the free fatty acid contents of the feedstock are esterified using the acidic catalyst such as ferric sulfate or sulfuric acid. In the second step, biodiesel are produced by the transesterification using the basic catalyst such as CaO or ZnO. The problem of the catalyst removal in the first step can be avoided by neutralizing the acid catalyst by using the extra alkaline catalyst in the second step. But the use of extra catalyst can increase the overall cost of the biodiesel production. The residues of the acidic or alkaline catalyst in the products of biodiesel can cause the engine problems because the acidic catalyst can attack the metallic parts of the engine. On the other hand, basic catalyst can produce higher level of incombustible ash. Therefore, both the catalyst must be removed properly from the biodiesel to avoid the aforementioned problems [49, 50]. Further, it can be concluded that there is substantial room for the development of an efficient and effective catalyst for profitable biodiesel technology (Figure 9).
Recently, bifunctional heterogeneous catalysts has been introduced to solve the drawbacks adhere with the solid base/acid catalyst and develop more economical biodiesel technology. The bifunctional heterogeneous solid catalyst can be used as an alternative for the biodiesel production that can promote both esterification and Transesterification simultaneously .
In recent years, bifunctional heterogeneous catalysts have been used widely for the production of industrial fine chemicals. The bifunctionality concept has been designed to drive complex reactions through the advance approach of combining two hostile functions, such as acid and base, with cooperative interactions between their active sites precisely positioned functional groups . Therefore, bifunctional heterogeneous catalyst can perform simultaneous esterification and transeseterification of free fatty acids and triglycerides respectively without being affected by the water content present or produced during the formation of biodiesel .
Generally, heterogeneous reactions involve three steps such as adsorption, surface reaction and desorption . In the first step, carbonyl group of free fatty acids (FFA) adsorbs on acid sites while methanol adsorb on the basic site of the catalyst to produce carbocation and oxygen anion for esterification and transesterification respectively. In the second step, at the surface of the catalyst, nucleophilic attacked carbocation and oxygen anion at each methanol hydroxyl group and triglyceride carbonyl group for esterification and transesterification reactions, respectively. The nucleophilic attack would generate tetrahedral intermediate. Finally, the product (FAME) is formed from desorption of hydroxyl group and alkyl triglycerides from catalyst surface after breaking the -OH and -C-O- bond respectively, while the deprotonated catalyst regenerated the active species for starting another catalytic cycle. Glycerol, H2O, are produced as by-product during esterification and transesterification reactions (Figure 10).
126.96.36.199 Transition metal-based catalysts
Transition metals such as Ni, Fe and Co based compounds have been extensively investigated as bifunctional heterogeneous catalyst for biodiesel production. The TiO and MnO have shown good catalytic activity for biodiesel production. These catalysts have been used for the simultaneous esterification of FFAs and transesterification of triglycerides under continuous flow conditions by using low grade feedstock with high fatty acids contents ofupto 15%.
Cannilla et al.  used a novel MnCeOx system for the transesterification of refined sunflower with the methanol. The performance of such catalyst was compared with that of common acid supported catalyst. The results showed that MnCeOx system have a superior activity especially by operating at low temperature i.e., ≤120°C. The catalytic performance was the result of synergic role played by the presence of both base/acid character and textural porosity.
188.8.131.52 Mixed metal oxides
Mixed metal oxides have shown potential applicationsin terms of their catalytic activity in various reactions due to their increased active acidic or basic sites and large surface area. As a result of these characteristic, the mixed metal oxides can simultaneously catalyze the esterification and transesterification and increases the yield of reaction under mild reaction condition .
Many researchers have investigated the catalytic activity of mixed metal oxide for biodiesel production. Furata et al.  prepared the Al2O3/ZrO2/WO3 solid catalyst by co-precipitation method for biodiesel production from soybean oil. The catalyst was compatible for both esterification and transesterification at 250℃ temperature and alcohol to oil molar ratio of 40:1, provided 90% methyl ester yield.
5. Feedstock used for the production of biodiesel
The feedstock is one of the key factor that plays vital role in the economics of the biodiesel technology. More than 350 oil-bearing crops have been identified as a potential feedstock for the production of biodiesel. The feedstock should fulfill two main requirements (i) large production scale (ii) low production cost . The feedstock availability for the production of biodiesel depends upon the geographical location, local soil conditions, regional climate and agricultural practices of any country. The suitability of feedstock depends upon various factors such as oil yield per hectare, production cost, oil content of the seeds and relevant product properties of the oil. It has been found that, the cost of the feedstock is about 75% of overall production cost of biodiesel . Therefore, selection of cheapest feedstock is a major problem and high relevant to the biodiesel industry. Biodiesel feedstocks are generally categorized into four classes as shown (Figure 11).
5.1 Vegetable oils
5.1.1 Edible vegetable oils
Resources of edible oil such as peanut , soybeans , sunflower , rapeseed , safflower, coconut and palm oil are extensively utilized for biodiesel production and are classified as first generation biofuels because these were the first crops used for production of biodiesel . Many countries of the World such as USA, Malaysia and Germany, have well off plantations of these vegetable oils. Currently, more than 95% of the world biodiesels are produced from the edible oils where rapeseed oil contributes 84%, sunflower 13%, 1% palm oil, 2% soybean and others. However, economic and social problems such as food versus fuel crisis and various environmental issues (such as destruction of vital soil resources), usage and deforestation of the available arable land are adhere with use of edible oils.
5.1.2 Non-edible vegetable oils
Due to the presence of some toxic components in the non-edible vegetable oils, they are not suitable to be used for human food. The use of non-edible vegetable oil for the production of biodiesel would pave the ways to overcome the economic, social and environmental problems and tackle the energy crises worldwide . Non-edible vegetable crops are grown on the lands that are largely unproductive, located in poverty-stricken areas and in degraded forests. These plants can also be planted on fallow lands, cultivator’s field boundaries and in public land such as roads, railways and irrigation canals. Plants of non-edible feedstocks are well adapted to arid, semi-arid conditions require low moisture and fertility. Moreover, these plants can grow and propagated through cutting or seeds . As these plants oilsdo not compete with food therefore the seed cake may be used as fertilizers for soil enrichment. Therefore, from economic and social prospective, edible oils must be replaced by some suitable feedstock for biodiesel production. Hence, non-edible feedstocks for biodiesel production could be considered as sustainable and alternative fuels.
184.108.40.206 Mazari palm (
Mazari is the local name for dwarf palm (
Mazarifibres are widely used for making ornamental products, ropes, mates, banns, different commodities for mosques, trays, baskets, grain bins, brooms, cupboards, hand fans and decoration pieces etc. (shown in Figure 13) [68, 69].
Fresh and dried leaves both are used for making products. Raw mazari production in the Pakistan is about 37,315 tons. Baluchistan is the biggest producer of the mazari with an average annual production of 27,265 tons . In 1991, the total exports of the products prepared from mazari by rural people were 126 milion rupees. Main buyer of these products are the local people because most of the products are used for domestic purposes and also these fascinating products attract both domestics and international tourists. Figure 14 shows the main buyer of the products.
The fruits of
Jatropha curcas is the bionomical name of Jatropha, belongs to spurge family. It is commonly known as Barbados, Purging or Physic nut. The height of Jatropha plant is about 6 m and is a flowering plant. The plant matures in 9–10 months and yield 2–3 times per year. On maturation, green rounded shaped seeds appeared on the plants and then turn into light blue or purple colored hard shells. The oil bearing mass located inside the shells known as meat or kernels. Oil content in the seeds varies from 20–60% by weight [74, 75]. J. curcas oil could be a valuable feedstock for the production of biodiesel in Pakistan (Figure 15).
Jatropha is a multipurpose drought resistant plant that is widely distributed in the wild or semi-cultivated areas in South East Asia, Pakistan, India and Central and South America. It is well adapted to arid and semi-arid conditions . Jatropha is rich source of hydrocarbons. Therefore, it is considered as commercial source for biofuel production all over the world. Jatropha oil contains 42% oleic, 35% linoleic, 14% palmatic and 6% stearic acid by composition .
In Pakistan, certain institutions are promoting Jatropha cultivation at the nursery level in various locations across Baluchistan, Punjab, and Sindh. In nurseries, these cultivated plants ranged in age from a few weeks to 18 months . However, after three years of private sector efforts in2008, oil bearing crop cultivation increased from 2 acres to over 400 acres. PSO (Pakistan State Oil) took a step in this direction in 2008, planting 20,000 saplings in farms. They’ve recently increased the number of samples taken for each transplantation, up to 20,000 or more. PSO’s initiatives aimed to plant more than 6 million trees produce 24 million kg of oil bearing seeds, and produce 7.2 million L of biodiesel worth 345 million PKR at a unit price of PKR 48 L−1 .
Other interested parties, such as the Karachi Forest Department and the Pakistan Army, have also successfully planted Jatropha plants in Sindh . So far, the Forest Department has been successful in cultivating 3000 samples on a trial basis in Malir Cantonment in 2010 for the cultivation of Jatropha seeds supplied by PS . Similarly, the Pakistan Agricultural Research Council (PARC) and KijaniEnergy, a Canadian company, are interested in establishing large-scale Jatropha cultivation for the production of biodiesel on marginal lands . Kijani Energy invested approximately US$ 150 million in2009, resulting in the use of 200,000 acres of land for Jatropha cultivation in Umerkot, Khairpur, Tharparker, Cholistan, and Sanghar.
220.127.116.11.2 Monotheca buxifolia (gurguray)
18.104.22.168.3 Date or date palm (
Date or date palm is a flowering plant species belongs to the palm family Arecaceae cultivated for its edible sweet fruit. It is a dioecious having separate male and female plants. It is a source of human nutrition rich with dietary fibers, carbohydrates, lipids, proteins, some vitamins and mineral matter . For millennia, the date palm tree has been cultivated in the Middle East and North Africa, and it is thought to be the world’s oldest domesticated fruit tree. Because of the variety of resources it provides, it has traditionally been the most valuable fruit crop in harsh arid or desert environments where water scarcity and extreme temperatures are common. Date palm trees are now grown in semi-arid climates and other parts of the world, including southern Europe, Australia and America. There are now over 100 million date palm trees in the world with around 2000 cultivars [87, 88]. A palm tree produces 500 kg of fresh dates per year on average, with production beginning at 5 years and lasting up to 60 years. Date production and consumption have increased rapidly, from 1.88 million t in 1965 to 3.43 million tons in 1990 and 8.46 million tons in 2016, with Middle Eastern and African countries dominating production . It’s a pitted fruit with a seed in the centre surrounded by a fleshy pericarp as shown in Figure 17.
The date seeds are very hard ranging from 5 to 15 mm in length with oblong shape with a ventral groove. The weight is about 11–18% of the total fruit mass and contain 4–13% of oil. Based on these digits, an estimated 1.3 million tons of date seeds and 127,000metric tons of date seed oil (similar amount of biodiesel) could be annually produced. In 2015, the total annual production of biodiesel was 38,700 tons in the Middle East and Africa [90, 91]. Date production in the world reached 9.07 million metric tons in 2019, up from 8.4 million metric tons in 2017. Similarly, date palm is widely distributed in different areas of Baluchistan, Sindh, KPK and Punjab. It has been reported that the annual production of date seed is around 600,000 metric ton per year in Pakistan . These seeds are used as feed for animals in some areas. However, most of these seeds degrade without any proper utilization. Therefore, the use of date seeds as biodiesel feedstock could be a promising to concern energy solution (Figure 18).
22.214.171.124.4 Karanja (
Many researchers have utilized karanja oil as feedstock for biodiesel production. It has been reported that the biodiesel obtained from karanja shows excellent properties such as low acid value, lower viscosity and higher flash point. Naik et al.  followed two steps process for the production of biodiesel from karanja oil with 20% free fatty acid. First, acid-catalyzed esterfication was applied using 0.5% (w/w) H2SO4, 6:1 methanol to oil ratio at 65℃. The acid treated oil was later transesterifiued with KOH using 1% (w/w) potassium hydroxide, 6:1 methanol to oil ratio to lower the FFA content. The yield of biodiesel obtained by dual step process from karanja oil was 96.6–97% at 65℃.
126.96.36.199.5 Neem (
Muthu et al.  produced the neem methyl ester from the neem oil in the presence of catalysts by two steps process of esterfication and Transesterification. Sulfated Zirconia was used as solid acid catalyst for esterfication, while alkali catalyst i.e., KOH was used for Transesterification. Optimum conversion of free fatty acid was achieved with 1 wt% of sulfated zirconia (acid) catalyst, at 65℃ temperature, 9:1 methanol/oil ratio and 2 h reaction time. The acid value of the raw oil was reduced by 94% (24.76 mg KOH/g) which show the successful conversion. The authors noted that when the pretreated oil was transesterified in the presence of KOH, 95% conversion efficiency was achieved (Figure 20).
5.2 Microalgae for biodiesel production
Microalgae are eukaryotic or prokaryotic photosynthetic micro-organism that can grow rapidly and live in harsh conditions due to their unicellular or simple multicellular structure . Examples of eukaryotic micro-organisms are green algae i.e., chlorophytaand diatoms i.e., bacillariophyta and prokaryotic micro-organisms are cyanobacteria. Microalgae are present in all existing ecosystem of the earth, not only in aquatic but also terrestrial ecosystem that lives in a wide range of environmental conditions . Interestingly, it is observed in small ponds and ditches in the villages and towns become fully green within a week during the rainy season in Pakistan. Although in Pakistan, the cultivation of oleaginous microalgae is in its infancy, however several species of algae are reported in the literature that can further process or cultivated for the production of oil . Microalgae can provide feedstock for several types of renewable fuels such as methane, biodiesel, ethanol and hydrogen. Biodiesel produced from algae contains no sulfur, reduce emissions of particulate matter, hydrocarbons, CO and SOx. However, NOx emissions may be higher in some types of engine.
Furthermore, a Pakistani researcher at Japan’s Mie University claims that the country could benefit from using its 27–28 million acre saline lands for algal farming, which would create jobs and benefit the rural community . Four algae strains suitable for cultivation in Pakistan’s deserts have been identified by other researchers. Other researchers have identified four strains of algae that are suitable for cultivation in Pakistan’s deserts and produce acceptable lipid yields, i.e. 40% by weight
To produce biodiesel, researchers at the National University of Sciences and Technology (NUST) cultivated Chlorella vulgaris in a closed photo-bioreactor (20 L) in a controlled environment and characterized its properties. At 5000 and 9000 psi and 50 and 80°C, the highest biodiesel yield (more than 99%) was achieved. The biodiesel produced was found to be of ASTM D6751 quality .
5.3 Waste cooking oil as feedstock of biodiesel
The term waste cooking oil (WCO) refers to vegetable oil that has been used in production of food and no longer viable for its intended use. Sources of waste cooking oil are domestic, industrial and commercial products . Waste cooking oils are problematic waste streams that need to manage properly because if WCO is disposed improperly, down streams of the kitchen, the oil solidifies and cause blockages of sewer pipes [98, 104]. Degraded waste cooking oil gets into sewage system and causes corrosion to metal and concrete elements . Thus, the waste cooking oil can be used as an effective feedstock for the biodiesel production via Transesterification .
In Pakistan, waste cooking oil sources include hotel chains, confectioneries, restaurants and domestic cooking. Pakistan is basically an agricultural country and has diverse ecological conditions, so the people mainly depend upon the agricultural products. Plants and crops that yield edible oils for cooking purposes are cultivated on extensive scale in the country. These oils are used in local shops, hotels, huts and every home of Pakistan . Pakistani people use meat of cows, buffaloes, camels, goats, poultry on large scale and use fats for cooking purposes. These all are the major sources for collection of waste cooking oil.
5.4 Waste animal fats
Animal fats and vegetable oils are of two types of biological lipid materials that are made up of mainly triacylglycerides (TAGs) and less diacyglycerides DAG and monoacylglycerides (MAGs) . Fats and oil have similar physical properties and chemical structures such as hydrophobicity, water-insolubility and solubility in nonpolar organic solvents. However, the high fatty acids content in fats and their different distributions make it different from oil. Oils are generally liquid at room temperature while fats and greases are solids due to their high content of saturated fatty acids (SFA). Different waste animal fats such as tallow (mutton tallow from sheep and beef tallow from domestic cattle), pork lard (rendered pork fat), chicken fats and grease. Since, many animal meat processing facilities, rendering companies of collecting and processing of animal mortalities, large food service and processing facilities create large amount of waste animal fats (WAFs), that will be a great opportunity to produce biodiesel from these very cheap raw material . The use of these waste animal fats as a feedstock for biodiesel production will eliminate the need of their disposal.
6. Pakistan’s energy scenario
Pakistan is the world’s sixth largest country in terms of population, (213 million) and an annual growth rate of 2%.A significant portion (63%) of this population lives in rural areas, while 37 percent live in urban areas . The recent economic growth and an ever-increasing population, has resulted in an increase in energy consumption. The country still depends on conventional resources of fossil oil.
Various initiatives to promote renewable energy in Pakistan have been taken over the years, but their outcomes are still pending due to a lack of sound policy . Recently, Alternative Energy Development Board (AEDB), was established in 2003 , in Pakistan to improve green technologies that can reduce greenhouse gas emissions and promote renewable technologies through a variety of projects that have been recognized on an international level by the International Solar Energy Society (ISES) and the World Wind Energy Association (WWEA) .
There is a significant gap between Pakistan’s energy production and energy demands, which is being bridged by the import of fossil fuels and requires substantial state revenue to be spent on these imports. Pakistan imported 13.57 Milliontons of oil equivalent (MTOE) of petroleum during fiscal year 2014–2015, ultimately putting tremendous pressure on the economy by increasing the import bills . Transportation and power generation are the main fossil fuel consuming sectors in Pakistan. Fuel price increases frequently, leading to increases in transportation costs and utility bills for both public and private consumers and pose socioeconomic challenges for the country. At present, Pakistan’s indigenous resources account for only up to 15 percent of the country’s energy requirements . Pakistan spends approximately 60% of its currency exchange on importing fuels to meet energy needs, and these import bills can be significantly reduced if indigenous alternative energy resources are used appropriately .
Pakistan’s government is searching for cost-effective, environmentally friendly alternative energy sources in order to address current energy crises and maintain economic stability .
7. Recommendations from the perspective of Pakistan
The use of agricultural residues as a renewable energy resource in Pakistan can provide a sustainable way to enhance the country’s energy mix in order to meet ever-increasing energy needs. Energy production through suitable and efficient technologies can have multiple positive economic impacts on Pakistan, (1) by saving huge investments in energy imports, (2) by reducing harmful gas emissions in order to protect the environment and (3) by empowering the people of the country in terms of social aspects . It can provide multiple job opportunities to people working in the agricultural, transportation and daily wagering sectors. Furthermore, public awareness campaigns emphasizing the importance of renewable energy resources, as well as basic education on how to effectively manage these resources, should be launched . This can be achieved by distinct financial assistance programs should be made available to encourage business investments in the renewable energy production sector .
Various important steps and measures must be taken as soon as possible, such as the establishment of generous research and development programs at the Country’s Universities and research institutions, with a focus on research activities involving renewable resources in the country.
This review presents an extensive analysis of the potential of biomass for renewable energy production in Pakistan. It also emphasizes the availability of local biomass resources as well as state-of-the-art of biomass conversion technologies. Heavy reliance on imported fossil fuels and global climate change are key factors contributing to Pakistan’s economic problems. To address these issues, relying on locally available renewable energy sources is a promising and cost-effective financial solution. The transportation sector is a major importer of petroleum fuels, accounting for the majority of the total import bill. Biodiesel and bio-ethanol, can supplement HSD/petrol, transportation fuels. To overcome this issue biodiesel production with full utilization of its by-products can provide a sustainable and environmentally friendly replacement of mineral high speed diesel (HSD).
Moreover, comprehensive detail of the locally abundantly available feedstocks for biodiesel production has also been discussed in this chapter. Overall, this study further concludes that Pakistan has the immense potential to produce economical viable biodiesel from the locally available feedstocks.
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