Chapter Twelve
Prospect of the Legume Tree Pongamia pinnata as a Clean and Sustainable Biodiesel Feedstock
Abstract
The demand for energy continues to increase due to population growth. In contrast, stocks for fossil fuel–based energy are limited and cannot be replenished, and therefore there is a need to find renewable resources to meet the increasing demand for energy around the world. With the declining reserves of fossil fuels, it has become apparent that biofuels are destined to make a substantial contribution to the future energy demands of the domestic and industrial economies. The use of vegetable oils from plants such as Jatropha curcas, canola, oil palm, and soybean has the potential to provide environmentally acceptable biodiesel, where production is greenhouse gas neutral, with reductions in current engine emissions. However, some of these feedstocks (e.g., oil palm and canola) are costly, making the production of biodiesel economically marginal due to their need for nitrogen fertilizer. The very high energy demands of nitrogen fertilizer production, transportation, and associated atmospheric pollution relating to NOx release from N fertilizer present a scenario where biofuels produced from legumes in association with nitrogen-fixing rhizobia may provide a major energy advantage over non-legume biofuel feedstock crops. Legumes may also produce NOx emissions at significantly lower levels than artificially fertilized crops. It thus appears strategic to utilize legumes and biological nitrogen fixation as a conduit for nitrogen input to enhance the sustainability of soils, whether it be for amelioration, rehabilitation, or for productive agricultural systems (e.g., biofuels), and to mitigate climate change. In the context of biofuels, legume-derived feedstocks that do not compete with other legumes or food crops for land, water, and fertilizer have the potential to provide a sustainable source of liquid fuels with minimal environmental impacts. A strong candidate legume species for biofuel feedstock is the tree legume Pongamia pinnata.
Keywords
Biofuel; Legume; Pongamia; Rehabilitation; Renewables; Sustainability
12.1. Introduction
The demand for reliable and affordable sources of energy across the globe is continuing to rise in parallel with a growing world population and a growing upper and middle class in previously poor socioeconomic populations. Currently over 7 billion people populate earth, with that number expected to rise to more than 9 billion by 2050. This is taking place at the same time that the long-term supply of conventional energy sources from the 20th century is being questioned along with the environmental impact of a fossil fuel–based economy. While the debate continues as to the reliable future supply of easily accessible fossil fuels, it is clear that the capacity of many countries to meet domestic demands for crude oil is not sustainable, a situation that is commonly referred to as “Peak Oil.”
Australia is one such country, with about 85% of its oil supply imported [1], the vast majority from Singapore via the Middle East. Similarly, while the adverse environmental impacts of continued fossil fuel consumption and its contribution to climate change is overwhelmingly accepted by the scientific community, there is still some debate among politicians and the lay community regarding the proportional role of anthropogenic activities toward global climate change. Nonetheless, the atmospheric chemistry surrounding fossil fuel mining, exploration, and consumption is understood. The predominant concern is with the generation of the so-called greenhouse gases (GHGs; i.e., CO2, CH4, and NOx) upon the combustion of fossil fuels. Despite these issues, exploration and mining continues unabated and with the short-term price of oil relatively low, perhaps artificially adjusted.
It is important to note at this time that much of the discussion around renewable energy among government policy makers and regulators is with technologies such as solar, hydro, and wind power that support, in part at least, the demand for electricity. The majority of demand for energy is at this stage still only met by energy-dense and readily storable liquid fuels. In light of the recognition of these impending issues, the search for environmentally sustainable biological sources of fuel has been underway for more than a decade now. The underlying assumption behind the search for suitable biofuel feedstocks has been the purported positive environmental impacts, primarily the “carbon neutral” effect, on atmospheric GHG concentrations.
Initially, biofuel feedstocks were sought from food crops such as soybean (Glycine max), corn (Zea mays), sugarcane (Saccharum officinarum), and rapeseed (Brassica napus) and were exploited for their already well-established farming systems and the ease with which their biomass could be converted into the two dominant forms of biofuel—ethanol and biodiesel. Ethanol is formed primarily by the fermentation of starch- or sugar-rich crops, such as corn, while biodiesel can be formed by the transesterification of plant seed oil of oil-rich crops, such as soybean.
The initial enthusiasm for these first-generation biofuel crops has subsequently been met with arguments that food crops and the land historically used for the cultivation of food crops should not be redirected to the production of biofuels, the so-called food versus fuel debate. In addition, the positive environmental impacts of these crops have been questioned by some [2]. Importantly, while the benefit of biofuels over fossil fuels has focused on the carbon cycle, little emphasis has been placed on the nitrogen cycle. This is particularly relevant when the economic, energetic, and environmental costs of nitrogen fertilizer production and application are taken into consideration [3]. Therefore, legumes are well placed to be examined as potential biofuel feedstocks with their ability to form symbiotic associations with soil bacteria, collectively known as rhizobia, which leads to the formation of root nodules and biological fixation of atmospheric nitrogen. One such legume under consideration and the subject of this chapter is the tree legume Pongamia pinnata (also called Millettia pinnata; hereafter referred to simply as pongamia).
12.2. Pongamia As Prospective Feedstock Candidate
Pongamia has gained attention as a strong candidate for sustainable biodiesel and aviation fuel production due to its large oil-rich seeds (40–50% by volume). The oil is nonedible, and the seed cake by-product (10–20%) following oil extraction has nutritional properties indicative of potential as an animal feed supplement [4,5]. Starch, seed pods, and indigestible fiber from seeds can be used for electricity cogeneration or fermentation (Fig. 12.1).
Pongamia is native to southern and southeast Asia, as well as northern Australia. It is a fast growing nonfood legume, capable of symbiotic biological nitrogen fixation (BNF), a process absent in other more well established biodiesel feedstocks (e.g., canola, mustards, oil palm, and jatropha). Importantly, pongamia has the ability to grow well on low agriculturally productive soils typically characterized by low water availability, low nutrient content, and high salinity [6–8]. Studies carried out by the Centre for Integrative Legume Research at the University of Queensland demonstrated that the growth performance of pongamia was equivalent to that of saltbush and rescue grass in saline soils [at 18 dS per meter (dS/m); [9]]. In contrast, in parallel experiments soybean perished at 5 dS/m. Unlike jatropha, another emerging biodiesel feedstock, pongamia is not listed as an invasive species in Australia and is not toxic to humans and animals [10]. In Australia, pongamia is currently grown predominantly in trial plantations in western Australia, the northern Territory, and Queensland [11].
Figure 12.1 The biological properties of pongamia. (A, B) Street trees in Brisbane, Australia, with abundant pods. Note the variability of canopy structure and leaf burst caused by genetic heterogeneity. (C) Pongamia flowers, pea-like, about 6–8 mm in length. (D) Pods of pongamia. (E) Mature pongamia seeds. Each is about 2–3 g dry weight. (F) Pongamia seed development: 10–12 months from fertilization to harvest of mature seed. (G) Pongamia seed oil analysis of three trees from different locations. Oil composition is broadly stable across trees and location. (H) Pongamia seed cake proteins. Left lane: soybean (Gm); right: pongamia (Pp). (a) lipoxygenase (90 kDa); (b) 7Sα prime (70 kDa); (c) 7Sα (66.4 kDa); (d) 7Sβ conglycinin (51 kDa); (e) 11SAβ (41 kDa); (f) 11SA1a, A1b, A2 (38.5 kDa); (g) sB1a, B1, B2, B3, 11SB4 (10 kDa). The genes encoding the dominant pongamia seed storage proteins (50 and 52 kDa) were cloned and sequenced and shown to be similar to the gene encoding the low-quality seed storage protein 7S β-conglycinin of soybean.
Pongamia has wide diversity in both genotypes and phenotypes. It exhibits fast growth and high seed production potential. Within 1 year, 50 cm pongamia saplings planted as a field trial at the UQ Gatton campus reached 3 m in height, up to 2000 g dry weight biomass shoot weight, and 6% of the trees had produced seed-bearing pods. The plant is estimated to sequester 25 ton CO2/ha/annum. Seeds can be mechanically harvested with a vibrating tree shaker. Trees bearing around 100,000 seeds have been noted. It is estimated that a tree can stay in full production for more than 35 years (note: 100-year old specimens are known in the Brisbane region). Based on conservative estimates from trees found in Brisbane, oil yields of 5 t/ha/annum are attainable. This compares well with soybean (0.8 t/ha/annum), canola (1.5 t/ha/annum), and oil palm (5 t/ha/annum). At present we estimate that a 20% diesel replacement with biodiesel for all of Australia would require 200 plantations of 6 × 6 km each. Planting of diverse germplasm using different elite clones is recommended to avoid potential disease problems arising from monoculture.
12.2.1. Pongamia and Nitrogen Fixation
Most biofuel feedstocks, including canola (B. napus), sugarcane (S. officinarum L.), sweet sorghum [Sorghum bicolor (L.) Moench], maize (Z. mays L.), and woody trees, such as eucalypts (Eucalyptus globulus Labill.) and willows (Salix spp.), require nitrogen fertilizer for their growth. The production and application of nitrogen fertilizers represent a large economic and energetic burden as costs have increased due to a dependence on fossil fuel and natural gas. Moreover, the application of nitrogenous fertilizer to crops results in resident soil bacteria producing NOx, a powerful GHG, possessing global warming potential 296 times that of CO2. This adverse scenario makes the supply of nitrogen to biofuel feedstocks a key issue when considering their sustainability on economic as well as ecological criteria [12,13]. In contrast, legumes are capable of forming symbiotic relationships with nitrogen-fixing rhizobia, which are housed in specialized root organs called nodules [14]. The use of perennial plants, such as pongamia, that are capable of symbiotic nitrogen fixation is a good strategy to sustain nitrogen for a more productive and diverse agroecosystem [15,16]. Pongamia as a legume can play an important role in sustainable agroforestry by fixing its own nitrogen from the atmosphere, minimizing the need for added nitrogenous fertilizers. It also enters into symbiosis with phosphate-mobilizing mycorrhizae [17].
Pongamia nodules have been reported to be determinate in nature with a spherical morphology [18,19]; however, Samuel et al. [20] reported that the “determinate-like” nodules progressed to “indeterminate-like” structures through activation of new cell divisions in the nodules of older plants. Therefore, older pongamia trees will exhibit a combination of spherical and coralloid nodules (Fig. 12.2A). This observation is consistent with a previous report by Sprent and Parsons [21] that most tree legumes do tend to have woody indeterminate nodules.
Despite being a perennial legume, the pongamia root symbiosis is very similar to that seen in annual legumes. Autoregulation of nodulation (AON) and nitrate (NO3) inhibition that are common in annual crop legumes were also displayed in pongamia [20]. Using split root experimental methods, Samuel et al. [20] showed that the initiation of late nodulation events is developmentally suppressed by the first-formed nodules. Rhizobia inoculation of one portion of the root system systemically suppressed nodule formation on a later-inoculated and physically isolated root. Likewise, seedlings inoculated at planting are characterized by significant nodule formation on the upper portions of the root system but no nodules on the lower parts of the root (i.e., crown nodulation; Fig. 12.2B).
Figure 12.2 Nodulation and nitrogen fixation of pongamia. (A, B) Nodules consist of both spherical and coralloid nodules. (C) AON on pongamia nodulation. (D) Effective nodules of B. japonicum strain CB1809. (E) Ineffective nodule of ineffective rhizobia strain. (F) Rhizobia inoculation significantly improve the growth of pongamia in soil: yellowing (light gray in print version) leaves on uninoculated pongamia, green (gray in print version) and healthy leaves on inoculated pongamia. (C) and (D) taken from Samuel S, Scott PT, Gresshoff PM. Nodulation in the legume biofuel feedstock tree Pongamia pinnata. Agriculture Research 2013;2:207–14.
Unlike many other legumes that form functional nodules in association with just one specific or select few strains of rhizobia, pongamia can form nodules with several strains of both Bradyrhizobium and Rhizobium [22–24]. However, the strains of rhizobia that infect pongamia vary in their efficacy for nodulation and nitrogen fixation. Therefore, selection of superior strains of rhizobia for pongamia is very important, as it will help to promote growth and potentially increase yields of oil-rich seeds. Toward this end, our laboratory [20] tested a wide range of bacterial strains from Australia and India, and established Bradyrhizobium japonicum strains CB1809 and USDA 110 as the best inocula tested. The nodules produced by these strains were larger and more extensively and uniformly filled with zones of infected bacteroids (Fig. 12.2C). In contrast, the nodules produced by less-effective strains had several infection zones with variable bacterial occupancy (Fig. 12.2D). The efficacy of pongamia nodules was demonstrated using the acetylene reduction assay, where C2H2 (acetylene) serves as a substrate for the bacterial encoded nitrogenase, and its reduction to C2H4 (ethylene) is quantified by gas chromatography. Recently, we successfully isolated two new strains of rhizobia that are more efficacious than B. japonicum strains CB1809 and USDA110 in vermiculite (Phoebe Nemenzo-Callica, unpublished data, 2015), and in soil (Fig. 12.2E). Moreover, these new strains were also more effective than B. japonicum strain CB1809 on pongamia grown in degraded mined soils (A. Indrasumunar, unpublished data, 2015).
12.2.2. Pongamia and Degraded/Marginal Lands
Currently, most biodiesel is produced from food crops growing on fertile land, for example, soybean in the United States and rapeseed in Europe [25]. Due to food security concerns, future biodiesel production should be produced from crop feedstocks that can be grown on marginal/degraded land and less profitable arable crop lands in order to ensure the establishment of a biodiesel industry that will not compete for land with other food crops. Marginal/degraded lands are usually associated with soil and water limitations and other environmental stresses (e.g., salinity and acidity) that require the selection of plant species adapted to such stresses.
In Australia, there are over 1 million km2 (100 million hectares) of marginal land, 15% of the total land area (7,687,147 km2) [26]. In addition, land clearing in areas such as the Murray–Darling River basin has resulted in dryland salinity problems. Affected lands are currently being reclaimed by planting salt-tolerant tree legumes, such as Acacia spp. [27]. Such marginal lands are excellent options for planting salt-tolerant biofuel crops, such as pongamia [28,29].
Pongamia is highly desirable as it has been reported to be highly tolerant to salinity (10 dS/m) [30] and drought (survived 4 months) without rain in Brisbane during 2007–08 drought [13], and can be grown in various soil textures (stony, sandy, and clayey). It can grow in humid subtropical environments with annual rainfall ranging between 500 and 2500 mm. Pongamia can survive maximum temperatures exceeding 45°C. Cuttings and saplings survived 65°C in a glasshouse when the temperature control unit failed during the January 2011 Brisbane flood, though ample water was available [13]. Although not considered as “frost tolerant,” pongamia can survive and recover from frost events [13].
12.2.3. Pongamia and Salinity
Salinity is a serious threat to agriculture in arid and semiarid regions [31,32]. Nearly 40% of the global land surface can be categorized as having potential salinity problems [33,34]. Salinity is a measure of the content of salts in soil or water. It is measured by the electrical conductivity (EC) of a solution or saturation extract of soil. Soils are considered saline when their EC exceeds 4 dS/m, and water exceeding 4.7 dS/m is unsuitable for the irrigation of most crop species [35]. However, 62% of pongamia trees survived in soil with EC values varying from 10 to 12 dS/m [7], and 13% could survive salinity values as high as 19 dS/m [8]. We have also shown that salinity of 20 dS/m did not adversely affect growth (shoot and root fresh weight, number of leaflets, root length, and plant height) of 12 week-old pongamia seedlings over an 8-week period of exposure. In addition, nodule number was not affected by salinity of up to 20 dS/m, but nodule mass and nitrogen fixation started to decrease at 4 dS/m. Efforts are required to improve nodulation and nitrogen fixation of pongamia on saline soils if it is to be a successful biofuel feedstock crop. Improvement of nodulation and nitrogen fixation in saline conditions have been shown in wattle (Acacia ampliceps Maslin) when inoculated with salt-tolerant strains of rhizobia [27,36]. Therefore, it is considered highly probable to improve nodulation and nitrogen fixation of pongamia in saline conditions by inoculation with salt-tolerant rhizobia [9]. Selection of rhizobia strains that are tolerant to environment stress is currently being conducted in our laboratory. We found two strains of rhizobia tolerant to higher levels of salinity than B. japonicum strain CB1809. These strains were able to grow on Yeast Mannitol Broth (YMB) medium containing 100 mM NaCl, while B. japonicum strain CB1809 was only able to grow on YMB medium containing 50 mM NaCl.
12.2.4. Pongamia and Drought
The increasing frequency and intensity of dry periods result in the consecutive occurrence of drought in Australia and other parts of the world [37,38]. Drought is a major abiotic stress limiting plant production in many countries. In 2006, more than three-quarters of Australia, encompassing 38% of the agricultural land, was affected by drought [9]. In addition, according to the Commonwealth Scientific and Industrial Research Organization, it is predicted that by 2030, rainfall in major capitals could drop by 15%. Pongamia has been reported as drought tolerant, possibly due to its dense network of lateral roots and thick, long taproot. Research in our laboratory supports the claim of this drought tolerance. Preliminary experiments indicate that seedlings are capable of withstanding extensive periods of water deprivation (25 days to 55% relative water content) without significantly affecting growth and biomass production. In the context of extensive drought periods, the vast Australian landscape should not provide an impediment to cultivation of pongamia [39]. However, further research on the effects of drought on seed and biomass production are still needed.
12.2.5. Pongamia and Mine Spoils
In addition to its ability to grow on poor quality soils for biofuel feedstock, it is worth considering the value of pongamia as a long-lived perennial plant for rehabilitation of mine spoil sites. An example of such an environment requiring rehabilitation is coal mine overburden and spoil. To date, the common practice in the rehabilitation of coal mine sites in Australia has been the planting of Eucalyptus spp., Acacia spp., or native grasses. This has been practiced at the Meandu Mine, Queensland (Stanwell Corp.) for many years. We have developed a new approach to establish an integrative rehabilitation planting scheme that contributes the restorative attributes of legume trees such as the A. spp., but also incorporates the planting of an economically valuable crop with multiple products and outcomes (i.e., biofuel, biochar, carbon sequestration, and carbon farming).
This new approach has some advantages. Firstly, mine rehabilitation to date has involved plantings that aim to restore any disturbed site to a landscape that resembles as close as possible undisturbed native vegetation. In this approach we integrated the capacity for rehabilitation of pongamia with its potential for the production of biomass (i.e., seed oil and associated by-products) of economic value. In promoting the value of pongamia it is worth noting that with the exception of soybean, all proposed biofuel feedstocks throughout the world to date have been nonlegume plant species. As such this approach addresses the neglected issue of the costs associated with nitrogen inputs to biofuel production through the exploitation of a perennial legume and the associated developmental process of nodulation and nitrogen fixation. Secondly, this project aims to enhance the growth performance, through improvements in nodulation and nitrogen fixation of a tree that is native to Australia and well suited to the role of a dedicated bioenergy crop. The planting of a perennial legume on coal mine spoil has an objective that directly targets the issue of long-term sustainability with both environmental and economic benefits via an agricultural biotechnology approach to complement and support the well-established coal-mining industry through restoration of disturbed mine sites.
It is widely acknowledged that pongamia can ameliorate and rehabilitate poor quality soils [29,40]. We have also shown that pongamia was successfully established in degraded mined soils (Fig. 12.3). More than 95% of pongamia seedlings survived well in the harsh environment of mine spoil. More evaluation on tree survival, growth rate, flowering, and seed production are still needed to measure the success of this mine rehabilitation. With this promising result, the potential of pongamia for mine rehabilitation needs to be extended to other mine sites.
12.3. Pongamia Improvement Program
The proposal that pongamia be considered as a future bioenergy crop has primarily been on the basis of observations of individual trees growing in forests or as street landscape specimens. While there is a long history in the Indian subcontinent and southeast Asia of the exploitation of pongamia by humans (e.g., heating fuel and traditional medicine; [18]), pongamia is yet to undergo the comprehensive domestication and selection that is normally associated with the development of modern dominant crop species, and so future deployment of plantations will require extensive genetic improvement of currently available germplasm. Importantly, pongamia adopts a predominantly outcrossing reproductive strategy [41], which presents both challenges and opportunities. The challenges revolve around the propagation of any elite germplasm generated in a domestication program. Propagation of pongamia is likely to be through methods such as rooted cuttings, grafted saplings, and tissue culture regeneration ([42–44]; Fig. 12.4). Rooted cuttings and grafted saplings are generated through essentially low technologies, but are labor-intensive and time-consuming. Tissue culture regeneration, if reliable methods are available, can generate the large numbers of saplings that are required for broad acre plantations. Unfortunately, while it is our experience that regeneration of whole plants is relatively easy from cotyledons, it seems to be much more difficult to regenerate whole plants through tissue culture from somatic tissue explant.
Figure 12.3 Pongamia establishment for rehabilitation and reforestation of mine spoils. Biodiesel production is seen as an additional outcome. (A) Soil and planting preparation, sculptured soil was intended to increase water retention. (B) Pongamia saplings at planting time, saplings were raised in nurseries for 10 months to get established saplings of 90 cm. (C) Most of trees dropped their leaves during winter. (D) One-year old pongamia grew well in degraded mined soil. Some trees will produce seeds within 3–4 years after planting, but in small, commercially insufficient quantity. Commercial use is expected to start in year 5 after trees' establishment.
Figure 12.4 Phenotypic and genotypic diversity of pongamia, and its clonal propagation. (A, B) The diversity of pongamia in spring foliage regrowth in trees from a field site near Roma, Queensland. (C) A selection of pongamia seedpods sourced from trees in and around the Brisbane area. (D) Variation in morphology of leaves from two Brisbane street trees. (E) Silver-stained polyacrylamide gel of PCR products amplified with pongamia inter-simple sequence repeat markers. (F) Clonal propagation from immature cotyledons of pongamia through subsequent developmental stages (G–L). (M) Pongamia tree derived from tissue culture grew well in the field.
The opportunities of an outcrossing mode of reproduction are the many and varied phenotypes derived from seed-borne progeny germplasm. Phenotypic variation has been observed in traits relevant to tree architecture, annual yield of seed, timing of flowering, seed oil content, and composition. With respect to clonal propagation and phenotypic variation it has been our experience that some germplasm is more amenable to propagation than others. This may present another challenge, particularly if germplasm exhibiting traits desirable for the agronomy of biofuel production is recalcitrant to clonal propagation. Nonetheless, the wide genotypic and phenotypic diversity should provide a germplasm pool that will enable the selection of elite planting material for extensive plantations. These plantations will most likely incorporate a mix of genotypes that will avoid the potential disease and problems that have seriously affected monoculture crops in the past.
Any future pongamia domestication program should be supported with appropriate genetic and genomic tools that help to characterize and define any elite germplasm. To date, a complete genome sequence of pongamia is yet to be constructed. However, the complete sequences of the mitochondrial and chloroplast genomes have been annotated [30]. A range of molecular marker technologies have been explored for future application in marker-assisted genetic improvement. These technologies have included RAPD, AFLP, ISSR, and SSR methods [41,45–48]. While they have been demonstrated to differentiate germplasm, they are yet to positively link markers with relevant phenotypes.
12.4. Quality Analysis and Advantages of Pongamia Oil for Biodiesel
According to the American Society of Testing and Material (ASTM), biodiesel is referred to as monoalkyl esters of long-chain fatty acids (fatty acid methyl esters, FAMEs) derived from renewable biological sources, such as vegetable oils or animal fats. The oil can be converted to biodiesel (i.e., FAMEs) by transesterification with CH3OH (methanol) in the presence of KOH [49] or NaOH [50] as catalyst. This reaction also produces glycerol, a low-value by-product for industry. The resulting biodiesel is quite similar to petroleum-based diesel fuel in its physicochemical characteristics and can be blended in any proportion with petroleum diesel to create a stable biodiesel blend [51]. It is well recognized as the best fuel substitute in diesel engines because its raw materials are renewable, it is biodegradable and more environmentally friendly than petroleum diesel, and it can be directly used in the compression ignition engines without significant modification of existing engines [52,53].
Many plants have emerged as sources of raw material for biodiesel, including canola and Indian mustard (B. napus and Brassica juncea), camelina (Camelina sativa; another mustard related to canola), soybean (G. max), oil palm (Elaeis guineensis), and sunflower (Helianthus annuus). However, it is not economically feasible to use food-grade vegetable oils to produce biodiesel because of the surge in feedstocks price as a result of competition between food and fuels. Therefore, the search for biodiesel feedstocks is focused on low-cost nonedible oils sources, and one of the most suitable species is pongamia due to favorable properties that include high oil recovery and quality of oil.
Pongamia produces an abundant supply of oil-bearing seeds with yields of more than 100 kg of seeds per tree [54]. The seeds of pongamia comprise 40–50% oil, the composition of which is dominated by oleic acid (C18:1), a fatty acid (FA) that is highly desirable for biodiesel production. It has been reported that biodiesel quality was affected by FA composition of the corresponding feedstock [54,55]. Different plant species produce oils with varying content and composition (Table 12.1). As shown in Table 12.1, pongamia is the best choice because it contains a high proportion of oleic acid but low levels of undesirable saturated and polyunsaturated fatty acids (PUFAs).
Table 12.1
Major Components of Oil of Several Plants Currently Used as Feedstock for Biodiesel Production
| Plant | Oil Yield (L/ha per annum) | Percent Oleic Acid (C18:1) | Percent Palmitic Acid (C16:0) | Percent Stearic Acid (C18:0) | Source or Reference(s)a |
| Corn | 172 | 30.5–43.0 | 7.0–13.0 | 2.5–3.0 | Dantas et al. (2007) |
| Soybean | 446 | 22.0–30.8 | 2.3–11.0 | 2.4–6.0 | Hildebrand et al. (2008) |
| Canola | 1196 | 55.0–63.0 | 4.0–5.0 | 1.0–2.0 | Masser (2009) |
| Jatropha curcas L. | 1892 | 34.3–45.8 | 13.4–15.3 | 3.7–9.8 | Becker and Makkar (2008) |
| Palm oil | 5950 | 38.2–43.5 | 41.0–47.0 | 3.7–5.6 | Sarin et al. (2007) |
| Algaeb | 59,000 | 1.7–14.3 | 3.7–40.0 | 0.6–6.0 | Hu et al. (2008) |
| Tallow | NA | 26.0–50.0 | 25.0–37.0 | 14.0–29.0 | Canakci and Sanli (2008) |
| Pongamia | 3000–6000 | 52–57 | 8–12 | 7–11 | Akoh et al. (2007), Mamilla et al. (2008), CILR |
Monounsaturated oleic acid is preferred over other saturated FAs for biodiesel production because of its relatively low cloud and pour point temperatures. The pour point of a fuel indicates the lowest temperature at which the oil can still flow, whereas the cloud point determines the temperature at which the dissolved solids in the oil will precipitate from the liquid [56]. The pour and cloud points of pongamia FAMEs are 2.1°C and 8.3°C, respectively, consistent with the presence of saturated oils (palmitic acid and stearic acid; Table 12.1). Lower cloud points are more desirable for engine performance and the cloud point of pongamia FAMEs is lower than biodiesel derived from other sources, such as oil palm (E. guineensis Jacq.; 10°C) and beef (Bos taurus) tallow (13°C) [57], but higher than biodiesel made from edible oil of soybean (–1°C), rapeseed (B. napus L.) (–7°C), and sunflower (H. annuus L.) (1°C).
Engine performance tests using pongamia FAMEs showed that blends up to 40% by volume with mineral diesel were successful in reducing exhaust emissions of CO, smoke density, and NOx without sacrificing the power output (torque, brake power, and brake thermal efficiency), and a reduction in brake-specific fuel consumption [58]. However, as the concentration of pongamia FAMEs in the blend was increased, deterioration in viscosity, cloud, and pour points (important for cold weather performance) of the fuel was detected. At low operating temperature, fuel may thicken and might not flow properly affecting the performance of fuel lines, fuel pumps, and injectors [52]. Therefore, improvement of oil composition is needed to make pongamia FAMEs suitable for markets in all climatic regions of the world [9].
Table 12.2
Biodiesel (B100) Fuel Quality Standard Based on ASTM D6751-09
| Property | Method | Limit |
| Flash point | D93 | >93°C |
| Water and sediment | D2709 | <0.05% vol |
| Kinematic viscosity, 40°C | D445 | 1.9–6.0 mm2/s |
| Sulfated ash | D874 | <0.02% mass |
| Sulfur S 15 grade | D5453 | <0.0015 ppm |
| Sulfur S 500 grade | D5453 | <0.05 ppm |
| Copper strip corrosion | D130 | <3 |
| Cetane number | D613 | >47 |
| Cloud pointa | D2500 | Report to customer |
| Carbon residue | D4530 | <0.05% mass |
| Acid number | D664 | <0.05 mg KOH/g |
| Free glycerine | D6584 | 0.02% mass |
| Total glycerine | D6584 | 0.24% mass |
| Phosphorus content | D4951 | <10 ppm |
| Vacuum distillation end point | D1160 | <360°C |
| Cold soak filtration | D6751 | <360 s |
Pongamia biodiesel must meet a set of criteria (Table 12.2) to achieve ASTM D6751-09 quality standard. Biodiesel is generally considered to be a compatible substitute for all or part of mineral diesel fuel, but some key physicochemical parameters need to be considered when considering pure biodiesel replacing conventional diesel or when it is blended with conventional diesel.
A comparison between pongamia biodiesel and mineral diesel is presented in Table 12.3 [50,52,59]. It shows that in general the properties of pongamia FAMEs are comparable to mineral diesel. The fuel properties—including viscosity, density, flash point, fire point, and calorific value—of the transesterified product (biodiesel) compare well with accepted biodiesel standards. The viscosity of pongamia biodiesel is close to that of mineral diesel, with the calorific value about 12% less than that of mineral diesel. It has higher flash point than mineral diesel, and hence is safe to transport and store. In addition, biodiesel from pongamia oil shows no corrosion on piston metal and piston liners, whereas biodiesel from Jatropha curcas has slight corrosive effects on piston liners [60].
Table 12.3
Comparison of Pongamia Biodiesel and Mineral Diesel
| Oil Properties | Pongamia Biodiesel | Mineral Diesel | ||||
| Imran et al. [59] a | Mamilla et al., [50] | Bobade and Khyade [52] b | Imran et al. [59] a | Mamilla et al., [50] | Bobade and Khyade [52] b | |
| Density at 15°C | 0.88 | 0.89 | 0.86 | 0.84 | 0.84 | 0.84 |
| Kinematic viscosity @40°C | 8.53 | 5.6 | 4.78 | 6.06 | 3.8 | 2.98 |
| Cloud point (°C) | – | – | 6 | – | – | −16 |
| Pour point (°C) | −4 | – | – | −2 | – | – |
| Flash point, (°C) | 158 | 217 | 144 | 70 | 56 | 74 |
| Fire point (°C) | – | 223 | – | – | 63 | – |
| Heating value (kJ/kg) | – | 36120 | – | – | 42800 | – |
| Acid value (mg KOH/g) | 0.42 | – | 0.42 | 0.34 | – | 0.35 |
| Sulfur content (% mass) | 0.02 | – | – | 0.91 | – | – |
| Cetane number | 58.22 | – | 41.7 | 51 | – | 49.0 |
| Calorific value (kcal/kg) | – | – | 3700 | – | – | 4285 |
| Specific gravity | – | 0.876 | – | – | 0.85 | – |
| Water content (%) | Trace amount | – | 0.02 | 0 | – | 0.02 |
| Carbon residue (%) | 0.39 | – | 0.005 | – | – | 0.01 |
| Ash content (wt%) | 0.003 | – | 0.005 | – | – | 0.02 |
Table 12.4
Co-ignition Characteristics of Pongamia Biodiesel With Mineral Diesel
| Diesel Type | Ratio | Duration (min) | Load Applied (kW) | Observation |
| Mineral diesel:pongamia biodiesel | 9:1 | 12 min | 2130 | Running smoothly |
| Mineral diesel:pongamia biodiesel | 8:2 | 11 min | 2130 | Running smoothly |
| Mineral diesel:pongamia biodiesel | 7:3 | 12 min 30 s | 2130 | Running more smoothly |
| Mineral diesel:pongamia biodiesel | 6:4 | 10 min 25 s | 2130 | Running smoothly |
| Mineral diesel:pongamia biodiesel | 5:5 | 9 min 32 s | 2130 | Running smoothly |
| Mineral diesel | 100% | 11 min 28 s | 2130 | Running smoothly |
| Pongamia biodiesel | 100% | 8 min 10 s | 2130 | Running smoothly |
Imran HM, Khan AH, Islam MS, Niher RS, Sujan A, Chowdhury AMS. Utilization of Karanja (Pongamia pinnata) as a major raw material for the production of biodiesel. Dhaka University Journal of Science 2012;60(2):203–7.
Co-ignition of pongamia biodiesel with mineral diesel was evaluated by Imran et al. ([59]; Table 12.4). They found that pongamia biodiesel can run diesel engines smoothly, but the performance was better when it was mixed with mineral diesel. The best performance was achieved when mineral diesel and pongamia biodiesel were mixed at the ratio of 7:3.
Using pongamia biodiesel has several advantages [50], such as (1) runs in any conventional diesel engine; (2) no need for engine conversion or modification; (3) can be stored anywhere that petroleum diesel fuel is stored; (4) reduces carbon dioxide emissions by up to 100%; (5) can be used alone or mixed in any amount with petroleum diesel fuel; (6) easy to handle because it is biodegradable and nontoxic; (7) safe to transport because it has a high flash point of about 150°C compared to the flash point of petroleum diesel fuel, which is 70°C; and (8) similar fuel mileage, auto ignition, power output, and engine torque to petroleum diesel fuel.
12.5. Conclusion
The demand for food, fuel, and fiber continues to increase due to an increasing world population. With the declining reserves of fossil fuels, there is a strong demand for the development of future fuel that is economically viable and environmentally sustainable. It has become apparent that biofuels are destined to make a substantial contribution to the future energy demands. Pongamia has immense potential as a biofuel feedstock for several reasons. (1) pongamia is drought- and saline tolerant; it can grow on marginal lands that are globally abundant and not suitable for most food crops, (2) as a legume, pongamia does not require supplemental nitrogen fertilizers, thereby increasing sustainability, (3) pongamia also enters into a symbiosis with mycorrhizal fungi, leading to reduced phosphorus demands, (4) pongamia oil has excellent properties; this gives assurance to mixtures of pongamia oil with other liquid fuels (e.g., a B20 mix containing 20% pongamia-derived biodiesel) against engine congestion, malfunction, or low effectiveness, (5) it has the potential to yield a number of commercially viable by-products that result from the biofuel production process, such as an animal feed supplement arising from the residual seed cake following oil extraction, a source of combustible energy from the waste seedpods, and biochar that could be produced from any or all the components of the waste biomass.
Like fossil fuel, biofuel also releases carbon dioxide when combusted. However, in contrast to fossil fuel, that CO2 is assimilated by the natural process of photosynthesis, leading to the synthesis of sucrose and subsequently FAs, as part of plant oil. Therefore, overall CO2 emissions to the atmosphere are neutral, compared to the negative effects of burning coal, gas, or oil that was deposited after CO2 capture millions of years ago. In addition, pongamia biodiesel is not plagued by undesired emission components, making it a prospective fuel of the future. To realize the eminent potential of this pongamia biodiesel, an optimistic and far-reaching investment and associate research support are needed.
As of 2016, the decrease in the current price of crude oil over recent years will not foster the necessary investment into renewable biofuels. However, we hope that the investors will recognize that the supply of liquid fossil fuel is limited and certainly will run out in the foreseeable future. We understand that biofuels, even those derived from the most promising plant crops, cannot supply in its entirety the global energy need. However, with proper management, they can be part of a diverse energy sources spectrum. The value of biofuels is especially valuable using feedstock such as pongamia, which has biological attributes lessening the energy-dependent inputs (N fertilizer).
There are scientific challenges to make pongamia a successful future biofuel feedstock. Research and development programs are needed to make this undomesticated and unimproved species a reliable and predictable source of oil and other valuable by-products [13]. Genetic and genomics tools in combination with traditional plant breeding should build a strong scientific foundation for genetic improvement of pongamia as a productive bioenergy crop.
Acknowledgments
We thank the Australian Research Council for ARC Linkage grants LP120200562 (in partnership with BioEnergy Plantation Australia, and Stanwell Corporation). ARC and UQ also provided funds through the ARC Centre of Excellence for Integrative Legume Research grant. Also thanked are TerViva (USA), the Brisbane City Council, and the Global Change Institute as well as the Queensland Smart Futures grant.
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