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| Fig. 1: 8-Cylinder, 25.8 MW Marine Diesel Engine. (Source: Wikimedia Commons) |
Petroleum-based diesel fuel (petrodiesel) forms an essential part of the world's economy. Its advantage lies in its high power output and efficiency compared to gasoline. It fuels larger engines, such as the marine diesel engine in Fig. 1, as well as personal vehicles, heavy trucks, city buses, farm equipment, generators, and trains. [1] However, the growing global energy demand, coupled with the negative environmental impact of fossil fuels, merit the consideration of renewable fuels. Biodiesel has the potential to offset global energy demand while reducing environmental impact.
Biodiesel is but one type of biofuel. In the broadest of terms, biofuels are fuels that are derived from biological material (biomass), be it animals or plants. They are usually classified by their biomass source: [2]
1st Generation: sourced from biomass that is edible (i.e. crops)
2nd Generation: sourced from biomass that is generally non-edible (e.g. woodchips, agricultural residues, municipal solid waste)
3rd Generation: sourced from algae
4th Generation: sourced from genetically engineered plants that will consume more CO2 during growth than the fuel will burn. [1]
Biodiesel is typically 1st or 2nd Generation. It is commonly derived from edible and non-edible oils, animal fats, and waste cooking oils, although future feedstocks could include algae. It is formed by transesterification - the reaction of the oils with alcohol in the presence of a catalyst to yield fatty acid esters and glycerol. The fatty acid esters are then purified to yield biodiesel. [3] There are various technologies which can achieve transesterification, including batch reactors, semi-continuous-flow reactors, and continuous-flow reactors. [4] In its pure form, biodiesel is designated B100 and, in the United States, must meet the requirements of the American Society for Testing and Materials D-6751. [1]
There is an energy budget associated with the entire manufacturing process. Energy is required to acquire the feedstock. Heat is frequently applied to quicken the reaction (averaging 50°C). Various catalysts can require both energy to make and energy to separate from the final products. Waste products require processing. These energy inputs are compared to the energy potential of the fuel in an energy balance ratio. For instance, soybean oil produces about 3 energy units for each unit consumed, typically in the form of fossil-based fuel. [5]
Biodiesel is similar to petrodiesel, but not identical. Biodiesel's exact qualities depend on its feedstock and treatment mechanisms. Petrodiesel consists of long-chain unbranched alkanes (C14-C24) with a cetane number of 40-50 and heat of combustion of 45,000 kJ/kg. By comparison, biodiesel has a similar chemical structure (C12-C22) except for the presence of the ester function, with a cetane number of 40-80 and heat of combustion of 40,000 kJ/kg. [6]
Biodiesel's viscosity is generally two times higher than that of conventional petroleum diesel and also has higher:
cloud points (temperatures at which wax crystals appear and fuel appears cloudy),
cold filter plugging points (temperatures at which wax crystals in the fuel begin to clog the fuel filter),
boiling ranges,
lubricities,
flash points (temperatures at which the fuel can produce a flammable vapor that can be ignited by a spark), and
NOx emissions.
Unlike petrodiesel, biodiesel does not contain sulfur and has lower emissions of fine particulate matter, polyaromatic hydrocarbons, and SO2. [6] It is also biodegradable, making spills at sea more acceptable. [3]
Biodiesel is miscible with petrodiesel in all concentrations. [6] It can be used as a direct substitute in most engines with little to no engine modifications with two key considerations. B100 can cause clogging because it is a better lubricant and solvent, thus releasing deposits from the engine walls. Also, engines made before 1993 require modifications to the rubber tubing and fittings due to the solvent properties of B100's esters. [3,6]
Numerous considerations should be taken into account when considering supplanting petrodiesel with biodiesel. Most considerations are based on the feedstock type and fuel processing technique, including feedstock availability, fuel performance (as detailed above), various emissions, efficiency of processing, and cost. Costs include operating (feedstock, catalyst, utilities, and labor) and capital (equipment and land). [3] Here I present a basic analysis of the land area required for plant feedstocks, minimum selling prices for economic breakeven, and energy balances of different biodiesels.
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| Table 1: Land area required for various plant feedstocks to supply global diesel demand. [3] |
Land Area: In 2021, the world's consumption of petrodiesel was 9.83 × 109 barrels. [7] Assuming proper engine modifications can be made for all diesel engines to run B100, the annual amount of biodiesel required to supplant petrodiesel globally would be:
| 9.83 × 109 bbl petrodiesel × | 45,000 kJ (kg petrodiesel)-1 40,000 kJ (kg biodiesel)-1 |
= | 1.11 × 1010 bbl biodiesel |
Table 1 gives the rough land area required per plant feedstock to provide this amount of biodiesel. These areas are based on the plants' average annual oil production rates. [3] Assuming an average oil density of 0.91 kg/L and 100% conversion from oil to biodiesel, the areas are calculated by
| 0.91 kg L-1 ×
1.11 × 1010 bbl ×
159 L bbl-1 1590 kg (ha Jatropha)-1 × 100 ha km-2 | = | 1.01 × 107 km2 Jatropha |
for jatropha, and similarly for other oils. For scale, the area of microalgae required is the size of Montana. Jatropha's area is the size of the United States (with Alaska). Corn's area is greater than all the land in the eastern hemisphere.
Minimum Selling Price:In addition to land use, the production costs must be considered for commercially-scaled biodiesel operations. Technoeconomic Analyses (TEAs) evaluate and optimize the economic feasibility for various bioprocess combinations based on empirical data from small-scale operations. [5] Table 2 gives minimum selling prices (in USD kg-1) as a result of TEAs of biodiesels of various feedstocks, processes, catalysts, and final quality. [5,8] Prices are then converted to dollars per Joule, for example in the case of Palm oil by
| 1.03 $ (kg Palm)-1 ×
1 kg Palm 4 × 107 J | = | 2.58 × 10-8 $ J-1 |
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| Table 2: Minimum selling prices of various biodiesels compared to fossil fuels. [5,7,8] | ||||||||||||||||||||||||||||||||||||
The dollar per Joule basis allows for direct comparison to fossil fuels. As Table 2 shows, crude oil, natural gas, and coal are all cheaper than even the cheapest biodiesels. [7]
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| Table 3: Energy balances of various biodiesels. [9-13] |
Energy Balance: Finally, Life Cycle Analyses (LCAs) are useful tools to analyze the energy and emissions balances of an entire production process. For biodiesel, this process can include feedstock cultivation/collection, oil extraction, conversion to fuel, processing of waste, and transportation between each of these phases. Methods for conducting LCAs vary and therefore results can even vary from LCAs conducted on the same biodiesel. Table 3 shows the energy balances of different biodiesels stemming from various LCAs. [9-13]
From Table 3 it can be seen that there are biodiesels ranging across different feedstocks and fuel processing techniques that are energy net positive (Output/Input - 1 > 0). While dried microalgae is shown as energy negative (Output/Input - 1 < 0), it should be noted that microalgae biodiesel energy balances are highly dependent on algae species, drying method, and oil extraction technique (accounting for 90% of the process energy input). [11] Wet algae's relatively large energy balance demonstrates the wide range of possible balances.
Biodiesel can potentially supplant global demand for petrodiesel, but careful consideration should be given to feedstock type and fuel processing technique. These two overarching areas encompass feedstock availability, land use, various emissions, fuel performance, speed of processing, energy balance, and cost. This report showed that cost, energy balance, and land use varies widely across biodiesels. Although minimum selling prices are lower than fossil fuel prices, continued investment to scale responsible processes could make biodiesel economically viable, let alone environmentally necessary.
© Peter Slye. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] A. Sarin, Biodiesel - Production and Properties, (Royal Society of Chemistry, 2012).
[2] R. A. Lee and J. Lavoie, "From First- to Third-Generation Biofuels: Challenges of Producing a Commodity From a Biomass of Increasing Complexity," Anim. Front. 3, No. 6, 6 (2013).
[3] M. Tabatabaei and M. Aghbashlo, eds., Biodiesel - From Production to Combustion (Springer, 2019).
[4] A. Islam et al, Advanced Technologies in Biodiesel, (Momentum Press, 2014).
[5] C. Drapcho, N. P. Nhuân, and T. Walker, Biofuels Engineering Process Technology, 2nd Ed., (McGraw Hill, 2020).
[6] R. Luque and J. Clark, eds., Handbook of Biofuels Production: Processes and Technology, 1st Ed., (Woodhead Publishing, (2010).
[7] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.
[8] B. Gurunathan and R. Sahadevan, eds., Biofuels and Bioenergy, 1st Ed. (Elsevier, (2022).
[9] A. Mohammadshirazi et al, "Energy and Cost Analyses of Biodiesel Production From Waste Cooking Oil," Renew. Sustain. Energy Rev. 33, 44 (2014).
[10] R. Piastrellini, A. P. Arena and B. Civit, "Energy Life-Cycle Analysis of Soybean Biodiesel: Effects of Tillage and Water Management," Energy 126, 13 (2017).
[11] L. Lardon et al, "Life-Cycle Assessment of Biodiesel Production from Microalgae," Environ. Sci. Technol. 43, 6475 (2009).
[12] A. Kumar, J. V. Tirkey, and S. K. Shukla, "Comparative Energy and Economic Analysis of Different Vegetable Oil Plants For Biodiesel Production in India," Renew. Energy 169, 266 (2021).
[13] R. Chen et al., "Life Cycle Energy and Greenhouse Gas Emission Effects of Biodiesel in the United States with Induced Land Use Change Impacts," Bioresour. Technol. 251, 249 (2018).
