Algal Biodiesel vs. Conventional Diesel: Comparisons and Tradeoffs

There is a lot of current research and development to push algal biofuels toward commercialization as a replacement for existing fossil fuels - namely diesel and jet fuel. But is algal biodiesel really the same as conventional diesel? As you probably know, biodiesel refers to fuels comprised of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats [1]. In general, biodiesel has similar properties to fossil-fuel derived diesel fuel and can be used in existing diesel engines. There are, however, some key differences and tradeoffs to be considered for the use of algal biodiesel. The purpose of this post is to highlight and describe those differences.

But first, some basics - an overview of the diesel engine and fuel production processes of diesel and algal biodiesel.

Compression Ignition Engine Operation

The Compression Ignition (CI) or diesel engine operates on a four-stroke cycle similar to a gasoline engine, except that the combustion takes place via autoignition of the fuel in compressed air instead of by a spark from a spark plug. The components of the combustion system in the CI engine include an intake and exhaust valve, to allow air to flow in and exhaust gases to flow out, respectively. It has a direct fuel injector to inject fuel into the combustion chamber. The valves and injector are located in the cylinder head, which is positioned above the piston. The piston is enclosed with cylinder walls, completing the boundary of the combustion chamber. When the position travels laterally up and down the cylinder, it transfers its linear mechanical energy to rotational mechanical energy via a rotating crankshaft. Figure 1 illustrates each component of the system. 

Figure 1 - Diagram of diesel combusion system [21]

The cycle starts with the piston at top dead center (TDC). The intake valve opens as the pistons moves down the cylinder, pulling in fresh air (and possibly some recirculated exhaust as described in later sections). The next stroke begins with the piston at bottom dead center (BDC). Both valves are closed as the piston moves up the cylinder, compressing all of the air into a small space, drastically increasing its temperature and pressure. The third stroke begins as the piston approached TDC again, and the fuel injector quickly pumps fuel into the chamber. The fuel autoignites from the high temperature in the compressed air, transforming its chemical energy into heat energy. The heat energy is transformed into mechanical energy as is pushes the cylinder back down to BDC. The fourth stroke begins with the piston at BDC. The exhaust valve opens as the piston moves up the cylinder, pushing the combustion products out of the chamber. Then the cycle starts all over from the beginning. An animation of the cycle can be found here.

Production of fossil-fuel derived diesel

Conventional diesel fuel (and other petroleum products) starts as crude oil pumped from the ground. Crude oil is a mixture of hydrocarbon molecules of varying sizes. A process called fractional distillation is used to separate the molecules using their inherent nature of having different boiling points. A fractional distiller is essentially a large vertical furnace in which crude oil is heated in progressively hotter chambers, each of which route the resulting gases into separate condensers and containers. In this way diesel, gasoline, kerosene, and other oil products are separated from the crude oil feedstock. The distillation temperature for diesel fuel is in the range of  250 ^oC - 350 ^oC [2].

Production of algal biodiesel

In the case of algal biodiesel, the fuel is harvested and processed from oils contained in three main types of algae: microalgae, cyanobacteria, and macroalgae. Algae are naturally composed of three main components: proteins, carbohydrates, and lipids. It is the lipids (fatty acids) that are the source of the fuel. While there are several extraction and conversion methods currently being researched, a common method is harvesting of microalgae via filtration, drying the algae to reduce water content, pressing the resulting algae to remove the oil, and then converting the oil to biodiesel through chemical transesterification.

Analysis of Fuel Considerations

The comparison of fuels used in transportation should include a wide variety of performance indicators that have a direct impact on engines, consumers, and the environment. A fuel that seems far superior in one category could have clear disadvantages when evaluated in another category. Societies must consider these tradeoffs when investing in the infrastructure required for the mass production and distribution of transportation fuels. Here, the performance of diesel and algal biodiesel characteristics are compared in seven categories: availability, cost, ease of use, energy properties, physical properties, safety, and emissions.

Availability

According to BP, global crude oil reserves are about 1333 billion barrels [3]. According to the same source, the world is consuming over 84 million barrels of oil per day. At the current rate of consumption (and assuming no new discoveries), this reserve will last 15,855 days, or about 43 years. In addition, the majority (56.6%) of proven reserves are in the Middle East, with only 5.5% of reserves in North America [3]. This is clearly an unsustainable trend. Energy is a major factor in the average standard of living, and upcoming scarcity of fossil fuels has the potential to cause major political upheaval and regional conflict in the next few decades. In the short term, diesel fuel is widely available (at least in the developed world). All gasoline stations have pumps dedicated to diesel fuel, and a widely developed infrastructure exists to produce and distribute diesel fuel to stations and consumers.

On the contrary, biodiesel derived from algae is not yet commercially available on a widespread basis, even as millions of research dollars and many young start-up companies are trying to develop cost-effective production methods. It is noteworthy that the US Navy recently purchased 1500 gallons of algal jet fuel for testing and certification [4]. Unlike traditional biofuel feedstocks which require lots of land and fresh water, algae is unique in that it doesn’t have to be grown on arable land or require much fresh water. Algae can be grown in bioreactors on arid or non-arable land or in large farms offshore. It can also be grown in brackish water. This means that it can be produced locally or regionally. This benefit would reduce the world’s reliance on a few nations for oil and reduce the need to transport crude around the world. If and when biodiesel from algae is ramped up to larger scale production, it could be distributed using the existing diesel infrastructure.

Although biodiesel yields vary by species, environment, and location, here is a rough order of magnitude calculation to determine how much area might be required to grow enough algae to replace all diesel consumption in the US. In 2008, the US consumed over 41 billion gallons of diesel for on-road transportation [5]. According to a recent study, producers can practically expect a yield of about 4350 – 5700 gallons per acre-year from algae [6]. Dividing 41 billion by this yield range shows that 10,900 – 14,400 square miles would be required to produce enough biodiesel to replace all on-road transportation needs in the US. This is roughly the size of the state of Maryland.

Cost

The cost of conventional diesel fuel in the last 16 years has ranged between $1/gallon and $4.70/gallon, with the current price just under $4/gallon. Figure 2 shows the pump price of diesel fuel from June 1994 through March 2011 [7].

Figure 2 - Average pump price of diesel fuel in the U.S. from June 1994 through March 2011 [7]

While the price has been volatile in recent years, it is still less than biodiesel production from algae. Since there are no large-scale production facilities of algal biodiesel, an accurate comparative cost per gallon is difficult to estimate. Some rough estimates put the current production price at around $50/gallon [8]. However, an anecdotal comparison can be made to algal jet fuel, which is farther along the path to commercialization. In February 2010, the Defense Advanced Research Projects Agency (DARPA) announced plans to build a large-scale algal jet fuel facility by 2013. It expects the production price to be under $3/gallon [9]. If this becomes a reality, and similar large-scale operations could be developed to produce biodiesel, it will be competitive in the marketplace.

Ease of Use & Compatibility

Since biodiesel fuel can be provided to the customer through the existing diesel infrastructure, there would be no difference in convenience or refueling time to a consumer.

Since most newer diesel engines are designed to accept low-sulfur diesel fuel and biodiesel blends up to 20% (B20), most consumers can use biodiesel in their existing diesel engines without any modification. However, in higher percent blends like B100, it is possible that certain natural rubber seals or hoses could degrade at an increased rate [10]. People who make the switch to B100 should check with the manufacturer to determine if any engine modifications are necessary. However, if it becomes clear that biodiesel is becoming more common in the marketplace, auto makers will inevitably start considering that in the design process so that diesel engines would be able to accept B100 blends with no modification.

Energy Properties

Energy-related properties of biodiesel should be considered when comparing to conventional diesel. One of the most important properties is cetane number. This is the measure of a fuel’s ignition delay, or the amount of time between the start of fuel injection and the start of combustion. A higher cetane number corresponds to a shorter ignition delay, so for diesel fuel a higher cetane number is generally better, since a shorter ignition delay will increase the cutoff ratio α and thus increase engine efficiency based on the equation for diesel cycle thermal efficiency:

The cetane number in conventional diesel varies between 40 and 45 in the US. The cetane number in biodiesel depends on feedstock, but usually varies between 45 and 67 [11]. A recent study of algal biodiesel reports a cetane number of 52 [12]. The increased cetane number in algal biodiesel will have a positive effect on engine efficiency.

Another important property of a fuel is its energy density. This matters because the higher the energy density, the better the fuel economy one can expect from the fuel, and the longer the range that can be achieved per tank. The energy density of conventional diesel is about 43.1 MJ/kg, or 35.9 MJ/L [13]. For biodiesel, this property also varies by feedstock. The same recent study of algal biodiesel reports an energy density of 40 MJ/kg, or 32.04 MJ/L [12]. This is about 7% less energy by mass, or 11% less by volume than conventional diesel. Some proponents of algal biodiesel report that some of these energy losses are offset by the higher combustion efficiency and better lubricity such that overall fuel economy (and range) is only decreased by 2% [14]. If this is true than decreases in fuel economy and range from algal biodiesel would be barely noticeable to the average driver.

I also performed some detailed calculations surrounding the adiabatic flame temperature of combustion in each fuel. The thermodynamics can get a little eye-glazing, so I’m not including the details – but they are here if you are interested. In summary, the flame temperature of conventional diesel is 2831 K, while the flame temperature from algal biodiesel is only slightly higher at about 2855 K. In theory, this slight increase in temperature indicates a slightly higher increase in efficiency, and might also result in increased NOx emissions (since NOx are formed in atmosphere at high temperatures). However, the relative temperature difference is so small that an actual difference in efficiency and NOx emissions may not be measurable.

Physical Properties

There are some important physical characteristics of biodiesel that should be considered when comparing with conventional diesel fuel. These properties include lubricity, API gravity, and cloud point.

In diesel engines, the fuel is what lubricates the fuel injection pumps and injectors. The lubricity of a fuel is the measure of the wear or scarring that occurs between two metal parts covered with the fuel as they contact each other. Better lubricity results in less wear and scarring. Lubricity is generally measured by the wear scar diameter (WSD) that results on a specific test rig. The lubricity of conventional diesel is 536 microns, whereas B100 has been measured at 314 microns, an indication of better lubricity [15].

The API gravity and its inverse specific gravity are basically a measure of fuel density, and can be used to provide an indication of how the density of a fuel varies with temperature. Studies have shown that the density of biodiesel varies in a linear fashion that is very similar to conventional diesel [16].

The cloud point is the temperature below which wax molecules in the diesel fuel begin to solidify, potentially resulting in gelling of the fuel. Gelling is a serious functional problem since it reduces or eliminates the ability of the fuel to flow through the injection system – essentially not allowing the engine to run. The cloud point of conventional diesel varies, but can be as high as -15 ^oC. Fuel additives, electric engine block or fuel warmers, and heated garages are commonly used in colder climates to work around this problem. The cloud point in biodiesel also varies, but is generally higher than conventional diesel. A recent study shows the pour point of algal biodiesel to be -14 ^oC, which corresponds to a cloud point of about -11 ^oC to -8 ^oC [12]. A switch to algal biodiesel would require the adoption of the same workarounds to a wider section of the population.

Safety

Safety is obviously an important consideration when analyzing a fuel. Important measures of fuel safety are its stability, flame visibility, and flash point.

Fuel stability can refer to thermal, oxidative, and storage stability. The most common comparison between biodiesel and diesel is oxidative stability, or the tendency of the fuel to react with oxygen at near-ambient temperatures. Biodiesel is more prone to oxidation because of the increased proportion of carbon-carbon double bonds. Oxygen atoms can more easily bond with adjacent carbon atoms, forming a hydroperoxide molecule. This reaction reduces molecule chain length, which can reduce the flash point, cause undesirable odors, and cause the formation of sediments in the fuel. ASTM D2274 test standards are usually used to measure stability in diesel fuel. However, this test is not adequate to measure biodiesel stability because of its incompatibility with the filters used in the test [17].  This instability should continue to be a focus of improvement for biodiesel. Stability can be improved through the manufacturing process and by the use of fuel additives or blends [18].

Flame visibility is simply how well you can see a flame resulting from the combustion of a fuel. It is more commonly a concern in industrial environments. The literature on flame visibility of diesel and biodiesel is thin. This suggests that there is likely not much of a difference in this factor.

At ambient temperatures, a fuel can mix with the surrounding air and become ignitable, causing a serious safety concern. The temperature at which this mixing begins to occur is called the flash point. A higher flash point is safer. The flash point of conventional diesel fuel is 52 ^oC [19]. The flash point of algal biodiesel is around 98 ^oC [12]. Biodiesel is therefore better in regards to the flash point, presenting a very low fire hazard.

Emissions

With so many vehicles on the road, tailpipe emissions are important to consider since they influence human health, quality of life, and global warming. The EPA produced a report in 2002 that compared emissions of various blends of biodiesel with conventional diesel. For B100, they found that biodiesel produced 67% less hydrocarbons, 48% less carbon monoxide, 47% less particulate matter, 80% less polyaromatic hydrocarbons, and 10% more NOx [20]. Figure 3 is from the EPA report and shows the relation between emissions and biodiesel blend percent.

Figure 3 - The effect on exhaust emissions for a range of biodiesel blends [20]

Biodiesel also produces no sulfate emissions since it does not contain sulfur, unlike conventional diesel. The higher oxygen content in biodiesel results in a higher combustion efficiency, which increases the combustion temperature and thus increases the amount of NOx emissions, since NOx are only formed above approximately 1500 ^oC. The CO₂ emissions from biodiesel combustion are the same as that from conventional diesel. However, since the CO₂ in biodiesel was captured through the process of photosynthesis in the feedstock, the algal biodiesel has an overall life-cycle CO₂ impact that is less than conventional diesel, assuming the CO₂ emitted in the processing of the biodiesel is less than the CO₂ saved. The relative value of each emission species is debatable, but overall the biodiesel has a clear advantage when it comes to emissions.

Regardless of the fuel used in a vehicle, there are two basic ways to reduce emissions: pretreatment or aftertreatment. It is noteworthy that diesel engines have traditionally not been treated, and therefore have higher polluting emissions than gasoline engines. In the last decade, the US has implemented low-sulfur regulations in diesel. Now all diesel fuel must meet the standard of having less than 15 ppm of sulfur. This is an example of pretreatment, where pollutants are removed from the fuel before it is burned. Another example of pretreatment is exhaust gas recirculation (EGR), where a portion of the exhaust stream is routed back into the intake of the engine and fed into the combustion chamber. This has the effect of lowering the flame temperature of the combustion event. Exhaust gas is able to absorb more heat energy from the combustion event than air since it is composed of inert triatomic molecules. This technology is common in diesels manufactured today. Another technology that increases the combustion efficiency and reduces pollutants is better fuel injection technology that uses higher pressures and pulsed injection, helping the fuel mix with the air more rapidly and evenly. Aftertreatment is when pollutants are removed after combustion, in the exhaust stream. One potential aftertreatment device for diesel engines are particulate matter traps. This comes with a tradeoff in efficiency and cost since traps block the flow of exhaust gases, thus the need for two separate traps and a mechanism to switch between the two when one is sufficiently full and needs to have the particulates burned off. Another potential aftertreatment method involves the addition of urea to the exhaust gas stream, followed by a Selective Catalytic Reduction (SCR) converter. Through a complex series of reactions, the “doping” urea and the catalyst turn much of the NOx emissions into water and nitrogen.

Conclusions

Based on this analysis, it is my opinion that the advantages of algal biodiesel outweigh the disadvantages, and that it shows promise as a replacement for conventional diesel fuel. Additional research and development should continue.

While not yet commercially available, algal biodiesel can use the same infrastructure as the existing diesel distribution network. And the potential for domestically sourced feedstock is a major advantage in order to avoid political instability and potential regional conflict due to scarcity of petroleum resources. While the technology is not fully commercialized, it has been shown that full-scale production of algal biofuel can be cost-competitive with conventional diesel.

The higher cetane number of biodiesel will increase the efficiency of the engine, while the lower energy density of the fuel will cause a very small decrease in fuel economy. The higher flame temperature in biodiesel combustion will slightly increase engine efficiency. Biodiesel is also much less of a fire hazard. A significant disadvantage of biodiesel is low-temperature performance and oxidative instability. While these factors are not significant enough to prevent wide adoption of biodiesel, further research should be conducted to help reduce or eliminate these drawbacks.

Finally, biodiesel has a clear advantage in terms of exhaust emissions. The only species that it performs worse than diesel is in NOx. Implementation of mentioned aftertreatment technologies can help reduce these and other emissions, along with further research to improve diesel engine emissions performance.

Works Cited

[1] NBB. The Official Site of the National Biodiesel Board. 2010. http://www.biodiesel.org/resources/definitions/default.shtm 

[2] HowStuffWorks. How Oil Refining Works. http://science.howstuffworks.com/environmental/energy/oil-refining2.htm 

[3] BP. Statistical Review of World Energy 2010. BP, 2010.

[4] Lane, Jim. Biofuels Digest. August 10, 2010. http://biofuelsdigest.com/bdigest/2010/08/10/solazyme-takes-lead-in-race-for-commercialized-algal-oil-raises-52m-for-algal-fuel-expansion/ 

[5] EIA. Estimated Consumption of Vehicle Fuels in the United States. April 2010. http:// www.eia.doe.gov/cneaf/alternate/page/atftables/attf_c1.xls 

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[7] EIA. Independent Statistics and Analysis. August 2010. http://www.eia.doe.gov/oog/info/wohdp/diesel.asp 

[8] Doty Energy. Micro-algae. May 5, 2010. http://www.dotyenergy.com/Markets/Micro-algae.htm 

[9] Goldenberg, Suzanne. Algae to solve the Pentagon's jet fuel problem. February 13, 2010. http://www.guardian.co.uk/environment/2010/feb/13/algae-solve-pentagon-fuel-problem 

[10] NBB. Biodiesel Performance. 2010. http://www.biodiesel.org/pdf_files/fuelfactsheets/Performance.PDF 

[11] Van Gerpen, Jon. National Biodiesel Board. September 1996. http://www.biodiesel.org/resources/reportsdatabase/reports/gen/19960901_gen-187.pdf 

[12] Vijayaraghavan, K. , Hemanathan, K. "Biodiesel Production from Freshwater Algae." Energy Fuels, 2009: 5448-5453.

[13] EUCAR. Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context. March 2007. http://ies.jrc.ec.europa.eu/uploads/media/TTW_Report_010307.pdf 

[14] Oilgae. Algal Biodiesel Characteristics & Properties. 2010. http://www.oilgae.com/algae/oil/biod/char/char.html 

[15] NBB. Lubricity. 2010. http://www.biodiesel.org/pdf_files/fuelfactsheets/Lubricity.PDF 

[16] Tat, M., Van Gerpen, J. "The Specific Gravity of Biodiesel and its Blends with Diesel Fuel." Journal of the American Oil Chemists' Society, 2000: 115-119.

[17] Becon. Fuel Stability. http://www3.me.iastate.edu/biodiesel/Pages/biodiesel19.html 

[18] Kenreck, Glenn. "Improving Biodiesel Stability with Fuel Additives." Biodiesel Magazine, February 2007.

[19] Engineering ToolBox. Flash Point - Fuels. 2005. http://www.engineeringtoolbox.com/flash-point-fuels-d_937.html 

[20] EPA. A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions. Ann Arbor, MI: US Environmental Protection Agency, 2002.

[21] DOE. "Just the Basics - Diesel Engine." US Department of Energy - Office of Energy Efficiency and Renewable Energy. August 2003. http://www1.eere.energy.gov/vehiclesandfuels/pdfs/basics/jtb_diesel_engine.pdf

- EIA. What fuels are made from crude oil? http://www.eia.doe.gov/kids/energy.cfm?page=oil_home-basics#oil_refining-basics 

- NFCRC. National Fuel Cell Research Center - dynamic analyses and control. 2009. http://www.nfcrc.uci.edu/2/ACTIVITIES/RESEARCH_STUDIES/Dynamic_Analyses_and_Control/Planar_and_Tubular_Solid_Oxide_Fuel_Cells/Index.aspx 

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- Yu-jie Fu, Yuan-gang Zu, Li-li Wang, Nai-jing Zhang, Wei Liu, Shuang-ming Li,. Determination of Main Fatty Acid Methyl Esters in Biodiesel Product from Yellow Horn. Harbin, China: Key Laboratory of Forest Plant Ecology, Ministry of Education, Northeast Forestry University, 2007.