Cetane Number of biodiesel

The ignition quality of a fuel can be deduced through its cetane number. A fuel with good ignition quality has a high cetane number, where the ignition delay period between the start of fuel injection and the onset of auto ignition is short. Cetane number of biodiesel varies with the feed stock used, but it is generally in the higher end of the typical diesel fuel range.

The value of cetane number is found to generally increase with increasing carbon chain length. Vegetable oils have a low cetane number. This is due to the presence of bulkier molecules in the triglycerides which have a high viscosity.

Methyl esters (biodiesel) are observed to possess a higher CN when compared to their corresponding oils. Methyl esters have a lower viscosity than their corresponding vegetable oils. Biodiesel records high CN values. These values are higher than that of diesel fuel with CN of 52. The high CN of biodiesel may be influenced by their characteristics of the feed stock.

Factors that affect the CN in the biodiesel are, e.g. the number of carbon atoms of the original fatty acids, the number of double bonds and the ester yield. Non edible oils blends have CN values close to those of the diesel fuel samples. This is due to the presence of diesel in blends. It is clear that, the higher the density the lower the Cetane number.

There exists a relation between cetane number and properties of biodiesel so a correlation was developed which is given as the following equation:

CN = K5 + K4 ν + K3 HV + K2 FP + K1 ρ

where:

K1, K2, K3, K4, K5 are constants,  ν is kinematic viscosity (mm2 /sec), HV is heating value (MJ/kg), FP is flash point (°C), ρ is density (kg/l).

The above equation shows the relationship between the Cetane number and the thermal properties. 

The average measured values of CN obtained compare well with the predicated values using the equation.  The equation is used to predict the CN values of different biodiesels consisting of different thermal properties. The coefficient of determination is 0.9, which indicates that the CN can be predicted with 90% accuracy based on thermal properties of biofuel.

Generations of Biofuels

First Generation Biofuels

First Generation biofuels are produced directly from food crops by abstracting the oils for use in biodiesel or producing bioethanol through fermentation. Crops such as wheat and sugar are the most widely used feedstock for bioethanol while oil seed rape has proved a very effective crop for use in biodiesel. However, first generation biofuels have a number of associated problems. There is much debate over their actually benefit in reducing green house gas and co2 emissions due to the fact that some biofuels can produce negative Net energy gains, releasing more carbon in their production than their feedstock’s capture in their growth. However, the most contentious issue with first generation biofuels is ‘fuel vs food’. As the majority of biofuels are produced directly from food crops the rise in demand for biofuels has lead to an increase in the volumes of crops being diverted away from the global food market. This has been blamed for the global increase in food prices over the last couple of years.

Second Generation Biofuels

Second Generation biofuels have been developed to overcome the limitations of first generation biofuels. They are produced from non-food crops such as wood, organic waste, food crop waste and specific biomass crops, therefore eliminating the main problem with first generation biofuels. Second Generation biofuels are also aimed at being more cost competitive in relation to existing fossil fuels. Life cycle assessments of second-generation biofuels have also indicated that they will increase ‘net energy gains’ over coming another of the main limitations of first generation biofuels.

Third Generation Biofuels

The Third Generation of biofuels is based on improvements in the production of biomass. It takes advantage of specially engineered energy crops such as algae as its energy source[3]. The algae are cultured to act as a low-cost, high-energy and entirely renewable feedstock. It is predicted that algae will have the potential to produce more energy per acre than conventional crops. Algae can also be grown using land and water unsuitable for food production, therefore reducing the strain on already depleted water sources. A further benefit of algae based biofuels is that the fuel can be manufactured into a wide range of fuels such as diesel, petrol and jet fuel.

Fourth Generation Biofuels

Four Generation Bio-fuels are aimed at not only producing sustainable energy but also a way of capturing and storing co2. Biomass materials, which have absorbed co2 while growing, are converted into fuel using the same processes as second generation biofuels. This process differs from second and third generation production as at all stages of production the carbon dioxide is captured using processes such as oxy-fuel combustion. The carbon dioxide can then be geosequestered by storing it in old oil and gas fields or saline aquifers. This carbon capture makes fourth generation biofuel production carbon negative rather then simply carbon neutral, as it is ‘locks’ away more carbon than it produces. This system not only captures and stores carbon dioxide from the atmosphere but it also reduces co2 emissions by replacing fossil fuels.  

Tobacco – the new biofuel?

There’s an interesting little piece in South African Airlines’ in-flight magazine this month, describing how the airline is partnering with Boeing and a company called SkyNRG to make a renewable jet fuel from the Tobacco plant. ‘Put this in your pipe – and fly it!’ they say, claiming a potential 50–80% reduction in carbon footprint.

The use of tobacco is an interesting way to sidestep a common criticism of biofuels; that they aren’t ethical because the biofuel replaces a food crop. Smoking is unfashionable now, and demand for tobacco is falling: so you find a hybrid tobacco plant, selected for yielding a high quantity of oil-rich seeds, and growers can continue to use their existing knowledge, practices and machinery. It’s a pretty neat idea.

A better one is the creation of biofuel from an algae that grows on sewage (Virgin and Air New Zealand were both involved in that as far back as 2008) but it’s not half bad.

How many tobacco seeds do you have to press to get a litre of oil? And how much of that oil can be fractioned into jet fuel? That is the big question, here.

In their paper, ‘Comparison of techniques for the extraction of tobacco seed oil’, Stanisavljevic et al (2009) report that in tobacco up to 40% of the total seed mass is oil. The best technique for oil extraction was cold pressing, delivering 93% of the oil content. The by-product ‘cake’ is useful for cattle-feed (don’t worry, the nicotine isn’t present in the seeds). 

Giannelos et al, 2002 has shown that tobacco seed oil can be used as a diesel substitute – not a bad use for it as the oil is high in cholesterol.

Having viewed the websites of enthusiasts who grow their own tobacco, I can report the following: a plant can have many flowers, and when these reach maturity they produce seed pods. The reported number varies, but around a eighty on a single plant appears to be common. Each seed pod contains a mass of seeds; hundreds or even thousands of the things… but each is smaller than a grain of sand.

Liao et al (2014) compares five strains of tobacco, and the most promising for our purposes appears to yield around 1,250 seeds per pod. Its average dry weight for a hundred seeds is also among the best… at just under 0.01g.

Let’s assume that SkyNRG’s tobacco hybrid is as oil-rich as it can possibly be. You can probably squeeze 19,750 tobacco plants into each hectare of land at your disposal. Let’s assume all plants survive. Each produces eighty pods. Each pod conforms to the highest yield seen by Liao et al (2014), and we use the most efficient cold pressing technique for extraction.

The number of seeds harvested is impressive, at almost two billion. That’s almost 188 kilos of tiny seeds… from which we get 61.7 litres of oil.

Exactly what fraction of that oil is suitable for use in a gas turbine, I don’t know, although jet engines aren’t particularly fussy, so let’s be generous and say it can all be used. To generate the 100,000 litres used up in a one-way trip between London and Johannesburg is going to require 1,620 hectares’ production – in other words a land area of 16.2 square kilometres… which doesn’t seem much when considered against the massive 1,221,037 km2 land area of South Africa. We have to remember, though, that the Airbus A330 in our study doesn’t just fly once a year; more likely hundreds of times a year. Perhaps you can get two tobacco crops from each plot of land, per year… but even so, the numbers for land use are adding up.

Furthermore, we have to keep in mind that agriculture doesn’t happen ‘for free’; there’s people to pay and to feed, machinery to run and water to find, all the while (it is to be hoped) preserving the spectacular biodiversity of the nation.

Is this anything more than a puff piece for South African Airlines, establishing their environmental credentials and perhaps making the passengers feel a little bit better about their carbon footprint? . It’s just about viable (although commercial success would require the economic use of the rest of the plant), and it is certainly worth conducting experiments of this kind. Most oilseed crops yield something like one metric tonne per hectare, which makes my estimate for tobacco’s contribution look absolutely puny at 61.7 litres… but it makes use of existing expertise in the growing community, and suggests a way to achieve continuity at a time when demand for tobacco is falling. That’s got to count for something.

How can a Diesel Engine be Converted to Biodiesel

A diesel engine is already designed to accept biodiesel fuels, so it does not have to be completely overhauled to become "green." The inventor of the diesel engine, Rudolf Diesel, originally envisioned peanut oil as the engine's fuel source, but a usable petroleum-based waste oil became more commercially available. Modern diesel engines still continue to run primarily on petroleum diesel, but a number of owners are now converting to alternative fuels such as used vegetable oils collected from restaurants and food processing plants. Biodiesel conversion kits are available for diesel car owners, although they can be somewhat expensive.

When most people think of converting their car to biodiesel, what they are really considering is how to utilize used vegetables oils as a fuel. Straight vegetable oil (SVO) itself is not biodiesel. Modifications to the engine must be made before a car can accept SVO. True biodiesel is processed before it goes into the car, and is added to the gas tank like regular fuel.

One of the main challenges with converting a diesel engine to accept SVO fuel is the nature of the fuel itself. Food-grade vegetable oils have a tendency to thicken or solidify as the air temperature drops. To be useful as a fuel, the vegetable oil must be heated. This heating is accomplished by installing a second fuel tank in the car's trunk and running lines from the radiator to provide radiant heat. Petroleum or mixed petroleum/biodiesel fuel is stored in another tank, since it does not need to be heated.

The SVO conversion kit also includes a thermostat and fuel tank switch placed near the driver's seat. When the vegetable oil has reached an acceptable temperature, the switch is activated and the heated oil is drawn through a second fuel line into the engine block. At this point the car is being completely operated on organic fuel. The standard petroleum fuel line is shut off until the switch is reversed.

Used vegetable oil may contain a number of food particles and other contaminants, so a SVO conversion kit also contains a special filter mounted near the engine block. This filtering process ensures that only heated vegetable oil reaches the engine, which reduces the risk of clogged fuel lines and contamination of the engine block. Biodiesel fuel blends containing both petroleum and organic materials have already been filtered, so that fuel is not sent through the filter. Biodiesel blends also use oils which have been processed to remain liquefied regardless of air temperature.

There are also kits available that will convert used vegetable oil to biodiesel fuel. These are usually free-standing systems that can be housed in a garage or other outbuilding. The process of converting vegetable oils to biodiesel takes a number of steps, and can be dangerous if the proper safety precautions are not followed. Fuel created using such kits can be used by a diesel car just like regular diesel fuel.

Converting a standard diesel engine to SVO is mostly a question of providing a second fuel tank and fuel line to deliver the organic fuel to the engine. If a stable form of vegetable oil with a lower point of solidification can be developed, the need to heat the tank may be eliminated in the future. It is important to understand that SVO and biodiesel work in diesel engines, not the gas-powered internal combustion engines found in many cars.

Biodiesel From Tallow

Biodiesel can be easily made from tallow using very similar processes to plant oils. However, and according to a paper by Miller-Klein Associates, it has some advantages and disadvantages that are agreed between different studies.

 Advantage

Biodiesel from tallow has a higher cetane number than plant oil biodiesel. This means cleaner and more efficient burning in diesel engines.

Cetane numbers rate the ignition properties of diesel fuels, just as octane numbers determine the quality and value of petrol. It’s a measure of a fuel’s ability to ignite when it’s compressed. The higher the cetane number, the more efficient the fuel. Biodiesel has a higher cetane number than petrodiesel because of its oxygen content. The ignition quality affects engine performance, cold starting, warm up and engine combustion roughness. A high cetane fuel also may lead to incomplete combustion and smoke if the fuel ignites too soon by not allowing enough time for the fuel to mix with air for complete combustion.

 Disadvantage

Higher cloud point. Because of the high levels of saturates, biodiesel from tallow tends to crystallise out at much higher temperatures than biodiesel from plant oils. In Northern Europe this makes tallow biodiesel unsuitable for winter use apart from blending at low rates into conventional diesel. Tallow diesel cannot meet the required DIN standard for 100% biodiesel, but as a 5% mix with conventional diesel it meets the required standards.

Conclusions

There are no environmental advantages to using tallow as a biodiesel feedstock instead of plant oils. Total availability of tallow as a feedstock is limited and cannot be increased, and in some instances is declining.

Use of tallow for biodiesel competes with existing commercial outlets where the properties of the specific fatty acid composition bring advantages.

Tallow is not the best feedstock for biodiesel, especially for fuel use in areas with cold winters as it congeals. High cloud is more of a problem than higher cetane number is a benefit.

Exploitation of tallow will make economic sense for companies who are vertically integrated into rendering, but it will always be a minor component of the whole biodiesel picture.

EU biofuels legislative overview

The EU 2020 climate and energy strategy sets a 10pc target of renewables in road transport fuels, as part of the Renewable Energy Directive, and a 6pc reduction of greenhouse gas (GHG) intensity of transport fuels, as part of the revised Fuel Quality Directive.

The EU Agriculture and Fisheries Council formally adopted the so-called "ILUC" Directive on 13 July 2015 in order to prepare "the transition from conventional biofuels to biofuels that deliver substantial greenhouse gas savings", implementing a quantification and reporting system to monitor emissions caused by indirect land use change (ILUC) from 2017.

The legislation, amending the Renewable Energy Directive as well as the Fuel Quality Directive, limits the share of crop-based biofuels to 7pc of the final consumption of energy in 2020.

Member-States have 18 months to set a sub-mandate for advanced biofuels that would be double counted towards the 10pc overall target, although the reference target of 0.5pc is non-mandatory.

The legislation was originally introduced by the Commission by 2012 and first set a 5pc cap on conventional biofuels. It failed to pass in the previous parliament in 2013.

In February 2015, the European Parliament's environment committee called for a 6pc cap, but Bulgaria, the Czech Republic, Estonia, France, Spain, Hungary, Poland, Romania and Slovakia opposed the adoption of any cap lower than 7pc.The parliamentary committee unsuccessfully argued for a binding sub-target of at least 2.5pc by 2020 on advanced biofuels.

Member States must transpose the Directive into their national legislation within 24 months of its adoption.

Germany
Since 1 January 2015, Germany has replaced its 6.25pc biofuel blend rate in energy terms, in which waste and residue-based biofuels were double-counted, with a 3.5pc quota for GHG reduction. The quota will be raised to 4pc in 2017 and 6 pc in 2020.

This new legislation has triggered an overall drop in German biofuels consumption in the first months of 2015. Under the new framework, first generation biofuels are less effective in achieving the necessary GHG savings. Conversely, it supported the incorporation of more second generation biofuels, although smaller volumes were needed to achieve the 3.5pc quota.

The drop was more marked for ethanol as its consumption was also driven down by decreasing gasoline consumption. Producers reported that the drop in the consumption of biodiesel could amount to 120,000-200,000t this year.

Belgium
Up until June 2015, Belgium had a blending mandate of 4pc of bioethanol in volume, and of 6pc for biodiesel. The latter had to be fulfilled by fatty acid methyl ester biodiesel, known as fame. Only 1.5pc of the 6pc mandate could be fulfilled with other types of biodiesel. This difference of treatment was struck down in May by the Belgian Constitutional Court, and the decision was effective from June 2015. Although there currently is no blending mandate in place for biodiesel, producers are still blending, with the expectation of a new mandate effective by the end of the year.

Double-counting was made possible by the 2013 legislation but any double-counting biofuel has to be approved by the government.

France
France's blending mandate is 7pc by energy content for ethanol, and 7.7pc for biodiesel. Advanced biofuels can be double-counted up to 0.25pc for bioethanol, while this cap is fixed at 0.35pc for biodiesel. Non-food-crop based biofuels, including used cooking oil and non-food grade tallow are included in this double counting system. Biofuels providers have benefited from fiscal incentives to support investments. But this system of incentives will end on 31 December 2015.

On 22 July, the National Assembly finally adopted the bill on Energy Transition for Green Growth. According to the bill, an advanced biofuels blending mandate will be part of the government's multi-year energy plan. The government will also be able to set the list of conventional and advanced biofuels. MPs defined "advanced biofuels" as biofuels that pose no threat to food production and that have no or low-ILUC impact. The bill set a 15pc renewable energy target in transport by 2030, although there was no indication as to how this would be achieved.

During discussions over the Growth and Activity Bill, MPs tried to trigger a more formal change in French biofuels policy, in order to further support the development of tallow and used cooking oils, considered as advanced biofuels. Although these attempts failed, the debate is likely to re-emerge during the Finance Bill for 2016, to be discussed in the autumn. The government was concerned that such changes would lead to increased imports, and require a change in biofuels fiscal policy in the middle of the fiscal year. Worries over a different treatment of advanced biofuels over conventional ones were also brought up by government representatives.

UK
The 2008 UK Renewable Transport Fuel Obligation (RTFO) is a scheme requiring that fuel suppliers of over 450,000 l/yr meet a 4.75pc quota of renewable supply, measured in volume. Renewable fuels have to comply with sustainability criteria in order to be taken into account under the scheme. Biofuels produced from wastes and residues are double counted.

To meet the obligation, fuel suppliers may redeem Renewable Transport Fuel Certificates (RTFCs) issued to fuel meeting the sustainability criteria, or pay a sum to ‘buy-out' of their obligation. For 2014-2015 — year 7 of the RTFO — 69pc of the 809mn RTFCs issued were to double-counted feedstocks.

A reform for implementation in time for year 10 of RFTO (April 2017) is planned, and ministers should agree on an approach by the end of 2015, according to the Department of Transport. But no clear direction has been announced yet. A combination of biofuels and electricity could be used to meet the 10pc RED requirement. Sub-targets for advanced biofuels, ranging from 0.5pc to 1.5pc, are among the scenarios considered.

Official statistics showed a drop in UK biofuels consumption for the first quarter of 2015, with biodiesel and bioethanol accounting for 2.9pc of all UK road transport fuel consumption.

Italy
Since 2015, Italy has enforced a blend rate of 5pc in energy terms for ethanol and biodiesel. Biodiesel from waste products, such as used cooking oils and tallow, are double counted. An advanced biofuels blending mandate, for waste and non-food feestocks biofuels, at 0.6pc by 2018 and 1pc by 2022, made Italy the first EU country to set a legally binding mandate for advanced biofuels.

But certification requirements do not allow substantial imports of used cooking oil to meet the sub-mandate, and this in turn is driving the prices of other waste-based biofuels up, and increasing the demand for tallow methyl ester (TME).

A 20pc cap on double-counted biofuels was removed in February 2014. It is expected that double-counted biofuels will account for up to 5pc in 2015.

Spain
Spain's biofuels target is 4.1pc of road transport fuel by energy content. Targets were reduced from 6.5pc in 2013. There are also sub-targets of 4.1pc of biodiesel in diesel, and 3.9pc of ethanol in gasoline. There is currently no double counting system in place.

A government bill proposes to raise the blend rate to 5pc in 2016 and 2017, 6pc in 2018, 7pc in 2019 and 8.5pc in 2020 in energy calorific terms. This is expected to be finalised and officially adopted this year.

The Spanish government said it wished to introduce a sub-mandate for biofuels made from waste oils and fats by 2020.

Portugal
A biofuels mandate of 7.5pc by energy content is in place for 2015 and 2016, set to increase to 9pc in 2017 and 2018 before reaching 10pc in 2019 and 2020. Fuels from waste, residues, non-food cellulosic material and lignocellulosic material count double.

Poland
Poland's biofuels blending mandate is 7.1pc in energy terms until the end of 2016. This can be reduced to around 6pc for blenders who buy 70pc or more of their feedstock from local suppliers.

A draft bill would introduce a double-counting system starting on January 2016 and running to the end of 2017. Waste and residue-based, including used cooking oil and tallow, biofuels could be concerned. The draft bill received mixed feedback from stakeholders and is expected to be reworked. Refineries and blenders judged the double-counting cap too low, while biofuels producers said it was too high and risked encouraging imports rather than strengthening the demand for domestically-produced biofuels.

Denmark
Denmark enforces a biofuels blending mandate of 5.75pc in energy terms, in place since 2010. No double-counting system is currently in place.

The June general elections saw the victory of the centre-right opposition over the Social Democrat government that had made declarations in favour of second generation biofuels, including agricultural waste based biofuels, and was considering the adoption of a sub-mandate for advanced biofuels. The change brings uncertainties as to the timeline and the extent of the Danish move towards advanced biofuels.

The Netherlands
The Netherlands currently imposes a biofuels quota on suppliers who sell more than 5,000 litres/yr of road transport fuel. The current quota is 6.25pc in energy terms and it is set to increase by 0.75pc each year in order to achieve 10pc in 2020. Fuel produced from wastes and residues, and from lignocellulose, counts double toward targets.

Biodiesel Blends

Biodiesel can be blended and used in many different concentrations. The most common are: B20 (6% to 20% biodiesel blended with petroleum diesel), B5 (5% biodiesel, 95% petroleum diesel) and B2 (2% biodiesel, 98% petroleum diesel). B100 (pure biodiesel) is typically used as a blend stock to produce B5 and B20 and rarely used as a transportation fuel.

Low-Level Blends

ASTM International develops specifications for a wide variety of products, including conventional diesel fuel (ASTM D975). This specification allows for biodiesel concentrations of up to 5% (B5) to be called diesel fuel, with no separate labeling required at the pump. Low-level biodiesel blends, such as B5 are ASTM approved for safe operation in any compression-ignition engine designed to be operated on petroleum diesel. This can include light-duty and heavy-duty diesel cars and trucks, tractors, boats, and electrical generators.

B20

B20 is a common biodiesel blend in the United States. B20 is popular because it represents a good balance of cost, emissions, cold-weather performance, materials compatibility, and ability to act as a solvent. Most biodiesel users purchase B20 or lower blends from their normal fuel distributors or from biodiesel marketers. Regulated fleets that use biodiesel blends of 20% (B20) or higher qualify for biodiesel fuel use credits under the Energy Policy Act of 1992.

B20 must meet prescribed quality standards as specified by ASTM D7467 (summary of requirements).

B20 and lower-level blends generally do not require engine modifications. Engines operating on B20 have similar fuel consumption, horsepower, and torque to engines running on petroleum diesel. B20 with 20% biodiesel content will have 1% to 2% less energy per gallon than petroleum diesel but most B20 users report no noticeable difference in performance or fuel economy. Biodiesel has some emissions benefits, especially for engines manufactured before 2010. For engines equipped with selective catalytic reduction (SCR) systems, the air quality benefits are the same whether running on biodiesel or petroleum diesel. However, biodiesel still offers better greenhouse gas (GHG) benefits compared to conventional diesel fuel. The emissions benefit is roughly commensurate with the blend level; that is, B20 would have 20% of the GHG reduction benefit of B100.

However, not all diesel engine manufacturers cover B20 use in their warranties. Users should always consult their vehicle and engine warranty statements before using biodiesel.

B100 and High-Level Blends

B100 and other high-level biodiesel blends are less common than B20 and lower blends due to a lack of regulatory incentives and pricing. B100 can be used in some engines built since 1994 with biodiesel-compatible material for certain parts, such as hoses and gaskets. B100 has a solvent effect, and it can clean a vehicle's fuel system and release deposits accumulated from petroleum diesel use. The release of these deposits may initially clog filters and require frequent filter replacement in the first few tanks of high-level blends.

When using high-level blends, a number of issues should be considered. The higher the percentage of biodiesel above 20%, the lower the energy content per gallon. High-level biodiesel blends can also impact engine warranties, gel in cold temperatures, and may present unique storage issues. B100 use could also increase nitrogen oxides emissions, although it greatly reduces other toxic emissions.

B100 requires special handling and may require equipment modifications. To avoid engine operational problems, B100 must meet the requirements of ASTM D6751, Standard Specification for Biodiesel Fuel (B100) Blend Stock for Distillate Fuels (summary of requirements). ASTM Specification D6751 now includes a No.1-B and a No.2-B grade. The No.1-B grade has stricter limits on monoglycerides and filterability than the No.2-B grade. The No.1-B grade is a special purpose biodiesel grade for use in applications where low temperature operability is needed.