BIODIESEL

PRODUCTION

AND ANALYSIS

Francesco Tavano Biofuels and Biomass Energy

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1 TABLE OF CONTENTS

2 Introduction ............................................................................................................................................... 2

3 Definition and production ......................................................................................................................... 2

3.1 Preliminary steps ............................................................................................................................... 2

3.2 The reaction ....................................................................................................................................... 3

3.3 Pre-treatment and Catalysts ............................................................................................................. 3

4 Features ..................................................................................................................................................... 4

4.1 Total acidity ....................................................................................................................................... 4

4.2 Distillation .......................................................................................................................................... 4

4.3 Density ............................................................................................................................................... 4

4.4 Cetane number .................................................................................................................................. 4

4.5 Iodine number ................................................................................................................................... 5

4.6 Calorific Value .................................................................................................................................... 6

4.7 Flash point ......................................................................................................................................... 7

4.8 Oxidation stability .............................................................................................................................. 7

4.9 Viscosity ............................................................................................................................................. 7

5 Analysis ...................................................................................................................................................... 8

5.1 Gas Chromatography ......................................................................................................................... 8

5.1.1 Goal ............................................................................................................................................ 8

5.1.2 Operating mode ......................................................................................................................... 8

5.2 NMR ................................................................................................................................................... 9

5.2.1 Goal ............................................................................................................................................ 9

5.2.2 Operating mode ....................................................................................................................... 10

5.3 Autoxidation .................................................................................................................................... 11

6 Final Considerations ................................................................................................................................ 11

7 References ............................................................................................................................................... 12

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2 INTRODUCTION

Particulate matter, acid rains and global warming are just some consequences of the negative impact that

the immoderate use of oil is carrying out on our planet.

Various ways of biocompatible energy sources have been studied: cars can nowadays run on dozens of

alternative systems. But it is well known that hybrid cars over only a fraction of currently circulating

vehicles since, for instance, despite progresses, we are still too far from an effective and cheap way of

harvesting hydrogen: a potential ultimate solution to energy harvesting.

This article is about to introduce an easier and more immediate alternative to conventional oil-based fuels;

something which constitutes already a reality in many EU countries and in the rest of the world: biofuels.

Starting from its definition, biofuel is a “liquid or gaseous fuel produced by plant material”, thus renewable.

Among the most commonly used ones, bioethanol and bio methanol, if added to gasoline, can dilute with

minimal loss of yield. These, however, show significant drawbacks: methanol is toxic so that any loss due to

a leak, for example, could cause severe pollution while ethanol is an azeotrope, feature that leads to a very

though time from an energy point of view (since it has to be purified from its excess of water).

In order to replace diesel, you can also use certain vegetable oils to be mixed with the diesel fuel itself but

all of them have high viscosity and numerous contraindications for usual engines (which have to be

accurately tuned).

This is the reason why a reaction of transesterification is needed to avoid or, at least, reduce to minimum

these drawbacks and make the fuel well-suited to unmodified cars. The result of these reaction is another

type of biofuel, which is also the main topic of this report: the Biodiesel, which can be diluted (as much as

80% in volume, or above) with common diesel without substantial adjustment in cars.

Moreover, production of Biodiesel does not involve particular difficulties of preparation to such an extent

that it can easily be made at home with simple tools and relatively little chemical knowledge.

In this report we will seek for understanding what the Biodiesel is and how it is prepared, as well as its

chemical and physical analysis. Conclusions will give an overview of biodiesel today.

3 DEFINITION AND PRODUCTION

Biodiesel belongs to the family of FAMEs (Fatty Acid Methyl esters) and, by definition, is a methyl ester

deriving from the transesterification of methanol with vegetable oils (mostly edible-grade oils such as

soybean, rapeseed and palm) or animal fats in alkaline catalysis providing lower yields but also lower

pollution coming from the toxicity of methanol. It is also possible to work in acid catalysis: this would

prevent the formation of soaps and would not make compulsory an anhydrous working environment, in

spite of what is typically required.

3.1 PRELIMINARY STEPS In order to get the best out of the chosen oil seeds, some pre-processing is necessary. Steps can be

summarized as follows:

Cleaning and grinding: useful to get rid of any metal part or any other impurities

Oil extraction: could be physical (by “crushing”) preserving vitamins, sterols antioxidants, or

chemical (performed by solvents) with higher yields and a faster and less expensive process

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Removing of solvent traces: In case solvents have been used in the process (like hexane) they can

be separated from the oil and recollected for reuse

Refining the oil: oil, in order to be ready for further processing, must be refined so that colour,

odour and bitterness are removed.

3.2 THE REACTION As it has been mentioned in the introduction, preparation of conventional biodiesel requires a

transesterification reaction consisting in lipids (or triglyceride oils) reacting with an alcohol in the presence

of a catalyst (NaOH usually), leading to the formation of esters and glycerol. As shown below:

Once the reaction is completed the result is a mixture of methyl esters that does not include any sulphur or

aromatics. Oxygen, instead, is present in such a high quantity that the mixture can be immediately used for

automotive or heating purposes.

As a by-product glycerine, once purified, can be used for cosmetics or pharmaceuticals purposes.

Requirements to make the reaction possible are:

Methanol must be present in excess at a ratio of 6:1 with respect to oil

Less than 3% of Free Fatty Acids on the reactants side

No water

3.3 PRE-TREATMENT AND CATALYSTS Considering the importance of what has just been stated, especially when talking about acidity and

moisture content, it is crucial to perform some pre-treatment in order to achieve the desired conversion

efficiency in the process.

A fundamental role, in this sense, is played by the type and the amount of selected catalyst: both excess

and lack of it may lead to soap formation. In particular the reaction of transesterification might be carried

out by Bronsted acyds or Lipase enzymes.

The first choice may be preferred in terms of overall outcome (yield in this case is very high) even though it

has some important drawbacks, such as the extreme slowness (more than 3 hours to complete the

conversion) and the need of high temperature (above 100 °C). On the other hand, Lipase-catalysed

biodiesel can be performed at lower temperature (therefore less energy consumption is expected), no pre-

treatment is required and no treatment of wastewater is needed. Still, high cost of the enzyme, technical

difficulties and small scale only feasibility are non-negligible factors.

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A third way is actually possible: a catalyst-free process. The most popular technique to work around the

lack of a catalyst takes advantages of a supercritical methanol at high temperature and pressure in such a

way that the reaction occurs spontaneously and very fast. Advantages in this case include the fact that the

process can tolerate water, there is no catalyst removal step and energy costs for reaching such a high

temperature are similar if not lower than common catalytic production routes.

4 FEATURES

Currently, technical characteristics of the biodiesel are defined by UNI, but a great effort has been made at

a European Level (CEN) in order to accurately identify different products. In this sense it turns out being

useful to take into account the features of those biodiesels involved in thermal power (heating) too, since

they can show slightly different features with respect to the ones used in diesel engines (vehicles) only.

The characteristics of methyl esters tend to be significantly different from those of the original raw oil

thanks to the transesterification process which profoundly changes all properties.

There are several parameters to describe biodiesel and a detailed list is offered above.

4.1 TOTAL ACIDITY This is a parameter which describe the amount (in milligrams) of potassium hydroxide (KOH) required to

neutralize the free fatty acids in one gram of oil: high values can be harmful to the engine since acids can

corrode the metal parts of the latter and the tanks where the fuel is stored.

A suitable value should be smaller than 0.5 mg KOH / g.

4.2 DISTILLATION It describes the fuel volatility that is, basically, its tendency to shift to steam phase. Therefore, high values

of the distillation temperature correspond to low volatility and the possibility of inadequate vaporization in

the burst chamber, with consequent risk of incomplete combustion, cracking, condensation and formation

of compounds such as the extremely dangerous PAH (Polycyclic Aromatic Hydrocarbons).

The starting point of vaporization trend is getting higher in comparison with the diesel fuel (280-320 °C)

while the ending point does not exceed 400 ° C, so problems due to cracking are significantly reduced.

4.3 DENSITY The density expresses the mass per unit of volume and it serves for tax purposes as well as for the

conversion from mass to volume. This value for any methyl ester depends on the density of the crude oil

source with an average of around 0.88 kg / dm3.

4.4 CETANE NUMBER It indicates the fuel behaviour at engine start up and therefore influences: cold start, the engine noise, the

quality of combustion, pollutants emission level and overall smoothness of operation. More specifically, the

cetane number is an indicator of the time between injection and combustion, so, the higher the value, the

greater the readiness of fuel power.

This parameter for a methyl ester varies from 46 to 51 depending on the starting oil. A typical value for

common diesel fuel is 40 (up to 55) with an average of 49 in temperate climates (according to EN 590

specifications) and slightly lower, 45-47, for Arctic climates.

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In the following table cetane numbers for different methyl esters are listed.

Fuel Cetane number

Methyl ester of soybean oil (average values) 46-51

Methyl ester of Rapeseed oil 54

Methyl ester of sunflower oil 49

Methyl ester of palm oil 62

Methyl ester of cottonseed oil 51

Ethyl ester of soybean oil 48-50

Oil 2D (average) 48

Source: US DOE-NREL 1998

The cetane number of the fuel depends not only on the oleaginous original species but, also, on the climatic

conditions of the area where the crop has been growing: different climates determine a different chemical

composition of the seeds. Another factor is the mixture of acids in the methyl ester: each fatty acid, in fact,

has its own Cetane number, as shown above:

Methyl ester Common name Purity [%] Cetane Number

Me 8: 0 methyl octane 98.6 33.6

Me 10: 0 methyl dean 98.1 47.2

Me 12: 0 methyl laureate 99.1 61.4

Me 14: 0 methyl myristate 96.5 66.2

Me 16: 0 methyl palmitate 93.6 74.5

Me 18: 0 methyl stearate 92.1 86.9

Source: US DOE-NREL 1998, Thompson, 1997

4.5 IODINE NUMBER It indicates the degree of unsaturation of the oil and of the methyl ester as well as the number of double

bonds. The amount of double bonds is a crucial value since each of them can undergo additional reactions

with the oxygen: a condition that can prevent fuel quality, as it will be addressed in the following sections.

In order to assess the unsaturation of the hydrocarbon chains, a common method is to measure the grams

of iodine (I2) that are able to react with 100g of product (basically reproducing the same reaction which

would happen with oxygen).

The higher the value, the greater the degree of unsaturation and the chances to compromise stability.

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Fuel Iodine

Methyl ester of soybean oil (Average value) 133

Rapeseed oil methyl ester 91.9

Methyl ester of sunflower oil 125.5

Methyl ester of cottonseed oil 105.7

Ethyl ester of rapeseed oil 96.7

Ethyl ester of soybean oil 123.0

Oil 2D 8.6

Source: US DOE-NREL 1998, Thompson, 1997

Since the number of iodine does not vary with refining and esterification processes, crude oil, oils and their

methyl esters can be classified by the degree of saturation and by chain length of the fatty acids, too:

Class of oils Examples of oils Iodine

Lauric Copra, down 5-30

Palmitic African palm 45-58

Stearic Cacao 50-60

Oleic olive, peanut, canola 80-100

Linoleic sunflower, soy, corn, cotton > 110

Highly unsaturated tobacco, flax

Source: US DOE-NREL 1998

4.6 CALORIFIC VALUE It is defined as the energy that a fuel releases during the process of combustion. You might refer to higher

calorific value (HCV) if considering all the energy produced by the fuel, or lower calorific value (LCV) when

not taking into account the fraction of energy used by the water to evaporate.

Esters have an average LCV of around 33 MJ / dm3 while diesel one is set at around 35.4 MJ / dm3. Values

per weight are shown below:

Unit pure Biodiesel Oil

Calorific value MJ / kg 37-38 42.0

Density kg / dm3 0,874 0,852

Source: US DOE-NREL 1998

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4.7 FLASH POINT It is the minimum temperature at which vapours of a compound light up in occurrence of flame at

atmospheric pressure condition. If the value is too low there may be volatile compounds such as methanol

in the biodiesel: technical European Standards fixed at 120 ° C the minimum value for biodiesel. The higher

the flash point, the safer the fuel storage.

Several values can be found in the following table:

Fuel Flash Point

Methyl ester of soybean oil (average value) 155 ° C

Methyl ester of sunflower oil 182 ° C

Methyl ester of cottonseed oil 110 ° C

Methyl ester of rapeseed oil 160 ° C

Ethyl ester of soybean oil 160 ° C

Oil 2D (average) 72 ° C

Source: US DOE-NREL 1998

4.8 OXIDATION STABILITY In the presence of oxygen the Biodiesel can oxidize and then deposit drains waxes and rubbers in the tank

or in the stationary elements of the engine. This parameter can be expressed in amount of rubber formed

in standard conditions.

The methyl ester is more stable than the corresponding ethyl ester. However, the simple adding (0.1% -

0.3%) of synthetic antioxidants can considerably increase stability (6.5 to 12 times).

An American study (Thompson, 1997) showed changes occurring in rapeseed oil methyl ester and ethyl

ester after two years of storage. Results are reported below:

Parameter methyl ester Ethyl ester

Density + 1.22% + 0.88%

Viscosity + 23.1% + 16.9%

Calorific value - 1.5% - 1.3%

Cetane number + 12% + 12%

Source: Thompson, 1997

The same study has shown that there are substantial performance differences into an engine fed with

biodiesel stored for two years or with the same fuel, but freshly produced.

4.9 VISCOSITY It is the resistance that the particles of a body encounter in flowing, one with respect to the others. The

unit of measurement is the centi-Stokes (mm2/s).

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The greater the saturation of the compound or the longer the Hydrocarbon chains or the less double

bonds, the higher the kinematic viscosity.

High viscosity generates problems to the injectors, leading to an increase in flow or pressure: maximum

value accepted for biodiesel is 5cSt.

As shown in the table, the transesterification is able to lower oil native viscosity, providing a product with

features similar to common diesel fuel. Throughout the reaction, triglyceride molecules get broken and

turn into three molecules of methyl esters: smaller and, consequently, less viscous.

Fuel Kinematic Viscosity

Methyl ester of soya oil (mean value) 4.01 cSt

Methyl ester of sunflower oil 4.6 cSt

Methyl ester of palm oil 5.7 cSt

Methyl ester of rapeseed oil 6 cSt

Ethyl ester of soybean oil 4.41 cSt

Oil 2D (average) 2.6 cSt

Source: US DOE-NREL 1998

5 ANALYSIS

Biodiesel quality and related characteristics can be assessed by several tools and lab experiments.

Among the most popular and effective ones we should mention the Gas Chromatography (GC), the Low

resolution Nuclear Magnetic Resonance (Lr-NMR) and the Autoxidation test. While the first two are

intended to be part of a preliminary examination of the produced fuel, autoxidation deals with the

monitoring phase in the moderately long term.

5.1 GAS CHROMATOGRAPHY

5.1.1 Goal

GC is a powerful tool to analyse fatty acids composition in the biodiesel. Most of the physical properties like

the ones listed in the previous chapter largely depends on FA composition of the chosen oil or on quality of

the biodiesel itself: GC is hence perfect for their identification.

In particular, GC is very useful when it comes to analyse viscosity properties: given the typical outcome of a

GC system (a chromatograph with its peculiar peaks), it is rather easy to recognize the mix of esters for the

selected biodiesel by simply considering that longer chains or less double bonds correspond to higher

viscosity. Thus, the later a peak shows up, the more viscous the ester.

5.1.2 Operating mode

Gas chromatography implicates a sample being vaporized and injected onto the head of the

chromatographic column, through which the sample is transported by the flow of an inert, gaseous mobile

phase. The column contains a liquid into an immiscible, immobile, stationary phase which is absorbed onto

the surface of an inert solid.

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All the samples are supposed to show different solubility in each phase and here is the fundamental

working principle of GC. A component which display a discrete solubility in the stationary phase will take

longer to travel through it. On the other hand, a substance which is not very soluble in the stationary phase

but very soluble in the mobile phase will travel along the column much faster.

Therefore, showing different motilities, each sample component will get separated as it goes through the

stationary phase.

The elution order depends both on the nature of the adsorbent solid in the column and on the boiling point

of the lipids in in mixture. A typical GC scheme is shown above:

Carrier gas: normally N2, must be inert.

Column: resolution of data acquisition is strictly correlated with the type of capillary columns used

in the device. The polarity of the stationary phase within the column itself influences the retention

times of FAMEs, especially when it comes to polyunsaturated types. Therefore we can state that

very polar phase based columns have the highest possible resolution while, on the other hand, non-

polar ones offer higher lifetime (but lower resolution).

Detector (Flame Ionization Detector): the effluent from the column is mixed with hydrogen and air,

and ignited. Organic compounds burning in the flame produce ions and electrons which can

conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a

collector electrode is located above the flame. The current resulting from the pyrolysis of any

organic compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this

gives the advantage that changes in mobile phase flow rate do not affect the detector's response. It

has high sensitivity, a large linear response range, and low noise. It is also robust and easy to use

but, unfortunately, it destroys the sample.

GC Oven: experiments can be performed either isothermally or under pre-set programs. Increasing

the oven temperature has two effects: shortened run time and resolution reduction.

5.2 NMR

5.2.1 Goal

Nuclear magnetic resonance is useful to acquire relaxation signals of oils and biodiesels which are then

analysed by exponential curve fitting. Data could then be used for generating viscosity prediction models.

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5.2.2 Operating mode

Following basic principles of nuclear magnetic resonance, this tool is capable of detecting changes in the

spin (angular momentum). This property is relevant only for atoms having odd number of protons/neutrons

such as H(1), C(13) and N(15).

After preliminary parallel or anti-parallel alignment with an external magnetic field “B0”, an external

radiation is applied exactly at Larmor Frequency (or spin characteristic frequency) so that an energy

transfer becomes possible between the base energy “α” and the higher energy level “β”. When the spin

returns to its base level (relaxation), energy is emitted at the same frequency. The signal which matches

this transfer is measured in many ways and processed in order to yield an NMR spectrum for the nucleus

under examination.

In particular, starting from the atoms excitation phase, the subsequent mechanism of relaxation is of

particular interest for data acquisition. It performs two main functions:

LONGITUDINAL MAGNETIZATION

Reconstructing the former Boltzman distribution of nuclear population (modified by the appliance of

a magnetic field) so that Nα > Nβ, where (α) and (β) refer to low and high energy level of the spin,

respectively. This phase relates with the resetting of original spin energy levels on the “z” axis and

is associated with the process of bringing enthalpy back to normal (a transfer of thermal energy to

the lattice occurs).

TRANSVERSE MAGNETIZATION

Destroying the phase coherence so that each single magnetic moment µ, when aligned with the “z”

axis, will once again have random orientation (the preliminary “B0” magnetic field is initially

applied in order to line up magnetic moments in parallel or perpendicularly to the direction field,

for low or high energy levels, respectively).

Dissipation of energy, in this case, is due to spin to spin interactions on the central axis: entropy

gets back to ordinary values.

Throughout the acquisition phase, the intensity at each time of the experiment is the sum of all the

resonating protons in the nucleuses.

An example of NMR data output is given below:

Two key parameters are now extrapolated: T1 and T2, describing the time for Longitudinal and Transverse

magnetization to return back to normality, respectively.

-1000

0

1000

2000

3000

4000

5000

6000

7000

0 2 4 6 8 10 12 14 16 18

Inte

nsi

ty

Time [s]

Castor

Linseed

Mustard

Olive

Rapeseed

Soybean

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Talking about T2, in particular, the higher the value and the better is the produced biodiesel in terms of

viscosity. We expect, in fact, that the process of transesterification has been effective and, thus, T2 value

for the obtained biodiesel is considerably higher than the initial oil which it has been derived from.

5.3 AUTOXIDATION Oxidation is a phenomenon that occurs between unsaturated fatty esters and atmospheric oxygen. It is

considered being a major issue especially when storing biodiesel over an extended period of time. Oxidized

fuel can block filters and injectors leading to fuel starvation, poor combustion and causing fuel injection

equipment failure due to production of an acidic, corrosive environment within the fuel delivery system.

Autoxidation is considered the primary mechanism of biodiesel degradation: monitoring the process is

therefore a crucial step for predicting performances and economic appeal of the fuel.

Biodiesel, despite being clean and reliable, displays a major drawback: when compared to commonly used

petro-diesel fuels, it has a poor oxidation stability, mainly originated by a comparatively larger number of

double bonds.

The main reason for this occurrence to take place must be identified in the allylic positions of the ester

chain, especially vulnerable to oxidation. Once the process has started (triggered by elevated temperature

and light exposure) a propagation sequence follows, ending up with the formation of hydro-peroxides.

These molecules, highly reactive, might undergo further decomposition into small chain acids but can also

form polymers with long chains and, therefore, higher viscosity.

In order to parametrically evaluate the level of decay of the fuel, it can be useful to compute the so called

Peroxide Value (PV), defined as the amount of peroxide oxygen per 1 kilogram of fat or oil: detection of

peroxide provides the initial evidence of rancidity.

6 FINAL CONSIDERATIONS

Biodiesel is a much cleaner fuel than conventional petroleum based diesel. Briefly:

Biodiesel burns up to 75% cleaner than petroleum diesel fuel.

Biodiesel reduces unburned hydrocarbons (93% less), carbon monoxide (50% less) and particulate

matter (30% less) in exhaust fumes, as well as cancer-causing PAH (80% less) and nitrated PAH

compounds (90% less). (US Environmental Protection Agency)

Sulphur dioxide emissions are eliminated (biodiesel contains no sulphur).

Biodiesel is plant-based and using it adds no extra CO2 greenhouse gas to the atmosphere.

The ozone-forming potential of biodiesel emissions is nearly 50% less than petro-diesel emissions.

Nitrogen oxide (NOx) emissions may increase or decrease with biodiesel but can be reduced to far

below petro-diesel fuel levels.

Biodiesel is environmentally friendly: it is renewable, and "more biodegradable than sugar and less

toxic than table salt" (US National Biodiesel Board).

Biodiesel is a much better lubricant than petro-diesel and extends engine life; even 1% biodiesel

added to petro-diesel will increase lubricity by 65%.

Biodiesel can be mixed with petro-diesel in any proportion, with no need for a mixing additive.

Biodiesel has a higher cetane number than petroleum diesel because of its oxygen content.

With slight variations depending on the vehicle, performance and fuel economy with biodiesel is

the same as with petro-diesel.

Biodiesel can be used in any diesel engine without modification.

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Western World desire to be less dependent on imported oil has rekindled interest in alternative fuels. This

need to be self-sufficient has led therefore to the enactment of laws by the governments of individual

States Members that are encouraging and pushing the biodiesel market.

"The biodiesel market has benefited from the support of the European Commission through the Kyoto

Protocol and the Directives 2003/30 / EC and 2003/96 / EC, which specifically seek to promote biofuels and

establish indicative targets for their use in the transport sector" says Frost & Sullivan analyst Robert Outram

(2007). It is expected that these regulations will lead to increased use of biofuels which would make them

competitive in terms of cost with respect to the other mineral fuels.

Encouraged by the EU legislation, individual member states have also approved a number of incentives

such as tax relief, RTFO (Renewable Transport Fuel Obligation, which entails suppliers a certain percentage

of sales from biofuels) and blending mandates: this legislation forces oil companies to blend a percentage

of biofuel in all its fuels, which for them represents a challenging task in terms of logistics.

"Oil companies will require large volumes of biofuel to meet the mandate levels, even if the rates are

usually low - explains the analyst of Frost & Sullivan -. This means that the oil companies are likely to join

the biofuel producers with long-term agreements or even invest in their plants. "

These blending mandates should, however, contribute to a rise in goods prices.

Since the production of vegetable oil in Western Europe has reached full capacity and has remained

constant over the past decade, the biodiesel market, highly competitive, is under pressure in the constant

quest for raw materials at competitive prices. Even with the support of 1 million additional tonnes from the

new EU member states, to meet the European Union Directives, whose purpose is to make sure that

biodiesel represents 5.75% of all transport fuels, will require approximately 9.5 million tonnes of biodiesel.

"Assuming a 1: 1 conversion of vegetable oil into biodiesel, in terms of volume, it would require 80% of all

the vegetable oil currently produced in Europe for the biodiesel market," emphasizes Outram. The huge

demand for ultimate products inevitably affects the prices of biofuels to such an extent that the marginal

profit of manufacturers will begin to decrease.

As the costs of raw materials account for 70% of all costs of operating a plant, the biodiesel producers, in

order to survive, will have to heavily rely on effective strategies for recovery of raw materials as well as

reducing logistics costs.

7 REFERENCES

1. S. Berman, Z. Wiesman, Biomass and Biofuels (2015)

2. R. Crozzi, T. Protti, P. Ruaro, “Elementi di analisi chimica strumentale” (2013)

3. Regulations UNI EN 14214 (2008)

4. R. Outram, “European Biodiesel and Feedstock Markets” (2007)

5. DOE - NREL “Life cycle inventory of biodiesel and petroleum diesel for use in an urban bus” (1998)

6. Peterson C.L., Reece D.L., Hammond B.L., Thompson J., Beck S.M., “Processing, characterisation and

performance of eight fuels from lipids” (1997)

7. Prankl H., Worgetter M. “Influence of the iodine number of biodiesel to the engine performance”

(1996)

8. Rickeard, D.J., N.D. Thompson, "A Review of the Potential for Bio-Fuels as Transportation Fuels",

SAE, paper number 932778 (1993)