Waste Vegetable Oil As A Diesel Replacement Fuel
Phillip Calais* and AR
(Tony)
* Environmental Science,
** Western Australian Renewable Fuels Association Inc, tony.clark_NO-SPAM@arach.net.au
Abstract
In the past, waste edible
oils and fats were often used in the production of animal feeds. However due to
links between BSE and this practice, the use of waste fats for animal feed is
not as common as it once was and this has resulted in surplus quantities
becoming available. This has led to significant disposal problems.
Waste oils and fats can be
used as renewable fuel resources. Conversion of waste oils and fats to
biodiesel fuel is one possibility but poses some difficulties such as in the
use of toxic or caustic materials and by-product disposal. Conversion to
biodiesel may also decrease the economic attractiveness of using waste oils as
fuels.
An alternative to the use of
biodiesel is the use of vegetable oils or rendered animal fats as a fuel.
Using relatively unmodified
oils or fats eliminates the problems associated with toxic and caustic
precursor chemicals and residual biodiesel alkalinity as the oil is used
without altering its chemical properties.
This paper discusses the use
of waste vegetable and animal oils and fats as unmodified fuels in compression
ignition engines.
Introduction
Waste edible oils and fats
pose significant disposal problems in many parts of the world. In the past much
of these waste products have been used in the production of animal feeds.
However due to possible links between BSE and this practice, the use of waste
edible animal fats for animal feed is not as common as it once was, resulting
in disposal problems. As it is often difficult to prevent the contamination of
waste vegetable oil with animal products during cooking, waste vegetable oil
often must be treated in a similar manner as is waste animal fats.
One possibility for the
disposal of these products is as a fuel for transport or other uses. Conversion
of waste oils and fats to biodiesel fuel has many environmental advantages over
petroleum based diesel fuel. However it is not commercially available in
Australia and the ‘back-yard’ production of biodiesel may present serious risks
as the process uses methanol, a toxic and flammable liquid, and sodium or
potassium hydroxide, both of which are caustic. By-product disposal may present
further difficulties and environmental considerations may preclude production
in sensitive areas.
An alternative to the use of
biodiesel is the use of vegetable oil or rendered animal fats as fuel.
Using unmodified oils not
only eliminates problems such as residual biodiesel alkalinity by-product
disposal, but also increases the economic viability of using the oil or fat.
While the use of vegetable
or animal oils and fats as fuels may be somewhat surprising at first, when
examined in an historical context we can see that the compression ignition
engine, first developed to a usable level of functionality by the French-born
Rudolf Diesel near the end of the 19th century, was originally
designed to operate on vegetable oil.
In 1900, Rudolf Diesel
demonstrated his new compression ignition engine at the World Exhibition in
Paris running on peanut oil. In 1911 he wrote “The engine can be fed with
vegetable oils and would help considerably in the development of agriculture in
the countries that use it.” [1]
It was about this time that
new drilling technology and exploration techniques were developed and together
these ushered in the age of cheap and plentiful fossil fuels. Consequently, the
use of vegetable and animal oils and fats as fuels has only been used for a few
special purposes such as in racing fuels or in environmentally sensitive areas
where petroleum spills tend to cause more serious problems than do spills of
animal and/or vegetable derived fuels.
After some one hundred years of using liquid petroleum fuels, we are now finding that there are unforeseen side effects, the foremost perhaps being the so-called Enhanced Greenhouse Effect.
In Australia, transport use
contributes some 16% of Australia’s greenhouse gas emissions. Of this, diesel
fuel contributed about 17% or 11,705,000 tonnes of CO2 equivalent.
An additional 1,622,000 tonnes is released from diesel fuel used for electricity
generation. [2] On top of greenhouse gas emissions is the vexing question of
how little – or much – is left.
However oils of vegetable
and animal origin, unlike fossil fuels, have to potential to be produced not
only on a sustainable basis but also could be greenhouse gas neutral, or at the
very least, emit substantially less greenhouse gases per unit energy than do
any of the fossil fuels.
Properties of Triglycerides as Fuels
A large amount
of research has gone into examining Diesel’s dream of using raw vegetable oils
as fuels and when one speaks of growing crops for liquid fuels it is often
assumed that the oil will be used after only basic extraction and filtering.
[3,4,5]
Work has been conducted to
examine these oils as fuel replacements or additives. For example in the late
1970’s and early 1980’s, research was undertaken at Murdoch University (Perth,
Australia) into the use of eucalyptus and other plant oils as a fuel component.
[6] In New Zealand, there are considerable problems with the disposal of
surplus tallow from the processed meat industry and a large amount of work was
conducted in the early 1980’s on the use of tallow as a fuel. [7]
Experience has shown that the use of unsaturated triglyceride oils as a fuel may cause significant problems that can affect the viability of their fuel use. [8] But this is not always the case and in many circumstances these problems can either be dealt with or are acceptable to the user.
While power output and
tailpipe emissions using plant or animal oils are in most cases comparable with
those when running on petroleum diesel fuel, the main problem encountered has
due to the higher viscosity of the triglyceride oils and their chemical
instability. These can cause difficult starting in cold conditions, the gumming
up of injectors and the coking-up of valves and exhaust. [3]
The viscosity of plant and
animal fats and oils varies from hard crystalline solids to light oils at room
temperature. In most cases, these ‘oils’ or ‘fats’ are actually a complex
mixture of various fatty acids triglycerides, often with the various components
having widely varying melting points. This may give the oil or fat a
temperature range over which solidification occurs, with the oil gradually
thickening from a thin liquid, through to a thick liquid, then a semi-solid and
finally to a solid.
High melting points or
solidification ranges can cause problems in fuel systems such as partial or
complete blockage as the triglyceride thickens and finally solidifies when the
ambient temperature falls. [3] While this also occurs with petroleum based
diesel, particularly as the temperature falls below about ~ -10 to -5° C for ‘summer’ formulations and ~ -20 to -10° C for ‘winter’ diesels, it is relatively easy
to control during the refining process and is generally not a major problem.
Many vegetable oils and some
animal oils are ‘drying’ or ‘semi-drying’ and it is this which makes many oils
such as linseed, tung and some fish oils suitable as the base of paints and
other coatings. But it is also this property that further restricts their use
as fuels.
Drying results from the
double bonds (and sometimes triple bonds) in the unsaturated oil molecules
being broken by atmospheric oxygen and being converted to peroxides.
Cross-linking at this site can then occur and the oil irreversibly polymerises
into a plastic-like solid. [9]
In the high temperatures commonly found in internal combustion engines, the process is accelerated and the engine can quickly become gummed-up with the polymerised oil. With some oils, engine failure can occur in as little as 20 hours. [10]
The traditional measure of the degree of bonds available for this process is given by the ‘Iodine Value’ (IV) and can be determined by adding iodine to the fat or oil. The amount of iodine in grams absorbed per 100 ml of oil is then the IV. The higher the IV, the more unsaturated (the greater the number of double bonds) the oil and the higher is the potential for the oil to polymerise.
While some oils have a low
IV and are suitable without any further processing other than extraction and
filtering, the majority of vegetable and animal oils have an IV which may cause
problems if used as a neat fuel. Generally speaking, an IV of less than about
25 is required if the neat oil is to be used for long term applications in
unmodified diesel engines and this limits the types of oil that can be used as
fuel. Table 1 lists various oils and some of their properties.
The IV can be easily reduced by hydrogenation of the oil (reacting the oil with hydrogen), the hydrogen breaking the double bond and converting the fat or oil into a more saturated oil which reduces the tendency of the oil to polymerise. However this process also increases the melting point of the oil and turns the oil into margarine.
As can be seen from Table 1, only coconut oil has an IV low enough to be used without any potential problems in an unmodified diesel engine. However, with a melting point of 25°C, the use of coconut oil in cooler areas would obviously lead to problems. With IVs of 25 – 50, the effects on engine life are also generally unaffected if a slightly more active maintenance schedule is maintained such as more frequent lubricating oil changes and exhaust system decoking. Triglycerides in the range of IV 50 – 100 may result in decreased engine life, and in particular to decreased fuel pump and injector life. However these must be balanced against greatly decreased fuel costs (if using cheap, surplus oil) and it may be found that even with increased maintenance costs that this is economically viable.
Table 1 Oils and their melting point and Iodine Values [11]
Oil Approx.
melting Iodine
point
°C Value
Coconut oil 25 10
Palm kernel oil 24 37
Mutton tallow 42 40
Beef tallow 50
Palm oil 35 54
Olive oil -6 81
Castor oil -18 85
Peanut oil 3 93
Rapeseed oil -10 98
Cotton seed oil -1 105
Sunflower oil -17 125
Soybean oil -16 130
Tung oil -2.5 168
Linseed oil -24 178
Sardine oil 185
All of these problems can be
at least partially alleviated by dissolving the oil or hydrogenated oil in
petroleum diesel. ‘Drying oils’ such as linseed oil for example, could be mixed
with petroleum diesel at a ratio of up to about 1:8 to give an equivalent IV in
the mid-twenties. Likewise coconut oil can be thinned with diesel or kerosene
to render it less viscous in cooler climates. Obviously the solubility of the
oil in petroleum also needs to be taken into account. [7]
Another method is to
emulsify the oil or fat with ethanol. Goering [12] found that eight parts of
soybean oil, when emulsified with two part ethanol and five parts of 1-butanol
as stabiliser, performed as well as diesel fuel and was able to start a cold
engine. The cost was calculated (in 1981) to be $0.40 a litre as compared to
$0.30 – 0.35 per litre for diesel.
Trans-esterifying
triglyceride oils and fats with monohydric alcohols to form biodiesel largely
eliminates the tendency of the oils and fats to undergo polymerisation and
auto-oxidation and also reduces the viscosity of the oil to about the same as
petroleum diesel.
However as previously
mentioned, the ‘back-yard’ production of use of does pose some risks,
particularly to those who are not familiar with the handling of toxic and
highly flammable liquids.
In many cases, it is
possible to use a variety of triglyceride fats and oils as a fuel. While engine
wear and maintenance may be increased, in some circumstances these problems are
not serious enough to prevent the use of the triglycerides as a fuel.
Conversion of a vehicle to operate on Waste Cooking Oil
An alternative
to the use of biodiesel is the use of vegetable oil or animal fats as a fuel.
The differences amongst fats and oils, whether of animal or vegetable origin,
relate mainly to the level of saturation in the carbon chain. Generally
speaking, the lower the number of double bonds, the higher the melting point of
the triglyceride and the greater the stability of the triglyceride to
polymerisation and spontaneous oxidation. From an engine use point of view, it
is preferable to use saturated fats as fuels as they are more stable and less
resistant to oxidation, particularly under the elevated temperatures and
pressures as found in an engine environment. However due to their higher
melting points, difficulty may be encountered in starting the engine without
pre-heating of the fat.
In order to test
the viability of using relatively unsaturated oils in engines, a 1990 Mazda with a 2.0 litre
indirect injection OHC diesel was obtained with a view of running it on various
types of triglyceride oils and fats. At that time of purchase, the vehicle had
covered 222,000 km.
The previous owner stated
that the engine head had been overhauled, but no further details were provided.
Since the purchase, and prior to the conversion to operate on triglyceride
oils, the injector pump was overhauled. Fuel consumption of the vehicle on
diesel was stable at about 6.9 L/100 km.
The vehicle is used as a
family vehicle in a 2-car household. At the time of conversion (October 2000), the
vehicle had covered 231,000 km.
Waste palm oil (a solid fat)
was used initially but the time delay in melting this oil prevented use of the
oil on short journeys. Consequently, waste canola oil was tried and has been
used exclusively for the last 7,500 km.
At about 80 cSt (at 20° C), the viscosity of used canola oil is
significantly greater than that of diesel which has a
viscosity in the range of 2 to 4.5 cSt. [7, 13] However, as canola oil is
warmed, its viscosity falls quite significantly and at about 70° C the viscosity is about 5 to 10 cSt. Thus
the viscosity is sufficiently low to allow its use as a replacement fuel for
diesel with out too much difficulty.
Table
2 Comparison of properties of diesel, canola oil and commercial US biodiesel.
[7, 13, 14]
Diesel Canola Oil Biodiesel
Density
kgL-1 @ 15.5° C 0.84 0.92 0.88
Calorific
value MJL-1 38.3 36.9 33 – 40
Viscosity
mm2s-1 @ 20°C 4 - 5 70 4
– 6
Viscosity
mm2s-1 @ 40°C 4 - 5 37 4
– 6
Viscosity
mm2s-1 @ 70°C 10
Cetane
number 45 ~ 40 - 50 45 – 65
The vehicle was
fitted with an additional 17 litre fuel tank under the bonnet together with the
necessary fuel lines, additional filter and a solenoid valve to control the
fuel source. Electrical connections to a thermostat, glow plug, run-on timer,
switches and the solenoid valve were also installed.
The
oil tank was
fitted with a heat exchanger comprising one metre of 12 mm copper tube. This
was connected to the engine coolant system and pre-heats the oil in the tank.
The tank was located in the engine bay to maximise heat transfer to the tank
and to keep the coolant lines short. The finished tank had a useable capacity
of seventeen litres. This gave a range of up to 240 km between refuelling.
Additional filtering was
installed with an internal preheater. The pre-heater, a 24 V diesel glow plug,
together with a relay and thermostat was installed so that if a solid fat was
used for fuel, any solidified fat in the filter chamber could be quickly
melted. The filter used (Ryco Z30) provides filtration to 30 micron.
A vacuum gauge was
fitted after the fuel filter and it was found that at start-up with cold canola
oil, fuel flow was insufficient causing a vacuum in the fuel line and filter.
An extra in-line fuel pump was added before the filter and this alleviated this
problem and has increased fuel filter life.
The heated oil fuel
line was one metre of 5/16” semi rigid nylon tube encased in a 5/8” rubber
coolant pipe. Brass fittings were used to ensure minimal corrosion and leakages
of coolant.
The three port, 12 Vdc
solenoid valve was mounted in close proximity to the fuel pump to minimise
changeover lag. The fuel return line to the diesel tank was redirected to the
fuel pump suction side between the solenoid valve and fuel pump. This was done
to prevent the oil in the return line going to the
diesel tank. A disadvantage of this is that the fuel system became rather
intolerant of air in the system.
A run-on timer was
installed using a modified ‘turbo timer’. After the solenoid valve is switched
back to diesel, the timer keeps the engine running for a period of time, even
if the vehicle is parked and the key removed from the ignition. During this
period, the oil in the injector pump is gradually replaced by diesel and after
several minutes, only diesel remains in the fuel pump, filter, fuel lines and
injectors. The correct time was found by a trial and error. A manual override
switch was also installed to allow emergency, or short duration stopping of the
engine.
The supplier of the used oil
(a fast food outlet) filters the oil and puts it into containers for collection.
To ensure that the oil is clean, the oil is heated and additionally filtered
through a 5 micron bag filter. It was found that the used oil usually becomes
cloudy and this was found to be a combination of the oil starting to solidify
due to partial de-unsaturation of the oil from use and minor water content. If
water content is suspected of being excessive, the oil is heated above 100’C to
evaporate the moisture.
Use of the oil and preliminary results
To date, the vehicle has
been driven over 7500 km using canola oil. In the morning, the oil in the tank
is cold and quite viscous and a particular start-up routine must be used. In
addition, if, at the end of the trip, the vehicle will not be restarted again
for several hours, then a shutdown sequence must be followed in order to allow
easy restarting.
Glow plugs are used for all
starts. When starting cold, the engine is started on diesel and the journey
commenced. When the engine temperature has reached ‘normal’ as shown on the
engine temperature gauge, the fuel solenoid is operated, and the journey
continues using vegetable oil. For hot or warm starts, the engine is started
using the vegetable oil.
Shutdown: In the cooler months, at about 5 km from
the end of each journey, the fuel solenoid is released. At the end of the
journey, the ignition switch is turned off. If the time delay has not expired,
the engine continues to idle, until the end of the delay. During warmer
periods, the shutdown delay override switch is used to stop the engine for all stops,
except for the last journey of the day.
Comparison of performance and economy
Using records
of fuel consumption and distance travelled, there has been no noticeable
difference in fuel consumption or engine power when operating on diesel, palm
or canola oil. The fuel consumption has been found to be approximately 6.9
L/100 km, regardless of what fuel is being used. It is planned that full
testing of performance will be carried out in the near future, taking into
account different driving conditions and different fuels.
Problems
Cold starting
with canola oil: If the engine has been allowed to cool
completely (eg overnight) and the shutdown routine not followed, then the
engine may be very difficult to restart. It has been found that heating the injectors
by, for example, pouring hot water over them or using a hair dryer, will allow
the engine to restart with no difficulty.
Saturated oils: The use of saturated (solid) fats in cooler
months would require significant improvements to the heat exchanger in the oil
tank. Palm oil tended to solidify in the filter (prior to a glow plug heater
being installed) and in the front half of the tank while travelling, reducing
useable capacity. This may be less of a problem if a 6 port solenoid valve were
to be used, circulating heated oil back to the tank, and improving the heating
of the oil in the tank.
Carbon build up causing
wear: Reports have been made of accelerated engine wear due to increased carbon
deposits in combustion chambers. [3, 4, 7] As the engine condition at the start
of the test was not accurately measured, no proper evaluation of the wear can
be made. However, at the end of the vehicle life, the engine will be dismantled
and evaluated for any abnormal evidence of wear or damage.
Galvanised Fuel tank: The
oil tank was made from scrap galvanised steel sheet metal. It was found that
the oil reacted with the zinc plating and this resulted in globules of reacted
oil blocking the fuel filter on many occasions. The oil tank was consequently
removed, cleaned and acid etched to remove the zinc coating. No reacted fuel
globules have been observed since.
Costs:
The tank was made
using scrap materials and consequently was of negligible cost. It was estimated
that if professionally made using new materials, it would have cost
approximately $400. Other expenses were: filter - $30, filter elements - $5,
heated fuel line - $120, solenoid valve - $95, sundry items - $100. The total
cost was $250. Operating costs are $5 per 10,000 km for new filter elements
Exhaust emissions:
At this stage
no exhaust emission tests have been done. The level of particulates as gauged
by simple visibility checks appears about the same whether running on diesel or
canola oil. However, as the oil contains no sulphur, SO2 emissions
are not present when running on triglyceride oils. Furthermore, the pungent
smell typical of diesel exhaust is not present. Rather the smell is similar to
that of a BBQ.
Life-cycle CO2
emissions are substantially reduced. Studies done by Sheehan et al, Beer et al
etc indicate that reductions by as much as 80 to 90% compared to fossil diesel
fuel can be expected, given the renewable nature of the oil, and that this is a
re-use of a spent product. [14, 15, 16, 17]
Nitrogen
oxides (NOx) emissions would most likely be similar or slightly
elevated by ~10% as compared to fossil diesel. In addition to atmospheric
nitrogen, most vegetable and animal oils contain small quantities of nitrogen
containing proteins, which upon combustion, release various nitrogen oxides.
[14, 15, 16, 17]
Unburnt
hydrocarbon emissions may or may not be increased. Previous research has shown
that this is very dependent of the vehicle’s state of tune, age and the
specific properties of the oil. [7, 14, 16, 17]
Cost efficiency:
Using free
waste canola oil, fuel costs have been only for the diesel fuel used in the
start-up and shutdown periods. Fuel purchase records for 10,000 km show the
vehicle has used 240 litres of diesel. Driving this distance on diesel only
would normally require approximately 690 litres of fuel. This represents a fuel
cost saving of 65%. The conversion has paid for itself with savings in diesel
purchases in excess of $400.
Different usage
patterns would give obviously give different results. Usage patterns for this
vehicle show mainly short trips, with one or two longer (>20 km) journeys
per week.
As vehicle use
increases, the diesel fuel savings would also be expected to increase. The only
requirement for diesel, is that the motor must be allowed to reach operating
temperature before operating on triglyceride oils, and that the oil must be
diluted or purged from the fuel pump before shutdown.
Due to higher
fuel usage larger vehicles would be able to have greater fuel cost savings
which would more than offset the increased costs of a remotely located oil
tank.
Obviously, if
there were to be a greater demand for used and waste cooking oil, the oil may
not be available free and the cost of purchasing waste oil must then be taken
into account. While this would extend the pay-back period, as long as there was
a reasonable difference between the cost of the waste oil and diesel fuel and
any extra maintenance costs were not too excessive, it would probably still be
economically viable to undertake the modifications and operate on used oil.
Possible improvements
Solenoid Valve: The use of a six port solenoid valve and an
alterative fuel line set-up would reduce the shutdown delay requirement, as the
return line would not be fed back into the fuel pump.
Fuel filter: The provision of a heated fuel filter, using filter
elements giving 5 micron filtration would protect the fuel pump from the
possible 5 – 30 micron particles not removed by the Ryco Z30 filter. Filter
heating would be most effective if heated by engine coolant.
Biodiesel: Starting the vehicle on biodiesel would further
enhance the environmental benefits obtainable.
Conclusion
Many vegetable
and animal oils can be used as diesel replacement fuels. The two ways of doing
this are to either use the oil as a straight fuel or to convert the oil to a
methyl or ethyl ester (biodiesel). Both of these ways have various advantages
and disadvantages. One of the authors (Clark) converted a Mazda 626 to operate
on straight vegetable oil and has done over 7500 km using this method. The
other author (Calais) has been using biodiesel in an unmodified Toyota Corolla
for over 20,000 km.
In converting
the Mazda 626 to operate on straight oil, a small tank was fitted under the
bonnet of the vehicle. In order to minimise fuel ‘cold-plugging’ problems due
to high fuel viscosity, both the tank, filter and fuel lines are heated. The
vehicle is first started on diesel and then when the engine has reached normal
operating temperature and the oil has been heated, a solenoid valve is operated
which switched the fuel system over to the oil.
To date there
has been no evidence of increased engine wear, lubricating oil dilution or
other problems. However the experience of others has shown that increased
engine wear may occur but as yet it is still too early to determine whether or
not this will occur in this example. Even so, the economic benefits obtained by
using waste canola oil may more than offset any extra engine maintenance costs.
It is hoped
that continuing research on this project may provide more information about
this in the future.
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