Bio-oils in Contact with Steels and Copper Surfaces

Bio-oil is a renewable energy source and can be used either in crude or processed forms. The crude bio-oil can be used in marine engines or conventional
combustors. Using bio-oils instead of petroleum fossil fuels in marine engines
reduces sulfur and phosphorus emissions. Bio-oils originate from animal fats,
agricultural crops, and pyrolysis of biomasses. Used cooking oils (UCOs) and fish
oils (FOs) are of increasing interest as economic feedstock for bio-oils or
biodiesel production. UCOs are non-edible oil residues collected from, e.g.,
restaurants. During the cooking or frying step, the temperature of the oils can
reach 190 °C, which causes the triglycerides in the oil to degrade thermally and
chemically. These reactions may form free fatty acids (FFAs), glycerol,
monoglycerides, and diglycerides. Some characteristics such as pre-existing
organic acids, water content, and any possible sediment in the bio-oil can lead to
corrosion of steels and copper in contact with the oil. However, the UCOs must be
used within a relatively short period of time after their collection and processing
to avoid, e.g., the formation of corrosive degradation components. Certain levels
of the acid number, viscosity, density, and water content are essential for
approving the bio-oils as fuels. However, the acid number and water do not
directly correlate with the bio-oil properties and corrosivity. The roles of
different bio-oil components and corrosion inhibitors on the corrosive
properties are not thoroughly understood.
In this work, we studied the physicochemical and thermal properties of
locally produced UCOs and FOs to determine their applicability as alternative
fuels for marine engines. The properties of these locally produced bio-oils were
compared to a commercial oil. The corrosive properties were studied by
immersing steels or copper rods at room temperature in the UCOs and FOs. Also,
the impact of water in oils on the dissolved iron concentration was investigated.
Oleic acid and glycerol were studied as corrosion inhibitors in UCOs. Further,
changes in the physicochemical and thermal properties were investigated as
functions of storage time for up to five years.
The corrosivity of different UCO batches was addressed with three-day
immersion tests of steel rods. Furthermore, the roles of contaminants, bio-oil
preservatives, and corrosion inhibitors in bio-oil-induced corrosion were
examined with oil samples containing added water, short-chain carboxylic acids,
and ten different amino acids.
The physicochemical and thermal properties of the bio-oils correlated with
their contents of different fats. The acid number of all bio-oils was relatively high
and slightly increased with their aging, likely due to the conversion of
unsaturated fatty acids. The concentrations of phosphorus and sulfur in the bio-
oils were below the limits specified for oils in marine engines. The bio-oils
decomposed at higher temperatures than the commercial reference oil but had
lower heat content than the commercial oil.
Detailed analysis of various physicochemical properties and the fatty acid
composition of the bio-oils suggested that the waste stream-based bio-oils are
potential sources of carbon dioxide neutral fuels in marine engines.
However, immersion tests with mild steel rods suggested an increased
dissolved iron concentration in the oil at 10 days. Adding oleic acid and glycerol
decreased the dissolved iron concentration in the oil. Water content, acid
number, and the overall oil composition substantially affected the corrosive
behavior of the oils. Among the tested oils, the FO and the reference commercial
oil product showed the highest and lowest amounts of dissolved iron,
respectively.
The results observed in this work imply that the immersion test of a steel rod
can be used as a reliable and cost-effective method to compare the corrosive
properties of bio-oils as well as other biofuels.
In general, the oils with the highest water concentrations showed the highest
corrosion properties, although their acid numbers were not the highest. The oils
with the highest acid numbers contained the highest concentrations of
unsaturated free fatty acids, such as oleic acid. The unsaturated free fatty acids
were assumed to form a protective layer on the rods, thereby preventing the
permeation of oxygen and water to the steel surface. When a very corrosive oil
was mixed with a less corrosive oil, the amount of dissolved iron decreased
notably. This suggests that mixing different bio-oils decreases the corrosion of
steel devices in contact with the oil. Among the ten studied potential corrosion
inhibitors, the amino acids L-lycine and L-arginine showed positive effects, also
when added at low concentrations. Short-chained carboxylic acids formic or
propionic acid also suggested minor corrosion inhibiting effect, most likely due
to the thin surface layer that formed on the steel. However, the simultaneous
presence of water and carboxylic acid led to corrosion. Furthermore, neither L-
lycine nor L-arginine could provide corrosion protection in the presence of both
water and a carboxylic acid. This suggests that UCOs containing short-chained
carboxylic acids and water increase the corrosion of steel.
The results of this work indicate that the bio-oils may be used as a
sustainable, locally sourced alternative fuel as long as they do not contain
carboxylic acids and water simultaneously. The impact of water content on the
corrosion might be decreased with amino acid-based inhibitors in the absence of
carboxylic acid.