Bio-Oil Production by Thermal Cracking in the Presence of Hydrogen
Abstract
This paper describes the bio-oil production process of a mixture of agricultural wastes: discarded soybean
frying oil, coffee and sawdust, by pyrolysis and thermal cracking in the presence of hydrogen. The fractions obtained in the
pyrolysis and/or cracking processes were divided into a light fraction and a heavy one. All the fractions were analyzed by
comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry detection (GC×GC/TOFMS).
The characteristics of the fractions obtained in the cracking process in the presence of hydrogen were similar to those of
petroleum-based naphtha, while the fractions obtained by pyrolysis contained significant quantities of compounds such as
furanmethanol, hexanol, and benzofuran, whose commercial value is high.
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Introduction
The recent environmental restrictions on the use of fossil fuels have intensified research into new alternative energy sources.
Many alternative technologies to produce cleaner fuels have been developed, including the use of biomass, which offers a
promising potential [1-4].
Biomass is a renewable source which has received attention due to various characteristics, particularly its low cost and wide
availability. Biomass can be converted into bio-fuel by means of different processes, e.g., reductive combustion, liquefaction,
pyrolysis and gasification [5]. The use of biomass is particularly interesting when it involves waste products such as waste
vegetable oil, fruit seeds, sugarcane bagasse, sugarcane straw, rice husks, coconut fibers, and coffee grounds, which are also
potential sources of energy [6-8].
Bio-oil from biomass pyrolysis, also known as pyrolysis oil, is a dark brown almost black liquid with a characteristic smoky
odor, whose elemental composition is analogous to that of the biomass from which it derives. It is a complex mixture of
oxygenated compounds with a significant amount of water originating from the moisture of the biomass and from cracking
reactions. Bio-oil may also contain small coal particles and dissolved alkali metals coming from the ash. Its composition
depends on the raw material and on the operating conditions used in its production. Pyrolysis oil is an aqueous
microemulsion resulting from the products of fragmentation of cellulose, hemicellulose and lignin [9-10].
Much attention has focused on pyrolysis, a biomass thermal decomposition process, for which the literature describes
numerous different reactors and conditions [11-13]. The presence of oxygen exerts a highly negative impact on the potential
uses for bio-oil. For example, oxygen lowers the heating value, gives rise to immiscibility with petroleum fuels, and leads to
corrosiveness and instability during long-term storage and transportation [14]. The biomass pyrolysis process is an
economically feasible option for producing chemicals and/or fuels [15,16]. The bio-oil resulting from the pyrolysis process
consists of a mixture of more than 300 organic compounds [17], but its processing, separation and characterization pose
technological challenges. In the thermal cracking process, the volatile compounds generated during pyrolysis also present a
promising potential for energy generation [18]. Moreover, the upgrading process, which involves the reduction of oxygenates
and is necessary to improve the quality of bio-oil, normally requires processes such as catalytic cracking, hydrogenation and
steam reforming [9-22].
Hydropyrolysis is an important technique for improving the quality of bio-oil produced from biomass pyrolysis. Hydrogen is
a reducing gas and cracking biomass in the presence of hydrogen can reduce the oxygen content in bio-oil [23]. This paper
discusses the characterization of bio-oil generated from the pyrolysis of a mixture of wastes: discarded soybean frying oil,
coffee grounds and sawdust.
Conclusion
In the fractions obtained by pyrolysis, 84 compounds were tentatively identified in the heavy fraction and 312 in the light
fraction. The vapors were subjected to thermal cracking in the presence of 5% hydrogen as a way to upgrade the bio-oil, and
214 compounds were identified in the heavy fraction and 324 in the light fraction. The thermal cracking process produced
mainly aliphatic, aromatic and cyclic hydrocarbons, yielding approximately 80% in weight of these compounds with
characteristics similar to those of naphtha derived from the atmospheric distillation of petroleum with potential applications
as fuels. The fractions obtained solely by pyrolysis consisted predominantly of hydrocarbons with nitrogen (46% in area) and
oxygen (47% in area) compounds. The oxygenated compounds included furanmethanol and hexanol alcohols, and
benzofuran and dioxyethane ethers. All the analyzed fractions contained phenolic compounds. When isolated, these
compounds are an excellent potential source of raw material for the preparation of pharmaceutical and industrial products of
high commercial and industrial value.