Full Length ArticleUpgrading of bio-oil from thermochemical conversion of various biomass – Mechanism, challenges and opportunities
Graphical abstract
Introduction
Over the years, demand and supply of energy in almost every sector are imbalanced as a result of economic development and population growth. Considering the world’s total final energy consumption by sector, significant increase in the energy consumption by transport sector from 23% in 1971 to 29% in 2017 is noticeable [49]. This high demand of energy (especially oil as dominate fuel) from the transport division are critical to manage in the subsequent years. Also, as per BP statistical review of world energy [100], the energy consumption and carbon emissions were found to increase at a rate of 2.9% and 2% respectively. Such urging energy demand due to inadequate supply from non-renewable fossil fuel sources can be fulfilled by renewable and sustainable biomass sources [4], [98]. In addition, biofuels from renewable sources decreases the level of pollutants in the atmosphere and helps in development of rural areas socio-economically [93]. In the pursuit of parent material for biofuels, algal biomass owing to its inherent properties such as higher biomass productivity, higher oil content, eco-friendly and carbon neutral nature [11], [78], [118]. In general, algae based biofuels consisting of solid, liquid and gaseous fuels are produced through i) lipid extraction followed by transesterification into biodiesel, ii) hydrothermal liquefaction (HTL) for bio-oil production, iii) pyrolysis for biochar, bio-oil and syngas [127] and anaerobic digestion, fermentation for biogas and biohydrogen production.
Among various fuel products from thermochemical processes, bio-oil is obtained through HTL and pyrolysis, which comprises of many non-identical molecules acquired from depolymerization and fragmentation of biological macromolecules [75]. Bio-oil is a highly preferred liquid fuel than solid (biochar) and gaseous (syngas) fuels because of its advantages of having higher energy density and its ease in transport and storage [53]. Examining the physical characteristics, bio-oil is a highly viscous black organic liquid with the pH ranging from 3.5 to 4.2, having typical smoky odour and near equivalent energy value of 70–95% to that of petro-crude [21], [92], [135]. However, the chemical constituents and physical properties of bio-oil slightly vary in quality and quantity depending on the operational conditions, types of biomass and conversion processes. To increase the yield of bio-oil through pyrolysis process, setting up certain operating conditions such as i) less than 3 mm particle size, ii) 0.5–2 s residence time, iii) 400–500 °C temperature would be favourable [82]. Bio-oil is claimed to be carbon neutral or greenhouse gas neutral with traceable or no zero SOX emissions because of the unsubstantial amount of sulfur content in the biomass [92]. Despite the fact of being carbon neutral, bio-oil has also certain limiting elements that are naturally unavoidable during conversion processes like acidic nature, oxidative instability, heteroatom existence, presence of water and oxygen content etc, which makes it unsuitable to be used as fuel [9], [85].
In view of the existing difficulties in utilizing bio-oil due to its invariable composition, upgradation is essential for its use in transportation. The above specified undesirable properties of bio-oil restrict its use in petroleum refinery for transportation application. Therefore, enhancing the quality of bio-oil with prerequisite characteristics is utmost important and in this regard, diverse physical and chemical upgrading methods are being investigated for upgrading bio-oil. These methods focused solely on reducing bio-oil viscosity, corrosiveness, oxygen, nitrogen, ash and water contents, phase separation, polymerization, coking and precipitation. Emerging bio-oil upgradation techniques that are widely practiced are i) Solvent addition for reducing viscosity, iii) Emulsification for improving ignition, iv) Supercritical fluids for increasing HHV, v) Hydrotreating for reducing N, S, O content in bio-oil, vi) Catalytic cracking for significant yield and better quality of bio-oil, vii) Steam reforming of bio-oil for H2 production [77], [135]. Considering the significance of the upgrading methods for bio-oil, this review aims to present all-inclusive upgrading methods for bio-oil improvement using copious literature survey.
Section snippets
Bio-oil composition and limitations
To make bio-crude to use in petroleum refinery, composition of bio-crude need to be analyzed. With this context, ultimate or elemental composition of bio-crude obtained from various algal strains has been given in Table 1. Carbon content of bio-oil occupied a predominant fraction in the vicinity of 63–78%. Oxygen is the second most predominant element that constitutes 7–26%. Further, Hydrogen, Nitrogen, Sulfur content of the bio-crude are in the range of 7.6–11.2%, 0.5–7.6% and 0.4–1.4%,
Bio-oil upgrading- a broad view on methods
Several physical and chemical methods are available for upgrading bio-oil [92]. The high water content and acidity lead corrosiveness of bio-oil when compare with fossil fuels make them not suitable for diesel locomotive engines. The combustion performance can be improved by upgrading process which enhances H/C rate and corrosion reduction. Numerous solutions in petrochemical industries with advanced machining technologies offer broad possibility of enhancement of bio-oil into petro crude level
Conclusion and recommendation
Biomass can be converted into liquid fuel or bio-oil by technical processes, though it is regarded as potential alternatives to petroleum fuels, direct application is limited due to unstable thermodynamic properties. In this review, upgrading techniques namely emulsification, steam reforming, catalytic cracking, zeolite cracking, alcoholysis/esterification, supercritical fluids, hydrogenation, hydrotreating/hydrodeoxygenation, and molecular distillation were discussed in detail on enhancing the
CRediT authorship contribution statement
Tharifkhan Shan Ahamed: Conceptualization, Data curation, Formal analysis, Writing - original draft. Susaimanickam Anto: Conceptualization, Data curation, Validation. Thangavel Mathimani: Supervision, Writing - original draft, writing - review & editing. Kathirvel Brindhadevi: Conceptualization, Data curation, supervision, Validation. Arivalagan Pugazhendhi: Validation, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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