Review articleA review on the pyrolysis of algal biomass for biochar and bio-oil – Bottlenecks and scope
Introduction
Concerns over fossil fuels availability and emission index increases research and development on renewable fuels and thus, biofuel has gained profound attention in recent times owing to its renewable and environmental friendly aspects [1], [2]. Biofuel from biomass is a promising source of renewable energy. Biomass evolution for biofuel typically classified into three generation. First and second generation fuels depend on the edible crops and non-food biomass. Although they impart various benefits, they have certain limitations of plantations [3], higher NOx emission [4] and surplus land requirement. In recent decades, the number of works investigating microalgae as biofuel feedstock had widely increased compared to lignocellulosic biomass owing to their various advantages. Algae gained lime light due to following reasons such as high growth rate, high yield, CO2 capture, and non-feed stock [5], [6]. Besides above-said merits, there are many other benefits associated with the utilization of algae as third generation feedstock [7], [8], which includes higher lipid productivity, rapid growth, less land requirement. Although there are series of studies conducted on the conversion of microalgae into useful energy still it was challenging due to the cultivation and post processing of algae. Microalgae can be used for biodiesel, jet fuels and Fischer-Tropsch fuels production as well. In addition to biodiesel production, algal biomass can also be converted into bio-oil and biochar through three main thermochemical routes such as gasification, liquefaction and pyrolysis [9]. Among them, pyrolysis has acquired prominent attention due to its operating conditions and yield [10].
Pyrolysis is a simple thermochemical process which operates in the oxygen free atmosphere from the temperature range from 300 to 700 °C. The main products of the pyrolysis are bio-oil, bio-char and gaseous component [11]. Liberation of the main product depends on the temperature, residence time and type of feedstock. By varying above two, the main product can be varied based on the requirement. Bio-oil and bio-char can be used as the essential source to generate heat by combustion process. Meanwhile, bio-char used as the activated carbon, fertilizers, soil compost and efficient catalyst for biofuel production. There are different pyrolysis based on the conditions such as slow, fast, flash, microwave assisted, and hydrolytic pyrolysis. Each pyrolytic process has its own merits and demerits. Composition and yield of biochar depends on the temperature and other conditions used in pyrolysis. Hence, it is imperative to understand the processing conditions of pyrolysis to ascertain the pertinent pyrolytic process for obtained desirable quality and yield of biochar. The key products bio-oil, bio-char and gas depends on the temperature, residence time, pressure, biomass nature, and other reactor conditions. Furthermore, the concentration of solid, liquid and gas can be altered by choosing the optimized pyrolysis process and the microalgae species. Therefore, this review was undertaken to review the different pyrolysis techniques. This study gives the recent trends in conversion of microalgae into useful bio-oil and bio-char through slow, fast, flash, microwave assisted, catalytic, and hydrolytic pyrolysis. The underlying mechanism of pyrolysis of biomass components such as carbohydrates, protein, lipids were comprehensively addressed using published literatures to comprehend the complete pyrolysis process. Eventually, conclusion and scope of the pyrolysis process was signposted to pave the way for foreseen research endeavours.
Section snippets
Pyrolysis of microalgae
Among the various thermochemical processes, pyrolysis has received considerable attention in recently due to its simplicity and speed and further, pyrolysis technique can yield versatile products (liquid), which are quite easy to handle unlike other thermochemical biomass conversion techniques [12], [13], [14]. Main products of the microalgae pyrolysis are biochar, bio-oil and non-condensable gases. Pyrolysis is classified into many types based on the difference in operating conditions namely,
Conclusion
Renewable energy production has become an inevitable research avenue among the various countries to satiate the energy demand and simultaneously reduce the emission of greenhouse gases from the combustion of conventional fossil fuel. Renewable energies are many viz., bioethanol, biobutanol, biodiesel, biogas, biohydrogen. In addition, biochar, bio-oil, and syngas from thermochemical processing of algal biomasses acquired much significance recently due to their application in various refineries.
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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