Introduction

Background

Growing concerns related to the availability of energy and emissions caused by fossil fuels have shifted the focus of researchers toward the search for an alternative fuel source. Biofuel produced from biomass is emerging as a potential alternative owing to its renewable and environment-friendly act (Mathimani & Pugazhendhi, 2019). Algae are considered third-generation biomass and are currently one of the focal points of bioenergy research; moreover, they have a high growth rate, increased yield, and significant CO₂-capturing ability (Das et al., 2021). Besides the above-mentioned benefits, algae have a significantly larger lipid content than other biomasses. Thus, biofuel from algae can be utilized in biodiesel, jet fuel, and Fischer-troops fuel production.

Bio-oil from algae can be produced via thermochemical or biochemical conversion; the latter procedure is less complicated and largely implemented. Based on the operational conditions and procedure, the thermochemical pathways could be classified into pyrolysis, liquefaction, and gasification. Among the above processes, pyrolysis has attracted significant attention from the research community due to its simpler operational conditions and higher product yield (Sekar et al., 2021). Pyrolysis is a process in which biomass is heated in the temperature range from 300 ℃ to 700 ℃ in an oxygen-limited environment. The key products of the pyrolysis process are biochar, bio-oil, and gases; the product distribution depends on the experimental conditions, such as pyrolysis temperature, heating rate, and residence time (Varma et al., 2018). Each pyrolysis method has impacts on the product yield, hence has its own pros and cons. Moreover, the biomass used during the pyrolysis procedure has a significant impact on product quality as well as product yields. Depending on the temperature range and other experimental parameters, multiple secondary reactions, including decomposition, compression, polymerization, and cracking, occur during the pyrolysis process (Gong et al., 2019). These secondary reactions could influence the various product yields.

The involvement of several secondary reactions makes the pyrolysis process somewhat complex, and thus, it is difficult to decide what parameters impact the bio-oil yield. Apart from yield, the calorific value of the produced bio-oil is a factor of economic importance. The calorific value of bio-oil influences its applicability at a large scale; for example, a higher bio-oil yield having a low calorific value might not be profitable from an industrial-scale application point of view (Khan et al., 2021). The calorific value of the bio-oil can be measured by its highest heating value (HHV); a high HHV indicates a better quality of bio-oil and vice versa. HHV measures the energy content of algae and has a unit of MJ/Kg. Thus, understanding the effect of pyrolysis conditions, biomass nature, and other process-related parameters on bio-oil production will help the commercialization of bio-oil produced from algae (Wang et al., 2022).

The product distribution can vary depending on the pyrolysis process conditions and the choice of algal biomass. Varying process conditions can impact bio-oil yield; moreover, the choice of appropriate algae can influence the quality of the produced bio-oil. The biochemical make-up of algal species affects the amount of bio-oil that can be produced from their biomass. Algal species such as Chlorella, Dunaliella, and Scenedesmus have higher lipid content and hence can be a good choice for bio-oil production (Udayan et al., 2022). In recent years, genetically modified algae have been prepared especially for their use in bioenergy applications (Shokravi et al., 2021). This indicates that bio-oil producers can benefit from the appropriate choice of algal biomass and optimized experimental conditions for maximizing bio-oil production, making bio-oil use as an alternative fuel economically viable.

Motive

Various experimental studies have explored the impact of various parameters on bio-oil production. However, in most studies, the analysis for understanding the effect of multiple parameters on bio-oil production is limited to a narrow range (focused on a smaller number of parameters). For example, studies analyzing the impact of experimental parameters do not include the effect of algal chemical composition. Similarly, most analyses include one or two algae to understand the effect. Moreover, most of the performed studies lack a comparison of bio-oil produced based on the type of algae and its chemical composition. Thus, summarizing the findings of the published articles will primarily serve two purposes: 1) it will help to generalize the results obtained from the individual study, which will be helpful for its application at the population level; 2) it will help to identify specific parameters with which have the highest impact on the bio-oil yield and its HHV.

Objectives of the study

The objective of this study is to analyze the data produced by various peer-reviewed articles and to analyze the impact of the algal biomass properties and pyrolysis experimental parameters on algal bio-oil production through pyrolysis. Moreover, the study will attempt to identify the critical parameters impacting bio-oil yield and HHV. The results produced from this analysis could help determine and identify suitable algal properties and experimental conditions for maximizing the bio-oil yield and HHV.