II: Biooils

There are six chapters on the research front of biooils. Chapter 5 (Konur, 202li) maps the research on biooils using a scientometric method comprising a sample of the 100 most-prolific sample papers and a population of over 20,000 papers.

Eight research fronts emerge from the examination of the sample papers: ‘biomass pyrolysis’, ‘biofuels from biomass pyrolysis’, ‘pyrolysis oil property and characterization’, ‘biomass conversion for biofuel production’, ‘biomass torrefaction’, ‘biomass liquefaction’, ‘biooil upgrading’, and ‘lignin conversion' (Table 5.8).

Chapter 6 (Konur, 202 Ij) presents the key findings of the most-cited 25 article papers on the properties and characterization of biooils. Table 6.1 provides information on the research fronts in this field. As this table shows, the primary research fronts of ‘characterization of biooils’ and ‘properties of biooils’ comprise 48 and 52% of these papers, respectively. These prolific studies on two complementary research fronts provide valuable evidence of the characterization and properties of biooils obtained from a variety of feedstock such as algae and wood.

Chapter 7 (Konur, 2021k) presents the key findings of the most-cited 25 article papers on the pyrolysis of biooils. Table 7.1 provides information on the research fronts in this field. As this table shows, the primary research fronts of ‘pyrolysis’ and ‘upgrading of pyrolysis oils’ comprise 72 and 28% of these papers, respectively. ‘Kinetic studies’ and ‘pyrolysis oils’ comprise 52 and 20% of the papers in the first group, respectively. These prolific studies on two different research fronts provide valuable evidence of biomass pyrolysis and upgrading of pyrolysis oils.

Chapter 8 (Zhang et al.. 2021a) provides an overview of catalytic biooil upgrading for processing aqueous-phase compounds. They first present the challenges and objectives in upgrading whole biooil. Then, they summarize the developed approaches for catalytic conversion of small oxygenates in the aqueous phase of biooil to produce valuable biofuels and biochemicals. They discuss the promising catalysts and proposed reaction mechanisms in an attempt to generate the relationships between catalyst properties (e.g. acid-base pairs) and catalytic performance which may guide the future rational design of catalysts in biooil upgrading.

Chapter 9 (Zhang et al., 2021 b) provides an overview of catalytic biooil upgrading for processing oil-phase compounds and real biooils, complementing Chapter 8

(Zhang et al., 2021a). Specifically, they focus on hydrotreating and cracking the phenolics, furanics, or heavier oligomers, as well as the approaches discussed in Zhang et al. (2021a) (e.g. steam reforming), to produce valuable biofuels and biochemicals. In addition, they discuss the advances of catalyst development and reaction mechanisms to correlate catalyst structure with performance (e.g. oxophilicity vs hydrodeoxygenation), serving as a basis for the design of selective and durable catalysts for biooil upgrading.

Chapter 10 (Cao et al., 2021) reviews the research on the biooil production through catalytic ’hydrothermal liquefaction' (HTL) of biomass, focusing on recent developments and future prospects. They note that efficient hydrothermal conversion of biomass to produce biooil is in line with the current concepts of environmental conservation and sustainable development. However, they caution that the research in this field is still at the lab-scale or pilot stage, and they assert that future research should focus on the development of highly active, hydrothermally stable, green, and easily recoverable new catalysts to realize the production of: high-quality biooils through hydrothermal conversion; the cost-effective separation of the main biomass as the route to achieve value-added products with high yield and selectivity; green and environmentally friendly biphasic or multiphasic solvents that should gain more emphasis in terms of both efficient conversion and product purification.

III: Biodiesel Fuels in General

There are seven chapters in the research stream of biodiesel fuels in general. Chapter 11 (Konur, 20211) maps the research on biodiesel fuels in general using a scientomet-ric method employing a set of 100 most-cited sample papers and a population paper set of over 6,500. There are three major topical research fronts for these sample papers; ‘production of biodiesel fuels’, ‘properties of biodiesel fuels’, and ‘emissions from biodiesel fuels’.

Chapter 12 (Goh et al., 2021) provides an overview of the research on biomassbased catalyst-assisted biodiesel production. Catalysts should be introduced in order to increase the overall rate of biodiesel production and reduce capital costs. They therefore illustrate the latest breakthroughs involved in the use of biomass-derived catalysts. In addition, they provide a better framework and methods to synthesize biomass-based catalysts for biodiesel production. This provides a solution to the catalyst separation problem in the current biodiesel field and enhances the economic viability of the industry, thus sustaining the environment while meeting energy demands.

Chapter 13 (Aguieiras et al., 2021) reviews the research on enzymatic biodiesel production focusing on the challenges and future prospects in this field. Many technologies have been developed in order to produce high performance biocatalysts and many of them have shown promising results towards the enzymatic synthesis of biodiesel. Alternative sources for enzyme and biodiesel production contribute to the advancement of this application. Among different raw materials, the use of low-cost agro-industrial waste as feedstock reinsert them in the production chain, reinforcing the circular economy bases, and approaching an economically feasible enzymatic biodiesel synthesis. Considering the economic aspects, Aguieiras et al. note that there are still some limitations in the industrial application of enzymes for biodiesel production and that this field still has numerous possibilities for development whilst enzymatic production becomes a more attractive and economically viable pathway.

Chapter 14 (Tomaret al., 2021) provides an overview of the research on biodiesel additives. In search of cleaner fuels and to make biodiesel usage economically viable, additives are becoming an indispensable tool in global trade. Additives covers a wide range of subjects and are categorized into various types according to their size, chemical compounds, state of matter and functionality. The amalgamation of additives in biodiesel and its blends has a significant effect on fuel properties such as viscosity, fire point, flash point, and calorific value, which furthermore influences the combustion, performance, and emission characteristics of biodiesel fuel. Various oxygenated fuel additives improve the combustion process and lower the in-cylinder pressure due to the higher latent heat of vaporization. Recent advancements in the field of biobased additives have also set a new benchmark in the era of fuel additives. Thus, as the energy sources are upgraded towards cleaner and renewable technology, the additives’ share in the world market will likely increase over the next ten years.

Chapter 15 (Kumar and Kalam, 2021) reviews the research on the qualitative characterization of biodiesel fuels. The properties of biodiesel depends largely on the choice of feedstock which often is dependent on a domestic source. The fatty acid compositions of the parent oil or fat, the nature of the alcoholic head-group of biodiesel, and the contaminants present in the produced biodiesel determine the characteristics of the biodiesel produced. Intense research in biodiesel has outlined the significance of its characteristics as the expected performance and emission characteristics of engines are by and large governed by these properties. The performance of biodiesel fuel engines is comparable to that of petrodiesel. However, a significant improvement in emissions, except nitrogen oxides (NOS), can be achieved by employing biodiesel as fuel. Hence, Kumar and Kalam propose that future research should focus on streamlining biodiesel properties and genetic modification of feedstock as a solution.

Chapter 16 (Hoang et al., 2021) reviews the research on the use of biodiesel fuels in diesel engines. The use of biodiesel or biodiesel-petrodiesel fuel blends for diesel engines may have significant effects on engine performance and emissions, deposit formation, problems relating to engine durability, and the corrosiveness of engine components. A small portion of biodiesel (< 15% of volume) in blends with petrodiesel fuel has been found to have better engine performance and emissions, except for NOX emissions, and it does not cause any noticeable issues for components, system, and lubricating oil in diesel engines. Higher NOX emissions could be solved through the application of advanced technologies such as ‘exhaust gas recirculation’ (EGR) or exhaust gas-assisted fuel reforming. The use of additives and inhibitors could bring a high effectiveness in the reduction of deposit formation and lubricating oil degradation, as well as corrosion of engine components.

Chapter 17 (Jain et al., 2021) discusses biodiesel promotion policies from a global perspective. Most of the countries around the world are not able to follow the policy guidelines for production and use of biodiesel. This can be due to unrealistic estimates regarding cost, availability of cheap and abundant feedstock, tax benefits for parity with petroproducts, improper support from established petroproduct suppliers/ producers and projections of high yields. Policy guidelines must be able to facilitate technology learning and the production scale-up necessary to reduce costs. Relevant policies should include advanced biofuel quotas and financial derisking measures, e.g. loan guarantees from development banks. These would be particularly effective in those countries which possess significant feedstock resources. Countries and regions should consider policies that specify reductions in the life-cycle carbon intensity of fuel and which are effective in boosting the demand for biodiesel from waste oil, fat and grease feedstocks.

IV: Glycerol

There are three book chapters on glycerol research. Chapter 18 (Konur. 2021 m) maps the research on the production of biofuels and biochemicals from glycerol, a by-product of biodiesel fuels, using a scientometric method with 100 sample and over 6,900 population papers. There are eight major topical research fronts for these sample papers concerning glycerol, namely its: ‘catalytic conversion’, ‘microbial conversion’, ‘hydrogenolysis’, ‘oxidation’, ‘fermentation’, ‘dehydration’, ‘reforming’, and other methods of conversion such as ‘transesterification’.

There are eight primary research fronts regarding the products of glycerol conversion: ‘hydrogen’, ‘chemicals’ in general, ‘acrolein’, ‘fuels’ in general, ‘propanediol’, ‘carbonate’, ‘ethanol’, and ‘other products’ such as ‘hydroxybutyrate’ or ‘docosahexaenoic acids’. There are also eight papers related primarily to the characterization of glycerol.

Chapter 19 (Ayodele et al., 2021) reviews the research on hydrogen-rich syngas production from glycerol focusing on the overview of the modeling and optimization strategies as a case study for the production of biofuels from glycerol. The various modeling and strategies that have been investigated for hydrogen-rich syngas production from the thermo-catalysis and bioconversion of biodiesel-derived glycerol are discussed. These various strategies have been proven to be effective in modelling and optimizing the various glycerol conversion processes.

Chapter 20 (Konur, 202In) presents the key findings of the most-cited 25 article papers in propanediol production from glycerol as a case study of biochemical production from glycerol. The primary research fronts of ‘catalytic propanediol production' and ‘microbial propanediol production’ comprise 44 and 56% of these papers, respectively.

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