Biofuel production from waste residuals: comprehensive insights into biomass conversion technologies and engineered biochar applications

Abstract

Biomass-derived residuals represent a vital renewable energy source, offering sustainable alternatives to mitigate fossil fuel dependency, address climate change, and manage waste. Although biomass generally has a lower calorific value (10-20 MJ kg^-1) compared to fossil fuels (40-50 MJ kg^-1), its energy recovery potential can be enhanced through advanced conversion technologies such as torrefaction, pyrolysis, and gasification. Additionally, biomass is considered carbon neutral when sourced sustainably, as the CO₂ released during combustion is reabsorbed by plants during their regrowth cycle, maintaining a balanced carbon flux in the atmosphere. This review explores the diverse sources of biomass and examines their chemical compositions and inherent properties, emphasizing their transformation into valuable energy carriers and bio-products. It provides a comprehensive analysis of thermochemical, biochemical, and physicochemical conversion technologies, detailing their mechanisms, efficiencies and applications. Special attention is given to biochar, a product of biomass pyrolysis, highlighting its potential in pollution mitigation, carbon sequestration, and as a catalyst in industrial applications. The review delves into synthesis processes of biochar and performance-enhancing modifications, illustrating its significant role in sustainable environmental management. Additionally, the economic and ecological advantages of biomass-derived energy, including reduced greenhouse gas emissions and waste reutilization, are critically evaluated, underscoring its superiority over conventional fossil fuels. Challenges limiting the scalability of biomass energy, such as technology costs, process efficiency, and market dynamics, are addressed, alongside prospective solutions. By consolidating extensive research on biomass conversion technologies and engineered biochar applications, this review serves as a valuable resource for researchers and policymakers. It aims to guide advancements in biomass utilization, fostering a transition toward sustainable energy systems and addressing global energy and environmental challenges.

Fig. 1. The carbon cycle in biomass production and utilization.

Fig. 2. Transforming various biomass sources into valuable products.

Fig. 3. Structure and key components of lignocellulosic biomass: a systematic overview.

Fig. 4. Mechanism of biochar preparation via cellulose decomposition.

Fig. 5. Formation pathways of hemicellulose (d-xylose) into biochar.

Fig. 6. Basic mechanism of lignin-derived biochar formation in hydrothermal carbonization

Fig. 7. Hydrothermal carbonization reaction pathways for lignocellulosic biomass, with emphasis on liquid biocrude formation.

Fig. 8. Schematic representation of biochar formation mechanism from sewage sludge.

Fig. 9. Biochar formation pathways from nitrogen-rich or nitrogen-free biomass.

Fig. 10. Different conversion techniques of biomass

Fig. 11. Thermo-chemical biomass processes and products.

Fig. 12. Main reactions during two-stage combustion of biomass.

Fig. 13. Types of gasifiers (A) entrained bed, (B) fixed bed, and (C) fluidized bed.

Fig. 14. Scheme of biomass pyrolysis reactions.

Fig. 15. Fixed bed reactor for biomass pyrolysis

Fig. 16. Bubbling fluidized-bed reactor.

Fig. 17. Schematic of circulating fluidized bed pyrolysis.

Fig. 18. Catalytic processing for biomass conversion towards fuels production.

Fig. 19. Electricity from biomass sources in 2020.

Fig. 20. Renewable energy use in transport in 2020.

Fig. 21. Biopower generation in continents in 2020.

Fig. 22. Renewable heat production in 2020.

Fig. 23. Comparison of CO₂ emissions from conventional energy sources biomass-derived energy sources along with their saving in GHGs emission.

Ahmed M. A. El Naggar

Adel A. El-Zahhar

Majed M. Alghandi

Hussien A. El Sayed