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This article reviews various waste decomposition technologies composed of thermochemical and biochemical conversion routes for the generation of biohydrogen from biomass wastes. Due to the escalation of global energy consumptions, concerns on the energy security fuelled increasing generation of energy processes to meet such demands. The development of hydrogen has always sustained interest due to its immense prospects as a clean energy source. Instead, the current hydrogen production process termed as grey hydrogen posed the main contributing factor for carbon-related emissions. Therefore, technological prospects for green hydrogen (biohydrogen) production in the transition towards a decarbonised energy sector are desirable and advantageous. Furthermore, current constraints associated to the production of biohydrogen, ranging from safety to transportation aspects, are also discussed to provide informative insights to researchers and decision makers for a better understanding of biohydrogen economy.

Microalgae cultivation, absorption, adsorption, and membrane separation are widely applauded as promising technologies to sequester CO2from flue gas. Herein, comparative carbon balance was used to elucidate their CO2capture performance in the aspects of CO2emission rates (direct, indirect, total, and net), CO2removal efficiencies (apparent and actual), and CO2removal rate per power input ratio. Screening criteria for effective CO2capture system rule out energy-intensive sorption processes, put forward low energy membrane separation, and disclose alterable competency of microalgae cultivation. For CO2capture from flue gas, microalgae (Chlorella vulgaris) cultivation in open raceway ponds was only inferior to membrane separation. To improve microalgal CO2capture, the sensitivity analysis was performed by replacing original microalgae species (C. vulgaris) or cultivation system (open raceway pond). The microalgal CO2capture in open raceway ponds became worse following the substitution of C. vulgariswith alternatives (Botryococcus braunii, Chlorella kessleri, Chlorella pyrenoidosa, Scenedesmus obliquus, Spirulinasp., or Tetraselmis suecica). For microalgal (C. vulgaris) CO2capture, the competent cultivation systems included open raceway pond and airlift photobioreactor, while the bubble column, flat panel, or tubular photobioreactors were classified as non-competent systems. In short, microalgal (C. vulgaris) CO2capture was technically feasible in open raceway pond or airlift photobioreactor; further, the use of airlift photobioreactor was preferred for better CO2capture and microalgae biomass production. Due to the necessity of a huge working volume, the low scalability of microalgae cultivation could hamper the industrial application of microalgal CO2capture from flue gas.

Sulfonated carbon‐based catalysts (SCC) are favorable heterogeneous acids for acid‐catalyzed reactions including esterification and transesterification for biodiesel production. They are covalently functionalized with SO3H groups via CPhSO3H or CSO3H linkages with special carbon structures. To date, the types of SCC for biodiesel production ranges from biochar (BC), activated carbon (AC), graphene, graphite oxides, multiwalled carbon nanotubes, order mesoporous carbon, and graphitic carbon nitride. Lignocellulosic and biomass wastes are important carbon precursors for low‐cost BC and AC production. This review critically reviews and summarizes the most up‐to‐date research progress in the evolution of SCC for biodiesel production. Systematic discussions and comparisons on the different carbon materials, preparation methods, and sulfonation preparation parameters which directly affect the physicochemical attributes and catalytic performance are provided. The applications and reusability studies of these materials in biodiesel production are also included. Finally, the challenges to be addressed and future prospects of the research direction on the applications of SCC for biodiesel production are discussed. This review reviews the research progress in the sulfonated carbon‐based catalysts (SCC) development for biodiesel production. This review covers the objectives: 1) identify the types of SCC used, 2) discuss different preparation techniques, 3) compare the influences of sulfonating parameters, 4) discuss the physicochemical properties alternations after the sulfonation functionalization, and 5) summarize the previously reported SCC‐catalyzed biodiesel production.