Waste-to-Energy Innovations and Advances in Hydrothermal Carbonization, Microwave, and Pyrolysis Processes: A Review

The accelerating global generation of municipal solid waste, coupled with escalating energy demands, presents one of the most pressing sustainability challenges of the 21st century. Conventional waste disposal pathways, such as landfilling and incineration, exacerbate environmental degradation and resource loss. In response, thermochemical technologies—specifically hydrothermal carbonization (HTC), microwave-assisted processing, and pyrolysis—have emerged as promising strategies for waste valorization and renewable energy recovery. This review critically examines recent advancements in these technologies, emphasizing their roles in converting diverse waste streams into high-value energy carriers and carbon-rich materials. Comparative analysis reveals that HTC operates efficiently under subcritical water conditions, producing hydrochar with tunable physicochemical properties ideal for soil amendment and carbon sequestration. Microwave processing offers rapid, uniform heating with lower energy input and shorter reaction times, enhancing process scalability and efficiency. Pyrolysis, a more established technology, provides high bio-oil and syngas yields suitable for fuel production and chemical synthesis. However, trade-offs in energy balance, process optimization, and emissions management persist. Key findings indicate that hybrid approaches, catalyst innovations, and integration with renewable systems can substantially improve overall energy efficiency and environmental performance. Future research should focus on techno-economic assessments, lifecycle sustainability analyses, and pilot-scale demonstrations to bridge the gap between laboratory success and industrial implementation, advancing the global transition from waste to wealth.