Available Pilot-Scale Technologies for Gasification of High-Ash-Content Biomass

The transition toward low-carbon energy systems and circular economy frameworks has intensified interest in biomass and waste valorization technologies that deliver reliable energy carriers while mitigating greenhouse gas emissions. Among the thermo-chemical pathways, gasification has emerged as a particularly flexible and robust option for transforming biomass resources into synthesis gas suitable for power generation, hydrogen production, and synthetic fuels. This review critically examines biomass gasification as a feasible alternative for valorizing waste and producing syngas. The manuscript discusses the physicochemical characteristics of biomass, highlights its influence on syngas quality, tar formation, and cold gas efficiency. The fundamental stages of the gasification process and the effects of different operating parameters were systematically reviewed. Special attention was given to the challenges posed by low-quality biomass, such as sewage sludge, digestates, and manures, which are characterized by high-ash content and high moisture levels. Syngas energy content reported across different experiences was usually around 4–5 MJ/m3 when operating with low-quality biomass, resulting in lower efficiencies than those reported for lignocellulosic biomass (around 30–70%, expressed as cold gas efficiency (CGE)). Current small-scale commercial gasification technologies were also reviewed, with emphasis on operational constraints. This review provides an integrated perspective on the operational challenges associated with low-quality biomass gasification and discusses technological pathways to enhance process efficiency and salability. Although biomass gasification cannot yet be regarded as a fully mature technology across all feedstocks, it nonetheless constitutes a technically significant pathway for strengthening energy system resilience and advancing the production of sustainable fuels within a net zero carbon framework.