Status overview of the torrefaction technology, its limitations, and prospective solutions: A reactor design and performance viewpoint

The goal of torrefaction is to transform lignocellulosic biomass into a solid product with better characteristics than raw biomass. Torrefaction is especially helpful when converting biomass into a final energy carrier via thermochemical processes like gasification and direct combustion. However, even though torrefied biomass has demonstrated advantageous quality, the prevailing torrefaction methods are not yet considered sufficiently advanced to overcome certain operational difficulties such as finding the proper balance between salient independent variables like temperature, reaction time and particle size; hence, much of the torrefied products obtained exhibit inconsistent properties. The technique also lacks the ability to efficiently handle a wide range of biomass feedstocks and exhibits scale-up issues that could be linked to poor temperature control and non-uniform heat distribution. Many of the existing torrefaction systems have been developed and validated for use in the processing of wood-based materials. Studies performed with non-woody materials yielded unsatisfactory results due to their inclination to ignite and carbonize readily during torrefaction. This review, therefore, presents a status overview of torrefaction technology and discusses the limitations of the technique from a reactor design perspective, as well as identifies system-level research that could potentially help to address the technology’s constraints. The basic principles of torrefaction and the mechanisms that affect the quality of torrefied products, including key process variables and kinetics are also discussed. Other expounded aspects include reactor design concepts, process monitoring and control, and specific technological barriers thought to hamper the technology’s attempt to achieve commercial success.