Alternatives for inert torrefaction to produce high-quality solid fuel: Review of available techniques, parameters, potentials and challenges

Dependence on inert gas such as nitrogen during torrefaction is not favourable due to the burden in terms of operational costs. Therefore, various alternatives have been introduced to reduce or eliminate the dependence on inert gas. The present article seeks to review the various alternatives to inert torrefaction for viable solid fuel production. The details on non -inert torrefaction, including experimental setup, effect of various essential parameters, and major findings, are reviewed and discussed. The primary trends in the torrefaction field were extracted from previous studies, were compiled and presented in several tables and graphs. The review findings showed that temperature, residence time, gas type, gas velocity, heating rate, and particle size can affect the performance, physicochemical, and combustion properties of torrefied biomass. The degree of the effectiveness of each parameter also depends on the biomass type and shape. Based on the van Krevelen diagram, the various alternative techniques can produce competitive torrefied solid products with viable fuel properties compared to commercial coals and products torrefied under inert torrefaction conditions. Overall, it is expected that the number of alternative techniques can be implemented on a larger scale after certain modifications are performed.