The influence of torrefaction temperature and reaction time on the properties of torrefied sun‐dried millet and sorghum straws from the arid and semi‐arid zones of western Africa

Abstract

There are increasing demands for syngas production. Fossil-based fuels are expensive and environmentally unfriendly. Affordable, sustainable, and environmentally healthy alternatives and sustainable fuel sources are needed. This study focused on producing tor-chars from two different tropical biomasses (from Nigeria, West Africa), which were torrefied and characterized to improve their properties for potential use as sustainable feedstocks in an entrained flow gasifier for syngas production. Torrefaction temperatures of 230, 240, 250, and 270 °C and residence times of 30 and 60 min were used to torrefy sundried sorghum straw (SS) and millet straw (MS) at particle size ≤5 mm. The properties of torrefied MS and SS improved significantly due to torrefaction. The optimum conditions for pretreatment of sundried MS and SS were 240 °C and 60 min, at which increased energy density significantly compensated for the severe mass loss and significant degradation of hemicellulose and cellulose. Under these conditions, the respective mass and energy yields were 53 wt% and 75% for millet straw and 49 wt% and 75% for sorghum straw. The O:C atomic ratio decreased to 0.3 in MS tor-chars and 0.2 in SS tor-chars resulting in higher heating values of 25 MJ kg −1 and 27 MJ kg −1 , respectively. Under optimum conditions, the ash content increased by 107% to be approximately 7 wt% in MS tor-char, and by 118% to about 7 wt% in SS tor-char. These properties are promising for potential feedstocks in co-firing plants or gasification systems.