Effect of temperature on thermochemical property and true density of torrefied coffee residue

•The temperature range of 563–593 K should be the window of torrefaction for coffee residue.•The maximal increase in calorific value was in high agreement with the TGA results.•True density can be used as an approximate indicator for the extent of torrefaction.

Torrefaction is a mild pyrolysis process in the relatively low temperature, which aims to upgrade the thermochemical and physical properties of biomass as a high energy-density fuel. In this study, coffee residue, a processing by-product from the soluble coffee industry, was torrefied at the temperature range of 503–623 K with residence time of 10 min by a commercial electric oven. The thermochemical properties of the torrefied biomass, such as elemental analysis and calorific value, were investigated. Furthermore, the information about microscopic and structural characteristics was also obtained by means of scanning electron microscopy (SEM) and true density. It was found that the calorific value of the torrefied biomass increased with temperature, while its maximal increase occurred at around 563 K. This result was in high agreement with the observation from the thermogravimetric analysis (TGA) of coffee residue. More consistently, the carbon content of torrefied coffee residue increased significantly as temperature increased from 563 to 593 K, which should be the temperature window of torrefaction for this biomass. By contrast, the contents of hydrogen and oxygen decreased slightly due to the devolatilization as a result of thermal decomposition reactions like dehydration and depolymerization. On the other hand, the true density of torrefied coffee residue increased with temperature at above 563 K, making it more observable in the structural shrinkage as also depicted in the SEM. However, it may involve light gases released and loose structure remained at temperature of below 563 K, leading to decreased true density with temperature from 503 to 563 K.