An integrated laboratory and industrial scale study of autothermal torrefaction of hardwood, softwood and Miscanthus

Abstract

Torrefaction research at an industrial scale is rarely reported in the literature. This study provides a unique combination of 1100 kgh⁻¹ torrefaction trials for autothermal operation in an industrial setting, with underpinning investigations from laboratory-scale thermogravimetric analysis (TGA). The feedstocks were softwood (pine and spruce) and hardwood (alder and ash) species as well as an herbaceous biomass (Miscanthus). The laboratory results were used to interpret their plant-scale torrefaction profiles and gave key insights on process optimisation and control. Industrial-scale trials on ash wood were challenging due to large fluctuations in both temperature and process gas generation. TGA studies indicated fast rates of torrefaction and a low temperature exotherm for this wood type, which can explain the observed behaviour. The hardwoods achieved autothermal operation in torrefaction more easily than the softwoods, and the Miscanthus showed the most promise for continuous, autothermal production. TGA provided nuanced insights into the relative rates of mass loss, characteristic decomposition temperatures and exo/endothermic thermal behaviours which were able to give perceptive interpretation of the plant scale observations. A dominant factor is the nature and reactivity of the hemicelluloses and the associated low temperature exotherm that exists for some feedstocks, particularly hardwoods. Another factor is the catalytic components, particularly potassium, and their availability to participate in catalytic torrefaction reactions. The novel integrated study highlighted the impact of highly reactive hemicelluloses in scale-up, whereby the small differences in thermochemistry identified by TGA are magnified and affect process control and ease of autothermal operation.