Influence of Torrefaction Pretreatment on Reactivity and Permanent Gas Formation during Devolatilization of Spruce

Torrefaction has shown potential for improving biomass properties and converting biomass to a more coal-like fuel. In this paper, both fast and slow devolatilization behaviors of untreated spruce and spruce torrefied at 290 °C and 20–30 min holding time have been studied by conducting experiments in a bench-scale heated foil reactor and a thermogravimetric analyzer, respectively. The former has been applied to estimate the permanent gas formation for biomass entering a fluidized bed reactor, and the latter has been used to determine the effect of torrefaction on (slow) devolatilization reaction kinetics. The reaction kinetics were derived using the Reaction Rate Constant Method and the Senum and Yang Temperature Integral Approximation. The gases produced during fast devolatilization, applying a constant heating rate and a final temperature in the range of 500–1000 °C, were analyzed by Fourier transform infrared spectroscopy, and tars were collected and quantified gravimetrically. Results show that the activation energy and pre-exponential factor increased for the global devolatilization reaction. The former increased by 25% due to torrefaction pretreatment. The yield of the produced permanent gases CO, CH 4, and CO 2 increased with increasing final devolatilization temperature. CO is the dominant gas at temperatures higher than 600 and 800 °C for untreated and torrefied spruce, respectively. At lower temperatures, CO 2 has the highest mass yield. CH 4 shows the lowest yield for each final temperature and fuel sample. The results reported in this paper provide basic information for thermochemical reactor design when using (torrefied) spruce as a feedstock. The findings confirm that torrefied spruce is more coal-like than the parent material with respect to activation energy, char production, respective evolution of CO and CO 2, and O/C and H/C atomic ratios.