Thermal Effects during Biomass Torrefaction
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mal Effects during Biomass Torrefaction V. M. Zaichenkoa,*, A. Yu. Krylovaa,**, G. A. Sytcheva,***, and A. L. Shevchenkoa,**** a
Joint Institute for High Temperatures of the Russian Academy of Sciences (JIHT RAS), Moscow, 125412 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] ****e-mail: [email protected] Received January 24, 2020; revised February 6, 2020; accepted March 30, 2020
Abstract—A study of the thermal effects observed during the low-temperature pyrolysis of various types of pelletized biomass (sunflower husk, coniferous wood, litter-litter mass) is presented. It is shown that during sunflower husk torrefaction the greatest exothermic effect is observed and the possible reasons for its occurrence are explained. The influence of a small oxygen content in the heating medium on the exothermic effects during biomass torrefaction is analyzed. It has been suggested that the use of internal energy resources of the process (waste heat of exhaust gases and internal energy released in the process of biomass torrefaction) significantly increases the torrefaction process energy efficiency and allows to consider secondary biomass as a real competitor to traditional fuels, a substitute for fossil coal. Keywords: low-temperature pyrolysis, torrefaction, biomass, exothermic reaction, energy efficiency DOI: 10.3103/S0361521920040084
INTRODUCTION Utilization of secondary biomass (various wastes of plant and animal origin) is an important and extremely relevant scientific and technical task, since the amount of waste has become disastrous. One of the secondary biomass utilization directions is using of granular (“pelletized”) fuel from biomass for energy purposes, as local raw materials for distributed energy. The production of pellets from various types of biomass (wood and agricultural waste, peat) has been mastered all over the world, including in Russian Federation. Pellets are pressed cylinders of crushed biomass with a diameter of 5–10 mm and a length of 10– 30 mm. In fact, this is a solid fuel with a narrow particle size distribution, which predetermines its using in layered fireboxes and fluidized-bed fireboxes. The main disadvantages of pellets are their low heating value and high hydrophilicity. Low heating value of such a fuel is associated with high oxygen content in biomass. During long-term storage, pellets absorb moisture and lose the benefits of high-quality solid fuels. In addition, the presence of various bacteria in raw biomass necessitates the creation of special storage conditions that prevent the occurrence of rotting and fermentation processes. Most of these problems can be solved by subjecting the pellets to heat treatment.
Torrefaction is a biomass low-temperature pyrolysis that occurs at 230–280°C in an oxygen-free medium [1–3]. In the torrefaction process, pellets acquire hydrophobic properties (surface capillaries are blinded), its heating value per unit mass increases by 20–25%, and its specific gravity increases by 25–30%. Torrefied pellets from
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