processes Article Heat and Mass Transfer during Lignocellulosic Biomass Torrefaction: Contributions from the Major Components—Cellulose, Hemicellulose, and Lignin Ken-ichiro Tanoue 1,*, Kentaro Hikasa 1, Yuuki Hamaoka 1, Akihiro Yoshinaga 1, Tatsuo Nishimura 1, Yoshimitsu Uemura 2,3 and Akihiro Hideno 4 1 Department of Mechanical Engineering, School of Sciences and Engineering for Innovation, Yamaguchi University, Tokiwadai 2-16-1, Ube, Yamaguchi 755-8611, Japan; [email protected] (K.H.); [email protected] (Y.H.); [email protected] (A.Y.); [email protected] (T.N.) 2 NPO Kuramae Bioenergy, Minato-ku, Tokyo 108-0023, Japan; [email protected] 3 Center for Biofuel and Biochemical Research, Universiti Teknologi PETRONAS, Seri Iskandar 32610, Malaysia 4 Paper industry innovation center of Ehime University, 127 Mendori-cho, Shkokuchuo 799-0113, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-836-85-9122 Received: 27 June 2020; Accepted: 6 August 2020; Published: 9 August 2020 Abstract: The torrefaction of three representative types of biomass—bamboo, and Douglas fir and its bark—was carried out in a cylindrical-shaped packed bed reactor under nitrogen flow at 573 K of the reactor wall temperature. As the thermal energy for the torrefaction was supplied from the top and the side of the bed, the propagation of the temperature profile of the bed is a crucial factor for discussing and improving the torrefaction reactor performance. Therefore, the temperature and gas flow rate (vector) profiles throughout the bed were calculated by model simulation so as to scrutinize this point. The measured temperature at a certain representative location (z = 30 mm and r = 38 mm) of the bed was well reproduced by the simulation. The volume faction of the bed at temperatures higher than 500 K at 75 min was 0.89, 0.85, and 0.99 for bamboo, and Douglas fir and its bark, respectively. It was found that the effective thermal conductivity is the determining factor for this difference. The heat of the reactions was found to be insignificant. Keywords: biomass torrefaction; packed bed reactor; biomass major components; reaction enthalpy; numerical simulation 1. Introduction Biomass is one of the representative renewable energy sources, and is one of the only energy sources that is tangible, as it consists of carbon, hydrogen, oxygen, and some minor atoms. Therefore, one of the near-future applications for biomass is the production of solid fuels. Torrefaction is a promising and simple technology for producing high-quality solid fuels from biomass [1–8]. Heat transfer within biomass is an important factor for determining the performance of torrefaction, as the size of biomass in the torrefaction reactor is larger than that for the other conversion technologies, such as gasification or liquefaction. Kinetic studies of heat and mass transfer are also being conducted experimentally and numerically regarding how much production can be expected and how much heat is required by torrefaction. A two-step parallel successive reaction model for all major components during pyrolysis, when the temperature was higher than 400 ◦C, has been reported by Miller et al. [9]. Their model, which was Processes 2020, 8, 959; doi:10.3390/pr8080959 www.mdpi.com/journal/processes Processes 2020, 8, x FOR PEER REVIEW 2 of 13 Processes 2020, 8, 959 2 of 13 pyrolysis, when the temperature was higher than 400 °C, has been reported by Miller et al. [9]. Their model, which was suggested by Di Blasi (1994) [10] for cellulose pyrolysis, was extended to hemicellulosesuggested by Dipyrolysis Blasi (1994) and [lignin10] for pyrolysis cellulose on pyrolysis, the basis was of extendedprevious toexperimental hemicellulose data. pyrolysis It has also and beenlignin reported pyrolysis by on Kawamoto the basis of that previous the cellulose experimental of biomass data. is It composed has also been of crystalline reported by (long Kawamoto block) and that amorphousthe cellulose (short of biomass block) is composedalternately, of like crystalline a block (long co-polymer block) and [11]. amorphous Decomposition (short block)starts alternately,at around 200like °C a block in the co-polymer amorphous [ 11state,]. Decomposition and the crystalline starts hardly at around decomposes 200 ◦C in at the that amorphous temperature. state, At andaround the 300crystalline °C, amorphous hardly decomposes decomposition at that is transmitted temperature. to Atthe around crystalline 300 ◦material,C, amorphous and the decomposition decomposition is progressestransmitted rapidly. to the crystalline material, and the decomposition progresses rapidly. The heat ofof thethe reactionsreactions during during pyrolysis pyrolysis or or torrefaction torrefaction have have been been summarized summarized in relationin relation to the to thechar char formation formation [12– 17[12–17].]. In particular, In particular, the heat the ofheat the of reactions the reactions during during pyrolysis pyrolysis were measured were measured directly directlyusing DSC using (Di ffDSCerential (Differential Scanning Calorimetry),Scanning Calorimetry), and it has beenand it suggested has been that, suggested firstly, anthat, endothermic firstly, an endothermicreaction will reaction occur, and will then occur, an exothermicand then an reaction exothermic will reaction successively will successively occur [12,14 –occur16]. The[12,14–16]. heat of Thethe pyrolysesheat of the of pyrolyses all of the of major all of components the major comp haveonents been reported have been qualitatively reported qualitatively using DTA (diusingfferential DTA (differentialthermal analysis) thermal [18]. analysis)[18]. Although direct Although measurement direct measurement by DSC for the by heat DSC of afor reaction the heat during of a cellulosereaction duringpyrolysis cellulose was reported pyrolysis by was Mok reported et al. [12 by], thereMok areet al no. [12], data there by DSC are no for data the heatby DSC of reaction for the heat during of reactionhemicellulose during pyrolysis hemicellulose and lignin pyrolysis pyrolysis. and As lignin heat and pyrolysis. mass transfer As heat during and pyrolysis mass transfer were reviewed during pyrolysisby Di Blasi were (2008) reviewed [19], it by has Di beenBlasi studied(2008) [19], using it has a chemical been studied reaction using model a chemical suitable reaction for the model heat suitableand mass for transfer the heat experimental and mass transfer results experimental [20–26], and results an analysis [20–26], using and the an heatanalysis of chemical using the reaction heat of chemicalcommensurate reaction with commensurate temperature changewith temperature has also been change conducted has also [20 been–23]. conducted [20–23]. In addition, the the effect effect of inorganic materials in biomass feedstock on torrefaction has been investigated by by S. S. Zhang et al. [27]. [27]. They They found found that that when when the the torrefaction torrefaction temperature temperature was was 270 270 °C,◦C, the product yields for raw rice husk were 55% in char, 23% in liquid, and 21% in gas, while the product yieldsyields for for rice rice husk husk with with a reduced a reduced potassium potassium concentration concentration of less of than less 1 wt%than of 1 allwt% inorganic of all inorganiccompounds compounds were 60% were in char, 60% 23% in inchar, liquid, 23% andin liquid, 17% in and gas. 17% in gas. There are manymany varietiesvarieties of of biomass, biomass, but but few few reports reports have have been been generalized generalized and and analyzed analyzed for thefor thethermochemical thermochemical reaction reaction of the of thermalthe thermal decomposition decomposition behavior, behavior, and and it is it not is clearnot clear about about heat heat and andmass mass transfer transfer in low in temperaturelow temperature regions, regions, like torrefaction.like torrefaction. In this study, study, we aimed to generalize the biom biomassass torrefaction behavior using the concentrations of the major constituents of cellulose, hemicellulos hemicellulose,e, and and lignin. lignin. Specifically, Specifically, as as shown shown in in Figure Figure 11,, the heat of the the reaction reaction for the the major major components components of the biomass during pyrolysis was measured. We We also experimentally investigatedinvestigated thethe biomassbiomass torrefaction torrefaction process process on on a a packed packed bed bed of of small small particles particles of ofbiomass, biomass, because because a big a big chunk chunk of a of biomass a biomass slab isslab di ffiiscult difficult to handle to handle and has and no has assurance no assurance for spatial for spatialuniformity. uniformity. Furthermore, Furthermore, the heat the and heat mass and transfer mass transfer simulation simulation during torrefactionduring torrefaction in the biomass in the biomasspacked bed packed were bed performed were performed using the using heat ofthe the heat chemical of the chemical reaction andreaction the pyrolysis and the pyrolysis model by model Miller byet al.Miller [9]. et The al. validity [9]. The ofvalidity the numerical of the numerical simulation simulation model was model compared was compared with that with of the that biomass of the biomasstorrefaction torrefaction process experiment.process experiment. Figure 1. ConceptConcept of of this this work work to to investigate the biomass torrefaction processes. 2. Materials and Methods Processes 2020,