processes

Article Heat and Mass Transfer during Lignocellulosic : Contributions from the Major Components—, , and

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 , Minato-ku, Tokyo 108-0023, Japan; [email protected] 3 Center for 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 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 , 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 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

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2. Materials and Methods

2.1. Sample Preparation Bamboo powder, Douglas fir powder, and the bark powder of Douglas fir were selected as representative biomass species. They were pulverized and sieved into #3.35 mm, #1.68 mm, #1.00 mm, and #500 µm using a motorized sieve (ANF-30, Nitto Kagaku Co., Ltd., Nagoya, Aichi, Japan). The size of about 400 particles for 500 µm or less was measured using a microscope. The constituent sugars and lignin in the bamboo and Douglas fir samples were analyzed using a modified method based on the technical report NREL/TP-510-42618 [28]. Table1 shows the mass percentages in the major components of biomass.

Table 1. Mass percentage of major components in the biomass powder.

Component (wt%) Biomass Species Hemicellulose Cellulose Lignin Others Xylan Arabinan Mannan Bamboo 34.1 25.2 2.3 0.2 24 14.2 Douglas fir 42.5 2.8 1.1 14 22 17.6 Bark of Douglas fir 25.4 3.3 0.9 3.9 51 15.5

Bamboo has three major components of cellulose, xylan in hemicellulose, and lignin, while Douglas fir consists of cellulose, mannan in hemicellulose, and lignin. On the other hand, there are only two major components of cellulose and lignin for the bark of the Douglas fir. The effect of the potassium concentration in the biomass on torrefaction was also investigated, but no significant difference was found in the product yield [27]. Therefore, in this study, we investigated the effect of the cellulose, hemicellulose, and lignin concentrations on the heat and mass transfer during the biomass torrefaction.

2.2. Thermogravimetric (TG) Analysis and Differential Scanning Calorimetry (DSC) In order to measure the heat of the reaction during the torrefaction of biomass, thermogravimetry analysis and differential scanning calorimetry analysis of pure cellulose powder (CAS RN 9004-34-6, Sigma Aldrich, Meguro-ku, Tokyo, Japan), pure xylan powder (CAS RN 9004-34-6, FUJIFILM Wako Pure Chemical Corporation, Osaka, Osaka, Japan) and pure dealkaline lignin powder (CAS RN 8068-05-1, Tokyo kasei, Chuo-ku, Tokyo, Japan) were conducted. For DSC (DSC3100s, MAC science, Chuo-ku, Tokyo, Japan) and TG (TG/DTA6300, Hitachi High-Tech Science Corporation, Minato-ku, Tokyo, Japan), the heating rate, nitrogen gas flow rate, and input mass were 10 K/min, 0.5 L/min, and 5 0.25 mg, respectively. The sample sizes of the cellulose, xylan, and lignin powder were 30, ± 45, and 64 µm, respectively. Figure2 shows the mass decrease profile from 300 K to 700 K from the TG analysis, as well as the relationship between the heat flow and temperature obtained by the DSC analysis for (a) cellulose, (b) xylan, and (c) lignin. The heat of reaction for these components was obtained by the following procedure.

(1) The time or temperature of the reaction start point tRS,i (TRS,i) and reaction end point tRE,i (TRE,i) were decided using the trend of the DSC and TG curves.

(2) The base line between tRS,i and tRE,i was linearly drawn in the DSC diagram. (3) The cross point tshift,i (Tshift,i) between the DSC curve and the base line was defined as the shifted point from the endothermic chemical reaction to the exothermic chemical reaction. (4) The endothermic and exothermic heats of reactions were given by Equations (1) and (2). Processes 2020Processes, 8, x2020 FOR, 8, PEER 959 REVIEW 4 of 13 4 of 13

(a) (b) (c)

Figure Figure2. Results 2. Results of ofthermogravimetory thermogravimetory (TG) and(TG) di fferentialand differential scanning calorimetry scanning (DSC) calorimetry experiments. (DSC) (a) Cellulose powder; (b) xylan powder; (c) lignin powder. experiments. (a) Cellulose powder; (b) xylan powder; (c) lignin powder.

Z 1 tshift,i   = t ∆HR,endo.,i 1 shift, iqbase,i qDSC,i dt (1) − m0 t − -ΔHR, endo., = RS,i q -q dt (1) m base, i DSC, i 0Z tRS, i 1 tRE,i   ∆HR,exo,i = qDSC,i qbase,i dt (2) − m0 − 1 t shift,tRE,i i - - Table2 shows the obtainedΔ heatHR, exo, of the i = reaction for celluloseqDSC, i pyrolysis,qbase, i d xylant pyrolysis, and lignin (2) m0 pyrolysis. From Table2, the absolute value of the endothermic tshift, i heat of the reaction was higher than that Tableof the 2 shows exothermic the heatobtained of reaction heat during of the cellulose reaction pyrolysis, for cellulose while the pyrolysis, absolute value xylan of thepyrolysis, exothermic and lignin heat of reaction was higher than that of the endothermic heat of reaction during xylan pyrolysis and pyrolysis. From Table 2, the absolute value of the endothermic heat of the reaction was higher than lignin pyrolysis. that of the exothermic heat of reaction during cellulose pyrolysis, while the absolute value of the exothermic heatTable 2.ofResults reaction for heats was of reactionhigher during than pyrolysis that of of the major endothermic components of biomass heat of by DSCreaction and TG. during xylan pyrolysis and lignin pyrolysis. ∆H ∆H Component i T (K) T (K) T (K) R,endo.,i R,exo.,i RS,i RE,i shift,i − (kJ/kg) − (kJ/kg) Cellulose 560 638 607 125.8 22.6 Table 2. Results for heats of reaction during pyrolysis of major components− of biomass by DSC and Xylan 371 602 477 56.4 245.0 − TG. Lignin 491 682 549 62.9 127.1 − Component i TRS,i (K) TRE,i (K) Tshift,i (K) −ΔHR,endo.,i (kJ/kg) −ΔHR,exo.,i (kJ/kg) 2.3. Experimental Apparatus and Procedure Cellulose 560 638 607 −125.8 22.6 XylanThe experimental 371 apparatus 602 is shown in477 Figure 3. The apparatus−56.4 consisted of anitrogen245.0 gas supply, a tubular reactor, furnace, data logger, thermocouples, cold trap for tar and water, and a Lignin 491 682 549 −62.9 127.1 wet-type gas flow meter. First, 150 g of biomass powder was placed into the reactor, of which the diameter and height were 108 mm and 230 mm, respectively. N2 gas of 0.5 standard liter per minute 2.3. Experimental(SLM) was Apparatus fed into the and reactor Procedure so as to avoid oxidation. In addition, the temperature in the packed bed and temperature at the reactor wall were measured throughout the experiment using twelve Thethermocouples, experimental and apparatus were recorded is shown in the hardin Figure disc of a3. computerThe apparatus through aconsisted data logger. of Duringanitrogen gas supply, pyrolysis,a tubular the reactor, generated furnace, moisture data and logger, tar were thermocouples, condensed thorough cold the stainless-steeltrap for tar and bend water, pipe with and a wet- type gasa flow silicon meter. tube forFirst, water 150 cooling g of biomass by the aspilator, powder and was were placed trapped into by the the reactor, egg-plant of shapedwhich flask.the diameter and heightThe were amount 108 of mm generated and 230 gas duringmm, respectively. torrefaction was N2 measured gas of 0.5 by standard the wet type liter gas per meter minute and was (SLM) was fed into recordedthe reactor by the so video as to camera. avoid The oxidation. temperature In ataddition, the reactor the wall temperature was set at 573 in K. Tablethe packed3 shows bed and the experimental conditions for the biomass packed bed. Although the particles were pulverized temperatureand sieved at the using reactor four sieves wall (#3.35mm, were #1.68 measur mm, #1.0ed mm,throughout and #0.5 mm), the theexperiment particle size diusingffered twelve thermocouples, and were recorded in the hard disc of a computer through a data logger. During pyrolysis, the generated moisture and tar were condensed thorough the stainless-steel bend pipe with a silicon tube for water cooling by the aspilator, and were trapped by the egg-plant shaped flask. The amount of generated gas during torrefaction was measured by the wet type gas meter and was recorded by the video camera. The temperature at the reactor wall was set at 573 K. Table 3 shows the experimental conditions for the biomass packed bed. Although the particles were pulverized and sieved using four sieves (#3.35mm, #1.68 mm, #1.0 mm, and #0.5 mm), the particle size differed depending on the biomass species. In our previous research [20], we investigated the effect of the particle size on gas generation, and the maximum gas generation error was 2.4% at Dp = 0.74 mm and Dp = 0.34 mm. Therefore, for these biomass samples, this work proceeded on the assumption that the

Processes 2020, 8, 959 5 of 13 depending on the biomass species. In our previous research [20], we investigated the effect of the Processesparticle 2020 size, 8, onx FOR gas PEER generation, REVIEW and the maximum gas generation error was 2.4% at Dp = 0.745 of mm 13 and Dp = 0.34 mm. Therefore, for these biomass samples, this work proceeded on the assumption particlethat the size particle dependence size dependence of the gas of thegeneration gas generation amount amount could be could small. be small.The biomass The biomass particles particles that werethat wereused were used those were thosethat had that been had dried been at dried 110 °C at 110for 12◦C h. for The 12 bulk h. The density bulk depended density depended on the type on ofthe biomass. type of biomass.So, the heights So, the of heights the packed of the packedbed HB bed were HB also were different. also different. In consideration In consideration of the of reproducibilitythe reproducibility of the of total the totalgenerated generated gas volume gas volume during during the biomass the biomass torrefaction, torrefaction, experiments experiments were conductedwere conducted twice twiceor more. or more. The Theexperimental experimental coeffici coeffientcient of the of thevalidation validation for for the the total total gas gas volume volume duringduring biomass biomass torrefaction torrefaction was was less less than than 2%. 2%.

FigureFigure 3. 3. ExperimentalExperimental apparatus apparatus for for measuring measuring heat heat and and mass mass transfer transfer during during the thetorrefaction torrefaction of biomass.of biomass. Table 3. Experimental conditions of the biomass packed bed. Table 3. Experimental conditions of the biomass packed bed. Bulk Density Height of Biomass Biomass SpeciesParticle Particle Size Size (µBulkm) Density 3 Height of Biomass Packed Bed, HB Biomass Species (kg/m ) Packed Bed, HB (mm) (µm) (kg/m3) (mm) Bamboo 196 254 85 Bamboo 196 254 85 Douglas fir 317 311 68 DouglasBark offir Douglas fir 317 222 311 122 68 170 Bark of Douglas 222 122 170 fir 2.4. Numerical Simulation 2.4. NumericalThe numerical Simulation simulation during pyrolysis was conducted in a two-dimensional cylindrical coordinate.The numerical Table4 showssimulation the governingduring pyrolysis equations was for conducted the numerical in a two-dimensional simulation. Figure cylindrical4 shows coordinate.the calculation Table domain 4 shows and the boundary governing conditions. equations for Although the numerical the cellulose simulation. of biomass Figure is4 shows composed the calculationof crystalline domain (long block)and boundary and amorphous conditions. (short Although block) alternately,the cellulose like of a biomass block co-polymer, is composed as hasof crystallinebeen reported (long by block) Kawamoto and amorphous [11], the co-polymer’s (short block) e ffalternately,ect of cellulose like a on block kinetics co-polymer, during pyrolysis as has been has reportednot been by investigated. Kawamoto In[11], this the paper, co-polymer’s Miller’s chemicaleffect of cellulose reaction modelon kinetics and kineticduring parameters pyrolysis has during not beenpyrolysis investigated. [9] were In adapted this paper, to take Miller’s into account chemical the reaction effect of model the major and componentskinetic parameters of the biomass.during pyrolysisThe pressure [9] were equation adapted was derivedto take into from account the mass the balances, effect of Ideal the major gas law, components and Darcy’s of equation.the biomass. The pressureThese governing equation wa equationss derived and from boundary the mass conditions balances, Ideal were gas discretized law, and by Darcy’s the control equation. volume method.These In governing order to stabilize equations the numericaland boundary simulation, conditions a hybrid were scheme discretized was adopted by the for control the convection volume method.terms in theIn order heat transfer to stabilize and massthe balances.numerical The simulation, temperature a hybrid and pressure scheme were was solved adopted by thefor Euler the convectionimplicit method. terms in The the material heat transfer balances and were mass solved balances. by the The fourth-order temperature Runge–Kutta and pressure method. were solved by the Euler implicit method. The material balances were solved by the fourth-order Runge–Kutta method. The dependence of all of the physical properties on temperature was described in our previous report [24]. The change of the porosity [24–26] during torrefaction was adopted. Grid sensitivity on the time course of the temperature and generated gas flow rate were investigated using (Nr, Nz) = (68, 102), (86, 128), (100, 149), and (114, 170), where Nr and Nz show the mesh number along the r

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The dependence of all of the physical properties on temperature was described in our previous report [24]. The change of the porosity [24–26] during torrefaction was adopted. Grid sensitivity on the time course of the temperature and generated gas flow rate were investigated using (Nr, Nz) = Processes 2020, 8, x FOR PEER REVIEW 6 of 13 (68, 102), (86, 128), (100, 149), and (114, 170), where Nr and Nz show the mesh number along the r component and along the z component, respectively. The maximum relative error of the growth rate component and along the z component, respectively. The maximum relative error of the growth rate distributions between (Nr, Nz) = (100, 149) and (Nr, Nz) = (114, 170) was about 1.6%. Therefore, (Nr, distributions between (Nr, Nz) = (100, 149) and (Nr, Nz) = (114, 170) was about 1.6%. Therefore, (Nr, Nz) N ) = (100, 140) was adopted for the bamboo torrefaction. As the heights of the packed beds were also z = (100, 140) was adopted for the bamboo torrefaction. As the heights of the packed beds were also different from the species of the biomass, (Nr, Nz) = (100, 119) and (Nr, Nz) = (100, 342) were adopted different from the species of the biomass, (Nr, Nz) = (100, 119) and (Nr, Nz) = (100, 342) were adopted for the Douglasfor the firDouglas torrefaction fir torrefaction and bark and torrefaction, bark torrefaction, respectively. respectively.

Table 4. ChemicalTable 4. Chemical reaction modelreaction and model governing and governing equations equati of theons numericalof the numerical simulation simulation during during the the biomass torrefaction.biomass torrefaction.

Chemical Reaction Chemical Reaction Model [9] Model [9]

n o ∂ ∂((ρC)ρ+Cε(ρ +C )ε)ρTC T    S S G G 1 ∂ 1 ∂ ∂ ∂ + r +(ρ C)GUTrρC+ UT(ρC +) GWTρC WT ∂t r ∂r G∂z G  ∂t  r ∂r 3 3   ∂z = 1 ∂ ∂T + ∂ ∂T + P P 3 (3 ) r ∂r rλ1e∂ff ∂r ∂zT λeff ∂z ∂T Rj, i ∆Hi = rλ + λ i=1 j= 1+ − R -∆H r ∂r eff ∂r ∂z eff ∂z j, i i where, i = 1 j =1 Energy balance P3   where,(ρC) = ρ C +ρ C +ρ C Energy balance S i i 3IM, i IM.i Char, i Char, i i=1 3 ρC = P ρ Ci + ρ CIM.i + ρ CChar, i (ρC) = ρ C S + ρi CIM, i +ρ Char,C i G N2 N2 i=1 Tar, i Tar, i Gas, i Gas, i i=1 3 R1, i= k1, i ρi, R2, i= k2, i ρIM, i, R3, i= k3, i ρIM,i ρC = ρ CN + ρ CTar, i + ρ CGas, i G N2 2 dρ Tar, i Gas, i Mass balance for component i i i=1dt = k1, i ρi R = k ρ ,R = k −ρ , R = k ρ 1, i 1, i i 2, i 2, i IM, i 3, i 3, i IM,i Mass balance for intermediate material dρ IM, i = k dρρ k ρ k ρ of component i Mass balance for dt 1, i ii 2, i IM, i 3, i IM, i − = -k ρ − 1, i i component i dρ dt Mass balance for char i Char, i Mass balance for = βi k3, i ρIM, i dρ dt IM, i intermediate material of ∂(ερTar, i)  = k  ρ -k  ρ -k ρ Mass balance for tar i + 1 ∂ rρ U1, i +i ∂2, iρ IM, iW3,= i kIM, i ρ = S component i ∂t r ∂r dt Tar, i ∂z Tar, i 2, i IM, i Tar, i     ∂(ερGas, i) d1ρ ∂ ∂ + Char,rρ i U + ρGas, iW = Mass ibalance for char i ∂t r ∂r Gas, = βi k3, i ρ ∂z Mass balance for gas  dt i IM, i 1 β k3, i ρ = SGas, i ∂ ερ − i IM, i Tar, i 1 ∂ ∂ Mass balance for tar i + ∂(εrPρ) U + ρ W = k ρ = S T Tar, i1 ∂ P Tar,∂ i P 2, i IM, i Tar, i ∂t r ∂r + r rU∂z + W = ∂t ∂r T ∂z T ! Pressure equation ∂ ερ 1 ∂ P3 ∂ P3 Gas, i R 1 S + 1 S Mass balance for gas i + 0rMρTar UTar, + i ρMGas W =Gas, 1-βi k3, i ρ = ∂t r ∂r Gas,i= i 1 ∂z Gas, i i=1 i IM, i S Darcy’s law Gas, i U = κ ∂P , W = κ ∂P P − µ ∂r − µ ∂z ∂ ε 1 ∂ P ∂ P T + rU + W = ∂t r ∂r T ∂z T Pressure equation 3 3 1 1 R0 STar, i + SGas, i MTar MGas i=1 i=1 κ ∂P κ ∂P Darcy’s law U = - , W = - µ ∂r µ ∂z

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FigureFigure 4. Boundary 4. Boundary conditions conditions of of the the numerical numerical simulation simulation during thethe biomassbiomass torefaction. torefaction.

3. Results and Discussion 3. Results and Discussion 3.1. Time Course of Temperature and Gas Flow Rate during Torrefaction 3.1. Time Course of Temperature and Gas Flow Rate during Torrefaction Figure5 shows the time course of the temperature at r = 38 mm and z = 30 mm in the biomass packedFigure bed 5 shows and the the generated time course gas of flow the rate temperature during the at biomass r = 38 mm torrefaction. and z = 30 The mm keys in the and biomass lines packedshow bed the and experimental the generated results gas and flow calculation rate during results, the biomass respectively. torrefaction. In Figure The5a, forkeys the and Douglas lines show fir thepowder, experimental the temperature results and at calculationr = 38 mm andresults,z = 30 respectively. mm in the bed In Figure gradually 5a, for increased the Douglas with time. fir powder, After thethat, temperature the temperature at r = 38 approached mm and z a= constant30 mm in temperature the bed gradually of about increased 573 K. The with calculation time. After temperature that, the temperatureagreed well approached with the experimental a constant temperature one. The gas of started about generating573 K. The atcalculation about 30min. temperature At t > 40 agreed min, wellthe with generated the experimental gas flow rate one. increased The gas adequately started generating and had a maximumat about at30 tmin.= 50 min.At t > At 40 this min, time, the generatedthe temperature gas flow was rateT =increased450 K, where adequately the gas generatedand had a could maximum be from at the t = thermal 50 min. decomposition At this time, ofthe temperaturethe mannan. was After T = 450 that, K, the where gas flowthe gas rate generated decreased could gradually be from with the time. thermal Although decomposition the calculation of the mannan.gas flow After rate that, of the the black gas solidflow linerate wasdecreased higher thangradually that of with the experimentaltime. Although one, the the calculation tendency ofgas flowthe rate time of coursethe black agreed solid quantitatively. line was higher Firstly, than that gas generationof the experimental could be startedone, the by tendency the hemicellulose of the time coursedecomposition agreed quantitatively. of the green line, Firstly, and had gas the generation maximum gascould generation. be started Then, by the decompositionthe hemicellulose of decompositionthe lignin and of the the cellulose green line, occurred. and had For the the maxi bamboomum powder gas generation. in Figure5 b,Then, the experimentalthe decomposition results of thefor lignin the timeand coursethe cellulose of the temperatureoccurred. For also the agree bamboo well withpowder the calculationin Figure 5b, results. the experimental The gas generation results for duringthe time the course bamboo of the powder temperature torrefaction also agree was higher well with than the that calculation during the results. Douglas The fir gas torrefaction, generation and could be started by the hemicellulose decomposition. Then, as the decomposition of the lignin and during the bamboo powder torrefaction was higher than that during the Douglas fir torrefaction, and cellulose occurred, the gas generation had a quasi-state value and decreased with time. For the bark of could be started by the hemicellulose decomposition. Then, as the decomposition of the lignin and the Douglas fir powder in Figure5c, the experimental results for the time course of the temperature cellulose occurred, the gas generation had a quasi-state value and decreased with time. For the bark also agree well with the calculation results. The maximum gas generation during the bark of the of the Douglas fir powder in Figure 5c, the experimental results for the time course of the temperature Douglas fir powder torrefaction was higher than that during the Douglas fir torrefaction, and could be also agree well with the calculation results. The maximum gas generation during the bark of the started by the lignin decomposition. Then, as the decomposition of the hemicellulose and cellulose Douglasoccurred fir powder successively, torrefaction the gas generation was higher had than a quasi-state that during value the andDouglas decreased fir torrefaction, with time. Theand totalcould be started by the lignin decomposition. Then, as the decomposition of the hemicellulose and cellulose occurred successively, the gas generation had a quasi-state value and decreased with time. The total generated gas of the bark of the Douglas fir torrefaction was lower than that of the bamboo powder torrefaction. Therefore, it was found that the time course of the temperature and gas generation

Processes 2020, 8, 959 8 of 13 Processes 2020, 8, x FOR PEER REVIEW 8 of 13 generated gas of the bark of the Douglas fir torrefaction was lower than that of the bamboo powder during the torrefaction of the biomass depended strongly on the mass percentage of the major torrefaction. Therefore, it was found that the time course of the temperature and gas generation during components. the torrefaction of the biomass depended strongly on the mass percentage of the major components. From Figure 5, the numerical calculation result of the gas generation flow rate during From Figure5, the numerical calculation result of the gas generation flow rate during torrefaction torrefactionwas higher was than higher the experimental than the experimental one for all ofone the fo biomassr all of the species. biomass Although species. the Although Miller model the Miller has model has been compared with the experimental results of several in a temperature range been compared with the experimental results of several biomasses in a temperature range of 400 ◦C of 400or higher °C or higher [9], there [9], are there no dataare no using data Miller’s using Miller’s reaction reaction model in model the low in the temperature low temperature range of range 300 of ◦300C. It°C. is alsoIt is necessaryalso necessary to study to study the heat the transfer heat transfer and gas and generation gas generation behavior behavior of cellulose, of cellulose, lignin, lignin,and hemicellulose,and hemicellulose, which arewhich the major are componentsthe major components of biomass at aroundof biomass 300 ◦ C.at Furthermore, around 300 it is°C. Furthermore,necessary to it reexamine is necessary the to pyrolysis reexamine model the with pyrolysis reference model to Di with Blasi’s reference experiment to Di [ 29Blasi’s], regarding experiment the [29],reaction regarding rate constantsthe reactionk2 and ratek 3constantsof the second k2 and step k3 of in the Millersecond model step in [9 ].the Miller model [9].

Figure 5. Time course of the temperature and generated gas flow rate during the biomass torrefaction. Figure 5. Time course of the temperature and generated gas flow rate during the biomass 3.2. Heat and Mass Transfer during Torrefactiontorrefaction. of Biomass Figure6 shows the calculation results for the special profile of the temperature, gas flow velocity 3.2. Heat and Mass Transfer during Torrefaction of Biomass vector (left), and the solid density (SD, right) at different reaction times. The rectangular blank at the top centerFigure in the6 shows right figurethe calculation represents results a stainless-steel for the special pipe. Theprofile temperature of the temperature, of the bed rose gas fromflow thevelocity top vectorand (left), left, which and the means solid the density side wall, (SD, with right) the torrefactionat different time.reaction The times. zone for The temperatures rectangular higher blank thanat the top540 center K started in the prevailing right figure (volume represents faction a= stainle0.83) atss-steel 180 min. pipe. Together The temperature with the high of temperaturethe bed rose zone from thepropagation, top and left, the which local SDmeans of the the bed side became wall, smallerwith the with torrefaction a similar profile. time. The Surprisingly, zone for thetemperatures decrease higherin SD than propagation 540 K started was not prevailing as significant (volume as the faction temperature = 0.83) propagation at 180 min. in theTogether bed. In with other the words, high temperaturethe decrease zone in SD propagation, propagation showedthe local a certainSD of timethe bed delay became of 60 to smaller 90 min inwith comparison a similar with profile. the Surprisingly,temperature the profile decrease propagation. in SD propagation This may bewas related not as to significant the sweep as gas the flow temperature profile, time propagation required in thefor completingbed. In other the words, reaction, the or decrease the diff erencein SD betweenpropagation primary showed decomposition a certain time (k1) delay and secondary of 60 to 90 mindecomposition in comparison (k2 withand k the3) in temperature the bed. In order profile to clarify propagation. this point This of “what may causedbe related this to delay?”, the sweep further gas investigation is required. At t = 30 min, the heat transfer due to the thermal conduction occurred from flow profile, time required for completing the reaction, or the difference between primary the reactor wall, and at the top surface of the backed bed, the temperature at the center and bottom decomposition (k1) and secondary decomposition (k2 and k3) in the bed. In order to clarify this point region was less than 400 K. No torrefaction occurred at 30 min. At t = 60 min, as the temperature at the of “what caused this delay?”, further investigation is required. At t = 30 min, the heat transfer due to wall and top surface of the packed bed was higher than 500 K, decomposition started from the corner the thermal conduction occurred from the reactor wall, and at the top surface of the backed bed, the temperature at the center and bottom region was less than 400 K. No torrefaction occurred at 30 min. At t = 60 min, as the temperature at the wall and top surface of the packed bed was higher than 500 K, decomposition started from the corner of the top surface. With the passing of time, the temperature near the region of the packed bed was higher than 500 K, the decomposition was propagated. At t =

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Processes 2020, 8, x FOR PEER REVIEW 9 of 13 of the top surface. With the passing of time, the temperature near the region of the packed bed was 180 min,higher the than Douglas 500 K, fir the packed decomposition bed at wasthe region propagated. of the At topt = and180 min,near thethe Douglas wall decomposed fir packed bed to atabout 70% theof the region initial of the packed top and bed, near while the wall the decomposed Douglas fi tor aboutpacked 70% bed of theat the initial other packed region bed, could while thenot be pyrolyzed.Douglas fir packed bed at the other region could not be pyrolyzed.

Figure 6. Calculation results for the time course of the temperature, velocity vectors, and total solid Figure 6. Calculation results for the time course of the temperature,3 velocity vectors, and total solid distribution during Douglas fir powder torrefaction. ρinit = 353 kg/m . distribution during Douglas fir powder torrefaction. ρinit = 353 kg/m3. Figure7 shows the calculation results for the heat and mass transfer at 75 min during various biomassFigure 7 torrefactions. shows the calculation As shown in results Figure 6forb forthe the he bambooat and mass powder transfer torrefaction, at 75 min the temperature during various biomassin the torrefactions. packed bed wasAs shown higher in than Figure that for6b fo ther the Douglas bamboo fir torrefaction, powder torrefaction, due to the the bulk temperature density. in theFurthermore, packed bed as was thehighest higher mass than percentage that for the ofthe Douglas major componentfir torrefaction, in the bamboodue to wasthe xylan,bulk density. the Furthermore,decomposition as the rate highest in the bamboomass percentage packed bed of was the higher major than component that in the in Douglas the bamboo fir packed was bed. xylan, As the decompositionshown in Figure rate 6inc for the the bamboo bark of thepacked Douglas bed fir was powder high torrefaction,er than that the in temperaturethe Douglas in fir the packed packed bed. bed was also higher than that for the Douglas fir torrefaction, because of the bulk density. However, As shown in Figure 6c for the bark of the Douglas fir powder torrefaction, the temperature in the as the most greatest percentage of the major component in the bark of Douglas fir was lignin, the packed bed was also higher than that for the Douglas fir torrefaction, because of the bulk density. decomposition rate was higher than that in the Douglas fir packed bed. The volume faction of the bed However,at temperatures as the most higher greatest than 500 percentage K at 75 min of was the 0.89, major 0.85, component and 0.99 for in bamboo, the bark and of Douglas Douglas fir andfir was lignin,its the bark, decomposition respectively. It rate was was found higher that the than effective that in thermal the Douglas conductivity fir packed was the bed. determining The volume factor faction of thefor bed this at di fftemperatureserence, because higher the eff ectivethan 500 thermal K at conductivity 75 min was of 0.89, the bed 0.85, at temperatures and 0.99 for higher bamboo, than and Douglas540 Kfir at and 75 min its wasbark, 0.0254, respectively. 0.0252, and It was 0.0303 foun W/d(m that2 K) the for bamboo,effective and thermal Douglas conductivity fir and its bark, was the determiningrespectively. factor Overall, for this the bamboodifference, torrefaction because was the the effective highest in thermal this study. conductivity of the bed at temperatures higher than 540 K at 75 min was 0.0254, 0.0252, and 0.0303 W/(m2 K) for bamboo, and Douglas fir and its bark, respectively. Overall, the bamboo torrefaction was the highest in this study.

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FigureFigure 7.7. Calculation resultsresults forfor temperature,temperature, velocityvelocity vectorsvectors andand solidsolid distributionsdistributions atat 7575 minmin duringduring torrefactiontorrefaction forfor variousvarious kinds kinds of of biomass. biomass. 4. Conclusions 4. Conclusions In this study, the torrefaction of three representative types of biomass, namely, bamboo, In this study, the torrefaction of three representative types of biomass, namely, bamboo, and and Douglas fir and its bark, was carried out experimentally and numerically, paying attention to the Douglas fir and its bark, was carried out experimentally and numerically, paying attention to the concentrations of the major constituents of cellulose, hemicellulose, and lignin in a cylindrical-shaped concentrations of the major constituents of cellulose, hemicellulose, and lignin in a cylindrical-shaped packed bed reactor under a nitrogen flow at 573 K of the reactor wall temperature. The following packed bed reactor under a nitrogen flow at 573 K of the reactor wall temperature. The following conclusions were obtained. conclusions were obtained. (1)(1) From DSC experiments, the cellulose pyrolysispyrolysis waswas progressedprogressed mainlymainly byby endothermicendothermic reactionreaction (s)(s) while xylan pyrolysis and lignin pyrolysis were proceeded mainly by exothermic reaction (s). (2)(2) Hemicellulose decomposition could be occurred firstlyfirstly duringduring DouglasDouglas firfir torrefactiontorrefaction andand bamboo torrefaction. And And then then lignin lignin and and cell celluloseulose decomposition decomposition would would be be occurred. occurred. So, So, a aquasi-state quasi-state gas gas flow flow rate rate could could be be observed. observed. On On the the other other hand, hand, bark bark of Douglas firfir torrefactiontorrefaction depends strongly on the ligninlignin decomposition.decomposition. (3)(3) The time course of the temperature inin thethe packedpacked bedbed agreedagreed wellwell withwith thatthat ofof thethe calculationcalculation byby taking into accountaccount thethe heatheat ofof reactionreaction forfor notnot only the Douglas fir,fir, butbut alsoalso forfor bamboobamboo andand thethe bark of DouglasDouglas fir.fir. On thethe otherother hand,hand, thethe numericalnumerical calculationcalculation resultresult ofof thethe gasgas generationgeneration flowflow rate during torrefaction waswas higherhigher thanthan thethe experimental oneone forfor allall of the biomass species, because therethere areare no no data data using using Miller’s Miller’s reaction reaction model model [9 ][9] in in the the low low temperature temperature range range of 300 of 300◦C. It°C. is It also is also necessary necessary to studyto study the the heat heat transfer transfer and and gas gas generation generation behavior behavior of of cellulose,cellulose, lignin,lignin, and hemicellulose, which are the major components of biomass atat aroundaround 300300 ◦°C.C. Furthermore, it isis necessarynecessary toto reexaminereexamine thethe pyrolysispyrolysis modelmodel withwith referencereference toto DiDi Blasi’sBlasi’s experimentexperiment [[29]29] regarding thethe reactionreaction raterate constantsconstants kk22 and k3 of the second step in the Miller model [9]. [9]. (4)(4) The zonezone at at temperatures temperatures higher higher than than 540 K540 prevailed K prevailed (volume (volume faction faction= 0.83) at= 1800.83) min. at Together180 min. withTogether the highwith temperaturethe high temperature zone propagation, zone propag theation, local the SD local of the SD bed of the became bed became smaller smaller with a similarwith a profile.similar profile. Surprisingly, Surprisingly, the decreased the decreased solid density solid (SD) density propagation (SD) propagation was not as was significant not as assignificant the temperature as the propagationtemperature in propagation the bed. In other in the words, bed. the In SDother decrease words, propagation the SD decrease showed apropagation certain time showed delay ofa certain 60 to 90 time min delay in comparison of 60 to 90 with min thein comparison temperature with profile the propagation.temperature Thisprofile may propagation. be related toThis the may sweep be gas related flow profile,to the sweep time required gas flow for profile, completing time therequired reaction, for orcompleting the difference the reaction, between or primary the difference decomposition between (k 1primary) and secondary decomposition decomposition (k1) and (secondaryk2 and k3) decomposition (k2 and k3) in the bed. In order to clarify this point of “what caused this delay?”, further investigation is required. As the temperature at the wall and top surface of the packed

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in the bed. In order to clarify this point of “what caused this delay?”, further investigation is required. As the temperature at the wall and top surface of the packed bed was higher than 500 K, the decomposition started from the corner of the top surface. With the elapsed time, the temperature near the region of the packed bed was higher than 500 K, and the decomposition was propagated. At t = 180 min, the Douglas fir packed bed at the region of the top and near the wall decomposed to about 70% of the initial packed bed, while the Douglas fir packed bed at the other region could not be pyrolyzed. (5) For the bamboo powder torrefaction, the temperature in the packed bed was higher than that for the Douglas fir torrefaction because of the bulk density. Furthermore, as the greatest mass percentage of the major component in the bamboo was xylan, the decomposition rate in the bamboo packed bed was higher than that in the Douglas fir packed bed. As the greatest mass percentage of the major component in the bark of Douglas fir was lignin, the decomposition rate was higher than that in the Douglas fir packed bed. 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 was the determining factor for this difference, because the effective thermal conductivity of the bed at temperatures higher than 540 K at 75 min was 0.0254, 0.0252, and 0.0303 W/(m2 K) for bamboo, and Douglas fir and its bark, respectively. Overall, the bamboo torrefaction was the highest in this study.

Author Contributions: Conceptualization, K.-i.T. and K.H.; methodology, K.-i.T. and Y.H.; software, K.-i.T.; validation, A.Y. and Y.U.; formal analysis, K.-i.T., K.H., and A.H.; investigation, K.H., Y.H., A.Y., and A.H.; resources, A.H.; data curation, K.H., Y.H., A.Y., and A.H.; writing (original draft preparation), K.-i.T.; writing (review and editing), K.-i.T. and Y.U.; visualization, A.Y.; supervision, T.N. and Y.U.; project administration, K.-i.T.; funding acquisition, T.N. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported, in part, by a Grant-in Aid for Scientific Research C (no. 17K06196) from the Japan Society for the Promotion of Science. Acknowledgments: Special thanks to Toshihide Irii and Hamza Bin Rahim for preparing the experimental equipment. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

C Heat capacity (J/(kg K)) e Emissivity HB Height of biomass packed bed (m) h Heat transfer coefficient between biomass and gas (W/(m2K)) ∆HR,endo.,i Endothermic heat of reaction during pyrolysis of biomass major component i (J/kg) ∆HR,exo.,i Exothermic heat of reaction during pyrolysis of biomass major component i (J/kg) i Major component i of biomass (i = cellulose, hemicellulose, and lignin) k1,i, k2,i, k3,i Reaction rate constant in Miller’s chemical reaction model for component i [9] (1/s) MTar Molecular weight of tar (= 0.11 kg/mol [22]) MGas Molecular weight of gas (= 0.38 kg/mol [22]) m0 Input mass of TG and DSC experiments (kg) N Maximum grid number P Pressure (Pa) Heat flow of the base line from the endothermic chemical reaction to the exothermic q base chemical reaction (J/s) qDSC Heat flow in DSC curve (J/s) 3 Ri Reaction rates in Miller’s chemical reaction model for component i [9] (kg/(m s)) R0 Universal gas constant (= 8.314 J/(mol K)) (J/(mol K)) r r coordinate in the packed bed (mm) 3 STar Reaction rate of tar for component i (kg/(m s)) 3 SGas Reaction rate of gas for component i (kg/(m s)) Processes 2020, 8, 959 12 of 13

T Temperature (K) TRS,i Temperature at reaction start time in TG curve for component i (K) TRE,i Temperature at reaction end time in TG curve for component i (K) Temperature at the shifted time from the endothermic chemical reaction to the T shift,i exothermic chemical reaction in TG curve for component i (K) tRS,i Reaction start time in TG curve for component i (s) tRE,i Reaction end time in TG curve for component i (s) Shifted time from the endothermic chemical reaction to the exothermic chemical t shift,i reaction in the TG curve for component i (s) U Volume averaged Darcy’s velocity along the r-axis in the packed bed (m/s) W Volume averaged Darcy’s velocity along the z-axis in the packed bed (m/s) z z coordinate in the packed bed (mm) Greek symbol βi kinetic parameter in Miller’s chemical reaction model for component i [9] ε Porosity in the packed bed κ Permeability in the packed bed (m2) λ Thermal conductivity (W/(mK)) µ Viscosity (Pa s) ρ Density (kg/m3) σ Stefan–Boltzman constant (= 5.669 10 8 W/m2K4) (W/m2K4) × − Subscript a Atmosphere Char Char eff Effective Gas Gas im Intermediate material init Initial value N2 Nitrogen r r component s Solid Tar Tar v Volatile wall Wall of the stainless-steel tube z z component Environmental condition on the wall of the stainless-steel tube ∞

References

1. Hisam, S.M.; Uemura, Y.; Tazli, A.M. Effects of Temperature and Concentration of Oxygen on Torrefaction of Empty Fruit Bunches. J. Jpn. Inst. Energy 2016, 95, 1110–1114. 2. Yoshida, T.; Kubojima, Y.; Kamikawa, D.; Kiguchi, M.; Tanaka, K.; Miyago, M.; Masui, M.; Ohyabu, Y.; Kobayashi, A.; Igarashi, H. Production Test of Torrefied Woody Biomass Solid Fuel in an Original Small-scale Plant. J. Jpn. Inst. Energy 2018, 97, 231–235. [CrossRef] 3. Chen, W.H.; Chen, J.; Peng, J.; Bi, X.T. A state-of-the-art review of biomass torrefaction, densification and applications. Renew. Sustain. Energy Rev. 2015, 44, 847–866. [CrossRef] 4. Ribeiro, J.M.C.; Godina, R.; Matias, J.C.D.O.; Nunes, L.J.R. Future perspectives of biomass torrefaction: Review of the current state-of-the-art and research development. Sustainability 2018, 10, 2323. [CrossRef] 5. Rodrigues, A.; Loureiro, L.; Nunes, L.J.R. Torrefaction of woody biomasses from poplar SRC and Portuguese roundwood: Properties of torrefied products. Biomass Bioenergy 2018, 108, 55–65. [CrossRef] 6. Nunes, L.J.; Matias, J.C. Biomass Torrefaction as a Key Driver for the Sustainable Development and Decarbonization of Energy Production. Sustainability 2020, 12, 922. [CrossRef] 7. Loureiro, L.M.; Nunes, L.J.; Rodrigues, A.M. Woody biomass torrefaction: Fundamentals and potential for Portugal. Silva Lusit. 2017, 25, 35–63. Processes 2020, 8, 959 13 of 13

8. Filipe dos Santos Viana, H.; Martins Rodrigues, A.; Godina, R.; Carlos de Oliveira Matias, J.; Jorge Ribeiro Nunes, L. Evaluation of the physical, chemical and thermal properties of Portuguese maritime pine biomass. Sustainability 2018, 10, 2877. [CrossRef] 9. Miller, R.S.; Bellan, J. A Generalized Biomass Pyrolysis Model Based on Superimposed Cellulose, Hemicellulose and Liqnin Kinetics. J. Comb. Sci. Tech. 1997, 126, 97–137. [CrossRef] 10. Di Blasi, C. Numerical simulation of cellulose pyrolysis. Biomass Bioenergy 1994, 7, 87–98. [CrossRef] 11. Kawamoto, H. Reactions and molecular mechanics of cellulose pyrolysis. Mokuzai Gakkaishi 2015, 61-1, 1–24. [CrossRef] 12. Mok, W.S.L.; Antal, M.J., Jr. Effects of pressure on biomass pyrolysis. II. Heats of reaction of cellulose pyrolysis. Themochimica Acta 1983, 68, 165–186. [CrossRef] 13. Milosavljevic, I.; Oja, V.; Suuberg, E.M. Thermal Effects in Cellulose Pyrolysis: Relationship to Char Formation Processes. Ind. Eng. Chem. Res. 1996, 35, 653–662. [CrossRef] 14. Ratha, J.; Wolfingera, M.G.; Steinera, G.; Krammera, G.; Barontinib, F.; Cozzanib, V. Heat of pyrolysis. Fuel 2003, 82, 81–91. [CrossRef] 15. Gomez, C.; Velo, E.; Barontini, F.; Cozzani, V. Influence of Secondary Reactions on the Heat of Pyrolysis of Biomass. Ind. Eng. Chem. Res. 2009, 48, 10222–10233. [CrossRef] 16. Joungmo, C.; Jeffrey, M.D.; George, W.H. The Intrinsic Kinetics and Heats of Reactions for Cellulose Pyrolysis and Char Formation. ChemSusChem 2010, 3, 1162–1165. 17. Richard, B.B.; Ahmed, F.G. Biomass torrefaction: Modeling of reaction thermochemistry. Bioresour. Technol. 2013, 134, 331–340. 18. Yang, H.; Yan, R.; Chen, H.; Lee, D.H.; Zheng, C. Characteristics of hemicellulose, cellulose and lignin pyrolysis. Fuel 2007, 86, 1781–1788. [CrossRef] 19. Di Blasi, C. Modeling chemical and physical processes of wood and biomass pyrolysis. Prog. Ener. Comb. Sci. 2008, 34, 47–90. [CrossRef] 20. Pyle, D.L.; Zaror, C.A. Heat transfer and kinetics in the low temperature pyrolysis of solids. Chem. Eng. Sci. 1984, 39-1, 147–158. [CrossRef] 21. Koufopanos, C.A.; Papayannakos, N. Modelling of the Pyrolysis of Biomass Particles. Studies on Kinetics, Thermal and Heat Transfer Effects. Can. J. Chem. Eng. 1991, 69, 908–915. [CrossRef] 22. Won, C.P.; Arvind, A.; Howard, R.B. Experimental and theoretical investigation of heat and mass transfer processes during wood pyrolysis. Combust. Flame 2010, 157, 481–494. 23. Okekunle, P.O.; Pattanotai, T.; Watanabe, H.; Okazaki, K. Numerical and Experimental Investigation of Intra-Particle Heat Transfer and Tar Decomposition during Pyrolysis of Wood Biomass. J. Therm. Sci. Technol. 2011, 6, 360–375. [CrossRef] 24. Tanoue, K.; Suetomi, T.; Nishimura, T.; Taniguchi, M.; Sasauchi, K. Thermal conduction and gas generation undergoing pyrolysis in the packed bed of woody biomass. J. Jpn. Inst. Energ. 2012, 91, 976–984. (In Japanese) [CrossRef] 25. Tanoue, K.; Suetomi, T.; Uemura, Y.; Nishimura, T.; Taniguchi, M.; Sasauchi, K. Effect of Tar Decomposition On Gas Generation During Pyrolysis in Packed Bed of Woody Biomass. Int. J. Biomass Renew. 2013, 2, 1–6. 26. Tanoue, K.; Hamaoka, Y.; Nishimura, T.; Taniguchi, M.; Sasauchi, K. Influence of Volume Shrinkage and Water Evaporation on Heat Transfer and Chemical Reactions During the Pyrolysis of A Cellulose-Powder-Packed Bed. Int. J. Biomass Renew. 2018, 7, 1–16. 27. Zhang, S.; Su, Y.; Ding, K.; Zhu, S.; Zhang, H.; Liu, X.; Xiong, Y. Effect of inorganic species on torrefaction process and product properties of rice husk. Bioresour. Technol. 2018, 265, 450–455. [CrossRef] 28. Sluiter, A.; Hames, B.; Scarlata, R.C.; Sluiter, J.; Templeton, D.; Crocker, D. Determination of structural carbohydrates and lignin in biomass. Lab. Anal. Proced. 2008, 1617, 1–16. 29. Di Blasi, C.; Lanzetta, M. Intrinsic kinetics of isothermal xylan degradation in inert atmosphere. J. Anal. Appl. Pyrolysis 1997, 40, 287–303. [CrossRef]

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