A comprehensive study of sawdust torrefaction in a dual-compartment slot-rectangular spouted bed reactor

Energy xxx (xxxx) xxx

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A comprehensive study of sawdust torrefaction in a dualcompartment slot-rectangular spouted bed reactor Ziliang Wang*, C. Jim Lim, John R. Grace Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, V6T 1Z4, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 February 2019 Received in revised form 19 July 2019 Accepted 7 October 2019 Available online xxx

A dual-compartment slot-rectangular spouted bed (DSRSB) reactor was designed and built to torrefy sawdust. The effects on torrefaction performance of temperature from 240 to 300
C, sawdust feedrate from 600 to 1400 g/h and oxygen concentration from 0 to 9 vol% were explored. The temperature difference between the adjacent compartments was limited. Pressure drop across the reactor was affected by biomass feedrate, and was higher for the downstream compartment than for the upstream one. The weight loss of the sawdust was 9.3e38.2 wt%, while the energy yield was 65.7e99.6%. Oxidative torrefaction resulted in greater weight loss and lower energy yield than non-oxidative torrefaction. The product properties were significantly affected by the reactor temperature and oxygen concentration. Solids, liquid and gaseous products were characterized. The results showed that the DSRSB is a promising reactor for biomass torrefaction. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Torrefaction Dual-compartment slot-rectangular spouted bed Temperature Sawdust Oxygen Reactor design

1. Introduction Biomass, an important sustainable and renewable resource, can partially replace fossil fuels to reduce greenhouse gas and fine particulate emissions [1]. Torrefaction, a thermal pre-treatment of biomass, can enhance chemical and physical properties of biomass [2,3]. Torrefaction is usually carried out at 200e300
C in the absence of oxygen at atmospheric pressure. The fibrous and tenacious biomass is broken down. Thermal degradation of biomass is characterized by deoxygenation, mainly from decomposition of hydroxyl, carbonyl and carboxyl groups [4,5]. Some studies replace some nitrogen by oxygen to torrefy biomass, referred to as oxidative torrefaction [6e9]. Since oxygencontaining gas is much easier to acquire (e.g. from flue gas), oxidative torrefaction is potentially attractive. Most studies recommend that the oxygen concentration be < 15 vol% [6e8,10]. The oxygen concentration in the flue gas generally varies from 6 to 14 vol% [11]. Torrefied biomass obtained with high oxygen concentration has lower solid yield and energy yield, reducing the torrefaction efficiency and the extent of densification [12]. In an

* Corresponding author. Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, Canada, V6T 1Z3. E-mail address: [email protected] (Z. Wang).

oxygen-containing environment, torrefaction reaction mechanisms include not only dehydration, decomposition and depolymerisation, but also oxidation reactions, providing heat needed for thermal degradation of the biomass, thereby reducing the heat demand [1,13]. Torrefaction primarily produces solid product, but also condensable liquid and gaseous by-products [14,15]. Thermal degradation is affected by temperature and reaction time. As the temperature increases and/or the reaction time is extended, more biomass is lost [2], while more liquid and gaseous products are generated [14]. The torrefied solid product has higher calorific value, lower moisture content, more hydrophobicity, less oxygen and more uniform properties than its parent feedstock. The condensable liquid product is composed of water, low-molecularweight aromatics and tar [16], whereas the gaseous product contains mostly CO, CO2, H2, CH4. Previous studies [7,10,12,17e25] show that torrefaction reactors can be categorized into two classifications: indirectly and directly heated reactors, on the basis of how heat is transferred to the biomass particles. Indirectly heated reactors include screwconveyors, rotary kilns, and rotary drums, while directly heated reactors are primarily fixed beds, moving beds, fluidized beds, and microwave reactors. Indirectly heated reactors are easier to operate and maintain, but may experience non-uniform heating. Moreover,

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directly heated reactors often have higher heat transfer than indirectly heated ones [17,18,26]. Hence, directly heated reactors tend to have better energy efficiency. It is critical that torrefaction be conducted with well-mixed particles and a uniform temperature distribution. Under these circumstances, the torrefied solid product has uniform properties, ensuring consistent quality. However, uniformity is often difficult to achieve in indirectly heated reactors. Spouted beds have excellent gas-solids heat and mass transfer, and vigorous cyclic particle [27]. Slot-rectangular spouted beds address the scale-up challenge by extending the width and/or length [28]. When applied to biomass torrefaction, the torrefied solid product is comparable with that produced by other directly-heated reactors [6,29,30]. Biomass particle size, biomass feedrate and reactor temperature are important parameters influencing the extent of torrefaction in slotrectangular spouted beds [29,30]. Previous results have shown that a single-compartment slot-rectangular spouted bed can torrefy sawdust, with a weight loss of 8e39 wt% and an energy yield of 61e99%. Oxidative torrefaction has also been conducted in a slotrectangular spouted bed [6]. Results indicated that the addition of oxygen can lead to a more uniform temperature distribution in the reactor, more mass loss of sawdust and retention of less energy in the solid product. Multiple-compartment slot-rectangular spouted beds facilitate scale-up of spouted beds, increasing their potential for industrial application [31]. Hydrodynamics of the solids and gas and the stability of a DSRSB can be significantly improved when a suspended partition is employed, and an independent windbox is provided for each compartment [32e35]. Solids mixing between adjacent compartments can approach a Lacey mixing index of 0.95, when the particle size >1 mm [36]. As a directly heated reactor with excellent solids mixing and stable hydrodynamic characterization, DSRSB is a promising alternative to overcome scale-up challenges associated with conventional spouted beds. However, dual-compartment beds have not been previously applied to torrefaction. In the present work, a torrefaction facility featuring a new dual-compartment slot-rectangular spouted bed (DSRSB) reactor was designed, built and operated as a biomass torrefier to evaluate and optimize its performance. The produced gas was partially recycled by a customized blower to reduce the operating cost. The effects of temperature, biomass feedrate and oxygen content on reactor performance, solid, gas and condensable liquid products and their properties were investigated. The results were compared to biomass torrefaction in a single-compartment slot-rectangular spouted bed (SRSB) and in other types of reactor. 2. Material and methods 2.1. Experimental set-up and operating conditions 2.1.1. Dual-compartment slot-rectangular spouted bed torrefaction reactor The carbon steel dual-compartment slot-rectangular spouted bed reactor, designed based on previous hydrodynamic and mixing studies [32,36], is shown in Fig. 1(a). The column dimensions were 1000  300  100 mm (height  width  thickness), with a W-type base of 60
inclination angles. A 6.4 mm thick partition, 100  100 mm in width  height, was suspended rigidly and centrally in the freeboard, with its bottom 320 mm above the base to stabilize the spouting [32,36]. Two identical slots of 4  30 mm (width  length), as shown in Fig. 1(b), were installed, one in each compartment, each connected to an independent windbox of dimensions 300  150  100 mm in height  width  thickness. Each compartment contained five quartz windows of 50.8 mm diameter

Fig. 1. (a) Schematic of dual-compartment slot-rectangular spouted bed reactor, (b) plan view of base. (T1-T7: K-type thermocouples, P: Pressure gauge with a range of 0e103.4 kPa, DP: differential pressure transducer with a range of 0e6.9 kPa)

for observation of particle motion in the reactor. Two thermocouples measured gas temperatures in the two independent windboxes, and five thermocouples were immersed in the spouted bed at Z ¼ 64, 165 and 267 mm above the top of the slot inserted 20 mm from the inside wall of the column. A thermocouple of Z ¼ 267 mm was present in the freeboard region. A feed port of diameter 25.4 mm was located on the right (upstream) side of the reactor, 180 mm above the base. Hereafter, the left and right compartments are designated the downstream and upstream compartments respectively. The pressure drop (DP) across the reactor was measured by two differential pressure transducers, with one end installed immediately above the distributor, as shown in Fig. 1(a).

2.1.2. Torrefaction facility and operating procedure The torrefaction facility, shown schematically in Fig. 2, includes a preheater, dual-compartment slot-rectangular spouted bed

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Fig. 2. Schematic diagram of dual-compartment slot-rectangular spouted bed torrefaction facility.

torrefaction reactor, cyclone, screw-feeder, filter, water cooling system, blower and after-burner. The experimental operating procedures for biomass torrefaction in the dual-compartment slotrectangular spouted bed reactor were the same as for the singlecompartment slot-rectangular spouted bed reactor utilized in previous torrefaction work [30]. However, 8.5e13.6 m3/h of the offgas was recycled to reduce the operating cost. The recycling blower was modified from an air compressor (Model: 5Z28B-2, SPEEDAIRE) by adding a frequency inverter (SMVector Series, Lenze/AC Tech, USA) to control the motor speed. Each torrefaction experiment began after pre-heating the column with air to the required temperature, requiring ~3 h. The reactor would take 30e90 min to cool down from the predetermined reaction temperature to 200
C. With respect of oxygen concentration in the carrier gas, a flue gas analyzer (PS-200, HORIBA) was connected to a gas sampling port #1 to ascertain the oxygen concentration prior to feeding biomass. Torrefied sawdust particles were carried over by the spouting gas and captured by the cyclone. Solid samples were collected immediately below the cyclone. Condensable liquid samples were collected from the liquid sampling port, whereas gas samples were collected from gas sampling port #2. 2.1.3. Operating conditions Table 1 summarizes the operating conditions for biomass torrefaction in the DSRSB facility. TD refers to torrefaction in the DSRSB reactor, while OTD refers to oxidative torrefaction in the same reactor. Runs TD1 to TD9, TD11 and OTD1-OTD5 were

repeated. Torrefaction temperatures (T) were nominally 240, 270 and 300
C, the same as those for the single bed SRSB facility [29,30]. Biomass feed rates (F) were nominally 600, 900 and 1400 g/ h. The oxygen concentration (CO2) in the feed-gas varied from 0 to 9 vol%, as in previous work in a single bed SRSB facility [6]. Temperature, biomass feedrate and oxygen concentration variations were less than 4% in repeated experiments. Cases TD1-TD9 and OTD1-OTD7 were conducted for 50 min. Runs TD10 to TD12 were longer, each lasting 120 min. As indicated in Table 1, some oxidative-torrefaction experiments were not conducted for safety reasons. Ums the minimum spouting velocity for the inert particles was measured at elevated temperatures with air prior to feeding biomass into the DSRSB reactor. Ums was 0.37, 0.39 and 0.41 m/s for 240, 270 and 300
C, respectively. The superficial gas velocity was set at 1.2Ums ensuring that the torrefaction experiments could last for at least 2 h. A preliminary test at 1.1Ums had shown excessive biomass accumulation in the reactor, whereas pre-tests at 1.3 and 1.4 Ums led to excessive entrainment. 2.2. Experimental materials Table 2 gives key properties of the experimental biomass, a spruce-pine-fir (SPF) sawdust from Tolko Industries Ltd., Vernon, BC, Canada. Based on results from a previous study [30], the 0.5e1.0 mm sieve size fraction of sawdust particles was selected for this study. This sawdust was pre-dried for 24 h prior to each experiment. The inert particles were glass beads of 1.0 mm Sauter mean diameter and 2530 kg/m3 density. As these glass beads were

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Table 1 Operating conditions for biomass torrefaction experiments in DSRSB reactor. Case

Nominal operating conditions

Actual operating conditions

T, F, CO2, t

T (
C)

F (g/h)

CO2 (vol%)

t (min)

600 g/h, 0 vol% O2, 50 min 600 g/h, 0 vol% O2, 50 min 600 g/h, 0 vol% O2, 50 min 900 g/h, 0 vol% O2, 50 min 900 g/h, 0 vol% O2, 50 min 900 g/h, 0 vol% O2, 50 min 1400 g/h, 0 vol% O2, 50 min 1400 g/h, 0 vol% O2, 50 min 1400 g/h, 0 vol% O2, 50 min 900 g/h, 0 vol% O2, 120 min 900 g/h, 0 vol% O2, 120 min 900 g/h, 0 vol% O2, 120 min

255 ± 4 278 ± 4 304 ± 3 242 ± 4 271 ± 3 301 ± 3 244 ± 2 269 ± 2 303 ± 3 249 271 ± 2 300

668 ± 30 610 ± 10 613 ± 12 930 ± 11 916 ± 10 957 ± 10 1408 ± 50 1334 ± 32 1408 ± 12 1091 907 ± 10 944

0 0 0 0 0 0 0 0 0 0 0 0

50 50 50 50 50 50 50 50 50 120 120 120

900 g/h, 900 g/h, 900 g/h, 900 g/h, 900 g/h, 900 g/h, 900 g/h, 900 g/h, 900 g/h,

241 ± 3 908 ± 15 243 ± 3 929 ± 12 250 ± 2 1046 ± 40 273 ± 2 920 ± 20 282 ± 4 946 ± 16 Not done for safety reasons 304 917 Not done for safety reasons Not done for safety reasons

3.3 ± 0.2 6.2 ± 0.3 9.0 ± 0.2 3.6 ± 0.2 6.4 ± 0.1

50 50 50 50 50

3.3

50

Non-oxidative torrefaction TD1 240
C, TD2 270
C, TD3 300
C, TD4 240
C, TD5 270
C, TD6 300
C, TD7 240
C, TD8 270
C, TD9 300
C, TD10 240
C, TD11 270
C, TD12 300
C, Oxidative torrefaction OTD1 240
C, OTD2 240
C, OTD3 240
C, OTD4 270
C, OTD5 270
C, OTD6 270
C, OTD7 300
C, OTD8 300
C, OTD9 300
C,

3 vol% 6 vol% 9 vol% 3 vol% 6 vol% 9 vol% 3 vol% 6 vol% 9 vol%

O2, O2, O2, O2, O2, O2, O2, O2, O2,

50 min 50 min 50 min 50 min 50 min 50 min 50 min 50 min 50 min

±value: Value of standard deviation.

Table 2 Key properties of raw SPF sawdust. Parameter (Units)

Value

Size range (mm) Sauter mean diameter, dsv (mm) Bulk density, rb (kg/m3) Moisture content, wet basis (wt.%) Proximate analysis a Volatiles (wt.%) Fixed carbon (wt.%) Ash content (wt.%) Fiber analysis a Hemicellulose (wt.%) Cellulose (wt.%) Lignin (wt.%) Extractives (wt.%) Elemental analysis a C (wt.%) H (wt.%) Ob (wt.%) N (wt.%) HHV (MJ/kg)

0.5e1.0 0.86 137.6 6.2

a b

84.55 15.06 0.39 14.6 49.0 27.6 8.4 46.2 6.4 46.9 <0.1 18.23

Dry basis. O ¼ 100-C-H-N-Ash.

spherical with high hardness, attrition was negligible. 2.3. Product characterization 2.3.1. Solid product Proximate analyses of the raw and torrefied sawdust were determined by a TGA (Shimadzu, TGA50) and a programmable furnace, following procedures outlined previously [30]. The volatile matter of the solid samples was measured by the TGA. The ash content was determined based on the NREL/TP-510-42622 method. The fixed carbon content was determined by difference. Elemental analyses of the raw and torrefied sawdust were provided by a Carlo Erba EA 1108 elemental analyzer, giving carbon (C), hydrogen (H) and nitrogen (N) elemental contents of the sample in wt.% on a dry basis. The oxygen (O) content was estimated by difference. The

higher heating value (HHV) was measured by a bomb calorimeter (Parr 6100), with sawdust pelletized prior to the measurement to ensure controlled combustion. The HHV was determined at least three times for each sample. To characterize the biomass torrefaction performance in the DSRSB reactor, the weight loss of biomass (X) and the true solid yield (Y) are defined, respectively, as

Weight loss

Y¼

    Mc þ Mr þ Mf  100% X ¼ 1 Mt

Mc  100% Mc þ Mr

(1)

(2)

where Mt is the total mass of sawdust fed (g), Mc is the mass of torrefied sawdust captured by cyclone (g), Mr is the mass of torrefied sawdust remaining in reactor (g) and Mf is the mass of torrefied sawdust captured by filter (g). The energy yield of sawdust is defined as

Energy yield ¼ ¼

Energy in torrefied productdry  100% Energy in raw biomassdry

Mc  HHVc þ Mr  HHVr  100% Mt  HHV0

(3)

where HHV is the higher heating value (MJ/kg); the subscripts 0, c and r represent initial status, cyclone and reactor, respectively.

2.3.2. Liquid product The chemical compositions of the condensable liquid were analyzed by a Gas Chromatograph (Agilent 7820 A GC system) coupled with a Mass Spectrometer (Agilent 5975 MS detector) from Agilent Technology (California, United State), with a HP-Innowax column (60 m  0.250 mm  0.25 mm). Prior to the GC-MS measurements, samples of the condensable liquid were diluted with acetone to assist in sampling, with an acetone-to-sample mass ratio of 2:1. The GC injector temperature was set at 250
C, with a split

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ratio of 100:1. During analysis, the GC oven temperature profile was set to follow the sequence: 50
C for 2 min, next increased to 150
C at a ramping rate of 15
C/min, next increased to 260
C with a ramping rate of 5
C/min, then held at 260
C for 5 min. The water content of the condensable liquid was determined by Karl Fischer titration (ASTM D1744). Each measurement was made twice at least for each sample, with the average and standard deviation then calculated. 2.3.3. Gas product The CO concentration in the off-gas was measured by a flue gas analyzer (PS-200, HORIBA) with a measurement range of 0e5000 ppm at 1 s intervals. The concentration of CO2 in the off-gas was measured by a CO2 detector (Model 906, Quantek Instruments) with a measurement range of 0e5000 ppm at 1 s intervals. Other compounds in the off-gas, namely H2, CH4 etc., were present only at low concentrations, outside the ranges of our GC and flue gas analyzer (H2 > 0.5 vol%, CH4 > 0.5 vol%). Therefore, only CO and CO2 concentration data were obtained. 3. Results and discussion 3.1. DSRSB reactor performance 3.1.1. Temperature profile of DSRSB for typical torrefaction Torrefaction reactor temperature is one of most important parameters affecting the torrefaction process, with the temperature profile playing a critical role in determining the product quality and consistency. Fig. 3 shows the temperature time-variation for case TD6 (for operating conditions, see Table 1). The torrefaction experiment began at time 0 min after pre-heating the column to the required temperature. Thermocouples T1 and T2 measured the temperatures of the carrier gas in the downstream and upstream windboxes. T3 and T5 measured the temperature at Z ¼ 64 and 165 mm above the slot in the downstream compartment, whereas T4 and T6 portray the temperatures at the same heights above the slot in the upstream compartment. T7 is the freeboard temperature on the upstream side, with its location (Z ¼ 276 mm) much higher than the initial static bed height of the inert particles (HB,0 ¼ 176 mm). The DSRSB reactor temperature was calculated as the average of temperatures from T3 to T6. For example, the average temperature for case TD6 was 299
C with a 4.9
C standard

5

deviation. Temperatures were sampled at 1 s intervals. In Fig. 3, the inlet temperatures, T1 and T2, were 318.7 ± 1.5
C and 318.8 ± 1.4
C. They were very similar, with the highest temperatures measured within the DSRSB reactor corresponding to the carrier gas heated by a preheater before entering the DSRSB windbox. In the upstream compartment, temperatures T4 and T6 first decreased and then levelled off. The initial decrease resulted from biomass at room temperature fed into the reactor on the upstream side. It is seen that T3 (297.3 ± 0.8
C) was greater than T4 (292.4 ± 0.5
C), associated with the biomass trajectory, with raw particles first fed into the upstream chamber, then transported to the downstream one. This also explains why T3 was only slightly affected by the biomass feeding. It was also found that T5 (305 ± 1.0
C) > T6 (300 ± 0.6
C), as T6 was close to the biomass feed port, where biomass at room temperature was continuously fed into the reactor. The standard deviations of the T1 to T6 temperature readings were all less than 3.5
C. The temperature difference between the two compartments was about 5
C. Temperature T7 increased with time, because the bed height gradually increased, causing more particles to contact the T7 thermocouple. 3.1.2. Pressure drop across DSRSB reactor Fig. 4 shows the upstream and downstream pressure drops, DP1 and DP2, across the DSRSB reactor for case TD6, measured at 1 s intervals. It is seen that the two pressure drops were similar for the initial ~15 min. After that, DP2 increased and was then higher than DP1, suggesting that more particles moved from the upstream to the downstream compartment. Over the rest of the experimental time, DP2 oscillated, with an amplitude of ~700 Pa, about double the amplitude of DP1 fluctuation. DP2 > DP1 was observed in all torrefaction runs, indicating that the downstream chamber had a higher inventory of biomass and inert particles. Note that biomass was continuously fed into the DSRSB from the upstream side, disturbing lateral solids mixing between the two compartments. At similar operating conditions, DP2 was higher than the pressure drop across the single-compartment SRSB reactor, while DP1 was similar to that across the SRSB reactor [30]. The bed pressure drop oscillated during the entire period of torrefaction, while observations through the quartz windows indicated that the biomass particles and inert particles mixed well. 3.2. Characteristics of torrefied product 3.2.1. Solid product 3.2.1.1. Solid product yield (a) Effects of temperature and biomass feedrate

Fig. 3. Temperature time-variation for case TD6. See Table 1 for operating conditions.

Table 3 gives the weight loss of biomass (X) and the mass yields in the reactor (Mr), cyclone (Mc) and filter (Mf) for torrefaction experiments with a nitrogen atmosphere in the DSRSB reactor. For non-oxidative torrefaction, cases TD1-TD9, the weight loss of biomass, defined by Equation (1), varied from 10.1 to 37.1 wt%. The weight loss increased with increasing temperature at a given biomass feed rate. On the other hand, the weight loss decreased as the biomass feed rate increased from 600 to 1400 g/h at a given temperature. The weight loss results were comparable to these obtained from the single-compartment SRSB reactor [29]. To obtain 30 wt% weight loss, a typical desired value for densification [37], our results suggested a combination of 300
C temperature and 0 vol% oxygen in the spouting gas with a 900 g/h sawdust feedrate. When the experimental duration was extended from 50 to 120 min, the weight losses of biomass were very similar at 240 and 270
C. However, the weight loss of biomass was greater for case

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Fig. 4. Pressure drop across DSRSB reactor for case TD6. See Table 1 for operating conditions. See Fig. 1 for definitions of DP1 and DP2.

Table 3 Torrefied product yield and weight loss in torrefaction experiments in DSRSB. Casea

Mt (g)

Mc (g)

Non-oxidative torrefaction TD1 557 ± 13 150 ± 3 TD2 508 ± 4 142 ± 3 TD3 511 ± 5 144 ± 3 TD4 775 ± 5 270 ± 7 TD5 763 ± 4 290 ± 6 TD6 798 ± 4 302 ± 6 TD7 1173 ± 21 572 ± 11 TD8 1112 ± 13 555 ± 11 TD9 1173 ± 5 655 ± 16 TD10 2192 1278 TD11 1814 ± 5 924 ± 18 TD12 1888 866 Oxidative torrefaction OTD1 757 ± 6 274 ± 5 OTD2 774 ± 5 277 ± 6 OTD3 872 ± 17 309 ± 8 OTD4 767 ± 8 298 ± 6 OTD5 788 ± 7 328 ± 7 OTD6 n/a n/a OTD7 764 229 OTD8 n/a n/a OTD9 n/a n/a a

Mr (g)

Mf (g)

Y (%)

X (%)

310 ± 8 221 ± 6 173 ± 4 419 ± 7 300 ± 8 232 ± 5 473 ± 12 339 ± 8 245 ± 6 700 495 ± 12 292

10 ± 0 8±1 5±0 5±1 5±0 5±0 10 ± 1 10 ± 0 10 ± 1 10 10 ± 1 10

32.6 ± 0.8 39.1 ± 1 45.5 ± 1.1 39.2 ± 1.0 49.2 ± 1.2 56.6 ± 1.4 54.8 ± 1.4 62.1 ± 1.6 72.8 ± 1.8 64.6 65.1 ± 1.6 74.8

15.5 ± 0.3 27.0 ± 0.6 37.1 ± 0.7 10.4 ± 0.2 22.2 ± 0.4 32.5 ± 0.7 10.1 ± 0.2 18.7 ± 0.4 22.4 ± 0.6 9.3 21.2 ± 0.4 38.2

382 ± 10 394 ± 10 419 ± 7 268 ± 6 218 ± 5 n/a 188 n/a n/a

9±0 9±0 10 ± 1 6±0 10 ± 1 n/a 9 n/a n/a

41.8 ± 1.0 41.3 ± 1.0 42.4 ± 1.1 52.7 ± 1.3 60.1 ± 1.5 n/a 54.9 n/a n/a

12.2 ± 0.3 12.2 ± 0.2 15.3 ± 0.3 25.4 ± 0.6 29.5 ± 0.3 n/a 44.2 n/a n/a

See Table 1 for actual operating conditions.

TD12 (120 min duration) than for case TD6 (50 min duration). The true yield Y, defined by Equation (2), was found to increase with increasing temperature, due to more reaction and more entrainment at higher temperature for a given feed rate. At a given temperature, Y increased as the biomass feed rate increased, with Y > 50% for a feed rate of 1400 g/h. Noted that a higher feed rate leads to a shorter residence time of biomass, reducing the effect of temperature on torrefaction. In order to obtain an optimal torrefied

product, both temperature and feedrate have to be considered at the same time. Fig. 5 summarizes the mass distribution of torrefied biomass in the DSRSB facility. As the biomass feed rate increased from 600 to 1400 g/h, the weight loss in the form of volatiles decreased, the mass percentage of torrefied product in the reactor decreased, that in the cyclone increased, and the mass percentage of torrefied product retained by the filter was barely affected. However, as noted above and shown in Table 3, the mass of torrefied product remaining in the reactor increased when the biomass feed rate increased. This is associated with reaction times of biomass particles, where a higher biomass feedrate results in a shorter reaction time. Furthermore, the filter captured less than 2 wt% of the torrefied sawdust, indicating that the cyclone was working very well during the torrefaction experiments. (b) Effect of oxygen concentration Compared to the non-oxidative torrefaction, it is seen in Table 3 that the oxygen-containing atmosphere led to increased weight loss of biomass, due to biomass oxidation and decomposition occurring simultaneously during the oxidative torrefaction [12]. Therefore, a lower heater demand was required for oxidative torrefaction to reach the same weight loss, compared to non-oxidative torrefaction. The weight loss of biomass also increased with increasing oxygen concentration at a specific temperature. Hence, to obtain 30 wt% weight loss, a typical desired value for densification [37], our results suggested that we use a combination of 270
C temperature and 6 vol% oxygen in the spouting gas with a 900 g/h sawdust feedrate. The true yield, Y, increased with increasing oxygen concentration, indicating that more torrefied biomass was entrained from the reactor when more oxygen was included in the feed-gas. The

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Fig. 5. Mass distribution of raw and torrefied sawdust for biomass torrefaction in DSRSB facility.

greater entrainment is explained by the particles losing more weight in the presence of oxygen. Fig. 5 plots the mass distribution of torrefied biomass in the oxidative torrefaction in the DSRSB facility. Comparison to the non-oxidative torrefaction results shows that adding oxygen to the feed-gas led to decreased product mass in the reactor, increased product mass in the cyclone, slightly increased mass captured by the filter, and greater weight loss. Therefore, it is feasible to increase the amount of the torrefied product captured by the cyclone by adding oxygen to the spouting gas. 3.2.1.2. Properties of solid product (a) Effects of temperature and biomass feedrate Tables 4e6 show relevant properties of torrefied product obtained from the cyclone and reactor. As shown in Table 4, it is expected that the volatile content of the torrefied sawdust is less than that of the raw sawdust. At a given biomass feed rate, the volatile content of the biomass decreased with increasing temperature, because decomposition of the biomass is strongly affected by the torrefaction temperature [30]. The fixed carbon content increased with increasing temperature from 240 to 300
C for feedrates of 600, 900 and 1400 g/h, respectively, as a consequence of the decreased volatile content. In addition, torrefied sawdust from the cyclone had greater volatile content and lower fixed carbon content compared to torrefied sawdust from the reactor, due to longer residence times of biomass particles remaining at higher temperature. Elemental analysis results appear in Table 5. The oxygen present in the biomass was reduced with increasing temperature due to

Table 4 Proximate analysis of torrefied sawdust on a dry basis. Casea

Cyclone-caught torrefied sawdust Torrefied sawdust from reactor VMc(wt.%) FCc(wt.%)

Non-oxidative torrefaction TD1 82.6 ± 0.1 17.0 ± 0.1 TD2 81.8 ± 0.1 17.8 ± 0.2 TD3 80.3 ± 0.1 19.2 ± 0.1 TD4 83.5 ± 0.2 16.1 ± 0.2 TD5 81.7 ± 0.1 17.8 ± 0.1 TD6 80.3 ± 0.8 19.2 ± 0.8 TD7 82.6 ± 0.1 17.0 ± 0.1 TD8 82.6 ± 0.2 17.0 ± 0.1 TD9 81.3 ± 0.3 18.2 ± 0.3 TD10 82.8 ± 0.5 17.2 ± 0.5 TD11 80.9 ± 1.2 18.6 ± 1.2 TD12 74.4 ± 1.4 25.6 ± 1.4 Oxidative torrefaction OTD1 82.8 ± 0.3 16.8 ± 0.3 OTD2 82.2 ± 0.6 17.4 ± 0.6 OTD3 81.7 ± 1.2 17.9 ± 1.2 OTD4 81.1 ± 1.3 18.4 ± 1.3 OTD5 76.4 ± 0.1 23.1 ± 0.1 OTD7 73.4 ± 1.2 26.0 ± 1.2 a

ACc(wt.%) VMr(wt.%)

FCr(wt.%)

ACr(wt.%)

0.4 ± 0.02 0.5 ± 0.06 0.5 ± 0.02 0.4 ± 0.01 0.5 ± 0.09 0.5 ± 0.04 0.4 ± 0.03 0.4 ± 0.02 0.5 ± 0.02 0.4 ± 0.01 0.4 ± 0.01 0.6 ± 0.01

78.1 ± 0.4 73.8 ± 1.2 69.0 ± 1.0 80.4 ± 0.2 74.3 ± 0.2 66.2 ± 0.9 79.3 ± 0.6 76.5 ± 0.6 60.9 ± 0.4 79.6 ± 0.3 75.0 ± 0.6 56.2 ± 1.9

21.4 ± 0.4 25.7 ± 1.2 30.4 ± 1.0 19.1 ± 0.2 25.2 ± 0.3 33.1 ± 1.0 20.3 ± 0.7 23.0 ± 0.7 38.2 ± 0.4 20.4 ± 0.3 24.2 ± 0.7 43.8 ± 1.9

0.45 ± 0.03 0.6 ± 0.02 0.6 ± 0.01 0.4 ± 0.04 0.5 ± 0.10 0.7 ± 0.04 0.4 ± 0.03 0.5 ± 0.02 0.9 ± 0.01 0.3 ± 0.03 0.5 ± 0.07 0.9 ± 0.01

0.4 ± 0.02 0.4 ± 0.00 0.4 ± 0.02 0.5 ± 0.03 0.5 ± 0.03 0.6 ± 0.02

79.1 ± 0.1 78.9 ± 0.1 77.4 ± 0.4 73.8 ± 0.8 63.5 ± 0.6 51.7 ± 0.16

20.4 ± 0.2 20.7 ± 0.1 22.1 ± 0.4 25.7 ± 0.5 36.0 ± 0.6 47.5 ± 0.2

0.4 ± 0.06 0.4 ± 0.02 0.4 ± 0.02 0.5 ± 0.01 0.5 ± 0.03 0.8 ± 0.01

See Table 1 for operating conditions.

decarboxylation and dehydration reactions involving hemicellulose and partially depolymerized cellulose [38,39]. Compared to raw sawdust, the oxygen content was significantly reduced for all these sawdust feedrate tests as the temperature increased from 240 to 300
C at otherwise identical operating conditions. On the other hand, the carbon content of the biomass increased with increasing

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Table 5 Elemental analysis of cyclone-caught torrefied sawdust on a dry basis. Casea

C(wt.%)

H(wt.%)

Non-oxidative torrefaction TD1 50.9 6.1 TD2 51.4 6.1 TD3 51.8 6.1 TD4 50.7 6.1 TD5 52.4 6.0 TD6 52.7 5.9 TD7 50.7 6.1 TD8 51.0 6.1 TD9 52.1 6.0 TD10 51.2 6.1 TD11 52.2 6.1 TD12 55.3 5.7 Oxidative torrefaction OTD1 50.8 6.1 OTD2 51.1 6.0 OTD3 52.2 5.9 OTD4 52.7 5.6 OTD5 53.6 5.7 OTD7 50.9 6.1 a

N(wt.%)

O(wt.%)

H/C

O/C

<0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1

42.5 41.9 41.5 42.7 41.0 40.8 42.7 42.4 41.3 42.2 41.2 38.3

0.12 0.12 0.12 0.12 0.11 0.11 0.12 0.12 0.12 0.12 0.12 0.1

0.83 0.82 0.80 0.84 0.78 0.77 0.84 0.83 0.79 0.82 0.79 0.69

<0.1 <0.1 <0.1 <0.1 <0.1 <0.1

42.6 42.4 41.4 41.1 40.1 42.3

0.12 0.12 0.11 0.11 0.11 0.12

0.84 0.83 0.79 0.78 0.75 0.83

See Table 1 for operating conditions.

Table 6 HHV of torrefied sawdust and energy yield on a dry basis. Casea

HHVc (MJ/kg)

Non-oxidative torrefaction TD1 19.98 ± 0.01 TD2 20.05 ± 0.01 TD3 20.26 ± 0.07 TD4 19.85 ± 0.07 TD5 19.96 ± 0.22 TD6 20.21 ± 0.19 TD7 19.56 ± 0.19 TD8 19.90 ± 0.02 TD9 20.29 ± 0.01 TD10 20.04 ± 0.12 TD11 20.52 ± 0.01 TD12 21.79 ± 0.01 Oxidative torrefaction OTD1 19.30 ± 0.65 OTD2 19.48 ± 0.56 OTD3 19.63 ± 0.12 OTD4 20.21 ± 0.04 OTD5 20.70 ± 0.15 OTD7 20.67 ± 0.72 a

the reactor was higher than that of the torrefied sawdust from the cyclone, as a result of longer residence times for biomass particles remaining in the reactor. Table 6 summarizes the energy yield results for non-oxidative torrefaction experiments. It was also found that energy yield was significantly affected by the temperature and biomass feedrate. As the temperature increased from 240 to 300
C, the energy yield decreased substantially for each sawdust feedrate. When the biomass feedrate increased from 600 to 1400 g/h, the energy yield increased, because higher biomass feedrates led to shorter residence times for biomass particles in the reactor. Tables 4e6 also present properties of torrefied sawdust obtained from extra experiments, TD10-TD12 over an extended 120 min time of operation, with the same operating conditions as for cases TD4, TD5 and TD6, expecting a longer operating time. Compared to the TD4, TD5 and TD6, cases with 120 min seem to experience more severe torrefaction, producing torrefied product with slightly higher fixed carbon and HHV, but lower volatile and oxygen contents. The relative difference in fixed carbon and HHV between the 50 min and 120 min duration cases was less than 8% and 10%, respectively. This demonstrated that the reactor would need a discharge port to maintain a constant inventory of particles in the reactor. This will be investigated in future work. (b) Effect of oxygen concentration

HHVr (MJ/kg)

Energy yield (%)

20.27 ± 0.10 21.32 ± 0.01 22.62 ± 0.02 20.09 ± 0.02 21.05 ± 0.04 23.40 ± 0.21 20.17 ± 0.12 21.08 ± 0.06 24.37 ± 0.06 20.52 ± 0.11 21.39 ± 0.13 25.95 ± 0.03

91.5 ± 0.3 81.6 ± 0.0 73.3 ± 0.3 97.6 ± 0.2 86.9 ± 0.4 79.2 ± 0.7 96.9 ± 0.8 89.7 ± 0.2 90.1 ± 0.1 99.6 89.4 ± 0.2 76.8

20.09 ± 0.30 20.17 ± 0.31 20.45 ± 0.12 21.00 ± 0.05 23.07 ± 0.04 23.50 ± 0.34

93.9 ± 0.5 94.5 ± 1.9 92.1 ± 0.1 83.3 ± 0.2 82.3 ± 0.3 65.7 ± 0.7

See Table 1 for operating conditions.

temperature, benefitting from more deoxygenation reactions. The hydrogen content of the biomass was reduced, resulting from reduction of hydroxyl substitutes [40]. Compared to the reduction of the oxygen content of the biomass, the reduction in hydrogen content was less. Compared to raw sawdust, the hydrogen content of torrefied sawdust was reduced by 3.91e5.01 wt%, 4.23e7.04 wt% and 4.54e6.10 wt% for 600, 900 and 1400 g/h feedrates when the temperature increased from 240 to 300
C, with otherwise similar operating conditions. The HHV of the torrefied sawdust was found to be much greater than that of the raw sawdust. This is regarded as an important advantage of torrefaction. As the temperature rose from 240 to 300
C, Table 6 indicates that the HHV of torrefied sawdust from the cyclone increased by 1.4, 1.8 and 3.7%, while the HHV of torrefied sawdust from the reactor increased by 11.6, 16.5 and 20.8% for the 600, 900 and 1400 g/h feed rates, respectively. The increased HHV in the torrefied product was due to the release of small molecules (H2O, CO, CO2 etc.), with low HHV being released in the torrefaction process [41]. Furthermore, the HHV of the torrefied sawdust from

Tables 4e6 also show the effect of oxygen concentration on the properties of the torrefied solid product obtained from the cyclone and reactor during oxidative torrefaction. Table 4 provides proximate analyses. As expected, the torrefied sawdust always had a lower volatile content than either the raw sawdust or torrefied sawdust produced during non-oxidative torrefaction. This was due not only to dehydration and devolatilization, but also to oxidation [13]. At a given temperature, when oxygen was added to the feedgas, the volatile content of the torrefied product decreased, while the fixed carbon content increased. As the oxygen concentration increased, the volatile content of the torrefied sawdust decreased, whereas its fixed carbon content increased. This suggested that small hydrocarbon molecules were more easily liberated in the presence of oxygen in the torrefaction experiments, because of biomass oxidation and decomposition [7]. An increase in temperature facilitates devolatilization and oxidation processes during torrefaction, especially for the hemicellulose component [6]. The impact of oxygen on the volatile and fixed carbon contents was more significant at higher temperature. Elemental analysis, shown in Table 5, illustrated that the torrefied product had higher carbon content than the raw sawdust, but lower oxygen and hydrogen contents, probably due to the release of moisture and decomposition of eCOOH and eOH groups in the biomass [7]. Moreover, the carbon content of oxidatively torrefied product was found to be higher than that of the non-oxidatively torrefied product at all three temperatures: 240, 270 and 300
C. This is believed to be because the oxidation of hydrocarbons in the biomass plays an important role in decomposing biomass [7,42]. In addition, the oxygen and hydrogen contents of the solid product decreased to some extent when the oxygen content of the feed-gas increased. These findings are associated with oxidation of hydrogen, carbonization of biomass [7] and combustion of volatiles released during torrefaction [43]. Whether the sawdust was torrefied in pure nitrogen or in an oxygen-containing environment, the HHV of the torrefied product was higher than that of the raw sawdust, as shown in Table 6. For operation at 240
C, adding oxygen to the feed-gas resulted in a slight decrease in HHV for the torrefied product under otherwise similar operating conditions. However, for temperatures of 270 and

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300
C, as oxygen was added to the feed-gas, the HHV of the torrefied product increased. This indicates that introduction of oxygen can improve the torrefaction performance at higher temperatures. This finding is consistent with our results obtained in the SRSB reactor [6], as well as data reported by Almeida et al. [44] and Wang et al. [12], but opposite to findings of Chen et al. [7]. Furthermore, as the oxygen concentration increased from 3 to 9 vol%, the HHV of the torrefied product increased slightly, likely because cellulose and lignin mainly contributed to the HHV of the torrefied sawdust [41], and these two constituents were insensitive to the oxygen concentration in the 3e9 vol% range covered. The energy yield decreased when oxygen was added to the feedgas during the torrefaction, with the effect of oxygen concentration variation more significant at higher temperature. This is because an increase in temperature has a positive effect on biomass oxidation and torrefaction in the oxidative torrefaction process. Although the energy yield drastically decreased with increasing oxygen content in the feed-gas, 98-66% energy yield is a still reasonable range for making high-quality pellets [37,45], leading to high-quality bio-oil or syngas [46]. In addition, compared to the torrefied sawdust obtained from the cyclone, the torrefied product remaining in the reactor had lower volatile content, higher fixed carbon content and greater HHV, due to the longer residence time of the torrefied sawdust remaining in the reactor. 3.2.1.3. Solid product consistency (a) Effects of temperature and biomass feedrate on ash content of torrefied product Fig. 6 shows the evolution of the ash content of torrefied product during several torrefaction experiments in the DSRSB reactor. It is seen that the ash content increased initially, then levelled off. The initial increase in ash content resulted mainly from biomass decomposition and release of volatiles during the torrefaction process, since pre-dried sawdust was used. The levelling off indicated that the torrefied sawdust properties were approaching constant values. The ash content of the torrefied sawdust increased as the temperature increased from 240 to 300
C at a given biomass feedrate. It has been previously reported [30] that the formation of volatiles during biomass decomposition is mainly influenced by temperature. In addition, at the same temperature, the ash content was lower for a higher biomass feedrate, due to a shorter residence time at the higher biomass feedrate. Furthermore, for 600 and 900 g/h biomass feedrates (Fig. 6 a and b), the ash content of the torrefied product needed ~10 min to reach a relatively constant value. However, for a feedrate of 1400 g/h (Fig. 6 c), the ash content of the torrefied product required ~20 min to approach a constant level. As shown in Fig. 6 d, when the operating time of the torrefaction experiment was extended to 120 min, the ash content of the torrefied product remained constant. During the experiments, biomass was continuously fed into the torrefaction reactor, and torrefied biomass was continuously entrained and captured by a cyclone. In such a reactor, the biomass particles have different residence times, which follow a nearly normal distribution. Thus some particles stay inside the reactor for long periods, while others leave the reactor in seconds. The general trend is that the ash content of the torrefied biomass first increases and then levels off with time. Note that the torrefaction temperature is in the range of 240e300
C, too low to volatize the mineral matter in the ash. (b) Effect of oxygen concentration on ash content of torrefied product

Fig. 6. Effects of temperature and biomass feedrate on evolution of ash content of cyclone-caught torrefied sawdust: (a) runs TD1 to TD3; (b) runs TD4 to TD6; (c) runs TD7 to TD9; (d) runs TD10 to TD12. For detailed operating conditions see Table 1.

Fig. 7 plots the effect of oxygen concentration on the evolution of the ash content of the torrefied product. The ash content evolution in the oxidative torrefaction follows the same trend as for non-oxidative torrefaction, as shown in Fig. 6. However, the increase in ash content of the torrefied sawdust mainly resulted not only from biomass decomposition and devolatilization, but also from oxidation reactions. The levelling off of the ash content of the torrefied sawdust indicated that the torrefied sawdust properties were approaching constant values. Fig. 7(a) shows that the ash content of the torrefied sawdust reached a maximum at ~20 min for all cases at 240
C. The ash content was higher after torrefaction in the oxygen-containing atmosphere than in the inert atmosphere. It was also found that the ash content increased as the oxygen concentration increased, reaching a maximum at ~10 min for all cases at 270
C, except for 6 vol% O2. The ash content for 6 vol% O2 and 270
C increased continuously beyond 20 min, consistent with the observed temperature variation of OTD5 in the DSRSB reactor. (See supplementary material,Figure A1(a)). In this case, the reactor temperature increased continuously starting from ~20 min, leading to more severe biomass decomposition, oxidation, and even carbonization. In Fig. 7(c), it is seen that the ash content kept increasing as 3 vol% oxygen was included in the feed-gas, consistent with the reactor temperature variation of OTD7 (See supplementary material,Figure A1b). As mentioned previously, to prevent the reactor temperature from running away, experiments at 270
C with 9 vol% O2, and 300
C with 6 and 9 vol% O2 were not carried out, so no OTD6, OTD8 and OTD9 results are available.

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Fig. 7. Effect of oxygen concentration on evolution of ash content of torrefied product captured by the cyclone: (a) CO2 ¼ 0-9 vol%, T ¼ 240
C and F ¼ 900 g/h; (b) CO2 ¼ 06 vol%, T ¼ 270
C and F ¼ 900 g/h; (c) CO2 ¼ 0-3 vol%, T ¼ 300
C and F ¼ 900 g/h. For detailed operating conditions see Table 1. Fig. 8. GC-MS spectra of condensable liquid obtained from (a) TD10; (b) TD11; (c) TD12. For operating conditions see Table 1.

3.2.2. Liquid product The condensable liquid collected during the 2 h torrefaction experiments (TD10 to TD12) was analyzed by GC-MS to determine its chemical composition. Fig. 8 shows GC-MS spectra of the condensable liquids obtained at 240, 270 and 300
C for a biomass feedrate of 900 g/h. 33, 42 and 44 chemical compounds of quality >60 were detected for the condensable liquid obtained at 240, 270 and 300
C, respectively. The complexity of the liquid product composition increased with increasing torrefaction temperature [14,47]. Detailed chemical components of the condensable liquid are identified in the Supplementary material (Tables A1-A3). It was found that the three liquids contained eight chemical compounds in common, listed in Table 7. These components included acids, furans, phenols, vanillin, and others, similar to those identified in previous studies [16,48,49]. Acetic acid was detected as the main acidic component; it was mainly formed from hemicellulose decomposition. As the temperature increased from 240 to 300
C, the acetic acid concentration decreased, as more organic compounds were formed at higher torrefaction temperature. Furan derivatives were formed from decomposition of both hemicellulose and cellulose. More 2furanmethanol was produced at higher temperatures, while less 2-furancarboxaldehyde and 5-hydroxymethylfurfural formed. 5Hydroxymethylfurfural is an intermediate product, which can further be converted into furan and formaldehyde during cellulose decomposition [50]. Phenols were mainly converted from hemicellulose and lignin depolymerisation, and could be further upgraded for the biofuels. Phenol concentrations increased with

increasing temperature. The simple structural phenol (phenol and its methyl-derivative) formed from hemicellulose decomposition due to the simple unit structure of hemicellulose [16]; while the phenol derivatives with methoxy substitutes were likely generated from lignin decomposition. Despite partial decomposition, it was found that the lignin contributed more phenol derivatives than hemicellulose. The relative content of vanillin decreased when the temperature increased from 240 to 300
C. Vanillin was obtained through the decomposition of lignin [50], characterized by cleavage of ether linkages such as b-O-4 and demethoxylation [51]. As many aromatics were detected in the condensed liquids, it is better to treat them as chemicals, rather than as fuels. The water in the condensable liquid mainly resulted from absorbed water, constitutive water and dehydration of hydroxyls in the raw sawdust. The last two sources were more important in the present study, because the biomass was pre-dried to a moisture content of <1 wt% before being torrefied. As shown in Fig. 9, the water content in the condensable liquid decreased from 16.4 ± 0.3 wt% to 8.7 ± 0.2 wt%, as the torrefaction temperature increased from 240 to 300
C, consistent with previous findings [49,52]. This occurs because more organic compounds are formed due to greater decomposition of biomass when the torrefaction temperature is increased; thus, the water content of the liquid products decreased with increasing temperature.

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Table 7 Main compounds identified in condensable liquid via GC-MS. No.

1 2 3 4 5 6 7 8

Residence Time (min)

10.45 10.80 13.10 16.32 17.93 24.81 27.28 28.58

Library/ID

Peak area (%)

Acetic acid 2-Furancarboxaldehyde 2-Furanmethanol Phenol, 2-methoxyPhenol, 2-methoxy-4-methylPhenol, 2-methoxy-4-(1-propenyl)2-Furancarboxaldehyde, 5-(hydroxymethyl)Vanillin

Fig. 9. Water content of condensable liquid collected for runs TD10: T ¼ 240
C; TD12: T ¼ 270
C; and TD10: T ¼ 300
C. For other operating conditions see Table 1.

3.2.3. Gas product Off-gas compositions from biomass torrefaction mainly include carbon monoxide, carbon dioxide and traces of hydrogen and methane [39,49]. Because the hydrogen and methane concentrations were below the lower limit of detection, 0.5 vol%, of our GC, and could not be measured accurately, only carbon monoxide and carbon dioxide concentration data are presented here. Fig. 10 plots these concentrations in the torrefaction off-gas. Because part of the tor-gas was recycled, the CO and CO2 contents were higher than the theoretical values that one would have if there were no recycle. The CO concentration increased from 24 ± 2 ppm to 231 ± 5 ppm and the CO2 concentration from 129 ± 4 ppm to 344 ± 17 ppm when the torrefaction temperature increased from 240 to 300
C. Carbon monoxide release was mainly caused by the cracking of carbonyl (CeOeC) and carboxyl (C]O) groups and secondary reactions of carbon dioxide and steam with porous char [39]. The carbon dioxide release was mainly contributed by the cracking and reforming of carboxyl (C]O) and carboxylic acid (COOH) groups in hemicellulose of SPF sawdust [39,53]. Previous studies [39,54] found that CO and CO2 releases at temperatures < 500
C could be attributed to decomposition of hemicellulose rather than cellulose and lignin decomposition. Due to secondary reactions, CO formed more aggressively with increasing temperature, resulting in the CO release rate being higher than the CO2 release rate. Release of these gases during torrefaction also led to a decrease of elemental oxygen in the torrefied sawdust, consistent with the results in Table 5.

240
C

270
C

300
C

5.113 1.526 1.061 0.986 0.192 0.751 2.972 4.107

3.455 0.905 2.245 1.417 0.585 2.202 2.925 4.241

1.595 0.727 3.085 2.114 1.767 2.255 0.621 0.912

Fig. 10. CO and CO2 concentrations in the off-gas during torrefaction at end of run TD10 T ¼ 240
C, run TD12: T ¼ 270
C, and run TD10: T ¼ 300
C. For operating conditions see Table 1.

3.3. Comparison of SRSB, DSRSB and other types of torrefaction reactor The DSRSB reactor used in this work was scaled-up from a single-compartment SRSB reactor of half the volumetric capacity. Fig. 11 compares the weight loss of biomass and energy yield obtained from torrefaction in the absence of oxygen in the SRSB and DSRSB reactors. At nominal temperatures of 240e300
C (241e298
C measured) and biomass feedrates of 220e710 g/h (210e803 g/h measured) in the SRSB reactor, the weight loss of biomass and the energy yield were 10.1e39.2% and 67.4e97.6% [29,30]. In the DSRSB reactor, the weight loss of biomass and the energy yield were 10.1e37.1% and 73.3e97.6% at very similar temperatures 240e330
C (242e303
C measured), but approximately twice the biomass feedrate 600e1400 g/h (610e1408 g/h measured). Furthermore, when 3e9 vol% oxygen was added to the spouting gas, the weight losses of biomass were 15.4e46.4 wt% and 12.2e44.2 wt%, and the energy yields were 61.2e99.4% and 65.7e94.5% for biomass torrefied in the SRSB reactor with a 440 g/h biomass feedrate and in the DSRSB reactor with a 900 g/h federate at temperatures of 240e300
C. Comparisons of the weight loss and energy yield from the SRSB and DSRSB reactors show that the performance of the DSRSB was very close to that of the SRSB. In addition, the primary torrefied sawdust captured by the cyclone in the SRSB apparatus had 19.7e20.8 MJ/kg HHV [6,29,30], while that produced in the DSRSB apparatus was very similar, 19.6e20.7 MJ/kg HHV. This indicates that it is feasible to scale up a slot-rectangular spouted bed torrefaction reactor by using multiple parallel slots

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types. 4. Conclusions Experimental results showed that sawdust can be feasibly torrefied in a dual-compartment slot-rectangular spouted bed reactor.  Weight loss of sawdust was 9.3e38.2 wt%, but 65.7e99.6% energy was preserved in the torrefied solid product for operating temperatures of 240e300
C and biomass feedrates of 600e1400 g/h.  The temperature difference between the two adjacent compartments was limited, but affected by the biomass feedrate.  The pressure drop of the downstream compartment was higher than that of the upstream compartment.  To obtain 30 wt% weight loss, a typical desirable value for densification, it is suggested to use a combination of 270
C temperature and 6 vol% oxygen in the spouting gas or 300
C temperature and 0 vol% oxygen with a sawdust federate of 900 g/h.  Increasing temperature and utilizing oxygen-containing gas were effective in producing more torrefied product with higher weight loss from the cyclone.  The condensable liquid product contained valuable aromatics, which could be used as chemicals. The gaseous product mainly included CO, CO2, with traces of H2 and CH4. The concentration of CO2 was much higher concentration than that of CO.  Comparison of the dual-compartment reactor with a single compartment spouted bed and other types of torrefaction reactor indicates that a DSRSB torrefaction reactor can be successfully scaled up from a SRSB reactor, and that this type of reactor is a feasible alternative for biomass torrefaction, producing torrefied product comparable with that produced by other types of reactors. Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and scholarships from the China Scholarship Council (CSC) are acknowledged with gratitude. Appendix A. Supplementary data

Fig. 11. Comparison of weight loss and energy yield of biomass obtained from torrefaction in the absence of oxygen in SRSB and DSRSB reactors.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.116306. References

and compartments. Other types of reactors used to carry out torrefaction include fixed beds [20,21], fluidized beds [12], screw conveyors [19], rotary drums [22], rotary kilns [23], electronic furnaces [7,24], and microwave reactors [25], operated in batch or continuous mode. Each reactor has its own characteristics, and different studies have utilized quite different operating conditions, e.g. torrefaction temperature, reaction time or material feedrate, biomass species, particle size and oxygen concentration. It is therefore extremely difficult to compare results and to judge whether one torrefaction technology is better than the others. In terms of torrefied product properties, previous studies [7,12,19e25] with different reactors and operating conditions show similar trends as in the present study. The weight loss of biomass varies widely from 5 to 75 wt%, the energy yield is from 29 to 99%, and the biomass HHV increased to a certain extent to 16e29 MJ/kg [2,8,11,13,22e24,48,55e57]. Overall, the slot-rectangular spouted bed reactor configuration produced high-quality torrefied biomass. Comparative experimental and CFD studies are needed with the same fuel and feed rates to allow full comparison of this type of reactor with the other

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Please cite this article as: Wang Z et al., A comprehensive study of sawdust torrefaction in a dual-compartment slot-rectangular spouted bed reactor, Energy, https://doi.org/10.1016/j.energy.2019.116306