Insights into Preventing Fluidized Bed Material Agglomeration in Fast

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Biofuels and Biomass

Insights into Preventing Fluidized Bed Material Agglomeration in Fast Pyrolysis of Acid-leached Pine Wood Xing Xin, Kirk M. Torr, Ferran De Miguel Mercader, and Shusheng Pang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04178 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 14, 2019

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Insights into Preventing Fluidized Bed Material Agglomeration in Fast Pyrolysis of Acid-leached Pine Wood Xing Xin,† Kirk M. Torr,*,‡ Ferran de Miguel Mercader,‡ and Shusheng Pang† †Department of Chemical and Process Engineering, University of Canterbury, New Zealand ‡Scion, Private Bag 3020, Rotorua 3046, New Zealand

KEYWORDS: acid-leaching, bed agglomeration, fast pyrolysis, fluidized bed, torrefaction.

ABSTRACT: Acid-leaching of lignocellulosic biomass prior to fast pyrolysis can increase bio-oil yield and improve bio-oil quality. However, fast pyrolysis of acid-leached biomass in fluidized bed reactors can lead to bed material agglomeration. A series of experiments were conducted to investigate how pyrolysis conditions and torrefaction pretreatment affected fast pyrolysis of acid-leached pine wood to gain insights into understanding and preventing this issue. Melting of the acid-leached wood during fast pyrolysis resulted in the formation of bed material agglomerates, which eventually caused defluidization of the reactor. Three approaches to preventing this issue were investigated in this study. Lowering the pyrolysis temperature to 360 C limited the conversion of the lignin and prevented bed agglomeration. Using a high sand feeding rate to continually refresh the bed material prevented melted biomass material from accumulating and causing agglomeration. Torrefaction pretreatment of the acid-leached wood resulted in structural changes that altered the pyrolysis behavior of the lignin and also prevented bed agglomeration. Each of these approaches had a different effect on the yield of the bio-oil product, but had a minimal impact on its quality.

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1. INTRODUCTION Biomass fast pyrolysis is a promising thermochemical process that can produce a liquid product referred to as pyrolysis oil or bio-oil.1,2 In order to maximize the bio-oil yield, the preferred conditions include pyrolysis temperatures from 425 to 500 oC, rapid heating of small feedstock particles, and short residence time in the reactor of less than 5 seconds for the hot vapors produced.2 Bio-oil can be directly used in boilers for heat generation, upgraded to transportation fuels, or used as a source of chemicals.1,3,4 In commercializing biomass fast pyrolysis, a fluidized bed reactor has advantages in terms of the potential for large throughput, high heat transfer rate and stability of reaction temperatures.2 Biomass pretreatment can alter the properties of biomass and be beneficial in fast pyrolysis. For example, acid-leaching of biomass can remove minerals from the biomass (typically alkali and alkali earth metals - AAEMs), leading to increased bio-oil yield and improved bio-oil quality.5-8 Oudenhoven et al. found that the yield of bio-oil organics increased from 48 to 58 wt.% in fast pyrolysis of acid-leached wood at 530 oC.6 Additionally, the production of levoglucosan was increased from 3 to 18 wt.%. Dalluge et al. found that passivation of AAEMs in biomass through acid infusion led to an increase in sugar yield and reduction in light oxygenates.9 Wigley et al. found that the acetic acid content of bio-oil produced from pine wood decreased from 3.5 to 1.9 wt.% after acid-leaching pretreatment.7 The increased yields of bio-oil and levoglucosan resulted from the absence of AAEMs, which are known to catalyze dehydration and ring fragmentation reactions of the sugars during pyrolysis.10 Despite improvements in bio-oil yield and quality, researchers working on fast pyrolysis of acid-leaching biomass have encountered problems with bed material agglomeration when using fluidized bed reactors.7,11 Bed agglomeration has been attributed to a sticky melt material that coats the bed particles adhering them together to form agglomerates.11 The formation of these bed agglomerates can result in operational issues, leading to complete defluidization and

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blockage of the reactor.11 Bed agglomeration issues have also been reported for fast pyrolysis of mineral acid-infused biomass,9,12 technical lignins,13,14 mallee leaf and bark,15,16 and pine wood bio-oil feedstocks.17 The exact nature and source of the sticky melt material that leads to the formation of agglomerates depends on the feedstock. Burton and Wu reported that bed agglomeration occurring during fast pyrolysis of mallee leaf was due to the production of sticky agents including both partially molten pyrolyzing biomass particles and organic matter produced from biomass pyrolysis reactions.15 In subsequent work, they indicated that extractives present in leaf and bark feedstocks played a critical role in the bed agglomeration.16 Nowakowski et al. reported on an international collaboration in which seven laboratories attempted to conduct fast pyrolysis of two technical lignins using fluidized bed or entrained flow reactors.13 Most of the laboratories found the processing of purified technical lignin nearly impossible due to lignin melting, resulting in the formation of adhered clumps of bed material and eventually leading to defluidization.13 Zhou et al. successfully prevented bed agglomeration during fast pyrolysis of technical lignin by pretreating the feedstock with calcium hydroxide.14 The calcium hydroxide was proposed to react with phenolic hydroxyl, aldehyde and carboxylic acid groups in the lignin to alter its melting behavior and inhibit agglomeration. In the case of acid-leached or acid-infused biomass, AAEMs appear to play a crucial role in the bed agglomeration issue. Kim et al. found that fast pyrolysis of sulfuric acid infused red oak wood led to the formation of large char agglomerates that clogged the fluidized bed reactor.12 Their previous work had shown that the acid reacted with the AAEMs to form thermally stable salts which are less catalytically active during pyrolysis.18 Oudenhoven et al. found that no bed agglomeration occurred when acid-leached wood back impregnated with AAEMs was pyrolysed.11 These studies indicated that the removal of the catalytic effect of the

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AAEMs appears to be the major cause of bed agglomeration. AAEMs are known to catalyst the conversion of cellulose, hemicellulose and lignin during pyrolysis, accelerating mass loss and affecting the composition of the products.19 Despite numerous reports of bed material agglomeration in fast pyrolysis for acid-leached or acid infused biomass, there is as yet no definitive solution to this important problem. Oudenhoven et al. found that pyrolyzing acid-leached wood at the lower temperature of 360 °C prevented bed agglomeration, however the reason for this was not investigated.11 Kim et al. successfully prevented bed agglomeration and improved sugar yield during fast pyrolysis of acid-infused wood by adding a small amount of oxygen into the nitrogen sweep gas.12 Torrefaction pretreatment of acid-leaching wood has been investigated as a way to improve pyrolysis oil quality, however, its effect on bed agglomeration was not studied.7,20 In this study, bed material agglomeration during fast pyrolysis of acid-leached pine wood was studied in a fluidized bed reactor with the goal of finding practical approaches to prevent this issue and to understand the impact of those approaches on bio-oil yield and composition. Current knowledge points towards a key role for lignin in the bed agglomeration process, whether it be melting of the lignin or the formation of sticky secondary pyrolysis products from the lignin. Building on this knowledge, the novelty of this study lies in mitigating this important issue using three different approaches and understanding their associated mechanisms. The three approaches investigated were 1) using low temperature pyrolysis to limit melting/pyrolysis of the lignin, 2) physical removal of problematic melt material before it accumulates and can agglomerate with the bed material, and 3) using thermal pretreatment of the biomass to modify lignin melting/decomposition behavior.

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2. EXPERIMENTAL SECTION 2.1. Biomass pretreatment. Fresh wood chips (Pinus radiata D. Don) were obtained from a local sawmill in Rotorua, New Zealand. These chips were dried to a moisture content below 10 wt.%, and then ground and sieved to wood particle with sizes in the range of 0.25-2 mm. Acid-leaching was performed by soaking and stirring the wood in a 1 wt.% acetic acid solution for 4 hours at 30 °C following the procedure proposed by Wigley et al.8 Acetic acid was selected as the demineralization agent as it can potentially be recovered from the pyrolysis oil and wood torrefaction condensate. Following acid-leaching, the wood particles were washed with deionized water and dried at 60 °C, and then at 105 °C. Torrefaction pretreatment was carried out for raw wood and acid-leached wood in a 316 stainless steel vessel which was heated in a high temperature oven. A steel tube inside the vessel was used to enhance thermal conductivity. Details of the apparatus are described elsewhere.8 During torrefaction, the vessel containing 650-700 g of biomass feedstock was heated in the oven to a preset point of 270 C for 260 minutes. The average heating rate was ca. 0.9 °C /min, with the vessel reaching a maximum temperature of 258 ± 3 °C. After pretreatment, the vessel and the wood samples were cooled down at a cooling rate of ca. 2.0 °C /min. This procedure followed the recommended torrefaction temperature range of 240-270 °C used in previous studies.7,21,22 During torrefaction, nitrogen was used as a sweep gas to remove volatiles which were condensed and collected. The four types of woody feedstocks prepared for this study were: raw wood without pretreatment (Rwood), acid-leached wood (ALwood), torrefied wood (Twood), and acidleached and torrefied wood (ALTwood).

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2.2. Feedstock Analysis. The four feedstocks were ground and dried at 105 °C before analysis. Ash content was determined by calcining the biomass in a muffle furnace at 575 °C for 12 hours and measuring the weight difference. Six replicate samples were tested, with standard deviations (stdev) lower than 0.01 wt.%. Elemental analysis was performed in triplicate using a FLASH 2000 CHN analyzer (Thermo Scientific, USA). A five-point calibration curve was constructed using acetanilide. The content of carbon (stdev < 0.9 wt.%), hydrogen (stdev < 0.1 wt.%), and nitrogen (all analyses showed nitrogen under 1 wt.%) were determined and the oxygen content was calculated by difference. Extractives content was determined in duplicate by Soxtec extraction with dichloromethane. Lignin and neutral carbohydrate contents were determined in duplicate following standard methods scaled down for the analysis of 0.25 g wood samples.23 Monomeric sugars in the filtrates from Klason lignin determinations were analyzed by ion chromatography.24 Wood samples were digested with concentrated HNO3 acid and microwave irradiation and trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) using a NexION 300X ICP-MS (Perkin Elmer, USA). Solid-state CP/MAS

13C

NMR spectroscopy was carried out using a Bruker 200 DRX

spectrometer at 50.3 MHz. The magic-angle spinning speed was set at 5 kHz. For the standard cross-polarization, each 1.5 s pulse delay was followed by a hydrogen preparation pulse of duration tp=4.6 μs, a 1 ms contact time and a 30 ms acquisition time. The hydrogen transmitter power was increased to a value corresponding to a 90o pulse width of 2.8 μs for hydrogen decoupled during 13C data acquisition. A Gaussian line broadening of 25 Hz was applied to all spectra prior to Fourier transform. All spectra were calibrated by assigning the C4 cellulose ordered peak (C4o) to a value of 89.3 ppm, and were assigned based on previously published work.22,25

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2.3. Bubbling fluidized bed reactor. A bubbling fluidized bed reactor was employed for the fast pyrolysis experiments. The design of this reactor was based on a similar setup used by the Sustainable Process Technology group at the University of Twente in The Netherlands.26 A schematic representation is shown in Figure 1. A calibrated screw transported the biomass into a mixing tube. The bed material (sand) was fed by another calibrated feeding screw into the mixing tube. Then the mixture of biomass and bed material was fed into the fluidized bed by a third screw. A water-cooled jacket at the end of this screw prevented the biomass from pyrolyzing before entering the bed. An overflow tube inside the reactor collected excess char and sand, keeping the bed level constant. The fluidized bed and the gas cleaning system (composed of a knock-out vessel and two cyclones in series) were located in two separately-heated ovens. The maximum heating rate of the two heating ovens was approximately 10 °C/min. The diameter of the bubbling fluidized bed reactor was 100 mm. Five thermocouples were located inside the reactor at different heights, four inside the fluidized bed, and one in the freeboard above the overflow tube. The four lower thermocouples were used to monitor the temperature profile in the bed. The difference in temperature between these thermocouples never exceeded 5 °C when the bed was fully fluidized. The average temperature of these four readings was defined as the pyrolysis temperature. Sudden variations of the temperatures from these thermocouples were used as an indication of bed defluidization. Preheated nitrogen, used as the fluidization agent, was fed through a sintered metal plate at the bottom of the reactor. Fine particles were removed by the gas cleaning set before quenching of the hot vapor. The quenching section was composed of an electrostatic precipitator (ESP) column and an intensive cooling (IC) column, both being jacketed with coolant (water and ethylene glycol mixture). Bio-oil was collected at the base of these columns. A cartridge filter

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located after the IC column collected any remaining condensable material. A gas flow meter measured the total flow rate of the non-condensed gas. In a typical experiment, the operating procedure was as follows: After assembly and successful leakage check, the pyrolysis reactor was heated to the targeted temperature. The temperature of the oven for the gas cleaning system was kept 5 °C lower than the pyrolysis reaction temperature. The temperatures of the ESP and IC columns were kept at -5 °C and -15 °C, respectively. Nitrogen with a flowrate of 17 NL/min was used as fluidizing gas. When the system conditions were stable, biomass and sand (particle size 0.08-0.4 mm) were continuously fed into the reactor. This was considered as the beginning of the experimental run (t = 0). The total gas flow rate exiting the system during the experiment was recorded by the gas flow meter. Non-condensable gases were sampled during the experiment and analyzed using gas chromatography. 2.4. Fast pyrolysis experiments. Three series of experiments were conducted in investigating how pyrolysis conditions and torrefaction pretreatment affected the issue of bed material agglomeration during fast pyrolysis of acid-leached pine wood (Table 1). Firstly, the effect of pyrolysis temperature was studied by conducting experiments at 360 °C, 450 °C, and 500 °C using a sand feeding rate of 3 kg/h. Secondly, three experiments at different sand feeding rates (3 kg/h, 4 kg/h, and 5 kg/h) were conducted to study operability at different rates at which the bed material was removed from the reactor. An additional experiment at lower temperature (450 °C) and very low sand feeding rate (1 kg/h) was conducted to support the findings. Lastly, the effect of torrefaction pretreatment on bed material agglomeration was studied at 450 and 500 °C. A control experiment at 450 °C using torrefied wood that had not been acid-leached was also performed. The impact of these experiments on bio-oil yield and composition was an important consideration.

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Table 1. Summary of the fast pyrolysis experiments Purpose Reproducibility Effect of temperature

Effect of removing bed material Effect of torrefaction pretreatment a

Feedstock Rwood ALwood ALwood ALwood ALwood ALwood ALwood ALTwood ALTwood

Temperature (C) 450 360 450 500 500 500 500 450 500

Sand feeding rate (kg/h) 3 3 3 3a 3a 4 5 3 3

Same experiment

A biomass feeding rate of 0.5 kg/h and a run time of 90 minutes were used for all experiments. Three pyrolysis experiments on Rwood at 450 °C and a sand feeding rate of 3 kg/h were used as control experiments to test the reproducibility of the system. Bed agglomeration was firstly detected in fast pyrolysis experiments by a sudden drop in temperature at the top of the reactor near the biomass feeding screw. This temperature drop was caused by the buildup of fresh biomass and sand entering the reactor due to defluidization. Upon cooling and disassembly of the reactor, bed material was collected and analyzed by scanning electron microscopy (SEM) to confirm the presence of bed material agglomerates. 2.5. Analysis of pyrolysis products. 2.5.1. Gas analysis. The composition of noncondensable gas was analyzed using an Agilent 490 Micro-GC with two analytical columns (10 m Molsieve 5 Å at 90 °C and 150 kPa, and 10 m PPQ column at 70 °C and 150 kPa). It was calibrated to measure N2, O2, H2, CH4, CO and CO2. Two gas samples were measured in each experiment and the averaged gas compositions reported. These values together with the volumetric gas flow rate and the temperature of the off-gas were used to calculate the mass flow of each gas.

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2.5.2. Bio-oil analysis. The water content of bio-oils was measured in triplicate by KarlFischer titration (Metrohm 870 KF Titrino plus). The solvent used was a mixture of methanol and dichloromethane at 3:1 (vol.), and the titrant was Hydranal Composite 5 (Riedel-de Haën). Elemental analysis was performed in triplicate using a FLASH 2000 CHN analyzer (Thermo Scientific, USA) as described in Section 2.2. 1H-NMR

spectroscopy was performed on a Bruker Avance III 400 MHz spectrometer

equipped with a 5 mm BBO probe. The spectra were obtained at 27 °C in acetone-d6. At least 64 transients were collected. A presaturation pulse was applied during acquisition to suppress the signal of water. The spectra were reprocessed, and the hydrogen signals from the bio-oils were classed into six groups based on their chemical shifts, following the method developed by Mullen et al.27 2.5.3. Char analysis. To isolate pyrolysis chars from the bed material, a char and sand mixture was first sieved to remove fine sand. The char enriched mixture was ground, transferred to a centrifuge tube and mixed with deionized water. After centrifuging at 3000 rpm for 10 minutes, the sand particles settled to the bottom of the tube and the fine char particles remained suspended. The char/water suspension was decanted and freeze-dried to obtain purified pyrolysis char. These char samples were analyzed by solid-state CP/MAS 13C NMR spectroscopy as described in Section 2.2. Samples of char and sand for scanning electron microscopy were mounted on carbon tabs, coated with chromium and examined on a JEOL 6700F field emission scanning electron microscope at an accelerating voltage of 15 kV using a backscattered electron detector.

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3. RESULTS AND DISCUSSION 3.1. Characterization of pretreated wood. Acid-leaching resulted in a 99 wt. % yield of pretreated wood. Acid-leached wood was chemically very similar to Rwood in terms of its elemental and biopolymer compositions (Table 2). Acid-leaching reduced the ash content of the wood from 0.26 to 0.03 wt.%, mainly through the removal of alkali and alkaline earth metals (i.e. K, Ca, Mg) and phosphorus (Table 2). The pretreated woods were also characterized by solid state

13C

NMR spectroscopy. The NMR spectra show signals arising

from C1 (anomeric carbon) to C6 carbons in cellulose and hemicellulose polymers, as well as acetyl, methoxy and aromatic lignin carbons (Figure 2). The C4 and C6 carbons have signals arising from ordered (C4o, C6o) and less-ordered (C4d, C6d) regions of the polysaccharides.25,28 The spectra of Rwood and ALwood were very similar confirming that the structural polymers in the wood were not affected by acid-leaching (Figure 2). The mild torrefaction pretreatment of Rwood and ALwood resulted in 17 % and 14 % mass loss, respectively. These mass losses were due to degradation of hemicelluloses as evidenced by the decrease in hemicellulosic sugars after torrefaction pretreatment (Table 2). Conversely, the apparent lignin content increased after torrefaction, due to the reduced hemicellulose content and hemicellulose degradation products analyzing as lignin. These chemical changes led to higher carbon and lower oxygen contents in the torrefied woods. The inorganic content of the wood slightly increased after torrefaction (Table 2), with K, Ca and Mg ion concentrations all increasing, possibly due to enrichment as a result of loss of hemicellulose components.

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Table 2. Elemental composition, trace elements and biomass chemical composition of pretreated biomass feedstocks Property Rwood Elemental composition, wt.% a N

Insights into Preventing Fluidized Bed Material Agglomeration in Fast

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