Biomass Fast Pyrolysis Using a Novel Microparticle Microreactor

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Biofuels and Biomass

Biomass fast pyrolysis using a novel micro-particle micro-reactor approach: Effect of particles size, biomass type and temperature Ali Zolghadr, Joseph James Biernacki, and Ronald J. Moore Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03395 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 3, 2018

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Biomass fast pyrolysis using a novel micro-particle micro-reactor approach: Effect of particles size, biomass type and temperature Ali Zolghadr*, Joseph J. Biernacki*† and Ronald J. Moore& *Department of Chemical Engineering, Tennessee Technological University, Cookeville, TN 38505, USA &Pacific Northwest National Laboratory, Richland, WA, 99354, USA

Abstract: Biomass fast pyrolysis is emerging as a front-running approach for the generation of renewable chemical and fuel resources. The pyrolysis temperature, solid and gas phase residence times, and biomass particle size and type have a substantial impact on char, oil and gas yields. A laboratory-scale fast pyrolysis technique was demonstrated using manufactured biomass microspheres. A unique single-particle (~10µg) micro-reactor technology coupled to a millisecond response flame ionization detector (fast-FID) was used to investigate the effects of relevant particle and process parameters and to capture the dynamic of real-time micro-scale single-particle pyrolysis for the first time. The manufactured biomass microspheres were produced by spray drying finely milled microcrystalline cellulose, switchgrass (Panicum virgatum) and tall fescue straw (Festuca arundinacea) flour. Keywords: biomass; fast pyrolysis; single particle; cellulose; switchgrass; tall fescue; flame ionization

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1. Introduction Biomass is the only long-term option for the production of renewable carbon-based fuels and chemicals. The use of biomass is not only renewable but can be carbon neutral.1 Pyrolysis, the thermal decomposition of organic matter in the absence of oxygen,2 is a front-running technology for transforming the sun-captured carbon in biomass into bio oil, biochar, and other basic chemicals. Pyrolysis is generally classified into fast and slow according to heating rate. Fast pyrolysis is favored when high liquid and gas and low char yields are desired.3 High heating and heat transfer rates and short residence times of products in the vapor phase are some of the essential conditions needed for industrial fast pyrolysis reactors to be more efficient and to produce more liquid products.4 Fundamental scientific information such as the temperature-time evolution of the decomposition chemistry (i.e., the entire reaction pathway) is required to design industrial fast pyrolysis reactors with optimal performance.5-10 The reactor is central to fast pyrolysis and a vast body of literature is presently available on reactor types. Much research has focused on semiindustrial (demonstration-scale) reactor designs and configurations that include systems based on (circulating) fluidized beds, 10 laminar entrained flow,11 countercurrent flow, 12 rotating blades, 13 and rotating cone reactors14, in addition to ablative, vortex, 15 and vacuum reactor configurations.16 On the laboratory-scale, research has focused on characterization of both weight loss, condensable and non-condensable volatiles. Various laboratory techniques such as radiant flash pyrolysis, 17 wire-mesh heating, 18 drop tube processing, 19 micropyrolyzers, 20-22 pyroprobes, 23-25 pyrolysis gas chromatography/mass spectrometry (Py-GC/MS)11-14 and most recently the Pulse-Heated Analysis of Solid Reactions (PHASR) technique, 26, 27 have all been used to achieve high heating rates.


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Figure 1. Timeline of experimental pyrolysis techniques showing pros and cons of recent new methods including the MSMR.

Figure 1, illustrates a timeline of experimental techniques which have been used to study pyrolysis beginning in the mid-20th century to present. A broad range of work has been published on biomass pyrolysis (especially cellulose) using bulk, batch, slow pyrolysis since the beginning of the 20th century.28-34 Likely the most used methods has been thermogravimetric analysis (TGA), which became widely used by about 1960.35-38 Py-GC/MS was introduced shortly after and has been a major tool used in the elucidation of reaction pathways.39-42 Techniques such as the free fall reactor (drop tube), radiant flash pyrolysis,17 wire-mesh heating and other laboratory-scale micropyrolyzers are less frequently cited among others. Gravimetric-based bulk, slow pyrolysis methods provide information only about the amount of oil, gas and char produced whereas, realtime gravimetric analysis (i.e. TGA) provides some, yet lumped, kinetic information. Methods that couple either batch or gravimetric-based reactors, e.g. pyroprobes, with gas analysis have provided much-needed information regarding product composition. In addition, pyroprobe-FTIR


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spectroscopy has been used to unravel the timescale of the diffusion-limited fast pyrolysis process.41,43 Until recently such methods were limited by the slow response of gas analyzers and so kinetic information was largely lost or at best lumped because of poor time resolution. The introduction of the PHASR reactor, however, is resolving many of these obstacles and is now providing time partitioned quantitative gas phase information. To achieve milli-second quantitative gas analysis the technique relies on quenching, i.e. (starting and stopping) and rapid thermal cycling of the reaction mass, and uses a metallic heating surface which might introduced catalytic effects. Nonetheless, the PHASR technique is a fast, milli-scale experiment. Based on these previous studies, it appears that there is yet no information on real-time fast pyrolysis of single particles. [Vinu and Broadbelt8 emphasize this point strongly. Thus, A major bottleneck towards a fundamental understanding of fast pyrolysis is the experimental difficulty in obtaining the time evolution of products and the total reaction time corresponding to complete conversion of the initial biomass in a typical micro-pyrolyzer or Pyroprobe reactors.] Since existing methods such as the free fall reactor and Py-GC/MS are suitable only for lumped quantitative analysis of evolved gases from fast pyrolysis of large amounts of biomass (mg to g sample sizes), there is yet no information

on the time evolution of single biomass particles at

the micro-scale (i.e. µg). Thus to forward the fundamental knowledge of kinetics and chemistry of biomass fast pyrolysis the biomass microsphere micro-reactor (MSMR) is introduced as an essentially new fast pyrolysis technique. The MSMR is designed to provide a real-time singleparticle quantitative lumped mass balance using fast gas phase detection. The pros of this technique include real-time quantitative kinetics response of single particles under fast pyrolysis condition, well-homogenized, water-free-based biomass microsphere particles and non-metalic reactor. However, no fast pyrolysis experiment is perfect. In the case of the MSMR, the events are


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inherently and deliberately transient. Since TGA is limited by heating rate and to some extent sample size, the MSMR turns to fast gas phase analysis instead. Heating rates for samples on the order 2 to 57 µg can be as high as 10,000 K/s. Dead volume and dilution also are problems with other techniques that are indeed minimized by the MSMR method. While the MSMR does not solve all problems, it was developed with the express intent of capturing the complete real-time reaction history for individual, geometrically similar spherical particles weighting nominally 30 µg. [This study includes an experimental investigation of fast pyrolysis using the MSMR that characterizes the real-time fast pyrolysis response of single particles of various size and biomass type as a function of reactor temperature and establishes reactor and particle characteristics such as reproducibility of experiments and uniformity particle-to-particle.] Three different types of biomass microsphere particles, microcrystalline cellulose, switchgrass and tall fescue, were made in various sizes with masses ranging between 2 and 57 µg by spray drying finely milled biomass flour. The effect of reactor temperature and type and size of biomass particles on the real-time amount of evolved pyrolysates and char shrinkage was investigated. The injection velocity of individual particles of different size was measured as a biomass microsphere particle uniformity metric.

2. Material and methods 2.1. Raw materials The switchgrass (Panicum virgatum L., variety “Alamo”) and tall fescue straw (Festuca arundinacea S., variety KY 31) used in this study were cut from well-established five-year-old stands that were grown in the summer and harvested in the fall from Shipley Farm, Tennessee Technological University, Cookeville, TN. Soil samples were taken at the same locations each


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year, with phosphorous (P), potassium (K) and lime applied at the levels recommended by the University of Tennessee’s Forage Testing Laboratory. Each plot also received annual springtime applications of 6.5 g/m2 of nitrogen (N)-based fertilizer for forage production. The crops were cut 25 cm above the ground. A Penn State Hay Sampler (45.72 cm long and 2.54 cm in diameter) attached to a “brace and bit” was used to sample the harvest after baling. A core measuring half the length of the bale was taken from one end from more than one bale to produce a representative sample. The tall fescue that was used in the study was harvested in the early part June when the seed heads were fully mature. Each bale was roughly 1 m long, 0.41 m high, and 0.46 m wide. The switchgrass plots were harvested only once each year, with the harvest used here completed in August. The hay was made into small, square bales varying from 23 to 27 kg each. All bales were stored inside of a barn, inside a room where they were protected from rainfall, animals, and deterioration due to climatic conditions, e.g. outdoor humidity and sun. All of the plant material was dried in a forced-air drying oven at 71℃ for a number of days. Microcrystalline cellulose, designated as “Sigmacell” (sold as product number S3504, with a 20 μm nominal particle size), was bought from Sigma-Aldrich Co., St Louis, MO, USA. Neat ethanol (Pharmco-Aaper, ACS/USP grade, 200 proof) was used as a spray drying vehicle to suspend the milled biomass materials. Ultra-high purity-grade nitrogen (UHP, Airgas, CO, USA, 99.999% pure) was used to atomize the spray drying slurry and to purge the reactor, and as a carrier gas in all of the pyrolysis experiments.

2.2. Preparation of biomass microspheres 2.2.1 Reduce the size of biomass


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The following procedure was used to reduce the size of raw whole biomass, switchgrass and tall fescue, materials. The raw biomass was oven-dried for 24 h at 60℃. The dried biomass was then cut into pieces of about 1 cm long. The cut pieces were comminuted in a coffee mill (Braun KSM2) and then using a high-intensity ball mill (SPEX Sample Prep, 8000M, Mixer/Mill, Metuchen, NJ, USA). During the coffee milling step, the material was processed in small portions of about 8 g for 10 min. Coffee grinding was done in intervals, mill-rest-mill-etc., to limit the mill temperature to no more than 55℃. The combined coffee ground materials were then ball milled using 9.525 mm diameter (3/8th inch) spherical alumina (99.95%) balls filling about 35 to 40% of the total volume of the milling vial (total volume of the vial was 45 ml). Again, milling was done in intervals to limit the temperature to no more than 55℃. The ball milled material was passed through a 400 mesh screen. The resulting biomass flours were found to have average particle sizes of 9.27 and 11.82 µm for tall fescue and switchgrass respectively using a Beckman Coulter LS 13 320 laser diffraction particle size analyzer. Since the as-received microcrystalline cellulose was already fine (nominally 20 μm) it was ball milled for 4 h and then sieved to produce an average particle size of of 16 µm. Table 1 shows the elemental analysis (C, H, O, N, S) of the three biomass materials (Galbraith Laboratories, Inc (GLI),. Knoxville, TN ).

Table 1. Chemical analysis of switchgrass, tall fescue and microcrystalline cellulose. Aluminum¥ Carbon

Hydrogen

Nitrogen

Oxygen

Sulfur

Switchgrass

0.89

42.64%

6.02%

< 0.50 %

44.89%

0.068%

Tall Fescue

1.60%

40.65%

5.93%

0.55%

42.93%

0.10%

40.32 %

7.18 %

< 0.5 %

49.97 %

na

Sample

Microcrystalline 51.3 ppm Cellulose

na Not Applicable. ¥A portion of the aluminum is contributed by the milling contamination.


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The carbon, hydrogen, and nitrogen contents were determined by Galbraith Laboratories, Inc. (GLI, Knoxville, TN), using a PerkinElmer 2400 Series II CHNS/O analyzer. The oxygen, aluminum, and sulfur contents were determined using a Thermo Finnigan FlashEATM elemental analyzers, GLI-ME-70, and LECO SC-432DR respectively.

2.2.2 Producing biomass microspheres The biomass flours prepared using the above procedure were then reconstituted into spherical particles using ethanol to form a slurry and then spray drying. The main components of the spray drying apparatus were a cylindrical drying chamber (5.1 m height and 0.51 m diameter) and a spray nozzle. The bottom of the cylindrical chamber was a conical section measuring 0.41 m height, with a 0.53 m inlet diameter and a 6 mm-diameter outlet. The particles, which entered the top of the chamber as slurry via the atomizing nozzle, were gathered in dried form on a paper plate at the cone outlet. A hot air blower (Master Heat Gun®, HG-301 A) sent warm air through an inlet 20 cm above the cone to produce an upwards draft. Three air inlets were evenly spaced around the hot air blower to allow airflow to be introduced by convection. An spray nozzle (Spraying Systems Co., Air Atomizing, 1/8 J, Full Cone, Round Spray, and Spray Set-up No. SU11) was used to atomize the slurry. A pressurized spray drying tank (7.57 kg, 140 psi (0.96 MPa), -29 to 38℃, Spraying Systems Co.) was used to force the slurry to the spray head at 20 psi (0.14 MPa). The slurry was kept at 60℃ in the spray drying tank and was stirred continuously. The procedure for producing biomass microspheres was as follows: The milled, screened biomass was mixed with ethanol in a weight ratio of 25% milled biomass to 75% ethanol to make the slurry.


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The slurry was continuously stirred for approximately one hour before it was transferred to the spray tank. The slurry was kept at 60℃ in the spray tank by heating the tank while constantly stirring it to prevent any settling. The spray drying tank was pressurized to 20 psi (0.14 MPa) with UHP-grade nitrogen. The outlet of the pressurized spray tank was connected to the slurry inlet of the atomizing nozzle and the atomizing gas (UHP grade nitrogen) pressure was set to 5 psi (0.03 MPa). Meanwhile, the spray column was heated for at least 30 minutes before any slurry was sprayed. The atomizing nozzle was pointed directly down into the spray column from overhead. While the atomizing gas was running, the slurry was introduced for three seconds in order to produce a small burst of droplets. This procedure was repeated until enough slurry was sprayed to produce several grams of particles in one lot. Finally, the biomass particles were collected at the bottom of the column. The spray column was carefully vented between spray bursts to prevent the ethanol content of the column purge gas from becoming flammable. Microscopic images and details about the manufacturing process for microsphere particles is described elsewhere.44

2.3. Fast pyrolysis micro-sphere micro-reactor (MSMR) 2.3.1. Experimental apparatus Figure 2 illustrates the fast pyrolysis MSMR set-up. A vertical pyrolysis reactor configuration with a small and well-defined hot zone was implemented. The micro-reactor consisted of a quartz tube (1 mm ID; 3 mm OD; 74 cm long). A short section of this tube was externally heated using a 6 mm ID spiral silicon carbide heating element (Surface Igniter LLC, Maryville, TN). Temperature was controlled using a J-type thermocouple mounted


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within a separate 3 mm OD×1 mm ID quartz tube adjacent to the reactor volume but inside the heating element. Closed loop temperature control with current measurement was used.

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10

9 8

4

6

7

Line 1

4

3 1

Line 2

5

Figure 2. Schematic of the micro-sphere micro-reactor (MSMR). (1) carrier gas (2) flow controller (3) electrical three-way valve (4) pressure transducer (5) biomass microsphere (6) clocking lasers (7) silicon laser detectors (8) ceramic fiber (9) thermocouple (10) silicon carbide heating element.

The actual reaction volume was defined by the active length of the quartz tube that was heated. A small, loose bundle of high alumina ceramic fiber (Fiberfrax®, Unifrax, Tonawanda, NY) 1 to 2 mm in length was inserted into the micro-reactor at the hottest point to trap the biomass microsphere particle, i.e., to prevent the particle from passing through the hot zone. Ultra-high purity (UHP) grade N2 carrier gas containing no more than 1 ppm O2 was diverted through an electrical three-way valve and split into one of two directions; each direction was equipped with a fast pressure transducer (1 ms response time from zero to full range) to indicate the line pressure just upstream of the particle injection and purge points. Laser and diode detectors were used at four different axial positions along the quartz tube to indicate the passage of an injected particle


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from which the particle velocity and time of arrival within the hot-zone (at the fiber trap) were determined. Finally, to indicate the rate of pyrolysates production, a fast flame ionization detector (fast-FID, ~1 ms response time) was used (Cambustion, Inc., HFR-400 Fast FID). All of the data was transferred to a computer via fast data acquisition and simultaneously recorded using a LabVIEW program and National Instruments interface electronics. To demonstrate the reliability of the FID analyzer, a GC Q Exactive mass spectrometer (without GC) (Thermo Scientific, San Jose, CA) with the source configured for electron impact (EI) ionization was used instead of the FID. A 70 cm length of 360 µm o.d., and 75 µm i.d. fused silica tube (Polymicro Technologies Inc., Phoenix, AZ) was installed in the EI source in place of the GC column. The inlet was then directly connected to the outlet of MSMR ~3 cm from the fiber trap so that sampling occurred as close as possible to the pyrolytic event. The sample flow rate into the EI source (70 eV) was estimated to be ~200 to 500 µL/min. The entire 70 cm length of fused silica tube was kept at 200℃. Orbitrap mass spectra were acquired from 50 to 500 m/z using a resolution setting of 15k, automatic gain control (AGC) target of 106, and a maximum ion accumulation time of 50 ms. The ion source was maintained at 225℃. Under these conditions, the data acquisition rate was ~11 Hz but could approach ~18 Hz during pyrolytic events when ion populations were high resulting in shorter ion accumulation times.

2.3.2. Experimental procedure Biomass microsphere particles were introduced one at a time into the micro-reactor tube lock at Point 5 (refer to Figure 2). The gas feed, Line 2, was then connected and the flow rate was gradually increased to 10 cm3/min to purge the reactor yet prevent the particle from moving. After 10 min of purging, the three-way valve was switched so that the carrier gas flowed through the by-pass,


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Line 1, and the flow rate was increased to 28 cm3/min. This established a hydrodynamic equilibrium at the injection flowrate prior to injecting the particle. The pressures were monitored until they become steady, typically a few seconds, after which the three-way valve was switched so that the gas again flowed through Line 2 (and Point 5, the lock) and the particle was carried into the furnace. As the particle moved through the quartz micro-reactor, it passed four lasers which clocked the velocity of the particle. The particle was finally trapped in the hot zone at Point 8 using a small piece of ceramic gauze. The fourth laser was positioned at the gauze and indicated the time of arrival of the particle in the hot zone. The particle was pyrolyzed and the gases pulled into the FID. The Cambustion HFT fast-FID use a unique sampling system that provides millisecond response time. High-speed data acquisition was used to record all of the data simultaneously.

3. Results and Discussion The MSMR was used here to explore particle uniformity and the effect of particle size, biomass type and reactor temperature on real-time pyrolysis response, i.e. the amount of pyrolysates produced and reaction rate dynamics.

3-1 Effect of biomass microsphere particle size In this study, spray drying was introduced for the first time as a way to produce homogeneous manufactured biomass microspheres in the range 160 to 400 µm in diameter and weighing roughly 2 to 57 g. These single particles are at least one-hundred to one-thousand times smaller than the smallest sample masses previously used, e.g. Py–GC/MS experiments typically use a 1 mg sample and TGA uses a 5 mg sample. Each biomass microsphere particle is a small, porous and homogeneous packet of biomass. Individual microspheres were selected and the projected area of


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each particle was measured from three random perspectives to ensure spherical uniformity, refer to Supporting Information Figure S-1 and Table S-1. Selected spherical particles were then pyrolyzed, and the fast-FID peak area plotted as a function of particle volume, refer to Figure 3.

Figure 3. Amount of detectable pyrolysates produced, i.e. area of the FID peak, as a function of particle size, i.e. particle volume, for tall fescue microspheres pyrolyzed at reactor temperatures of 500, 600, 700, 800, and 900℃.

The plot demonstrates exceptional linearity for tall fescue microsphere particles between 160 to 400 µm in diameter when pyrolyzed at 500, 600, 700, 800, and 900℃. This result shows that the ratio of FID-detectable pyrolysates produced to particle volume is independent of the size of particle for microsphere particles smaller than 400 µm at a constant temperature. Previous work shows that for fast pyrolysis of biomass particles smaller than 200 m (in a free fall reactor) the yield of tar is about 3%, while the yield of gas is ~ 83%.45 Since the MSMR has much faster heating rates and much lower gas-phase residence times it is expected that the majority of produced


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pyrolysates remains in the vapor phase and is detected by the FID. In a previous study, the effect of particle size on gas yield was investigated by pyrolyzing particles of pine sawdust and apricot stones (pits) of different size in a free fall reactor.45 It was found that pyrolysis outcomes for biomass particles above 200 m is controlled by mass and heat transfer and that for particle sizes smaller than 200 m, outcomes are kinetically controlled and that gas compositions are dominated by the reactor temperature. It was also observed that a small difference in hydrocarbon yield for particles smaller than 100 m up to 450 m persists. Another study, however concluded that there is no effect of particle size on gas composition and that biomass completely pyrolyzed in a free fall reactor at high heating rates and temperatures (1000℃, 1200℃ and 1400℃ ).46 In yet another study, no significant effect of particle size in the range from 44 m to 2 mm on the yield of product was found for flash pyrolysis (~1000℃/s).47Finally, a resent review paper investigated the effect of heat transfer and mass transfer on different particle sizes and product yield concluding that there is a lack of information on the effect of the particle size on product distribution and reaction mechanism.46-50 The results of the present MSMR study, however, suggests that there is no effect of particle size on product yield when the influence of mass transfer and intra-particle secondary conversion of volatiles is eliminated. Similar results are shown for switchgrass and microcrystalline cellulose microspheres in the Supporting Information, refer to Figures S-2 through S-6. Also, a simple analysis using the Biot (Bi) number and dimensionless pyrolysis (Py) numbers is provided to estimate the reaction condition for different particle sizes at different temperatures (See Appendix C of the Supporting Information).


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Figure 4. The velocity of tall fescue microspheres of different particle sizes as measured in the reactor cold-zone using Lasers 2 and 3 (refer to Figure 2) as a function of particle radius; note that the reactor hot-zone temperature was 700℃.

The particle velocity as a function of particle radius was also found to be highly linear, further supporting the assertion that manufactured biomass microspheres are very uniform and the experimental techniques very reproducible, refer to Figure 4. Similar figures for switchgrass and microcrystalline cellulose microspheres are included in the Supporting Information, refer to Appendix A. It is compelling that the velocity of the particle is a linear function of particle radius in the cold-zone of the reactor, i.e. the portion of the reactor that is not heated. The actual particle trajectory is very complex and involves the body force, drag force and shape of the flow profile in the reactor tube. Studies of particle motion in laminar liquid flow through tubes show that small particles migrate into a flow streamline off the axial center of the tube and that prediction of such is quite challenging.51 Nonetheless, in the present work, it was observed that particle velocity is


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linear with respect to particle radius suggesting particle uniformity and experimental repeatability, factors critical to the interpretation of pyrolysis outcomes in the MSMR system. The MSMR technology introduced in this paper enables the capture of real-time reaction dynamics of individual particles under fast pyrolysis conditions. This novel capability was used here to characterize the reaction dynamics as a function of particle size, biomass type and reactor temperature. Figure 5a shows the real-time rate of pyrolysates production for tall fescue microspheres of different particle size at 800℃ and Figure 5b shows the normalized data. Normalization was done by dividing the y-values by the area under the curve of each FID peak, thus the area of the FID curve will be equal to one for all particle sizes. Time zero for the raw FID data is the time at which the particle triggered (was indicated by) Laser 4, i.e. the time when the particle reaches the fiber trap, the hottest point in the reactor. As Figure 5 illustrates, the time delay (the time needed for the pyrolysis products to reach the detector) is about 60 ms at 800℃. As a result, the heat transfer effect on the rate of reaction for different particle sizes is realized. Figure 5b shows that the rate of pyrolysates production for small particles is much faster than for large particles. Particles of different size are expected to have different temperature histories, smaller particles experiencing higher heating rates than larger particles. The difference in the dynamic behavior of particles of different size is a reality in reactors of all scale which process particles in mass, i.e. many particles simultaneously either in a batch (e.g. drop tube and pyro-probe reactors) or continuously (fluid or moving bed reactors). Refer also to the complete dataset of FID signals and normalized FID signals for switchgrass, tall fescue and cellulose at 500, 600, 700, 800, and 900℃ in the Supporting Information, Figures S-7 through S36.


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Figure 5a. The raw FID signal (instantaneous amount of pyrolysates in vent gas) versus time for tall fescue microspheres of various size at a reactor temperature of 800℃.

Figure 5b. The normalized FID signal (i.e. peak area is normalized to unity) versus time for tall fescue microspheres of various size and a reactor temperature of 800℃.


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The advantage of the MSMR technique is that individual particles of known geometry (i.e. spherical) can be studied and datasets such as Figure 5 developed and used to quantify the timedomain fast pyrolysis dynamics of individual particles for the first time. The main outcome of this work indicates that the total area under the FID curve, i.e. the amount of pyrolysates produced, is a linear function of particle volume (i.e. particle mass), refer to Figure 3. Though the rate of reaction is vastly different for different particle sizes (Figure 5b), nonetheless, the amount of pyrolysates produced per unit mass of matter is a constant. The amount of pyrolysates produced is a linear function of particle volume (i.e. particle mass and particle size at constant particle bulk density and processing conditions), refer to Figure 2. This observation suggests, at least qualitatively, that there is no mass transfer limitation; under conditions of mass transfer limiting reaction, less pyrolysate would be expected for the larger particles for which secondary reactions would be detectable. Vinu and Broadbelt8 calculated rate curves for reactions of cellulose under idealized isothermal conditions at 500 oC which suggest likewise idealized reaction times. Their results indicate reaction times on the order of seconds – outcomes that are not inconsistent with reaction times observed experimentally using the MSMR and particles, for example, between 80 and 156 um (refer to Figure 10(d) – thus supporting the assertion that manufactured microspheres are free of mass transport effects under MSMR conditions of reaction. Notably, however, heat transfer effects are present.. This indicates that the amount of pyrolysates is an intrinsic property of the biomass and that heat transfer affects the rate of reaction only whereas mass transfer affects intra-particle secondary reactions which in-turn affects the amount of observed pyrolysates. Manufactured biomass microspheres reacted in the MSMR apparatus appear to be virtually devoid of mass transfer limitations as indicated by these results.

The same trend is observed for

microcrystalline cellulose, switchgrass and tall fescue, refer to the Supporting Information, Figures


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S-2 through S-36.

3-2 Effect of biomass types Whereas the effect of particle size was discussed above, the focus is now turned to differences in pyrolysis response due to biomass type. Again, two grass types, switchgrass and tall fescue, and microcrystalline cellulose were considered. Again particle velocity as a function of size for all material types was found to be very linear, indicating a high level of particle-to-particle uniformity, refer to the Supporting Information, Figures S-2 through S-6. Switchgrass particles moves faster than tall fescue which move faster than microcrystalline cellulose particles, consistent with previously reported particle density values for switchgrass (~0.52 g/cm3), tall fescue (~0.55 g/cm3) and the microcrystalline cellulose (~0.61 g/cm3). To correct FID peak areas for these differences in particle density, the FID peak areas were plotted as a function of normalized volume relative to cellulose, i.e. Normalized Volume Switchgrass = Volume Switchgrass × (Density of Cellulose /Density of Switchgrass). The results indicate that per gram of matter, pure microcrystalline cellulose produces more detectable gas than tall fescue and switchgrass. Likewise, tall fescue produces more detectable gas than switchgrass, refer to Figure 6. The same trend is observed at other temperatures, refer to Figures S-2 through S-6 in the Supporting Information. Other studies also report a higher production of gas for microcrystalline cellulose as compare to whole biomass.52-54 Figure 7. illustrates a comparison between normalized FID signals (i.e. the time-domain FID integral is unity) versus time for the three different types of biomass with near the same particle sizes (it was impossible to pick particles with exactly the same size).


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Figure 6. FID peak area as a function of normalized particle volume for microcrystalline cellulose, switchgrass and tall fescue pyrolyzed at a reactor temperature of 800℃.

Figure 7. Density-normalized FID peaks for different types of biomass with kind of the same sizes and a reactor temperature of 700℃.


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These real-time gas analysis results show that there is clear evidence of multiple reaction processes for microcrystalline cellulose, whereas for tall fescue and switchgrass the same is not as evident. This result is surprising since TGA results for the same materials, e.g. at 10℃/min, show clear evidence of multiple reaction processes for whole biomass types and no evidence of such for microcrystalline cellulose9, 55. Thus, multiple processes were not expected for the pure-component cellulose material. To demonstrate repeatability, the results for a second microcrystalline particle of nearly identical size is also shown on Figure 7. Furthermore to demonstrate that the FID peak shape is not an analyzer artifact, the MSMR was coupled to a Q Exactive mass spectrometer with a resolvable data acquisition rate of about 10 Hz and the total mass ion signal was compared with the fast-FID data, refer to Figure 8. While the signal time resolution is not as good for the mass spectrometer (about 1/60 s) as it is for the FID (about 1/1000 s), the total mass signal and the fastFID data are nominally the same. Notably, the FID and MS were not performed simultaneously, i.e. the FID and MS were performed on different particles of nominally same size. This makes the outcomes even more compelling, showing that real-time gas dynamics can be captured for single particles by more than one technique, thus validating the outcomes.

Figure 8. Comparison between FID result and Q Exactive mass spectrometer for pyrolysis of switchgrass, tall fescue and microcrystalline cellulose microspheres at 500℃. Note that traces are


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for different particles of nominally same size, i.e. that FID and MS were not performed simultaneously.

It would be difficult to conclude that the observed behavior for cellulose is not due to multiple reaction events. The result of two recent studies clearly support the hypothesis that cellulose undergoes some form of shift in reaction mechanism between 400 and 500℃. In one study the sample temperature was directly measured during the fast pyrolysis of cellulose using a “captive sample pyrolysis reactor (CSPR)”. In the CSPR the sample was heated at a constant heating rate, however, the presence of a strongly endothermic reaction between 400 and 500℃ limited the heating rate to no more than 165℃/s, even when the set-point heating rate was 20,000℃/s.23 It was reported that the slowest heating rate was observed between 420℃ and 430℃. The second study concluded that the mechanism of thermal decomposition of cellulose shifts from chain-end cleavage (unzipping) to intra-chain scission at a temperature of 467℃ 26 For temperatures below 467℃ decomposition is dominated by end-chain unzipping with apparently negligible heat effects. However, above 467℃ there is rapid random depolymerization (intra-chain scission) associated with melt (liquid) formation and volatilization and clear endothermic effects. There is a notable difference (~30℃) in the reported temperature at which the transition occurs, however, in general, there is a good agreement. In the present MSMR research, an alternative method was used in which the gas phase data was the primary indicator of reaction rates as opposed to thermal or kinetic indicator experiments. In this case, a distinct transition occurs during the heating of single microcrystalline cellulose microspheres, particularly noticeable (quantifiable) for lower target reactor temperatures, i.e. lower heating rates, consistent with the prior research which finds that end member unzipping is not observed when the sample is rapidly heated above 467℃ since the


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sample does not spend enough time below this temperature to exhibit significant end member decomposition. At present, it is unclear as to why this multiple mechanism is not observed for whole biomass. Notably, all biomass microspheres were produced from materials that were milled for 4 h. Such milled cellulosic materials contain largely amorphous cellulose independent of origin. Thus, the bifurcation noted for the micro-crystalline cellulose microspheres is not likely due to the crystallinity of the materials which were produced from a crystalline source but was effectively reduced to amorphous cellulose by milling. The peak bifurcation observed for the cellulose particles may, however, be due to the difference in mechanism of cellulose fast pyrolysis (parallel series dependent reaction) in comparison to whole biomass (series reaction). This unanswered question needs to be the subject of continued inquiry.

3-3 Effect of temperature on the fast pyrolysis process To investigate the effect of temperature on the amount of pyrolysates, the amount of pyrolysates was regressed as a function of temperature and particle volume for each type of biomass. The result show a good linear correlation between temperature and pyrolysates amount for same particle size (R2=99), refer to Appendix B of the Supporting Information, Figure S-37, S-38 and S-39. The amount of pyrolysates increases linearly as the temperature increases. To further analyze the effect of temperature the slope of the FID peak area versus particle volume lines was plotted as a function of temperature for microcrystalline cellulose, switchgrass and tall fescue, refer to Figure 9. For example, for tall fescue, the slope of the lines on Figure 2 were determine, i.e. the FID peak area per volume, and then the slope values were plotted as a function of temperature, refer to Figure 9.


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Figure 9. The slope of FID peak area versus volume as a function of temperature for microcrystalline cellulose, switchgrass and tall fescue.

Figure 9, thus represents the effect of temperature on the amount of pyrolysates produced for the different biomass types. Although a straight-line through the data provides a good fit (R2=0.97), the collective data suggests an inflection between 500 and 900℃ which might relate to char decomposition8,50 and the secondary tar reactions.12,52 Furthermore, these results are in good qualitative agreement with the yield of gas at different reactor temperatures for other biomass types reported in vastly different studies.56-62 The MSMR experiments are inherently dynamic i.e. events are always non-isothermal. For constant reactor temperature, small particles heat faster. For constant particle size, particle are heated faster at higher reactor temperature. Thus, temperature effects are discernible by both changing the reactor temperature and holding particle size constant, or by changing particle size and holding the reactor temperature constant. Here, both approaches are demonstrated.


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The effect of temperature on real-time pyrolysates production rate was also assessed for particles of roughly same size, refer to Figure 10 (a) and (b) for switchgrass and crystalline cellulose respectively. As expected, as the reactor temperature increases the reaction rate increases. The same trend is observed for tall fescue, see the supporting information Figure S-39.

Figure 10 (a). FID signal versus time for switchgrass microsphere particle with close particle size at the different temperatures.

Figure 10 (b). FID signal versus time for microcrystalline cellulose microsphere particle with close particle size at the different temperatures.


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Notably, for microcrystalline cellulose the reaction rate peak bifurcates into two peaks at higher temperatures. This behavior was also noted when particles of different size were pyrolyzed at the same temperature, refer to Figure 10 (c). For small particles, the peak appears as a single homogeneous event, whereas for larger particles, the peak is bifurcated, refer to Figure S-27 through S-36 in Supporting Information for additional data sets. This peak bifurcation (separation) is particularly notable as temperature decreases, compare Figures 10c and 9d at 700℃ and 500℃ respectively. At the lower temperature, the pyrolysis event is characterized by two distinct peaks, even for very small particles. FID effectively counts of the number of carbon atoms present in the vapor phase and provides a calibratable signal proportion of the number of carbon atoms in the vapor.63 In this way, FID is semi-quantitative. Although, most fast pyrolysates (83%) 45 are vapors, the composition of the pyrolysates cannot be determined using FID. In this way, FID is only qualitative. The authors recognize these limitations and have already mapped a pathway for coupling the MSMR to fast mass spectroscopy instruments.

Figure 10 (c). FID signal versus time for microcrystalline cellulose at different particle sizes at 700℃.


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Figure 10 (d). FID signal versus time for microcrystalline cellulose at different particle sizes at 500℃. 3-4 Shrinkage of biomass microspheres in fast pyrolysis process. In addition to the real-time gas analysis, char particles were also retrieved after complete pyrolysis. Particles were pyrolized for a total time of 60 s after which time they were cooled in situ to room temperature before removing from the reactor. The size of the resulting charred microspheres were measured using an optical microscope. The results, refer to Figure 11 for tall fescue experiments, shows that the final diameter varies linearly with initial diameter, i.e. shrinkage is not a function of particle size for a fixed pyrolysis temperature. This direct proportionality is maintained for a 100-fold change in particle volume (a 5-fold change in diameter), again indicating that manufactured biomass microspheres are extremely uniform and devoid of mass transferinduced char forming secondary reactions.


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Figure 11. Shrinkage of biomass for tall fescue microspheres at different pyrolysis temperatures.

Shrinkage was also observed to increases as the pyrolysis temperature increases. Similar results were observed for switchgrass, see the Supporting Information Figure S-40. This result is striking since we have already demonstrated, as expected, that fast pyrolysis of particles as large as 100 to 400 µm in diameter are burdened by heat transfer, as indicated by Figure 9. The combined observations indicate that char formation is an intrinsic property in the absence of mass transfer limitation, and is only a function of final temperature and not a function of heating history. There was no detectable (retrievable) char for microcrystalline cellulose. This however, does not imply zero char, only that no char could be retrieved in these experiments. Finally, electron microscope images of chars produced at three different temperatures (500, 700 and 900℃) support a hypothesis for the formation of a liquid-phase intermediate at high heating rates, refer to Figure 12. The microstructure of spheres pyrolyzed at 500℃ retains the size and shape of the parent milled biomass flour from which the microspheres were manufactured.


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Figure 12. Scanning electron microscope (SEM) images of switchgrass microspheres at different temperatures: a) TGA at 10℃/min up to 1000℃. b) MSMR at 500℃. c) MSMR at 700℃. d) MSMR at 900℃. A progressively more fused (sintered) appearing microstructure emerges as the pyrolysis temperature is increased from 500℃ to 900℃. At the higher temperatures, 700℃ and 900℃, the surface of the charred microspheres appears smooth, glassy and as if a liquid phase was present at some point during the pyrolysis event. It seems that heating rate plays a central role. Similar particles pyrolyzed in a TGA at 10℃/min up to 1000℃ show no evidence of a persistent liquid phase, refer to Figure 12a.

4- Conclusion The microsphere-micro-reactor (MSMR) technique introduced here captures the real-time dynamics of individual spherical biomass microparticles. This approach overcomes problems associated with dead volume and extensive gas dilution by controlling the flow rate of carrier gas into a minimal effective reactor volume. Since the reactor is made of quartz glass, there is no


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potential for secondary heterogeneous catalysis prior to detection. Thus, the MSMR system provides an opportunity to uniquely study the impact of particle size, temperature and type of biomass on single microsphere particles having a controlled geometry (spherical) in real-time under fast pyrolysis conditions. The results show that the amount of pyrolysates (fraction) produced is an intrinsic property of biomass, constant for different particle sizes independent of existent heat transfer limitations (i.e. despite the existence of heat transfer effects) yet in the absence of mass transfer constraints. Furthermore, highly linear results indicate that manufactured biomass microspheres are very consistent particle-to-particle. Microcrystalline cellulose produces more pyrolysates in comparison to whole biomass materials (switchgrass and tall fescue) and exhibits at least two distinct pyrolysis events. This observation appears to be consistent with the recently proposed chain-end cleavage to intra-chain scission transition at about 460℃ and furthermore appears to be dependent upon heating rate and coincides with a change in the thermisity of the reaction, i.e. becomes more endothermic. Whole biomass does not exhibit these characteristics. Finally, based on SEM images of charred microspheres processed in the MSMR, the microstructure supports a liquid-phase intermediate hypothesis for whole biomass materials whereas no char could be collected for cellulose. Supporting Information. Microscopic image of switchgrass microsphere particle; The volume of six switchgrass microsphere particle from three different perspectives; FID peak area as a function of normalized particle volume for crystalline cellulose, switchgrass and tall fescue pyrolyzed and a reactor temperature of 500, 600, 700, 800 and 900 ˚C.; The raw and normalized FID signal (amount of pyrolysates) versus time for switchgrass, tall fescue and microcrystalline cellulose microspheres


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of various size and a reactor temperature of 500, 600, 700, 800 and 900 ˚C; FID peak area for similar crystal cellulose, switchgrass and tall fescue particle size compare with linear recreation; FID signal versus time for tall fescue microsphere particle with close particle size at the different temperatures; Shrinkage of biomass for different switchgrass microsphere at the different temperatures; The velocity of different crystal cellulose, tall fescue and switchgrass microsphere particle sizes at different zone versus radius of biomass while the maximum temperature of the reactor is 500, 600, 700, 800 and 900 °C; Summery of output of linear regression.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel.: (931) 372-3667 † Tennessee Technological University, Cookeville, TN 38505, USA

Funding Sources National Science Foundation (NSF) under grant number CBET-1360703. Acknowledgements The authors acknowledge support from the National Science Foundation (NSF) through grant award No. CBET-1360703. A portion of the research was performed using the Pacific Northwest National Laboratory (PNNL) Environmental Molecular Science Laboratory (EMSL, Ringgold ID 130367), a Department of Energy (DOE) Office of Science User Facility sponsored by the Office of Biological and Environmental Research.


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