Modified Pyroprobe Captive Sample Reactor: Characterization of

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Modified Pyroprobe Captive Sample Reactor: Characterization of Reactor and Cellulose Pyrolysis at Vacuum and Atmospheric Pressures M. Brennan Pecha, Jorge Ivan Montoya, Cornelius F. Ivory, Farid Chejne, and Manuel Garcia-Perez Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 19 Apr 2017 Downloaded from http://pubs.acs.org on April 20, 2017

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Industrial & Engineering Chemistry Research

Modified Pyroprobe Captive Sample Reactor: Characterization of Reactor and Cellulose Pyrolysis at Vacuum and Atmospheric Pressures M. Brennan Pecha1,2, Jorge Ivan Montoya1,3, Cornelius Ivory2*, Farid Chejne3, Manuel GarciaPerez1* 1 2

*

Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120, USA The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164-6516, USA 3 Grupo Tayea, Facultad de Minas Universidad Nacional de Colombia, Medellín, Colombia

[email protected]; [email protected]

1 ACS Paragon Plus Environment


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Abstract

With existing analytical pyrolysis systems, e.g., TGA, pyroprobe, drop tube furnace, etc., available on the market, it is not possible to monitor the real temperature of a sample or visualize the transformation of the solids. Furthermore, typical sample preparation methods, e.g., using a powder, result in poor mass and heat transfer due to large sample thickness and the explosion of particles during the heating regime. To study and address these problems 1) a new experimental setup was designed to allow temperature measurement and pyrolysis sample visualization using a CDS Pyroprobe 5000; 2) the system was completely characterized with cellulose (85.7 µm thick) as the solid sample; and 3) preliminary results showed the propulsion of cellulose particles out of the quartz sample tube which could be a major source of error. As a result, a thin sample film was used for further experimentation. Pyrolysis experiments using cellulose were performed at near-vacuum (3.5 mbar) and at near-atmospheric (950 mbar) pressures to gain insight into the effect of pressure on pyrolysis. The temperature of the samples, and the yield of solid residue (char) were measured. Results showed that the heating rate of the sample was much slower than the heating rate of the platinum coil. Furthermore, endothermic reactions suppressed the heating rate as the temperature rose above 400°C. Data was extracted from videos of pyrolysis at both pressures to track the darkening of the solid samples. A computer simulation of these experiments including heat, mass and momentum transport at 3.5 and 950 mbar shows how operation at different pressures and heating rates affect the generation and distribution of pyrolysis products. These simulations indicate that convective heat transfer dominates over radiative heat transfer to the cellulose sample at high and low pressures if the heating coil is wrapped tightly around the quartz sample tube. However, radiative heat transfer is somewhat


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more important at lower pressures where the gas-phase thermal conductivity is smaller, especially if the heating coil is loosely wrapped around the sample tube.

Keywords: Pyroprobe, temperature profiles, captive sample reactor, visualization, simulation



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1

Introduction

Pyrolysis is a technology which is used to thermally degrade polymers into oil, char, and gas.1 A detailed understanding of the reaction mechanisms and kinetics is required in order to design more selective reactors.2 The study of biomass pyrolysis reaction mechanisms and kinetics is a contentious topic among researchers in the field.3,4 One of the major challenges for the thermochemical conversion community is to isolate the effect of a reaction of interest from undesirable reactions and transport phenomena (mass and heat transfer) that simultaneously occur under conditions of practical relevance. This task is extremely difficult in a pyrolysis system where most of the primary reactions happen in solid phase leading to the formation of depolymerized products (typically oligomers) which form a reactive liquid intermediate that bubbles when volatile products form by secondary reactions.

Secondary reactions can be

homogeneous reactions in the vapor or liquid intermediate phase or heterogeneous at the particle surfaces.5-8

To study chemical reactions in heterogeneous systems, the reaction of interest must be kinetically controlled (heat or mass transport phenomena should not influence the outcome) 9. The actual temperature of the reacting biomass sample needs to be known. The most common instrument used for pyrolytic kinetic analysis, i.e., the thermogravimetric analyzer (TGA), is limited to the study of reactions with slow heating rates (less than 250 K min-1).3, 10, 11 Heating this slowly is not realistic for collecting data for fast pyrolysis reactors like fluidized beds, which heat particles faster than 100 °C/s. There are some new reactor systems which are capable of very rapid heating including the heated plate reactor,12,

13 14

the wire mesh reactor,15 and the


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radiative flash reactor,16 Today, the most versatile experimental system is the plate reactor, which features infrared temperature measurement of the sample and continuous flow sample collection.13

Although it is very difficult to experimentally define the nature of the first group of reactions that occurs when a material is heated, in the literature it is common to classify pyrolysis reactions as primary and secondary.5-8,

17-19

Primary reactions in pyrolysis have been the focus of many

studies, both experimentally and with molecular modeling.6, 11, 20-22 One of the major challenges when studying pyrolysis reactions is how to distinguish the different reactions.8 From the point of view of an experimentalist, the primary reactions are defined as those that result in the first reaction products that we can effectively identify and quantify.6

The secondary reactions are those that subsequently convert the first products into new products, including homogeneous and heterogeneous reactions that occur in the gas phase after evaporation or thermal ejection of primary products).7, 8, 17, 23-29 To isolate and study the kinetics of the “primary reactions”, the following are necessary: (1) fast heating, (2) even heating, (3) a thin sample layer to maximize heat and mass transfer, and (4) rapid removal of volatiles from the heated region of the reactor as soon as they are produced. The last requirement can be achieved by applying a vacuum to the system.30

Historically, slow pyrolysis, fast pyrolysis, and “flash” pyrolysis are characterized by heating rates of 1-100, 100-1000, and >1000˚C/min, respectively.1 However, the best measure of a reactor’s heating rate is by its power density (MW m-2), which is reflected in the temperature



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rise. Based on a literature survey, fast pyrolysis reactors are those capable of heating at 0.1-1.0 MW m-2 with an external heat transfer coefficient far above 50 W m-2 K-1.

1,31

To achieve

isothermal heating conditions, a prerequisite for easy kinetic calculations, the sample thickness on a heated surface should be on the order of 10 µm,9 which also greatly minimizes mass transport limitations.

One of the major challenges in studying fast flash-pyrolysis reactions (heating rates of about 100-1000oC/min or >1000oC/min) is how to simultaneously heat and track the temperature of the samples. As mentioned before, the poplular TGa is only capable of slow heating rates less than 250°C/min.3, reactions.32,

10, 11

33

Spoon reactors have been also extensively used to study slow pyrolysis

There are some new reactor systems which are capable of very rapid heating

including the heated plate reactor,12-14 the wire mesh reactor,15, 34 and the radiative flash reactor.16 However, by far most of the studies of biomass thermochemical reactions published today are conducted with commercial analytical pyrolyzers (furnace pyrolyzer, curie-point pyrolyzer, and resistive filament pyrolyzer).35-39 These pyrolyzers are commercially available from Frontier Laboratories

(http://www.frontier-lab.com),

Pyromat

(http://www.Gsg-

analytical.com/english.pyromat.Htm), Gerstel Inc. (www.gerstel.com), and CDS Analytical (http://www.Cdsanalytical.com). A major challenge of using commercial pyrolyzers is that most them typically focus on the analysis of volatile pyrolysis products detectable by GC/MS, disregarding important reaction products such as the water, the gases, and the oligomers.

Another major challenge of using Py-GC/MS to study fast pyrolysis reactions is that most of the commercial systems can only study very volatile pyrolysis products while other important


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reaction products such as water vapor, expelled gases, and oligomeric products cannot be studied with this system. Furthermore, with these systems, it is not currently possible to measure the sample temperature directly or achieve different pressures, let alone capture all the volatiles released during pyrolysis. Many studies are carried out using a packed bed of powder with particle sizes less than 100 µm to assure isothermal conditions where the Biot number is less than 0.1, and minimize the intraparticle gas residence time.15, 40, 41 The Biot number is defined as Bi=hL/k where h is the heat transfer coefficient between gas and the particle, L is the characteristic length (thickness in this case) and k is the thermal conductivity of the solid. However, Paulsen et al. investigated the role of sample dimensions over product distribution in cellulose pyrolysis and found that pyrolyzing very thin films (~10µm) is critical for fundamental pyrolysis studies.9 Lédé and Authier31 similarly concluded that samples need to be on the order of 10s of µms to minimize heat and mass transfer in a non-ablative pyrolysis reactor. Using a thin film ensures isothermal sample conditions and superior mass transfer in comparison with powder pyrolysis. Also, the thin film ensures that sample distribution is uniformly heated, thus avoiding temperature gradients. The gas residence time in thin films can be reduced by two orders of magnitude in comparison with packed bed powder pyrolysis.

Surprisingly, there are very few studies describing the reaction conditions in commercially available analytical pyrolyzers. Researchers from Iowa State University performed detailed experimentation combined with CFD modeling of the gas flows to characterize the Frontier drop-tube furnace micro-pyrolyzer system, finding maximum rates on the order of 200˚C/s.42 In this manuscript, the authors have focused their attention on the widely-used CDS Pyroprobe 5000, which allows the user to set the nominal heating rate to 20oC/ms (20,000oC/s) on the



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platinum coil; the coil surrounds a quartz tube that holds the sample. This heating rate does not represent the heating rate on the sample itself 31.

Nevertheless, many researchers do not explicitly separate the heating rate of the filament and the heating rate achieved by the biomass particles.43-45 Some authors have measured the heating rate at the center of a pyrolyzed plug sample and have reported a maximum heating rate of 50 oC/s with the same pyroprobe system.46 In one thorough study, researchers developed a numerical model for the CDS pyroprobe heating coil using the internal current and voltage measurements; they calculated that the highest heating rate possible at the center of a sample plug is 270˚C/s, and it took 10 s to reach 1500oC.47 It is common to use a particle sample with 1-2 mm Feret diameter or a plug of powder with glass wool above and below the sample. However, there is still uncertainty on the temperature profile and reaction conditions in the CDS pyroprobes.

The main objective of this paper is to study the pyrolysis of thin cellulose films in order to characterize char evolution and yields as well as temperature and pressure excursions, under near-vacuum and at atmospheric pressure using various heating rates. In this work, the authors created a closed reactor system capable of fast pyrolysis with sample and gas temperature measurement, pressure measurement, and sample visualization at different pressures, for reasons which will be explained in this manuscript. An example is provided for cellulose pyrolysis at two pressures: 3.5 mbar and 950 mbar to show how the system can be used, as well as to gain fundamental insight into this material’s behaviors. A secondary objective was to prepare a detailed computer simulation of the Pyroprobe apparatus, making it as precise as possible within the constraints of our computational resources and our knowledge of the kinetic, thermodynamic and transport parameters which govern pyrolysis. This would not only give us a chance to 8 ACS Paragon Plus Environment

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compare our experimental observations with the predictions of the model, but would also allow us to explore phenomena which occur in the reactor but which are not observable with our existing instrumentation, e.g., the convective flow patterns, temperature profiles, and the eventual fate of our pyrolysis products.

2

2.1

Experimental Section

Modified Pyroprobe for captive sample pyrolysis reactor

Our team designed a captive sample reactor that uses the CDS Pyroprobe 5000 (CDS Analytical, Oxford, PA) with a programmable power supply to heat a helical platinum coil which tightly surrounds a quartz tube (OD 2.1 mm, thickness 250 µm, length 1.75 cm) that holds cellulose samples (0.5-20 mg depending on the material). Unlike the typical Py-GC/MS setup, this pyroprobe was not connected to a GC/MS system. Instead, we designed and constructed a custom glass housing for the pyroprobe to allow for a closed vacuum system for measurement of temperatures and pressures as well as sample visualization to allow recording of the cellulose combustion and gas evolution (Figure 1). The housing is a solid aluminum cylinder (A1) with holes, one of which seals the pyroprobe rod, and two stainless steel pipes on the side connected to the pressure gage (A2) and valve (A3) for purging and evacuating the glass culture tube housing as well as gas sampling, and a glass culture tube (Pyrex No 9825, coleparmer.com) encloses the pyroprobe (A4, B1) and is sealed with a rubber gasket to the surface of the aluminum housing. Two thermocouples (C1, C2) were fed through the rubber gasket. A copper cooling loop was used to cool the reactor walls to ~7˚C. Two different chiller configurations



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were used: a full loop insulated with tape for vapor condensation (A5) and a partial loop for recording videos (B2). Please see Figure 2 for close-up of solid sample thermocouple.

Figure 1. A. Modified pyroprobe captive sample reactor used for this study. The custom aluminum housing (A1) holds the pyroprobe inside a glass culture tube (A4, B1), sealed with a rubber gasket to the body. The glass tube is chilled by insulated 3/8” copper chilling loop (A5) at -7 °C. There are lines leading to the vacuum pump and gas sampling port (A3) as well as a vacuum gage (A2). B. Alternative chilling loop (B2) to allow for video recording of the solid sample, shown in the vertical orientation. C. Pyroprobe exposed horizontally to display thermocouple placement. Two thermocouples measure temperature in the gas (C2) and sample wall (C1).


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Figure 2. Close-up of pyroprobe with empty capillary. The thermocouple end physically touches the thin-film sample inside the capillary during pyrolysis. The pressure sensor is a Druck DPI 104, calibrated to output absolute pressure (GE Measurement and Control). The subminiature thermocouples used to measure gas temperature and quartz sample tube wall temperature were type K with PFA Insulation, 40 AWG (Omega Engineering Inc.) connected to a Lenovo Z40-70 laptop via USB-TC01 thermocouple measurement devices (National Instruments Corp., Austin, TX). One thermocouple was suspended in the gas phase and the other in contact with the inside surface of the quartz tube to measure the temperature adjacent to the sample during pyrolysis (Figure 1C and Figure 2). MatLab was used to record and save the temperature and pressure data simultaneously. The recording rate was precisely 4.16 Hz.

2.2

Reactor operation

Samples were first loaded into quartz tubes (see next sections for more details). The quartz tubes were inserted into the pyroprobe and re-dried for 5 minutes at 110˚C under vacuum while the glass culture tube housing and cooling loop were attached. In this manuscript, the word “vacuum” refers to approximately 3 mbar, as is traditional in the field of pyrolysis.48-52 The 11 ACS Paragon Plus Environment


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temperature was monitored to assure that it was placed correctly (i.e. 100˚C output) in the middle section of the quartz tube (see Figure 2). The system was then re-pressurized with nitrogen. For atmospheric pressure studies, the pressure was adjusted to 950 mbar. For low pressure studies, a vacuum pump brought the pressure down to 3.5 mbar. The platinum coil temperatures were set to values such that the final temperatures were 400, 450, 500, 550, and 600˚C. The nominal heating rate of the platinum coil, unless otherwise specified, was set to 20˚C/ms to heat the coil as fast as possible until the final temperature was reached and held such that the total heat-up time plus hold time is equal to 60 s. Results show that the sample temperature rose at a much slower rate than the coil, so the measured heating rate on the wall of the sample appears more complex and much lower than the nominal heating rate.

For visualization, a Fastec Sportscam 1000 ME (Fastec Imaging Corporation, San Diego, CA) black and white camera was used in conjunction with a type C Nikon adapter followed by a Tokina AT-X Pro macro 100 lens (Tokina, Tokyo, Japan). The frame rate was set to 60 frames per second (fps), resolution to 640x480, shutter to 1.0, and gamma to 1.0. The light source was a Solarc LB-50 (Ushio Corp., Cyprus, CA)

Raw data were collected to obtain pressure, pixel intensity on the solid sample, and temperature along the inside of the quartz tube wall. The temperature measurement was taken separately due to the difficulty of keeping the thin thermocouple in the correct position inside the sealed vessel. However, multiple experiments showed that this protocol provides an accurate temperature.


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2.3

Sample preparation: thin cellulose film inside quartz sample tube

Powder samples have poor heat transfer, and also can explode out of the quartz sample tube (see Supplementary Material). To address this issue, a preparation methodology was developed to create cellulose films 72s

0

Gas Flows: The gas phase, including pyrolysis products, is assumed to be an ideal gas consisting of an inert background gas, i.e., N2, a condensable pyrolysis product and a non-condensable pyrolysis product. The distribution of the pyrolysis products, which contribute to the condensable and non-condensable gases, is assumed constant across all temperatures and pressures and the make-up of these gases is discussed below in the reactions section. These pyrolysis gases are 19 ACS Paragon Plus Environment


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assumed to be expelled from the inner surface of the cellulose phase into the quartz capillary tube as a mass flux as soon as they are created and the condensable product gases are assumed to condense out onto the cold glass wall as soon as they come in contact with it. The noncondensable gases do not condense on any surface but continue to mix and to circulate with the inert background gas after pyrolysis (See Figure 7). A: 950 mbar

B: 3.5 mbar

Figure 7. A: 950mbar streamlines (white), pressures contours (horizontal lines) and temperatures (-6 to 800˚C) for pyroprobe initially at 25˚C and 950 mbar N2 visualized at (A) t=11 s, just before the Pt wire is turned on; (B) t=12.5 s, at the highest power input; (C) t=18 s, at the largest Reynolds number averaged over the quartz tube; (D) t=30 s, approaching steady state. Generally speaking, a strong vortex forms above and to the right of the quartz tube and a weak vortex forms below the quartz tube. Note that there is very little distortion of the pressure due to the flow; and only a very small pressure variation along the vertical axis due in the gas phase. B: 3.5mbar streamlines (white), pressures contours and temperatures (-6 to 800˚C) for pyroprobe initially at 25˚C and 3.5 mbar N2 20 ACS Paragon Plus Environment

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Even at pressures as low as 3 mbar, the mean free path of molecules in the ideal gas phase is less than 20 microns so the compressible Navier-Stokes equations can be used to model gas flows outside of the cellulose layer. The flow is assumed to be laminar and to be driven by natural convection due to spatial density variations as well as by forced convection due to the efflux of gaseous pyrolysis products out of the cellulose phase and by the condensation of the condensable gases on the cold glass wall. The assumption of laminar flow is borne out in the calculations since the Reynolds number, Re ≡

ρD U , averaged over the interior of the quartz capillary tube µ

is always less than 20, even when the efflux of pyrolysis products is at its peak, and the Rayleigh number, Ra ≡

ρ 2C p gD 4 1 d ρ , in the quartz capillary tube is always less than 60. In these µk ρ dr

formulas, U is the magnitude of the velocity in the vapor phase, ρ is the density, µ is the viscosity, D is the inner diameter of the quartz tube, Cp is the heat capacity, k is the thermal conductivity and g is the acceleration due to gravity.

The Reactions: As the cellulose layer on the inside surface of the quartz capillary tube is heated by the helical platinum coil, the cellulose is first converted to an activated state and then the activated cellulose can follow either one of two reaction pathways,57 i.e., conversion to levoglucosan, which is a condensable gas, or conversion to char, non-condensable gases and condensable gases other than levoglucosan, according to the reaction scheme shown in Table 2. Assuming the reactions are elementary, their mathematical formulas and the kinetic parameters are given in Table 2. As the temperature in the cellulose layer increases, the condensable and non-condensable gases formed by reactions 2 and 3 are immediately expelled from the inner



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surface of the cellulose layer into the inert gas phase in the quartz capillary tube, leaving behind activated and unactivated cellulose as well as char. The Arrhenius expression for the kinetic coefficients is assumed to react at a rate governed by the average temperature of the cellulose layer.

Table 2. Reaction scheme used for cellulose pyrolysis and Arrhenius kinetic parameters used in the pyrolysis simulation. Adapted from Cho et al.57 Note that the pre-exponential coefficients have each been reduced by a factor of 100 compared with Cho et al57 to accelerate convergence. Reaction R1 = -k1 [Cellulose] R2 = -k2 [Activated Cellulose] R3 = -k3 [Activated Cellulose]

k(1/s)

A(1/s) 1019.6 103.5 1012.8

k1 k2 k3

EA(kJ/mole) 257.72 102.94 198.91

∆Hr(kJ/mole) -1. -170.17 +121.38

Following Cho et al57, reaction R3 produces only levoglucosan while reaction #2 produces condensable gases, non-condensable gases and char with mass of 87%, 5% and 8%, respectively. The non-condensable gases include CO, CO2, H2, CH4 and small amounts of higher molecular weight gases with an average molecular weight of 34.9 g/mole while the condensable gaseous products, not including levoglucosan, contain glycoaldehyde, 5-hydroxymethyl furfural, cellobiosan and water with an average molecular weight of 41.6 g/mole. The make-up of the condensable gases, excluding levoglucosan, and non-condensable gases is assumed constant across all temperatures and pressures with only the relative rates of production by reactions R2 and R3 varying with temperature.

Mass Transport: As the activated cellulose reacts, condensable and non-condensable gases are expelled from the inner surface of the cellulose layer into the quartz capillary tube where they mix with the inert background gas. Transport of each species in the gas phase is governed by the equations of conservation of mass which take the forms shown in Table 3, where N2 is the inert 22 ACS Paragon Plus Environment

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carrier gas, [CG] is the collective set of condensable gases and [nCG] is the set of noncondensable gases in the gas phase. It should be noted that gaseous products are not created or destroyed in the gas phase: they are created in the cellulose layer and ejected into the gas phase. The condensable gases are allowed to condense out of the gas phase on the inner surface of the cold glass containment vessel. Condensation is considered to be instantaneous and complete wherever condensable gases come into contact with the cold containment vessel walls.

Table 3. Conservation of mass equations for the species described in this model.

Boundary Conditions: Since heat transfer is included in all of the geometrical elements of the computational domain, continuity of both the temperature and the energy fluxes is enforced on all of the interior surfaces while the outer surface of the hemisphere at the top of the glass containment tube is taken as adiabatic and the temperature on the outer surface of the side of the glass containment tube is taken as -6˚C due to the nearby copper cooling coil.

The Navier Stokes equations apply only to the gas phase. The no-slip condition applies to all solid surfaces in contact with the gas except (1) the inner surface of the cellulose film, which expels gases into the quartz heater tube and (2) the flat portion of the inner surface of the glass containment tube which condenses some gases out of the vapor phase. The latent heat released upon condensation is assumed to be drawn through the glass containment wall and is not 23 ACS Paragon Plus Environment


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included in these calculations. The equations of conservation of mass also apply only to the gas phase. In this case all solid surfaces support no-flux boundary conditions except the cellulose surface where gases enter, as a result of pyrolysis, or the glass surfaces where they leave, i.e., condense out of, the gas phase.

The cellulose reactions apply only in the cellulose layer on the interior surface of the quartz heating tube. The cellulose, activated cellulose and char remain in that phase which is not allowed to change volume or to generate an interior flow. All condensable and non-condensable gases formed by the reactions at height, z, are immediately expelled into the gas phase at the height z, where they are created, through the inside surface of the cellulose layer. The pressure was estimated by the momentum equations.

Initial Conditions: Before the start of each simulation at t=0, and immediately after the copper cooling coil has been placed over the glass containment tube, every interior part of the pyrolysis apparatus is assumed to be at 25˚C, i.e., room temperature. After the cooling tube is placed over the glass tube and the vapor phase is adjusted to operating pressure, the apparatus is allowed 12 seconds to cool and equilibrate before electrical power is applied to the heating coil to start heating the cellulose layer. The vapor phase is initially made up of motionless, inert gas, i.e., N2, with trace amounts of condensable water vapor and no other non-condensable gases. The cellulose layer is assumed initially to contain no activated cellulose or char.


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4 4.1

Experimental Results Reactor profile: Heating characteristics

The thermal profile of the reactor was characterized by varying the placement of the thermocouple, changing the heating rates, and measuring various temperatures. These data, collected at atmospheric pressure, should be useful to other experimentalists using the pyroprobe in traditional settings (e.g. Py-GC/MS) (see Figure 8). Even more critically, the heating rate diverged hugely from the nominal heating rate. Whereas the set point for the heating rate in the coil was 20,000˚C/s, the fastest measured heating rate at center point C was 216.4˚C/s and the average heating rate was around 131.1˚C/s at atmospheric pressure. This result clearly shows that the heat-transfer mechanism is not solid conduction from the coil to the quartz, but gas-phase convection and conduction as well as radiation. The heat transfer mechanisms are explored in more detail in the modeling section of this manuscript.

Figure 8. Measured temperature vs. time at center of tube on wall at various heating rate set points with a final temperature of 600 °C at atmospheric pressure without a sample. Notice that at heating rate set points 500 °C s-1 and above have the same measured heating rate time to reach their final temperatures. Some differences were observed due to placement of the thermocouple. Note that it takes nearly 8 seconds to reach 600 °C.



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To further illustrate the heating rate delay, a variety of heating rates were set on the controller with a final set temperature of 600˚C at the center of the quartz tube (Figure 8). In essence, setting the heating rate in the coil above the maximum measured heating rate of 337˚C/s did not speed the heating. This indicates that phenomenon limiting the heating of the tube is not solidsolid conduction from the platinum coil to the quartz tube. In fact, due to thermal expansion, the platinum coil only actually makes contact with the quartz tube at a few small points and in the cool top and bottom regions of the coil.

Finally, the heating time was compared for different final temperature set points with 20˚C ms-1 at the center point of the quartz tube. It took nearly 8 seconds to reach within 2˚C of the final temperature at 600˚C. Nearly the same heating times were observed for heating to final temperatures of 400 and 200˚C. Other experiments showed that the temperature of the inside wall of the quartz tube is identical both with and without the cellulose pyrolysis sample because the thermal mass of the quartz tube was large compared with the cellulose sample (55 mJ/K : 7 mJ/K).

4.2

Thermal profiles of thin film cellulose pyrolysis under vacuum and atmospheric pressures

The temperatures on the cellulose films were measured for pyrolysis under two pressures (vacuum and 1 atm) and five temperatures (400, 450, 500, 500, and 600˚C). The thermocouple tip was placed on the cellulose in the vertical middle of the quartz tube (see Figure 2). As there was a chiller wrapped around the glass chamber, the coil set point needed to be set higher than the desired final hold temperature under atmospheric pressure with cellulose samples (e.g. for 26 ACS Paragon Plus Environment

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600˚C sample temperature, the coil was set to 642˚C). For cellulose pyrolysis under vacuum, the coil temperature needed to be significantly higher than the desired sample temperature (e.g. for 600˚C sample temperature, the coil was set to 715˚C). Figure 9 shows the calibration curves developed for coil set points versus final sample temperature. The requirement for such a high temperature under vacuum indicates that convection is a dominant heat transfer mechanism, as it is dependent on pressure. This may seem illogical as it looks like the quartz tube physically touches the quartz tube at every wrap. However, closer inspection reveals that only a few small points physically touch the quartz tube, especially as thermal expansion makes the coil diameter grow.

Figure 9. Platinum coil temperature setpoints versus final (equilibrium post-reaction) temperature at the centerpoint with cellulose film samples. Notice that under vacuum, the coil needs to be much hotter to maintain the sample temperature. This implies that convection is a dominant heat transfer mechanism because of the dependence on pressure.

Analysis of the heating profiles at both pressures reveals new insights into the thermal behavior of cellulose during pyrolysis. Figure 10 shows the temperatures and temperature change with time, dT/dt, for cellulose film pyrolysis at atmospheric pressure. In Figure 10A, the first observation is that the heating is slower with the sample than it is for the blank performed at 27 ACS Paragon Plus Environment


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600˚C. Secondly, there is a very clear slow-down of the heating from around 400 to 500˚C. To analyze the precise temperatures more carefully, Figure 10B shows the derivatives for the same temperature excursions, plotted versus temperature. Here, there is a very clear endothermic zone that starts at 400˚C and ends at 460˚C, with the slowest heating between 420 and 430˚C. Furthermore, the heating rate when the sample is present never exceeds 165˚C/s for any sample, and is slower when the coil temperature is lower. For the 600˚C sample, an artifact appears after 500˚C that could be due to thermocouple movement as the char disappears and the thermocouple touches the quartz. Also interestingly, there appears to be a speed-up of the heating rate at 200˚C which appears in all samples. The 400˚C sample does not exhibit distinct endothermicity because it is below the temperature at which reactions accelerate exponentially.

950 mbar

950 mbar

Figure 10. (A) Temperature of cellulose film the center region of the quartz tube during pyrolysis with a starting pressure of N2 of 950 mbar in the captive sample reactor held at various temperatures for 60 s. The curve “Blank 600 °C” was performed on a capillary without cellulose. Notice that the heating rate is consistently low at 425 °C, and plateaus at the final temperature once pyrolysis is complete for samples above 400 °C. The sample at 400 °C exhibits one long delay until reaching its final temperature at ~ 35 s. Samples at higher temperatures take less and less time and the 600 °C sample is completely heated by 13 s, but all exhibit a slower rate between 400 and 50 °C. (B) Time derivatives for cellulose pyrolysis at atmospheric pressure plotted versus temperature. 28 ACS Paragon Plus Environment

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The same analysis was performed under near-vacuum conditions with cellulose film pyrolysis, shown in Figure 11. Under near-vacuum, the heating rate (Figure 11B) is much slower than at atmospheric pressure (even for the blank at 600°C). As discussed earlier, convection appears to be a critical heat transfer mechanism. In Figure 11A, one can see a distinct endothermic region between 400 and 500°C, with the slowest heating between 425 and 435°C. The endothermic reactions of cellulose depolymerization and evaporation of anhydrosugars occur in this region. The reader should be reminded that the coil temperatures under vacuum are set much higher than at atmospheric pressure to achieve the same equilibrium sample temperatures (see Figure 9). The depressed heating rate region is visually more evident for pyrolysis at atmospheric pressure because it heats faster.

Figure 11. (A) Temperature of cellulose during pyrolysis with a starting pressure of 3.5 mbar in the captive sample reactor held at various temperatures for 60 s. The heating rate of the coil was set to 20 K ms-1. (B) Time derivatives for cellulose pyrolysis under vacuum plotted versus temperature. This plot illustrates that the capillary heats much more slowly when the sample is present, and there is an endothermic reaction that reaches its highest rate between 410 and 430 °C. The 600 °C sample heats up faster than the “blank” at 510 °C.



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Recent work by the research group of Dauenhauer using a heated plate reactor has concluded that at 467˚C there is a transition region to a melt phase where random depolymerization reactions occur rapidly.13 Before this region, the authors propose, only end-chain unzipping reactions occur, which would have a less dramatic impact on the sample temperature. However, in this work the endothermic reactions clearly take place at their fastest rates between 410°C and 430˚C. Because each pressure resulted in a different heating rate, this could lead to differences in the temperature at which the maximum endothermicity is observed.

4.2.1 Pressure profile over time

The gas phase temperature and pressure were recorded simultaneously with the data presented in the previous section to distinguish between increase in the moles of gas, or simply heating of that gas. Their respective curves are presented in Figure 12 and Figure 13.

B

A

Figure 12. Gas pressure and temperature for cellulose thin film experiments performed with a starting pressure of 950 mbar. Temperatures in the legends correspond to approximate final pyrolysis temperature of the solid


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B

A

Figure 13. Gas pressure and temperature for cellulose thin film pyrolysis experiments performed with a starting pressure of 3.5 mbar (vacuum). Temperatures in the legends correspond to approximate final pyrolysis temperature of the solid

Pressure and temperature for cellulose pyrolysis performed at atmospheric pressure is shown in Figure 12A and B. The largest drop in heating rate can be seen for the 600˚C sample. Here, there is a clear increase in pressure between 3 seconds and 11 seconds, corresponding to solid temperatures of 400 to 600˚C (see Figure 10A). The interesting feature of the pressure decrease after it increases can be associated with heavy molecules which are vaporized during pyrolysis and which subsequently condense on the walls of the chilled glass reactor body. The gas temperature (Figure 12B) does not noticeably increase when the sample wall is between 400 and 500 °C, and only dramatically rises after 10 seconds due to gas heating by the platinum coil.

The pressure and gas temperature measurements for cellulose film pyrolysis under vacuum (Figure 13) reveal different, though similar, behaviors to those at atmospheric pressure. Take, for example, the 600˚C curve. There is an initial jump in the pressure of ~1 mbar between 3 and 6 seconds (200 to 280˚C cellulose temperature) that could be associated with desorption of residual 31 ACS Paragon Plus Environment


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water, which remains a gas at this pressure. Based on an ideal gas calculation, a release of just 0.04 mg water could contribute to a 1 mbar increase in pressure in the reactor. After this, there is a dramatic pressure increase from 8 to 18 seconds (400-560˚C cellulose temperature) which should be caused by the release of volatile compounds by pyrolysis. Due to the low pressure, a large portion of those compounds remain in the gas phase and the pressure does not reduce again. Notably, the gas temperature rises much more dramatically for the 600˚C sample than for any of the other samples. This is expected because the coil is at a higher temperature than in the other conditions.

4.3

Char yields versus temperature for vacuum and atmospheric pressure pyrolysis of cellulose

The char yield at atmospheric pressure is always higher than char yields for pyrolysis of cellulose under vacuum at all temperatures (See Figure 14). This phenomenon is due the fact that vacuum helps to evaporate some of the intermediate species from the melt phase, it is evident that the low pressure could mitigate some of the secondary, char-forming reactions when compared with atmospheric pressure. In the literature there are very few comparisons of pyrolysis at different pressures.52, 58-60 In general, pyrolysis under vacuum reduced the char yields by at least 50 % compared with atmospheric pressure.


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Figure 14. Char yields versus final temperature (hold time 60s, heating rate 10 °C ms-1). From 623 K (350 °C) to 873 K (600 °C). Error bars represent one population standard deviation in each direction. 4.4

Analysis of pyrolysis videos of cellulose at atmospheric pressure and vacuum, 600˚C

Because the sample could be seen between the platinum coil, it is possible to record videos of the cellulose film on the wall of the quartz tube undergoing pyrolysis. In this analysis, pyrolysis was performed with a final temperature of 600˚C for two starting pressures: 3.5 mbar and 950 mbar (~ atm). Figure 15 shows the pictures at different times. In Figure 16 the white intensity of the cellulose film is plotted. Still frames captured from the videos can be seen in Figure 15 (950 mbar) and Figure 17 (vacuum, 3.5 mbar). The first observation is that darkening of the cellulose occurs in less time at 950 mbar than under vacuum. This is mostly due to the more rapid heating at the higher pressure (see Figures 8 and 9); however it should be noted that the atmospheric pressure pyrolysis also yields more char (see Figure 14) which could accelerate the effect of darkening.



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Figure 15. Still frames from videography of cellulose pyrolysis under nitrogen (950 mbar starting P) with a final temperature of ~600 °C. The char formation can be observed inside the quartz capillary, with darkening beginning below 400 °C. The yellow box shows the pixel region used to calculate light intensity to monitor char formation (See Figure 14)

Figure 16. Relative white intensity of cellulose film and sample temperature versus time for pyrolysis under atmospheric pressure N2. Notice that the sample begins to darken at approximately 280 °C. 34 ACS Paragon Plus Environment

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In the 950 mbar experiment (Figure 15), the cellulose appears completely black by five seconds and condensed vapors can be seen on the glass housing at 7.5 s and beyond. In the video (supplementary material), vapors can be seen dramatically leaving the quartz tube and pooling below the heater before condensing. In the low pressure experiment (Figure 17 and 18), significant darkening cannot be seen until about 15 s, at which point condensate can also be seen on the chilled glass condenser reactor housing. With the vacuum experiment, cannot be seen floating around the vessel because diffusion is much more rapid.

Figure 17. Still frames from videography of cellulose pyrolysis under vacuum with a final temperature of ~600 °C and a starting pressure of 3.5 mbar. The char formation can be observed inside the quartz capillary, with darkening beginning below 400 °C. The yellow box shows the pixel region used to calculate light intensity to monitor char formation in Figure 16.



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Figure 18. Relative white intensity of cellulose film and sample temperature versus time for pyrolysis under vacuum. Notice that the sample begins to darken at approximately 237 °C.

Analysis of the raw image data allows for a more advanced examination of the rate of darkening of the cellulose during pyrolysis. This was achieved by accessing the image data for a small portion of the samples. In the first frames of Figure 15 and Figure 17, there are small yellow boxes indicating the pixel region where data was analyzed. This region is in the middle of the quartz tube, where temperature was measured in separate experiments. In these boxes, the brightness values (from 0 to 256 in 8-bit format) for all the pixels are summed for each frame and then normalized from 0 to 1. After this, the normalized pixel data was smoothed (15 point moving average) and the first derivative was calculated to visualize the times at which darkening is occurring. Sample temperatures from separate experiments were overlaid for comparison.


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For the 950 mbar sample (Figure 16), the relative light intensity begins to drop at 213˚C and continues to do so until the sample reaches 440˚C. At this point, the brightness increases again because vapors condense on the glass housing and reflect more light into the camera. The vacuum sample (Figure 18), although it heats more slowly, exhibits nearly identical initial behavior: darkening begins at 237˚C (variation could be due to the fact that temperature was measured in separate experiments). However, in the vacuum sample, darkening continues until the sample reaches 600˚C, at which point the brightness slightly increases again as the material disappears and condensation reflects light into the camera. One possible reason for the extended reaction time under vacuum is that the vacuum video was taken under brighter lighting conditions, which allow for more sensitive measurements by the camera. However, the darkening initiation time should not be significantly altered by the brightness of the scene.

4.5

Simulation Results

The numerical simulation clearly shows the role of natural convection in the cool-down period before pyrolysis begins and, during and after pyrolysis, in the transport of hot gases from the quartz pyrolysis tube to the cold glass containment vessel, a process that takes just a few seconds. It also clearly shows, with respect to the pyrolysis reactions, that the activation step begins at a temperature of about 330˚C, persists for about 3-4 seconds at these heating rates, and is complete or nearly complete before the generation of char and pyrolysis gases begins. These reactions also last about 3-4 seconds and favor the production of levoglucosan.



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During pyrolysis, the radial temperature difference across the 85.6 micron cellulose sample is always less than about 1˚C with the quartz interface being the warmer surface except when the pyrolysis gases are being generated. During a brief period that lasts less than 2 s, the temperature difference between the inner and outer surfaces of the cellulose reaches about ∆T~6˚C with the cellulose surface in contact with the quartz tube being cooler than the outer surface. At the same time, the vertical variation in temperature along the cellulose film is generally about 30˚C from either edge to the tube center while the Pt heating coil is powered. This is somewhat consistent with the axial temperature variation of the Pt coil illustrated in Fig. 4, but is specific to the cellulose sample itself.

Simulation Results at 950 mbar: Shortly after the cooling coil is placed over the glass containment tube, the gas adjacent to the glass containment wall cools and begins to flow vertically down the wall, starting a large vortex which accelerates pre-ignition cooling of the pyroprobe. At 12 seconds, the heating coil is turned on following the profile in Figure 6 which features a very rapid 1.5 s heat-up followed by a lower-power, steady heat input that allows the pyroprobe to establish a steady-state temperature very quickly. The low-power heat is maintained for 58.5 s and is then turned off, at which time the system as a whole begins to cool down through the walls of the glass containment tube wall.

During the 1.5 s rapid heating, the temperature of the cellulose rises quickly and, at approximately 400˚C the cellulose is converted to activated cellulose. The further conversion of activated cellulose to levoglucosan begins near 450˚C and consumes an overwhelming majority of the activated cellulose in the next 5 s. Using the kinetic parameters provided by Cho et al.57


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the conversion of activated cellulose to levoglucosan is much faster than the conversion to char at these temperatures, so very little of the original cellulose is converted to char in our simulations.

Starting at a pressure of 950 mbar, the pressure decreases to about 875 mbar in the 12 s cooldown before pyrolysis, rises to 910 mbar during the rapid heat-up and then continues to rise beyond 1020 mbar as non-condensable gaseous products are expelled from the cellulose. At the higher temperatures where reactions are fast, the rapid generation of gases causes an overshoot in the pressure but, once the reactions die down, the pressure stabilizes near 1020 mbar as the condensable gases precipitate onto the glass wall. At the end of the 58.5 s heating program, power to the heater is turned off and the pressure returns close to its original 950 mbar as the non-condensable gases contract on cooling.

Simulation Results at 3.5 mbar: Again the gas adjacent to the glass tube wall flows vertically down the cold glass wall during the 12 s cooling period and causes a large vortex to form in the gas phase prior to pyrolysis (see Figure 19). At 12 seconds, the heating coil is turned on, delivering a 1.5 second high-power pulse followed by 58.5 seconds of maintenance power. The temperature excursion in the cellulose phase at low pressure is nearly indistinguishable from the high pressure case so the reactions follow the same pattern with virtually all of the cellulose converted to levoglucosan.



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Figure 19. Cellulose pressure from the COMSOL simulation with different power schedules for the 3.5 mbar and 950 mbar gas pressure (A and C, respectively), and cellulose temperature (B and D, respectively) plotted with t=0 as the timestamp when the power is first applied to the coil (in the heating schedule this is at 12s). Four different power values were applied to the coils to achieve different final temperatures: (A) – (D) for and (E) – (H) for the 3.5 and 950 mbar pressures, respectively. Notice that the heat of reaction slows the heating rate as the reactions kick on between 300 and 600 °C. In the 950mbar pressure, there is an overshoot as the reactions release vapors which take time to condense; this phenomenon was not observed in the experimentation.

Starting at a pressure of 3.5 mbar, the N2 pressure decreases slightly during the 12 s cooling period but surges to about 90 mbar as the conversion to levoglucosan kicks in at about 16.5 s and finishes at 17.5 s, much faster than the high pressure case. The main reason for the longer 40 ACS Paragon Plus Environment

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conversion time in the higher pressure case appears to be associated with the presence of the inert N2 gas, which contracts during cooling and then expands quickly with the temperature rise in the vapor phase, masking the conversion of cellulose to gaseous products.

Heat Transfer: While the temperature of the Pt heating wire tracks within about 15˚C of the temperature reading on the thermocouple touching the surface of the cellulose, the remote thermocouple suspended away from the quartz tube grossly underreports the temperature excursion, registering less than 1/3 of the temperature rise in the cellulose during heating. The main reason for this is likely that, in the simulation, the heating coil is in very close proximity to the quartz: close enough that the small vapor gap does not interfere with conduction through the quartz. The remote thermocouple, on the other hand, relies primarily on convection to bring hot pyrolysis gases into contact with the bimetallic sensor tip and these hot gases apparently cool as they transit from the cellulose to the remote thermocouple.

The original model (where the coil to quartz distance is 18 µm) also indicates that radiative heat transfer plays a minor role in the heating, in contradiction to experimental observations which suggest that radiation plays a significant role in heating the cellulose (See Figure 20). In particular, in our experiments, the low-pressure system needed significantly more electric power to the coil to reach operating temperatures while in the simulations, the opposite was true. Again, in the simulations shown in Figure 20A this is most likely due to the close proximity of the heating coil to the quartz tube (18 µm, the standard distance used for other presented results) which allows conduction through the quartz tube to dominate heat transfer during heating of the cellulose (>90%). However, as shown in Figure 20B at a distance of 370 µm (one extra coil



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diameter away), radiation contributes nearly 40 % and 30 % of the heat transfer for the 1 atm and 3.5 mbar cases, respectively. The reason that the total value of radiation is higher is because the coil is actually at a higher temperature to achieve the same effective cellulose temperature at a larger coil distance. The additional power required in the simulation of the high-pressure system is likely due to heat losses from the coil due to natural convection in the vapor phase near the coils. At high pressures, the flow will be stronger and the circulating gas phase can carry away more heat.

Figure 20. Total heat transfer and radiative heat transfer inwards toward the cellulose at the quartz interface for platinum coil to quartz distance of A. 18 µm (this was the standard distance used for other data) and B. 370 µm (one extra coil diameter away). For case A., radiation only contributes a small amount compared with heat transfer from convection and conduction (

Modified Pyroprobe Captive Sample Reactor: Characterization of

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