Anaerobic Pyrolysis Chamber: An atmosphere-controlled furnace for controlled-atmosphere thermal decomposition

TL;DR

A new furnace called the Anaerobic Pyrolysis Chamber allows controlled thermal decomposition of materials without combustion, enabling detailed study of partially decomposed samples.

Contribution

The novel Anaerobic Pyrolysis Chamber enables controlled pyrolysis in an inert atmosphere, allowing for the isolation of partially decomposed material samples.

Findings

The APC successfully produces gram-scale samples of partially decomposed materials.

The APC maintains precise control over temperature, pressure, and oxygen concentration during pyrolysis.

The APC supports property measurement in subsequent research studies.

Abstract

As some materials are heated they undergo thermal decomposition, resulting in a change in the material’s physical structure and chemical composition through a process called pyrolysis. During pyrolysis, flammable vapors are driven off the material; if those vapors are released in the presence of oxygen, flaming combustion may occur. Combustion can accelerate the decomposition process, making it difficult to isolate and harvest partially decomposed material samples for subsequent analysis. Due to this difficulty, the thermo-physical changes to materials undergoing pyrolysis have not been extensively studied. A new apparatus, termed the Anaerobic Pyrolysis Chamber (APC), was designed to produce gram-scale samples of partially decomposed materials by pyrolyzing those materials under a defined heating protocol and in an inert atmosphere. The APC consists of a decomposition chamber positioned within a muffle furnace, and an instrumentation package designed to monitor and control the temperature, pressure, and oxygen concentration within the decomposition chamber. The driving purpose of the APC was to produce solid products of pyrolysis for study in other measurement apparatus. The performance of the apparatus has been characterized, and the APC has been used to support property measurement in research studies.

Figures

Schematic of decomposition chamber.Fig. 2Assembled decomposition chamber.

Schematic of Bubble Vent.

Assembled Bubble Vent.

Schematic of Oxygen Sensor Module.

Wiring Diagram for Electrical Components in SC.

Gas flow diagram.

System control module exterior.Fig. 9System control module interior.

Benchtop assembly of the APC.

Example of Temperature History of APC for Eucalyptus Hardboard Decomposition.

Full text

Specifications table

Table 1. Specifications table.Hardware nameAnaerobic Pyrolysis ChamberSubject areaEngineering and material scienceHardware typeExperimental solid material preparationClosest commercial analogThe closest commercial analog are a class of products called inert atmosphere furnaces.Open source licenseCreative Commons Attribution Share Alike 4.0 International licenseCost of hardware$5,996.00Source file repositoryhttps://doi.org/10.5281/zenodo.10215409

Hardware in context

1

As materials become involved in fires they undergo thermal decomposition in a process called pyrolysis. As a material pyrolyzes the physical structure and chemical composition of the materials change as flammable volatiles are driven off the solid material and participate in combustion reactions to produce a flame. The physical and chemical changes to the solid materials result in changes to the thermo-physical (thermal conductivity, specific heat capacity, density) and optical (emissivity, absorption coefficient) properties [1]. An understanding of the properties of a material involved in a fire is critical to a complete understanding of the fuel generation rate of materials and the fire dynamics of a burning scenario [2].

When a material undergoes flaming combustion, fuel vapors mix with air to form a diffusion flame that is seated on the surface of the material. One side of the flame sheet is characterized by a concentration gradient of volatile flammable gases that decreases from the material surface to the flame sheet; the concentration of the gas mixture within this region is above the upper flammability limit. The opposite side of the flame sheet is characterized by ambient air and gaseous products of combustion. At the interface of these two regions is a flame sheet. Because oxygen is completely consumed in combustion reactions at the flame sheet, no oxygen diffuses through the flame and the region on the fuel-rich side of the flame may be modeled as inert.

The systems that are typically utilized to effectively simulate a burning material sustaining a diffusion flame when thermal decomposition processes are studied or measurement of the gaseous or solid products of decomposition is required involve controlled heating in a well-characterized inert atmosphere. At present there are few commercially-available products that are specifically designed for generating an inert atmosphere in a controlled high temperature environment. The commercially-available systems that are capable of generating conditions consistent with these requirements include thermal analysis instruments and inert atmosphere furnaces.

Thermal analysis techniques, including thermogravimetric analysis, differential scanning calorimetry, microscale combustion calorimetry, etc., typically utilize well-controlled heating regimes and afford the ability to adjust the gas atmosphere based on the desired measurement. These techniques and the apparatus designed to conduct such experiments are typically limited to sample masses on the order of a few milligrams, so they may only be used to investigate thermal decomposition when transport processes are not rate-limiting. Because of these limitations, the solid products of decomposition are not representative of those produced when real-scale materials are involved in fires. Additionally, the smaller scale of the material results in smaller residence times within the material matrix for the gaseous effluent which may affect the composition of gas mixture.

Commercially-available inert atmosphere furnaces are primarily used for heat treating in manufacturing processes that require inert atmosphere protection (i.e., shielding from oxidation). These furnaces are designed with various form factors and temperature ranges to provide flexibility for a wide range of applications, but, critically, these products are not designed to accommodate materials that decompose to produce flammable gases. A gas mixture that is at a high temperature and a concentration above the upper flammability limit poses a detonation or deflagration hazard if any oxygen is introduced to the mixture. Safeguards are necessary to ensure safety when decomposing materials that release flammable vapors, and this is not a primary design consideration in inert atmosphere furnaces which are not designed for such a use case.

The Anaerobic Pyrolysis Chamber (APC) was designed to be a low-cost and configurable apparatus to safely pyrolyze materials in a temperature and atmosphere controlled environment. It was designed to be compatible with any commercial muffle furnace having a suitable port opening.

Hardware description

2

The APC is comprised of four major components: the decomposition chamber, bubble vent, oxygen sensor module, and control module.

The decomposition chamber (DC) is used to house samples during the heating process. The DC maintains a sealed environment, free of oxygen, via continuous nitrogen injection for the duration of the heating process. The DC is positioned inside a muffle furnace, wherein samples may undergo pyrolysis during the heating process. The small form factor of the DC (5 in. by 5 in. by 2-9/32 in.) aids in its versatility to be used in a wide range of furnace sizes. The two-piece design (consisting of a base and lid that are bolted together, as opposed to a hinged design), ensures a tight seal that can withstand high pressures in the case of oxygen intrusion and a subsequent deflagration.

The DC key features are the inert gas port, thermocouple port, exhaust port, and copper sealing gasket. The 1/8 in. inert gas inlet port is used to supply nitrogen to the DC and create an anerobic environment for material decomposition to occur. A 1/16 in. ungrounded K-type Inconel thermocouple is mounted in the thermocouple port to monitor temperatures inside the DC. The DC chamber thermocouple is also used as the thermocouple sensor input for the furnace temperature controller. By incorporating a thermocouple to collect temperature internal of the DC, users are able to ensure the decomposed samples are at the minimum required temperature to produce stable, solid products of decomposition. A 1/2 in. exhaust port is used to evacuate gaseous products of decomposition and route exhaust gases to the oxygen sensor module to determine oxygen concentration inside the chamber. A copper sealing gasket is placed between the chamber base and chamber lid. With eight through-bolt fastening locations, the copper gaskets malleability allows deformation and creates a tighter seal with the stainless-steel base and lid.

The bubble vent (BV), depicted in Fig. 3, is a pressure control device used to force positive pressure in the DC, cool hot exhaust gases, and filter out cooled products of decomposition. Positive pressure is ensured by restricting the exhaust gas flow, maintaining exhaust back pressure at a minimum of 5 inH_2_O (0.18 psig). Constructed from off-the-shelf components including PVC pipe, polypropylene tubing, stainless steel fittings, and mesh filters, the BV is a simplistic, easy to manufacture, and effective device to promote positive pressure. The main reservoir of the BV is constructed of 2 in. clear PVC pipe. Exhaust gases from the DC are transported to the BV inlet via 1/2 in. stainless steel tubing. A 30  $[eqn]$  30 stainless steel mesh filter is included in the BV exhaust outlet to aid in removal of cooled gas condensate before the gases are transported to the oxygen sensor module. With repeated sample decompositions, the BV will require periodic maintenance of replacing the water and cleaning of the stainless steel mesh filter. The maintenance schedule will be dependent on the quantity and material type being pyrolyzed.Fig. 3. Schematic of Bubble Vent.

The oxygen sensor module (OSB), depicted in Fig. 5, is a sealed chamber equipped with an electrochemical oxygen sensor and an environmental sensor to measure the temperature, relative humidity, and pressure within the instrument. The purpose of the OS is to measure the oxygen concentration of the exhaust stream after it exits the DC and travels through the BV. 3D printed from Accura ClearVue filament and seal by an o-ring, the two-piece design allows operators to replace the electrochemical oxygen sensor when it reaches the end of its usable life. Additionally, operators can quickly identify condensate build up, if any, within the oxygen sensor module. Utilizing sealed electrical connections, power is delivered and data signals are transferred to and from the Arduino while maintaining an airtight seal. The OSB is the primary component used to ensure no leakage is present during the decomposition process, certifying an anaerobic environment within the DC.Fig. 5. Schematic of Oxygen Sensor Module.

The system control module (SC), seen in Fig. 8, Fig. 9, is used to control and monitor all electrical and gas flow components of the Anerobic Pyrolysis Chamber. Nitrogen is supplied to the SC via a gas bottle regulated to approximately 15 psig. The SC contains a secondary regulator to reduce the nitrogen supply pressure to 8 inH_2_O. The volume flow rate of nitrogen is controlled by a 0-1 L/min rotameter with a needle valve. The SC contains two ball valves: the first valve is used to turn the nitrogen supply to the DC on and off, while the second valve is used to divert the exhaust gas from the BV to one of two locations: the OS (for monitoring of the DC gas environment) or to laboratory exhaust (to bypass the OS and directly exhaust gases). The SC contains an Arduino Uno, which is used to collect and print measurements to the built-in displays. The displays on the SC provide an instantaneous readout of the DC temperature as well as the temperature, relative humidity, pressure, and oxygen concentration measured in the OS.Fig. 8. System control module exterior.Fig. 9. System control module interior.

Program files to monitor and log the APC measurements are available in the APC Zenodo repository [3]. The Furnace_Data_ Collection.ino file is an Arduino sketch program (C++) that sets up the Arduino Uno to collect APC atmospheric condition data and display it on the LCD screens of the SC. The Furnace_Temp_Data_Log.py file is a Python script that is used to log the APC atmospheric condition data. When the Arduino Uno is connected to a computer communication port, the python script will intercept and connect to the Arduino. When the Python script is run, data logging will begin and will continue until the Python script is forced, by the user, to stop running. The Python script will output a .csv file containing all measurements. The Python script is not necessary for the Arduino to collect data, it is only needed for data logging, if data logging is desired by the user.

The highlights of this hardware are summarized as follows:

• $[eqn]$ Enables well-defined temperature and gas composition control in a safe and low-cost implementation for heat treatment or reactive studies.
• $[eqn]$ Allows for controlled thermal decomposition of gram-scale material samples when the solid products of decomposition must be studied.
• $[eqn]$ Provides data logging abilities to accurately report temperature conditions post sample decomposition.

Design files summary

3

All design files for the Anaerobic Pyrolysis Chamber can be found here https://doi.org/10.5281/zenodo.10215409.

Table 2. Design files summary.Design filenameFile typeOpen source licenseLocation of the fileBubble_Vent.stpCAD fileCC BY-SA 4.0https://zenodo.org/records/10215409Chamber_Gasket.stpCAD fileCC BY-SA 4.0https://zenodo.org/records/10215409Decomposition_Chamber.stpCAD fileCC BY-SA 4.0https://zenodo.org/records/10215409LCD_Display_Mounting.stpCAD fileCC BY-SA 4.0https://zenodo.org/records/10215409Oxygen_Sensor_box.stpCAD fileCC BY-SA 4.0https://zenodo.org/records/10215409Thermocouple_Amplifier_Mounting_Bracket.stpCAD fileCC BY-SA 4.0https://zenodo.org/records/10215409Bubble_Vent_Schematic.pdfImageCC BY-SA 4.0https://zenodo.org/records/10215409Decomposition_Chamber_Schematic.pdfImageCC BY-SA 4.0https://zenodo.org/records/10215409Gas Flow Diagram.pdfImageCC BY-SA 4.0https://zenodo.org/records/10215409Oxygen_Sensor_Box_Schematic.pdfImageCC BY-SA 4.0https://zenodo.org/records/10215409Wiring Diagram.pdfImageCC BY-SA 4.0https://zenodo.org/records/10215409

Bill of materials

4

Table 3. Bill of materials.DesignatorComponentNumberCost per unitTotal costSource of materialsMaterial typeDC1Chamber Base1$1445.71$1445.71XometryMetalDC2Chamber Lid1$547.42$547.42XometryMetalDC3Copper Gasket1$47.20$47.20McMaster-CarrMetalDC4High Hex Nut12$0.08$0.96McMaster-CarrMetalDC5Hex Head Screw12$0.20$2.40McMaster-CarrMetalDC6Split Lock Washer12$0.08$0.96McMaster-CarrMetalDC7Washer12$0.14$1.68McMaster-CarrMetalDC81/2 $[eqn]$ Tube OD × 1/2 $[eqn]$ NPT Male1$25.96$25.96McMaster-CarrMetalDC91/16 $[eqn]$ Tube OD × 1/8 $[eqn]$ NPT Male1$16.30$16.30SwagelokMetalDC101/8 $[eqn]$ Tube OD × 1/8 $[eqn]$ NPT Male1$12.55$12.55McMaster-CarrMetalOSB1Oxygen Sensor Module Lid1$203.78$203.78XometryPolymerOSB2Oxygen Sensor Module1$609.64$609.64XometryPolymerOSB3Through-Wall Straight Connector2$27.04$54.08McMaster-CarrMetalOSB4Flange Nuts4$0.22$0.88McMaster-CarrMetalOSB5Hex Head Screw4$0.47$1.88McMaster-CarrMetalOSB6Square-Profile Oil-Resistant O-Ring1$0.47$0.47McMaster-CarrPolymerOSB78-pin Electrical Receptacle Kit1$26.88$26.88RSMetalOSB88-pin Electrical Plug1$55.55$55.55RSMetalOSB9Plug Gasket1(Included With OSB7)$0.00[RS](https://www.alliedelec.com/product/amphenol-sine-systems/rt00128pknh03-k/71998153/)InorganicOSB10Hex Nut4$0.06$0.24[McMaster-Carr](https://www.mcmaster.com/91828A211/)MetalOSB11Hex Head Screw4$0.06$0.24[McMaster-Carr](https://www.mcmaster.com/92800A537/)MetalOSB12Washer8$0.03$0.24[McMaster-Carr](https://www.mcmaster.com/98689A112/)MetalOSB13Oil-Resistant Gasket2$0.03$0.06[McMaster-Carr](https://www.mcmaster.com/9452K23/)PolymerBV1Straight Connector1$17.90$17.90[McMaster-Carr](https://www.mcmaster.com/4596K57/)PolymerBV2Plug with External 8-Point Drive1$8.35$8.35[McMaster-Carr](https://www.mcmaster.com/4596K78/)PolymerBV3Plug with External 8-Point Drive, Modified1$8.35$8.35[McMaster-Carr](https://www.mcmaster.com/4596K78/)PolymerBV4Tee Connector1$21.09$21.09[McMaster-Carr](https://www.mcmaster.com/4596K37/)PolymerBV5NPT Threaded Pipe1$54.75$54.75[McMaster-Carr](https://www.mcmaster.com/4677T45/)PolymerBV6Male NPT Connector1$6.16$6.16[McMaster-Carr](https://www.mcmaster.com/46825K71/)PolymerBV7NPT Female Reducer1$15.84$15.84[McMaster-Carr](https://www.mcmaster.com/46885K228/)PolymerBV83/8$[eqn]$Tube OD × 1/2$[eqn]$NPT Male1$25.07$25.07[McMaster-Carr](https://www.mcmaster.com/5182K126/)MetalBV9Through-Wall Straight Connector1$55.16$55.16[McMaster-Carr](https://www.mcmaster.com/5182K378/)MetalBV10Wire Cloth Discs1$1.30$1.30[McMaster-Carr](https://www.mcmaster.com/9317T147/)MetalBV11Smooth-Bore Seamless Tubing1$86.25$86.25[McMaster-Carr](https://www.mcmaster.com/89785K845-89785K27/)MetalSC1Slant-Top Enclosure1$293.85$293.85[McMaster-Carr](https://www.mcmaster.com/70015K13/)MetalSC2Hex Drive Screw2$0.10$0.20[McMaster-Carr](https://www.mcmaster.com/92949A264/)MetalSC3DIN 3 Rail2$3.67$7.34[McMaster-Carr](https://www.mcmaster.com/8961K85/)MetalSC4Screws for Plastic10$0.28$2.80[McMaster-Carr](https://www.mcmaster.com/99461A915/)MetalSC5Hex Nut10$0.04$0.40[McMaster-Carr](https://www.mcmaster.com/91841A005/)MetalSC6Washer10$0.01$0.10[McMaster-Carr](https://www.mcmaster.com/92141A005/)MetalSC7Screw10$0.06$0.60[McMaster-Carr](https://www.mcmaster.com/91772A118/)MetalSC8Washer10$0.04$0.40[McMaster-Carr](https://www.mcmaster.com/95610A310/)MetalSC9Hex Head Screw10$0.22$2.20[McMaster-Carr](https://www.mcmaster.com/92314A830/)MetalSC10Serrated Flange Locknut10$0.14$1.40[McMaster-Carr](https://www.mcmaster.com/93776A401/)MetalE1Barrel-Style DC Connector1$1.24$1.24[McMaster-Carr](https://www.mcmaster.com/8320N117/)MetalE2Toggle Switch1$9.4$9.40[McMaster-Carr](https://www.mcmaster.com/7343K253-7343K254/)MetalE3Panel Light1$5.72$5.72[McMaster-Carr](https://www.mcmaster.com/2779K11-2779K111/)PolymerE4Cooling Fan1$19.29$19.29[McMaster-Carr](https://www.mcmaster.com/1939K17/)PolymerE5Cooling Fan Guard2$2.50$5.00[McMaster-Carr](https://www.mcmaster.com/19155K2/)PolymerE6Red Wire1$3.58$3.58[McMaster-Carr](https://www.mcmaster.com/8054T13-8054T167/)MetalE7Black Wire1$3.58$3.58[McMaster-Carr](https://www.mcmaster.com/8054T13-8054T123/)MetalE8Orange Wire, 300V AC, 22 Gauge1$3.58$3.58[McMaster-Carr](https://www.mcmaster.com/8054T13-8054T123/)MetalE9Yellow Wire1$3.58$3.58[McMaster-Carr](https://www.mcmaster.com/8054T13-8054T178/)MetalE10Blue Wire1$3.58$3.58[McMaster-Carr](https://www.mcmaster.com/8054T13-8054T128/)MetalE11Terminal Block Adapter1$34.95$34.95[Adafruit](https://www.adafruit.com/product/4556)PolymerE12Terminal Block End Cover4$0.43$1.72[McMaster-Carr](https://www.mcmaster.com/7641K31/)PolymerE13Red Terminal Block3$1.41$4.23[McMaster-Carr](https://www.mcmaster.com/7641K921-7641K919/)PolymerE14Black Terminal Block3$1.41$4.23[McMaster-Carr](https://www.mcmaster.com/7641K921-7641K929/)PolymerE15Yellow Terminal Block3$1.41$4.23[McMaster-Carr](https://www.mcmaster.com/7641K921-7641K927/)PolymerE16Orange Terminal Block3$1.41$4.23[McMaster-Carr](https://www.mcmaster.com/7641K921-7641K928/)PolymerE17Terminal Block End Stop2$0.76$1.52[McMaster-Carr](https://www.mcmaster.com/7641K35/)PolymerE18Noninsulated Jumper2$7.39$14.78[McMaster-Carr](https://www.mcmaster.com/7641k925/)MetalE19Crimp Connector Housing1$0.13$0.13[Pololu](https://www.pololu.com/product/1903)PolymerE20Crimp Connector Housing1$0.17$0.17[Pololu](https://www.pololu.com/product/1905)PolymerE21Female Crimp Pins1$0.08$0.08[Pololu](https://www.pololu.com/product/1930)PolymerE22Male Crimp Pins1$0.12$0.12[Pololu](https://www.pololu.com/product/1931)PolymerE23Cat6 Right Angle Cable1$6.006.00[Cable Matters](https://www.amazon.com/Cable-Matters-Combo-Pack-Degree-Ethernet/dp/B01MTMV49V/ref=sr_1_14?keywords=90%2Bdegree%2Bethernet%2Bcable%26qid=1644500363%26sr=8-14%26th=1)PolymerE24Adapter Cord114.84$14.84[McMaster-Carr](https://www.mcmaster.com/70235K593/)PolymerA1Thermocouple Amplifier1$14.95$14.95[Adafruit](https://www.adafruit.com/product/5165)PolymerA2Oxygen Sensor1$58.00$58.00[Arduino](https://store-usa.arduino.cc/products/gravity-i2c-oxygen-sensor)PolymerA3Arduino Uno Rev31$25.30$25.30[Arduino](https://store-usa.arduino.cc/collections/boards/products/arduino-uno-rev3)PolymerA4LCD Display Module2$5.30$10.60[GeeekPi](https://www.amazon.com/GeeekPi-Character-Backlight-Raspberry-Electrical/dp/B07S7PJYM6/ref=pd_lpo_2?pd_rd_i=B07S7PJYM6$14.95$14.95[Adafruit](https://www.adafruit.com/product/2652)PolymerA6Male Headers3$0.95$2.85[Adafruit](https://www.adafruit.com/product/4209)PolymerA7Expandable Sleeving10$2.24$22.40[McMaster-Carr](https://www.mcmaster.com/2656K11/)PolymerA8Thermocouple Probes1$45.45$45.45[Omega](https://www.omega.com/en-us/temperature-measurement/temperature-probes/probes-with-integral-connectors/kmqxl-nmqxl/p/KMQXL-062U-12)MetalA9Thermocouple Connectors1$58.65$58.65[Omega](https://www.omega.com/en-us/temperature-measurement/temperature-connectors-panels-and-block-assemblies/panel-connectors-and-assemblies/mjp/p/MJP1-06-K)PolymerA10K Type Thermocouple Wire1$29.92$29.92[Omega](https://www.omega.com/en-us/temperature-measurement/temperature-wire-and-cable/thermocouple-wires/xc-k-tc-wire/p/GG-K-24-25)MetalA11K Type Thermocouple Extension Wire1$47.69$47.69[Omega](https://www.omega.com/en-us/temperature-measurement/temperature-wire-and-cable/thermocouple-wires/exgg-kx-wire/p/EXPP-K-20S-25)MetalA12Insulated Wire Ferrules1$9.12$9.12[McMaster-Carr](https://www.mcmaster.com/7950K17/)MetalA13Heat Shrink Tubing1$13.9913.99[Wirefy](https://www.amazon.com/Wirefy-200-Heat-Shrink-Tubing/dp/B089D82FLG/ref=sr_1_4?keywords=heat%2Bshrink%2Btubing%26qid=1655493054%26s=industrial%26sprefix=heat%2Bshrink%2Cindustrial%2C59%26sr=1-4)PolymerP110 in WC Pressure Regulator1921.34$921.34[Equilibar](https://www.equilibar.com/product/pressure-reducing-regulators/lpr2-ultra-low-pressure-reducing-regulator/)MetalP2Tube × NPT Adapter4$11.03$22.06[McMaster-Carr](https://www.mcmaster.com/5182K807/)MetalP3Through-Wall Straight Connector5$27.04$135.20[McMaster-Carr](https://www.mcmaster.com/5182K375/)MetalP4Panel-Mount Valve1$128.38$128.38[McMaster-Carr](https://www.mcmaster.com/45775K57/)MetalP5Panel-Mount Flow Meter1$124.69$124.69[Omega](https://www.dwyeromega.com/en-us/5-acrylic-variable-area-flow-meters-for-air-or-water/FL2000-Series/p/FL-2011-SS)PolymerP6Diverting Valve1$177.05$177.05[McMaster-Carr](https://www.mcmaster.com/45165K42/)MetalP7Semi-Clear Tubing1$30.00$30.00[McMaster-Carr](https://www.mcmaster.com/52355K57/)PolymerP8Smooth-Bore Tubing1$12.32$12.32[McMaster-Carr](https://www.mcmaster.com/89785K826-89785K227/)MetalP990 Degree Elbow Reducer1$57.97$57.97[McMaster-Carr](https://www.mcmaster.com/5182K407/)MetalP10Smooth-Bore Tubing1$49.72$49.72[McMaster-Carr](https://www.mcmaster.com/89785K812-89785K13/)MetalP1190 Degree Elbow Connector1$51.05$51.05[McMaster-Carr](https://www.mcmaster.com/5182K417/)MetalP12Right-Angle Tee1$80.25$80.25[McMaster-Carr](https://www.mcmaster.com/5182K193/)MetalP13High-Pressure Valve1$28.11$28.11[McMaster-Carr](https://www.mcmaster.com/45395K214/)MetalP14Soft Tubing1$9.50$9.50McMaster-CarrPolymer

Build instructions

5

Decomposition chamber

5.1

A schematic of the decomposition chamber is shown in Fig. 1. An assembled example of the decomposition chamber is shown in Fig. 2. Three bores through the lid of the decomposition chamber (DC2) are machined with NPT threads. One of these threaded bores has 1/2 in. NPT threads through the entire depth of the bore and the other two have 1/8 in. NPT threads through the depth of the bore. A 1/2 in. tube by 1/2 in. NPT through-wall connector (DC8) is screwed into the bore with the 1/2 in. NPT threads, a 1/8 in. tube by 1/8 in. NPT through-wall connector (DC10) is screwed into one of the bores with 1/8 in. NPT threads, and a 1/16 in. tube $[eqn]$ 1/8 in. NPT through-wall connector (DC9) is screwed into the other bore with 1/8 in. NPT threads.Fig. 1. Schematic of decomposition chamber.Fig. 2. Assembled decomposition chamber.

A nickel-based anti-seize lubricant is applied to the threaded interface areas of all the decomposition chamber fasteners and fittings. The copper gasket (DC3) is positioned between the decomposition chamber lid (DC2) and the decomposition chamber base (DC1) such that all holes are aligned. Before each use, inspect the copper gasket (DC3) for oxidation and defects. DC3 is designed to be a consumable part. The life expectancy of DC3 is a factor of how much it is used. If the gasket sealing surfaces are cleaned and leaks are still present, it is time to replace DC3. Twelve hex-head screws (DC5) are inserted through the holes from the base to the lid. A washer (DC7) is placed around the threaded end of each screw, and a lock-washer (DC6) is placed on top of each washer. A hex-head nut (DC9) is screwed onto each screw to a torque of at least 11 ft-lbs.

Bubble vent

5.2

A schematic of the bubble vent is shown in Fig. 3. An assembled example of the decomposition chamber is shown in Fig. 4. The ferrules from one end of a 1/2 in. through-wall connector (BV9) are compressed onto a 12 in. long 1/2 in. diameter tube with 1 in. gradations marked on the exterior. A 3/4 in. diameter hole is drilled through the center of a 2 in. NPT male pipe plug (BV3). The 1/2 in. through-wall connector (BV9) is connected through the drilled hole in the pipe plug with the 12 in. tube (BV11) connected to the fitting extending in the direction of the threads of the pipe plug. Silicone sealant is used to ensure an airtight seal between the through-wall fitting and the pipe plug.Fig. 4. Assembled Bubble Vent.

The male pipe plug with the through-wall fitting and the graduated tubing is screwed into a 2 in. female NPT tee fitting (BV4). An additional 2 in. NPT male pipe adapter (BV6) is screwed into the 2 in. female NPT tee fitting (BV4) with a mesh filter (BV10) places inside the tee fitting (BV4). A 2 in. NPT male pipe plug (BV2) is screwed into a 2 in. NPT female straight connector (BV1). A transparent 6 in. long 2 in. NPT pipe nipple (BV5) is attached to the opposite end of the straight connector (BV1). The opposite end of the transparent pipe nipple is screwed into the NPT tee fitting (BV4) such that the graduated tube is concentric with the pipe nipple.

A 1/2 in. NPT to 2 in. NPT adapter (BV7) is screwed onto the 2 in. NPT male pipe adapter (BV6). A 3/8 in. tube to 1/2 in. NPT male fitting (BV8) is installed in the 1/2 in. NPT female end of the pipe adapter (BV7). Teflon thread sealant tape is used to create an airtight seal on all NPT thread connections. 3/8 in. soft PVC tubing (P14) is used to connect the BV to the SC. Soft tubing (P14) length will be determined by the proximity of the BV to the System Control Module. The bubble vent is placed in line between the decomposition chamber and oxygen sensor module to function as a flame arrestor, preventing damage to downstream components.

Oxygen sensor module

5.3

A schematic of the oxygen sensor module is provided in Fig. 5. Two 1/4 in. through-wall connectors (OSB3) are installed through the holes in the sides of the 3D-printed oxygen sensor module base (OSB2) with an O-ring (OSB13) between each through-wall connector and the module base on the interior side of the module base to create an airtight seal.

An 8-pin electrical receptacle (OSB7) is installed through the wall of the module base (OSB2). 14 mm long stainless steel M3 × 0.5 mm machine screws (OSB11), stainless steel washers (OSB12), and M3 × 0.5 mm nuts (OSB10) are used to secure the receptacle (OSB7) and the receptacle gasket (OSB9) to the exterior of the module base. An oxygen sensor (A2) and an environmental sensor (A5) are mounted to the underside of the oxygen sensor module lid (OSB1). The oxygen sensor (A2) is intended to be a consumable component and should be replaced in accordance with manufacture guidelines. The oxygen and environmental sensors are connected to the 8-pin electrical receptacle (OSB7) via a 8-pair cable (E23).

The oxygen sensor module lid (OSB1) is secured to the oxygen sensor module base (OSB2) by passing four 3-1/4 in. long 1/4-20 machine screws (OSB5) through the four holes through the corners of the base. A square-profile o-ring (OSB6) is installed between the base and the lid to ensure an airtight seal. Flange nuts (OSB4) are tightened onto the machine screws on the lid side of the module to generate the downward pressure necessary for the airtight seal.

System control module

5.4

Photographs of the system control module are provided in Fig. 8, Fig. 9. The system control module is comprised of a slant-top enclosure (SC1) with various mounting hardware, plumbing fittings, and electrical connections. Holes were drilled through the slant-top enclosure to accommodate mounting for electrical and plumbing components. Precise locations of these holes is not important and have been excluded from this description. Rectangular holes, one measuring 3-1/4 in. wide by 1 in. tall, and another measuring 3-5/16 in. wide by 3-1/8 in. tall, are cut into the slant-top enclosure to allow the thermocouple connectors (A9) and LCD Displays (A4) to be installed through the wall of the enclosure. Circular holes with a diameter of 1-1/2 in. were drilled into the slant-top enclosure to allow for installation of the cooling fan (E4) and ventilation intake (E5). The thermocouple connector assembly, the ventilation intake, and the cooling fan are attached to the enclosure with four screws (SC7), washers (SC6), and nuts (SC5). The LCD display was connected to the enclosure with four hex head screws (SC9), washers (SC8), and serrated flange lock-nuts (SC10).

A wiring diagram which shows all connections and components in the SC is provided as Fig. 6. Each of the six thermocouple connectors in the assembly is connected to the thermocouple adapter module (A1) with type K thermocouple wire (A10). The thermocouple amplifier is connected to the Arduino (A3) with an adapter cord (E24). The Arduino is mounted to a DIN 3 rail (SC3) with a terminal block adapter (E11). A set of color-coded terminal blocks (E13, E14, E15, E16), separated by terminal block end covers (E12) and terminated on each end by end stops (E17) are mounted on a DIN 3 rail (SC3). The color-coded terminal blocks are connected to the Arduino with matching color solid core wire (E6, E7, E8, E9).Fig. 6. Wiring Diagram for Electrical Components in SC.

A gas flow diagram which shows all plumbing connections and directions of gas flow is provided as Fig. 7. Five 1/4 in. through-wall straight compression fitting connectors (P3) are installed through the enclosure. The nitrogen supply is connected to a low pressure gas regulator (P1) with two compression to NPT adapters (P2). The low pressure side of the regulator is connected to a through-wall connector (P3). The interior side of this through-wall connector is connected to a panel-mount valve (P4) via 1/4 in. flexible tubing (P7). The outlet of the panel-mount valve is connected to a 1 L/min panel-mount rotameter (P5) using two compression to NPT adapters (P2). The outlet of the rotameter is connected to a through-wall connector (P3) which directs the nitrogen flow to the DC.Fig. 7. Gas flow diagram.

The outlet of the BV is attached to smooth-bore stainless steel tubing (P8) which is bent to reduce tension on the tubing connecting the BV to the SC. The bent tubing is attached to a through-wall connector (P3) which is connected to a panel-mounted diverting valve (P6) with flexible tubing (P7). One outlet of the diverting valve is directed to the OSB, while the other is directed out of the SC via a through-wall connector (P3). The outlet of the OSB is also directed to out of the SC via a through-wall connector (P3).

Assembly

5.5

Fig. 10 illustrates an example of a fully assembled APC setup. The material samples are placed in the DC, and the DC is sealed as described in Section 5.1. The purge gas outlet connects to 1/4 in. smooth bore tubing (P8) which is connected to a reducing elbow (P9) with 1/8 in. smooth bore tubing (P10) which is directly connected to the lid of the DC. The elbow directs the 1/8 in. tubing downward through the furnace exhaust hole to allow for connection and purge gas flow to the DC.Fig. 10. Benchtop assembly of the APC.

A 1/2 in. smooth bore tube is connected to the exhaust on the lid of the DC to allow the products of decomposition and the purge gas to flow out of the DC. The tube is connected to an elbow compression fitting (P11) and additional 1/2 in. tube. The tube connects to a right-angle adapter (P12) that is attached to the 1/2 in. tubing in the BV (BV11). The adapter (P12) is also connected to a valve (P13) that allows the user to bypass the BV if necessary. The outlet of the BV is connected to the SC via 3/8 in. flexible tubing (P14).

A 1/16 in. diameter thermocouple (Inconel sheathed, K-type, 24 gauge, ungrounded) with a molded mini connector (A8) is inserted into the DC through the 1/16 in. tube compression fitting (DC9) on the lid to the chamber. The thermocouple is attached to the SC via duplex type-K thermocouple wire (A10). A second length of thermocouple wire is wired between the molded mini connector and the furnace control input connection to ensure that the furnace provides the necessary power to maintain the set point temperature or temperature ramp profile in the DC.

Operation instructions

6

The APC is intended to initiate thermal decomposition of potentially combustible materials. Because this process can generate flammable fuel vapors, there is the potential for ignition of those fuel vapors within the DC, and consequently an explosion hazard, if there is oxygen present in the DC. The APC was designed to resist the intrusion of air from the surrounding atmosphere and to mitigate this explosion risk by purging oxygen from the DC, but the operator must be aware of the possibility of an over-pressure in the system. Under the intended operating procedure, the DC is continuously purged with an inert gas to force non-oxidative thermal decomposition, eliminating the possibility of an ignitable fuel-air mixture forming in the DC. Operating instructions are included here for this procedure.

The first step in producing pyrolzed samples with the APC is to record the mass and dimensions of each sample, before sealing the samples inside the DC. The DC is then positioned in the muffle furnace. The thermocouple and tubing (which carry purge gases into and out of the DC) are routed through the port in the top of the muffle furnace and are attached to the DC. The BV is filled with water to the 5 in. level, and the nitrogen pressure regulator is adjusted to a pressure that is slightly greater than the pressure generated from the BV water level. This will allow purge gas to flow through the DC and bubble through the water inside the BV.

Purge gas flow is initiated to the decomposition chamber by adjusting the rotameter to a flow rate of 1 L/min. Flow of the purge gas through the system is verified through observation of bubbles in the bubble vent and a rapid decrease in the oxygen concentration measured in the purge gas flow. When the oxygen concentration in the purge gas is effectively zero (0 ± 0.01 vol.%), this will take approximately two minutes. Next, the furnace temperature controller is adjusted to the desired set point temperature. The temperature measured in the decomposition chamber is monitored to ensure that the temperature approaches the set point and the oxygen concentration in the purge gas path is monitored to ensure that no leaks are present in the decomposition chamber. The APC is allowed to reach the set point temperature and soak at that temperature for the desired soak duration. To bring the system back to room temperature, the set point temperature is decreased to room temperature and the measured temperature is monitored to ensure proper operation.

Validation and characterization

7

The use case for which the APC was initially developed involved production of structurally stable thermally decomposed material samples to allow for measurement of the thermal diffusivity of the produced samples through a range of environmental temperatures [1]. The APC was used to control the environment at an elevated temperature in a nitrogen atmosphere to produce the thermally decomposed samples. The initial plan was for the samples to be tested over the temperature range that the material would be expected to experience in a fire environment using transient plane source [4], modified transient plane source [5], and laser flash [6] apparatuses to measure the thermal diffusivity. Due to challenges encountered measuring thermal diffusivity with the transient plane source and modified transient plane source at elevated temperatures, only the laser flash apparatus was used to characterize the sample. This sample preparation process was performed for a eucalyptus hardboard to validate the capabilities of the APC.

The thermocouple installed in the APC was connected to the temperature controller such that the APC temperature was the controlled variable. Characterization of the instrument required an understanding of the thermal inertia and rise time of the APC temperature as well as characterization of the ability of the device to fully purge oxygen from the DC. Validation of the instrument required validation of the temperature and oxygen concentration of the environment generated within the chamber. The oxygen measurement was validated by flowing nitrogen through the system and monitoring the decrease in oxygen concentration as the system was purged of air. A soap solution was applied at all plumbing fittings to ensure no leaks were present while nitrogen flowed through the system.

To prepare the eucalyptus hardboard samples with the APC, the chamber was heated to 500 °C at a rate of approximately 16 °C/min and held at 500 °C for 90 min while purging the chamber with ultra high purity nitrogen at a rate of 0.9 L/min. While any heating program may be applied, in this case it was desired to increase the temperature linearly at this rate. A plot of the chamber temperature measured during the ramp up and soaking phases of the thermal decomposition process are provided in Fig. 11.Fig. 11. Example of Temperature History of APC for Eucalyptus Hardboard Decomposition.

The soak temperature and duration were determined based on isothermal thermogravimetric analysis (TGA) tests [7]. The TGA data were collected on a eucalyptus hardboard prepared as a powder through cryogenic grinding. TGA tests were conducted with a heating rate of 30 °C/min up to the set point temperature and held at the set point temperature for various lengths of time to establish a relationship between soak time and residual mass. The fraction of the initial mass remaining at the time where mass loss became negligible in TGA tests served as a benchmark for the performance of the APC. The TGA residual mass at 500 °C after one hour was 22.0 ± 0.2% and the APC residual mass at 500 °C after one hour was 25 ± 4%. The results of these experiments indicated that the APC was able to decompose the eucalyptus hardboard to a state that was consistent with the residue remaining after a thermal analysis experiment to allow for additional experimentation that required larger-scale sample specimens.

An additional study which used the APC to prepare samples for further measurement was conducted to investigate the evolution of thermal conductivity with exposure temperature and exposure duration for char formed from densified wood samples [8]. In this work, the flash method [6] was used to measure the thermal diffusivity of the partially decomposed samples. The flash method directly measures heat flow through the sample, so accurate measurement requires a thermally stable sample, i.e. a sample that does not continue to decompose at a given set point temperature.

In this work, the temperature of the APC was increased along a prescribed temperature ramp of 1 °C/min up to a maximum set point temperature of 500 °C. Samples prepared according to this protocol were tested in dynamic thermogravimetric experiments in nitrogen to verify thermal stability and it was found that the samples lost approximately 2% of their initial masses up to 500 °C, which effectively confirmed the samples prepared in the APC would have sufficient thermal stability to be reliably tested according to the flash method. The study revealed the importance of the temperature history of a char sample on its thermal transport properties.

CRediT authorship contribution statement

Ryan T. Chaffer: Writing – original draft, Investigation, Conceptualization. Joseph Johnson: Methodology, Conceptualization. Matthew J. DiDomizio: Writing – review & editing, Supervision, Conceptualization. Mark B. McKinnon: Writing – original draft, Supervision, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.