A Study of Charring Behavior of Woods Based on Internal Temperature and Ct-Scan Measurements

Home , Charring, Flash point

A STUDY OF CHARRING BEHAVIOR OF WOODS BASED ON INTERNAL TEMPERATURE AND CT-SCAN MEASUREMENTS

YE TIAN

Submitted in partial fulfillment of the requirements for the degree of

Master of Science

Department of Mechanical and Aerospace Engineering

CASE WESTERN RESERVE UNIVERSITY

May, 2019 CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis/dissertation of

Ye Tian

candidate for the degree of Master of Science*.

Committee Chair

Dr. Fumiaki Takahashi

Committee Member

Dr. James Ti’en

Committee Member

Dr. Ya-Ting Liao

Date of Defense

April 2, 2019

*We also certify that written approval has been obtained

for any proprietary material contained therein.

Table of Contents

List of Tables ...... 4

List of Figures ...... 5

Acknowledgements ...... 10

Abstract ...... 11

1 Introduction ...... 12

2 Literature review ...... 13

2.1 Flammability of Wood ...... 13

2.2 Char and Microstructure of wood ...... 15

3 Experimental method ...... 17

3.1 Sample preparation ...... 17

3.2 Cone calorimeter test ...... 19

3.2.1 Apparatus ...... 19

3.2.2 Data Collection and Calculations ...... 24

3.3 Computer tomography aided morphological observations ...... 26

4 Results and Discussion ...... 27

4.1 Flammability Data from Cone calorimeter ...... 27

4.1.1 Heat Release Rate (HRR) and Smoke Production Rate(SPR) ... 27

4.1.2 Ignition Phenomena ...... 43

4.1.3 Fire Endurance Time ...... 49

4.2 Charring Phenomena ...... 51

4.2.1 Temperature Distribution during Combustion ...... 51

4.2.2 Mutation Point in Temperature ...... 58

4.2.3 Charring rate ...... 67

4.3 CT-scan Results ...... 85

5 Conclusion ...... 91

References ...... 93

List of Tables

Table 3-1: Depth of thermocouples below the surface within wood of different thickness

...... 22

Table 4-1:Time to ignition of pine with different thickness and moisture content ...... 48

Table 4-2:Time to ignition of oak with different thickness and moisture content ...... 48

Table 4-3:Fire endurance time of pine with different thickness and moisture content ..... 50

Table 4-4:Fire endurance time of oak with different thickness and moisture content ...... 50

Table 4-5: Jumping points of pine at heat flux of 35 kW/m2 with different moisture

contents and thicknesses...... 59

Table 4-6: Jumping point of pine at heat flux of 25 kW/m2 with different moisture content

and thickness...... 60

Table 4-7: Pyrolysis of three main wood components ...... 62

Table 4-8: Charring rate for pine under 35 kW/m2 heating ...... 69

Table 4-9: Charring rate for pine under 25 kW/m2 heating ...... 70

Table 4-10: Charring rate for oak under 25 kW/m2 heating ...... 70

Table 4-11: Charring rate for oak under 35 kW/m2 ...... 71

List of Figures

Figure 2-1: Micrographs of cross-section of different type of wood [5] ...... 16 Figure 3-1:(a) Soaking wood (b) Oven ...... 18 Figure 3-2:Wood sample (a) Pine (b) Red oak ...... 19 Figure 3-3: Schematic view of the Cone Calorimeter [12] ...... 20 Figure 3-4: Cone calorimeter ...... 20 Figure 3-5: Sample holder ...... 21 Figure 3-6: Schematic view of the specimen holder [20] ...... 21 Figure 3-7: (a) Bottom view photograph of a specimen. (b) Instrumentation of the specimen...... 23 Figure 3-8:Example of thermocouple arrangement (∆s is same for all zones) ...... 24 Figure 3-9:(a) CT reconstructed wood with char removed, (b) cross-section surface of scanned sample ...... 27 Figure 4-1: Temporal variations of HRR for pine with 0 % moisture contents. Specimen thickness,19.05 mm; and the incident heat flux, 35 kW/m2 ...... 29 Figure 4-2: Temporal variations of HRR and SPR for pine with 0% moisture content. Specimen thickness,19.05mm; and incident heat flux: 35kW/m2...... 30 Figure 4-3: Temporal variations of HRR and SPR for oak with 0% moisture content. Specimen thickness,19.05mm; and incident heat flux: 35kW/m2...... 30 Figure 4-4:Comparison of HRR and SPR for pine with 0% and 40% moisture content; thickness: 19.05mm, and incident heat flux: 35kW/m2...... 31 Figure 4-5: Temporal variations of HRR and SPR for pine with moisture content (a) 0%(b) 10%(c) 20%(d) 30%(e)40%; thickness: 19.05mm; and incident heat flux: 35kW/m2...... 34 Figure 4-6: Temporal variations of the HRR for pine with various moisture contents. Specimen thickness,19.05 mm; and the incident heat flux, 35 kW/m2...... 34 Figure 4-7: Temporal variations of the HRR for oak with various moisture contents. Specimen thickness,19.05 mm; and the incident heat flux, 35 kW/m2...... 35 Figure 4-8:Comparison of combustion status of pine with different moisture content (a)0%

(b)40%...... 36 Figure 4-9: Temporal variations of HRR and SPR of pine with 0% moisture content under incident heat flux: 35kW/m2; thickness: (a) 6.35mm (b) 12.7mm (c)19.05mm (d) 38.1mm...... 38 Figure 4-10: Temporal variations of the heat release rate for pine with various thickness. Specimen moisture content 0%; and the incident heat flux, 35 kW/m2...... 39 Figure 4-11: Temporal variations of the heat release rate for oak with various thickness. Specimen moisture content 0%; and the incident heat flux, 35 kW/m2...... 40 Figure 4-12: Temporal variations of HRR and SPR of pine with 0% moisture content; specimen thickness 19.05mm; incident heat flux: (a)25kW/m2 (b) 35kW/m2;...... 41 Figure 4-13: Temporal variations of the heat release rate for pine with various incident heat flux. Specimen moisture content 0%; and thickness 19.05mm ...... 42 Figure 4-14:ignition state of pine under 25 kW/m2 incident heat flux with different moisture content and thickness...... 43 Figure 4-15: Temporal variations of heat release rate and smoke production rate of pine with thickness: 19.05mm;, and incident heat flux: 25kW/m2; 20% moisture content...... 44 Figure 4-16:Heat release rate and smoke production rate. pine ; thickness: 19.05mm;, and incident heat flux: 25kW/m2; 40% moisture content...... 45 Figure 4-17: Heat release rate and smoke production rate. oak ; thickness: 19.05mm;, and incident heat flux: 25kW/m2; 40% moisture content...... 46 Figure 4-18:(a) Ignition time of pine with different thickness and moisture content at heat flux 35 kW/m2, (b) ignition time of pine with different thickness and moisture content at heat flux 50 kW/m2...... 47 Figure 4-19:Fire endurance time of pine with different thickness and moisture content. 49 Figure 4-20: Heat release rate and temperature distribution for 19.05 mm pine at heat flux of 35 kW/m2 with 0% moisture content ...... 52 Figure 4-21: Heat release rate and temperature distribution for 19.05 mm pine at heat flux of 35 kW/m2 with 40% moisture content...... 52

Figure 4-22: Heat release rate and temperature distribution for 19.05 mm oak at heat flux of 35 kW/m2 with 0% moisture content...... 53 Figure 4-23: Heat release rate and temperature distribution for 19.05 mm oak at heat flux of 35 kW/m2 with 40% moisture content...... 54 Figure 4-24:Triplicate measured temperature of TC6 for 19.05 mm pine with 40% moisture content under 35 kW/m2...... 54 Figure 4-25: Temperature and changing rate of temperature of TC6 for 19.05 mm pine with 40% moisture content under 35 kW/m2...... 56 Figure 4-26: HRR and changing rate of temperature of TC6; 19.05 mm Pine of 40% moisture content under 35 kW/m2 ...... 57 Figure 4-27: Comparison of increasing rate of temperature of TC6; 19.05 mm Pine of 0% and 40% moisture content under 35 kW/m2...... 57 Figure 4-28: Comparison of changing rate of temperature of TC6; 19.05 mm Pine and Oak of 40% moisture content under 35 kW/m2...... 58 Figure 4-29: Example of mutation points for 19.05 mm pine with 0% moisture content at a heat flux of 35 kW/m2 ...... 59 Figure 4-30: Time and temperature of pine’s jumping point with different moisture contents and thicknesses...... 60 Figure 4-31: Temporal variations of HRR and temperature distribution for 38.10 mm pine with 0% moisture content at a heat flux of 35 kW/m2 ...... 63 Figure 4-32: Temporal variations of HRR and temperature distribution for 19.05 mm oak at heat flux of 35 kW/m2 with various moisture content (a) 0% (b) 20% and (c) 40%...... 66 Figure 4-33: Charring line for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 67 Figure 4-34: Charring rate for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 68 Figure 4-35:Charring rate for 12.70mm and 19.05mm pine at different heat flux and moisture content...... 71

Figure 4-36:Charring rate for 19.05mm pine at heat flux of 35 kW/m2 with 0% moisture content under two methods...... 72 Figure 4-37:Charring rate for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 73 Figure 4-38:Temperature distribution for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 73 Figure 4-39:Charring rate for 12.7 mm pine with 10% moisture content at heat flux of 35 kW/m2...... 74 Figure 4-40:Temperature distribution for 12.7 mm pine with 10% moisture content at heat flux of 35 kW/m2...... 74 Figure 4-41:Charring rate for 12.7 mm pine with 20% moisture content at heat flux of 35 kW/m2...... 75 Figure 4-42:Temperature distribution for 12.7 mm pine with 20% moisture content at heat flux of 35 kW/m2...... 75 Figure 4-43:Charring rate for 12.7 mm pine with 30% moisture content at heat flux of 35 kW/m2...... 76 Figure 4-44:Temperature distribution for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 76 Figure 4-45:Charring rate for 12.7 mm pine with 40% moisture content at heat flux of 35 kW/m2...... 77 Figure 4-46:Temperature distribution for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 77 Figure 4-47:Charring rate for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 78 Figure 4-48:Temperature distribution for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 78 Figure 4-49:Charring rate for 19.05 mm pine with 10% moisture content at heat flux of 35 kW/m2...... 79 Figure 4-50:Temperature distribution for 19.05 mm pine with 10% moisture content at heat

flux of 35 kW/m2...... 79 Figure 4-51:Charring rate for 19.05 mm pine with 20% moisture content at heat flux of 35 kW/m2...... 80 Figure 4-52:Temperature distribution for 19.05 mm pine with 20% moisture content at heat flux of 35 kW/m2...... 80 Figure 4-53:Charring rate for 19.05 mm pine with 30% moisture content at heat flux of 35 kW/m2...... 81 Figure 4-54:Temperature distribution for 19.05 mm pine with 30% moisture content at heat flux of 35 kW/m2...... 81 Figure 4-55:Charring rate for 19.05 mm pine with 40% moisture content at heat flux of 35 kW/m2...... 82 Figure 4-56:Temperature distribution for 19.05 mm pine with 40% moisture content at heat flux of 35 kW/m2...... 82 Figure 4-57:Charring rate for 38.1 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 83 Figure 4-58:Temperature distribution for 38.1 mm pine with 0% moisture content at heat flux of 35 kW/m2...... 83 Figure 4-59:Charring rate for 19.05 mm pine with 20% moisture content at heat flux of 35 kW/m2...... 84 Figure 4-60:Temperature distribution for 19.05 mm pine with 20% moisture content at heat flux of 35 kW/m2...... 84 Figure 4-61: Heat release rate for 19.05 mm 10% moisture content pine at heat flux of 35 kW/m2 with (a) interior temperature distribution and (b) charring rate...... 86 Figure 4-62: CT scanned 19.05 mm pine wood samples with 10% moisture content exposed to a 35 kW/m2 heat flux that was extinguished at (a) the post ignition stage, (b) the charring propagation stage, and (c) throughout the combustion stage ...... 88 Figure 4-63: CT scanned 19.05 mm Oak wood samples with 10% moisture content exposed to a 35 kW/m2 heat flux that was extinguished at (a) the post-ignition stage,

(b) the charring propagation stage, and (c) throughout combustion stage...... 89

Acknowledgements

I would like to express my gratitude to all those who helped me during the writing of this thesis. I gratefully acknowledge the help of my primary advisor, Dr. Fumiaki Takahashi, who has offered me valuable suggestions in the academic studies. In the preparation of the thesis, he has spent much time reading through each draft and provided me with inspiring advice. I also owe a special debt of gratitude to Dr. James T’ien and Dr. Ya-Ting Liao for their instructive advice and useful suggestions on my project.

I am thankful for the support that was provided by Jiyuan Kang in teaching me how to use the cone calorimeter and CT-scanner. I am also greatly indebted to Dr. Mike Johnston,

Yumi Matsuyama and Chenran Wen, who have instructed and helped me a lot in the past two years.

This project would have never been possible without the support provided by

Underwriters Laboratories and the U.S. Department of Homeland Security, Federal

Emergency Management Agency, Assistance to Firefighters Grant Program, Fire Prevention and Safety grant (No. EMW-2014-FP-00688).

Last my thanks would go to my beloved family for their loving considerations and great confidence in me all through these years. I also owe my sincere gratitude to my friends and my roommate who help me work out my problems.

A Study of Charring Behavior of Woods Based on Internal Temperature and CT-Scan Measurements

Abstract

Ye Tian Recent trends of increasingly tall wood buildings call for further research on the combustion and carbonization of wood. Burning, charring, and smoke characteristics of two common building woods (pine and oak) with different thicknesses (6.35 to 38.1 mm) and moisture contents (0 to 40 %) were investigated in a cone calorimeter under different incident radiant fluxes (25, 35, and 50 kW/m2). Besides the standard cone calorimeter outputs (the heat release rate, time to ignition, and the concentrations of O2, CO, CO2, and smoke), real-time temperature variations in the interior of the specimens were measured by six thermocouples embedded at different depths. The charring rate of the wood was determined by tracking the time when each thermocouple reached 300 ℃. The heat-release rate curve exhibited two peaks at the initial and last flares and a reduced-level charring period in-between, typical of wood burning. The second heat-release rate peak was initiated (abruptly for pine) when the charring wave reached the back side of the wood specimen. Furthermore, computed tomography (CT)- scanning of the specimens, quenched at different times after ignition, revealed the internal structure and yielded the charring rate independently, which was consistent with that obtained from the temperature measurement. With an aim at reducing the risk of fire, this study demonstrates the laboratory-scale methods useful to understand the physical processes taking place in the interior of wood specimens and how they relate to the various burning characteristics of woods.

1 Introduction

As one of the important raw materials for building construction, wood has been used since thousands years ago. With the development of modern times, noncombustible building materials such as steel and concrete have become common to meet the demand for higher and stronger buildings. However, now, with the increase in strength brought by engineered wood, tall buildings with wooden structures are showing a global trend. Fire performance becomes an essential consideration in the safety of wood construction. On the other hand, wildland fires have caused increasingly devastating damage to communities in recent decades. Like urban conflagrations a century ago, wildfire in urban and suburban settings poses one of the greatest fire challenges of our time [1]. Therefore, studying the mechanisms of wood combustion is of essential importance in the fire safety of wooden structures.

Engineered timber has better portability compared with usual building materials. Tall buildings constructed from wood are becoming more prevalent globally [2]. With the increase of building height, the possibility of serious consequences caused by fire increases. Fire safety of tall wood buildings need to be studied with the aim of identifying the knowledge gaps which will reveal the relationship between wood properties and combustion. Moisture content of wood is the key to deterioration and fire problems.

The combustion processes of wood are complex phenomena. The burning process includes heat transfer, chemical reactions and escape of volatiles through and from the particles [3].

Although previous research has studied the overall fire-performance characteristics of wood using various fire testing technology, the effects of microstructure of wood and its charring

12 process during burning still need to be investigated.

Wet or thicker wood is less flammable. When the wood particle is wet, the whole burning process is delayed. Additional heat must be supplied for water to evaporate before pyrolysis temperatures are reached. Thick wood is sometimes seen as noncombustible, as it generates a char layer to block the contact between air and fuel during burning. Thus, the effect of moisture content and thickness are significant for studying the combustion characteristics of wood [4].

The objectives of this study are: (1) to gain a better understanding of the physical processes taking place in the interior of burning wood, and (2) to determine the burning, charring, and smoke characteristics using two common building woods (pine and oak) with different (soft and hard) microstructure, thicknesses, and moisture contents under different incident heat fluxes.

2 Literature review

2.1 Flammability of Wood

There is a large number of experimental studies on wood as a building material. The information is recorded in the wood handbook [5], including its physical and mechanical properties. Fire resistance is another important part of construction with wood and wood-based products. Previous researches have tabulated the flammability data for construction purpose and provided empirical formulas for calculation. Fire Protection Research Foundation published a report to introduce the fire safety performance of wood [6]. Several researches also

13 have been done to estimate the fire safety in timber buildings by Birgit and Tondi [7, 8].

There are a variety of different types of wood. Choosing right experimental materials is essential for testing. Carlton introduced the first candidate material, red oak, which is widely used as the calibration standard for fire testing [9], and pine is considered as an alternative.

Both red oak and pine are widely distributed in the United States and are easily acquired. The difference in composition between hardwood (oak) and softwood (pine) is discuss by Popescu

[10].

Three main chemical composition of woods are cellulose, lignin, and hemicelluloses [11].

The basic elements of wood are carbon, hydrogen and oxygen. And the pyrolysis process of these two types of wood is discussed by Müller-Hagedorn and Tillman [12, 13]. The wood combustion process is divided into five stages: heating, thermal decomposition, ignition, combustion and spread of flame. The combustion products are varied and experiments on analysis of the emission components are done in detail by McDonald [14]. The burning of wood is related to its thermal conductivity, thermal diffusivity, density and chemical composition [15]. This research focuses on the effect of moisture content and thickness on wood combustion. The thermal decomposition of wood with different chemical composition is highly correlated with temperature [16, 17].

In the process of wood combustion, hydrogen combustion, carbon monoxide combustion and hydrocarbon combustion are the main chain reactions [18, 19]:

1) H+3H2+O2→2H2O+3H;

2) OH+CO→CO2+H CO+O+M→CO2+M

3) RH+O2→R’H+CO+H2O

Heat release rate is the most important characteristic among the flammability parameters.

A cone calorimeter (ASTM E1354) is used to measure the burning characteristics of the tested samples. Laura compared the quality and accuracy of the heat release rate measurements of a cone calorimeter and mass loss calorimeter [20] and the cone calorimeter is more accurate for obtaining the heat release rates than the mass loss calorimeter.

Several tests are based on the cone calorimeter. Kim et al. [21] estimated the fire behavior of wood with different flooring and thickness. Harada [22] measured the time to ignition, heat release rate, and fire endurance time of wood in a cone calorimeter test. Spearpoint [23] provides a detailed model to predict the piloted ignition of wood in the cone calorimeter. The integral model helps to solve the heat transfer process and apply the approximate solutions for ignition. Considering the moisture content, Shen [24] carried out experiments on the thermal decomposition of wet wood in air and stated a one-dimensional pyrolysis model to calculate the results. When the wood particles are wet, the whole combustion process is delayed [25].

Water evaporation absorbs heat before the wood reaches its pyrolysis temperature. Drying and pyrolysis occurred in the same time, which could not be modeled separately. Bryden, Enrico , and Bilbao [26-28] studied the pyrolysis of wet wood and applied respective modeling tool to simulate the combustion process.

2.2 Char and Microstructure of Wood

In addition to the flammability characteristics of wood, charring rate is a critical factor in the fire endurance of wood. Changes in the internal wood structure also affect its combustion

15 characteristics. Several investigators have studied the charring rates for fixed heat fluxes and examined the microstructure of the wood. Robert [29] found the charring rate, when the wood is exposed to a constant external heat flux, can be considered as a linear function of time, but it may change to nonlinear at high heat flux levels. Silcock [30] estimated the fire severity condition with char depth, applying a practical method for fire safety. Zicherman [31] used scanning electron microscopy (SEM) to study the microstructure of char and indicated a probable relationship between the microstructural features of wood and its behavior in fire.

Cutter [32] studied the char structure by taking SEM photographs of burnt wood by electron microscope and showed the thermal degradation in different temperature.

Two micrographs of a representative cross-section of pine and oak are shown below [5].

The magnifications of both micrographs are approximately 15×. Anatomically, the significant difference between hardwoods and softwoods is the vessel elements. Wood cell with open ends forms the vessel elements. Clearly vessels or pores can be easily found in the micrographs of oak, which affect the char formation after burning.

(a)Oak (b) Pine

Figure 2-1: Micrographs of cross-section of different type of wood [5].

3 Experimental methods

3.1 Sample preparation

Ponderosa Pine and red oak are chosen to be the testing material. Ponderosa Pine, a softwood easily found in the USA, has excellent workability for variety of uses. The wood specimens were cut into 10×10cm standard sized plate with the thickness of 6.35 mm (0.25 inch), 12.7 mm (0.5 inch), 19.05mm (0.75 inch), 38.1 mm (1.5 inch) for pine samples and 6.35 mm (0.25 inch), 12.7 mm (0.5 inch), 19.05mm (0.75 inch) for oak samples. In order to reduce the error and increase the measurement accuracy, all experiments are repeated at least twice. If the differences between the two experimental results are significant, a third experiment is added.

The wood samples tested under the same experimental conditions are cut from the same lumber.

The wood sample shared the similar density are used in this study without any knot or damage.

The density of air-dried pine is 0.378±0.05 g/cm3. The density of air-dried oak is 0.708±0.05 g/cm3.

The oven-drying method [33] has been the most universally accepted method for determining the moisture content. The specimen is prepared to be free from knots and other irregularities, such as bark and pitch pockets. To prevent drying or uptake of moisture, each specimen is immediately sealed in a plastic bag or tightly wrapped in metal foil after weighted to protect it from moisture change until it can be tested.

The moisture content is defined as follows:

MC =（A - B）/B  100 [%] (3-1) where: A = original mass, g, and B = oven-dry mass, g.

The specimen is placed in a convection oven with 104℃ by following the drying method in ASTM D4442-16. After 24 hours of drying, net mass of totally dry wood was recorded. The mass difference of each specimen with same thickness is less than 5%. Totally wet specimen is obtained by submerging the wood in water for a homogenization period not less than 2 weeks until the mass of the specimen stopped changing. Soaked specimen is then put in the oven again for re-drying to match the final moisture content to 0 %,10 %, 20 %, 30 %, and 40 %. Shrinkage of wood with different moisture content is less than 2% which can be ignored.

Figure 3-1 shows the equipment for soaking wood and the oven for drying wood. Figure

3-2 shows photographs of the samples of pine and oak.

(a) (b)

Figure 3-1:(a) Soaking wood (b) Oven.

(a) (b) Figure 3-2:Wood sample (a) Pine (b) Red oak.

3.2 Cone calorimeter test

3.2.1 Apparatus

A cone calorimeter (ASTM E1354) is used to measure the burning characteristics of woods in this work. Incident heat fluxes of 35 Kw/m2, 35 Kw/m2 or 50 Kw/m2 are used as heat exposure to moisture-controlled wood specimens with different thicknesses. All test specimens are exposed in their horizontal orientation with the standard plot ignitor operating, and the incident heat flux was parallel to the grain. All of the sides except the top side of the specimens are wrapped with 0.03-0.05 mm thick aluminum foil. The specimens are placed in the same holder and exposed to a cylindrical heater located 25 mm from the top surface of the specimen.

During the heating process, the imposed heat flux is kept constant. The heat release rate (HRR) is measured from cone calorimeter tests as one of the key parameters to describe the wood specimen flammability. In addition, the smoke production rate (SPR), ignition time, and burning duration are collected for performing burning mechanism analyses of the woods. 19

Schematic view of the Cone Calorimeter is showed in Figure 3-3.

Figure 3-3: Schematic view of the Cone Calorimeter [12]

Smoke funnel a conical radiant electric heater

Shutter Spark igniter

Load cell Ceramic cover

Specimen holder

Figure 3-4: Cone calorimeter

Figure 3-4 shows the cone calorimeter consists of the following parts: a smoke funnel, shutter, load cell for measuring specimen mass loss, conical radiant electric heater, spark igniter, and specimen holder.

Figure 3-5: Specimen holder

Figure 3-6: Schematic view of the specimen holder [20] 21

Figures 3-5 and 3-6 show a photograph and a drawing of a bottom part of the specimen holder, modified for inserting six thermocouples in order to measure the temperatures within the specimen. Six holes were drilled symmetrically distributed around the center of the wood specimen with isometric positions within the wood of 19.05 mm and 12.70 mm thicknesses from the bottom surface of the specimen. Figure3-7 shows a photograph of the specimen and locations of holes for the thermocouples. Thermocouples (labeled TC1, TC2, TC3, TC4, TC5, and TC6) are connected to a National Instruments data acquisition module, which is connected to a laptop computer with a running LabVIEW program. Real-time temperature readings from the interior of the burning specimen can be collected and stored.

For all of the tests, thermocouple TC6 was always inserted through the wood and had its tip exposed to the cone heater directly, while TC1 was always attached to the bottom surface of the wood, The other thermocouples were arranged uniformly in the interior of the sample while the same distances between thermocouple tips were maintained. A summary of all thermocouple positions is provided in Table 1, and an example of a thermocouple arrangement is shown in Figure 3-7.

Table 3-1: Depth of thermocouples below the surface within wood of different thickness

Thickness(mm) TC1(mm) TC2(mm) TC3(mm) TC4(mm) TC5(mm) TC6(mm)

38.10 0 7.62 15.24 22.86 30.48 38.10 19.05 0 3.81 7.62 11.43 15.24 19.05 12.70 0 2.54 5.08 7.62 10.16 12.70 6.35 0 2.54 5.08 6.35

(a) (b)

Figure 3-7: (a) Bottom view photograph of a specimen. (b) Instrumentation of the specimen.

In this study, the position of the charring front is taken as the position of the 300℃ isotherm. Because the six thermocouples are equally spaced in their depths inside the test specimen, thus dividing the specimen into 5 zones (Fig.3-8), the charring rate can be determined by the following equation:

Δs Ch = (3 − 2) 푡퐿표푤푒푟 푇퐶 − 푡푢푝푝푒푟 푇퐶

Where Ch is the charring rate, Δ푠 is the distance between thermocouple tips, and tupper TC and tlower TC are the times when the upper and lower thermocouples of each zone reach 300℃ (the charring front), respectively. With this method, the rate of charring process of a burning wood can be measured.

Figure 3-8:A schematic of thermocouple and zone arrangements (∆s is same for all zones).

3.2.2 Data Collection and Calculations

3.2.2.1 Heat Release Rate

As a widely used bench-scale instrument for fire testing, the cone calorimeter uses O2 consumption technique to measure the heat release rate. The method based on the changes in the oxygen concentration to calculate the heat release rate was first developed in 1968 [34].

The National Institute of Standards and Technology refined the standard of this technology

[35]. The HRR is calculated following the standards described in detail in ASTM E1354 [36] and ISO 5660-1 [37].

Gas analysis equipment of O2, CO2 and CO is used in this study. The equations are as follows.

The following symbols are used in this section.

푚̇ e = exhaust duct mass flow rate (kg/s).

∆푃 = pressure differential, Pa.

Te = absolute temperature of gas, K.

푋 0 = Measured mole fraction of O2 in the incoming air. 푂2

푋 0 = Measured mole fraction of CO2 in the incoming air. 푐표2

푋퐶푂2 = Mole fraction of CO2 reading after experiment.

푋퐶푂 = Mole fraction of CO reading after experiment.

푋푂2 = Mole fraction of O2 reading after experiment.

Δhc = net heat of combustion, kJ/kg. ro = stoichiometric oxygen/fuel mass ratio

The exhaust duct flow shall be determined as follows [35]:

∆P ṁ e = C√ (3 − 3) 푇푒

The rate of heat release is as follows [36]:

( ) ⁄ ∆ℎ푐 휑 − 0.172 1 − 휑 푋퐶푂 푋푂2 ( ) 표 [ ] HRR = 1.10 푋푂2 푚̇푒 (3 − 4) 푟0 (1 − 휑) + 1.105휑

Where

푋푂0(1 − 푋퐶푂 − 푋퐶푂) − 푋푂 (1 − 푋푐표0) φ = 2 2 2 2 (3 − 5) 푋 0(1 − 푋 − 푋 − 푋 ) 푂2 퐶푂2 퐶푂 푂2

3.2.2.2 Smoke Measurement Instruments

When the wood undergoes combustion or pyrolysis, smoke is generated. Smoke is a collection of airborne solid and liquid particulates and gases [38]. In the cone calorimeter test, however, the definition is limited and incapable of explaining the experimental phenomena in detail. "Smoke" denotes the degree to which gaseous products block the optical properties of laser signals; and “soot” has been used to describe the mass properties of these particles [39].

The cone calorimeter uses a flow-through measurement in conjunction to measure the smoke production rate. And soot mass has been measured by a disk filter.

3.3 Computed tomography aided morphological observations

The computed tomography (CT)-scanning method is used to observe the morphological structure of the burned and quenched specimens. Wood specimens under cone calorimeter tests were removed from the instrument at selected times, and then the fire was extinguished by spraying a small amount of water on the burning surface. The heat-exposed specimens (after a short time to cool down) were then brought to an Inveon-PET-CT scanner, which can generate up to 960×960×960 pixel (front, top, and side views) 16-bit grayscale images, where each pixel represents a 0.098995×0.098995 mm area and the increment between images is also 0.098995 mm. The scanned data were exported as an image sequence with the Digital Imaging and

Communications in Medicine (DICOM) format and then 3D-reconstructed to process using

ImageJ software.

In the CT scanned images, the grayscale values of pixels represent the X-ray absorption of materials at the location, and the value depends on the density of the material. High X-ray absorption resulting from high density materials stands for high grayscale values (bright), and low density materials result in low grayscale values (dark) in the image. Hence, the 16-bit grayscale images obtained from the CT scanner possess a unique set of grayscale values assigned to char and original wood; these values allow the boundary between original wood and char – or in other words, the char propagation line – to be clearly visualized. With the CT scan technology coupled with image processing tools embedded in the ImageJ software, the morphological structure of wood specimens at different burning time were studied.

Figure 3-9 shows the CT scanned picture for the same sample with the char removed by image processing and the cross-sectional surface and a CT-scanned 3D reconstructed model 26 for the same sample.

(a) (b) Figure 3-9:(a) CT reconstructed wood with char removed, (b) cross-section surface of scanned sample

4 Results and Discussion

4.1 Flammability Data from Cone Calorimeter

4.1.1 Heat Release Rate (HRR) and Smoke Production Rate(SPR)

The heat release rate (HRR) is one of the key parameters to describe the flammability of a material and smoke production rate is found to be correlated closely with HRR. In order to study the basic relationship between HRR and SPR, completely dried wood is studied first.

Figure 4-1 shows the HRR curve of 19.05 mm pine with 0% moisture content under 35kW/m2 incident heat flux of three repeated experiments in the same condition. For repeatability of the data, it can be seen how a set of three repeated HRR data similar to each other. Unlike other experimental materials (e.g., polymers), wood is not a homogeneous compound. It is normally difficult to ensure the repeatability of the tests and overlapped curves due to the deformation of the wood structure. The differences are within an acceptable range. Moreover, average values are calculated and shown in black dash line.

For wood, there are typically two peaks of the HRR curve due to charring. Figure 4-2 shows the HRR and SPR curve of 19.05 mm pine with 0% moisture content under 35kW/m2 incident heat flux. While burning, wood will decompose to form a layer of porous char which act as an insulation layer to reduce the heat transfer rate from flaming zone and the gasification rate, thus blocking the vaporized fuel from reaching with oxygen. The time to ignition is recorded by the experimenter. Fire endurance time is regarded as the time when the HRR curve reaching its second peak. It shows the back side of wood sample may begin to combust.

Figure 4-3 shows the HRR curve of oak under the same conditions with Figure 4-2. The whole burning period for oak is extended compared to that for pine due to the larger mass of oak in the same volume, which can provide more fuel for combustion. For pine, the value of two HRR peaks is closer compared with oak. So pine is more easily to generate large amount of heat after ignited. While first HRR peak of oak (175 kW/m2) is somewhat lower than that of pine (200 kW/m2), the second peak (330 kW/m2 vs. 260 kW/m2) is higher, thus vigorously burning after a long time of preheating. For the smoke production, pine exhibits approximately twice the SPR compared to oak over the entire burning period. This may relate to the wood composition. Pine is softwood which generally has more lignin than oak, a kind of hardwood.

Figure 4-1: Temporal variations of HRR for pine with 0 % moisture contents. Specimen thickness,19.05 mm; and the incident heat flux, 35 kW/m2

Figure 4-2: Temporal variations of HRR and SPR for pine with 0% moisture content. Specimen 2 thickness,19.05mm; and incident heat flux: 35kW/m .

Figure 4-3: Temporal variations of HRR and SPR for oak with 0% moisture content. Specimen thickness,19.05mm; and incident heat flux: 35kW/m2.

Figure 4-4:Comparison of HRR and SPR for pine with 0% and 40% moisture content; thickness: 19.05mm, and incident heat flux: 35kW/m2.

4.1.1.1 HRR and SPR with Different Moisture Content

Figure 4-5 shows the HRR and SPR with increasing moisture content. It is obvious that the increasing moisture content reduces the value of heat release rate and extends the burning time. An increase in the moisture content requires more energy as well as longer time to complete the water vaporization process under the same incident heat flux and the ignition conditions, and thus the charring propagation delays and the burning period extends. The second peak is always higher than the first peak.

As the moisture content is increased from 0 % to 40 %, the first peak of the HRR decreases from 200 kW/m2 to 75 kW/m2 and the second peak decreases from 250 kW/m2 to 140 kW/m2.

The burning period extends with increasing the moisture content and the SPR reduces to almost

0 m2/s during the stable combustion period between two peaks. The addition of water suppresses the smoke generation.

(a)

(b)

(c)

(d)

(e)

Figure 4-5: Temporal variations of HRR and SPR for pine with moisture content (a) 0%(b) 10%(c) 20%(d) 30%(e)40%; thickness: 19.05mm; and incident heat flux: 35kW/m2.

Figure 4-6: Temporal variations of the HRR for pine with various moisture contents. Specimen thickness,19.05 mm; and the incident heat flux, 35 kW/m2.

Figure 4-7: Temporal variations of the HRR for oak with various moisture contents. Specimen thickness,19.05 mm; and the incident heat flux, 35 kW/m2.

The heat release rate of pine and oak combustion with different moisture content and thickness are shown in Figures 4-6 and 4-7. Even the different fuels, the trend in the moisture effect during burning is similar.

(a) (b) Figure 4-8:Comparison of combustion status of pine with different moisture content (a)0% (b)40%.

Figure 4-8 shows the combustion status of pine with different moisture content. For completely dry wood (Fig. 4-8a), combustion process is intense with bright flame. The sound of breaking wood structure inside is made throughout the whole burning period. The combustion process for wet wood (Fig. 4-8b) is much gentler and silent. Blue flame is generated above the surface of wood sample probably at lower temperature.

4.1.1.2 HRR and SPR with Different Thickness

(a)

(b)

(c)

(d) Figure 4-9: Temporal variations of HRR and SPR of pine with 0% moisture content under incident heat flux: 35kW/m2; thickness: (a) 6.35mm (b) 12.7mm (c)19.05mm (d) 38.1mm.

Figure 4-9 shows the HRR and SPR of pine with different thickness under same incident heat flux and moisture content. For 6.35mm pine with 0% moisture content (Fig. 4-9a), the first peak of heat release rate is not obvious as the other three thicker cases. The heat release rate

38 continuously increases right after the ignition. Because the sample is thin and dry, the 6.35mm- thick pine is easily heated and the charring does not slow down burning much to form the sharp first peak. The thin char layer formed above the unburned fuel does not block the contact between oxygen and wood pyrolyzates. Therefore, the whole wood involves in the combustion process to form a large second HRR peak. For 12.70 mm and 19.05 mm pine, the HRR and

SPR curves are similar though the value is different. The pine of proper thickness can undergo the common burning process of two peaks.

On the other hand, for the thickest (38.10 mm) pine, the heat release rate trend is totally opposite; i.e., the first peak is larger than the second one. For the thick wood, after combusting for a long time after ignition, a thick char layer impedes the combustion of the whole wood and the wood remains intact on the bottom. The smoke production rate also shows that the second

HRR peak of the 38.10mm pine is different with the other three thinner specimens. The SPR curve does not follow the HRR. After the initial peak, the SPR decreases and stays nearly zero throughout the burning period.

Figure 4-10: Temporal variations of the heat release rate for pine with various thickness. Specimen moisture content 0%; and the incident heat flux, 35 kW/m2. 39

Figure 4-11: Temporal variations of the heat release rate for oak with various thickness. Specimen moisture content 0%; and the incident heat flux, 35 kW/m2.

Figure 4-10 shows the heat release rate of pine with different thickness, replotted from the data points in Fig. 4-9. In summary, increasing thickness prolong the burning period of wood with the increasing fuel mass. And the second peak of HRR decreases as the char layer accumulates over the thicker wood. The first peak of HRR is in the range of 130 to 200kW/m2.

Figure 4-11 shows the heat release rate of burning oak with two different thicknesses. The trend is similar to that of pine. The first peak of HRR is also in the range of 130 to 200kW/m2, and as the specimen thickness is increased, the second peak of HRR decreases and the burning period becomes longer. Hence, the second peak is somewhat larger than pine.

4.1.1.3 HRR and SPR with Different Incident Heat flux

(a)

(b) Figure 4-12: Temporal variations of HRR and SPR of pine with 0% moisture content; specimen thickness 19.05mm; incident heat flux: (a)25kW/m2 (b) 35kW/m2;

Figure 4-13: Temporal variations of the heat release rate for pine with various incident heat flux. Specimen moisture content 0%; and thickness 19.05mm

Figure 4-12 shows the heat release rate and smoke production rate of pine under different incident heat flux, 25 kW/m2 and 35 kW/m2. The ignition time for lower incident heat flux is longer, so the smoke production rate before the ignition is much more obvious compared with the higher incident heat flux. Before ignition, it takes more than 100 seconds to reach the ignition point for pine. Combustible gases including CO, formic acid [40], and noncombustible water vapor must be generated above the surface of the sample. Large amount of smoke

(aerosol) must be produced before ignition. After the ignition, the consumption of flammable volatiles results in a sudden drop in the SPR curve. Overall burning process must be similar between two conditions with different incident heat flux. The HRR has two peaks and SPR is correlated with HRR. Figure 4-13 shows the effect of the incident heat flux on HRR of pine.

The HRR curves are similar, but the two peaks are smaller for 25 kW/m2 than for 35 kW/m2.

4.1.2 Ignition Phenomena

Irrespective of synthetic materials and flame retardant additives, moisture content and thickness affect the wood combustion process. Fig 4-14 shows the ignition state of pine under

25 kW/m2 incident heat flux with different moisture content and thickness. Because wood is combustible, it can be ignited after being heated for a long time whatever the moisture content and thickness. A limit for 5 minutes is defined if as wood can’t be ignited within 5 minutes, it is non-ignited in this experiment. In Fig 4-14, circle dot represents the ignited condition.

Triangle dot represents that wood is ignited, but the flame extinguished after a short period.

Rectangle dot represents wood can’t be ignited within 5 minutes.

1.75 1.5 1.25 1 0.75

0.5 Thickness(inches) 0.25 0 0 10 20 30 40 50 60 70 Moisture Content(%) Figure 4-14:ignition state of pine under 25 kW/m2 incident heat flux with different moisture content and thickness.

Fig 4-14 shows that both the moisture content and thickness may affect the combustion process, especially the ignition condition. Higher moisture content requires more energy as well as longer time to complete the vaporization process under the same incident heat flux. For

12.7 mm pine, thin wood can be burned through as long as it is ignited. The ignited wood can generate enough energy to maintain the burning process while the water is still vaporizing. For 43 thicker wood like 19.05 or 38.1 mm pine, flame may be extinguished with high moisture

content.

Figure 4-15: Temporal variations of heat release rate and smoke production rate of pine with thickness: 19.05mm;, and incident heat flux: 25kW/m2; 20% moisture content.

Figure 4-15 shows the HRR and SPR when pine is ignited (flaming) and then smoldering

(non-flaming). The HRR curve only has one peak compared with Fig 4-1. It takes more than

300 s for pine to be ignited with higher moisture content and then fail to reach its second peak.

For pine of 0% moisture content, smoke production rate is found to be correlated closely with

HRR. While for pine of 20% moisture content, the SPR curve rises before the HRR curve.

Smoldering is the initial stage of combustion of wet wood. A large amount of aerosols of water and pyrolyzates are generated before the wood’ flaming ignition. After ignited, SPR has a sudden drop because of vaporization of aerosols and combustion of wood reduce the production of smoke.

Figure 4-16:Heat release rate and smoke production rate. pine; thickness: 19.05mm;, and incident heat flux: 25kW/m2; 40% moisture content.

Figure 4-16 shows the HRR and SPR of non-ignited (non-flaming) pine. The HRR has a small peak, although there is no visible flame can be recorded during the experiment. The peak may be due to the temperature reaching the flash point of wet wood. The pyrolyzates of pine must have reacted and generate heat in a short time. While temperature does not reach the fire point, pine can not keep burning after being ignited. Figure 4-17 shows the non-ignited oak.

The HRR is bouncing within the margin of error. The oak is smoldering and generating smoke including water vapor and pyrolysis products. So oak is harder to be ignited than pine with the same moisture content.

Figure 4-17: Heat release rate and smoke production rate. oak ; thickness: 19.05mm;, and incident heat flux: 25kW/m2; 40% moisture content.

Figure 4-18 shows the ignition time of wood samples with 6.35 mm, 12.7 mm and

19.05mm initial thickness and different moisture content exposed to (a) 35 kW/m2 and (b) 50 kW/m2 incident heat fluxes. According to the test result, ignition time of wood samples increased almost linearly with respect to the increase of the moisture content for both incident fluxes. The higher incident heat flux results in shorter ignition times. When exposed to heat, water at the very top surface immediately vaporizes due to the rapid temperature rise, and the heat transferred into the interior of the wood causes water in the lower region to be vaporized.

Therefore, the specimen dries from top to bottom as described in the previous section. With an increase in the exposure time, the heat loss from the specimen’s top surface decreases due to the loss of moisture and allowed the wood material to reach the ignition temperature eventually.

The wet wood with a higher more moisture content exposed to the same heat exposure condition requires longer time to complete this process and thus results in a longer ignition delay.

(a)

(b)

Figure 4-18:(a) Ignition time of pine with different thickness and moisture content at heat flux 35 kW/m2, (b) ignition time of pine with different thickness and moisture content at heat flux 50 kW/m2.

Table 4-1 and Table 4-2 shows the ignition time of pine and oak. Slash represents the wood is not ignited.

Table 4-1:Time to ignition of pine with different thickness and moisture content Time to ignition (s)

Moisture content Thickness Heat flux 0% 10% 20% 30% 40% (mm) (kW/m2)

35 15 22 24.5 33 42.5 6.35 50 7 10.5 13 15 18

25 76 91.5 171.5

12.70 35 22 32 40 49 61

50 10 15.5 19.5 23.5 26

25 75 218 365.5

19.05 35 16.5 20 31 39.5 50

50 13 18 21.5 30 45

25 120.5 181 \ 38.10 35 51 59 \

Table 4-2:Time to ignition of oak with different thickness and moisture content

Time to ignition (s)

Moisture content Thickness Heat flux 0% 20% 40% (mm) (kW/m2) 25 115.5 780 885 12.70 35 45 80.5 170.5

25 111 \ \ 19.05 35 73.5 126 164

4.1.3 Fire Endurance Time

In order to study the flame resistance of different type of wood, we defined the time when heat release rate of the wood sample reaching its second peak as the fire endurance time. Fire endurance time helps us to estimate the ability of wood to persist in a fire. The fire endurance time prolongs with increasing moisture content. The time differences between heat fluxes are much smaller than the differences between different thicknesses. After ignition, spontaneous exothermic reaction inside the test specimen is dominated the burning characteristics compared with the outside radiation flux. The longer fire endurance time of thicker wood is because of the larger combustible mass. Figure 4-19 shows the fire endurance time of wood with different thickness and moisture content. And table 4-3 and 4-4 shows the time of fire endurance time.

Figure 4-19:Fire endurance time of pine with different thickness and moisture content.

Table 4-3:Fire endurance time of pine with different thickness and moisture content Time to ignition (s)

Moisture content Thickness Heat flux 0% 10% 20% 30% 40% (mm) (kW/m2)

35 180 235.5 285 300 320 6.35 50 120 145.5 235.5 250.5 265

25 440 \ 550 \ 800

12.70 35 365 505 535 595.5 730

50 300.5 415.5 460 550 585.5

25 765 \ \ \ \

19.05 35 565.5 950 1120 1320 1335.5

50 540 720.5 1060 1220.5 1440

25 2475.5 38.10 35 1780 2770

Table 4-4:Fire endurance time of oak with different thickness and moisture content Time to ignition (s)

Moisture content Thickness Heat flux 0% 20% 40% (mm) (kW/m2) 25 520.5 1235 1420 12.70 35 430 735.5 950

25 865 \ \ 19.05 35 815.5 1615 2190.5

4.2 Charring Phenomena

4.2.1 Temperature Distribution during Combustion

Using thermocouples to measure the temperature within the wood while burning can help us draw the temperature curve along with the HRR. Figures 4-20 and 4-21 shows the heat release rate together with the interior temperature distribution for 19.05 mm wood samples with

0% and 40% moisture content, respectively. The temperature recorded by the upper thermocouple 0 mm below the surface increases rapidly after the test begins.

For 0% moisture content, the temperature measured by the thermocouples in the wood increases monotonously while burning. For wet wood with a 40% moisture content, the

3.81mm thermocouple measured a uniformly increasing temperature, while the rest of the thermocouples exhibit a temperature plateau at 100℃ for approximately 300 s. The plateau is a result of the latent heat of vaporization at the boiling point of water (100℃). The lower thermocouples then measured temperature increases in sequential order as the wood “dries” from the top to bottom.

At temperatures below 100℃, depolymerization reactions without carbohydrate weight loss occur [5]. The reaction would reduce the strength of the wood while the chemical bonds start to break when the temperature is above 100℃. Therefore, wet wood can’t be regarded as an entire object while burning. The thermal degradation happens layer by layer. Water must be consumed completely as vapor or reactant first before the temperature exceeds 100 ℃. Then, the wood becomes dehydrated and generates noncombustible gases. Thus, the moisture content and thickness affect the flammability characteristics considerably.

Figure 4-20: Heat release rate and temperature distribution for 19.05 mm pine at heat flux of 35 kW/m2 with 0% moisture content .

Figure 4-21: Heat release rate and temperature distribution for 19.05 mm pine at heat flux of 35 kW/m2 with 40% moisture content.

The temperature measured by the thermocouples placed at the rear surface of the sample showed a jump during the burning process. The time of sudden change of the bottom temperature coincides with the beginning of the steep rise in the second peak of the HRR. It

52 shows the whole wood starts to undergo significant pyrolysis and exothermic reactions. The temperature of the bottom thermocouple reaches 300℃ when the HRR’s steep rise is at the second peak. The wood burning reaction reaches its peak when the whole wood turns to char and the pyrolyzates oxidize in the gas phase. The time to the second HRR peak is defined as the fire endurance time. Fire endurance time is studied with different moisture contents and thicknesses under 35 kW/m2 and 50 kW/m2 incident heat fluxes.

For oak, the temperature is measured under the same condition as the pine. Figure 4-22 and 4-23 show the heat release rate together with the interior temperature distribution for

19.05 mm oak samples with 0% and 40% moisture content, respectively.

Figure 4-22: Heat release rate and temperature distribution for 19.05 mm oak at heat flux of 35 kW/m2 with 0% moisture content.

Figure 4-23: Heat release rate and temperature distribution for 19.05 mm oak at heat flux of 35 kW/m2 with 40% moisture content.

The combustion of oak can reach higher internal temperatures and the whole combustion period is longer than those of pine. Comparing all the experimental results, all the temperature curves of TC6 have a clear jump (200℃ rise within 20 seconds) in each test for pine, whereas for oak, there is only find one case among all twelve tests. The temperature within the oak rises more gently and gradually.

Figure 4-24:Triplicate measured temperature of TC6 for 19.05 mm pine with 40% moisture content 54

under 35 kW/m2.

The temperatures are measured at least twice for each experimental condition. Clearly, the temperature traces exhibit good repeatability as shown in Figure 4-24. Ordinary averaging would smooth out the curve suttle features needed for the analysis. Therefore, one of these three results has been studied.

The temperature of the bottom surface of the wood sample is a crucial parameter during burning. Several heat release rate changes are based on the temperature of the rear surface. We study the temperature curve measured by the thermocouple TC6 separately. Figure 4-25 shows the temperature (dashed gray) curve and the slope of the tempetature (solid red) curve, i.e., the rate of temperature change per second, for pine with 40% moisture content. In the temperature changing rate shows four clear turning points. Before point 1 (T < 100 C), the temperature of the rear surface increased slowly, i.e., the slope of temperature curve was positive and small.

After point 1 (t = 400 s), the temperature maintained at b.p. of water (100 C) for ~240 s, while the water evaporated with a large amount of heat of vaporization was absorbed. Then the temperature increased monotonously as the bottom of the pine was involved in the pyrolysis and combustion process. At point 2, the temperature exceeded 300 C and the temperature changing rate jumped to 10 degrees per second. At Point 3, the temperature of the rear surface of the pine reached its peak (~600 C) so that the temperature changing rate was zero and turned to negative.. After point 4, the temperature leveled off to a ~500 C level with the near-zero temperature changing rate afterwards.

Figure 4-25: Temperature and changing rate of temperature of TC6 for 19.05 mm pine with 40% moisture content under 35 kW/m2. Figure 4-26 shows the changing rate of temperature and HRR. When the changing rate curve of the temperature was combined with the heat release rate, four points divided the combustion process into several parts. Before point 1, the pine was ignited and the HRR reached its first peak with the initial flare. After point 1, the HRR began to undergo a low-level charring period, during which heat is transferred to the bottom of the wood sample. Point 2 represents the beginning of the second HRR peak. The time of point 3 is called the fire endurance time. After point 4, the HRR went down and was lower than at the combustion period between point 1 and 2.

Figure 4-26: HRR and changing rate of temperature of TC6; 19.05 mm Pine of 40% moisture content under 35 kW/m2

Figure 4-27: Comparison of increasing rate of temperature of TC6; 19.05 mm Pine of 0% and 40% moisture content under 35 kW/m2.

Figure 4-28: Comparison of changing rate of temperature of TC6; 19.05 mm Pine and Oak of 40% moisture content under 35 kW/m2.

Figure 4-27 shows a comparison of the temperature changing rate of pine with the moisture contents of 0% and 40%. For the 40% moisture content, the temperature changing rate stayed zero while the water vaporization (400 s < t < 640 s), which does not exist for 0 %. Figure 4-

28 shows the comparison of pine and oak under the same moisture content (40 %), thickness, and incident heat flux. The the temperature changing rate for oak was much smaller thant that for pine. The temperature changes are gentler in oak.

4.2.2 Soring Point in Temperature

In all the pine experiments, the temperature curve has a clear jump at the beginning of the steep increase in the HRR to form the second peak. We define the time when TC6 begin to rise suddenly as the turning point. All the turning points are recorded in the same plot for samples with different moisture contents and thicknesses. For thicker wood, such as 1.5-inch pine, the

HRR curve doesn’t have a clear second peak, so the temperature distribution curve doesn’t yet

58 have a turning point. All the pine samples were burnt through at thicknesses of 6.35, 12.7, and

19.05 mm and have clear turning points under a 35 kW/m2 incident heat flux.

Soring Point

Figure 4-29: Example of mutation points for 19.05 mm pine with 0% moisture content at a heat flux of 35 kW/m2

Figure 4-29 shows an example of a turning point. From the figure, the temperature of the turning point and the time when the TC6 turning a sudden rise are recorded. Table 4-5 shows the turning point of pine at heat flux of 35 kW/m2 with different moisture contents and thicknesses.

Table 4-5: Turning points of pine at heat flux of 35 kW/m2 with different moisture contents and thicknesses. Pine Moisture content(%) Thickness(mm) Data 0 10 20 30 40 Time(s) 114.5 164 234 261.5 277 6.35 Temperature(℃) 249 281.5 302 289.5 250 Time(s) 323 443.5 461 491 781 12.70 Temperature(℃) 269.5 272 267 274.5 282 Time(s) 503 770 1076.5 1123 1234 19.05 Temperature(℃) 250.5 270 283 287 298.5 Time(s) 38.10 None \ None \ Nonignited Temperature(℃)

For a lower heat flux of 25 kW/m2, except for the non-ignited and extinguished condition, all the temperature curves of 12.70 and 19.05 mm burn-through pine have turning points, while the turning point disappeared for 38.10 mm pine. At the same time, the HRR for 38.10 mm pine doesn’t have an obvious second peak, so the thickness is the main effect influence on the second peak and turning point.

Table 4-6: Turning point of pine at heat flux of 25 kW/m2 with different moisture content and thickness. Pine Moisture content(%) Thickness(mm) Data 0 20 40 Time(s) 391 506.5 786 6.35 Temperature(℃) 251.5 259 287.5 Time(s) 648 12.70 Extinguished Extinguished Temperature(℃) 267 Time(s) 38.10 None Extinguished Nonignited Temperature(℃)

Figure 4-30: Time and temperature of pine’s turning point with different moisture contents and thicknesses.

Figure 4-30 shows the time and temperature of each turning point for pine samples with different moisture contents and thicknesses. The arrow above the line represents the trend of 60 the increased moisture content from 0% to 40%. The time when pine reaches its turning point almost extends with increased moisture content and the temperature changes within a range between 250℃ to 320℃. Because the wood sample is heated during the whole experiment, the

0.25-inch pine is too thin to ignore the external heat influence. The heat transfer process turns out to be stable in the thicker 12.70 and 19.05 mm pine. If we ignore the 6.35 mm pine, the turning point temperature is between 250℃ to 300℃. By checking the wood handbook, we can find the thermal degradation process of wood.

Wood is a combination of various chemical components, such as lignin, cellulose, hemicellulose, and so on. Each chemical component has its own temperature at which it undergoes thermal degradation [5]. Inquiring the data, chemical bonds begin to break at temperature above 100℃. The main reactions between 100℃ to 200℃ are dehydration and water vaporization. There, wood becomes charred and noncombustible gases escape from the wood, such as CO2, formic acid, and acetic acid. The pyrolysis process of wood is exacerbated with the rise in temperature. Between 200℃ to 300℃, beside the production gases listed above, large quantities of CO and tar are formed. The pyrolysis of hemicelluloses happened in 200℃ to 300℃, generating much of the acid and lignin was pyrolyzed at temperatures approximately

225℃ and 450℃ and the residue turned to be mostly char. Between 300℃ to 450℃, cellulose begin to depolymerize and produce flammable volatiles and the aliphatic side chains and carbon-carbon linkage split. At approximately 450℃, all the wood components turn into char.

Above 450℃, the pyrolysis of wood changes into degradation of activated char. The pyrolysis process of the three main wood components is shown in Table 4-7.

Table 4-7: Pyrolysis of three main wood components [5] Temperature Wood components Reaction Products range(℃) Hemicellulose 200-300 deacetylation Acetic acid significant Production of Cellulose 300-350 depolymerization flammable volatiles A high char 225-450 Dehydration yield for wood Aliphatic side chains start splitting Approximately 300 off from the Lignin aromatic ring Carbon–carbon linkage between 370-400 lignin structural units is cleaved

On the other hand, the turning point appears when the temperature of the rear surface reaches approximately 300℃ and the second HRR peak appears at temperatures approximately

450℃. The cellulose content in the pine is 50-60%, lignin is 25%-30%, and the rest is hemicellulose and others compounds. Cellulose and lignin occupy most of the wood, at approximately 80%. When the whole pine was preheated at approximately 300℃, the three components are all involved in the pyrolysis process. 300℃ is regarded as the charring line, as the hemicellulose finishes its pyrolysis process and cellulose and lignin start undergoing significant depolymerization. The degradation reaction of lignin has a peak between 225℃ and

450℃, dependent on the external air condition, while the second HRR peak is approximately

450℃. Above 450℃, all the wood components turn to char and the glowing combustion process changes into smoldering for the residue. All the thin pine with different moisture contents thinner than 19.05 mm have a clear second HRR peak and temperature turning point .

Figure 4-31: Temporal variations of HRR and temperature distribution for 38.10 mm pine with 0% moisture content at a heat flux of 35 kW/m2 Figure 4-31 shows the temperature distribution for 38.10 mm pine with 0% moisture content at a heat flux of 35 kW/m2. Even though the HRR curve has a second peak, the turning point is missing. Compared with other conditions, for thicker pine, the second HRR peak is lower than the first one. The temperature of the bottom thermocouple is under 300℃ when the

HRR reaches its second peak, while in the other condition, the temperature of the bottom thermocouple is approximately 450℃ after the sudden jump. Because 300℃ is the charring line of wood as we previously mentioned, the whole 38.10 mm pine changes into char after the second peak. The increasing thickness of the pine also add the thickness of the char after the pyrolysis process. The char blocked the contact between the air and fuel, retarding pyrolysis.

At the same time, the char also slowed down the heat transfer process due to its low thermal conductivity compared with the other components of wood. Thus, the rear surface stays at a lower temperature and undergoes incomplete combustion after the glowing burning is done.

For oak, the cellulose content is 50% and lignin is 15%. Compared with pine, the 63 hardwood generally has less lignin content than softwood. Figure 4-32 shows the heat release rate and temperature distribution for 19.05 mm oak with 0%, 20%, and 40% moisture content at a heat flux of 35 kW/m2. Unlike all the pine thinner than 19.05 mm, oak with 12.7 mm and

19.05 mm thicknesses hardly has a turning point. Among all 12 experiments for heat fluxes under 25kW/m2 and 35 kW/m2, only 3 of them show the clear temperature jump at the start of the second HRR peak. Even though the temperature curve shows a steep slope after the beginning of second HRR rate, the temperature still increases gently and continuously during most of the experiment on oak compared with pine. For hardwood such as oak, the smaller amount of lignin reduces the possible fuel sustained for burning above 300℃. The compact structure of oak also influences the burning process under the CT-scan study discussed in the next chapter.

(a)

(b)

(c)

Figure 4-32: Temporal variations of HRR and temperature distribution for 19.05 mm oak at heat flux of 35 kW/m2 with various moisture content (a) 0% (b) 20% and (c) 40%.

4.2.3 Charring rate

The position of the char-line can be taken as the position of the 300 ℃ isotherm [41].

Because the wood samples used in this study have limited thicknesses, the charring rate is affected considerably when the char-line nearly reaches the bottom of the sample. The bottom temperature has a sudden change approximately 300 ℃ due to the heat release rate reaching a second peak. For wood of 12.7 mm and 19.05 mm, the last 3 thermocouples reach 300 ℃ almost in the same time. Therefore, the first four points were used to calculate the charring rate.

Figure 4-33: Charring line for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2.

After we know the exact time of each thermocouple reaching 300℃, the distance between each thermocouple is fixed. The charring rate can be calculated using the temperature. Figure

4-33 shows the charring line for 19.05 mm pine with 0% moisture content at a heat flux of

35 kW/m2. Figure 3-16 shows the charring rate curve along with the HRR. The time on horizontal axis means the time when the charring rate is calculated.

Figure 4-34: Charring rate for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2.

For each wood sample, six thermocouples divided the wood sample into 5 zones from the top surface to the bottom. Five charring rates can be obtained by the temperature measurements.

In Figure 4-34, the first three charring rates are close compared with the fourth one, while the fifth charring rate is calculated by the last two thermocouples near the bottom surface of the wood sample. The temperature differences are tiny, so the charring rate is relatively huge compared to the realistic condition. Thus, the fifth charring rate is ignored. The fourth charring rate increased as the HRR begins to approach its second peak. As we previously discussed, the wood sample has already turned to char before the HRR reaches its second peak. Therefore,

68 the fourth charring rate can’t represent the real charring process during burning. We can calculate the average charring rate from the first three charring rates.

Table 4-8: Charring rate for pine under 35 kW/m2 heating Thickness Moisture First Second Third Fourth Average (mm) content (%) Charring Charring Charring Charring Charring rate rate rate rate rate(1,2,3) (mm/s) (mm/s) (mm/s) (mm/s) (mm/s) 12.7 0 0.0249 0.0229 0.0462 0.0726 0.0313 10 0.0118 0.0251 0.0322 0.1104 0.023 20 0.0125 0.0124 0.0291 0.1411 0.018 30 0.0174 0.00613 0.027 0.1494 0.0168 40 0.00917 0.0159 0.0431 0.0770 0.0227 19.05 0 0.0171 0.0428 0.0240 0.1089 0.028

10 0.0083 0.036 0.0247 0.053 0.023 20 0.01 0.0014 0.00126 0.0529 0.0122 30 0.01067 0.01055 0.00992 0.0334 0.0104 40 0.00843 0.009 0.01581 0.0646 0.0111 38.10 0 0.0204 0.0104 0.0267 0.021 0.0192 20 0.0098 0.0092 0.0108 0.0252 0.0099 40 Nonignited

Table 4-9: Charring rate for pine under 25 kW/m2 heating Thickness Moisture First Second Third Fourth Average (mm) content (%) Charring Charring Charring Charring Charring rate rate rate rate rate (mm/s) (mm/s) (mm/s) (mm/s) (mm/s) 12.70 0 0.017 0.0446 0.0279 0.0941 0.0298 20 0.022 0.0166 0.027 0.054 0.0219 40 0.0127 0.0207 0.0217 0.0529 0.0184 19.05 0 0.0097 0.0259 0.0756 0.0454 0.0219

20 0.005 0.0185 Flame out \ 40 0.0055 0.0054 Flame out \ Table 4-10: Charring rate for oak under 25 kW/m2 heating Thickness Moisture First Second Third Fourth Average (mm) content (%) Charring Charring Charring Charring Charring rate rate rate rate rate (mm/s) (mm/s) (mm/s) (mm/s) (mm/s) 12.70 0 0.021 0.021 0.0198 0.0462 0.0206 20 0.0125 0.021 0.0431 0.041 0.0255 40 0.0363 0.0134 0.014 0.0508 0.0212 19.05 0 0.0058 0.0187 0.0318 0.0318 0.0188 20 Nonignited \ 40 Nonignited \

Table 4-11: Charring rate for oak under 35 kW/m2 Thickness Moisture First Second Third Fourth Average (mm) content (%) Charring Charring Charring Charring Charring rate rate rate rate rate (mm/s) (mm/s) (mm/s) (mm/s) (mm/s) 12.70 0 0.0279 0.0379 0.0285 0.0686 0.0314 20 0.0174 0.0235 0.0097 0.094 0.0169 40 0.0178 0.0191 0.0119 0.0498 0.0163 19.05 0 0.0142 0.0175 0.0235 0.0615 0.0184

20 0.0063 0.00702 0.0171 0.0977 0.0101 40 0.0055 0.0054 0.0095 0.0276 0.0068

For both pine and oak, the charring rate calculated for the case of a 35kW/m2 incident flux has clear regulation compared with the results for the case of a 25kW/m2.

Figure 4-35:Charring rate for 12.70mm and 19.05mm pine at different heat flux and moisture content. Figure 4-35 shows the charring rate of pine with different thickness, moisture content and incident heat flux. As moisture content is increased, charring rate is decreased. The upper 2 lines indicate the charring rate of 12.70 mm and lower 2 lines indicate of 19.05mm. The thinner

71 sample has a larger charring rate. The charring rates are almost the same with the same thickness and moisture content despite the heat flux rate is different.

In order to calculate the error of the charring rate calculated by temperature. Charring rate is also measured by experiment. In addition, CT-scan is used to measure the exact charring layer depth of each wood sample extinguished in 5, 10 and 15 minutes. The average charring rate of 2.5,7.5 and 12.5 minutes are acquired. The calculated charring rates are compared with experimental data from CT-scan in Figure 4-36. Calculate the average charring rate measured by CT-scan method, the error for two methods is less than 15%. The results are acceptable.

Figure 4-36:Charring rate for 19.05mm pine at heat flux of 35 kW/m2 with 0% moisture content under two methods.

Following figures shows the charring rate and temperature distribution of pine with different experimental condition.

Figure 4-37:Charring rate for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-38:Temperature distribution for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-39:Charring rate for 12.7 mm pine with 10% moisture content at heat flux of 35 kW/m2.

Figure 4-40:Temperature distribution for 12.7 mm pine with 10% moisture content at heat flux of 35 kW/m2.

Figure 4-41:Charring rate for 12.7 mm pine with 20% moisture content at heat flux of 35 kW/m2.

Figure 4-42:Temperature distribution for 12.7 mm pine with 20% moisture content at heat flux of 35 kW/m2.

Figure 4-43:Charring rate for 12.7 mm pine with 30% moisture content at heat flux of 35 kW/m2.

Figure 4-44:Temperature distribution for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-45:Charring rate for 12.7 mm pine with 40% moisture content at heat flux of 35 kW/m2.

Figure 4-46:Temperature distribution for 12.7 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-47:Charring rate for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-48:Temperature distribution for 19.05 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-49:Charring rate for 19.05 mm pine with 10% moisture content at heat flux of 35 kW/m2.

Figure 4-50:Temperature distribution for 19.05 mm pine with 10% moisture content at heat flux of 35 kW/m2.

Figure 4-51:Charring rate for 19.05 mm pine with 20% moisture content at heat flux of 35 kW/m2.

Figure 4-52:Temperature distribution for 19.05 mm pine with 20% moisture content at heat flux of 35 kW/m2.

Figure 4-53:Charring rate for 19.05 mm pine with 30% moisture content at heat flux of 35 kW/m2.

Figure 4-54:Temperature distribution for 19.05 mm pine with 30% moisture content at heat flux of 35 kW/m2.

Figure 4-55:Charring rate for 19.05 mm pine with 40% moisture content at heat flux of 35 kW/m2.

Figure 4-56:Temperature distribution for 19.05 mm pine with 40% moisture content at heat flux of 35 kW/m2.

Figure 4-57:Charring rate for 38.1 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-58:Temperature distribution for 38.1 mm pine with 0% moisture content at heat flux of 35 kW/m2.

Figure 4-59:Charring rate for 38.1 mm pine with 20% moisture content at heat flux of 35 kW/m2.

Figure 4-60:Temperature distribution for 38.1mm pine with 20% moisture content at heat flux of 35 kW/m2.

4.3 CT-scan Results

To study the microstructure of the wood during burning, four wood samples with the same thickness and moisture content are prepared for the cone calorimeter test. One of the sample is burned through and the temperature within the wood was measured. The other three are extinguished during the burning at the post ignition stage, charring propagation stage, and throughout the combustion stage. 19.05 mm pine with 10% moisture content was tested under a 35 kW/m2 heat flux. Figure 3 shows the heat release rate and interior temperature for 19.05 mm pine with 0% moisture content at a heat flux of 35 kW/m2. Figure 4-59 shows the HRR curve of 19.05 mm pine with 10% moisture content under a 35kW/푚2 incident heat flux.

(a)

(b) Figure 4-61: Heat release rate for 19.05 mm 10% moisture content pine at heat flux of 35 kW/m2 with (a) interior temperature distribution and (b) charring rate.

After the wood specimens were first exposed to the cone heater, a thermocouple placed on top surface (TC1 0 mm) starts to measure a temperature jump. Ignition happens after a certain time of heat exposure while TC1 measures temperature approximately 300℃, which indicates a charring process has started on the top surface of wood material. During the post ignition stage, the reaction between wood and oxygen on top sample surface releases large

86 amount of heat, which caused the first peak in heat release rate while burning wood decompose to form a layer of porous char that acts as an insulation layer to reduce the heat transfer rate from the flaming zone and gasification rate, thus blocking the vaporized fuel from reaching the oxygen (Fig. 4-60a). The measured charring rate from the top two thermocouples (zone 1) appears to be the lowest of the entire burning process. It took 370 s for the charring process to reach TC2, which is placed 3.81 mm below the top surface. This is because of the heat loss to the environment and the heat transfer to the lower part of wood specimen carries away a significant amount of energy, which effectively slows down the propagation of the char line.

After a complete layer of char is formed and has covered the specimen to protect it from the heat, the heat release rate goes into a relatively stable stage and the burning process enters the charring propagation stage. The char line continuously propagates toward the bottom of the wood specimen (Fig. 4-60b) and the thermocouples placed at the interior of the wood sample starts to measure temperature rises in a sequential order (TC2, TC3, and 4). Charring rates measured at zone 2 and zone 3 are close and slightly higher than zone 1. When temperature measured by the thermocouples placed at the rear face of the sample showed a jump (TC5 and

6), which indicates the char line has propagated to the bottom, the wood is observed to have a dramatic deformation. The specimen has shrunk by approximately 15% in its horizontal direction and expanded by 36% in its vertical direction. A large amount of void space is created at the interior of specimen (void fraction: 50.3%) and linked to the outside environment, which can significantly assist the fuel-oxygen contact. The whole wood starts to undergo significant pyrolysis and exothermic reactions, while the second heat release rate peak is reached. The charring rate accelerated one order of magnitude faster than in the upper regions and penetrated

through zone 4 and 5 within 100 s

extinguish

(a)

extinguish

(b)

extinguish

(c) Figure 4-62: CT scanned 19.05 mm pine wood samples with 10% moisture content exposed to 35 kW/m2 heat flux that was extinguished at (a) the post ignition stage, (b) the charring propagation stage, and (c) throughout the combustion stage

Extinguished

(a)

Extinguished

(b)

d Extinguishe

(c) Figure 4-63: CT scanned 19.05 mm Oak wood samples with 10% moisture content exposed to a 35 kW/m2 heat flux that was extinguished at (a) the post-ignition stage, (b) the charring propagation stage, and (c) throughout combustion stage.

Figures 4-60(c) and 4-61(c) show the microphotographs of the side view of fire-exposed pine and oak. The width and shape of typical fissures can be measured on the image. After the second HRR peak, the whole wood turned to char, which was clearly arranged layer by layer in pine and oak. Compared with oak, the holes inside pine were obvious and large. In addition, these holes were generated between the charring propagation period and throughout combustion period. Reasonable assumptions can be drawn that these holes were generated while the HRR reaching its second peak. Beside the charring process, the breakage of wood structure intensified the burning reaction. Oxygen can easily attach the interior material during burning, leading to an increase in temperature. Thermocouples inside pine measure the flame temperature instead of wood temperature. For oak, the wood structure was much harder to damage, so the temperature curves measured by thermocouples inside oak were less likely to show the increase.

5 Conclusion

Beyond the scope that the standard cone calorimeter testing method provides (the heat release rate, time to ignition, and the concentrations of O2, CO, CO2, and smoke), the internal wood specimen temperature measurement and CT-scan observations shed light on physical insights into the wood burning, charring, and smoking behaviors.

An increase in the moisture content of the wood specimen delays the ignition time and burning duration significantly, while largely reducing the peak heat-release rate. The measured temperature in the lower region of the burning wood reaches its peak in approximately 200 s after ignition before the flame extinguishes for all cases. The measured peak temperature

(600 ℃) indicates a complete carbonization of the wood, which is nearly coincident with the second heat release peak as a result of the completion of the char oxidation processes. The increasing thickness of wood affects the complex combustion processes. The fuels available for combustion increase as the thickness of wood increases, while the HRR peak is decreases.

A thicker char layer is generated from thicker wood, reducing the rates of heat feedback, pyrolysis, gasification, and thus the exothermic reaction of combustible wood with oxygen. If the thickness of the wood increases to a certain level, the second HRR peak becomes lower than the first peak. An increase in the incident heat flux reduces the ignition time.

The temperature measurements bring about significant information on the combustion processes. For pine, the beginning of the second HRR peak occurs when the temperature at the rear surface of wood reaches approximately 250℃ to 300℃. This temperature range corresponds to the point when lignin and cellulose undergo significant decomposition. The lack

91 of lignin in oak, compared with pine, seems to lead to a gentle temperature rise during the second peak. The microphotograph also indicates the breakage of the wood structure influences the combustion process. Pine generates a large amount of void spaces during burning, forming a porous structure which is beneficial to increase the contact area between the fuel and oxygen.

Thicker pine and most of the oak do not have the temperature jumping point due to incomplete burning and components of different types of wood.

The charring rates determined from the temperature measurement and the CT-scanner are generally consistent. The charring rate decreases as the moisture content is increased.

Microphotographs help us to take images of fissure and holes inside wood after burning.

The structures of pine and oak shows clear differences after being exposed to fire. The porous interior structure of the pine leads to a sharp change in combustion during burning. A certain thickness of pine may cause secondary damage in a fire. Thicker pine or hardwood have a better flame resistance compared with thin softwood.

The present approach provides significant information useful in understanding the burning processes of woods, as well as in developing numerical models of pyrolysis and wood burning that can cover more complicated environmental and material conditions.

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