Charred Wood Analysis Using Cone Calorimeter and Char Depth Estimation

Euy-Hong Hwang; Jihoon Han; Jae-Sung Oh

doi:10.7731/KIFSE.20d9575f

Abstract

This study aimed to improve data consistency and establish baseline data for wood ignition sources in standard tests. Fire safety standards were reviewed to guide tree species selection and property analysis, and methodologies including cone calorimeter testing and charring depth measurement were examined. Combustion experiments were conducted on four selected wood species using a cone calorimeter to evaluate thermal performance, and charring depth was measured using a corresponding method. Char volume, charring rate, char mass, char density, and combustion efficiency were calculated and compared as indicators of charring level based on experimental results and direct measurements. The comparison identified correlations and limitations of these parameters for assessing charring level, including thermal performance and charring depth. The findings provide reference data to reduce variability in ignition source results among tree species by supporting the simultaneous application of charring-level parameters when characterizing standard wood ignition sources.

Keywords: Wood charring; Charring parameter; Cone calorimeter

1. Overview and Purpose

Wood has been widely used as a standard ignition source in fire and fire suppression experiments and simulations [1,2]. In particular, a standard ignition source comprises two physical components: a crib and a panel. A crib arranged as a porous array facilitates fire development and dissipation, whereas a panel allows detailed observation of surface fire behavior and charring characteristics [3,4]. However, generalizing the charring characteristics of standardized wood ignition sources is limited because different tree species are used across a wide range of sectors, including construction materials and furniture [5,6]. Several studies on fire experiments involving wood materials have reported variability in fire characteristics, such as the heat release rate of test specimens, regardless of whether the material is solid wood or a composite combustible [7]. Furthermore, as the scale of the ignition source increases to full scale, pronounced differences in fire characteristics have been observed [8]. Other studies have analyzed fire characteristics based on wood charring level, an approach applied to investigations of fire causes, damage assessment, and building structural safety [9,10]. In the fire safety domain, wood materials are subjected to performance testing for limited combustibility and flame-retardant properties when used as construction or interior decorative materials [11]. In particular, to evaluate limited combustibility, a cone calorimeter test is conducted to verify that the total heat of combustion remains below a specified threshold within a defined time. Test homogeneity and suitability are assessed by measuring the moisture content and mass changes of wood specimens. Oxygen consumption is also recorded to verify data collection and the accuracy of heat release rate calculations by the analysis system, thereby ensuring equipment reliability.

Thus, this study aims to establish baseline data to support measures for improving the consistency of wood ignition source data in standard tests. To achieve this objective, the research scope included, in sequence, tree species selection and property analysis, measurement and analysis of experimental results, and parameter calculation. The research methodology is outlined as follows. First, four wood species used as standard ignition sources were selected in accordance with fire safety standards and preprocessed. Second, the selected species were tested according to cone calorimeter combustion performance standards. Third, using the experimental results and a charring depth measurement method commonly applied in fire investigations, charring-level parameters were compared for each wood species based on both experimental data and direct measurements. Finally, considerations and limitations related to ensuring uniformity in ignition source data were discussed, and conclusions were drawn.

2. Methodology

2.1. Review of fire safety standards and selection of tree species

In Korea the fire safety sector classifies wood as a construction and interior decorative material subject to fire safety performance testing. Fire safety standards are applied across a range of fire detection, fire-fighting, and related equipment performance criteria based on fire safety performance results. Within these standards, the rate or total amount of combustion products generated from wood is standardized, a practice that is also reflected in international fire-fighting and building fire standards. Table 1 summarizes the wood ignition sources specified in fire safety standards, including formal approval standards, technical standards, and excellent quality certification standards for fire-fighting equipment as notified by the National Fire Agency.

As shown in Table 1, the wood materials used as standard ignition sources belong to the coniferous family, predominantly pine. Coniferous species are primarily used because they exhibit more uniform density and moisture content, as well as more consistent fire performance, than broadleaf species [12]. Although testing standards are broadly similar across countries, the tree species specified differ. For example, the sprinkler real-fire test defined in the U.S. civil standard UL 199 requires spruce or fir, whereas the corresponding Korean testing standard specifies cedar [13,14]. The Japanese fire extinguisher formal approval standard also specifies cedar for fire-fighting tests, while the corresponding Korean standard requires pine or alder [15,16]. These differences can be attributed to variations in tree species production volumes, import and export conditions, and cost efficiency during the development and adoption of national standards. Based on these considerations, four tree species commonly used worldwide—fir, pine, spruce, and cedar—were selected for analysis of their characteristics.

2.2. Review and establishment of cone calorimeter testing and charring level measurement methods

A cone calorimeter is a test method used to measure combustion characteristics, such as heat release rate and total heat generation, based on the mass loss rate of the ignition source specimen and the corresponding oxygen consumption rate [17]. It is commonly applied to construction and interior decorative materials, for which performance is evaluated in terms of flame retardancy or limited combustibility. The standard specimen size is a square area of 4 in. (approximately 100 mm) with a thickness of up to 2 in. (approximately 50 mm). In Korea, KS F ISO 5660-1 specifies the relevant test methods and performance evaluation criteria. In this study, the test duration was 10 min, corresponding to the limited combustibility performance threshold. The specimen thickness was set to 30 mm (approximately 1¼ in.), reflecting the typical dimensions of square lumber used in fire and fire suppression tests. The total effective heat of combustion, average heat release rate per unit surface area, and peak heat release rate were recorded.

Charring level is commonly used in fire investigations to determine the cause of a fire. Charred wood collected from fire scenes serves as critical evidence for identifying fire spread paths and the initial ignition location. One of the most widely used methods for this purpose is charring depth measurement [9]. This method determines charring depth by combining the portion of wood completely lost due to thermal decomposition with the partially charred portion and is also used as a parameter for evaluating the fire resistance of construction timber [10]. In this study, charring depth was measured using a dedicated measuring device, and char volume was estimated by analyzing its relationship with the heated portion of the specimen. The charring-level evaluation results for the four tree species were used to examine their relationship with changes in wood mass and to compare charring-level parameters.

2.3. Preprocessing and property analysis of the selected tree species

Figure 1 presents the preprocessing images of the four tree species selected for the cone calorimeter experiment on the left (a, c, e, g) and the corresponding post-experiment images on the right (b, d, f, h). The experiments were repeated three times to identify consistent trends. Specimen properties—including dimensions (width, length, and height), moisture content (top and bottom), and mass—were measured and compared before and after the experiments. Table 2 summarizes the average values obtained from the property analysis of the wood species used in the study.

3. Results and Measurements

3.1. Performance testing results of tree species using a cone calorimeter

Figure 2 presents the results of the cone calorimeter performance tests for each tree species. The data represent average values obtained from three repeated experiments. The derived parameters include mean heat release rate (MHRR), average heat release rate per unit area, peak heat release rate (PHRR), time to PHRR (PHRR_time), and total heat release (THR), which were analyzed sequentially.

First, MHRR and THR characterize fire progression, including growth and decay, and are known to complement each other when comparing the performance of different materials [18]. Among the four tree species, the MHRR values in Figure 3(a) and the THR values in Figure 3(c)-(f) are ordered as fir, pine, cedar, and spruce. In addition, PHRR and PHRR_time reflect specimen-specific characteristics, such as the magnitude of energy release and the timing of peak fire development. For these parameters, the four tree species are ordered as fir, cedar, pine, and spruce. These results indicate that cedar and pine exhibit the greatest similarity in fire growth patterns, timing, and maximum heat release based on the performance test data.

3.2. Charring level review and depth estimation by tree species

Figure 3(a) illustrates the charring scope defined after reviewing the experimental charring levels for each tree species [8,9,19]. This method, commonly applied in both theory and practice, defines charring regions as the completely burned, pyrolyzed, and charred portions of wood and analyzes their characteristics. In this study, the depth measurement point was defined at the location of maximum charring depth by identifying the groove formed at the boundary between the wood core and the carbonized layer, as shown in Figure 3(b). The average charring depth was calculated using the estimated maximum value and measurements taken within a 5 mm radius (red line area) from the four vertices.

Table 3 presents the estimated charring depths of each specimen. The estimated charring depth decreased in the following order: cedar, fir, pine, and spruce. Greater charring depth indicates a higher calorific value per unit density and, consequently, higher combustion efficiency. Conversely, insufficient charring may reflect variations in wood density or moisture content, potentially introducing measurement errors.

3.3. Parameter comparison for charring level by tree species

Table 4 presents the charring-level parameters derived from cone calorimeter tests and charring depth measurements. These parameters include char volume, charring rate, char mass, char density, and combustion efficiency [20]. Char volume represents the volume of charred material, while charring rate indicates the time required to form that char volume. These parameters exhibited trends similar to those observed in the PHRR results. Char mass, defined as the total mass of charred material, and char density, calculated as char mass per unit volume, were also analyzed, with their trends corresponding to the measured charring depth. Finally, combustion efficiency, defined as the ratio of the initial wood density to the char density measured after the experiment, was analyzed and showed trends similar to MHRR and THR.

3.4. Correlation between charring level and thermal property parameters

The correlation between charring-level parameters and thermal properties can be examined using various theoretical approaches. Char density is calculated from char mass and char volume, while charring rate is determined from char volume and time, as it represents a volumetric flow rate. Heat release rate (HRR) represents the amount of heat generated per unit time and is therefore related to specific heat, mass, temperature change, and time. It can be calculated as shown in Eq. (1):

(1)

W(=Qt)∝ΔmVt(=Δρt)

where, W is the calorific value, Q is the HRR, t is time, m is mass, V is volume, and ρ represents density. The correlation was examined based on the above equation. Depending on the sign of the slope obtained from linear regression analysis of the collected data, correlations can be either directly proportional (positive) or inversely proportional (negative). An R2 value closer to 1 indicates a stronger correlation.

Figure 4 illustrates the correlations among the parameters derived using Eq. (1). The R2 values were 0.8772 for PHRR and charring rate, 0.9843 for char mass and charring depth, 0.9586 for MHRR and combustion efficiency, and 0.9486 for THR and combustion efficiency, indicating a relatively strong correlation. Notably, char mass and charring depth were inversely proportional. This is attributed to differences in initial density, moisture content, and combustion rate among tree species. Char mass is defined as the sum of the remaining charred portion and the portion already pyrolyzed and destroyed. Therefore, if the pyrolyzed and destroyed portion is relatively large, the measured charring depth is correspondingly small [20].

4. Discussion and Limitations

The charring level of the selected tree species was analyzed based on experiments repeated three times. Thermal properties—PHRR, AHRR, and THR—were measured using a cone calorimeter, followed by measurement of charring depth for the same specimens. Char volume, charring rate, char mass, char density, and combustion efficiency, which indicate charring level, were analyzed using both experimental results and direct measurements. Correlations among these parameters were then derived. These results are expected to serve as reference data for reducing variability in ignition source data among tree species by supporting the simultaneous application of charring-level parameters when characterizing standard wood ignition sources.

To accurately establish charring-level parameters, the bonding structure, arrangement, and moisture content of tree species must be carefully considered. However, this study assumed that destruction, charring, and pyrolysis occurred only in the upper portion of the specimens, based on measured moisture content in the top and bottom sections. This assumption represents a major limitation because it may introduce data discrepancies. In addition, wood may undergo deformation during combustion, such as warping, due to impurities (e.g., knots) or variations in cross-sectional features, including growth rings. Despite these factors, the analysis relied solely on measurement data. These limitations will be addressed in future studies.

5. Conclusions

Thus, this study aimed to establish baseline data to support measures for improving the consistency of wood ignition source data in standard tests. The research scope and methodology were designed to achieve the study objectives.

The approach involved selecting tree species, analyzing their properties, conducting experiments and analyzing the results, and deriving parameters, leading to the following conclusions.

(1) The peak heat release rate (PHRR) measured using the cone calorimeter was correlated with char volume and charring rate, both of which represent charring level. Among the tree species, fir exhibited the highest PHRR (260.17 kW/m²), along with the highest char volume (84.82 cm³) and charring rate (1.41 cm³/s).

(2) The measured charring depth was correlated with char mass and char density, both of which represent charring level. Among the tree species, spruce exhibited the lowest charring depth (7.75 mm) while recording the highest char mass (52.15 g) and char density (0.6876 g/cm³).

(3) The average heat release rate (AHRR) and total heat release (THR) measured using the cone calorimeter were correlated with combustion efficiency, which represents charring level. Among the tree species, fir showed the highest AHRR (89.33 kW/m²) and THR (53.6 MJ/m²), along with the highest combustion efficiency (0.9208).

(4) Correlations among the parameters were evaluated using the coefficient of determination (R²). The R² values were 0.8772 for PHRR and charring rate, 0.9843 for char mass and charring depth, 0.9586 for mean heat release rate (MHRR) and combustion efficiency, and 0.9486 for THR and combustion efficiency, indicating strong correlations.

(5) The parameters used to indicate charring level were derived from species-specific characteristics and thermochemical reactions and are therefore considered reliable. These findings are expected to serve as reference data for reducing variability in ignition source data among tree species by supporting the simultaneous application of charring-level parameters when characterizing standard wood ignition sources.

(6) The primary limitations of this study include insufficient consideration of bonding structure, arrangement, and moisture content of the tree species, as well as wood cross-sectional shape and the presence of impurities. These limitations will be addressed in future studies.

Notes

Contributions

“Conceptualization, EH.H.; methodology, EH.H.; validation, EH.H.; formal analysis, EH.H.; investigation, JS.O.; resources, EH.H.; data curation, JH.H.; writing—original draft preparation, EH.H.; writing—review and editing, EH.H.; visualization, EH.H.; supervision, EH.H.; project administration, EH.H.; funding acquisition, EH.H. All authors have read and agreed to the published version of the manuscript.”

Conflicts of Interest

“The authors declare no conflict of interest.”

Acknowledgments

"Not applicable."

Figure 1.

Pre- and post-experiment images of wood specimens in cone calorimeter tests.

Figure 2.

Cone calorimeter test results for each tree species.

Figure 3.

Methods for measuring wood charring level.

Figure 4.

Correlations (R2) among charring-level and thermal property parameters for the four tree species.

Table 1.

Standard Wood Ignition Sources Used in Fire-Fighting Tests

Contents	Cedar	Pine	Spruce	Fir
Type Approval	2	9	3	-
Excellent Quality Certification	-	2	1	1

Table 2.

Characteristics of Wood Ignition Sources

Properties	Cedar	Pine	Spruce	Fir
Initial width (mm)	97.25	98.10	98.71	99.11
Final width (mm)	95.83	95.24	96.53	98.34
Initial length (mm)	101.30	101.47	101.83	101.30
Final length (mm)	101.21	101.50	101.44	101.22
Initial height (mm)	29.96	30.12	30.16	30.05
Final height (mm)	27.80	29.25	28.74	29.90
Initial mass (g)	110.01	140.31	147.84	147.60
Final mass (g)	69.71	97.97	105.68	105.69
Initial moisture content on top (%)	8.83	7.97	9.80	8.97
Final moisture content on top (%)	Unmeasurable	Unmeasurable	Unmeasurable	Unmeasurable
Initial moisture content on bottom (%)	9.37	9.17	9.93	9.80
Final moisture content on bottom (%)	8.45	8.40	9.40	9.03

Table 3.

Measurement Result of Wood Charring Depth

Properties	Cedar	Pine	Spruce	Fir
Maximum depth (mm)_Point 1	10.62	10.53	10.71	10.30
Node 1 depth (mm)_Point 2	7.19	6.79	7.45	7.79
Node 2 depth (mm)_Point 3	7.41	7.09	6.48	7.89
Node 3 depth (mm)_Point 4	8.82	9.06	7.78	8.14
Node 4 depth (mm)_Point 5	9.54	7.01	6.31	8.49
Total Average depth (mm)	8.72	8.10	7.75	8.52

Table 4.

Comparison of Charring-Level Parameters

Properties	Cedar	Pine	Spruce	Fir
PHRR by cone calorimeter (kW/m²)	241.99	228.09	225.82	260.17
Char volume (cm³)	84.53	78.28	75.84	84.82
Charring rate (cm³/s)	0.1409	0.1305	0.1264	0.1414
Charring depth (mm)	8.72	8.10	7.75	8.52
Char mass (g)	41.75	46.93	52.15	45.07
Char density (g/cm³)	0.4940	0.5995	0.6876	0.5313
MHRR by cone calorimeter (kW/m²)	77.37	78.99	73.89	89.33
THR by cone calorimeter (MJ/m²)	47.0	47.9	44.7	53.6
Combustion efficiency (-)	0.7544	0.7805	0.7123	0.9208

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