Combustion Characteristics of Moxa Floss Under Nitrogen Atmosphere

Abstract

To investigate the combustion characteristics of moxa under a nitrogen atmosphere, this study employed an integrated approach combining experimental and theoretical analysis. Twelve moxa floss samples with different leaf-to-floss ratios, geographical origins, and storage durations were selected for thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) of their carbonized products in nitrogen environment. Through TG-DTG analysis, the thermal degradation patterns of the twelve moxa floss samples under nitrogen atmosphere were systematically examined to elucidate their pyrolysis behaviors, with particular emphasis on the influence of pyrolysis temperature and leaf-to-floss ratio on combustion characteristics. The pyrolysis process occurred in three distinct stages, with the most significant mass loss (120–430 °C) observed in the second stage. Higher leaf–fiber ratios and longer storage years were found to promote more complete pyrolysis. Kinetic analysis was performed to fit thermogravimetric data, establishing a reaction kinetic model for moxa pyrolysis. Results indicated that samples with higher leaf–fiber ratios required greater activation energy, while storage duration showed negligible impact. Notably, Nanyang moxa demanded higher pyrolysis energy than Qichun moxa. FTIR analysis identified the primary components of carbonized products as water, ester compounds, flavonoids, and cellulose. These findings suggest that moxa carbonization products retain chemical reactivity, demonstrating potential applications in adsorption and catalysis processes.

1. Introduction

Amid the accelerating global energy transition toward low-carbon and sustainable systems, biomass energy—abundant in reserves, renewable, and environmentally friendly—is emerging as a critical solution to address the dual challenges of energy crises and environmental pollution. It not only significantly reduces reliance on fossil fuels but also mitigates environmental contamination through the resource utilization of waste materials, playing an irreplaceable role in advancing sustainable energy development [1].

Thermogravimetric analysis (TGA) [2,3,4,5,6], a precise and efficient thermal analysis technique, enables the real-time monitoring of mass changes in materials under programmed temperature conditions, providing critical technical support for the in-depth exploration of reaction mechanisms, reaction kinetics, and product characteristics during biomass pyrolysis. Through TGA, researchers can clearly elucidate the pyrolytic behavior of biomass across different temperature intervals and obtain key parameters of pyrolysis reactions, thereby laying the theoretical foundation for the efficient conversion and utilization of biomass energy.

Moxa floss, as an indispensable core material in traditional Chinese medicine (TCM) acupuncture therapy, exhibits unique therapeutic effects such as warming meridians to dispel cold and reinforcing yang to stabilize collapse [7,8,9]. It has played a vital role in thousands of years of clinical TCM practice. The thermal stimulation generated by its combustion synergizes with volatile medicinal components to act on human acupoints, regulating the circulation of qi and blood in meridians, thereby achieving disease treatment and preventive healthcare [10,11,12,13,14,15,16]. With increasing global recognition and appreciation of TCM culture, the application value of moxa floss has become increasingly prominent.

However, existing studies predominantly focus on the combustion characteristics and pharmacological effects of moxa floss, with limited research on its pyrolytic behavior, kinetic characteristics, and the properties of solid char products during pyrolysis [17,18,19,20,21,22,23,24,25]. The solid char generated from moxa floss pyrolysis not only contains rich chemical information but also exhibits unique physicochemical properties, demonstrating potential application value in fields such as adsorption and catalysis. Huifang Wang et al. [26] found that moxa ash extracts contain flavonoids with antibacterial activity, Jiawei Chen et al. [27] investigated the effects of moxa ash treatment on the properties of cotton fabrics, and Xuelin Zhang et al. [28] discovered that carbonized moxa floss products exhibit good capacitive performance. The in-depth investigation of moxa floss pyrolysis-derived char will help further unlock its latent value and expand its application domains.

In light of this, the present study aims to systematically investigate the combustion and kinetic analysis of moxa floss by comprehensively considering key factors such as its geographical origin, storage duration, and leaf-to-floss ratio. Concurrently, Fourier transform infrared (FTIR) spectroscopy is utilized to perform the in-depth analysis of the solid pyrolysis-derived char, thereby fully elucidating the pyrolysis characteristics of moxa floss [29,30]. This work provides more comprehensive and reliable theoretical foundations and technical support for enhancing the efficient application of moxa floss in traditional Chinese medicine and advancing its development and utilization in emerging fields such as renewable energy and advanced materials.

2. Materials and Methods

2.1. Materials

The experiment collected a total of twelve moxa floss samples, each with varying years of production, origins, and leaf-to-floss ratios. Detailed information can be found in

Table 1. Experimental sample.

Sample	District	Storage Years	Leaf-to-Floss Ratio	Brand
L5	Nanyang	5 years	5:1	LvYing
L10	Nanyang	5 years	10:1	LvYing
L15	Nanyang	5 years	15:1	LvYing
L30	Nanyang	5 years	30:1	LvYing
Z5	Qichun	3 years	5:1	ZhongKang
Z10	Qichun	3 years	10:1	ZhongKang
Z15	Qichun	3 years	15:1	ZhongKang
Z30	Qichun	3 years	30:1	ZhongKang
H5	Nanyang	3 years	5:1	HuaKang
H10	Nanyang	3 years	10:1	HuaKang
H15	Nanyang	3 years	15:1	HuaKang
H30	Nanyang	3 years	30:1	HuaKang

. All experimental samples in this study were sourced from commercially available moxa floss, with production years, origins, and leaf-to-floss ratios being supplier-labeled information.

2.2. Proximate and Ultimate Analysis

The analytical base is air dry basis [2]. The industrial analysis is based on the Test Method for Fracture Resistance of Biomass Forming Fuel and Industrial Analysis Method (LY/T 3243-2020) and the GB/T 28731-2012 Methods for Industrial Analysis of Solid Biomass Fuel. Elemental analysis is based on the Method for the Determination of Hydrocarbon in Solid Biomass Fuel GB/T 28734-2012; the Method for the Determination of Nitrogen in Solid Biomass Fuel GB/T 30728-2014; and the Method for the Determination of Whole Sulphur in Solid Biomass Fuel GB/T 28732-2012.

2.3. Thermogravimetric Analysis

The thermogravimetric analysis was carried out using a thermogravimetric analyzer. The carrier gas was nitrogen with a flow rate of 50 mL/min. Before the experiment was started, nitrogen was introduced for 20 min. A total of 10 mg of mugwort was weighed and placed in the crucible of the thermogravimetric analyzer and heated up to 900 °C at a rate of 20 °C/min, and at the same time, nitrogen was introduced to ensure the anaerobic or low-oxygen environment. The crucible had a volume of 70 microliters and was made of ceramic. The thermogravimetric analyzer was a comprehensive thermal analyzer produced by Beijing Hengjiu Experimental Equipments Co, Ltd., model HCT-4. The recommended nitrogen flow rate was set at 50–100 mL/min to purge residual gasses from the furnace chamber. Selecting 50 mL/min minimized the impact of instrumental parameter fluctuations on experimental results.

The parameters involved in the pyrolysis process analysis were defined and processed as follows:

1.

Initial Temperature (T_i)

The temperature at which the TG curve begins to deviate from the baseline.

2.

Pyrolysis Onset Temperature (T_e)

The reaction onset of the rapid weight-loss stage refers to the starting temperature point. The extrapolated onset temperature of the TG (thermogravimetric) curve serves as the reference starting temperature for this stage. To determine the pyrolysis onset temperature, first draw a vertical line from the peak on the DTG (derivative thermogravimetric) curve to intersect the TG curve, identifying the inflection point. A tangent line is then drawn at this inflection point, and its intersection with the horizontal baseline of the TG curve prior to the weight-loss step is defined as T_e. This parameter is primarily used to characterize the thermal stability of materials: a higher T_e indicates better thermal stability of the moxa floss.

3.

Pyrolysis End Temperature (T_c)

Pyrolysis end temperature refers to the terminal point of the rapid weight-loss stage. The extrapolated end temperature of the TG (thermogravimetric) curve is used as the reference temperature endpoint for this stage. To determine T_c, a vertical line is drawn from the peak on the DTG (derivative thermogravimetric) curve to intersect the TG curve, identifying the inflection point. A tangent line is then drawn at this inflection point, and its intersection with the horizontal baseline of the TG curve after the weight-loss step is defined as the pyrolysis end temperature T_c.

4.

Maximum Mass Loss Rate (V_p) and Corresponding Peak Temperature (T_p)

The peak temperature (T_p) corresponds to the temperature point at the maximum of the DTG curve. The maximum weight-loss rate (V_p), expressed in %/min, is calculated as the product of the DTG peak value and the heating rate. A higher V_p indicates a more intense reaction and a greater release of volatile matter during pyrolysis.

2.4. Kinetic Analysis

The Coats–Redfern integration method [2,31,32] was chosen to analyze the experimental data dynamically and to solve its kinetic parameters.

Firstly, according to the Arrhenius equation, the pyrolysis reaction rate can be expressed as

$${\displaystyle \frac{\mathrm{d}\alpha }{\mathrm{dt}}}=\mathrm{kf}\left(\alpha \right)$$

(1)

where k is the Arrhenius reaction rate constant, and

$$\mathrm{k}=A{\mathrm{e}}^{-\mathrm{E}/\mathrm{RT}}$$

(2)

where E is the activation energy, kJ/mol;

• A is the pre-exponential factor, s⁻¹;
• R is the gas constant, 8.314 J/(mol-K);
• T is the thermodynamic temperature, K.

f(α) is the reaction mechanism function, which for a general reaction can be expressed by the following equation:

$$\mathrm{f}(\alpha )={(1-\alpha )}^{\mathrm{n}}$$

(3)

where α is the rate of change and

$$\alpha ={\displaystyle \frac{W_0-W}{W_0-W_{\infty }}}$$

(4)

where W₀ is the starting mass of the specimen;

• W is the starting mass of the specimen at T °C;
• W_∞ is the maximum mass loss.

In this experiment, a fixed heating rate β = 20 °C/min was used and

$$\beta ={\displaystyle \frac{\mathrm{dT}}{\mathrm{dt}}}$$

(5)

Simultaneously, (1), (2), (3), (4), and (5), can be obtained.

$${\displaystyle {\int }_0^{\alpha }\frac{\mathrm{d}\alpha }{{(1-\alpha )}^{\mathrm{n}}}}={\displaystyle \frac{A}{\beta }}{{\displaystyle \frac{RT}{E}}}^2{(1-{\displaystyle \frac{2RT}{E}})}^{-{\displaystyle \frac{E}{RT}}}$$

(6)

According to other studies, n = 3 [33], integral Equation (6), sorting, and taking logarithms of both sides, can be achieved.

$$\ln [{\displaystyle \frac{(1-\alpha {)}^{-2}-1}{2T^2}}]=\ln {\displaystyle \frac{\mathrm{A}}{\beta }}{\displaystyle \frac{R}{E}}(1-{\displaystyle \frac{2RT}{E}})-{\displaystyle \frac{E}{RT}}$$

(7)

For the general reaction temperature region and most of the E values, $1-{\displaystyle \frac{2RT}{E}}\approx 1$, ${\displaystyle \frac{E}{RT}}\ge 1$, so (3)–(7) can be reduced to

$$\ln [{\displaystyle \frac{(1-\alpha {)}^{-2}-1}{2T^2}}]=\ln {\displaystyle \frac{\mathrm{A}}{\beta }}{\displaystyle \frac{R}{E}}-{\displaystyle \frac{E}{RT}}$$

(8)

Plotting $\ln [{\displaystyle \frac{(1-\alpha {)}^{-2}-1}{2T^2}}]$ against ${\displaystyle \frac{1}{T}}$ gives a straight line with a slope of $-{\displaystyle \frac{E}{R}}$ and an intercept of $\ln {\displaystyle \frac{\mathrm{A}}{\beta }}{\displaystyle \frac{R}{E}}$, which allows the kinetic parameters of the reaction for the pyrolysis of mugwort to be found.

2.5. FTIR Analysis

Moxa samples were weighed and loaded into the tube furnace. Nitrogen gas purged the system for over 10 min to eliminate air. Carbonization proceeded under nitrogen atmosphere following a set temperature program. A flow meter maintained the nitrogen flow at 0.6 L/min to stabilize the process. The temperature increased to 900 °C. The wavenumber range of the FTIR analysis was 4000–500 cm⁻¹.

Post-carbonization, the product was analyzed using a Fourier transform infrared (FTIR) spectrometer. Carbonized material was placed on the spectrometer stage, and parameters were calibrated to collect spectral data.

The tube furnace model was OTF-1200X and the FTIR spectrometer was tensor27.

3. Results and Discussion

3.1. Proximate and Ultimate Analysis

The results of proximate and ultimate analyses for the twelve moxa floss samples are presented in

Table 2. Results of the proximate and ultimate analyses of moxa samples.

	Proximate Analysis/%				Ultimate Analysis/%
	Mad	Aad	Vad	Cad	Cad	Had	Oad	Nad	Sad
L5	7.89	7.27	67.50	17.34	43.52	4.12	50.92	1.29	0.15
L10	7.74	6.36	69.09	16.81	43.41	4.20	51.40	0.86	0.12
L15	7.33	5.71	70.10	16.86	43.66	4.25	51.23	0.75	0.11
L30	7.23	3.83	73.30	15.64	42.21	4.13	53.55	0.05	0.06
Z5	8.41	6.91	67.83	16.85	44.19	4.16	49.85	1.64	0.16
Z10	9.20	6.69	66.17	17.94	45.13	4.14	49.18	1.40	0.15
Z15	8.68	6.50	68.52	16.30	42.59	4.13	51.73	1.40	0.15
Z30	7.64	6.47	69.25	16.64	43.58	4.16	50.94	1.19	0.14
H5	8.24	7.65	67.35	16.76	43.42	6.13	48.23	2.05	0.17
H10	8.76	6.84	67.87	16.53	42.84	6.49	48.94	1.58	0.14
H15	9.67	6.38	68.08	15.87	41.75	6.70	50.14	1.29	0.12
H30	7.12	4.41	73.57	14.90	41.08	6.57	51.93	0.37	0.04

Note: Proximate analysis: M_ad: moisture; A_ad: ash; V_ad: volatile component; C_ad: fixed carbon; ultimate analysis: C_ad: carbon; H_ad: hydrogen; O_ad: oxygen; N_ad: nitrogen; S_ad: sulfur.

. The analytical base was air drying.

Proximate analysis can determine the composition of fuel, dividing it into combustible and non-combustible components. The combustible portion consists of volatile matter and fixed carbon, while the non-combustible portion includes ash and moisture [2]. Based on this, the organic content and combustion performance of moxa floss can be evaluated under different leaf-to-floss ratios, production years, and origins. The analysis of volatile matter, ash, fixed carbon, and moisture content shows that, in general, moxa floss samples with higher leaf-to-floss ratios and longer storage years ignite more easily and burn more completely. The ignition difficulty and combustion completeness vary by origin, but no absolute positive or negative correlation is observed, likely due to differences in growing and storage conditions across production regions.

Elemental analysis reveals the composition and content of elements in moxa floss, primarily carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S) [2]. Different elements release varying amounts of heat upon complete combustion. Based on elemental analysis, the calorific value of moxa floss can be estimated, providing further insight into its fuel characteristics. The analysis indicates that higher leaf-to-floss ratios and longer storage years lead to increased carbon content, though volatile carbon remains relatively consistent. Hydrogen content is largely unaffected by the leaf-to-floss ratio, but the H-group exhibits significantly higher hydrogen levels than other groups. Oxygen content shows a positive correlation with both leaf-to-floss ratio and storage year, while nitrogen and sulfur exhibit negative correlations. Based on these findings, it is inferred that moxa floss with higher leaf-to-floss ratios likely has a slightly lower calorific value and emits fewer harmful gasses upon combustion, though the differences are expected to be marginal. The H-group is predicted to have a higher calorific value than the other groups.

3.2. Thermogravimetric Analysis

Thermogravimetric analysis is a commonly used thermal analysis method that can be used to study the relationship between the mass of a substance and its temperature. The experimental data were processed by Origin2022 software to obtain the thermogravimetric (TG) curves and the differential (DTG) curves of twelve samples of moxa under a nitrogen atmosphere (see Figure 1).

Calculated pyrolysis parameters for twelve groups of mugwort samples are shown in Figure 2 and Table 2,

Table 3. Pyrolysis parameters of moxa samples at the first stage.

Sample	Mass Loss (%)	Peak Temperature (°C)	Maximum MASS Loss Rate (%/min)	Remaining Mass (%)
H5	10.72	71.62	3.87	89.28
H10	11.20	77.15	2.84	88.80
H15	11.66	81.22	2.74	88.34
H30	10.92	75.58	2.62	89.08
L5	9.42	81.53	2.37	90.58
L10	11.14	81.08	2.57	88.86
L15	9.86	78.78	2.41	90.14
L30	9.22	79.80	2.13	90.78
Z5	9.82	83.52	2.36	90.18
Z10	9.06	81.31	2.39	90.94
Z15	9.57	87.37	2.16	90.43
Z30	10.10	81.57	2.22	89.90

,

Table 4. Pyrolysis parameters of moxa samples at the second stage.

Sample	Mass Loss (%)	Initial Temperature (°C)	Peak Temperature (°C)	Pyrolysis Onset Temperature (°C)	Pyrolysis End Temperature (°C)	Maximum Mass Loss Rate (%/min)	Remaining Mass (%)
H5	48.86	135.28	333.09	271.61	370.90	10.11	40.42
H10	52.68	152.50	321.10	271.17	358.02	12.02	36.13
H15	52.83	139.33	341.44	279.64	367.77	13.85	35.51
H30	60.25	133.50	345.65	303.46	362.83	21.86	28.83
L5	49.70	141.56	330.94	265.33	366.43	10.15	40.89
L10	54.87	148.21	334.31	287.48	361.82	13.17	33.99
L15	53.29	149.71	336.30	285.30	367.03	13.00	36.85
L30	60.21	144.84	344.18	304.59	363.02	21.27	30.57
Z5	46.44	151.88	313.48	251.68	354.42	9.59	43.74
Z10	48.24	152.31	317.06	266.40	350.72	11.33	42.70
Z15	50.01	153.85	332.36	273.26	369.67	10.51	40.42
Z30	48.04	151.05	334.71	282.83	362.18	11.71	41.86

and

Table 5. Pyrolysis parameters of moxa samples at the third stage.

Sample	Mass Loss (%)	Initial Temperature (°C)	Peak Temperature (°C)	Maximum Mass Loss Rate (%/min)	Remaining Mass (%)
H5	25.59	422.34	\	\	14.83
H10	35.91	403.86	448.14	3.96	0.21
H15	21.73	435.06	\	\	13.78
H30	17.55	410.86	\	\	11.28
L5	36.05	411.82	462.97	3.02	4.83
L10	26.99	415.78	\	\	7.00
L15	26.53	414.68	\	\	10.32
L30	17.98	416.57	\	\	12.58
Z5	38.94	383.62	432.64	6.02	4.80
Z10	37.08	393.89	430.73	5.96	5.62
Z15	29.86	417.18	\	\	10.56
Z30	19.27	411.07	\	\	22.59

.

The analysis of thermogravimetric behavior is as follows.

• Pyrolysis Process

Figure 2 and Table 3, Table 4 and Table 5 show data from three distinct stages in moxa floss pyrolysis [6,22,34].

The first stage of moxa floss pyrolysis is the drying and dehydration stage, with a temperature range from room temperature to 120 °C. During this stage, both free water and bound water in the moxa floss gradually evaporate under heating, leading to a progressive mass loss. The TG curve shows a gentle slope with relatively slow mass loss rates—the maximum mass loss rate reaches 3.87%/min for sample H5, while remaining below 2.90%/min for other samples. All twelve moxa floss samples maintained high mass retention rates above 88.00%, with total mass loss not exceeding 12.00%. However, the actual mass loss slightly exceeded the moisture content determined by proximate analysis, indicating minor volatile release also occurred during this stage.

The second stage of moxa floss pyrolysis is the main pyrolysis stage, with a temperature range of approximately 120 °C to 430 °C. During this stage, as temperature increases, organic components such as hemicellulose, cellulose, and lignin in the moxa floss thermally decompose to produce small-molecule gasses, liquid bio-oil, and solid char. This stage shows significant mass loss, with all twelve samples exhibiting mass loss rates exceeding 48.00% and maximum mass loss rates ranging between 9.50%/min and 22.00%/min. The mass loss is dramatic and displays distinct variations among different samples.

The third stage of moxa floss pyrolysis is the carbonization stage, with a temperature range of approximately 430 °C to 900 °C. In this stage, as deeper volatiles further diffuse outward, they form a porous but more stable carbon structure. Typically, both TG and DTG curves tend to flatten during this phase. However, compared with other samples, H10, L5, Z5, and Z10 exhibit a third peak in their DTG curves. This phenomenon may be attributed to either higher lignin content in these moxa floss samples or the presence of more impurities, resulting in more complex compositions that increase the difficulty of thermal decomposition, consequently leading to reduced mass loss rates and additional mass loss peaks.

2.

Thermal Stability

When the leaf-to-floss ratio varies, the pyrolysis onset temperatures (T_e) of groups H, L, and Z increase with higher ratios, showing a positive correlation. For samples with identical leaf-to-floss ratios but different storage years, group L demonstrates higher T_e values than group H. Among samples with matching leaf-to-floss ratios but distinct geographical origins (groups H and Z), group H exhibits consistently higher T_e values than group Z. These results indicate that moxa floss with larger leaf-to-floss ratios and longer storage years possess enhanced thermal stability. Furthermore, Nanyang moxa floss shows superior thermal stability compared to Qichun counterparts.

3.

Reaction extent

The analysis of the three pyrolysis stages of moxa floss identifies the mass loss rate in the second stage and the maximum mass loss rate (V_p) as key parameters for evaluating reaction extent.

When the leaf-to-floss ratio differs, the mass loss of groups H and L shows a positive correlation with increasing ratios. The four samples in group Z exhibit minimal variation in second-stage mass loss, likely due to their higher impurity content. All three groups demonstrate increased maximum mass loss rates with higher leaf-to-floss ratios. For samples with identical ratios but different storage years, group L displays slightly higher mass loss rates and maximum mass loss rates than group H. For samples with identical ratios but different origins (groups H and Z), group H consistently achieves higher mass loss rates than group Z. These results indicate that higher leaf-to-floss ratios and longer storage years lead to more complete and intense pyrolysis reactions with fewer impurities. Additionally, Nanyang-origin moxa floss outperforms Qichun-origin moxa floss in terms of reaction extent.

3.3. Kinetic Analysis

The calculation and fitting of experimental data from twelve sample groups were performed, with the linear fitting plot shown in Figure 3. The obtained parameters are listed in

Table 6. Pyrolysis kinetic parameters of moxa floss samples.

Sample	Intercept	Slope	Activation Energy E	Pre-Exponential Factor A	Correlation Coefficient r
H5	6.33449	−11,480.60284	95.45	107,856.3316	0.9583
H10	4.2362	−10,525.58922	87.51	12,129.7601	0.9437
H15	7.30086	−12,052.676	100.21	297,613.9678	0.9629
H30	11.01056	−14,304.09489	118.92	14,425,516.3137	0.9745
L5	4.02953	−10,300.41925	85.64	9653.9516	0.9428
L10	7.22257	−12,019.6974	99.93	274,449.5166	0.9632
L15	7.16895	−11,917.78927	99.08	257,915.7023	0.9611
L30	11.13215	−14,380.52373	119.56	16,377,647.4220	0.9772
Z5	5.76553	−11,372.31246	94.55	60,483.0542	0.8898
Z10	6.55716	−11,901.76537	98.95	139,700.0699	0.8904
Z15	5.94554	−11,297.28822	93.93	71,934.3685	0.9568
Z30	8.20149	−12,471.55645	103.69	757,929.5872	0.9659

.

As shown in Table 6, the high correlation coefficients r indicate excellent fitting performance.

When leaf-to-floss ratios differ, activation energy (E) generally increases with higher ratios, except for a minor deviation in Z15. The comparative analysis of Groups H and L reveals irregular variations in activation energy across storage years. Geographically distinct samples show higher activation energy in Group H (Nanyang) than Group Z (Qichun). These results demonstrate that larger leaf-to-floss ratios require greater energy for moxa pyrolysis, while storage years exert minimal influence on energy requirements. Nanyang-origin moxa floss consistently requires greater thermal decomposition energy than Qichun-origin counterparts.

3.4. FTIR Analysis

The thermal decomposition of moxa floss under nitrogen atmosphere produces carbonized residues. The analysis of these residues is critical for understanding combustion mechanisms and optimizing pyrolysis processes. The FTIR spectra of twelve carbonized samples were processed using Origin 2022 software (Figure 4), with major absorption peaks, corresponding vibrational modes, and functional group assignments detailed in

Table 7. The main absorption peaks of different moxa samples and their attribution.

Wavenumber (cm−1)												Vibrational Modes	Assignment
H5	H10	H15	H30	L5	L10	L15	L30	Z5	Z10	Z15	Z30
3414	3402	3431	3425	3408	3431	3427	3412	3417	3431	3429	3431	ν(O-H)	Water
\	\	\	\	\	2923	2921	2921	2923	2925	2925	2923	νas(-CH2)	Esters
\	\	\	\	\	\	\	\	\	2854	2856	2854	νs(-CH2)	Esters
1657	\	1625	1659	\	1670	1663	1643	1664	\	\	\	ν(C = O)	Flavonoids
1448	1448	\	1456	1448	1458	1456	\	\	\	1458	1456	Aromatic ring skeletal vibration	Flavonoids
1381	1377	1404	1377	1379	\	\	1381	1402	1383	1385	1385	δ(C-H)	Cellulose
1033	1033	1118	1031	1022	1035	1035	1026	1110	1043	1043	1045	ν(C-O)	Flavonoids

Note: ν: stretching vibration; δ: bending vibration; ν_s: symmetric stretching; ν_as: asymmetric stretching.

.

The analysis of the FTIR spectra reveals characteristic peaks corresponding to distinct functional groups and chemical bonds. The systematic identification and analysis of these peaks enables the determination of the types and distribution patterns of functional groups present in the carbonized moxa products [35,36].

The infrared spectra of three carbonized moxa groups exhibit characteristic absorption peaks. First, the peak at 3400–3450 cm⁻¹ corresponds to O-H stretching vibrations, likely originating from bound water generated during reactions and surface-adsorbed moisture during storage. Second, the peaks near 2923 cm⁻¹ and 2854 cm⁻¹ are attributed to asymmetric and symmetric -CH₂ stretching vibrations, respectively, assigned to ester compounds. The aromatic ring skeletal vibration at 1450 cm⁻¹ and C-O stretching vibrations at 1000–1200 cm⁻¹ indicate flavonoids. The C-H bending vibration near 1377 cm⁻¹ suggests residual cellulose in the carbonized products.

It can be observed that the flavonoid retention rate in ash derived from 5-year-aged moxa floss showed little variation with leaf-to-floss ratios, while ash from 3-year-aged moxa floss exhibited an increasing trend in flavonoid retention rate as the leaf-to-floss ratio rose. The hydroxyl retention rate decreased with prolonged aging periods.

These aromatic compounds in the carbonized moxa likely derive from volatile components and inherent aromatic constituents preserved or transformed during carbonization. The identified functional groups demonstrate chemical reactivity in the carbonized residues, suggesting potential applications in adsorption and catalysis.

4. Conclusions

The thermal decomposition of moxa floss under nitrogen atmosphere occurs in three stages. In the first stage (room temperature to 120 °C), moisture and light volatiles are released. The second stage (120–430 °C) involves the pyrolysis of organic components, generating gaseous and solid products. The third stage follows, where residual inorganic components dominate, and the reaction stabilizes during carbonization. The second stage exhibits the highest mass loss rate, representing the primary decomposition phase. Thermogravimetric analysis confirms that higher leaf-to-floss ratios and longer storage years enhance thermal stability, with Nanyang-origin moxa floss outperforming Qichun-origin counterparts. Additionally, larger leaf-to-floss ratios and extended storage promote more complete pyrolysis with fewer impurities, while Nanyang-origin moxa demonstrates superior reaction extent. A higher leaf-to-floss ratio indicates a greater proportion of active components derived from mugwort leaves in the moxa floss. With longer aging periods, the content of volatile components decreases, while non-volatile components (e.g., thujone, caryophyllene) increase. This suggests that aged and refined moxa floss contains higher concentrations of therapeutically active constituents essential for moxibustion efficacy [37].

The combustion of moxa floss exhibits multistage characteristics, with approximately 60% mass loss occurring between 200 and 400 °C, primarily attributed to the thermal degradation of hemicellulose, cellulose, and partial lignin. This aligns with the temperature range of main component degradation reported in the study by Lim et al. [22]. However, discrepancies exist in specific parameters: the average peak temperatures for 3-year and 10-year stored samples in Lim et al.’s study were 317.85 °C and 324.50 °C, respectively, whereas in our study, the average peak temperatures for 5-year and 3-year stored moxa floss samples differed, with 5-year moxa floss reaching the maximum mass loss rate at 336.4 °C and 3-year moxa floss at 335 °C. These differences may be attributed to variations in sample origin, preparation methods, and experimental conditions.

Regarding kinetic data, due to the limited availability of studies directly addressing moxa floss pyrolysis kinetics under experimental conditions identical to those in this work, we referred to related research on biomass pyrolysis kinetics. In general biomass combustion processes, the decomposition temperature ranges of hemicellulose and cellulose show certain commonalities across studies. For example, in some studies [6], hemicellulose decomposition primarily occurs at 160–240 °C and cellulose decomposition at 240–360 °C. These ranges correlate with the decomposition temperature ranges of the main components observed in our study of moxa floss pyrolysis, further indicating that the combustion behavior of moxa floss conforms to the general principles of biomass combustion. However, the unique chemical composition of moxa floss imparts distinct characteristics to its combustion kinetics.

Kinetic analysis using the Coats–Redfern integral method (200–600 °C) reveals that higher leaf-to-floss ratios require greater activation energy. Storage years show minimal impact on energy requirements. Nanyang-origin moxa demands higher decomposition energy than Qichun-origin samples. High correlation coefficients (r > 0.95) confirm reliable fitting.

The carbonized residues retain oxygen-containing functional groups. FTIR analysis identifies characteristic peaks corresponding to water (3400–3450 cm⁻¹), esters (2923/2854 cm⁻¹), flavonoids (1450 cm⁻¹; 1000–1200 cm⁻¹), and residual cellulose (1377 cm⁻¹). These aromatic compounds, likely derived from volatiles and inherent aromatic components preserved during carbonization, suggest potential applications in adsorption and catalysis due to their chemical reactivity. Furthermore, a quantifiable relationship exists between the aging duration and the retention rates of functional groups (e.g., flavonoids, hydroxyl groups) in solid-phase products.

This study provides pivotal experimental data for investigating combustion behavior under varied atmospheres and offers a theoretical foundation for optimizing moxibustion practices. Furthermore, it contributes valuable insights into biomass combustion behavior, advancing research on bioenergy utilization.