Comparable Study on Celadon Production Fueled by Methanol and Liquefied Petroleum Gas at Industry Scale

Abstract

As a major contributor to industrial energy consumption and carbon emissions, the kiln industry faces increasing pressure to adopt cleaner energy sources. This study investigated the combustion characteristics, redox processes in celadon firing, product quality, and pollutant emissions for an industry furnace with methanol and liquefied petroleum gas (LPG) as kiln fuels. Methanol combustion reduced firing time by 17.4% due to the faster temperature rise during oxidation and holding phases and provided a more uniform and stable flame, compared with LPG cases. Significant reductions in emissions were observed when methanol is used as fuel. For example, NO concentration is reduced by 70.89%, 37.43% for SO₂, 93.67% for CO, 45.07% for CO₂, and 85.89% for CH₄. The methanol-fired celadon exhibited better quality in terms of the appearance and threshold stress–strain value. The chemical analysis results show that K/O element ratio increased from 8.439% to 11.706%, Fe/O decreased from 4.793% to 3.735%, Al/O decreased from 33.445% to 31.696%, and Si/O increased from 76.169% to 89.825%. These findings demonstrate the potential of methanol as a sustainable kiln fuel, offering enhanced combustion efficiency, reduced emissions, and improved ceramic quality.

1. Introduction

Ceramics have played an irreplaceable role in international cultural and trade exchanges as a traditional Chinese art form or a modern household commodity [1]. Over time, the diversity of ceramic types and their widespread distribution across China have expanded, as illustrated in Figure 1. However, kilns powered by fossil fuels including coal, oil, and liquefied petroleum gas (LPG) face significant challenges related to high energy consumption and environmental pollution. In China, the ceramic industry annually consumes approximately 150–200 million tons of raw materials and 40–60 million tons of standard coal energy [2], resulting in carbon dioxide emissions ranging from 0.9 to 140 million tons [3]. Therefore, reducing emissions from industrial kilns is imperative to achieve carbon neutrality.

The overall emissions of the kiln system are significantly influenced by fuel selection. Green methanol, hydrogen, and ammonia have been identified as the most promising renewable energy carriers and potential alternative fuels [4]. Among these, green methanol is considered the most viable near-term solution due to its compatibility with existing combustion devices, favorable storage and transportation characteristics, combustion properties, and economic feasibility [5,6]. For instance, green methanol has already been adopted as a marine fuel by Maersk [7,8] and as a fuel for passenger vehicles by Geely Auto [9]. These developments highlight its potential as a sustainable alternative for reducing emissions from industrial applications.

Significant progress has been made in methanol combustion research, particularly in fundamental combustion properties and engine applications [10,11,12]. Laminar flame speeds of methanol have been measured using a shock tube, and the results indicate that flame speed increases at higher temperatures [13]. The structure and combustion stability mechanisms of methanol spray flames have also been investigated using optical phenomenology and chemical kinetics [14], revealing that methanol spray flames exhibit significantly lower stability compared to those of isooctane and n-heptane. Additionally, the stability and combustion characteristics of methanol/biodiesel engine flames have been analyzed, demonstrating that pressure variations and combustion characteristics become more pronounced with increasing methanol doping ratio [15,16].

Furthermore, studies have shown that the addition of methanol significantly reduces emissions of nitrogen oxides (NO_x) and particulate matter from internal combustion engines [17]. When a methanol–gasoline blend with a 30% substitution rate was used as engine fuel, NO_x emissions were reduced by 45%, hydrocarbon (HC) emissions by 30%, and particulate matter emissions by 50% compared to pure gasoline [18]. Another study reported that carbon emissions were reduced by up to 80% when pure methanol was used in internal combustion engines, with thermal efficiency improving by approximately 15% [19]. Recent studies further emphasize methanol’s potential in mitigating emissions. The adoption of methanol in passenger vehicles has been shown to significantly decrease carbon monoxide (CO) and carbon dioxide (CO₂) emissions due to the variation in ignition delay time at higher methanol blend ratios [20]. Additionally, a review involving methanol production and application in internal combustion engines highlighted an overall improvement in emission profiles, reinforcing methanol’s viability in enhancing the sustainability of internal combustion engines within the context of carbon neutrality [21].

While methanol has demonstrated significant potential as a green alternative fuel in reducing pollution and carbon emissions for combustion applications, research on its use in kilns remains limited [22,23,24,25,26]. The choice of fuel plays a crucial role in determining the quality of celadon. During the firing process, the color of celadon is highly sensitive to redox reactions, oxygen atmosphere, and temperature, with variations in oxygen content significantly influencing glaze color. Therefore, precise control of oxygen levels and temperature within the kiln is essential for achieving the desired celadon glaze. Methanol, when used as a fuel, produces oxygen atoms during combustion, which affects the oxygen atmosphere in the kiln [27,28]. As a result, the use of methanol fuel may alter the combustion environment, potentially leading to a brighter glaze color [29,30]. However, its impact on the overall quality of celadon products has not yet been reported. To ensure the stability of celadon’s color and texture, further research on the application of methanol fuel in the firing process is necessary. Additionally, a deeper investigation into the influence of the kiln’s oxygen atmosphere on the glaze’s structural and chemical composition would provide valuable insights on fuel selection in kilns industry. The results from this work may extend to other high-temperature industrial processes, such as glass and brick production, where the control of combustion conditions and emission reduction are critical.

This study comprehensively investigates the combustion behavior, emission characteristics, and product quality in the kiln industry with Zhejiang Longquan celadon as a case study (Figure 2). Both methanol and liquefied petroleum gas are considered. The temperature distribution, concentration profiles of major gas-phase species, elemental composition of the celadon glaze, microstructure, stress intensity of the products, and overall pollutant emissions were systematically examined through experimental analysis. The findings contribute to the advancement of the green transition by integrating green fuel and advanced combustion control processes into ceramic production. Furthermore, the results provide valuable reference data for the application of methanol as a fuel in industrial kilns and other high-temperature industrial processes.

2. Experimental Method

The experimental setup is presented in Figure 3. The experiments can be divided into two sections: process monitoring and product analysis. Process monitoring involves capturing flame images, recording temperature profiles at 30 min intervals, monitoring fuel flow at various firing stages, analyzing flue gas composition at 30 min intervals, and assessing the gas composition within the combustion chamber. Product analysis includes evaluating the appearance of the finished kiln products, conducting product characterization, performing microstructural and elemental composition analyses, as well as assessing stress–strain properties and microstructural features.

In the study, the reference fuel is liquefied petroleum gas (LPG) consists of 60% methane, 30% propane, 5% n-butane, and 5% isobutane by volume. The chemical formula of methanol is CH₃OH, and the physicochemical properties of both fuels are detailed in

Table 1. Comparison of Fuel Properties.

Substance	Calorific Value (kJ/kg)	Density (kg/m3)
liquefied petroleum gas (g)	53,340	1.24
methanol (l)	19,500	0.791 × 103

. To ensure the accuracy and repeatability of the tests, the fuel flow rates of the LPG methanol groups were controlled to maintain the same energy input.

Table 2. Test flow condition.

Mass Flow (kg/H)	LPG	Methanol	Calorific Value of Flow (MJ/Hour)
oxidation stage	4.69	12.82	250
First insulation stage	6.09	16.67	325
Reduction stage	10.22	27.95	545
Second insulation stage	7.78	21.28	415

presents the corresponding mass flow rates for different celadon firing stages under equivalent thermal input conditions.

The celadon kiln used in this study has dimensions of 86.0 cm in length, 60.0 cm in width, and 88.0 cm in height, and belongs to the Longquan kiln type, which is characterized by its ability to maintain a consistent temperature distribution and control the oxygen atmosphere, essential for achieving the desired glaze effects in celadon production. Temperature monitoring and gas analysis were conducted at three designated locations: the kiln flue gas outlet, the kiln top, and the kiln bottom (locations shown in Figure 3). Three K-type thermocouples were employed for temperature measurement, while a MRU MGA6 (MRU Instruments, Neckarsulm-Obereisesheim, Germany) infrared flue gas analyzer was used for gas composition analysis The measurement error of MRU MGA6 infrared flue gas analyzer is estimated within ±1% after calibration. Flame images were captured using a Canon EOS R7 camera with an RF-S18-150mm F3.5-6.3 IS STM lens (Canon, Tokyo, Japan).

For product analysis, celadon plates with a diameter of 16.8 cm were used for characterization. A DWJ3020-BB five-axis gantry waterjet cutter (Dardi International Corporation, Nanjing, China) was utilized to shape the celadon sample. Microstructural and elemental analyses were conducted using an SU8010 (Hitachi High-Tech Corporation, Tokyo, Japan) cold field emission scanning electron microscope (SEM), with system settings controlled by SEM software(application version as 10.03.06.0100), achieving a resolution of 1.3 nm at a landing voltage of 1 kV. The sample dimensions for microstructural analysis were 1.0 cm in length, 0.6 cm in width, and 0.5 cm in height. Stress–strain analyses of different samples were performed using an INSTRON 5966 universal testing machine(Instron Corporation, Norwood, MA, USA), with sample dimensions of 5.0 cm in length, 1.0 cm in width, and 0.5 cm in height. (

Table 3. Test condition.

Experimental Parameter	Details
Temperature Measurement Method	Three K-type thermocouples
Gas Composition Analysis	MRU MGA6 infrared flue gas analyzer
Flame Image Capture	Canon EOS R7 camera with RF-S18-150mm F3.5-6.3 IS STM lens
Cutting Method for Celadon	DWJ3020-BB five-axis gantry waterjet cutter
Microstructural and Elemental Analysis	SU8010 cold field emission scanning electron microscope (SEM)
Stress–Strain Analysis	INSTRON 5966 universal testing machine

).

This system employs the principle of a Bunsen burner, incorporating the venturi effect to simplify air supply (Figure 4). A negative pressure zone is formed in the fuel gas passage (with a throat diameter of 22.07 mm), naturally drawing in ambient air to mix with the fuel gas. The mixture then travels 23.0 mm to reach the burner outlet. This design eliminates the need for an external gas line to achieve a combustible mixture. By adjusting the valve opening, the ratio of fuel gas to air can be precisely controlled, thereby allowing accurate regulation of the flame. In the experiment, the air-fuel ratio was controlled at 1.7.

3. Result and Discussion

3.1. Comparative Analysis of Flame Appearances

From the perspective of flame color analysis, the observed variations in flame appearance can be attributed to the combined effects of chemiluminescence and soot radiation. The blue reaction zone, governed by CH* chemiluminescence, serves as an indicator of active radical reactions in the high-temperature region. When soot particles are present, this zone is visually obscured, resulting in a white appearance due to broadband scattering and thermal emission. The yellow to dark red light, primarily emitted by incandescent soot particles, is influenced by local soot temperature and particle density [31]. As illustrated in Figure 5, increasing air flow induces significant changes in flame transparency and coloration. In methanol flames, transparency decreases while the dark red hue remains, suggesting enhanced flame luminosity without substantial soot formation. In contrast, the LPG flame undergoes a gradual color transition from orange to white and ultimately to deep blue, indicating a shift from incomplete to complete combustion regimes. The presence of orange coloration is indicative of soot formation, typically associated with oxygen-deficient conditions [32]. Comparative analysis confirms that soot generation is intensified under low air flow conditions when the air valve is fully closed, whereas full opening of the air valve facilitates complete combustion and suppresses soot formation through increased oxygen availability for the burner used here.

Figure 6 presents flame images at various stages of celadon firing—holding, oxidation, and reduction (Figure 7). As fuel flow rate increases, a corresponding rise in flame height is observed, with the LPG flame extending from 15.0 cm to 24.0 cm and the methanol flame from 10.0 cm to 24.0 cm. During the holding stage, both flames appear deep blue, indicating a predominantly soot-free combustion environment. At the oxidation stage, deep blue coloration persists with the emergence of localized orange hues, signifying the onset of soot formation under transitional combustion conditions. Flame behavior diverges during the reduction stage. Specifically, the LPG flame exhibits minimal variation compared to the previous stage, while the methanol flame shift to an orange-dominant appearance with a retained deep blue base, suggesting a shift toward oxygen-deficient, soot-forming conditions [33].

In terms of flame shape and stability, distinct differences between the two fuel types are observed. The methanol flame generally exhibits a conical configuration with lower height and greater stability, reflecting a more homogeneous distribution of combustion products. In contrast, the LPG flame displays an irregular shape with higher flame height and noticeable flickering or wavering. Comparative analysis highlights that the methanol flame achieves a more complete and stable combustion state, whereas the LPG flame tends to exhibit less uniform combustion characteristics under the conditions in this work.

3.2. Assessment of Celadon Firing Process

3.2.1. Temperature Changes During the Firing Process

The temperature information is provided in Figure 8, and a significantly faster temperature rise and greater thermal output were observed by the methanol flame compared to the liquefied petroleum gas (LPG) flame, particularly during the oxidation and reduction stages. During the holding stage, temperature was also marginally higher in the methanol case, suggesting enhanced baseline thermal stability. This thermal behavior reflects the higher combustion reactivity and superior heat transfer characteristics of methanol. The temperature-time profiles reveal that during the reduction stage, a pronounced disparity emerged: the case using LPG required 330 min to raise the kiln bottom temperature from 1141.15 K to 1474.15 K, whereas the case using methanol achieved a larger temperature increment (from 1190.15 K to 1543.15 K) within 180 min. The observations here imply more efficient heat release and higher rate of energy transfer to the ceramic body and surrounding atmosphere can be achieved if methanol is used as fuel.

This phenomenon is primarily governed by the intrinsic physicochemical properties of methanol. As previously reported [34], methanol combustion involves a shorter ignition delay, a higher laminar flame speed, and more complete oxidation of intermediates compared to LPG. Furthermore, methanol’s low carbon-to-hydrogen ratio suppresses soot formation. According to studies [35,36], methanol combustion not only delivers a higher calorific value per unit of oxygen consumed but also facilitates a more uniform thermal field, contributing to faster heating rates and reduced temperature gradients within the kiln chamber. These factors collectively account for the observed rapid thermal response during firing.

Moreover, the superior temperature behavior of methanol has been corroborated by previous research [37], which emphasized its consistent thermal output and combustion stability across various operating regimes. The oxidation stage, characterized by intensified exothermic reactions, particularly benefits from methanol’s high reactivity and enhanced flame propagation dynamics. Such characteristics may explain the higher heat efficiency and less firing duration for methanol case. The comparative analysis further validates methanol’s suitability as an alternative fuel in celadon firing, where precise thermal control and energy efficiency are critical [38]. Thus, the integration of methanol not only enhances thermal utilization efficiency but also optimizes firing kinetics.

3.2.2. Pollutant Emissions During the Firing Process

The pollutant emissions are provided in Figure 9, Figure 10 and Figure 11, it was observed that the emission profiles at the kiln vent, top, and bottom exhibited comparable trends, indicating a relatively uniform spatial distribution of combustion by-products within the kiln chamber. Therefore, emissions from the vent were selected as representative for detailed analysis. The including CO₂ and CH₄ emission was mainly released from the high-temperature reduction phase, where incomplete combustion are more likely to occur. Notably, the methanol flame exhibited a substantially lower emission intensity than the liquefied petroleum gas (LPG) flame during this stage. This difference can be attributed to distinct combustion mechanisms. Methanol (CH₃OH) undergoes a complete oxidation reaction as follows:

$${\mathrm{CH}}_3\mathrm{OH}+{\displaystyle \frac{3}{2}}{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(1)

In contrast, LPG, primarily consisting of CH₄ and C₃H₈, follows reactions such as:

$${\mathrm{CH}}_4+2{\mathrm{O}}_2\to {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{C}}_3{\mathrm{H}}_8+5{\mathrm{O}}_2\to 3{\mathrm{CO}}_2+4{\mathrm{H}}_2\mathrm{O}$$

(3)

Under oxygen-deficient conditions like the reduction stage, partial oxidation and pyrolysis dominate the process, which leads to the formation of CO and unburned hydrocarbons (UHCs). Integral emission analysis demonstrated that, compared to the LPG case, the methanol case reduced CO₂ emissions by approximately 47.05% and CH₄ emissions by approximately 85.89% (Figure 9), a result consistent with previous findings [39]. Furthermore, the firing duration during the reduction stage was 120 min shorter when methanol is used, contributing to a lower cumulative emission load.

Regarding NO_x and SO_x emissions, the differences are rooted in fuel composition and flame temperature characteristics. NO formation predominantly occurs through the thermal (Zeldovich) mechanism:

$${\mathrm{N}}_2+\mathrm{O}\to \mathrm{NO}+\mathrm{N}$$

(4)

$$\mathrm{N}+{\mathrm{O}}_2\to \mathrm{NO}+\mathrm{O}$$

(5)

This pathway is highly sensitive to temperature-, with the NO generation rate following an exponential dependence on flame temperature ($\propto \exp (-Ea/RT)$). As demonstrated previously, the combustion of methanol shows a lower flame peak temperatures due to its more uniform thermal distribution, thus suppressing thermal NO_x formation. The detected NO emission was reduced by 70.89% in the methanol group compared to the LPG (Figure 9). The reduction in NO emission is consistent with the results from literature [39]. Similarly, SO₂ emissions were reduced by 37.43% in the methanol group. This is attributed to the sulfur-free nature of methanol, while LPG contains trace sulfur compounds. The reaction pathway is expressed below:

$$\mathrm{S}+{\mathrm{O}}_2\to {\mathrm{SO}}_2$$

(6)

A significant reduction in CO emission was also observed, with a 93.67% decrease in the methanol case compared to the LPG case (Figure 9). This improvement reflects the enhanced oxidation completeness. The CO/CO₂ ratio, a key indicator of combustion efficiency, showed a marked decline in the methanol group. In sub-stoichiometric conditions, incomplete oxidation can yield CO through:

$$2{\mathrm{CH}}_3\mathrm{OH}+3{\mathrm{O}}_2\to 2\mathrm{CO}+4{\mathrm{H}}_2\mathrm{O}$$

(7)

However, the high flame propagation speed and superior air-fuel mixing efficiency at elevated temperature suppress this pathway. Moreover, the oxygenated molecular structure of methanol facilitates secondary oxidation:

$$\mathrm{CO}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\to {\mathrm{CO}}_2$$

(8)

This exothermic reaction not only enhances thermal output but also improves combustion stability. In contrast, LPG flames exhibit higher CO accumulation concentration due to slower reaction kinetics and greater combustion heterogeneity. These findings reinforce the conclusion that methanol could effectively reduce pollutant generation during celadon firing due to its favorable combustion chemistry, efficient thermal behavior, and simple fuel composition [39].

3.3. Celadon Product Analysis

3.3.1. Analysis of Celadon Product Appearance

Figure 12 displays the appearance of origin celadon samples prior to firing, celadon samples fired with methanol as fuel, and the celadon samples fired with liquefied petroleum gas (LPG) as fuel. From a chromatic perspective, the methanol celadon samples exhibit a uniformly distributed deep green hue, whereas the LPG samples present a lighter green coloration with lower chromatic purity and visible unevenness. The formation of celadon color is primarily governed by the redox state of iron oxide (Fe₂O₃), which acts as the key chromophore, and is strongly influenced by the prevailing atmosphere during firing [40]. In a sufficiently reducing atmosphere, Fe³⁺ ions in Fe₂O₃ are partially reduced to Fe²⁺, which subsequently contributes to the characteristic bluish-green color. The deeper and more homogeneous coloration observed in the methanol-fired samples indicates that a better environment is reached during the reduction state. In contrast, the less pronounced and uneven coloration in the LPG-fired samples suggests localized oxidation or insufficient reduction, leading to partial retention of Fe³⁺ and reduced color development.

From the perspective of glaze surface quality, the methanol celadon samples exhibit a finer microstructure, a smoother glaze surface, and noticeably higher glossiness compared to those fired with LPG. The smoother glaze texture is likely attributed to more complete melting and vitrification of the glaze components, facilitated by the stable and intense combustion environment provided by methanol. The formation of a dense and defect-free glaze layer may be attributed to the higher thermal efficiency and shortened firing duration. This will be further discussed in the subsequent analysis. In contrast, the LPG-fired samples, affected by greater combustion fluctuations and lower firing temperatures, exhibit relatively lower glaze uniformity and surface brightness. These findings collectively suggest that the use of methanol as a fuel not only optimizes the reduction conditions essential for chromophore development but also enhances the esthetic qualities of the final celadon product [41].

3.3.2. Comparative Analysis of Microstructure and Elements in Celadon

Figure 13 presents the microstructural characteristics of the liquefied petroleum gas (LPG) celadon glaze (Figure 13a,b) and the methanol celadon glaze (Figure 13c,d). The measurements were conducted by the field emission scanning electron microscopy (FE-SEM) at magnifications of 10.0 k and 20.0 k, respectively. The LPG celadon samples exhibit a glaze structure characterized by a high density of bubbles, with diameters predominantly ranging from 0.5 to 10 μm, and an looser microstructure. In contrast, the methanol celadon samples display a denser texture with no apparent bubbling, indicating a more compact glaze phase and improved structural homogeneity. These observations further support the previously discussed findings on surface smoothness and gloss.

The formation of different microstructure can be attributed to the redox behavior of iron oxides and the thermodynamic conditions prevailing during firing. In an oxidizing atmosphere, iron predominantly exists as Fe³⁺, favoring yellow or brown glaze coloration; whereas, in a reducing atmosphere, Fe²⁺ is stabilized, resulting in the characteristic lime green celadon hue. The redox dynamics are strongly dependent on both oxygen partial pressure and firing temperature. Around 1200 °C, variations in the redox equilibrium and metal oxide solubility become pronounced, influencing glaze color and structure. Under reducing conditions, the conversion of Fe₂O₃ to FeO, represented by the reaction:

$${\mathrm{Fe}}_2{\mathrm{O}}_3+3\mathrm{CO}\to 2\mathrm{FeO}+3{\mathrm{CO}}_2$$

(9)

is thermodynamically favored, enhancing the production of Fe²⁺ and contributing to the development of the green glaze color. Conversely, in an oxidizing environment, FeO may be re-oxidized to Fe₂O₃, shifting the glaze color toward yellowish tones. The control of oxygen concentration and thermal profile is, therefore, critical for achieving a consistent and high-quality celadon glaze appearance.

In the LPG-fired kiln, the decomposition of Fe₂O₃ in the iron-bearing ceramic body during the oxidizing phase—typically occurring between 1150 °C and 1250 °C—results in substantial oxygen release. As the firing atmosphere transitions to a reducing state, the premature reduction of Fe₂O₃ to FeO further contributes to gas evolution. These gas-phase reactions facilitate bubble nucleation and growth within the glaze matrix. As firing progresses, smaller bubbles coalesce into larger voids due to thermal buoyancy and reduced viscosity, which consequently impairs the compactness and optical smoothness of the glaze. The presence of these defects in the LPG-fired samples supports the previously discussed concerns regarding inadequate thermal control and inefficient fuel combustion [40,42,43].

In contrast, the absence of visible bubble structures in the methanol-fired celadon glaze indicates a more stable reduction process, characterized by a slower and more controlled gas release. The superior combustion efficiency and uniform heat distribution inherent in methanol combustion mitigate thermal gradients and prevent localized overheating, thereby suppressing the conditions that promote gas entrapment. The resultant microstructure not only reflects enhanced firing kinetics but also indicates a higher degree of technological maturity in process control. This microstructural evidence further substantiates the advantages of methanol-based firing in producing celadon with superior glaze quality, color consistency, and structural integrity, marking a significant advancement in firing technology and ceramic material processing [44].

Table 4. Comparative Analysis of Elements.

Atomic (%)	Unfired Celadon	Celadon Fired Using Liquefied Petroleum Gas	Celadon Fired Using Methanol
O	43.402	44.874	42.201
Al	13.768	15.008	13.376
Si	37.977	34.180	37.907
K	3.386	3.787	4.940
Fe	1.467	2.151	1.576

presents the elemental composition of the glaze surfaces for glazed celadon bisque samples prior to firing, along with the liquefied petroleum gas (LPG)-fired celadon samples and the methanol-fired celadon samples, with elemental ratios normalized to oxygen for comparative analysis. Post-firing results reveal that the K/O ratio in the methanol-fired celadon increased from 8.439% to 11.706%, from 4.793% to 3.735% for Fe/O, and from 33.445% to 31.696% for Al/O. Concurrently, the Si/O ratio exhibited a substantial increase from 76.169% to 89.825%, signifying a compositional trend characterized by higher potassium and silicon content, coupled with lower iron and aluminum levels. This shift in composition highlights a more refined melting behavior and improved thermodynamic balance within the glaze system under methanol combustion conditions.

The increase in potassium content is crucial in glaze chemistry. Potassium, primarily in the form of potassium oxide (K₂O), acts as a fluxing agent that lowers the eutectic point of the glaze system. The formation of potassium silicate phases such as:

$${\mathrm{K}}_2\mathrm{O}+{\mathrm{SiO}}_2\to {\mathrm{K}}_2{\mathrm{SiO}}_3$$

(10)

facilitates the melting and spreading of the glaze at lower temperatures, enhancing surface uniformity and promoting the formation of a glassy phase with improved gloss and transparency. The higher K/O ratio in methanol samples implies more extensive potassium-silicate network formation, which also contributes to the formation of amorphous silicate matrices, thereby enhancing glaze continuity and surface compactness [45].

The lower Fe/O ratio observed in methanol samples is indicative of limited reduction of Fe₂O₃ into FeO, and consequently, less interaction with silica to form iron silicates, such as:

$$\mathrm{FeO}+{\mathrm{SiO}}_2\to {\mathrm{FeSiO}}_3$$

(11)

Iron silicates are commonly associated with darker, less transparent glaze appearances and can contribute to surface heterogeneity. The suppression of such phase formation in methanol-fired glazes enhances the visual purity and uniformity of color. Moreover, the reduced diffusion of iron into the glaze matrix is correlated with a decrease in oxygen release during firing, thereby limiting gas entrapment and bubble formation, as substantiated by microstructural analysis. These findings are consistent with previous studies, which suggest that excessive FeO content promotes phase separation and bubble formation, resulting in optical defects within the glaze layer [46].

From a thermodynamic perspective, the Gibbs free energy change (ΔG) associated with the reduction of Fe₂O₃ under different atmospheres provides further insight. The reduction of Fe₂O₃ to FeO is governed by:

$${\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CO}\to 2\mathrm{FeO}+{\mathrm{CO}}_2$$

(12)

$$\Delta G=-RT\ln K$$

(13)

where K is the equilibrium constant. Under methanol combustion, the partial pressure of CO is lower due to more complete oxidation, shifting the equilibrium toward Fe₂O₃ and inhibiting excessive reduction, which supports the preservation of a stable glaze structure with reduced porosity [42].

The higher Si/O ratio observed in methanol glazes also indicates a greater incorporation of silica into the glaze matrix. SiO₂ is a primary network former in vitreous phases, and its increase promotes the polymerization of silicate tetrahedra, enhancing the mechanical strength and chemical durability of the glaze. The degree of polymerization (Qⁿ units) of silicate structures increases with SiO₂ content, favoring Q³ and Q⁴ units over Q⁰ or Q¹, which is characteristic of a denser, more interconnected glass network. This can be represented schematically as:

$${\mathrm{SiO}}_4^{4-}{+\mathrm{SiO}}_4^{4-}\to (\mathrm{Si}\text{-}\mathrm{O}\text{-}\mathrm{Si})network$$

(14)

Moreover, the moderate decrease in Al/O ratio suggests a reduction in aluminosilicate formation, such as:

$${\mathrm{Al}}_2{\mathrm{O}}_3+6{\mathrm{SiO}}_2\to 2{\mathrm{Al}}_2{\mathrm{Si}}_3{\mathrm{O}}_{12}$$

(15)

Aluminum oxide (Al₂O₃) plays a crucial role in the formation of aluminosilicate phases, which are more refractory and less glass-forming than pure silicate phases. When present in a glaze, Al₂O₃ increases its viscosity, reduces fluidity, and hinders the smooth flow and melting of the glaze at high temperatures. In contrast, a lower Al₂O₃ content promotes better glaze flow and improves melt homogeneity during the firing process. This results in a more uniform formation of the glassy phase, essential for achieving a smooth and well-vitrified surface. Consequently, the glaze becomes more uniform in texture and composition, leading to enhanced surface quality with improved gloss and transparency.

The reduction in Al₂O₃ content in methanol-fired samples facilitates the formation of a more fluid, low-viscosity melt at elevated firing temperatures. This not only enhances glaze uniformity but also reduces surface defects such as uneven gloss or rough textures. Additionally, a lower Al₂O₃ content encourages the incorporation of more network formers, such as silicon dioxide (SiO₂) and potassium oxide (K₂O), which strengthens the glass network, improving the mechanical properties of the final product. As a result, the glaze exhibits a smoother, more durable surface, as observed in methanol-fired celadon samples, which have a more vitrified and defect-free surface compared to those fired with conventional fuels.

In summary, the elemental composition data demonstrate that methanol-fired celadon glazes exhibit a thermochemically optimized balance between flux and network formers, reduced structural disruption induced by iron, and enhanced silicate polymerization. These factors are directly associated with the improved esthetic and physical properties of the glaze. When combined with previous structural and morphological analyses, these findings further validate the technical and material advantages of methanol combustion in the production of high-quality celadon [46].

3.3.3. Comparative Analysis of Stress–Strain of Products

In the northeastern regions of China and other high-latitude areas, low ambient temperatures during winter significantly increase the risk of thermal shock-induced damage in celadon products when subjected to rapid heating. As a result, evaluating the stress–strain properties is crucial for assessing the mechanical reliability and thermal resilience of celadon ceramics. As illustrated in Figure 14, a comparative analysis of the stress–strain behavior between methanol-fired and liquefied petroleum gas (LPG)-fired celadon samples reveals no significant difference in the average bending displacement at the point of maximum bending stress; however, the methanol-fired samples exhibit slightly superior mechanical performance. Specifically, the methanol-fired celadon samples demonstrate an average fracture stress of 51.82 MPa and an average strain of 0.238 mm; whereas, the LPG-fired samples exhibit values of 44.79 MPa and 0.198 mm, respectively. This enhanced performance suggests that microstructural densification and optimized internal stress distribution during the methanol firing process contribute to improved mechanical resistance.

The superior stress–strain response observed in the methanol-fired celadon samples can be attributed to several intrinsic and extrinsic factors. First, the more uniform and compact microstructure, as evidenced by previous scanning electron microscope (SEM) analyses, reduces the presence of internal flaws such as pores, microcracks, and intergranular voids—commonly identified as critical sites for crack initiation under mechanical loading. A denser glaze-body interface minimizes stress concentration effects, as described by:

$${\sigma }_{\max }={\sigma }_0(1+2a/r)$$

(16)

where a represents crack length and r denotes the radius of curvature at the crack tip. This reduction in stress concentration delays crack propagation and enhances fracture resistance.

Second, differences in thermal expansion behavior and heat distribution between the two firing atmospheres significantly influence the development of residual stresses. The green methanol flame, characterized by higher thermal uniformity and a shorter firing duration, promotes more homogeneous thermal gradients within the ceramic body, resulting in lower thermal mismatch stresses between the glaze and the ceramic matrix. In contrast, the LPG-fired samples may experience localized overheating or cooling zones due to combustion instability, which leads to tensile residual stresses that compromise mechanical integrity under external loading.

Furthermore, the superior bonding and continuity of the glass phase in the methanol-fired samples—enhanced by the fluxing action of potassium silicates, as previously discussed—contribute to improved load transfer across the glaze-body interface. The more integrated silicate network structure reduces the likelihood of interfacial debonding under stress, thereby enhancing strain tolerance and elastic recovery capacity.

From a materials mechanics perspective, the slightly greater bending displacement observed in the methanol-fired group may also indicate improved fracture toughness (K_IC), which governs the material’s resistance to crack propagation. This behavior aligns with recent studies on celadon ceramics that exhibit restorative structural responses under loading [47], where enhanced microstructural connectivity and energy dissipation mechanisms contribute to improved mechanical resilience.

Although the bending strength and elastic modulus values are broadly similar between the two groups, minor discrepancies in strain behavior may be attributed to variations in sample preparation parameters, localized compositional gradients, or slight differences in porosity distribution. Nevertheless, the overall performance trend supports the conclusion that methanol-fired celadon demonstrates superior stress–strain characteristics, driven by optimized thermal processing, refined microstructural evolution, and improved glaze-body cohesion [43,47]. These attributes enhance its suitability for applications in thermally demanding environments, particularly in cold climate regions where mechanical durability is of critical importance.

4. Conclusions

This study evaluates the application of methanol as an alternative fuel to liquefied petroleum gas (LPG) in industrial-scale celadon production. Through the comprehensive assessment of combustion characteristics, firing processes, pollutant emissions, and product quality, the following conclusions can be drawn:

• Combustion Performance: Under identical production conditions (400 cups/day per kiln), the methanol flame was observed to be deeper in color, more transparent, and hotter than the LPG flame. Greater stability in combustion was also recorded across various firing stages, including oxidation, reduction, and holding.
• Firing Process Efficiency: A faster temperature increase was recorded in the methanol-fired kiln compared to the LPG-fired kiln, particularly during the oxidation and holding stages, resulting in a 17.4% reduction in total firing time.
• Pollutant Emissions: A substantial decrease in pollutant emissions was observed with methanol combustion, with reductions of 70.89% in NO, 37.43% in SO₂, and 93.67% in CO compared to LPG. Additionally, CO₂ and CH₄ emissions were reduced by 45.07% and 85.89%, respectively.
• Celadon Quality: Celadon fired with methanol exhibited a more uniform and vivid glaze color, a smoother and glossier surface, and a denser microstructure with minimal bubble formation. Elemental analysis confirmed a compositional shift, with the K/O ratio increasing from 8.439% to 11.706%, the Fe/O ratio decreasing from 4.793% to 3.735%, the Al/O ratio decreasing from 33.445% to 31.696%, and the Si/O ratio increasing from 76.169% to 89.825%. These changes align with the characteristics of high-quality celadon, featured by lower iron and aluminum content and higher potassium and silicon content.
• Mechanical Properties: Bending stress analysis indicated that the mechanical properties of celadon produced using methanol and LPG were comparable.

Looking ahead, further research into alcohol fuels, such as methanol and ethanol, is essential for optimizing their utilization across various industrial applications. Specifically, the integration of machine learning techniques for data processing offers significant potential in enhancing combustion control, predicting system behavior, and improving fuel efficiency [48]. These advancements are poised to optimize the combustion process, reduce resource consumption, and minimize operational costs, thereby enabling more sustainable industrial practices.