Influence of Deposition Time and Location on the Pyrolysis Performance of Grease in Kitchen Flues

Abstract

In the high-temperature cooking process of Chinese-style catering, the oil fume accumulates on the inner wall of the flue during the cooling process, forming grease stains, which can easily trigger flue fires and cause a kitchen fire. Statistics indicate flue fires are a primary cause of kitchen fires in China. The changes in the composition of grease stains are due to different freezing points, which will adhere to different parts of the flue and be repeatedly heated and cooled if not cleaned in time. This leads to changes in combustion performance, subsequently affecting the progression of flue fire propagation. This paper takes grease deposits with different deposition times and locations in the flue of commercial kitchens as the research object. The research selected a medium-sized commercial kitchen flue (kitchen chimney) in Langfang City, with deposition times of the parts of the inlet and outlet for 2 months and grease in the inlet for a deposition time of 7 days, 60 days, and more than 1 year. This paper analyzed the grease deposits at different deposition positions at the flue inlet and outlet using a thermogravimetric analyzer and a gas-mass spectrometer. It is found that the primary components of the grease at the outlet have low molecular weight, thermal decomposition starting temperature ignition temperature, and activation energy in the first stage and will catch fire first; the grease at the inlet has a high comprehensive combustion performance, and the combustion is violent with little effect from the oxygen supply. Then, the pyrolysis analysis of grease stains located at the entrance of the flue is performed at different deposition times under air and nitrogen atmosphere. The results showed that the pyrolysis process of grease stains with a more than 1 year deposition time consists of two stages. One stage is the first weightlessness stage, which has the lowest activation energy, the longest combustion process, and the greatest fire risk; the other is the pyrolysis combustion process of grease stains with a deposition time of 7 days. Its activation energy is the highest, and the fire risk is the smallest. The research results can be a reference for the setting of the fire dampers and the cleaning time for the flue.

1. Introduction

Fires that involve commercial cooking equipment play a crucial role in China’s fire issues. Regardless of the type, each restaurant presents distinct fire hazards due to its cooking operations and the potential for large crowds of customers to assemble simultaneously [1]. Most accidental structural fires from commercial cooking appliances occur due to inadequate construction and maintenance practices. The Code for fire protection design of buildings in China outlines the construction and installation standards necessary to endure and extinguish a fire using kitchen fire suppression systems. The challenging truth is that commercial cooking areas continue to pose a significant fire risk due to insufficient construction clearances, poor maintenance of the extinguishing system, and the persistent buildup of grease vapor deposits.

During cooking, oils and fats change from a solid or semi-solid state into a liquid form. They then atomize and form grease-laden vapors that drain off as altered oils. The hot grease vapors mixed with evaporated fats and oils and solid particles gradually cool during exhaust to the outdoors via ducts and will be deposited on the inner wall of the duct, a substance known as grease. Grease is one of the most serious hazards to a commercial cooking area because it may be ignited when encountering flames or hot gases [2]. Over 55% of restaurant fires originate from commercial kitchens [3]. Therefore, the grease fire risk in restaurants should be studied.

Grease can also be regarded as biomass waste, which has a large share in the energy generation matrix. Ruoso, A.C. et al. [4] evaluated the parameters used in the manufacture of briquettes produced with forest residues and the economic engineering for the manufacturer. The parameters discussed in the paper include moisture content, bulk density, chemical analysis, calorific value, and electric energy consumption.

Grease is one of the special combustible materials in a specific limited space. Current research focuses on the harm kitchen cooking fumes pose to the personnel and environment and fume purification. The research on grease fires primarily includes the cause of the fire, the grease fire characteristics, and management measures. However, the pyrolysis and combustion characteristics of grease were less studied [5,6,7].

Omidghane M [8] and Pratt L M [9] investigated the pyrolysis characteristics of grease, with Omidghane M utilizing gas chromatography (GC) coupled with a mass spectrometer for peak identification and Pratt L M employing a Tech-Zoom 50 mL high-pressure reactor for the study. Based on the orthogonal test and the Large Eddy Simulation, GUO Zidong [10] used FDS to study exhaust hood fires under different wind velocities, cross-sections, grease thickness, and grease types. The study was based on the assumption that grease has a consistent composition and uniform thickness. In the commercial kitchen, because of the soot complex composition, different components of the freezing point will be attached to different flue parts in the grease formation process. Thus, the deposition thickness will be different, affecting the flue fire spread development process. Hence, the performance of the combustion flue grease in other locations is analyzed.

Since grease is not cleaned up after deposition, it will be heated and cooled repeatedly during cooking operations, causing changes in the sedimentary distribution and composition and the development of flue fires. In China, there is a requirement to clean food and beverage flues in densely populated places no less than once a quarter [11], and some provinces have previously recommended a two-month or one-quarter cleaning cycle [12,13]. According to the survey, there is a considerable difference in reality since cleaning time is not mandatory. The oily fume in large high-end restaurants is cleaned at least once every two months, and the thickness of flue grease at the inlet of the exhaust system is usually no more than 2 mm. Medium-sized and small restaurants are poorly equipped to clean the lampblack. About 30% of restaurants are cleaned once every six months, and about 40% of restaurants have not been cleaned. Grease has been deposited for more than 1 year, and the thickness of grease at the inlet of the exhaust system can reach more than 5 mm. About 30% of the restaurants clean once every half a year, and about 40% have not cleaned grease deposits for more than a year. At the inlet of the exhaust system, most oil dirt thickness can reach more than 5 mm. There is limited understanding of the impact of deposition duration on grease combustion performance. Therefore, it is worthwhile carrying out corresponding investigations.

The pyrolysis process is the subject of several works. The characteristics of expanded polystyrene (EPS) were comprehensively studied using thermogravimetry, cone calorimeter, and fame spread experiments, and its fire risk was characterized using the analytic hierarchy process method [14].

This research selected a medium-sized commercial kitchen flue (kitchen chimney) in Langfang City, whose deposition time of the parts of the inlet and outlet for 2 months and grease in the inlet to a deposition time of 7 days, 60 days, and more than 1 year. The thermogravimetric analysis method is used to compare and study the effect of the location and time of the deposition of pyrolysis performance grease. The objective is to determine how deposition time and location affect the spread of flue fire. This provides basic data on the position of the Chinese commercial kitchen fire damper setting, flue cleaning time, and fire protection design.

2. Experimental

2.1. Materials

To explore the effects of different deposition sites and times on the pyrolysis of performance grease, this study selected a medium-sized commercial kitchen restaurant in LangFang, which is involved in Chinese cooking. The experiment is conducted within the same flue of four different dirties attachment positions, sediment thickness, deposition time, and other characteristics appearance, as depicted in

Table 1. Characteristic parameters of cooked grease sample.

Sample	Attachment Position	Sedimentary Thickness/mm	Deposition Time/Day	Traits
1#	Flue outlet to the outside	2.1	60	Gray and black, low viscosity, low gloss
2#	Exhaust system inlet	1.5		Light brown, gummy solid, Poor light transmittance
3#		0.6	14	Yellow, viscous liquid, good light transmission
4#		3.5	>365	Black, gummy solid, opaque

. The schematic diagram of the flue and the sample position is shown in Figure 1. The sampling point at the flue inlet is located at the 0.3 m corner of the inlet. The sampling point at the outlet is within 0.2 m of the outlet.

2.2. Methods

Pyrolysis experiments are performed in the TG analyzer at atmospheric pressure using a differential scanning calorimetry (DSC) module under a nitrogen flow (60 mL/min), with a recording period of 1 s.

(1) TGA (Thermogravimetric Analysis). The TGA model is the Mettler Toledo TGA/SDTA851e. Thermogravimetric analysis studies the changes in the mass of the tested material with temperature to analyze the composition changes of the material, the oxidation cracking process, and the combustion cracking residues, etc. The TGA device is mainly composed of a microbalance, a heating furnace, a furnace temperature controller, a temperature monitor, and an atmosphere controller. The furnace temperature controller allows the temperature inside the furnace to rise according to the pre-set program. The furnace temperature is monitored by a thermocouple, and the sample quality is measured and recorded by a balance.

Place about 5.8 mg samples in a 70 μL Al₂O₃ crucible. In the experiments, the protective gas is high-purity nitrogen, with a flow rate of 30 mL/min. The reaction gas is air and nitrogen; their flow rate is 100 mL/min. They are heated from 100 °C to 800 °C with a rate of 20 °C/min.

(2) GC-MS (Chromatography-mass spectrometry). It is taken as a supplement to thermogravimetric analysis results. In this study, the organic components of 1# and 2# samples are analyzed by GC-MS. The GC-MS model is STAR 3400CX. A 0.2 g sample is dissolved in 5 ml of acetone solution and dehydrated by anhydrous sodium sulfate. The sample is put in the cracker. The sample temperature is 260 °C and the column temperature is 30–220 °C. The temperature is raised to 230 °C over 4 °C/min and maintains for 10 min. The carrier gas is high-purity helium.

3. Results and Discussion

3.1. Effects of the Deposition Site on Pyrolysis Performance Grease

3.1.1. TGA Results

Figure 2 and Figure 3 depict the pyrolysis characteristics of 1# and 2# sample curves in an air and nitrogen atmosphere. The weight loss procedures for both greases are divided into three stages under an air atmosphere [15,16,17,18]. The first stage is about 190–416 °C, mainly the volatilization of light components (such as acetaldehyde, glycolic acid, formic acid, and acetone hydroxy ketone) and degradation of part heavies. The second stage, at about 416–518 °C, is mainly the volatilization and cracking of the macromolecule compound. The third stage is the combustion of polymer fatty acids and carbon residue. Finally, the leftovers are light gray ash. Notably, the emergence of a small peak in the 2# sample pyrolysis combustion curve is only around 416 °C. This may be due to more intermediate products in the 2# Sample weight loss process, and weightlessness segmentation is not obvious [17].

In a nitrogen atmosphere, the weightlessness phase of Sample 1# can be categorized into two distinct stages, and the weightlessness process is consistent with the first two stages in an air atmosphere, which are the volatilization of light components, pyrolysis of heavy components, and the volatilization of macromolecular compounds. However, the first stage of the corresponding time lags relatively under the air atmosphere, which implies that the oxygen reaction has a certain promoting role in this stage. Then, the pyrolysis rate is the same as that in the atmosphere of air, which is due to the higher oxygen content of the pyrolysis at this phase [19]. With the increase in temperature, residue slowly decomposes under a nitrogen atmosphere. Weight loss of peak corresponding to what is in air atmosphere does not appear. Eventually, the residue is black coke, and the percentage of remaining mass is higher than in the air atmosphere. For Sample 2#, in a nitrogen atmosphere, the weightlessness process is one stage in which the small shoulder peak under the air atmosphere and oxidation peak in high-temperature conditions disappear. The entire process of weightlessness is continuous.

According to Figure 2 and Figure 3, the tangent method is used to obtain the initial decomposition temperature (T_s) of the main stage. The temperature is the end of the pyrolysis temperature when weight loss reaches 99%. A complete weight-loss peak corresponds to the temperature range of the decomposition temperature segment. According to the DTG in Figure 3, the eigenvalues of two samples are obtained in the pyrolysis process under an air and nitrogen atmosphere, as illustrated in

Table 2. Characteristic values of pyrolysis and combustion of cooked grease samples in different atmospheres.

Atmosphere	Sample	Ts/°C	Weight Loss Peak Temperature and the Corresponding Peak Traits		Decomposition Temperature and the Proportion of Total Weightlessness		Th/°C	X∞/%
			Temperature/°C	Peak/%·min−1	Temperature Segment/°C	Weightlessness Ratio/%
Air	1#	184	373	7.25	184–416	54.93	684	36.32
			467	5.72	416–518	29.17
			612	1.81	518–684	15.91
	2#	192	402	12.01	192–416	52.31	619	8.27
			438	16.17	416–518	41.51
			568	1.20	518–619	6.18
Nitrogen	1#	220	366	7.28	220–416	58.76	528	47.81
			467	5.43	416–525	33.07
	2#	235	438	13.3	235–530	80.65	518	19.65

Note: T_s is the main stage of initial decomposition temperature, T_h is the end of the decomposition temperature, and X∞ is the final percentage of remaining mass.

:

Table 2 depicts that the first two weight loss stages of Sample 2# under an air atmosphere and a nitrogen atmosphere are higher than that of Sample 1#, indicating that the reaction of Sample 2# is stronger during the pyrolysis process. The max weight loss peak of Sample 2# appearing in the second weight-loss stage is 16.17%/min, nearly twice that of Sample 1#, and the temperature appears 65 °C higher than that of Sample 1#. In the air, the conclusion can be drawn that the weight-loss process of the two samples is the same at every temperature segment of pyrolysis. In the nitrogen, there is a continuous weight-loss peak; thus, it cannot be compared in sections. The weight-loss temperature stages of the two samples are similar, but there is an obvious difference in the weight-loss percentage of each stage.

For a further explanation of this phenomenon, the components of two samples are analyzed by GC-MC. The results are shown in Figure 4 and Figure 5.

Table 3. GC-MS analysis results of cooked grease sample.

Sample	Residence Time/min	Compound Name	Molecular Formula	Molecular Weight	Percentage P%
					1#	2#
1	3.73	Caproaldehyde	C6H12O	100.16	1.56	6.41
2	4.49	Diacetone alcohol	C6H12O2	116.2	87.42	15.24
3	6.86	Caproic acid	C6H12O2	116.16	3.69	4.12
4	9.11	Nonanal	C9H18O	142.24	2.07	2.98
5	12.52	Trans-2,4-decadinenal	C10H16O	152.23	0.29	4.09
6	20.47	hexadecanoic acid (Palmitic acid)	C16H32O2	256.42	4.97	67.16

presents the component analysis.

Table 2 displays that two samples have the same chemical components. The organics are primarily organic acids, aldehydes, ketones, and lipids, but the percentages vary significantly. The major component of Sample 1# at the flue inlet is hexadecanoic acid (palmitic acid), which has a molecular weight of 256.42 and accounts for 67.16%. However, the major component of Sample 2# at the flue outlet is diacetone alcohol, which has a molecular weight of 116.2 and accounts for 87.42%

The primary components of Samples 1# and 2# are identical, resulting in similar phenomena in the same temperature range of thermal gravimetric analysis, as shown in Table 3. In Sample 1#, the higher percentage of light components (87.42%) results in a greater proportion of weight loss in the low-temperature segment, whereas Sample 2#, with a higher percentage of heavy components, exhibits the opposite trend. The law of smoke movement indicates that lighter components accumulate more at the chimney outlet. This is likely due to their lower gasification points, which allow them to condense for an extended period during the ascent and cooling of oil fumes.

In nitrogen and air atmospheres, the residual mass ratios after pyrolysis are 47.81% and 36.32%, respectively, for Sample 1#, higher than 19.65% and 8.27% for Sample 2#. In the air atmosphere, the burnout temperature of Sample 1# is higher, while under the nitrogen atmosphere, the end pyrolysis temperature differs from that of Sample 2#. This is probably because Sample 1# has more impurities, such as dust and combustibles, which slowly burn, leading to a higher burnout temperature observed in the last weight-loss temperature segment and the last weight-loss percentage.

3.1.2. Pyrolysis Kinetic Analysis

The Arrhenius rate equation: When the sample mass increases from the initial mass $m_0$ to $m$ at a certain time $t$, its decomposition rate under the control of the temperature rising can be expressed as follows [20]:

$$d\left(\alpha \right)/dt=kf\left(\alpha \right)$$

(1)

where $k=A\mathit{exp}(-E/RT)$; $f\left(\alpha \right)=(1-\alpha {)}^n$; $\alpha ={\displaystyle \frac{m_0-m}{m_0-m_{\mathrm{\infty }}}}\times 100\%$

$$\beta =dT/dt$$

(2)

Combining Equations (1) and (2) together provides:

$$d(\alpha )/f(\alpha )={\displaystyle \frac{A}{\beta }}\mathit{exp}(-E/RT)dT$$

(3)

Integrating Equation (3), letting $g\left(\alpha \right)={\int }_0^{\alpha }d\left(\alpha \right)/f\left(\alpha \right)$

Hence:

$$g(\alpha )={\int }_{T_0}^T{\displaystyle \frac{A}{\beta }}\mathit{exp}(-E/RT)dT$$

(4)

where m_∞ is the final mass of the sample, mg; m is the percentage of the material weightlessness at t time; A is the frequency factor, min⁻¹; E is the activation energy, kJ/mol; R is the universal gas constant, 8.31 × 10⁻³ kJ/(mol·k); β is the heating rate, 20 °C/min in this study; T is the reaction temperature, K.

Using the Coats–Redfern integral method to approximately derive temperature integral, the integral equation can be drawn as follows:

$$\mathit{ln}[{\displaystyle \frac{g\left(\alpha \right)}{T^2}}]=\mathit{ln}\{{\displaystyle \frac{AR}{\beta E}}[1-{\displaystyle \frac{2RT}{E}}]\}-{\displaystyle \frac{E}{RT}}$$

(5)

In the formula, ${\displaystyle \frac{2RT}{E}}\ll 1$, for the correct g(α) form, the graph of $\mathit{ln}[{\displaystyle \frac{g\left(\alpha \right)}{T^2}}]$ with respect to $1/T$ should be a straight line with a slope of $-E/R$ and the intercept containing the frequency factor $A$.

Table 4. Common solid pyrolysis reaction models and g(α) expression [20,21,22].

Reaction Models			$\mathit{g}\left(\mathsf{\alpha }\right)$
N order simple model	$n=1$	Number	$-\mathit{ln}(1-\alpha )$
	$n\ne 1$	F1	$[{\displaystyle \frac{1-(1-\alpha {)}^{1-n}}{(1-n)}}]$
Contracting cylinder		R2	$1-(1-\alpha {)}^{{\displaystyle \frac{1}{2}}}$
Contracting sphere		R3	$1-(1-\alpha {)}^{{\displaystyle \frac{1}{3}}}$
One-dimensional diffusion		D1	${\alpha }^2$
Two-dimensional diffusion		D2	$(1-\alpha )\mathit{ln}(1-\alpha )+\alpha$
Three-dimensional diffusion (Jander)		D3	${\left[1-(1-\alpha {)}^{{\displaystyle \frac{1}{3}}}\right]}^2$
Three-dimensional diffusion (Ginstling-Brounshein)		D4	$(1-{\displaystyle \frac{2\alpha }{3}})-(1-\alpha {)}^{{\displaystyle \frac{2}{3}}}$

Note: To apply the method, four different temperature heating rates (β), as described above, were chosen.

depicts several common $g\left(\alpha \right)$ used to study solid reaction kinetics.

The temperature ranges of weight loss in higher and lower ranges are regarded as individual reactions, according to the kinetic analysis of the three samples. Equation (5) is used to determine the temperature range of the entire weight-loss process to build the kinetics mode.

As described in

Table 5. Pyrolysis and combustion kinetic parameters during the main reaction stage in air and N₂ atmosphere of cooked grease samples.

Atmosphere	Sample	Temperature/°C	Activation Energy E/kJ·mol−1	Frequency Factor A/mol−1	Goodness of Fit r	$\mathit{g}\left(\mathsf{\alpha }\right)$
air	1#	184–416	77.06	3.18 × 1002	0.993	${\alpha }^2$
		416–518	239.98	1.00 × 1014	0.997	$-\mathit{ln}(1-\alpha )$
		518–684	150.81	3.94 × 105	0.992
	2#	192–416	139.29	1.98 × 107	0.991	${\alpha }^2$
		416–518	125.88	6.29 × 105	0.992	$-\mathit{ln}(1-\alpha )$
		518–619	227.11	6.80 × 1010	0.995
N2	1#	235–416	156.09	1.90 × 107	0.990	$-\mathit{ln}(1-\alpha )$
		416–532	201.23	2.37 × 1014	0.994	$2\left[\right(1-\alpha {)}^{-0.5}-1]$
	2#	220–416	125.54	3.51 × 108	0.997	$-\mathit{ln}(1-\alpha )$
		416–525	267.57	1.04 × 1016	0.995	$2\left[\right(1-\alpha {)}^{-0.5}-1]$

, the pyrolysis and combustion kinetic model of grease from different positions in the flue can be described as follows:

$$g\left(\alpha \right)=[-\mathit{ln}(1-\alpha ){]}^m{\alpha }^{2n}\left[2\right(1-\alpha {)}^{-0.5}-2{]}^k$$

(6)

where $m$, $n$, and $k$ values are 0 or 1; we can get different pyrolysis and combustion kinetics models of grease.

The calculation results showed that during the pyrolysis process under an air atmosphere, the activation energy of Sample 1# at the first stage is minimum, i.e., 77.06 kJ/mol, and they are most easily ignited in case of fire. The activation energy of the second stage is significantly higher than that of other stages.

For Sample 2#, the activation energy of the first combustion stage is low, and they are easy to ignite because the molecular weight of their major components is lower than that of Sample 1#. When comparing the activation energy in air and nitrogen, it is evident that oxygen has a substantial impact on the first stage of Sample 2#, while its influence on the subsequent stages is considerably diminished. This discrepancy may be attributed to the high oxygen levels present in their composition. This indicates that decomposition will occur within the relatively closed flue, even in the absence of oxygen during the burning process [19].

3.2. The Influence of Deposition Time on Grease Pyrolysis Performance

Figure 6 and Figure 7 depict the pyrolysis curves of Samples 2#, 3#, and 4# in air and N₂ atmospheres.

3.2.1. Pyrolysis Process Analysis of Samples

Figure 7 also presents the weight loss of three samples, mainly between 150 °C and 600 °C in air. In the nitrogen atmosphere, the subject weight loss of three samples occurred in the temperature range of 150–540 °C. With the increase in deposition time, the number of weight loss peaks and the temperature range of weight loss gradually increase, the maximum value of the weight loss peak decreases, and finally, the percentage of residual mass increases.

The analysis of Figure 2 and Figure 3 and Table 2 indicates that three samples have the same composition; however, the percentages of each ingredient vary due to differing deposition times.

The combustion characteristic values of the two samples are derived using the tangent method, as illustrated in Figure 7.

Table 6. Characteristic values of pyrolysis and combustion of cooked grease samples in different atmospheres.

Environment	Sample	Ts/°C	The Weight-Loss Peak		The Weight-Loss Proportion of Each Temp Stage		Th/°C	X∞/%
			Temp/°C	Peak Value/%·min−1	The Range of Temp/°C	The Percentage of Total Weight Loss/%
Air	2#	232	402	12.01	192–416	52.31	593	2.18
			438	16.17	416–518	41.51
			568	1.20	518–619	6.18
	3#	286	438	30.1	286–496	98.11	496	1.07
	4#	128	395	12.61	128–438	58.77	561	9.64
			467	5.81	438–561	20.90
	2#	268	414	15.37	268–515	90.19	515	13.32
Nitrogen	3#	336	438	29.28	339–496	99.80	467	2.15
	4#	148	396	12.42	148–438	75.04	532	22.85
			467	5.72	438–525	20.49

Note: T_s is the main stage of initial decomposition temperature, T_h is the end of the decomposition temperature, and X∞ is the final percentage of remaining mass.

presents the results.

Table 4 illustrates that as deposition time increases, the primary weight-loss process changes from one phase to three phases and then reverts to two phases. The initial weight-loss temperature, ignition temperature, maximum value of the weight-loss peak, and final temperature during the weight-loss phase decrease progressively; conversely, the main weight-loss end temperature and residual mass increase steadily.

During the pyrolysis in the nitrogen atmosphere, compared to the combustion process, the number of weight-loss phases of Sample 2# becomes 1, but the tendency of the two other samples does not have an obvious change. The three samples show an increase in the initial decomposition temperature; however, the decomposition temperature after the main stage decreases. Notably, the maximum value and temperature of the weight loss peak remain unchanged.

The conclusion can be drawn that during the long-term repeated heating and cooling process, macromolecule components of flue oil split into lighter molecules or combine again for new polymers. The main ingredients of grease change from macromolecules to lighter molecules, and the macromolecule content decreases, making the ingredients of grease complex. With the increase in deposition time, the content of lighter molecules, dust, and impurities increases, and the initial decomposition temperature decreases. During the burning in the air atmosphere, the ignition temperature of grease decreases to be ignited easily with the increase of deposition time. The effect of oxygen on the oil weight-loss process first increases and then decreases. The cause of the effect could be that the grease has a lower oxygen content [18]. Macromolecules and intermediate products cannot be oxidized completely in a short time. In other words, when the deposition time is long enough, the demand for oxygen for burning grease in the flue decreases, with the fire risk increasing. Therefore, the inner grease should be cleaned regularly to prevent flue fire.

3.2.2. Analysis of Pyrolysis and Combustion Dynamics

The weight loss process of grease under different atmospheres was fitted and calculated using different reaction models g(α). Figure 8 illustrates the comparison of the two primary pyrolysis stages of sample 3# in air across different models. In this figure, n = 1 − D4 represents 11 distinct reaction models. Similar calculations were conducted for the weight loss data of other samples, yielding comparable curve trends. Based on the fitting results, the g(α) model with the highest fitting degree (closest to 1) during the pyrolysis and combustion processes of the samples was identified. Figure 9 displays the curves corresponding to the maximum fitting degree for the three samples during the pyrolysis and combustion processes, with all fitting degrees exceeding 0.98.

Here, g(α) with different function types [20] is applied to fit the oil-weight-less processes under different environments. The g(α) fitting close to 1 is adopted.

Table 7. Pyrolysis and combustion kinetic parameters during the main reaction stage in air and N2 atmosphere of cooked grease samples.

Environment	Sample	Sample	Activation Energy E (kJ/mol)	Frequency Factor A (1/mol)	Fitting Degree r	$\mathit{g}\left(\mathsf{\alpha }\right)$
Air	1#	286–496	237.84	1.70 × 1013	0.99323	${\left[1-(1-\alpha {)}^{{\displaystyle \frac{1}{3}}}\right]}^2$
	2#	228–377	99.81	1.92 × 105	0.99167	$-\mathit{ln}(1-\alpha )$
		382–485	245.96	3.92 × 1015	0.99527	${\displaystyle \frac{\alpha }{1-\alpha }}$
		492–583	446.10	7.63 × 1026	0.98685	${\displaystyle \frac{\alpha }{1-\alpha }}$
	3#	148–438	72.87	9.93 × 105	0.99259	${\alpha }^2$
		445–532	360.99	6.67 × 1022	0.98989	${\displaystyle \frac{\alpha }{1-\alpha }}$
Nitrogen	1#	344–481	311.20	7.83 × 1018	0.98389	${\left[1-(1-\alpha {)}^{{\displaystyle \frac{1}{3}}}\right]}^2$
	2#	268–515	139.89	1.56 × 106	0.99532	${\left[1-(1-\alpha {)}^{{\displaystyle \frac{1}{3}}}\right]}^2$
	3#	150–438	77.51	1.55 × 106	0.98054	${\alpha }^2$
		440–525	364.68	3.85 × 1024	0.99478	${\displaystyle \frac{\alpha }{1-\alpha }}$

shows that the pyrolysis and combustion dynamical parameters of the three samples at the main reaction phase are gained by fitting calculations.

Based on the data presented in Table 3, the pyrolysis and combustion reaction of Sample 1#, which has the shortest deposition time, can be described by one-dimensional diffusion model D3, and the reaction function adapts. The pyrolysis and combustion reaction of Sample 3#, which has the longest deposition time, can be described by one-dimensional diffusion model D1 and two-dimensional simple model O2. The lower temperature phase of the reaction function adapts and the higher temp phase of the reaction function adapts. One-dimensional simple model O1 can describe the combustion model of Sample 2#, two-dimensional simple model O2, and two-dimensional simple model O2. The pyrolysis model is a one-dimensional diffusion model D3; the reaction function corresponding with the model changes significantly.

Moreover, the pyrolysis and combustion activation energy of Sample 1# are highest at 311.20 kJ/mol and 237.84 kJ/mol. Sample 1# required the highest activation energy for pyrolysis and combustion, indicating superior stability. The pyrolysis and combustion activation energy of Sample 2# are 139.89 kJ/mol and 99.81 kJ/mol, respectively. In contrast, Sample 3# showed the lowest energy requirements, suggesting a higher propensity for thermal degradation and combustion. The pyrolysis and combustion activation energy of Sample 3# at the first phase is 77.51 kJ/mol and 72.87 kJ/mol, and they decreased by 75.1% and 69.4%, respectively. The activation energy of Sample 1# during the process of burning is lower than that during the process of pyrolysis in nitrogen, which confirms that the oxygen in the air decreases the activation energy and promotes reactions. As for Sample 2#, the activation energy of the first burning phase is the lowest, and the energies of the last two phases are lower than that during the pyrolysis process in the nitrogen, confirming that the oxygen promotes the first phase of the reaction and inhibits reactions during the last two phases. As for sample 3#, the activation energies of the two phases have little difference, confirming that oxygen in the air has little impact on the reaction process of sample 3# with a long deposition time. The fire risk of flue oil increases with the increase in time. Different pyrolysis characteristics will lead to different combustion behaviors and, subsequently, different thermal radiation behaviors. The next step can be to study its thermal radiation characteristics according to the method of Caetano N R [23] to further understand the spread mechanism of different types of grease in the flue.

4. Conclusions

This study used thermogravimetry to examine the pyrolysis and combustion processes of flue oil, considering various deposition times in a commercial kitchen setting. The conclusions are as follows.

(1) In the long-term process of repeatedly heating and cooling, the primary ingredients of grease change include the range of macromolecules to lighter molecules. The macromolecule content decreases, and grease ingredients become complex. The content of dust and impurities increases, and the initial decomposition and ignition temperature decrease.

(2) The oil with various deposition times has different pyrolysis and combustion mechanisms. The pyrolysis and combustion reaction of Sample 1# with a deposition time of 7 days can be described by one-dimensional diffusion model D3. The combustion reaction of Sample 2#, which has a deposition time of about 60 days, can be described by one-dimensional simple model O1, two-dimensional simple model O2, and two-dimensional simple model O2. The pyrolysis model is a one-dimensional diffusion model D3 where the reaction function corresponding with the model changes significantly. The pyrolysis and combustion reaction of sample 3#, which has a deposition time of over 365 days, can be described by one-dimensional diffusion model D1 and two-dimensional simple model O2.

(3) The longer the grease deposition time, the lower the activation energy at the burning process. The activation energy of grease with a deposition time over 365 days is 72.87 kJ/mol. The activation energy is reduced by 70% compared to the grease, with a deposition time of 7 days. The effect of oxygen in the air on the thermal degradation process decreases gradually when deposition time reaches a certain level.

(4) The fire risk of flue oil increases with time. Therefore, the inner grease should be cleaned regularly to prevent flue fire.