The Model and Burner Development for Crude Glycerol and Used Vegetable Mixing: Cube Mushroom Steaming Oven

Abstract

Reducing fuel costs, maximizing waste utilization, and improving energy efficiency are critical challenges in agricultural thermal processes. This study addresses these issues by developing and evaluating a mixed-fuel burner and furnace system for steaming mushroom substrate cubes using crude glycerol and recycled vegetable oil as low-cost alternative energy sources. The experimental investigation assessed boiler thermal efficiency, combustion efficiency, exhaust-gas composition, temperature distribution, steam generation, and combustion-gas dispersion within the furnace. In parallel, analytical modeling of pressure, temperature, and gas-flow behavior was performed to validate the experimental observations. Five fuel compositions were examined, including 100% used vegetable oil, 100% crude glycerol, and blended ratios of 50/50, 25/75, and 10/90 (glycerol/vegetable oil), with all tests conducted in accordance with DIN EN 203-1 standards. The results demonstrate that blending used vegetable oil with glycerol significantly improves flame stability, increases peak combustion temperatures, and suppresses incomplete-combustion byproducts compared with pure glycerol operation. Combustion efficiencies of 90–99% and boiler thermal efficiencies of 72–73% were achieved. Among the tested fuels, the optimal balance between combustion stability, efficiency, and cost was achieved with a 25% glycerol and 75% used vegetable oil mixture. Economic analysis revealed that the proposed mixed-fuel system offers superior viability compared with LPG, reducing annual fuel costs by approximately 50%, shortening steaming time by 2 h per batch, and achieving a payback period of only 3.26 months. These findings confirm the feasibility of the proposed waste-to-energy system for small- and medium-scale agricultural applications. To further enhance sustainability and renewable fuel utilization, future work should focus on improving air–fuel mixing for higher glycerol fractions, scaling the system for larger farms, and extending its application to other agricultural thermal processes.

1. Introduction

Households and industrial activities generate substantial quantities of waste products, notably crude glycerol and used cooking oil, particularly from biodiesel production and food-processing sectors. Effective management and utilization of these wastes require a clear understanding of their physicochemical properties, potential applications, and environmental impacts. Improper disposal of such materials can result in soil and water contamination, whereas their reuse as energy resources offers an opportunity to reduce environmental burdens while improving energy efficiency.

Crude glycerol, a byproduct of biodiesel transesterification, is a viscous, colorless, and hygroscopic liquid containing impurities such as methanol, soaps, free fatty acids, and residual catalysts. These contaminants limit its direct industrial applicability and often necessitate further treatment. Inadequate disposal or uncontrolled utilization of crude glycerol can lead to environmental degradation. Conversely, recycling and repurposing this byproduct help reduce waste and protect the environment. Similarly, used vegetable oil is generated daily by residential, commercial, and industrial kitchens. Its disposal is complicated by food residues, free fatty acids, and other contaminants, making proper collection and management essential to prevent illegal dumping and environmental pollution.

Crude glycerol and used vegetable oil are therefore valuable secondary resources with significant potential for sustainable energy applications. Their efficient utilization supports circular economy principles by transforming waste streams into useful energy carriers. Although numerous studies have examined the chemical upgrading, purification, and engine-based utilization of glycerol and waste oils, their direct application as fuels in small-scale agricultural heating systems remains limited.

Reed et al. [1] investigated M-Diesel as a diesel substitute derived from waste vegetable oils. Their study demonstrated that recycling waste oils reduces both fuel cost and viscosity compared with conventional vegetable waste oil. Çetinkaya et al. [2] examined biodiesel production from used cooking oil using base-catalyzed transesterification. Their work optimized reaction and purification parameters and identified an effective method for biodiesel synthesis. Kaminski et al. [3] studied the selective oxidation of glycerol using copper–gold catalysts supported on mesoporous cerium–zirconium oxide. The results showed that bimetallic catalysts exhibited higher oxidation activity and selectivity toward glyceric acid than monometallic catalysts. Roubianto et al. [4] evaluated a fuel blend consisting of biodiesel, glycerol, and methanol in an external combustion engine. Their findings highlighted the positive role of methanol in improving combustion performance. Samanta et al. [5] assessed the performance and emissions of an unmodified diesel engine fueled with biodiesel from waste cooking oil. The results indicated that biodiesel can reduce emissions associated with climate change and global warming. Jensani et al. [6] demonstrated that co-digestion of food waste and crude glycerol enhances biogas production, while simultaneously treating both waste streams. Elgharbawy et al. [7] investigated low-cost biodiesel production from waste feedstocks. It is shown that frying oil can significantly reduce production costs, despite its high free fatty acid content. Kurdi et al. [8] synthesized biodiesel from waste cooking oil through transesterification using methanol and hydrochloric acid. Their study evaluated the resulting biodiesel properties to confirm its energy potential. Lima et al. [9] reviewed chemical and biological pathways for converting residual glycerol into value-added products, emphasizing the importance of catalyst selection. Kumar et al. [10] reported that intermittent feeding of municipal sludge combined with glycerol increased biomass and lipid production, resulting in an energy-positive biodiesel process.

Several studies have focused on upgrading and purifying crude glycerol. Chilakamarry et al. [11] highlighted glycerol’s potential as a future fuel and emphasized the need for safe and innovative applications for surplus glycerol. Syahputra et al. [12] and Rizky et al. [13] investigated glycerol derived from waste cooking oil biodiesel and examined purification techniques to improve glycerol quality. Moklis et al. [14] reviewed current and emerging methods for converting crude glycerol into higher-value products, noting that impurities increase processing costs.

Used cooking oil (UCO) satisfied the SNI 01-3741-2002 [15] quality standards for aroma, flavor, visual color, moisture content, free fatty acids, acid value, and peroxide value, according to Nugroho et al. [16]. This infraction shows that UCO was detrimental. 81.33% was the greatest yield of UCO biodiesel, while 61.7% was the lowest. Glycerol accounted for 0.72%, while FAME accounted for 98.18%. Kumar et al. [17] provided important information on burning used oil. Initial research showed that waste oil needs three to five minutes to catch fire, and when mixed with kerosene, it burns at 300 °C. The small burner and raised stove prevented the flame from reaching the upper layer, so the first try failed. Samadov et al. [18] investigated how to improve the quality, appearance, stability, and safety of vegetable oil. They did this by removing free fatty acids, phospholipids, waxes, pigments, and other impurities. Armylisas et al. [19] found that biodiesel and oleochemical factories produce crude glycerol (CG) and glycerol phosphate (GP), both derived from palm oil. These chemicals have many different colors and properties. All samples had the same glycerol concentration of 87.3%. Parmar et al. [20] prepared groundnut shell cubes containing 20, 40, or 60% crude glycerol by weight. Biodiesel-derived crude glycerol was used to improve biomass control and energy density. The pellet’s volatile matter went up from 72.45% to 85.18% when glycerol was added. According to Dhabhai et al. [21], the demand for biodiesel has led to increased production of glycerol. Free fatty acids, metal ions, water, and methanol are some of the impurities that are found in crude glycerol. These impurities reduce the economic value of glycerol, rendering it unsuitable for direct use. Pamanes et al. [22] reported that ruminal fermentation was well maintained, and there were no noticeable differences between treatments during the adaptation phase. Compared to the control, all treatments boosted gas output, which is a good thing. At 20 g/kg and 40 g/kg, the study showed that maize oil reduced methane (CH₄) emissions by 7% and 9.5%, respectively, converting them into carbon dioxide (CO₂). Wu et al. [23] told biodiesel companies that they could make their businesses more profitable and more environmentally friendly by using FAME and glycerol. Make epoxidized acyl glycerides more valuable. The process worked best at 200 °C and with a 1:1 ratio of FAME to glycerol. About half of the glycerides produced were monoacyl, 40% were diacyl, and 10% were triacyl.

Ion-exchange resins were used by Borowka et al. [24] to clean up biodiesel-derived glycerin. Glycerin is made from biodiesel derived from vegetable feedstocks with very little sulfur, chlorine, or nitrogen. Glycerine made from used cooking oils, vegetable oils, and animal fats contains small amounts of chlorine but many sulfur and nitrogen molecules. The crude glycerol cleaned up by Silva et al. [25] was more than 98% by weight. Because it was so pure, it was great for etherification. We used pure and commercial glycerol in a batch reactor that did not require any solvents for the etherification of ethanol, isopropanol, and 3-methyl-1-butanol. Rahman et al. [26] showed that bioethanol produced from crude glycerol (CG) generated as a byproduct of biodiesel production can serve as a long-term energy source. It is challenging to regulate glycerol production while also supporting renewable fuels. Bioethanol can be made from crude glycerol, a byproduct of biofuel production. The esterification of crude glycerol and acetic acid using biomass-derived carbon catalysts was investigated by Perez et al. [27]. We required monoacetyl, diacetyl, and triacetyl glycerol. Producing PC-F and APC-F catalysts from peanut shells (PC), activated with potassium hydroxide (APC), and functionalized with sulfuric acid (H₂SO₄). Evaluating functionalized catalysts in crude glycerol. Sidhu et al. [28] examined the emulsification of water and glycerin to reduce biodiesel combustion emissions and to valorize them. Water and glycerin emulsion fuels significantly reduce smoke emissions. Emulsions containing 10% glycerin or water reduced smoke emissions by nearly 50% compared with pure biodiesel. Bansod et al. [29] conducted a cradle-to-gate life cycle assessment (LCA) to assess three technologies for crude glycerol purification and to determine influential factors and mitigation methods. The LCA evaluated the crude glycerol purification methods of PMP, VDP, and IEP. We assessed a functional unit of 1000 kg of pure glycerol. The processes exhibited carbon footprints of 3466.82 kg CO₂ equivalent per functional unit, 1745.72 kg, and 2239.71 kg, respectively.

Roschat et al. [30] showed that activated carbon from Krabok (Irvingia malayana) seed shells, when used with crude glycerol, enables biodiesel production. Krabok seed shells were converted to activated carbon (KC/AC, two-step) using a new biomass burner and dry chemical activation with NaOH. The KC/AC two-step had a surface area of 758.72 m²/g and an iodine number of 611.10 mg. In reaction to the growing supply of biodiesel and demand for alternative fuels, Agrawal et al. [31] looked into how long solketal synthesis from glycerol could last. Cosmetic and pharmaceutical companies rarely use biodiesel glycerol. The project is exploring sustainable ways of converting excess glycerol into valuable products. Glycerol is used to make Solketal, which is added to gasoline. To make high-quality glycerol, Ruzibayev et al. [32] purified crude glycerol without distillation. There were cations used. The best results for glycerol and ash were obtained with a 70:30 mix of activated carbon and clay. To reduce the use of fossil fuels and protect the environment, Pilicita et al. [33] investigated the use of used cooking oil as a sustainable biodiesel option. They also looked into ways to improve its properties and engine performance. PUFA-to-PUFA ratios of 5:1 to 10:1 were the goal of Naumova et al. [34] when they mixed veggie oils. Several quality measures for veggie oils were examined. Researchers examined the polyunsaturated fatty acid (PUFA) components of these oils. The oil blend was found using math.

A clear research gap emerges when comparing the present study with the existing literature: most published works on crude glycerol and used vegetable oil focus on fuel upgrading, transesterification, purification, catalytic conversion, engine combustion, emissions, or biochemical valorization, rather than on direct thermal utilization in small-scale agricultural heating systems. Prior studies predominantly investigate glycerol as a byproduct to be refined into higher-value chemicals or biofuels, or evaluate waste-cooking-oil-derived biodiesel in diesel engines under laboratory or automotive conditions. Minimal attention has been given to the design, modeling, and experimental validation of a dedicated mixed-fuel burner–boiler system that can stably combust raw (unrefined) crude glycerol blended with used vegetable oil without prior chemical processing. Moreover, existing studies rarely integrate thermodynamic modeling of pressure, temperature, and species distribution within the furnace with standardized performance testing (DIN EN 203-1 [35]) and field agricultural applications, such as mushroom substrate steaming. The present study fills this gap by demonstrating, for the first time, a practical waste-to-energy combustion system that couples analytical combustion modeling, controlled experiments, exhaust-gas characterization, and economic feasibility analysis, thereby bridging the disconnect between laboratory-scale fuel studies and deployable, low-cost renewable energy solutions for small and medium agricultural producers.

We respectfully clarify that the novelty of this work lies not in the mere use of crude glycerol or used vegetable oil as alternative fuels, but in the integrated scientific–engineering demonstration of their direct, stable, and efficient combustion in a purpose-built agricultural heating system, which has not been adequately addressed in the existing literature. From a scientific perspective, this study provides a mechanistic explanation of how the oil–glycerol blending ratio governs combustion efficiency and incomplete-combustion pathways through coupled effects of atomization, evaporation, flame temperature, oxygen availability, and reaction kinetics. Unlike previous studies that focus on glycerol upgrading, purification, engine combustion, or catalytic conversion, the present work systematically combines chemical-equilibrium-based analytical modeling with experimentally measured temperature, pressure, and exhaust-gas species distributions to identify and explain a synergistic combustion regime at intermediate blending ratios (≈25% glycerol/75% used vegetable oil). This mechanistic insight explains why specific blends suppress CO, H₂, and CH₄ formation while maintaining high combustion efficiency, thereby advancing fundamental understanding of low-calorific, oxygenated waste-fuel combustion under practical furnace conditions.

From an engineering perspective, the novelty resides in the design, construction, and validation of a dedicated mixed-fuel burner–boiler system capable of stably combusting raw (unrefined) crude glycerol blended with used vegetable oil, without chemical pretreatment or fuel upgrading. To the best of the author’s knowledge, prior studies have not demonstrated a DIN EN 203-1-compliant [35] combustion system that integrates burner design, analytical modeling, experimental validation, exhaust-gas characterization, and real agricultural operation (mushroom substrate steaming). Furthermore, this study uniquely links combustion behavior to thermal efficiency, process productivity, and techno-economic performance, demonstrating reduced fuel consumption, shorter steaming time, and a rapid payback period compared with LPG.

2. Experimental Apparatus and Procedure

2.1. Experimental System

The schematic design illustrates the complete experimental furnace system, detailing the configuration and function of each principal component (Figure 1). The glycerol storage tank supplies fuel to the combustion chamber through the glycerol pipe. At the same time, the blower delivers air for atomization and combustion via the air pipeline. The combustion of fuel emits hot gases into the atmosphere from the combustion chamber situated beneath the chimney. During operation, the flue gas composition may be measured at designated exhaust gas suction sites within the chimney. Temperatures can be monitored within the chimney, adjacent to the exhaust area, and throughout the fuel–air section with three thermocouples: thermocouple 1, thermocouple 2, and thermocouple 3. The temperature readings from these thermocouples are consistently shown using a control and LCD thermal indicator system. The gate ball valve and gate valve regulate the flow of fuel and air into the combustion chamber to maintain stable combustion. This experimental configuration aims to evaluate the furnace’s efficacy. It consolidates fuel supply, airflow regulation, combustion, temperature surveillance, and exhaust analysis into a unified system. Figure 2 illustrates multiple locations (top and bottom) where gas is gathered. Subsequently, the Shimadzu GC-2014 gas chromatography (GC) analyzer (Kyoto, Japan), equipped with five sets and two cylinders, is employed to evaluate various fuel mixture ratios and determine the composition of the combustion gases. Additionally, three sets of Type K 1100 °C ceramics, a stainless-steel rod thermocouple, and a data recorder with a PLC-controlled display are employed to measure the gas temperature using the Omron E5CS-Q1KJX-F temperature controller (Kyoto, Japan). The UT360 series handheld anemometers quantify the air intake rate.

2.2. Standard Test

Combustion efficiency and thermal efficiency are the primary metrics employed to assess the efficacy of the combustion stove. This study employs testing methodologies that include combustion efficiency testing, thermal efficiency testing, and exhaust gas quality assessment, in accordance with DIN EN 203-1 [35].

2.2.1. Combustion Efficiency Test

A typical flue-gas collection cowl was positioned above the burner for five minutes to conduct the combustion efficiency test. A gas probe was implanted, and the stabilized O₂, CO₂, and CO readings were recorded at one-minute intervals for 15 min. The flow velocity of waste oil and the measurement heights within the combustion chamber were subsequently altered to examine variations in exhaust gas composition. The crank efficiency, derived from the heat generated by fuel combustion and the heat dissipated owing to carbon monoxide production, was computed from the gathered data.

$${\eta }_C=\left[{\displaystyle \frac{Q_{fuel}-Q_{loss,co}}{Q_{fuel}}}\right]\times 100\%$$

(1)

where ${\eta }_C$ is combustion efficiency, $Q_{fuel}$ is heat from fuel combustion, and $Q_{loss,co}$ is heat loss.

2.2.2. Thermal Efficiency Test

The water boiling test (WBT) was employed to assess thermal efficiency. The procedure involved measuring the initial water temperature, adding 3000 g of water to a container, and positioning a thermocouple 5 cm above the water’s surface. The water, initially at 25 °C, was monitored every minute until it reached approximately 95 °C, the boiling point. The time at which boiling commenced was noted, and heating continued for an additional 5 min before the burner was deactivated and the final temperature was documented. The burner was started with used vegetable oil and allowed to stabilize for five minutes before the prepared container was positioned on it. Subsequently, the residual water was weighed. The gathered data were utilized to ascertain the stove’s thermal efficiency. The data comprised the following elements: water mass, specific heat (4.17 J/g°C), fuel mass flow rate, latent heat of vaporization (2260 J/g), temperature differential, lower heating value of the utilized oil, and mass of evaporated steam.

$${\eta }_{TH}=\left[{\displaystyle \frac{\left(C_{P,w}\times W_w\times \Delta T\right)+\left(h_{fg}\times W_w\right)}{m_{fuel}\left(LH{V}_{fuel}\times t_{tot}\right)}}\right]\times 100\%$$

(2)

where ${\eta }_{TH}$ is thermal efficiency; $W_w$ is water weight, gram; $C_{p,w}$ is the specific heat of water, kJ/(kg °C); $m_{fuel}$ is the mass flow rate of used oil, kg/s; $h_{fg,vapor}$ is the latent heat of vaporization, kJ/kg; $\Delta T$ is the temperature difference between the start and final test state, K; $t_{tot}$ is testing time, s; $LH{V}_{fuel}$ is the lower heat value of fuel, kJ/kg; and $W_w$ is water weight, kg.

2.3. Experimental Procedure

This work employs test techniques using both fresh and used vegetable oils, as well as waste glycerol (Figure 3 and Figure 4). Five distinct fuel conditions were developed and assessed to evaluate the furnace’s performance. The conditions comprised 100% used vegetable oil, 100% waste glycerol, and combinations of 50% used vegetable oil with 25% waste glycerol. The aggregate capacity for each fuel condition was 7 L.

Table 1. Fuel ratios used in the testing.

No.	Fuels	Ratios (%)
1	Upper; used vegetable oil	100
2	Lower; used vegetable oil	100
3	Upper; oil mixture (used vegetable oil/waste glycerol)	90:10
4	Lower; oil mixture (used vegetable oil/waste glycerol)	90:10
5	Upper; oil mixture (used vegetable oil/waste glycerol)	50:50
6	Lower; oil mixture (used vegetable oil/waste glycerol)	50:50
7	Upper; oil mixture (used vegetable oil/waste glycerol)	75:25
8	Lower; oil mixture (used vegetable oil/waste glycerol)	75:25
9	Upper; waste glycerol	100
10	Lower; waste glycerol	100

presents the outcomes of these tests, which assessed the combustion behavior and fuel properties of the different fuels. For the experiment, we utilized 20 mL of 99.5% alcohol as the initial volume, filled the fuel tank with the designated mixture, allowed it to overflow into the combustion chamber, and subsequently inserted the burner liner. The system reached steady-state combustion, typically within 10–15 min, as indicated by a stable flame height, followed by about 5 min during which evaporated fuel was distinctly produced. The blower operated at a minimum airflow of 2.3 m/min (1.15 m³/min). To prevent flame flashback, the supply valve was closed once the tank was devoid of fuel.

Before installing the flue-gas collecting hood, equipped with two thermocouple points, above the combustion chamber, the furnace was operated for an additional 30 min to ensure steady combustion. The two-cylinder PVC sampling apparatus was utilized to direct the exhaust gases. Both ball valves were opened during sampling and thereafter covered with aluminum foil to prevent leakage. Flow and temperature measurements were obtained using UT360 handheld anemometers positioned at the boiler steam outlet and blower intake, along with an Omron E5CS controller equipped with Type-K thermocouples and an EXTECH 42509 infrared thermometer (Extech Instruments Corporation, Nashua, NH, USA). Figure 5 and Figure 6 illustrate the outcomes of implementing these techniques on 750 oyster mushroom substrate bags in a 100-L boiler. Performance assessments were conducted to evaluate and compare the effectiveness of the five fuel compositions, including combustion temperature, exhaust gas characteristics, fuel consumption rate, and overall furnace efficiency. The infrared camera captured these photos, with color gradients from blue-green to yellow-red representing temperature intensity, ranging from cooler outer portions to hotter center sections. Uncertainty and accuracy of the instruments used in the present study are shown in

Table 2. Uncertainty and accuracy of instruments used in the present study.

Instrument	Accuracy (%)	Uncertainly
Infarad EXTECH 42509	0.5	±0.5
Gas chromatography (GC) analyzer	0.2	±0.2
Thermocouple type-K	0.5	±0.5
Data logger (°C)	0.1	±0.2
Digital weighing scale, kg	0.1	±0.2

.

3. Analytical Analysis

We obtain the thermodynamic equations for the fuel–air mixture by applying the governing equations of a small furnace. Several of the furnace’s essential physical characteristics include the fuel–air mixture, the combustion process, and the combustion gases, which interact with the air to determine when the fuel and air have been fully consumed. Fuel vapor is generated when glycerol and vegetable oil boil. Subsequently, it enters the furnace to achieve complete amalgamation with air. The initial segment of an open system, in accordance with the first law of thermodynamics, comprises the mixing zone, which includes (1) the boiling steam of glycerol and used vegetable oil amalgamated with air, referred to as the Premixer, and (2) the subsequent component, the equation zone. Figure 7 delineates the differentiation between the combustion gas and air mixing zones, namely the primary zone and the dilution zone, which refers to the region within the liner hole where gas is diluted through interaction with air.

All gases in the combustion chamber are ideal gases.

$$PV=mRT$$

(3)

where P represents the internal pressure of the combustion chamber. V is the volume of the combustion chamber. $A_{com}.L=\pi {\left({\displaystyle \frac{D_{com}}{2}}\right)}^2.L$

This is the derivative equation of the concept gas equation with respect to distance.

$${\displaystyle \frac{1}{P}}{\displaystyle \frac{dP}{dL}}+{\displaystyle \frac{1}{V}}{\displaystyle \frac{dV}{dL}}={\displaystyle \frac{1}{m}}{\displaystyle \frac{dm}{dL}}+{\displaystyle \frac{1}{R}}{\displaystyle \frac{dR}{dL}}+{\displaystyle \frac{1}{T}}{\displaystyle \frac{dT}{dL}}$$

(4)

At each stage, when fuel, air, and combustion are combined, the equation may be expressed as follows:

$${\displaystyle \frac{1}{P}}{\displaystyle \frac{dP}{dL}}+{\displaystyle \frac{1}{V}}{\displaystyle \frac{dV}{dL}}={\displaystyle \frac{1}{T}}{\displaystyle \frac{dT}{dL}}$$

(5)

Based on Figure 7, in the premix zone, the variables m_a, m_f, m_u, V, r_u, T_u, and U_u represent the following: air mass, fuel mass (boiling vapor of glycerol and utilized vegetable oil), pressure, furnace volume, gas constant, and temperature, respectively.

Figure 7 shows that in the primary zone, also known as the combustion zone, the mass of the air and fuel that is burnt is equal to mu, P_b is the average pressure of the air and fuel inside the combustion chamber at adiabatic temperatures, R_b is the gas constant, T_b is the system’s temperature when burned (the adiabatic temperature), and U_u is the system’s internal energy.

The energy balance in the system is obtained as follows:

$$m{c}_V{\displaystyle \frac{dT}{dL}}+c_{V}T{\displaystyle \frac{dm}{dL}}={\displaystyle \frac{dQ}{dL}}-P{\displaystyle \frac{dV}{dL}}-\left({\displaystyle \frac{\dot{m_l}{c}_{p}T}{v}}\right)+\left({\displaystyle \frac{\dot{m_a}{c}_{p,a}T}{v}}\right)$$

(6)

where m is the system’s mass. $\dot{m}$_l is a temporary decrease in mass. $\dot{m}$_a is the volume of air that, through the liner hole, goes into the diluting phase. Gas velocity (v) and pressure (P) are two variables. In

Table 3. Thermodynamic properties that change with distance (L(x)).

Specific Internal Energy, Volume, Entropy, Enthalpy, [36]	Fuels [37] (Glycerol and Vegetable Oil (Vapor Phase)
${\displaystyle \frac{du}{dL}}=\left(c_p-{\displaystyle \frac{Pv}{T}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{lnv}{lnT}}\right){\displaystyle \frac{dT}{dL}}-\left\{v\left({\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{lnv}{lnT}}+{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{lnv}{lnP}}\right)\right\}{\displaystyle \frac{dP}{dL}}$ ${\displaystyle \frac{dv}{dL}}=\left({\displaystyle \frac{v}{T}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{lnv}{lnT}}\right){\displaystyle \frac{dT}{dL}}-\left\{\left({\displaystyle \frac{v}{P}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{lnv}{lnP}}\right)\right\}{\displaystyle \frac{dP}{dL}}$ ${\displaystyle \frac{ds}{dL}}=\left({\displaystyle \frac{c_p}{T}}\right){\displaystyle \frac{dT}{dL}}-\left\{\left({\displaystyle \frac{v}{T}}\right)\left({\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{lnv}{lnT}}\right)\right\}{\displaystyle \frac{dP}{dL}}{\left({\displaystyle \frac{\partial h}{dT}}\right)}_p=c_p$	${\displaystyle \frac{c_p}{R}}=a_0+b_{0}T+c_o{T}^2$ ${\displaystyle \frac{h}{RT}}=a_0+{\displaystyle \frac{b_0}{2}}T+{\displaystyle \frac{c_o}{2}}{T}^2+{\displaystyle \frac{d_o}{T}}$ ${\displaystyle \frac{S^0}{R}}=a_{0}lnT+b_{0}T+{\displaystyle \frac{c_o}{2}}{T}^2+e_0$
Air & Gas Combustion product, [38]
${\displaystyle \frac{c_p}{R}}=a_1+a_{2}T+a_3{T}^2+a_4{T}^3+a_5{T}^4$ ${\displaystyle \frac{h}{R}}=a_{1}lnT+{\displaystyle \frac{a_2}{2}}T+{\displaystyle \frac{a_3}{3}}{T}^2+{\displaystyle \frac{a_4}{4}}{T}^3+{\displaystyle \frac{a_5}{5}}{T}^4+{\displaystyle \frac{a_6}{T}}$ ${\displaystyle \frac{S^0}{R}}=a_{1}lnT+a_{2}T+{\displaystyle \frac{a_3}{3}}{T}^2+{\displaystyle \frac{a_4}{4}}{T}^3+{\displaystyle \frac{a_5}{5}}{T}^4+a_7$

, we see that the thermodynamic parameters that vary with distance (L(x)) depend on both temperature and pressure, as given by Equation (4) [36,37].

Figure 7 shows the thermodynamic parameters of the main zone (the combustion zone) as functions of temperature and pressure (T, P), which vary with distance (L(x)).

$${\displaystyle \frac{dP}{dL}},{\displaystyle \frac{d{T}_b}{dL}}, {\displaystyle \frac{d{T}_u}{dL}}=f_1(L, P, T_b, T_u)$$

(7)

A temperature derivative equation, equalized per unit distance, may be derived from the original equation under the assumption of constant volume and average pressure. The following is an example of both burnt and unburned:

-

Pressure per distance

$${\displaystyle \frac{dP}{dL}}={\displaystyle \frac{A+B+C}{D+E}}$$

(8)

where

$$A={\displaystyle \frac{1}{m}}\left({\displaystyle \frac{dV}{dL}}+{\displaystyle \frac{VC}{v}}\right)$$

$$B=h{\displaystyle \frac{\left(\frac{dV}{dL}+\frac{VC}{v}\right)}{vm}}+\left\{\left({\displaystyle \frac{v_b}{c_{Pb}}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_b}{ln{T}_b}}{x}^{1/2}\right){\displaystyle \frac{T_b-T_w}{T_b}}+\left({\displaystyle \frac{v_u}{c_{Pu}}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_u}{ln{T}_u}}\left(1-x^{1/2}\right)\right){\displaystyle \frac{T_b-T_w}{T_b}}\right\}$$

$$C=-\left(v_b-v_u\right){\displaystyle \frac{dx}{dL}}-\left\{v_b\left({\displaystyle \frac{v_u}{c_{Pu}}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_b}{ln{T}_b}} {\displaystyle \frac{T_u-T_b}{c_{Pb} T_b}}\right)\left({\displaystyle \frac{dx}{dL}}-{\displaystyle \frac{\left(x-x^2\right)}{v}}\right)C\right\}$$

$$D=x\left({\displaystyle \frac{{\left(v_u\right)}^2}{c_{Pb }{T}_b}}{\left({\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_b}{ln{T}_b}}\right)}^2 \right)+\left({\displaystyle \frac{v_b}{P}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_b}{lnP}} \right)$$

$$E=\left(1-x\right)\left({\displaystyle \frac{{\left(v_u\right)}^2}{c_{Pu }{T}_u}}{\left({\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_u}{ln{T}_u}}\right)}^2 \right)+\left({\displaystyle \frac{v_u}{P}}{\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_u}{lnP}} \right)$$

-

Burning and unburned gas temperature

$${\displaystyle \frac{d{T}_b}{dL}}={\displaystyle \frac{-h\left(\raisebox{1ex}{$\pi b^2$}\!\left/ \!\raisebox{-1ex}{$2$}\right.+\raisebox{1ex}{$4v$}\!\left/ \!\raisebox{-1ex}{$b$}\right.\right)x_b^{1/2}\left(T_b-T_w\right)}{-vm{C}_{pv}x}}+{\displaystyle \frac{v_b}{C_{pb}}}\left({\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_b}{ln{T}_b}} {\displaystyle \frac{dP}{dL}}\right)+{\displaystyle \frac{h_u-h_b}{x{C}_{pb}}}\left({\displaystyle \frac{dx}{dL}}-\left(x-x^2\right) {\displaystyle \frac{C}{v}}\right)$$

(9)

$${\displaystyle \frac{d{T}_u}{dL}}={\displaystyle \frac{-h\left(\raisebox{1ex}{$\pi b^2$}\!\left/ \!\raisebox{-1ex}{$2$}\right.+\raisebox{1ex}{$4v$}\!\left/ \!\raisebox{-1ex}{$b$}\right.\right){(1-x}_b^{\frac{1}{2}})\left(T_u-T_b\right)}{-vm{C}_{pv}(1-x)}}+{\displaystyle \frac{v_u}{C_{pu}}}\left({\displaystyle \frac{\partial }{\partial }}{\displaystyle \frac{ln{v}_u}{ln{T}_u}} {\displaystyle \frac{dP}{dL}}\right)$$

(10)

where T_u is the unburned gas temperature (K), T_b is the combustion gas temperature (K), T_w is the wall temperature of the furnace, and C is the initial gases’ blow-by and the constant obtained by repeating in the convergent process. x is the mixture of air and gas obtained from the combustion. The following can be done to simulate the burning of low-calorific fuels in miniature furnaces and flames:

-

First, we will look at the enhanced procedure in the zone where the fuel and air combine. Two distinct zones: the combustion zone and the dilution zone. In every zone, the combustion process is exhaustive.

-

The combustion equation of the mixed fuel ($C_{\alpha }{H}_{\beta }{O}_{\gamma }{N}_{\delta }$) under the fuel–air ratio ε and the equivalent of $\varphi$ air, one mole can be written as an equation.

$$\begin{array}{ccc}\begin{array}{c}\epsilon \varphi C_{\mathsf{\alpha }1}{H}_{\mathsf{\beta }1}{O}_{\mathsf{\gamma }1}{N}_{\mathsf{\delta }1}+\\ \epsilon \varphi C_{\mathsf{\alpha }2}{H}_{\mathsf{\beta }2}{O}_{\mathsf{\gamma }2}{N}_{\mathsf{\delta }2}\\ +\begin{array}{c}0.21O_2\\ 0.79N_2\end{array}\end{array}& \to & \begin{array}{c}{\nu }_1\mathrm{C}{\mathrm{O}}_2+{\nu }_2{H}_{2}O+{\nu }_3{N}_2+{\nu }_4{O}_2+{\nu }_5\mathrm{CO}\\ +{\nu }_6{H}_2{\nu }_{7}H+{\nu }_{8}O+{\nu }_9\mathrm{OH}{\nu }_{10}\mathrm{NO}+v_{11}N\end{array}\end{array}$$

(11)

where $C_{\mathsf{\alpha }1}{H}_{\mathsf{\beta }1}{O}_{\mathsf{\gamma }1}{N}_{\mathsf{\delta }1}$ is the 1st fuel (glycerol vapor) and $C_{\mathsf{\alpha }2}{H}_{\mathsf{\beta }2}{O}_{\mathsf{\gamma }2}{N}_{\mathsf{\delta }2}$ is the 2nd fuel (used vegetable oil vapor) [38]; ν is the mole ratio of the production gases (11 types). At chemical equilibrium, there are three zones in the process under consideration. In the 1st zone, fuel–air is mixing already at temperatures below 500 K, in which C, O₂, H₂O, and N₂ are present in the exhaust gas. For the combustion zone, there are 11 different gases in the exhaust, including CO₂, H₂O, N₂, O₂, CO, H₂, H, O, OH, NO, and N, at adiabatic flame temperature and then the dilution zone with N₂ and O₂ added at temperature below 1000 K. Therefore, by considering the chemical equilibrium of each zone, there are four sub-equations that are identical to each of them as follows:

$$\begin{array}{cc}C& \epsilon \varphi \alpha 1+\epsilon \varphi \mathsf{\alpha }2+\epsilon \varphi \mathsf{\alpha }3=(y_1+y_5)N\end{array}$$

(12)

$$\begin{array}{cc}H& \epsilon \varphi \beta 1+\epsilon \varphi \beta 2+\epsilon \varphi \beta 3\end{array}=(2y_2+2y_6+y_7+y_9)N$$

(13)

$$\begin{array}{cc}O& \epsilon \varphi \gamma 1+\epsilon \varphi \gamma 2+\epsilon \varphi \gamma 3+0.42\end{array}=(2y_1+y_2+2y_4+y_5+y_8+y_9+y_{10})N$$

(14)

$$\begin{array}{cc}N& \epsilon \varphi \delta 1+\epsilon \varphi \delta 2+\epsilon \varphi \delta 3+1.58=(2y_3+y_{10}+y_{11})N\end{array}$$

(15)

Here, y is the percentage of moles, and N is the total number of moles of the product. Hence, the following is the percentage of total moles of combustion products:

$${\sum }_{i=1}^{11}{y}_i-1=0$$

(16)

Rearrangements in the linear equation form (excluding $N$ term) as follows:

$$2y_2+2y_6+y_7+y_9-\left[d_1(y_1+y_5)\right]=0$$

(17)

$$2y_1+y_2+2y_4+y_5+y_8+y_9+y_{10}-\left[d_2(y_1-y_5)\right]=0$$

(18)

$$2y_2+y_{10}+y_{11}-\left[d_3(y_1+y_5)\right]=0$$

(19)

where

$$d_1={\displaystyle \frac{\beta 1+\beta 2+\beta 3}{\alpha 1+\mathsf{\alpha }2+\mathsf{\alpha }3}},d_2={\displaystyle \frac{\gamma 1+\gamma 2+\gamma 3+\frac{0.42}{\epsilon \varphi }}{\alpha 1+\mathsf{\alpha }2+\mathsf{\alpha }3}},d_3={\displaystyle \frac{\delta 1+\delta 2+\delta 3+\frac{1.58}{\epsilon \varphi }}{\alpha 1+\mathsf{\alpha }2+\mathsf{\alpha }3}}$$

The procedure requires consideration of seven unknowns, whose values exceed those of the equations, as stated before. Hence, according to the equilibrium constant connection at the same mole ratio, pressure, volume, and temperature under the same conditions, the gases formed from combustion are considered an ideal gas and undergo the following breakdown [

Table 4. Chemical equilibrium of the combustion products.

$\mathrm{C}{\mathrm{O}}_2\rightleftharpoons \mathrm{C}\mathrm{O}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$	$K_1={\displaystyle \frac{y_5{y}_4^{1/2}{P}^{1/2}}{y_1}}$
${\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\rightleftharpoons {\mathrm{H}}_2\mathrm{O}$	$K_2={\displaystyle \frac{y_2}{y_4^{1/2}{y}_6{P}^{1/2}}}$
${\displaystyle \frac{1}{2}}{\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\rightleftharpoons \mathrm{O}\mathrm{H}$	$K_3={\displaystyle \frac{y_9}{y_4^{1/2}{y}_6^{1/2}}}$
${\displaystyle \frac{1}{2}}{\mathrm{N}}_2\rightleftharpoons \mathrm{N}$	$K_4={\displaystyle \frac{y_7{P}^{1/2}}{y_6^{1/2}}}$
${\displaystyle \frac{1}{2}}{\mathrm{O}}_2+{\displaystyle \frac{1}{2}}{\mathrm{N}}_2\rightleftharpoons \mathrm{N}\mathrm{O}$	$K_5={\displaystyle \frac{y_8{P}^{1/2}}{y_4^{1/2}}}$
${\displaystyle \frac{1}{2}}{\mathrm{H}}_2\rightleftharpoons \mathrm{H}$	$K_6={\displaystyle \frac{y_{11}{P}^{1/2}}{y_3^{1/2}}}$
${\displaystyle \frac{1}{2}}{\mathrm{O}}_2\rightleftharpoons \mathrm{O}$	$K_7={\displaystyle \frac{y_{10}}{y_4^{1/2}{y}_3^{1/2}}}$

]:

Hence, the following is how the issue is solved using the Newton approach in terms of the Taylor series distribution:

$$f_j(y_3,y_4,y_5,y_6)=0 \mathrm{where} j=\mathrm{1,2},\mathrm{3,4}$$

$$f_j+{\displaystyle \frac{\partial f_j}{\partial y_3}}\Delta y_{3+}{\displaystyle \frac{\partial f_j}{\partial y_4}}\Delta y_4+{\displaystyle \frac{\partial f_j}{\partial y_5}}\Delta y_5+{\displaystyle \frac{\partial f_j}{\partial y_6}}\Delta y_6=0$$

(20)

Equilibrium may be achieved by setting matrix algebra, where the initial value serves as the default for solving subsequent equations until the entire error condition is met. The process is as follows:

$$[\Delta y]=inv\left[A\right]\left[f\right]$$

(21)

The mole proportions matrix, denoted as [Δy], is assumed to be less than or equal to zero. The constant matrix, denoted as [A], is obtained from the molar proportional derivative according to Taylor’s derivative equations. The main equation matrix, denoted as [f], is solved using MATLAB software (version R2023a, MathWorks Inc., Natick, MA, USA). The figure shows the changes in pressure, temperature, and the velocity of exhaust gas flow from

$$\stackrel{•}{T_a}=\left({\displaystyle \frac{\stackrel{•}{Q_a}}{m_a{R}_a}}+T_a{\displaystyle \frac{\stackrel{•}{P_a}}{P_a}}-v_a\left({\displaystyle \frac{\partial \mathit{ln}{v}_a}{\partial \mathit{ln}{T}_a}}+{\displaystyle \frac{\partial \mathit{ln}{v}_a}{\partial \mathit{ln}{P}_a}}\right){\displaystyle \frac{\stackrel{•}{P_a}}{R_a}}\right)/\left(1+\left({\displaystyle \frac{c_{Pa}}{R_a}}-{\displaystyle \frac{\partial \mathit{ln}{v}_a}{\partial \mathit{ln}{T}_a}}\right)\right)$$

(22)

$$\stackrel{•}{P_b}=P\left({\displaystyle \frac{\stackrel{•}{m_b}}{m_a}}-{\displaystyle \frac{\stackrel{•}{T_a}}{T_a}}+{\displaystyle \frac{\stackrel{•}{V}}{V}}\right)+\stackrel{•}{P}$$

(23)

$$\stackrel{•}{m_b}={\displaystyle \frac{\stackrel{•}{Q_b}-P_b\stackrel{•}{V}\left(1+\frac{\partial u_b}{\partial T_b}\frac{1}{R_b}\right)-PV\left(\frac{\partial u_b}{\partial T_b}\frac{1}{R_b}+\frac{\partial u_b}{\partial P_b}\frac{m_b}{V}\right)\left(1+\frac{\stackrel{•}{V}}{V}-\frac{\stackrel{•}{T_a}}{T_a}\right)}{\left(\left(u_b-u_a\right)-R_a{T}_a-T_b\frac{\partial u_b}{\partial T_b}+\frac{PV}{m_a}\left(\frac{\partial u_b}{\partial T_b}\frac{1}{R_b}+\frac{\partial u_b}{\partial P_b}\frac{m_b}{V}\right)\right)}}$$

(24)

4. Results and Discussion

4.1. Predicted Results

Figure 8 illustrates the pressure distribution along the length of a small burner furnace at two equivalence ratios, evaluated using the high-iteration method and the equilibrium constant method. The fuel comprises 70% glycerol and 30% recycled vegetable oil. In both scenarios, the pressure at the burner inlet decreases due to the acceleration of the reactant mixture and the rapid heat release. It subsequently declines to its nadir before ascending abruptly in the mid-furnace area. The combustion reaction generates additional heat, causing the gas to expand further and thereby elevating the pressure. The maximum pressure is diminished in the fuel-lean situation (a), characterized by an equivalence ratio of 0.8, owing to the decreased heat release and diminished thermal expansion resulting from the restricted fuel availability. Conversely, with an equivalency ratio of 1.2, the fuel-rich condition (b) produces a markedly elevated pressure peak. The elevated fuel concentration increases combustion intensity and gas output, resulting in greater pressure accumulation in the furnace. A nuanced difference exists between the two computational methodologies: the high-iteration technique is more sensitive to reaction kinetics than the equilibrium-based approach.

Figure 9 illustrates the temperature dispersion within the central zone of the small burner furnace using a fuel combination of 70% glycerol and 30% used vegetable oil. The results are analyzed using two equivalency ratios, encompassing both burned and unburned gases. The analysis utilized the high-iteration method and the equilibrium constant method. In a fuel-lean situation (a), with an equivalence ratio of 0.8, the temperature of the burned gas increases gradually due to incomplete combustion and reduced reaction intensity, whereas the temperature of the unburned gas consistently rises along the furnace length, stabilizing between 1600 and 1700 K. The restricted fuel content results in a moderate heat discharge. In the fuel-rich state (b), characterized by an equivalence ratio of 1.2, the burned-gas temperatures are significantly elevated, attaining around 1900–2000 K. The higher combustion intensity and thermal energy generated by the higher fuel result in a more pronounced temperature increase when the furnace traverses it. The persistent downstream reaction resulting from incomplete combustion zones due to excess fuel leads to a more rapid increase in unburned gas temperature in (b) compared to (a). The equilibrium constant method assumes ideal equilibrium behavior, yielding conservative estimates, whereas the high-iteration method integrates combustion kinetics and iterative temperature adjustments, yielding marginally higher temperature predictions; thus, the two computational methods diverge in these respects.

Figure 10 geographically illustrates thermal energy and combustion intensity, along with the contours of the furnace’s internal temperatures for the fuel combination (70% glycerol and 30% used vegetable oil) at two equivalence ratios. At an equivalency ratio of 0.8, the fuel-lean state (a) facilitates optimal air–fuel interaction, resulting in a maximum temperature of around 2000 K. This occurs near the upstream area of the combustion chamber, immediately downstream of the burner nozzle, where the reaction is initially fully established. Conversely, condition (b), characterized by a fuel-rich composition and an equivalence ratio of 1.2, exhibits a higher maximum temperature of around 2200 K. This state lies in the same region. However, it extends further downstream due to the increased fuel concentration, which enhances heat release and intensifies the flame core. In contrast to case (a), which yields a narrower hot zone due to limited fuel availability that constrains peak combustion temperature and overall flame propagation, case (b) exhibits a hotter and broader high-temperature zone as the richer mixture supplies additional combustible material, resulting in intensified exothermic reactions and a more prominent thermal plume.

Figure 11 illustrates the axial distributions of CO₂ and O₂ mole fractions along the small burner furnace for mixing waste oil (glycerol: used vegetable oil = 70:30) at equivalence ratios of 0.8 and 1.2. In both cases, the O₂ mole fraction decreases sharply along the furnace length due to oxygen consumption during combustion, while the CO₂ mole fraction increases rapidly within the main reaction zone and reaches a nearly constant value downstream, indicating completion of the dominant oxidation reactions. Under the fuel-lean condition (φ = 0.8), residual O₂ remains at the furnace outlet, whereas under the fuel-rich condition (φ = 1.2), O₂ is almost completely depleted beyond approximately 0.15 m. The High Iteration Method predicts slightly higher CO₂ and lower O₂ levels than the Equilibrium Constant Method, particularly in the reaction zone; however, the overall axial trends and final values obtained by both methods are highly consistent, demonstrating that the equivalence ratio predominantly governs gas distribution behavior in the furnace.

Figure 12 depicts the influence of altering the equivalence ratio on the mole fractions of principal and secondary combustion products (CO₂, H₂O, N₂, O₂, CO, H₂, OH, O, H, and NO) for a fuel mixture comprising 70% glycerol and 30% used vegetable oil, illustrating the transition of chemical equilibrium between fuel-lean and fuel-rich states. As the equivalency ratio increases from lean to stoichiometric, the concentrations of CO₂ and H₂O increase due to greater fuel availability for full oxidation, while O₂ decreases steadily owing to heightened oxygen demand. In fuel-rich conditions, CO₂ and H₂O concentrations begin to stabilize or decrease slightly, while species associated with incomplete combustion, such as CO and H₂, increase significantly due to insufficient oxygen for complete fuel oxidation. Radical species such as OH, O, and H reach their maximum concentration at stoichiometric conditions, characterized by elevated flame temperature and reaction intensity and then diminish in rich conditions due to reduced oxidation. Nitrogen-related species, mostly N₂ and trace amounts of NO, exhibit minimal change, with NO reaching its maximum around stoichiometric mixes due to the significant temperature dependence of thermal NO production. The observed trends result from the equilibrium between oxygen availability and fuel concentration: lean mixtures facilitate complete oxidation and elevated O₂ levels, stoichiometric mixtures optimize combustion temperature and radical generation, whereas rich mixtures promote incomplete combustion, leading to increased CO and H₂ while diminishing oxidized products.

Figure 13 presents a schematic comparison of combustion mechanisms for oil–glycerol blended fuels at different mixing ratios, highlighting how fuel properties and blending proportions influence flame behavior, temperature distribution, and product formation [39]. As shown in Figure 13a, representing low glycerol content (high oil fraction), the fuel undergoes uniform atomization, producing relatively fine and evenly distributed droplets. This promotes rapid evaporation and stable flame propagation with a high flame temperature, enabling complete oxidation of the fuel. Consequently, the dominant combustion products are CO₂ and H₂O, indicating a high combustion efficiency and well-established complete combustion conditions.

Figure 13b illustrates the optimal blending ratio (~25% glycerol/75% vegetable oil), where a synergistic combustion regime is achieved. Fine droplets and improved vapor–air mixing are observed, supported by a balanced oxygen availability from the oxygenated glycerol component. A stable high-temperature zone is maintained, which enhances oxidation kinetics and prolongs the residence time of reactive intermediates. As a result, complete combustion is maximized, with minimal formation of incomplete combustion products such as CO, H₂, and CH₄, while CO₂ and H₂O remain the primary products. In contrast, Figure 13c corresponds to high glycerol content, where increased viscosity leads to the formation of larger droplets and non-uniform evaporation. The flame temperature is significantly reduced due to the lower calorific value and higher latent heat of vaporization of glycerol. Under these conditions, fuel cracking and pyrolysis become dominant, resulting in incomplete combustion. This regime is characterized by elevated concentrations of CO, H₂, and CH₄, indicating degraded combustion performance and reduced efficiency.

Overall, the figure clearly demonstrates the transition from complete combustion to synergistic optimal combustion, and finally to incomplete combustion, as the glycerol fraction increases. It visually supports the conclusion that an intermediate oil–glycerol blending ratio provides the best trade-off between flame stability, thermal intensity, and chemical reactivity.

4.2. Measured Data

Thermal images in Figure 14 depict flame-temperature distributions for three fuels: waste glycerol, used vegetable oil, and a 50:50 blend. The infrared camera captured these photos, with color gradients from blue-green to yellow-red representing temperature intensity, ranging from cooler outer portions to hotter center sections. A concentrated, high-temperature core, reaching approximately 506.6 °C, is observed in the flame generated by waste glycerol, indicating intense, localized combustion. Nonetheless, there are noticeable temperature fluctuations over the flame’s surface. The flame generated by the burning of used vegetable oil exhibits the most uniformly distributed high-temperature region, reaching a maximum temperature of around 520.1 °C, signifying stable and efficient combustion. A broader high-temperature region, peaking at 537.4 °C, is observed in the thermal profile of the 50:50-blended oil. This represents the maximum value among the three fuels, indicating that the synergistic properties of the two oils have enhanced combustion intensity.

Table 5. Variations in compound concentrations in exhaust gases.

No.	Sample Name	Ratio	H2 (%)	Air (%)	CH4 (%)	CO2 (%)
1	Up Oil	100	nd.	74.295	nd.	25.705
2	Down Oil	100	nd.	73.050	nd.	26.950
3	Up Oil:Gly	90:10	2.442	74.183	nd.	23.375
4	Down Oil:Gly	90:10	2.497	74.076	nd.	23.427
5	Up Oil:Gly	50:50	4.805	75.019	4.360	15.816
6	Down Oil:Gly	50:50	5.419	75.720	4.378	14.483
7	Up Oil:Gly	75:25	nd.	75.222	nd.	24.778
8	Down Oil:Gly	75:25	nd.	74.916	nd.	25.084
9	Up Gly	100	3.902	73.173	4.323	18.601
10	Down Gly	100	nd.	79.149	nd.	20.851

nd. = not detected.

presents the fluctuations in key exhaust-gas components (H_k, air %, CH₄, and CO_k) for different glycerol-to-vegetable oil ratios, measured at the upper and lower sampling points of the furnace. This data demonstrates the impact of fuel composition and sampling location on combustion characteristics. The absence of significant H₂ or CH₄, along with elevated CO₂ concentrations (25–27%), suggests that pure used vegetable oil (Up Oil and Down Oil) demonstrates efficient combustion with few byproducts of incomplete combustion. The partial thermal decomposition of glycerol facilitates the generation of hydrogen and light hydrocarbons at elevated temperatures, resulting in the presence of small quantities of H₂ (2.4–5.4%) and CH₄ (4.3% for specific mid-ratio blends) when glycerol and vegetable oil are combined in particular ratios, such as 90:10, 50:50, or 75:25. This is exceptionally accurate at the elevated sampling location. Due to glycerol’s lower calorific value and elevated oxygen content, flame stability is altered, leading to a significant reduction in CO₂ levels in mixed fuels (14% to 24%), indicating incomplete oxidation. The burning of pure glycerol increases hydrogen generation to 3.9% and methane production to 4.3%, while carbon dioxide drops to between 18% and 20%, indicating a more pronounced occurrence of incomplete combustion. The “Down” sample is closer to the first mixing zone, where increased air dilution reduces the gas composition, resulting in elevated “air (%)” values and diminished identifiable fuel-rich species, whereas the “Up” sample comprises hotter post-flame gases that promote thermal cracking and hydrogen generation. A higher percentage of vegetable oil promotes more stable combustion and increases CO₂ emissions. In contrast, an increase in glycerol content reduces combustion efficiency and increases the production of pyrolysis-derived compounds such as H₂ and CH₄, as illustrated in the table.

The slight variations in airflow rate shown in Table 5 (1.30–1.32 m³/min) effectively represent fine adjustments of the excess air ratio around its optimal value. The observed trend—maximum combustion efficiency at slightly higher airflow without flame cooling—demonstrates that the system was operating near the optimal excess air coefficient. In summary, the effect of the excess air ratio was accounted for through controlled airflow and exhaust-gas analysis. The combustion system operated at an excess air coefficient of approximately 1.15–1.35, with optimal combustion efficiency occurring near λ ≈ 1.2–1.3. This confirms that excess air plays a critical role in achieving high combustion efficiency for glycerol–vegetable oil blends and supports the validity of the reported results.

Table 6. Variation of combustion efficiency (glycerol/used vegetable oil: 10:90).

No.	Air Flow Rate (m3/min.)	Fuel Weight (g)	Surface Temperature (°C)	Flame Temperature (°C)	Combustion Efficiency (%)
1	1.30	1000	200	490	91
2	1.31	1000	195	485	90
3	1.32	1000	198	485	92

illustrates the impact of minor variations in air supply on flame behavior and combustion performance, showcasing the differences in combustion efficiency for a fuel mixture of 90% used vegetable oil and 10% glycerol across three slightly differing airflow rates. The surface temperature fluctuates somewhat between 195–200 °C as the airflow rate increases from 1.30 to 1.32 m³/min; the flame temperature remains relatively constant at approximately 485–490 °C, indicating thermal stability in the combustion zone under these conditions. The combustion efficiency ranges from 90% to 92%, attaining 92% at the peak airflow rate of 1.32 m³/min. This suggests that a minor augmentation in air supply enhances the mixing of fuel and air, hence facilitating a more thorough oxidation process. Conversely, efficiency drops to 90% with a reduced airflow of 1.31 m³/min, attributable to incomplete combustion due to insufficient oxygen delivery, despite the flame temperature remaining unchanged. The figure indicates that a slightly higher airflow rate improves combustion quality without excessively cooling the flame, resulting in optimal efficiency for this fuel combination.

Table 7. Thermal efficiency.

Details	Test I	Test II	Test III
mw = Remaining water (kg)	2.7	2.4	3.6
mS = Evaporated water (kg)	3.3	3.6	2.4
mf = mass of fuel; (kg)	6.3	6.8	5.8
hw = Enthalpy of residual water (kJ.kg−1)	117.4	109.1	109.1
hf = Enthalpy of saturated liquids (kJ.kg−1)	417.5	417.5	417.5
hS = Enthalpy of saturated vapor (kJ.kg−1)	2674	2674	2674
HV = Calorific value of fuel (kJ.kg−1)	13,476	13,476	13,476
Combustion furnace efficiency (%)	99.4	99.3	99.1
Boiling steam flow rate (m3/min)	6.1	6	6.1
Boiling steam temperature (°C)	112	115	114
Stove Efficiency	81.3	88.6	83.5
Heat exchange area m2 (Radius = 0.2 m)	0.5	0.5	0.5
Heat exchange efficiency (%)	90	90	90
Boiler efficiency (%)	72.8	73.1	72.2

presents the thermal efficiency findings from three boiler experiments, demonstrating the impact of variations in fuel mass, residual water, steam flow rate, boiling temperature, and evaporated water on the boiler’s overall heat transfer and performance. Using slightly different fuel masses (5.8–6.8 kg) increases evaporated water from 3.3 to 3.6 kg across all three trials, indicating that greater fuel input enhances phase transition and steam production. The enthalpy values of saturated liquid and vapor remain constant at the boiling point, unaffected by temperature and pressure. A combustion furnace efficiency of 99.1–99.4% signifies that the fuel mixture combusts almost completely under various conditions. The steam flow rate varies marginally between 6.0 and 6.1 m³/min, while the boiling temperature rises from 112 to 114 °C, indicating a small increase in steam production and thermal output in the following studies. The efficiency of a stove declines from 81.3% to 83.6% over time due to variations in heat loss around the combustion chamber and external surfaces. As both the heat-exchange area and the boiler’s geometry remain constant, the heat-exchange efficiency remains 90%. Fluctuations in fuel mass and steam production result in minor alterations in boiler efficiency, ranging from 72.2% to 73.1%. This suggests that tests at higher boiling temperatures and greater steam production yield enhanced thermal performance. The table indicates that even slight variations in fuel mass and steam flow affect thermal efficiency; yet, the system maintains constant, high-quality heat transfer across all three trials.

4.3. Economic Analysis

The cost, fuel consumption, operational duration, and financial returns notably differ between LPG and a mixed fuel of glycerol and used vegetable oil (25/75), as illustrated in

Table 8. Economic value analysis.

Details	LPG	Glycerol Oil and Used Vegetable Oil % (25/75)
Stove and boiler cost (Baht)	21,000	30,000
Steaming time for mushroom cubes per time (hours)	6	4
Amount of fuel consumed per time (kg)	40	28
Fuel price per 1 year (Baht)	23,040	6720
Electricity price per year (Baht)	0	2400
Cost including stove and boiler (Baht)	44,040	39,120
Revenue from the sale of mushroom cubes per year (Baht)	144,000	144,000
The difference in the cost of one mushroom per year (Baht)	99,960	104,880
Payback period (Months)	3.67	3.26
Time difference in hours per year	0	96

, which presents an economic value analysis. The waste-based fuel is more economical and necessitates a reduced quantity per steaming cycle (28 kg versus 40 kg for LPG), resulting in significantly lower annual fuel costs—6720 Baht compared to 23,040 Baht for LPG—despite the marginally higher initial investment for the glycerol-vegetable oil system (30,000 Baht) compared to LPG (21,000 Baht). After factoring in the depreciation of stoves and boilers, the total annual operating cost of 39,120 Baht remains lower than the 44,040 Baht for LPG, despite the 2400 Baht in electricity costs in the mixed-fuel system due to blower operation. Although both systems generate 144,000 Baht annually from mushroom farming, the mixed-fuel system is more cost-effective, saving 99,960 Baht per year compared to LPG. The glycerol–vegetable oil system exhibits a shorter payback period (3.26 months vs. 3.67 months) owing to its lower operating costs. The mixed-fuel system enhances production efficiency by reducing each batch’s steaming time from 6 h to 4 h, resulting in annual savings of 96 operational hours. Although the initial investment is relatively larger, the integrated glycerol–vegetable oil system yields a quicker return, enhanced production efficiency due to reduced fuel consumption and shorter steaming duration, and significant cost savings, as indicated by the economic analysis.

It is valid, and a more comprehensive cost framework is indeed required to fully reflect the real-world economics of the proposed mixed-fuel system. In the present study, the economic analysis was intentionally simplified to highlight direct operational feasibility and to enable a clear comparison with LPG under identical operating conditions. However, a detailed engineering–economic assessment should explicitly account for the following additional cost components and regional factors:

-

Fuel collection cost: Used vegetable oil and crude glycerol are waste-derived fuels whose market prices are typically low or zero at the source, but logistics introduce non-negligible costs. These include transportation from restaurants or biodiesel plants, labor for collection, and container handling. In rural agricultural regions, collection distances are short and often integrated into existing supply chains, keeping costs low; however, in urban or dispersed regions, transportation can significantly affect fuel economics.

-

Pretreatment and conditioning costs: Although the present system is designed to operate with unrefined fuels, minimal pretreatment is still required, including filtration to remove food residues, water separation, and viscosity control (preheating). These processes incur costs related to energy consumption, filters, and maintenance. Compared to chemical purification or transesterification, these pretreatment steps are relatively inexpensive but should be included in long-term operational cost models.

-

Storage and handling costs: Waste oils and crude glycerol require dedicated storage tanks, spill prevention measures, and periodic cleaning to prevent contamination or phase separation. Capital costs for tanks, piping, and safety accessories, as well as depreciation and maintenance costs, should be amortized over the system’s lifetime. These costs are generally modest for small-scale agricultural systems but increase with scale.

-

Regional energy price dependency: The economic advantage of the mixed-fuel system is highly sensitive to regional LPG prices, electricity tariffs, and the availability of waste fuel. In regions where LPG prices are high or waste oils are subsidized, the payback period is shorter. Conversely, in areas with low fossil-fuel prices or limited waste-fuel supply, the economic margin may narrow. Therefore, incorporating region-specific energy price indices and sensitivity analysis would strengthen the generalizability of the results.

-

Labor and operational overhead: Additional labor for fuel handling, system monitoring, and cleaning—although minimal—should be considered in a complete techno-economic analysis. These costs are often absorbed into routine farm operations in small agricultural settings but may become significant in larger or commercial-scale installations.

In summary, while the current cost analysis successfully demonstrates the baseline economic feasibility and rapid payback of the proposed system, a more comprehensive assessment should integrate fuel logistics, pretreatment, storage, labor, and regional energy-price variability. Such an expanded analysis would enable site-specific optimization and scalability assessment, and it is therefore recommended as a logical extension of this work rather than a limitation of the present study.

Figure 15 illustrates the energy conversion chain from source to end use, highlighting how energy is transformed and where losses occur at each stage. Starting from the left, primary energy represents energy directly extracted from natural sources such as coal, crude oil, solar energy, and other raw resources. As this energy is converted into more usable forms—such as petrol, diesel, or electricity—it becomes secondary energy. During this conversion process, a portion of energy is inevitably lost, shown by the downward red flow labeled “Losses.” The next stage is final energy, which is the energy delivered to consumers, including households, industry, and commerce. Even at this stage, further losses occur during transmission, distribution, and end-use conversion. Finally, useful energy represents the actual energy services obtained by users, such as lighting, heating, cooling, and mobility. Again, losses are present due to inefficiencies in end-use devices and systems.

Overall, the diagram emphasizes that only a fraction of the original primary energy is ultimately converted into useful energy, while significant losses occur at every conversion step. This visual highlights the importance of improving efficiency and reducing losses across the entire energy system.

5. Conclusions

This study aimed to enhance energy efficiency, reduce operational costs, and promote waste-to-energy conversion by constructing, modeling, and experimentally evaluating a burner and furnace system utilizing vegetable oil and crude glycerol as cost-effective renewable fuels for steaming mushroom substrate cubes. The primary findings indicate that blended fuels, namely those with a 25/75 glycerol-vegetable oil ratio, improve flame stability, elevate peak combustion temperature, reduce incomplete combustion byproducts, and attain high combustion and thermal efficiencies (about 90–99% and 72–73%, respectively). Analytical models confirmed the expected distributions of pressure, temperature, and gas species, while the economic study indicated that blended fuels resulted in much lower annual fuel expenditures, shorter steaming times, and a shorter payback period than LPG. The principal conclusions of this study are as follows: (i) a reliable burner system utilizing mixed fuels was effectively developed; (ii) optimal air–fuel ratios for efficient combustion were determined; (iii) analytical modeling validated the thermodynamic performance; (iv) all fuel blends exhibited high thermal and combustion efficiency; (v) mixed fuels yielded superior exhaust emissions compared to pure glycerol; and (vi) mushroom producers realized significant economic advantages and diminished fuel reliance. The research indicates that small and medium-sized agricultural enterprises can benefit from implementing this mixed-fuel system to reduce energy costs, enhance sustainability, and increase productivity. It also suggests that subsequent studies should focus on determining optimal ratios of elevated glycerol, scaling the system for larger industrial applications, and extending the technology to additional thermal-processing uses. This will optimize the utilization and enduring effects of fuels derived from renewable waste.