Operational and Environmental Tests of Nanomembrane-Produced Rapeseed Biodiesel at a Small Biomass-Fuelled Power Plant

doi:10.21203/rs.3.rs-4112712/v1

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Research Article

Operational and Environmental Tests of Nanomembrane-Produced Rapeseed Biodiesel at a Small Biomass-Fuelled Power Plant

https://doi.org/10.21203/rs.3.rs-4112712/v1

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published 12 Sep, 2024

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The research tests an environmentally sustainable rapeseed biodiesel produced by transesterification in an electrospinning-manufactured nanomembrane reactor from polyvinylidene fluoride and a copolymer of vinylidene fluoride with tetrafluoroethylene. The Kalman filter analyzed the following hydrodynamic and kinematic characteristics of the atomized flow of diesel fuel, fatty acid methyl esters (FAME) of rapeseed oil and their mixtures B6 and B12: droplet mean diameter and number, their absolute velocity and the distributions of their velocity and trajectories in streamlined and forced swirling gas-liquid flows. Fuel combustion tests took place in a test rig equipped with a swirl burner while maintaining the equivalence ratio equal to 1 and the air/liquid ratio – 14.5. They determine effects of injection pressure, FAME content, and gas-liquid flow swirling on the flame structure and temperature, the fuel ignition delay. The emissions of CO, CO2, and NOx from burning biodiesels B6 and B12, FAME, and diesel fuel are compared. The effect of exhaust gas recirculation on concentrations of the main combustion gases (O2, CO, CO2, NOx) is estimated. The findings treated by the weighted sum method made it possible to suggest the optimal biofuel blend. An affordable green energy technology for small-scale power generation is proposed.

environmental sustainability

green energy

rapeseed biodiesel

nanofiltration membrane

atomization

combustion

Biofuels have a high potential for the energy sector and internal combustion engines, as they can be made from a wide range of available resources (oils, fats, agricultural waste, etc.). The advantages of using biofuels are reduced particulate matter (PM), carbon neutrality, reduced greenhouse gas emissions, waste management, energy and economic security, less dependence on oil [1].

Due to the global trend of switching to renewable energy sources and reducing carbon emissions, biofuels can be considered as the best choice for the transport and energy sectors [2, 3]. Due to the non-renewable nature and depletion of oil resources, biofuels have become an unavoidable alternative choice [4, 5]. Considering that fuel production should not compete with food products (i.e. the first-generation biofuel [6–9]), biofuel production is focused on increasing the share of alternative sources [10–12]. Examples of alternative sources are agricultural waste and industrial bio-waste (the second-generation biofuel), as specified in the Renewable Energy Directive (RED) II [13]. Algae biofuel or the third-generation biofuel is one of the most promising and has a huge potential in the aspect of sustainable energy production. While most of the ocean is not cultivated. Therefore, it is unlikely that this type of fuel will become widespread in the next few years with existing technologies [14]. This study uses biofuel from rapeseed oil obtained by transesterification and nanofiltration in a nanomembrane reactor, and thus more environmentally sustainable, since long stages of the classic transesterification method are excluded [15–19].

1.1. Biodiesel atomization

The quality of fuel atomization depends on its physical and chemical properties, mainly such as density and kinematic viscosity [20–22]. Fuel atomization characteristics have a significant impact on combustion characteristics and exhaust gases. In recent years, experimental and numerical studies [23–27] have been published on the characteristics of biodiesel atomization. Park et al. [23] determined the effect of ambient temperature on the characteristics of soy biodiesel atomization. The authors concluded that biodiesel can create a high-quality mixture with air at high temperatures of the fuel and the external gas-air environment. Guan et al. [24] found out that when di-n-butyl ether (DBE) is added to biodiesel, atomization characteristics are improved, and the Sauter mean diameter (SMD) is reduced. The characteristics of soy biodiesel (B100) and its blend with diesel fuel and di-n-butyl ether (DBE15) are compared with those of diesel fuel. The comparative analysis elucidated that compared to DBE15 and diesel fuel, B100 has a deeper spray tip penetration (STP), an increased velocity, and a smaller spray cone angle (SCA). Agarwal et al. [25] examined the atomization characteristics of Karanja biodiesel and diesel fuels at various pressures of an external gas-air environment. The characteristics of biodiesel fuel are inferior to those of the DBE-based fuel under the same injection and environmental conditions. A significant influence of the ambient pressure on the fuel atomization characteristics is determined. Azevedo et al. [26] sprayed soy biodiesel using three types of injectors: cylindrical, conical, and conical-cylindrical. The mean droplet diameter clearly decreases with increasing air-liquid ratio (ALR), while SCA did not change with increasing ALR. At a given mass flow rate of liquid, the conical nozzle demonstrated a higher spraying quality with smaller mean diameters than the cylindrical and conical-cylindrical nozzles. Zhang et al. [27] explored the effect of the nozzle orifice size and injection pressure on the spray morphology, velocity distribution, and SCA. The experimental study was carried out on a pressure-swirl nozzle and reported that a higher pressure increases SCA and the spray velocity. It is proved that too small or large size of the nozzle orifice affects fuel atomization.

The component composition of diesel and biodiesel fuels is different. Therefore, their physical and chemical properties also differ. Studies ^20,24–27 shed light on that biodiesel has a higher density, viscosity, and surface tension, and its evaporation rate is lower than that of diesel fuel. These data indicate a deterioration in the quality of biodiesel atomization. Thus, determining the qualitative and quantitative characteristics of spraying new samples of liquid biofuels, from the point of view of hydrodynamics and kinematics, is an essential engineering task.

1.2. Biodiesel combustion

With a lower oxygen concentration in the external gas-air environment, the heat release rate of biodiesel contributes to an increased ignition delay and longer combustion [32]. The ignition delay of diesel-pine oil blends increases due to the lower cetane number of the oil. Thus, the fuel-air mixing time increases, which favorably affects the excess air factor and the mixed air content, as well as the heat release rate [33]. An exception is the shorter ignition delay time of hydrogenated catalytic biodiesel compared to diesel fuel [34].

The amount of PM in the combustion process of biodiesel compared to diesel fuel is lower [35, 36]. In addition, biodiesel fuels have increased oxidation rates due to the shorter flame lifetime. This is mainly due to the lower equivalence ratio λ (defined as the ratio of mass flow rate of fuel to that of air divided by the same ratio for the stoichiometry of the reaction in question) due to the presence of oxygen in the biofuel molecules. Moreover, the lower content of aromatics and sulfur in biodiesel prevents the formation of PM [37].

Due to the increased calorific value and density, biodiesel has high values of cumulative heat release, combustion pressure, and heat release rate [38].

With due regard for the above factors, the exploration of the effect of the bio-additive content in liquid fuel on the combustion characteristics of biofuel blends in model combustion chambers, including those with swirl burners, remains an urgent task.

1.3. Analyzing pollutions from biodiesel combustion

Significant interest in biodiesel is associated with the possibility of reducing greenhouse gases. In addition, the use of biodiesel and biodiesel blends has a positive impact on the emissions of hydrocarbons (HC), carbon monoxide (CO), and PM [39], i.e. contributes to their reduction. Compared to conventional fuels, biodiesel is more environmentally friendly, but not with respect to NO_x. In general, the concentration of NO_x in combustion products is higher when using biodiesel. This increase happens due to “biodiesel NO_x effect” [40]. Since the magnitude of NO_x exposure is typically much smaller than that of CO, HC, and PM, it is often difficult to determine quantitively or even qualitatively this value when using low-concentration biodiesel blends, such as B20 or lower. In Refs. ^26–33, the concentrations of NO_x that release when using B0 and B20 with a large number of test cycles and engines do not differ significantly. However, the “biodiesel NO_x effect” can be mitigated by using exhaust gas recirculation (EGR) [40].

Khiraiya et al. [41] compared CO emissions for diesel and palm oil-based biodiesel fuels. It is proved that the usage of the blends B25, B50, B75, and B100 atomized at a pressure of 11 bar contributes to lower CO emissions compared to diesel fuel. However, when the pressure is increased to 15 bar, the CO emissions from the combustion of B100 are 21% higher than those from diesel fuel. Shen et al. [42] revealed that CO emissions are minimized when λ = 1. As this ratio increases, there is a monotonous growth in emissions. In turn, the opposite trend is typical of CO₂ emissions. Diesel fuel combustion in a single-injection engine is characterized by knocking due to rapid heat release, high rate of pressure rise, weak mixing, and increased NO_x emissions in the exhaust gases [43]. The combustion of biodiesel and diesel fuels with a separate injection is characterized by the gradual heat release with minimal fluctuations, low combustion temperature and NO_x emissions due to improved fuel-air mixing.

Based on the above, an urgent additional task of this research is to minimize the concentrations of NO_x in the exhaust gases during the combustion of biofuels through the introduction of an EGR system.

1.4. Novelty, practical relevance and purpose of the research

The results presented in the cited papers on atomization, ignition, combustion, and gas analysis of the combustion products from biodiesel fuels relate mainly to diesel engines. The issues of using biofuels in existing power plants, including small-scale ones, are relevant. This study provides a comparative analysis of the atomization, combustion and exhaust gas emission characteristics for B100 biodiesel from rapeseed oil, biodiesel-diesel blends B6 and B12, and diesel fuel in a swirl burner equipped with an optical access combustion chamber and EGR system. In particular, we analyze the features of the combustion process, as well as the hydrodynamic and kinematic characteristics of the spray flow of rapeseed oil biofuel produced by an environmentally sustainable method – transesterification and nanofiltration in an electrospinning-manufactured nanomembrane reactor from polyvinylidene fluoride and a copolymer of vinylidene fluoride with tetrafluoroethylene. Moreover, the harmful emissions of NO_x, CO, and CO₂ are determined in the exhaust gases of such fuel and compared with diesel fuel and biodiesel-diesel blends. The scientific novelty of the study is to characterize the weighed sum model-based efficiency of the rapeseed oil biodiesel produced by the proposed environmentally sustainable method. The practical relevance is related to the development of a simple and affordable green energy technology for small-scale power generation using existing facilities based on the swirl combustion of liquid fuel.

2.1. Materials

The liquids under examination are diesel fuel, fatty acids methyl esters (FAME) of rapeseed oil (B100), and diesel-FAME blends (B6 and B12, i.e. the concentration of FAME is 6 wt.% and 12 wt.%, respectively, from the mass of diesel fuel). Table 1 lists the mass concentrations of components for the production of FAME and diesel-biodiesel blends.

Table 1

Components for producing FAME and blends of biodiesel samples
Fuel	Components	Mass Concentration (wt.%)
FAME (B100)	Methanol	24.75
	Potassium hydroxide	1
	Rapeseed oil	74.25
B6	FAME	6
B6	Diesel fuel	94
B12	FAME	12
B12	Diesel fuel	88
Diesel	Diesel fuel	100

FAME is produced from refined rapeseed oil, methanol (chemically pure) as an esterifying agent, and potassium hydroxide (chemically pure) as a catalyst. The choice of substances is due to the fact that methyl esters, unlike ethyl esters, show higher power output and torque in engine tests. When using ethanol, problems arise when washing biodiesel from excess alcohol, since ethanol forms a stable emulsion. Methyl esters are stored much longer than ethyl esters. Potassium hydroxide (KOH) is used as the catalyst, since the base catalysts are preferable to acid ones due to increased activity and reduced reaction temperature.

2.2. Biodiesel preparation using a self-developed nanofiber-based membrane reactor

At the first stage of preparation, 4 g of KOH is dissolved in 100 ml of methanol. Then, 300 ml of rapeseed oil is added to the resulting mixture. After that, this mixture is poured into a nanomembrane reactor manufactured at the in-house multichannel electrospinning unit “NIKE-1” from a spinning solution – a mixture of N, N-dimethylformamide (DMF) and acetone in a ratio of 15/85 wt.%, to which polyvinylidene fluoride (PVDF) and a copolymer of vinylidene fluoride with tetrafluoroethylene (VDF-TeFE) are added in a ratio of 50/50 wt.% with a total polymer ratio in solution of 8 wt.%. Cylindrical polymer nanomembrane has the following dimensions: inner diameter of 100 mm, 150 mm in length, 320 µm in thickness. According to scanning electron microscopy (SEM) performed using the TESCAN VEGA 3 SBH instrument (Tescan, a.s., Czech Republic), the nanomembrane is formed by randomly intertwining fibers with an average diameter of 1.03 ± 0.04 µm. The surface area of pores is 147.41 ± 10.13 µm², and the surface porosity of nanomembranes is 34.57 ± 2.41%. According to the Brunauer–Emmett–Teller (BET) method for the mathematical characterization of the physical adsorption of gas molecules on a solid surface, the specific surface area of the nanomembrane is 4.6498 m²/g, the pore volume is 0.008549 cm³/g, and their average size is 7.35403 nm. In the reactor, the liquids are mixed using an overhead stirrer US-8100 (a speed of rotation of 100-3,000 rpm is set with an accuracy of ± 2%) at a given rotation speed of 650 rpm. To maintain a constant liquid temperature of 70°C, a coiled heat exchanger is placed in the reactor. An external circuit of a cryostat (an operating temperature range from − 30°C to + 200°C, an accuracy of ± 0.1°C) is connected to the exchanger. As a result, during the chemical reaction, FAME of rapeseed oil – biofuel B100 – pass through the porous walls of the reactor and collect in a laboratory beaker for further use.

2.3. Liquid properties

To determine the cetane number, an octane meter with a measuring range from 30 to 60 units and a measurement error ± 2.0 units is employed. The octane meter sensor is immersed in a test tube with a biofuel sample, after which the cetane number of the test sample is measured. Three series of measurements are performed for each sample, and the average value is calculated.

The flash point is determined by the Pensky-Martens closed-cup test. The measurements are performed at regular intervals when the temperature of the test sample increases by 1°C. The lowest temperature, at which the fuel vapors are ignited, is the flash point under atmospheric pressure.

To measure fuel viscosity at 40°C, an external circuit of the cryostat is connected to the Brookfield DV3T-LV viscometer (an accuracy of ± 1%, a measuring range of 1...6·10⁶ mPa·s). In particular, to maintain a fuel temperature of 40°C, this circuit passes through the Small sample adapter (SSA) in which the fuel sample is located at the measurement. Temperature is monitored by a thermocouple built in the SSA (an operating temperature range from − 15°C to + 100°C). Measurements at 40°C are carried out in a range of shear rates from 100 s^− 1 to 250 s^− 1 in 1 s^− 1 increments. In this range of shear rates, the torque is 10–100 N\(\cdot\)m. For all biofuels, the three measurements are made for each shear rate.

The density of biofuel is measured by the pycnometric method using the average density values in three measurements using the Vibra AF 225DRCE analytical scales with a sensitivity of 10^− 6 g.

The surface tension factor is measured using the Kruss K20 tensiometer with a measuring range of 1...999 mN/m and a resolution of 0.01 mN/m by the Wilhelmy plate method. For each biofuel, a series of five measurements is performed with a further automatic calculation of the average values.

Table 2 reports the results of measuring the properties of the studied biofuels. The properties of diesel fuel are documented in the product passport received at the local gas station “Gazpromneft". The detailed properties are given in Table A1 in Supplementary materials.

Table 2

Properties of the considered liquid fuels
Parameter	Fuel
Parameter	Diesel	B6	B12	B100 (FAME)
Cetane number, units	53	55.7	57.6	71
Closed-cup flash point, °C	66	62	63	> 170*
Viscosity at 40°C, mm²/s	3.07	2.53	2.54	3.81
Density, kg/m³	841	830	834	869
Surface tension factor, mN/m	28.1	27.4	27.5	30.5

* The upper limit of the thermometer measurement is 170°C. The flash point was not recorded up to this temperature.

2.3. Test rig

Figures 1a and 1b demonstrate the scheme and photograph of a test rig with a swirl burner, respectively. It is equipped with a combustion chamber with optical access (see design information in Fig. A1 in Supplementary materials) and the EGR system. The test rig allows the analysis of the characteristics of atomization (number, size, velocity and trajectory of liquid droplets) and combustion (flame temperature and structure), and the concentrations of such gaseous combustion products as CO, NO_x, CO₂, O₂).

The fuel storage tank (8) is a metal cup (see design information in Figs. A2 and A3 in Supplementary materials). The following components are installed in the upper part of the cup: a pressure gauge (5), a fuel loading unit (6), a gas outlet pipe to the atmosphere with a control valve (4) and a gas tank filling pipe with a control valve (3). The tank is filled with gas using a nitrogen cylinder (2) and a pressure regulator (1). Fuel is atomized through a Danfoss 0.4 x 60 S 030F6904 diesel nozzle with a spray angle of 60° (13), above which ignition electrodes (12) are installed to form a spark. To control the fuel atomization, an electric valve (10) and a ball valve (9) are used. A designed axial-vane swirler of a gas-liquid flow (25) is installed in front of the combustion chamber (23) to enhance air-fuel mixing (see the insert of Fig. 1a). An oxygen concentration sensor (14) is placed behind the combustion chamber, showing the air/fuel ratio AFR. This ratio is regulated by means of a centrifugal fan regulator (11). To reduce the exhaust gas temperature, a water heat exchanger of the spiral type (15) made of stainless steel is used. This heat exchanger cools the exhaust gases by heat transfer between the heated (gases) and cold (water) media. Recirculation of water from tank (21) through the heat exchanger is carried out using a circulation pump (22) with a maximum volume flow of 2.5 m³/h. After cooling, the exhaust gases are released into the atmosphere through pipe (19). However, when the EGR system needs to be operated, throttle valve (18) opens. To determine the component composition of the gas combustion products, a gas analyzer (20) and a probe (16) installed in the heat exchanger casing (15) are utilized. The location of the probe is determined depending on the exhaust gas flow velocity; for the most reliable measurement, the location with the lowest flow velocity is required. Therefore, the most suitable location of the probe is the heat exchanger casing, namely, behind the heat exchanger relative to the motion direction of exhaust gases. Magnetic stirrer (27) with a heating option is employed to maintain a constant and homogeneous fuel temperature in the storage tank (8) at a level of 20°C ± 1°C.

2.4. Biodiesel atomization characterization

To study fuel atomization with and without a gas-liquid flow swirl, a series of three experiments is performed at injection pressures (P) of 0.6 MPa and 1.2 MPa. At the same time, the air flow velocity varies in the range from 11 m/s to 13 m/s. The spray process is recorded by shadow video recording at a distance of 7 cm from the nozzle orifice in the central part of the viewing window of the combustion chamber. Video recording is carried out using a high-speed video camera (scale factor 0.0248 mm/px, sample rate 25,000 fps, resolution 384\(\times\)288 px) with a lens (focal length 200 mm, aperture 11), a LED spotlight and a PC with software for processing and post-processing the captured video material. Post-processing of the resulting image is performed in the Matlab software package using the in-house code that implements the Kalman filtering method [44]. This filter allows tracking individual droplets in the flow, while determining their number, size, trajectory, and velocity according to the DIN SPEC 91325 [45]. The Kalman filter also allows predicting the future location of a droplet based on the equation of motion. To perform post-processing using the Kalman filter, the original video recording consisting of 500 frames is uploaded to Matlab. In each video frame, droplets are detected and assigned an individual track number, which allows tracking the detected droplets on subsequent video frames. For each detected droplet, the area S in pixels is calculated, and then the droplet diameter is calculated using the following formula: \(d=\sqrt {{{4S} \mathord{\left/ {\vphantom {{4S} \pi }} \right. \kern-0pt} \pi }}\), where S is the area of a spherical droplet. Multiplying by a scale factor, the diameter is converted from a pixel value to a millimeter value. Calculating the droplet diameters on all video frames enables the determination of the arithmetic mean diameter (d₁₀), Sauter mean diameter (d₃₂), and droplet velocity magnitude (U) estimated as \(U=\sqrt {U_{x}^{2}+U_{y}^{2}}\), where U_x and U_y are the droplet velocities along the X-axis and Y-axis, respectively. Diameter d₁₀ is calculated as: \({d_{10}}={{\sum d } \mathord{\left/ {\vphantom {{\sum d } N}} \right. \kern-0pt} N}\), where d is the droplet diameter, N – total number of droplets. The diameter d₃₂ is the one of a droplet that has the same volume-to-surface ratio as the total volume (V) of all droplets relative to their total surface (A), and is calculated as: \({d_{32}}={{d_{{\text{V}}}^{3}} \mathord{\left/ {\vphantom {{d_{{\text{V}}}^{3}} {d_{{\text{S}}}^{{\text{2}}}}}} \right. \kern-0pt} {d_{{\text{S}}}^{{\text{2}}}}}\) where d_S is the surface diameter of droplet set, d_V – volume diameter of droplet set. The values of d_S and d_V are determined as: \({d_{\text{V}}}={\left( {{{6V} \mathord{\left/ {\vphantom {{6V} \pi }} \right. \kern-0pt} \pi }} \right)^{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-0pt} 3}}}\) and \({d_{\text{S}}}=\sqrt {{A \mathord{\left/ {\vphantom {A \pi }} \right. \kern-0pt} \pi }}\). Averaged droplet velocity fields and droplet trajectories are also constructed using a Kalman filter based on video fragments consisting of 10 consecutive frames. Figure 2a depicts a region of the atomized flow and the averaged velocity field. The size of the analyzed region for the averaged velocity field is 2.232\(\times\)2.248 mm.

2.5. Biodiesel combustion characterization

Measurement of the flame temperature in the combustion chamber T when EGR is on and off, as well as visualization of the flame (no EGR) of diesel and biodiesel fuel with and without a swirl of the gas-liquid flow are carried out at P = 0.6 MPa and P = 1.2 MPa; the air flow velocity ranges from 11 m/s to 13 m/s. The temperature T is measured in the combustion chamber using a multifunctional thermometer (error ± 1.5%, operating conditions 0–40°C, temperature range 250-1,767°C) and a type-K temperature probe (temperature range 200-1,372°C). Before each measurement, the combustion chamber is cooled to room temperature. The flame structure is recorded at a distance of 7 cm from the nozzle orifice in the central part of the viewing window of the chamber using a high-speed color video camera (sample rate 4,200 fps, resolution 1,280\(\times\)800 px) with a lens (focal length 200 mm, aperture 11) and a PC with software for processing and post-processing of video data.

The delay time of gas-phase ignition (t_delay) refers to the time interval from the beginning of the thermal effect on the sample to the implementation of the flame combustion of the vapor-gas shell around the fuel droplet. The values of t_delay is carried out on an auxiliary experimental setup, which consists of the following elements: a tubular muffle furnace (maximum heating temperature up to 1,100°C, muffle pipe inner diameter 30 mm), a high-speed monochrome video camera (sample rate 2,500 fps, resolution 1,280\(\times\)800 px) with a lens (focal length 50 mm, aperture 2.8), and a robotic linear displacement device. A fuel droplet with a diameter of 2 ± 0.2 mm is moved by the device in a muffle furnace, and a high-speed camera records the fuel ignition process from the opposite side. For more information, see Ref. [46].

To determine the stoichiometric mixture in the combustion chamber, at which λ = 1, the mass flow rates of fuel Q_m and air Q_V are measured. Figure 3 provides the mass flow rates at λ = 1 and ALR = 14.5.

2.6. Analysis of exhaust gases

Tests on fuel combustion product analysis are carried out on a test rig (Fig. 1) with the EGR system turned on and off in the presence of a gas-liquid flow swirl. The injection pressure P is 0.6 MPa and 1.2 MPa. The EGR system is a device that allows returning some of the exhaust gases to the combustion chamber for mixing with air and fuel. As a result, concentrations of nitrogen oxides (NO_x) are reduced, which helps to reduce their emissions into the atmosphere [30, 32, 47]. The role of the EGR system is played by a throttle valve and a recirculation path. When the throttle valve is opened, some of the exhaust gases are fed through the path to the combustion chamber for reuse. The gas analyzer used has the following characteristics: a measurement accuracy of ± 5% and a sensor response time of 5–15 s. It is equipped with the following sensors: electrochemical for O₂ (± 0.2 vol%, 0–25%), NO (0–2000 ppm, ± 5%), and NO₂ (0–500 ppm, ± 7%), as well as optical ones for CO₂ (0–30%, ± 2%) and CO (0–50%, ± 5%). During measurements, the gas analyzer plots real-time curves of exhaust gas concentrations. A universal broad-band lambda probe (Bosch, accuracy 0.1 ALR) with a diameter of 52 mm and a measuring range of 11–17 is employed to determine an ALR. It is located at the exit of the combustion chamber to monitor a proper stoichiometric ratio. During the tests, the ALR is maintained at a constant value of about 14.5. This ratio ensures complete combustion of diesel fuel according to the findings from Ref. [48].

3.1. Nanomembrane-prepared biodiesel atomization

This section presents the hydrodynamic and kinematic characteristics of the droplets in a spray flow of diesel fuel, biofuel B100, blends B6 and B12, which include N, d₃₂ and d₁₀, U, the averaged velocity fields of droplets and their trajectories. The values of Q_V are 17.23 g/s and 18.83 g/s at P = 0.6 MPa and P = 1.2 MPa, respectively. The pressure of the external gas-air environment is atmospheric. The ambient temperature is 20°C ± 1°C. For the reliability of the results, three experiments are performed under identical conditions. The atomization characteristics are analyzed using video fragments with a duration of 0.02 s.

Figure 4. Relationships of the number of droplets N (a) and the arithmetic mean diameter d₁₀ (b) with the injection pressure (P = 0.6 MPa and P = 1.2 MPa) and the corresponding mass air flow rate (Q_V = 17.23 g/s and Q_V = 18.83 g/s) in the presence of a gas-liquid flow swirl and without it

The results in Fig. 4 report the effects of such factors as P, the mass fraction of FAME in a fuel, and the swirl of the gas-liquid flow on N and d₁₀. With an increase in P, an increase in the values of N from 10–40% is noticeable in both swirling and non-swirling flows (Fig. 4a). Due to an increase in N, a growth of P results in decreasing the values of d₁₀ by about 20% (Fig. 4b), since the number of larger droplets is reduced [25]. With a rise of the FAME fraction, a moderate increment of d₁₀ occurs by about 10%, which is consistent with the conclusions from Refs. [49, 50]. This effect results from the elevated viscosity and surface tension of biodiesel B100 (Table 2), as well as the lowest mass of gas entrainment and poor air-fuel mixing. Together with an increase in d₁₀, the values of N diminish quite significantly – by approx. 30%. This is caused by the combined effect of fuel viscosity and surface tension: a lower biodiesel injection rate results in a larger droplet size [32]. Similarly, when using an additive of FAME the parameter of d₃₂ escalates (Fig. 5) due to higher viscosity and surface tension of FAME (Table 2); similar conclusions on increasing d₃₂ are made for palm and cooked oil biodiesel under ultra-high injection pressures up to 300 MPa by Wang et al. [31]. This represents a clear trend for biodiesel from various raw materials, sprayed at both low and high injection pressures.

The parameter N declines by about 25% in the swirling flow relative to the non-swirling due to the combined effect of droplet sticking to the swirler (see the arrangement of nozzle and swirler in Fig. 1) and the coalescence of droplets due to their contact in the swirling flow (Fig. 6). This effect is most noticeable at P = 0.6 MPa (Figs. 4a and 4b).

At P = 1.2 MPa, the difference in N values for swirling and non-swirling flows is not so significant (the difference is less than 15%), since due to the high flow velocity, liquid droplets stuck to the swirler are immediately carried away by the air flow. To estimate the gas-liquid flow velocity, Fig. 7 provides the averaged velocity fields of liquid droplets at P = 0.6 MPa and P = 1.2 MPa.

The averaged velocity fields (Fig. 7) exhibit that N in the video recording area rises with increasing P, but their average size d₁₀ declines, as noted earlier (Fig. 4b). The parameter N can be estimated from local maxima in the averaged velocity field, and the parameter d₁₀ – from the size of the characteristic region around the local maximum (Fig. 4b). The local maximum characterizes the velocity of the liquid droplet in the flow, and the area around the droplet is the air flow that flows around this droplet. Accordingly, the larger the area around the local maximum, the larger the liquid droplets should be. In a swirling flow, the regions of local maxima are rather blurred (Fig. 7b and Fig. 7d). However, the number of local velocity maxima increments with rising P, and the sizes of characteristic regions around local maxima contract. Although the velocity field of the swirling flow still retains a feature in the form of blurred local maxima (Fig. 7d). Both individual liquid droplets and some agglomerates of several droplets in the flow are also noticeable (Fig. 7d). This is explained by the chaotic motion of droplets in a swirling flow, which eventually leads to coalescence of droplets with each other and the formation of larger liquid fragments (Fig. 6). When the droplets collide, they change their initial direction of movement, and the velocity in the flow can also falls off locally. Figure 8 illustrates the trajectories (velocity vectors) of the movement of B100 droplets in non-swirling (Fig. 8a) and swirling (Fig. 8b) gas-liquid flows at P = 0.6 MPa.

The velocity vectors of B100 fuel droplets in a non-swirling flow (Fig. 8a) are very different from those in a swirling one (Fig. 8b) in a number of important aspects. The direction of the vectors characterizes the nature of the forced perturbed flow (Fig. 8b), while the vectors of the non-swirling flow (Fig. 8a) are mainly co-directed. The absolute value of the droplet velocity and the degree of inhomogeneity of the velocity field can be characterized by the size of the vectors. Differences in the velocity and its vectors for droplets in the swirling flow indicate a vortex character of movement (Fig. 8b) [51]. The absolute velocity of fuel droplets in a gas-liquid flow can be estimated using the velocity magnitude U, which represents the velocity vector. Figure 9 provides a histogram with the absolute velocities of liquid droplets U in swirling and non-swirling gas-liquid flows at different P.

The data in Fig. 9 allow us to distinguish the effect of forced perturbation of the gas-liquid flow on U. Due to mechanical impact on the flow (artificial turbulization), the absolute velocity of liquid droplets in the flow declines by \(\sim\)25% at P = 0.6 MPa, which is well consistent with the conclusions in Refs. [51–53]. However, at P = 1.2 MPa, the effect is more neutralized, and the difference is no more than 10% compared to the case at P = 0.6 MPa. Differences in U in a non-swirling gas-liquid flow (Fig. 9) are less than 5%, while in the swirling one (Fig. 9) they are more than 25%.

3.2. Nanomembrane-prepared biodiesel combustion

3.2.1. Visualization of the flame structure

To illustrate the effect of swirling the flow on the flame structure, we provide for the frame sequences of the flame generated while burning biodiesel B6 at P = 0.6 MPa and P = 1.2 MPa under conditions of swirling and non-swirling gas-liquid flows (Fig. 10).

Based on the flame frames, a qualitative difference in the flame structure under artificial turbulization is distinguished relative to the case with an initially unperturbed gas-liquid flow. At P = 0.6 MPa (Figs. 10a and 10b), it is seen how the swirler works. Figure 10a demonstrates the effect of laminarization and homogeneity in the flame structure in the absence of artificial disturbance of the gas-liquid flow, see white horizontal directional arrows. While in Fig. 10b (see white rounded multidirectional arrows) the movement of flame vortices is noticeable, which characterizes the operation of the gas-liquid flow swirler. At an increase in P to 1.2 MPa (Figs. 10c and 10d), heterogeneity and vortex formation are similarly recorded with respect to the flame structure only when the gas-liquid flow swirler is activated (Fig. 10d). In the case with a deactivated swirler, there are bends in the air duct along the path of the air flow from the fan to the combustion chamber, as a result of which the flow having a weak vortex structure becomes unstable. The structure is nevertheless not explicitly revealed in high-speed video recording (Figs. 10a and 10c). When using a gas-liquid flow swirler, such instabilities become much larger, which is reflected in the obvious heterogeneity of the flame structure – the formation of multiple flame vortices, judging by the frame sequences in Figs. 10b and 10d.

3.2.2. Flame temperature in the combustion chamber. Fuel ignition delay time

To assess the effect of EGR on the flame temperature in the combustion chamber T, Fig. 11 shows a histogram with the values of T at a variable pressure P and an activated gas-liquid flow swirler. The temperature T is recorded 30 s after the fuel-air mixture combustion is initiated. This time period determines the stabilization of the combustion process according to the steady-state data on the dynamics of the exhaust gas emissions in Fig. A4 in Supplementary materials.

The readings of T demonstrate the effect of EGR on the temperature regime in the model combustion chamber. Due to the displacement of the oxidizer in the combustion chamber by recirculated gases, the combustion temperature of the fuel decreases, which positively affects the NO_x release during combustion. Thus, EGR helps to reduce the concentration of NO_x [30, 32, 40, 47]. The minimum value of T is fixed for biodiesel B100. Accordingly, the admixture of FAME into diesel fuel reduces the temperature in the combustion chamber (Fig. 11), which corresponds well to the conclusions in Refs. [40, 54]. However, when EGR is on (Fig. 11), the differences in T become less.

The efficiency of the ignition system in the combustion chambers can be evaluated not only by the corresponding starting characteristics, but also by other indicators, in particular by the ignition delay time t_delay of the air-fuel mixture after switching on the ignition [55]. In a swirl burner (Fig. 1), the ignition occurs due to an electric discharge that occurs between high-voltage wires. Further, the fuel is ignited by providing a high temperature of the oxidizing medium in the combustion chamber. Therefore, it is proposed first of all to study the effect of the FAME additive on t_delay of fuel samples when heated in a muffle furnace under close operating conditions of the combustion chamber burner. To assess the effect of the fuel component composition, single droplets with a size exceeding ~ 10 times the size of fuel droplets sprayed by the nozzle into the combustion chamber are utilized. Based on the data in Fig. 12, it is concluded that the addition of FAME to diesel fuel in the amount of 6 wt.% and 12 wt.% has no significant effect on t_delay. With an increase in the heating temperature (T_c) in the muffle furnace in the range from 500°C to 900°C (covers the recorded flame temperatures T in the combustion chamber), t_delay of the studied fuels decreases by 1.4-8 times due to the intensification of the droplet heating. At T_c = 900°C, the difference between t_delay does not exceed 10%, indicating that an increase in the oxidizer temperature minimizes differences in the component composition of fuel mixtures. The ignition delay of biodiesel B100 is less in comparison with other fuels at T_c from 500°C to 800°C due to the higher cetane number (Table 2); a similar approach to explaining chemical autoignition delay for soybean biodiesel blends with ethanol and butanol additives is given by Liu et al. [56].

The temperature in the combustion chamber T exceeds 640°C (Fig. 11). At such temperatures of the gas-air environment, t_delay of biodiesels B6 and B12 differs by no more than 8% from the same indicator for diesel fuel (Fig. 12). Thus, it can be assumed that under the ignition conditions considered in this paper (Fig. 1), varying the fuel component composition should not make a noticeable change in t_delay, especially at T > 640°C. Therefore, additional improvements to existing power plants with similar ignition systems are not required, i.e. typical fuel ignition systems can be used. A similar result was obtained by Gaidukova et al. [57] when analyzing t_delay of various hydrocarbon fuels.

3.3. Exhaust gas when burning a nanomembrane-prepared biodiesel

Analysis of the exhaust gas emissions during the combustion of biofuels is an important aspect that allows us to prove the feasibility of using biofuels and the efficiency of the power plant [58]. To assess the influence of such factors as P, the mass fraction of FAME in a fuel, and EGR, Fig. 13 reports averaged emission concentrations in a quasi-stationary combustion mode at P = 0.6 MPa and P = 1.2 MPa. The dynamics of emissions of O₂, CO₂, NO_x, and CO in quasi-stationary combustion mode at P = 0.6 MPa and P = 1.2 MPa, when EGR is off and on, is presented in Supplementary materials (Figs. A4a–A4h).

Based on the analysis of the dynamics of O₂ in combustion products, an increment in P combined with an increase in Q_m to maintain λ = 1 results in a decline of the O₂ concentration from 70–20%. However, for the sample B100, a growth of the O₂ concentration by 20% is recorded with rising P both in the case of EGR and without EGR. The growth of the oxidizer concentration is caused by the presence of oxygen molecules in the FAME [37]. This feature is also characteristic of the biodiesel B12 (Fig. 13a). It consists of 12 wt.% of FAME (Table 1), which may contribute to a certain increase in the oxidizer content in the exhaust gases.

As P increases, fuel consumption increases, as does air consumption. In this regard, Fig. 13b demonstrates an increase the CO₂ concentration during the transition from P = 0.6 MPa to P = 1.2 MPa. The fuel burns more completely over the same time interval and, accordingly, the CO₂ release becomes larger. The values of the CO₂ concentration shown in Fig. 13b indicate that the completeness of fuel combustion in all cases is quite close. EGR in the combustion chamber does not significantly affect the completeness of fuel combustion. The presence of FAME in a fuel has little effect on the release of CO₂, the difference is less than 5%.

Data on the NO_x concentration in the exhaust gases allows us to formulate a conclusion about a decrease in it by 3–20% when EGR is on (Fig. 13c). This stems from a decrease in T (Fig. 11) and an increase in the completeness of combustion. However, there is a slight increase in the NO_x concentration in the fuels with FAME relative to diesel fuel [39]. The “biodiesel NOx effect" appears. It consists in a high level of unsaturated compounds during the combustion of FAME, as a result of which more hydrocarbon radicals are formed than during the combustion of diesel fuel, leading to a larger release of NO_x [40, 59].

With an increase in P, the CO concentration rises (Fig. 13d), which, together with a decrease in the O₂ concentration, indicates the incomplete fuel combustion [32]. The utilization of EGR also induces the CO concentration rise. However, when burning the biodiesel B12, the CO concentration reduces both with an increase in P and with the use of EGR. Based on this, it can be concluded that the positive effect of FAME on the completeness of fuel combustion. This is caused by the carbon neutrality of FAME and the oxidizer content in it. These results may explain the lower CO/CO₂ values for the combustion of biodiesel B12 (Fig. 14). For further analysis of the CO/CO₂ ratio, its values are given when the mass fraction of FAME in a fuel changes (Fig. 14). This ratio indicates the incomplete fuel combustion, since a high CO concentration is a reason for underburning. The higher this ratio, the greater the fuel underburning.

Since the biodiesel B12 has the lowest CO/CO₂ values (Fig. 14), this fuel is characterized by the best indicator of the completeness of fuel combustion, which is also seen earlier in Fig. 13d. However, when using EGR, an increase in the CO/CO₂ ratio can be observed for all the fuels studied. This indicates a lack of oxidizer in the combustion chamber at the same ALR. Nevertheless, for the biodiesel B12, the completeness of combustion enhances with an increase in P, based upon a decrease in CO/CO₂. Khiraiya et al. [41] note the maximum decrease in the CO concentration relative to diesel fuel for blends B50 (by 12%) and B25 (by 9%). This effect, as in our case (Fig. 11), is attained by reducing the temperature in the combustion chamber [41]. However, too low a temperature in the combustion chamber can cause high concentrations of CO emissions [41]. That is why the effect of reducing the CO/CO₂ ratio is not a case when burning B100 in our study; when burning B100, reduced T values are recorded (Fig. 11).

3.4. Biofuel efficiency factor. Future practical development of the research

The preparation of a bio-additive for diesel fuel using nanomembrane technology optimized for labor and time costs has shown efficiency and robustness in atomization and combustion tests, as well as environmental sustainability. To highlight the most efficient biodiesel composition, we will refer to the results of atomization, combustion, and exhaust gas analysis. Based on the combination of three major factors, the biodisel B12 can be distinguished. When analyzing the spray characteristics, the differences in N from diesel fuel do not exceed 11% in the swirling flow; in d₁₀, they are less than 10%, similarly for d₃₂. The gas-phase ignition delay of the biodiesel B12 differs from diesel fuel by no more than 3%, and the concentrations of NO_x and CO emissions in the exhaust gases are lower than those of diesel fuel by 10% and 44%, respectively. Biodiesel B12 shows similar atomization and combustion characteristics to diesel fuel, and at the same time allows reducing the amount of harmful emissions in exhaust gases. To quantify the effectiveness of the fuels under consideration (Table 3), the weighted sum method is applied. An analysis of the values in Table 3 allows the highlighting in green the benchmark fuel with the best indicator within each of the rows. In such a case, the indicator is taken as equal to 1. For the remaining three fuels, an indicator is calculated against the best value for each of the analyzed characteristics. A similar approach is used, for example, in recently published studies [60, 61].

Table 3

Generalized data on the analyzed characteristics and results of calculating the fuel efficiency using the weighted sum method; the benchmark fuel with the best indicator is highlighted in green within each of the rows
		Weight			Sample
Type of parameters	Characteristic	V1	V2	V3	Diesel fuel	Rapeseed biodiesel fuel B6	Rapeseed biodiesel fuel B12	Rapeseed biodiesel fuel B100 (FAME)
Hydrodynamic parameter (with flow swirling, at P = 1.2 MPa)	N (pcs)	0.2	0.1	0.1	135 000	105 000	97 000	88 000
Kinematic parameter (with flow swirling, at P = 1.2 MPa)	Absolute values of liquid droplet velocity U (m/s)	0.2	0.1	0.1	20	19	20.5	20.8
Energy parameters	Ignition delay time t_delay (s) at T_c = 700°C	0.1	0.05	0.05	1.50	1.58	1.37	1.25
Energy parameters	Flame temperature in the combustion chamber T (°C) when EGR is on and P = 1.2 MPa	0.1	0.05	0.05	650	648	645	640
Environmental parameters, EGR operates at an equivalence ratio λ = 1 and P = 1.2 MPa	NO_x (ppm)	0.1	0.05	0.3	55.1	68.6	49.7	46.7
	CO / CO₂	0.1	0.05	0.3	0.47	0.44	0.27	0.37
Price in RUB for one liter of fuel in Russia	for FAME – the costs for nanomembrane manufacturing included; data for commercial diesel fuel from Ref. [62].	0.2	0.6	0.1	59.52	112.95	166.38	950
Weighted sum of criteria		Yes	No	No	0.92	0.75	0.80	0.72
		No	Yes	No	0.96	0.64	0.58	0.39
		No	No	Yes	0.81	0.70	0.88	0.79

It is important to note that Table 3 considers the most popular conditions for determining fuel characteristics from a technological point of view (EGR, swirling gas-liquid flow, increased injection pressures) with the maximum change in the weighted sums of criteria from 0.39 to 0.96. Consideration of other conditions will make this range even wider, starting from ~ 0.15–0.2. Table 3 examines three variants of the distribution of weight factors within the framework of the weighted sum method: variant 1 (V1) assumes a basic calculation and an equal distribution of factors by types of analyzed parameters; variant 2 (V2) is the calculation with a strong dominance of the weight factor (0.6) for the criterion of the cost of one liter of fuel; variant 3 (V3) is the calculation with a strong dominance for the environmental criterion. Within the framework of V1, the biodiesel B12 is closest to diesel fuel in terms of efficiency (weighted sum of criteria). In conditions where the cost of fuel is the determining criterion, the weighted sum of criteria for biodiesel is greatly reduced, almost 2–3 times, and the amount of bio-additive to diesel fuel proportionally affects the efficiency of biofuels. Although, the price for the FAME production already includes the reduced labor and time costs compared to the multi-stage transesterification-based method. From the standpoint of environmental sustainability (V3), the weighted sum of criteria for rapeseed biodiesel B12 exceeds this indicator for diesel fuel, which certainly confirms the feasibility of optimized nanofiber-based biofuel production.

The findings demonstrate a strong foundation for a simple and affordable green energy technology for small-scale power generation. Work on the development of this technology can be certainly continued in the future within the following main vectors:

(a) analysis of energy indicators of the energy production process;
(b) further optimization of the FAME production for subsequent scaling to increased productivity in order to provide a small-scale test facility with green fuel;
(c) development of a closed-loop system both in terms of raw materials for biofuels and in terms of the efficiency of the power plant.

An analysis of the main results of the study, indicating the specific obtained values of the measured quantities and the observed effects, is given below in the form of a bulleted list:

An increase in the injection pressure P from 0.6 MPa to 1.2 MPa has a noticeable effect on the hydrodynamic characteristics of the spray flow of all fuel samples considered. Regardless of the type of flow (swirling and non-swirling), we revealed an increment in the number of droplets N by 25%, a decline in their arithmetic mean diameter d₁₀ by 20%, and a decrease in their Sauter mean diameter d₃₂ by 8%. Based on the Kalman filtration method, an approach is proposed for analyzing the hydrodynamic characteristics of the spray flow through the kinematics of the process, i.e. by processing data from the averaged velocity and trajectory fields of liquid droplets.
As the mass fraction of FAME in a fuel increases, the arithmetic mean diameter of liquid droplets d₁₀ rises by 10%, and N reduces by 30%. This is caused by the combined effect of the viscosity and surface tension of the FAME.
Perturbation of the gas-liquid flow by forced swirling using an axial-vane swirler, on the one hand, significantly reduces the number of droplets in the flow N (by ~ 25%) and contributes to an increase in the arithmetic mean diameter d₁₀ (by ~ 10%) when compared with similar characteristics in the case of a non-swirling flow. On the other hand, it minimizes differences in the blend composition of the sprayed fuel.
Under similar operating conditions of the swirl burner provided in the muffle furnace chamber, the ignition delay time t_delay of the studied fuel blends B6 and B12 differs by no more than 8% from the same indicator for diesel fuel. This fact allows us to recommend the operation of the developed power plant (test rig) on the produced biodiesel fuel without re-engineering.
The positive effect of EGR on the flame temperature in the combustion chamber and the concentration of harmful emissions in the exhaust gases is detected. The flame temperature in the combustion chamber decreases depending on the fuel composition and injection pressure P up to 50°C. The CO₂ concentration does not change significantly, which indicates the completeness of fuel combustion. The NO_x concentration in the exhaust gases decreases by an average of 11%. However, with respect to the CO release, the EGR operation tends to increase the CO concentration by ~ 14% due to a lack of oxidizer.
The fuel efficiency is estimated using the weighted sum method; the performance of biodiesel produced using nanomembrane technology is analyzed within the framework of three variants of distribution of weight factors by types of analyzed parameters: basic calculation with equal weight factors, calculations with strong dominance of price and environmental criteria. Biodiesels B6 and B12 demonstrates competitive performance in the basic calculation and calculation with a focus on environmental sustainability.

A Surface area of particle set, m²

d Particle diameter, mm

d_SSurface diameter of particle set, mm

d_V Volume diameter of particle set, mm

d₁₀Arithmetic mean diameter, mm

d₃₂Sauter mean diameter, mm

d^av₁₀Average value of d₁₀, mm

d^av₃₂Average value of d₃₂, mm

N Total number of particles, pcs

P Injection pressure, MPa

S Area of a spherical particle, m²

T Flame temperature in the combustion chamber, °C

T_c Temperature of the gas-air environment in the muffle furnace, °C

t Time, s

t_delay Ignition delay time, s

U Absolute particle velocity, m/s

U_x Particle velocity along the X-axis, m/s

U_y Particle velocity along the Y-axis, m/s

V Volume of particle set, m³

Q_v Mass air flow rate, g/s

Q_m Mass flow rate of fuel, g/s

λ Equivalence ratio

Acronym

ALR Air/liquid ratio

HC hydrocarbons

EGR Exhaust gas recirculation

FAME Fatty acids methyl esters

PM Particulate matter

DBE Di-n-butyl ether

SCA Spray cone angle

SMD Sauter mean diameter

STP Spray tip penetration

Funding

The authors are grateful for the financial support received from the Tomsk Polytechnic University (TPU) development program Priority 2030 (Priority-2030-NIP/EB-038-1308-2024).

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

Alexander Ashikhmin: Conceptualization, Writing – original draft, Methodology, Data curation, Investigation, Formal analysis, Visualization. Danila Verkhodanov: Writing - review & editing, Methodology, Investigation, Visualization, Software. Maxim Piskunov: Conceptualization, Supervision, Project administration, Formal analysis, Visualization, Methodology, Writing - review & editing. Pavel Strizhak: Supervision, Writing - review & editing, Resources, Funding acquisition.

Data Availability

Data will be made available on reasonable request.

Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary information

Supplementary materials (docx) contained properties of summer diesel fuel, drawings of combustion chamber and fuel storage tank and additional experimental details of the exhaust gas emissions in quasi-stationary combustion of the studied fuels.

Acknowledgments

The authors express their gratitude to Evgeny Nikolaevich Bolbasov ([email protected]) for manufacturing and characterization of nonwoven membranes kindly provided for the research.

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Operational and Environmental Tests of Nanomembrane-Produced Rapeseed Biodiesel at a Small Biomass-Fuelled Power Plant

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Abstract

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1. Introduction

1.1. Biodiesel atomization

1.2. Biodiesel combustion

1.3. Analyzing pollutions from biodiesel combustion

1.4. Novelty, practical relevance and purpose of the research

2. Experimental methods

2.1. Materials

2.2. Biodiesel preparation using a self-developed nanofiber-based membrane reactor

2.3. Liquid properties

2.3. Test rig

2.4. Biodiesel atomization characterization

2.5. Biodiesel combustion characterization

2.6. Analysis of exhaust gases

3. Results and discussion

3.1. Nanomembrane-prepared biodiesel atomization

3.2. Nanomembrane-prepared biodiesel combustion

3.2.1. Visualization of the flame structure

3.2.2. Flame temperature in the combustion chamber. Fuel ignition delay time

3.3. Exhaust gas when burning a nanomembrane-prepared biodiesel

3.4. Biofuel efficiency factor. Future practical development of the research

4. Conclusions

Abbreviations

Declarations

References

Supplementary Files

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