Upgrading Esterified

Pyrolysis Bio-Oil through a Two-Step Process and Assessing the Performance and Emissions of Diesel–Biodiesel–Transesterified Pyrolysis Bio-Oil Blends in Diesel Engines

Authors: Jarernporn Thawornprasert, Sutthichai Khamhuatoey, Krit Somnuk

Abstract

This study primarily aimed to investigate the effect of diesel–biodiesel–transesterified pyrolysis bio-oil (TPB) blends on performance and exhaust gas when used in diesel engines at various speeds and loads. The pyrolysis bio-oil (PBO) was obtained from fresh palm fruits using a pyrolysis process and then converted into biodiesel using first-step esterification, followed by second-step transesterification. The response surface methodology was used to investigate the optimal conditions for producing TPB from esterified pyrolysis bio-oil in the second step. The methyl ester (ME) purity of the TPB was optimized by varying three methanol content (13.2–46.8 wt %), KOH loading (1.6–18.4 g/L), and reaction time (26–94 min). The experimental results revealed an ME purity of 74.35 wt % under the recommended conditions of 38.3 wt % methanol, 15.1 g/L KOH, and 62 min reaction time at 60 °C with a 300 rpm stirring speed. This study’s workflows included observing phase behavior and evaluating the gas emissions and performance of the diesel–biodiesel–TPB blends in a diesel engine. The results presented that the 30 wt % diesel, 60 wt % biodiesel, and 10 wt % TPB blend (D30B60TPB10) achieved similar performance to that of the diesel in terms of performance under low-emission conditions. Compared with diesel, the D30B60TPB10 blend's brake-specific fuel consumption and brake thermal efficiency rose by 7.19% and 3.88% at maximum load and engine speed, respectively, while CO2 and NO~x~ emissions reduced by 8.73% and 31.37%. Furthermore, D30B60TPB10 and D30B60EPB10 blends were compared to assess the possibility of using TPB after upgrading its properties. The test results indicated that the diesel–biodiesel–TPB blends are an attractive potential alternative biofuel for agricultural engines that would benefit rural communities and farmers.

Introduction

Petroleum fuels were used in several countries to stimulate the economy and develop industries, including transportation, power generation, and agriculture, resulting in higher demand and prices for petroleum resources. However, relying on petroleum fuels has environmental impacts, causing climate change and global warming.^1^ Because of these concerns, several governments are seeking ways to reduce greenhouse gas emissions and increase renewable energy adoption. In recent years, there has been an upward trend in the adoption of and funding for renewable energy sources, such as solar, wind, geothermal, wave, and biomass energy.^2^ Biomass has become an attractive alternative to conventional petroleum fuels because of its renewable energy and potential to reduce greenhouse gas emissions.^3^ Postharvest crop residues, agricultural byproducts, and household waste—such as rice husks, cassava leaves, bagasse, corn cobs, sawdust, palm fibers, and food waste—can be sources of biomass.^4,5^ Biomass can be converted into useful energy using several methods, including direct combustion, thermochemical, chemical, and biological processes.^5^ Thermochemical methods such as pyrolysis, liquefaction, gasification, and supercritical fluid extraction are employed to enhance potential for biomass potential for energy.^6^ The pyrolysis process has received much interest because of its high biomass conversion efficiency at low pressure and its independence from other chemicals. Many studies have highlighted the promise of pyrolysis as a thermochemical conversion process for producing pyrolysis bio-oil (PBO) from agricultural crop residue with a low heating value.^7,8^ PBO is a highly oxygenated organic liquid that includes alcohols, ketones, aldehydes, and carboxylic acids.^9,10^ Many researchers have focused on producing PBO from the solid biomass byproducts of palm oil mills, such as empty palm fruits, oil palm trunks, oil palm leaves, and palm kernel shells.^2,11−15^ In addition fresh palm fruit has been used as a biomass source in our previous research to produce PBO, which is used as fuel for diesel engines.^16^ The benefits of PBO could include improving the engine combustion efficiency and minimizing the environmental impact through lower emissions of NO~x~~, CO, and CO~~2~. Pandey et al.^17^ investigated the effect of diesel–PBO blends (5%, 10%, 15%, 20%, 25%, and 30% PBO blends) on emissions and performance in diesel engines under various load conditions. They found that the bio-oil–diesel blend with 30% bio-oil content emitted the least NO~x~~. The 30% PBO blend also showed the lowest CO and CO~~2~ emissions at the maximum load out of all fuel blends because the high oxygen component of PBO improved fuel combustion. Prakash et al.^18^ studied the performance and exhaust gas of fuel blends used in direct injection (DI) diesel engines. Wood pyrolysis oils (10 and 15 vol %) were blended into methyl esters (ME) of Karanja (KOE10 and KOE15) and Jatropha (JOE10 and JOE15), adding 20 vol % Span as a surfactant to improve phase separation. They found that the CO emissions of JOE10 and KOE10 were 25% lower than those of diesel fuel at all engine loads. Compared with diesel, at maximum load, JOE15’s NO~x~ emissions decreased by 3.0%, while those of KOE15, JOE10, and KOE10 increased by 0.5%, 3.1%, and 2.2%, respectively. At maximum load, the smoke opacity percentages of KOE10, JOE10, KOE15, and JOE15 were 16.4%, 18.5%, 13.5%, and 18.2% less than diesel, respectively. Paramasivam et al.^19^ studied the use of PBO produced from Aegle marmelos seed cake (AM) in a diesel engine, testing the performance and emissions of various AM contents (10%, 15%, and 20% AM). They reported that increasing the bio-oil content in diesel could reduce CO and NO~x~ emissions; at maximum load conditions, the CO emissions of 10%, 15%, and 20% AM blends were 0.21%, 0.45%, and 0.57% lower than those of diesel, respectively, and the NO~x~ emissions of 10%, 15%, and 20% AM blends were 74, 13, and 10 ppm lower than those of diesel, respectively.

To investigate engine performance when using fuels with PBO, Pandey et al.^17^ tested the performance of diesel engines using diesel–PBO blends (5%, 10%, 15%, 20%, 25%, and 30% PBO blends) under various loads. They observed that the brake-specific fuel consumption (BSFC) of all fuel blends decreased with increasing engine load. At full load, the BSFC values were 0.24, 0.25, 0.27, 0.26, 0.26, and 0.25 kg/(kW·h) for the 5%, 10%, 15%, 20%, 25%, and 30% PBO blends, respectively. The BTE of all fuel blends increased with increasing engine load and then gradually decreased after 75% load. At maximum load, the BTE values were 34.69%, 31.24%, 32.03%, 32.36%, 34.85%, and 35.02% for 5%, 10%, 15%, 20%, 25%, and 30% PBO blends, respectively. Paramasivam et al.^19^ investigated the performance of diesel–bio-oil blends (with 10%, 15%, and 20% AM contents) in a diesel engine. The results suggested that adding bio-oil improved BTE and reduced BSFC compared with that of conventional diesel. The highest BTE was achieved with a 10% AM blend. At maximum load conditions, 10%, 15%, and 20% AM blends had BTE values of 29.67%, 26.62%, and 25.18%, respectively. Sukumar et al.^20^ tested PBO produced from sweet lime empty fruit bunches (BOSL) mixed with jatropha ME in a DI diesel engine. The fuel blends of jatropha ME and BOSL (at contents of 5%, 10%, 15%, and 20%) were tested under different engine loads and were compared with diesel. The results demonstrated that all fuel blends had a higher specific fuel consumption than diesel, while the BTE of all blends was lower than that of diesel, although the BTE of the fuel blend containing 5% BOSL was close to those of diesel.

The primary objective of this study was to improve the physical properties of esterified pyrolysis bio-oil (EPB), specifically its density, viscosity, acidity, and ME purity. First-step esterification reduced EPB from PBO to FFA, and then EPB was used as the raw material in base-catalyzed transesterification to produce transesterified pyrolysis bio-oil (TPB). To the best of our knowledge, no prior research has investigated improving the properties of PBO obtained from the pyrolysis of palm fruit using a two-step biodiesel production process. Therefore, this research fills a knowledge gap by enhancing the ME purity and properties of PBO through a two-step process. As previously described,^16^ the esterification process involved mixing PBO with reactants (methanol and sulfuric acid) in the batch process. The optimum conditions for the first step were 44.8 wt % methanol, 13.6 wt % sulfuric acid, and a reaction time of 61 min. To obtain the highest purity of ME in TPB, this research focused on optimizing three methanol, KOH, and reaction time for the transesterification reaction. The effect of these factors on the ME purity of TPB were investigated using the response surface methodology (RSM) and central composite design (CCD).

Research on diesel–biodiesel–TPB blends in DI diesel engines is limited with respect to both performance and emissions testing. Khamhuatoey et al.^16^ previously detailed an investigation of the phase stability, emissions, and performance of diesel–biodiesel–EPB blends under various engine loads and speeds. Few studies have been conducted on the stability, emissions, and performance of TPB derived from second-step blended diesel–biodiesel under different engine loads and speeds. To fill this knowledge gap, the performance (Pb, BSFC, and BTE) and emissions [O2, CO, CO2, NO~x~, smoke opacity, and exhaust gas temperature (EGT)] of diesel–biodiesel–TPB blends in diesel engines were assessed performing at various speeds and loads. Finally, diesel–biodiesel–TPB blends were compared with diesel–biodiesel–EPB blends to assess performance, emissions, and production costs, with the goal of determining the feasibility of these fuels as a sustainable alternative energy source for future fuel development.

Materials and Methods

Materials

The first part of our study on the FFA reduction of PBO used the first-step esterification process in the presence of a sulfuric acid catalyst to produce EPB. In the PBO production process, the oil was extracted from palm fruits through slow pyrolysis. PBO was upgraded to EPB via the esterification process, as shown in eq 1. The EPB was used as the raw material for the first stage, the details of this process are described in a previous paper.^16^ The current study focused on the second step, the transesterification process for synthesizing ME from EPB, in order to complete the entire process of biodiesel production from PBO. A base-catalyzed transesterification process was used to convert the components of EPB into TPB to improve the physical properties and purity of the ME, as described in eq 2. This study used the liquid fuels TPB, diesel (B10, denoting 10% ME blended into 90% diesel), and biodiesel (ME of crude palm oil), blending them to study the phase stability of diesel–biodiesel–TPB (D–B–TPB) blends. The schematic diagram of the whole process, which included slow pyrolysis, biodiesel production, blending, and an engine testing process, is presented in Figure 1. Commercial-grade chemicals with purities of 98% for KOH and 99% for methanol were used in the transesterification reaction of the TPB production process. Table 1 presents the properties of EPB, TPB, diesel, biodiesel, and D–B–TPB blends of D30B60EPB10, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60.12

Figure 1: Schematic diagram of the whole process, which
included slow pyrolysis,
biodiesel production, blending, and an engine testing process.

Upgrading EPB Using the Transesterification

Process

Experimental Design for Producing TPB

The different conditions were manipulated to assess the feasibility of converting ME in the preparation of TPB for blending into the D-B-TPB blended fuel. To assess the optimal conditions for producing ME from EPB, CCD was used for the experimental design and RSM for the analysis. The objective was to determine each parameter of the transesterification reaction to obtain the highest purity of ME in TPB. The experimental design matrix, including the three variables of methanol, KOH loading, and reaction time, was set at five levels with rotatability encoded as −α, −1, 0, +1, and +α, where −α and + α represent the lowest and highest values of each parameter, respectively. The number of variables (k) in the rotatable CCD determines the position of the star points (α~x~~) at an alpha distance from the center. In this study, the k value was equal to three, calculated based on eq 3. Consequently, the α~~x~ levels for this experiment were set at five distinct values for each −1.682, −1, 0, +1, and +1.682. The optimal conditions for achieving the highest ME purity in TPB were investigated by studying factors within the ranges of methanol content (13.2–46.8 wt %), KOH concentration (1.6–18.4 g/L), and reaction time (26–94 min), as detailed in Table 2. Subsequently, the predictive model of ME purity in TPB was assessed using multiple quadratic polynomial regression, as expressed in eq 4. The ME response parameter was modeled using multiple regression with Excel software and an add-in solver analysis tool to determine the optimal conditions for ME production.34where Y is the response variable, xi and xj are the factors, β0 represents the constant coefficients, β~i~~, β~~ii~~, and β~~ij~ are the coefficients of the factors, k is the number of factors, and ε is the error term.

Experimental Procedure for Producing Biodiesel

from TPB

To prepare the transesterification process, 100 g of EPB was poured into a 250 mL beaker. The EPB was stirred and heated to a constant temperature of 60 °C by using a magnetic stirrer (IKA RW 20 digital) at 300 rpm. The methanol and KOH were mixed in a beaker to form a potassium methoxide (CH3OK) solution for the transesterification reaction and stirred with a magnetic stirrer. The experimental conditions are listed in Table 2. After maintaining the reaction temperature at 60 °C, the CH3OK solution was carefully added to the beaker to start the transesterification process and monitoring of the reaction time parameter. After the reaction was complete, the samples were rapidly collected and cooled with cold water to prevent any forward or backward reactions. They were then washed with water to eliminate any remaining impurities. After being washed, the TPB samples were dried at 105 °C for 15 min to remove any remaining moisture before the ME purity was analyzed using the nuclear magnetic resonance (NMR) analyzer.

Blending of Diesel–Biodiesel–TPB

For D–B–TPB blended fuel production, the three fuels were mixed with a magnetic stirrer at 400 rpm for 10 min at 30 °C room temperature to obtain a homogeneous phase and to enable the study of the phase separation of these three components. After fuel blending, the D–B–TPB blended fuel was sealed using plastic wrap to prevent oxidation of the fuel blend and was placed on a shelf for observation of its physical characteristics. The different phases of the fuel blend were observed for diesel (10–80 wt %), biodiesel (10–80 wt %), and TPB (10–80 wt %) blends. The ternary diagram illustrates the stable phases of the diesel–biodiesel–TPB blended fuel. The stability of all blended fuel phases was observed for 30 days at 30 °C. Finally, the blended fuels were studied as a homogeneous phase for density and viscosity in accordance with diesel specifications before being tested in diesel engines.

Fuel Property Testing and Diesel Engine Testing

The properties and testing standards of all of the fuels are shown in Table 1. A Julabo MD-16G Visco Bath (model: Julabo Labortechnik GmbH; Seelbach; Germany) measured viscosity in accordance with ASTM method D445-17a, while a hydrometer measured density in conformance with ASTM D1298-12b. A Herzog CPP 97-2 (Germany) instrument measured pour and cloud points in accordance with ASTM-D97 and ASTM-D2500 standards, respectively. A titration conducted in accordance with ASTM D664-09 tested the acid value. Conforming to ASTM D130-04, Herzog HZ9011 measured copper strip corrosion. A CHNS/O analyzer (model: Flash 2000; Thermo Scientific; Italy) was used to determine the HHV and LHV. The NMR analyzer was used for determining the ME purity of both EPB and TPB fuels in accordance with EN 14103. The interfacial tension (IFT) of the fuel was analyzed using an optical contact analyzer (OCA25, Dataphysics, Germany). A diesel engine (model: Kubota RT 100 DI, Siam Kubota Corporation Co., Ltd., Thailand) and dynamometer (model: DW16, Jiangsu Lan Ling Test Equipment Co., Ltd., China) were used for diesel engine performance and exhaust gas testing. The engine was tested under 25%, 50%, and 75% loads at 1100, 1400, 1700, 2000, and 2300 rpm to compare the performance and emissions of diesel, TPB, and blended fuel. An electronic balance (model: AND EK-300i) measured the fuel consumption in kg/h. During engine testing, the performance and emissions parameters were recorded using a data logger and a gas analyzer. During engine testing, performance parameters were recorded in real-time over a long period using a data logger with various sensors. Emissions were measured three times using a gas analyzer, with data recorded every 10 s for 2 min during each measurement. The results from these repetitions were then averaged to ensure accuracy. The units of emission variables for exhaust gas analysis were converted from vol % and ppm to g/kW h using the following equation.^22,23^5678

Uncertainty Analysis

The uncertainty of the experiment was determined to measure the variability in data results caused by instrumentation errors. Table 3 presents the measuring technique, measurement range, accuracy, and percentage uncertainty of each measured parameter. The percentage uncertainties for brake power (Pb), fuel consumption, BSFC, and BTE were 0.18%, 0.31%, 0.49%, and 1.18%, respectively. These percentage uncertainties were calculated from the techniques for measuring several parameters (rotary encoder, strain gauge load cell, electronic balance, stopwatch, temperature sensor, and CHNS/O analyzer), as listed in the footnotes in Table 3. The overall uncertainty of the performance and emissions experiments was 1.73%, calculated using the overall experimental uncertainty expressed by eq 9.9

Results and Discussion

Prediction Model and Statistical Analysis

of the RSM

The experimental results of the transesterification process used to convert EPB from the first step to a high purity ME in TPB are shown in Table 2. The ME purity in TPB varied between 73.41 and 77.92 wt % under 18 actual experimental conditions. To create a predictive model from those tests, the findings were examined using multiple regression analysis with a 95% confidence level. The prediction model was developed to describe the relationship between ME purity and the three factors of methanol, KOH loading, and reaction time, as presented in eq 10.10where ME represents the ME purity in TPB (wt %), M is the methanol (wt %), K is the KOH loading (g/L), and T is the reaction time (min).

The RSM conducted an analysis of variance (ANOVA) for the predictive model impacting ME purity, as shown in Table 4. Each term’s significance was assessed with p-values. A term with p < 0.05 is considered significant, with the smallest p-value indicating the most significant effect on the prediction model. The predictive model eliminated terms with p > 0.05, deeming them sufficiently insignificant.^24^ The terms β1M and β4M^2^ had the lowest p-values, indicating that methanol content is highly significant for ME purity in the transesterification process. The terms KOH content (β2K) and reaction time (β3T) ranked third and fourth, respectively, with respect to their influence on ME purity. This suggests that KOH catalyst loading and reaction time also play important roles in determining ME purity in TPB. The predictive model’s accuracy was evaluated using the coefficient of multiple determination (R^2^) and the adjusted coefficient of multiple determination (R^2^~~adjusted~~). The R^2^ and R^2^~~adjusted~~ values for the ME purity prediction model were 0.976 and 0.962, respectively, indicating a high predictive capability. Additionally, F-tests were conducted to test hypotheses and assess the significance of the variation within the regression model. The F-test resulted in an F0 value of 73.17, which is significantly higher than the critical F value of 3.09 (F~~0.05, 6, 11~~). Therefore, the prediction model for ME purity in the transesterification process was statistically significant. Figure 2 illustrates the correlation between the predicted and actual experimental ME purity. The number of experiments in this study is sufficient to analyze the effects of various variables on the ME purity. The results validated that the model is suitable for predicting increases in the ME purity. In addition, the validity of the model was assessed using the R^2^ and R^2^~~adjusted~~ values. The model’s high significance and accurate representation of the data are confirmed by both coefficients being very close to 1. These statistical tests thus demonstrated that the model equation can accurately predict the ME purity over all ranges of the experimental variables.

Figure 2: Relationship between predicted and actual experimental ME purity
of TPB in the transesterification process.

Response Surface Plots and Optimal Condition

The relationships between the factors (methanol, KOH loading, and reaction time) and ME purity in TPB via transesterification in a batch process are shown as 3D response surface plots in Figure 3. Figure 3a shows that the methanol content range of 35.6–46.8 wt % and the KOH content range of 7.2–18.4 g/L can produce TPB with an ME purity exceeding 78.00 wt %. Figure 3b illustrates that the methanol content range of 31.9–46.8 wt % and the reaction time range of 48.8–93.6 min can produce TPB with an ME purity of over 78.00 wt %. It can be observed from Figure 3c that the highest ME purity in TPB is achieved within a KOH content range of 7.2 to 18.4 g/L and a reaction time range of 48.8–93.6 min. The independent variables affecting ME purity in TPB were optimized using the solver function in Microsoft Excel. The highest ME purity predicted by the model was 78.29 wt %, achieved under the optimal conditions of 41.8 wt % methanol content, 13.5 g/L KOH content, and 75 min reaction time at a temperature of 60 °C. To confirm the accuracy of the predicted model, these optimal conditions were tested in the actual experiments. This experiment produced TPB with an ME purity of 76.63 wt %, resulting in an error of 2.12% compared to the predicted ME purity. Under optimal conditions, large amounts of methanol are used to achieve the highest ME purity, leading to increased chemical costs. Consequently, the predictive model was set up with an ME purity of 78.00 wt % in eq 10 as it was quite close to the optimal condition. To obtain the appropriate conditions from that prediction model, the three factors (methanol, KOH loading, and reaction time) were resolved. The optimal conditions for achieving 78.00 wt % ME purity in TPB, as recommended by the resolving model, are 38.3 wt % methanol, 15.1 g/L KOH, and 62 min reaction time at 60 °C. These recommended conditions were proved through actual experiments to confirm the ME purity calculated by the model. The results showed an ME purity of 74.35 wt % with an error of approximately 4.6%. As a result, the recommended condition decreases the methanol content and reaction time by 3.5 wt % and 13 min, respectively, while increasing the KOH by 1.6 g/L compared to the optimal conditions. Table 5 presents the optimal and recommended conditions for producing TPB using the transesterification process. Even though a two-step process and more chemicals were used to produce biodiesel from PBO, the purity of TPB was not higher than 74.35 wt %. This is a limitation of the two-step procedure for producing high-purity biodiesel from pure bio-oil derived from fresh palm fruits. Since bio-oil is mostly composed of triglycerides and FFAs, it also contains a variety of other chemicals, including ketones, aldehydes, and carboxylic acids.^9,10^ Therefore, these compounds cannot be converted via esterification and transesterification processes. However, the analysis of TPB properties, including density (860–900 kg/m^3^), viscosity (1.9–8.0 cSt), and copper strip corrosion (<no. 3), meets the requirements of the community biodiesel standard in Thailand for use in agricultural engines.^25^

Figure 3: Three-dimensional
response surface plots illustrate the influence
of three factors on ME (a) methanol and KOH, (b) methanol
and reaction time, and (c) KOH and reaction time.

Table 6 presents the chemical costs used in producing biodiesel from PBO per batch for the first- and second-step esterification and transesterification processes. The esterification process achieved the highest ME purity of 73.26 wt % in EPB under the optimal conditions of 44.8 wt % methanol, 13.6 wt % sulfuric acid, and a reaction time of 61 min, as previously detailed by Khamhuatoey et al.^16^ The total chemical cost of the esterification process was 0.71 USD/batch. After that, EPB was utilized as a feedstock in the transesterification reaction to enhance ME purity and improve the bio-oil properties, such as density, viscosity, and acid value. For the second-step transesterification process, the highest ME purity in TPB of 74.35 wt % was obtained under the recommended conditions of 38.3 wt % methanol, 15.1 g/L KOH, and a reaction time 62 min. The total chemical cost of the transesterification process was 1.08 USD/batch. The total chemical production cost for the whole two-step reaction process was 1.79 USD/batch. Figure 4 shows the percentage of chemical costs in the two-step biodiesel production process. The chemical cost of the two-step production process is primarily attributed to methanol (60%), followed by KOH (33%) and sulfuric acid (7%). It was found that there was no significant difference in the purity of the methyl ester with the esterification and transesterification processes. Thus, it may not be required to produce TPB in the second step for blending with diesel and biodiesel in order to minimize chemical costs and production time. However, the density and viscosity of diesel-biodiesel blends can be enhanced by adding TPB from the transesterification process, which improves the long-term phase separation stability of the blended fuel in comparison to EPB. Chanphavong^26^ studied the cost assessment of biodiesel production from used cooking oil to reduce dependence on petroleum. The results showed that the optimal conditions of the transesterification process were 20 vol % methanol and 12 g/L KOH at 60 °C with a reaction time of 30 min. For the cost analysis, the primary expense in biodiesel production from used cooking oil is the cost of chemicals, particularly methanol, which is essential for ensuring a complete reaction. Deng et al.^27^ reported similar findings in their study on biodiesel production from Jatropha curcas L. oil using ultrasonication in a two-step process. They reported that the catalyst concentration and the oil-to-methanol ratio were the important factors affecting the conversion efficiency of the reaction. The oil-to-methanol ratio should be higher than the theoretical requirement to drive the reaction to completion and produce more methyl esters. Their experimental results showed that the optimal conditions for esterification were 40 vol % methanol and 4 vol % sulfuric acid at 60 °C for 1 h, while the optimal conditions of transesterification were 24 vol % methanol and 1.4 wt % KOH at 60 °C for 30 min in an ultrasonic reactor. These results showed that a two-step process with a high methanol content (64 vol %) was required to produce biodiesel from oil that had a high FFA content.

Figure 4: Percentage of chemical costs in two-step biodiesel production.

Phase Stability of Diesel–Biodiesel–TPB

Blends

The long-term phase stability of the D-B-TPB blends was investigated using the ternary phase diagram, as shown in Figure 5a. After mixing, these fuel blends were kept stationary at 30 °C (room temperature) for 30 days to observe their phase separation behavior. Two different behaviors were used to classify their physical a single-phase liquid and a two-phase liquid. The phase separation behavior of D–B–TPB blends showed that the fuel blends exhibited a single-phase liquid under 18 conditions (three black triangles, 14 white triangles, and one white dot) and a two-phase liquid under 18 conditions (one black square and 17 white squares) out of 36 total conditions. The single-phase behavior in the ternary phase diagram was observed for fuel blends with proportions of 10–30 wt % diesel, 10–80 wt % TPB, and 10–80 wt % biodiesel. The two-phase behavior occurred when the diesel proportion exceeded 30 wt %, leading to faster phase separation. The phase separation behavior indicated that TPB had better compatibility with biodiesel than diesel, resulting in prolonged phase stability for fuel blends with less than 30 wt % diesel. In the surface tension analysis, the IFT values for diesel, biodiesel, and TPB were found to be 33.45, 34.97, and 35.30 mN/m, respectively. These results were not significant and did not correlate with the observed trend of phase separation. The solubility of the fuels, which depends on the polarity of the molecules, may influence the phase separation of the fuel blends. In general, bio-oil contains highly polar compounds,^28−30^ whereas diesel consists of nonpolar compounds, leading to low solubility. To address this issue, biodiesel was used as an emulsifier to prevent phase separation. Consequently, TPB enhances emulsification and may serve as an emulsifier in diesel–biodiesel blends.^31,32^ Additionally, the phase separation of the fuel blend was carefully observed for a period of up to 30 days to ensure accurate and reliable results. The results showed that the single-phase blends remained stable and did not separate, even after 30 days. These single-phase fuel blends were evaluated for density and viscosity to ensure that they met the physical requirements for testing diesel engines. The physical results showed that the density of all 18 single-phase fuel blends (three black triangles, 14 white triangles, and one white dot) was within the diesel standard range of 810–870 kg/m^3^ at 15 °C. For the viscosity property, only three conditions (three black triangles for D30B30TPB40, D30B20TPB50, and D30B10TPB60) met the diesel standard range of 1.8–4.1 cSt at 40 °C. Table 1 details the properties of diesel, TPB, biodiesel, and their blends.

Figure 5: Phase behavior of (a)
D–B–TPB blends and (b) D–B–EPB
blends at 30 °C, (○) liquid one phase, (□) liquid
two phase, (△) acceptable limits of density for blended fuel,
(▲) acceptable limits of density and viscosity for blended
fuel, and (■) conditions selected for comparison with D30B60EPB10.

Our previous study discussed the phase behavior of D–B–EPB blends. Only the single-phase separation condition of D30B60EPB10 was accepted for testing in a diesel engine to assess performance and gas emissions at different speeds and loads.^16^ However, TPB was produced from EPB to improve its physical properties compared to D–B–EPB blends. Therefore, the same proportions of D–B–EPB (D30B60EPB10 blend for the first step) and D–B–TPB blends (D30B60TPB10 blend for the second step) were selected to compare performance and emissions when the three fuel blends had the same concentrations, and the only change was substituting TPB for EPB. Although the D30B60TPB10 blend separated after 15 days, this duration was sufficient to conduct engine tests and compare its performance with D30B60EPB10. For this reason, the D30B60TPB10 blend (black square in Figure 5a) and the D30B60EPB10 blend (black triangle in Figure 5b), both of which include 10% TPB and 10% EPB in each condition blend, were chosen for further comparisons. Finally, the four blends of D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 will be evaluated in terms of their performance and emissions (NO~x~~, CO, CO~~2~, O2, EGT, and smoke opacity) in a diesel engine operating at various speeds and loads. Furthermore, these results were used to compare the 10% TPB in blend D30B60TPB10 with blend D30B60EPB10 from our previous research.

Engine Performance

Brake Power

The dynamometer measures torque at different engine speeds to determine the Pb, which is the actual power driven by the engine crankshaft. When an increase in either torque or angular speed resulted in an increase in Pb, Pb was calculated by multiplying these two variables.^33^ The Pb levels of blended fuels, diesel, and biodiesel at different engine speeds are presented in Figure 6. The Pb values of all fuels increased with increasing engine speed. The diesel showed the highest Pb level at all engine speeds, followed by D30B60TPB10, biodiesel, D30B30TPB40, D30B20TPB50, and D30B10TPB60, respectively. At 2300 rpm, the Pb values of D30B60TPB10, biodiesel, D30B30TPB40, D30B20TPB50, and D30B10TPB60 decreased by 1.17%, 2.49%, 4.95%, 5.80%, and 7.26%, respectively, compared to diesel. Because biodiesel and fuel blends have higher density and viscosity than diesel,^34,35^ this resulted in the inability of the fuel to mix well with air, leading to incomplete combustion of the diesel.^34^ Another consequence of fuels with high viscosities is poor fuel atomization, leading to reduced combustion efficiency and power output.^34,36^ Furthermore, the lower LHV of biodiesel and fuel blends in comparison with diesel results in a lower Pb at all engine speeds.^34,37^ Because the blended fuel contained less oxygen than biodiesel, the test results revealed that it had a lower Pb concentration. As a result, the engine had incomplete combustion, which contributed to the reduced Pb.^38^ In addition, the performance and emissions of EPB and TPB in blended fuels were compared after EPB fuel was upgraded by using the transesterification process. Comparing EPB and TPB blends, the Pb value of the D30B60TPB10 blend was slightly higher than D30B60EPB10 at all speeds. The Pb value of D30B60TPB10 was higher than that of D30EPB10B60 by 0.25%, 0.08%, 0.28%, 0.50%, and 0.31% at 1100, 1400, 1700, 2000, and 2300 rpm, respectively.

Figure 6: Brake power of a diesel engine in operation
with all fuels at different
engine speeds.

Brake-Specific Fuel Consumption

Figure 7a shows the BSFC variations for diesel, biodiesel, and fuel blends at various engine speeds and loads. The BSFC of all fuels tends to decrease as the engine load increases and increases with higher engine speeds. The BSFC of biodiesel and fuel blends was greater than that of diesel because of their lower LHV value. This indicated that the engine required more fuel to maintain the same Pb level as diesel as engine speed increased.^39^ Midhun Prasad and Murugavelh^40^ observed similar results in their study on the performance of bio-oil blends from tomato peel pyrolysis used in a diesel engine. They reported that the BSFC of fuel blends (tomato pyrolysis oil blended with diesel) decreased with increasing load and was higher than that of diesel at all loads due to the higher viscosity and lower LHV of the fuel blends. The increased viscosity and density of biodiesel and fuel blends resulted in larger fuel droplets. Injecting fuel into the combustion chamber became more difficult, and the BSFC level increased.^41^ Subramanian et al.^42^ found a similar result, suggesting that the BSFC of the bio-oil blends was higher than that of diesel because of bio-oil’s heavy phase. The effect of the blended fuel’s TPB content on BSFC showed that the BSFC level of blended fuels increases with higher proportions of TPB. When comparing D30B60TPB10 with biodiesel, the BSFC of D30B60TPB10 was less than that of biodiesel because the LHV of D30B60TPB10 was higher than biodiesel. However, the BSFC of D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends was higher than for biodiesel due to TPB’s lower oxygen content than biodiesel. The higher TPB content in the blended fuels indirectly increased the BSFC due to incomplete combustion in the engine. Compared with diesel, at 25% load and 2300 rpm, the BSFC levels of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 increased by 9.76%, 3.50%, 10.99%, 11.85%, and 14.28%, respectively. At 50% load, the BSFC of biodiesel increased by 15.32% compared to diesel at 2300 rpm. The results showed that the BSFC levels of D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends were higher than those of diesel by 5.53%, 16.45%, 17.14%, and 22.27%, respectively. When compared with diesel at 75% load and 2300 rpm, the BSFC levels of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 increased by 9.71%, 5.61%, 9.45%, 10.78%, and 12.21%, respectively. When D30B60TPB10 from the second-step and D30B60EPB10 from the first-step processes were compared, the BSFC of D30B60TPB10 was similar to that of D30B60EPB10 at all engine loads. At maximum speed, the BSFC of D30B60TPB10 was 0.36%, 0.85%, and 0.93% lower than that of D30B60EPB10 at engine loads of 25%, 50%, and 75%, respectively.

Figure 7: Performance
of diesel engines in operation with all fuels at different
engine speeds and (a) BSFC and (b) BTE.

Brake Thermal Efficiency

Figure 7b illustrates the BTE for all fuels at various engine speeds and loads. At all engine speeds and loads, the BTEs of biodiesel and D30B60TPB10 blends were higher than those of diesel. The oxygen content of biodiesel and D30B60TPB10 enhances combustion and oxidation efficiency, leading to an increase in BTE.^43^ However, the BTE levels of D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends were lower than those of diesel for all engine speeds and loads. Because the oxygen content in the fuel blend decreased as the proportion of TPB in the fuel blend increased, this resulted in a decrease in the combustion efficiency. In addition, compared with diesel, these fuels have a lower capacity to convert released heat energy into mechanical energy, resulting in lower efficiency and increased fuel consumption.^44^ At 25% load and maximum speed, the BTE levels for biodiesel and D30B60TPB10 were 7.14% and 7.25% higher than those of diesel, while the BTE levels for D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends were 1.82%, 3.17%, and 5.81% lower than those of diesel, respectively. At engine loads of 50% and 2300 rpm, the BTE levels for biodiesel and D30B60TPB10 were 1.98% and 5.18% higher than those of diesel, respectively, while D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends decreased by 6.43%, 7.55%, and 11.96% when compared to diesel. At 2300 rpm and 75% load, biodiesel and D30B60TPB10 had BTE levels 7.19% and 5.10% higher than those of diesel, respectively. On the other hand, the D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends had BTE levels that were 0.44%, 2.24%, and 4.07% lower than those of diesel. Comparing the BTE levels of the D30B10TPB60 and D30B10EPB60 blends, D30B10TPB60 provided a slightly higher BTE than D30B10EPB60. At 2300 rpm speed, the BTE levels of D30B10TPB60 increased by 0.60%, 1.09%, and 1.16% when compared to the BTE levels of the D30B10EPB60 blend at engine loads of 25%, 50%, and 75%, respectively.

Engine Emission

Oxygen Gas

The oxygen (O2) content of exhaust gas is an indicator of the air/fuel ratio, which affects the engine’s combustion efficiency.^23^Figure 8a shows the O2 release in the exhaust gas of all fuels under various speeds and loads. The O2 content in the exhaust gas after combustion decreased as the load increased because the engine required more fuel and air to complete combustion. Ağbulut et al.^23^ reported similar results in which engines achieved high volumetric efficiency at lower speeds, resulting in higher O2 content in the fuel. With increasing engine speed, turbulence in the airflow within the chamber rises, enhancing the fuel blend’s combustion efficiency. As a result, the content of excess O2 released into the environment decreases due to increased O2 consumption. The O2 released from biodiesel and all fuel blends was found to be higher than that from diesel, attributed to the increased O2 content from adding biodiesel to the fuel blend.^45^ The concentrations of O2 gas released during the combustion at 25% and 2300 rpm for biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 were 8.74%, 3.88%, 3.67%, 2.00%, and 1.95% higher than those of diesel, respectively. At 50% load and 2300 rpm, it was found that the biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends produced 8.11%, 5.65%, 4.62%, 2.91%, and 2.15% more oxygen than diesel. Compared with diesel, at a maximum load of 75% and 2300 rpm, the O2 released by the biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends increased by 32.46%, 21.35%, 19.47%, 18.92%, and 7.82%, respectively. At a maximum load and 2300 rpm, the biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends emitted O2 increased by 32.46%, 21.35%, 19.47%, 18.92%, and 7.82% more O2 than diesel, respectively. The O2 emissions from D30B10TPB60 and D30B10EPB60 blends at 2300 rpm were 1.26%, 0.72%, and 0.25%, respectively. These were slightly lower than those from D30B10TPB60 at 25%, 50%, and 75% loads.

Figure 8: Emissions of diesel engines in operation with all fuels at different
engine speeds and (a) O2, (b) CO, (c) CO2, (d) NO~x~, (e) EGT, and (f) smoke opacity.

Carbon Monoxide Emission

Figure 8b illustrates the CO emissions in the exhaust gases of all fuels at various engine speeds and loads. The CO emissions of fuels decreased as load of the engine increased from 25% to 50%, and they rose at 75% load due to higher fuel consumption, leading to a richer air-fuel mixture.^46^ During all condition testing engine loads, the CO emissions of biodiesel were found to be lower than those of diesel. This is because of the higher oxygen content of biodiesel, which promotes fuel combustion in the chamber. For the D30B60TPB10 blend, the CO emissions were lower than those of diesel due to the higher oxygen content in the high biodiesel proportion of the fuel blend, which promoted more efficient fuel combustion.^35,47^ Similar results have been reported by Tarangan et al.,^48^ who suggested that the increased oxygen content from adding biodiesel to the fuel blend was a critical factor in achieving complete combustion and reducing CO emissions. However, the D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends had higher CO emissions than diesel due to the low biodiesel content in the proportional blended fuel. At 25% load and 2300 rpm speed, the CO emissions of biodiesel and D30B60TPB10 decreased by 29.58% and 14.55% compared with diesel, while those of D30B30TPB40, D30B20TPB50, and D30B10TPB60 were 39.35%, 38.69%, and 90.60% higher than the CO emissions of diesel, respectively. At 50% load and maximum speed, the CO emissions of biodiesel and D30B60TPB10 were 26.08% and 8.35% lower than those of diesel, while D30B30TPB40, D30B20TPB50, and D30B10TPB60 CO emissions were 36.34%, 77.80%, and 127.36% higher than those of diesel, respectively. At a maximum load of 75% at 2300 rpm, biodiesel and D30B60TPB10 emitted 55.19% and 8.73% less CO than diesel, while D30B30TPB40, D30B20TPB50, and D30B10TPB60 emitted 229.80%, 399.66%, and 477.13% more CO than diesel, respectively. The CO emission of D30B60TPB10 was higher than that of D30B60EPB10 at all engine speeds and loads. At the maximum speed of 2300 rpm, the CO emissions of D30B60TPB10 were 2.33%, 33.26%, and 5.50% higher than those of D30B60EPB10 at engine loads of 25%, 50%, and 75%, respectively.

Carbon Dioxide Emissions

Figure 8c presents the CO2 emissions in the exhaust gases of all fuels at various engine speeds and loads. Typically, CO2 emissions rise when the engine speed and load increase.^49^ All fuels emitted high CO2 emissions when the engine load increased due to the high fuel consumption necessary for combustion without excess air; results similar to those described by Freitas et al.^50^ When compared with diesel, TPB blends emitted less CO2 due to the reduced hydrogen-to-carbon ratio and higher O2 concentration from adding biodiesel to blends, which improved the combustion and emission characteristics of the blends.^51^ At 25% load and 2300 rpm speed, the CO2 emissions of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 were lower than those of diesel by 23.33%, 11.13%, 9.89%, 9.98%, and 9.82%, respectively. At 50% load and 2300 rpm, the CO2 emissions from biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 decreased by 8.21%, 6.43%, 5.26%, 0.99%, and 0.56%, respectively, when compared with diesel. At 75% load and 2300 rpm, the CO2 emissions of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 were 31.62%, 14.09%, 12.19%, 9.96%, and 9.02% lower, respectively, than those of diesel. Comparing the D30B10TPB60 and D30B10EPB60 blends, the CO2 emission of D30B60TPB10 was higher than that of D30B60EPB10 at all loads and speeds. At a maximum engine speed, the CO2 emissions of D30B60TPB10 were 3.22%, 8.75%, and 18.68% higher than those of D30B60EPB10 for 25%, 50%, and 75% loads, respectively.

Nitrogen Oxides Emissions

Figure 8d shows the NO~x~ emissions from exhaust gases for all fuels with various engine speeds and loads. The NO~x~ emissions in exhaust gases increased with increasing load.^52^ For all fuel blends, the O2 content was a crucial factor in NO~x~ emissions.^17^ The O2 content of a fuel blend depends on the proportion of biodiesel in the mixture, with a higher biodiesel content leading to higher O2 levels. Increased O2 content enhances combustion, resulting in higher NO~x~ emissions, which aligns with the analysis of O2 content in the fuel.^17,47,53^ The NO~x~ emission of D30B60TPB10 is higher than all fuel blends due to the greater O2 content, resulting from the highest biodiesel content in the fuel blend. However, NO~x~ emissions from all fuel blends decreased when compared with diesel due to the blends having lower combustion temperatures than diesel.^54,55^ At 25% load and 2300 rpm, the NO~x~ emissions from the biodiesel and D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends were 43.76%, 32.98%, 17.79%, 12.60%, and 10.08%, respectively, all of which were lower than the emissions from diesel. At 50% load and 2300 rpm speed, the NO~x~ releases of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 were lower than those of diesel by 30.66%, 28.69%, 26.15%, 21.16%, and 18.49%, respectively. For 75% load and 2300 rpm engine speed, the NO~x~ emissions from biodiesel and D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 were 37.37%, 31.86%, 37.82%, 34.79%, and 30.29% lower than those from diesel. Comparing the NO~x~ emissions from EPB and TPB blends, the NO~x~ emissions of D30B60TPB10 were higher than those of D30B60EPB10 in all engine conditions. At a maximum speed, D30B60TPB10 emitted NO~x~ emissions of 6.24%, 3.63%, and 8.35%, which were higher than those of the D30B60EPB10 blend at 25%, 50%, and 75% loads, respectively.

Exhaust Gas Temperature

Figure 8e presents the EGT from all fuels at various engine speeds and loads. At 25% load and 2300 rpm speed, the EGT of D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 decreased by 16.33%, 7.60%, 6.25%, 1.78%, and 0.94%, respectively, compared with diesel. At 50% load and 2300 rpm speed, the EGT of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 decreased by 8.32%, 10.42%, 4.58%, 2.73%, and 1.35%, respectively, compared to diesel. Compared with diesel, at maximum load and 2300 rpm, the EGT of biodiesel, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 decreased by 19.76%, 13.39%, 8.09%, 3.93%, and 1.89%, respectively. Biodiesel has a lower LHV than diesel, which lowers the combustion chamber temperature of the engine during operation, resulting in biodiesel having a lower EGT than diesel.^53^ A similar result was reported by Sukumar et al.,^20^ who compared the use of sweet lime pyrolysis oil blended with biodiesel and diesel in a diesel engine. They found that using bio-oil blends affected the ignition delay period, with a shorter ignition delay period resulting in delayed combustion and reduced EGT. When comparing the EGT of TPB and EPB blends, the D30B60TPB10 blend had a higher EGT than the D30B60EPB10 blend for all engine conditions. When compared with the D30B60EPB10 blend, the EGT of D30B60TPB10 increased by 2.49%, 2.36%, and 1.53% for 25%, 50%, and 75% loads, respectively, at a speed of 2300 rpm. Therefore, the extended ignition delay of the D30B60TPB10 blend could potentially slow down combustion, resulting in a slight increase in EGT.^54^

Smoke Opacity

Figure 8f presents the smoke opacities from all fuels at various engine speeds and loads. Smoke opacity is the measurement of the opacity of exhaust smoke emitted from an engine, which is caused by soot particles and unburned hydrocarbons.^42^ When all fuels were measured, it was found that biodiesel and D30B60TPB10 released less smoke opacity than diesel; however, D30B30TPB40, D30B20TPB50, and D30B10TPB60 blends emitted greater smoke opacity than diesel at 75% load and 2300 rpm. The higher oxygen percentage of biodiesel in blended fuels led to more complete combustion, which was followed by decreased smoke emissions.^45^ For 75% maximum load at 2300 rpm, the biodiesel and D30B60TPB10 emitted 87.67% and 55.61% less smoke opacity than diesel, while D30B30TPB40, D30B20TPB50, and D30B10TPB60 emitted 15.47%, 89.46%, and 121.52% more smoke opacity than diesel, respectively. When D30B60TPB10 was compared with D30B60EPB10 at 2300 rpm, the smoke opacity of D30B60TPB10 was higher by 10.69%, 10.47%, and 9.34% at 25%, 50%, and 75% loads, respectively.

Summary of Engine Performance and Emission

Figure 9 presents a comparison of the BSFC and BTE for different engine loads at 2300 rpm. At all engine loads, biodiesel, D30B60EPB10, and D30B60TPB10 exhibited significantly higher BTE than diesel, indicating that these fuels convert more combustion energy into mechanical energy under identical engine loads. However, biodiesel, D30B60EPB10, and D30B60TPB10 also demonstrated higher BSFC than diesel because these fuels have lower heating value than diesel, resulting in increased fuel consumption for the same power output. However, three fuel blends of D30B30TPB40, D30B20TPB50, and D30B10TPB60 showed lower BTE than diesel at all engine loads due to incomplete combustion in the engine. Consequently, these blends exhibited higher BSFC, reflecting a reduced capacity to convert released heat energy into mechanical energy, leading to decreased efficiency and elevated fuel consumption. Figure 10 also illustrates a comparison of emissions for all fuels under different engine loads at 2300 rpm using radar charts. The CO emissions of biodiesel, D30B60EPB10, and D30B60TPB10 were lower than those of diesel at all engine loads due to the high oxygen content in the fuel blends, which promotes more efficient combustion. At 75% engine load, D30B30TPB40, D30B20TPB50, and D30B10TPB60 emitted significantly higher CO levels than diesel. For results of CO2 and NO~x~ emissions, biodiesel, D30B60EPB10, D30B60TPB10, D30B30TPB40, D30B20TPB50, and D30B10TPB60 exhibited lower CO2 and NO~x~ emissions than diesel at all engine loads. The high oxygen content in the fuel blends enhanced combustion, leading to higher NO~x~ emissions. However, the NO~x~ emissions of all fuel blends were lower compared to diesel because the fuel blends had lower combustion temperatures than diesel. Finally, smoke opacity measurements indicated that D30B60EPB10 and D30B60TPB10 had higher values than diesel at 25% and 50% engine loads. However, at the maximum engine load of 75%, the smoke opacity of D30B60EPB10 and D30B60TPB10 was lower than that of diesel because the higher oxygen content of biodiesel in the fuel blends resulted in more complete combustion.

Figure 9: Comparison of BSFC and BTE results for different engine loads at
2300 rpm.

Figure 10: Radar charts showing results in terms of emissions of
all fuels
at 2300 rpm for (a) 25%, (b) 50%, and (c) 75% engine loads.

Conclusions

In the first part of this study, upgraded TPB was successfully produced from EPB using a batch transesterification reaction and optimizing three parameters using the RSM technique. The recommended conditions were 38.3 wt % methanol, 15.1 g/L KOH, 62 min reaction time, 300 rpm stirrer speed, and 60 °C reaction temperature. These conditions achieved ME purity in TPB, converting from 73.50 to 74.35 wt %. In the second part of this study, produced TPB was subsequently employed as a blended fuel to decrease the proportion of diesel in the biodiesel and diesel blending process. A ternary diagram was employed to examine the phase stability of the diesel-biodiesel-TPB blends at 30 °C. A single-phase behavior was observed for the fuel blends with a proportion of 10–30 wt % diesel, 10–80 wt % TPB, and 10–80 wt % biodiesel. For the final part of this study, we tested four single-phase fuel blends that met some diesel specifications in a diesel engine to analyze their performance and emissions. With respect to the BTE values, D30B60TPB10 had higher BTE than diesel due to its higher O2 content, while D30B30TPB40, D30B20TPB50, and D30B10TPB60 had lower BTE than diesel under all engine conditions. The CO emissions of D30B60TPB10 were lower than those of diesel, while D30B30TPB40, D30B20TPB50, and D30B10TPB60 had higher CO emissions than diesel because of the lower proportion of biodiesel in the blended fuel. Additionally, the CO2 and NO~x~ emissions of all fuel blends were lower than those of diesel in all conditions due to the fuel blends’ increased O2 content, which promotes fuel combustion. The comparison between D30B60TPB10 and D30B60EPB10 revealed that their performance and exhaust gas were not significantly different. Therefore, to save chemical costs and production time, TPB does not need to be produced and added to the mixture because adding EPB to diesel–biodiesel fuels is sufficient in terms of emissions and performance. However, TPB exhibited better long-term phase separation stability than EPB due to the improved density and viscosity achieved during the transesterification process. Therefore, adding a greater proportion of TPB than EPB to diesel–biodiesel blends allows for the use of more pretreated bio-oil in the blends. However, the sufficiency potential of EPB from the first-step process is comparable to that of diesel, and its environmental friendliness makes it suitable for use in agricultural diesel engines. Therefore, when considering only diesel engine performance and emissions, if using EPB in diesel–biodiesel blends, it is recommended that the biodiesel is produced from PBO using a single esterification process. Further research and development in the future can also investigate the stability and long-term effects of fuel blends on diesel engine components.