Emissions from a dual-fuel agricultural tractor engine operating on diesel-biomethane blendsABSTRACT:
Agricultural production is essential for global food security, and Brazil ranks among the world’s four largest grain producers. Mechanization has transformed agricultural production by replacing manual and animal labor with productivity-enhancing machinery. Despite these technological advancements, modern agriculture remains heavily dependent on fossil fuels. The adoption of renewable energy sources, particularly biomass-derived biofuels; has therefore gained increasing attention. Biomethane produced from biogas derived from organic waste, especially animal manure, may represent a viable alternative. This study evaluated emissions of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) from an agricultural tractor engine fueled by Diesel S10 (containing 14% biodiesel) blended with biomethane at substitution levels of 2.5%, 5.0%, 7.5%, and 10%. Partial substitution of diesel with biomethane did not reduce CO emissions at higher engine speeds, whereas CO emissions decreased with increasing engine speed. Hydrocarbon emissions rose proportionally with biomethane content, and NOx emissions exhibited a non-linear relationship with biomethane content. Particulate matter emissions were negligible, remaining below the detection limit of the measuring equipment.
Key words:
biogas; agricultural machinery; sustainability; pollutants.
RESUMO:
A produção agrícola é essencial para a segurança alimentar global e o Brasil se destaca como um dos quatro maiores produtores de grãos do mundo. Para ser um dos maiores produtores a mecanização agrícola acaba por transformar profundamente o setor, substituindo o trabalho que antes era manual e animal, por máquinas capazes de aumentar significativamente a produtividade. No entanto, a agricultura moderna ainda depende fortemente de combustíveis fósseis. Nesse contexto, o setor agrícola tem voltado esforços para a adoção de fontes de energia renováveis, com destaque para os biocombustíveis derivados da biomassa. Entre essas alternativas, o biometano obtido a partir do biogás gerado por resíduos orgânicos, especialmente de dejetos de animais, torna-se como uma solução promissora. Portanto, o objetivo do estudo foi avaliar os níveis de emissões de monóxido de carbono (CO), hidrocarbonetos (HC), óxidos de nitrogênio (NOx) e material particulado (MP) de um motor de trator agrícola operando com Diesel S10 (com 14% de biodiesel) e percentuais de 2,5, 5,0, 7,5 e 10% de biometano. Os resultados mostram que a substituição de percentuais de Diesel por biometano não resultou em diminuição das emissões de CO em regime de rotação mais alta, no entanto, o aumento da rotação do motor indicou uma tendência de queda nas emissões. Quanto aos HC, aumentou proporcionalmente em relação ao percentual de biometano. Já as emissões de NOx não seguiram uma tendência linear com o aumento dos percentuais de biometano. O MP teve emissões com tendência a zero ficando abaixo da escala registrada pelo equipamento.
Palavras-chave:
biogás; máquina agrícola; sustentabilidade; poluentes.
INTRODUCTION
The COVID-19 pandemic and ongoing international conflicts have contributed to global instability (TELES, 2022). This has led to shortages of raw materials, particularly fossil fuels, with significant impacts on global markets (NEFF et al., 2011). Increased demand for petroleum-based fuels, exacerbated by geopolitical conflicts, has driven fuel prices upward, with cascading effects on goods, services, and agricultural production (UMAR et al., 2021).
According to the United Nations (2016), the global population is projected to reach approximately 8.5 billion by 2030. Biomass-derived energy sources have gained importance from both economic and sustainability perspectives, particularly in countries with extensive agricultural production areas, such as Brazil. The development of innovative technologies is; therefore, essential to reduce dependence on fossil fuels and support long-term sustainability.
Many farms that treat animal production waste fail to utilize the biogas generated, which is often released into the atmosphere, thereby contributing to environmental impacts (CASTRO et al., 2022). Methane, the primary component of biogas, is typically emitted into the atmosphere and has high global warming potential, remaining in the atmosphere for extended periods and exerting a warming effect several times greater than that of CO2.
Brazil has an estimated annual biogas production capacity of 84.6 billion Nm³, the highest worldwide, with corresponding potential for biomethane production (LEMOS et al., 2024). However, current production is only about 1.5% of the national capacity.
Brazil could also generate 17,095.66 GWh annually from biogas in the agro-industrial sector, significantly increasing its share in the national energy matrix (VAZ, 2023). The use of biogas could also reduce CO2 emissions by approximately 86.5 million tons per year, equivalent to the carbon sequestration capacity of about 554.5 million reforested trees.
The use of renewable fuels in agricultural machinery engines and the evaluation of pollutant emissions have received increasing attention. Further studies are needed to assess the use of biomethane as an alternative fuel in agricultural systems, where it is readily available and exhibits strong potential for use in agricultural machinery engines.
This study evaluated emissions of gaseous pollutants and particulate matter from an agricultural tractor engine operating in dual-fuel mode with Diesel S10-biomethane blends.
MATERIALS AND METHODS
The experiment evaluated Diesel S10 fuel with partial substitution by biomethane at levels of 2.5%, 5.0%, 7.5%, and 10%. The original engine electronic control unit (ECU) was replaced with a programmable ECU to enable real-time control of injection parameters and monitoring of engine operating variables via a computer interface. The system, integrated with dedicated FuelTech software, allowed continuous monitoring of intake air temperature (°C), fuel temperature (°C), crankcase oil temperature (°C), rail pressure (bar), and engine speed (rev min-¹). It also provided direct control of fuel injection duration (ms) and injection timing (°), both essential parameters for engine operation.
Injection duration and timing were adjusted to reproduce the torque and power curves obtained with the original ECU (Figure 1A and Figure 1B), while monitoring the lambda parameter (Figure 1C). Under these conditions, variations in operating parameters were evaluated, including emissions of carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons (HC), and particulate matter (PM).
Torque, power, and lambda curves for the original engine and diesel-biomethane blends at different biomethane levels.
A four-stroke diesel engine (AGCO Power, model 44CWC3) equipped with a Bosch Common Rail injection system was used. The turbocharged engine featured four in-line cylinders, a displacement of 4,400 cm³, and 64 hours of prior operation. Rated power was 96 kW (130 hp) at 2,000 rev min-¹, and maximum torque was 565 N·m at 1,500 rev min-¹.
Diesel S10 fuel (10 ppm sulfur), containing 14% biodiesel, was obtained from commercial fuel stations. Biomethane was supplied by Folhito Fertilizante Orgânico (Estrela, Rio Grande do Sul state, Brazil). The biogas was produced via anaerobic digestion of organic waste, including swine manure, urban waste, and agroforestry residues. Supplier analysis at the time of collection indicated a composition of 93.4% methane (CH₄) and 0.07% oxygen (O2).
Engine tests were conducted using a water-cooled dynamometer (Horiba, Schenck Pegasus, WS 400 F), with a maximum capacity of 400 kW and 2,000 N·m and precision of ± 1.0 rev min-¹. The dynamometer applied resistive loads and recorded data to determine engine power.
Exhaust emissions were measured using an AVL SESAM FTIR gas analyzer, capable of detecting more than 31 combustion-related gases, and a partial-flow opacimeter (Saxon, Opacilyt ELD 1030). A sampling probe was installed in the exhaust pipe to collect a portion of the exhaust gases for analysis.
Fuel injection was controlled by a programmable electronic module (FuelTech, model 550), which managed both diesel injection into the combustion chamber and gas injection into the intake manifold. A dedicated gas injection system, comprising four injectors, was installed in the intake manifold to ensure accurate gas dosing. The manifold was modified to accommodate the injectors.
Fuel consumption was measured using three Coriolis flow meters (Endress + Hauser, Proline Promass A 300), with two dedicated to liquid fuel and one to gaseous fuel. The experimental setup and primary electronic instrumentation are shown in figure 2. Multiple sensors and electronic systems were used to monitor operating variables that could affect engine performance and mechanical integrity.
Schematic of the experimental setup for engine performance evaluation. dynamometer (1), diesel engine (2), opacimeter (3), gas analyzer (4), diesel tank (5), biomethane cylinder (6), liquid fuel flow meter (7), gaseous fuel flow meter (8), pressure regulator (9), computer (10), dynamometer controller (11), data acquisition software (12), programmable injection module (13), and driveshaft (14).
Engine speed, fuel flow rates, and temperatures were recorded at multiple points in the engine using a low-frequency data acquisition board integrated into the dynamometer control system, developed by the Research Group on Engines, Fuels and Emissions (GPMOT). Data acquisition and monitoring were conducted using three software programs: MW IELD 01030 for particulate matter analysis, FTManager for communication with the electronic injection module, and DAQMOT (SANTOS, 2023) for real-time combustion analysis.
The experiment followed a completely randomized design in a 4 × 5 factorial arrangement, with engine speed and biomethane concentration as the independent factors. Engine speed included four levels (1,250, 1,500, 1,750, and 2,000 rev min-¹), and biomethane concentration five levels (0%, 2.5%, 5.0%, 7.5%, and 10%), with three replicates, totaling 60 experimental units.
The dependent variables evaluated were carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). Data were tested for normality and homoscedasticity using the Shapiro-Wilk test. Quantitative variables were subjected to analysis of variance (P ≤ 0.05) and regression analysis using Sisvar software (FERREIRA, 2014).
RESULTS AND DISCUSSION
Analysis of variance revealed a significant interaction between fuel and engine speed. Both factors significantly influenced CO, HC, and NOx emissions (P ≤ 0.05) (Table 1).
Partial substitution of diesel with biomethane affected CO emissions, particularly at higher engine speeds. CO emissions peaked at 1,250 rev min-¹; where increasing biomethane content reduced CO concentration (Figure 3A).
Carbon monoxide (CO) (a), hydrocarbon (HC) (b), and nitrogen oxide (NOx) emissions from an agricultural tractor engine operating on biodiesel-biomethane blends at 2.5, 5.0, 7.5, and 10% biomethane.
At 1,750 and 2,000 rev min-¹, CO emissions rose with biomethane addition. The lowest values occurred with Diesel S10 only (0.66 and 0.51 g kWh-¹, respectively), increasing to 1.59 and 2.27 g kWh-¹ with 10% biomethane. No significant differences were observed at 1,500 rev min-1.
These findings suggested incomplete combustion, likely due to lean mixtures, ignition timing differences, or reduced peak combustion temperatures. Methane addition in compression-ignition engines alters combustion behavior. Under these conditions, CO formation is governed by combustion temperature and fuel availability, with higher emissions under oxygen-deficient or low-temperature conditions (WEI & GENG, 2016). Oxygen-limited incomplete combustion also increases CO emissions (LEYKUN & MAKONEN, 2022).
In dual-fuel diesel engines operating with diesel and biogas, CO emissions increased with higher biomethane content due to reduced oxygen availability caused by intake air displacement (LEYKUN & MEKONEN, 2022).
Hydrocarbon emissions rose with increasing biomethane content compared with diesel-only operation (Figure 3B). The highest HC emission (0.68 g kWh-¹) occurred at 1,250 rev min-¹ with the maximum biomethane level. Similar results have been reported for turbocharged diesel-gas engines, where HC and CO emissions were considerably higher with increasing gas substitution compared with diesel-only operation (EGÚSQUIZA et al., 2009).
This pattern indicated an increase in unburned hydrocarbon formation with methane addition to the diesel mixture, potentially due to poor atomization. The presence of gas may alter diesel spray dispersion in the combustion chamber, creating rich and lean zones that promote HC formation. Only mixtures within flammability limits are effectively burned; overly rich mixtures lack sufficient oxygen, while overly lean mixtures do not sustain combustion, resulting in HC formation (FERREIRA, 2013).
Biomethane’s low cetane number can suppress combustion temperatures and exacerbate HC formation. Diesel fuel generally yields lower HC emissions than blended mixtures, with higher biomethane content associated with increased emissions (WEI et al., 2022).
NOx emissions did not show a linear correlation with increasing biomethane content (Figure 3C). Values fluctuated, with a slight decrease at higher engine speeds (1,750 and 2,000 rev min-¹). Peak NOx emissions were recorded at 2.5% and 5.0% biomethane at 1,500 rev min-¹, whereas the lowest values occurred at these same levels but at 2,000 rev min-¹.
NOx formation is primarily governed by the thermal (Zeldovich-Keck) mechanism (RIDENTI & PINO, 2018) , and strongly influenced by combustion temperature (above 1,800 °C), oxygen availability, and gas residence time (WANG & YANG, 2024). Biomethane addition can alter combustion temperature, oxygen availability, and gas residence time, thereby affecting NOx formation. Variations in air-fuel ratio and ignition timing may further influence these effects, contributing to the observed variability and the absence of a clear trend. ZHENG et al. (2022) reported similar results, observing reduced NOx emissions under most of the tested operating conditions, with different decreasing trends depending on excess air ratio and ignition timing.
The lambda value (Figure 1C) increased with engine speed, indicating a leaner mixture at higher speeds. At 2,000 rev min-1, biomethane addition reduced lambda proportionally. Particulate matter emissions were negligible. The opacimeter measures exhaust opacity using the light absorption coefficient (K value) on a scale from 0.0 to 10.0. Measured values were close to zero, precluding PM quantification because they fell below the equipment’s detection limit.
CONCLUSION
Biomethane addition did not reduce carbon monoxide (CO) emissions at higher engine speeds. While CO emissions decreased with increasing engine speed, hydrocarbon (HC) emissions rose proportionally with biomethane content. Conversely, NOx emissions did not follow a linear correlation with increasing biomethane concentrations. The use of biomethane in agricultural diesel engines requires careful evaluation, considering both energy performance and its impact on atmospheric emissions. This alternative fuel requires specific control strategies for injection parameters, ignition timing, and air-fuel mixture composition to ensure that potential environmental benefits are not offset by adverse emission effects.
ACKNOWLEDGMENTS
This study was partially funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil - Funding Code 001. The authors also acknowledge the Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS) for funding the research project (No. 22/2551-0001631-6), under which this study was conducted.
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Edited by
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ASSOCIATE EDITOR:
Alessandro Dal’Col Lúcio (0000-0003-0761-4200)
All data generated or analyzed during this study are included in this published.
