Evaluation of Greenhouse Gas Emission Levels during the Combustion of Selected Types of Agricultural Biomass

Abstract

This paper presents the results of an experimental study of the emission levels of selected greenhouse gases (CO₂, CH₄, NO_x) arising from the combustion of different forms of biomass, i.e., solid biomass in the form of pellets and liquid biomass in the example of engine biofuel (biodiesel). Both types of biomass under study are rape-based biofuels. The pellets are made from rape straw, which, as a waste product, can be used for energy purposes. Additionally, biodiesel contains rape oil methyl esters (FAME) designed to power diesel engines. The boiler 25 kW was used to burn the pellets. Engine measurements were performed on a dynamometer bench on an S-4003 tractor engine. An analyzer Testo 350 was used to analyze the exhaust gas. CO₂ emission studies do not indicate the environmental benefits of using any alternative fuels tested compared to their conventional counterparts. In both the engine and boiler tests for NO_x emissions, no environmental benefits were demonstrated from the use of alternative fuels. The measured average NO_x emission levels for biodiesel compared to diesel were about 20% higher, and for rapeseed straw pellets, they were more than 60% higher compared to wood pellets. Only in the case of engine tests was significantly lower CH₄ (approx. 30%) emission found when feeding the engine with rape oil methyl esters.

1. Introduction

The most serious global threats to the natural environment include the intensification of so-called greenhouse gas (GHG) emissions. The effect of this phenomenon’s intensification is climatic changes of global character, caused by an increase in the temperature of the lower layers of the atmosphere as well as the surface of the Earth and surface waters [1].

Fossil fuel consumption is a major cause of climate change. In China, where coal-fired power plants dominate, the carbon emission factor is about 1.1 kg CO₂·kWh⁻¹. In the Tokyo area of Japan, the carbon emission factor is about 0.4 kg CO₂·kWh⁻¹, and in regions using hydropower, such as Brazil, it is 0.2 kg CO₂·kWh⁻¹ [2]. NO_x emission values are higher when burning coal fuels compared to other fuels, while CO₂ emissions are the highest when burning lignite [3]. A review of the environmental impact of electricity generation based on combustion technologies of different fuels [4] clearly indicates that hard coal combustion has the highest impact on global warming and ecotoxicity. Among fossil fuels, the highest CO₂ emission factors are characterized by lignite combustion (1300 kg·MWh⁻¹), and the lowest are characterized by natural gas (550 kg·MWh⁻¹). In contrast, the highest NO_x emissions are associated with the combustion of diesel fuel (75,000 kg·MWh⁻¹), and the lowest are associated with natural gas and lignite (15,000 kg·MWh⁻¹). For CH₄ emissions, the high emissions are associated with the combustion of hard coal and natural gas (15,000–20,000 kg·MWh⁻¹).

Interest in bioenergy has increased in recent decades due to the increased awareness of climate change issues and ambitions to reduce the dependence on fossil fuels [5]. The use of biomass is one of the options for an emission-neutral greenhouse gas as an energy source. However, as [6] noted, carbon neutrality in terms of carbon emissions is not the same as climate neutrality.

Biofuels can reduce the use of fossil fuels and thus reduce greenhouse gas emissions. The impact of biofuel use on mitigating climate change and reducing dependence on fossil fuels is the subject of intense debate in the scientific community. The actual benefits may be limited by local geographic factors, biofuel production technology, and the energy system used.

The use of biomass for energy production carries the risk of increasing emissions of GHG into the atmosphere. According to [7], biomass combustion, including residential biomass combustion (RBC), is a significant global source of gaseous emissions. Biomass combustion is the third largest source of CH₄, contributing to global methane emissions of approximately 40 Tg per year [8] and thus having a direct impact on the global CH₄ balance due to its long residence in the troposphere. Biomass combustion also emits nitrous oxide N₂O. Reactive nitrogen compounds have a significant impact on the chemistry of the atmosphere. This gas has a higher ability to absorb and remit terrestrial radiation than CO₂ or CH₄, so it is a more potent greenhouse gas.

Another use of biomass is biofuels for transportation. Vegetable oils or animal fats are converted to biodiesel through the transesterification with methanol, resulting in a mixture of fatty acid methyl esters (FAME) [9], which are used as an alternative fuel for feeding internal combustion engines [10]. Although the new guidelines mandate that biodiesel be derived from non-food raw materials, for Central Europe and countries with similar climatic conditions, the most promising is the use of biodiesel based on rape oil [11]. In the process of obtaining rapeseed oil, there is waste generated in the form of rapeseed straw, which can be utilized for energy purposes. Some researches [12,13] found out that rape straw is a valuable energy raw material of high calorific value. Redundant rape straw can become raw material for the production of compact solid biofuels [14] as an alternative to wood pellets used in low-power boilers with automatic loading. This policy is in line with one of the objectives of the EU Common Agricultural Policy, which is to promote agricultural practices that help protect the environment and climate.

The paper [15] outlines a number of positive and negative sustainability considerations associated with the removal of crop residues for expanded uses. As the authors point out, before using crop residues for biofuel production, it should be verified that neutral or positive sustainability impacts can be maintained under site-specific conditions. Crop residues from primary crops are available in significant quantities and do not compete with food production, and to some extent, they are created by virgin cereals production. As a result, there is no need for land conversion. However, this potential largely depends on the development of sustainable and efficient bioenergy systems [15].

Although there are many publications available on the emissions from both biodiesel and rape straw pellets, there is a lack of a comprehensive reference to the impact of the use of these biofuels on GHG emissions in exhaust gases concerning traditional fuels. Therefore, this study aimed to analyze the emissions of selected greenhouse gases (CO₂, CH₄, NO_x) generated during the combustion of different types of biofuels derived from the same plant, i.e., rape (rape oil methyl esters and rape straw pellets), in comparison with conventional fuels (diesel and wood pellets). The obtained results of the emission studies can demonstrate the environmental benefits of using alternative fuels in comparison with their typical counterparts

2. Materials and Methods

2.1. Boiler Tests

The fuels tested were rape straw pellets and A1-grade wood pellets available on the market. The physicochemical properties of rape straw pellets and wood pellets are presented in

Table 1. Physicochemical properties of wood pellets and rape straw pellets.

Parameter	Symbol	Unit	Wood Pellets	Rape Straw Pellets
Moisture	Wtr	%	5.7	9.4
Ash	Aa	%	0.3	10.4
Volatile matter	Vdaf	%	84.45	64.7
Carbon	Ca	%	49.5	40.1
Hydrogen	Ha	%	6.06	5.8
Sulphur	SaA	%	0.02	0.31
Nitrogen	Na	%	0.17	0.8
Oxygen *	O	%	38.25	33.19
HHV	Qsa	kJ∙kg−1	19,953	15,972
LHV	Qir	kJ∙kg−1	17,893	14,763

* Oxygen was calculated as a complement.

.

In order to test the combustion of rape straw pellets and wood pellets, a 25 kW boiler adapted for burning pellets was used. The boiler was equipped with a furnace, to which fuel was fed from a reservoir in an automated way. Boiler operation was controlled by a programmed electronic controller. A diagram of the boiler stand is shown in Figure 1.

Boiler operation was controlled by a programmed electronic controller. The amount of fuel fed for combustion as well as the amount of air required for proper combustion was automatically selected by the controller, based on the results of measurements of the oxygen content in the flue gas provided by the lambda probe and the temperature sensor at the boiler outlet. Combustion tests were carried out under fixed boiler operating conditions at rated settings. Before starting the measurements, the boiler was warmed up for a period of 1 h, the time required to stabilize the boiler was not included in the test duration. The combustion test of individual pellets lasted for 1 h. The fuel consumption was determined by weighing the fuel fed into the reservoir before and after the test for each fuel. The fuel mass flux was, for wood pellets, 6.15 kg·h⁻¹ and, for rapeseed straw pellets, 7.63 kg h⁻¹. The flue gas temperature was 138 °C and 134 °C, respectively.

2.2. Engine Testing

B100 biodiesel (fatty acid methyl esters FAME) and ON Efecta Diesel were used for the engine testing (

Table 2. Selected physicochemical properties of diesel fuel and methyl esters rape oil.

Parameter	Symbol	Unit	B100	DF
Ester Content	FAME	% (m/m)	98.8	6.8
Density at 15 °C	ρ	kg/m3	883	835
Viscosity at 40 °C	η	mm2/s	4.47	2.6
Cetane Number	CN	-	52.1	51.4
Flash point	FP	°C	120	69
Carbon	Ha	% (m/m)	76.9	85.7
Hydrogen	SaA	% (m/m)	11.9	10.6
Oxygen	O	% (m/m)	10.3	2.4
HHV	Qsa	kJ/kg	40,365	45,839
LHV	Qir	kJ/kg	37,918	43,511

).

The tests were carried out on a type S-4003 internal combustion engine installed on a dynamometer (Figure 2). The characteristic technical data of the tested engine are presented in

Table 3. Basic technical data of the S-4003 engine.

Parameter	Unit	Characteristics
Type	-	Self-ignition engine
Cylinder arrangement	-	Vertical in-line
Number of cylinders	-	4
Operating system	-	Four-stroke
Injection system	-	Direct injection
Compression ratio	-	17:1
Engine displacement	dm3	3.12
Rated power	kW	38.3
Rated speed	rpm	2200
Maximum torque	Nm	186
Maximum torque speed	rpm	1500–1600

. Despite the successive replacement of the machine park in Polish agriculture with modern tractors characterized by advanced operation and emission parameters, engines like the one under study are still being used to a considerable extent and pose potential ecological problems [16].

Engine load shifting was accomplished with an electric brake type K1-136B-E (asynchronous ring generator), which was also used to start the engine. Emissions were measured based on engine load characteristics at two characteristic speeds (maximum torque and rated power) over the full load range [17,18].

2.3. Measuring Apparatus for Emission Tests

The concentrations of nitrogen oxides (NO_x), carbon dioxide (CO₂), and methane (CH₄) were measured using Testo 350. Testo 350 is a portable exhaust gas analysis system for the measurement of exhaust gas emissions.

2.4. Statistical Analysis

The obtained results were analyzed with the use of analysis of variance (ANOVA). The data analysis was carried out using the Statistica ver. 13 software (TIBCO Software Inc., Palo Alto, CA, USA, 2017) at a significance level of α < 0.05.

3. Results and Discussion

3.1. Boiler Test Results

Figure 3, Figure 4 and Figure 5 show the time courses of changes in CO₂, NO_x, and CH₄ concentrations accompanying the combustion of rape straw pellets and wood pellets.

Table 4. Average concentrations of the measured flue gas components.

Specification	Unit	Wood Pellets	Rape Straw Pellets
CO2	%	4.10	3.15
NOx	ppm	41.4	119.2
CH4	ppm	275.3	579.6

lists the average concentrations of the measured flue gas components, calculated from the time courses of their changes. Moreover,

Table 5. Average emissions of GHG components (g·kWh⁻¹).

Emitted Fumes’ Component	Units	Wood Pellets	Rape Straw Pellets
CO2	g·kWh−1	432	439
NOx		0.33	0.96
CH4		1.09	2.29

compares the emissions of these compounds during the combustion of the analyzed fuels in relation to the obtained heat energy.

By comparing the concentrations of the tested gases presented in Figure 3, Figure 4 and Figure 5 and the average concentrations presented in Table 4, it was found that, in the case of burning pellets from rape straw, the concentrations of CO₂ were lower those of than wood pellets, while the concentrations of NO_x and CH₄ were higher. Especially high concentrations were found for NO_x (almost three times higher) and CH₄ (two times higher for rape straw pellet combustion). The lower values of CO₂ content may be due to the lower calorific value and the higher moisture content of rape straw pellets compared to wood pellets. On the other hand, the higher NO_x concentration was caused by high nitrogen and oxygen contents in rape fuel. The high concentration of CH₄ in the flue gas also indicates an imperfect combustion process—incomplete combustion.

The results of the study presented in Table 5 confirm that the combustion of rape straw pellets is associated with higher NO_x and CH₄ emissions and lower CO₂ emissions in relation to the thermal energy obtained. By comparing the nitrogen content of rape straw pellets (Table 1) with the values typical for wood biomass, which, according to the literature, are below 0.2% [19], it is possible to notice a relatively high content of this element in rape straw pellets. This may be associated with the use of nitrogen-containing mineral fertilizers during rape cultivation, which adversely affects the NO_x emission during the combustion of the investigated biomass. This fact is confirmed by the results of energy and emission studies (NO 421.7 mg∙m_n⁻³, NO₂ 664.8 mg∙m_n⁻³) (Table 4 and Table 5, Figure 2, Figure 3 and Figure 4).

The NO_x concentration found in the study [20] did not exceed 400 mg∙m⁻³. If the temperature in the furnace is relatively low, the NO_x concentration depends mainly on the nitrogen stream supplied to the furnace with the fuel, and nitrogen oxides are formed from nitrogen contained in the fuel [21]. In small power boilers, the combustion temperature very often does not exceed 1300 °C, and NO_x are not formed due to the oxidation of atmospheric nitrogen [22,23,24]. In this case, NO_x emissions should be directly related to the nitrogen content of the fuel.

However, due to the very high ash content in agricultural biomass, when considering the formation of NO_x during its combustion, the catalytic effects of the ash surface must also be taken into consideration. The different NO_x emissions during the combustion of pellets from agricultural biomass may be due to the varying nitrogen content of the biofuel as well as the ash catalyzing the formation of NO_x [25].

Table 6. ANOVA results for the emission levels (volumetric shares) of GHG components by fuel (rapeseed straw pellets, wood pellets).

GHG Component	Factor	Degrees of Freedom df	Totals of Squares SS	Medium Square MS	Test Function Value F	Calculated Significance Level p
CO2	fuel	1	163.2181	163.2181	15,593.84	0
NOx	fuel	1	1,080,143	1,080,143	156,755.5	0
CH4	fuel	1	16,528,891	16,528,891	151,205.4	0

shows the ANOVA results obtained for the measured emission levels of greenhouse gases, and in

Table 7. ANOVA results for the emission levels of GHG components (g·kWh⁻¹) by fuel (rapeseed straw pellets, wood pellets).

GHG Component	Factor	Degrees of Freedom df	Totals of Squares SS	Medium Square MS	Test Function Value F	Calculated Significance Level p
CO2	fuel	1	1.1 × 1010	1.1 × 1010	2554.695	0
NOx	fuel	1	70,670,641	70,670,641	37,051.95	0
CH4	fuel	1	16.279	16.279	24,133.99	0

, they were converted to the unit of mass referred to as kWh in boiler tests.

In both cases, the results of the statistical analysis calculated using the analysis of variance method for all analyzed GHGs due to the type of pellet showed significant differences between the average values (at the significance level of α = 0.05). Similar dynamics of changes in the content of the studied flue gas components were observed.

3.2. Engine Test Results

During the tests of the S-4003 engine on a dynamometer bench using FAME fuel and diesel fuel, emissions were measured under various speed-load conditions. Figure 6 and Figure 7 show the waveforms of changes in the level of CO₂ emissions in the exhaust gas of the engine operating according to load characteristics, performed at 1600 rpm and 2200 rpm, respectively.

The study showed a slight decrease in CO₂ emission levels for biofuel, compared to ON, at both engine speeds (Figure 6 and Figure 7). The largest difference between the average values of the concentration of this component in the exhaust gas over the entire engine load range (2.92%), was recorded between B100 and ON at the rated speed (2200 rpm). Figure 8 and Figure 9 show the waveforms of changes in NO_x content in the exhaust gas as a function of engine load (effective power) for 1600 rpm and 2200 rpm, respectively. The study showed that feeding the engine with biodiesel increased in the concentration of nitrogen oxides in the exhaust gas compared to diesel fuel. Averaged over the entire engine load range, the increase in exhaust NO_x concentration was 19.8% at 1600 rpm and 18.9% at 2200 rpm. The level of CH₄ emissions in the exhaust gas (Figure 10 and Figure 11) was found to be significantly lower for the ester-fueled engine compared to the diesel powertrain, averaging over the entire power range by 26.8% at 1600 rpm and by 29.3% at 2200 rpm.

Effective power is one of the most important indicators of an engine’s operation, as it determines the amount of energy given up to the consumer at each moment. As the effective power changes, the components of the exhaust gas change significantly, as demonstrated in the study (Figure 6, Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11). The measurements further showed that feeding the engine with biodiesel, compared to the ON drive, resulted in a decrease in effective power (evident at a full engine load) by 4.1% at 1600 rpm and by 6.1% at 2200 rpm. A particularly important engine operating parameter is effective (overall) efficiency, which characterizes the amount of heat energy supplied to the engine that is converted into useful (effective) work. Internal combustion engines typically have efficiencies of 30–40%. The highest effective efficiency (33%) was achieved by the tested biodiesel-fueled engine in the range of maximum torque characteristics.

GHG emissions are closely related to the temperature inside the cylinders and then, in the exhaust system, to the temperature of the exhaust gas. Exhaust gas temperatures measured during the tests varied from more than 100 °C at no-load engine operation to nearly 600 °C at full-load operation. A slightly higher average exhaust gas temperature, on the order of 1–2%, was recorded when the engine was fueled with biodiesel.

Table 8. Level of emissions of GHG components (g·kWh⁻¹) due to fuel and engine speed (rpm) and power output (kW).

B100					ON
Speed rpm	Power Output kW	CO2 g·kWh−1	NOx g·kWh−1	CH4 g·kWh−1	Speed rpm	Power Output kW	CO2 g·kWh−1	NOx g·kWh−1	CH4 g·kWh−1
1600	0	-	-	-	1600	0	-	-	-
	3.5	2161.0	24.5	3.4		3.5	2063.0	20.9	5.9
	7.1	1275.0	15.4	1.7		7.1	1244.5	12.3	2.6
	10.6	1024.7	13.7	1.4		10.6	998.6	10.4	1.9
	14.1	912.5	12.4	1.2		14.1	896.2	10.0	1.5
	17.7	863.7	12.1	1.1		17.7	881.3	10.3	1.5
	21.2	871.0	11.8	1.2		21.2	868.0	10.0	1.5
	23.5	883.7	10.6	1.3		23.5	881.2	9.0	1.6
	26.1	1019.2	10.8	1.8		27.2	950.1	8.8	2.1
2200	0	-	-	-	2200	0	-	-	-
	4.9	2364.8	21.2	4.9		4.9	2193.8	14.2	6.1
	9.7	1480.0	12.2	2.3		9.7	1400.1	10.3	3.1
	14.6	1195.7	11.1	1.6		14.6	1146.2	9.1	2.2
	19.4	1059.2	11.4	1.5		19.4	1011.5	8.8	1.9
	24.3	988.6	10.6	1.4		24.3	985.2	8.8	1.9
	29.1	1019.5	10.3	1.7		29.1	1001.5	8.6	2.4
	32.2	1106.8	8.8	2.1		34.3	1070.1	7.0	2.9

shows the emissions of measured GHG components converted to g·kWh⁻¹ according to the calculation methodology in the works of [26,27].

It can be seen from Table 8 that NO_x and CH₄ exhaust emissions expressed in g·kWh⁻¹ showed the same trend of change as the measured values (in ppm), i.e., an increase and a decrease, respectively, in these two exhaust compounds when the engine was fed with B100 biofuel compared to ON. The average over the entire engine power range increase in NO_x was 21.8% at 1600 rpm and 26.5% at 2200 rpm, while the average decrease in CH₄ was 25.4% (1600 rpm) and 25.1% (2200 rpm). In the case of CO₂, however, higher mass emissions were obtained for biodiesel than for diesel, averaging 2.2% at 1600 rpm and 4.0% at 2200 rpm. This is mainly due to the higher fuel consumption and lower heating value of the B100 bioester compared to ON.

Table 9. ANOVA results for emission levels (volume shares) of GHG components due to fuel and engine speed (biodiesel, ON).

GHG Component	Factor	Degrees of Freedom df	Totals of Squares SS	Medium Square MS	Test Function Value F	Calculated Significance Level p
CO2	fuel	1	0.057647	0.057647	0.010419	0.919335
	speed rpm	1	2.359477	2.359477	0.432062	0.515679
NOx	fuel	1	100,118.4	100,118.4	1.151727	0.291214
	speed rpm	1	140,155.9	140,155.9	1.63585	0.210093
CH4	fuel	1	2647.059	2647.059	4.195462	0.048811
	speed rpm	1	1152.941	1152.941	1.701444	0.201403

shows the ANOVA results obtained for the measured GHG emissions in the engine tests.

The results of the statistical analysis calculated by the analysis of variance showed a significant effect of fuel type (p < 0.05) only for CH₄. For all the other cases analyzed, the differences in the averages are not statistically significant.

Table 10. ANOVA results for the GHG component emissions (g·kWh⁻¹) due to fuel and engine speed (biodiesel, ON).

GHG Component	Factor	Degrees of Freedom df	Totals of Squares SS	Medium Square MS	Test Function Value F	Calculated Significance Level p
CO2	fuel	1	13,402.76	13,402.76	0.070184	0.79301
	speed rpm	1	200,098.5	200,098.5	1.121286	0.298382
NOx	fuel	1	49.152	49.152	3.352122	0.077781
	speed rpm	1	29.16259	29.16259	1.924114	0.175973
CH4	fuel	1	3.675	3.675	2.322869	0.138701
	speed rpm	1	2.417044	2.417044	1.537817	0.224884

shows the ANOVA results obtained for the emission levels of the studied GHG components converted to g·kWh⁻¹ in the engine tests.

The results of the statistical analysis calculated using the analysis of variance method for all the cases analyzed (Table 9) showed that the differences in the averages are not statistically significant (at the significance level of α = 0.05). A valuable advantage of using biodiesel compared to diesel fuel is the reduced emission of particulate matter and the gaseous components of the exhaust gas (CO, HC), along with the excessive NO_x emissions, as demonstrated in numerous studies, both domestic and foreign. For example, in the work of the authors [28,29], the increase in NO_x concentration for B100 fuel compared to ON was about 10%, confirming the upward trend of this component of the exhaust gas in the completed studies (an increase of about 20%). In turn, the same researchers found a significant decrease in hydrocarbon emissions (about 60%) for B100 fuel compared to ON, which also confirms the results of the present study concerning methane (a decrease in the range of 25–30%). It should be noticed that the performance of an engine depends on its design features (shape of the combustion chamber, design of the fuel injection system, design of the intake system) and operational features (type and characteristics of the fuel, technical condition of individual engine systems, adopted control settings) [30,31,32,33]. According to the authors [34], comparing the Life Cycle Assessment (LCA) of biodiesel to that of diesel, the use of this biofuel is more beneficial in terms of reducing the overall greenhouse effect, CO₂ emissions, or carcinogenic compounds. In terms of CO₂, the researchers found that burning each ton of diesel fuel emits 2.8 tons of CO₂ into the atmosphere, while burning biodiesel emits 2.4 tons of CO₂/ton of bioesters. Energy crops are expected to expand significantly in the very short term, bringing significant social and environmental benefits. However, many studies indicate either very positive or negative environmental effects of energy crop cultivation and processing, so there is still a lot of uncertainty regarding these issues [35]. When considering the highest degree of greenhouse gas emission reductions accompanying the use of biomass for energy [36], first-generation biodiesel was found to have less of an impact than first-generation bioethanol concerning bioenergy systems. In addition, for first-generation biodiesel, sunflower showed a lower energy impact than rape. To minimize greenhouse gas emissions from energy systems, an analysis was conducted [37], which indicates that, with adequate biomass availability, liquid fuel production should be based on agricultural residues. Electricity production should be based on forest residues and other woody biomass, and heat production should be based on forest and agricultural residues.

The targets set by the 2009 Renewable Energy Directive for renewable energy make the EU a major global source of demand for biomass. Demand for biomass energy is likely to increase as EU member states set increasingly ambitious renewable energy targets. While biomass power generation is steadily being displaced by other renewable energy sources (mainly wind and photovoltaic power), biomass is likely to remain a major source of renewable heat and transportation biofuels in the short term [38]. The closed-loop economy allows for treating waste biomass as a potential source of valuable energy raw materials. The transition to a closed-loop economy requires, among other things, new ways of transforming hitherto unused waste into new products that constitute resources such as energy. Nevertheless, sustainability criteria should be considered to distinguish raw materials with different climate impacts, as burning different types of biofuels can generate GHG emissions. It is therefore important to control the types of biomass used in order to reduce their negative impact on the climate. Financial and regulatory support should be limited to those raw materials that reduce GHG emissions in the short term, such as lumber residues, agricultural production waste, and post-consumer waste. The overarching goal is to develop sustainable energy systems that do not contribute to further climate change or negatively impact other aspects of sustainability.

4. Conclusions

The novelty of the paper is a comparative study of GHG emissions from the combustion of biodiesel and diesel in an internal combustion engine and from the combustion of various solid biofuels (rapeseed pellets and wood pellets) in a low-power boiler. Such a comparison has not yet been encountered in the literature. As the paper proves, biomass can be used for energy purposes in a variety of ways, and the benefits vary greatly depending on the system used. Bioenergy systems can contribute to climate change mitigation, but the use of biomass resources requires careful consideration of how to target the actions taken in relation to available resources.

The engine test results showed significant reductions in CH₄ emissions when burning B100 biodiesel compared to burning conventional fuel, both at maximum torque and rated speed. Higher NO_x emissions were found for the biofuel burned relative to diesel combustion. In addition, higher CO₂ emissions expressed in g·kWh⁻¹ were recorded for the combustion of bioester compared to the combustion of diesel, which is mainly due to an increase (about 10%) in the B100 fuel consumption, measured on the dynamometer bench. The engine test results showed a significant reduction in CH₄ emissions when burning B100 biodiesel compared to burning conventional fuel, both at maximum torque and rated speed. The boiler test results indicate that the combustion of rapeseed pellets is associated with higher CH₄ and NO_x emissions compared to the combustion of wood pellets. In contrast, comparable values were found for CO₂ emissions expressed in g·kWh⁻¹ during the combustion of rapeseed and wood pellets.

Thus, the results obtained from the CO₂ emissivity studies do not clearly indicate the environmental benefits of using the two alternative fuels tested compared to their conventional counterparts. Neither in engine tests nor in boiler tests for NO_x emissions have any ecological benefits been shown from the use of alternative fuels. Only in the case of engine tests were significantly lower CH₄ emissions found when fueling the engine with methyl esters of rapeseed oil.

In conclusion, although the studies did not show significant environmental benefits of using rapeseed-derived fuels, they should not be disqualified. Further research into the combustion process of these fuels can help improve emission factors. In addition, the management of waste biomass such as rapeseed straw, thanks to the possibly longest retention of its economic value, will allow for the sustainable use of rapeseed crops and attempt to close the CO₂ cycle.

The agricultural sector is a sizable emitter of GHGs and a consumer of energy derived mainly from fossil sources. Hence, it is particularly important to use energy (in various forms) from renewable sources in agriculture as much as possible. The results of the authors’ research can provide recommendations for the use of, for example, biomass-derived pellets or the more environmentally friendly biodiesel, given the significant amounts of fuel consumed by tractors and other agricultural vehicles. The authors intend to continue this type of research using various forms of agricultural biomass (e.g., biodiesel derived from frying oils) in terms of energy parameters, emissivity, combustion residues, etc.

The research results obtained in this paper are promising and indicate that biomass can play a key role in the diversification of raw material resources and sustainable management based on biotechnology. It is reasonable and interesting to conduct further research on the conversion to energy of different types of waste biomass in terms of greenhouse gas emissions, considering different energy systems. This will allow for an assessment of the environmental impact, selecting and popularizing the best solution.