Production and Quality of Biodiesel under the Influence of a Rapeseed Fertilization System

Abstract

Compared to fossil fuels, biodiesel is a clean fuel, does not pollute the environment, and can be produced from inexhaustible natural sources. The objectives of our research are to study how increasing doses of complex fertilizers, applied to rapeseed oil culture, affect the production of rapeseeds and oil; the comparative study between the quality parameters of refined rapeseed oil (RRO) and fatty acids (FA); and the analysis of the quality of the biodiesel obtained from them (BRO and BFA). The experimental field is set-up in a Latin rectangle format and is placed on a total area of 400 m² divided into 16 parcels, randomized for uniformity with four replications and four fertilizer graduations (N₀P₀K₀—Control, N₁₆₀P₁₆₀K₁₆₀, N₃₂₀P₃₂₀K₃₂₀, N₄₈₀P₄₈₀K₄₈₀). To obtain biodiesel form oil (BRO) and fatty acids (BFA), the processes of esterification, transesterification, refining, washing, sedimentation, and drying were applied. A comparison between biodiesel quality parameters from RRO and FAs were made, and we highlighted the differences in quality compared to samples from the experimental field. The use of large amounts of complex fertilizers leads to high yields of rapeseed (13.3–47.0 q ha⁻¹) and oil (629.8–2130.8 L ha⁻¹), which are statistically significant only for high doses (N₃₂₀P₃₂₀K₃₂₀ and N₄₈₀P₄₈₀K₄₈₀). For most of the qualitative parameters studied, the differences in values between BFA and BRO have positive values, which means a higher quality for BRO. Both BRO and BFA generally fall within the quality parameters imposed by European standards (ES). Although the quality of BRO is superior to BFA, it is produced on a smaller scale due to the nutritional importance of refined rapeseed oil. This study is of particular importance in the optimization of rapeseed fertilization, with a view to the efficient conversion of crude oil, a feedstock for chemical fertilizers and an environmentally friendly fuel.

1. Introduction

Energy represents a key source for economic growth, and since their discovery, fossil fuels have been the major contributor to fulfilling energy needs [1]. However, due to environmental issues related to the overuse and combustion of these fuels, together with the increase in price and depletion of natural energy, researchers have been forced to find cost-effective, sustainable, renewable, and efficient alternative energy sources [2].

Sustainable development means more than simply protecting the environment; it requires long-term structural change in our economic and social systems, with the aim of reducing environmental and resource consumption to a sustainable level, while maintaining economic performance and social cohesion [3]. Uncertainty about the availability and price of petroleum has focused attention on alternative fuels for transportation [4,5].

Biomass production takes place in different natural ecosystems or agro-ecosystems under the influence of a complex of natural factors (light, heat, precipitation, relief, lithological, hydrological, pedological, etc.), which change over time and space under the influence of anthropogenic factors [6,7]. The sustainable management of natural resources represents a modern form of land/soil management, with the main objective of increasing their fertility, with the aim of producing high quality food [8,9]. Therefore, the main objective of practicing sustainable agriculture is to maintain and increase the quality and fertility of the soil [10], including through fertilization with mineral fertilizers [11].

Rapeseed (Brassica napus L.) is now a particularly important source of vegetable oils in the world economy [12]. Rapeseed is high in monounsaturated oleic acid and low in saturated and unsaturated fatty acids [13,14]. Rapeseed oil is an ideal raw material for fuels due to its calorific value, oxidation stability, and low temperature behavior [15]. Given that the raw material for the production of mineral fertilizers is crude oil, (fossil fuel) there is the possibility of converting it into biofuel, which is an environmentally friendly fuel.

Rapeseed cultivation has multiple phyto-technical advantages: it is sown and harvested outside the crowded periods; it has a favorable reaction to fertilization; it allows for the complete use of the same set of machines as for cereals; it can be used as an excellent precursor for successive crops or for autumn cereals; it raises soil fertility and prevents erosion on sloping lands; it is a good honey plant, with the cakes being rich in protein (38–41.9%), carbohydrates (31.5–36.6%) and mineral salts (8–9.8%); it has a good feed value; and the epigenous part of the plant (straw) can be used in the manufacture of particle board [16].

Umberto KWS is an intensive hybrid with the best recommendations to become the “backbone” of the rapeseed culture in farms aiming for the highest levels of production. The wide-ranging ecological plasticity gives it adaptability to the most diverse crop areas, even in those prone to periods of intense drought. The high tolerance to winter frosts is noted as the best among the multitude of hybrids sold in Romania [17].

Rapeseed is one of the crops with the highest specific consumption. For a ton of seeds and the related biomass production, the specific consumption is 50–60 kg nitrogen, 30–60 kg phosphorus, 40–50 kg potassium, 50–60 kg calcium, 20–30 kg sulfur, and important quantities of microelements. The complex fertilizer NPK 16:16:16 was used in the fertilization process, is based on the content of nitrogen, phosphorus, potassium and sulfur, is easily absorbable, and ensures a balanced fertilization ratio [18].

Considering the climatic conditions, agricultural potential and socio-economic aspects, rapeseed is one of the most profitable crops for biofuels [19]. A very important issue is the efficiency of using mineral fertilizers to increase the quantity and quality of the resulting biofuel.

The acidity of the oils is mainly due to the existence of free FAs resulting from the hydrolysis of triglycerides in the presence of water [20]. Such hydrolysis reactions can occur in the biomass if it is stored under poor conditions (humidity), during pressing if temperatures are too high, and during oil storage in the presence of water and light. The acidity of the oil is responsible for damage to the engine supply circuits (hose, gasket, etc.), and engine corrosion [21]. The water present in oils comes directly from biomass that has been poorly dried or from condensation in poor oil storage conditions [22,23].

Reflecting the sustainability criterion, biofuel is one of the most relevant renewable alternatives [24]. This includes several types of fuel, such as biodiesel and bioethanol [24]. These types of fuel are attractive targets for the transport sector due to their inherent and similar properties compared to fossil fuels [25,26,27], in particular self-ignition [28,29]. The remarkable advantage of using biodiesel as a fuel is its adaptability to a diesel engine without any major modifications [27,29]. Biodiesel is an alternative fuel that can be used in pure form or blended with diesel when burned in internal combustion engines [29].

Pure biodiesel, referred to as B100, is a mono alkyl ester of FAs derived from vegetable or animal oils [30]. It is obtained chemically by the trans-esterification reaction, where a glycerol reacts with an alcohol (methanol) in the presence of a catalyst (potassium hydroxide) to form fatty acid alkyl esters (crude biodiesel) and an alcohol (crude glycerol) [30,31,32,33]. To achieve good quality biodiesel, several purification steps are applied [34].

The FAs used in this study are the result of separating the soapstock from the oil production process. To obtain biodiesel from FAs, we applied the following processes: the esterification and transesterification of fatty acids with methane sulfonic acid, and purification (acid wash, neutral wash, drying) [35].

Biodiesel plays the most important role in reducing emissions and is in fact the most sustainable liquid fuel, reducing carbon emissions by an average of 50% to 81% compared to fossil fuels [35,36,37]. Biodiesel has a tangible impact on the reduction of carbon dioxide emissions, and should therefore be at the forefront of global transport decarbonization [38,39].

Biodiesel viscosity influences engine starting characteristics and injection system performance. The fuel-air mixture has a strong impact on the efficiency of the fuel combustion process in a diesel engine. The quality of this mixture depends, in turn, on the fluidity of the fuel, which is directly related to viscosity [40].

One of the main possibilities to increase rapeseed production is the administration of mineral fertilizers, which use crude oil as a raw material, a less environmentally friendly fossil fuel. Under these conditions, rapeseed can convert fossil fuels (crude oil) into biodiesel, a natural, environmentally friendly fuel. With the aim of optimizing the use of fertilizers in the culture of rape, we propose to follow the process of obtaining biodiesel (the production of grains, RRO, FA, BRO and BFA) and its qualitative characteristics, from the sowing of rape until the last phase of processing.

The objectives of our research are to study how increasing doses of fertilizer applied to rapeseed affect grain and oil yields; the comparative study between the quality parameters of RRO and FAs; and the analysis of the quality of the biodiesel obtained from them (BRO and BFA) and the influence of the administered doses of fertilizers.

2. Materials and Methods

2.1. Experience Field

The experimental field for the study is located in the commune of Nojorid, Bihor County, Romania, with the geographical coordinates of 47.0012° North latitude and 21.8843° East longitude.

We used the hybrid Umberto KWS for the rapeseed culture. The particularities of this hybrid are: semi-late; medium-high waist; very good strength at germination, followed by a superior growth rate into winter. It has an almost total polygenic resistance to black rot, Phoma lingam, provided by the RML-3 gene; an extremely high branching capacity of the stem (in the experience field the plants had between 10–12 branches); a high number of seeds/silica; and high oil content 44–47%, with a low glucosinolate content [17].

The experimental set-up is of the Latin rectangle type and is placed on a total area of 400 m² divided into 16 parcels. Each parcel is 10 m long and 2.5 m wide. In the experimental field the 16 parcels are randomized for uniformity, with four replications and four fertilizer graduations. (Figure 1).

The doses of complex fertilizers administered in the field are:

N₀P₀K₀—Control—no complex fertilizers administered;

N₁₆₀P₁₆₀K₁₆₀—dose of complex fertilizers: N—160 kg ha⁻¹ active substance; P—160 kg ha⁻¹ active substance; K—160 kg ha⁻¹ active substance;

N₃₂₀P₃₂₀K₃₂₀—dose of complex fertilizers: N—320 kg ha⁻¹ active substance; P—320 kg ha⁻¹ active substance; K—320 kg ha⁻¹ active substance;

N₄₈₀P₄₈₀K₄₈₀—dose of complex fertilizers: N—480 kg ha⁻¹ active substance; P—480 kg ha⁻¹ active substance; K—480 kg ha⁻¹ active substance.

2.2. Fertilization System

The NPK 16-16-16 complex fertilizer was used in the study. It is a fertilizer that contains the three essential macro elements: nitrogen (N), phosphorus (P), and potassium (K), which ensures a balanced ratio of nutrients necessary for plant development. This type of fertilizer has total nitrogen 16%; total phosphorus pentoxide 16%, and is 60% soluble in water; potassium chloride 16% and is soluble in water; and the pH is minimum 4.5 [41].

Fertilizer rates applied per parcels are calculated per hectare [18,42] and in relation to the results of soil analysis. The doses of complex fertilizers administered on each repetition parcel supplemented the basic fertilization carried out on the entire surface of the parcel. All other treatments were carried out in accordance with rapeseed cultivation techniques (

Table 1. Fertilization system for autumn rape.

Specification	Dose	Unit of Measurement
Specific consumption (per tone of seed)
Nitrogen (N)	50–60	kg
Phosphorus (P)	30–60	kg
Potassium (K)	40–50	kg
Calcium (Ca)	50–60	kg
Sulfur (S)	20–30	kg
Specific consumption (per 100 kg seed)
N	2	kg
P2O5	2.5	kg
K2O	10	kg
Fertilization with N
Nitrogen	35–45	kg ha−1
Mineral nitrogen (from soil)	45–60	kg ha−1
Fertilization with P
P2O5	50–80	kg ha−1
Fertilization with K
K2O	200–400	kg ha−1
Fertilization with S
(NH4)2SO4	125	kg ha−1

). They were administered in the spring, after the snow had melted.

In the 2020–2021 agricultural year, the experimental field (Figure 2) was cultivated with rapeseed.

2.3. Natural Conditions

From a climatic point of view, the commune of Nojorid falls into the moderate temperate continental climate, with a slight Mediterranean nuance. According to the records from the Oradea Meteorological Station, (47.06° North latitude, 21.93° East longitude and an altitude of 148 m compared to the level of the Black Sea), the multiannual average precipitation is 580 mm and the multiannual average air temperature is 15 °C. Within the territory studied, based on the morphological and physical-chemical characteristics, the soil unit Gley Haplic Luvisol, weakly gleyed, with loess character, dusty loam/medium clay loam, on loams was delimited [42].

2.4. Production of Rapeseed Grains

Rapeseed grains from the experimental field were collected with the harvester for each parcel repetition. The production achieved by each parcel was weighed and then converted to production per unit area (q ha⁻¹).

2.5. Rapeseed oil (RRO) Production

To determine the quantity of RRO obtained from each replicate plot, the same quantity of seeds was used for extraction for each plot. To obtain raw oil from the rapeseed before pressing, it is necessary to clean, dry, peel (resulting in husks, hulls, and furfural), grind, and roast the seeds. Crude press oil and broken cakes are obtained by pressing, after which the crude extraction oil is refined. The refining process includes degumming, neutralization, washing, drying, bleaching, winterization, and deodorization. These processes were carried out at the oil factory S.C. Ardealul S.A.

The resulting amount of oil was then related to the rapeseed yield of each replicate parcel. This oil was then converted into the resulting amounts per unit area (liters ha⁻¹).

2.6. Quality of RRO and FAs

To determine the quality of RRO and FAs obtained from the experimental field, the amount obtained from the four replicate parcels fertilized with the same quantity of complex fertilizers (NPK) was used.

To differentiate them from a qualitative point of view, the following parameters were considered:

• Free FAs (%), chemicals: 0.5 Mol/L methanolic KOH by Carl Roth; 2-Propanol 99.5% purity by Carl Roth, neutralized, analytical system: automatic titration system by Metrohm, Herisau, Switzerland;
• Water content (%), chemicals Hydranal-solvent, 34800 by Fluka, Hydranal-Titrant 5, 34801 by Fluka; the analytical system was a Metrohm 870KF Titrino plus with an 803 Ti stirrer.
• Density at 15 °C (kg m⁻³) with an Anton Paar DMA 4100M analytical system;
• Phosphorus content (mg kg⁻¹) using the ICP-OES System: an Agilent 5110 with an SPS 4 Autosampler;
• Sulfur content (mg kg⁻¹) using the ICP-OES System with an Agilent 5110 with an SPS 4 autosampler;
• Kinematic viscosity at 40 °C in mm² s⁻¹ using an Anton Paar DMA 3001 analytical system;
• FAs chain (%), using chemicals TMSH 0.2 M: Macherey-Nagel, 0.2 M Trimethylsulfoniumhydroxid diluted in Methanol Purity 99%, 15% Boron trifluoride in Methanol, Sigma-Aldrich, n-Heptan: Bern Kraft Laborchemikalien; analytical system GC 2030 with AOC 20i autosampler by Shimadzu, Kyoto, Japan.

The test methodology was carried out according to the European Standards (ES) developed by the German Institute for Standardization (DIN) [40].

The methods used for the analysis of these parameters are presented in

Table 2. Test Methods for oil and fatty acids.

Parameters	Unit	Test Method
Free fatty acids	% (m/m)	DIN EN ISO 660 [43]
Water content	% (m/m)	DIN 51777-1 [44]
Density at 15 °C	Kg m−3	DIN EN 12185 [45]
Phosphorus content	mg kg−1	DIN EN 14538 [46]
Sulfur content	mg kg−1	DIN EN 14538 [46]
Kinematic viscosity at 40 °C	mm2 s−1	DIN EN ISO 3104 [47]

.

2.7. Transesterification of RRO

The processes of esterification, transesterification, and analysis of the resulting biodiesel for this scientific paper were carried out at the certified laboratory of the Tecosol GmbH biodiesel plant in Germany, Jahnstrasse, Ochsenfurt. The methods and parameters of the European Biodiesel Quality Standards were applied.

For transesterification, 300 g refined oil, 45 g of methanol (Wistema GmbH, with a purity of 99%+) and 15 g of potassium methylate (BASF Ludwigshafen, 32% (in methanol)) catalyst were used. The mixture was kept at 35 °C for one hour at 400 rpm.

After the transesterification process, the samples were put to sedimentation, where due to the density differences, the glycerin phase (glycerol) with the higher density descends to the bottom of the container and the methyl ester remains on the top. The next step was the washing of the biodiesel with 200 g of glycerol (Topchem Chemikalienhandel, with a purity of 95%) to remove the remaining free fatty acids, then the separation of the phases. The biodiesel was then washed with 500 g of distilled water and mixed for between 5 and 10 min, and then it was put to sedimentation until the phases separated. This was followed by the drying of the biodiesel, after which it was mixed, until it reached a temperature of 130 °C, and when it reached this temperature, all of the water was evaporated.

2.8. Esterification and Transesterification of FAs

2.8.1. Esterification of FAs

The esterification process was carried out in a reactor in the laboratory. A total of 300 g of FAs, 150 g of Methanol Wistema GmbH, with a purity of 99%+), and 15 g of methane sulfonic acid (Arkema Thiochimie, with a purity of 70%), was added. The reaction took place for 60 min at a temperature of 60 °C.

After esterification, the crude ester from the reaction was put to sedimentation until the water settled.

2.8.2. Crude Ester Refining

The refining of crude ester consists of adding a mixture of 20% KOH (Potassium Hydroxide), 30% methanol, and 15% glycerin; this mixture is also called the glycerin phase, and results from trans-esterification. The crude ester is placed on the mixer with a pH meter (Mettler Toledo Seven Compact with pH-sensor Inlab Science PRO ISM) and the glycerin phase is added until a pH of 14 is obtained [48].

After refining, the samples were subjected to sedimentation, where the phase of glycerin and free fatty acids was removed.

2.9. Transesterification of Crude Ester

After the refining process, 3% Potassium Methylate (KM 32) and 6% Methanol were added to all 4 samples. Each sample was mixed for 60 min at a temperature of 35 °C.

To obtain a higher quality after the transesterification process, the following processes were carried out:

• Washing with 5% glycerol (mixing for 5–10 min)
• Sedimentation (the separation of crude biodiesel from the glycerin phase)
• Washing with acid water (10% distilled water and 2% sulfuric acid (Brenntag GmbH, with a purity of 96%) were added)
• Drying (the samples were mixed on the heater until they reached a temperature of 130 °C, and the water evaporated).
• To determine the quality of BRO and FAs, the following parameters were analyzed [48]:
• Acid value mg KOH g⁻¹, chemicals: 0.5 Mol/L methanolic KOH by Carl Roth; 2-propanol with 99.5% purity by Carl Roth, neutralized, analytical system: automatic Titration system by Metrohm, Herisau, Switzerland;
• Water content (%), used chemicals: Hydranal-Coulomat AG, 34836 by Fluka,; analytical system: Metrohm Karl-Fischer Coulomat;
• Density at 15 °C (kg m⁻³) analytical system Anton Paar DMA 4100M;
• Phosphorus content (mg kg⁻¹); analytical system ICP-OES System: Agilent 5110 with Autosampler SPS 4;
• Sulfur content; a Trace Elemental Instruments Xplorer TN/TS analytical system with the Archie Autosampler,
• Kinematic viscosity at 40 °C in mm² s⁻¹ with an Anton Paar DMA 3001 analytical system;
• Free glycerol and Mono-, Di- and Triglycerides% (m/m) method: EN 14105, Chemicals: n-Heptane with 99% purity by Bern Kraft; a Pyridine purity of 99.8% by Sigma-Aldrich; MSTFA by Macherey-Nagel; Monononadecanoin purity > 99% (by Larodan AB; 1,3-Dinonadecanoin purity > 99% by Larodan AB; Trinonadecanoin purity > 99% by Larodan AB; Analytical system: GC 2030 with AOC 20i autosampler by Shimadzu, Kyoto, Japan;
• Ester content% (m/m) method: EN 14103, chemicals: Toluol of 99.9% purity by Carl Roth; Methyl Nonadecanoate > 99.5% purity by ASG AG Neusäß, using a GC 2010 analytical system with an AOC 20i autosampler by Shimadzu, Kyoto, Japan.

2.10. Statistical Analysis

For the statistical processing of the elements determined in the experimental field (crops and oil), the statistical processing model indicated for the Latin rectangle type, arranged in blocks, with plots in three repetitions was used [49,50]. The regressions between the analyzed factors were tested after the graphical representation of the measured values using Microsoft Excel 2016.

3. Results and Discussions

The differences in rapeseed production and rapeseed oil and oil extraction efficiency achieved by the variants studied in the experimental field, showed statistical significance according to a Fisher’s test

3.1. Production of Rapeseeds

The average productions obtained from rapeseed, on the fertilization variants studied in the experimental field from Nojorid, are between 8.30 q ha⁻¹ and 55.9 q ha⁻¹. The average values escalate with the increase in the doses of complex fertilizers administered, from the control parcel without fertilizer addition (N₀P₀K₀), where the average production is 10.98 q ha⁻¹, to the variant with the maximum dose (N₄₈₀P₄₈₀K₄₈₀), where the average production is 47.00 q ha⁻¹.

The standard deviation (SD) has values between 2.34 and 14.84, the lowest value of 2.17, recorded for the variant N₁₆₀P₁₆₀K₁₆₀, showing a higher accuracy of determining the average production. The mean squared error (MSE) has a variation range of 1.17–7.42, with the lowest value of 1.09 for the same variant N₁₆₀P₁₆₀K₁₆₀. These values show a better uniformity of production from the repetitions of the N₁₆₀P₁₆₀K₁₆₀ variant and the highest accuracy in determining the average production.

In

Table 3. Analyzing the statistical significance of production differences using Limit Differences (LD).

Variant	Average Productions		Differences		Signification
	(q ha−1)	(%)	(q ha−1)	(%)
N0P0K0 (Control)	10.98	100.00	-	-	-
N160P160K160	13.30	121.18	+2.33	+21.18	-
N320P320K320	38.53	351.03	+27.55	+251.03	**
N480P480K480	47.00	428.25	+36.02	+328.25	***
		LD 5%	13.48
		LD 1%	20.41
		LD 0.1%	32.79

** distinct significant statistically positive; *** very significant statistically positive.

we observed that the productions relative to the control, in rapeseed, are between 121.18% for the variant N₁₆₀P₁₆₀K₁₆₀ and 428.25% for the variant N₄₈₀P₄₈₀K₄₈₀. The differences in the production compared to the control parcel (without fertilizer), grow with the increase in the fertilizer doses, from 2.33 q ha⁻¹ to 36.02 q ha⁻¹.

According to the calculated LD, the difference of the N₁₆₀P₁₆₀K₁₆₀ variant, compared to the control, is statistically insignificant (2.33 q ha⁻¹ < LD 5% = 13.48 q ha⁻¹). The other production differences of 251.03% (2.5 times higher production) for N₃₂₀P₃₂₀K₃₂₀ and 328.25% (more than three times and 75% higher) for N₄₈₀P₄₈₀K₄₈₀ are distinctly significant (27.55 q ha⁻¹ > LD 1% = 20.41 q ha⁻¹) and highly statistically significant (36.02 q ha⁻¹ > LD 0.1% = 32.79 q ha⁻¹).

Woźniak et al. reported average rapeseed productions of 32.60 q ha⁻¹ for European Union countries, 34.38 q ha⁻¹ for Germany, and 27.34 q ha⁻¹ for Poland in 2017 [51]. The average productions obtained in our experimental field, at the N₁₆₀P₁₆₀K₁₆₀ variant, are below the level shown above. Given that, in the case of N₃₂₀P₃₂₀K₃₂₀ and N₄₈₀P₄₈₀K₄₈₀, the yields are much higher, and it follows that the use of mineral fertilization maximizes rapeseed production.

In a study on the optimization of agronomic practices in China, Zhang et al., mentioned that the use of balanced fertilization, including N, P, K, the potential production of rapeseed (30.34 q ha⁻¹–40.04 q ha⁻¹) improved by 21.1% [51]. In our study, the average production obtained for the maximum dose of 47 q ha⁻¹ shows that there are still possibilities for increasing production.

3.2. Rapeseed Oil Production (l ha⁻¹)

The average production of RRO obtained in the experimental field are between 390.6 l ha⁻¹ and 2497.3 l ha⁻¹. The average values increase with the intensification in the doses of administered fertilizer, from the control without added fertilizer (N₀P₀K₀), in which the average production is 522.7 l ha⁻¹, to the variant with the maximum dose (N₄₈₀P₄₈₀K₄₈₀), in which the average production is 2130.8 l ha⁻¹.

The values of the SD and of the MSE are the lowest in the variant N₁₆₀P₁₆₀K₁₆₀, indicating a high accuracy with regard to determining the average, and the values were 101.7 and 50.9. It is worthy of note that the accuracy and uniformity of determining the average RRO production is similar to that recorded for rapeseed yields.

The transgression precision, P%, determined with the help of calculated t-values, indicates that the production differences of RRO are distinctly significant and highly statistically significant for the fertilizer doses N₃₂₀P₃₂₀K₃₂₀ and N₄₈₀P₄₈₀K₄₈₀. (

Table 4. Statistical significance analysis of yield differences in RRO using a Student’s t-test (transgression).

Variant	Average Oil Production (l ha−1)	Relative Oil Production (%)	Differences ± l ha−1	t	P%	Significance
N0P0K0 (Control)	522.7	100.00	-	-	-	-
N160P160K160	629.8	120.51	107.2	0.44	70.6	-
N320P320K320	1794.4	343.32	1271.7	5.16	0.2	**
N480P480K480	2130.8	407.69	1608.2	6.53	<0.1	***

** distinct significant statistically positive; *** very significant statistically positive.

).

The differences in oil production from the variant N₃₂₀P₃₂₀K₃₂₀, of +1271.7 l ha⁻¹ and from the variant N₄₈₀P₄₈₀K₄₈₀, of +1608.2 l ha⁻¹ are distinctly significant (p > 0.01) and very significant (p > 0.001) from a statistical point of view.

The differences in oil production highlighted above show proportional increases with the doses of fertilizers administered, as with rapeseed production. Therefore, increases in oil production could be the result of increases in rapeseed production. The same trend of increasing fat production is reported by research carried out in rapeseed, cultivated with a high technological level (autumn fertilization 30 kg ha⁻¹ N, 80 kg ha⁻¹ P, 150 kg ha⁻¹ K and spring fertilization in 3 stages, with 400 kg ha⁻¹ N and 80 kg ha⁻¹ S) [52].

3.3. Efficiency of Oil Extraction (%)

The average values of the yields for oil extraction vary between 43.75% for the control and 42.13% for the variant (N₄₈₀P₄₈₀K₄₈₀) with the maximum administered dose (Figure 3).

The variation range of the efficiency values is between 40.6% for the (N₄₈₀P₄₈₀K₄₈₀) and 44.4%, for the N₃₂₀P₃₂₀K₃₂₀ variant, respectively. The maximum SD = 1.17 corresponds to the same variant N₃₂₀P₃₂₀K₃₂₀, while the lowest value SD = 0.30 corresponds to the variant with the lowest dose of fertilizers administered (N₁₆₀P₁₆₀K₁₆₀). For the same variant, the MSE = 0.15 indicates the best accuracy for determining the average yield value.

The relative efficiency of oil extraction from rapeseed, compared to the control, is between 98.88% for the variant N₁₆₀P₁₆₀K₁₆₀ and 94.00% for the variant N₄₈₀P₄₈₀K₄₈₀. (

Table 5. Statistical significance analysis of efficiency differences in rapeseed oil using Limit Differences (LD).

Variant	Average Efficiency		Differences		Significance
	(%)	(%)	(%)	(%)
N0P0K0 (Control)	43.75	100.00	-	-	-
N160P160K160	43.23	98.80	−0.53	−1.20	-
N320P320K320	42.80	97.82	−0.95	−2.17	-
N480P480K480	41.13	94.00	−2.63	−6.00	ooo
		LD 5%	1.00
		LD 1%	1.51
		LD 0.1%	2.43

ooo—very distinctly statistically significant negative.

).

The percentage efficiency differences, compared to the control without fertilizer, are reduced with the increase of the administered dose, from 43.75% to 41.13%. The differences are negative, and for the maximum dose this is highly statistically significant.

Stepien et al. do not report statistically significant differences between the three levels of researched imputations (low, medium and high) [52]. This situation can be explained by the fact that the applied fertilization systems do not differ very much. However, the differences between the doses of N applied in spring can be noted: low level, 320 kg ha⁻¹; medium level, 360 kg ha⁻¹; high level, 400 kg ha⁻¹. These are very close to the maximum level used by us in the experimental field (320–480 kg ha⁻¹ N).

The specialized literature explains the reduction of oil extraction efficiency by the exaggerated increase of N doses, and mentions the possibility of improving them by applying a balanced fertilization by adding some P doses [53].

In our case, the doses of complex fertilizers have a balanced content of N, P, K; however, in the case of the maximum dose, the reduction of oil extraction efficiency is very significant (p > 0.001).

3.4. Regressions between Applied Doses of Complex Fertilizers and Productions of Rapeseed and Oil

The correlative link existing between the parcels tested in the experimental field and the grain rape production (q ha⁻¹) achieved on the repetition plots is of the exponential type (y = 9.97e0.53x). The regression coefficient R² = 0.7965 indicates that this relationship is distinctly statistically significant. (Figure 4).

The graphic representation of the evolution of rapeseed production shows a continuous escalation in production, with the increase in doses of administered complex fertilizers. The exponential shape of the curve suggests that at low doses the increase in production is less, and at high doses the increase is greater. For the first dose of N₁₆₀P₁₆₀K₁₆₀, the production increases by about 6 q ha⁻¹, while between the last two parcels the increase is about 20 q ha⁻¹. Instead, the standard error bars for productions, which are larger for high doses, show that, in this area, the average simulated productions have a lower precision.

The linear regressions established by Wang et al., for a long period of time between grain rape yields and applied N doses show that yields increase with increasing doses. Although the maximum dose applied is 360 kg ha⁻¹ N, they consider the optimal dose to be 120–160 kg ha⁻¹ N [54].

The evolution of rapeseed oil production according to the fertilizer doses is of the same type as that of grain rapeseed productions, specifically the exponential type, indicating the tendency of production to increase with the increase in fertilizer doses. (Figure 5).

The regression link between the two factors is also distinctly statistically significant (R² = 0.7957), indicating the same level of confidentiality and uncertainty. The close sizes of the standard error bars indicate a uniform prediction of the average oil productions, regardless of the applied fertilizer dose. However, the regression coefficient is lower for oil production due to the efficiency of oil extraction.

The regression established between the doses of complex fertilizers and the percentage of rapeseed oil extraction is of the II-degree polynomial type and shows the tendency to reduce the yields with an increase in the administered doses of fertilizers (Figure 6).

The correlational link shows a regression coefficient of R² = 0.693; it is lower than for productions (grains and oil), but still distinctly statistically significant (p < 0.01). If the efficiency reductions between the control and the first dose of fertilizers of 0.26% and 0.83% between the first and second dose, respectively, are acceptable, the reduction between the second and the third dose, at 1.41%, is too high.

Research on agronomic management in different tillage systems for rapeseed culture shows statistically significant regressions between administered N doses and oil extraction efficiency [55]. These confirm the results obtained by us, in that, the values of oil extraction efficiency are reduced from 494.5 g kg⁻¹ for the dose of 160 kg ha⁻¹ N administered to 483.8 g kg⁻¹ for the dose of 240 kg ha⁻¹ N.

3.5. RRO and FAs Quality

3.5.1. Quality of RRO

Table 6. RRO quality.

Parameters	Unit	N0P0K0 (Control)	N160P160K160	N320P320K320	N480P480K480
		Values	Values	Values	Values
Acidity	mg KOH g−1	0.099	0.102	0.092	0.089
Water content	% (m/m)	0.03	0.04	0.03	0.03
Density at 15 °C	kg m3	919.9	920.0	920.6	919.8
Phosphorus content	mg kg−1	2.76	2.80	2.62	2.73
Sulfur content	mg kg−1	7.43	7.03	7.03	3.00
Kinematic viscosity at 40 °C	mm2 s−1	35.010	34.950	34.961	34.996
C18:1 Oleic acid	% (m/m)	65.8	65.8	66.7	66.7
C18:2 Linoleic acid	% (m/m)	19.3	19.2	19.3	19.3
C18:3 Linolenic acid	% (m/m)	7.4	7.3	7.0	6.9

shows the results of the analysis carried out to determine the quality of RRO obtained from the experimental field.

The acidity of rapeseed oil for the control sample (N₀P₀K₀) is 0.099 mg KOH g⁻¹, the others having higher values, except for the N₁₆₀P₁₆₀K₁₆₀, for which the determined value was 0.102 mg KOH g⁻¹. The frame limit for this analysis is a maximum of 0,6 mg KOH g⁻¹. In a similar study, the values obtained were 2.7 mg KOH g⁻¹ [56,57].

The ES limits for water content in oil is a maximum of 0.1% (m/m). The values obtained in the analyzed samples are below the limit; for N₀P₀K_0, the water content is 0.03% (m/m), and the N₁₆₀P₁₆₀K₁₆₀ shows a lower difference of 0.01% (m/m).

High density may indicate high water or impurity content. Acceptable limits for rapeseed oil are between 912.0 and 918.0 kg m⁻³. The values obtained show a small crossing over of the limit, but are still acceptable. The highest value was registered for the variant N₃₂₀P₃₂₀ K₃₂₀ 920.0 kg m⁻³.

The phosphorus content shows differences compared to the control at N₁₆₀P₁₆₀K₁₆₀ with −0.4 mg/kg, at N₃₂₀P₃₂₀K₃₂₀ with +0.14 mg kg⁻¹, and at N₄₈₀P₄₈₀K₄₈₀, with +0.03 mg kg⁻¹.

Sulfur content has the highest value in the N₀P₀K₀ control sample, at 7.43 mg kg⁻¹. The differences obtained compared to the control sample are smaller for the other samples, with N₁₆₀P₁₆₀K₁₆₀ with 0.4 mg kg⁻¹, N₃₂₀P₃₂₀K₃₂₀ with 0.4 mg kg⁻¹ and for the N₄₈₀P₄₈₀K₄₈₀ variant with 4.43 mg kg⁻¹, respectively. In other similar studies carried out by other researchers, the value of the sulfur content in rapeseed oil was 11.2 mg kg⁻¹ [57].

The kinematic viscosity is 35.010 mm² s⁻¹ for the control variant. The recorded values were lower than the control. Jia et al., in their study [23], show that the viscosity value at 40 °C is 36.3 mm² s⁻¹, a value very close to the one we found for the control.

The FAs chain saturated FAs do not have C=C carbon double bonds. Unsaturated fatty acids have one or more C=C double bonds. C=C double bonds can give cis or trans isomers [48]. When determining the FAs chain in the refined oil, the nominal values are C18:1 51–70%, C18:2 15–30% and C18:3 5–14% (trans content); the results obtained for C18:1 are between 66.7% (m/m) and 65.8% (m/m), and for C18:2 they are between 19.2 and 19.3% (m/m).

The resulting differences, compared to the control sample, for the composition of FAs are as follows. For C18:1 oleic acid + isomers, variants N₃₂₀P₃₂₀K₃₂₀ and N₄₈₀P₄₈₀K₄₈₀ show the same differences compared to the control, at 0.9% higher. For C18:2 linoleic acid + isomers, the N₁₆₀P₁₆₀K₁₆₀ variant, at 0.1% lower, presents the only difference. For the C18:3 linolenic acid + isomers, there are smaller differences compared to the control in variants N₁₆₀P₁₆₀K₁₆₀ by 0.1%, N₃₂₀P₃₂₀K₃₂₀ by 0.4% and N₄₈₀P₄₈₀K₄₈₀ by 0.5% (Figure 7.)

Rapeseed oil typically contains about 98% triglycerides. The main components of this oil are oleic acid (monounsaturated), which is present in a proportion of more than 60%, and linoleic acid (double unsaturated), which exceeds 20% [58].

3.5.2. FAs Quality

The quality parameters for the FAs in the experimental field are presented in

Table 7. FAs quality.

Parameters	Unit	N0P0K0 (Control)	N160P160K160	N320P320K320	N480P480K480
		Values	Values	Values	Values
Free FAs	% (m/m)	59.83	59.88	60.07	59.95
Water content	% (m/m)	1.27	1.21	1.22	1.23
Density at 15 °C	kg m−3	919.3	919.4	918.8	918.6
Phosphorus content	mg kg−1	906	904	900	893
Sulfur content	mg kg−1	145	144	143	138
Kinematic viscosity at 40 °C	mm2 s−1	36.858	36.176	36.221	35.972
C16:0 Methyl Hex decanoate	% (m/m)	6.1	6.2	6.1	6.1
C16:1 Methyl cis-Palmitoleate	% (m/m)	0.4	0.4	0.4	0.4
C17:0 Methyl Heptadecanoate	% (m/m)	0.1	0.1	0.1	0.1
C18:0 Methyl Octadecenoate	% (m/m)	1.9	2	1.9	1.9
∑C20-C24 (Area%)	% (m/m)	2.3	1.9	2.4	2.5

.

The FAs analyzed come from the residues (soapstock) of the oil obtained from the studied experimental field, through the separation process. The reaction takes place in the presence of an acid (concentrated sulfuric acid 96%) and steam, until a temperature of 100 °C or more is reached; depending on the available installation, the steam favors the separation of the molecules and causes the soapstock to mix with the acid. The reaction time depends on the temperature of the raw material and the capacity of the steam generator.

Free FAs for all analyzed variants present negative differences in relation to the control variant. Free FAs expressed in % (m/m) have the value of 59.88 for the control sample, while the other samples have higher values of 0.05 to 0.12% (m/m).

For the analyzed fertilization variants, the water content of the FAs has the highest value of 1.27% for the control sample, while the other samples have lower values. Considering that the limit of the ES for the water content of fatty acids was 3% (m/m), for the conditions in our experimental field the resulting values were much lower.

The density at 15 °C has values between 920.6 and 919.8 kg m⁻³. For density, all values are lower than the Nominal FA Limit, which is 925.0 kg m⁻³. After the biodiesel production processes, the density decreased.

The phosphorus content shows differences compared to the control between 2 mg kg⁻¹ for the N₁₆₀P₁₆₀K_{160 sample} and 13 for the N₄₈₀P₄₈₀K_{480 sample}. Sample N₃₂₀P₃₂₀K₃₂₀ has a difference of 6 mg kg⁻¹, which increases with increasing doses of the complex fertilizers administered.

The sulfur content for the analyzed variants is reduced proportionally with the doses of fertilizers, the biggest difference compared to the control variant being recorded for the N₄₈₀P₄₈₀K₄₈₀ variant, which is 13 mg kg⁻¹.

The kinematic viscosity at 40 °C shows lower values than the control sample (36.86 mm² s⁻¹); for all cases, the differences being 0.682 mm² s⁻¹ (N₁₆₀P₁₆₀K₁₆₀), 0.637 (N₃₂₀P₃₂₀K₃₂₀) and 0.886 mm² s⁻¹ (N₄₈₀P₄₈₀K₄₈₀), respectively.

The C 16:0, C 16:1, C17:0, C18:0, and ∑C20–C24 values give an indication whether the analyzed FAs, which are a carboxylic acid with a long aliphatic chain, are saturated or unsaturated. Unsaturated FAs have one or more C=C double bonds. C=C double bonds can give cis or trans isomers. For FAs, the nominal values indicated by ES are C16:0 10.0%, C16:1= 1.3%, C17:0 = 0.3%, and C18:0 =3.0%, ∑C20–C24 =5.0%.

The values resulting from our analyses are in all cases below the limit of the nominal values. In the composition of FAs C16:0, C16:1, C17:0, C18:0 and ∑C20–C24, we have low or no differences. At C16:0, the only difference compared to the control is registered at the variant N₁₆₀P₁₆₀K₁₆₀ of 0.1% (m/m). C16:1 and C17:0 show no differences. For the C18:0 parameter, the only difference we have in the N₁₆₀P₁₆₀K₁₆₀ sample is 0.1% (m/m). For ∑C20–C24 we have a smaller difference compared to the control at N₁₆₀P₁₆₀K₁₆₀ of 0.4% (m/m) and higher differences of N₃₂₀P₃₂₀K₃₂₀ and N₄₈₀P₄₈₀K₄₈₀ of 0.1% (m/m) and 0.2% (m/m), respectively (Figure 8).

3.5.3. Statistical Processing of Quality Parameters for Refined Oil and Fatty Acids

Table 8. Statistical processing of quality parameters for RRO and FAs.

Parameters	Unit	Mean	Min.	Max.	SD	MSE
The quality parameters for RRO
Acidity	mg KOH g−1	0.10	0.089	0.102	0.01	0.003
Water content	% (m/m)	0.03	0.03	0.04	0.01	0.003
Density at 15 °C	kg m−3	920.08	919.8	920.6	0.36	0.18
Phosphorus content	mg kg−1	2.73	2.62	2.80	0.08	0.04
Sulfur content	mg kg−1	6.12	3	7.43	2.09	1.05
Kinematic viscosity at 40 °C	mm2 s−1	34.98	34.950	35.010	0.03	0.01
C18:1 Oleic acid	% (m/m)	66.25	65.8	66.7	0.52	0.26
C18:2 Linoleic acid	% (m/m)	19.28	19.2	19.3	0.05	0.03
C18:3 Linolenic acid	% (m/m)	7.15	6.9	7.4	0.24	0.12
The quality parameters for FAs
Free FAs	% (m/m)	59.93	59.8	60.07	0.10	0.05
Water content	% (m/m)	1.23	1.21	1.27	0.03	0.01
Density at 15 °C	kg m−3	919.03	918.6	919.4	0.39	0.19
Phosphorus content	mg kg−1	900.75	893	906	5.74	2.87
Sulfur content	mg kg−1	142.50	138	145	3.11	1.55
Kinematic viscosity at 40 °C	mm2 s−1	36.31	35.972	36.858	0.38	0.19
C16:0 Methyl Hex decanoate	% (m/m)	6.13	6.1	6.2	0.05	0.03
C16:1 Methyl cis-Palmitoleate	% (m/m)	0.40	-	-	0	0
C17:0 Methyl Heptadecanoate	% (m/m)	0.10	-	-	0	0
C17:0 Methyl Heptadecanoate	% (m/m)	0.10	-	-	0	0
∑C20–C24 (Area%)	% (m/m)	2.28	1.9	2.5	0.26	0.13

shows a statistical processing of the averages, minimums, maximums, SD, and the MSE of the quality parameters for RRO and FAs.

Small values of SD and MSE indicate a high precision of determination of the mean value. We have the lowest values of SD for oil parameters at the water content of 0.01, a kinematic viscosity of 0.03 and 0.05 for C18:2. For MSE, the smallest values are 0.003 for water content, 0.003 for acidity, and a kinematic viscosity of 0.01.

The FAs parameters for SD and MSE are 0 in the case of C16:0 and C17:0. C16:0 shows SD values of 0.03 and an MSE of 0.03. We observed a water content of 0.03 at SD and 0.013 at MSE.

3.6. Quality of BRO and BFA

3.6.1. BRO Quality

The qualitative characteristics of the BRO, resulting from the plots of the experimental field on which increasing doses of complex fertilizers were applied, are presented in

Table 9. Quality of BRO.

Parameters	Unit	N0P0K0 (Control)	N160P160K160	N320P320K320	N480P480K480
		Values	Values	Values	Values
Acid value	mg KOH g−1	0.130	0.055	0.150	0.146
Water content	% (m/m)	0.014	0.035	0.041	0.013
Density at 20 °C	kg m−3	882.6	882.5	882.9	882.6
Phosphorus	mg kg−1	0.05	0.01	0.05	0.03
Sulfur content	mg kg−1	3.79	3.75	3.57	3.23
Kinematic viscosity at 40 °C	mm2 s−1	4.414	4.397	4.427	4.397
Monoglycerides	% (m/m)	0.24	0.23	0.23	0.23
Diglycerides	% (m/m)	0.18	0.15	0.15	0.15
Triglycerides	% (m/m)	0.36	0.31	0.31	0.31
Free glycerol	% (m/m)	0.001	0.003	0.003	0.003
Ester content	% (m/m)	97.1	98.2	98.2	98.2

.

The acidity value of biodiesel is a measure of the corrosive properties and an indication of the origin of the FAs, which result from the raw material used. Our analyses indicate that the highest value is obtained for the variant N₃₂₀P₃₂₀K₃₂₀, at 0.150 mg KOH g⁻¹, and the lowest for N₁₆₀P₁₆₀K₁₆₀, at 0.055 mg KOH g⁻¹. As a result, the corrosive properties increase for high doses of fertilizers. Encinar et al obtained a value of 0.49 mg KOH g⁻¹ for BRO [59]. Compared to the acidity limit value for biodiesel [60], which is 0.500 mg KOH g⁻¹, in addition to the result reported by Encinar et al., our values are much lower.

The water content of the BRO has values between 0.013% (m/m) for the variant with the highest dose of complex fertilizers studied, and 0.041% (m/m) for the variant N₃₂₀P₃₂₀K_320. The maximum is registered in the area of average doses of fertilizers. The study mentioned above states a value 0.06% (m/m) for the water content of BRO, half of our value for the control [59]. However, all of our values, determined for all analyzed variants, are lower than the nominal limit value in ES [61], which is 0.050% (m/m).

The density at 20 °C shows very close values, between 882.9 kg m⁻³ and 882.5 kg m⁻³, indicating that the doses of fertilizers do not essentially influence this characteristic. Compared to the results of the research carried out by Encinar et al., which were 861.9 kg m⁻³ our values are slightly lower, but fall within the limits imposed by the European Norms [45], which are between 860 and 900 kg m⁻³.

The phosphorus content of BRO shows very low values, between 0.01 for N₁₆₀P₁₆₀K₁₆₀ and 0.05 mg kg⁻¹ for the control variant and N₃₂₀P₃₂₀K₃₂₀. N₄₈₀P₄₈₀K₄₈₀ has a value of 0.03 mg kg⁻¹.

The values obtained in our experiment for the kinematic viscosity were: 4.414 mm² s⁻¹ for the control sample, and 4.397 mm² s⁻¹ for N₁₆₀P₁₆₀K₁₆₀ and N₄₈₀P₄₈₀K₄₈₀, as well as for N₃₂₀P₃₂₀K₃₂₀. At 4.427 mm² s⁻¹, these are higher than the values obtained by Khan et al. [61]. It can also be noted that all of the determined values fall within the ES [47], which are between 3.5–5.0 mm² s⁻¹.

The sulfur content of biodiesel comes mainly from the raw materials used, with animal fats contributing to a higher proportion than vegetable oils. The reduced content is important for limiting carbon dioxide emissions into the atmosphere. The values of the sulfur content in the biodiesel obtained from the refined oil are very good, the highest value being recorded in the control variant, which was 3.79 mg kg⁻¹, a much lower value in relation to the maximum value specified by the ES [46], which is 15 mg kg⁻¹. Thus, increasing the doses of fertilizers determines the reduction of sulfur content and therefore the reduction of gas emissions into the atmosphere.

Monoglycerides, diglycerides and triglycerides in BRO have values that fall within the limits. The ES [62,63], for monoglycerides has a maximum limit of 0.70% (m/m) and a maximum rejection limit of 0.82%; for diglycerides the maximum limit is 0.20% and the rejection limit is 0.24%, and for triglycerides the maximum limit is 0.20% and the rejection limit is 0.27%.

Our determinations show an ester content of 97% (m/m) for the control variant (N₀P₀K₀), while for the variants to which complex fertilizers were administered, the purity increased to 98.2% (m/m). The ES [63] had a value of 96.5% (m/m) as the limit for the minimum content of esters.

In conclusion, BRO generally falls within the quality parameters imposed by ES. The parameters analyzed to evaluate the quality show close values between the variants of complex fertilizers administered to the rape crop. In general, the variants N₁₆₀P₁₆₀K₁₆₀, N₃₂₀P₃₂₀K_320, and N₄₈₀P₄₈₀K₄₈₀ register lower values compared to the control variant (N₀P₀K₀). Higher values of fertilizers produce an increase in corrosive power, water content, and free glycerol, which indicates a possible contamination with impurities.

3.6.2. BFA Quality

To be commercialized, biodiesel must meet the requirements for fatty acid methyl esters as a fuel component. Parameters and threshold values were selected to ensure functionality. In

Table 10. Quality of the BFA.

Parameters	Unit	N0P0K0 (Control)	N160P160K160	N320P320K320	N480P480K480
		Values	Values	Values	Values
Acid value	mg KOH g−1	0.535	0.207	0.328	0.493
Water content	% (m/m)	0.013	0.006	0.013	0.007
Density at 20 °C	kg m−3	890.3	881.3	881.8	882.5
Phosphorus	mg kg−1	2.68	0.51	1.27	1.51
Sulfur content	mg kg−1	14.4	11.4	11.8	11.6
Kinematic viscosity at 40 °C	mm2 s−1	4.985	4.479	4.612	4.558
Monoglycerides	% (m/m)	0.53	0.37	0.30	0.22
Diglycerides	% (m/m)	0.21	0.19	0.17	0.15
Triglycerides	% (m/m)	0.23	0.04	0.19	0.02
Free glycerol	% (m/m)	0.008	0.006	0.006	0.004
Ester content	% (m/m)	90.8	95.5	95.5	97.1

the results of the BFA are displayed.

The acidity in BFA can be increased by the saponification of fats and acid processing, and they form additional free FAs. Under the conditions of our study, the acidity values are between 0.207 mg KOH g⁻¹ (N₁₆₀P₁₆₀K₁₆₀) and 0.535 mg KOH g⁻¹ (control sample N₀P₀K₀). The value obtained by Kim et. al., for a fatty acid biodiesel acidity of 0.36 mg KOH g⁻¹ falls within our values [58]. Both sets of values fall within the limits imposed by [60], which are 0.500 mg KOH g⁻¹, and less for the control variant.

The water content of BFA is due to its washing with distilled water, and its partial removal in the last step of the process, which is drying. The solubility of water in biodiesel is approximately 1000 mg kg⁻¹, as it is hygroscopic. The increased water content is the main cause of microbial contamination, and this can cause corrosion and filter blockage. The values recorded in our research are 0.006–0.013% (m/m). The maximum reported by Kim et. al., regarding the water content, is 0.0183% (m/m) [58], which is higher than our values. The maximum allowed by ES [61] is 0.050% (m/m).

The density of biodiesel is slightly higher than that of fossil diesel. A lower density indicates contamination, such as a high methanol content, for example. The recorded values are similar, except for the control variant, which has a slightly higher value of 890.3 kg m⁻³, but is still within the limits of the ES for density [45].

The phosphorus content is higher in the control variant (2.68 g kg⁻¹), so in the differently fertilized variants it is reduced by 1.17–2.17 g kg⁻¹, which is directly proportional to the size of the doses.

The sulfur content is between 11.4 mg kg⁻¹ for the N₁₆₀P₁₆₀K₁₆₀ variant and 14.4 mg kg⁻¹ for the N₀P₀K_{0 one}. The results are within nominal limits.

The kinematic viscosity is also higher for the control (4.985 mm² s⁻¹), and it is decreasing for the other variants; they do not exceed the limit values indicated by the ES.

Monoglycerides, diglycerides, and triglycerides maintain the same tendency as the phosphorus and sulfur content, with the recorded values being within the limits of European Norms.

Free glycerol is preserved in all the variants studied: (0.008–0.004% (m/m) below the maximum value specified by ES (0.020% (m/m).

The ester content ranges from 90.8% (m/m) (N₀P₀K₀) to 97.1% (m/m) at N₄₈₀P₄₈₀K₄₈₀. The variants N₁₆₀P₁₆₀K₁₆₀ and N₃₂₀P₃₂₀K₃₂₀ have a value 1% (m/m) less than the limit accepted by the European Norms, but after distillation the content increases above the minimum limit.

3.6.3. Statistical Processing of the Quality Parameters of BRO and BFA

The averages of SD, range of variation, and MSE for quality parameters of the BRO and BFA are listed in

Table 11. Statistical processing of the quality parameters for BRO and BFA.

Parameters	Unit	Mean	SD	Variation Range		MSE
The quality parameters for BRO
Acid value	mg KOH g	0.12	0.04	0.16	−0.08	0.02
Water content	% (m/m)	0.03	0.01	0.04	−0.01	0.01
Density at 20 °C	kg m−3	882.65	0.17	882.82	−882.48	0.09
Phosphorus	mg kg−1	0.04	0.02	0.05	−0.02	0.01
Sulfur content	mg kg−1	3.59	0.26	3.84	−3.33	0.13
Kinematic viscosity at 40 °C	mm2 s−1	4.41	0.01	4.42	−4.39	0.01
Monoglycerides	% (m/m)	0.23	0.005	0.24	−0.23	0.003
Diglycerides	% (m/m)	0.6	0.02	0.17	−0.14	0.01
Triglycerides	% (m/m)	0.32	0.03	0.35	−0.30	0.01
Free glycerol	% (m/m)	0.003	0.001	0.004	−0.002	0.001
Ester content	% (m/m)	97.93	0.55	98.48	−97.38	0.28
The quality parameters for BFA
Acid value	mg KOH g−1	0.39	0.15	0.54	−0.24	0.08
Water content	% (m/m)	0.01	0.004	0.01	−0.01	0.002
Density at 20 °C	kg m−3	883.98	4.25	888.22	−879.73	2.12
Phosphorus	mg kg−1	1.49	0.90	2.39	−0.59	0.45
Sulfur content	mg kg−1	12.30	1.41	13.71	−10.89	0.70
Kinematic viscosity at 40 °C	mm2 s−1	4.66	0.22	4.88	−4.43	0.11
Monoglycerides	% (m/m)	0.36	0.13	0.49	−0.22	0.07
Diglycerides	% (m/m)	0.18	0.03	0.21	−0.15	0.01
Triglycerides	% (m/m)	0.12	0.11	0.23	−0.01	0.05
Free glycerol	% (m/m)	0.01	0.002	0.01	−0.004	0.001
Ester content	% (m/m)	94.73	2.72	97.45	−92.00	1.36

.

The means at BRO are between 0.003 (free glycerol) and 882.65 (density). The lowest SD values are 0.001 for free glycerol and 0.55 for the ester content. For the MSE, the lowest value is 0.00050 in the case of free glycerol.

The averages for BFA are between 0.01 (glycerol content) and 883.98 (density). The lowest registered value for SD is 0.0010 for the glycerol content, while the highest value is recorded for the sulfur content, with 0.26. The MSE presents the lowest value of 0.001 for glycerol content and the highest value for sulfur content, at 0.13.

3.6.4. Comparison of the Differences between the Quality Parameters of BRO and BFA

To highlight the differences compared to the control samples, we analyzed each quality parameter separately. We took the control samples difference at 100% value to see the percentage alterations for the other samples.

In the case of acidity, we notice that the values of the differences compared to the control samples are small, which indicates better quality (

Table 12. Differences (BFA—BRO) for the acidity parameter.

Acidity	BFA (mg KOH g−1)	BRO (mg KOH g−1)	Differences (mg KOH g−1)	%
N0P0K0 (Control)	0.535	0.130	+0.405	100
N160P160K160	0.207	0.055	+0.152	37.5
N320P320K320	0.150	0.328	+0.178	44.0
N480P480K480	0.493	0.146	+0.347	85.7
Average	0.391	0.120	0.271	67

).

The acidity values are higher in BFA than in BRO. For the same dose of fertilizers, they are reduced in BRO compared to BFA, indicating an improvement in quality. The greatest effect on the improvement of acidity conditions, is given by the maximum dose of fertilizers (N₄₈₀P₄₈₀K₄₈₀) compared to the control variant, at 85.7%.

The differences in water content between BFA and BRO have average values of 0.016% (m/m), these being higher at BRO (Figure 9).

The differences at the same fertilization level are −0.001 (N₀P₀K₀) and −0.029% (N₁₆₀P₁₆₀K₁₆₀). The highest values of the differences, compared to the control samples, of 2800 and 2900%, are found in N₁₆₀P₁₆₀K₁₆₀ and N₃₂₀P₃₂₀K₃₂₀ variants. The maximum fertilization dose induces a difference of only 600%.

The higher water content for BRO than BFA can be explained by the fact that in the production of BFA, the technological process includes a drying stage, with the aim of removing water.

For density, the value of the control sample BFA is 7.7 times higher than the BRO, while the other results have minimal differences (

Table 13. Differences (BFA—BRO) for density parameter.

Density at 20 °C	BFA (kg m−3)	BRO (kg m−3)	Differences (kg m−3)	%
N0P0K0 (Control)	890.3	882.6	+7.7	100
N160P160K160	881.3	882.5	−1.2	−15.6
N320P320K320	881.8	882.9	−1.1	−14.3
N480P480K480	882.5	882.6	−0.1	−1.30
Average	884.0	882.7	1.3	17

).

In general, the doses of fertilizers reduce the density of biodiesel, so they become negative. The biggest effect is registered for the N₁₆₀P₁₆₀K₁₆₀ variant, the reduction being 15.6%. Given that a lower density indicates a possible contamination, the reduction of its values, along with the increase in the doses of fertilizers, indicates an increase in the quality of biodiesel.

BFA-BRO differences in phosphorus content are positive, with an average value of 4.55 mg kg⁻¹. The rapeseed fertilization system reduces the BFA—BRO differences for the phosphorus content from 100% for the control to 19.01–52.27%. (Figure 10). The best effect in the re-reduction of phosphorus is registered with the N₁₆₀P₁₆₀K₁₆₀ variant.

For sulfur content (Figure 11), the lowest value compared to the control is at N₁₆₀P₁₆₀K₁₆₀, with 72.1%. The mean value for BFA is 12.3 mg kg⁻¹, and for BRO it is 3.59 mg kg⁻¹. Reductions due to the fertilization system are between 72.1% and 78.88%.

In both cases the quality of biodiesel is superior in the case of BRO. The best quality improvements result for the N₄₈₀P₄₈₀K₄₈₀ variant, the reductions being 1.48 mg kg⁻¹ for phosphorus and 8.37 mg kg⁻¹ for sulphur.

The kinematic viscosity has higher values for BFA (4.6585 mm² s⁻¹) than for BRO (4.4086 mm² s⁻¹), indicating a better quality for BRO (

Table 14. Differences in the kinematic viscosity parameter.

Kinematic Viscosity 40 °C	BFA (mm2 s−1)	BRO (mm2 s−1)	Differences (mm2 s−1)	%
N0P0K0 (Control)	4.9846	4.4141	+0.57	100
N180P160K160	4.4793	4.3972	+0.08	14.4
N320P320K320	4.6122	4.4272	+0.19	32.4
N480P480K480	4.5579	4.3972	+0.16	28.2
Average	4.6585	4.4086	0.25	44.0

).

In the case of BFA the kinematic viscosity values are lower for the fertilized variants than for the control. In the case of BRO, the highest value is recorded for the N₃₂₀P₃₂₀K₃₂₀ variant, which is 4.4272 mm² s⁻¹, a value that is still lower than all of those for BFA. The effect in reducing the differences has the same variant, for which there is an improvement compared to the control, which is 32.4%.

Figure 12 shows the percentage values of BFA–BRO differences for monoglycerides, diglycerides, and triglycerides.

Monoglycerides generally have higher values for BFA, at 0.355% (m/m), than for BRO, at 0.235% (m/m). If the differences are positive for variants N₁₆₀P₁₆₀K₁₆₀ and N₃₂₀P₃₂₀K₃₂₀, for variant N₄₈₀P₄₈₀K₄₈₀ the difference is negative, at −0.01% (m/m). Compared to the control, the differences are reduced in proportion to the doses of admitted fertilizers, from 48.3% to −3.4% (Figure 12a).

Furthermore, in the case of diglycerides, higher values are encountered for BFA. BRO reductions to BFA are 0.00–0.04% (m/m). The percentage decreased is reduced with the increase of the doses of fertilizers, these being null for the maximum dose (Figure 12b).

In the case of triglycerides, BRO values are higher than the BFA values. The increases of the differences in relation to N₀P₀K₀ are more than twice as large for N₁₆₀P₁₆₀K₁₆₀ (207.7%) and N₄₈₀P₄₈₀K₄₈₀ (223.1%) (Figure 12c).

The presence of glycerides in BFA is related to the conventional process of biodiesel synthesis from FAs. Although some of the glycerol is removed by sedimentation, the remaining glycerol phase is estimated at 10% [63]. Triglycerides are the fatty acids of glycerol, the main constituents of lipids, mostly represented by vegetable oils from which BFA is obtained [30,64]. For this reason, they are found in higher amounts in rapeseed oil and BRO.

The differences in BFA–BRO for free glycerol and ester contents are presented in Figure 12.

Regarding the free glycerol content, the difference values are very close, being 0.007% (m/m) and 0.003% (m/m) for the control, respectively,, except for the variant with the maximum fertilizer, for which it is reduced to 0.001% (m/m) (Figure 13a). The percentage reductions of the fertilized variants reach 42.9% of those of the variant without fertilization.

In the case of the content of ester, the negative differences BFA–BRO, increase for the fertilized variants compared to the control variant, for which it registered −6.3% (m/m) (Figure 13b). Thus, the differences reach 42.9% of the value of the control variant, in the case of the N₁₆₀P₁₆₀K₁₆₀ and N₃₂₀P₃₂₀K₃₂₀ variants, and 17.5% for variant N₄₈₀P₄₈₀K_480, respectively.

The higher values for free glycerol from BFA are explained by the residues left after the purification step by separating and removing the glyceride phase [65].

The ester content is an indicator of the purity of biodiesel. A low ester content results from contaminants present in the oil or improper reaction conditions, as well as incomplete reactions. Contaminants include sterols, residual alcohols, glyceride fractions, non-degradable glycerol, and unsaponifiable materials [66,67]. However, these contaminants can be removed through the distillation process.

The technological process of obtaining biodiesel from residues is less costly than the process involved with refined vegetable oil; it has the highest carbon dioxide savings of all biodiesels. This type of biodiesel (BFA) is considered an advanced biofuel that does not compete with food products and releases fewer gases into the atmosphere [35].

4. Conclusions

Rapeseed (Brassica napus oleifera L.) is now a particularly important source of vegetable oils. Rapeseed oil is an ideal raw material for fuels due to its calorific value, oxidation stability, and low temperature behavior.

Rapeseed is a crop that responds well to the application of mineral fertilizers. The use of large amounts of complex fertilizers leads to high yields of rapeseed (13.3–47.0 q ha⁻¹) and oil (629.8–2130.8 L ha⁻¹), which are statistically significant only for the high doses (N₃₂₀P₃₂₀K₃₂₀ and N₄₈₀P₄₈₀K₄₈₀).

The average values of the oil extraction efficiency decrease with the increase in the administered doses of fertilizers. The efficiency differences, in relation to the control without administered fertilizers, are negative, and for the maximum dose (N₄₈₀P₄₈₀K₄₈₀) this is −2.63% and is statistically very significant.

The quality of the RRO and FAs does not differ much, depending on the fertilization variants. However, the values of water content (+1.20% (m/m), phosphorus (+918.02 mg kg⁻¹), and sulfur (+136.38 mg kg⁻¹) are much higher for FAs compared to RRO.

The differences between BFA and BRO, of the qualitative parameters analyzed, demonstrate a higher quality for BRO, but these are within the limits of the ES regarding the quality of biodiesel.

The influence of administered complex fertilizers on quality is positive, and increasing the administered doses determines the improvement of the quality, both for BRO and BFA. With all the advantages of using complex fertilizers for rapeseed fertilization, we do not recommend excessive increases in applied doses due to the fact that high doses reduce the efficiency of oil extraction.

Although the quality of BRO is superior to BFA, it is produced on a smaller scale due to the nutritional importance of RRO. BFA production is preferred, although the technological process is more laborious due to the need to improve some of its quality characteristics.

The lack of a sufficient amount of RRO and FAs for the qualitative analysis of four repetitions of BRO and BFA, for each analyzed fertilizer dose, required the averaging of samples for the same fertilizer dose. Under these conditions, it was not possible to statistically analyze the results and respectively establish their uncertainty. Considering the ES in terms of quality, the analysis methodology used the latest generation equipment and statistical analyses, and it is believed that there are no special problems related to uncertainties. To eliminate uncertainty, the experiment will be repeated with the harvesting of a larger amount of rapeseed, which will ensure the oil and fatty acids for the biodiesel corresponding to each analyzed dose of fertilizer.

At the same time, we believe that it is very important to know the mechanism by which fossil fuels, namely crude oil and raw material for chemical fertilizers, can be transformed into biodiesel, an environmentally friendly fuel, with the help of rapeseed.