Experimental Testing of Combustion Parameters and Emissions of Waste Motor Oil and Its Diesel Mixtures

: The production of hydrocarbon fuel from waste engine oil is an excellent way to produce alternative fuels. The aim of the research in this paper is obtaining fuel with a mixture of waste engine oil (WMO) and diesel fuel that can be used as an alternative fuel for internal combustion engines and low power heat generators. With this goal in mind, tests were conducted to estimate the combustion parameters and emissions at a low heat output of 40 kW. Waste motor oils (WMO) and four of its diesel mixtures were used, varying in weight from 20% WMO to 50% WMO. Test results were analysed and compared with diesel fuel. Higher NO, CO and CO 2 emissions were determined for WMO and its mixtures compared to diesel fuel. The ﬂue gas temperature in the kiln was high for all WMO and diesel blends, which indicates the efﬁciency of the input energy. The absorption of ﬂue gases in the scrubber with distilled water showed higher presence of sulphates, sulphides, nitrates and nitrites compared to allowable values.


Introduction
Increasing energy consumption, strict emission standards and consumption of oil resources have led a large number of researchers to explore the possibility of using alternative fuels for internal combustion engines and heat generators. Waste oils present a serious problem for the environment due to their disposal and are considered useful energy sources due to their high calorific value.
Motor oils have multiple roles in internal combustion engines. They lubricate the engine, reduce friction and wear of its moving parts, ensure good cylinder-piston assembly sealing, protect the engine from corrosion, ensure efficient engine cooling, keep the engine in a clean and functional state and prevent the formation of sediment and impurities on engine parts. During use, there are changes in the initial characteristics and contamination with different materials (water, products fraying and other impurities) so that, after a certain period of use, they do not satisfy the lubrication requirements and must be replaced with new ones. Legal regulations concerning waste disposal are increasingly violated in Bosnia and Herzegovina, and that especially applies to the disposal of motor oil (WMO) waste. Inadequate disposal of WMOs can lead to great environmental damage and ripening of soil and water pollution. In order to protect the environment, the following activities should be undertaken: • Reduce the amount of WMO for lubrication (by extending the service life of lubricating oil using rapidly biodegradable lubricating oils); • Organize WMO collection (oil waste collection is part of the overall waste management process); • Recycling of WMOs (conversion of waste lubricating oils into commercially usable products, for example, refining processes to base oils).
In addition to the negative environmental impact, WMOs also have caloric properties (energy value), so when mixed with other fuels such as diesel fuel, they can be used as fuels for agricultural machinery and individual combustion plants and for the operation of larger marine engines and various combustion plants [1]. They could also be used in various branches of process engineering (heating plants, dryers, cement plants, brick production, asphalt bases, etc.). There is insufficient data in the literature on the feasibility of using WMO and its blends with diesel as fuel in a higher-power heat generator. Recently, many researchers have been exploring the possibility of using waste cooking oil and waste transformer oils as an alternative fuel for diesel engines [2][3][4]. The studies were performed on a single cylinder four-stroke 4.4 kW and 5.7 kW maximum cylinder pressure of about 8.6 × 10 6 Pa [5][6][7][8][9][10]. Many researchers [11][12][13][14][15][16] have been exploring the possibility of using various types of waste oils (transformer, palm, rapeseed, sunflower, various lubricants) and their mixtures as an alternative fuel in internal combustion engines. They obtained similar results, i.e., waste oils and their mixtures can be used in internal combustion engines. It has also been shown that engine performance has improved in terms of fuel economy and environmental impact. There is insufficient data in the cited literature on the environmental impact of the composition of flue gas resulting from the combustion of WMO and its mixtures with diesel fuel.
In this study, an attempt was made to evaluate the combustion and emission characteristics using WMO and its mixtures in a 40 kW furnace at atmospheric pressure with respect to diesel fuel combustion. WMO can be used in 40 kW heat generators and can reduce problems relating to the protection of the environment. This is especially important for the development of a circular economy that uses renewable energy sources, which is especially important for Bosnia and Herzegovina, a highly resource-dependent country.

Motor Oil
WMOs are all mineral or synthetic oils that are unusable for the purpose for which they were originally intended, such as: engine lubrication, engine cooling, friction reduction on moving engine parts, protecting the engine from corrosion and more.
Mineral motor oils are obtained by adding certain additives to the base mineral oil to achieve its desired physical and chemical properties. There are various additives such as: various enhancers for viscosity, oxidizing inhibitors, dispersants, high pressure additives, antifoaming agents, emulsifiers and de-emulsifiers. Due to their low prices, they are the most represented on the market.
Synthetic motor oils are derived from artificially produced base oils with additives to improve their chemical and physical properties. Synthetic motor oils are much more expensive than mineral oils. Their application is justified only in areas where there are cooling and lubrication problems that cannot be technically solved by the use of conventional lubricants. Hydrocarbon and ester synthetic lubricants are most prevalent in motor oils.

Physical and Chemical Properties of WMO
WMO used as fuel in this test was collected in the units of Electric Power Company "Elektro-Bijeljina" a.d. Bijeljina, Bosnia and Herzegovina. Diesel fuel was purchased at the gas station of the company "OPTIMA GRUPA" d.o.o. Banja Luka, Bosnia and Herzegovina. This is why, for experimental research purposes, WMO is mixed with diesel fuel in different mass ratios, 20% WMO and 80% diesel fuel, 25% WMO and 75% diesel fuel, 33% WMO and 67% diesel fuel and 50% WMO and 50% diesel fuel, designated as WMO20, WMO25, WMO33 and WMO50, respectively. The composition of the mixture of WMO and diesel fuel in the considered mass ratios was determined using the expression where: X (mixture)-the mass fraction of component X in the mixture, % X (WMO)-mass fraction of component X in WMO, % X (diesel fuel)-mass fraction of component X in diesel fuel, % x-mass fraction of WMO in the mixture (0.20, 0.25, 0.33, 0.50), kg kg −1 1 − xmass fraction of diesel in the mixture, kg kg −1 .
The thermal power of the WMO-diesel fuel mixture was determined using the expression [17] H A special fact is that the use of pure WMO is made difficult due to its density and viscosity ( Table 2).

Experimentation
The scheme of the laboratory facility used in this research and the list of measurement sites are shown in Figure 1. Diesel combustion parameters and WMO-diesel fuel blend parameters were tested in the various mass ratios, and it was performed in a rotary kiln (Figure 1). WMO and its diesel fuel mixtures were well mixed to give a homogeneous stable mixture. Prior to combustion, samples of various fuel mixtures were held in an open-air atmosphere (2). Mass consumption of fuel was measured directly by a scale (1). The fuel from the mensuration was injected using a burner (3) and burned in a rotary kiln (4). Using a pump (11) located at the end of the sampling line, a partial flow of flue gas (6) was sampled, and in the vessel (7), the composition of the flue gas (CO, CO2, O2, NO) as well as the coefficient of excess air and the temperature of the sampled gas were determined. After determining the composition of the flue gas sampled, the flue gas was passed through a flue gas rinse with distilled water (9) to remove sulphate. The gas flow was measured using a rotameter (10). In order to obtain reliable results of testing the parameters of the fuel combustion process, the experiment was repeated four times for each fuel used. The results were compared, and no major discrepancies were observed. Values of standard deviation (S) and variation coefficient (Cv) of measurement results for gaseous components NO, CO and CO2 caused by combustion of fuel D4 and its mixtures with WMO are shown in   Diesel combustion parameters and WMO-diesel fuel blend parameters were tested in the various mass ratios, and it was performed in a rotary kiln ( Figure 1). WMO and its diesel fuel mixtures were well mixed to give a homogeneous stable mixture. Prior to combustion, samples of various fuel mixtures were held in an open-air atmosphere (2). Mass consumption of fuel was measured directly by a scale (1). The fuel from the mensuration was injected using a burner (3) and burned in a rotary kiln (4). Using a pump (11) located at the end of the sampling line, a partial flow of flue gas (6) was sampled, and in the vessel (7), the composition of the flue gas (CO, CO 2 , O 2 , NO) as well as the coefficient of excess air and the temperature of the sampled gas were determined. After determining the composition of the flue gas sampled, the flue gas was passed through a flue gas rinse with distilled water (9) to remove sulphate. The gas flow was measured using a rotameter (10). In order to obtain reliable results of testing the parameters of the fuel combustion process, the experiment was repeated four times for each fuel used. The results were compared, and no major discrepancies were observed. Values of standard deviation (S) and variation coefficient (Cv) of measurement results for gaseous components NO, CO and CO 2 caused by combustion of fuel D4 and its mixtures with WMO are shown in  Diesel combustion parameters and WMO-diesel fuel blend parameters were teste in the various mass ratios, and it was performed in a rotary kiln (Figure 1). WMO and it diesel fuel mixtures were well mixed to give a homogeneous stable mixture. Prior to com bustion, samples of various fuel mixtures were held in an open-air atmosphere (2). Mas consumption of fuel was measured directly by a scale (1). The fuel from the mensuratio was injected using a burner (3) and burned in a rotary kiln (4). Using a pump (11) locate at the end of the sampling line, a partial flow of flue gas (6) was sampled, and in the vesse (7), the composition of the flue gas (CO, CO2, O2, NO) as well as the coefficient of exces air and the temperature of the sampled gas were determined. After determining the com position of the flue gas sampled, the flue gas was passed through a flue gas rinse wit distilled water (9) to remove sulphate. The gas flow was measured using a rotameter (10 In order to obtain reliable results of testing the parameters of the fuel combustion process the experiment was repeated four times for each fuel used. The results were compared and no major discrepancies were observed. Values of standard deviation (S) and variatio coefficient (Cv) of measurement results for gaseous components NO, CO and CO2 cause by combustion of fuel D4 and its mixtures with WMO are shown in      During the experimental testing of the combustion process of diesel fuel and WM diesel fuel mixture, the following measuring instruments were used: digital scale QZfor measuring fuel consumption (M.P.1.); measuring range: 0-5000 g, measurement er ±1 g; WL2V burner kg h −1 1.6-3 A −1 ; digital temperature gauge Testo 925 with type K pro (NiCr-Ni) (M.P.2.); measuring range: 50-1000 °C, measurement error ±0.2%; and Te 330-2 LL gas analyser to measure the composition of the sampled gas (CO, NO, O2, C λ, t) (M.P.3).

Combustion Analysis
Fuel consumption during the test ranged from 2.09 kg h −1 for diesel fuel to 3.50 kg for WMO50. The increased fuel consumption of the WMO20, WMO25, WMO33 a WTO50 relative to diesel fuel was compared to the diesel fuel density due to the hig density of the considered fuels (Table 3).     During the experimental testing of the combustion process of diesel fuel and WM diesel fuel mixture, the following measuring instruments were used: digital scale QZ-1 for measuring fuel consumption (M.P.1.); measuring range: 0-5000 g, measurement err ±1 g; WL2V burner kg h −1 1.6-3 A −1 ; digital temperature gauge Testo 925 with type K pro (NiCr-Ni) (M.P.2.); measuring range: 50-1000 °C, measurement error ±0.2%; and Tes 330-2 LL gas analyser to measure the composition of the sampled gas (CO, NO, O2, CO λ, t) (M.P.3).

Combustion Analysis
Fuel consumption during the test ranged from 2.09 kg h −1 for diesel fuel to 3.50 kg for WMO50. The increased fuel consumption of the WMO20, WMO25, WMO33 a WTO50 relative to diesel fuel was compared to the diesel fuel density due to the high density of the considered fuels (Table 3).  During the experimental testing of the combustion process of diesel fuel and WMO diesel fuel mixture, the following measuring instruments were used: digital scale QZ-161 for measuring fuel consumption (M.P.1.); measuring range: 0-5000 g, measurement error ±1 g; WL2V burner kg h −1 1.6-3 A −1 ; digital temperature gauge Testo 925 with type K probe (NiCr-Ni) (M.P.2.); measuring range: 50-1000 • C, measurement error ±0.2%; and Testo 330-2 LL gas analyser to measure the composition of the sampled gas (CO, NO, O 2 , CO 2 , λ, t) (M.P.3).

Combustion Analysis
Fuel consumption during the test ranged from 2.09 kg h −1 for diesel fuel to 3.50 kg h −1 for WMO50. The increased fuel consumption of the WMO20, WMO25, WMO33 and WTO50 relative to diesel fuel was compared to the diesel fuel density due to the higher density of the considered fuels (Table 3). This resulted in a reduction in the excess air ratio from 2.07 for diesel to 1.74 for WMO50. The sampled gas flow during the test was kept approximately constant and ranged from 1.06 m 3 h −1 to 1.10 m 3 h −1 .
The flue gas temperature in the rotary kiln ranged from 589 • C to 719 • C, and the exchange temperature (gas sampling temperature) ranged from 157 • C to 214 • C. We measured higher flue gas temperatures in the case of mixing WMO with diesel fuel compared to diesel fuel with higher WMO density results, and WMO measurement was higher than diesel density, resulting in higher heat release rates ( Figure 5). This resulted in a reduction in the excess air ratio from 2.07 for diesel to 1.74 f WMO50. The sampled gas flow during the test was kept approximately constant an ranged from 1.06 m 3 h −1 to 1.10 m 3 h −1 .
The flue gas temperature in the rotary kiln ranged from 589 °C to 719 °C, and t exchange temperature (gas sampling temperature) ranged from 157 °C to 214 °C. W measured higher flue gas temperatures in the case of mixing WMO with diesel fuel com pared to diesel fuel with higher WMO density results, and WMO measurement w higher than diesel density, resulting in higher heat release rates ( Figure 5).

Nitric-Oxide (NO) Emission
Variations in NO emissions from diesel fuel combustion, WMO-diesel fuel blends a shown in Figure 3. It can be observed that NO emissions for WMO diesel mixtures a higher than NO emissions from diesel combustion. Higher NO emissions are the cons quence of higher gas temperatures in a rotary kiln for mixtures than the gas temperatu generated by combustion of diesel. Compared to diesel fuel, by burning fuel WMO50, t flue gas temperature in the rotary kiln increased by about 22%. Most literature data su as [1,6,19] show a slight increase in NO by burning mixtures of waste oils and diesel fue in internal combustion engines.

Carbon-Monoxide (CO) Emission
Variations of CO emission during combustion of a mixture of WMO with diesel fu and diesel are shown in Figure 4. CO emissions for WMO diesel fuel mixtures are signi cantly higher than diesel combustion emissions. The CO emission for diesel fuel w 0.0032%, and for blends WMO20, WMO25, WMO33 and WMO50, was 0.0064%, 0.0086

Nitric-Oxide (NO) Emission
Variations in NO emissions from diesel fuel combustion, WMO-diesel fuel blends are shown in Figure 3. It can be observed that NO emissions for WMO diesel mixtures are higher than NO emissions from diesel combustion. Higher NO emissions are the consequence of higher gas temperatures in a rotary kiln for mixtures than the gas temperature generated by combustion of diesel. Compared to diesel fuel, by burning fuel WMO50, the flue gas temperature in the rotary kiln increased by about 22%. Most literature data such as [1,6,19] show a slight increase in NO by burning mixtures of waste oils and diesel fuels in internal combustion engines.

Carbon-Monoxide (CO) Emission
Variations of CO emission during combustion of a mixture of WMO with diesel fuel and diesel are shown in Figure 4. CO emissions for WMO diesel fuel mixtures are significantly higher than diesel combustion emissions. The CO emission for diesel fuel was 0.0032%, and for blends WMO20, WMO25, WMO33 and WMO50, was 0.0064%, 0.0086%, 0.0094% and 0.0124%, respectively. The CO emissions for WMO and its mixtures are higher because of poor mixture formation due to less oxygen in the chamber for combustion. In the cited literature [1,6,19] an increase in CO for all types of tested fuels can be found.

Carbon-Dioxide (CO 2 ) Emission
Combustion of WMO and its diesel blends recorded CO 2 emissions in the interval of 8.12% to 9.20% for WMO20 and WMO50, respectively. CO 2 emissions for diesel WMO blends are slightly higher than diesel combustion emissions of 7.44% ( Figure 5). In the flue gas relative to the volume fraction of CO for all types of combustion, the CO 2 :CO ratio indicates lower CO 2 concentrations for WMO-diesel blends compared to diesel. For combustion of WMO50, CO 2 :CO = 0.74·10 3 :1, and for combustion of diesel, CO 2 :CO = 2.32 × 10 3 :1; they are as expected ( Figure 6).
Energies 2021, 14, x FOR PEER REVIEW 7 of combustion. In the cited literature [1,6,19] an increase in CO for all types of tested fue can be found.

Carbon-Dioxide (CO2) Emission
Combustion of WMO and its diesel blends recorded CO2 emissions in the interval 8.12% to 9.20% for WMO20 and WMO50, respectively. CO2 emissions for diesel WM blends are slightly higher than diesel combustion emissions of 7.44% ( Figure 5). In the flu gas relative to the volume fraction of CO for all types of combustion, the CO2:CO rat indicates lower CO2 concentrations for WMO-diesel blends compared to diesel. For com bustion of WMO50, CO2:CO = 0.74·10 3 :1, and for combustion of diesel, CO2:CO = 2.32 10 3 :1; they are as expected (Figure 6).

Rinsing the Flue Gases
The flue gas absorption was performed in the washer (9) with distilled water (Figu 1). The rinsing was performed to determine the composition of the liquid mixture afte the combustion process of the various fuels. Parameters determined: sulphates, pH, su phides, nitrates and nitrites [20]. The results are shown in the diagrams of Figures 7-1 In the washer (9), the presence of all considered parameters was detected. The highe concentration of sulphate (SO4) in the washer was observed during WMO20 combustio of 45 mgL 1 , and the lowest in distilled water was less than 2 mg l −1 (Figure 7).

Rinsing the Flue Gases
The flue gas absorption was performed in the washer (9) with distilled water (Figure 1). The rinsing was performed to determine the composition of the liquid mixture after the combustion process of the various fuels. Parameters determined: sulphates, pH, sulphides, nitrates and nitrites [20]. The results are shown in the diagrams of Figures 7-11. In the washer (9), the presence of all considered parameters was detected. The highest concentration of sulphate (SO 4 ) in the washer was observed during WMO20 combustion of 45 mgL 1 , and the lowest in distilled water was less than 2 mg l −1 (Figure 7). combustion. In the cited literature [1,6,19] an increase in CO for all types of tested fu can be found.

Carbon-Dioxide (CO2) Emission
Combustion of WMO and its diesel blends recorded CO2 emissions in the interva 8.12% to 9.20% for WMO20 and WMO50, respectively. CO2 emissions for diesel WM blends are slightly higher than diesel combustion emissions of 7.44% ( Figure 5). In the f gas relative to the volume fraction of CO for all types of combustion, the CO2:CO ra indicates lower CO2 concentrations for WMO-diesel blends compared to diesel. For co bustion of WMO50, CO2:CO = 0.74·10 3 :1, and for combustion of diesel, CO2:CO = 2.3 10 3 :1; they are as expected (Figure 6).

Rinsing the Flue Gases
The flue gas absorption was performed in the washer (9) with distilled water (Fig  1). The rinsing was performed to determine the composition of the liquid mixture af the combustion process of the various fuels. Parameters determined: sulphates, pH, s phides, nitrates and nitrites [20]. The results are shown in the diagrams of Figures 7-In the washer (9), the presence of all considered parameters was detected. The high concentration of sulphate (SO4) in the washer was observed during WMO20 combust of 45 mgL 1 , and the lowest in distilled water was less than 2 mg l −1 (Figure 7).   air ratio of 1.95 during the combustion process. These concentrations of sulphate, whe found in the atmosphere, contribute to the formation of acid rain, so that through precip itation, they can degrade the soil and reach groundwater, which has a detrimental effe on the environment. The presence of sulphate in the water increases the acidity of th solution as observed in Figure 8.   The nitrate content of the solution ranges from 1 mgL −1 when using WMO50 fuel t 1.8 mgL −1 when using distilled water. The decrease of nitrate in WMO and its mixtures observed in the yield on diesel and distilled water ( Figure 10). All recorded nitrate value in the solution are 20 to 36 times higher than the allowable values of 0.05 mgL −1 [21].    The nitrate content of the solution ranges from 1 mgL −1 when using WMO50 fue 1.8 mgL −1 when using distilled water. The decrease of nitrate in WMO and its mixture observed in the yield on diesel and distilled water ( Figure 10). All recorded nitrate val in the solution are 20 to 36 times higher than the allowable values of 0.05 mgL −1 [21]. The highest nitrite concentration of 0.19 mgL −1 was observed when using diesel was 6.33 times higher than the allowed value of 0.03 mgL −1 [21]. The concentration o trite when using WMO and its blends with diesel is within the permissible limits of Fig   Figure 10. Proportion of nitrate in the solution during washing of the flue gas to different types of fuel. The highest nitrite concentration of 0.19 mgL −1 was observed when using diesel an was 6.33 times higher than the allowed value of 0.03 mgL −1 [21]. The concentration of n trite when using WMO and its blends with diesel is within the permissible limits of Figur 11.

Conclusions
Waste motor oil was used as fuel by mixing with diesel fuel in mass proportions o 20%, 25%, 33% and 50%. Since these mixtures are close to the properties of diesel fuel i terms of calorific value (Table 2), combustion was performed in a rotary kiln with therma power of 40 kW without any difficulties, and the following conclusions were reached: • The flue gas temperature in the furnace is significantly higher for all WMO and dies fuel blends compared to diesel, which indicates the efficiency of the input energy; • Used diesel WMO blends can be used as fuel in low power heat generators, agricu tural machinery, low power diesel engines as well as in various branches of the pro cess industry; • NO emissions for the used WMO and diesel fuel blends are significantly higher tha NO emissions from diesel combustion; This was expected, given the sulphur content of WMO of 0.15% and the high excess air ratio of 1.95 during the combustion process. These concentrations of sulphate, when found in the atmosphere, contribute to the formation of acid rain, so that through precipitation, they can degrade the soil and reach groundwater, which has a detrimental effect on the environment. The presence of sulphate in the water increases the acidity of the solution as observed in Figure 8. Figure 9 shows a decrease in the sulphide in the flush when combusting WMO and its mixtures with respect to diesel. The highest concentrations of sulphide in diesel and distilled water are expected given the physical and chemical properties of diesel and distilled water.
The nitrate content of the solution ranges from 1 mgL −1 when using WMO50 fuel to 1.8 mgL −1 when using distilled water. The decrease of nitrate in WMO and its mixtures is observed in the yield on diesel and distilled water ( Figure 10). All recorded nitrate values in the solution are 20 to 36 times higher than the allowable values of 0.05 mgL −1 [21].
The highest nitrite concentration of 0.19 mgL −1 was observed when using diesel and was 6.33 times higher than the allowed value of 0.03 mgL −1 [21]. The concentration of nitrite when using WMO and its blends with diesel is within the permissible limits of Figure 11.

Conclusions
Waste motor oil was used as fuel by mixing with diesel fuel in mass proportions of 20%, 25%, 33% and 50%. Since these mixtures are close to the properties of diesel fuel in terms of calorific value (Table 2), combustion was performed in a rotary kiln with thermal power of 40 kW without any difficulties, and the following conclusions were reached:

•
The flue gas temperature in the furnace is significantly higher for all WMO and diesel fuel blends compared to diesel, which indicates the efficiency of the input energy; • Used diesel WMO blends can be used as fuel in low power heat generators, agricultural machinery, low power diesel engines as well as in various branches of the process industry; • NO emissions for the used WMO and diesel fuel blends are significantly higher than NO emissions from diesel combustion; • Emissions of CO mixture used for WMO and diesel fuels are significantly higher than the emissions of CO produced by burning diesel fuel; • CO 2 emissions are higher for used WMO mixture and diesel fuel than CO 2 emissions resulting from the combustion of diesel fuel; • The CO 2 :CO ratio in the flue gas is lower for WMO and diesel fuel blends compared to diesel; • Absorption of flue gases have recorded concentrations of sulphate, sulphide, nitrate and nitrite and their presence in the flue gas, when found in the atmosphere, can adversely affect the environment. The nitrate concentrations in the solution are much higher than the allowable values and the nitrite concentrations in the solution are much higher than the allowable values for diesel fuel only. Research on fuel blending procedures and the use of mineral additives, as ways to mitigate these problems, are being implemented.
The conclusion is that WMO and its mixtures with diesel fuel can be used as an alternative fuel in different mass ratios in internal combustion engines as well as in various low heat power combustion plants. Taking into account particularly low fuel consumption values obtained for the WMO-diesel fuel ratio, this represents a significant value and reduces additional costs.