Energy recovery of waste plastics into diesel fuel with ethanol and ethoxy ethyl acetate additives on circular economy strategy

An investigation on the performance and emission characteristics of waste plastic fuel in a diesel engine has been conducted. Using diesel as the baseline fuel and quaternary fuel blends made up of 10{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} ethanol and 10{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} ethoxy ethyl acetate on a volume basis, blended with various ratios of 20{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 30{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 40{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} of WPF, the test was carried out on a diesel engine. The assessment was carried out on an unaltered single-cylinder diesel engine operating at a 25{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} incremental load from 0 to 100{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} of its maximum capacity. Hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), and smoke are among the exhaust gases produced by engines that are being analyzed.

Performance characteristics

Engine performance was observed with the brake thermal efficiency (Fig. 2) of 27.61{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 24.12{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 28.92{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 26.26{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 25.45{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} at full load for the Diesel, WPF, and quaternary fuel blends of WPF with oxygenates additives. The brake thermal efficiency ranges from 17.84 to 28.92{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 16.81 to 26.26{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 16.1 to 25.45{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} at various loads with oxygenates additives. When evaluated at maximum load, the BTE of WEE20 was about 4.74{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} greater than that of diesel and almost 20{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} higher than that of WPF. Improved BTE outcomes of 22{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 12{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 8{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} have been observed with quaternary blends WEE20, WEE30, and WEE40 when compared to waste plastic fuel under various loading. The utilization of waste plastic fuel mixes resulted in a higher BTE being achieved.

Figure 2
figure 2

Performance of Brake thermal efficiency on engine loads.

The addition of ethanol and ethoxy ethyl acetate to waste plastic fuel positively affected fuel burning. An increase in oxygen content due to the presence of oxygen molecules in additives may be responsible for this, which would result in more efficient burning due to the presence of oxygen molecules in the additives34. Because less energy is lost in the combustion process due to a lower exhaust temperature, higher engine performance may be achieved. Plastic fuel contains a larger concentration of aromatic compounds, it takes a lot of energy to break the polymerization chain of plastic fuel. Fuel injection issues and poor spray quality may also be blamed on WPF’s higher viscosity for its worse thermal efficiency under different load circumstances compared to other evaluated fuels3.

Specific fuel consumption of each engine are unique and vary depending on the engine’s speed and load. The highest efficiency of a reciprocating engine is achieved only when the engine receives unthrottled air and when the engine is moving close to its torque peak. Figure 3 depicts the change in specific fuel consumption as a function of ternary blends under various loading scenarios. Specific fuel consumption decreases for WEE20 about 3.16{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} to 7.77{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} at various load conditions with diesel. There was a considerable reduction in fuel usage ranging from 14.1{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} to 23.8{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} at various load circumstances compared to the WPF. An increase in efficiency is obtained by the use of highly oxygenated conditions that require less fuel to provide the same amount of power output. More importantly, the calorific value of the blends influences engine output.

Figure 3
figure 3

Performance of Specific fuel consumption on engine loads.

The waste plastic fuel has low calorific value than the pure diesel. It has an impact on the development of the fuel spray formation, which results in partial combustion. This results in the reduction in thermal efficiency, and the higher specific fuel consumption. It was observed that 275 g/kW-hr, 312 g/kWhr and 322 g/kWhr for quaternary blends WEE20, WEE30, and WEE40 respectively when compared to waste plastic fuel, which recorded 361 g/kWhr at maximum load. Due to the lower calorific value and higher viscosity of WPF and its blends, the fuel usage is greater than that of diesel operation19. The lower heating valve of mixed additives resulted in higher fuel consumption to provide the same amount of power, increased ignition delay owing to the low cetane number, and reduced combustion temperature due to ethanol’s quenching action18.

The exhaust gas temperature in diesel engines varies significantly depending on the quantity of heat released in the engine chamber during the combustion cycle. Exhaust Gas Temperature (EGT) can also provide a strong overview of performance, air–fuel ratio, combustion heat, and available oxygen levels. The combustion temperature influences the EGT, contributing to an increase in the exhaust temperature with increasing load35. The WPF and quaternary blends’ EGT observed was more than the diesel at all loads conditions (Fig. 4). WEE20 recorded a 5.3{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} increased EGT, and other blends also observed higher temperature around 9–10{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} over the diesel. WPF shows incomplete combustion due to the higher viscosity and lower volatility, resulting in higher EGT. The higher EGT has been observed because some gases undergo combustion at the end of expansion stroke36. The quaternary blends contain more oxygen content which promotes combustion resulting in higher EGT in all load conditions.

Figure 4
figure 4

Variation of Exhaust gas temperature on engine loads.

Emission characteristics

Carbon monoxide emissions are mostly produced by inadequate oxygen availability or poor oxygen usage in the combustion process. When compared to diesel, quaternary blends emit much less carbon monoxide at all loads. Figure 5 shows that carbon monoxide emissions drop gradually from lower load to half load and then rise till reaching full load for all of the mixes presented. Testing at maximum loads found CO emissions from WEE20 is to be 13.41{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} lesser than diesel and about 20.22{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} lesser than from WPF. Compared to diesel, the quaternary blends WEE20, WEE30, and WEE40 showed significant CO reductions of 13.41{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 6.21{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 3.73{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, respectively, when the loading was varied. Comparing quaternary blends to WPF, it is shown that there is a 9{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} to 23.6{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} decrease in carbon dioxide emissions. The quaternary blends have a higher concentration of oxygen, which allows for greater effective burning of the fuel. It results in a reduction in CO emissions as more fuel particles get oxidized. Addition of alcohols with low cetane numbers increases the ignition delay time during combustion. As a result of the OH group’s effect, the majority of the alcohols undergo H-abstraction by OH radicals from the carbon position, as had previously been reported37. As a result of the ignition delay time, more fuel–air mixing occurs, resulting in improved combustion and CO reduction. A negative impact on carbon monoxide emissions was shown by waste plastic oil, whose increased viscosity results in inefficient atomization of fuel mixes, ultimately leading to increased carbon monoxide emissions34.

Figure 5
figure 5

Variation of Carbon monoxide emissions on engine loads.

Hydrocarbon emissions are mostly generated by inadequate mixing of fuel and air particles inside the combustion process, as well as by incomplete combustion of the fuel itself. When comparing diesel with quaternary blends of WEE20, WEE30, and WEE40 (Fig. 6), the hydrocarbon emission decreases about 11.76 to 16.39{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 4.41 to 8.82{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 1.47 to 1.72{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} at various load conditions respectively. Due to the higher oxygen concentration in the fuel blends and appropriate mixing of the fuel and air takes occurs within the combustion chamber when the fuel is burned. The amount of fuel particles burnt in the combustion chamber is thus greater than diesel. The hydrocarbon emissions of WEE20 were about 16{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} less than diesel and 21.5{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} less WPF at maximum load. Low engine speed and low fuel injection pressure are present during idle conditions, resulting in slightly rich combustion conditions required for the combustion’s stability. When ethanol is mixed with diesel, the reduction of partial fuel-rich areas caused by the effect of oxygen and the improved atomization caused by the lower viscosity of the injected fuel are the main reasons for reducing HC emissions. Mani et al.38 found that the HC was 15{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} greater at maximum load when comparing plastic fuel to diesel. Because of unsaturated aromatic combinations in waste plastic, it has an imperishable character, resulting in a rise in hydrocarbon emissions36. The low cetane number of WPO and its lower auto-ignition characteristics contribute to the enhancement of the quenching effect in the leaner mixture region of the cylinder, which in turn contributes to the rise in the quantity of hydrocarbons emitted.

Figure 6
figure 6

Variation of hydrocarbon emissions on engine loads.

In diesel engines, NOx is produced primarily via the thermal mechanism and, to a lesser degree, through the prompt mechanism. At elevated temperatures, the thermal process results in an exponential rise in NOx levels. Nitrogen oxide emissions increased by 12.06{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, 22.13{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and 35.85{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, respectively, when quaternary blends WEE20, WEE30, and WEE40 were compared to diesel fuel at different loadings (Fig. 7). Increased NOx emissions from quaternary blends are caused mainly by increased fuel blend’s combustion temperature. Completion of combustion occurs as a result of the increased oxygen concentration in mixed fuel. As a consequence, the combustion temperature rises, increasing the amount of NOx emitted. When oxygenates are added to diesel fuel, the fuel becomes more oxygenated. As a result, the combustion chamber was running lean. Oxygenated fuel provides the additional oxygen needed to oxidise the nitrogen. Oxygenated fuel’s NOx emissions increase as a consequence.

Figure 7
figure 7

Variation of nitrogen oxide emissions on engine loads.

Mani et al.38 found that, the NOx emissions were 25{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} higher for plastic fuel was compared to diesel at full load. As shown in Fig. 6, the NOx emission levels of all tested fuels are rising. Excess oxygen concentration has the most significant impact on the generation of NOx emissions in the cylinder. Nitrogen chains break down and disintegrate when exposed to high temperatures. Following that, these nitrogen bonds interact with the oxygen molecules trapped inside the cylinder’s monotonic configuration. Emissions from waste plastic fuel were found to be between 12 and 50{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} more than those from diesel. WPF has more carbon-number compounds, which decreases surplus air availability, which leads to increased temperatures, which results in a rise in NOx.

The engine exhaust is a visual indication of the engine’s combustion process. Smoke is produced when fuel is burnt inefficiently, resulting in unburned carbon particles. Smoke is formed in engines during the diffusion combustion stage. All the fuel atomized droplets are split into elementary carbon atoms and are subsequently oxidized in the combustion zone. Smoke emissions also occur in the combustion-rich zone due to a shortage of air, a more excellent carbon-to-hydrogen ratio, a higher viscosity of the fuel, insufficient atomization, and an excessive fuel buildup inside the combustion chamber. According to Fig. 8, compared to diesel, the amount of smoke generated by WEE20 and WEE30 blends decreases by between 8 and 9.38{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} and 4.44 to 7.69{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, respectively. WEE40, on the other hand, reported a modest increase of 2{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} in smoke.

Figure 8
figure 8

Variation of smoke emissions on engine loads.

In comparison to diesel fuel, quaternary blends emit less smoke. It is primarily due to the synergistic impact of a higher cetane number and the presence of oxygen in fuel mixes. The cetane number indicates the quality of the ignition: the higher the cetane number, the more flammable the fuel. As the cetane number rises, the fuel’s ignition quality improves as well. When the ignition quality of the fuel improves, the fuel burns more efficiently within the combustion chamber. As a result, the engine produces fewer unburned carbon particles. Additionally, the oxygen in the fuel aids in fuel combustion, reducing smoke output. Ravikumar and Senthilkumar39 found an 8.6{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} to 21.28{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} reduction in smoke in the coated engine than a standard diesel engine. Compared to diesel, waste plastic fuel produced an 18.8{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} to 39{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} greater amount of smoke. WPF has a more significant proportion of aromatic components, which results in incorrect fuel mixture development and spray production, resulting in incomplete combustion and significant smoke emission13. Another reason for incomplete combustion is that WPF has a higher viscosity and is less volatile12.

Environmental impact of waste plastics and ethanol

Plastic goods are ubiquitous in the workplace and home surroundings of humans. Plastic pollution has the potential to harm and pollute the terrestrial ecosystem. Additionally, plastic contributes to global warming. Plastic remains in the environment for an extended period, endangering animals and spreading poisons. Each year, plastics kill millions of animals, from birds to sea livings (Okunola et al.40). On the other hand, diesel emissions cause cancer, cardiovascular and respiratory diseases, air, water, soil pollution, soiling, reduced visibility, and global climate change. Carbon monoxide affects the number of greenhouse gases related to climate change and global warming. CO causes acute poisoning when combined with hemoglobin to form carboxy-hemoglobin (COHB), preventing adequate oxygen transport from the lungs to human tissues. As a COVID-19 symptom, excessive CO concentrations impair proper respiratory system function (Adefeso et al.41). Hydrocarbons are very harmful to humans. Intake of hydrocarbons affects the immune system, hepatic, respiratory, reproductive, circulatory, and renal systems. Human effluents contaminated by hydrocarbons also cause cancer and hormonal issues that may disrupt development and reproduction (Srivastava et al.42).

Because ethanol is water-soluble, biodegradable, and easily evaporated, it may provide some safety benefits over fossil fuels. Ethanol fuel is the most cost-effective energy source since it can be produced in almost any country. Ethanol is a type of fuel derived from corn and other plants. There are many various forms of ethanol, but the most common is E10, and the blend ratio varies from 10 to 15{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} over the world. Many nations, like Brazil and the United States, allow for the use of a high-level ethanol fuel blend containing 50–85 percent ethanol43. Because ethanol is easily produced, it is less expensive than fossil fuel. The primary by-products of ethanol fuel combustion are carbon dioxide and water. In terms of pollution, the carbon dioxide emitted has little impact. The burning of ethanol made from biomass such as corn and sugarcane, on the other hand, is regarded to be “atmospheric carbon neutral”. This is due to the fact that when biomass grows, it absorbs CO2, which may offset the CO2 emitted when ethanol is burned44.

The linear economy focuses on the feedstock, the manufacturing process, and the delivery of the final product. The product’s afterlife was never given the consideration it deserved. The condition of the product after it has reached the end of its useful service life has been overlooked. Plastic products might be disposed of in landfills or incinerated as an alternative to recycling. Gong et al.45 and Zhang et al.46 have developed alternative solutions for the energy recovery of waste polymers into electrochemical storage and steam evaporation systems, which are the ideal polymer waste recovery methods.

A refinery using recycled plastic decreases oil consumption, lowering capital expenditure on exploration and increasing oil reserves. Plastic production uses around 8{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} of the world’s oil, with roughly half of that usage going to the creation of monomers and the other half going to the production of energy. Physical and chemical treatments must be widely implemented in order to be commercially sustainable. The proposed approach by Palos et al.47 suggests establishing a new waste management business network. The oil industry would gain from the business network’s commitment to sustainable development.

This research focuses on recovering energy from plastic waste and utilizing bio-cropped ethanol to achieve the circular economy approach as a potential fuel for transportation vehicles. Because of the high energy density of hydrocarbons found in plastic, they are excellent fuel sources. The quantity of recyclables that can be recycled without degrading the strength is one of the issues facing the circular economy. When it comes to offering a cost-effective end-of-life solution, plastic pyrolysis and combustion are viable options since they allow for the production of value-added goods while also reducing environmental impact. Recycling and reusing discarded plastic has the potential to save and recover a great deal of energy. Similarly, in the period of 2020–2021, India’s net petroleum imports were 185 Mt at $551 billion, the ethanol blending E20 programme that works may save the government $4 billion annually. Ethanol is also less polluting and cheaper than fossil fuels. E20 is a national need and a strategic demand due to abundant arable land, expanding food grain and sugarcane output, and the capacity to convert automobiles to ethanol mixed fuel. In two-wheelers, the CO emission drop was 50{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}, and in four-wheelers reduction up to 30{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}. Also, blends of ethanol and gasoline lower hydrocarbon emissions by 20{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157}48,49.

The objective of this study is to determine the performance of waste plastic fuel generated from the pyrolysis of HDPE in a diesel engine. A quaternary fuel blend of WPF was developed to combat the high-value emissions of WPF during diesel engine performance. The blends included 10{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} ethanol and 10{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} ethoxy ethyl acetate as an oxygenated additive to reduce the harmful emissions. The outcome of the WPF blends results in better fuel economy of up to 20{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} better than diesel and a reduction in tailpipe emissions of around 13{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} of CO and 16{09e594db938380acbda72fd0ffbcd1ef1c99380160786adb3aba3c50c4545157} of HC compared to fossil fuel. Similarly, the ethanol blending programme in India will reap numerous benefits, including annual savings of Rs.30,000 crore in foreign exchange, energy security, lower carbon emissions, better air quality, self-reliance, the use of damaged foodgrains, an increase in farmer incomes, the creation of new jobs, and increased investment possibilities48. The usage of energy recovered WPF and oxygenated additives is possible to combat climate change by reducing greenhouse gas emissions from engines via better fuel efficiency, enhancing the country’s energy needs and boosting the economy48,49.