Exploring Performance of Pyrolysis-Derived Plastic Oils in Gas Turbine Engines

Abstract

This study explores the intersection of waste management and sustainable fuel production, focusing on the pyrolysis of plastic waste, specifically polystyrene. We examine the physicochemical parameters of the resulting waste plastic pyrolytic oils (WPPOs), blended with kerosene to form a potential alternative fuel for gas turbines. Our findings reveal that all WPPO blends lead to increased emissions, with NO_X rising by an average of 61% and CO by 25%. Increasing the proportion of WPPO also resulted in a higher exhaust gas temperature, with an average rise of 12.2%. However, the thrust-specific fuel consumption (TSFC) decreased by an average of 13.8%, impacting the overall efficiency of waste-derived fuels. This study underscores the need for integrated waste-to-energy systems, bridging the gap between waste management and resource utilization.

1. Introduction

Fuels generated from plastic waste represent a potentially powerful approach to waste management, additionally serving as an alternative to traditional fossil fuels. The development of this alternative fuel sector bolsters energy autonomy, considering that waste materials and biomass are regionally accessible, storable, available in predictable quantities, and adaptable to a wide spectrum of energy requirements via numerous energy conversion technologies [1,2].

A predominant development in waste management and fuel production has been the exploration of waste polymers as potential alternative fuels, with attention to their environmental safety. Polymers, with plastics taking the lead, are extensively used across numerous industries, including electronics, logistics, automotive, and healthcare. In 2019, the global plastic production was approximated at 368 million tons, with Europe contributing nearly 57.9 million tons [3]. Notably, it was estimated that 38% of this plastic waste was destined for landfills, with 26% recycled and 36% processed to generate energy.

Polystyrene (PS) is a widely used plastic, with demand and production likely increasing from the 14.9 million metric tons produced in 2019 [4]. PS is primarily derived from petroleum and, similar to other plastics, consists of long chain hydrocarbon molecules. Its main feature is its expandability, making it a lightweight and effective insulation material. Its high resistance to heat, impact, and moisture also makes it a preferred choice for food packaging [5].

Polystyrene is challenging to recycle due to its physical and chemical properties, which often result in it being sent to landfills. This presents a significant environmental problem. However, PS has excellent fuel properties when processed through pyrolysis, yielding a high-quality oil rich in styrene. This oil can be used as an alternative fuel, providing a valuable use for PS waste and helping to reduce dependency on virgin fossil fuels.

Chemical recycling, particularly pyrolysis, shows promise in converting PS waste into reusable styrene oil, reducing the need for virgin materials and landfill waste. Strategies are also being explored to use PS waste in energy recovery processes, although these bring challenges around emissions and toxicity. Efforts to improve waste segregation and sorting technologies are underway to enhance PS recycling. Also, the high hydrocarbon content of discarded plastics makes them a potential source of alternative fuels. Pyrolysis, a thermal degradation process performed in an oxygen-free environment, yields oil, gas, and solid residue or char. The product quality depends on several factors, including raw materials, pyrolysis reactor, and process parameters. The liquid byproduct of pyrolysis—pyrolytic oil—finds potential application in a range of settings, including furnaces, boilers, turbines, and diesel engines, without necessitating upgrades or treatments [6,7,8,9].

Through rapid heating, thermal degradation of PS can yield a liquid fraction composed mainly of styrene monomers. It was found in [10,11], respectively, that the yield of oil at a temperature of 420 °C was equal to 98% and at 450 °C it was equal to 90.7%, whereas the solid fraction which was also formed amounted to about 1–5%. The aim is to obtain a large amount of oil for potential fuel use, keeping a focus on reliability and environmental safety.

In the literature, several studies have explored the potential of waste plastic pyrolysis oil (WPPO) as alternative fuels in diesel engines, often with intriguing outcomes.

The research [12] under discussion explores energy production from polypropylene (PPO) and polystyrene (PSO) waste via pyrolysis. The authors achieved high liquid yields, suitable for energy-intensive applications. However, combustion properties and emissions differ significantly between PPO and PSO. PPO, featuring various hydrocarbons, exhibits similar combustion to diesel with minimal emissions’ impact. In contrast, PSO shows poor auto-ignition properties and increases emissions, especially particulate matter at high loads. Despite challenges, diesel blending mitigates issues, though PSO’s high aromatic content necessitates reducing polystyrene in mixed plastic feedstock for diesel substitutes. The study underscores PPO’s potential as a diesel alternative, with manageable combustion delay.

Research by [13] examines the utilization of WPO, procured from the catalytic pyrolysis of high-density polyethylene, in diesel engines. The study identifies a decrease in brake thermal efficiency alongside an increase in exhaust gas temperature and fuel consumption with escalating WPO ratios. Nevertheless, mechanical efficiency enhanced with rising brake power. With respect to emissions, an increase in NO_X and CO was observed with higher WPO blends, although CO₂ emissions were lower in comparison to diesel. Despite certain performance and emissions trade-offs, the study postulates WPO as a potentially viable substitute for diesel.

Investigation presented in [14] focuses on WPO derived from waste plastics pyrolysis, with Zeolite-A employed as a catalyst. The performance of single-cylinder DI four-stroke diesel engines was tested using pure WPO and a blend of WPO with diesel. Interestingly, a 20% blend of WPO with diesel showed a slight improvement in brake thermal efficiency (BTE) and a substantial decrease in brake-specific fuel consumption (BSFC) relative to pure diesel. Emission analysis revealed reduced NO_X and HC emissions at lower loads, although these increased with load, compared to diesel. The study concludes with the promising potential of a 20% WPO–diesel blend as a substitute for diesel engines.

In the study [15], mixed plastic waste undergoes pyrolysis at 450 °C to generate plastic pyrolytic oil (PPO), which is then blended with diesel. This high-quality fuel, resembling conventional fuels, shows enhanced BTE and reduced-specific fuel consumption (SFC) when up to 50% PPO is blended with diesel, particularly under elevated load conditions. The oxygenated nature of PPO contributes to the reduction in emissions. The research concludes that a blend of up to 50% PPO with diesel could be a viable substitute in diesel engines, albeit with a marginal efficiency loss and slight increase in exhaust emissions.

The work by [16] investigates PPO derived from various types of plastic waste through a fast pyrolysis process. The resultant oil, exhibiting properties similar to diesel, was tested in a four-cylinder direct injection diesel engine. The results suggest that at higher loads, the engine performs similarly to diesel, while longer ignition delays induce stability issues at lower loads. Despite marginally lower brake thermal efficiency and higher NO_X emissions, blends of 60–70% PPO operating at 80–90% engine load demonstrate promise. However, further optimization of engine parameters, such as injection timing, pressure, and cetane number improvers, requires exploration.

The study [17] investigates the energy extraction from low-density polyethylene (LDPE) plastic waste via catalytic pyrolysis and its use in a diesel engine. Optimization of reaction parameters resulted in plastic oil, which was tested in a single-cylinder diesel engine. The performance, combustion, and emissions were analyzed, showing that up to 40% of diesel fuel could be replaced by plastic oil. The addition of methanol and diethyl ether improved efficiency and reduced emissions.

The study [13] examines the performance and emissions of diesel engines using blends of waste plastic oil (WPO) from high-density polyethylene (HDPE) and diesel. Results indicate that brake thermal efficiency is lower, and NO_X and CO emissions are higher for WPO blends, while CO₂ emissions are lower than diesel. The research highlights the trade-offs and potential of WPO as a diesel substitute.

Turbine engines offer several advantages over conventional piston combustion engines. Some of these benefits include fewer moving components and the lack of components subject to friction. This translates to lower maintenance requirements and costs, which could provide significant savings over the life of the engine. Furthermore, gas turbines are characterized by lower levels of vibration and noise, a crucial factor when considering the comfort and safety of the working environment and the potential impact on surrounding areas. Also, the low consumption of lubricating oil, a necessary component for the smooth operation of engines, offers both financial and environmental advantages, adding another reason for the preference of gas turbines over their piston combustion counterparts.

However, despite the promising potential of gas turbines, the use of alternative fuels such as pyrolysis oils derived from used plastics has been more extensively researched in the context of diesel engines, with encouraging results. Pyrolysis oil, a liquid product obtained from the thermal degradation of waste plastics in the absence of oxygen, exhibits properties similar to fossil fuels, making it a potential alternative energy source. Using this type of fuel could offer both environmental and economic benefits, such as waste reduction and energy recovery.

Nevertheless, the exploration of this sustainable fuel option in the application of gas turbines is surprisingly scarce. Only a handful of scientific publications in recent years have reported experimental tests involving gas turbines fueled by waste- or biomass-based fuels. This suggests a gap in the current research landscape, highlighting the need for more extensive experimental investigations into the performance and emission characteristics of gas turbines operating on alternative fuels like pyrolysis oil. Such studies could pave the way for more sustainable and economical power generation practices in the future.

While surveying the existing body of literature, it became evident that a noticeable gap exists in the research regarding the combustion of fuels composed of waste plastic pyrolytic oil, specifically within the context of gas turbines. This discovery underscores the novelty of the present study, which aims to investigate the performance and emission characteristics of a GTM-140 miniature gas turbine engine manufactured by the Polish company JETPOL from Poznań, Poland, fueled by mixtures of kerosene JET A and polystyrene pyrolysis oil (PSO).

The PSO under consideration is derived from polystyrene, a common plastic material used in a wide variety of applications, highlighting the potential for repurposing a significant waste stream. The investigation explores blends with compositions ranging from 25% to 100% PSO in PSO/JET A mixtures, providing a comprehensive view of how increasing concentrations of PSO might impact the gas turbine’s operation.

Given the pressing need to transition towards sustainable energy practices, the outcomes of this research could hold valuable insights for enhancing the environmental compatibility of power generation systems. Equally, understanding the effects of these alternative fuels could offer information on how best to optimize turbine performance, contributing towards more efficient and economically viable power generation strategies.

Measurements of turbine exhaust gas temperatures, static thrust, fuel mass flow, and thrust-specific fuel consumption (TSFC), in addition to thrust-specific emissions of nitrous oxides (NO_X) and carbon monoxide (CO) are captured across an extensive spectrum of turbine load. These measures are then comparatively analyzed within the projected range of fuel blend composition.

2. Fuel

The fuel used in the study was prepared by the authors through a thermal pyrolysis process of polystyrene. The plastic material was re-granulated and subjected to pyrolysis in a fixed bed reactor with electric heating, reaching a final temperature of 500 °C at a controlled heating rate of 10 °C/min. No catalyst was used to avoid additional costs, and the process achieved a 92% yield of the liquid fraction. Detailed conditions and procedures of the pyrolysis process are described in [12].

The crude polystyrene pyrolytic oil (PSO) was analyzed using Gas Chromatography—Mass Spectrometry (GC–MS) to identify its organic compounds. Hydrocarbons were categorized by chain length, varying between C6–C9 and C10–C24. The chromatogram of PSO indicated that 88.53 wt% consisted of C6–C9 compounds, with the concentration of C10–C24 hydrocarbons at 11.47 wt%. This low presence of long-chain hydrocarbons is attributed to the primary degradation of polystyrene to its monomer—styrene (C8H8) at 300 °C.

Distillation curves (Figure 1) for PSO and the reference fuel JET A show distinct differences. JET A fuel distillates uniformly with an increase in temperature, due to its diverse composition of various chain length hydrocarbons. PSO’s distillation curve, on the other hand, exhibits a unique breakdown due to the predominance of the monomer styrene, which boils at about 145 °C.

A type of fuel with predominantly aromatic content displays a higher carbon-to-hydrogen ratio than another type of fuel. It has a calorific value of 40.5 MJ/kg, indicating that this fuel is about 6.4% less calorific than kerosene. The increased presence of monocyclic aromatic hydrocarbons in this fuel leads to a higher adiabatic flame temperature, which contributes to an elevated heat release rate and NO_X emissions [16]. The flash point for this fuel falls below those of aviation fuel and standard diesel, with a value of 24 °C. The fundamental physico-chemical properties of the examined oil were assessed using the standards and methods outlined in

Table 1. ASTM standards and methods of determination of fuel properties.

ASTM Standard	Unit	Property	Method
D1298 [18]	kg/m3	Density	density meters with U-tube oscillators (U-tube)
D445 [19]	m2/s	Kinematic Viscosity	Rheometer
D4809-95 [20]	MJ/kg	Calorific value	Calorimeter
D92 [21]	°C	Flash point	Persky–Martens flash point test

. Comparative assessments of specific fuel properties with reference to JET A and diesel can be found in

Table 2. Physical characterization of PSO and comparison with aviation fuel and pure diesel fuel.

Property	JET A	PSO	Diesel [16]
Molecular weight [kg/kmol]	142	117.8	-
Chemical formula	C10H22	C9,51H9,66
Density [kg/m3]	821 (at 15 °C)	943.5	840
Viscosity [cP]	1.5–2.6 (at 20 °C)	1.012	2.62 (at 40 °C)
HHV [MJ/kg]	43.28	40.5	42.9
Flash point [°C]	42	>24	59.5
Sulfur [wt%]	0.3	-	0.00135
C content [wt%]	86.15	92	86.57
H2 content [wt%]	13.85	8	13.38
O2 content [wt%]	0.10	-	0.05
Aromatic content [wt%]	26	98	29.6

.

A set of consistent blends of each studied pyrolytic oil with kerosene (JET A1) was created, comprising 25, 50, and 75 wt% of the given fuel. These blends were named in accordance with the volume of the pyrolytic oil in the JET A1 mixture, such as PS25, PS50, PS75, and PS100 for pure pyrolytic oil. These blends remained chemically stable, whether stored under refrigeration or at room temperature. The oil samples were uniformly mixed and clear, showing no noticeable color changes even after exposure to room temperature for several weeks.

3. Test Stand and Methodological Approach of GTM-140 Engine

Due to the significant expenses involved in carrying out experimental research on gas turbines, some research groups have chosen to focus on smaller-scale gas turbine engines [22,23,24,25]. The detailed performance specifications of the gas turbine are provided in [26]. This particular study was conducted using a JETPOL GTM-140 gas turbine engine, located in the Turbine Department of the Institute of Fluid-Flow Machinery at the Polish Academy of Sciences in Gdansk [24,25].

Comprehensive technical performance information pertaining to the analyzed miniature gas turbine engine can be found in [27]. The GTM-140 consists of—located on the same shaft—single stage centrifugal compressor, single stage axial turbine, and reverse flow annular combustion chamber with six fuel injectors. The experimental setup was fitted with measurement tools designed to record the flow and performance parameters of the gas turbine, in addition to emission levels. Measurement details are gathered in

Table 3. Gas turbine performance and emission measurements.

Measured Parameter	Range	Unit	Device/Sensor Type	Resolution	Uncertainty
Temperatures of inlet/exit compressor/turbine sector (T1–T4)	0–1100 °C	°C	Thermocouple K-Type	1 °C	±1 °C
Pressure of inlet/exit compressor and turbine sector (P1–P4)	P1 0–0.98 bar(a) P2–P4 0–9.8 bar(a)	bar	Digital pressure transducers	0.01 bar	±1.0%
Static Thrust	0:200	N	Strain gauges	1 N	1 mV/V
Fuel volumetric flow	0.5–100	LPH	Oval-gear flowmeter	0.01 LPH	±0.5%
Rotational speed	0:200,000	rpm	Rotational speed sensor—magnetic pick-up	1 rpm	±0.5%
Gas Emissions	Range	Unit	Device/Sensor Type	Resolution	Uncertainty
Oxygen	0:20.95	%	Electrochemical	0.01%	±0.2% absolute
or 5% rel.
Carbon monoxide	0:5000	ppm	Electrochemical	1 ppm	±5 ppm absolute
or 5% rel.

. The table provides the applying range of variation of the main parameters and emission indexes, including the measurement.

Stationary test conditions were maintained throughout the experimental procedure, with engine speeds ranging from 50,000 rpm to 100,000 rpm using standard aviation kerosene. The engine’s operational data were captured once a steady state was achieved, as indicated by stable temperature and flow rates. The testing commenced with the engine running on JET A as the baseline fuel. Subsequent tests were conducted with alternative fuels, keeping the throttle position constant to ensure that the engine can adapt to the new fuel. The gathered data are depicted through line diagrams that illustrate the performance over the entire range of engine speeds and through stacked column charts that detail the results at specific engine speeds. Within the established range of rpm, the experiment analyzed critical performance metrics and indicators, including static thrust, fuel consumption, thrust-specific fuel consumption (TSFC), and emission indices for CO, CO₂, and NO, as well as overall environmental performance characteristics.

4. Performance Characteristics of the GTM-140 Engine

In this experimental investigation, an in-depth analysis was conducted to evaluate the performance characteristics of a miniature gas turbine utilizing various fuel compositions, with a particular emphasis on polystyrene pyrolytic oil (PSO) as an alternative fuel. Figure 2 provides an illustrative representation of the static thrust and mass flow rate of fuel, which are quintessential parameters in assessing the performance of gas turbines. The experimental protocol was meticulously designed to ensure consistency, with the throttle valve settings being analogous to those employed for JET A, a conventional aviation fuel. PSO, characterized by its higher density and concomitantly lower calorific value, necessitated an augmented mass flow rate in comparison to aviation kerosene. The thrust curves, which are indicative of the relationship between the thrust produced and various operational parameters, did not manifest significant discrepancies for PSO/JET A blends. This observation is indicative of the compatibility of PSO as an alternative fuel. However, to achieve a commensurate static thrust, the engine required an escalated fuel input, which is attributable to the lower calorific value of PSO.

Thrust-specific fuel consumption (TSFC), a critical metric defined as the ratio of fuel consumption to static thrust, is instrumental in evaluating the efficiency of the engine in converting fuel into thrust. Figure 3 delineates the variation of TSFC with engine load, quantified as rotor rotational speed. An inverse relationship between TSFC and thermal efficiency was observed, with the latter being a measure of the engine’s ability to effectively convert the heat energy from the fuel into mechanical work.

Within the ambit of lower rotational speeds (up to 60,000 rpm), an incremental addition of PSO to the blend culminated in an increment in TSFC, thereby implying a decrement in the thermal efficiency of the gas turbine [28]. This is attributable to the lower calorific value of PSO, which necessitates a higher volume of fuel to achieve the same energy output. Notwithstanding the significantly lower heating value of PSO, the engine demonstrated comparable performance characteristics to those achieved with the baseline fuel.

Exhaust gas temperature (EGT), a pivotal parameter indicative of the temperature of the gases emanating from the turbine, is depicted in Figure 4. The EGT oscillated between 525 and 625 °C (798 and 889 K) across the entire spectrum of speed ranges considered. For all the blends examined, the temperature exhibited an almost linear decrement with engine load. This trend is emblematic of this genre of gas turbines and is indicative of the efficiency of the combustion process for higher load [29]. Furthermore, the PSO/JET A blends manifested elevated temperatures at the engine outlet compared to the base fuel at analogous engine speeds. While these temperature variations are marginal from an operational standpoint, they have a non-trivial impact on NO_X emissions. This is attributable to the fact that higher temperatures facilitate the formation of nitrogen oxides through the Zeldovich mechanism.

5. Emission Indexes of the GTM-140 Engine

In the study of exhaust gas emissions from combustion processes, a crucial metric used is the emission index, which represents the mass of a specific pollutant emitted per unit mass of fuel burned. This can be expressed with the following Equation (1) [30]:

$${EI}_i={\displaystyle \frac{m_{i, emitted}}{m_{f, burned}}},$$

(1)

The emission index is employed to gauge the efficiency of the combustion process. In the context of hydrocarbon fuel combustion with air, the emission index can be computed for the concentrations of individual emission components (molar fraction) in conjunction with all constituents containing a carbon (C) atom. For a more specific analysis that considers the engine’s output, the thrust-specific emission index is used. This index relates the quantity of a pollutant produced to the unit of static thrust generated by the engine. Incorporating the molar fractions of the pollutants and the molecular weights of the pollutant and the fuel, the thrust-specific emission index can be calculated as follows (2) [25,28]:

$${EI}_{i,\tau }={\displaystyle \frac{\left(\frac{{\chi }_i}{{\chi }_{CO}+{\chi }_{{CO}_2}}\right)\left(\frac{{xMW}_i}{{MW}_F}\right){\dot{m}}_f}{\tau }},$$

(2)

where ${\chi }_i$ denotes the molar fraction of the pollutant in the exhaust, ${xMW}_i$ is the molecular weight of the pollutant, ${MW}_F$ is the molecular weight of the fuel, ${\dot{m}}_f$ is the mass flow rate of the fuel, and $\tau$ is the static thrust of the engine. This formula gives a clear indication of the environmental impact of the engine in relation to its performance, allowing for the optimization of engine design and operation to minimize emissions while maintaining efficiency.

The thrust-specific NO_X emission index for the investigated micro gas turbine fueled by PSO and JET A blends found from Equation (2) is depicted in Figure 5. An increase in the proportion of PSO in the blends with JET A increases the total emissions, although there is an exception for the rotational speed of 60,000 rpm for PS25 and PS50 fuels, for which the emission values are at a similar level as for JET A.

${EI}_{i,\tau }$ for NO_x is higher for PSO/JET A blends for almost the whole range of the tested turbine load. The primary mechanism for the formation of nitrogen oxides is the Zeldovich mechanism, or thermal mechanism of oxidation of neutral atmospheric in the high temperature areas of the gases. The process occurs under endothermic conditions and intensifies at temperatures above 1850 K. With the increasing share of PSO in the blends with JET A, a considerable rise in NO_X emission index of the micro gas turbine is observed for the entire tested range of rotational speed. Averaging the determined curves, the specific emission index for PS100 is greater than that of the base fuel by 71.3%.

The CO emission index taking into account the fuel efficiency for the investigated PSO/JET A blends is illustrated in Figure 6. For PSO/JET A blends, the thrust-specific CO emission indexes are higher in relation to JET A, typically higher by up to 5–20% depending on the content of PSO and operational range. The research conducted suggests that the emission rates of nitrogen oxides and carbon oxides are usually higher when using polystyrene pyrolysis oils. The increase in emissions is especially evident for pure polystyrene oil; however, this can be accepted as a method of utilization of waste plastics and turning them into energy, since a large part of plastics is not viable for recycling. The average increase for pure polystyrene pyrolysis fuel with respect to the reference fuel is 13%.

6. Conclusions

Distinct mixtures of polystyrene oil with aviation kerosene JET A were formulated, comprising 25%, 50%, 75%, and 100% of PSO. The research extensively assessed the impact of the PSO proportion in PSO/JET A fuel blends on the performance and emission features of a small-scale gas turbine, GTM-140. Charts portraying exhaust turbine temperature, fuel flow, static thrust, thrust-specific fuel consumption (TSFC), and thrust-specific emissions of NO_X and CO over an extensive range of micro gas turbine loads were displayed.

The findings of this study on the use of pyrolysis oils derived from polystyrene waste as fuel for gas turbines align with previous research on alternative fuels. For example, a study on the use of oils derived from polypropylene [31] shows similar emission trends.

The work by [12], which investigated blends of pyrolysis oils from tires in modern diesel engines, also reported increased emissions of NO_X and CO when using alternative fuels. Similar results were observed by [13] in their study of high-density polyethylene pyrolysis oil blends with diesel, where they noted a decrease in thermal efficiency and an increase in exhaust gas temperature. Research on the use of pyrolysis oils in diesel engines by [16] also confirmed higher emissions and the necessity for further optimization of engine parameters. Our results in the context of gas turbines are novel, as the application of pyrolysis oils from plastic waste in this type of engine has been scarcely explored, highlighting the need for further investigation in this area.

The experimental deductions drawn from the micro gas turbine engine setup led to several key findings:

• The thermal productivity of the micro gas turbine engine, as suggested by thrust-specific fuel consumption, was found to be comparable to standard fuel when operated on polystyrene oil, across a majority of the characteristics. For lower rotational speeds up to 60,000 rpm, the inclusion of more PSO in the blend with JET A resulted in an elevation in TSFC by up to 20% relative to pure JET A. However, despite a significantly lower heating value of up to 10%, the TSFC of the engine functioning on polystyrene pyrolytic oil was observed to be similar to that powered by the standard fuel for the rest of the TSFC traits.
• The exit temperatures of the gas turbine engine registered a rise by 10–15 °C for PSO/JET A blends in comparison to pure JET A.
• A pronounced escalation in the thrust-specific NO_X emission index of the micro gas turbine was noticed with the introduction of polystyrene oil to the blend with JET A. This trend remained constant for all examined PSO blends across the entirety of speeds probed, with the maximum average increase being 70% for pure PSO in contrast to pure JET A.
• The thrust-specific CO emission index for PSO/JET A blends was typically elevated by around 5–20% in comparison to that of JET A, contingent on the PSO content and operational range.