Impact statement

Mismanaged plastic waste is a significant crisis in the Global South, where facilities to properly recycle or sequester postconsumer waste are often non-extant. Local solutions that are low cost, safe, can be easily implemented, and benefit the community are therefore needed. This research focuses on utilizing appropriate technology-based slow pyrolysis to convert plastic waste into a clean burning fuel oil that can be used locally. The primary impact of this research is found in the approach taken to enable communities in the Global South to take control of plastic waste management in a way that benefits people and the environment.

Background

Plastic to fuel chemistry

The chemistry for the conversion of polyolefin plastic to liquid fuel oil is well-established (Pinto et al., Reference Pinto, Costa, Gulyurtlu and Cabrita1999; Demirbas, Reference Demirbas2004; Miskolczi et al., Reference Miskolczi, Bartha, Deak and Jover2004; Al-Salem et al., Reference Al-Salem, Lettieri and Baeyens2009; Panda et al., Reference Panda, Singh and Mishra2010; Kumar and Singh, Reference Kumar and Singh2011; Sarker, Reference Sarker2011; Sarker et al., Reference Sarker, Molla, Rashid and Rahman2012; Wong et al., Reference Wong, Ngadia, Abdullahb and Inuwac2015; Singh and Ruj, Reference Singh and Ruj2016; DeNeve et al., Reference DeNeve, Joshi, Higgins and Seay2017; Patil et al., Reference Patil, Varma, Singh and Mondal2017; Santaweesuk and Janyalertadun, Reference Santaweesuk and Janyalertadun2017). Polyolefin plastic consists of long-chain polymers consisting of thousands of monomer units. These plastics can be decomposed into shorter chain lengths of 12–14 carbons. At these chain lengths, these polyolefins remain in the liquid phase and can be used as a substitute for diesel fuel or kerosene. The total energy required for the conversion of LDPE, HDPE, and PP was calculated by Joshi and Seay (Reference Joshi and Seay2020) and is listed in Table 1.

Table 1. Pyrolysis energy for polyolefins (Joshi and Seay, Reference Joshi and Seay2020)

To approximate real operation of the plastic to fuel oil processor in the Global South where the process will be run, experiments were carried out with a mix of LDPE, HDPE, and PP. To approximate this mode of operation, an average value for the pyrolysis energy of 8,000 kJ/kg was be used in all calculated values.

Design of the plastic-to-fuel process

The plastic-to-fuel pyrolysis processor has been designed using the principles of AT. The process consists of an electrically heated 30-liter, 30.5-cm OD inner retort made of mild steel. The operating temperature of the electric heater is controlled using a PID controller. The retort is also equipped with an analog thermometer to indicate the internal temperature. The outer housing is constructed of mild steel. The 7.6 cm annular space is filled with insulation. The vapor product is collected overhead and condensed with a 3-m aluminum coil submerged in a water bath. The processor operates at ambient pressure; however, it is equipped with a pressure relief valve to mitigate any inadvertent overpressure. A schematic of the processor previously published by Foong et al. (Reference Foong, Browning and Seay2022) is illustrated in Figure 1. This design is flexible and so that substitutions with locally available materials of construction are possible.

Figure 1. Schematic of electric plastic to fuel oil processor (Foong et al., Reference Foong, Browning and Seay2022). R1, Fabricated retort; R2, High-temperature gasket; R3, 3/4″ CS tee fitting; R4, High-temperature thermocouple; R5, 3″ Blind flange; R6, 12″ × 3/4″ CS Pipe; R7, Pressure relief valve; R8, High-temperature wire; R9, 3/4″ CS pipe nipple; R10, 3/4″ × 3/8″ pipe reducer; R11, Analog bi-metal thermometer; R12, Fabricated processor housing; R13, Heating element, 240 V, 2,200 W (2); R14, PID controller; R15, Insulation; R16, NEMA plug w/wire leads; C1, 3/8″ Easy Bend Aluminum Tubing (10 feet); C2, Fabricated condensing vessel; C3, 3/8″ Brass Ball Valve; C4, 3/8″ compression × 3/8″ NPT fitting; C5, 3/8″ compression × 3/8″ NPT fitting; C6, 3/8″ Clear Tubing; C7, 3/8″ Hose Barb.

Global energy price review

To be economically viable, the value of the fuel oil produced must be greater than the cost of the electricity needed to operate the process. Selected electricity and traditional diesel fuel prices for regions in the Global South as well as the global average are listed in Table 2.

Table 2. Selected global energy and fuel prices (Global Petrol Prices, 2022)

Average income in Global South countries

The plastic to fuel oil processor is designed to provide sufficient supplementary income for operators in the Global South. Average daily incomes (2019 and 2022 Data) for selected regions are listed in Table 3.

Table 3. Selected average daily wages for selected Global South countries (Our World in Data, 2022)

Experimental methods

The experimental runs were conducted in the pyrolysis retort described by Foong et al. (Reference Foong, Browning and Seay2022). Mixed plastics consisting of LDPE, HDPE, and PP in equivalent amounts were used. The plastics used in these experiments were sourced from household waste. Previous results by Jangid et al. (Reference Jangid, Miller and Seay2022), indicated that temperatures between 375°C and 425°C were appropriate for waste plastic pyrolysis. For the experimental runs conducted in this research, two temperatures in the mid-range were selected, 385°C and 395°C. Temperatures below 375°C produced almost no product and temperatures above 400°C tended to generate wax.

To assess the efficiency of the processor, the measured electricity input in kWh, the fuel oil production in grams, the heater temperature, temperature at the surface, and the temperature in the retort were all recorded at 30-minute (0.5-hour) intervals. Data for each run were collected for 390 minutes (6.5 hours). This time was selected based on an 8-hour workday, allowing time for shredding plastic, loading the processor, and cooling and clean-up time at the end of the workday. Two insulation materials were tested, mineral wool (thermal conductivity = 0.055 W/m-K at 400°C) and Pyrogel XTE Aerogel composite blanket (thermal conductivity = 0.046 W/m-K at 400°C). In both cases, 7.65 cm of total insulation thickness was used.

Results and discussion

Experimental results are reported in Figures 2 and 3. These results indicate that higher R-value insulation results in improved efficiency, reduced heat loss, and higher profitability.

Figure 2. Average profit versus time for mixed plastic pyrolysis using Mineral Wool insulation.

Figure 3. Average profit versus time for mixed plastic pyrolysis using Aerogel insulation.

Based on the measured experimental data, the average daily profitability has been calculated and is reported in Table 4. Profitability is based on an average diesel fuel price of 1.63 USD/liter and an electricity price of 0.12 USD/kWh. Calculations are based on an 8-hour workday with a total processor operating time of 6.5 hours. The profitability calculation is based on the assumption that in the Global South, waste plastic can be purchased from waste pickers at 0.15 USD per kg. If the waste plastic is collected from the environment by the operator at no cost, the profitability will be increased.

Table 4. Summary of pyrolysis profitability experimental results

Based on the measured data from the experimental results, the heat loss to the environment can be calculated. This heat loss to the environment is calculated as follows:

(1) $$ Q=h\bullet A\bullet \Delta T, $$

where: Q is the average heat loss in kJ/h, h is the convective heat transfer coefficient in W/m²-K, and ΔT is the temperature difference between the processor surface and the ambient air.

The heat transfer coefficient for convective heat loss to the environment in still air at an ambient temperature of 25 °C is estimated to have an average value of 5.0 W/m²-K (Kosky et al., Reference Kosky, Balmer, Keat and Wise1999). The surface area for heat transfer from the outer housing is 1.2 m². The heat loss calculation results are listed in Table 5.

Table 5. Summary of heat loss from pyrolysis runs

The efficiency, η, of the AT plastic to fuel processor is calculated as follows:

(2) $$ \eta =\frac{\mathrm{Theoretical}\ \mathrm{pyrolysis}\ \mathrm{energy}}{\mathrm{Total}\ \mathrm{electric}\ \mathrm{energy}\ \mathrm{input}}\bullet 100\%. $$

The results of the efficiency calculations are reported in Table 6. As can be seen, there is a significant efficiency improvement with the use of Aerogel.

Table 6. Summary of efficiency from pyrolysis runs

It should be noted that Aerogel is significantly more expensive than mineral wool insulation and may not be easily available in countries in the Global South. This highlights the trade-offs that must often be made when designing processes using the principles of AT. However, the insulating benefits of Aerogel can be achieved by increasing the thickness of the mineral wool insulation used. This research demonstrates that plastic to fuel oil by slow pyrolysis can be carried out in an economically viable manner in the Global South. Furthermore, the design flexibility allows room for future improvement.

Conclusions and future work

This work has demonstrated that polyolefin can be successfully converted into liquid fuel oil using AT. The results are significant in that they demonstrate that converting plastic waste into liquid fuel can be a viable endeavor for people in communities in the Global South. Based on these results, it can be observed that the average profitability provides sufficient income for many Global South countries. The AT design requires trade-offs; however, this proposed design does provide sufficient income for operators in the Global South. As shown in the results, the effectiveness of the insulation is critical to the efficiency and profitability of the plastic-to-fuel oil process. Additional work to improve the efficiency is therefore needed to improve the overall sustainability of the process.