Sustainable Development Solutions for the Medical Waste Problem Using Thermal Plasmas

Abstract

Waste-to-energy (WTE) conversion is a vital process in the Middle East, especially in the Kingdom of Saudi Arabia (KSA), which is considered sustainable development for the environmental strategic project. Previous publications dealt with the environmentally friendly plasma treatment of wastes such as municipal waste, scrap tire waste, plastic waste, and grey water, using thermal plasmas produced by the cornerstone part of plasma reactors, namely the air plasma torch. In the present paper, with a view to energy recovery from medical waste, the thermodynamic properties of air plasma torches with a flow rate of air ranging from 10 mg/s to 30 mg/s and plasma jet temperatures ranging from 1500 °C to 5000 °C were investigated; these include power loss, enthalpy, plasma flux, and torch efficiency variation with plasma input power and air flow rate. The measured electrothermal efficiency of the plasma torch is in the range of 42% to 80% and increases with the increasing input power and gas flow rate. In Makkah, the number of beds in the hospitals is 10,500; the average annual weight of the medical waste in the hospitals is 2835 × 10³ t, with an extracted amount of pyrolysis oil equivalent to 2268 × 10³ t and an equivalent energy of 90 × 10⁹ M J. In the proposed plasma treatment project, the amount of diesel oil after the distillation process will reach up to 1928 × 10³ t, with an estimated sale profit of the electricity reaching up to 21 × 10⁶ MW·h. The sale profit of pyrolysis oil in 2022 reaches up to USD 34.44 million, and the sale profit of electricity extracted by using the diesel oil in 2022 reaches up to USD 1020 million for households and USD 1445 million for factories.

1. Introduction

Until 1879, people thought that there were only three states of matter (the solid, liquid, and gaseous states) in the universe, but then, William Crookes, Joseph Thompson, and Irving Langmuir discovered the properties and the nature of the fourth state of matter, which they called plasma. Plasma is a quasi-neutral ensemble of positively and negatively charged particles and neutral particles, featuring a collective behavior. Plasma can be formed in different ways, but the most common way is electric discharge. Applying an electrical voltage leads to the breakdown of the gas and its ionization. Plasmas are generally divided into two large groups. The first is the cold, non-thermal or non-equilibrium plasma, which was studied in our previous articles for applications such as etching for industrial applications [1,2], disinfection applications [3,4], environmental process, textile treatment [5,6], and nano-technology applications [7]. The second group is the hot, thermal or equilibrium plasma used in many applications which were studied and outlined in our previous articles on using the plasma treatment system (PST) to convert waste to energy (WTE) using, e.g., municipal solid waste (MSW) and scrap tires [8,9].

Thermal plasma produced in a plasma torch has helped the development of industrial applications at very high temperatures and degrees of conversion due to its high energy density and fast processes and low emissions in comparison with the conventional methods. In these plasma reactors, organic compounds are decomposed at very high and fast degrees of conversion, whereby the waste is converted into different energy products, such as syngas, synthetic fuels, hydrocarbons, biofuel, carbon black, and electricity [10,11].

The plasma gasification reactor to obtain energy recovery from different types of medical waste is based on our previous stages of the plasma treatment projects (PTP) in Makkah in the seasons of the pilgrims: (i) the plasma treatment project for municipal solid waste to energy (PTPMSW) [12], from waste sources or from landfill with a view to their sustainable energetic or chemical valorization and the product gas efficiency of waste to gas. The electrical power generation system for the plasma treatment process required was estimated at 5000 kW: 2000 kW for the operation of the system and 3000 kW sold. The second was the (ii) plasma treatment project for scrap tires (PTPST) [13] to produce synthetic fuels and syngas, hydrocarbons, biofuels, and carbon black. The main products are 40% slag, 8% synthetic syngas, and 52% pyrolysis oil. Annually in Makkah, after treatment by plasma, the heat energy extracted from scrap tires is 15.5 TJ per ton.

Huge quantities of medical waste are collected every day from the hospitals, health-care units, and maternity and nursing homes in Makkah. Medical waste is considered to be environmentally hazardous, dangerous, and highly infectious because of possible contamination with pathogenic micro-organisms. According to official KSA reports, hospitals in Makkah contain 10,500 beds, and each bed generates 750 g/day of medical waste [14,15,16,17].

There are many categories of medical waste in these hospitals, such as chemical waste, infectious waste, and radioactive waste, and the majority contain different types of plastics, such as plastic pallets, plastic drums, plastic buckets, plastic film, waste appliances shell, plastic garbage drums, IBC drums, IBC barrels, garbage bags, rubbish bags, and plastic pipes. The plastics are classified as follows: HDPE (high-density polyethylene), PVC (polyvinyl chloride), PET (polyethylene terephthalate), LDPE (low-density polyethylene), (PS) polystyrene, and miscellaneous plastics [18,19].

The proposed plasma gasification process for Makkah will be considered as an energy recovery transition process for medical waste and an environmentally friendly technology because in comparison to conventional technologies it converts organic waste pollutants into commercially useful products. Thermal plasma treatment provides a faster destruction process of the plasma jet emerging from the plasma torch, high-value energy products (syngas, pyrolysis oil, and slag), high profit, and a low production of pollutants. In the present article, another target is to study the characteristics of the emerging plasma jet required for the plasma gasification of medical waste. Moreover, it is to study the conversion process of medical waste into fuel oil as well as the environmental and economic aspects. Furthermore, energy recovery and the profit from the sale of the produced electricity are analyzed.

The experimental set-up to test and validate the plasma treatment of the medical waste is described in Section 2. Section 3 presents the characteristics of the plasma jet exciting the plasma torch and the production of pyrolysis oil, diesel, and electricity. Finally, the conclusions and outlook are given in Section 4.

2. Experimental Set-Up and Procedures

2.1. Plasma Torch

The air torch is shown in Figure 1. It is powered by direct current, and it is considered as the main component of the plasma furnace. In our previous article [20], a brief discussion of the air torch’s schematic diagram (non-transferred arc) is provided. The plasma torch’s characteristics, which are listed in

Table 1. Air plasma torch parameters.

Parameter	Amount
Power	Direct Current
The working active gas	Air
Vmax	500 V
Imax	250 A
Pmax. out	125 kW
Tplasma jet	500–1000 °C
Air flow rate	30 mg/s
Cooling system	Inlet and outlet water

, make it possible to create a plasma jet that is both compatible with waste-to-energy operations and easily adjustable for the conversion of electrical energy into the thermal energy needed for various environmentally friendly processes.

An electrical discharge is generated between the electrodes; the cathode and the anode are large enough to resist gradual erosion, considering the water cooling to handle the high temperatures. In the plasma torch, electrical energy is converted into heat, and a thermal plasma jet is ejected from the torch due to the pressure of the compressed air flow when it passes between the gas column and the surface of the inner anode [21,22].

The flow rate of the reaction gas is detected using a Pitot tube flow meter, while the temperature at the reactor’s inner wall and the temperature of the gas are measured using thermocouples. The air flow rate varies between 10 mg/s and 30 mg/s, and the high-temperature plasma of the jet at the nozzle of the torch significantly impacts the oxidation in the plasma treatment reactor for the current investigation (present study) [23]. The operating characteristics of the air torches are presented. In Section 3, the operating characteristics of the air plasma torch are presented, such as the applied power, the applied flow rate, the torch efficiency variation, and the characteristics of the emerging jet (mean temperatures, velocity, radiation wavelength, power loss, enthalpy, and plasma flux).

The enthalpy probe is a device commonly used in experiments [12,13,24]. The advantages are that it is a low-cost appliance, and the velocity and enthalpy of the plasma can be measured directly. The disadvantage is that it may perturb the plasma. Experiments have shown that the plasma is only cooled down about 3% in the centerline by inserting the enthalpy probe. This error is comparable with that found in other techniques, such as emission spectroscopy. It was found that the enthalpy probe is accurate for temperatures up to 10,000 K and even more accurate than emission spectroscopy.

The enthalpy probe consists of three concentric tubes and is water-cooled. The probe measures the stagnation pressure and takes gas samples. There are three thermocouples present. One measures the temperature of the gas at the probe end, and the two others measure the temperature rise in the cooling water The local specific enthalpy of the plasma h_tip can be derived from the combined energy balance of the cooling-water flow and the gas sample. The plasma enthalpy can be given by the following Equation (1) [25]:

h_tip = m_water m_gas C_p (∆T_GF − ∆T_NGF) + h_exit

(1)

h_tip is the enthalpy at the tip of the probe, and h_exit is the enthalpy at the exit of the probe. ∆T_GF is the temperature rise of the cooling water if there is a gas flow, and ∆T_NGF is the temperature when there is no gas flow (tare measurement). m_water is the mass flow of the cooling water, m_gas is the mass flow of the gas, C_p is the specific heat of the water and is assumed to be constant. The flow of the gas must be in accordance with the isokinetic sampling law. This law states that the velocity in the measuring tube must be as close as possible to the free stream velocity. In this equation, m_water must be altered to make sure the temperature difference (∆T_GF − ∆T_NGF) is large enough to measure, but it is constant during the measuring cycle [26]. If the mass flow of the gas is increased, the ratio of the mass flows decreases, but the heat flux to the cooling water increases. These two effects cancel each other out; so, the plasma enthalpy is independent of the mass flow of the gas within a certain range. h_exit is calculated from the temperature at the exit of the probe and it does not vary significantly with the mass flow. If the gas composition is known at the probe exit, the mass of the gas can be calculated through the mass flow rate. The plasma temperature T can be calculated from the dependency of h_tip on the temperature [27].

2.2. Plasma Gasification Reactor

The plasma treatment project in the present case study stands for: (i) the pre-treatment of medical waste for rapid processing using chopping and grinding processes before feeding it to the plasma treatment reactor. (ii) The conversion process of medical waste into the three products of syngas, pyrolysis oil, and slag occurs in a combustion-like chamber at temperatures as high as 1500 °C and in the presence of the working active gas air, which has a significant impact on oxidation in the plasma treatment reactor without producing environmental contaminants from the produced gases. (iii) The synthesis gas (syngas) is cleaned, the industrial fuel (pyrolysis oil) is gathered, distillated, and purified, and the slag is occasionally or continuously tapped out at the reactor’s bottom [28].

The experimental reactor for the treatment of medical waste is shown in Figure 2. The inner surface of the reactor is covered with a layer of thermal ceramics and acts as an insulating layer with a thickness of 400 mm to reduce energy loss to the outer walls. To prevent the destruction of the ceramic insulation, before carrying out the experiments, the reactor is pre-heated for 24 h to about 1500 °C and, in addition, by a plasma torch at a power level of 125 kW. All parts of the reactor chamber are cooled with water, and calorimetry measurements are made on the cooling circuits. The medical plastic waste to be treated is continuously supplied, and the feeding rate is controlled.

3. Results and Discussion

3.1. Plasma Jet Characteristics

Plasma is usually generated in an electric arc by running a current through a gas or a vacuum, whereby a conducting path is formed between the electrodes; this process is called breakdown.

The mean temperature of the jet is (500–1500) °C at the torch nozzle with an applied power of (25–125) kW. The analysis of the obtained results shows that this plasma technology could be easily employed for environmentally friendly applications, such as the decomposition of scrap tires into their products: carbon, syngas, pyrolysis oil, and slag.

The corresponding jet Length images, with a gas flow rate of G = 10 L/min and 20 L/min and an arc current of I = 100 A, as in Figure 3a,b, show that as the energy supplies increases, the plasma jet length increases. The jet temperature increases and the heat exchange between the plasma jet and the atmosphere also increases. This may be because the plasma jet is in a laminar flow state at a slower gas flow rate. At this state, the jet length increases when the energy supplied to the plasma jet is increased by adjusting the arc length.

When increasing the gas flow rate to G_gas = 30 L/min and 40 L/min and an arc current of I = 100 A, as in Figure 3c,d, the plasma jet entrains more air, which leads to a larger heat loss, and the plasma jet gradually changes from a laminar flow state to a turbulent flow state [29].

The relation between the applied power and the electrothermal efficiency ratio $\left(\eta \right)$ of the plasma torch was investigated as follows [30,31]:$\left(\eta \right)$ is the ability of the plasma torch to convert electric energy into thermal energy. The efficiency depends on several parameters, such as torch design, plasma gas flow rate, the nature of the plasma gas, torch input power, etc. The $\eta$ of the plasma torch can be calculated as in (2)

$$\eta =\left(1-\frac{P_{loss}}{P_{in}}\right)\times 100$$

(2)

where P_in and P_loss are the input electrical power flow into the jet (I_ampere × V_volt) and the power loss by the cooling system, respectively. The power loss represented by the dissipated power (heat transfer to electrodes and walls and waste material) due to conduction, convection, and radiation can be calculated according to the following Equation (3)

$$P_{loss}\left(\mathrm{kW}\right)=P_{loss by cond.}+P_{loss by conv.}+P_{loss by rad.}$$

(3)

Neglecting the power loss due to convection and radiation for the plasma torch, Equation (3) can be written as in (4):

$$P_{loss}\left(\mathrm{kW}\right)=C_p{m}_{\mathrm{H}2\mathrm{O}} \left(T_2-T_1\right)$$

(4)

Substituting (4) in (2), we obtain (5):

$$\eta =\left(1-\frac{C_p{m}_{\mathrm{H}2\mathrm{O}} \left(T_2-T_1\right)}{P_{in}}\right)\%$$

(5)

where C_p represents the specific heat of water (kJ/kg K), $m_{{\mathrm{H}}_2\mathrm{O}}$ represents the mass flow rate of the cooling water (kg/s) to prevent the torch from overheating too rapidly, $T_1$ and $T_2$ are the inlet and outlet water temperatures (K), and ΔT = $T_2-T_1$ represents the heat losses to the walls and electrodes which are taken away by the cooling water. By using Equation (3), the electrothermal efficiency of the plasma jet (%) as a function of the measured input power (kW), and for the different values of the air flow rate (10–20–30) mg/s, can be plotted as shown in Figure 4, where $\eta$ increases with the increasing input power and is found to be in the range from 80% to 42%, which always increases with the gas flow rate increases because of the high gas flow, which carries more heat out of the torch channel [26,27,28,29].

The different effective parameters of $\eta$ will be measured at the low air flow rate of 10 mg/s.

Equation (2) can be written as follows:

$$\begin{gathered} \eta =\left(1-\frac{\beta \Delta t }{P_{in}}\right) \\ \mathrm{where}{P}_{loss}=\beta \Delta t \\ \mathrm{and}\beta =C_p{m}_{\mathrm{H}2\mathrm{O}} \end{gathered}$$

(6)

Then, Equation (6) can be written as follows:

$$\eta =-\beta \frac{1 }{\left[\raisebox{1ex}{$P_{in}$}\!\left/ \!\raisebox{-1ex}{$\Delta T$}\right.\right]}+1$$

(7)

By using Equation (6), Figure 5 shows the electrothermal efficiency of the plasma torch for the low air flow rate of 10 mg/s as a function of $\frac{1 }{\left[\raisebox{1ex}{$P_{in}$}\!\left/ \!\raisebox{-1ex}{$\Delta T$}\right.\right]}$ to give a straight line with negative slope $\beta$, represented by $y=-0.3852x+1$, where, as $\Delta T$ increases, the efficiency of the torch will be reduced because of the losses due to the heat transfer to the electrodes and walls (conduction and convection). From Equations (3)–(5), (ΔT) is directly proportional to P_in and P_loss. Figure 6 shows P_in and P_loss for the air flow rate of 10 mg/s as a function of (ΔT). Furthermore, the plasma flux of the jet as a difference between $P_{in}\mathrm{and}{P}_{loss}$ can be calculated as follows:

$${\mathrm{F}}_{\mathrm{plasma}}=P_{in}-P_{loss}$$

(8)

Figure 7 shows that the plasma flux for the low air flow rate of 10 mg/s is directly proportional to input power with the linearity relation.

As thermal plasma is in local thermodynamic equilibrium (LTE), it may be described as a fluid with specific thermodynamic and transport properties [32,33]. Therefore, heat transfer in thermal plasmas can be described in terms of heat convection, conduction, and radiation [30]; compared with other fluids, thermal plasma is characterized by very high enthalpy (MJ/kg), thermal conductivity, and radiation intensity, and thus, the heat transfer in thermal plasma is extremely high.

At high temperatures, the power losses due to different processes (discussed briefly in Appendix A) increase the heat transfer from the plasma jet [34,35,36].

3.2. Conversion Process and Fuel Type from Medical Waste Treatment

Plasma gasification is considered as an environmentally friendly process to convert medical waste into fuel oil [37,38,39,40]. The proposed plasma treatment system is based on the above-presented characteristics of the plasma torch mounted on a plasma furnace with a capacity of 200 t per day, as schematically shown in Figure 2.

After the quenching process of the hot emerging gas, pyrolysis oil condensates as a fuel oil, but it cannot be used directly as source of energy due to its high ash and wax content, as shown in Figure 8. The collected pyrolysis oil product is very similar to crude oil and is brownish-black and viscous [41,42,43,44], which is an indication that it needs many distillation steps to be a source of hydrocarbons in the form of naphtha products, with characteristics modified with the standard fuel products expected to be used in an engine. The condensation process for the pyrolysis oil at different temperatures gives different types of fuel: (i) gasoline fuel at 500–560 °C; (ii) kerosene fuel at 580 °C and 600 °C; and (iii) diesel oil at 620 °C and 650 °C.

3.2.1. Energy from Pyrolysis Oil

We use the following data from the official KSA reports.

Table 2. The amount and profits of pyrolysis oil in MAKKAH annually within 1440.

Parameter	Amount
Capacity of plasma reactor	72 × 103 t
No. of beds in hospitals	10,500
The average weight of Md.W. per bed	270 t
The average weight of Md.W in hospitals	2835 × 103 t
Amount of pyrolysis oil extracted	2268 × 103 t
Amount of diesel oil extracted	1928 × 103 t
Number of gallons of pyrolysis oil	54,500 × 103 gallon
Number of liters of pyrolysis oil	205 million liters
Energy produced from pyrolysis oil	90 × 109 M J
Energy produced from diesel oil	76 × 109 M J
The amount of sale of electricity	21 × 106 MW·h

shows the following annual data for Makkah: (i) the number of beds in the hospitals is 10,500, with an average weight of medical waste per bed of 270 t, and the average weight of medical waste in the hospitals is 2835 × 10³ t; and (ii) the amount of pyrolysis oil extracted is 2268 × 10³ t with the equivalent energy of 90 × 10⁹ M J of plastic waste [45] using the plasma proposed above; the analysis of the medical plastic waste using the plasma gasification system proposed above [45] shows that fuel oil from the pyrolysis of medical waste plastics is an important source of energy. In this pyrolysis process, the medical plastic waste is decomposed thermochemically using the plasma gasification furnace system at temperatures of up to 1500 °C and converted into fuel oil, slag, and syngas (oil, solid, and gaseous) fractions.

3.2.2. Electricity from Diesel Oil

Table 3. The amount and profits of diesel oil extracted from pyrolysis oil in MAKKAH annually within 1440.

Parameter	Amount
The unit cost of electricity sale	12 USD/MW·h for home (2022) 17 USD/MW·h for factories (2022)
Sale profit of electricity	USD 255 million for home (2022) USD 361 million for factories (2022)
Price of each liter of pyrolysis oil	USD 0.042
Sale profit of pyrolysis oil liters	USD 8.61 million (2022)

shows the output electricity produced from diesel oil annually, with a lower sulphur and higher cetane value in comparison with traditional diesel, after the distillation process for the pyrolysis oil (where the diesel oil amount reaches to 85% of the pyrolysis oil, as shown in Table 2, as follows: (i) the equivalent diesel oil after the distillation process reaches to 1928 × 10³ t, with an extracted sale amount of electricity reaching to 21 × 10⁶ MW·h.

Although the destruction of plastics requires more input electricity than that required for other waste such as paper or wood, the treatment of plastics produces the largest energy recovery per ton of the input feedstock, due mainly to their higher HHV [46,47,48].

3.3. Economical Vision

The sale profit of pyrolysis oil liters in 2022 reaches to USD 8.61 million, and the sale profit of electricity extracted by using the diesel oil in year 2022 reaches to USD 255 million for households and USD 361 million for factories, as shown in Table 3.

Petroleum [49,50,51,52] is a crucial resource for commercial and large-scale infrastructure and is used in many facets of daily life. When synthetic fuel is made from medical plastic waste that has been processed in a plasma reactor, it gives the equivalent energy and costs less than that taken from petroleum and produces a gross profit, as shown in Table 3.

4. Conclusions and Outlook

The plasma reactor of the thermal plasma treatment system with a flow rate of air ranging from 10 to 30 mg/s into the plasma torch and an electrothermal efficiency in the range from 42% to 80% was analyzed. When the plasma torches operate at low power levels, the temperature increases, and there are losses of power due to the conduction and convection processes. This in turn leads to a faster destruction process and increases the heat transfer to the thermal-treated products, such as grey water, municipal solid waste, and scrap tires.

In Makkah, the average weight of medical waste in hospitals annually is 2835 × 10³ t; the extracted amount of pyrolysis oil reaches up to 2268 × 10³ t with an equivalent energy 90 × 10⁹ M J; the amount of diesel oil reaches up to 1928 × 10³ t.

The estimated sale profit of electricity from the medical waste treatment project reaches up to 21 × 10⁶ MW·h. The sale profit of pyrolysis oil in 2022 reaches up to USD 8.61 million, and the sale profit of electricity extracted by using the diesel oil in 2022 reaches up to USD 255 million for households and USD 361 million for factories.

The perspective is to use the investigated plasma torch in an experimental set-up to test and validate the conversion of slaughterhouse waste in Makkah into useful products.