Chemical and pharmaceutical waste disposal with thermal plasma pyrolysis-melting

الموضوعات : Journal of Theoretical and Applied Physics

Shahrooz Saviz ¹ , Davoud Dorranian ² , Amir Hossein Sari ³

1 - Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
2 - Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
3 - Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran

تاريخ الإرسال : 24 الإثنين , محرم, 1444 تاريخ التأكيد : 04 الجمعة , ربيع الأول, 1444 تاريخ الإصدار : 07 الخميس , جمادى الأولى, 1444

الکلمات المفتاحية: Plasma Reactor, Plasma torch, Gasification, Covid-19 wastes, slag,

ملخص المقالة :

Thermal plasma treatment is considered as a suitable alternative for the treatment of highly-hazardous wastes such as industrial, radioactive and medical waste. Therefore, a Plasma-Gasification-Melting (PGM) system for treatment of Chemical and Pharmaceutical Wastes (CPW) with a capacity of 1 ton/day is developed using a melting and gasification furnace equipped with two non-transferred thermal plasma torches. In this article, the whole method of chemical and pharmaceutical waste disposal is presented along with exhaust gas analysis, and slag and energy balance approach for improving the relevant technology process. It is successfully demonstrated that the thermal plasma process converts chemical and pharmaceutical wastes into harmless slag. Also, the associated emission level of air pollutants is shown to be very low. The synthetic gas produced can be used as a source of energy. (11.7 Nm3 / hr for CO and 16.4 Nm3 / hr for H2). The total power consumption of the system is 120 k W including 90 kW for thermal plasma torch and 30 kW for utilities with natural gas flow rate of 1.3 Nm3/hr.

نص كامل:



Chemical and pharmaceutical waste disposal with thermal plasma pyrolysis-melting

Shahrooz saviza,¹, Davoud Dorranianb, Amirhossein Saric

a Plasma Physics research center, Science and research Branch, Tehran, Iran

b Plasma Physics research center, Science and research Branch, Tehran, Iran

c Plasma Physics research center, Science and research Branch, Tehran, Iran

 

Abstract

     Thermal plasma treatment is considered as a suitable alternative for the treatment of highly-hazardous wastes such as industrial, radioactive and medical waste. Therefore, a Plasma-Gasification-Melting (PGM) system for treatment of Chemical and Pharmaceutical Wastes (CPW) with a capacity of 1 ton/day is developed using a melting and gasification furnace equipped with two non-transferred thermal plasma torches. In this article, the whole method of chemical and pharmaceutical waste disposal is presented along with exhaust gas analysis, and slag and energy balance approach for improving the relevant technology process. It is successfully demonstrated that the thermal plasma process converts chemical and pharmaceutical wastes into harmless slag. Also, the associated emission level of air pollutants is shown to be very low. The synthetic gas produced can be used as a source of energy. (11.7 Nm3 / hr for CO and 16.4 Nm3 / hr for H2). The total power consumption of the system is 120 k W including 90 kW for thermal plasma torch and 30 kW for utilities with natural gas flow rate of 1.3 Nm3/hr.

Keywords: Plasma torch, Gasification, Slag, Plasma Reactor, Covid-19 wastes.

 

 

 

 

 

1. Introduction

     Hazardous wastes such as pathology, chemical and pharmaceutical and radioactive wastes pose many hazards to human health and living organisms [1-6]. There is currently no balance between the production of hazardous waste and the methods of its disposal [7-8]. The diversity of hazardous wastes is increasing though no specific methods have yet been proposed for their disposal. The most recent example is the Covid-19 virus. The horrific number of patients infected with the virus and the daily increase in the number of hospitalized patients have led to a dramatic increase in hazardous waste. Any medications, equipment, vaccines etc. that are relevant to patients with Covid-19 can be a potential source of hazardous waste contaminated with the covid-19 virus. In third world countries, due to the lack of separation of corona patients' waste from other hazardous wastes, it is more likely for all wastes to be infected with this virus. One type of hazardous waste that is difficult to dispose of is chemical and pharmaceutical waste. This waste cannot be decontaminated by methods such as autoclaving. Therefore, one of the possible methods to eliminate them is the application of thermal methods. The presence of chemicals, outdated drugs, and a variety of attenuated viruses and large volumes of minerals in vial containers require high temperature systems. Fossil fuels are not able to create adequately high temperature for melting minerals, thus large amounts of ash remain and many chemicals are left unchanged inside the ash. The new method of using thermal plasma solves the temperature problem [9-13]. Plasma torches [14] can produce temperatures of up to 15,000 °C. In this method, plasma furnaces reach a temperature of about 1800 degrees Celsius. Therefore, all minerals are removed from the furnace in the form of melt. The volume of waste is significantly reduced and the remaining melt is completely safe. On the other hand, organic matter is released as a synthetic gas from the top of the furnace and can be used to generate energy.

     In this paper, the system of chemical and pharmaceutical waste disposal with thermal plasma technology is analyzed. In the second part, the generalities of the process in the system and its configuration are examined. In the third part, the output results and system performance are examined and, finally, the conclusion is presented in the fourth part.

2. Experimental Section

2.1. Process overview and plasma reactor configuration

     The process of design and construction of the disposal plant for thermal-plasma-based and chemical and pharmaceutical waste was implemented in three six-month periods. In the first six months, the whole plant was designed by professional consulate teams led by PPRC specialists. Second period was dedicated to technical works on construction of the plant and the third period was devoted to tests and optimizations. During the optimization period, attempts were made to improve the operation process of the plant. Some of the main activities performed during this period include normal operation of the plant, decreasing the amount of pollutants in the exhaust gas, reducing the operational cost and upgrading of the feeding system.

The schematic setup of the constructed thermal plasma-based medical waste disposal plant is presented in Fig. 1.

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[1] * correspondence author

   Email address: [e-mail] (Shahrooz saviz)

 

Fig. 1. (a), (b) System setup: 1- main shaft furnace, 2- plasma torch, 3- feeding system, 4- power supplies, 5- control room, 6- air compressor, 7- after-burner, 8- water quench, 9- wet scrubber, 10- jet fan, 11- stack, 12- water reserve, 13- chiller, 14- Air purification system.

 

     The components of the plasma disposal system are shown in Figure 1. As seen, the system consists of three main sections for PGM process on CPW: 1- waste feeding system 2- Long shaft furnace with two non-transfer plasma torches 3- Exhaust gas purification system including afterburner system and water scrubber.

2.2. Specification of CPW

     The CPWs are taken from Farhikhtegan Hospital, Tehran, Iran. The physical properties of CPW in terms of weight percentage of different compounds are given in Table 1.

Table 1. Physical composition of the CPW

	Test 1	Test 2	Test 3	Test 4	Test 5
Vial of medicine (glass) (%)	49.3	42.2	50.2	39.7	43.2
Plastic (%)	28.4	32.6	26.1	30.4	28.7
Paper (%)	2.2	1.9	2.9	1.7	2.1
Metal (%)	1.1	1.3	0.4	0.6	0.8
Residues of pharmaceutical powders (%)	9.2	8.8	5.1	11.2	10.8
Residues of drug solutions (%)	8.4	7.9	10.2	12.2	10.3
Others (%)	1.4	5.3	5.1	4.2	4.1

     One of the important parameters in examining the mass and energy balance of a reactor is lower heating value (LHV) of the waste. Equation 1 is related to the calculation of this parameter. The Lower Heating Value (LHV) of the CPW is obtained from the following equation:

                                                                (1)

The definitions and mean values of the coefficients in Equation 1. are as follows:

A: Heating value of the combustible materials=18400,

B: Heat loss due to water in the waste=2636,

C: Heat loss due to glass in the waste=628,

D: Heat loss due to metals in the waste=544,

E (): Average heat loss due to other inorganic materials in the waste=450,

: Weight percentage of combustible materials,

: Water weight percentage,

: Glass weight percentage,

: Metals weight percentage.

: Weight percentage of other inorganic materials.

The percentages of combustible and non-combustible materials are given in Table 2. Using Eq. 1 and the given weight percentages of combustible and non-combustible materials as well as the coefficients A, B, C, D and E, the LHVs of the waste are calculated for five samples as given in table 2.

Table 2. The LHV of the CPW for five tests

 

2.3. CPW feeding system

     Considering the hazardous nature of CPW, it is necessary to inject it into the furnace without opening the bags. A special injection system must be designed and built for each type of waste. Therefore, after testing several methods, the special system is designed for CPW (Fig. 2). This mechanism consists of several pneumatic jacks that are used to transfer waste through a cylinder into the furnace. The steps of injecting chemical and pharmaceutical waste into the furnace are as follows:

1- Airlock Gate No.1, opened by the pneumatic jack, 2- Waste is thrown into waste container No.1, 3- Airlock Gate No. 1 closes, 4- Airlock Gate No. 2 is opened by the pneumatic jack, and the CPW first enters the waste container No. 2 and then gradually enters the waste injection cylinder, 5- The piston is moved inside the cylinder by a pneumatic jack and pushes the waste into the furnace.    

 

 

Fig. 2. CPW feeding system

2.4. Long shaft furnace with two plasma torches

     The reactor is the heart of the CPW disposal plant. The reactor is a vertical cylinder of 4 m height. The diameter of the upper part is 1 m while the diameter of the lower part is 1.5 m. The lateral surface of the reactor consists of three layers. The inner layer is made of refractory bricks covered by thermal insulation bricks with 20 cm thickness as the thermal insulator. The outer surface is made of steel plate with a thickness of 5 cm. The reactor is equipped with two 70 kW thermal plasma torches in its lower part. They are installed symmetrically around the cylinder at a height of 0.3 m from the base of the reactor (Fig.3). As shown in Figure 3a, the angle of the plasma torches with the furnace axis is 120 degrees. Figure 3b shows that, the plasma torches are placed tangentially in the furnace circumference. This causes: 1- the heat of the plasma torches circulates inside the furnace environment, 2- the retention time of the heat of the plasma torches inside the furnace is maximized, 3- the furnace reaches faster the desired temperature, and 4- the efficiency of the system increases. Two of the three torches are on duty and one of them is standby. To drain the slags, a gate is installed at the base of reactor. Two gates at the top of the reactor serve as the feeding gate and exhaust. The waste pyrolysis and gasification process is performed in the furnace shaft and melting is done in the lower part of the furnace (Fig. 2 and Fig.4). This design is based on the fact that minerals make up a large portion of the waste composition. In the melt chamber, the temperature reaches 1800 degrees Celsius. As high temperatures pass through the waste in the furnace shaft, organic matter is converted to synthetic gas and minerals flow to the melt chamber. Waste is continuously fed into the furnace shaft and the melt is removed from the bottom of the furnace. The furnace shaft and the furnace melt chamber can be separated. This design is very useful due to the fact that the molten part needs to be repaired in some cases as a result of the high temperature. An air sheath between the plasma torch and the furnace is used to protect the plasma torches from the impacts of furnace temperature.

     Thermal plasma torches (Fig. 3) are non-transferred DC torches with a maximum current of 200 A current and voltage of 350 V. They are water-cooled with copper electrodes and use pressured air as the carrier gas. For each thermal plasma torch, a pressure of 10 bars is provided by high-pressure air compressor. Through the use of the two torches, the temperature in the lower part of the reactor reaches 1800 degrees Celsius. These plasma torches were designed and manufactured by the authors. In these plasma torches, the cathode consists of a rod electrode and the anode is a hollow copper electrode. The cathode life is 100 hours and the anode life is 400 hours.

2.5. Effluent Gas Purification System

     The exhaust system which is actually a gas cleaning system, consists of an afterburner (Fig.4) and water scrubber which are installed in series. The fly ashes are collected and removed in the afterburner. The natural gas burner is employed to complete the combustion of H2 and CO and fly ash molecules. Furthermore, Caustic Soda solvent is fed into the scrubber to remove acidic gases. The water quencher and scrubber are located at the outlet of the afterburner. The role of the water quencher is to quickly cool the gas to 60 °C. It also partially removes the remaining acidic gases but the total removal of all acidic gases is achieved in the scrubber using NaOH solution. The scrubbing solution with controlled pH is constantly recirculated. Finally, the output gas of the afterburner is sent to stack through a gas cooler system. The temperature of the gas flowing in the stack is 60 degrees Celsius.

 

 

 

 

 

 

 

 

Fig. 3. (a) Position of the plasma torches respect to furnace axis, (b) Position of the plasma torches respect to furnace circumference, (c) Plasma torch in operation.

 

 

 

 

Fig. 4. Molten material leaking from the furnace bottom

3. Results and discussion

     In the process of CPW disposal, solid, liquid and gaseous by-products are produced. The solid by-product comes out of the bottom of the furnace in the form of a slag. The liquid byproducts are generated in the scrubber wastewater treatment system and the gaseous byproducts are related to the gasification of CPW.

3.1. Slag byproduct

  The results of the slag composition analysis are shown in Table 3. The highest percentage is related to silicon dioxide. There were other elements that were not included in the table due to their very low percentage. The weight percentage (wt%) of slag produced in proportion to the total injected CPW is measured in five steps. As shown in Table 4, the average weight percentage (wt %) of slag is 59.4%. The density of the destroyed CPW was about 283 kg/m3 which is higher than the municipal solid waste (MSW) density of about 100 kg/m3. This high density is attributed to the high content of minerals in chemical and pharmaceutical wastes. The measured density of the slag is about 2400 kg/m3. Therefore, the reduction in CPW volume is about 89%.  Further, Toxicity Characteristic Leaching Procedure (TCLP) is performed on the five slag samples. The results show that there is no heavy metal detected in slag.

Table 3. Physical composition of the CPW

						LHV
Test 1	30.6	8.4	49.3	1.1	10.6	5045.7
Test 2	34.5	7.9	42.2	1.3	1.41	5804.2
Test 3	29	10.2	50.2	0.4	10.2	4703.8
Test 4	32.1	12.2	39.7	0.6	15.4	5262.9
Test 5	30.8	10.3	43.2	0.8	14.9	5053
Average	31.4	9.8	44.92	0.84	10.5	5173.9

	Test 1	Test 2	Test 3	Test 4	Test 5
SiO2	57.6	58.3	54.1	55.4	59.2
Al2O3	8.26	9.2	9.8	8.4	9.5
Na2O	6.56	10.4	6.9	6.4	5.7
MgO	0.64	0.52	0.72	0.51	0.65
CaO	4.85	4.2	3.9	5.1	4.8
TiO2	0.35	0.21	0.45	0.12	0.24
MnO	0.12	0.08	0.21	0.17	0.3
Fe2O3	0.38	0.31	0.28	0.14	0.25

 

 

 

Table 4. Percentage of remaining slag to the injected CPW (wt %)

	First Step	Second Step	Third Step	Forth Step	Fifth Step
Remaining slag (wt%)	58.5	61.4	59.4	57.3	60.4

     Given that the slag is not toxic and does not contain hazardous substances, it can be used in various industries including road infrastructure, construction industries etc.

3.2. Liquid Byproducts

     The water scrubber is located after the combustion system. Treatment is necessary for the water used for rapid gas cooling and de-acidification

3.3. Gaseous Byproducts

     The most important requirement for a hazardous waste disposal system is the standard amount of exhaust gases. For this purpose, the concentration of air pollutants before the afterburner (Table 5) and in the stack (Table 6) is measured. The gas flow rate before afterburner and in the stack is about 43 Nm3/hr and 95 Nm3/hr, respectively. This flux difference is due to the presence of the afterburner system. Gas chromatography results of the CPW plasma-pyrolysis before afterburner (Table 5) indicate that the produced gases are rich in hydrogen and carbon monoxide and also contain some other compounds. The total quantity of H2 and CO in the gaseous mixture is above 66% so we expect the afterburner temperature to rise. Possible reactions which take place during the pyrolysis of CPW include:

+heat               for cellulose

     for plastic

     As shown in Table 1, a significant constituent of organic matter in chemical and pharmaceutical wastes is plastic. Therefore, the percentage of hydrogen is expected to be higher than carbon monoxide. This prediction is clearly seen in Table 5. As seen in this table, the concentrations of O2 before afterburner is 0.3 which is small. This shows that oxygen starvation has happened inside the furnace.

 

 

 

Table 5. Exhaust gas composition before afterburner chamber

Gas	CO	H2	CO2	N2	CH4	H2O	HCL	H2S	O2
Molar Distribution (%)	27.2	38.9	13.94	6.6	4.8	8.2	0.04	0.02	0.3

     In this system, our goal is not to produce energy from synthetic gas. Synthetic gas is burned in the afterburner system. Results of exhaust gas analysis after the afterburner system is given in Table 6. The results are indicative of the standard amounts of hazardous pollutants.

Table 6. Exhaust gas composition at the stack

Gas	SOx (ppm)	NOx (ppm)	CO (ppm)	Cl2 (ppm)	NH3 (ppm)	HCl (ppm)	F (ppm)	H2S (ppm)	Dust (mg/Nm3)
Molar Distribution (%)	0.1±0.1	9.8±2.2	7.9±2.8	0.03±0.02	1.3±0.5	0.3	0.9±0.3	0.02±0.01	11.1±1.3

 

4. Conclusions

     Thermal plasma technology is the best and most complete method for disposing of chemical and pharmaceutical wastes. In this method, both organic and inorganic wastes are paralyzed, gasified and melted to ensure their complete elimination and the least environmental pollution. The exhaust gas from the furnace is full of energy and can be recycled to increase the energy efficiency of the system. Also, with the provision of the combustion system, the amounts of dangerous exhaust gases are within the standard range.

Statements and Declarations

The authors did not receive support from any organization for the submitted work.

The authors have no relevant financial or non-financial interest to disclose.

 

 

 

 

 

 

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