DEGREE PROJECT IN MATERIALS SCIENCE AND ENGINEERING, SECOND CYCLE, 30 CREDITS
STOCKHOLM, SWEDEN 2016
Pyrolysis of medical waste and the Pyro gas combustion system
SIYUAN SHUI
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
Abstract
This report reviews the different types of medical waste and associated medical waste generation data by geographic regions. Incineration methods and non-incineration methods, together with their associated technologies, are reviewed in detail. Among all the methods, pyrolysis technologies are, in principle, technically and politically attractive due to less pollution and toxic products emissions as compared to other methods (especially traditional incineration methods). In this report, the data are organized and analyzed from a series of pyrolysis tests carried out by KTH according to a technology concept developed by Bioincendia AB.
A combustion system for the pyro gas treatment is built based on the small-scale induction pyrolysis machine. The concept of the pyro gas combustion system is expressed through the block diagram and the boundary conditions are estimated according to the test data and the literature. The result of theoretical calculation indicates the boundary conditions of system are in reasonable range. The critical parameters of heat exchange unit increase the building of whole system.
1 Introduction and Objective ...1
1.1 Introduction ...1
1.2 Objective ...1
2 Review ...2
2.1 Medical Waste ...2
2.2 Medical waste treatment ...6
2.2.1 Incineration method ...6
Controlled air incinerator ...7
2.2.2 Non-incineration method ...7
Thermal Process ...8
Low-heat thermal Process ...8
Medium-heat thermal process ...10
High-heat thermal process ...10
Chemical process ...15
Chlorine based technology ...15
Non-Chlorine technology ...15
Irradiation process ...16
2.3 Waste gas control system ...16
2.3.1 SO2 and Acid gas cleaning system ...16
Wet scrubber ...17
Spray scrubber ...18
Ejector venturi gas scrubber ...19
Packed tower scrubber ...19
Dry scrubber ...20
2.3.2 Carbon monoxide cleaning system ...21
Low temperature oxidation ...21
2.3.3 Particulate cleaning system ...22
Fabric filter ...22
Electrostatic precipitator (ESP) ...22
Cyclonic separation ...23
2.3.4 Nitrogen oxide cleaning system ...23
2.3.5 Hydrocarbon cleaning system ...24
Distribute incinerator ...24
2.4 Pyrolysis end products and treatment ...25
2.4.1 Pyrolysis off-gas treatment ...25
2.4.2 Pyrolysis solid products and treatment ...25
2.4.3 Pyrolysis liquid products and treatment ...25
2.5 Summary of review ...26
3 A novel concept to treat medical waste ...27
3.1 Process of Bioincendia AB ...27
3.2 Experiment setup ...27
3.3 Experiment test data analyzing ...29
4 Pyro gas combustion system ...31
4.1 System boundary condition ...32
4.1.1 Adiabatic temperature ...32
4.1.2 Airflow rate ...34
4.1.3 Heat Recovery ...37
4.1.4 Water Cooling ...37
4.1.5 Mass of sodium hydroxide and water ...38
4.2 Summary of the pyro gas combustion system ...38
5 Discussion ...40
6 Summary and Conclusion ...42
7 Reference ...43
1 Introduction and Objective
1.1 Introduction
Nowadays, low-pollution processes and applications are in increased demand by industry and governments to reduce environmental impacts and to obtain economic benefits. The medical waste disposal industry is around the world and has a significant influence on the relevant industries and technology sectors the environment. In recent years, with the development of technology and the improvement of medical service, medical waste generation has continued increasing in most of the regions. However, in some regions especially the developing countries, the impropriate treatment of medical waste is not an unusual case (e.g. the mixing treatment with other general waste and open-bit burning).
The medical waste does not only contain the plastic that has a challenging in treating through a simple method but also contains the hazardous waste that is harmful to the human and other creatures. The pretty common and simple method adopted today is incineration due to its ability in decomposing different materials and destroy of organisms and pathogens. However, on the other hand, some pollution problems associated with incineration methods must be avoided or minimized. So other methods are developed to supplement or replace traditional incineration method and the autoclave method prior to their landfill.
1.2 Objective
The objective of this project is to review the relevant data of medical waste and different technologies of medical waste treatment and focus on the pyrolysis technology. A type of pyrolysis waste gas control system is described that matches an overarching concept developed by Bioincendia AB.
2 Review
2.1 Medical Waste
Analyzing the medical waste is the principle objective prior to deciding the specific type of medical waste treatment technology.
A large amount of hazardous medical wastes is produced by hospitals, nursing homes and research facilities daily. According to the World Health Organization (WHO), about 85% medical waste generated by health facilities is general waste while the remaining 15% is considered to be the hazardous waste that is toxic, infectious or radioactive.1 Medical waste is typically sorted into four categories according to their properties and different disposal regulations. They are general waste, infectious waste, chemical waste and low-level radioactive waste.
General waste always does not require special recycling treatment and disposal because of its non-harmful properties. However, infectious waste, chemical waste and radioactive waste could be included into hazard waste due to their potential danger and contamination to the environment and healthy risk to animals and humans. The hazardous waste always contains pathogenic organisms and toxic chemical therefore appropriate collection, classification and treatment are required. For example, the average amount of solid medical waste generated is 2.7kg per bed/day in Amsterdam and 2.5kg per bed/day in France.2 3 A medium-sized hospital could produce about 300kg solid medical waste per day.
The storage and the disposal of these medical wastes should be appropriate and timely in order to prevent contamination or infection. The general medical waste consists of different plastic, paper, glass, metal and other waste. Figure 1 shows the medical waste composition in four different hospitals in different regions. It can be seen from the figure that the different hospitals have different medical waste compositions.
However, the main components of medical waste are very similar and the plastic account for around half in total mass.
Figure 1 Medical waste composition in Sivas, Turkey, National Taiwan University Hospital, Phitsanulok, Thailand and Inisfahan, Iran4567
Besides the composition of medical waste, the generation rate is another typical parameters relate to medical waste disposal industry and the generation rate in different countries is showed in Table 2. It is obvious that the generation rate is not a constant value and the fluctuation can be observed due to several factors (e.g. the change of the policy and expansion of department).
The plastic is used widely in medical instrument because of its desirable properties.
However, the suitable treatments vary according to different plastic and unwanted products are easily generated when impropriate methods are selected. Thus the treatment for plastic medical waste always attracts much attention. The Table 1 shows commonly used plastic in medical instrument.
Table 1 Different types of plastic medical waste8 9 10
Typical plastic medical waste Harmful products after combustion/pyrolysis
Suitable treatment
methods
Application
PVC (Polyvinyl chloride) Methyl chloride, hydrogen chloride, benzene, etc.
Low-temperature plasma sterilization
Surgical gloves, Inhalation mask, etc.
PE (polyethylene) 1-Hexene, Carbon monoxide EtO and e-beam sterilization
Container and breather patches, etc.
PP (polypropylene) Carbon monoxide Autoclave Hypodermic
syringes PS (Polystyrene) Carbon monoxide, Polycyclic
aromatic hydrocarbons (PAH)
Gamma radiation, UV light sterilization
Flask and pipette
ABS (Acrylonitrile Butadiene Styrene copolymers)
Carbon monoxide, Hydrogen cyanide (HCN)
Gamma Radiation, Electron beam
Blood access device
PTFE (Polytetrafluoro ethylene) Ethylene Oxide
Table 2 Medical waste generations in different countries11
Country Hospital Bed Waste generation
(kg/bed/day)
USA - - 3-4.5
China - - 2.5-4
Sivas, Turkey Sosyal Sigortalar
Kurumu Hospital
362 2.6
China (Taiwan)
National Taiwan University Hospital
1180 2.8
With the development of economic, the range and the scope of the medical service should increase continuously. The relevant data is collected from the National Bureau of Statistics of China and showed in the Figure 2.
Figure 2 Number of health facilities and visits in China from 2008 to 201412
Figure 3 Medical waste generations in China from 2008 to 2015 12
The Figure 2 indicates the increasing trend of health facilities and the visits in China from 2008 to 2014, which grow by around 15000 and 4 billion every year respectively. With the increasing of health facilities and visits, the medical waste rises have averaged 6% every year that indicates in Figure 3. The medical waste generation reaches 200 billion in 2015. However, with the consideration of large population in China, the mass of medical waste is probably keep increasing in next few years.
2.2 Medical waste treatment
2.2.1 Incineration method
Incineration is always considered as the typical method of medical waste treatment due to the quite large mass and volume reduction of waste and the various type of medical waste can be treated. Generally, hospital incinerators deal with the medical waste and the final products are deposited at landfill sites.
Incineration means the combustion of medical waste. The incineration process can be grouped into low temperature incineration（300oC－400oC）, middle temperature incineration(800oC-900oC) and high temperature incineration(>1000oC) according to the heating temperature. Middle and high temperature-range incineration are always favored because of its reliable destructive effects of various organisms. However, some pathogenic organisms could still survive if the incineration is incomplete and that could cause the disease spread. Moreover, toxic pollutants are produced during the incineration process such as dioxins and furans when the airflow is insufficient or temperature is not high enough. High temperature incineration could handle most of the medical waste and destroyed them completely. However, this means more energy and fuel are required and high quality of the incinerator. The ash generated is also potentially hazardous which may cause water and soil pollution under the improper treatment situation (much literature has been generated on the generation, behavior and handling of medical fly ash in medical waste disposal). The possible environmental challenges associated with traditional incineration methods have prompted further research into non-incineration methods. With the improvement of health care service, more and more medical waste are generated. If there is a suitable and timely treatment of the medical waste in the hospital and research center, the efficiency and the safety will be improved. In order to increase the efficiency and flexibility, other non-incineration methods are developed which should be smaller and much easier to handle.
Controlled air incinerator
Controlled air incineration is widely used in hospital and research facilities nowadays to deal with various medial wastes such as injector, injection bag and general medical waste. The combustion process in the controlled air incinerator is a two-stage process.
In the first stage, the amount of air injected into the primary combustion chamber is under the required level. Therefore, the air-fuel ratio (AFR) is low in the first combustion chamber and most of the carbon burns. In the second stage, extra air is injected into the secondary combustion chamber with the volatile gases from the primary chamber. The combustion is completed in the secondary chamber and the resultant gas stream mainly contains carbon dioxide and water vapor. The gas temperature range in first chamber is approximately from 760oC to 980oC and the secondary chamber gas temperature is higher from 980oC to 1095oC. The feed capacity of the incinerator is adjustable (e.g. from 25kg/h up t0 1000kg/h intermittently or continuously according to the Verantis company).13
2.2.2 Non-incineration method
Non-incineration means there is no combustion process existed during the treatment of medical waste. Non-incineration methods are applied is because the control of harmful emission gas and residue solid are better than incinerator. Non-incineration methods can be sorted into a thermal process, chemical process, irradiative process and biological process according to the differences of their fundamental behaviors.
Some pre-treatment are applied such as mixing, compaction, shredding. This helps to reduce the volume and weight and make the waste more even in terms of composition for the follow-up treatment.
Thermal Process
Thermal process is based on heat application to treat the medical waste. They also could be divided into low-heat process, mid-heat process and high-heat process according to the energy supplied.
Low-heat thermal Process
Generally, the temperature range of low-heat thermal process is between 93oC and 177oC. Due to the rather low temperature, the combustion or pyrolysis process would not occur and the chemical properties of waste would be stable. Both steam and dry air can be used in low-thermal process and the typical technologies are autoclaves and hot air ovens respectively.
Autoclave
The autoclave is widely used nowadays in hospital and research center to disinfect the instrument or medical waste and the Figure 4 shows the autoclave for disinfection.
The water is heated to its saturation temperature and turn into steam. The saturation temperature of water depends on the pressure (e.g. the saturation temperature is 100oC at atmospheric pressure). The autoclave comprise an inside metal chamber and an outside steam jacket. Steam exists in both chamber and steam jacket in order to balance the high pressure. After the collection of medical waste, the metal chamber is pre-heated to the required temperature, and then waste is loaded into the chamber.
Checking the sealing before introducing the steam is necessary. The air should be removed in order to improve the efficiency of heat conduction and the removal of air could be realized through using a vacuum pump. After the steam sterilization, more time is needed to cool down the medical waste. Furthermore, some mechanical shredding may be applied in order to facilitate the follow-on treatment. In order to
prevent the harmful emissions, some hazardous waste should be separated (e.g. waste containing Hg). The post-process of emissions from some hazardous should be evaluated prior to the steam disinfection.
The advantages of the autoclave are cost-effective comparing to other non- incineration method and capability is high and well established. Although the type of autoclave is various, it is quite straightforward for the staff to handle. Because there is little change of the mass and the volume, the transportation and the storage of waste would still be a problem. The properties of waste would affect the efficiency of disinfection (e.g. materials with low thermal conductivity may need longer time to complete the disinfection process).
Figure 4 Autoclave for disinfection14
Hot air oven
The temperature control device of a hot air oven is a thermostat and the typical temperature range is between 50oC and 300oC. The inner layer and outer layer are made of different materials. The air between the layers facilitates the heat isolation.
There is a fan inside the oven helping the circulation of hot air. The hot air oven is much more safer and stable than autoclave due to the absence of water and low pressure inside. Although its size is smaller the efficiency is quite high. The drawback of hot air oven is the incomplete destruction of some organisms because of the absence of moisture sterilization.
Medium-heat thermal process
The temperature range of medium-heat thermal process is between 177oC and 371oC.
The range of medical waste can be treated of a medium-heat process is wider than low-heat thermal process such as sharps, plastic, glass and biological wastes.
During the medium-heat thermal process, microwaves supply the energy to break down the organic material. The process is called depolymerisation that means large molecules break down to small molecules here. Because the microwave energy is first absorbed by the inside part of medical waste and then spread out. The inside temperature of medical waste reach a high temperature but the outside of the waste keeps a lower temperature level. When the temperature is high enough, the combustion process could happen. So the N2 or other inert gases are introduced into the system to prohibit the combustion during the thermal process. During the medium-heat thermal process, there are some chemical reactions with the organic waste, but some wastes are chemical stable such as metal and glass. The off-gas of the medium-heat process may contain some light hydrocarbons and hydrogen chloride (HCl) that, in turn, can be eliminated through combustion and the use of an alkaline filter or solution.
High-heat thermal process
The temperature range of the high-heat thermal process is above 371oC and the temperature could go up to 8300oC or even higher. During the high-heat thermal process, the medical wastes are destroyed completely due to some chemical and physical reactions involved.
Pyrolysis process is considered as one of the non-incineration methods although the heating temperature of it is quite high. Pyrolysis can be defined as the decomposition
of organic material at elevated temperature and this process happens when the oxygen is depleted. Some chemical reactions are involved during the pyrolysis and the products vary such as glassy material, hydrocarbon and carbon residue. Except the products are different from incinerator, the final pollutants are at a lower level comparing to the incineration methods.
Plasma pyrolysis
Through breaking down atoms into electrons and ions, the plasma state is obtained.
By the means of plasma method, the temperature can reach 10000oC easily and quickly. This technology can treat both liquid and solid medical waste due to the high energy supplied. Furthermore, the organic chloride can be handled rely on the ultraviolet radiation. The basic part of this technology is the plasma torch that consists of water-cooled anode and cathode surrounded by magnetic field coil. The DC or microwave power source provide the energy and the nitrogen gas flow is introduced into troch for stabilizing the plasma arc. Because of the high resistance of conductive ionized gas, the electric energy is transformed to heat and the temperature range is above 1650oC. The Figure 5 indicates a schematic of commercial plasma system. The medical waste enters the system through the feeder and then reaches the primary chamber. The primary products enter the secondary chamber to finish the pyrolysis process.
After plasma pyrolysis, most of the medical are completely destroyed. Hydrogen and carbon monoxide are produced as byproduct and heat from the combustion of these gases can be recycled. Other toxic gases produced are under the limit. Another advantage of plasma pyrolysis is the complete destroy of the pathogenic bacteria under the elevated temperature and radiation condition.
According to the plasma pyrolysis system developed FCIPT of the Institute for Plasma Research, the electricity required per Kg of charge is approximate less than
1kWh. With other co-system addition, the cost is still quite low comparing to the other conventional waste treatment system in the market nowadays.
There is no doubt that plasma pyrolysis has a large potential to take the place of conventional incineration method. However, the extreme high temperature, complex chemistry and corrosion problem increase the difficulty of commercialization. There is no sufficient information of the small-scale plasma pyrolysis equipment for medical waste treatment in the market.
Figure 5 Schematic of Plasma pyrolysis15
Pyrolysis-Oxidation
This technology contains two steps. The medical waste is treated in the pyrolysis chamber first and then transported to the combustion chamber to complete the combustion process. During the oxidation process, some oxygen is added into the chamber as oxidizer. Post treatment of the off-gas is necessary so that pollution is effectively controlled. Because of the effective treatment and the control of waste gas, this technology is used commercially e.g. Bio-Oxidizer. Although the cost is pretty high, the potential of this technology cannot be ignored.
Induction based pyrolysis
When the properties of medical waste are evident or there is only one kind of waste,
the heating process can be adopted according to the medical waste properties.
However, the medical waste always contains different types of wastes and this is the reason why pyrolysis method is always adopted. Comparing to other heating process, the induction heating is always considered as the most optimal method due to its flexibility and high efficiency.
The current could be induced in a conductor when there is coil carrying alternative current couple with it. As a result, the magnetic field is created and the heat is generated with every drop of voltage. The schematic in Figure 6 illustrates the induction heating principle. A lager amount of heat can be generated when the current is high enough. The advantage of this technology is the heating process is pretty fast and the temperature control is precise and flexible. Decreasing the heating time to low temperature range (200oC-300oC) is beneficial to the control of dioxin. This technology has a high efficiency so it can be used to treat large amount of medical waste and work continuously. The Figure 7 shows a schematic of machine based on the induction heating principle that mainly consists of input unit, feeding unit and heating unit.
Figure 6 schematic of induction heating16
The operating mode of commercial pyrolysis system can be designed as batch or continuous. The batch mode is always adopted in smaller system i.e. lower capacity.
The initial investment of batch is quite low comparing to the continuous pyrolysis system. Only a small fraction of manufacturer provides the batch pyrolysis system to
the market nowadays, but it would be capable of the medical waste from hospital and research center.
Figure 7 schematic of induction-based pyrolysis technology17
The research from Paul T. Williams et al state the gas yield shows an increasing trend when the pyrolysis temperature and heat rate increase.18 Some research shows the similar effects of the temperature on the pyro gas generation e.g. when the temperature increases from 500oC to 800oC, the pyro gas composition in end products goes up to 96.5% from 5.7%.19
Advanced Thermal Oxidation
This technology is not processed under pyro lytic conditions which means pyrolysis process is not involved. Instead of pyrolysis process, combustion process is the main process involved. Unlike the normal incineration, advanced thermal oxidation needs the pretreatment of medical waste. The waste is always treated into small particles and injected into combustion chamber using high-speed vortex. The temperature range of advanced thermal oxidation is always higher than normal incineration so the efficiency is increased and the toxic products such dioxins and furans are better controlled.
Chemical process
Chemical treatment of medical waste can destroy the bacteria effective but the contact between chemical and medical waste is a premise. Therefore chemical based technology always contains a shredding and mixing system. The medical waste can be treated are various such as sharps, blood, body fluid and surgery waste. The appropriate treatment is always necessary when use the chemicals although some are harmless to human.
Chlorine based technology
Chlorine based method are used in hospital and research center to disinfect the infectious waste or reusable instrument. Chlorine and sodium hydroxide react with water produce sodium hypochlorite (NaOCl) that is normally used for disinfection and the chlorine dioxide (ClO2) is alternative chemical commonly used. However, several disadvantages go against the availability in medical waste treatment. The consumption of chemicals lead to regular supplement and the store and the usage of hazardous chemicals increase the risk. This technology is supposed to care more on operation due to the danger of chemicals i.e. skin and eyes injury.
Non-Chlorine technology
The non-chlorine system can be various due to the different types of chemicals they rely on such as ozone (O3), alkali solution and solid calcium oxide (CaO). The main advantage of the non-chlorine technology is that the products are harmless i.e. no dioxins or toxic chlorine compounds generated. However, some chemicals still do harm to human body. Therefore appropriate storage and usage are necessary.
Irradiation process
The electron beams or UV irradiation are applied in irradiation process and the advantage of it is complete destruction of pathogens and microorganisms. In order to ensure the disinfection efficiency, the pretreatment e.g. shredding is needed. The disadvantage of this technology is the volume and mass reduction at a low level.
2.3 Waste gas control system
Before decide an air pollution control system, some factors ought to be considered such as required elimination efficiency, original pollutant concentration, capacity of the system and appropriate cost.20 The basic idea to increase the elimination efficiency is to enlarge the contacting area and increase the liquid-gas ratio. When the elimination process is mainly chemical absorption, the scrubbing liquid selection is the priority. The high absorption efficiency, low viscosity and low cost are favored properties of solvent. Among different flow direction, the countercurrent of gas and scrubbing liquid is considered the most appropriate way because its high theoretical elimination efficiency. When the countercurrent is settled the liquid-gas ratio could be lower comparing to other methods under the same condition.
2.3.1 SO
2and Acid gas cleaning system
Sulfur is always used as primary vulcanizing in medical gloves production and this is the main source of sulfur dioxide after pyro gas combustion. There is an estimation of 0.2% sulfur contains in normal medical waste.21 Most of the HCl comes from the degradation of PVC. Because of the existence of acid gas e.g. HCl; the elimination of the acid gas is necessary in order to prevent the equipment corrosion and environment problem. Some plastic pyrolysis research shows the HCl gas has a lower concentration when the pyrolysis temperature at an elevated level. However, the
pyrolysis temperature is always decided by the efficiency. Although the temperature range may not favor reducing the HCl concentration, other methods should be adopted to eliminate the HCl. The HCl gas elimination process could be done either before the combustion or after the combustion. If the alkaline solution is used to remove the HCl, the drying process should be done after the removal in order to prohibit the water from entering the combustion chamber. In addition, the gas should be preheated in order to improve the efficiency and productivity. The additions of preheat and drying process definitely increases the investment and reduces the efficiency. However, it could avoid the corrosion risk from the HCl gas. If the HCl gas amount is within the acceptable range, the removal of HCl gas is preferable after the combustion process that means lower cost and higher efficiency.
Although the concentration of HCl showed in experiment is not high, the accumulation of it in the real operation also could lead to corrosion and healthy problem. HCl gas has an Immediately Dangerous to Life and Health (IDLH) concentration of 50 ppmv with an OSHA Permissible Exposure Limit (PEL) of 5 ppmv. Therefore periodic replacement of alkaline solution or sodium carbonate solid is necessary. Other HCl gas elimination methods are developed recently (e.g.
CO3·Mg−Al LDH shows a high efficiency in incinerator steam treatment).22
Wet scrubber
The water without any chemical additions shows an average elimination efficiency of 60% and 30% of hydrogen chloride and sulfur dioxide respectively. The removal efficiency will increase dramatically with the addition of neutralizer (e.g. calcium hydroxide). The gas flow rate and the solution flow rate will also have an influence on the HCl removal efficiency. It shows that the percentage removal of HCl decreases with the gas flow rate and increases with the liquid flow rate.23 However, the appropriate range of these parameters ought to be determined on the basis of actual
full-scale trials and actual operation.
Spray scrubber
The liquid droplets fall from the top enter the tower and contact the gas at the bottom of reaction vessel. The structure of spray tower is quite simple compared to other methods and thus the cost of it is lower. Although the efficiency of it is lower compared to other methods but it is still used commonly in many cases and fulfill the basic efficiency requirement. The Figure 8 shows the basic concept of spray tower.
The waste gases enter from the bottom of the tower and the water always with the addition of other chemicals e.g. calcium hydroxide, in order to increase the removal efficiency, is sprayed from the top i.e. counter flow. Another advantage of the spray tower is decreasing the temperature of waste gas. Water recycling system is always installed inside the spray tower in order to decrease the cost.
Figure 8 Schematic of spray scrubber24
Ejector venturi gas scrubber
The ejector venturi scrubber is a commonly used wet scrubber in air pollution control
and abatement processes. It is always designed for large furnaces. However, improvements can still be carried out for optimization in small size incinerators. The merit of ejector venturi scrubber is the lack of a fan or blower by taking advantage of high velocity scrubbing liquid to transport the waste gases. The removal efficiency of an ejector scrubber system can exceed 90%. The Figure 9 shows a schematic of ejector venture gas scrubber.
Figure 9 Ejector venturi gas scrubber25
Packed tower scrubber
The packed tower scrubber is a kind of wet scrubber designed according to the countercurrent principle and Figure 10 shows the principle of packed tower. The gas enters the tower from the bottom and contact the liquid from the top and the packing increases the contact area between liquid and gas. The liquid absorbing the waste gas and leave the tower from the liquid drain at the bottom. The following process depends on the off-gas emission requirement and it is always used cooperatively with particulate cleaner in the end of the process.
When an appropriate packing material is used, the removal efficiency of a packed tower can reach 99.9%. The packed tower can handle a strong gas flow fluctuation,
from 0 to a maximum value. This is useful in case of an emergency. On the other hand, the cost of a packed tower is much higher as compared to the spray method.
The packing bed material has a defined service life and in order to ensure the cleaning efficiency, the packing material therefore requires periodic maintenance.
Figure 10 Packed tower scrubber26
Dry scrubber
Dry scrubber use alkaline solid power instead of liquid to neutralize the acid gas (e.g., CaO power). The alkaline powders are injected with the gas carriers into a chamber that has a particulate elimination system. The fabric filter is inside the chamber to increase the contact time and area between the powder and gases and control the PM.
The dry scrubber is commonly used in some developed countries with high standard industrial systems (e.g., Japan to neutralize the acid gases). The solid waste after elimination is recycled, while the remainder of the treated materials is taken to landfills. The whole process is likely to consume more time compared to other methods .If the design for powder distribution is sub-optimal, part of the powders may fail to react with acid gas and a chemicals recycling process is therefore necessary in order to reduce the cost. An example of SO2 gas elimination in dry scrubber is indicated in Figure 11 and the possible reaction in reactor is Eq.1. Some CaSO3
product reacts with oxygen in the reactor and generates CaSO4. The cost of reagent may have a significant difference (e.g. the lime’s cost is several times the limestone’s cost). The selection of reagent powder therefore depends on the efficiency requirement and estimate of cost.
Figure 11 circulating dry scrubber26
Ca(OH)2 + SO2 = CaSO3 + H2O Eq.1
2.3.2 Carbon monoxide cleaning system
Low temperature oxidation
The low temperature oxidation method has been used to eliminate carbon monoxide in many different systems such as automobile exhaust and enclosed system.
The catalyst selection is critical when the low temperature oxidation of waste gas is considered. A noble metal catalyst always has the better activity, stability and longer lifetime. Using a noble metal catalyst e.g. Au during the low temperature oxidation may fulfill the requirement, but it definitely will increase the cost. Identifying an appropriate catalyst without noble metals used in low temperature oxidation of waste gas is quite challenging. The catalyst selection will increase the difficulty of low temperature oxidation method. Both the temperature and the moisture composition
also affect the activity and stability of the catalyst so that increases the difficulty in oxidation operation.
The flow rate of gas has a significant effect on the oxidation efficiency of pyrolysis gas. Therefore finding an appropriate balance point between the time and efficiency is quite necessary. Due to the difficulty in analyzing the kinetic data, the assessment of the low temperature oxidation process is sufficient reliable.
2.3.3 Particulate cleaning system
Fabric filter
Fabric filters are contained within specially designed individual filter bags that capture the particulate when waste gas containing particulate pass through the filter.
The fabric filter has a quite high efficiency to eliminate the particulate as compared to other methods. In addition to the high particulate removal efficiency, fabric filters could be used to capture fine particulates. The periodical cleaning of filters is necessary due to particulate accumulation.
Electrostatic precipitator (ESP)
The electrostatic precipitator (ESP) has a quite high efficiency when remove the fine particulate. The basic concept of electrostatic precipitator (ESP) is showed in the Figure 12. The basic ESP consists of thin vertical wires and metal plates. There is a negative voltage between the wires and metal plates and the particles pick up a negative charge. When the particles enter the zone between collecting plates, the particles are attracted to the collecting plates.
Figure 12 Schematic of electrostatic precipitator27
Cyclonic separation
The Cyclonic removal of particulates makes use of vortex separation instead of fabric filters. Comparing with ESP and fabric filters, the removal efficiency of cyclonic separation is lower. There is a critical size of the particulates above which the system has a quite high efficiency while smaller particulates do not have desired removal result. The small cyclonic separation system is widely used today and its cost is similar with other methods.
2.3.4 Nitrogen oxide cleaning system
The nitrogen oxide in the waste gas might be from two different ways: nitrogen element in the waste or the combination between nitrogen and oxygen at elevated temperature. Selection of nitrogen oxide control system is decided by the amount of generation and the removal efficiency requirement.
2.3.5 Hydrocarbon cleaning system
Distribute incinerator
COSTAIR technology
The COSTAIR technology is based on the continuously staged air to obtain a stable combustion in combustion chamber and lower nitrogen oxide emission. H. Rahms, et al. proposed a strategy to take advantage of the low caloric gases using the COSTAIR method.28 The Figure 13 indicates the basic concept of COSTAIR. The air is injected by the tube in the middle and then distributed by the porous distributor. The fuel gases from the gas inlet ring are injected near the air distributor through the gas nozzles.
Figure 13 Schematic of COSTAIR combustion29
Combustion of waste gas would be more cost effective and relatively easier to handle when compared with the low temperature oxidation. On the other hand, the gas products from the pyrolysis process include hydrogen, carbon monoxide and light hydrocarbon—all of which possess high heat values. When the medical waste only consists of polyethylene, the combustible gas generated from the plasma pyrolysis account for more than 70% of the total volume. Although the LHV of the gas mixture is low as the nitrogen composition is pretty high. The utilization of the heat through the combustion process still could be used to preheat the supplied air and aid the pyrolysis process of medical waste. In order to make sure the complete combustion, enough time, space, turbulence and temperature must be provided.
2.4 Pyrolysis end products and treatment
2.4.1 Pyrolysis off-gas treatment
After the pyrolysis process, some gases are produced such as CO, H2, CH4, CO2 and HCl. A proper treatment of these gases could have energy recycling benefits, as well as helping to prevent air pollution. Two possible methods could be used to treat the pyro gas: low temperature oxidation and combustion. Some gases are combustible such as carbon monoxide, methane, hydrogen and other hydrocarbons. Therefore, the combustion method could recycle the heat. The heat generated can be used as a partial energy source supplying the pyrolysis process and thus decreasing the cost.
2.4.2 Pyrolysis solid products and treatment
The principal solid products of medical waste pyrolysis are carbon black and in order to prevent the accumulation of it in the chamber, quartz sand can be placed at the bottom of chamber and removed after several cycles. Because of stability of the quartz sand within the pyrolysis temperature range and the low cost, the practicality and flexibility of gathering solid pyrolysis products is enhanced. After the collection, the sand can be disposed in landfill. Besides the sand addition method, water could also be injected into the chamber to remove the solid residual.
2.4.3 Pyrolysis liquid products and treatment
Research by Qiang Lu, et al. indicates that liquid products yield of cellulose increase with the pyrolysis temperature between 400oC and 700oC.30 Research by Williams shows the liquid products yield of plastic mixture reduces with the pyrolysis temperature between 550oC and 700oC while the gas products amount increases all the way. It is therefore reasonable to hypothesize that the liquid products break down
and form the gas products at elevated temperature. The liquid product is a mixture of useful substance and it can be collected to use as the fuel. The direct combustion of liquid and gas products is desired in medical waste treatment because of the simplified process and heat recycling.
2.5 Summary of review
The increasing trend of medical waste generation indicates the importance of developing of medical waste treatment technologies, especially those with fewer pollution control and abatement challenges.
Compared with the incineration method, the non-incineration methods have the advantages of less pollution and similar efficiency. Among the non-incineration method, the pyrolysis process indicates a large potential in dealing with medical waste. Perhaps the main component of chemical reaction importance of typical medical waste is plastic and this is a major reason why pyrolysis is suitable. Although the pyrolysis could not reduce little volume of glass and metal, it is still an appropriate choice. Most of the metal medical waste can be recycled (and excluding single-use medical consumables) that could be disinfected through other methods.
Some pyrolysis-based technologies are already on the market (e.g. plasma pyrolysis) and some are still been developed (e.g. induction-based pyrolysis). Usability enhancement of the pyrolysis-based technology attracts more attention i.e. decrease the size of the system so that it could be used in hospital and research center.
3 A novel concept to treat medical waste
3.1 Process of Bioincendia AB
Bioincendia AB developed a novel concept of treating medical waste by means of thermal process. This concept takes the advantage of pyrolysis process to convert hazardous and dangerous medical waste into low risk general waste. The medical waste contains different materials and this method avoids classification process to some extent. Different kinds of medical waste are gathered and fed into the pyrolysis chamber. The main process is pyrolysis and it is taken in a medium temperature range due to the cost and efficiency considerations. After the pyrolysis process, the products are gathered and cleaned by the cleaning unit. The aim of Bioincendia AB is to enhance the usability of medical waste treatment method, so the size and the efficiency are prior aspects. Unlike the medical waste treatment at a facility scale, the concept from Bioincendia AB is trying to introduce the machine into the hospital and medical workspaces. The data from some hospitals indicates the medical waste generation is usually under 5kg/bed/day and a small hospital that have beds less than 100 could generate medical waste no more than 200kg per day. Compared to traditional methods of treating medical waste, the small machine used inside the hospital provides greater convenience and some safety advantages. Evaluating the possibility of the concept from Bioincendia AB could start from the test with different parameters in the assumptive range and the test could be done through the KTH pyrolysis unit.
3.2 Experiment setup
According to the concept developed by Bioincendia AB, the test has been done by the KTH to study how the temperature affects the pyrolysis process of typical medical waste.31 The schematic of apparatus used in the experiment is showed in the Figure 14
and different parts of the apparatus are illustrated.
Figure 14 Schematic of the apparatus
1.N2 supply 2.Gas regulator 3.Flow meter 4.Three way valve 5.Reactor 6.Sample and mesh support 7.Heater 8.Insulation 9.Gas washing unit 10.Cooling system 11.GC 12.Data recording unit
13.Thermocouple31
The nitrogen supplement unit is to ensure the deficiency of oxygen environment and transport the pyrolysis gas and liquid product. The medical waste sample is placed inside the crucible and heated to the aim temperature by heating unit. Before entering the GC unit, the acid gases in the pyrolysis products are removed through the gas- washing unit in order to avoid the corrosion problem. The GC is used to detect the pyro gas every three minutes.
The typical medical waste is analyzed and the medical waste sample with similar composition is used in test and the composition is shown in Table 3.
Table 3 Medical waste compositions
PVC HDPE LDPE PP Latex Paper Metal Glass
Wt% 3.3 20.9 20.9 27.6 13.3 2.0 1.2 10.7
3.3 Experiment test data analyzing
According to the data from KTH experiment, the pyrolysis products composition at different pyrolysis temperature is shown in Figure 15.
Figure 15 Products composition of medical waste pyrolysis at different temperature
When increasing the pyrolysis temperature, the gas product indicates a growing tendency probably because liquid molecules tend to break down at higher temperature and form smaller gas molecules.9The solid composition does not show an apparent difference at different temperature.
The detector of the research could detect the mass of H2, CH4, CO, C2H4, C2H6, C3H6
and C3H8 gases. The total amount of gas products during the pyrolysis at different temperature is showed in Figure 16. It can be seen from the Figure 16 that the generation rate of gas products is not a constant value while its trend is similar with the normal distribution. It also clearly indicates the increasing generation rate of gas products with increasing temperature. Both temperature and time have an influence on the pyro gas generation, while the temperature has a more significant effect on the pyro gas generation.
Figure 16 Mass of pyro gas generated at different temperature
4 Pyro gas combustion system
One of the main tasks of this project is to propose a scenario of the pyro gas treatment system. The aim of the system is to develop a pyro gas recycle unit to match the concept developed by Bioincendia AB.
The combustion unit is adopted as the main process in this system. Among different treatments of waste gas, the combustion method is suitable in this case due to the composition of pyro gas and simplicity of combustion process. In order to match the concept of Bioincendia AB, the pyro gas treatment system should also consider the usability and the efficiency.
The pyro gas combustion process is shown in Figure 17. The pyro gas combustion system consists of combustion unit, heat recovery unit and air pollution control unit.
Figure 17 Pyro gas combustion system
The pyro gas produced by pyrolysis enters the combustion chamber and the preheated air aid the combustion. After combustion, the heat recovery unit recycles the heat to
preheat the air. The water-cooling system is adopted before the spray to cool down the waste gas. A spray tower is used to eliminate the acid gases and cool down the waste gas further. Because the pyro combustion system is based on the small pyrolysis machine, the unit in the system should be simplified as much as possible.
4.1 System boundary condition
In order to establish the boundary condition of pyro gas combustion system, some assumptions are made according to the research data. The 530oC of pyrolysis temperature is selected due to the composition of medical waste. The main component of medical waste is plastic and the temperature range of decomposition of different plastic showed in the Figure 18 indicates a high decomposition rate between 500oC and 600oC. The higher pyrolysis temperature the more cost needed, so 530oC is an appropriate selection here to establish the boundary condition of pyro gas combustion system.
Figure 18 The mass reduction of different plastic at different temperature 17
4.1.1 Adiabatic temperature
In order to estimate the adiabatic temperature of pyro gas mixture, the estimation of
every kind of gas could be done first and make the estimation of gas mixture through weighted a The Cantera software is used for calculations. The initial condition assumed is 300oC, 1 atm and stoichiometric air-fuel ratio. The adiabatic temperature and composition of every gas component is calculated and showed in the Table 4.
Table 4 Composition and adiabatic temperature of pyro gas
H2 CH4 CO C2H4 C2H6 C3H6 C3H8
Composition (wt%)
0.7 13.2 3.4 22 16.5 44 0.2
Adiabatic
temperature (oC)
2237 2081 2103 2205 2110 2107 2116
So the adiabatic temperature of pyro gas mixture could be estimated from the Eq.2 and the result is 2126oC.
Ta=wt1%*T1+ wt2%*T2+ wt3%*T3+ …... +wtn%*Tn Eq.2
Assume the heat loss of combustion chamber is 10%, the maximum flame temperature is estimated as 1913oC.
Except the gas composition, the initial temperature also has an influence on the adiabatic temperature. Since the initial temperature of the pyro gas is not a constant value, the estimation is taken in the range of 300oC to 400oC. Without changing the gas composition, the estimation result is showed in Figure 19. The diagram indicates that the adiabatic temperature of waste gas increases with the initial temperature of pyro gas.
Figure 19 Adiabatic temperature of waste gas with different initial temperature
4.1.2 Airflow rate
Before the combustion of pyrolysis gas, the mixing of the air and pyrolysis gas should be done so as to increase the oxygen composition.
Due to the change of gas products generation, the airflow rate in the following combustion process is not a constant value either. Assume the air consist of 21% O2
and 79% N2 at standard condition. The stoichiometric air-fuel ratio of pyro gas is calculated and showed in the Table 5.
Table 5 stoichiometric air-fuel ratios
Fuel Stoichiometric air-fuel ratio (air m3/fuel m3)
Stoichiometric air-fuel ratio (air kg/fuel kg)
H2 2.38 34.32
CH4 9.53 17.18
CO 2.38 2.45
C2H4 14.28 14.71
C2H6 16.66 16.02
C3H6 21.42 14.71
C3H8 23.81 15.61
According to the stoichiometric air-fuel ratio in the Table 5 and the mass of pyro gas detected at 530oC. The mass of air provided to the combustion process is calculated and showed in the Figure 20. The mass of air needed shows a similar trend to the pyro gas generation line.
Figure 20 Air mass for pyro gas generated at 530oC
If the oxygen composition is under a certain level, the combustion of pyro gas is incomplete and the emission gas may contain undesirable products and that is why Air-to-Fuel ratio is significant. In real combustion process, excess air is always required to ensure the sufficient burning. Most of the incinerator shows an appropriate balance between the efficiency and energy loss when extra 5%-20% air is supplied.32 Adjustment of the airflow to match the requirement is necessary with the aid of flow meters. However, increasing the airflow will cause the energy loss from the exhaust stack and this is significance of managing airflow.
In order to establish the boundary condition of pyro gas combustion system to match the real case, the mass of medical waste disposed is assumed 25kg per batch and total amount is 100kg per day. The medical waste also has the same composition with the previous research in Table 3, so the data could be enlarged proportionally.
The amount of air provided to the combustion process is showed in the Figure 21. The
highest point is chosen as the boundary condition and the air mass flow rate for the pyro gas combustion ṁair1 can be calculated from the Eq.3 and the AFR is the air fuel ratio. The calculation result is 5kg/min.
ṁair=ṁfuel*ΣiAFR*wt% Eq.3
Figure 21 Air mass for pyro gas combustion at 530oC
The combustion process also contains the liquid product combustion, but the previous research does not include the mass generation data of liquid products. In order to estimate the air needed for the liquid products combustion, the mass conservation is considered although there might be some errors. The medical waste composition is showed in the Table 3. Except the PVC, PP and PE, the composition of other materials is unknown. In order to simplify the estimation, the latex is assumed contains 70wt% polybutadiene and paper content is neglected.
Through the mass conservation, the molecular formula of liquid products is estimated as C67H120. According to the Eq.4, the stoichiometric air-fuel ratio (kg/kg) of liquid products is estimated as 16. Thus the air mass flow rate for the liquid products is
estimated as ṁair2 3.8kg/min. The total mass flow rate of air is ṁair1+ ṁair2 equal to 8.8kg/min.
CxHy+(x+y/4)O2 = xCO2 + y/2H2O Eq.4
4.1.3 Heat Recovery
In order to reduce the cost as much as possible, the heat from the exhaust gas is possibly used to preheat the combustion air. The combustion air preheat could be realized through the heat exchanger. The heat recovery efficiency between 30%-90%
is common in different heat exchanger. The cold air inlet and outlet temperatures are estimated as 25oC and 400oC respectively. The waste gas inlet and outlet temperature are estimated 1913oC and 700oC respectively. Assume the heat exchanger adopt the counter flow method. The Log mean temperature difference is calculated by Eq.5.
The ΔT1 is the temperature difference of inlet and ΔT2 is the temperature difference of outlet. The ΔTLM is estimated as 1038oC.
ΔTLM =(ΔT1-ΔT2)/ln(ΔT1/ΔT2) Eq.5
4.1.4 Water Cooling
The waste gas passing the heat exchanger still has a high temperature and not allowed to enter the spray tower as it may cause corrosion problem. In order to decrease the temperature of waste gas, water-cooling system is adopted. The waste gas enter the water cooling system is estimated as 700oC according to the estimation of heat exchanger. After the water-cooling system, the temperature is decreased to around 100oC.
4.1.5 Mass of sodium hydroxide and water
The data in the precious test does not include the hydrogen chloride and sulfur dioxide gas, so assume all the chlorine and sulfur elements (0.7%wt in latex) in medical waste form the HCl and SO2. The reactions between acid gases and reagent are Eq.6 and Eq.7. Assume all the acid gases are neutralized by the sodium hydroxide.
The 25Kg medical waste could generate 92.8g SO2 gas and 479g HCl gases, so the total mass of sodium hydroxide is 590g.
HCl + NaOH = NaCl + H2O Eq.6
SO2 + 2NaOH = Na2SO3 + H2O Eq.7
The concentration of sodium hydroxide solution to remove acid gas is various and determined by the specific case, so 5% weight is assumed here when considering the system efficiency. The mass of water used in spray tower is estimated as 11.8kg to treat the waste gas from pyrolysis of 25kg medical waste.
4.2 Summary of the pyro gas combustion system
Considering all the estimation and calculation of the system boundary conditions, the system is indicated in Figure 22.
The heat recovery unit is this system decreases the cost through recycle the heat to preheat combustion air. According to the estimation of boundary conditions in heat recovery unit, the Log mean temperature difference is a quite large value, which means the selection of the heat exchanger is critical.
Because there is a lack of data of similar device, the evaluation of mass consumption is impossible here.
Figure 22 Schematic of pyro gas combustion system
5 Discussion
The theoretical calculation is used to design the pyro gas combustion system based on the experimental data from KTH. Pyro gas combustion system consists of three main units and boundary conditions of every unit are calculated to develop the system.
Because using pyrolysis treating medical waste and pyro gas combustion system is a quite novel concept, it is quite difficult to find similar equipment in the market and make a comparison. Considering the boundary condition of temperature, the calculation results such as 100oC in water-cooling unit and 25oC in spray tower unit is in a reasonable range. The inlet and outlet temperature of waste gas in heat exchanger is 1913oC and 700oC and this temperature difference is quite large. The calculation result of mean temperature difference also shows a quite large value and this means very few heat exchangers meet the requirement and more cost in developing this unit.
Heat exchanger unit plays a vital role in the whole system because it is used to recycle the heat and decrease the temperature of waste gas further for the next cooling step.
Although the calculation result indicates the difficulty in selecting heat exchanger and may increase the cost, removing of this unit ought to consider efficiency, space, cost and other factors in the real case. The water and chemicals consumption indicates a pretty low level and match the concept of low cost. After adding the recirculation unit, the consumption of water and chemicals is likely to decrease to a lower level.
From social and ethical aspect, the treatment of medical waste has certain significance. With the economic growth of many developing countries, especially the countries with large population e.g. China, the amount of medical waste increases fast in these years. In most of developing countries, there is a lack of safe and effective treatment of medical waste. The common method is landfill and combustion and these methods are potential dangerous. Some medical wastes are recycled as normal plastic and metal without correct classification and this may be harmful to human and animals because these medical wastes may carry pathogenic bacteria. Through
pyrolysis method, especially the development of small and medium scale equipment, it would simplify the classification, storage and treatment process of medical waste and improve the safety.
6 Summary and Conclusion
Through the review of medical waste, the different regions show a different composition and generation rate of medical waste, especially between developing and developed countries. The generation of medical waste is very likely keeping increasing in next few years.
The pyrolysis method shows more advantages than incineration method in medical waste treatment and the pyrolysis technology probably, to a degree, supplant incineration method in the future.
Although the plasma technology is already on the market, its cost is not suitable to the facilities with small amount of medical waste treatment target. The induction-based pyrolysis indicates a large potential in the market in term of small pyrolysis machine.
The pyro gas combustion system established by this project could fulfill the basic requirement of a small pyrolysis machine and match the concept developed by Bioincendia AB.
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