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

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

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.

3

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, Iran4 5 6 7

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.

4

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.

5

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.

6

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.

7

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.

8

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

9

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.

10

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

11

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

12

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,

13

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

14

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.

15

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.

16

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 SO2 and 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

17

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

18

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

19

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,

20

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

21

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

22

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.

23

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.

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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.

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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

26

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.

27

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

28

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

29

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.9 The 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.

30

Figure 16 Mass of pyro gas generated at different temperature

31

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

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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

33

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.

34

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

35

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

36

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

37

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.

38

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.

39

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

40

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

41

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.

42

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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7 Reference

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