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Type of Waste to be treated
Source Separated Organic Waste
Source Separated Packaging
Mixed MSW
Min Capacity (in tonnes)
>5.000
>12.500
>50.000
ALL
Secondary Products or Energy Production
FE
AL
Plastic
Paper
Glass
Wood
RDF
SRF
Compost
Biogas
Max Residues (%)
0-10%
10-20%
20-30%
30-40%
Capital Expenditure € tn of total feedstock on wet basis
0-100
101-200
301-400
601-700
Annual operation and maintenance cost € tn of total feedstock on wet basis
21-40
41-60
61-80
81-100
Land requirement m2 per tn of total feedstock
0-0.2
0.21-0.4
0.41-0.6
0.61-0.8
0.81-1
Water consumption m3 per tn of total feedstock
0-0.05
0.051-0.1
0.11-0.15
Plasma Gasification Technology PDF Print E-mail
1.  General
2.  Mass and energy balances
3.  Market potential for products
4.  Environmental impacts
5.  Economic data
6.  Applicability in the target area
References

1. General

Plasma refers to every gas of which at least a percentage of its atoms or molecules is partially or totally ionized. In a plasma state of matter, the free electrons occur at reasonably high concentrations and the charges of electrons are balanced by positive ions. As a result, plasma is quasi-neutral. It is generated from electric discharges, e.g. from the passage of current (continuous, alternate or high frequency) through the gas and from the use of the dissipation of resistive energy in order to make the gas sufficiently hot. Plasma is characterized as the fourth state of matter and differs from the ideal gases, because it is characterized by ‘collective phenomena’. ‘Collective phenomena’ originate from the wide range of Coulomb forces. As a result, the charged particles do not interact only with neighboring particles through collisions, but they also bear the influence of an average electromagnetic field, which is generated by the rest charges. In a large number of phenomena, collisions do not play important role, as ‘collective phenomena’ take place much faster than the characteristic collision time (Blachos, 2000).

 

Plasma Technology can be used as a tool for green chemistry and waste management (Mollah et al., 2000). Thermal plasmas have the potential to play an important role in a variety of chemical processes. They are characterized by high electron density and low electron energy. Compared to most gases even at elevated temperatures and pressures, the chemical reactivity and quenching rates that are characteristic of these plasmas is far greater. Plasma technology is very drastic due to the presence of highly reactive atomic and ionic species and the achievement of higher temperatures in comparison with other thermal methods. In fact, the extremely high temperatures (several thousands degrees in Celsius scale) occur only in the core of the plasma, while the temperature decreases substantially in the marginal zones (Gomez et al., 2009).

 

Five distinct categories of processes are used as the basis for the plasma systems catering for waste management (Juniper, 2006). These are:

  • Plasma pyrolysis (Huang & Tang, 2007; Sheng et al., 2008)
  • Plasma combustion (also called plasma incineration or plasma oxidation)
  • Plasma vitrification
  • Plasma gasification in two different variants (Malkow, 2004)
  • Plasma polishing using plasma to clean off-gases

 

Plasma gasification is the most common plasma process. It is an advanced gasification process which is performed in an oxygen-starved environment to decompose organic solid waste into its basic molecular structure. Plasma gasification does not combust the waste as incinerators do. It converts the organic waste into a fuel gas that still contains all the chemical and heat energy from the waste. Also, it converts the inorganic waste into an inert vitrified glass (Moustakas et al., 2005; Moustakas et al., 2008).

 

Mixed solid waste is shredded and fed into a reactor where an electric discharge similar to a lightning (the plasma) converts the organic fraction into synthesis gas and the inorganic fraction into molten slag. Typically temperatures are greater than 7,000°F achieving complete conversion of carbon-based materials, including tars, oils, and char, to syngas composed primarily of H2 and CO, while the inorganic materials are converted to a solid, vitreous slag. The syngas can be utilized in boilers, gas turbines, or internal combustion engines to generate electricity while the slag is inert and can be used as gravel.

The advantages of the process include: Good environmental performance, production of about 400 KWh net of electricity per tonne of waste treated, no by-products going to landfill.

 

The disadvantages of the process include: Relatively high cost, high level of maintenance and skilled labor required for operations.

Electricity is fed to a torch, which has two electrodes, creating an arc. Inert gas is passed through the arc, heating the process gas to internal temperatures as high as 25,000 degrees Fahrenheit. The following diagram illustrates how the plasma torch operates.

The temperature a few feet from the torch can be as high as 5,000-8,000oC. Due to these high temperatures, the input waste is completely destroyed and broken down into its basic elemental components. At these high temperatures all metals become molten and flow out the bottom of the reactor. Inorganics, such as silica, soil, concrete, glass, gravel, etc. are vitrified into glass and flow out the bottom of the reactor. There is no ash remaining to go back to a landfill and the produced vitrified residue called slag is the only material that can end up at landfills if no suitable markets (e.g. as construction material) are found for that.


The plasma technology is flexible, since it can be used for the thermal treatment of a variety of waste streams. The only variable is the amount of energy that it takes to destroy the waste. Consequently, no sorting of waste is necessary and any type of waste, except nuclear waste, can be processed.


The plasma reactor operates at a slightly negative pressure, meaning that the feed system is simplified, because the gas does not want to escape. The gas has to be pulled from the reactor by the suction of the compressor. Because of the size and the negative pressure, the feed system can handle bundles of material up to 1 meter in size. This means that sizeable waste can be fed directly into the reactor and pre-processing of the waste is not needed. Also, the performance of the plasma gasifier is not affected by the moisture of the waste (during incineration, the moisture of waste consumes energy to vaporize and can impact the capacity and economics of the process) (WASTESUM, 2006).

 

An indicative list of initiatives to apply plasma technology in the field of waste treatment is given in the table below.

Commercial Plasma Waste Processing Facilities (Circeo, 2007)

Location

Waste

Capacity (TPD)

Start Date

Mihama-Mikata, JP

MSW/WWTP Sludge

28

2002

Utashinai, JP

MSW/ASR

300

2002

Kinuura, JP

MSW Ash

50

1995

Kakogawa, JP

MSW Ash

30

2003

Shimonoseki, JP

MSW Ash

41

2002

Imizu, JP

MSW Ash

12

2002

Maizuru, JP

MSW Ash

6

2003

Iizuka, JP

Industrial

10

2004

Osaka, JP

PCBs

4

2006

Taipei, TW

Medical & Batteries

4

2005

Bordeaux, FR

MSW ash

10

1998

Morcenx, FR

Asbestos

22

2001

Bergen, NO

Tannery

15

2001

Landskrona, SW

Fly ash

200

1983

Jonquiere, Canada

Aluminum dross

50

1991

Ottawa, Canada

MSW

85

2007 (demonstration)

Anniston, AL

Catalytic converters

24

1985

Honolulu, HI

Medical

1

2001

Hawthorne, NV

Munitions

10

2006

Alpoca, WV

Ammunition

10

2003

2. Mass and energy balances

In general, the mass and energy balances have similarities with the respective ones referring to gasification. A typical energy balance assumes that from one tonne of waste treated more than 400 KWh net electricity is produced.

3. Market potential for products

There are a number of applications for the plasma gasification syngas. For example, it can be utilized as fuel source to produce electric power (e.g. in a simplified steam-cycle configuration consisting of a conventional boiler/steam generator with steam turbine) or in a gas engine, configured to accept lower heat value gas. The gas can be used in a gas turbine, both in simple cycle and in combined cycle operations. It can also be used as a feedstock for chemical processes, e.g. the production of methanol.

The use of lower heat value plasma gasification syngas as a fuel source for gas engines has been successfully demonstrated with syngas generated from various feedstocks, including the gasification of biomass. Other applications for the utilization of the plasma gasification syngas are as follows: separation of hydrogen from the syngas, which can provide an excellent source of hydrogen for use with fuel cells, using the syngas as a feedstock for the production of liquid fuels, such as ethanol.

Applications for the glassy product include roadbed/fill construction and concrete aggregate. Any reclaimed valuable metal could be sold to metal dealers and processors. Metal alloy is bought and sold based on a commodity-based pricing system.

4. Environmental impacts

Plasma gasification uses an external heat source to gasify the waste. Almost all of the carbon is converted to fuel gas. Plasma gasification is the closest technology available to pure gasification. Because of the temperatures and drastic conditions involved all the tars, char and dioxins are broken down. The exit gas from the reactor is cleaner and there is no ash at the bottom of the reactor, while there are no by-products that end up to landfills provided that there are available markets for the produced slag. On the other hand, the use of plasma gasification processes reduce methane emissions produced from the disposal to landfill sites, while as a waste to energy treatment method, enables the displacement of CO2 that would have been emitted had the electricity been generated from fossil fuels.

5. Economic data

According to the Carbon Finance Unit of the World Bank in 2008, the capital cost of a plasma gasification system with a capacity of 900 tonnes per day is 40-60 /tonne, while the operation and maintenance cost is 55-100 /tonne. Nevertheless, most sources estimate that the cost is a little bit higher than other thermal methods due to the use of electrical energy.

6.  Applicability in the target area

The application potential of gasification and plasma gasification is also considered high, since these methods have recently proved that they are effective and flexible, since they can also be used for the treatment of other waste streams (e.g. sludge, hospital waste, etc.) apart from municipal waste. That is why the gasification practices are considered as suitable alternative especially in the case of isolated areas, such as islands. The relevant cost is similar to that of other thermal management practices, higher than that of biological options, the relevant land demand is limited and the energy yield is also considered of vital importance. The experience from the operation of such plants is less than that from incineration units.

The first attempt to apply gasification process in the target region and more specifically in Greece was made he National Technical University of Athens, with a unit that was installed in Mykonos in order to treat all types of waste generated on the island with emphasis on municipal solid waste. The unit had been initially designed and developed in the framework of the LIFE project entitled: “Development of a demonstration plasma gasification / vitrification unit for the treatment of hazardous wastes” and later was modified in order to cater for the treatment of municipal solid waste, too. The scope was to investigate the use of this innovative technique in an isolated area like an island in order to provide a solution to the overall management of waste. General views of the whole demonstration facility are available below:

The primary waste feeding system consists of a hopper intended for feed of solid material having maximum moisture content of 50% and a maximum particle size of 2.5 cm. The screw conveyor solid feeder has a maximum capacity of about 85 kg/h of waste and the feeding capacity varies depending on the feed waste bulk density. The feed rate is adjustable by varying the speed of the screw conveyor. Waste is manually loaded into the hopper connected to the screw conveyor. The feed rate is continuous and very steady, compared to a hydraulic feeder.

 

Waste is fed from a hopper through a screw feeder to the top of the furnace and dropping down is passing through the very hot and free of oxygen region between the two electrodes.

The furnace is comprised of a crucible, with approximately 130 litters capacity. It also includes a start-up natural gas burner for preheating and idle operation, a port for gasification air injection, a water-cooling mechanism for the graphite electrodes, an external surface water-cooling for the furnace walls and a tapping hole for periodical or continuous slag removal. During the operation of the plasma unit, the bottom part of the furnace contains the molten slag, while the upper section of it contains the process gases and is lined with a suitable high-temperature refractory. The required gasification air fed to the furnace is supplied by a compressed air system. Adjusting the valves on the compressed air line can control the flow rate.

In the pilot unit the furnace in which waste gasification is taking place is preheated at 600-800°C by burning propane in its interior. After preheating, two cylindrical graphite electrodes are inserted in the furnace and their ends are approached to a close distance. Two graphite electrodes are used to supply an electrical arc to the furnace. The current flows from the anode (+) to the molten bath and from the bath to the cathode (-). The cathode is grounded at zero (0) potential.

 

Graphite electrodes with male/female threads are used. The electrode dimensions were 7.6 cm in diameter and 106.7 in length. Electrodes are installed with the female end down, in order to avoid dust accumulation in the threads. Two electrodes were screwed together on each side (anode and cathode) and are mounted on flexible joints, which allow them to be moved over the slag pool and improve mixing. The mechanism also permits the electrodes’ extension into the furnace to be adjusted during operation (Carabin & Holcroft, 2005; Carabin et al., 2004; Gagnon & Carabin, 2006).

.

The DC power supply for the electrodes has a maximum power output of 200 KVA (Plasma arc power supply, input: 600 VAC-3f-60HZ, 3 X 200A fuses).

 

Then, a high voltage is applied between them producing an electrical arc which is raising locally the temperature up to values as high as 5,000°C and creating a plasma atmosphere. Air is not permitted to enter the furnace. Under these conditions it is ensured that from the volatile part of the waste syngas is produced consisting mainly of H2, CO, CO2 and H2O and containing in very low proportions H2S and HCl, but without significant presence of NOx. A camera is installed in front of a window on the top of the furnace, connected with a laptop, by which we can watch or make video recording of the electrical arc and the decomposition of the organic matter taking place in the interior of the furnace.

 

The slag could be tapped out periodically from the tap hole located on the front side of the crucible, close to the bottom of the furnace. The slag was either poured in a slag mold to form ingots or quenched in a water tank to produce granulated slag.

The inorganic part of the waste used is melted, drops to the bottom of the furnace and from time to time is removed through a hole in the lower part of the furnace, is collected to a fire resistant pan and is taken to the laboratory for analysis and investigation of its toxicity.

 

The hot cyclone was designed to remove dust in the synthesis gas. The produced gases, while entering the cyclone, are put in circular movement and the centrifugal force makes particulate matter contained in the gases to be removed to a high degree.

The result of its operation is the oxidation of the components of the furnace gases. The secondary combustion chamber was designed to combust H2 and CO in the synthesis gas. In order to combust CO and H2 into CO2 and H2O, air is added into the secondary combustion chamber. Propane burners are used to maintain the chamber temperature at 1,100oC. The operator can check local regulations to determine the required temperature in secondary combustion chamber. This temperature is required to fully combust CO and H2 in a region where no hazardous by-products are created. In normal operation, the gas residence time in the secondary combustion chamber is about two seconds. A single blower provides the combustion air for the burners and the combustion air for the synthesis gas.

 

It is located at the outlet of the secondary combustion chamber. Its role is to cool the combustion gases quickly to approximately 75oC so as to minimize any production of dioxins, furans or other organic compounds. The shock-like cooling avoids the formation of the aforementioned compounds from elementary molecules in the synthesis gas due to the de novo Synthesis back reactions (Calaminus & Stahlberg, 1998). These reactions are known to occur in waste heat boilers where a slow cooling in the range from 400oC to 250oC of flue gases with chlorine compounds, non combusted organic molecules and catalysts such as dust will result in dioxin formation. The quench vessel uses two atomizing nozzles to quench the gas from the secondary combustion chamber. These nozzles are capable of providing 2 litters per minute of flow. Regulating the amount of the quenching water can control the gas temperature exiting the vessel.


It removes water-soluble components of the off-gas including hydrochloric acid and most oxides of sulphur, prior to discharge. Since the synthesis gas may contain acid gases (such as HCl or SO2), a packed tower type wet scrubber uses caustic soda to neutralize the acid gas from the quench vessel. The pH of the scrubbing solution is controlled at 9.0. The scrubber liquor is re-circulated through a wet bagfilter in order to remove suspended particles. The bagfilter is a cartridge unit having series of cylindrical filters that are cleaned periodically by an automatic sequence using pulses of compressed gas.

 

The pilot unit has a maximum hourly capacity of only 50 kg of waste and the quantity of the syngas produced is too low for a gas engine to convert it in electrical energy; therefore, the syngas has to be released in the atmosphere but in a safe way. Hence, CO and H2 have to be transformed to CO2 and H2O and for this purpose a Secondary Combustion Chamber (SCC) has been added in the installation, which is maintained at high temperature (around 700-800°C) by combusting propane with air and in which CO and H2 are burnt to CO2 and H2O. The SCC in our installation is situated after the furnace and between the two units is interceded a cyclone to remove the solid particles. After the SCC the flue gases are objected to quenching by coming in contact with a big quantity of cold water and this takes place in a pipe where flue gases and cooling water are moving opposite each other. After quenching, the flue gases are passing for cleaning through a scrubber with NaOH solution, then through a filter and finally before they are released to the atmosphere via a stack are cooled in a heat exchanger to condense and recirculate the maximum quantity of water vapors. The results of the pilot application were positive and encouraging for future applications using this technology. It is hoped that a full scale unit will operate soon in Mykonos and other Greek islands using gasification or plasma gasification technology. However, it is true that the existing severe economic crisis in Greece will cause significant delays is these management plans.

 

No other similar applications have been made in Romania, Bulgaria or Slovenia.

REFERENCES

Blahos, L. (2000). Plasma Physics, the Fourth State of Matter, Giolas Editions, 1–12.

Carabin, P. & Holcroft, G. (2005). Plasma resource recovery technology converting waste to energy and valuable products, in: Proceedings of the 13th Annual North American Waste to Energy Conference, NAWTEC13, 71–79, Article number NAWTEC13-3155

Carabin, P., Palumbo, E. & Alexakis, T. (2004). Two-stage plasma gasification of waste, in: Proceedings of the 23rd International Conference on Incineration and Thermal Treatment Technologies, Phoenix, AZ, USA, May 10–14.

Circeo, L. (2007). Plasma Arc Gasification of Municipal Solid Waste, EPA Region 4 Clean and Sustainable Energy Conference Embassy Suites Hotel at Centennial Olympic Park, Atlanta, GA

Gagnon, J. & Carabin, P. (2006). A torch to light the way: plasma gasification technology in waste treatment, Waste Management World 1, 65–68

Gidarakos E. (2006). Hazardous Waste, Management, Treatment, Disposal, Zigos Editions, Thessaloniki

Gomez, E., Rani, D.A., Cheeseman, C.R., Deegan, D., Wise, M. & Boccaccini, A.R. (2009). Thermal plasma technology for the treatment of wastes: A critical review, Journal of Hazardous Materials, 161, 2-3, 614-626

Huang, H. & Tang, L. (2007). Treatment of organic waste using thermal plasma pyrolysis technology, Energy Conversion and Management, 48, 1331–1337

Juniper Consultancy Services Limited. (2006). Independent Waste technology Reports, Bathurst house, Bisley GL6 7NH, England

Malkow, T. (2004). Novel and innovative pyrolysis and gasification technologies for energy efficient and environmentally sound MSW disposal, Waste Management 24, 53-79.

Mollah, M.Y.A., Schennach, R., Patscheider, J. Promreuk, S. & Cocke, D.L. (2000). Plasma chemistry as a tool for green chemistry, environmental analysis and waste management, Journal of Hazardous Materials B 79 301-320

Moustakas, K., Fatta, F., Malamis, S., Haralambous, K.-J. & Loizidou M., (2005). Demonstration plasma gasification/vitrification system for effective hazardous waste treatment, Journal of Hazardous Materials B123 120-126

Moustakas, K. & Loizidou, M., (2010). Solid Waste Management through the Application of Thermal Methods, Waste Management, Er Sunil Kumar (Ed.), ISBN: 978-953-7619-84-8, INTECH

Moustakas, K., Xydis, G., Malamis, S., Haralambous, K.-J. & Loizidou M. (2008). Analysis of results from the operation of a pilot gasification / vitrification unit for optimizing its performance, Journal of Hazardous Materials, 151, 473-480

Sheng, H., Wang, R., Xu, Y., Li, Y. & Tian, J. (2008). AC plasma arc system for pyrolysis of medical waste and POPs: Paper #77 Air and Waste Management Association - 27th Annual International Conference on Thermal Treatment Technologies 2, 605-612

Wastesum (2006). Management and Valorisation of Solid Domestic Waste for the Small Urban Communities in Morocco, LIFE-3rd Countries, 2007-2009, European Commission

World Bank Carbon Finance Unit (2008), Municipal Solid Waste Treatment Technologies and Carbon Finance, Thailand.