technological aspects for thermal plasma treatment of municipal-main

8/10/2019 Technological Aspects for Thermal Plasma Treatment of Municipal-main

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Review

Technological aspects for thermal plasma treatment of municipalsolid wasteA review

Biswajit Ruj a,, Subhajyoti Ghosh b

a Thermal Engineering Department, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, Indiab Mechanical Department, GDGWI-Lancaster University, India

a b s t r a c ta r t i c l e i n f o

Article history:

Received 19 February 2014Received in revised form 5 May 2014

Accepted 12 May 2014

Available online 7 June 2014

Keywords:

Thermal plasma

MSW

Syngas

The 21st century earth is a new world, with numerous urban areas, exponentially growing population, global

warming, global markets and with it, increased consumerismwhich has led us to amass huge amountsof munic-ipal solid waste (MSW). This waste is difcult to manage using conventional methods and is ever increasing,

blocking essential space that has become an expensive commodity in today's world. Conventional techniques

such as combustion/incineration have been the conventionally preferred method of waste management for sev-

eral nations in lieu of land-lling, releasing toxic emissions onto an already over polluted environment. In this

paperwe shall explore a novelMSW management technologyin the form of plasma torches and thermal plasma

treatment thatenables us to reduce waste density by as muchas 95%,without any toxic emissions, while produc-

ing a synthetic gas as by-product. Synthetic gas or syngas is presently being used to generate energy. Some re-

searchers are also exploring the possibility of hydrogen extraction through this route. This paper discusses the

current limitations of this technology and highlights a few researches that are being conducted around the

world that may soon take this concept from technical feasibility to practical reality.

2014 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

1.1. MSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

1.2. Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

1.3. Plasma generators (torches) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

2. Thermal plasma treatment of MSW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

2.1. Plasma gasication & industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

2.2. Plasma gasication: future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

1. Introduction

Since thebeginningof industrialrevolution in the18th century there

hasbeen a steadygrowthin urban populationas more peoplefrom rural

areas were migrating into cities to be part of a revolution that would

provide people with jobs, food and clothing. This was the beginning of

the creation of an urban consumer market. The concept of consumerism

grew with the development of new technologies that gave people ac-

cess to a variety of products in huge quantities with substantially consis-

tent quality and by 1939 the concept of consumerism grew on a global

scale as more countries such as Germany, Franceand the USA, following

the example of the British Empire, had rapidly developed their industri-

al capabilities. The end of the Second World War and the rise of theUSA

as a new superpower, saw a new form of consumerismthe consump-

tion of products in huge quantities, not just limited to those that are

considered essentials to fuel economic growth[1].

The growth of consumerism meant that the supply of products

must be unhindered. Industry grew and along with it the demandfor la-

bour. The World Health Organisation reports that in the beginning of

Fuel Processing Technology 126 (2014) 298308

Corresponding author at: Principal Scientist Thermal Engineering Department CSIR-

Central Mechanical Engineering Research Institute (CMERI) M.G. Avenue, Durgapur-

713209 India. Tel.: +91-343-6452156.

E-mail address:[email protected](B. Ruj).

http://dx.doi.org/10.1016/j.fuproc.2014.05.011

0378-3820/ 2014 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

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the 20th century, 20% of the population dwelled in urban areas, by 1990

that number rose to a little less than 40% and is expected to rise to a

staggering 70% by 2050. These statistics show that there is a growing

trend in people migrating into urban areas for better job, lifestyle and

livelihood[2]. The increase in urban population and the steady rise in

consumption have adverse effects on the environment such as rapid

global population increase (currently the global population stands at

7.2 billion people and rising as per United Nation's Department of Eco-

nomic and Social Affairs[3]) and the generation of huge quantities ofmunicipal solid waste (MSW) is increasing along with the increasing

numbers of urban dwellers (Table. 1). While most countries do not

regard population increase as an immediate threat, the excessive accu-

mulation of MSW has led to major concerns in the developed and de-

veloping nations[4,6]as conventional methods[4,810]are not able

to effectively dispose off the waste at rates at which they are being

generated. While MSW recycling is essential it is dependent on the

government's motivation to take the necessary measures to promote

awareness. However the generation of waste will continue to grow

making it essential for us to formulate a solution to effectively manage

waste regardless of geographical or income of a country, factors that

play an important role.

Accumulation of waste results in decomposition and harmful emis-

sion of gases and some methods of storage require large tracts of land

which are becoming increasingly valuable with increase in population.

The World Bank reports that there are presently three billion urban res-

idents generating 1.2 kg per person per day of MSW and that number is

projected to grow to 4.3 billion urban residents generating 1.42 kg per

person per day of MSW by 2025 [4]. Hence an unconventional yet effec-

tive solution is required which can be found in theform of thermal plas-

ma pyrolysis which this paper seeks to explore.

1.1. MSW

MSW has various compositions, varying from region to region,

country to country and from people to people based on their income,

lifestyle/culture, climate, energy sources and economic afuence. Devel-

oping countries such as India and China, with a rapidly growing urban

population, produce MSW which is mostly organic in nature, such asfood scraps, wood, leaves, and process residues from farms whereas de-

veloped countries with a wealthier population show higher consump-

tionsin inorganic materials suchas plastic,paper,metal, ande-wastes[4].

E-wastes are essentially discarded electronic appliances such as

computers, cellulardevices,televisions or components suchas discarded

mother boards, and processors (this may consist of carcinogenic heavy

metals such as lead, mercury, chromium, which dees other forms of

processing andmay enterour food cycle through water andsoil contam-

ination, if not treated/neutralised effectively), due to e-waste high de-

gree of mercury contamination can be expected in MSW[5].

The MSW composition cannot be simply categorised as organic and

inorganic wastes. Industrial wastes, mostly inorganic such as plastic,

tyres, metal components and medical wastes such as soiled bandages,

syringes, cotton, and plastics are infectious wastes or red bag wastes

which may be contagious and pose health and environmental hazards

[1416], and therefore are required to be segregated from the typical

waste pile gathered from residential areas. The World Bank reports

that while countries with high income have a collection rate of 98%,

low income countries have a very low collection rate of a mere 48%

even though a substantially large amount of their municipalities'

waste management budget goes into collection; separation of varioustypes of wastes is generator dependent, however in regions with low-

income, the generators have insufcient knowledge and motivation to

categorise and separate their waste and their governments lack funding

and/or the inclination to prioritise sorting, post collection. So it must be

assumed that the presence of industrial and medical wastes is highly

probable in MSWs if sourced from regions of low income or with poor

waste managementpolicies [4]. Although medical wastes and industrial

wastes mayrequire special processing, it has been established that ther-

mal plasma pyrolysis techniques can be used to treat both medical

wastes[10,12,14,16]and industrial wastes[11,13,1620], generating

syngas without producing any toxic by-product and using it for energy

generation.

There are no specic data available on the composition of MSW,

makingit difcult to determinea standard.However theWorldBank re-

ports in [4] that a global MSWcomposition estimate can be represented

in the form of a pie chart,Fig. 1.

As shown inFig. 1, MSW is pre-dominantly composed of organic

wastes. As mentioned earlier organic waste can be food scraps, yard

trimmings, and process residues; its composition will vary from region

to region basedon theincome of theregion,geography, etc. C. Ducharme

in[6]noted that organic component of MSW can be approximated by

the formula C6H10O4, an observation stated by Themelis et al. in[30]

on his study of New York City MSW. The formula can guide researchers

when considering the organic component of the MSW sample, and de-

termine its composition percentage.

1.2. Plasma

After solid, liquid and gas, plasmais considered to be the fourth stateof matter; plasma is essentially composed of electrons, ions and neutral

particles. However, plasma in its entirety is electrically neutral.

Plasma has a long history of utility in industry. It wasrst employed

formetallurgical processes in the19th century andlater in the20th cen-

tury. It was used for acetylene extraction from natural gases in the

chemical industry. Thereason for using plasma was its ability to provide

high temperatures. The very same reason saw NASA develop this tech-

nology extensively for simulating the high temperatures that missiles

and space-crafts routinely face upon re-entry into earth's atmosphere

due to the rapid ionisation. The technologies that we currently use in

waste processing are derivatives of the technology initially developed

by NASA[10].

Table 1MSW Generation by country[4].

Current available data Projections for 2025

Country Total urban population Total MSW generation

(tonnes/day)

U rban population Total MSW generation

(tonnes/day)

India 321,623,271 109,589 538,055,000 376,639

China 511,722,970 520,548 822,209,000 1,397,755

USA 241,972,393 624,700 305,091,000 701,709

Russia 107,386,402 100,027 96,061,000 120,076

United Kingdom 54,411,080 97,342 59,738,000 110,515

France 47,192,398 90,493 53,659,000 107,318

Germany 60,530,216 127,816 61,772,000 126,633

Brazil 144,507,175 149,096 206,850,000 330,960

Israel 5,179,120 10,959 8,077,000 16,962

South Korea 38,895,504 48,397 41,783,000 58,496

Japan 84,330,180 144,466 86,460,000 146,982

299B. Ruj, S. Ghosh / Fuel Processing Technology 126 (2014) 298308


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A. Gutsol in[7] states that plasma can be categorised into three

types, thermal plasma, cold plasma and warm (intermediate) plasma.

Thermal plasma attains high temperatures, although not as high as

hot plasmafound in thermo-nuclear research and astrophysics, and

is in thermal equilibrium. Thermal equilibrium infers that all the species

of the plasma, such as ions, atoms, electrons and neutral species, all re-

tain thesame temperature. A. Bogaerts et al.in [21] classiedthistypeof

plasma as fusion plasma, a type of plasma that is commonly found in

stars with a temperature range of 4000 K to 20,000 K. The other two

types of plasma are classied as non-thermal equilibrium plasma.

Plasma is created through the application of energy sourced from

electric discharges of frequencies ranging from Direct Current (DC) to

the optical range which is in the order of 1015. The energy absorbed

by the electrons is spent in excitation of atoms and molecules, non-

elastic collisions for ionisation and for elastic collisions for direct gas

heating. This spent energy is subsequently dissipated into the environ-ment. Plasmas considered by A. Gutsol have low ionisation degree

thereby the degree of energy dissipation depends on the translational

gas temperature T0. A plasma becomes thermal plasma if the energy

transfer from the electrons to gas heating occurs fast enough for T0to

equal the electron temperature Tethereby attaining thermal equilibri-

um. In order for theelectrons to be capableof ionising the gas molecules

with ionisation energy in the order of 10 eV, it must attain an energy in

thelevelof1eVorTe of 10,000K. A. Gutsol infers that plasma must have

a temperature of 10,000 K or above to be stated as thermal plasmas,

which is within the temperature range of plasmas found in stars as

stated by A. Bogaerts et al. in [21].

The third type of plasma, warm plasma has high translational

temperatures of around 2000 K, although it is signicantly lower than

thermal plasmas. This plasma dissipates energy into the environment

through non-equilibrium discharges. Microwave plasmas are one such

typeof plasmawith physical properties that allow for a stable condition

to generate, under a range of external parameters.

The second type of plasma, or the cold plasma is another example of

non-equilibrium plasma, with low energy levels as the energy transfer

from electrons into gas heating is very slow. The energy level is lowenough for the molecules of the plasma to rapidly cool to thesurround-

ing temperatures. Corona discharges, whether AC, DC or pulsed, are ca-

pable of producing this kind of plasma, at atmospheric pressure.

In Fig. 2 a segregation on the types of plasma is shown based on def-

initions provided byA. Bogaerts et al. in[21]and A. Gutsol in[7].

In this paper, we are going to discuss low temperature plasmas, es-

pecially thermal plasmas which have been used extensively in several

researches dealing with MSWprocessing, as they have higharc temper-

ature, high intensity and energy density and most importantly high

non-ionising radiation which is useful in destroying highly toxic com-

pounds and dehydrogenate organic chlorine in an eco-friendly manner

[10].

1.3. Plasma generators (torches)

The fundamental concept of plasma generation is, when huge

amounts of electrical energyare provided to a gasat certain temperature

and pressure, it tends to excite and ionise it, generating electrons that

further collide with consequent atoms in-elastically thereby generating

more ions and electrons. This process continues in a self-sustaining

manner, provided a steady source of energy is continually applied.

High temperature is generateddue to thesignicant electrical resistivity

that generates across the system.

Thermal plasma can be generated by various methods of discharges

which A. Bogaerts et al. have elaborated upon in[21], however we shall

look into the two methods of thermal plasma discharges that are being

extensively used in concerned experiments, arc generated plasma using

Direct Current (DC) and Radio Frequency (RF) inductively coupled dis-

charges[16,21,22].Arc generated plasma using Direct Current (DC) involves the use of

DCelectriccurrentsas high as1 105 A, depending on thespecications

of the torch, across two electrodes which create a potential difference

across the input gas. The gas is forced to pass through the conned

space between the two electrodes which provides the energy required,

beginningthe electrical breakdownthat leads to plasma generation.The

plasma leaves the torch through a circular opening in one of the elec-

trodes, usually the anode (non-transferred arc generators). The plasma

arc that comes out is unstable. Therefore, an external magneticeld is

used to stabilise the arc. The stabilisation of the arc can also be done

by controlling the ow rate of the plasma gas.

Fig. 1.Pie-chart illustrating the global solid waste composition [4].

Plasma

Low temperature

Thermal/Equilibrium

Direct Current

discharge

Radio Frequencydischarge

Non-thermal/Non-equilibrium

Corona discharge(cold plasma)

Micro Waveplasma (warm

plasma)

High temperature

Laser fusionplasma / Hot

plasma

Fig. 2.Types of plasma.

300 B. Ruj, S. Ghosh / Fuel Processing Technology 126 (2014) 298308
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However the designs of DC plasma arc generators differ greatly

depending on whether they are non-transferred [Fig. 3] or whether

they are transferred. In transferred arc generators, one of the electrodes,usually theanode has a large separation with respect to the cathode. It is

usually a conducting material such as graphite, which also has refracto-

ry properties and does not require to be water cooled. It can have a hole

through it to allow the plasma gas to pass through or the gas could be

made to pass through the cathode externally, guided by a constrained

wall [Fig. 3]. Transferred arc reactors can utilise multiple rod electrodes

to generate a plasma arc. Non-transferred DC arc torches are used

popularly for their high temperature plasma arcs and better mixing of

the reactants (e.g. MSW) with plasma, although some designers and

researchers have opted fortransferred plasma arcs dueto economic rea-

sons as cheaper nitrogen gas can be used instead of argon as the work-

ing gas[11] [6,10,16,20,21].

There is one major drawback with DC thermal plasma arc generators

which A. Bogaerts et al.[21]have mentioned, a phenomenon called

sputtering where the discharged ions and atoms from the plasma gas

collide with cathode surface causing the release of secondary electrons

and some atoms from the cathode which later either deposits alongthe circular anode surface or passes through the opening, along with

the arc and contaminates the reactants. Due to this phenomenon the

cathodes have a denitelife span and require time-bound replacements

which increase maintenance cost and frequency of maintenance. In ad-

dition, more than 50% of electrical energy fed into thermal plasma is

wasted through cooling water which is necessary for stable arc opera-

tion. Otherwise, metallic electrodes are readily corroded or melted.

This is the major drawback that results in the energy efciency of ther-

mal plasma to be poor.

In the case of an RF inductively coupled discharges of thermal plas-

ma, which is being increasingly considered as their design prevents

any contact between the plasma gas and the electrodes, the energy nec-

essary to generate the plasma is provided by the RF induction coils and

allows the feed to be injected directly through the plasma region [9],

Non-transferred Arc plasma torch

Type: Direct Current Plasma torch.

Temperature: 10000K-14000K [non-transferred]

12000-20000K [transferred].

Electrode erosion: takes place, has a life span

Cathode

AnodePlasma jet

Water

Jacket

Working

gas

Transferred Arc plasma torch

ranging from 1000-3000h in inert gas, lesser in

oxidative gas ranging from 200-500h.

Heat sinking: Required to cool the electrodes.

Stabilise the arc operation and prevent corrosion or

melting of electrodes.

Ignition of plasma: Easy.

Volume of plasma: Small.

Efficiency of power supply device: 50%

Influence of solid feeding on plasma stability: No.

Type: Radio Frequency plasma torch.

Temperature: 3000-8000K.

Electrode erosion: No erosion takes place.

Heat sinking: Cooling water flowing inside the coil

Ignition of plasma: Difficult

Volume of plasma: Medium

Efficiency of power supply device: 40-70%

Influence of solid feeding on plasma stability:

Yes.

Anode

Cathode

Plasma jet

Water

Jacket

Working

gas

Carrying

Gas

Fig. 3.Plasma generators (torches) characteristics and schematic diagrams [16,20].



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27.541 kW. A. Vaidyanathan et al. concluded, based on the results ob-

tained, that the experiment was not successful as efciency was not

optimised and the amount of gas obtained gave a heating value much

lower than the90 kW power supplied to thetorch. However it is expect-

ed thatwith certainmodicationsin the process such as longerprocess-

ing time and improved feed delivery system can provide better results.

The conclusion drawn by A. Vaidyanathan et al. has highlighted an

essential problem that exists with several researchers who have

attempted to simulate successful experiments related to plasma gasi

-cation, to replicate the data and to device new experiments based on

that data. While some researchers are successful at replication, they

nd several complications while contemplating and executing new ex-

periments. These complications can only be removed by trial and error

methods through repeated experimentation. The essential problem is

that plasma based experimentation is a time consuming and costly pro-

cess, as operating a plasma torch requires huge amounts of electricity.

This often limits the number of experiment capabilities a researcher

can conduct, due to budget constraints within which they all operate.

Plasma gasication is a thermo-chemical process and the plasmafur-

nace is thecentralpart of theprocesswithin which several chemical con-

versions take place that can be dened by the following formulas[27]:

C(s)+ H2O = CO + H2[heterogeneous water gas shift reaction

endothermic]

C(s)+ CO2= 2CO [Boudouard equilibriumendothermic]

C(s)+ 2H2= CH4[hydrogenation gasicationexothermic]

CH4+ H2O = CO + 3H2[methane decompositionendothermic]

CO + H2O = CO2+ H2[water gas shift reactionexothermic].

These chemical conversions are the basis of an equilibrium model

designed by A. Mountouris et al.[27] to aid the researcher in predicting

the performance of a plasma gasication process, called the GasifEq.

This model has been created using recent thermodynamic data taken

from various sourcessuch as National Instituteof Standard andTechnol-ogy (NIST) and Design Institute for Physical Properties (DIPPR) consid-

ering all operational parameters such as moisture content, oxygen

amount, and gasication temperature and deduce its effects on the

composition of the syngas produced as well as providing the energy

and energy efciency analysis. The model GasifEq is a possible solution

to the problems that researchers such as A. Vaidyanathan et al. have

faced, by predicting the optimum operational conditions required and

the corresponding syngas composition, for operational parameters set

by the researchers thereby resulting in greater experimental success

and creating avenues for further research to improve efciency of the

processes as well as discovering more effective process techniques

and process variables.

2.1. Plasma gasication & industry

A major drawback of the use of thermal plasma torches based on

DC discharge is that they consume huge amounts of electricity. While

some researchers such as M. Punochet al.[11]and S. K. Nema and

Ganeshprasad[10]have proposed the generation of electricity using

syngas produced from gasication of plastic waste and medical waste

respectively, heterogeneous wastes such as MSW are a greater chal-

lenge as they contain a mixture of various products ranging from

organic to inorganic, of varying proportions, hence the output syngas

composition would vary and thereby its caloric value,Fig. 5.

In order to make thermal plasma treatment of MSW an industrially

feasible process, we need to be able to simulate the process in the

form of an experiment. Y. Byun et al. [23]have developed a working

MSW plant capable of processing 10 ton of waste per day to observe

the feasibility of the process in real-time. The pilot plant consists of

ve important sections:

1) MSW storage unit and feeding system;

2) Integrated furnacetted with two thermal non-transferred torches

and an assistant LPG gas burner;

3) Steam generator;

4) Efuent gas treatment system which contains a bag lter, water

quencher and scrubber;

5) Secondary combustion chamber; and

6) Air pre-heater/gas cooler.

A schematicdiagram of thepilot plant is shown in Fig. 6. The waste is

stored in the storage unit which has an air curtain that prevents any

odour from escaping. The waste is then sorted using magnetic separa-

tors to remove metals, processed by crushers to reduce their size and

then continuously fed into the integrated furnace, pre-heated to about873 K using LPG burners. The burners are also responsible for igniting

the waste. The entering feed is oxidised immediately by hot air which

is being fed into the reactor simultaneously. This reduces the electricity

consumption of the torches. The MSW undergoes gasication at a tem-

perature of 1673 K. The slag produced is tapped out from underneath

the furnace and water cooled to produce granules. Thesyngasproduced

is then taken to the steam generators where the gas temperature is re-

duced from 1673K to 453 K and theresultant thermal transfer isusedto

generate steam. Thecooledsyngasis then made to pass through the bag

lters where any residualy ash is removed. Here the gas is doped with

Ca(OH)2which reduces acidic gases present and increase the efciency

ofy ash capture. The syngas is subsequently passed through a water

Fig. 5.Shows the effects of moisture and different components on the caloric value of

MSW, extract from[29].

MSW storageunit

Magneticseperator &

Crusher

Hydraulicfeeder

IntegratedFurnace

Steamgenerators

Bag filtersWater

quencherScrubber

SecondaryCombustion

chamber

Air pre-heater/ gas

cooler

Stack

Fig. 6.Schematic diagram of the pilot plant for thermal plasma treatment of MSW[23].

http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B6%80http://localhost/var/www/apps/conversion/tmp/scratch_2/image%20of%20Fig.%E0%B5%80


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quencher which rapidly cools the gas to 303 K with 40% NaOH solution

following which a scrubber (pH 9 maintained) removes any remaining

acidic gases that might be present. The syngas isnally burned in a

secondary combustion chamber, where the temperature is maintained

at 1173 K and the output gas (syngas) is passed through an air pre-

heater/gas cooler. The air pre-heater/gas cooler collects the air from

MSW storage unit and heated using the output gas (syngas) from the

secondary combustion chamber, to raise its temperature to 873 K (out-

put gas temperature reduces to 473 K). The output gas (syngas) is thenstored in a stack.

Y. Byun et al. have concluded that their setup has been successful

in producing syngas with little or no trace of any poisonous or

hazardous gases, as shown inTable 2, the power consumption is

1.14 MWh/MSW-ton [thermal plasma torch (0.817 MWh/MSW-ton) +

utilities (0.322 MWh/MSW-ton)] and the amount of LPG used to pre-

heat the furnace is 7.37 Nm3/MSW-ton, respectively. The authors

conclude that the recoverable electricity from the syngas produced is

only 0.79MWh/MSW-ton, assuming that the Integrated Gasication

Combined Cycle (IGCC) has an efciency of up to 35%. This is due to

the fact that there are excess of heat loss in several sections of the

IGCC such as the steam generator where it is estimated that 70% of

the input energy of the gasfrom thefurnace was lost as theheat gener-

ated was not reused. Y. Byun et al. believe that by increasing the MSW

capacity and re-using the heat lost at the steam generator, the process

can be made more economically viable.

In order to make plasma gasication industrially effective many

companies have tried to combine traditional gasication with plasma

torches, such as Europlasma and Plasco, using DC plasma torches

to clean the gas produced from auto-gasiers before introducing them

to several scrubbers, then to a Gas engine optimised to use syngas as

fuel, to generate electricity; another company called InEnTec, proposed

the concept of Plasma Enhanced Melter (PEM) which combined the

concepts of plasma gasication and glass melting technologies. The

technology composed of three components, a downdraft pre-gasier,

a PEM process vessel and a thermal residence chamber. The MSW is

fed into the pre-gasier which is responsible for the gasication of

80% of the waste to syngas, while the remainder is processed in the

PEM vessel attached to the gasier. The PEM vessel uses a DC poweredplasma arc and an AC powered resistance heating system, to reduce the

load on the transferred arc. The inorganic materials are vitried in the

form of a slag and are collected. The design, although innovative, was

unable to reduce emissions as opposed to a classic grate combustion

plant[6].

C. Ducharme in[6] has done a comprehensive review of all the

present industrial scale plasma assisted waste to energy (WTE) process-

es including one process developed by Alter Nrg using torches and

cupola designed by Westinghouse Plasma Corporation (WPC), which

is an effective plasma gasication unit that can process MSW directly

without any pre-sorting or pre-gasication, and the syngas produced

is then used to generate electricity using customised turbines. This

setup is very similar to the experimental setup by Y. Byun et al.,Fig. 6.

The cost analysis in this report indicates that the model proposedby Alter Nrg/WPC is benecial for processing MSW, provided they

implement the Integrated Gasication Combined Cycle (IGCC) model

as shown inFig. 7,as opposed to the processes proposed by the other

companies,Table. 3.

Thecost to setup a traditional grate combustion WTE plant is around

$60/ton of MSW as opposed to $76.8/ton of MSW required to set up a

plasma gasication WTE plant. The values mentioned are shown in

Table. 3. The cost of setting up a classic grate combustion WTE plant is

compared to the cost of setting up a base plant, which is essentially a

cost assumption made by C. Ducharme in[6], on the capital requiredto set up a plasma gasication WTE plant. The base plant cost estimates

are developed keeping the components of the grate combustion plant

constant; omitting the cost of components that is irrelevant, such as

stoker, furnace, boiler, turbine, condenser and stack and including the

costs of plasma gasicationvessel, plasmatorches, and waterquenching

vessel and engine generators. The cost estimates reveal that the cost of

setting up a combustiongrate plant is signicantlycheaper,also thecost

involved in maintenance and operation, or variable cost, is signicantly

higher for plasma gasication plants. Although Westinghouse Plasma

Corporation'sIGCC model provesto be a cost effectivemodel as opposed

to other alternate plasma gasication models, it is still producing sig-

nicantly less benet, $12.33 less than the classic grate combustion

plants[6].

2.2. Plasma gasication: future

Plasmagasication technology has proven to be an effective method

for waste disposal, being environmentally friendly while providing en-

ergy in the form of syngas or hydrogen which is later used in generating

electricity using specially designed generators or as fuel in hydrogen IC

engines[28]and fuel cells. Current technologies in the eld of thermal

plasma treatment are limited to the two types of plasma discharges,

RF discharge and DC discharge, which are either in the research stage

or in industry. While the industry focuses on DC plasma arc technology

currently, it is proving to be incapable of competing with traditional

grate combustion WTE plants in terms of cost, reducing its economic

feasibility. However other than Alter Nrg/WPC technology no other

company has been able to test an economically viable waste to energy

project; the factors that inuence the economic andnancial viabilityof a project, as elaborated by L. Yang et al. in[29]are:

1. The composition of the waste, its caloric content;

2. The plant reactor size;

3. The competitive commercial tipping fees for the waste streams;

4. Ratio of organic to inorganic content;

5. Local equipment cost;

6. Local labour cost;

7. Local regulation/laws;

8. Sale price per unit of electricity generated; and

9. Design of the plant. Some designs produceenough electricity to meet

process requirements such as in[10].

The most important factor that affects the viabilityof a project is the

technology that is being implemented. While the technology currentlybeing pursued by the industry is environmentally viable and barely

meeting the operation cost, researchers are looking into alternate tech-

nology in line with thermal plasma technology thatcan address the cost

factor by increasing theprocess efciencyand units of power generated.

One such alternate technology is being experimented in Israel by Q.

Zhang et al.[31,32], called Plasma Gasication Melting (PGM) where

MSW gasication and plasma melting of the residues from the gasica-

tion are achieved in a single moving-bed counter current up-draft gas-

ier in a continuous one-step process. The process involves feeding of

air into the melting chamber of the reactor by the plasma torches

which are placed at the bottom of the reactor. The air enters at high

speed and high temperature in the form of plasma jet which effectively

melts the inorganic components of the waste and the air with its resid-

ual heat mixes withsteam, which is injected through theside walls. This

Table 2

Composition of syngass output from the integrated furnace in[23].

CO2(%) 9.9 3.0

CO (%) 14.2 4.5

O2(%) 0.4 0.2

H2(%) 10.4 3

HCL (ppm) 0.5 0.4

THC (ppm) 23.2 5.2

SOX(ppm) Not detected

NOX(ppm) Not detected

N2(ppm) Not detected

PCDDs/DFs (NG-TEQ/Nm3) 1.04 0.75



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