klemen ambrozic-seminar 1b ver2

8/18/2019 Klemen Ambrozic-Seminar 1b Ver2

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University of Ljubljana

Faculty of Mathematics and Physics

Department of physics

Seminar I b - 1. year, II. stage

MICROWAVE STEAM

PLASMA GASIFICATION

Author:Klemen Ambrožič

Mentor:Dr. Tomaž Gyergyek

May 5th, 2015

Abstract

Principles of gasification have been developed for more then a cen-tury, and widely used during WWII, during fuel crisis in northern Eu-rope. With fuel prices increasing, development on biomass gasificationhas again regained interest. With development of new technologies inhigh temperature physics, it has become possible not only to use biomass,

but also municipal waste as synthetic gas production material. With thedevelopment of plasma incineration, a more environmentally sustainableoptions of waste management have become available.Microwave steam plasma gasification in particular offers all the benefits of current plasma gasification processes, with additional emphasis on hydro-gen production and long continuous opeartion intervals, it presents itself as a solution to many problems of today’s society.


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Contents

1 Introduction 1

2 Gasification of hydrocarbons 2

3 Microwave plasma 3

3.1 Theoretical model . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2 Microwave waveguide and cavity resonator . . . . . . . . . . . . . 5

4 Pure steam microwave plasma torch 7

5 Current work 9

6 Conclusion 10

1 Introduction

Steam plasma gasification of solid municipal waste gives a prominence in energyindependence and sustainable waste disposal, eliminating the need for landfills,as the slag can be further used as raw material in other industrial processes.Gasification of biomass and solid waste offers significant increase in power out-put, while reducing the pollution to a bare minimum, comparing to incineration.Due to high gasification temperatures comparing to incineration, a reduction of N Ox formation is observed. Moreover, the volume of produced syn-gas is muchlower than that of flue gas from incineration.Previously, plasma gasification was not a viable option, due to shot cycles be-tween service intervals of plasma torches, while using DC or AC discharge plas-mas, where electrodes are in close contact with highly reactive plasma at hightemperatures. Now days, a new type of electrode-less high power plasma torchesare developed, utilizing microwave and RF discharge plasmas. Microwave plas-mas seem to be more suitable, using steam as a working medium, which in gasifi-cation terms inherently increases hydrogen production, as described in section 2.The produced gas can be further filtered and cleaned using cyclone separators,catalytic converters and filters, to ensure a desirable output gas composition,which can be either stored, burned or used as fuel for internal combustion en-gines, to produce electricity.Due to temperatures higher than melting temperatures of most materials, theresidual vitrified molt, called slag, can easily be compacted, and afterwards re-

processed using the same plasma torch technique to separate metals and othermaterials, thus ensuring sustainability.Several processes must be discussed for understanding microwave steam plasmagasification: microwave plasma discharge, gasification of biomass and microwaveresonance. This are all broad areas of science. This paper gives a basic overviewon all of them.

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2 Gasification of hydrocarbons

Gasification is a process in which compounds made of complex hydrocarbonmolecules are decomposed to simple gaseous molecules by thermal treatment athigh temperatures (T > 1200K ). At maximum efficiency, the only two prod-ucts are gaseous CO , which can be burnt in oxygenated environment, and H 2,which can be either stored for further use or burnt as well, producing very lit-tle contaminants. Noncarbohydrate contaminants, such are glass, ceramics andmetals are also disintegrated into a melt mixture and can be poured into moldsfor further processing.Gasification process is thoroughly described by Littlewood [1].The first step of gasification of hydrocarbon rich materials is pyrolysis, which decomposes feed-stock material into gaseous and liquid products, leaving solid carbon. Equation1 illustrates pyrolitic decomposition of the feedstock:

C aH bOc + Heat →∑liquid

C a,gH b,gOc,g +∑liquid

C a,lH b,lOc,l +∑solid

C (1)

Solid carbon can then be burnt completely in adequate oxygen environment,producing 394 kJ of heat per kmol of carbon and carbon dioxide. The reactionis described with equation

C + O2 → CO2 − 393.7kJ /kmol (2)

Where - sign means that energy exits the system, and + sign denotes additionof energy to system. If steam is introduced to solid carbon, it is also gasifiedin an endothermic reaction, forming carbon monoxide and hydrogen. The gas

mixture is known as producergas. The process is described by equation 3:

C + H 2O → CO + H 2 + 131.0kJ /kmol (3)

On the other hand, solid carbon can also be gasified by restricting the supplyof oxygen to the reaction, producing carbon monoxide (CO). The reaction isdescribed by equation 4.

C + 1/2O2 → CO − 110.5kJ /kmol (4)

If gasification product CO is burnt in oxygenated environment, the rest of theenergy (283 kJ/kmol) is released.To further increase hydrogen production, high temperature steam is introduced

into the process, which reacts with carbon monoxide, as described by equation5.

CO + H 2O → H 2 + CO2 − 41.0kJ /kmol (5)Steam induced gasification at ultra high temperatures (T > 1200K ) can

therefore increase the proportion of lighter hydrocarbons. The process differsfrom hydro-gasification of carbon, as it hydrogenates the free radicals, before there-polymerization process occurs. That is why steam plasma is more suitable,as steam also decomposes to H and OH radicals at temperatures exceeding6000 K, that are highly reactive. If operated at a high enough temperature andadequate steam quantity, the only products are CO and H 2.Steam temperature plays an important role in the gasification process. On

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figure 1 we can observe the composition of gas produced in gasification versus

the steam temperature:

Figure 1: Steam-argon plasma gasification produced gas composition on hardwood feedstock [2].

3 Microwave plasma

Development of microwave plasma sources date back to Second World War withintroduction of high power microwave sources for radar and communication

systems. This research led to development of various kilowatt-level microwavesources, ranging from steady-state to pulsed mode microwave sources.Microwave plasmas have much higher electron temperature, compared to DC orRF plasmas. At kilowatt lever, the electron number density can be 7 · 1016 1

m3

at typical microwave frequency of 2.45 GHz. Microwave plasmas can operatein a variety of gas pressures, ranging from 0.1 mPa to 105 Pa, depending onthe application. Due to their high electron densities, working gas in microwaveplasmas is highly dissociated and therefore chemically very reactive. The mi-crowave plasma discharge is induced without any need for electrodes, reducingcontaminants and ion sputtering of the electrode, insuring a long service inter-val for such a device.Microwave plasma generators are usually microwave coupled reactors, wheremicrowave power is fed into a tapered waveguide resonator (applicator), sur-rounding a dielectric tube (usually quartz), filled with working gas. Intenseelectric fields in the applicator cause the gas to break down and maintain theplasma. A typical microwave plasma setup can be observed in figure 2.

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Figure 2: Typical microwave coupled reactor, with tapered microwave resonator.[3]

3.1 Theoretical model

Microwave frequencies are usually higher then electron plasma frequencies, in-fluencing electron in plasma as a collective, so it is necessary not only to accountfor Newton dynamics of the electron, but to incorporate Maxwell’s equations

as well. These interactions are described by cold plasma theory , thoroughlydescribed by Heald and Wharton [4], which is an extensive subjects. Only con-vections and results will be presented in this paper.First, we must write down the equation of motion for electron, acted on byelectromagnetic wave as described by equation 6.

⃗ F = md⃗v

dt = −e⃗ E − υcm⃗v − e(⃗v × ⃗ B) (6)

where m is electron mass, υc is the effective electron collision frequency, ⃗ E isthe electric field of the electromagnetic wave, and ⃗ B static background magneticfield when present. We must also write down Maxwell’s equations:

∇ · ⃗ E ≈ 0 (7)

∇ · ⃗ B = 0 (8)

∇× ⃗ E = ∂ ⃗ B

∂t (9)

∇× ⃗ B = µ0⃗ J + µ0ε0∂ ⃗ E

∂t (10)

where the induced current can be described by Ohm’s law:

⃗ J = σ⃗ E (11)

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The solution can be expressed in the form of an EM wave propagation equa-

tion in terms of electric field, with exponential attenuation coefficient, given byequation 12. The exponential attenuation is related to imaginary part of thecomplex refractive index, given by equation 13.

E (z, t) = E 0exp(−αz)exp[i(ωt − kz)] (12)

where ω = 2πυ, k = 2πλ

and α = 1δ

, where α is the attenuation coefficient,related to δ , which is the skin depth. We also define complex refractive indexµ̄ = µ − iχ, where µ is the real refractive index and χ the attenuation index.This is due to frequency dependent dielectric function, which has both real andimaginary component, as described by Podgornik [5].Attenuation coefficient α is given by:

α = ω

c χ (13)

and propagation constant β given by:

β = ω

c µ (14)

The propagation of an electromagnetic wave in an unmagnetized plasma can bedescribed in terms of complex refractive index:

µ =

1

2

1− ω

2 p,e

ω2 + υ2c

+

1

2

1− ω

2 p,e

ω2 + υ2c

2+

υ2cω2

ω2 p,e

ω2 + υ2c

2

1/2

1/2

(15)

χ =

−

1

2

1− ω

2 p,e

ω2 + υ2c

+

1

2

1− ω2 p,e

ω2 + υ2c

2+

υ2cω2

ω2 p,e

ω2 + υ2c

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(16)In equation 16 we can observe the linkage between collision frequency υc andattenuation χ. As the colission frequency becomes negligible in comparison tothe frequency applied (υc ≪ ω), the attenuation index goes to 0.

3.2 Microwave waveguide and cavity resonator

Microwave energy is transmitted from microwave source to the working gas via

microwave transmission waveguides. Those are usually rectangular channels,with highly conductive walls. Microwave propagate trough them by current in-duction in the walls of the waveguide, so they have to be made of best possibleconductors to reduce losses. Mostly they are made of copper, with thin film of silver on the inside surface.Inside the waveguide, several modes of EM-field can propagate. In general weseparate electric TE and magnetic TM modes, which means that appropriatefield is always transverse to the waveguide. In the waveguide, several modescan be transversed, depending on the waveguide cross-section size, denoted byT E n,m or T M n,m accordingly, where m and n denote number of half-wave vari-ations of electric or magnetic field, m for the broad side of the waveguide, and

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n for the narrow side of the waveguide. A schematic of the waveguide can be

observed in figure 3.

Figure 3: Schematic of a rectangular waveguide, where a is the length of thelong side, and b length of the short side[6].

The dominant mode, which has the longest wavelength is T E 10 mode. If thedimension of the long end of the waveguide cross-section is less than 1/2 of thewavelength, no propagation will occur.

The waveguide therefore acts as a high-pass filter, where minimal, cut-off fre-quency is defined by length a:

f c = 1

2c

√ ma

2+n

b

2(17)

Resonant circuits in microwave transmission can be analogously described byelectric resonant circuits.When current flows trough the wall of the waveguide, it induces a magneticfield, which delays the voltage, thus having impedance, due to Biot-Savart law.Because the conductor is made of materials with some resistance, a parallelresistor can be added to the circuit. Having two conductors, a finite distanceapart also gives capacitance to the system. The resonant frequency in an electric

circuit is given by Thompson’s formula:

f 0 = 1

2π√

LC (18)

characteristic impedance of such a circuit is given by

Z 0 =

√ L

C (19)

and input admittance by

Y in = 1

R + i

f

f 0− f 0

f

√ C

L (20)

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In equation 20 we can observe, that at frequencies above resonance frequency,

input impedance is capacitive, and below resonance it is inductive.In terms of waves, a resonance is achieved, when a traveling wave is reflected insuch a way, that the phase of the reflected wave is in sync with incident wave,forming a standing wave formation, with well defined peaks and lows of electricfield. This produces well defined position for maximum electric field position of the sample, as well as maximum microwave source yield, and minimum reflectedinterference with the microwave source.

4 Pure steam microwave plasma torch

Steam microwave plasma torch is a microwave driven plasma discharge at at-mospheric pressure, which uses high temperature steam as working gas. In

comparison with other types of plasma torches, microwave plasma offers a morestable discharge at higher rates of dissociation and ionization of the working gas.In the past, steam plasma torches also used an inert noble carrier gas, such asargon, mixed with steam. This meant that the efficiency of water disintegrationwas much lower.

Figure 4: Pure steam microwave plasma torch schematic with main components.

A pure steam plasma would therefore be the best solutions. Such a devicewas recently developed and it’s operation described by Han S. Uhm, Jong H.Kim and Yong C. Hong [7], [8]. The torch mainly consists of a 2.45 GHz mi-crowave magnetron with power output up to 2 kW, coupled via tapered waveg-uide resonator to the quartz tube, where high temperature steam is dischargedin a swirl by graphite or steel block, to create a vortex flow in the dischargetube. The torch schematic, with its main components, can be observed in figure4. Plasma temperatures of over 6000 K were measured and plasma density inorder of 1013 reached. Steam was generated by a commercially available steamgenerator, originally intended as a carpet cleaner. The steam temperature atthe exit of the steam generator was around 160◦ C. The torch itself exhibitedtwo distinctive temperature regions: a bright, whitish high-temperature zone,

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where steam is dissociated, and a dimmer, reddish low-temperature zone, where

hydrogen is burnt with oxygen.The quartz tube with 3 cm diameter, sits on graphite or steel block, with ma-chined vanes, which swirl the steam, prior to entering the discharge tube. Whenplasma ignites, the swirl block gets super heated, serving as steam preheater, en-hancing steam plasma performance. Prior to plasma discharge, additional heatmust be supplied to the steam, as it cools down below ignition point, whichis the critical factor. These blocks must endure extremely high temperatures.In case of graphite block, the surface evaporates slowly at rate of 5 · 10−4 g

min.

Steel block however operated without any noticeable surface change for over 50hours.The plasma region inside the torch is quite large, and the flame volume increasesalmost linearly with applied electrical power, as it can be observed in figure 5.

Figure 5: Flame size inside discharge tube vs. applied microwave power[8].

Figure 6: Plasma flame temperature vs. axial distance from the discharge [8]..

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Figure 7: Relative water disintegration parts vs. temperature [8].

Plasma flame temperatures inside the discharge tube, depending on the ax-ial distance from the actural discharge were also measured, using optical spec-troscopy for temperatures T > 2000 K, and thermocouple device at lover tem-peratures T < 2000 K. Temperature measurement data are potted on figure6.

Due to the extreme temperatures inside the steam plasma torch, water itself disintegrates, mainly to to atomic hydrogen H and hydroxide OH . Both arehighly reactive, and enhance biomass gasification rate. These relative product

densities vs. temperature are plotted on figure 7.Similar plasma torches have been used for gasification of powdered coal, as

described by Shin, Honh, others [9]. Powdered coal (as solid carbon represen-tative), mixed with air is introduced just after steam plasma discharge point.This gasification setup produced synthesis gases with relative concentrations of 52% of H 2, 23% of CO and 25% of CO2, at mass ration of 0.55 of steam tocoal. It was also acknowledged, that such gasification methods might be viablefor combustion of biomass materials, such as wood chips and municipal waste.

5 Current work

Several test facilities have displayed successful operation in biomass and munic-

ipal waste conversion to syngas. Here, only few are described.A performance analysis on solid waste gasification with plasma melting reactorwas described by Q.Zhang, L. Dor, L. [10]. However steam was added sep-arately, not as a plasma working medium, which decreased overall efficiency.Nevertheless, useful data for further development of pure steam plasma gasifi-cation plant were obtained.A solid bed, counter current updraft gasification reactor with capacity of 12-20tons of municipal solid waste gasification per day was constructed in NorthernIsrael, with a chamber on the bottom for vitrified slag. Plasma torch was placedat the bottom of the fixed bed, where residual carbon was gasified, and slag re-moved to bottom chamber via gravitational pull. Air was injected as a mixture

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with argon to the plasma torch for higher temperatures, and lower torch power

consumption. Temperatures were monitored, and feeding rates of air and steamcontrolled by a central control system. In figure 8a a schematic of the plasmareactor and on figure 8b a operation schematic is displayed.

(a) Plasma reactor schematic [10].(b) Plasma reactor operationschematic [10].

Figure 8

On the other hand, pure steam microwave plasma gasification facilities arebeing developed by L. Ricketts and A. Shaw at Stopford Ltd. [11]. A successfuldemonstration of a 2 kg/hr reactor was constructed and tested, for a large scaleoperation reactor.In 2012, a pilot plant was built in Monterey, Mexico by Plasma GasificationCorp., utilizing microwave steam plasma gasification of biomass, such as mu-nicipal waste, and crop leftovers. The plant uses microwave plasma to createcarbon monoxide and hydrogen, with as much as 52% of the total output. Hy-drogen is then used in liquid fuel production trough various chemical processes,yielding fuel at prices of 30 $ per barrel, or even less, when waste if gasified.The plant operates in a 10 ton per day trial, and yields a 3-fold increase in en-ergy output, comparing to conventional incineration plants. All of the carbonis gasified and minimal vitrified slag leftover.

6 Conclusion

Microwave steam plasma gasification offers many benefits for waste disposal,both comparing to incineration and to landfill disposal. It also offers retrieval of raw material as well as producing combustible and strategically important gasesat minimal cost. Comparing to other types of biomass plasma gasification, it is

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also more reliable and less complicated, also suitable for smaller scale gasification

plants. Producing high hydrogen quantities from biomass and municipal waste,where hydrogen being the fuel of the future, this type of gasification may be thefuture solution for both energy and waste management crisis of today. This isan incentive for further research and development.

References

[1] Kenneth Littlewood. Gasification: Theory and application. Progress in Energy and Combustion Science , 3(1):35 – 71, 1977.

[2] G. Van Oost, M. Hrabovsky, V. Kopecky, M. Konrad, M. Hlina, andT. Kavka. Pyrolysis/gasification of biomass for synthetic fuel productionusing a hybrid gas–water stabilized plasma torch. Vacuum , 83(1):209 – 212,2008.

[3] J.R. Roth. Industrial Plasma Engineering: Volume 1: Principles . Indus-trial Plasma Engineering. CRC Press, 1995.

[4] M.A. Heald and C.B. Wharton. Plasma diagnostics with microwaves . Wileyseries in plasma physics. Wiley, 1965.

[5] R. Podgornik and A. Vilfan. Elektromagnetno polje . Matematika - fizika: zbirka univerzitetnih učbenikov in monografij / DMFA - založništvo.DMFA - založništvo, 2012.

[6] S.F. Adam. Microwave theory and applications, 21 laboratory experiments .

Prentice-Hall, 1969.[7] Han S. Uhm, Jong H. Kim, and Yong C. Hong. Microwave steam torch.

Applied Physics Letters , 90(21):–, 2007.

[8] Han S. Uhm, Jong H. Kim, and Yong C. Hong. Disintegration of watermolecules in a steam-plasma torch powered by microwaves. Physics of Plasmas (1994-present), 14(7):–, 2007.

[9] Dong Hun Shin, Yong Cheol Hong, Sang Ju Lee, Ye Jin Kim, Chang HyunCho, Suk Hwal Ma, Se Min Chun, Bong Ju Lee, and Han Sup Uhm. Apure steam microwave plasma torch: Gasification of powdered coal in theplasma. Surface and Coatings Technology , 228, Supplement 1(0):S520 –S523, 2013. Proceedings of the 8th Asian-European International Confer-

ence on Plasma Surface Engineering (AEPSE 2011).

[10] Qinglin Zhang, Liran Dor, Lan Zhang, Weihong Yang, and WlodzimierzBlasiak. Performance analysis of municipal solid waste gasification withsteam in a plasma gasification melting reactor. Applied Energy , 98(0):219– 229, 2012.

[11] Stopford Energy & Environment Ltd. Microwave-Induced Plasma Gasifi-cation & Pyrolisis for Treatment of Solid Fuels . IchemE, 2014.

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klemen ambrozic-seminar 1b ver2

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