1. home and garden
2. appliances
3. microwaves

University of Ljubljana
Faculty of Mathematics and Physics
Department of physics
Seminar Ib - 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 century, and widely used during WWII, during fuel crisis in northern Europe. With fuel prices increasing, development on biomass gasification
has again regained interest. With development of new technologies in
high temperature physics, it has become possible not only to use biomass,
but also municipal waste as synthetic gas production material. With the
development of plasma incineration, a more environmentally sustainable
options 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 hydrogen production and long continuous opeartion intervals, it presents itself
as a solution to many problems of today’s society.
Contents
1 Introduction
1
2 Gasification of hydrocarbons
2
3 Microwave plasma
3.1 Theoretical model . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Microwave waveguide and cavity resonator . . . . . . . . . . . . .
3
4
5
4 Pure steam microwave plasma torch
7
5 Current work
9
6 Conclusion
1
10
Introduction
Steam plasma gasification of solid municipal waste gives a prominence in energy
independence 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 output, 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 much
lower than that of flue gas from incineration.
Previously, plasma gasification was not a viable option, due to shot cycles between service intervals of plasma torches, while using DC or AC discharge plasmas, where electrodes are in close contact with highly reactive plasma at high
temperatures. Now days, a new type of electrode-less high power plasma torches
are developed, utilizing microwave and RF discharge plasmas. Microwave plasmas seem to be more suitable, using steam as a working medium, which in gasification 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 engines, to produce electricity.
Due to temperatures higher than melting temperatures of most materials, the
residual vitrified molt, called slag, can easily be compacted, and afterwards reprocessed using the same plasma torch technique to separate metals and other
materials, thus ensuring sustainability.
Several processes must be discussed for understanding microwave steam plasma
gasification: microwave plasma discharge, gasification of biomass and microwave
resonance. This are all broad areas of science. This paper gives a basic overview
on all of them.
1
2
Gasification of hydrocarbons
Gasification is a process in which compounds made of complex hydrocarbon
molecules are decomposed to simple gaseous molecules by thermal treatment at
high temperatures (T > 1200K). At maximum efficiency, the only two products are gaseous CO, which can be burnt in oxygenated environment, and H2 ,
which can be either stored for further use or burnt as well, producing very little contaminants. Noncarbohydrate contaminants, such are glass, ceramics and
metals are also disintegrated into a melt mixture and can be poured into molds
for further processing.
Gasification process is thoroughly described by Littlewood [1].The first step of
gasification of hydrocarbon rich materials is pyrolysis, which decomposes feedstock material into gaseous and liquid products, leaving solid carbon. Equation
1 illustrates pyrolitic decomposition of the feedstock:
∑
∑
∑
Ca Hb Oc + Heat →
Ca,g Hb,g Oc,g +
Ca,l Hb,l Oc,l +
C
(1)
liquid
liquid
solid
Solid carbon can then be burnt completely in adequate oxygen environment,
producing 394 kJ of heat per kmol of carbon and carbon dioxide. The reaction
is described with equation
C + O2 → CO2 − 393.7kJ/kmol
(2)
Where - sign means that energy exits the system, and + sign denotes addition
of energy to system. If steam is introduced to solid carbon, it is also gasified
in an endothermic reaction, forming carbon monoxide and hydrogen. The gas
mixture is known as producergas. The process is described by equation 3:
C + H2 O → CO + H2 + 131.0kJ/kmol
(3)
On the other hand, solid carbon can also be gasified by restricting the supply
of oxygen to the reaction, producing carbon monoxide (CO). The reaction is
described by equation 4.
C + 1/2O2 → CO − 110.5kJ/kmol
(4)
If gasification product CO is burnt in oxygenated environment, the rest of the
energy (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 equation
5.
CO + H2 O → H2 + CO2 − 41.0kJ/kmol
(5)
Steam induced gasification at ultra high temperatures (T > 1200K) can
therefore increase the proportion of lighter hydrocarbons. The process differs
from hydro-gasification of carbon, as it hydrogenates the free radicals, before the
re-polymerization process occurs. That is why steam plasma is more suitable,
as steam also decomposes to H and OH radicals at temperatures exceeding
6000 K, that are highly reactive. If operated at a high enough temperature and
adequate steam quantity, the only products are CO and H2 .
Steam temperature plays an important role in the gasification process. On
2
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 hard
wood feedstock [2].
3
Microwave plasma
Development of microwave plasma sources date back to Second World War with
introduction of high power microwave sources for radar and communication
systems. This research led to development of various kilowatt-level microwave
sources, ranging from steady-state to pulsed mode microwave sources.
Microwave plasmas have much higher electron temperature, compared to DC or
RF plasmas. At kilowatt lever, the electron number density can be 7 · 1016 m13
at typical microwave frequency of 2.45 GHz. Microwave plasmas can operate
in a variety of gas pressures, ranging from 0.1 mPa to 105 Pa, depending on
the application. Due to their high electron densities, working gas in microwave
plasmas is highly dissociated and therefore chemically very reactive. The microwave plasma discharge is induced without any need for electrodes, reducing
contaminants and ion sputtering of the electrode, insuring a long service interval for such a device.
Microwave plasma generators are usually microwave coupled reactors, where
microwave power is fed into a tapered waveguide resonator (applicator), surrounding a dielectric tube (usually quartz), filled with working gas. Intense
electric fields in the applicator cause the gas to break down and maintain the
plasma. A typical microwave plasma setup can be observed in figure 2.
3
Figure 2: Typical microwave coupled reactor, with tapered microwave resonator.
[3]
3.1 Theoretical model
Microwave frequencies are usually higher then electron plasma frequencies, influencing electron in plasma as a collective, so it is necessary not only to account
for Newton dynamics of the electron, but to incorporate Maxwell’s equations
as well. These interactions are described by cold plasma theory, thoroughly
described by Heald and Wharton [4], which is an extensive subjects. Only convections and results will be presented in this paper.
First, we must write down the equation of motion for electron, acted on by
electromagnetic wave as described by equation 6.
d⃗v
⃗ − υc m⃗v − e(⃗v × B)
⃗
F⃗ = m
= −eE
dt
(6)
⃗ is
where m is electron mass, υc is the effective electron collision frequency, E
⃗ static background magnetic
the electric field of the electromagnetic wave, and B
field when present. We must also write down Maxwell’s equations:
⃗ ≈0
∇·E
(7)
⃗ =0
∇·B
(8)
⃗
⃗ = ∂B
∇×E
∂t
(9)
⃗
⃗ = µ0 J⃗ + µ0 ε0 ∂ E
∇×B
∂t
where the induced current can be described by Ohm’s law:
⃗
J⃗ = σ E
4
(10)
(11)
The solution can be expressed in the form of an EM wave propagation equation in terms of electric field, with exponential attenuation coefficient, given by
equation 12. The exponential attenuation is related to imaginary part of the
complex refractive index, given by equation 13.
E(z, t) = E0 exp(−αz)exp[i(ωt − kz)]
(12)
1
where ω = 2πυ, k = 2π
λ and α = δ , where α is the attenuation coefficient,
related to δ, which is the skin depth. We also define complex refractive index
µ̄ = 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 and
imaginary component, as described by Podgornik [5].
Attenuation coefficient α is given by:
α=
ω
χ
c
(13)
ω
µ
c
(14)
and propagation constant β given by:
β=
The propagation of an electromagnetic wave in an unmagnetized plasma can be
described in terms of complex refractive index:
µ=

(

1

2
1−
2
ωp,e
2
ω + υc2
)

(
)2
(
)2 1/2 1/2

2
2
2
ωp,e
ωp,e
1
υ

+  1− 2
+ c2
2
2
2

2
ω + υc
ω
ω + υc

(15)

(
1/2 1/2
(
)
)
(
)
2
2


 1

2
2
2
ωp,e
ωp,e
ωp,e
1
υc2

χ= −
1− 2
+
1
−
+


ω + υc2
2
ω 2 + υc2
ω 2 ω 2 + υc2
 2

(16)
In equation 16 we can observe the linkage between collision frequency υc and
attenuation χ. As the colission frequency becomes negligible in comparison to
the 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 induction in the walls of the waveguide, so they have to be made of best possible
conductors 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 we
separate electric TE and magnetic TM modes, which means that appropriate
field is always transverse to the waveguide. In the waveguide, several modes
can be transversed, depending on the waveguide cross-section size, denoted by
T En,m or T Mn,m accordingly, where m and n denote number of half-wave variations of electric or magnetic field, m for the broad side of the waveguide, and
5
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 the
long side, and b length of the short side[6].
The dominant mode, which has the longest wavelength is T E10 mode. If the
dimension of the long end of the waveguide cross-section is less than 1/2 of the
wavelength, no propagation will occur.
The waveguide therefore acts as a high-pass filter, where minimal, cut-off frequency is defined by length a:
√( )
1
m 2 ( n )2
fc = c
+
(17)
2
a
b
Resonant circuits in microwave transmission can be analogously described by
electric resonant circuits.
When current flows trough the wall of the waveguide, it induces a magnetic
field, which delays the voltage, thus having impedance, due to Biot-Savart law.
Because the conductor is made of materials with some resistance, a parallel
resistor can be added to the circuit. Having two conductors, a finite distance
apart also gives capacitance to the system. The resonant frequency in an electric
circuit is given by Thompson’s formula:
f0 =
1
√
2π LC
(18)
characteristic impedance of such a circuit is given by
√
L
Z0 =
C
and input admittance by
Yin
1
+i
=
R
(
f
f0
−
f0
f
6
)√
C
L
(19)
(20)
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 in
such 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 electric
field. This produces well defined position for maximum electric field position of
the sample, as well as maximum microwave source yield, and minimum reflected
interference with the microwave source.
4
Pure steam microwave plasma torch
Steam microwave plasma torch is a microwave driven plasma discharge at atmospheric pressure, which uses high temperature steam as working gas. In
comparison with other types of plasma torches, microwave plasma offers a more
stable 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 as
argon, mixed with steam. This meant that the efficiency of water disintegration
was 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 device
was 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 microwave magnetron with power output up to 2 kW, coupled via tapered waveguide resonator to the quartz tube, where high temperature steam is discharged
in a swirl by graphite or steel block, to create a vortex flow in the discharge
tube. The torch schematic, with its main components, can be observed in figure
4. Plasma temperatures of over 6000 K were measured and plasma density in
order of 1013 reached. Steam was generated by a commercially available steam
generator, originally intended as a carpet cleaner. The steam temperature at
the exit of the steam generator was around 160◦ C. The torch itself exhibited
two distinctive temperature regions: a bright, whitish high-temperature zone,
7
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 machined vanes, which swirl the steam, prior to entering the discharge tube. When
plasma ignites, the swirl block gets super heated, serving as steam preheater, enhancing steam plasma performance. Prior to plasma discharge, additional heat
must be supplied to the steam, as it cools down below ignition point, which
is the critical factor. These blocks must endure extremely high temperatures.
g
In case of graphite block, the surface evaporates slowly at rate of 5 · 10−4 min
.
Steel block however operated without any noticeable surface change for over 50
hours.
The plasma region inside the torch is quite large, and the flame volume increases
almost 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]..
8
Figure 7: Relative water disintegration parts vs. temperature [8].
Plasma flame temperatures inside the discharge tube, depending on the axial distance from the actural discharge were also measured, using optical spectroscopy for temperatures T > 2000 K, and thermocouple device at lover temperatures T < 2000 K. Temperature measurement data are potted on figure
6.
Due to the extreme temperatures inside the steam plasma torch, water itself
disintegrates, mainly to to atomic hydrogen H and hydroxide OH. Both are
highly 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 representative), mixed with air is introduced just after steam plasma discharge point.
This gasification setup produced synthesis gases with relative concentrations of
52% of H2 , 23% of CO and 25% of CO2 , at mass ration of 0.55 of steam to
coal. It was also acknowledged, that such gasification methods might be viable
for combustion of biomass materials, such as wood chips and municipal waste.
5
Current work
Several test facilities have displayed successful operation in biomass and municipal waste conversion to syngas. Here, only few are described.
A performance analysis on solid waste gasification with plasma melting reactor
was described by Q.Zhang, L. Dor, L. [10]. However steam was added separately, not as a plasma working medium, which decreased overall efficiency.
Nevertheless, useful data for further development of pure steam plasma gasification plant were obtained.
A solid bed, counter current updraft gasification reactor with capacity of 12-20
tons of municipal solid waste gasification per day was constructed in Northern
Israel, with a chamber on the bottom for vitrified slag. Plasma torch was placed
at the bottom of the fixed bed, where residual carbon was gasified, and slag removed to bottom chamber via gravitational pull. Air was injected as a mixture
9
with argon to the plasma torch for higher temperatures, and lower torch power
consumption. Temperatures were monitored, and feeding rates of air and steam
controlled by a central control system. In figure 8a a schematic of the plasma
reactor and on figure 8b a operation schematic is displayed.
(a) Plasma reactor schematic [10].
(b) Plasma reactor
schematic [10].
operation
Figure 8
On the other hand, pure steam microwave plasma gasification facilities are
being developed by L. Ricketts and A. Shaw at Stopford Ltd. [11]. A successful
demonstration of a 2 kg/hr reactor was constructed and tested, for a large scale
operation reactor.
In 2012, a pilot plant was built in Monterey, Mexico by Plasma Gasification
Corp., utilizing microwave steam plasma gasification of biomass, such as municipal waste, and crop leftovers. The plant uses microwave plasma to create
carbon monoxide and hydrogen, with as much as 52% of the total output. Hydrogen 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 energy output, comparing to conventional incineration plants. All of the carbon
is 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 gases
at minimal cost. Comparing to other types of biomass plasma gasification, it is
10
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 the
future solution for both energy and waste management crisis of today. This is
an 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, and
T. Kavka. Pyrolysis/gasification of biomass for synthetic fuel production
using a hybrid gas–water stabilized plasma torch. Vacuum, 83(1):209 – 212,
2008.
[3] J.R. Roth. Industrial Plasma Engineering: Volume 1: Principles. Industrial Plasma Engineering. CRC Press, 1995.
[4] M.A. Heald and C.B. Wharton. Plasma diagnostics with microwaves. Wiley
series 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 water
molecules 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 Hyun
Cho, Suk Hwal Ma, Se Min Chun, Bong Ju Lee, and Han Sup Uhm. A
pure steam microwave plasma torch: Gasification of powdered coal in the
plasma. Surface and Coatings Technology, 228, Supplement 1(0):S520 –
S523, 2013. Proceedings of the 8th Asian-European International Conference on Plasma Surface Engineering (AEPSE 2011).
[10] Qinglin Zhang, Liran Dor, Lan Zhang, Weihong Yang, and Wlodzimierz
Blasiak. Performance analysis of municipal solid waste gasification with
steam in a plasma gasification melting reactor. Applied Energy, 98(0):219
– 229, 2012.
[11] Stopford Energy & Environment Ltd. Microwave-Induced Plasma Gasification & Pyrolisis for Treatment of Solid Fuels. IchemE, 2014.
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