NON THERMAL PLASMA CONVERSION OF PYROGAS INTO
SYNTHESIS GAS
Fela Odeyemi, Alexander Rabinovich, and Alexander Fridman
Mechanical Engineering and Mechanics Department, Drexel University, Philadelphia PA
Abstract
This paper discusses plasma assisted conversion of
pyrolysis gas (pyrogas) fuel to synthesis gas
(Syngas). Pyrogas is a product of biomass,
municipal wastes or coal - gasification process.
Pyrogas usually contains hydrogen (H2), carbon
monoxide (CO) as well as unreacted light and
heavy hydrocarbons (especially methane (CH4),
ethane (C2H6), propane (C3H8) and tar. These
hydrocarbons diminish the fuel value of pyrogas
thereby necessitating the need for the conversion of
the hydrocarbons. Various conditions and
reforming reactions were considered for the
conversion of pyrogas into syngas (a combination
of H2 and CO). Non thermal plasma is an effective
homogenous process which makes the use of
catalysts unnecessary for fuel reforming. The
effectiveness of gliding arc plasma has a nonthermal plasma discharge is demonstrated in the
fuel reforming reaction processes with the aid of a
specially designed low current device called glid
arc plasma reformer. Gliding arc plasma is a nonequilibrium discharge with multiple advantages
over other reforming techniques which will be
further discussed in this paper. Results obtained
from thermodynamic simulations were compared
with experimental results with emphases on yield,
molar concentration, and enthalpy at different
reaction temperatures.
Keywords: Pyrogas, Dry CO2 reforming, Glid arc
plasma
1. Introduction
Biomass, municipal wastes, hydrocarbon fuels
or coal can be reformed via one or a
combination of pyrolysis, combustion and
gasification processes. A series of chemical
reactions during the course of these processes
usually result in the formation of a complex
mixture of combustible gases such as CH4,
CO, H2, unreacted heavy hydrocarbons; tar
and a noncombustible gas - CO2. A
combination of all these gases constitutes what
is known as pyrolysis gas or pyrogas.
The presence of heavy hydrocarbons and tar
diminishes the quality of pyrogas from the
perspective of its use as an intermediate for
synthetic fuel production. This draw back
therefore necessitates the removal of the
unreacted hydrocarbons. A reasonable
approach is the use of non-equilibrium gliding
arc plasma for the chemical reformation of
pyrogas into synthesis gas or syngas.
Catalytic partial oxidation and plasma assisted
fuel reforming are two different fuel reforming
techniques. Non equilibrium gliding arc
plasma reforming is a homogenous process of
fuel reforming which eliminates the need for
catalysts[1]. Catalytic partial oxidation has
been known to have extensive drawbacks such
as high cost, poisoning problem, large size and
significant carbon footprint. Gliding arc
plasma on the other hand has smaller reactors,
fast start-up time, higher efficiency and low
electrical energy cost to produce plasma; about
2 % – 5 % of total power produced by the
system [3]. The reforming reactions
considered for pyrogas reforming are steam
reforming reaction and dry CO2 reforming
reaction, both of which are endothermic[2].
These two main reforming reactions produce
hydrogen rich synthesis gas which can be used
for power generation, utilized for fuel cells to
produce electricity[3] and as a building block
for production of synthetic fuels via the fischer
tropsch process[4]. Syngas is a gas comprising
a varying quantity of hydrogen (H2) and
carbon monoxide (CO). Partial oxidation
reaction will be inappropriate in this situation
due to the presence of hydrogen in the pyrogas
mixture. Oxygen reacts with hydrogen to form
water; this oxidation process diminishes the
concentration of the hydrogen in the final
mixture. Gliding arc plasma serves as a
resource for active species and radicals such as
O and OH which are necessary to stimulate the
desired chemical reactions and reduce the
initial temperature required to jump start fuel
reforming chemical reactions[5-6].
The basis for this paper is the detailed
description of the plasma catalytic reforming
of a complex mixture of hydrocarbons Pyrolysis gas; in the presence of nonequilibrium gliding arc plasma with the aid of
a gliding arc reactor which is essentially a
plasma reforming device. Experimental results
obtained from the plasma assisted reforming
of pyrogas process were compared with
thermodynamic predictions. Conversion rates
were also quantified based on hydrogen yield,
carbon monoxide yield, hydrocarbon
conversion. Energy costs of the dry CO2
reforming and steam reforming reactions
considered were also compared.
2. Experimental Apparatus
The set up for the pyrogas reforming
experiments essentially comprises a gliding
arc plasma reformer, mass flow controllers,
thermocouples, dc power supply, gas
chromatograph, data acquisition unit with
labview program, steam generator, furnace and
heaters.
2.1 Gliding arc plasma reformer
Gliding arc plasma discharge was
selected for pyrogas reforming experiments
due its relative high local temperature, low
power and non-equilibrium plasma catalysis
which are essential for stimulation of
hydrocarbon reforming reactions. The gliding
arc plasma reformer discussed in this paper is
specially designed to function at temperatures
above 850 degree celsius and atmospheric
pressure. The gliding arc reformer also has the
capability to work under auto thermal
conditions. The gliding arc reactor shown in
fig. 1 consists of a high voltage electrode made
out of stainless steel material, a stainless steel
ground electrode designed with multiple
tangential jets to provide a vortex shaped
discharge with gas flow; this system helps
prevent heat losses to the walls of the
electrode by propelling the gliding discharge
over the electrodes. Other reactor parts include
a fuel atomization nozzle, a glass filled teflon
dielectric material which separates the high
voltage electrode from the ground electrode.
The spark gap (discharge breakdown gap)
between the high voltage electrode and ground
electrode is 3 mm.
Fig. 1: The Gliding Arc Plasma reformer which consists of 2
stainless steel electrodes separated by glass filled Teflon which
serves as a dielectric material. The ground electrode consists of
multiple tangential gas jets which helps provide a vortex effect.
The pyrogas composition that the
experiments and results presented in this paper
are based on is provided in table 1. The
pyrolysis gas composition was determined
after several laboratory tests were carried out
on various pyrogas samples; it should be noted
that the percentage composition provided in
table 1 is the mean molar concentration (%) of
the different constituent gases of pyrogas. The
pyrolysis gas used in the experiments is a
mixture of the gases listed in table 1 with
corresponding percentage molar
concentrations. Definite molar concentrations
of individual gases were used during the
experiments to ensure consistency during the
course of the reforming experiments. The
molar concentrations of the gases making up
the pyrogas composition stated in table 1 are
within the range of concentrations obtained
from coal gasification processes. The mean
value of the molar concentrations of the
respective gases stated in table 1 was used for
the steam reforming and dry CO2 reforming
experiments of pyrogas. The total flow rate of
the pyrolysis gas fuel mixture used for the
experiments is 30 SLPM. The experiments
were carried out at atmospheric pressure
conditions. It should be noted that the
parameters and conditions under which the
reforming experiments were conducted were
constant throughout to ensure a stable
experimental set up. Water vapor was not
included in the pyrogas composition used in
the reforming experiments due to the
limitation of the gas chromatography
equipment in detecting and measuring water
vapor molar concentrations.
Table 1
Pyrogas Composition
Gas
Mole %
H2
32-37
CO
10-14
CH4
20-25
CO2
19-24
C2H6
6-10
C3H8
1-2
C3H6
< 1
C4H10
< 1
The main hydrocarbon reforming reactions
usually considered include partial oxidation
reaction, steam reforming and dry CO2
reforming. Partial oxidation will be
inappropriate for syngas production due to the
presence of hydrogen and CO in the pyrogas
mixture. Oxygen will easily oxidize hydrogen
and CO, thereby reducing the concentrations
of hydrogen and CO which is
counterproductive to synthesis gas (syngas)
formation. Hence, experiments were carried
out with steam reforming and dry CO2
reforming reactions. Since CO2 is already
contained in the pyrogas mixture in significant
quantities; an external source of CO2 will not
be required for the dry CO2 reforming
reaction.
A summary of some experiment parameters
and working specifications of the plasma
reformer is provided in Table 2.
Table 2
Operating conditions of the
plasma reformer
Fuel
Maximum pressure
Max Temperature
Oxidants
Pyrogas Flow
Steam : Carbon Ratio
Max Power
Spark Gap
Pyrolysis Gas
1 atm
850 C
Water or CO2
30 SLPM
0 - 1.5
3 KW
3 mm
3. Results and Discussion
3.1 Thermodynamic Model
The pyrolysis gas plasma steam
reforming process was simulated with all the
constituent gases that make up pyrogas.
Chemical reactions calculations were
conducted based on the supposition that the all
the participating reactants completely mix in
the reactor. The thermodynamic simulation
was carried out using the reaction design
software - CHEMKIN 4.1 package. The
physical input parameters included in the
steam reforming model conditions include
temperature (degree celsius), reactor area and
volume, pressure (atm), and mass flow rate.
The reaction temperature was varied from 0 to
950 C. The output parameters are the gas
composition and molar fractions of the
resulting gases.
3.2 Plasma - Dry CO2 reforming
Plasma – Dry CO2 reforming is a
reforming reaction which involves the reaction
of hydrocarbons with CO2 in the presence of
plasma discharge to form Syngas. With the
already available CO2 present in the pyrolysis
gas composition, plasma - dry CO2 reforming
of pyrogas experiments were conducted at
conditions stated in Table 2. The experimental
setup for dry carbon dioxide CO2 reforming of
pyrogas is identical to that described in the
plasma – steam reforming experiments without
the steam generator. One of the advantages of
this chemical reaction process is that the need
for a supplementary source for carbon dioxide
(CO2) is unnecessary due to the existing CO2
already present in the original pyrogas
concentration. This reduces the cost and
increases of efficiency of the chemical
reaction process. The non equilibrium plasma
catalysis – dry CO2 reforming process of
pyrogas experiments were conducted at a
temperature range of 800 C - 900 C. Exhaust
gases from the plasma reforming process were
analyzed with a gas chromatograph. Results
show increases in the concentrations of
hydrogen and carbon monoxide (CO) after the
plasma chemical catalysis process. Also, the
concentrations of methane, ethane, propane
and carbon dioxide (CO2) reduced when
compared to their initial individual
concentrations in the pyrogas mix.
Fig 2: Thermodynamic simulation of Steam reforming of
Pyrolysis gas
Fig 3: Graph shows the variations in gas concentrations
compared to initial concentrations of individual gases that
constitute Pyrogas. Changes in the concentration of individual
gases with increase in enthalpy (KWhr/M3) can be observed.
Conclusion
The main purpose of this work is to
demonstrate and compare the effectiveness of
the gliding arc plasma assisted reformer in
removing light and heavy hydrocarbons
contained in pyrogas by converting the
hydrocarbons and carbon dioxide to syngas
using both steam reforming reactions and dry
CO2 reactions at atmospheric pressure
conditions. The data collected and results
analyzed during the non equilibrium plasma
reforming experiments indicate the plasma
reforming of pyrogas into synthesis gas
(syngas) using plasma - dry CO2 reforming
reaction. The analyzed experimental results
from plasma - dry CO2 reforming were also
similar to thermodynamic predictions. Note
that the pyrogas from an industrial gasifier is
usually at a relatively high temperature (800900C); hence plasma energy is mainly
required to stimulate chemical reactions and
not as a source of heat energy in the reforming
processes discussed in this paper. This
therefore makes energy consumption for
pyrogas very low. Conversion rates of the
different gases were higher with plasma -dry
CO2 reforming reaction when compared to
plasma - steam reforming reaction. Lower
conversion rates observed with plasma – steam
reforming reaction may be attributed to the
inability to maintain steam temperature
required for homogenous reaction. This
ultimately demands an increase in energy
required to maintain steam at the same
temperature of other reactants in the system.
As a result, steam reforming occurs at a much
lower temperature.
The data presented in this work further
suggests that pyrogas conversion with gliding
arc plasma is stimulated by plasma catalytic
effect. The results also show the ability to
produce hydrogen rich syngas with the gliding
arc plasma technology effectively. Gliding arc
plasma does not just provide energy but
stimulates attainment of thermodynamic
equilibrium. The experimental results
presented here further support some of the
advantages of gliding arc plasma which
include fast start-up time, compactness,
robustness, relatively low power consumption
and adaptability for various fuel types.
Further work is required to optimize
the gliding arc plasma reformer system for
steam reforming conditions. Future
optimization will also include minimizing heat
losses at both pre-plasma and post plasma
zones of the reformer; decreasing the enthalpy
of the system while maintaining Syngas yield
and conversion rates; and increasing residence
time.
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