Mechanisms of conversion of heavy
hydrocarbons in biogas
initiated by pulsed corona discharges
V.A. Bityurin, E.A. Filimonova, G.V. Naidis
Institute for High Temperatures of
Russian Academy of Sciences,
Moscow, Russia
e-mail: helfil@mail.ru
ACKNOWLEDGMENT
The work was partially supported by
NWO-RFBR Grant No 04-02-89006
and
RFBR Grant No 05-02-16590.
Urgency of problem
The contribution of biomass to the world’s energy supply is
ranging from 10-14%.
Biogas can be used in power generation and production of
synthetic fuels as methanol, hydrogen or bio-diesel.
Gasification of biomass produces significant amounts of heavy
hydrocarbons (TAR).
TAR tends to condense in the downstream equipment leading to
several operational problems.
Main problem is to remove TAR from biogas and reduce the
energy consumption in cleaning process.
The modeling of removal process of TAR is a difficult task
because of absence of many data for simulation of plasmachemical kinetics at intermediate temperatures.
Composition of biogas
TAR specification
Main components:
15-21% H2, 10-22% CO,
11-13% CO2, 1-5% CH4,
and N2
C10H8 =~ 800-1000 ppm
Naphthalene Structure
Components Formula
Mass %
in tar
Benzene
C6H6
67.56
Toluene
C7H8
1.28
Naphthalene
C10H8
17.86
Penol
C6H6O
0.12
Pyrene
C16H10
2.94
Phenanthrene
C14H10
4.27
Fluorene
C13H10
0.53
Indene
C9H8
1.35
Fluoranthene
C10H10
2.83
Characteristic properties of the work
1)
2)
3)
4)
consideration of several gas compositions without O2,
gas temperature is close to 500 K;
simulation of positive streamer propagation in biogas;
consideration of primary plasma-chemical processes at the
discharge stage;
5) consideration of naphthalene removal as a one of the most
stable component;
6) using the chemical kinetics code taking into account nonuniform distribution of components in the discharge reactor;
7) comparison with experimental results obtained at the pilot
setup.
The goal of work is
to reveal the main mechanisms governing
naphthalene cleaning process
Experimental setup
Eindhoven University of Technology, Eindhoven, The Netherlands
High-Voltage
FTIR
Heating tapes +
insulation
U = 40-100 kV
Stack
Corona
reactor
Vsyst = 300 L
Vacuum pump
FTIR
N2
VR = 150 L
H2
Condensor
f = 50 Hz
CO2
GC
Q = 240 Nm3/hr
CO
Biogas
T ~ 500 K
Ep= 1-1.4 J/pulse
tar
injection
Recirculation
fan
Cooler
Pressure
gauge
Cleaning process
One pulse
Discharge stage: ~10-8 s
2D streamer model
E  ,  2  4π
N j
  ( N j  j E)  F j  S j
t
N2, CO2, CO, H2O, CH4, H2,
N4+, N2+, CO2+, H2O+,
e, N2(A3Σ), N, N(2D),
O, O(1D), OH, H, CH3
G-values
Afterglow stage: 4*10-2 s
Chemical kinetics
modeling
dn (t,r)
i
dt
  Qij  Q
dif
j
79 components and
350 reactions
Initial concentrations
Number of pulses is 400-1000 and more
Simulation of positive streamer propagation
The ionization coefficient (α/n) and mean energy (<ε>)
of electrons versus E/N.
p = 1 bar
T = 600 K
Solid red line is
51% N2+20% CO +
12% CO2+17% H2 mixture
Blue dashed line is
80% N2+20% O2 mixture
Cyan dash dotted line is N2.
Simulation of positive streamer
propagation
Electric field at the streamer axis
Gas composition:
50% N2 + 20% CO +
12% CO2 + 17% H2 +
1% CH4
p = 1 bar
T = 600 K
Simulation of positive streamer
propagation
G-values
G j    X i Fij
i
Fij 
Eh
1
eEh
2
 Rij dE
Ec
Gas composition:
50% N2 + 20% CO +
12% CO2 + 17% H2 +
1% CH4
p = 1 bar
T = 600 K
Reaction mechanisms for pure N2
Processes by direct electron
impact
N2+e —>
N2+e —>
N2+e —>
N2+e —>
N2+e —>
N2+e —>
N2+e —>
Species
N2(A)+e
N2(B)+e
N2(C)+e
N2(a)+e
N2(a’)+e
N(2D)+N+e
N2+ +2e
+
N2 + N2 + M —> N4 + M,
N4+ + e —> N2 + N2(A),
N2(A)
2.94
N
0.25
N(2D)
0.25
Species Concentration
Ion – molecule reactions and
dissociative recombination
+
G-values
10-10
τ~
s
τ ~ 10-7 s
N2(A)
7.8e+14 cm-3
N(2D)
6.7e+13 cm-3
N2+
1.8e+14 cm-3
Comparison of experimental and
calculation results in pure N2
Chemical kinetics
N2(A) + C10H8 —> products
N(2D) + C10H8 —> products
N2(A) + N2(A) —> N2 + N2(A)
Remaining fraction of C10H8
1.0
0.8
N2
0.6
0.4
diffusion —> C10H8
0.2
0.0
0
50
100
150
200
250
Input energy, J/l
300
350
Reaction mechanisms for
90%N2+10%CO2 mixture
G-values
Processes by direct electron impact
Species
N2+e —> N2(A)+e
N2+e —> N2(B)+e
N2+e —> N2(C)+e
N2+e —> N2(a)+e
N2+e —> N2(a’)+e
N2+e —> N(2D)+N+e
N2+e —> N2+ +2e
CO2+e —> CO+O+e
Ion – molecule reactions and
dissociative recombination
N2+ + N2 + M —> N4+ + M,
N4+ + e —> N2 + N2(A),
N4+ + CO2 —> N2 + N2 + CO2+,
CO2+ + e —> CO + O(1D),
τ ~ 10-10 s
τ ~ 10-7 s
τ ~ 10-9 s
τ ~ 10-7 s
I
II
N2(A)
2.94
2.02
N
0.25
0.22
N(2D)
0.25
0.22
O

0.05
O(1D)

0.7
CO

0.75
Comparison of experimental and
calculation results in 90%N2+10%CO2 mixture
Chemical kinetics
N2(A) + C10H8 —> products
N(2D) + C10H8 —> products
N2(A) + N2(A) —> N2 + N2(A)
CO + N2(A) —> N2 + CO
O + C10H8 —> H + C10H7O
CO2 + N —> CO + NO
CO2 + N(2D) —> CO + NO
CO2 + N2(A) —> CO+O+N2
NO + N —> O +N2
NO + NH —> H + N2O
NO + NH —> OH + N2
NO + NH —> O + N2H
diffusion —> C10H8
Remaining fraction of C10H8
1.0
90%N2+10%CO2
0.8
0.6
100%N2
0.4
0.2
0.0
0
50
100
150
200
250
Input energy, J/l
300
350
Comparison of experimental and
calculation results in 80% N2+10% CO2+10% CO
mixture
Processes by direct electron impact
N2+e —> N2(A)+e
N2+e —> N2(B)+e
N2+e —> N2(C)+e
N2+e —> N2(a)+e
N2+e —> N2(a’)+e
N2+e —> N(2D)+N+e
N2+e —> N2+ +2e
CO2+e —> CO+O+e
CO +e —> C+O+e
Remaining fraction of C10H8
1.0
80%N2+10%CO2+10%CO
0.8
0.6
100%N2
0.4
0.2
0.0
0
50
100
150
200
250
Input energy, J/l
300
350
Reaction mechanisms for
50% N2+20% CO+12% CO2+17% H2+1% CH4 mixture
Processes by direct electron impact
N2+e —> N2(A)+e
N2+e —> N2(B)+e
N2+e —> N2(C)+e
N2+e —> N2(a)+e
N2+e —> N2(a’)+e
N2+e —> N(2D)+N+e
N2+e —> N2+ +2e
CO2+e —> CO+O+e
CO +e —> C +O+e
H2+e —> H +H +e
Ion – molecule reactions and
dissociative recombination
N2+ + N2 + M —> N4+ + M, τ ~ 10-10 s
N4+ + e —> N2 + N2(A),
τ ~ 10-7 s
N4+ + CO2 —> N2 + N2 + CO2+, τ ~ 10-9 s
CO2+ + e —> CO + O(1D),
τ ~ 10-7 s
G-values
Species
I
II
III
H


0.5
C


0.12
N2(A)
2.94
2.02
N
0.25
0.22
0.12
N(2D)
0.25
0.22
0.12
O

0.05
0.18
O(1D)

0.7
0.7
CO

0.75
0.76
CH3


0.06
1.12
Comparison of experimental and calculation results in
50% N2+20% CO+12% CO2+17% H2+1% CH4 mixture
Chemical kinetics
N2(A) + C10H8 —> products
N(2D) + C10H8 —> products
N2(A) + N2(A) —> N2 + N2(A)
CO + N2(A) —> N2 + CO
O + C10H8 —> H + C10H7O
CO2 + N —> CO + NO
CO2 + N(2D) —> CO + NO
NO + N —> O +N2
NO + NH —> H + N2O
NO + NH —> OH + N2
NO + NH —> O + N2H
O + H2 —> H + OH
O(1D) + H2 —> H + OH
OH + C10H8 —> H2O +C10H7
H2 + C10H7 —> H + C10H8
diffusion —> C10H8
Remaining fraction of C10H8
1.0
50%N2+12%CO2+20%CO+17%H2+1%CH4
0.8
0.6
100%N2
0.4
0.2
0.0
0
50
100
150
200
250
Input energy, J/l
300
350
Conclusions
•
The results of simulation on naphthalene removal in biogas, pure
nitrogen and mixtures of N2 with CO, CO2 and H2 agree with the
experimental data rather well.
•
A original multifactor self-consistent approach for modeling of cleaning
process on the base of pulse corona discharge has been presented.
•
It has been found that the reaction of naphthalene with exited nitrogen
molecules plays a key role in the removal process. Addition to N2 of such
gases as CO, CO2 and H2 worsens the removal efficiency.
•
It is necessary to take into account the ion-molecule, electron-molecule
reactions and dissotiative recombination under the high electric filed
because of an appreciable N2(A) influence on the removal C10H8.
•
N2(A) is the best component for destruction of TAR because its energy is
enough to destroy TAR. For this reason a streamer type of discharges is
very suitable for TAR decomposition in biogas.
The knowledge of products and the channels of reactions with
participation of TAR is open problem today. Without the decision of this
problem we cannot talk about a toxicity level of cleaning.
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