from downdraft gasification of wood in thermal … · shown by blue line. other part of feed was...
TRANSCRIPT
Title [1/13]
Vadim A. Kuznetsov
Institute for
Electrophysics
and Electric
Power RAS
FROM DOWNDRAFT GASIFICATION OF WOOD IN THERMAL PLASMA OF AIR TO DEVELOPMENT OF MULTI-
GAS PLASMA TORCH
Role of Plasma in Gasification [2/13]
0.41 1.53
0.59 1.53 0.41
Paraformaldehyde could be gasified by air at 1500 K…
but efficiency will be about 67.7 % and syngas will be diluted.
To avoid these setbacks often oxygen is used instead of air...
but energy consumption will increaseand syngas will not stop being diluted.
0.29 0.71 0.29
LOW BIOMASS ENERGY
CONVERSION RATE
HIGHER BIOMASS ENERGY
CONVERSION RATE
AND INDIRECT CONSUMPTION
OF ELECTRICAL ENERGY
Role of Plasma in Gasification [3/13]
142.2 kJ/mol
Direct energy supply to the gasification process
makes possible to produce undiluted syngas.
HIGHEST BIOMASS ENERGY
CONVERSION RATE
AND DIRECT CONSUMPTION OF
ELECTRICAL ENERGY
Plasma gasification of wood
(LHV ~13.9 MJ/kg) by air plasma
at 1500 K & 1 atm.
The more energy is
supplied the less
oxygen is required to
maintain to process
temperature.
Plasma provides new controlled process
parameter – ENERGY CONSUMPTION.
High efficiency of energy transfer is a main
advantage of plasma. The only direct energy
supply is heating of oxidizers which is limited to
temperatures below 1000 °C due to efficiency
reduction.
Main Advantages of Plasma [4/13]
An electricity consumed by
plasma torch will be reproduced
with a surplus by combined cycle.
An electrical energy consumed by
plasma torch will be transformed
to the energy of liquid fuels.
Disbalance of energy prices in US
(average of 2013):
Electricity – 18.9 $/GJ
Liquid fuels – 34.8 $/GJ
Plasma can stabilize gasification
of wastes with variable
composition and LHV.
Plasma could be supplied to any
part of the gasifier and thus
increase local temperature.
Plasma is a highly reactive
oxidizer due to high content of
radicals and monatomic gases.
Plasma could be generated from
industrial gases (Air, H2O, CO2).
Experimental Installation [5/13]
1 – Gasifier; 2 – Plasma Torch; 3 – Afterburner; 4 – Duty Torch; 5 – Cyclone; 6 – Spray Scrubber;
7 – Packed Bed Scrubber; 8 – Exhaust Fan; 9 – Stack; 10 – Mass Spectrometer; 11 – Feeding System;
12 – Syngas Output Pipe; 13 – Ash Removal and Quenching Device. Process Steps: I – Accumulation;
II – Evaporation; III – Pyrolysis; IV – Oxidation; V – Reduction; VI – Unreactive; VII – Ash Removal.
Case:
D = 1.6 m
H = 4.2 m
Shaft:
D = 0.6 m
(IV-VI) = 1.9 m
Experimental Results [6/13]
Wood loading started load after 8 hours of charcoal
gasification at average air flow rate ~169.2 kg/h and
plasma energy supply ~67.7 kW.
Four distinct regimes could be considered: I - 8.00-9.16 h,
II - 9.23-10.06 h, III - 10.18-11.43 h, IV - 11.65-12.69 h.
Regimes I & II are not stable because hydrogen content
shows steady growth.
~351.55 kg of
wood was loaded
since 8 hours.
There are only ~2.29 hours of data for plasma gasification.
Oxygen and argon contents are near the detection level.I
II
III
IV
I
II III
IV
Experimental Results [7/13]
Evolution of temperature suggests unstedy thermal
balance on all regimes despite stable syngas composition.
Which means that gasification process takes place much
closer then 77 cm to the plasma injection zone.
I
II
III IV
At the outlet pipe syngas still contain some H2O and CO2
which means that charcoal bed is affected.
Syngas flow rate was calculated assuming that feed’s
nitrogen content is neglectable which is true for a wood.
I
II
IIIIV
One part of feed which was transformed to syngas is
shown by blue line. Other part of feed was transformed to
steam. Steam content of raw syngas was not measured.
Experimental Results [8/13]
If dashed lines are higher than solid lines of the same
color and the distance between black lines is twice bigger
than distance between red lines (if mass balance
suggests water as a unaccounted mass flow) regime
could be considered as a unaffected gasification of wood.
This is true for 13.10-13.22 hours with 10 % accuracy.
I
II III IV
This means either the presented results show rather
plasma gasification of mixed feedstocks than a wood
processing or there is an error in syngas measurements.
Regime III IV
Air/Syngas, kg/m3 0.525 0.573
Energy/Syngas, MJ/m3 1.323 0.851
H2 30.2 27.8
CO 25.7 23.8
CO2 7.8 8.9
N2 35.4 38.7
LHV, MJ/m3 5.83 5.37
Summarized process parameters (m3 at 25 °C and 1 atm).
The distortion in mass balance could be caused by
charcoal bed.
Discussion on Energy and Mass Disbalance [9/13]
Two major factors afflict results.
1. Oxidation by CO2 and H2O of coke bed which
was not originated from pyrolysis of wood.2. Heat losses in the reduction zone.
Gasification was not in stoichiometric regime and it
rarely is thus presence of CO2 and H2O unavoidable.
Volume between plasma inlet and syngas outlet is
about 0.67 m3 (contain about 126 kg of charcoal).
The sharp drop of temperature at the transition from
charcoal to wood gasification implies high reactivity of
wood and high temperatures in the mixing zone (about
1500-1700 °C considering oxidation of volatiles).
8 hours is not enough to reach stationary thermal
regime on this gasifier of at a low throughput.
The following steps should be done to resolve these issues:
- Decrease volume of the reduction zone;
- Increase both power of plasma torches and oxidant flow rate.
Unfortunately the increase of power and flow rate will took a full revamp of auxiliary systems.
Alternatively the CO2 and H2O could be implement instead of air.
The energy consumption will increase while syngas mass flow could be the same or even lesser.
Steam-air AC Plasma Torch [10/13]
The stabilization of direct current arc is achieved
by ballast resistance thus the efficiency of power
supply system is about 50-60 %.
WHY AC?DC plasma torches usually have several time lower arc
voltage than AC plasma torches and thus to achieve
have the same power they have to increase current.
Arc current directly
afflict electrode
erosion.
AC power supply system suppress arc instabilities by current-limiting inductors
and a capacitive reactive power compensators (efficiency is about 98-99 %).
Due to high
stabilization
capabilities of AC
power supply system
arc could have higher
length and with higher
variation of length .
Plasma torch
Power
supply
The arc length is
stabilized by gas flow
and is depended on
gas composition.
Gas Composition Influence on Plasma Torch Power [11/13]
Current = 28.12-29.27 A
Voltage = 1.03-1.58 kV
Efficiency = 94.3-95.3 %
Steam + Air = 6.55-6.80 g/s Coulomb scattering defines arc
conductivity at temperatures
higher than ~9000 K.
The higher hydrogen content in plasma forming gas
leads to higher thermal conductivity, higher energy
losses and higher thermal capacity of plasma.
The increase of hydrogen
content in plasma forming
gas leads to decrease in
arc temperature and
consequently decrease of
arc conductivity. At a
constant current this leads
to increase of voltage drop
to sustain the current.
Arc diameter ~4.47 mm
Arc length ~798 mm
Arc diameter and length are weakly
depend on the steam/air flow-rate ratio.
Coulomb cross-
sections are on
2-3 orders higher
than elastic.
Steam Plasma Torch Power Applications [12/13]
Apart from gasification steam plasma torch could be
implemented for methane conversion, tar reforming, ash
melting and other processes demanding high energy input.
Stoichiometric plasma conversion of methane.
Estimate for constant thermal energy flux ~1 MW/m3.
3
Energy consumption on CO
conversion to H2 was neglected.
Conclusions [13/13]
Process temperature is not limited anymore by adiabatic flame
temperature. It is now possible to produce syngas almost
completely free from CO2, N2, H2O and tars.
Plasma technologies provide a
brand-new way to control
gasification process.
THANK YOU.
Additional H2+CO in syngas carry 1.5-1.7 more energy than was
consumed on their generation thus if high efficiency combined
cycle is used then net efficiency grows. Additional gains surpass
expenses ~2 times for liquid fuels production using plasma.
Electrical energy spent on plasma
formation is recovered with surplus or
transformed to more valuable energy.
Plasma process shows ~1000 °C on the gasifier wall at 77 cm
distance downstream from plasma injection zone.High temperature zone is limited and
consequently gasification rate is high.
The higher enthalpy the more energy is transformed to syngas
LHV and higher temperature in injection zone.Plasma process parameters are enhanced
with increase of plasma enthalpy.
Highest plasma energy content was ~12 MJ/kg for steam-air operation and up to
~20 in multi-gas regime while efficiency was ~95 %, power ~50-120 kW.
Plasma enthalpy of about 20 MJ/kg of H2O is optimal for methane conversion at
stoichiometric regime.
New plasma torch has
high enough enthalpy to
consider it for industrial
applications.