preliminary investigations into the interactions between fe...
TRANSCRIPT
Preliminary Investigations into the
Interactions Between Fe-based CLC Oxygen
Carrier Materials and Biomass TarMatthew E. Boot-Handford, Nick Florin, Paul S. Fennell*
Dept. of Chemical Engineering
Imperial College London
6th High Temperature Solid Looping Cycles Network Meeting
Politecnico Di Milano, Milan, Italy
1st - 2nd September 2015.
Presentation Overview
1. Brief introduction the application of chemical-looping technology for biomass processing
2. Overall Project Objectives
3. Reactor Setup and Experimental Procedure
4. Preliminary Results
5. Conclusions from preliminary investigations
6. Future research objectives
Chemical-Looping utilising Biomass
Biomass is a renewable source of energy, chemicals and fuels which is often available in large quantities and at a low cost
as a by-product of agriculture and forestry
The use of biomass coupled with CCS (BECCS) can potentially result in negative CO2 emission
The efficiency for biomass conversion using conventional fuel conversion techniques is constrained by the relatively low
energy density and high moisture content of biomass.
Integrated Gasification Chemical-Looping Combustion IGCLC could provide a highly efficient method for the production of
electricity, syngas (CO + H2) and/or H2.
Biomass gasification produces a syngas containing large quantities of tar (typically between 0.1-10 wt.% of initial biomass
input for a conventional gasification process).
What is Tar?
Tar is a broad term used to describe all organic compounds formed during pyrolysis, gasification and combustion of solid
fuels that are condensable at standard temperature and pressure.
Tar is a complex mixture of different hydrocarbons but typically consists of long chain, aromatic and polycyclic aromatic
hydrocarbons (PAHs).
Most of the primary pyrolysis tar compounds are cracked in subsequent high temperature gasification/ combustion
processes to combustible gases (H2, CO, CH4, light HCs), CO2 and H2O.
Small amount of stable secondary and tertiary tar compounds are also formed which exit with the syngas from a
conventional gasification process (typically 0.1-10 wt.% of the initial biomass input).
Problems Associated with Tar in the CLC Process
1. Tar cause clogging and blockages in gas lines, filters and downstream process equipment
– expensive and time consuming to remove
2. Reduces fuel conversion efficiency – energy content of the tar is not recovered
In the context of CLC:
3. The presence of tar can lead to coking of OCs – Temporary deactivation
– Reduced CO2 capture efficiency
• Study the interactions between biomass tar and oxygen carrier materials in chemical-looping processes.
• Develop mechanistic models to describe the observed thermochemical breakdown pathways of tar in a
CLC process.
• Establish whether the presence of tar has any effect on oxygen carrier deactivation
• Develop novel dual-functional oxygen carriers with enhanced tar cracking capabilities
• Investigate the application of chemical-looping technology to the upgrading of fuel gas produced from
biomass gasification.
Overall Project Objectives
Preliminary investigations
This project is still in its early stages.
Here, results from preliminary experiments investigating the interactions between biomass tar and two OC
materials are presented: 100 wt.% Fe2O3 OC material
60 wt.% Fe2O3/40 wt.% Al2O3 OC material.
Effect of OC oxidation state and carrier gas composition on the following properties were measured
(i) the pyrolysis product distribution,
(ii) extent of carbon deposition
(iii) reactivity of the OC after exposure
High shear mixing and homogenisation of the dry
powders.
DI water added to form a paste.
Paste extruded through a sieve (425 µm) to form extrudates of
regular and controlled diameter.
Extrudates partially dried at 35 ˚C.
Partially dried extrudatesspheronised to achieve
spherical, uniform particles of the target size fraction (300 –
425 µm).
Dried particles sintered at 1000 ˚C for 4 hours to improve
mechanical strength.
Description of Wet Granulation Fabrication Technique
300 µm
300 µm
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
1 10 100 1000
dV
p/d
(lo
g(D
p))
[cm
3.g
-1]
Pore Diameter, Dp [nm]
100FeMM
60Fe40AlMM
Oxygen Carrier + 100FeMM 60Fe40AlMM
Surface Area [m2.g-1] 1.06 28.96
Cumulative vol. of pores (1.7 - 300 nm) [cm3.g-1] 0.017 0.213
Cumulative Volume of pores (0.05-10 µm) [cm3.g-1] 0.15 0.47
Skeletal density x10-6 [g.m-3] 4.00 4.83
Porosity (0.05-10 µm) 0.37 0.69
Envelope density x 10-6 (0.05-10 µm) [g.m-3] 2.52 1.48+ All morphological data for unreacted OC particles
Oxygen Carriers
100FeMM* – 100 wt.% Fe2O3
60Fe40AlMM – 60 wt.% Fe2O3 / 40 wt.% Al2O3
* MM = Prepared via Mechanical Mixing Technique
Precursor Powders
Fe2O3 Powder (5 µm, Sigma Aldrich)
Al(OH)3 Powder (Sigma Aldrich) 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1 10 100 1000 10000 100000
dV
p/d
(lo
g(D
p))
[cm
3.g
-1]
Pore Diamer, Dp [nm]
100FeMM
60Fe40AlMM
Schematic of 2-Stage Fixed Bed reactor system
Why use this reactor system?
• Can be utilised to emulate CLC with pre-gasification of
solid fuels (biomass pyrolysis used to emulate fuel gas
containing large quantities of tar)
• Allows independent control of 1st stage (fuel
pyrolysis/gasification zone) and 2nd stage (CLC/tar cracking
zone) temperatures (up to 1000 ºC).
• Unwanted interactions that may interfere with results are
inherently avoided (ie interactions involving tar and ash; tar
and chars; ash and OC (or tar cracking catalyst).
• Can be operated at pressures up to 30 bar.
Schematic of 2-stage Fixed-Bed Reactor
Operating Parameters for a 5 Cycle CLC Experiment with Reduction by CO
only
Operating Conditions 1st Stage
Temperature (1st / 2nd Stage) 500 ºC / 700 ºC
Oxygen Carrier (2nd Stage) 100% Fe2O3 (0.750 g, 300 - 425µm)
Reducing Gas 3 mol.% CO/ 15 mol.% CO2 (5 mins)
Oxidising Gas 5 mol.% O2 / N2 (5 mins)
Purge Period 100 mol.% N2 (2 mins)
Pressure 1.5 bar
5 Cycle Simulated CLC Experiment Concentration Profile with reduction by CO only at 973 K
0 1000 2000 3000 4000 5000 6000 7000 8000 90000
5
10
15
20
25
30
35
Time / s
Gas C
oncentr
ation [vol.%
]
CO2
CO
Calibration CyclesReduction phase Oxidation phase
Simulated CLC experiments with exposure to biomass pyrolysis products
Operating Conditions Description
Temperature (1st / 2nd Stage) 500 ºC / 700 ºC
Oxygen Carrier (2nd Stage) 100FeMM or 60Fe40AlMM (0.750 g, 300 - 425µm)
Reducing Gas 3 mol.% CO/ 15 mol.% CO2 /N2 Bal. (5 mins)
Oxidising Gas 5 mol.% O2 / N2 Bal. (5 mins)
Purge Period 100 mol.% N2 (2 mins)
Pressure 1.5 bar
Biomass 0.1 g Beechwood (106 – 150 µm)
Gas Composition during biomass feed
(1) 100 mol.% N2
(2) 15 mol.% CO2, bal. N2 (15%CO2)
3 mol.% CO, 15 mol.% CO2, bal N2 (3%CO15%CO2)
Cycle position of biomass feed
3rd cycle before reduction with CO (PreRed)
3rd cycle after reduction by CO (PostRed)
3rd cycle during reduction with CO (InRed)
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000-5
0
5
10
15
20
25
30
35
Time / s
Ga
s C
on
ce
ntr
atio
n [vo
l.%
]
CO2
CO
2 Reduction / oxidation activation cycles prior to the biomass feed
Combined CLC and Biomass pyrolysis – Condition (1) N2_PreRed
8350 8400 8450 8500 8550 8600 8650 8700-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
Time / s
Ga
s C
on
ce
ntr
atio
n [vo
l.%
]
CO2
CO
CH4 Biomass fed into reactor
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000-5
0
5
10
15
20
25
30
35
Time / s
Ga
s C
on
ce
ntr
atio
n [vo
l.%
]
CO2
COBiomass fed into reactor with 3% CO / 15% CO2
7000 7100 7200 7300 7400 7500 7600 7700-2
0
2
4
6
8
10
12
14
16
18
Time / s
Ga
s C
on
ce
ntr
atio
n [vo
l.%
]
CO2
CO
CH4
Combined CLC and Biomass pyrolysis – Condition (5) 3%CO15%CO2_InRed
Baseline Experiments: Tar production in the 1st stage via fast pyrolysis at 773 K
0.0
0.2
0.4
0.6
0.8
1.0
1.2
200 250 300 350 400 450 500
No
rma
lise
dS
ign
al
Wavelength [nm]
0
1
2
3
4
5
0 5 10 15 20 25 30
No
rm
ali
sed
Sig
na
le
Time [mins]
x 103
SEC
UVF
Carrier Gas Composition
N2 15%CO2 3%CO15%CO2
Fast pyrolysis of beech wood in the single stage reactor produced between 43.0-47.4 wt.% tar.
The composition of the carrier gas had negligible effect on the pyrolysis product distribution- variation in both tar and gas yields were within
the experimental error range
The presence of CO2 and CO in the carrier gas had a small effect on the molecular size distribution (MSD) - small shift to larger
molecular mass species (SEC)
- Possible enhancement of
condensation and polymerisation reactions
Baseline Experiments: Tar cracking at 973 K over an inert sand bed
Subjecting the tar to elevated temperatures of 973 K (empty 2nd stage) caused a substantial decreased in the tar yield by 59 %.
0
1
2
3
4
5
6
0 5 10 15 20 25 30
No
rma
lise
dS
ign
al
Time [mins]
Addition of a small layer of inert sand (0.75 g) caused a further small decrease in tar yield
The presence of CO2 (15 mol.%) or CO (3 mol.%) had no additional effect on the amount of tar recovered although some differences in the
molecular size distribution were observed (SEC).
The presence of CO2 (15 mol.%) or CO (3 mol.%) caused a small increase in the total amount of CO2 and CO produced compared with
pyrolysis experiments in N2 – onset of gasification reactions at the elevated T of 973 K
0.0
0.2
0.4
0.6
0.8
1.0
1.2
200 300 400 500 600
No
rma
lise
d S
ign
al
Wavelength [nm]
x 103
N2_1Stage 15%CO2_Sand
N2_Empty 3%CO15%CO2_Sand
N2_Sand
SEC
UVF
Tar reduction accompanied by significant increase in CO yield and much smaller increases in the CO2 and CH4 yields.
~300 amu
0
1
2
3
4
0 5 10 15 20 25 30
No
rma
lise
dS
ign
al
Time [mins]
CLC with exposure to biomass pyrolysis products: Tar yields
60Fe40AlMM
0
1
2
3
4
0 5 10 15 20 25 30
No
rma
lise
dS
ign
al
Time [mins]
100FeMM
100FeMM 60Fe40AlMM
The presence of the 100FeMM and 60Fe40AlMM caused further reductions in the tar yield compared with analogous experiments with an inert
sand bed.
The tar cracking effect of both OC materials appeared to be independent of the oxidation state and composition of the carrier gas
- enhanced cracking due to thermal or catalytic cracking on the surface of the OC material rather than the result of tar directly
reacting with the OC material.
60Fe40AIMM has greater reduction than the pure oxide
SEC analysis of the MSD of the recovered tar indicates there may be differences in the tar cracking/destruction mechanism.
N2_Sand N2_PreRed N2_PostRed
Carrier Gas = 100 mol.% N2
Tar Yield from equivalent
experiments with sand
x 103
0
1
2
3
4
0 5 10 15 20 25 30
No
rma
lise
dS
ign
al
Time [mins]
0
1
2
3
4
0 5 10 15 20 25 30
No
rma
lise
dS
ign
al
Time [mins]
CLC with exposure to biomass pyrolysis products: Tar yields
60Fe40AlMM
100FeMM
100FeMM 60Fe40AlMM
The presence of the 100FeMM and 60Fe40AlMM caused further reductions in the tar yield compared with analogous experiments with an inert
sand bed.
The tar cracking effect of both OC materials appeared to be independent of the oxidation state and composition of the carrier gas
SEC analysis of the MSD of the recovered tar indicates there may be differences in the tar cracking/destruction mechanism.
5%CO2_Sand 15%CO2_PreRed 15%CO2_PostRed
Carrier Gas = 15 mol.% CO2, 85 mol.% N2
Tar Yield from equivalent
experiments with sand
x 103
CLC with exposure to biomass pyrolysis products: Carbon Deposition(a) (b)
(c) (d)
15%CO2PreRed
* Wire in images is 200 µm in diameter
15%CO2PostRed 15%CO2PreRed 15%CO2PostRed
100FeMM 60Fe40AlMM
Carbon deposition on the surface OC material accounted for 2.7-9.9 wt.% of the initial
biomass feed mass.
The extent of carbon deposition was slightly higher for the 60Fe40AlMM across all
conditions studied
The increase in carbon deposition was roughly equivalent to the difference in tar yield
observed between the two OC materials
Carbon deposition was higher when the OC material was exposed to the biomass tar in
its reduced (Fe3O4) formDetermined via CHN elemental analysis (Assumes
carbon was uniformly distributed across all the
surfaces of the OC particles)
CLC with exposure to biomass pyrolysis products: Rates of reduction with CO
0
2
4
6
8
10
12
1 2 3 4 5 6
Rate
[x
10
5m
ol.
s-1.g
-1]
Cycle No.
3.0
4.0
5.0
6.0
7.0
8.0
9.0
1 2 3 4 5 6
Rate
[x 1
05
mol.
s-1.g
-1]
Cycle No.
100FeMM 60Fe40AlMM
There appears to be a small increase in the rate of reduction of the 100FeMM OC after exposure to the biomass pyrolysis products in the
3rd reduction cycle phase.
- lack of experimental data points it is not possible to determine the significance of this variation.
- however, no deterioration in the reactivity of the 100FeMM was observed.
Variation in the rate of reduction for the 60Fe40AlMM was within experimental error
Reduction by CO only
N2_PreRed
N2_PostRed
15%CO2_PreRed
15%CO2_PostRed
3%CO15%CO2_InRed
Conclusions
• Both the 100FeMM and 60Fe40AlMM OC materials caused enhanced tar cracking compared with the tar cracking
induced by sand.
• The tar cracking effect was independent of the oxidation state of the OC material
- additional tar cracking was a result of enhanced thermal or possibly catalytic cracking
interactions on the larger surface area provided by the OC materials
• Exposing the pyrolysis products to the OC in its oxidised form resulted in a greater conversion to CO2 as is desirable
in a CLC process.
• A lower conversion to CO2 was observed when the pyrolysis products were exposed to the reduced OC materials with
interactions appearing to favour production of CO.
- desirable for applications of chemical-looping reforming (CLR) and biomass syngas upgrading
Conclusions (Continued)
• Both materials were effected by carbon deposition.
• The extent of carbon deposition was greater for the 60Fe40AlMM OC material.
• The difference in carbon deposition between the two OC materials was roughly equivalent to the difference in tar yield.
• No obvious deleterious effects were observed in the reactivity of the OC materials after the OC materials were exposed to
the biomass tar
• This work serves provides a proof-of-concept for the use of the new 2-stage fixed-bed reactor for studying interactions
between CLC oxygen carrier materials and biomass tar.
Future work
• Continue screening different OC materials
• Develop dual-functional OC materials with tar cracking functionality, we will also test some CLOU OC materials.
• Identify promising OC materials with superior tar cracking capabilities and test at more relevant conditions for CLC
applications:
T = 1073-1273 K
P = 1 – 30 bara
Effect of steam
• Extended cycle tests to determine oxygen carrier performance over repeated oxidation and reduction cycling with
multiple exposures to gases contained large quantities of tars.
Acknowledgements
We gratefully acknowledge funding from the EPSRC under grants
EP/I010912/1: Multi-scale evaluation of advanced technologies for capturing the CO2: chemical looping applied to solid fuels.
EP/K000446/1: UKCCSRC - The United Kingdom Carbon Capture and Storage Research Centre
And UKCCSRC-C1-39: Chemical looping for low-cost oxygen production and other applications
The Energy Programme is a Research Councils UK cross council initiative led by EPSRC and contributed to by ESRC, NERC, BBSRC and STFC
CLC with exposure to biomass pyrolysis products: Gas yields100FeMM 60Fe40AlMM
Interactions between the oxidised OC materials and biomass pyrolysis products favours CO2 production whilst interactions with the
reduced OC materials favours CO production
Production of CO in N2_PostRed with 100FeMM – further reduction of OC to FeII and possibly Fe0.
Production of CO in 15%CO2_PostRed with 100FeMM – gasification reactions between CO2 in carrier gas and
pyrolysis products
– further reduction to FeII And Fe0 prevented by inclusion of 15 mol.% CO2
in carrier gas
Reduction of Fe3O4 to FeAl2O4 not prevented by 15 mol.% CO2