Preliminary Investigations into the

Interactions Between Fe-based CLC Oxygen

Carrier Materials and Biomass TarMatthew E. Boot-Handford, Nick Florin, Paul S. Fennell*

p.fennell@imperial.ac.uk

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