Environmental and Energy Engineering Research Group

Department of Chemical and Biological Engineering

March 2014

Peter Weston, MEng

Supervisors: Professor Vida Sharifi

Professor Jim Swithenbank

Design and Development of a

Coandă Tar Cracker for Tar

Destruction in Biomass Gasification

Main Objective

To design and develop a novel

Coandă tar cracker for tar

destruction in biomass

gasification systems using a

two-stage system (pyrolysis-

Coandă tar cracker).

1. Pyrolysis 2. Tar Cracking 3. Gasification

Background:

Biomass Gasification

• A clean and efficient process that can utilise a range of different biomass materials

• Considered to offer the solution to providing clean sustainable energy in both

developed countries using advanced techniques and in developing countries to

provide rural electrification (Kirkels and Verbong, 2011)

• Gasification of biomass produces low calorific value gas, normally between 4.0 and

6.0 MJ/Nm3 (Stassen and Knoef, 1993)

• Tar formation is unavoidable and remains a key hindering factor in the technical

progression of biomass gasification

Gasification technology (Kirkels and Verbong, 2011)

Feedstock

• Wood

•Waste

•Crop residues

Gasifiers

• Fixed bed

• Fluidised bed

• Entrained flow

Gas Cleaning

Applications

• Electricity

• Heat

• Transport fuels

Background: Tar

Removal Systems

Secondary removal methods (Devi et al., 2003)

Gasifier

Tar

Removal

Gas

Cleaning

Producer

Gas

+ Tar Biomass

End-Use

Gasifier

& Tar Removal

Air,

Steam,

Oxygen

Air,

Steam,

Oxygen

Biomass

Primary removal methods (Devi et al., 2003)

Tar Free

Gas Gas

Cleaning End-Use

Background: Partial

Oxidation

Micro-Swirl Burner (Houben, 2004)

Partial oxidation offers tar reduction

without waste or soot and the

additional benefit of maintaining the

heating value of the tars in the gas

product, although at the loss of a

proportion of the heating value of

the treated gas which is combusted

(van der Hoeven, 2007)

Low primary rates of air (λ=0.2) in a

partial combustion system resulted

in a reduction of over 90% of the

total tar content (Houben et al.,

2005)

Background: Coandă

Ejectors

The Coandă effect can be described as a phenomenon in which a jet blown over a

surface, that has a convex curvature, adheres to that surface (Gregory-Smith and

Gilchrist, 1987)

1. High pressure air inlet, air enters through an annular slit

2. Secondary, surrounding fluids entrained

3. Mixing between primary flow and secondary flow along the length of the reactor

(Lien & Davis 2008)

Defining the Problem

Blue flame Air in

‘Tarry’

pyrolysis gas

Sub-stoichiometric

combustion

Entrainment of the

pyrolysis gas at the inlet

of the ejector device

High mixing potential

between primary air and

pyrolysis gas

Recycling of

flame product

Experimental

Operation

Wood Pellets - Ø 6 mm

• C.V(MJ/kg) 18.0

• Moisture wt% 7.7

• Volatiles wt% 73.8

• Fixed Carbon wt% 9.9

• Ash wt% 0.4

Fuels and Feeding

• Wood pellets are fed

into the pyrolyser

using a batch twin-

valve feeding system

• Feed rate: 3 – 4.5

kg/hr

• Ten 100g batches

were loaded

• Total of 1 kg of

pellets pyrolysed per

experiment

Experimental

Operation Pyrolysis Unit

• 316 stainless steel

• Internal diameter – 200 mm, Length - 1300 mm

• Temperature range (500 – 800 °C)

• Purge gas: Nitrogen – 8 LPM

Experimental

Operation Tar Cracking

• Air flow rate to tar cracker: 4 – 40 LPM

• 5 K-type and 1 R-type thermocouples to monitor temperature

• Sub-stoichiometric, high temperature (> 1000 °C) blue flame

combustion of pyrolysis gas

Experimental Rig

Schematic Diagram of the Experimental Rig

Results: Pyrolysis

Only

500 600 700 800

0.0

0.1

0.2

0.3 Carbon Monoxide

Carbon Dioxide

Hydrogen

Methane

Pyrolysis Temperature (°C)C

O,

CO

2 &

CH

4 (

kg

/kg

of

fue

l)

0.000

0.002

0.004

0.006

0.008

H2 (

kg

/kg

of

fue

l)

500 600 700 800

0

100

200

300

400

500

Pro

du

ct

Yie

ld (

g/k

g o

f fu

el)

Pyrolysis Temperature (°C)

Char

Gas

Tar

• Char yield decreases

• Gas yield increases

• Tar yield decreases at 800 °C

• Increase in pyrolysis temperature

caused an increase to the gaseous

yields of CO, CO2, CH4 and H2

Results: Pyrolysis

Only

Pyrolysis Temperature (ºC) 500 600 700 800

Char Yield Analysis

Char weight (g) 293.4 227.8 158.9 156.3

Char Yield (%) 29.34 22.78 15.89 15.63

Calorific Value (MJ/kg) 31 29 31 31

Proximate Analysis (wt.%)

Moisture 1.0 1.2 1.0 0.9

Volatiles 13.9 12.9 6.8 6.4

Fixed Carbon 85.7 85.4 91.3 92.7

Ash 0.1 0.3 0.6 0.2

Ultimate Analysis (wt.%)

Carbon 80.7 86.5 87.3 93.7

Hydrogen 2.7 2.0 1.1 1.0

Nitrogen 0.3 0.3 0.3 0.3

Pyrolysis Temperature vs. Char Characteristics

Results: Tar

Cracking

500 600 700 800

0

2000

4000

6000

8000

10000

12000

Be

nze

ne

(m

g/n

m)

Pyrolysis Temperature (ºC)

Pyrolysis Only

Pyrolysis & CTC

500 600 700 800

0

500

1000

1500

2000

2500

3000

3500

4000

4500

To

lue

ne

(m

g/n

m)

Pyrolysis Temperature (ºC)

Pyrolysis Only

Pyrolysis & CTC

• Hydrogen, oxygen, hydroxyl and hydroperoxy radicals are formed at higher

levels. These radicals increase the decomposition of tar species due to an

increase in the rate of H-atom abstraction

Results: Tar

Cracking Gravimetric Tar Comparison

500 600 700 800

0

10

20

30

40

50

60

70

Gra

vim

etr

ic T

ar

Yie

ld (

g/n

m3)

Pyrolysis Temperature (°C)

Pyrolysis and CTC

Pyrolysis Only

CFD Analysis

Coanda gap, e, (mm) 0.08

Nozzle length, a, (mm) 1.2

Radius, R, (mm) 5

Nozzle Ratio, R/a 4.2

Diameter, D, (mm) 26.6

Air velocity at inlet, (m/s) 75

Pressure of air at inlet, (Barg) 6.6

(1) Flow field in the

ejector (2) High

velocity region (3)

Geometry

Coandă

gap, e

Industrial

Applications Three Stage Gasifier Biochar Generation

• Recycle hot gases to drive

pyrolysis reaction

• Continuous operation

• Produces biochar for a number of

applications

Main Conclusions

• At each of the tested pyrolysis

temperatures, the yield of gravimetric tar

decreased when the producer gas was

treated by the Coandă tar cracker

• Key tar species were reduced when

comparing the yields of the gas treated

by the tar cracker with the un-treated

gas; naphthalene was reduced by 98%,

benzene was reduced by 95% and

toluene reduced by 96% when the

pyrolyser was operated at 800 ºC

Acknowledgements and Contact

I would like to thank the Engineering and Physical Science Research Council

(EPSRC) for their financial support for this PhD project.

Peter Weston

Department of Chemical and Biological Engineering

The University of Sheffield

Sheffield

S1 3JD

Tel: +44 (0)114 222 4910

Email: peter.weston@sheffield.ac.uk

References

Devi, L., Ptasinski, K.J., Janssen, F.J.J.G., 2003. A review of the primary measures for tar elimination in biomass

gasification processes. Biomass and Bioenergy 24, 125–140.

Gregory-Smith, D.G., Gilchrist, A.R., 1987. The compressible Coanda wall jet—an experimental study of jet

structure and breakaway. Int. J. Heat Fluid Flow 8, 156–164.

Han, J., Kim, H., 2008. The reduction and control technology of tar during biomass gasification/pyrolysis: An

overview. Renew. Sustain. Energy Rev. 12, 397–416.

Houben, M., Delange, H., Vansteenhoven, A., 2005. Tar reduction through partial combustion of fuel gas. Fuel 84,

817–824.

Houben, M.P., 2004. Analysis of tar removal in a partial oxidation burner. Technische Universiteit Eindhoven.

Kirkels, A.F., Verbong, G.P.J., 2011. Biomass gasification : Still promising ? A 30-year global overview. Renew.

Sustain. Energy Rev. 15, 471–481.

Lien, T.K., Davis, P.G.G., 2008. A novel gripper for limp materials based on lateral Coanda ejectors. CIRP Ann. -

Manuf. Technol. 57, 33–36.

Stassen, H.E.M., Knoef, H.A.M., 1993. Small Scale Gasification Systems, in: The Netherlands: Biomass Technology

Group, University of Twente.

Van der Hoeven, T.A., 2007. Partial product gas combustion for tar reduction. Eindhoven University of Technology.