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Improving understanding of gasification of woody biomass in a downdraft gasifier

Researcher Sanna Oikari1

Professor Jukka Konttinen2

Research Assistant Roshan Budhathoki2

1 Kokkola University Consortium Chydenius

2 University of Jyväskylä, Dept. of Chemistry

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Material balance

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Picture reference: Prof.Animesh Dutta, 2007, Asian Institute of Technology

http://www.soi.wide.ad.jp/class/20070041/slides/05/index_42.html

Fuel kg/h

Air kg/h Product gas kg/h

Ash kg/h

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Energy balance

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Fuel heating value

Sensible heat of gases

Product gas heating value

Product gas sensible heat

Heat loss from cylinder

Ash residue heating value

Ash residue sensible heat

Measurements in Energy- and mass balance evaluations

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5

Balance measurements

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Inputs

Fuel

Mass of fuel

-Wood species

Properties of fuel

- Moisture content

- Heating value

- Composition (ash and elements carbon,

hydrogen, nitrogen and oxygen)

Air

Flow

Ambient temperature

6

Balance measurements

Outputs

Product gas Gas composition CO2, CO, H2, H2O, N2 and CH4

Calorific value

Flow

Product gas temperature

Bottom ash Mass and composition Moisture content Calorific value of the ash

Tars

Mass and composition

Fly ash Mass and composition

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Balance measurements

General

• Duration of experiment

• Pressure in process

• Measuring devices used

• Temperatures in furnace (measured at a specific point)

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Energy- and mass balance measurements

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Place

Date

Duration of

experiment

Measuring

devices used

Fuel

• Mass of fuel

Air

flow

Ambient

temperature

Gasifier

Temperatures in

furnace (measured

at a specific point)

Properties of fuel

Moisture content

Calorific value

Wood species

Composition (C, H, N, O and ash)

Product gas

temperature

Pressure in process

Mass and

composition of the

tars

Heat losses

Bottom ash

Mass

Moisture content

Calorific value

ash composition

Fly ash

weight and composition

Product gas

Gas composition

H2O, CO2, CO, H2, N2 and

CH4

Calorific value

Flow

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Balance calculation Results

Measurements made in Sievi pilot-gasifier 15.11.2012

Evaluations like 30 % of air of amount needed in combustion

Balance calculations

Output/Input

relation calculation

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94 92 101

0

20

40

60

80

100

120

Energybalance

Overall massbalance

Carbonbalance

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Modeling benefits • Simulation • A working model can also be applied in design of a

commercial process • Also operator training can become possible

• Understand phenomenon more deeply optimization of gasification process Model needs to be validated with experimental results

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Three zone equilibrium modeling of Fixed bed Gasifier

Research Assistant Roshan Budhathoki

University of Jyväskylä, Dept. of Chemistry

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Model scheme

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• Model consist of three zones;

– Drying and Pyrolysis

– Oxidation

– Reduction

• Finally composition of gas and temperature is determined and used as input for next zone

• Predicts values of product gas composition and temperature of each zone

• Using chemical equilibrium and energy balance

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Model References

• GILTRAP D.L. et al. A steady state model of gas-char reactions in a downdraft biomass gasifier. Solar Energy, January 2003, Vol. 74, Issue 1, pp. 85-91.

• WANG Y and KINOSHITA C.M. Kinetic model of biomass gasification. Solar Energy, 1993, Vol. 51, pp. 19-25.

• BABU B.V. and SHETH P.N. Modeling and Simulation of Reduction Zone of Downdraft Biomass Gasifier: Effect of Char Reactivity Factor. Energy Conversion and Management, September 2006, Vol. 47, Issues 15-16, pp. 2602-2611

• BABU B.V. and SHETH P.N. Modeling and Simulation of Reduction Zone of Downdraft Biomass Gasifier: Effect of Air to Fuel Ratio. Journal on Engineering and Technology, 2007, Vol. 2, Issue 3, pp. 35-40

• GAO N. and LI A. Modeling and simulation of combined pyrolysis and reduction zone for a downdraft biomass gasifier. Energy Conversion and Management, December 2008, Vol. 49, Issue 12, pp. 3483-3490.

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Model References

• KOUFOPANOS C.A., LUCCHESI A. and MASCHIO G. Kinetic modelling of the pyrolysis of biomass and biomass components. The Canadian Journal of Chemical Engineering, February 1989, Vol. 67, Issue 1, pp. 75–84.

• ROY P.C., DATTA A. and CHAKRABORTY N. Modelling of a downdraft biomass gasifier with finite rate kinetics in the reduction zone. International Journal of Energy Research, July 2009, Vol. 33, Issue 9, pp. 833-851.

• ZAINAL Z. A. et al. Prediction of performance of a downdraft gasifier using equilibrium modeling for different biomass materials. Energy Conversion and Management, August 2001, Vol. 42, Issue 12, pp. 1499-1515

• RATNADHARIYA J.K. and CHANNIWALA S.A. Three zone equilibrium and kinetic free modeling of biomass gasifier – a novel approach. Renewable Energy, April 2009, Vol. 34, Issue 4, pp. 1050-1058.

• THUNMAN H. et al. Composition of Volatile Gases and Thermochemical Properties of Wood for Modeling of Fixed or Fluidized Beds. Energy Fuels, 2001, Vol. 15, Issue 6, pp. 1488-1497.

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Assumptions

• Process is isobaric

• The char is modeled as graphite carbon

• Char carry over is negligible

• No heat transfer between the zones

• The overall heat loss is assumed to be 10% of the product of equivalence ratio and HHV of fuel. – Q Tot = 10% × φ ×HHV

• Heat loss in pyrolysis, oxidation and reduction are assumed to be 25, 40 and 35% respectively

• Lamda assumed to be 0,27

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Assumptions

• Assumptions for pyrolysis zone

– 4/5 of oxygen is associated with fuel H in the form of H2O

– 1/5 of fuel oxygen is associated with fuel carbon and released as CO and CO2

– Etc

• Assumptions for oxidation zone

– Acetylene formed during pyrolysis is fully oxidized

– Hydrogen is also oxidized and converted into water

– Remaining oxygen is consumed in the char reduction to form CO and CO2

– Etc

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Pyrolysis zone

• The reaction in pyrolysis zone is assumed as;

C1HhOo + wH2O → ncC + nco2CO2 + ncoCO + nCH4CH4 + nH2H2 + nC2H2C2H2 + nH2OH2O

• Constituent balance: carbon balance; 1 = nc + nco2 + nco + nCH4 + 2nC2H2

hydrogen balance; h + 2w = 4nCH4+ 2nH2 + 2nC2H2 + 2nH2O oxygen balance; o + w = 2nco2 + nco + nH2O

• Energy balance: [ho

f+∆H]BM + w[hof+∆H]H2O = ∑ ni,p[hofi + ∆Hp]ni + Q1

– where hof = heat of formation

– ∆H = change in enthalpy

– ni, = pyrolysis products

– Q1 = heat loss at pyrolysis zone

– BM = biomass

– P = pyrolysis

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Oxidation zone

• The reaction in Oxidation zone is assumed as;

ncC + nco2CO2 + ncoCO + nCH4CH4 + nH2H2 + nC2H2C2H2 + nH2OH2O + a(O2 + 3.76N2) →

nc_oxC + nco2_oxCO2 + nco_oxCO + nCH4_oxCH4 + nH2O_oxH2O + nN2_oxN2

• Constituent balance: carbon balance; nc + nco2 + nco + nCH4 + 2nC2H2 = nc_ox + nco2_ox + nco_ox + nCH4_ox hydrogen balance; 4nCH4+ 2nH2 + 2nC2H2 + 2nH2O = 4nCH4_ox + 2nH2O_ox oxygen balance; 2nco2 + nco + nH2O + 2a = 2nco2_ox+ nco_ox + nH2O_ox

• Energy balance: ∑ ni,p[hofi + ∆Hp]ni_pyrolysis = ∑ ni,ox[h

ofi + ∆Hox]ni_oxidation + Q2 – where ni = gas composition in the respective zone

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Reduction zone

– The model is based on reduction zone model proposed by Giltrap. – The reaction in the reduction zone are considered as;

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Reaction Equation

Boudouard reaction C + CO2 = 2CO

Water-gas reaction C + H2O = CO + H2

Methane formation C + 2H2 = CH4

Steam reforming

reaction CH4 + H2O = CO + 3H2

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Reduction zone

Relation for constituent change:

Temperature change:

Flow change:

Pressure change:

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1.62% 21.26%

5.64%

10.39%

7.0%

0

10

20

30

40

50

60

H2 CO CO2 CH4 N2

Co

mp

osi

tio

n %

(d

ry)

Gas composition

Comparison of experimental vs. modeled

Present model

Sievi experimental

Results

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05

1015202530354045505560

0 5 10 15 20 25 30

Co

mp

osi

tio

n (

%)

Moisture content (%)

Gas composition (d.b) (φ = 0.27)

H2

CO

CO2

CH4

N2

Influence of moisture content on dry composition of product gas

Results

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• Influence of moisture content on carbon conversion and cold gas efficiency

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0%

20%

40%

60%

80%

100%

0 5 10 15 20 25 30

Effi

cie

ncy

(%

)

Moisture content (%)

Conversion Efficiency

Cold gas effficiency

Results

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0

5

10

15

20

25

30

35

40

45

50

55

60

0,2 0,25 0,3 0,35 0,4 0,45

Co

mp

osi

tio

n (

%)

Equivalence ratio

Gas composition (db.) (MC = 15%)

H2 CO

CO2 CH4

N2

Influence of equivalence ratio on dry gas composition of product

gas

Results

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0%

20%

40%

60%

80%

100%

0,2 0,25 0,3 0,35 0,4 0,45 0,5

Effi

cie

ncy

(%

)

Equivalence ratio

Conversion Efficiency Cold gas effficiency

Influence of equivalence ratio on carbon conversion and cold gas

efficiency

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References

1. PIENINIEMI, K. ja MUILU, Y.; Kaasutus ja tuotekaasun analysointi, in book: Lassi, Ulla ja Wikman, Bodil (ed.), Biomassan kaasutus sähköksi, lämmöksi ja biopolttoaineiksi, HighBio projektijulkaisu, University of Jyväskylä, Kokkola University Consortium Chydenius, Kokkola 2011

Also available in swedish:

Lassi, Ulla ja Wikman, Bodil (ed.), Förgasning av biomassa till värme, elektricitet och biobränslen : publikation för HighBio-projektet, Jyväskylä universitet, Karleby universitetscenter Chydenius, Karleby 2011

2. MUILU, Y.; EK-puukaasu Loppuraportti, Centria tutkimus ja kehitys, Ylivieska, 15.09.2007

3. LAMPINEN, A.; Uusiutuvan liikenne-energian tiekartta. Pohjois-Karjalan

ammattikorkeakoulun julkaisuja B:17, Joensuu 2009, 437 s.4. ALAKANGAS, E. 2000.

Suomessa käytettävien polttoaineiden ominaisuuksia, Otamedia Oy, Espoo, Valtion

teknillinen tutkimuskeskus, VTT tiedotteita 2045.

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References

5. SOUZA-SANTOS, M.L. 2010. Solid Fuels Combustion and Gasification, Modeling, Simulation and Equipment Operations, CRC Press Taylor &Francis Group, London

6. David R. Lide (editor). CRC Handbook of Chemistry and Physics, 83rd edition 2002 – 2003. CRC Press LLC, Boca Raton, Florida, USA. p. 8 – 111. 2002

7. Picture reference: Prof.Animesh Dutta, 2007, Asian Institute of Technology http://www.soi.wide.ad.jp/class/20070041/slides/05/index_42.html

8. RATNADHARIYA J.K. and CHANNIWALA S.A. Three zone equilibrium and kinetic free modeling of biomass gasifier – a novel approach. Renewable Energy, April 2009, Vol. 34, Issue 4, pp. 1050-1058.

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Conclusions

Calculated mass and energy balances were rather near 100 %.

Model predicted product gas composition reasonable well.

A working model can also be applied in design of a commercial process.

Material and energy balances are one way to analyze and optimize gasification.

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