Computational fluid dynamic simulation of a solid biomass combustor: modelling approaches

  • Martin Miltner
  • Aleksander Makaruk
  • Michael Harasek
  • Anton Friedl
Original Paper

Abstract

The importance of biomass in combustion processes for the combined production of electrical power and district heat is still rising. In the presented work, CFD is used for the development and optimisation of an innovative combustion chamber for a solid stem-shaped biofuel in the form of compressed biomass bales. The main focus of this investigation is the maximisation of the thermal output of the combustor by an optimisation of the bale burnout and the minimisation of gaseous emissions such as VOCs, carbon monoxide and nitrogen oxide. For this purpose the functionality of a commercial CFD-solver has been extended in terms of the solid phase description and the solid–gas interactions. These sub-routines comprise the description of the solid biomass fuel as a porous bed, the biomass drying, the degradation during devolatilisation and char burnout, as well as the generation of gaseous species and the release/consumption of energy during these three steps. Moreover a simplified model for the prediction of NOx-emissions emanating from the fuel-bound nitrogen has been implemented. The results of this work show that the application of CFD enables a significant reduction of the development costs and the time-to-market of innovative chemical engineering concepts such as solid biomass combustion.

Keywords

Biomass Combustion Emission reduction NOx Straw CFD 

List of symbols

A

Arrhenius pre-exp. Factor

Aspec

specific surface (m2/m3)

D

molecular diffusion coefficient (m2/s)

dp

particle diameter (m)

E

activation energy (J/mol)

hm

heat transfer coefficient (W/(m2 K))

hrs

heat transfer coefficient for radiation at the contact surface (W/(m2 K))

hrv

effective radiation heat transfer coefficient of the voids (W/(m2 K))

J

Colburn factor

k

kinetic constant

kEff

effective reaction rate constant

kFilm

film diffusion coefficient

ls

equivalent thickness a layer of fluid should have to represent the same thermal resistance as the sphere (m)

lv

equivalent thickness a layer of fluid should have to represent the same thermal resistance as the fluid film (m)

Δl

distance between char particles (m)

m

mass (kg)

MG

molecular weight (g/mol)

Nu

Nusselt number

\( p_{{{\text{O}}_{{\text{2}}} }} \)

oxygen partial pressure (Pa)

R

universal gas constant (J/(mol K))

r

chemical reaction rate (kmol/(m3 s))

Re

Reynolds number

Sc

Schmidt number

Sh

Sherwood number

T

temperature (K)

t

time (s)

VCel

cell volume (m3)

vGas

gas velocity (m/s)

x

species mass content (wt%)

ε

char bed porosity

Φ

char combustion stoichiometric ratio

λEff

effective thermal conductivity of a packed bed (W/(m K))

λ

thermal conductivity (W/(m K))

μGas

gas viscosity (kg/(m s))

φGas

gas density, (kg/m3)

ξ

emissivity

References

  1. Adanez J, Gayan P, de Diego LF, Garcia-Labiano F, Abad A (2003) Combustion of wood chips in a CFBC: modelling and validation. Ind Eng Chem Res 42(5):987–999CrossRefGoogle Scholar
  2. Bassilakis R, Carangelo RM, Wojtowicz MA (2001) TG-FTIR analysis of biomass pyrolysis. Fuel 80(12):1765–1786CrossRefGoogle Scholar
  3. Bech N, Wolff L, Germann L (1996) Mathematical modelling of straw bale combustion in cigar burners. Energy Fuels 10(2):276–283CrossRefGoogle Scholar
  4. Brink A, Kilpinen P, Hupa M (2001) A simplified kinetic rate expression for describing the oxidation of volatile fuel-N in biomass combustion. Energy Fuels 15:1094–1099CrossRefGoogle Scholar
  5. Dwivedi PN, Upadhyay SN (1977) Particle-fluid mass transfer in fixed and fluidized beds. Ind Eng Chem Process Des Dev 16(2):157–165CrossRefGoogle Scholar
  6. Fjellerup J, Henriksen U (2003) Heat transfer in a fixed bed of straw char. Energy Fuels 17(5):1251–1258CrossRefGoogle Scholar
  7. Fletcher DF, Hayes BS, Chen J, Joseph SD (2000) A CFD based combustion model of an entrained flow biomass gasifier. Appl Math Model 24(3):165–182CrossRefGoogle Scholar
  8. FLUENT 6 Users Guide (2005) FLUENT IncGoogle Scholar
  9. Glassman I (1996) Combustion, 3rd edn. Academic Press, San DiegoGoogle Scholar
  10. Hill SC, Smoot LD (2000) Modelling of nitrogen oxides formation and destruction in combustion systems. Prog Energ Combust Sci 26(4–6):417–458CrossRefGoogle Scholar
  11. Jordan C, Miltner M, Potetz A, Harasek M (2005) Modellierung turbulenter Freistrahlen mit numerischer Strömungssimulation (CFD) (in German). Chemie Ingenieur Technik 77(8):1061–1062CrossRefGoogle Scholar
  12. Kaer SK (2005) Straw combustion on slow-moving grates—a comparison of model predictions with experimental data. Biomass Bioenergy 28(3):307–320CrossRefGoogle Scholar
  13. Magnussen BF, Hjertager BH (1976) On mathematical modelling of turbulent combustion with special emphasis on soot formation and combustion. In: 16th International symposium on combustion, Cambridge. The Comb. Inst, Pittsburgh pp 719–729Google Scholar
  14. Menter FR (1994) Two-equation Eddy-viscosity turbulence models for engineering applications. AIAA J 32(8):1598–1605CrossRefGoogle Scholar
  15. Miltner M, Jordan C, Potetz A, Harasek M (2005) Behandlung von turbulenten Drall-Freistrahlen mit CFD (in German). Chem Ingenieur Technik 77(8):1061–1062Google Scholar
  16. Miltner M, Miltner A, Harasek M, Friedl A (2006) Process simulation and CFD calculations for the development of an innovative baled biomass-fired combustion chamber. Appl Therm Eng (in press)Google Scholar
  17. Reid RC, Prausnitz JM, Poling BE (1987) The properties of gases & liquids, 4th edn. McGraw-Hill, New YorkGoogle Scholar
  18. Russell NV, Beeley TJ, Man CK, Gibbins JR, Williamson J (1998) Development of TG measurements of intrinsic char combustion reactivity for industrial and research purposes. Fuel Process Technol 57(2):113–130CrossRefGoogle Scholar
  19. Van der Lans RP, Pedersen LT, Jensen A, Glarborg P, Dam-Johansen K (2000) Modelling and experiments of straw combustion in a grate furnace. Biomass Bioenergy 19(3):199–208CrossRefGoogle Scholar
  20. Wilcox DC (1998) Turbulence modelling for CFD. DCW Industries, CaliforniaGoogle Scholar
  21. Winter F, Wartha C, Hofbauer H (1999) NO and N2O formation during the combustion of wood, straw, malt waste and peat. Bioresour Technol 70(1):39–49CrossRefGoogle Scholar
  22. Yagi S, Kunii D (1957) Studies on effective thermal conductivities in packed beds. AIChE J 3(3):373–381CrossRefGoogle Scholar
  23. Zhou H, Jensen AD, Glarborg P, Jensen PA, Kavaliauskas A (2005) Numerical modeling of straw combustion in a fixed bed. Fuel 84(4):389–403CrossRefGoogle Scholar
  24. Zolin A, Jensen AD, Jensen PA, Dam-Johansen K (2002) Experimental study of char thermal deactivation. Fuel 81(8):1065–1075CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Martin Miltner
    • 1
  • Aleksander Makaruk
    • 1
  • Michael Harasek
    • 1
  • Anton Friedl
    • 1
  1. 1.Institute of Chemical EngineeringVienna University of TechnologyViennaAustria

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