modeling of biomass combustion in grate fired fuel bed

Fatehi & Bai Lund 2010-02-11

Modeling of biomass combustion in grate fired fuel bed

Hesam Fatehi, Xue-Song BaiDivision of Fluid Mechanics

Dept. of Energy SciencesLund University


Outline

• Solid Fuel

• Governing Equation

• Numerical Discreetization

• Results

• Future work


Solid Fuels


Combustion of Solid Fuels

• Three main types of combustion systems for firing the solid fuels are

– Fixed-bed Combustion

– Suspension

– Fluidized-bed Combustion

• behavior of these three combustion system is related to the behavior of a single fuel particle.

• When a solid fuel particle is exposed to a hot flowing gas stream, it undergoes three stages of mass loss:

– Drying

– Devolatilization or Pyrolysis (generally refers to the anaerobicthermal decomposition of solids )

– Char Combustion


Combustion of Solid Fuels

• The Time and Sequence of these three processes depends on

– Fuel Type

– Fuel Moisture

– Size

– Heat and Mass Transfer


Typical boiler


Governing Equation


Two phases


Conservation of mass

��(��+ (1 − ��)��)

��+ ��∙ ��+ ��∙ ��= 0

��

• Conservation of mass

• Conservation of mass for the solid phase

• Conservation of mass for the gas phase

��(��)

��+ ��∙ ��=

�� ω� g��

��

��((1 − ��)��)

��+ (��∙ ��)��= � ω� s

��

��


Conservation of mass

�� = �� = �� = �� = �� = �� = �� = �� ω� s = ω� drying + ω� dev + ω� charcomb

ω� s + ω� g = 0


Conservation of Energy

• where is the enthalpy of formation at the reference temperature (T=25ºc)

��(��ℎ��+ ��(1 − ��)ℎ��)

��+ ��∙ ��ℎ��+ ��∙ ��ℎ��

��

− ��∇��+ (1 − ��)��∇��

= � ��dV��

ℎ�� = " ��ℎ��

ℎ��= # ��

��0+ ℎ��°

ℎ°


Enthalpy of particle

• particles contain different materials (ash, moisture, char and DAF wood)

• The enthalpy of the particle can be written as sum of the enthalpy of these different materials

ℎ��= ��ℎ ℎ��ℎ + ��ℎ�� + ��ℎ��ℎ��ℎ��+ ��ℎ��

ℎ�� = % ��0 + (1 − ��)ℎ��° + ��(ℎℎ2��° − ��)

�� = �� +4190 ��1−��

1+ ��1−��

+ )23.5��0 − 1320 ��1−��

− 6191. ��1−��

�� = 3.87��+ 103.1


Wood CP

�� = ��ℎ ��ℎ + �� + ��

��ℎ = 800, �� = 4200, �� = 2400 ��/��


Wood enthalpy of formation

• assume wood (formed only from Carbon, Hydrogen and Oxygen) on a one step complete combustion

• The products of combustion are only the carbon dioxide and water(in gas form)

��+ ��2 → ��2 + ��2��

ℎ��° = �� 2��

ℎ��2° + �� 2��

��ℎ��2��° + ��


Kinetic rates

�� 456 �� + ��

�� 456 ��ℎ��+ ��ℎ + ��

��ℎ�� 45556 ��2 + ��2��


Drying

• the mass change between two phases due to drying is equal to the mass rate of H

2O extracted from the solid going to the gas

phase.

• Two different drying models can be used. The first one is based on the Arrhenius expression of chemical reaction model

• The second drying model calculates the evaporation rate of a particle using convective mass transfer between the solid and gas phases. This model expresses drying as a function of moisture content of the wood and the surrounding air and hot gases

• km is the mass transfer coefficient (m/s), Ap is the area of particle cross section (m2), and are the concentration of moisture on gas flow and on particle surface, respectively (kg/m3).

�� drying = �� , �� = ��exp(−��ℛ��)

ℳ� ��drying = −��(��∞ − ��)

��∞ ��


Devolatilization

• where the subscribe DAF means Dry Ash Free particle and is the fraction of each of tar, char and gas inside DAF particles

�� dev = �� , ��= ��exp(−��ℛ��)

�� dev �� = ∅��∗ ℳ� ��dev

�� dev �� = ∅�� ∗ ℳ� ��dev

�� dev ��ℎ��= ∅��ℎ��∗ ℳ� ��dev (��ℎ�� ℎ�� )

∅


Char Combustion Rate

�� charcomb = −60 ��?��2@ 11

�� + 1��

, ��= 2.3��(−11100/��)

�� ℎ�� (��)


Numerical Modeling


Drying, devolatilizationDevolatilization, combustion

combustion

ash

fuelModeled fuel bed shape

Fuel bed and primary air and flue gas supply

several zones supplying hot flue gas + air


Input Data: Fuel supply

• The fuel - waste

– High ash content: 21.448%

– Low heating value of fuel as DAF: 9.09 MJ/kg

– Moisture content: 30.248%

– Mass rate: 2.933 kg/s

– Energy input = mass rate x LHV = 2.933x9.09x0.483 = 13 MW

• DAF fuel composition (mass fractions)

– C%: 51.76

– H%: 6.87

– O%: 40.52

– N%: 0.842

• DAF fuel reaction mechanism

– DAF Fuel = 0.293 Gas + 0.557 Tar + 0.15 Char

– Gas = 0.22CO2 + 0.51H2O + 0.22CO + 0.05CH4


Input Data: Air supply

• Air supply to the furnace

– Total mass flow rate: 16.58 kg/s

– Primary air flow rate (50.3% of total air) = 8.34 kg/s

– Primary air temperature: 298.1 K

– Energy input through primary air: 0

• Primary air and flue gas distribution

– 8 zones


Mp_tar represents the

potential volatile in the solid.

Less Mp_tar indicates that the volatileshave been ’running’ outside of the solid particles

More volatile

at the fresh wood

Less volatiles in the

final particles. Here

the particle is mostly made of ash


Temperature


Mass of gas extracting from bed


Nitrogen


CO2


Drying


Hydrocarbons


Carbon Monoxide


Particle


Species


Mass balance

– Input from fuel: 2.933 kg/s

– Input from flue gas: 0. kg/s

– Input from primary air: 8.34 kg/s

– Output through volatile: 10.05 kg/s

– Output through Particle: 0.07 kg/s

– Loss through ash + char: 1.07 kg/s


Energy balance

– Input from fuel: 24.7 MW

– Input from flue gas: 0 MW

– Output through volatile: 6.6 +11.9 = 18.5 MW

– Loss through ash + char: 6.1 MW


CFD Modeling of Upper Part


Modeling Problem


• OpenFOAM

• Solver:

– Flow: buoyantSimpleFoam (Steady-state solver for buoyant, turbulent flow of compressible fluids)

• Prepare boundary condition

– Combustion: ?

• Radiation is an important issue


Expected Results


Future Work

• More detail kinetic rates; drying, devolatilisation, char combustion, volatile combustion, particle shrinkage

• Radiation models (more detail heat transfer models)

• Coupling bed model with upper part combustion CFD model

modeling of biomass combustion in grate fired fuel bed

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