COMBUSTION IN POROUS MEDIA

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COMBUSTION IN POROUS MEDIA

• BY
• Prof. Dr. Mohammed Hamdan
• Eng. NABEEL ABO SHABAN

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CONTENTS

• PART 1 Theoretical background Introduction
• Combustion Chemical reactions
• Heating values of fuel
• fundamentals of combustion
• 
  
• PART 2 Combustion in porous media
• Porous materials
• Porous burners
• Flame propagation
• Modeling of flame in porous media

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Terminology

• Fuel
• Flame
• Porous Media

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Fuel
is a substance whose oxidation reaction is
strongly exothermic.
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FLAME
Is the visible oxidation (combustion) of gaseous
material There is no flame unless you have
burning gases, therefore a liquid or solid
material can only burn with a flame if it is
converted into gaseous fuel.
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POROUS MEDIA

• Is presence of both a persistent
• solid matrix and avoid space
• which is occupied by one or more
• fluid phases within porous media
• domain .
• Rocks,ceramics,Human tissue are
• examples of porous media.

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Fundamentals of combustion

• Steps of the combustion process
• Heating and drying (evaporation of free and
  bonded water of the fuel.
• Pyrolytic decomposition (heat induced
  decomposition of solid fuel to gaseous, liquid
  and solid products)
• Gasification (the solid carbon is converted to
  gaseous carbon-monoxide)
• Oxidation (combustion of the produced
  intermediate gaseous (and liquid) products
  releasing heat)
• These different steps are timely and spatial
  interconnected in existing combustion devices.
• The combustion technology has to account for
  these conversion characteristics to ensure an
  environmentally sound combustion
• (i. e. complete oxidation).

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Combustion Chemical reaction Fundamental laws

• Combustible Elements in Fuels
• C S H2 CO CH4 C2H4 C2H6 C3H8 ......
• Alcans CnH2n2
• Alcens CnH2n
• Aromatics ...
• Combustion
•  
• C O2 ? CO2
• S O2 ? SO2
• H2 ½ O2 ? H2O
• CO ½ O2 ? CO2
• CH4 2 O2 ? CO2 2 H2O
• C2 H4 3 O2 ? 2 CO2 2 H2O
• ........
• CnH2n2 ½ ( 3n 1 ) O2 ? n CO2
  ( n 1 ) H2O

? Chemical Reaction of Oxidation HEAT
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II. Heating values of fuels
Combustible Elements in Fuels C S H2 CO
CH4 C2H4 C2H6 C3H8 ...... Alcans
CnH2n2 Alcens CnH2n Aromatics ...
Combustion
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Combustion in Porous Media

• COMBUSTION IN POROUS MEDIA HAS RECENTLY ATTRACTED
  ATTENTION IN COMPARISON WITH FREE FLAME BURNERS
  DUE TO
• CLEAN AND HIGLY EFFICIENT CHARACTERISTICS.
• THE HIGLY CONDUCTIVE NATURE OF THE POROUS BED AND
  STRONG RADIATIVE EMMISION FROM THE SOLID MATRIX.
• STRONG FEED BACK MECHANISM ALLOWS TO SIGNIFICANT
  AMOUNTS OF ENERGY FROM THE FLAME ZONE TO PREHEAT
  THE UNBURNED MIXTURE OF FUEL AND AIR CALLED
  EXCESS ENTHALPY BURNING.
• REDUCING NOx and CO EMMISIONS.
• COMBUSTION WITHIN POROUS MEDIA INCREASES THE HEAT
  TRANSFER PROPRTIES OF THE GAS MIXTURE.

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• Inert porous media combustors may offer high
  modulation range, low emission and high compact
  very small scale sizes, which correspond to the
  desired characteristics of industrial
  applications or household heating combustion.

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Classification of porous burners

• MATRIX-STABILIZED BURNER
• (Inside the porous matrix)
• SURFACE- STABILIZED BURNER
• (Near or on the porous material surface
  significant part of combustion taking place out
  side the matrix).

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FLAME PROPAGATION IN POROUS CUMBUSTION

• THE MAIN FACTOR THAT CONTROLS THE HEAT TRNSFER
  COEFFICIENT BETWEEN SOLID AND THE GAS IS THE PORE
  SIZE OF THE SOLID MATRIX WHICH IS USED IN THE
  POROUS BURNER.
• FLAME PROPAGATES WHEN THE RATE OF HEAT RELEASED
  FROM THE REACTION IS HIGER THAN THAT OF HEAT
  TRANSFER TO THE SURROUNDING MATERIAL.
• MODIFIED PECELT NUMBER (Pe) BY WHICH CRITICAL
  PORE SIZE CAN BE ESTIMATED

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Table A Flame Propagation regimes in porous media
Regime Speed of combustion wave m/s Mechanism of flame propagation
Low velocity 0-.0001 Heat conduction and inter phase heat exchange
High velocity .1-10 High convection
Rapid combustion 10-100 Convection , low pressure gradient
Sound velocity 100-300 Convection with significant pressure gradient
Low velocity detonation 500-1000 Self ignition with shock wave
Normal detonation 1500-2000 Detonation with momentum and heat loss
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Possibilty Of Combustion In Porous Media

• Combustion in inert porous media is possible if
• The modified Peclet-number defined with the
  typical diameter dm
• of the porous cavity size and the laminar flame
  velocity SL is high
• enough (gt65), for methane /air mixtures.
• The combustion inside the porous medium is very
  intense and the reaction zone in the porous
  matrix has an elongated form with a length of
  several centimeters in the stream wise direction
  when using premixed natural gas with air under
  atmospheric pressure.

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Materials for Porous Medium Combustion

• It is a special feature of this technology that
  it is dependent on special high-temperature
  resistant porous components.
• The most important material and forms for porous
  burners are
• SiC foams as well as mixer-like structures made
  of Al2O3 fibers, ZrO2 foams (can be used at
  temperatures above 1650C). and C/SiC structures.
• Iron-chromium-aluminum alloys and nickel-base
  alloys can be used. (Temperature resistant metal
  alloys may be used for temperatures below
  1250C).
• Ceramic foams of different base materials are
  also used for porous burners. Independent on the
  base material such structures feature good
  conduction heat transport, a rather long start-up
  phase, low radiation heat transport properties,
  intermediate dispersion properties, and a
  relatively high pressure drop.
• .

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POURS MATRIALS SHAPES AND BURNERS
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SOME OF POROUS MATERIALS

• Different ceramic porous materials
• AL2O3 fiber structure.
• (b) C/SiC structure
• (c) static mixer made of zirconium foam
• (d) Fe-Cr-Al-alloy wire mesh

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SOME OF POROUS BURNERS
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ASSUMPTIONS TO 1D FLAME MODEL

• (l) The burner is adiabatic without heat losses
  at exit.
• (2) The flame structure and heat transfer
  mechanism are one-dimensional.
• (3) Potential catalytic effects of the high
  temperature solid are negligible.
• (4) The Dufour effect, 'bulk' viscosity and body
  forces are negligible.
• (5) The flow speed is sufficiently low that the
  process is isobaric.
• (6) The mixture gas is non radiating.

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PHYSICAL MODEL TO STUDY THE COMBUSTION IN NON
HOMEGENOUS POROUS MEDIA

• Figure shows a schematic diagram of a
  one-dimensional physical model. It consisted of
  two porous ceramic cylinders stacked together and
  insulated around the circumference. The upstream
  and downstream ceramic cylinders were referred to
  as preheating region (PR) and the stable burning
  region (SBR).
• The porous ceramic was a reticulated matrix that
  consisted of
• alumina oxide (Al2O3). Pore densities as
  specified of l0 pores per inch (PPI) was used for
  the SBR and the PR had 66 PPI. The unit porosity
  was 87 for l0 PPI and 83.4 for 66 PPI
• The actual mean pore diameter was 1.52 mm for l0
  PPI and 0.29 mm for 66 PPI length of PR was 5 cm,
  and the SBR was 10 cm.

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THANK YOU VERY MUCH