COMBUSTION IN POROUS MEDIA☼ BY Prof. Dr. Mohammed Hamdan & Eng. NABEEL ABO SHABAN

CONTENTS Combustion = Chemical reactions 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

Terminology Fuel Flame Porous Media

Fuel is a substance whose oxidation reaction is strongly exothermic.

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.

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.

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).

Combustion – Chemical reaction : Fundamental laws Combustible Elements in Fuels : C S H2 CO CH4 C2H4 C2H6 C3H8 ...... Alcans : CnH2n+2 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 ........ CnH2n+2 + ½ ( 3n + 1 ) O2  n CO2 + ( n + 1 ) H2O  Chemical Reaction of Oxidation + HEAT

II. Heating values of fuels Combustible Elements in Fuels : C S H2 CO CH4 C2H4 C2H6 C3H8 ...... Alcans : CnH2n+2 Alcens : CnH2n Aromatics ... Combustion :

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.

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.

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).

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:

Table A :Flame Propagation regimes in porous media 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

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 (>65), 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.

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 1650°C). 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 1250°C). 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. .

POURS MATRIALS SHAPES AND BURNERS FIG. 8 PACKED BEDS FROM AL2O3 SPHERES AND RINGS FIG. 9A) CERAMIC SiC FOAMS B). 20 kW BURNER WITH SiC foam FIG. 10 A. CERAMIC FIBER STRUCTURE B. 10 kW BURNER WITH AL2O3 FIBER STRUCTURE

SOME OF POROUS MATERIALS (b) (c) (d) 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

SOME OF POROUS BURNERS Figure 2: Air cooled porous medium burner with a maximum thermal power of 25 kW for the use in air heating systems Figure1: Independent vehicle heating system on porous burner basis Figure 3: Ring-type porous medium combustor

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.

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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