Packed Bed Combustion:An Overview

William HallettDept. of Mechanical Engineering

Université d’Ottawa - University of Ottawa

Packed Bed Combustion - University of Ottawa - CICS 2005

Introduction

air

fuel feedproducts

xbed

grate

Packed Bed Combustion: fairly large particles of solidfuel on a grate, air supplied from beneath

- also called mass burning or fixed bed combustion

- bed not fluidized, particles remain at rest

Packed Bed Combustion - University of Ottawa - CICS 2005

ApplicationsHistorically, this was the main means of burning coal,wood, coke and charcoal

- up to 7 tons/hr (. 50MW heat release) in locomotives,15 tons/hr (. 120 MW) stationary

CPR 890, North British Loco. Works 1903

Packed Bed Combustion - University of Ottawa - CICS 2005

Modern industrial applications:- wood/wood waste combustionin pulp and paper plants- trash incineration- power gen. (eg Denmark)- small scale district energysystems using biomass

Related processes:- gasification of coal orbiomass - main difference is amuch thicker bed- smelting iron in blast furnaceor cupola

Boiler for 50 t/h woodwastes and sludge (Bowater, Gatineau, 1993)

Applications

Packed Bed Combustion - University of Ottawa - CICS 2005

Process Types

fuel

fuelfeed

products

Overfeed (spreader stoker - counterflow)

fuel

fuelfeed products

ash

Travelling (chain) grate (crossflow)

air

air

Packed Bed Combustion - University of Ottawa - CICS 2005

air

products

ash

Inclined grate (crossflow)

Process Types

www.vonrollinc.com

fuelfeed

www.vonrollinc.com

Packed Bed Combustion - University of Ottawa - CICS 2005

fuel

fuel feed

air

products

air

fuel

Retort-type underfeed stoker(cocurrent flow/crossflow)

Process Types

Pure Underfeed(cocurrent flow - not practical)

products

Underfeed Combustion

fuelfeed

air

Packed Bed Combustion - University of Ottawa - CICS 2005

Process TypesUnderfeed Combustion - multiple retort stoker

fuel feedfeed channel

“tuyeres”(air feed - grates)

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Underfeed Bed?

Danish “cigar burner” for straw - 1 million tons/yr

straw bale “cigar” air inlet at mouth

www.videncenter.dk

Packed Bed Combustion - University of Ottawa - CICS 2005

Why Packed Bed Combustion?

- simple to operate

- largely insensitive to fuel composition, moisture(up to 60%), particle size, impurities

- minimum of fuel preparation required

Packed Bed Combustion - University of Ottawa - CICS 2005

Overfeed Bed

fuel motion

products

air

fuelfeed

- simplest fuel bed configuration, most commonobject of tests and models

- transient overfeed bed is simple model forcombustion of a “slice” of a travelling orinclined grate bed

Packed Bed Combustion - University of Ottawa - CICS 2005

Solid Fuel Combustion Processes

drying devolatilization(pyrolysis) - CO, CO2,hydrocarbons evolved,char (C) remains

H2Ovolatiles - burnin gas phase

ASH

C 6 CO

CO

CO 6 CO2

char (C)combustion

Q Q

O2

Packed Bed Combustion - University of Ottawa - CICS 2005

surface

reduction

oxidation

primary air feed

Ash

products

grate

C + CO2 6 2CO

C + 0.5 O2 6 COCO + 0.5 O2 6 CO2

devolatilization

drying

charcombustion

secondary air

fuel feed

Overfeed Bed - Structure

particlemotion

- overfeed = counterflow combustion

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Overfeed Char Bed - Operating Diagram

0

0.04

0.08

0.1

0 0.4 0.8 1

0

primary air mass flux (kg/m2 bed/s)

upper limit of airflux - blowout(convective)

lower limit of airflux - extinction(radiative/kinetic)

- for 3.2cm cokeparticles (carboncombustion only)

Packed Bed Combustion - University of Ottawa - CICS 2005

Overfeed Bed - Characteristics- thin bed - primary product CO2

- thick bed - ( > 8 particles thick) - primaryproduct CO - essentially a gasifier

- volatiles almost always burn above bed withsecondary air

Stoichiometry of an overfeed bed is determinedONLY by bed thickness. Changing air flow only

changes combustion rate, not stoichiometry!

- bed behaviour as function of bed thickness scaleswith particle size - determining parameter is ratioof thickness to particle size.

Packed Bed Combustion - University of Ottawa - CICS 2005

Overfeed Bed - Characteristics

Light firing ... will produce the highest combustionefficiency. ... Maintain a thin, level, bright fire anduniform distribution of fuel over the entire gratesurface.

- instruction manual for the Standard BK Locomotive Stoker,The Standard Stoker Company, New York, 1942.

Packed Bed Combustion - University of Ottawa - CICS 2005

Overfeed Bed - CharacteristicsNYC tests - 1940

- elutriation losses athigh firing ratescause efficiency todrop substantially

Practical grateloading limits:

40 lb/ft2hr (200 kg/m2hr)

NYC J1 Hudson

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Travelling or Inclined Grate

bed motion

“green” fuel

pyrolysis front

char

ash

ignition/volatiles combustion

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Underfeed Bed

products

fuelfeed

air

0.04

0 0.2 0.4

primary air mass flux (kg/m2 bed/s)

0.02

rate ofignition

rate ofburning

rate of ignitionand burning

unsteady steady state

depth

Pure Underfeed(cocurrent flow)

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Underfeed Bed

- smaller particles -operating diagram shiftsto higher combustionrates while roughlypreserving stoichiometry

smallerparticles

stoich. line (all C to CO2)

- stoichiometry of bedchanges sharply with airflow (unlike overfeed bed)

Packed Bed Combustion - University of Ottawa - CICS 2005

Packed Bed Quantitiesparticles

voids

Particle size d- volume-equivalent diameter for non-spherical

and/or non-uniform particles - determine frombed mass, ,, and density

Void fraction ,- vol. fraction of interparticle

space - range 0.33 (close-packed

spheres) to 0.7

Packed Bed Combustion - University of Ottawa - CICS 2005

Packed Bed QuantitiesSpecific surface area aB = particle surface area/unit

total bed volume (solids + voids)

Sphericity

Superficial velocity v = flow rate divided by totalbed cross-section (solids plus voids) - actualvelocity in voids is called interstitial velocity.

Packed Bed Combustion - University of Ottawa - CICS 2005

C + ½ O2 6 COCO2 + C 6 2 CO6

CO + ½ O2 6 CO2

mass transfer (CO, O2, CO2)heat transfer to/from surface

particle motion

gas flow heat conduction in solid

species diffusion and heatconduction in gas phase

radiation,conductionbetweenparticles

particle burnout, shrinkage,ash release

volatiles release

pressuredrop

Packed Bed Combustion Processes

pyrolysis

Packed Bed Combustion - University of Ottawa - CICS 2005

Modelling Packed Bed Combustion Usual assumptions:

- continuum treatment of packed bed (no trackingof individual particles)

- one dimensional (uniform properties intransverse directions)

Governing equations:1. Gas and solid phase continuity

Packed Bed Combustion - University of Ottawa - CICS 2005

Governing equations:2. Gas and solid phase energy

Modelling Packed Bed Combustion

3. Gas phase diffusion

Packed Bed Combustion - University of Ottawa - CICS 2005

Modelling Packed Bed Combustion

5. Properties (from correlations in literature):- kSeff - effective solid conductivity - includes inter-

and intra-particle conduction and radiation- kG eff, Di eff - effective (turbulent) gas conductivity

and diffusivity (back diffusion) - oftenneglected, but significant

- hSG - heat transfer solid to gas

Governing equations:4. Particle consumption

- from particle number balance or particle tracking

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Modelling Packed Bed Combustion Analytical solution

- possible for diffusion-controlled comb’n withmany simplifying assumptions (Mayers 1945,Spalding 1954)

Numerical solutions- possible simplifications:

- equal solid and gas temps.- fast reaction- neglect back-diffusion in gas- neglect particle motion

- many recent examples in literature

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Modelling Packed Bed Combustion

0

20

40

1200

2400

0 5 10 15 height above grate (cm)

TGTS

CO2

CO

O2

T

- sample: 15 cm overfeed bed of 0.76 cm coke

ash ox’n reduction of CO2

Packed Bed Combustion - University of Ottawa - CICS 2005

Modelling - Details

Pyrolysis- many models for single-particle pyrolysis, but

only a few have integrated it into bed model(Peters et al.; Würzenberger et al.)

Fuel

gas flow

Ash

masstransfer

Ash- accumulates in voids, affects

heat and mass transfer- can play major role in extinction- little work as yet

Packed Bed Combustion - University of Ottawa - CICS 2005

Experimental Techniques

grate (dropped tosample bed)

secondary air

gas samplingprobe

window

fuelbed

flowstraightener

thermocouples

primary air

Gas-chromato-graph

refractory lining

- most common tool: the “pot” combustor (overfeed)

Packed Bed Combustion - University of Ottawa - CICS 2005

Experimental Techniques

Factors to consider:- bed / particle size ratio should be small to reduce

- void fraction anomalies near wall- “bridging” and cavity formation

- shape of fuel particle - to avoid regular packing- fuel preparation- fuel feeding- ignition mode and boundary conditions must be

related to modelling- steady state or transient?

Packed Bed Combustion - University of Ottawa - CICS 2005

Experimental Techniques

Measurements:- optical access impossible, therefore all

measurement techniques invasive- thermocouple probes- gas sampling probes (water-cooled - disturbance!)- tar collection probe- bed properties: void fraction from water

displacement, sphericity by permeametry

Bed Sampling (after run)- fuel particle size, chemical or ASTM analysis- ash particle size distribution, unburnt C

Packed Bed Combustion - University of Ottawa - CICS 2005

Experimental Techniques

“Fuel particle” with data storage- Swithenbank (Sheffield) - for local temp. and

motion measurement in incinerator bed

Packed Bed Combustion - University of Ottawa - CICS 2005

Current Work

- mostly focussed on combustion of wood andagricultural wastes- mostly European (Denmark, Sweden, Norway,Finland, Germany, Austria, Switzerland, Italy,Spain, France)

- trash incineration - large scale work in Sheffield,U.K. and in Karlsruhe, Germany

- some coal stoker work in Britain, Eastern Europe

Packed Bed Combustion - University of Ottawa - CICS 2005

Research FrontiersTwo-dimensional beds (travelling or inclined grate)- currently treated as moving transient 1-D bed

0 0 0 0

5

10

15

x - cm

10 min. 30 min. 46 min. 68 min

1 1 1 1

TS

C

ash

ash

C

TS

ashC

TS C TSash

ash and carbon fractions

1800 1800 1800 1800300 300 300 300temperature - K

Packed Bed Combustion - University of Ottawa - CICS 2005

Pollutant formation/destruction- particularly important for incineration

Non-uniform particle size- most work with mono-sized fuel particles- mono-sized fuel + ash (two sizes)- effect of distribution of fuel particle sizes??

Two-dimensional beds- tranverse gas mixing, heat transfer- particle motion and mixing on inclined grate

Research Frontiers