packed bed combustion: an overview - engineeringby.genie.uottawa.ca/~hallett/cics05plenary.pdf ·...
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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)
Packed Bed Combustion - University of Ottawa - CICS 2005
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
Packed Bed Combustion - University of Ottawa - CICS 2005
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
Packed Bed Combustion - University of Ottawa - CICS 2005
Travelling or Inclined Grate
bed motion
“green” fuel
pyrolysis front
char
ash
ignition/volatiles combustion
Packed Bed Combustion - University of Ottawa - CICS 2005
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)
Packed Bed Combustion - University of Ottawa - CICS 2005
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
Packed Bed Combustion - University of Ottawa - CICS 2005
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
Packed Bed Combustion - University of Ottawa - CICS 2005
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