Packed Bed Combustion: An Overview. William Hallett Dept. of Mechanical Engineering Université d Ottawa - University of Ottawa

Transcription

1 Packed Bed Combustion: An Overview William Hallett Dept. of Mechanical Engineering Université d Ottawa - University of Ottawa

2 Introduction Packed Bed Combustion: fairly large particles of solid fuel on a grate, air supplied from beneath products fuel feed bed x grate air - also called mass burning or fixed bed combustion - bed not fluidized, particles remain at rest

3 Applications Historically, 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

4 Applications Modern industrial applications: - wood/wood waste combustion in pulp and paper plants - trash incineration - power gen. (eg Denmark) - small scale district energy systems using biomass Related processes: - gasification of coal or biomass - main difference is a much thicker bed - smelting iron in blast furnace or cupola Boiler for 50 t/h wood wastes and sludge (Bowater, Gatineau, 1993)

5 Process Types fuel feed products fuel air Overfeed (spreader stoker - counterflow) fuel feed products fuel air ash Travelling (chain) grate (crossflow)

6 Process Types Inclined grate (crossflow) fuel feed products air ash

7 Process Types Underfeed Combustion products products fuel fuel air fuel feed Pure Underfeed (cocurrent flow - not practical) air fuel feed air Retort-type underfeed stoker (cocurrent flow/crossflow)

8 Process Types Underfeed Combustion - multiple retort stoker tuyeres (air feed - grates) fuel feed feed channel

9 Underfeed Bed? Danish cigar burner for straw - 1 million tons/yr straw bale cigar air inlet at mouth

10 Why Packed Bed Combustion? - simple to operate - largely insensitive to fuel composition, moisture (up to 60%), particle size, impurities - minimum of fuel preparation required

11 Overfeed Bed fuel feed products fuel motion - simplest fuel bed configuration, most common object of tests and models - transient overfeed bed is simple model for combustion of a slice of a travelling or inclined grate bed air

12 Solid Fuel Combustion Processes H 2 O volatiles - burn in gas phase CO CO 2 CO O 2 C CO ASH drying Q Q devolatilization (pyrolysis) - CO, CO 2, hydrocarbons evolved, char (C) remains char (C) combustion

13 Overfeed Bed - Structure - overfeed = counterflow combustion fuel feed particle motion surface drying devolatilization reduction C + CO 2 2CO oxidation C O 2 CO CO O 2 CO 2 Ash products secondary air char combustion grate primary air feed

14 Overfeed Char Bed - Operating Diagram for 3.2cm coke particles (carbon combustion only) 0.04 upper limit of air flux - blowout (convective) primary air mass flux (kg/m 2 bed/s) 0 lower limit of air flux - extinction (radiative/kinetic)

15 Overfeed Bed - Characteristics - thin bed - primary product CO 2 - thick bed - ( > 8 particles thick) - primary product CO - essentially a gasifier - volatiles almost always burn above bed with secondary air Stoichiometry of an overfeed bed is determined ONLY by bed thickness. Changing air flow only changes combustion rate, not stoichiometry! - bed behaviour as function of bed thickness scales with particle size - determining parameter is ratio of thickness to particle size.

16 Overfeed Bed - Characteristics Light firing... will produce the highest combustion efficiency.... Maintain a thin, level, bright fire and uniform distribution of fuel over the entire grate surface. - instruction manual for the Standard BK Locomotive Stoker, The Standard Stoker Company, New York, 1942.

17 Overfeed Bed - Characteristics NYC tests NYC J1 Hudson - elutriation losses at high firing rates cause efficiency to drop substantially Practical grate loading limits: 40 lb/ft 2 hr (200 kg/m 2 hr)

18 Travelling or Inclined Grate green fuel ignition/volatiles combustion pyrolysis front char ash bed motion

19 Underfeed Bed products unsteady steady state depth 0.04 fuel feed rate of ignition rate of ignition and burning air 0.02 Pure Underfeed (cocurrent flow) rate of burning primary air mass flux (kg/m 2 bed/s)

20 Underfeed Bed - stoichiometry of bed changes sharply with air flow (unlike overfeed bed) stoich. line (all C to CO 2 ) - smaller particles - operating diagram shifts to higher combustion rates while roughly preserving stoichiometry smaller particles

21 Packed Bed Quantities Void fraction - vol. fraction of interparticle space - range 0.33 (close-packed spheres) to 0.7 particles voids Particle size d - volume-equivalent diameter for non-spherical and/or non-uniform particles - determine from bed mass,, and density

22 Packed Bed Quantities Specific surface area a B = particle surface area/unit total bed volume (solids + voids) Sphericity Superficial velocity v = flow rate divided by total bed cross-section (solids plus voids) - actual velocity in voids is called interstitial velocity.

23 Packed Bed Combustion Processes pressure drop radiation, conduction between particles volatiles release pyrolysis mass transfer (CO, O 2, CO 2 ) heat transfer to/from surface C + ½ O 2 CO CO2 + C 2 CO CO + ½ O 2 CO 2 particle burnout, shrinkage, ash release particle motion species diffusion and heat conduction in gas phase gas flow heat conduction in solid

24 Modelling Packed Bed Combustion Usual assumptions: - continuum treatment of packed bed (no tracking of individual particles) - one dimensional (uniform properties in transverse directions) Governing equations: 1. Gas and solid phase continuity

25 Modelling Packed Bed Combustion Governing equations: 2. Gas and solid phase energy 3. Gas phase diffusion

26 Modelling Packed Bed Combustion Governing equations: 4. Particle consumption - from particle number balance or particle tracking 5. Properties (from correlations in literature): - k Seff - effective solid conductivity - includes interand intra-particle conduction and radiation - k G eff, D i eff - effective (turbulent) gas conductivity and diffusivity (back diffusion) - often neglected, but significant - h SG - heat transfer solid to gas

27 Modelling Packed Bed Combustion Analytical solution - possible for diffusion-controlled comb n with many 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

28 Modelling Packed Bed Combustion - sample: 15 cm overfeed bed of 0.76 cm coke ash ox n reduction of CO 2 T G T S T CO O 2 CO height above grate (cm)

29 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.) Ash - accumulates in voids, affects heat and mass transfer - can play major role in extinction - little work as yet Fuel mass transfer Ash gas flow

30 Experimental Techniques - most common tool: the pot combustor (overfeed) gas sampling probe window Gaschromatograph secondary air refractory lining grate (dropped to sample bed) fuel bed primary air thermocouples flow straightener

31 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?

32 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

33 Experimental Techniques Fuel particle with data storage - Swithenbank (Sheffield) - for local temp. and motion measurement in incinerator bed

34 Current Work - mostly focussed on combustion of wood and agricultural 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

35 Research Frontiers Two-dimensional beds (travelling or inclined grate) - currently treated as moving transient 1-D bed 15 x - cm 10 ash 10 min. 30 min. 46 min. 68 min T S 5 C ash C T S ash C T S C T S ash ash and carbon fractions temperature - K

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

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