combustion simulations of pre-dried greek lignite at
TRANSCRIPT
Combustion simulations of pre-dried Greek lignite at experimental and industrial scale facilities
Clean Coal Technologies Conference 2009 Dresden, 18. May 2009
Michalis
Agraniotis, Dimitris
Stamatis, Panagiotis
Grammelis, Emmanuel Kakaras
National Technical University of Athens Laboratory of Steam Boilers and Thermal Plants (NTUA / LSBTP)
Center of Research & Technology Hellas Institute of Solid Fuel Technology & Applications (CERTH / ISFTA)
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ContentsIntroduction:
Methodology:
Discussion - Results:
Industrial scale boiler
Summary - Conclusions
-
The potential of pre drying for the new generation of brown coal power plants
- Applied models in a comprehensive CFD combustion code - Simulation approach for the semi industrial scale facility (1MWth)-
Simulation approach for the industrial scale boiler (90MWth)
Semi industrial scale facility
-
Evaluation of influence of dry coal co-firing on combustion behaviour
in terms of: Temperature, O2
-
profiles burnout, heat flux, NOx
emissions
-
Evaluation of boundary conditions based on available experimental data
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Introduction
Needed heat for drying is obtained from a very high temperature level (high exergylosses)
The 110-120˚C vapour from the drying process is recirculatedinto the furnace and not further utilised
The potential of the pre drying concept for the new generation of brown coal power plants
Conventional drying with hot flue gas - Energetic drawbacks:
Brown coal pre drying – Energetic benefits
Low exergy losses due to the low temperature heat used for drying (low temperature steam)
Further efficiency increase through utilization of the produced vapour as a heating medium in the water pre-heaters
The Fluidized Bed Drying Concept with Internal Waste Heat Utilization (WTA concept) is currently being demonstrated in industrial scale
Brown coal pre-drying is a necessary step for the future application of the oxyfuel combustion technology also in brown coal power plants
The traditional firing concept
Boiler milling and drying with recirculated flue gas in the beater fan mills
Flue gas
1000˚C recircu- lated flue gas
Raw brown coal
Dried brown coal+ flue gas+ vapour
Flue gas
Boiler
The integration of the pre drying concept in a brown coal fired boiler
Dried brown coal (remaining moisture 12-15%)
Obtained vapour from WTA pre drying
Low temperature steam from the steam turbine
condensate
Raw brown coal
WTA dryer
milling
Source: www.rwe.com
Expected plant efficiency increase with WTA predrying, Source: RWE, VGB Powertech, 12-2006
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Activities of NTUA and CERTH/ ISFTA on lignite pre drying
Simulation of the performed tests at the 1MWth semi industrial scale facility, validation of the determined parameter set
Simulation of the 90 MWth boiler at reference and co-firing conditions. Evaluation of the expected change of combustion behaviour for increased co-firing thermal shares
Scope of the present numerical investigations
Already performed investigationsComparison of different pre drying technologies based on thermodynamic calculations. The fluidized bed dryer has been proven as one of the most promising concepts.
Thermodynamic calculations on the integration of a WTA dryer in two existing Greek power plants.
Drying tests of Greek lignite in a lab scale fluidized bed dryer. Determination of the equilibrium curves.
Dry lignite combustion tests in a semi industrial scale facility. Performed investigations: temperature profiles, emission measurements, fly ash sampling, investigation of slagging and fouling tendency.
Dry lignite co-firing tests in a 90MWth industrial scale boiler for a thermal share up to 5%. Performedinvestigations: temperature profiles at the superheater section, stack emission measurements, fly ash sampling, operational data monitoring.
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Simulated facilities: 1MWth semi industrial scale combustor
Technical characteristics of the semi industrial combustion facility
Temperature and emission profile measurements along the central furnace axis
Isokinetic fly ash particle sampling along the central axis
Fuel and ash sampling
Investigation of the slagging and fouling behaviour by using dedicated deposition probes in 4 different levels of the furnace
Fig. Schematic diagram of the 1MWth combustion facility Furnace exit
Facility
exit
Cylindrical top fired furnace
Milling and drying system comparable to the large scale plants with a scaled model of an industrial beater fan mill. Control of the flue gas mass flow recirculated in the furnace by separating the coal dust from the flue gas and vapour through a dedicated cyclone.Simulation of “raw coal” and “dry coal” combustion conditions is possible
Performed experimental investigations
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Simulated facilities: 90MWth industrial boiler
the boiler’s operational parameters including-
steam parameters, produced electric power -
operation of the feeding systemflue gas temperatures in the furnace exitflue gas emissions in the stackfuel burnout and residues’ quality
Fig: Drawing of the Liptol
boiler 1st
dry coal burner, level +5.5m
2nd
dry coal burner, level +8.3m
Furnace exit, level +24.5m
Investigation of the effects of dry coal co-firing on
SH2 exit, level +26.5m
Performed industrial scale measurements
Technical characteristics of PPC’s Liptol boiler
Dry coal burner levels at two different levels in the left and right boiler side Feeding of dry coal dust produced in a nearby drying plant with a pneumatic transport systemA dry lignite co-firing thermal share of up to 5% can be realised
In operation since 1960 Thermal input of the simulated boiler: about 90MWth, Steam production 80t/h and superheated steam parameters 485°C/ 64bar.Front wall fired with two jet burners each consisting of a lower and upper part
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Methodology (1 of 3) : Overview of the applied combustion models
Commercial code Fluent is used for the CFD simulations.
Realizable k-ε turbulence model used for the swirl dominated flow in the experimental combustor.
For radiation, the P1 model in the large scale boiler and the discrete ordinate model at the 1MWth combustor.
About 12.000 particles injected and tracked at the experimental scale facility, and about 50.000 particles at the industrial scale boiler.
Volatiles’ combustion according to the two steps global reaction mechanism and the Eddy-Break Up (EBU) model.
For NOx post-processing, ‘‘thermal NOx” (Zeldovich mechanism) and ‘‘fuel NOx”(nitrogen in fuel) submodels are considered.
Particle locations, number density and radiative properties
Turbulent, Chemically Reactive, Gas Flow Field Solution (Continuous)
• Continuity (mass conservation)
• Momentum Conservation
• Turbulence model
• Energy Conservation
• Species Conservation with chemical mechanisms
Particle Field solution (Discrete Phase)
• Heating, Drying, Devo-latilization and char oxidation (mass conservation)
• Convection, conduction, radiation (energy conservation)
• Trajectories (momentum conservation)
Radiative Transport Equation Solution
Problem Solution
Initialization
• Grid specification
• Boundary Conditions
• Model and solution options
Pollutant formation Solution
Converged flow field variables
Local gas temperatures, velocities, species composition
Mass, Energy and momentum source terms
Local volumetric radiative energy source terms
Local gas temperatures, species composition
Radiative energy exchange
Particle locations, number density and radiative properties
Turbulent, Chemically Reactive, Gas Flow Field Solution (Continuous)
• Continuity (mass conservation)
• Momentum Conservation
• Turbulence model
• Energy Conservation
• Species Conservation with chemical mechanisms
Particle Field solution (Discrete Phase)
• Heating, Drying, Devo-latilization and char oxidation (mass conservation)
• Convection, conduction, radiation (energy conservation)
• Trajectories (momentum conservation)
Radiative Transport Equation Solution
Problem Solution
Initialization
• Grid specification
• Boundary Conditions
• Model and solution options
Pollutant formation Solution
Converged flow field variables
Local gas temperatures, velocities, species composition
Mass, Energy and momentum source terms
Local volumetric radiative energy source terms
Local gas temperatures, species composition
Radiative energy exchange
Fig. Component models of a comprehensive combustion code
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One quarter of the facility is simulated due to symmetry in order to save computational time. The developed hexahedral structured grid is composed of about 350.000 cells. Boundary conditions of the simulated cases are taken from the performed experimental campaign.
1 MWth semi industrial scale combustor
Methodology (2 of 3): Simulation of the 1MWth combustor
Fig. Numerical mesh at the near burner region
Parametric investigations for a number of boundary conditions that were not determined during the measurements:
Simulated casesB0, reference case: Recirculation of the maximum achievable flue gas mass flow into the furnace.B1, dry coal co-firing case: Recirculation of about 50% of the maximum flue gas mass flow into the furnace.B2, dry coal firing: case: Recirculation of the minimum flue gas mass flow into the furnace.
Temperature distribution and emissivity of the refractory lined furnace wall.
Swirl number of the secondary combustion air.
Devolatilization and char combustion kinetics of the specific lignite.
Parametric investigations
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Methodology (3 of 3): Simulation of the industrial scale boiler
Optimised 3D geometry of 357.000 hexahedral cells (furnace and superheaters).No data regarding the distribution of combustion air between the different burner levels and the hopper, due to the difficulty in measuring the specific air mass flows at site.Different cases representing possible distributions of the combustion air set and simulated.
Raw lignite mills
Recirculated flue gas
Furnace
(1)
(4)
(2)
(3)
(5)A mass balance of the facility is performed, in order to determine the flue gas composition at the furnace outlet and the carrier gas composition after the mill.
Investigated cases:Reference case5% thermal substitution
10% <<20% << Energy and mass balance
Proximate analysis (% a.r.) Ultimate analysis (% daf)
Water 56,25 C 63,81
Ash 13,35 H 4,87
Volatiles 18,33 N 2,07
Fixed C 12,07 O 27,6
Hu (kJ/kg K) 5656 S 1,68
Table: Proximate and ultimate analysis of the Greek lignite
Industrial scale boiler
Fig. Boiler’s numerical mesh
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Results: the 1MWth combustor (1 of 2)
Parametric investigations of wall boundary conditions
100200300400500600700800900
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0.00 1.00 2.00 3.00 4.00 5.00 6.00
axial position (m)
T (˚C
)
B0, exp. data
twalls 1100 deg C
twalls 1000 deg C
twalls 900 deg C
twalls 800 deg C
100200300400500600700800900
10001100120013001400150016001700
0.00 1.00 2.00 3.00 4.00 5.00 6.00axial position (m)
T (˚
C)
B0, exp. data
q=-14/10 kW-m2
q=0, adiabatic walls
q=-10 kW-m2
q=-20 kW-m2
Setting constant temperature as wall boundary condition leads to a strong dependency of the axial temperature profile on the chosen temperature value
More realistic results for constant heat flux boundary conditions, especially at the co-firing cases
Optimum distribution of wall heat flux determined by parameter variation
Fig. Reference case validation: constant wall temperature
Fig. Reference case validation: constant wall heat flux
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Results: the 1MWth combustor (2 of 2)
100200300400500600700800900
10001100120013001400150016001700
0.00 1.00 2.00 3.00 4.00 5.00 6.00
axial position (m)
T (˚C
)
B0, exp. data
B1, exp. data
B2, exp. data
B0 - simulated
B1 - simulated
B2 - simulated
adiabatic walls - simulated
Simulation cases with constant temperature wall boundary conditions
Shift of the temperature pick in the near burner region to upstream, indicating faster ignition.
Increase of axial temperature throughout the whole furnace axis.
Increase of furnace outlet temperature up to 100K between the reference case (B0) and the dry coal combustion case (B2). 100
200300400500600700800900
100011001200130014001500
0,00 1,00 2,00 3,00 4,00 5,00 6,00
axial position (m)
T (˚
C)
B0, measured
B1, measured
B2, measured
B0, simulated
B1, simulated
B2, simulated
Fig. Co-firing cases validation: constant wall temperature
Fig. Co-firing cases validation: constant wall heat flux
Co-firing cases: simulation results
Co-firing cases: experimentally observed trends by increasing dry coal co-firing share
-
The increase of the temperature pick and the shifting to upstream is successfully reproduced
-
The increase of furnace outlet temperature cannot be reproduced
Simulation cases with constant wall heat flux boundary conditions
-
Improved results for all examined cases-
The temperature increase at the furnace outlet is slightly under-predicted
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Higher temperature levels in the dry coal co-firing cases at the near burner region (levels of 5 – 10m), comparing to baseline case. Temperature increase of about 100K at the level of the lower raw lignite burners (z=4.6m).Temperature increase of up to 70K at the level of the upper raw lignite burners (z=7.2m).A raise of 35K at the furnace exit temperature.
570,00
610,00
650,00
690,00
730,00
770,00
810,00
850,00
890,00
930,00
970,00
1010,00
3 6 9 12 15 18 21Z (m)
Ave
rage
Tem
pera
ture
s (o C
)
dry coal 20%
dry coal 10%
dry coal 5%
baseline
Fig. Average temperature profiles along furnace height
Results: Industrial scale boiler (1 of 4)
Temperature profiles
lower raw lignite burners
upper raw lignite burners
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Results: Industrial scale boiler (2 of 4)
Temperature contours ( °C)
Reference case 5% co-firing 10% co-firing 20% co-firing
Due to the particular burner geometry no central vortex is obtained like in the tangential fired boilers resulting to an inhomogeneous flow field
Cold and hot recirculation regions are predicted
Dry coal co-firing leads to an average higher temperature in the near burner zones as also noticed in the T-profiles
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Results: Industrial scale boiler (3 of 4)
O2 profiles
0,0450
0,0570
0,0690
0,0810
0,0930
0,1050
0,1170
0,1290
3 6 9 12 15 18 21
Z (m)
O2 (
vol.
% d
ry)
dry coal 20% dry coal 10% dry coal 5% baseline
Lower values of unburnt carbon when increasing the dry lignite co-firing share.In the 20% dry coal co-firing case the unburned carbon
is 3 times lower compared to the reference case.
Fig. Average O2
along furnace height
Fig. simulated unburnt
carbon
Unburnt carbon
0
0,4
0,8
1,2
1,6
2
2,4
2,8
3,2
3,6
4
BASELINE dry coal 5% dry coal 10% dry coal 20%
Percen
tage
of u
nburnt
carbo
n (%
)
O2 profilesDecrease of mean O2 concentrations at the main burner region when co-firing dry coal
Indication for improved ignition and fuel burnout.
Burnout
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Results: Industrial scale boiler (4 of 4)
Moderate increase of the total and the radiative heat flux observed for the co-firing cases
Relative increase between the maximum co-firing share and the baseline: below 4% indicating low influence of co-firing on the evaporator heat flux
The specific results: characteristic only for the particular boiler, may not be fully reproducible in modern boilers with higher specific heat fluxes (kW/m2)
A clear increase of NOx concentrations by raising the percentage of dry coal co-firing is observed.
The predicted increase is not confirmed by the performed emission measurements, possibly due to the low co-firing share tested during the co-firing tests (lower than 5%)
Fig. Average NOx
concentration along furnace height (ppm)
150,00
200,00
250,00
300,00
350,00
3 6 9 12 15 18 21Z (m)
NO
x C
once
ntra
tion
(ppm
)
dry coal 20% dry coal 10% dry coal 5% baseline baseline measured value
0300060009000
1200015000180002100024000
heat
flux
(kw
)
baseline 19487 24276
dry coal 5% 19830 24955
dry coal 10% 20113 25235
dry coal 20% 20310 25174
Radiative Heat Flux Total Heat Flux
Fig. Heat flux integral (kW) to furnace walls
Simulated NOx emissions
Heat flux integral to the overall furnace wall surface
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Conclusions
Temperature profiles-
Higher temperature values in dry coal co-firing at the near burner region.-
By 20% co-firing, temperature increase of 35K in furnace outlet.
Burnout-
Improvement of burnout by increasing co-firing thermal share.
Heat flux-
No specific tendency in low dry coal co-firing share.
NOx concentrations-
Increase of NOx
concentrations in the dry coal co-firing cases.
The pre drying and dry coal firing technology will play a key role in the new generation of brown coal power plantsExperimental investigations regarding the combustion behaviour of dried Greek lignite have been performed on a semi industrial scale combustor and a large scale boilerBoth facilities are simulated in the present work
Future work
Main simulation results for the large scale boiler
Dry coal co-firing simulations in a modern Greek lignite boiler representing
the current state of the art
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The support of Research Fund for Coal and Steel on the Demonstration Project “Drycoal”
(RFCP-CT-2004-00002) is
gratefully acknowledged!
Acknowledgements