combining discrete volume and systems level a cogeneration … · 2014. 7. 28. · feedwater...
TRANSCRIPT
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Combining discrete volume and systems level analysis techniques in the design process of a cogeneration power stationRyno Laubscher
July 2014
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Contents
1. Introduction to the to a small coal-fired Boiler.2. Outline of techniques utilized.3. Modelling of Rankine/cogeneration cycle using SCFD
(systems computational fluid dynamics).4. Numerical modelling of boiler combustion, heat transfer
and fluid mechanics.5. Modelling of the homogenous two-phase natural
circulation system (steam drum, furnace wall, etc.).
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1. Introduction
• This presentation discusses the advanced modelling techniques used to develop detailed mathematical models for the thermodynamic cycles, furnace combustion and steam/water circulations.
• The small coal-fired boiler under consideration is a modular 20t/h power generation boiler that can produce steam conditions up to 62bar(a) and 470°C. The steam inturn can be used to drive Rankine or Cogeneration vapour-power cycles.
• The name of the product is: MicroGen Boiler
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MicroGen Small Power Boiler
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Design techniques
• Detailed engineering analysis can be divided into the following areas:– Power systems modelling and optimization of the relevant
vapour-power cycle using thermodynamic analysis.– Analysis of the fuel, oxidizer and products’ evolution
through the boiler and their interaction with each other and the heat transfer surfaces.
– Natural circulation of the water/steam mixture.
Lumped system analysis
Rankine/cogeneration cycle modelling using SCFD
CFD modelling Two-phase circulation modelling
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Lumped system analysis
BOILER OVERALL PERFORMANCE UNITS SA Grade B Coal
Boiler Maximum Continuous Rating % MCR 100
Boiler Peak Load (max 30 mins in every 8 hrs) % MCR 110
MCR Evaporation kg/h 20,000
Final Steam Temperature @ MSSV(±5) °C 455*
Final Steam Pressure @ MSSV kPa(g) 6,100*
Steam temperature Turndown Capability % MCR 70-110*
Feedwater Temperature ex Deaerator °C 130
Final Gas Temperature °C 179
Primary Air Temperature (Ex Airheater) °C 150
Efficiency on GCV % 83.66
Efficiency on NCV % 86.65
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2. Thermodynamic cycle modelling• Utilization of 1D – conservation equation of mass, momentum
and energy to setup model and solve using a state-of-the-art nonlinear steady-state/transient solver ideally setup with speed and accuracy for pipe network problems (Flownex SE Software was used for the modelling).
• Systems can be divided into the following building blocks: – Boilers– Superheater– Turbines– Condensers/Factories– Pumps
What is a thermodynamic vapour-power cycle?:
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Ideal Rankine Cycle (full condensing):• Fully condensing turbine (Ideal Rankine) simulation:
11 MW WASTED
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Cogeneration Cycle:
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Flownex applications:
• Flownex is also used to develop detailed thermal-hydraulic models of various subsystems (superheaters, furnace, economiser etc.). The models can quickly become very complex.
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Computational fluid dynamics
• CFD – takes boiler engineering to the next level• Determining the flame shape at different loads• Calculate the heat flux for circulation studyBoiler CFD modeling can be subdivided into the following modeling sections:
1) Combustion – chemical kinetics hetero-/homogenous combustion
2) Fluid dynamics 3) Heat and mass transfer – Heat transfer
and species transport equations
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• The fluid dynamics is modeled using the conservation of momentum equation in a Eulerian frame. The Reynolds stress term is solved using the k- two equation model. The continuity and energy equations are also solved.
• In addition to solving the transport equations for the continuous phase a discrete second phase (solid fuel particles – coal) is solved in a Langrangian frame.
• Combustion of coal:1) Initial heating (EVAP) 2) Devolatilisation3) Homogenous comb.4) Char burning
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CFD Results
Flame profile - temperature Temperature profile in combustion zone
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CFD Results
Heat flux on furnace walls – CFD = 65.6 kW/m2 (average)
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3. Circulation Modelling• U = overflow tube• G = mixture tube• R = unheated return tube• Z = header• S = heated risers• V = feeder manifold
Advantages of water/steam separation boiler:1. Quick circulation on start-up – no long overflow distances to heating surface2. Mono-drum configuration (fewer amount of tubes connected to steam drum)3. Self-supporting construction plus bottom-to-top thermal expansion4. Higher circulation due to unheated return lines5. Quick turn down response (no significant changes in pressure and water level
How does flow boiling work?
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Principles of natural circulation
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Pressure part design
Furnace
Furnace
Evap. Flags
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Modelling of natural circulation.
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Microgen Circulation model
Furnace
SH Cavity
Evap. FlagsDowncomers
Preseparationheaders + collector tubes
Unheated returnsFeeder tubes
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Pressure distribution
Furnace
SH Cavity
Evap. Flags
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Heat input distribution
Parameter Heat inputFurnace 8665.6 kWSH Cavity 150 kWEvap. Bank 1390 kW
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Mass flow distribution
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Quality distribution
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Flow regimes of individual circuits
• We use the Gas and Liquid Froude number analogy to plot the flow regime on a map to determine in what working range we are:
• We must ensure that the flow regime is not in the annular flow regime (vertical) or in any stratified (horizontal) regions.
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Vertical circuits – flow regimes
Red – Furnace rearwallBlue – Furnace sidewallBlack – Furnace frontwallPurple – Cavity sidewall
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Horizontal circuits – flow regimes
Red – Furnace roofBlue – Rearwall inclined section
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END
• Thank you very much for your time.• Any questions.