Combining discrete volume and systems level analysis techniques in the design process of a cogeneration power stationRyno Laubscher

July 2014

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.).

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

MicroGen Small Power Boiler

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

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

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?:

Ideal Rankine Cycle (full condensing):• Fully condensing turbine (Ideal Rankine) simulation:

11 MW WASTED

Cogeneration Cycle:

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.

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

• 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

CFD Results

Flame profile - temperature Temperature profile in combustion zone

CFD Results

Heat flux on furnace walls – CFD = 65.6 kW/m2 (average)

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?

Principles of natural circulation

Pressure part design

Furnace

Furnace

Evap. Flags

Modelling of natural circulation.

Microgen Circulation model

Furnace

SH Cavity

Evap. FlagsDowncomers

Preseparationheaders + collector tubes

Unheated returnsFeeder tubes

Pressure distribution

Furnace

SH Cavity

Evap. Flags

Heat input distribution

Parameter Heat inputFurnace 8665.6 kWSH Cavity 150 kWEvap. Bank 1390 kW

Mass flow distribution

Quality distribution

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.

Vertical circuits – flow regimes

Red – Furnace rearwallBlue – Furnace sidewallBlack – Furnace frontwallPurple – Cavity sidewall

Horizontal circuits – flow regimes

Red – Furnace roofBlue – Rearwall inclined section

END

• Thank you very much for your time.• Any questions.