Super Crit Boilers

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  • Topic: Boiler, Coal, Supercritical fluid
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  • Published : February 19, 2013
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TECHNICAL PRESENTATION
TECHNICAL

CETHAR LIMITED

Types of boilers
Sub critical boilers
Drum type natural circulation boilers
Drum type controlled circulation boilers
Once through boilers

Super critical boilers
Once through boilers

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Evaporator systems

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Design Criteria
Description

OT Super critical

Sub-Critical

Furnace sizing

Decided by Fuel and Gas side
parameters

Same

Evaporator

Supercritical fluid;
Variable evaporation end
point; Superheated steam at
evaporator outlet;

Water and Steam mixture;
Fixed evaporation end point;
Saturated steam at evaporator
outlet

(Supported by BENSON
Technology)

Natural / controlled circulation

Drum

Replaced by smaller
Separators and collecting
vessel. Required only during
low loads (about 35-40%)

Required for water and steam
separation at all loads

Start up and low
load circulation
system

Required.
(Supported by BENSON
Technology)

Not applicable

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Design Criteria
Description

OT Supercritical

SH and RH sizing

Same for both sub critical and supercritical

Economiser

Same for both sub critical and supercritical

Pulveriser sizing

Same for both sub critical and supercritical

Fans. RAPH & ESP
and ducts
and

Same for both sub critical and supercritical

Controls

Feed water flow is
controlled based on load
SH steam temperature is
controlled by spray followed
by firing rate.

Feed water flow is controlled
by drum level.
SH outlet temperature is
maintained by spray.

Water treatment
and water
chemistry

AVT and Oxygenated water
treatment (mandatory)

Generally AVT

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Sub-Critical

Furnace Design
FEGT = IDT - 50 K

FEGT = Furnace exit gas temperature
IDT = Initial deformation temperature of ash

Furnace design and size are
decided by coal and ash quality

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Evaporator wall design
Key Issues
Heat absorption variation and resulting
temperature difference at furnace wall outlet

Boiling crisis like DNB and Dry out and associated
wall temperatures

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Evaporator systems

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Steam temperature at furnace wall outlet

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Heat flux variation in furnace

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Sliding pressure operation

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Operating regimes

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Fluid Density

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Flow pattern
Flow pattern at different pressures:
Supercritical
Subcritical

Insignificant density difference
at Supercritical pressure results
in single phase supercritical fluid.
Consequently there is no DNB and
Dry Out which exist in sub-critical
pressure regime of operation.

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Boiling Crisis

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Evaporator Designs
High mass flux design (2000 to 2400 kg/ m2 s)
Spiral wall arrangement with smooth tubes

Medium mass flux design (1500 to 2000 kg/m2 s)
Vertical wall arrangement with rifle tubes and orifices

Low mass flux design (900 to 1000 kg/m2 s)
Vertical wall arrangement with optimised multilead
rifled tubes

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Furnace wall designs

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CETHAR’s 660 MW boiler
HTSH

HTRH

Screen

Low mass flux design

LTSH

Furnace with Vertical
tubes
RSH
LTRH
Econ

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Optimised Multi Lead
Rifled tubes for lower
furnace
(OMLR tubes)

Spiral wall arrangement

Spiral wall construction reduces the number of evaporator wall tubes and thereby increases mass flux through the tubes.
This ensures a minimum water wall flow and protects water wall tubes.

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Changes in mass flux
.

. ∆m
mm ean

16

%
12
Change of m ass flow
in a tube with 25%
increased heat input

8
4
0
Nat ural
circulation

-4
-8

Oncethrough

-12
-16

600

800

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1
000

1200

1
400
kg/m2 s
1800
Mass flux (at full load)

Impact of flow characteristic on fluid temperatures...
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