boilers_2013.pdf

Academic Year 2013-14

School of Industrial Engineering

Lecture Notes for Energy Systems and Low Carbon Technologies

Boilers

prof. Stefano Consonni

Department of Energy

AY 2013-14 – Steam Generators – prof. S. Consonni

2 Physical process

combustion

fuel

oxidizing agent

flue gases

heat transfer

heat carrier

(cold)

heat carrier

(hot)

The physical process includes two steps:

1) combustion

2) heat transfer

Physically simultaneous.


3 example: fireplace and stove

a simple fireplace… …and a more evolved stove.


4 example: heat recovery fireplace


5 Steam Generators

Steam generators and heat recovery system include many subsystems:

1. furnace and combustion system,

2. boiler or steam generating bank,

3. steam superheaters/reheater,

4. economizer,

5. steam drum,

6. attemperator and steam temperature control system,

7. air heater,

8. fuel handling system,

9. flue gas treatment systems

10. ash handling system

11. gas-side cleaning equipment

12. ducts, fans, etc.


6 Steam Generators

Steam generators can be classified by various criteria:

1.Fuel (gaseous, liquid, solid)

2.Circulation method (natural, forced, once through)

3.Main heat transfer process (radiation, convection, indirect heating)

4.End use (heating, industrial process, electric utility)

5.Capacity


7 Small capacity boilers

smoke tube boiler - single pass -


8 Small capacity boilers

Smoke tube boilers

P evap. < 15 bar


9 Smoke tube boiler

http://www.boiler.de/eng/steamboiler/system.html

why the number of tubes of 2nd pass > 3rd pass ?


10 Smoke tube boiler

Smoke-tube boiler with wet end plate

to the stack


11 Smoke tube boiler

different views of a smoke tube boiler: internal and external


12 Water tube boiler


13 Water tube boiler

air

flue gases


14 Membrane walls of combustion chamber

Most of the heat transfer takes place by RADIATION


15 Boiler casing


16 Convective section

Convection heat transfer becomes the dominant heat transfer mode for gas temperatures below 900-1000°C Given the small gas-side heat transfer coefficient and the relatively small ΔT between the gas and steam/water, it requires large heat transfer areas


17 Diathermic oil boilers – ORC

- In case of small size applications, water does not work very well since

small scale steam turbines are characterized by low efficiency (size of

the blades) and excessive complexity (multi-stage expansion).

- A solution use of Organic Rankine Cycle where high-molecular-

mass organic fluids are utilized. Their thermodynamic properties

guarantee the absence of moisture during and at the end of the

expansion.

- But these organic fluids have a low decomposition temperature

they can not exchange heat directly with the flue gases need to

exchange with an intermediate heat transfer fluid. Diathermic oils suit

this necessity properly, as:

• they are thermally stable

• they are not chemically aggressive

• they do not change phase in the range of working temperatures

( safety)

• they can bare high temperatures being at low levels of pressure


18 Diathermic oil boilers

Elementary setup for diathermic oil boiler Organic Rankine Cycle


19 Diathermic oil boilers

Organic Rankine Cycle


20 Spiral tubes


21 Small capacity boilers - pellet

smoke tube water tube


22 Large boilers for power production


23 Large water tube boiler with membrane walls

550 MW Radiant boiler for pulverized coal.


24 Large water tube boiler with membrane walls

Economizer with

steel finned tubes


25 Heating surface configuration


26 Natural Circulation

R = Circulation factor

Ratio between

Water mass flow rate in the downcomer

and

Steam mass flow rate at risers outlet


27 Boiling conditions

R = Circulation factor

Ratio between

Water mass flow rate in the downcomer

and

Steam mass flow rate at risers outlet


28 Boiling conditions


29 Forced/Assisted Circulation

Once through

Assisted/Forced circulation

Natural circulation

Assisted/Forced circulation


30 Steam Drum

Vertical cyclone separator


31 Water drum with riser tubes


32 Supercritical boilers

Increasing efficiency requires to produce steam at supercritical pressure (>220 bar) in a once through boiler instead of natural circulation boilers used for subcritical power plants.

A critical aspect of this class of boilers is that in case of unbalanced flow rates among different pipes thermal stress may arise in pipes with the lower heat transfer coefficient (the ones with the lower flow rates)


33 Circulation systems


34 Closed domes for coal storage


35 Coal-fired boiler

vibrating grate

traveling grate


36 Coal-fired boiler


37 Pulverizer

the oldest pulverizer design: ball–and-tube

horizontal high speed pulverizer


38 Pulverizer

vertical air swept wheel pulverizer grinding mechanism

pulverized coal recirculation


39 Ljungstrom air pre-heater


40 Membrane walls with burner housings


41 Conventional Burners

Cell Burners Circular Burners


42

S type burner: larger swirl components and better secondary air control

Conventional Burners


43 Low NOx burners (3rd generation)


44

Low NOX burner

Staged combustion

Reburning

Low NOX burner


45 Combustion chamber


46

CAMINO

~ LP IP HPLP VHP

RH2

SH

preriscaldatore aria

ARIA

CROSSOVER, 3 bar

Turbina T/pompa

TURBOPOMPA DI ALIMENTO

RIGENERATORI HPRIGENERATORI LP

POMPA ESTRAZ.CONDENSATO

Trafil.

0.05 bar CONDEN- SATORE

580 °C, 26 bar

580 °C, 90 bar

580 °C, 300 bar

polverino di carbone

315 °C

SCR

high dust

iniezione di ammoniaca

ESP

scamb.coda

FGD

calce / calcare

gesso

Trafila-

menti

DEGASATORE

RH1

Plant configuration


47 Waste-to-Energy plant

Membrane walls covered with refractories

Convective section

Open passes


Waste Combustion – grate furnace 48

1

2

3

4

5

6

7

8

9

1 FEED HOPPER

2 FEEDER

3 GRATE

4 ASH DISCHARGER SCORIE

6 STEAM HETAED AIR PREHEATER

7 UNDERFIRE AIR

8 FLUE GASES RECIRCULATION

9 OVERFIRE AIR

5 FURNACE

Fonte: Martin Gmbh


Waste handling system 49

Fonte: Martin Gmbh


Sistema di combustione

The grate is designed to support and to transport the refuse while simultaneously stoking or mixing the refuse during the combustion process

Different types of grate depending on fuel LHV and manufacturer:

• horizontal/inclined

• stationary or moving

• Water or air cooled

• reciprocating, roller, travelling, vibrating, etc.

Combustion Grate 50


Sistema di combustione Vibrating Grate 51


Sistema di combustione Travelling Grate 52



“Reverse Acting” Grate

Gravity

Thrust of moving bars

Mixing of waste bed

Fonte: Martin Gmbh

Reciprocating Grate 53



Fonte: Martin Gmbh

Grate furnace 54


Combustion air distribution 55

SA: secondary air

PA: primary air

Fonte: Chungen Yin,, Lasse A. Rosendahl, Søren K. Kær 2008


56

Primary air

Fonte: Martin Gmbh

Underfire air distribution


57

4 nozzles rows 2 nozzles rows

Fonte: Martin Gmbh

Overfire air distribution

Secondary air



Fonte: Martin Gmbh

From grate end

T = 300°C

T = 90°C

Water level= air seal for combustion

chamber To vibrating conveyor

Bottom ashes discharger 58

boilers_2013.pdf

Documents

steam superheatersreheater

steam drum

small scale steam turbines

heat transfer coefficient

smoke tube boiler http

heat recovery system

heat recovery fireplace

dominant heat transfer