Performance Evaluation of A Steam Generator

P M V SubbaraoProfessor

Mechanical Engineering Department

A Measure of Efficient Combustion …..

First Law Analysis of Furnace at Site

• CXHYSZOK + 4.673 (X+Y/2+Z-K/2) AIR + Moisture in Air + Ash Moisture in fuel→ P CO2 +Q H2O +R SO2 + T N2 + U O2 + V CO + W C + Ash

• Mass of air: *4.673* (X+Y/2+Z-K/2) *28.89 kg.

• Mass of Coal: 100 kg.

• Excess Air: -1)4.673* (X+Y/2+Z-K/2) *28.89 kg.

Fuel of Mass

Air of Mass RatioFuelAir

CXHYSZOK + 4.673 (X+Y/2+Z-K/2) AIR + Moisture in Air + Ash + Moisture in fuel→ P CO2 +Q H2O +R SO2 + T N2 + U O2 + V CO + W C + Ash

First Law Analysis of Furnace:SSSF

Conservation of Mass:

m fuel

m air

m fluegas

Q

Q

Wfans0

ashfluegasfuelair mmmmm ash

CV

ashfluegas

fuelair

CV

WgzV

hmgzV

hm

gzV

hmgzV

hmQ

22

22

22

22

First Laws for furnace in SSSF Mode:

Flue Gas Analyzer

Ps CO2 +Qs H2O +Rs SO2 + Ts N2 + U sO2 + Vs CO

Measurements of Gas Analyser

• Dry Exhaust gases: Ps CO2 +Rs SO2 + Ts N2 + Us O2 + Vs CO kmols.

• Volume of gases is directly proportional to number of moles.

• Volume fraction = mole fraction.

• Volume fraction of CO2 : x1 = Ps * 100 /(Ps +Rs + Ts + Us + Vs)

• Volume fraction of CO : x2= V s* 100 /(Ps +Rs + Ts + Us + Vs)

• Volume fraction of SO2 : x3= Rs* 100 /(Ps +Rs + Ts + Us + Vs)

• Volume fraction of O2 : x4= U s* 100 /(Ps +Rs + Ts + Us + Vs)

• Volume fraction of N2 : x5= T s* 100 /(Ps +Rs + Ts + Us + Vs)

• These are dry gas volume fractions.

• Emission measurement devices indicate only Dry gas volume fractions.

Analysis of Ash & Measurement of UC

100

AshUC

UC

mm

mR

• Measurements:

• Volume flow rate of air.

• Volume flow rate of exhaust.

• Dry exhaust gas analysis.

• x1 +x2 +x3 + x4 + x5 = 100 or 1

• Ultimate analysis of coal.

• Combustible solid refuse.

nCXHYSZOK +n 4.673 (X+Y/4+Z-K/2) AIR + Moisture in Air + Ash & Moisture in fuel

→ x1 CO2 +x6 H2O +x3 SO2 + x5 N2 + x4 O2 + x2 CO + x7 C + Ash

Moisture due to Combustion air

OHsat

OHact

p

pRH

2

2

,

, atmOHair

OHOHact p

nn

np

2

2

2,

OHair

OHOHsat nn

npRH

2

2

2,

atmOHOHairOHsat pnnnpRH 222,

OHsatatm

airOHsatOH pRHp

npRHn

2

2

2

,

,

OHsatatm

OHsat

airOH pRHp

kz

yxnpRH

n2

2

2

,

,

,

24673.4

Moisture due to fuel

18,2

Mnn fuelOH

nCXHYSZOK +n 4.76 (X+Y/4+Z-K/2) AIR + Moisture in Air + Ash & Moisture in fuel → x1 CO2 +x6 H2O +x3 SO2 + x5 N2 + x4 O2 + x2 CO + x7 C + Ash

•x1, x2,x3, x4 &x5 : These are dry volume fractions or percentages.

•Conservation species:

•Conservation of Carbon: nX = x1+x2+x7

•Conservation of Hydrogen: nY = 2 x6

•Conservation of Oxygen : nK + 2 n (X+Y/4+Z-K/2) = 2x1 +x2 +2x3 +2x4+x6

•Conservation of Nitrogen: n 3.76 (X+Y/4+Z-K/2) = x5

•Conservation of Sulfur: nZ = x3

nCXHYSZOK +n 4.76 (X+Y/4+Z-K/2) AIR + Moisture in Air + Ash & Moisture in fuel → x1 CO2 +x6 H2O +x3 SO2 + x5

N2 + x4 O2 + x2 CO + x7 C + Ash

• Re arranging the terms (Divide throughout by n):

CXHYSZOK + 4.76 (X+Y/4+Z-K/2) AIR + Moisture in Air + Ash & Moisture in fuel → (x1 /n)CO2 +(x6/n) H2O +(x3/n)

SO2 + (x5/n) N2 + (x4/n) O2 + (x2/n) CO + (x7/n) C + Ash

CXHYSZOK + 4.76 (X+Y/4+Z-K/2) AIR + Moisture in Air + Ash Moisture in fuel

→ P CO2 +Q H2O +R SO2 + T N2 + U O2 + V CO + W C + Ash

Specific Flue Gas Analysis

• For each kilogram of fuel:• Air : 4.76 (X+Y/2+Z-K/2) * 29.9 /100kg.

• CO2 : P * 44/100 kg.

• CO : V * 28/100 kg.

• Oxygen in exhaust : 32 * U/100 kg.

• Unburned carbon: 12*12/100 kg.

Various Energy Losses in A SG

• Heat loss from furnace surface.• Unburned carbon losses.• Incomplete combustion losses.• Loss due to hot ash.• Loss due to moisture in air.• Loss due to moisture in fuel.• Loss due to combustion generated moisture.• Dry Exhaust Gas Losses.

•Heat loss from furnace surface.

•Unburned carbon losses.

•Incomplete combustion losses.

•Loss due to hot ash.

•Loss due to moisture in air.

•Loss due to moisture in fuel.

•Loss due to combustion generated moisture.

•Dry Exhaust Gas Losses

•~ 4.5%

Heat gained by superheater & reheater

40%

Heat gained by economizer & air preheater

12%

Fuel Energy100%

Heat gained by boiling water40%

Hot gas

Flue gas

Energy Credits

• Chemical Energy in the fuel.

• Energy credit supplied by sensible heat in entering air (recycling of energy).

• Energy credit supplied by sensible heat in the fuel(Recycling of energy).

• Energy credit supplied by auxiliary drives.

Furnace Energy Balance

• First Laws for Furnace in SSSF Mode (in molar form):

Q +n air hair + n fuel hfuel = n fluegas hfluegas + W

n fuel

n air

n fluegas

Q

Q

Wfans