Selection of Optimal Air Fuel Ratio

P M V SubbaraoProfessor

Mechanical Engineering Department

Efficient Combustion Requires Sufficient Air…..

Model Testing for Determination of important species

Air Flow Rate

Fuel Flow Rate

Water Flow Rate

Flue gas Analysis

Results of Model Testing.

• For a given fuel and required steam conditions.

• Optimum air flow rate.

• Optimum fuel flow rate.

• Optimum steam flow rate.

• Optimum combustion configuration!!!

Air Supply Systems

Direct Method of SG Performance Analysis

• Energy balance:

• Fuel Energy = Steam Enthalpy + Losses.

• Measurements:– Steam Flow Rate

– Steam properties

– Fuel flow rate.

• Difficulties:

• Measurement of steam flow rate.

• Measurement of fuel flow rate.

• Errors in measurements.

CreditsEnergy Energy Fuel

Enthalpy SteamGenerator Steam of

Efficiency

Selection of Optimal air for Best Performance

Results for Best Efficiency

Effect of Excess Air on Emissions

Selection of Optimal air for Eco-Friendliness

Study of Impact of Excess Air

Unified Optimization Methods

Methods to Apply the Optimization

Optimal Excess Air

Air Ingression Due to Better Combustion Conditions

Effect of Air Ingression on Actual Available Excess Air

First Law Analysis of Furnace at Site

• CXHYSZOK + 4.76 (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: *4.76* (X+Y/2+Z-K/2) *28.89 kg.

• Mass of Coal: 100 kg.

• Excess Air: -1)4.76* (X+Y/2+Z-K/2) *28.89 kg.

Fuel of Mass

Air of Mass RatioFuelAir

CXHYSZOK + 4.76 (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 air + m fuel - mfluegas = 0

• First Laws for SG in SSSF Mode:

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

m fuel

m air

m fluegas

Q

Q

Wfans

Performance Testing of SG

Air Flow RateDry Flue gas Analysis

Ex. Gas Flow Rate

Indirect Method of SG Performance Analysis

• For every 100 kg of Coal.

• 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

Measurements of Gas Analyser

• Dry Exhaust gases: P CO2 +R SO2 + T N2 + U O2 + V CO kmols.

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

• Volume fraction = mole fraction.

• Volume fraction of CO2 : x1 = P * 100 /(P +R + T + U + V)

• Volume fraction of CO : x2= VCO * 100 /(P +R + T + U + V)

• Volume fraction of SO2 : x3= R * 100 /(P +R + T + U + V)

• Volume fraction of O2 : x4= U * 100 /(P +R + T + U + V)

• Volume fraction of N2 : x5= T * 100 /(P +R + T + U + V)

• These are dry gas volume fractions.

• Emission measurement devices indicate only Dry gas volume fractions.

• 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 4.76 (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

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.