Estimation and Selection of Air for a Fuel

P M V SubbaraoProfessor

Mechanical Engineering Department

A Criteria for Sizing of Furnace & Furnace Accessories.

Chemical Models for Real Fuels

Wood 5106 OHCn

5108 OHCnPeat

41635 OHCnLignite

41949 OHCnSub-bituminours

Production of Useful Fuels from Crude

Boiling range, and molecule size for typical refinery

• BOILING RANGE # CARBON ATOMS

• Refinery Gas <25 oC 3

• Gasoline 40-150 oC 4-10

• Naptha 150-200 oC 10-12

• Kerosene 200-300 oC 12-16

• Diesel Fuel 300-400 oC 16-25

• Residual Oil >400 oC >25

Generation of Fuel Model• Practical fuels are complex mixtures of compounds.

• Some useful for generation of heat & some are useless.

• Some are threat to environment.

• Proximate Analysis & Ultimate Analysis.

• Proximate analysis - to determine the moisture, ash, volatiles matter and fixed carbon

National Capital Power Station Dadri

Equilibrated BasisSample MOIS. ASH V.M. FC UHV

(%) (%) (%) (%) K.CaI/Kg.

Wash Coal 7.1 33.7 25.0 34.2 3270

Raw Coal 6.4 35.2 25.8 32.6 3159

Commercial Use of Proximate Analysis

The Energy content -- CFRI Formulae --Low Moisture Coal(M < 2% ) -- CV (Kcal/kg) = 71.7 FC + 75.6 (VM-0.1 A) - 60 MHigh Moisture Coal(M > 2%) -- CV(kcal.kg) = 85.6 {100 - (1.1A+M)} - 60 M

Where, M, A, FC and VM denote moister, ash , fixed carbon and Volatile mater (all in percent), respectively.

Ultimate Analysis

• Ultimate or elementary analysis - to determine the elemental composition of the fuel

Ultimate Analysis: On Air Dried Basis

Wash Coal

%

Gross Calorific Value 4370

Carbon Content 51.28

Hydrogen Content 3.18Oxygen Content 9.72

Nitrogen Content 0.45

Sulfur Content 0.30

Ash Content 35.07

Raw Coal

Gross Calorific Value 4250

Carbon Content 51.00

Hydrogen Content 3.14Oxygen Content 8.20

Nitrogen Content 0.35

Sulfur Content 0.35

Ash Content 36.96

Ultimate Analysis: On Air Dried Basis

Raw Coal

Gross Calorific Value 3884

Carbon Content 42.6

Moisture Content 6.0

Hydrogen Content 3.3

Oxygen Content 11.5

Nitrogen Content 0.7

Content 0.35

Ash Content 35.7

Fuel Model

Ash Model

Fuel Model

• Ultimate Analysis of fuel: Gravimetric

• Percentage of carbon : x --- Number of moles, X = x/12

• Percentage of combustible hydrogen : y --- Number of atomic moles, Y = y/1

• Percentage of combustible oxygen: k --- Number of atomic moles, K = k/16

• Percentage of sulfur: z – Number of atomic moles, Z = z/32

• Equivalent chemical formula : CXHYSZOK

• Equivalent Molecular weight : 100 kgs.

Ideal Combustion

• Ideal combustion

• CXHYSZOK + 4.673 (X+Y/4+Z-K/2) AIR → P CO2 +Q H2O + R N2 + G SO2

• Ideal Air- Fuel Ratio:

1

10024

9.28773.4

stoistoi A

FK

ZY

X

F

A

Second law limit on possibility of extent of reaction

• Reactants Products

• At any time a reactor contains a combination of reactants and products.

• A reaction is said to be complete when the entropy of an adiabatic furnace reaches its maximum value.

• The value of maximum entropy will vary with the pressure and temperature of the reaction.

• A reaction system and parameters of reaction should be designed such the the maximum entropy is obtained when the reaction is almost complete (>98%).

i jPjjRii

i jjjii

sYsXMaximize

PYRX

:

Mathematical Model for Completion of Reaction

For every fuel, a designer should know all possible reactants !!!Some products will influence the efficiency of reaction.Few other may not influence the efficiency of reaction but severely affect the environment.The optimal parameters for efficient reaction may not be optimal for safe reaction !!

Generalized Theory of Extent of Reaction Possible

Extent of Reaction

Ent

ropy

of

Uni

vers

e

P1,T1 P2,T2

P3,T3

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!!!

Realization of MATt Theory• Mixing: Fuel preparation systems.

• Air: Draught systems.

• T : Preheating of fuel.

• t : Dimensions of combustion chamber. : Turbulence generation systems.