Fuel Combustion 2 (17%)

 

Power Plant Technology Fuel Fu el and Combustion (Lecture 2)

by Mohamad Firdaus Basrawi, Dr (!ng) Mechanical !ngineering !ngineeri ng Faculty Faculty m"irdausb#um$edumy

 

COMBUSTION REACTION Combuston: hea release wih high emperaure oxidaton of fuel. Combustble elemens in fuel: C, H and S. Basic chemical equaton for complee combuston:

C + O 2 � CO2 2 H 2 + O 2 � 2 H 2O S + O 2 � SO2 The formaton of CO happened when oxygen is insucien and C will be burned incompleely.

2C + O2     2CO

 

Four asic con!iton con!itonss "MA "MATT# TT# $or com%le&e comuston'  

•

Mi(&ure: To have enough &urulence for hrough mi(in) of fuel+air.

• • •

Air: Adeuae supply of air for complee combuston. Tem%era&ure : !ucien &em%era&ure inside chamber o ignie he incoming fuel air mixure. Time: "rovide sucien tme for complee combuston.

S&oichiome&r S&oichiome&ric ic air "O* ase!#

The ultmae analysis of he fuel # C + H + O + N + S + M + A  -. O* nee!e! $or &he o(i!aton can be calculaed as follows: C

+

$% 

& C

%$'% (g

)%(g

** (g

' (g

%., (g

)., (g

C /)

/)  *.01 C /) 

2.01 C /)

 

% H% 

+

$% 

&

%-%$

* (g

)% (g

) (g

' (g

 (g

/ (g

H /)

/)   3 H /)

+

S

)% (g

$% 

4 H /)

&

!

%$)% (g

* (g

' (g

' (g

% (g

S /)

/)  S /) 

* S /)

$% needed for complee combuston of ' (g fuel is:

5   *.01 C + 3 H + S 6 O O*

$: oxygen in he fuel

 

Air conens %).%0 oxygen by mass. Thus# &heoretcall7 air "s&oichiome&ric 8 air# needed for complee combuston of ' (g fuel is

WT  =

W O2  

=

2.67

8

C+

H+

1

0.232 0. 0.232 0.232 0.232 WT  = 11.5C + 34.5H + 4.31S - 4.31O

Ho9e:er,, Ho9e:er

S-

1 0.232

O

heoretcal or s&oichiome&r s&oichiome&ric ic air is insucien for complee combuston

E;CESS AIR "s&oichiome&ric 8 air 8 e(cess air# is needed for complee combuston.

The %ercen&a)e o$ e(cess air supplied is:

%excessair  =

W A - W T 

 

W T 

1A : acual amoun of air supplied.

�100

 

The !iluton coeue gas analysis is measured is on dry basis which is: C$% + C$ + $% + 8% & ';;0

Ey Folume

Assume ha he ?@ analysis of a gas sampling is: '%0 C$ %# )0 C$# 0 $% and ;0 8% by volume. Therefore ' mole of ?@ conains ;.'% mole C$ %# ;.;) mole C$# ;.; mole $ % and ;. mole 8%. The Gass of he ?@ hen becomes eual o : ** 6;.'%7 + % 6;.;)7 + )% 6;.;7 + % 6;.7 & );.'% (g(g mole fuel.

2n general he Gass of dry >ue gas 6dfg7 is given by: Gdfg & **C$% + %C$ + )%$% + %8%

 

Therefore# he percenage of each combuston produc is given by 44CO2

%CO by mass   2

O2 by mass   %N2 by mass   %CO by mass  

100

44CO2

28CO 32O2 32O 32 O2

28N 2

44CO2

28CO 32O2 28N 2

28N 2

44CO2

28CO 32O2

28N 2

44CO2

28CO 28CO 32O2

28N 2

100 100 100

Gass of Carbon per (g ?@ is deermined as below:

C

+

'% (g

@or ** (g of C$% gas# i has '% (g

$%  H C$% )% (g

** (g

%C +

$%  H %C$

%* (g

)% (g

 (g

'% (g

' (g

% (g

Carbon

$I

@or % (g of C$ gas# i has '% (g Carbon

 

1 2 C O2

 asss !" Ca  as Carb! rb!# # $e $err & '()

44C O2

28 C O

 

CO

3 2 O2

28 N 2 

Je Cab be he mass fracton of carbon C in he fuel which has beed oxidiKed eiher o C$% o C$. Then 6C4Cab7 is he mass fracton of unburn carbon in he refurse. @or '(g fuel burn# here should be Cab (g of carbon in he dfg. Therefore# Gass of dfg produced per (g fuel. Cab   44CO2   28CO 32O2 12 CO2

28 N2 

CO

.

@rom ue

 

The excess air is han adNused by conrolling he amoun air supply o show he optmum value of C$% and $%. The optmum value of excess air for he bes combuston eciency is as below 6Lg. 7.

@ig. . $ptmum excess air for maximum combuston eciency