Bunsen Burner - icetpu

rangebeaverMécanique

22 févr. 2014 (il y a 7 années et 5 mois)

212 vue(s)

Combustion Calculations

Calculate the theoretical flame temperature of
combustion of hydrogen with theoretical amount of
air assuming

(a)
No dissociation of combustion product (i.e. H2O)

(b)
Assuming 4 % dissociation of H2O

NCV of H2 = 10.16 MJ/m3

Assume that both the hydrogen and air are dry at 0 C

Mean value of Cp (of combustion product H2O):


Temp C

2000

2100

2200

Cp kJ/m3C

1.643

1.651

1.657

Combustion Calculations


H2 + ½ O2 = H2O


Air required for 1 m3 of H2= 2.38 m3


Total flue gas= N2+ H2O= 0.79 x 2.38 +1



= 2.88 m3 / m3 of H2


(a) Theoretical flame temp T = (10.16 x 1000)/(2.88 xCp)



Assuming T=2100 C (Cp=1.651)


Calculated T= 2136 C which is quite close to assumed


(b) T =(10.16 x 1000 x (1
-
0.04))/(2.88 xCp)


Assuming T=2000 C calculated T = ?


Assuming T =2100 calculated T= ?


Combustion Process




The requirements for Combustion are
:


Fuel ( Solid, liquid, gaseous already discussed)


Oxygen (Normal source is air)


The 3 Ts time



temperature



turbulence


time: sufficient time must be available for complete


combustion



Temperature: The fuel/air mixture must be heated to ignition




temperature to promote combustion


Turbulence: turbulent mixing is the best approach for combustion to


complete



Major Efficiency Losses


Gas exit temperature
:



A reduction of 22 C in flue gas temperature results in 1%
increase in efficiency


If gases cooled below dew point, sulphuric acid will
condense on the surfaces


The acid dew point temperature limits the amount of heat
which can be safely recovered


Losses due to excess air:


To reduce the mass of flue gas, we must reduce excess
air If too little air, then incomplete combustion (just
enough air to burn all the fuel)


Burners for Gaseous Fuels


There are two methods for
burning gaseous fuels



(i) The gas and air is pre
-
mixed
and then fired ( premix or
inside mixing type of burners
e.g. Bunsen burner)



(ii) The gas and air flow
separately and mix together as
combustion proceeds ( Outside
mixing type or diffusion flame
burners)


Bunsen Burner

The device is named after Robert
Bunsen, the German chemist who
introduced it in 1855.

The kinetic energy of the gas is used
to draw primary air from the atmosphere
into a mixing tube which has burner
head at its end

The primary air gas mixture velocity is
kept more than the flame speed

Secondary air is supplied from
atmosphere to the flame so that flame
does not flash back/backfire/strike back
down the bunsen tube




The parts of a Bunsen burner.

Base
:
The Bunsen must also be
placed on a heat mat before it is lit.

Collar
:
This can spin if
the holes are to be
opened or closed.

Chimney
:
A mixture of
air (containing oxygen)
and methane flows
through here.

Air holes
:
Will allow
air to mix with fuel in
different proportions

Gas inlet
:
Will be
attached to
the gas
supply with a
rubber tube.

Bunsen Burner


If the velocity of the
primary air gas mixture
velocity is much greater
than the flame speed the
flame can be blown off the
tube and the burner get
extinguished


With insufficient primary air
supply the flame produced
is long, lazy and luminous
which gives low heat
release

Flame stability


Reading assignment


Burners for gaseous fuels from the book