5 replies to this topic

Hi. I'm a chemical engineer student from singapore. Could you help me on these few questions:
1) why does air for combustion needs to be preheated first? Is it just to recover waste heat from hot flue gases, or is there any certain purposes/advantages that could be achieved?

2) if the combustion air was not preheated first, could there be any trouble/disadvantages when it came to combustion with natural gas?

3) does the combustion air preheating has anything to do with the furnace's efficiency? and if it does, how do you calculate it?

thanks in advance for all comments, answers, and suggestions. love u all..

[Adriaan]

Preheating air is usually done to make the process more efficient, recouping energy from the reaction products.

A second reason is that with preheated air you get higher (flame) temperatures which is a requirement for some processes (such as thermal cracking).

[engware]

Hi Dahniel:

Adriaan gave you a few good answers to to your questions.

I have a free spreadsheet type demo where you can get different combustion output values for given input values. The URL is as follows: http://engware.i-den...m/ec-p-demo.xls.

I do hope that the free demo will help you observe the basic engineering combustion trends.

Thanks,

Gordan

dear adriaan n gordon,

thx for the input n solution from both of you. hopes it could make easier to me to solve the problem i had at present.

best regard



Dahniel

[engware]

Dahniel:

No problem.

Thanks,

Gordan

[Adriaan]

Dahniel,

to answer your question 2), no problems are to be expected in NG burning when the air is NOT preheated. Here is the Phillips method for calculating the theoretical maximum flame temperature which includes the combustion air temperature:

Ht = 0.03457 x ( ta – 65.6 ) + LHV x ( F / A )

for gas
tF = 456 + 17.2 Ht + 0.0020 ta (Ht – 1)

for non aromatic oils
tF = 450 + 18 Ht + 0.0032 ta (Ht – 34)

for aromatic oils (air temperature > 500 °C)
tF = 386 + 20 Ht

LHV lower heating value in Btu per unit of fuel
tF theoretical flame temperature in °C
ta air temperature in °C
F / A fuel to air ratio

For natural gas the minimum F / A ratio is 10 for Dutch natural gas, 9 for German NG to achieve complete combustion.

The range of applicability for this method is limited to certain ranges for Ht.



re question 3) the temperature can be a big influence on a furnaces' efficiency, the bigger the temperature difference the more energy will get transferred (in any type of heat exchanger).