Steam production-Different flue gases

[MedicineEng]

Dear All:
I am with a problem that I would like to have your oppinion about:
-Imagine a situation that you have a hot combustion gas heating up a boiler for steam production.
-This gas is produced by burning a stream of secondary products that changes in its composition along the time, namely its water content.

My doubt is:
If the combustion gas is stable at a determinate temperature, say 1000C, will the steam production rate in the boiler change if the composition of the burned stream is for instance 30% water or 50% water?

Thanks a lot for your help.

 

[25362]

You say you have a problem, but don't care to define it.
Anyway, here are some comments regarding combustion.

It appears this is an incinerator of liquid waste or refuse, the temperature of the combustion gases apparently controlled by the amount of excess air admitted.

More water in the fuel stream would absorb more heat and involves less air dilution to keep a constant combustion gas temperature.

It seems heat transfer by radiation would improve with increased water presence because diatomic molecules (nitrogen, oxygen) have very little radiation capacities when compared with CO₂, H₂O, SO₂-SO₃.

 

[MedicineEng]

25362:
Thanks a lot for your reply. You are right, I didn't define very well my problem, my apologies.
Here it goes with some more details:
You are right about your assumptions regarding this system, it burns secondary products at a constant temperature. The temperature control is made by the flow of burning stream and by controlling hte excess air, but mainly by the first one. After that, this hot gas stream passes trough a boiler to produce steam. This is the main steam used in our processes, since it is free. We have a diesel fired boiler to back up this one if the steam production is not sufficient. What I am facing now is a skyrocketing consumption of diesel in the backup boiler without a direct reflection in Production. I already checked and rechecked the system and there are only 2 things left: check burning efficiency of the diesel boiler (that I am dealing with) and assess the impact of different streams of secondary products in steam production. That's why I made my original question, since I don't have a clue if there is any relationship or not.

 

[25362]

What about the secondary stream flow rate ? If it drops or its calorific value diminishes (at constant temperature), the amount of raised steam is bound to drop. Am I right ?

 

[MedicineEng]

The secondary product flo rate is variable to maintain the temperature, meaning that if the overall calorific value is lower, it will burn more to maintain the temeprature inside the combustion chamber.
My issue is afterwards, in the boiler: will the thermal transfer (and consequent steam production) be the same if the gas composition resultant from the combustion changes?

 

[quark]

If I take it correctly, you mean to say you are using the heat of flue gas. i.e combustion is done elsewhere and the flue gases are run through a boiler for steam production.

Are you condensing the flue gas or only using the sensible portion of heat? There shouldn't be much difference, in this case. You should note that specific heat of water vapor will be a bit higher than fule gases at that temperature.

In case of condensing type boiler, there will be a huge difference and 50% moisture will give you better steam flowrate.

 

[MedicineEng]

Quark:
Your assumptions are correct, we burn in a equipment and then these combustion gases pass trough a boiler. This boiler doesn't condense the flue gas since the gas still exits the boiler at around 400C.

So I see by your reply that in this case, there should not be so much difference in the flow rate if the water content in the flue gas changes, am I correct?

 

[25362]

If possible, I suggest you read the pertinent article on incineration in Cheremisinoff's Handbook of Heat and Mass Transfer, Vol 1: Heat Transfer Operations (Gulf).

In this book you'll find that, for a given HHV of refuse and a constant gas temperature, lower excess air % result in lower gas heat content per lb of refuse (table 7, page 1360).