5 replies to this topic

[ahmed gomaa]

Dear, 

 

I am working in a petrochemical plant and we have a thermal oxidizer which produce waste gases at atmospheric pressure, 980 c, and about  1,500 kg/hr and i am thinking to recover these energy in to an air pre-heater. 

we have a heater using combustion air at ambient temperature, natural draft, natural gas at 6 bar, ambient temp. and  flow rate of 700 Nm3/h.

since the heater is natural draft i have no measure of the combustion air but we have an Oxygen analyzer on the exit of the heater so i think it could be calculated from the combustion equation with some assumptions (i don't know how if someone could tell me or by assuming A/ F)

 

Assuming A/F = 15 , 

 

700 Nm3/hr (density of natural gas at 6 bar is 4 kg/m3 and normal condition is close to the ambient here so so we can use 2800 kg/h. 

 

then air flow rate= 42000 kg/h

 

if we make a quick heat balance assuming the waste gases and combustion air have the same specific heat

then temperature drop in the waste gases will be 30 time the temperature rise in the air

let us say temp. of waste gases drops 700 c then we will have 25 increase in air combustion temperature

 

My questions are:

1- How could i calculate the air quantity or A/F in the heater by knowing the composition of the fuel gas and the oxygen content in the product.

2- How can i calculate the flame temperature now.

3- How can i calculate the effect of increasing the air combustion on the efficiency or % of fuel saving.

4- Could this be reasonable, dropping temp 700 at a side and 25 rise in the other side, as thermal and material considerations.

5- Can i use a tubular pre-heater with waste gases inside the tubes and the air in the box with very large flow rates differences (1,500 kg/h-40,000 kg/h).

 

i am a beginner in this field so please understand these silly questions and my poor language.

 

I highly appreciate your help.

Thank you. 

[PingPong]

700 Nm3/hr (density of natural gas at 6 bar is 4 kg/m3 and normal condition is close to the ambient here so so we can use 2800 kg/h.

That is not correct.

700 Nm3/hr natural gas is only roughly 500 kg/h.

This corresponds to a lower heating value (LHV) of roughly 7000 kW.

 

Cooling down 1500 kg/h oxidizer fluegas by 700 ^oC will give about 330 kW heat duty.

 

So energy saving could be in the order of 4 to 5 % on natural gas flow.

 

Exact numbers depend very much on the composition of the natural gas, and that of the oxidiser flue gas.

 

Before you decide to recover heat from the thermal oxidiser flue gas you need to know its composition, especially content of water vapor and any corrosive compounds. The new heat preheat exchanger has to be corrosion resistent to that oxidiser flue gas.

Moreover you will need a forced draft fan to push the combustion air through the new preheat exchanger.

[ahmed gomaa]

Dear Pingpong,

 

Thanks for your response,

 

- I converted the cubic flow rate into mass flow rate by dividing by the density which i found in an online calculator at 25 c and P=6 bar is 4 kg/m3.

- Yes i make a roughly estimation that recovery of these waste gases could reduce fuel consumption by about 3 % (the actual LHV is 8400) 

- The composition of the thermal oxidizer is the combustion products of natural gas + air + process gas (Nitrogen 90% - styrene 7% - - Ethylbenzene 0.5 % - water 4.5% )

- I take into my consideration that i will have to make the heater forced draft

- I am asking about the equation that i can calculate the fuel saving precisely which i should do by calculating the flame temp. and the new fuel required to obtain the same flame temp. by using preheated air by about 25 c. and also type of equipment could be used to deal with the large difference in the flow rate between the two fluids, and material consideration.

 

Thank you

[PingPong]

- I converted the cubic flow rate into mass flow rate by dividing by the density which i found in an online calculator at 25 c and P=6 bar is 4 kg/m3.

700 Actual m3/h with a density of 4 kg/m³ is equal to 2800 kg/h.

 

However 700 Nm³/h, which is what you specified, is only about 500 kg/h.

[ahmed gomaa]

'' 700 Actual m3/h with a density of 4 kg/m³ is equal to 2800 kg/h.

 

However 700 Nm³/h, which is what you specified, is only about 500 kg/h.''

 

You are absolutely right, my mistake, so what heat transfer equipment do you advise me to use?

 

thanks 

[PingPong]

Problem is that you want to use very hot flue gas of 980 ^oC to preheat air of ambient temperature.

Normal air preheaters that are for sale are not designed for such extremely hot flue gas.

You would have to let a vendor build a very special and very expensive air preheater.

 

Moreover you need a new forced draft fan for the combustion air and a new induced draft fan for the oxidiser fluegas (downstream the air preheater).

And you have to add control systems and safeguarding systems on both the heater and the oxidiser that can interfere if there is a problem or hick-up in one of them.

 

The amount of heat that you can recover, say 330 kW, is in my opinion far too little to go to so much trouble.

 

It would make more sense to preheat the combustion air with fluegas from the heater itself. In that way you can use a cheaper conventional air preheater and you avoid that the control and safeguarding systems of heater and oxidiser need to be connected.

 

If one wants to use waste heat of 980 ^oC it is much better to use it to heat up a stream to high temperature. For example: use it to generate and superheat steam, or the heat up hot oil, or further heat a hot process fluid, or ......

In that case however I am still sure that 330 kW is not worth the cost of such system.