Economic transfer of acidic flue gas 1

[tuanjim]

Hi,

I am a process/chemical engineer with very little experience in moving gases and am hoping that some of the experts out there might be able to help me out with what I see as a fairly tough design problem.
The process I am involved with requires that we transport flue gas from an exhaust stack which is under slightly negative pressure about 450 ft to another part of the process which will remain under pressures just above atmospheric (i.e. approx. 0.05 inches of water). Due to the fact that gas is the by-product of coal combustion it has quite a high moisture content along with other acid gases which will condense on cooling and potentially chew up most types of equipment. We have quite severe economic constraints on how we can run this system but in an attempt to mitigate most of the acid gases (which also interfere with our end product) we were planning on trying to condense the moisture prior to the blowers via cooling loops and a knock-out/ moisture collection drum in which we may control the liquid level such that flue gas is bubbled through it at a low depth in order to promote dissolution of the acid compounds. At the pilot level the gas flow rate would be around 150 lpm but at full scale would probably be around 1000 times this. The gas will flow through the next stage of the process and then return to the exhaust stack.
You are probably already realizing I have no idea regarding the subtleties of gas handling but what I am really hoping is that:
a) People might be able to comment on the overall suitability of the gas condensing stage and whether or not this makes good engineering sense (i.e. do you foresee major operational problems?)
b) People might also be able to comment on what type of blower/compressor system we should be looking at in order to optimize the efficiency of this type of operation (i.e. high flow, negative intake pressure, low delivery pressure) which will also be able to handle the potential aggressive environment. Energy costs are a big issue in all processes but it's critical to our longevity I think that we get this right.

Anyway, I plan on working with local gas handling experts on this but I also thought that I'd see what all the old-hands here would suggest!

Thanks very much for any suggestions or help people can offer up. It's much appreciated (you also deserve credit just for getting through this question!).

Cheers

 

[zdas04]

As various countries begin implementing/strengthening carbon-emissions rules, this question comes up quite often. I have a couple of observations:

First, every millibar of dP upstream of the blower results in a substantial increase in energy cost. You want your blower to be as close to the stack as physically possible.

Second, while the corrosiveness of the stack gas is a real concern, the lengths people go to reduce the risk can be significantly overdone.

A simple fin-fan cooler on the discharge side of the blower followed by a mechanical separator can be very effective at lowering the water content from over 20,000 lbm/MMCF to under 1,000 lbm/MMCF which is a lot easier to deal with.

There are several blowers on the market that all would work fine in this application. I've looked at Roots Blowers for this application and the specs have looked to be adequate to go from a slight vacuum to over 1 bar(g). Much higher discharge pressure than that gets really expensive.

David

 

[crjones]

The suggestion of a PD (positive displacement) blower is the place to start. They will go from 15inHg vacuum on up. Limited by compression ratio. They are also routinely made in non-corrosive materials or coated. Possible US sources Gardner-Denver, Roots, and Tuthill. In Europe Kaeser, Aerzen, and Robuschi.

 

[dcasto]

I believe that the sizes of the units used for CO2 removal at power plants will be in the range of axial compressors.

I guess you could take the hot exhaust and send it to the axial compressor and compress it. The compressor would have to be made of lots of SS because of the temperatures and not the acid gases since you'd be above any acid dew point. A quench cooler would condense the water and acids.

Another method would require a huge quenching tower or system to cool the exhaust before entering the compressor and a scrubber. The compressor inlet would always be close to the dew point of acids and the first and second stage (or rows) of blades would be SS.

 

[chief]

It has been common practice on oil tanker ships to use the ships exhaust,after scrubbing, to put a gas blanket over the cargo tanks,when loading and discharging. I should think your system will benefit from research in this area, as they have many years of experience with this operation.

Offshore Engineering&Design