Superheated steam?

[joshBaldwin]

My plant produces steam for many processes. There is a lot of unused steam left over. We want to take that steam and generate electricity. However, our steam is so wet and sillica rich that we have to take a geothermal style turbine that has a really bad effeciency.

Would it be worth it to add a superheater before the turbine such as the effeciency increase enough to make it worth it ?

Is there plants doing such things ?

 

[danberry]

Please clarify:

Explain why you have excess steam.
What happens to the excess steam.
Process steam requirements P,T flow
Steam generating equipment type
Available steam pressure temperature flow
fuel(s) used

 

[joshBaldwin]

We have processes that generate steam no matter what. the steam is produced to evacuate heat generated. The steam production is relatively steady.

On the other hand. We have processes that need steam. However, Their requirement is smaller then what is produced, + they are seasons dependent.

For now, The excess steam is simply vented. its 189degC 1150kPa saturated steam. We have between 45 and 70 t/h available.

We were thinking of buying a geothermal style turbine for their wet/silica resistance...but they are expensive and unefficient.. (30% carnot effeciency)

What is common to improve effeciency ?

 

[EdStainless]

Don't use a turbine, use a screw expander. They look like a screw compressor, but they are built to run backwards. They run at lower speeds and like wet steam. It seals them and improves efficiency. I have seen them used in geothermal and heat recovery applications. There is a company that makes packaged units with generator and controls. You might search through the GRC, try ElectaTherm.
Normally these units transfer the heat to a low boiling organic like butane and then run the expander with that. Packaged units are available.

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Plymouth Tube

 

[danberry]

Your pressure is very low and there isn't that much steam. I estimate you could get 7 to 11 MW with your 45 to 70 t/h waste steam. That is a maximum, using an efficient condensing machine. Superheating the steam would increase power and reduce the back-end moisture but it may not be necessary or justifiable.

I think the economics will be marginal depending on the value of power in your part of the world, but it is certainly worth investigating. Perhaps mechanical drives could be interesting if you have big enough electric drives.

I recommend you have a competent consultant do a pre-feasibility study. The silica issue should be discussed with turbine manufacturers.

 

[joshBaldwin]

7 to 12 is around what we estimated for a geothermal turbine
what is the tonnes/h needed per MW of a typical screw expander ?
How is the maintenance of those ??

 

[KevinNZ]

What are you defining as a geothermal turbine? Geothermal steam turbines are same as other steam turbines but usally have a low inlet pressure to match the wells. To allow for the saturated steam from the wells the turbines have extra details for drainage but apart from that they are mostly the same.

Geothermal plants most have very good steam srubbing upstream of the trubine to remove contaminants. the steam at turbine inlet is usally dry saturated and will little water.