utilizing waste heat to produce electricity 1

[AppleChris]

I work in pulp & paper industry. We loose a lot of energy (~150°F hot air and condensing steam), from dryers, through the roof vents.

Could I use Kalina-cycle (ammonia/water) turbine or a Stirling engine to recover this heat and utilize it to produce electricity? Any suppliers of the equipment?

 

[dcasto]

There isn't much energy in hot air.

The condensing steam should be used to preheat water or air in other processes.

 

[MintJulep]

I agree, probably not economically viable to use low grade waste heat to generate electricity.

Use it to preheat combustion air for boilers, or the make-up air for the driers.

 

[trashcanman]

Alas, I also agree the previous posts. Payback costs for the capital investments required would be too long.

 

[PowerSmith]

I have seen heat pumps used in this type of waste heat application to recover and boost the temperature so the the energy could be used in a practical mechanical application.

Pete PowerSmith

 

[davefitz]

Lowest practical commercial temp I know of is 90 C ( 194 F) to drive a lithium bromide absorption air conditioner ( Yazaki).

 

[willard3]

Yazakis (lithium bromide)will produce rated tonnage at 175F if condensing temp is kept low, well within the range of existing heat-recovery equipment.

 

[stanlsimon]

There is a company based in Phoenix Arizona which is commercializing sterling engines. That design uses co2 as the working fluid.

However as a general rule it is more economical to conserve energy than to find new sources.

If you could convert this thermal waste to electricity you could hope to get an efficiency of 30 or 40%. On the other hand if you were to install an air to air heat exchanger to preheat your drier make-up air you would be able to get an efficiency of around 80%, depending on what the outside air temperatures are and also how saturated the exhaust stream is. You would need a strategy to handle defrosting of the HX if you are in a cold climate area. The cost effectivenes of the HX will be tempered by the amount of ductwok modifications needed to connect the HX to intake and exhaust.

 

[omarbakr]

Well steam and air coming from the top threw vents and dryers will not give you that much energy for the installation costs and the required systems, and besudes most of the time they are sources of getting rid of contamination and cleaning the system

 

[MJCronin]

Apple.....

Give up on the idea of using "low-grade" heat to generate power.... your capital investment will be huge and the amount of power generated will be minimal

Perhaps you should consider a re-generative type of heater for the incoming(cold) air ?

A "pipe within a pipe" type heat exchanger (of fairly long length) for 150F might prove economic if prpoerly constructed.

At 150F, you are somewhat in the range where heat must be "thrown away" simply to get it out of the building.

If this application were the flue duct from a boiler, about 220F would be the minimum limit for the exhaust gas temperature (and even higher temps should be considered for high-sulfur fuels).

My opinion only

-MJC