Pop quiz: After unwrapping a package you received, you have an empty box. What do you do with it?
- Throw it in the trash.
- Put it out with the newspapers and magazines to be recycled.
- Break it up for kindling the next time you use the fireplace.
- Store it away to be reused when you have a package to send someone.
Apart from the first choice, all these are forms of recycling. You’re deciding whether to recycle the box as paper or cardboard, energy or, well…a box. There is no “right” answer to this question because your best choice is determined by circumstances.
From the efficiency standpoint, the best choice is to reuse the box. Apart from peeling off a label or two, no extra energy or cost was involved in the recycling effort. Converting it to new paper is probably preferable to burning it because a cardboard box is not really a concentrated form of energy. However, if there’s no recycling system where you live, burning it might be better than consigning it to a dump or landfill. Oh, wait a minute -- there’s some sort of plastic foam stuck to the inside. Maybe it’s not such a good idea to burn it after all.
So efficiency considerations favor reusing the box, but what if you live in a cramped apartment, where storage space is at a premium? What if you don’t expect to have a package to ship for several months? Is it worth your while to have an empty box taking up closet space for who-knows-how long? What if the box came to you so battered in shipment that putting it back in shape for reuse will consume yards of tape and a half hour of your time? (If you think nobody does this, you haven’t spent much time in the post office line.)
The point of this is not to turn the fate of a cardboard box into one of your life’s Great Decisions -- it’s to illustrate how complex the decision can be. Let’s see how this can apply to the issue of waste energy, something all process heating operations have in abundance.You have a similar set of choices to consider in the evaluation of a waste heat stream. You can:
- Just let it go.
- Recycle it
- by preheating combustion air.
- by raising steam.
- by generating electrical energy via a turbine.
- by generating warm air for space heating.
- by heating process water.
In this list, preheating the incoming product is the equivalent of reusing the box. It is the most efficient and elegant way to use the waste energy because it goes directly to the product. There’s no need to convert it into a different form or to pass it off to a different flow stream. All the other choices are thermally less efficient.
Suppose preheating the product is out. It would require completely reconfiguring your process line, or it might have a harmful effect on the product’s time-temperature cycle. So, you look at preheating the combustion air instead. Good thought, but the oven exhaust temperature is only 350oF (180oC), and it won’t give you a combustion air preheat higher than, say, 250oF (120oC). You run the numbers and find the efficiency improvement is marginal, you won’t see a payback in your lifetime, and to make matters worse, the exhaust gets cooled so much you may run into a problem with moisture condensation in the ductwork.
You look at waste heat boilers and turbines. Same problem -- the exhaust gases are too cool to make the process worthwhile. Besides, your plant has no use for the steam, and you can’t generate enough electricity to pay for the installation.
Putting the exhaust through a heat exchanger for space heating will work -- you don’t need high temperatures for this -- but just how much need do you have for it if your plant is in Phoenix? OK -- process water, then. You use lots of that. What, the exhaust stream contains contaminants that can corrode or foul a heat exchanger? Awwwww….
I’m not trying to be a wet blanket here. We all want to recycle waste energy, for reasons that range from noble to utterly selfish. But if it’s to be more than an expensive mistake, you have to begin with a careful, frank assessment of your circumstances. Otherwise you may exchange a waste of energy for a waste of money, product quality or maintenance effort, and lose your shirt in the bargain.