By Raynald Labrecque and Kiari Goni Boulama, Institut de Recherche d’Hydro-Québec (Laboratoire des Technologies de l’Énergie) |
The process industries are extensive users of energy. But it is now clearly established that a large amount of this energy is wasted, essentially in the form of heat rejected to the environment through gaseous or liquid effluents. The temperature level of this heat makes it of no use by the process; and at the same time, in many cases, it cannot be economically upgraded. Examples of industries rejecting considerable thermal energy include: thermal power plants, metallurgical processing, the cement industry, pulp and paper, and petrochemicals. Table 1 gives typical exhaust temperatures for some such industries.
On the basis of the second law of thermodynamics, the quality of an energy source is determined by its ability to perform work. In the specialized literature, this work potential is referred to as availability, or more precisely "available energy" or exergy. From this standpoint, heat (thermal energy) is considered to be the most degraded form of energy.
A simplified approach to evaluate the exergy content of a hot waste stream is presented here. The objective is to provide engineers a means to easily estimate the amount of useful energy they may recover from industrial heat exhausts. Such energy can be…