Multiple hearth furnace exhaust has been collected and utilized in different forms for many years. Charcoal manufacturers routinely use exhaust directly for drying their feedstock before it is introduced into the furnace. The exhaust from production of activated carbon is very high in combustibles so it is usually burned in a thermal oxidizer and then put through a heat exchanger to extract the high quantity of available energy in the form of superheated steam. The steam has been used for plant process requirements, or for production of electrical power.
Reacting to regulatory changes in California in the 1980’s, two multiple hearth furnaces were specifically built for power production. They successfully burned agricultural wastes and produced up to 10 megawatts of power for a number of years.
The municipal sludge industry also has some examples of heat recovery, even as early as the 1970’s, producing either electrical power or steam for process or heating use, although in limited instances.
Today’s Environment For Energy Recovery
The last few years have seen dramatic changes in the atmosphere surrounding MHF energy recovery and power generation. Like Mark Twain’s weather, everyone seems to be talking about it, but in this case, at least some are doing something about it. We believe the change is not so much one of attitude, but of focus. Industrial and municipal management has always considered energy savings a good thing, but other priorities usually came ahead of anything more complicated than simple endeavors like more efficient lighting. Recently the public has become more vocal and enthusiastic about recycling, saving energy, and utilizing our waste streams. This translates to more political incentive in the municipal area, but industrial installations are also seeing pressure to be “Good Citizens” where it comes to energy and sustainability.
It Does Not Have To Be Steam!
Often plant managers and owners have reservations about adding a steam power generation system when they do not currently have any high-pressure steam in their plant. Concerns expressed typically are for safety (Plant workforce not familiar with or untrained for high-pressure steam), and regulatory restrictions requiring certified or licensed boiler workers on site at all times.
The good news is that there is a viable option called the “Organic Rankine Cycle” (ORC), which substitutes an organic fluid for water. Fluids now available are non-flammable, have low global warming potential, and operate at much lower temperatures than steam (less than 600 °F). The inherent safety of these systems, coupled with proven very low maintenance requirements, make them a very attractive option for utilizing MHF waste heat.
ORC systems, as applied to the typical multiple hearth furnace, have thermal efficiencies that are equal to or better than steam systems. While steam may be better for some high-volume, high-temperature applications, the organic fluid can be chosen to be more efficient at the typically lower temperatures available from MHF exhaust.
Build on Furnace Experience for Working Solutions
IFCO has been involved in varying capacities with several municipal power production projects recently, and one thing has been very clear – Specific experience with multiple hearth furnace systems is critical. We have the years of multiple hearth furnace experience required to guide a heat recovery project past the potential pitfalls in design, construction, and startup.