Combined heat and power systems -- more often referred to as CHP or cogeneration systems -- are systems that employ the simultaneous generation of thermal and electrical energy at or near the point of consumption. They are nearly always sited on the processor's premises, usually within or directly adjacent to the plant or facility. Depending upon the nature of the individual project and specifics of the integration, the system may supply all or just a fraction of the processor's total thermal and electrical requirements. A few of the more common prime movers for industrial CHP applications are natural-gas-fired microturbines, combustion gas turbines and reciprocating engines. These three are proven and widely utilized technologies. When engineered properly, total efficiencies in the range of 80 percent to 90 percent are realized.
Few applications lend themselves quite so nicely to economically sound integration of CHP systems as do industrial process heating applications. Direct- and indirect-fired convection, hot water and steam process heating systems alike can be easily designed or retrofitted with efficient, energy-saving and environmentally friendly CHP systems. Recovered heat can and has been used effectively for these and other common process heating applications:
- Convective parts drying.
- Convective paint curing (liquid and powder).
- Hot water supply.
- Heating of oils and other fluids.
- Steam boiler makeup water preheating.
- Combustion air preheating.
Let's now take a closer look at some of the more common industrial CHP applications.
Direct Vent ApplicationOne Ingersoll Rand MT70 microturbine is capable of generating upwards of 90 kW of grid-parallel electrical power while co-producing 500,000 to 600,000 BTU/hr of useful thermal energy in its 5,760 lbm/hr, 450°F (232°C) exhaust stream (1,370 scfm). This exhaust stream is quite clean (with NOX emissions less than 0.15 lbm/MWh and CO emissions less than 0.5 lbm/MWh) and rich in oxygen. As such, the hot products of combustion from a microturbine may be utilized at a lower temperature industrial heating process, so as to reduce -- or in some cases even eliminate -- fuel consumption at the targeted process. The exhaust stream may be injected directly into the return plenum of the oven air recirculation system to serve as a supplemental heating source. Or, if fitted with a suitable burner, the CHP system may serve as a viable substitute for the traditional burner set. Under this arrangement, the CHP system would serve as the primary heating source. Care must be exercised to maintain proper flow of air through the oven to which the CHP system is being retrofitted, so as not to upset the precisely designed balance of pressure between the oven interior and exterior zones. Operating efficiencies of 80 percent to 90 percent are attainable for systems integrated using this method of heat recovery.
Consider such a system (figure 1) at Vestil Manufacturing Corp. in Angola, Ind. Vestil, a manufacturer and distributor of custom material-handling equipment, operates two MT70 microturbines to partially offset electricity purchases from the local utility as well as to provide supplemental heat for the following energy intensive processes:
- Convection type powder coat curing oven.
- Convection type parts drying oven.
- Parts wash and rinse line.
The exhaust from both microturbines is diverted for use at the 350°F (177°C) powder coat curing oven to assist the original heat source -- a 4.5 MM BTU/hr direct fired burner. For increased system efficiency, the powder coat cure oven exhaust stream is recycled for use at the 225°F (108°C) parts drying oven to supplement its existing 1.7 MM BTU/hr direct-fired burner. In a final effort to further increase fuel efficiency, the parts drying oven exhaust stream is routed through a fin and tube heat exchanger. This provides free thermal energy for the five-stage heated wash-and-rinse process, assisting its original 3.5 MM BTU/hr indirect-fired tube-type burner set.
Fluid Heating ApplicationOne Capstone C60 microturbine with on-board or remote air-to-fluid heat recovery systems is capable of producing up to 60 kW of grid-parallel or stand-alone electrical power along with 375,000 BTU/hr of thermal energy in the form of hot water (design flows and temperatures vary widely depending upon particular details of the application). This 375,000 BTU/hr of heated fluid (water or propylene glycol mix) is derived from the unit's 3,816 lbm/hr, 580°F (304°C) combustion exhaust stream. With exhaust emissions of less than 9 ppm NOX, this technology is currently unmatched in the field of fossil fueled power generation.
The heat recovered from a microturbine's exhaust stream (figure 2) is being utilized in several ways at Stripco Inc.'s plant in Mishawaka, Ind. Stripco Inc. is a manufacturer of production-ready steel coils and strips. Their microturbine-based CHP system maximizes the use of recovered heat energy by:
- Offsetting energy consumption of electrical heating elements used to maintain the temperature of process oils used during production.
- Providing localized space heating for certain work stations around the plant.
- Warming the used-oil recovery system.
- Supplying hot water for a clean and safe work environment.
In the event of an extended electrical grid power failure, the system becomes a source of emergency power for the administration building as well as for critical stations within the plant itself.
Combination: Direct Vent and Fluid Heating ApplicationUtilimaster Corp. of Wakarusa, Ind., a builder of quality custom commercial vehicles, employs an Ingersoll Rand MT70 based CHP system (with onboard heat exchanger) to supply partial base load power and process heating energy to their product preparation and finishing plant.
Recovery of heat from the microturbine system allows Utilimaster to achieve higher fuel utilization efficiency -- all while greatly improving the overall effectiveness of some of their most crucial process heating applications. Processes directly affected by integration of the CHP system include:
- Vehicle drying line (figure 3).
- Convection type paint curing oven.
- Convection type parts drying oven.
- Vehicle masking and finishing line.
This solution involves the recovery and transfer of waste heat from the microturbine exhaust stream to a circulating fluid loop (the working fluid being water, in this case). Upon exiting the microturbine's onboard heating coil, the fluid -- now heated to approximately 190°F (88°C) -- is routed directly through a heat exchanger mounted in the regeneration section of a newly installed split-stream rotating wheel desiccant dehumidification unit located inside the vehicular drying room (figure 4).
This provides enough heat to sufficiently dry out (regenerate) the desiccant wheel following exposure of the wheel to the wet process airstream. The process repeats itself as the regenerated desiccant wheel once again is exposed to a stream of very humid air from the drying room. Moisture readily adheres to the surface of the wheel via adsorption to the silica gel material of which the desiccant wheel is constructed. Moisture driven from the desiccant wheel during the regeneration process is discharged completely from the drying room. Whereas Utilimaster's original method of drying vehicles involved the firing of radiant-tube burners along with the continuous exhaust of tremendous amounts of air and heat from the drying room, this new method maximizes moisture removal while minimizing energy consumption. Microturbine-driven desiccant dehumidification has significantly improved the effectiveness of the vehicular drying process.
At the small parts paint curing booth, a tube-and-fin heat exchanger placed at the fresh air intake transfers energy from the CHP circulating fluid loop (presently at 170 to 175°F [76 to 80°C]) to the entering ambient makeup airstream. A direct-fired burner is used here to maintain the desired zone temperature within the convection oven.
The small parts drying booth is heated directly using the remaining energy contained in the microturbine's exhaust stream. A temperature probe in the duct controls the amount of energy delivered (i.e., exhaust stream will bypass oven to atmosphere if oven setpoint temperature is exceeded).
Finally, heated regeneration air from the desiccant system is routed to other areas of the plant as required. This reduces the total heating load for existing plant space heaters.
Both technologies -- microturbine and reciprocating engines -- are suitable for such applications, although each may possess certain advantages over the other in some key areas (technically as well as financially). This should be considered carefully during the preliminary energy study. A few of the major factors to consider for preliminary determination of the economic viability of a CHP project are:
- Price of utility purchased electricity (current and future).
- Price of natural gas (current and future).
- Energy usage profile (electric and gas).
- Plant and process operating profile (days and hours of operation).
- Estimated efficiency of incumbent, in-plant heat sources.
Each of the profiled installations demonstrates the flexibility and diverse applicability of CHP systems to the industrial process heating community. Could CHP benefit your energy-intensive process heating needs?