05-03-2013, 02:31 PM
Waste Heat Recovery
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Abstract
Many industrial processes generate large amounts of waste energy that simply pass out of plant stacks and into the atmosphere or are otherwise lost. Most industrial waste heat streams are liquid, gaseous, or a combination of the two and have temperatures from slightly above ambient to over 2000 degrees F. Stack exhaust losses are inherent in all fuel-fired processes and increase with the exhaust temperature and the amount of excess air the exhaust contains. At stack gas temperatures greater than 1000 degrees F, the heat going up the stack is likely to be the single biggest loss in the process. Above 1800 degrees F, stack losses will consume at least half of the total fuel input to the process. Yet, the energy that is recovered from waste heat streams could displace part or all of the energy input needs for a unit operation within a plant. Therefore, waste heat recovery offers a great opportunity to productively use this energy, reducing overall plant energy consumption and greenhouse gas emissions.
INTRODUCTION
Waste heat recovery methods used with industrial process heating operations intercept the waste gases before they leave the process, extract some of the heat they contain, and recycle that heat back to the process.
Common methods of recovering heat include direct heat recovery to the process, recuperators/regenerators, and waste heat boilers. Unfortunately, the economic benefits of waste heat recovery do not justify the cost of these systems in every application. For example, heat recovery from lower temperature waste streams (e.g., hot water or low-temperature flue gas) is thermodynamically limited. Equipment fouling, occurring during the handling of “dirty” waste streams, is another barrier to more widespread use of heat recovery systems. Innovative, affordable waste heat recovery methods that are ultra-efficient, are applicable to low-temperature streams, or are suitable for use with corrosive or “dirty” wastes could expand the number of viable applications of waste heat recovery, as well as improve the performance of existing applications.
Recovery of Waste Heat in Cogeneration and Trigeneration Power Plants
In most cogeneration and trigeneration power and energy systems, the exhaust gas from the electric generation equipment is ducted to a heat exchanger to recover the thermal energy in the gas. These heat exchangers are air-to-water heat exchangers, where the exhaust gas flows over some form of tube and fin heat exchange surface and the heat from the exhaust gas is transferred to make hot water or steam. The hot water or steam is then used to provide hot water or steam heating and/or to operate thermally activated equipment, such as an absorption chiller for cooling or a desiccant dehumidifer for dehumidification.
WHAT IS "TRIGENERATION"?
Trigeneration is the simultaneous production of three forms of energy - typically, Cooling, Heating and Power - from only one fuel input i.e. trigeneration power plants produce three different types of energy for the price of one.
Trigeneration power plants reach overall system efficiencies of 86% to 93%. Typical "central" power plants, that do not need the heat generated from the combustion and power generation process, are only about 33% efficient.
Trigeneration plants are installed at locations that can benefit from all three forms of energy. These types of installations that install trigeneration power plants are called "onsite power generation" also referred to as "decentralized energy." .
CONCLUSION
In order to minimize the adverse impacts of waste heat discharges, support should be continued for programs which foster energy conservation, either through the development of new, more efficient processes or through the use of efficient, low cost waste heat recovery devices. A variety of conventional waste recovery devices are currently available. However, there are two areas where the development and demonstration of new devices could significantly enhance the technology of waste heat recovery. The first application is for the utilization of low-grade waste heat for the direct conversion to electric energy through the use of various heat engine devices. The development of low-cost efficient heat pumps and Organic Rankine Cycle Heat engines are two devices which could foster increased waste heat reduce the flue gas temperature beyond recovery of low grade waste heat the condensation temperature, while sources, i.e., less than 35OOF (1 77OC). avoiding the high cost of corrosion