Published on March 3, 2014
Low Quality Waste Heat Recovery • Waste Heat recovery is the collection of the heat created as an undesired by-product of the operation of the piece of an equipment or machinery to fulfill a desired purpose elsewhere. • Fossil fuels are the world’s dominant energy supply. As these resources are nonrenewable, we must utilize it prudently. Hence Waste Heat Recovery is the need of the hour. • It is estimated that about 20-50% of the Industrial Heat is lost as waste heat in the form of hot exhaust gases, cooling water, heat lost from the hot equipment surfaces and heated products. • There are 3 essential components in Waste heat recovery as shown in the figure-
Sources of Waste Heat • Waste heat losses arises from equipment inefficiencies and from thermodynamic limitations on the equipments and processes.
Factors to be considered for Waste Heat Recovery • Quantity of Waste Heat:- It is the measure of how much energy is contained in a waste stream. It is given by Q = m*Cp*ΔT. • Waste Heat Temperature/Quality:- It is a key factor which is used to determine the waste heat recovery feasibility. For effective utilization of waste heat, it must be kept in mind that as the heat transfer takes place from a hot body to a cold body, the temperature of the waste heat source must be higher than the heat sink. Depending upon these two factors, the thermodynamic assessment of any power plant or other equipment can be done. • Suppose a system is at 1000 K & the atmosphere is at 300 K, then the exergy (maximum useful work) of the system is 70%. • 30% of the energy available is not useful, as it is unavailable energy and cannot be converted to work.
This is a very simplistic diagram of a coal-fired power plant. •Here, there are two major sources of waste heat. 1. Flue gases/stack gases 2. The steam after having passed through the turbine The temperature in both the cases is around 100-150 0C. But the atmospheric temperature is around 25 – 30 0C. Hence the exergy available is around (150 – 30) = 120 0C •This is a schematic diagram of a coal-fired power plant. •First of all, coal is pulverized in a mill, and it is mixed with the air from the atmosphere, which is then sent to the boiler where it is heated at a very high temperature to obtain the flue gases. •These flue gases come in contact with the very pure water which is passed through the section of water, which is then converted into steam. Now, this steam drives the turbine, generates the electricity. The steam is sent back to the condenser to be condensed back to water
Qualitative thermodynamic assessment with an aging boiler • The Heat balance calculations we perform are based on the first law of thermodynamics. • The shortcoming of this analysis is that it does not take into account properties of the system environment, or degradation of energy quality through dissipative processes. In short it doesn’t take into consideration, the irreversibility of the process. • Hence exergy analysis/thermodynamic assessment is done which is based on the second law of thermodynamics.
Ways and Means to use the Waste Heat • Heat Exchangers Recuperator Regenerator Heat Wheel Passive Air Preheaters Regenerative/Recuperative Burners Economizers Waste Heat Boilers • Heat Pumps Most of these changes have been employed by the power plants. Air has to be preheated and the steam from the HP turbine has to be sent through the boiler. As in the regenerative Rankine cycle, even the feed water is preheated. Now these Heat Exchangers and economizers are employed to make certain design changes in the plant. Now Power plants are based on the Rankine Cycle. We can have following cycles to replace it:1) Regenerative Rankine Cycle 2) Rankine Cycle with Reheat 3) Organic Rankine Cycle
But does this really solve our problem? • These Recuperators, Regenerators, Air Preheaters, Economizers, Waste Heat Boilers work in the High to Medium Temperature Range. • What we are really concerned is about the low temperature range at which the waste is directly thrown into the atmosphere. So these equipments won’t come in use then. • The real difficulties in using the Low Temperature Waste Heat Recovery:1. Corrosion on H.E. surface : As the water vapor in the exhaust gas cools, some droplets will condense on the H.E. surface. These are highly corrosive in nature. 2. Large H.E. Surface required : As the heat transfer is dependent on the H.E. area as well as the temperature gradient, due to lower temperature gradient in low temperature waste heat, large heat transfer area is required. 3. Find a use for the low temperature heat
Heat Pumps Organic Rankine Cycle • We use an organic fluid which has a boiling point much lower to that of the temperature of the condensate water at 150 0C. • The relatively low temperature heat is converted into useful work, which in turn is converted into electricity. • The major requirement in this process is that the organic vapor is cooled and condensed by air or water at a low temperature (typically near 30° C).
Carbon Capturing using Waste Heat : A Future Potential • • • • • Coal-fired power plant is a major source of Carbon Dioxide, and because majority of the power plants in India are Coal-based, it is a major source for global warming. Waste heat can be employed for Carbon Capture Solvents such as MEA, DEA are used to absorb the CO2 from the flue gases generated in the boiler where the solvent is passed from the top, flue gases from the bottom. The CO2 so captured would then be passed through a heater (utilizing the waste heat), which would separate it into solvent and CO2. The solvent is recycled, whereas CO2 is stored underground.
References •Waste Heat Recovery : Technology and Opportunities in U.S. Industry prepared by U.S. Department of Energy •Exergy Analysis and Efficiency improvement of a Coal-fired Thermal Power Plant in Queensland by R. Mahamud, M.M.K. Khan, M.G. Rasul and M.G. Leinster •For Carbon Capture, http://www.greentechmedia.com/articles/read/couldwaste-heat-be-the-secret-for-carbon-capture • A Power Point Presentation on Exergy Analysis This Presentation has been prepared by :Vismay Harani Samay Parikh Fp ID :
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