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Alfred Piggott 2012.05.31 Industrial Waste Heat Recovery Thermal Literature Review
 

Alfred Piggott 2012.05.31 Industrial Waste Heat Recovery Thermal Literature Review

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Literature Review of Industrial Waste Heat Recovery with Many Parallels to Automotive Waste Heat Recovery

Literature Review of Industrial Waste Heat Recovery with Many Parallels to Automotive Waste Heat Recovery

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    Alfred Piggott 2012.05.31 Industrial Waste Heat Recovery Thermal Literature Review Alfred Piggott 2012.05.31 Industrial Waste Heat Recovery Thermal Literature Review Document Transcript

    • Credit: Department of Energy Industrial Waste Heat Recovery Alfred Piggott 4/20/2012 MEEM 4220 – Internal Combustion Engines
    • 1.0 Introduction ....................................................................................................................................... 3Table of Contents2.0 Waste Heat Grades .......................................................................................................................... 33.0 Systems for Waste heat Recovery ............................................................................................. 5 3.1 Heat Exchangers .......................................................................................................................... 5 3.2 Load Preheating .......................................................................................................................... 5 3.3 Low Grade Recovery .................................................................................................................. 5 3.3.1 Deep Economizers.............................................................................................................. 6 3.3.2 Indirect Contact Condensation Recovery.................................................................. 6 3.3.3 Direct Contact Condensation Recovery ..................................................................... 6 3.3.4 Transport Membrane Condenser ................................................................................. 6 3.3.5 Heat Pumps ........................................................................................................................... 6 3.3.6 Closed compression cycles ............................................................................................. 6 3.3.7 Open Cycle Vapor Recompression ............................................................................... 6 3.3.8 Absorption Heat Pumps ................................................................................................... 6 3.4 Power Generation ....................................................................................................................... 7 3.4.1 Generating Power via Mechanical work .................................................................... 7 3.4.2 Direct Electrical Conversion Systems ......................................................................... 74.0 Conclusion .......................................................................................................................................... 85.0 Bibliography ...................................................................................................................................... 9Waste Heat Recovery Page 2 of 9
    • 1.0 IntroductionAs fuel prices rise, supplies decrease, and concerns about environmental impact intensify, westart looking for new and better energy sources. A pollution-free source of energy that isoften overlooked is waste heat. Waste heat is a byproduct of converting energy from oneform to another. As governed by the second law of thermodynamics, no process of energyconversion is 100% efficient. Typical fossil fuel energy conversion processes includeconverting coal or natural gas to electricity or gasoline to vehicle power.In 2008 world energy consumption was roughly 505 quadrillion BTU (505 X 1015 BTU) (1).Conversion of fossil fuels to usable energy accounts for roughly 84% (1) of the world energyconsumption. The efficiency of these fossil fuel conversion processes tends to be around 28-43% (2). This means 57-72% or 217-288 quadrillion BTU is turned to heat and not part ofthe usable output. This equates to roughly 4-5 times more energy going to waste heat than allthe renewable energy (Wind, Solar, Hydropower, Biomass, Geothermal) usage which wasabout 50 quadrillion BTU in 2008 (1).2.0 Waste Heat GradesThe second law of thermodynamics also governs the amount of waste heat that can berecovered. The higher the temperature of the waste heat, the greater the proportion that canbe recovered. This can be seen with the equation for Carnot efficiency (equation 1). TH is thetemperature of the waste heat and TL is the temperature of the environment, for example theambient air temperature or the temperature of a lake or river where a portion of heat notrecovered will be “dumped”. 𝑇𝐿 𝜂 𝐶𝑎𝑟𝑛𝑜𝑡 = 1 − 𝑇𝐻 Equation 1: Carnot EfficiencyWaste heat temperatures are generally classified into three categories (3). These categorieswere chosen based on typical industrial waste heat temperatures and the commerciallyavailable equipment to recover the waste heat.High-Grade 1100 ≤ TH ≤ 3000◦F (590-1650◦C)Medium-Grade 400 ≤ TH ≤ 1100◦F (205-590◦C)Low-Grade 80 ≤ TH ≤ 400◦F (27-205◦C)Table 1 shows various sources of high-grade waste heat. Although high-grade waste heat canbe recovered at a higher efficiency than the lower grades, the cost to do so will be higher dueto special materials and equipment design needed to withstand the higher temperatures.Waste Heat Recovery Page 3 of 9
    • Table 1: Sources of High Grade Waste Heat [Source (3)]Table 2 shows sources of medium grade waste heat. This is a temperature range that can stillbe economical (3) without the higher cost associated with high-grade recovery conversionequipment. Table 2: Sources of Medium Grade Waste Heat [Source (3)]Table 3 shows sources of low-grade heat. Due to low efficiency at these temperatures, it istypically not economical to extract work from these sources. Some applications includepreheating process gases, liquids, solids, or space heating.Waste Heat Recovery Page 4 of 9
    • Table 3: Source of Lowe Grade Waste Heat [Source (3)]3.0 Systems for Waste heat Recovery3.1 Heat ExchangersIn medium to high temperature heat recovery systems, heat exchangers use heat fromcombustion exhaust gases to preheat pre-combustion incoming air. This reduces the amountof heat taken from combustion to heat the air and thus more combustion heat is available torun the intended process. There are many types of heat exchangers used, these includerecuperators, regenerators, heat wheels, passive air preheaters, heat pipes, waste heat boilersand finned tube heat exchangers / economizers. Each of these has advantages anddisadvantages for a given application.3.2 Load PreheatingLoad preheating refers to the preheating solid materials entering a plant with the waste heatfrom the plant process. An example of solid preheating is using the waste heat from a brazefurnace to preheat the parts that will be brazed. This reduces the load on the furnaces andthus reduced energy consumption.3.3 Low Grade RecoveryAs in high and medium grade waste heat recovery, low-grade waste heat recovery also usesheat exchangers to accomplish the task. Low-grade recovery has a different set of challengesthan medium and high-grade waste heat recovery. The main challenges are corrosion, largeheat transfer surfaces, and finding a use for recovered heat. Corrosion becomes a challengebecause these heat exchangers cool the gases to a low enough temperature that vaporscondense. These combustion vapors are highly corrosive. Another challenge for low-gradeWaste Heat Recovery Page 5 of 9
    • recovery is the size of the heat exchanger. The laws of heat transfer require a larger surfacearea for heat transfer if the difference in temperature of the hot side and cold side is smaller.3.3.1 Deep EconomizersDeep economizers are corrosion resistant heat exchangers designed to cool exhaust gases tolow-grade 150-160ºF. The heat recovered from the exhaust gas can then be used for anotherprocess.3.3.2 Indirect Contact Condensation RecoveryIndirect contact condensation recovery units are corrosion resistant shell and tube heatexchangers that can cool gases enough (100-110ºF) to completely condense vapor which inturn increases their efficiency.3.3.3 Direct Contact Condensation RecoveryDirect contact condensation recovery heat exchangers mix process “waste” steam withcooling fluid that is used to heat or preheat an external system. The direct contact of steamwith the cooling fluid makes this process more efficient than an indirect contact heatexchanger.3.3.4 Transport Membrane CondenserA transport membrane condenser uses capillary action to condense combustion gas vaporand recover latent heat for use in another process.3.3.5 Heat PumpsHeat pumps can increase the temperature of low-grade waste heat for usage in a process thatrequires a higher temperature. In certain cases this can be done economically depending onthe temperature rise needed and the cost of fuel and electricity.3.3.6 Closed compression cyclesThe closed compression cycle is essentially a heat pump. This cycle removes heat from onefluid loop where cooling is needed and adds that heat to another fluid loop where heating isneeded.3.3.7 Open Cycle Vapor RecompressionThese systems use either mechanical or thermal compression to increase the pressure andthus the temperature of a waste side vapor. This allows the heat to be used in processeswhere a higher temperature is needed.3.3.8 Absorption Heat PumpsThe operation of an absorption heat pump is similar to the closed cycle compression systembut instead of using mechanical compression it uses chemical means driven by heat.Waste Heat Recovery Page 6 of 9
    • 3.4 Power GenerationPower generation from waste heat typically involves using waste heat to generate mechanicalenergy, which subsequently drives an electrical generator. Prevailing technologies foraccomplishing this are the steam Rankine cycle, organic Rankine cycle and the Kalina cycle.Other types of power generation that currently have not been demonstrated for large-scaleindustrial use are thermoelectric, piezoelectric, thermionic and thermal voltaic powergeneration. These types convert heat directly to electrical energy.3.4.1 Generating Power via Mechanical work3.4.1.1 Steam Rankine CycleThe most common system that converts heat to mechanical work is the steam Rankine cycle.This system is typically used for medium grade waste heat as it becomes less economical forlow-grade heat. Furthermore, if temperatures are too low, superheat will not be achieved andif superheat is not achieved, condensation and erosion of turbine blades will occur.3.4.1.2 Organic Rankine CycleOrganic Rankine cycle is much more suitable for low temperature waste heat recovery. Thissuitability comes from the organic working fluid, which has a higher vapor pressure and alower boiling point than water. The higher molecular mass of the organic working fluid alsoallows for smaller turbine design due to more energy imparted on the turbine blade per unitarea.3.4.1.3 Kalina CycleThe Kalina cycle is basically a Rankine cycle that uses a mixture of two non-reacting fluids.The benefit of using two fluids is better thermal matching to the waste heat source. Thisthermal matching allows the Kalina cycle to achieve significant efficiency gains over the onefluid Rankine cycle.3.4.2 Direct Electrical Conversion Systems3.4.2.1 Thermoelectric GenerationThermoelectric generators (TEG) utilize the Seebeck effect to convert heat directly toelectricity. When two different semiconductors are connected electrically in series and atemperature differential is applied, a voltage is created across the series. Thermoelectricmaterials are suitable for medium and high-grade waste heat recovery. Currently the costs arehigh and efficiencies are relatively low compared with the Rankine cycles. Advances inmaterials will make thermoelectric power generation more competitive.Waste Heat Recovery Page 7 of 9
    • 3.4.2.2 Piezoelectric Power GenerationPiezoelectric Power Generation (PEPG) converts mechanical vibrations into electricity.These vibrations come from oscillating gas expansion processes. PEPG are suitable for low-grade heat recovery. These devices are currently very low efficiency and high cost.3.4.2.3 Thermionic GenerationThermionic generation devices operate on the principle of thermionic emission. Thermionicemission is produced when a temperature difference across two metal oxide plates separatedin a vacuum causes electrons to flow through the vacuum gap. These devices are suitable forhigh and low-grade heat sources.3.4.2.4 Thermophotovoltaic (TPV) GeneratorTPV generators operate by converting radiant heat into electricity. The heat source heats anemitter, which gives off electromagnetic radiation. This radiation travels through a filter andon to the Photovoltaic cell that converts the radiation into electricity.4.0 ConclusionWaste heat is a large potential source of pollution free energy. The maximum efficiency ofthe system used to recover waste heat depends on the temperature of the heat and isgoverned by laws of thermodynamics. There are many types of waste heat recoveryequipment, each with its own pros and cons depending on the characteristics of the wasteheat source. In the future, it will be possible to convert heat directly to electricityeconomically on a larger scale with solid-state devices.Waste Heat Recovery Page 8 of 9
    • 5.0 Bibliography 1. "International Energy Outlook 2011." U.S. Energy Information Administration (EIA). Sept. 2011. Web. 7 Mar. 2011. <http://www.eia.gov/forecasts/ieo/pdf/0484(2011).pdf>. 2. "Environmental Footprints and Costs of Coal-Based Integrated Gasification Combined Cycle and Pulverized Coal Technologies." United States Environmental Protection Agency. Nexant, Inc., July 2006. Web. 7 Mar. 2012. <2. http://www.epa.gov/air/caaac/coaltech/2007_01_epaigcc.pdf>. 3. Doty, Steve, and Wayne C. Turner. Energy Management Handbook - Seventh Addition. 7th ed. Lilburn: Fairmont, 2009. Print. 4. "Waste Heat Recovery: Technology and Opportunities in the U.S. Industry." U.S. Department of Energy Efficiency and Renewable Energy. BCS Incorporated, Mar. 2008. Web. 10 Mar. 2012. <http://www1.eere.energy.gov/industry/intensiveprocesses/pdfs/waste_heat_recovery.pdf>.Waste Heat Recovery Page 9 of 9