Biochemical and Thermochemical Conversion of Biomass.pptx

Unit -5 Cogeneration and waste heat recovery

1. KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY (Autonomous) Department of Agricultural Engineering Course: Biochemical and Thermo chemical Conversion of Biomass Topic : Cogeneration and Waste Heat Recovery By Mr. M.Prabhu , Assistant Professor, Department of Agricultural Engineering, Kongunadu College of Engineering and Technology

2. Cogeneration is on-site generation and utilization of energy in different forms simultaneously by utilizing fuel energy at optimum efficiency in a cost-effective and environmentally responsible way.

3. Heat-to-Power ratio Definition of heat-to-power ratio is thermal energy to electrical energy required by the industry. Cogeneration technologies Steam turbine-based cogeneration system Gas turbine-based cogeneration system Combined steam/gas turbine-based cogeneration system Reciprocating engine-based cogeneration system Most widely used cogeneration systems in the chemical process industrial plants are based on steam turbine, gas turbine or combined steam/gas turbine configurations. Steam turbine-based cogeneration system This system works on the principle of Rankine cycle of heat balance. In Rankine cycle, the fuel is first fired in a suitable boiler to generate high-pressure steam at predetermined parameters. The steam so produced is then expanded through a steam turbine to produce mechanical power/ electricity and a low-pressure steam. The steam turbine could be of backpressure type, extraction-cum-condensed type or extraction-cum-back pressure type depending on different levels and parameters at which the steam is required by the chemical process in that particular plant.

4. In a conventional fossil fuel fired power plant, maximum fuel efficiency of about 35% is achieved. Maximum heat loss occurs by way of the heat rejection in a steam condenser The steam so extracted could be supplied to either process consumer or to heat the feed water before it enters into boiler. The overall efficiency of around 80-85% is achieved in this type of plant configuration. The selection of steam turbine for a particular cogeneration application depends on process steam demand at one or more pressure/temperature levels, the electric load to be driven, power and steam demand variations, essentiality of steam for process, etc. The steam to power ratio also plays a role in selection of the steam turbine. Steam turbine-based cogeneration systems can be fired with variety of fossil fuels like coal, lignite, furnace oil, residual fuel oil, natural gas or non-conventional fuels like biogas , bagasse , municipal waste, husk

5. Gas turbine based cogeneration system This type of system works on the basic principle of Bryton cycle of thermodynamics. Air drawn from the atmosphere is compressed and mixed in a predetermined proportion with the fuel in a combustor, in which the combustion takes place. The flue gases with a very high temperature from the combustor are expanded through a gas turbine, which drives electric generator and air compressor. The exhaust flue gases from the gas turbine, typically at a high temperature of 480-540 C Industrial gas turbine-based power plants installed to generate only electric power operate at the thermal efficiency of 25-35% only depending of type and size of gas turbine. With recovery of heat in exhaust flue gases in a waste heat recovery boiler (WHRB) or heat recovery steam generator (HRSG) to generate the steam, overall plant efficiency of around 85-90% is easily achieved.

6. Gas turbine-based cogeneration system gives a better performance with clean fuels like natural gas, or non-ash bearing or low ash bearing liquid hydrocarbon fuels like Naphtha, High speed diesel, etc. Another major drawback is that when the demand of power drops below 80% of gas turbine capacity, the specific fuel consumption increases and the steam output from WHRB also drops . Combined steam/gas turbine-based cogeneration system It is clear from the title of system itself that it works on the basis of combination of both Rankine and Bryton cycles, and hence it is called combined steam/gas turbine-based cogeneration system. In this system, fuel energy is first utilised in operating the gas turbine as described in Gas turbine-based cogeneration system. Waste heat of high temperature exhaust flue gases from the gas turbine is recovered in WHRB to generate a high pressure steam. This high-pressure steam is expanded through a back-pressure steam turbine, or an extraction-cum-back pressure steam turbine, or an extraction-cum condensing steam turbine to generate some additional electric power. The low-pressure steam available either from the exhaust of back-pressure steam turbine or from extraction is supplied to the process consumer.

7. When the ratio of electrical power to thermal load is high The process in which the demand of electricity remains very high even when the demand of steam is very low, then extraction-cum-condensing steam turbine can be used instead of back pressure steam turbine. Combined gas-cum-steam turbine system-based cogeneration achieves overall plant efficiency of around 90% with optional fuel utilisation . Major drawback of this system is less fuel flexibility as in case of gas turbine-based cogeneration system. Reciprocating engine-based cogeneration system The reciprocating engine is fired with fuel to drive the generator to produce electrical power. The process steam is then generated by recovery of waste heat available in engine exhaust in WHRB. The engine jacket cooling water heat exchanger and lubeoil cooler are other sources of waste heat recovery to produce hot water or hot air. The reciprocating engines are available with low, medium or high-speed versions with efficiencies in the range of 35 - 42 %.

8. The engines having medium and high speeds are widely used for cogeneration applications due to higher exhaust flue gas temperature and quantity. When diesel engines are operated alone for power generation, a large portion of fuel energy is rejected via exhaust flue gases. With this, the overall system efficiency of around 65-75% is achieved . The heat rates of reciprocating engine cycles are high in comparison to that of steam turbine and gas turbine-based cogeneration systems. This system is particularly suitable for application requiring a high ratio of electric power to steam. Reciprocating engines can be fired only with hydrocarbon-based fuels such as High-speed diesel, Light diesel oil, residual fuel oils, Natural gas, etc. Factors for selection of cogeneration system Normal as well as maximum/minimum power load and steam load in the plant, Anticipated fluctuations in power and steam load and pattern of fluctuation, sudden rise and fall in demand with their time duration and response time required to meet the same.

9. Type of fuel available - whether clean fuel like natural gas, naphtha or high-speed diesel or high ash bearing fuels like furnace oil, LSHS, etc or worst fuels like coal, lignite, etc Commercial availability of various system alternatives, life span of various systems and corresponding outlay for maintenance. Influence exerted by local conditions at plant site, i.e. space available, soil conditions, raw water availability, infrastructure and environment. Project completion time Project cost and long term benefits. Rankine Cycle

10. Advantages of Reheat Rankine Cycle Improved the thermal efficiency of the cycle as steam condenses during expansion reducing the damage to turbine blades. Increases the total work output of the turbine considering the total work input. ntages of Reheat Rankine Cycle Disadvantages of Reheat Rankine Cycle These cycles need a long piping setup. Hence, high initial installation costs are followed by high maintenance cost. As there is reheating, the size of the condenser may increase. Regenerative Rankine Cycle In the cycle using a regenerator. This process is mixed with a fluid to convert it to a saturated liquid. Thus, the process is known as ‘Regeneration’ and the cycle is called the ‘regenerative Rankine cycle’. Supercritical Rankine Cycle

11. Classification of Cogeneration Systems Topping Cycle Bottoming-Cycle Thermal energy exhausted from the electricity generating equipment is captured and used for a variety of useful purposes, such as manufacturing, space heating and cooling, water heating, and drying. Steam Turbine Topping Systems For typical coal-fired power plants, less than 40% of the energy in the fuel is converted to electricity. Because conventional power plants only use fuel energy to produce electricity The primary difference between a conventional power plant and a steam turbine toppingcycle CHP system is the manner in which waste heat is handled. Depending on the relative amount of electricity that is generated, steam turbine CHP systems are generally able to use between 65% and 85% of the energy in the fuel

12. Steam turbines used in topping-cycle CHP systems are usually either back-pressure systems or extraction-condensing systems.

13. Gas Turbine Topping Systems Gas turbine topping-cycle CHP systems are used throughout the world as an effective way to simultaneously produce power and heat from a single fuel source. Ranging in size from 500 kW to hundreds of megawatts, gas turbines (also referred to as combustion turbines) turn generators to produce electricity while providing useful thermal energy. Solid fuels such as coal, municipal waste, and biomass can also be gasified to produce gaseous fuel suitable for gas turbines. Most open-cycle gas turbine CHP systems burn relatively clean fuels like natural gas. High-temperature exhaust gas that contains impurities (e.g., sodium, potassium, calcium, vanadium, iron, sulphur , and particulates) can corrode some metals used in turbine construction, and certain residual solids in the exhaust gas can erode the turbine blades.

14. Reciprocating Engine Topping Systems Principal components in reciprocating engine topping-cycle CHP systems include the reciprocating internal combustion engine, heat-recovery equipment, and electrical generator. Internal combustion engines reject heat from the following sources: radiation from the engine block and other hot surfaces, exhaust gases, lubricating oil, jacket water, and, for turbocharged and aftercooled engines, the engine aftercooler . By recovering heat rejected from the exhaust and cooling systems, approximately 70 to 80% of the energy in the fuel can be recovered and used for power production or process heating applications. Engine exhaust up to 1,000°F or higher represents from 30 to 50% of the available waste heat that can be used to produce hot water, and low- or high-pressure steam. Heat in the engine coolant system accounts for up to 30% of the energy input. separate heat exchangers are sometimes used to recover lower temperature heat (usually less than 160°F) rejected from the engine by the lubricating oil system and aftercoolers .

15. Bottoming-Cycle Systems In a bottoming cycle, the primary fuel produces high temperature thermal energy and the heat rejected from the process is used to generate power through a recovery boiler and a turbine generator. Bottoming cycles are suitable for manufacturing processes that require heat at high temperature in furnaces and kilns, and reject heat at significantly high temperatures. Typical areas of application include cement, steel, ceramic, gas and petrochemical industries. Bottoming cycle plants are much less common than topping cycle plants. Waste heat from the thermal process becomes the heat source for the CHP system. Heat rejected from the steam turbine is then made available for other process heating applications. Most CHP systems are based on either the topping cycle or combined cycle, which uses heat from a topping cycle as the energy source for a bottoming cycle.

16. Applications All continuous process chemical plants such as fertilizers, petrochemicals, hydrocarbon refineries, paper and pulp manufacturing units, food processing, dairy plants, pharmaceuticals, sugar mills, etc always require an uninterrupted input of energy in the form of electric power and steam to sustain the critical chemical processes. Selection Normal as well as maximum/minimum power load and steam load in the plant Anticipated fluctuations in power and steam load and pattern of fluctuation Under normal process conditions, the step-by-step rate of increase in drawl of power and steam as the process picks up - whether the rise in demand of one utility is rapid than the other Type of fuel available - whether clean fuel like natural gas, naphtha or high-speed diesel or high ash bearing fuels like furnace oil, LSHS, etc or worst fuels like coal, lignite, etc., long term availability of fuels and fuel pricing. Influence exerted by local conditions at plant site, Project completion time Project cost and long-term benefits

17. Waste Heat Recovery Waste heat is heat, which is generated in a process by way of fuel combustion or chemical reaction, and then “dumped” into the environment even though it could still be reused for some useful and economic purpose. The essential quality of heat is not the amount but rather its “value”. Heat Losses – Quality Depending upon the type of process, waste heat can be rejected at virtually any temperature from that of chilled cooling water to high temperature waste gases from an industrial furnace or kiln. Usually higher the temperature, higher the quality and more cost effective is the heat recovery. Heat Losses – Quantity In any heat recovery situation it is essential to know the amount of heat recoverable and also how it can be used. Classification And Application High Temperature Heat Recovery Temperatures of waste gases from industrial process equipment in the high temperature range. All of these results from direct fuel fired processes.

18. Medium Temperature Heat Recovery Temperatures of waste gases from process equipment in the medium temperature range. Most of the waste heat in this temperature range comes from the exhaust of directly fired process units. Low Temperature Heat Recovery Low temperature waste heat may be useful in a supplementary way for preheating purposes. Benefits of Waste Heat Recovery Direct Benefits This is reflected by reduction in the utility consumption & costs, and process cost. Indirect Benefits Reduction in pollution. Reduction in equipment sizes. Reduction in auxiliary energy consumption.

19. Commercial Waste Heat Recovery Devices Plate heat exchanger The cost of heat exchange surfaces is a major cost factor when the temperature differences are not large. One way of meeting this problem is the plate type heat exchanger, which consists of a series of separate parallel plates forming thin flow pass. Each plate is separated from the next by gaskets and the hot stream passes in parallel through alternative plates whilst the liquid to be heated passes in parallel between the hot plates. Hot liquid passing through a bottom port in the head is permitted to pass upwards between every second plate while cold liquid at the top of the head is permitted to pass downwards between the odd plates. When the directions of hot & cold fluids are opposite, the arrangement is described as counter current. Typical industrial applications are Pasteurisation section in milk packaging plant. Evaporation plants in food industry.

20. Run Around Coil Exchanger It is quite similar in principle to the heat pipe exchanger. The heat from hot fluid is transferred to the colder fluid via an intermediate fluid known as the Heat Transfer Fluid. It is more useful when the hot land cold fluids are located far away from each other and are not easily accessible. Typical industrial applications are heat recovery from ventilation, air conditioning and low temperature heat recovery.

21. Shell and Tube Heat Exchanger When the medium containing waste heat is a liquid or a vapor which heats another liquid, then the shell and tube heat exchanger must be used since both paths must be sealed to contain the pressures of their respective fluids. The shell contains the tube bundle, and usually internal baffles, to direct the fluid in the shell over the tubes in multiple passes. Tube and shell heat exchangers are available in a wide range of standard sizes with many combinations of materials for the tubes and shells.

22. Waste Heat Boilers Waste heat boilers are ordinarily water tube boilers in which the hot exhaust gases from gas turbines, incinerators, etc., pass over a number of parallel tubes containing water. The water is vaporized in the tubes and collected in a steam drum from which it is drawn off for use as heating or processing steam. If the waste heat in the exhaust gases is insufficient for generating the required amount of process steam, auxiliary burners which burn fuel in the waste heat boiler or an afterburner in the exhaust gases flue are added. Waste heat boilers are built in capacities from 25 m3 almost 30,000 m3 / min. of exhaust gas. Typical applications of waste heat boilers are to recover energy from the exhausts of gas turbines, reciprocating engines, incinerators, and furnaces.

23. Heat Pumps In the various commercial options previously discussed, we find waste heat being transferred from a hot fluid to a fluid at a lower temperature. It has been taken as a general rule of thumb in industrial perations that fluids with temperatures less than 120°C (or, better, 150°C to provide a safe margin), as limit for waste heat recovery because of the risk of condensation of corrosive liquids. Thermic Fluid Heaters ` Thermic fluid heaters are very efficient equipment used in process heating and it uses high viscous synthetic oil as a heating medium. Process Industry can achieve the maximum process heat temperature with low pressure in a Thermic fluid heater.

24. Thermic Fluid Heater Comprises of following Heater coils Burner/Furnace Expansion cum Deaerator tank Air-cooled, Circulating oil pump with mechanical seal suitable for high temperatures Primary and secondary fans (Depends on fuels) Heat recovery units Pollution control equipment (Includes only with solid fuel fired systems) Chimney for stack dispersal Control panel for ease of operation Types of Thermic Fluid Heater Solid Fuel Fired Fluid Fuel Fired

25. Advantages of Thermic Fluid Heater The thermic fluid heater can cover temperature that ranges from 0° C to 400° C, whereas the standard steam system can cover the temperature that ranges from 121° C to 250° C but at very high pressure. The standard hot water can work between the temperature 0° C and 140° C. Attain High Temperature at Low Pressure Low Maintenance Easy to Operate Easy Installation No Need for a Licence Every Year Less Capital and Operating Cost Industrial Applications of Thermic Fluid Heater Road Construction Industry Food & Beverages Industry Plywood & Laminate Industry Chemical Industry Pharmaceutical Industry Rubber Industry Oil & Lubricant Manufacturing Refineries Textile Industry