• Cogeneration

    The conventional method of power generation and supply to the customer is wasteful in the sense that only about one third of the primary energy fed into the power plant is actually made available to the user in the form of electricity. In conventional power plant, efficiency is about 35% and remaining 65% of energy is lost. Following two separate processes are required by an Industry: one for producing electricity and one for producing thermal energy.

    Commonly, a utility will use conventional fossil fuels –coal, oil, or natural gas – to generate electricity which is then purchased by the consumer. Also requiring heating and cooling, the consumer will, in addition, have an on-site boiler or furnace to generate thermal energy. In this two approach, two separate fuel supplies are utilized and a substantial amount of heat and energy “waste” is lost to the environment. The major source of loss in the conversion process is the heat rejected to the surrounding through cooling towers, flue gases or by some other means. This waste heat may be used for some useful purpose such as heating or producing steam for industrial applications. Ex. – Paper mills, Textile mills, Sugar mills, Plastic manufacturing, Laundries, Hotels, etc.

     

    Cogeneration is a technology that can be used to replace or complement traditional power generation processes. In cogeneration, thermal “waste” from electric power generation is recovered and used in a highly-efficient process to supply useable thermal energy for heating or cooling systems.  Typically onsite or in close proximity to the consumer, cogeneration plants use a single fuel source – opting for clean fuels such as natural gas or biomass – to produce both electricity and thermal energy in one efficient process. In this process, an internal combustion engine, such as a gas turbine, uses the primary fuel source to generate electricity. In turn, a large amount of heat is generated as a by-product. Typically lost in traditional power generation, this thermal energy is recovered by a heat recovery steam generator (HRSG) that produces steam or hot water for heating or other uses.

    Figure: Cogeneration

    CHP technology have higher efficiencies than traditional power generators. Higher efficiencies result in reduced green house gas (GHG) and other pollutant emissions. A cogeneration system can either be an inplant power generation system or a reject heat utilization system.

     

    Inplant power generation system

    The inplant power generation is employed  in  industries which need both steam and electricity. In conventional industries, steam is produced by a boiler and electricity is either purchased or produced by diesel engine. In inplant power generation, the boiler is made to produce steam at higher temperature and pressure that needed for manufacturing process. This steam is first used in a turbine generator set to produce electricity and then the exhaust steam is used for manufacturing process.

    Figure: Inplant Cogeneration

     

    Reject heat utilization system

    In reject heat utilization system, which is used in power plants, some steam extracted from the turbine, at suitable temperature and pressure, is supplied to an adjacent industry for manufacturing purposes. There are two main configuration of cogeneration: i)  Topping cycle and  ii)  Bottoming cycle.

    Figure: Reject heat utilization system

     

    Topping Cycles

    In a topping cycle, the primary fuel produces high temperature thermal energy which is used to generate power and the heat rejected from the turbine is used for manufacturing processes. Topping cycles are suitable for manufacturing processes that require heat at low temperature in furnaces.

    Figure: Topping Cycle

     

    Bottoming Cycles

    In a bottoming cycle (also known as “waste heat to power”), 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. The waste gases coming out of the furnace is utilized in a boiler to generate steam, which drives the turbine to produce electricity. Note that bottoming cycle plants are much less common than topping cycle plants.

    Figure: Bottoming Cycle

     

    Cogeneration offers many benefits for municipal, industrial, commercial, institutional and even residential facilities.

    A. Fuel Economy: Less consumption of primary fuel. Fuel economy results from higher thermodynamic efficiency of a cogeneration system as compared to separate power producing and heat producing systems.

    B. Lower capital cost: Industry needing steam has to invest on boilers, the extra investment needed to upgrade boiler so that same electricity can also be generated is pretty small (about 50 %) as compared to the cost of boiler. Thus cogeneration results in enormous savings in capital costs.

    C. Low gestation period: A utility needs 6 years to add a thermal generation system. Whereas, installation of a cogeneration system by an industry needs only 3 years. The lower gestation period results in saving in interest, early utilization of facility, early return on investment.

    D. Low supply interruptions: Saving to industry from power cuts and power supply interruptions.

     

    Cogeneration Technologies

    The cogeneration technologies used for topping cycles are:

    1. Steam Turbine Cogeneration System

    It consists the equipments similar to that in thermal plant i.e., boiler, steam turbine, generator, etc. Fuel may be oil, coal, natural gas or wood, i.e., fuel flexibility. The higher pressure steam produced by the boiler drives turbine and then generate electricity. The lower pressure exhaust form steam turbine is used for industrial process application. Energy Savings of about 15% is obtained using this cogeneration.

     

    Figure: Steam Turbine Cogeneration System

     

    2. Gas Turbine Cogeneration System

    It consists the equipments that are shown in figure. Fuel is burnt in combustion chamber & is used for heating the compressed air. This hot pressurised gas expands in a turbine which drives generator. The exhaust from gas turbine is used as process heat. If necessary the hot gases can be used to raise steam in waste heat recovery boiler where heat of exhaust gasses is transferred to water. Energy Savings of about 25% is obtained using this cogeneration. Due to less availability and high cost of petroleum products, the cost of electricity produced is high. Gas turbines need more maintenance than steam turbines.

     

    Figure: Gas Turbine Cogeneration System

     

    3. Combined Cycle Cogeneration system

    It is a combination of gas turbine and steam turbine systems. The gas turbine exhaust is used in waste heat boiler to raise steam. The steam from recovery boiler is used in a steam turbine which drives another alternator to produce electricity. The low pressure exhaust from steam turbine is used as process steam. Energy saving of about 35% is done by combined cycle cogeneration system.

     

    Figure: Combined Cycle Cogeneration system

     

    4. Diesel Engine Cogeneration System

    This system uses a diesel engine which drives the electric generator.  The engine exhaust and jacket cooling are used as heat sources for the waste heat boiler which converts water into process steam. This system has higher electricity to thermal ratio than that required by most industries. It can be used only if excess electricity can be sold. Generation cost per unit is higher than other system, but environmental pollution is minimum. Energy saving is about 26% with diesel engine cogeneration.

    Figure: Diesel Engine Cogeneration System