COGENERATION

• Cogeneration is the simultaneous production of electricity and useful heat from the single fuel. It is also known as CHP i.e. combined heat and power.

• In one manifestation the energy of fuel is converted into heat in the boiler to produce steam and this steam is used to generate electricity.

• Besides producing electricity, it provides heat for manufacturing process.

• Facilities with cogeneration systems use them to produce their own electricity, and use the waste heat for process steam, hot water heating, space heating, and other thermal needs.

• They may also use excess process heat to produce steam for electricity production.

FIG. - Typical Cogeneration System

FIG. - Typical Conventional Power Generation Station

• Electricity generation into the environment through cooling towers, or by other means.

• CHP captures the excess heat for domestic or industrial purposes.

• Cogeneration is an energy-efficient environmentally-friendly method of producing electricity, steam or hot water at the same time from one fuel.

• Fuels used in cogeneration include natural gas, bio gas (gas fuels), oils, gasoline (liquid fuels), coal, wood and solid waste (solid fuels) etc.

FIG. - Cogeneration Capacity By Fuels

• These "primary" fuels are used to make electricity, a "secondary" fuel.

• In fact all fuels (given above) can be used for cogeneration but natural gas is the most common fuel for cogeneration.

• Because it is a fuel source which emits less than half the greenhouse gas (methane), per unit of energy produced than the cleanest available thermal power station.

• Conventional power plants do not convert all of their available energy into electricity, with the excess being wasted as excess heat whereas in CHP excess heat is captured.

• CHP is most efficient when the heat can be used on site or very close to it. Osborne is the largest (180 MW) most modern cogeneration plant in Australia.

TYPES OF PLANTS

• Cogeneration projects are typically represented by two basic types of power cycles, topping or bottoming.

• The topping cycle has the widest industrial application.

• Topping cycle plants produce electricity first, then the exhaust is used for heating.

• A topping cycle cogeneration plant always uses some additional fuel, beyond what is needed for manufacturing, so there is an operating cost associated with the power production.

• Bottoming cycle plants, which are rare, produce heat for an industrial process first, then electricity is produced using a waste heat recovery boiler.

• A cogeneration system can be inplant or reject heat utilization system.

• Inplant system is used in industries and steam is mainly used as process steam and remaining steam is used for generating electrical energy.

Fuel

Boiler

Water

Steam

Condensate

Generator

Electric Power

Turbine

Steam For Manufacturing Process

FIG. – In plant power Generation system

Fuel

Boiler

Water

Steam

Condensate

Generator

Electric Power

Turbine

Exhaust Steam to Adjacent Industry

FIG. - Reject Heat Utilization System

COGENERATION TECHNOLOGIES

• A typical cogeneration system consists of an engine, steam turbine, or combustion turbine that drives an electrical generator.

• Cogeneration produces a given amount of electric power and process heat with 10% to 30% less fuel than it takes to produce the electricity and process heat separately.

• There are four types of topping cycle cogeneration systems

STEAM TURBINE SYSTEM:

• This type of system burns fuel to produce high-pressure steam that then passes through a steam turbine, which is coupled, to generator. So, electrical power is generated.

GAS TURBINE SYSTEM• Fuel (mostly natural gas) is burnt in the combustion chamber and heats the

compressed air.

• The hot pressurized gas expands in gas turbine, which drives a generator.

• The exhaust gas goes to a heat recovery boiler that makes process steam and process heat.

• Waste heat recovery boiler is defined as a heat retrieval unit using hot by-product from chemical processes, to produce steam in a boiler type system i.e. system for recovering heat from stream of heated gases.

• Heat recovery boilers can produce either hot water or steam by recovering energy in the waste heat contained in the exhaust fumes of a gas turbine or of an engine.

Electricity

Generator

Gas Turbine

Exhaust

Water

Process

Steam

Air Gas Turbine

Compressor

Combustion

Chamber

Fuel

Waste Heat recovery

Boiler

COMBINED CYCLE SYSTEM• This system burns fuel in a gas turbine to produce electrical or mechanical power.

• The exhaust provides process heat, or goes to a heat recovery boiler to create steam to drive a secondary steam turbine.

• This is a combined-cycle topping system, energy saving is 35%.

FIG. - Combined Cycle System

DIESEL ENGINE SYSTEM

• This system uses a diesel engine which is coupled to generator. The input to diesel engine is fuel and air.

• The hot water from the engine jacket cooling system flows to a heat recovery boiler, where it is converted to process steam and hot water for space heating.

• This system can be used only if excess electricity can be sold because the generation cost per unit is higher than other systems.

Gas Turbine Combined Cycle

Diesel Engine Steam Turbine

Suitable applications

Where high pressure steam required

Where wide fluctuations in the amount of energy required

Where high pressure steam not needed

Where low-cost solid fuel available

Suitable size for applications

1 MW to 250 MW Medium to large Up to about 3 MW Low MW upwards

Electrical efficiency

Moderate to high High High to Medium Moderate

Initial Cost Moderate High Low because of less pressure

High

Maintenance and Operating Costs

Moderate Moderate High High

Advantages Heat recovery steam generator can have supplementary firing

High efficiency Good operational characteristics with small setsHigh electricity to thermal ratio

Can use most types of fuel including solid fuels such as wood,

Disadvantages Not suitable below about 3MWe

Expensive to make and to maintain

High purchase cost and expensive to maintain

Low power-to-heat ratio and a poor match between process steam and electrical requirements

ADVANTAGES OF COGENERATION• Lower energy costs

• Excellent return on investment

• More environmentally friendly than purchased electricity from your utility

• Highly efficient

• Installation incentives available in some areas

• Best defense against rising electricity prices

FIG. - Graph showing the CO2 emissions

• Cogeneration technology provides greater conversion efficiencies than traditional generation methods as it harnesses heat that would otherwise be wasted.

• The heat by-product is available for use without the need for the further burning of a primary fuel.

• This can result in up to more than a doubling of thermal efficiency or higher heat values.

• Cogeneration systems predominantly use natural gas, a fuel source that emits less than half the greenhouse gas, per unit of energy produced than the cleanest available thermal power station.

• NOx emissions are lower, 25% in comparison to electricity produced in coal-fired power plants and heat production in boilers. CO2 emissions are 30

DRAWBACKS OF COGENERATION

• In some cases, cogeneration can increase emissions of nitrogen oxides

and noise.

• CHP is most efficient when the heat can be used on site or very close to it.

• Overall efficiency is reduced when the heat must be transported over

longer distances whereas electricity can be transmitted along a

comparatively simple wire, and over much longer distances for the same

energy loss

APPLICATIONS

• Cogeneration plants are commonly found in district heating systems of big towns, universities, hospitals, hotels, prisons, oil refineries, paper mills, wastewater treatment plants, thermal enhanced oil recovery wells and industrial plants with large heating needs.

• The industries that need both power and steam for their working are most suitable for cogeneration.

• Paper, pulp, chemical, petroleum refining, textile, sugar, cement and iron industries are some examples of cogeneration.

• Cogeneration plants are also found in district heating systems of big towns, universities, hospitals, hotels, prisons, oil refineries, paper mills, wastewater treatment plants etc.

SALE OF ELECTRICITY TO UTILITY AND IMPACT ON COGENERATION

• Some industries demand a huge amount of process steam for manufacturing and a limited electricity requirement.

• Basically it is the rate of purchasing electricity by electric utilities that encourages industrialist to adopt cogeneration.

• Let the total annual cost of cogeneration system be Rs.C. if system produces a units of electricity and b units of steam then a(x) + b(y) =C

Where x = prices of per unit of electricity generated

& y = prices of per unit of process steam

• To recover total cost C, the electricity price OE determines the steam price OS provides a, band C are kept constant.

• The rate of purchasing electricity by electric utility may be equal to OE or more than OE or less than OE.

• Industry will neither be encouraged nor discouraged i.e. neither be subsidized nor taxed.

• If rate of purchasing is low, industries would have to produce more electricity to recover the cost. If they are not doing so, then they have to pay tax indicated by area XYX2.

• They need to produce less electricity. If the unit produces same amount of electricity then the area XYX1 indicates the amount of subsidy paid to industries going for cogeneration.

Steam Price Rs/KCal

XX2E0

S

Y

X1

Electricity Price Rs. /KWh

a1x+b1y=C

a2x+b1y=C

a3x+b1y=C

FIG. - Impact of Pricing