Cogeneration

It is defined as, "Sequential generation of two different forms of energy from a single source (generally mechanical and thermal energy, Combined Heat & Power Generation)".

For E.g.: Warm pool water with Lower Costs and Lower Emissions of Conventional powerplant.

Technical Options for Cogeneration

1. Cogeneration is not used by large industries.
2. For constant temperature heating or drying, steam is very good (since isothermal conditions can be maintained allowing saturated steam to condensed (at that temp) utilizing latent heat.
3. Reduction of Carbon Emissions

1. Reciprocating Engine Cogeneration Systems

It is defined as, "Utilizing one source (diesel/gas engine) to produce electricity energy converted into heat water (process heat)".

Steps:

Burning of Fuel in engine  ↦  Produce heat  ↦  Produce electricity  ↦  engine is cooled by coolants  ↦  which transfer heat contents into circulating water  ↦  which is further heated in boiler.

2. Gas Turbine Cogeneration Systems

Steps:

Fuel + air  ↦  Compressed + Burn in compressor  ↦  hit to turbine blades (expands)  ↦  which make shaft to rotate, which produce electricity  ↦  remaining heat is used to heat water.

3. Steam Turbine Cogeneration Systems

It is similar to Rankine Cycle but Condenser is replaced by Process Heater (which gives process heat as per our requirement).

• Additional Components ↦ process heater + pump.
• Continuously bled steam in process heater.

Steps:

Fuel burn in boiler  ↦  Steam produced  ↦  hit turbine blades  ↦  Electricity generated  ↦  Steam enter process heater  ↦  Exchange heat with cold chemicals.

There are two types of Steam Turbine, namely:

• Back Pressure Turbine
• Pass Out Turbine

Back Pressure Turbine

It is defined as, "When the exhaust steam from cogeneration of the turbine is utilized for process work".

• We can modify initial steam pressure or exhaust pressure to get required steam of desired pressure and temp.
• Process heater is running Above or At atmospheric pressure.
• Ideally, we want 100% of heat extracted to transform steam into liquid .
• Pump cannot work with 2 phase.

Pass Out Turbine

It is defined as, "The back pressure turbine through which a certain quantity of steam is continuously exhausted for the process heating at desired pressure and temp".

• Process Heater  ↦  has Higher Power-to-Heat ratio.
• Feasibility Criteria  ↦  Cost of generation, No utility present.

References:

• Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).