Processes of the Rankine cycle

 The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electric power used throughout the world. including It is named after William John Rankine, a Scottish scientist.

A Rankine cycle describes a model of the operation of steam Turbine most commonly found in Industrial Power Plants. Common heat sources for power plants using the Rankine cycle are Coal, Natural Gas ot Liquid Fuels.

The Rankine cycle is sometimes referred to as a practical Carnot Cycles, when an efficient turbine is used, the TS diagram will begin to resemble the Carnot cycle. The main difference is that a pump is used to pressurize liquid instead of gas. This requires about 100 times less energy than that compressing a gas in a compressor. (as in the Carnot Cycle).

The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure going super critical the temperature range the cycle can operate over is quite small, turbine entry temperatures are typically 565°C (the creep limit of stainless steel) and condenser temperatures are around 30°C. This gives a theoretical Carnot efficiencyof around 63% compared with an actual efficiency of 42% for a modern coal-fired power station.

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The water vaporoften seen billowing from power stations is generated by the cooling systems (not from the closed loop Rankine power cycle) and represents the waste heat that could not be converted to useful work. Note that steam is invisible until it comes in contact with cool, saturated air, at which point it condenses and forms the white billowy clouds seen leaving cooling tower. While many substances could be used in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties.

One of the principal advantages it holds over other cycles is that during the compression stage relatively little work is required to drive the pump, due to the working fluid being in its liquid phase at this point. By condensing the fluid to liquid, the work required by the pump will only consume approximately 1% to 3% of the turbine power and so give a much higher efficiency for a real cycle. 

Ts diagram of a typical Rankine cycle operating between pressures of 0.06bar and 50bar

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the right.

• Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.
• Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.
• Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.
• Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase change.

In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the Ts diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine, which reduces the energy removed by the condensers.

Real Rankine cycle (non-ideal)

Rankine cycle with superheat

In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not isentropic. In other words, these processes are non-reversible and entropy is increased during the two processes. This somewhat increases the power required by the pump and decreases the power generated by the turbine.

In particular the efficiency of the steam turbine will be limited by water droplet formation. As the water condenses, water droplets hit the turbine blades at high speed causing pitting and erosion, gradually decreasing the life of turbine blades and efficiency of the turbine. The easiest way to overcome this problem is by superheating the steam. On the Ts diagram above, state 3 is above a two phase region of steam and water so after expansion the steam will be very wet. By superheating, state 3 will move to the right of the diagram and hence produce a dryer steam after expansion.

Introduction to Steam turbine

 A rotor of a modern steam turbine

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion.

Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermomodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process..

 Types

Industrial steam turbines employ both impulse and reaction. Their capacities vary from 0.5 MW to 1000 MW.

 Steam Supply and Exhaust Conditions

These types include condensing, non condensing, reheat, extraction and induction.

process steam are available.

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.

Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.

Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

Casing or Shaft Arrangements

These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.

Principle of Operation and Design

An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.

Schematic diagram outlining the difference between an impulse and a reaction turbine

To maximize turbine efficiency, the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as impulse or reaction turbines. Most modern steam turbines are a combination of the reaction and impulse design. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type.

Impulse Turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from steam chest pressure to condenser pressure (or atmosphere pressure). Due to this relatively higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss".

Reaction Turbines

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

Speed regulation

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials.

Calculation of turbine efficiency and Rankine cycle efficiency for the given plant.

 

Steam Turbine efficiency

 

Efficiency= Energy output/ Energy input

 

Heat input = Steam inlet flow in TPH * Enthalpy of inlet steam (Kca/Kg)

                = 168*811

                = 136248 Kcal

 

Heat out put  = Power in MW* Heat equivalent of Power

                =30*860

                =25,800 Kcal

 

 

Efficiency of Steam Turbine =   Heat output/ Heat input

                                             = 25,800/136248

                                              = 18.93%

 

 

 

Rankine cycle efficiency for given plant:

 

W1=30 MW

 

W2= 1MW (assumed)

 

Q1= Heat input to a Boiler

     =m*(h1-h4)

     =336*(811-150)   (where 336 is total steam generation in Boilers)                                                     .    =222096

     =246773 Kcal(assuming Boiler efficiency of 90%) .

 

In a power plant that produces both Steam and power, we must give credit to Steam export also

 

Useful output as steam export= Steam export flow*Heat value (Kcal/Kg) 

                                            =150* 760

                                            =114000 Kcal                                                      

 

Efficiency of cycle is = ((W1-W2)+( Heat value of export steam)) /Q1

                                 =((60-2)*860)+114000)/ (246773) 

                                  =66.40%    

Typical Fuel quality requirements for a Gas Turbine:

A separate starting fuel is required for applications with single oil conventional burner if the fuel viscosity is above 4.1 mm²/s at 40°C.

The viscosity of the fuel at the injectors must not exceed 6.0 mm²/s (2nd gen. DLE burner) or 7.0 mm²/s (conventional burner) for good atomisation. A fuel with a higher viscosity than 6.0 respectively 7.0 mm2 /s, at the lowest temperature the fuel system may experience, requires preheating and trace heating of fuel piping in order to achieve correct viscosity at the fuel injectors.

Fuels with viscosity less than 2.0 mm2/s at highest fuel operating temperature, such as naphtha or kerosene, require dosage of anti-wear additives in order to prevent pump failures. 

The supply temperature of minimum 5°C prevents eventual free water from freezing in pipes and valves. The supply temperature shall also always be higher than the cloud point.

Fuels that are handled at temperatures close to or above the flash point require explosion proof equipment and a separate starting fuel. Special safety regulations will apply which vary from country to country.

The fuel must be kept at least 10°C above the cloud point.

In the specification polyaromatic compounds are defined as tricyclic compounds and higher, i.e. no di-aromatics included.

For fuels with higher water and/or sediment content separators should be installed.

Generally less than 0.5 ppm(w) heavy metals is required in order to conform to the condition for the ash sticking point. Somewhat higher contents can often be handled by employing corrosion inhibition additives. The quantity of the additive shall be enough to keep the ash sticking temperature above 950°C. However, a high content of additives requires additional cleaning of the turbine. Siemens will advise on equipment, additive type and dosage.

Generally less than 0.5 ppm(w) sodium + potassium is required in order to conform to the condition for the ash sticking point. Somewhat higher concentrations of sodium, potassium and calcium can be reduced to acceptable limits by fuel washing and/or separation. However, the washability of the fuel has to be checked. Siemens will advise on equipment.

The use of a fuel having an ash sticking point temperature below 950 °C introduces a great risk of high temperature corrosion for the gas turbine and must not be permitted even for a short period of time. The sticking point temperature can be raised by additive treatment of the fuel. Any such additive must be approved by Siemens 

The smaller or “micro” (20- to 500-watt) turbines

The smaller or “micro” (20- to 500-watt) turbines are used in a variety of applications such as charging batteries for recreational vehicles and sailboats.

One- to 10-kW turbines can be used in applications such as pumping water.

Turbines used in residential applications can range in size from 400 watts to 100 kW (100 kW for very large loads), depending on the amount of electricity you want to generate

Wind energy has been used for centuries to pump water and grind grain.Although mechanical windmills still provide a sensible, low-cost option for pumping water in low-wind areas,

farmers and ranchers are finding that wind-electric pumping is a little more versatile and they can pump twice the volume for the same initial investment.

Working Model of Coal Based Power Plant

How an Inlet Guide Vanes looks like?

Bottoming Cycle

In a bottoming cycle cogeneration system, the thermal energy is used in first place later the electricity energy is generated.

For example in the above figure, the thermal energy is produced in the process and this is used for steam generation. Later this steam is used for power generation.

Schematic Representation of an Extraction-Condensing Turbine

Schematic Representation of a Back Pressure Turbine

Water gets converted to steam in boiler using the heat energy of fuel. This steam drives the turbine and gets expanded, later the steam at a relatively lesser pressure and temperature enters the process heaters/heat exchangers to give away the balance heat. The condensed steam will be reused again as a heat carrying media for turbine and process

Gas Turbine Animation - Showing inlet air and fuel through compressor, combustion in combustion chamber and expansion in turbine

Schematic Diagram of Gas Turbine Cogeneration

ENERGY BALANCE IN TYPICAL COAL FIRED POWER STATION

Heat to Power Ratio of various power and steam producing cogeneration System along with power output as percent of fuel input and overall efficiency