PRESENTATION ON HYDROGEN GENERATION PLANT

MAIN TOPICS TO BE DISCUSSEDKNOWING HYDROGEN GASWHY GENERATORS NEED TO BE COOLED ?WHY HYDROGEN IS USED AS COOLANT?TRADE OFF BETWEEN AIR COOLING AND HYDROGEN COOLING.EFFECT OF HYDROGEN PURITY ON GENERATOR PERFORMANCE.TYPICAL GENERATOR HYDROGEN COOLING SYSTEMDIFFERENT HYDROGEN GENERATION METHODSSAFETY MEASURES TO BE TAKEN WHILE HANDLING HYDROGEN.OUR EXPERIENCE REGARDING VISIT TO NTPC VINDHYACHAL SUPER THERMAL POWER STATION

KNOWING HYDROGEN GASHydrogen Gas Is Dangers At any mixture of Hydrogen and Air, Ranging from 4% to 74%, it is flammable (High Flammability Range). In dry oxygen, the flammability limits are 4% to 96% by volume. Hydrogen flame is almost invisible to the naked eye. It burns with a light blue or pale blue flame or no colour at all. While the flame temperature is very high, relatively little heat is radiated. It is almost too late before you can detect a hydrogen flame.Hydrogen being light gas (1st element of the periodic table) always leaks out. So a minor leakage of hydrogen is very common.

HYDROGEN FLAMMABILITY RANGE

WHY GENERATORS NEED TO BE COOLED ?

As current flows in a conductor, heat is generated. A generator has a lot of conductors and a lot of current flowing through the conductors, generating a lot of heat. If that heat isn't "removed" then the windings will be damaged. In a synchronous generator (alternator), there are high currents flowing in the rotor windings, which also generates heat which must be "removed".So, the purpose of the generator hydrogen cooling system is to maintain the generator rotor and the stator iron within their proper operating temperature ranges under all operating conditions.

WHY HYDROGEN IS USED AS COOLANT?

The density of Hydrogen is only 0.07 compared to that of air (1). Windage loss and power loss are minimized, Increase in generator efficiency.Specific heat capacity of hydrogen is about 14 times than that of air. less quantity of hydrogen for same rating of generator Reducing fan power.Hydrogen does not react with any metals inside the generator No danger of oxidation of metals inside the generators.

TRADE OFF BETWEEN AIR COOLING AND HYDROGEN COOLING.Generally for small generator up to 100 MW Air Cooling is preferred Cost of transport of Hydrogen or Generation of Hydrogen is much higher than cost of low Generator efficiency.For 100 MW and above Hydrogen Cooling is commercially viable.

EFFECT OF HYDROGEN PURITY ON GENERATOR PERFORMANCE. Decrease in Hydrogen's purity results in an increase in density Increase in generator windage friction lossesDecrease in Generator efficiencyDecrease in Hydrogen's purity results in an increase inFan power Increase in Auxiliary Power consumption.For a 800MW unit with a 1% increase in hydrogen purity will results in 366kW increase in power output which is shown graphically.A 3% increase in hydrogen purity translates to an additional 1MW output without any increase in fuel consumption. Considering selling price of $.04/kWh, a 3% purity increase can save $320,000 per year (based on 8000 operating hours).

EFFECT OF H2 PURITY ON GEN EFFICIENCY AND OUTPUT

OPERATIONAL REQUIREMENTS OF THE GENERATOR HYDROGEN COOLING SYSTEM.To continuously re-circulate the hydrogen gas within the generatorTo cool the hydrogen to the required temperature.To dry the hydrogen to the required dew point.To maintain the correct hydrogen gas pressure in the generator by providing make-up hydrogen to compensate for leaks.To provide an alarm for liquid oil or water within the generator cavity.To monitor the hydrogen gas purity.To monitor CO2, Air, and H2 for purging and charging the generator.

Major Components of the Generator Hydrogen Cooling System:-Centrifugal FansThe centrifugal fans located at each end of the rotor draw hydrogen from the air-gap between the rotor and stator and blow it through the coolers located within the generator yoke. From the coolers the hydrogen is directed to both the stator iron and to the rotor. The cool hydrogen passes through the ducts in the stator irons and enters the air gap from the centre portion of the stator iron.

Major Components of the Generator Hydrogen Cooling System:-Hydrogen CoolersThe hydrogen coolers are long, finned, U-Tube units mounted axially in compartments located in the generator yoke. The hot hydrogen passes over the finned tubes, losses its heat to the service water and then flows on to cool the stator iron and rotor conductors. Gas Supplies H2, CO2 and AirHydrogen from a bulk storage system is fed to the generator via a pressure regulating valve. The H2 pressure within the generator is held relatively constant by the hydrogen makeup system. CO2 from the portable bottles is used to purge hydrogen or air from the generator when required. The CO2 is then displaced by clean, dry air if the generator is to be opened for maintenance.

Hydrogen Dryer Typically, hydrogen dryer will be a twin tower type using beds of activated alumina. Cycle times are adjusted to suit the drying load. Cycle times are adjusted to suit the drying load. Hydrogen Gas Analyzers The gas analyzer unit analyzes the H2 purity when the generator is at operating speed. A low purity alarm is provided. A portable gas analyzer is used when charging and discharging the generator. The primary significance of the hydrogen purity is the requirement to avoid an explosive H2/Air Mixture, i.e. H2 content must be above 80% or below 3%.

Major Components of the Generator Hydrogen Cooling System:-

The three Methods used in Power Industry for generation of Hydrogen are:-Unipolar DesignLiquid Bipolar (Potassium Hydroxide as Electrolyte) DesignSolid Bipolar Design (Proton Exchange Membrane Technology)The three methods are discussed briefly :

DIFFERENT HYDROGEN GENERATION METHODS

Unipolar Design:-In unipolar design electrolysis of water uses the tank electrolyzer in which a series of electrodes, anodes and cathodes alternately, are suspended vertically and parallel to one another in a tank partially filled with electrolyte. A single tank-type cell usually contains a number of electrodes, and all electrodes of the same polarity are connected in parallel, electrically, as pictured in Figure. Advantage : - Extremely simple to manufacture and repair. Disadvantages of Unipolar Design: - It usually operates at lower current densities and lower temperatures.

Bipolar Design (Potassium Hydroxide as Electrolyte):-

Bipolar Design (Potassium Hydroxide as Electrolyte):-

The second type of electrolyzer construction is the bipolar design. Often called the filter-press design, this has alternating layers of electrodes and separation diaphragms that are clamped together. The cells are connected in series and can result in higher stack voltages. Since the cells are relatively thin, the overall stack can be considerably smaller than the unipolar design. This method is widely used in the industry. The space requirement for this type of generation is more. The H2 Generation plant is often placed outside at some distance. Hydrogen is supplied by H2 gas cylinders to the generator.

Bipolar Design (Proton Exchange Membrane) :- The other electrolyzer technology is called PEM or solid polymer electrolyte. In a PEM, the electrolyte is contained in a thin, solid ion-conducting membrane rather than the aqueous solution in the alkaline electrolyzers. This allows the H+ ion (i.e., proton) to transfer from the anode side of the membrane to the cathode side and separates the hydrogen and oxygen gases. Oxygen is produced at the anode side and hydrogen is produced on the cathode side. PEM electrolyzers use the bipolar design and can be made to operate at a high differential pressure across the membrane. The space requirement in this type of H2 generation is pretty less. Practically no maintenance is required in this type, neither a full time operator is required.

The PEM Electrolysis Process

Advantages of Bipolar Design: - Reduced stack footprints, higher current densities, and its ability to produce higher pressure gas.Disadvantages of Bipolar Design: - It cannot be repaired without servicing the entire stack. Previously asbestos was used as a separation diaphragm, but manufacturers have replaced or are planning to replace this with new polymer materials. And since Asbestos is a carcinogenic material it is important for the manufacturers to replace them with different materials.

SAFETY MEASURES TO BE TAKEN WHILE HANDLING HYDROGEN.

General Precautions:- When hydrogen is contained within process and storage vessels, the concentration should be checked regularly to ensure the hydrogen is outside the explosion range.Before hydrogen is introduced into a process or storage vessel, a purging or evacuation process must be performed to remove the oxygen.Before a process or storage vessel containing hydrogen is opened to atmosphere, perform a hydrogen purge using an inert gas. (e.g:- Nitrogen)Maintain a positive pressure in the hydrogen vessel to prevent the introduction of air.Open Flames, smoking or welding is not permitted in areas where hydrogen is present.

Stator Cooling Water ConductivitySince the large generators operate at 18000 volts AC or above, it is absolutely essential that the stator cooling water conductivity be kept low enough to provide adequate electrical insulation. Hydrogen/Air ConcentrationsThe hydrogen/air concentration must be kept outside the explosion range (4-74 %) to avoid serious damage to equipment and possible fire injury to personnel.Hydrogen to Seal Oil Differential PressureThe seal Oil pressure must be greater than the hydrogen pressure to prevent the leakage of hydrogen from the generator.

Hydrogen to Stator Water Differential PressureTo prevent leakage of liquid Water from the stator system into the hydrogen, the hydrogen pressure must be greater than the stator cooling water pressure. Hydrogen DrynessTo prevent condensation and possible ground faults within the generator, the hydrogen gas which is circulating in the generator must be kept dry enough to always above the dew point. DrainsAny leakage of liquid, oil or water in to the generator can cause severe physical damage. The drains from the bottom of the generator must be in operation.

SAFETY PRECAUTIONS RELATED TO LAYING OF H2 PIPE LINES.

H2 Pipelines should not preferably be underground. If it is in a trench, care should be taken that the trench is properly ventilated.H2 Pipelines should preferably run on the top of all the other pipelines and not along with them.All the H2 pipes to be properly earthed.Flanged and screwed joints to be avoided. Welded joints shall be used.Provision for thermal expansion to be kept.The H2 piping shall be coloured as per code and at regular interval distance shall be marked with a warning.

OUR EXPERIENCE REGARDING VISIT TO NTPC VINDHYACHAL SUPER THERMAL POWER STATION

NTPC ranked 341st in the 2010, Forbes Global 2000 ranking of the Worlds biggest companies. With a current generating capacity of 34,194 MW, NTPC has embarked on plans to become a 75,000 MW company by 2017.The plant is located on the north western bank of Govind Ballabh Pant Sagar (GBPS) at Vindhyasagar of Sidhi district in the state of Madhya Pradesh.

Our observations :

The electrolyzer for Stage I have been replaced 2-3 times duo to leakage in the vessel and through adjoining fittings.For Stage II and III no of Hydrogen Cylinder required is 7 per 500 MW Unit per day.For Stage I (6X210 MW) Hydrogen is fed to Generator through direct Pipe line.Temperature of DM Water after electrolysis is 48 CFor Stage II and III there was no need of Mechanical maintenance of the Hydrogen Generation Plant (using PEM Technology). For Stage II and III no UPS was provided.

Some Photos from the Vindhyachal NTPC Visit H2 generator PEM

Diaphragm Compressors

Inside the PEM Unit

Electrolysis Cell - PEM

Hydrogen Tank Yard

Electrolysis by KOH

Hydrogen and Oxygen Seperator Tanks

KOH leakage from Electrolysis Cell

KOH Electrolyzer Cell

Hydrogen Filling Manifold

Generator

**Methods for dealing with program risksIntense review of processFully discuss with management the risk to get added resourcesspecial oversight of component engineeringfrequent reviews and oversight by senior engineersspecial analysis and testingadditional analysis and/or fabrication carried outrapid prototypingconstruct and test prototype to ensure validity relieving of requirementsadjustment of performance or compatibility requirementsparallel development fallbackParallel effort to supply adequate alternative with reduced performance.*Operational requirements must be described in terms of operational OUTCOMES rather than system performance. They should not be stated in terms of implementation.They should not be biased toward one conceptual approach.All requirements should be measurable and testable.The rationale for all requirements must be stated or reference to let developers understand the user needs.Customers and developers should understand that requirements will be refined during the development process.