Power Plants

Power Plant engineering, Master Course 2006

Seite 1

OVERVIEW OF POWERPLANTS.......................................................................................................2 FUELS....................................................................................................................................................2

Important properties ...........................................................................................................................2 Peat .....................................................................................................................................................3 Lignite.................................................................................................................................................3 Hard Coal............................................................................................................................................3

Bituminous Coal .............................................................................................................................3 Anthracite .......................................................................................................................................3

Crude-Oil ............................................................................................................................................4 Fuel-Oil...............................................................................................................................................4 Natural Gas .........................................................................................................................................4 Nuclear Fuel .......................................................................................................................................4

Mining Uranium .............................................................................................................................5 Fuel factory I ..................................................................................................................................5 Fuel factory II: Enrichment ............................................................................................................6 Fuel factory III................................................................................................................................8

FUEL USING.........................................................................................................................................9 Necessary Air .....................................................................................................................................9 Ignition................................................................................................................................................9 Pollutants ..........................................................................................................................................11

NOX...............................................................................................................................................11 CO.................................................................................................................................................11 Soot (Ruß).....................................................................................................................................12 Ashes ............................................................................................................................................12

WHATER TREATMENT....................................................................................................................12 Pretreatment......................................................................................................................................13

EXHAUST GAS...................................................................................................................................14 BOILER................................................................................................................................................15

Natural circulation ............................................................................................................................17 Forced circulation .............................................................................................................................19 Once Through Boiler ........................................................................................................................19 Once –Trough with part load Recirculation .....................................................................................20 Benson Boiler ...................................................................................................................................20 Sulzer Boiler .....................................................................................................................................20 Small stoker-fired boiler for e.g. Chemical factories .......................................................................20 Furnace wall heat transfer.................................................................................................................20 Superheater, Reheaters and economizers .........................................................................................21 Steam-Generating or Boiler Bank ....................................................................................................21 Heat transfer in Combustion Loop ...................................................................................................21 Preheating .........................................................................................................................................22 Design diagram for boilers ...............................................................................................................23 Pressure Parts....................................................................................................................................25

GAS TURBINE....................................................................................................................................29 Control ..............................................................................................................................................29


Seite 2

OVERVIEW OF POWERPLANTS Net-efficiency of: Hydro-: 95% Gas: 40-50% Steam: 30-40%, through district heatening the efficiency could increase but large network is

needed and the heat is used in a very low level (60-70%), comes back at 40°C (Remark: the efficiency of the Generator is 95-99%, the rest are heat losses, must be cooled down!)

FUELS Calorific value [MJ/kg] (value of stored Energy) Hard-coal 29 MJ/kg (varying quality!) 25% volatile content Lignite 15 MJ/kg (var. qual. -> 7-24 MJ/kg) 35% Anthracite 33 MJ/kg >90% C, best hard coal 7-10% Peat 5-10 MJ/kg 50-60 % C (dry) >70% Fuel-Oil 30 MJ/kg Nature-Gas 40 MJ/m³ Table 1.Aggrates of fuels SOLID LIQIUD GASES Peat,…, coal Fuel-oil Natural gas Wood, charcoal Biofuels Weak-gas(city gas) Waste Biogas Propane Butone

Important properties • Sulphur content • Carbon content • Hydro content • Oxygen content • Nitrogen content • Ash content • Water content SCHON AW • Ignition behaviour

- Take coal, heat up without O2 - Degassing -> important for ignition (Steam, Methan, SO2)

� How much gas comes out? � Mixes easier with air -> better ignition

• Volatile contents [Coal]


Seite 3

• Ash softening Temp. [Coal]

Peat Only from special region, good burnable (Finland, Ireland, Denmark)

• From swamp, drying needed • Comes from plants, it forms out of plant material under following conditions:

- no oxygen - pressure - heat increased

• pressing out water is increasing the carbon density/content • quality determines the application

Lignite • Water content: 50% and more (problem: it could freeze! Oil must possible be fired to run the

PP in winter -> expensive!) • Very high ash content compared to bituminous coal

Hard Coal Coal is a mixture of minerals, metals which are not burnable -> Ashes Ashes in burner are mostly out of metal-oxides, depending on metal it is

-melted: slag (the melt solidifies again at coolest area in the heater -> heat exchanger -> rock hard layer

-not melted: powder Ash-Softening temperature Test: Press ash into cube, heat up and check when it starts deforming -> Softening temp.

• High activation energy needed to start the burning process. PPs are mostly start up with oil • By choosing the diameter of coal particle you can predict the fast burning

Bituminous Coal • Relatively hard coal containing a tar-like substance called bitumen • Better quality than lignite coal but poorer quality than anthracite coal • Carbon content: 60-80% • Calorific value: 24-35 MJ/kg

Anthracite • Carbon content is between 92-98% • Fewest impurities of all coals • But lower calorific content: 26-33 MJ/kg


Seite 4

Crude-Oil • Very high viscosity, nearly solid • Needed to be heated up in tubes • Cheap • Burned in PPs and Ships • Low ignition point – 250-260°C (Coal: 500°C) • Havy oil will evaporate at 370°C • Last compound that evaporates during the refining process

Fuel-Oil 1. LPG, naphter 40-90 °C 2. Kerosene 80-140 3. Petrol 150-300 4. Diesel 300-360 5. heavy Oil 370 -> Contents varies from place to place

Natural Gas Is used since 2nd world-war before it was just burned in ancient times, gas from coal for lightning (heat up coal without O2)

1. Picture: Coak manufacturing Coak is used for iron-melting (very high C-content)

• Strong gas: Natural gas > 90% Methane • Weak gas: CH4, CO (CO -> CO2) from coak or other processes

Nuclear Fuel U-235 Pu-239 Pu-241

COAK GAS

COAL

Molecular weight


Seite 5

Mining Uranium Only 1/1000 from rock is Uranium. It is spread all over the world (but is there in the same amount like Ni or Co) Uranium ore has 1 0/00 U δ f (b) In natural U U-235 600 0.72 U-238 0 99.275 Classical reactors need 3-5 % U-235, (Research reactor in Munich needs 80%)

Fuel factory I Conversion, take the rest of the dirt, change the chemicals Chemical treatment -> U3O8, “yellow cake” 70% U; produced by early mining operations.

- Bath with sulphuric acid and so on, you have to build the chem. Factory by the mine. The milled U3O8 has to be converted into UF6


Seite 6

For enrichment you need a chemical compound, the pure Element F, no two types of F exist because there is only one isotope. Produce UF6: colourless crystalline solid which forms a vapour at temperatures above 56,4 °C. UF6 is the compound of uranium used for the two most common enrichment processes, gaseous diffusion enrichment, and gas centrifuge enrichment. It is simply called "hex" in the industry. It is corrosive to many metals and reacts violently to water and oils.

Fuel factory II: Enrichment Means: come from 0.7% U -235 to 5% U-235! We start with: 0.7%

CENTRIFUGES

0.77%

0.63%

0.84%

0.70%

5%

0.25% (put back in some mines, not used)

0.70%


Seite 7

MEMBRANES

V~ 1/sqrt(m) – so the lighter one are a little faster -> it will hit the wall more often SEPARATION NOZZLE PROCESS

• Many steps in row.

• Every step creates a enriched and a depleted fraction

• 1000 rpm


Seite 8

Fuel factory III UF6 + CO2 + NH3 reacts to Ammonium Uranyl Carbonate (NH3)*UO2 (CO2)3 (UO2CO3·2(NH4)2CO3) is known in the uranium processing industry as AUC and is also called uranyl ammonium carbonate. This compound is important as a component in the conversion process of uranium hexafluoride (UF6) to uranium dioxide (UO2). The ammonium uranyl carbonate is combined with steam and hydrogen at 500-600ºC to yield UO2. In another process aqueous uranyl nitrate, known as uranyl nitrate liquor (UNL) is treated with ammonium bicarbonate to form ammonium uranyl carbonate as a solid precipitate. This is separated from the solution, dried with methanol and then calcinated with hydrogen directly to UO2 to obtain a sinter able grade powder. The ex-AUC uranium dioxide powder is free-flowing, relatively coarse (10 µ) and porous with specific surface area in the range of 5m2/g and suitable for direct pelletisation, avoiding the granulation step. Conversion to UO2 is often performed as the first stage of nuclear fuel fabrication. The AUC process is followed in South Korea and Argentina. In the AUC route, calcination, reduction and stabilization are simultaneously carried out in a vertical fluidized bed reactor. In most countries, sinterable grade UO2 powder for nuclear fuel is obtained by the ammonium diuranate (ADU) process, which requires several more steps. Ammonium uranyl carbonate is also one of the many forms called yellowcake in this case it is the product obtainded by the heap leach process. Or

Together with 95% H2, to become higher speeds (realized in Brazil)


Seite 9

MOX = mixed oxide (aqueous solutions, or taking UO2 * PuO2 milling that grains fine enough for mixture, but this process has the disadvantage of forming lots of radioactive dust. An alternative is to mix a solution of Uranyl nitrate and plutonium nitrate in nitric acid. This can be converted using a base into a solid. The solid can then be calcined into mixed uranium and plutonium oxide.) is a blend of plutonium and natural uranium, reprocessed uranium, or depleted uranium which behaves similarly (though not identically) to the low enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors that predominate nuclear power generation. An attraction of MOX fuel is that it is a way of disposing of surplus weapons-grade plutonium, which otherwise would have to be handled as a difficult-to-store nuclear waste product, and a nuclear proliferation risk. END OF PROCESS When the process in PP is over, they need to load it with new fuel –preprocesses GB, France only two in Europe).Everything is stretched in pieces and given in chemistry solution.

FUEL USING For fuel (liquid coal) you need atomizer. For coal there are stoker systems, old systems from 1920, but there are still some working.

Necessary Air How much air do we need? For every fuel atom you have exactly 1 atom to combine! But they have to meet each other. For lower speed you have o build everything larger, so we have to increase the meeting rate, increase the oxygen rate for the last stadium of combustion (away from the main flame, remember 1400°C) The tasks are:

- Burn fuel completely - Low concentration of NOX - Low usage of air (looking for humidity -> dry air)

λ= air ration: (Spent air flow)/(stöchiometric air flow)

Combuster Lambda Motors 1,1 ( ~1 with cat)

Diesel 1,1 … 1,15 Coal with man. Fed stokers 1,5 … 2,0 Coal with mech. Fed stokers 1,1 …. 1,3 Burner firing (pulverized coal) 1,1 …1.3 Oil furnace 1,06 ... 1,2 Gas furnace 1,03 ... 1,1

Table: Lambda for different combustion forms

Ignition Ignition is a process that has to be started, pushed. If we can control the ignition than we can control the timing.


Seite 10

List of materials and necessary ignition energy [mJ]: Cotton 40-80 Sugar 40 Aluminum 50-120 CH4 0.28 H2 0.0019 Propane 0.26 This is not practical for realization because we measure the temperature in the burner not the energy. List of materials and necessary ignition temperature [°C] Paper 240 Butane 430 P(white) 60 H2 560 CH4 610 Lignite 400 Hard coal 500 Fuel oil 250 So we have to preheating the oil without air!

Table: velocities

To get the flame out -> increasing the velocity of Fuel! So what is necessary :

- Ignition temperature - The speed of both: Air/fuel and the flame - The mixture

H2

Stadtgas

Benzin

Quantity of gas in air 10 20

Rea

ctio

n v

elo

city

cm

/s

220

50

Air / fuel flame combustion

We have to balancing the speed!

Feeding speed = v_flame -> non moving flames


Seite 11

Pollutants

NOX Form only by high Temp about 1500 °C, above this temp. N2 is broken up. So it is desired to have an average of 1400 °C without any peaks in temperature allowed. There is a need of good mixing, this could realized through many nozzles which are burning the fuel. But if it happened, it has to remove afterwards, you can get rid of the NOX braking up the molecules. Many measurements in the burner, if there is a locally hot spot -> bring in local water to cool.

CO You have to measure exhaust gas: If CO2 is low -> not enough air If CO2 is max -> best combustion

Diagram: Lignite

You can measure only one, the other you get from the diagram

Diagram: fuel oil

20 22

18

20

O2 in exhaust gas [Vol-%]

CO

2 in

exh

aust

gas

[Vol

-%]

λ –air ratio

1.0

1.1

1.2

8%

6%

Give the content of CO

20 22

18

20

O2 in exhaust gas [Vol-%]

CO

2 in

exh

aust

gas

[Vol

-%]


Seite 12

Soot (Ruß) unburned carbon Molecule that consisting mostly from Carbon. If you do nothing it will contribute to emissions and you are not using the fuel completely. The efficiency and income will decrease. Candle flame: in the yellow part is more soot. The soot is helping the heat transfer it transports heat! (Black body radiation) You can get in from less air low lambda-zones. Solution to use it: First producing it with lambda < 1 Then burning it with lambda >1

Ashes Burning minerals leads to ashes.

WHATER TREATMENT Water has to be purified by some means (e.g. limestone out) before using. What can be in Water? Fluorine, Mg, Ca ….. [IONS] Oxygen [GAS] Particles [sands,…] -> careful: look for sedimentation (hydropower) Organic particles [plants…] Organic compounds [oil,….] Some things go out through filtering, container with pebbles -> adsorption of particles What do the substances? Build deposits, layers -> they degrades –heat flow -fluid flow Corrosion

- ions - CaCo3 to be removed (water hardness) - Turbidity ~ particles 10.000 ppm particles - Gases (solved in water, problematic one is O2 (drink water ~ 10ppm)

Water is a great factor for planning the PP. You have to know the quantity and also handle the quality! The problem by chemical treatment: you might end up with another chemical in the water and spend a lot of money.


Seite 13

Pretreatment Sedimentation (10µm-100µm can be removed), large quantities of bigger particles, but is sometimes not quick enough. + Filtration is catching particles by some other material -you have to clean the filter

� a help is clueing the particles which can then get out by sedimentation / filtration [COAGULATION AND FLOCUATION]

Chemical treatment Oxidation, chlorinication for organic matter, but you have to remove the Oxygen afterwards. (for portable water only use UV-light only for portable water) Clarification Removing of water hardness -> softening the water Ca(OH)2 which you can add to treat Ca(HCO2)2 aq and this has to be removed Ca(OH)2 + Ca(HCO2)2 -> 2H2O + 2CaCO3 (s) this precipitate and go down Similar compound to magnesium: Mg(HCO3)2 Large quantities can removed out that way, pretreatment. Filtration can be done in order of pretreatment They are different filters, you are not spending material, washing and reusing ! Demineralization

- Demineralization water - Deionate

Exclude evaporation Membranes: may pass larger ions, not salts Osmosis, RO (reverse osmosis)

- ion exchange

Table: Use of membranes to demineralization

Clear water low conc. of ions

dirty water dirty water

Clear water



Seite 14

Water movement in order to equalize concentrations But we want to purify the water -> pressure (Osmotic pressure) if you exhort that pressure you can stop the water movement

Table: RO, reverse osmosis

The forced water movement leads to different concentrations. Or: support by electric fields (dielectrophoresies) ions can move through membran (like dyalyse)- its not often used. ION-Exchange To be removed e.g. Mg 2+, Ca+ (because this precipate) To be replaced e.g. Na+, or H+ depends on pH-value (this not)

Table: ion exchange Seawater treatment: heat water up, catch steam -> this is possible in warm areas.

EXHAUST GAS

• T under 100°C -> water condensates -> more heat out -> efficiency increases - But then are sulphur-acids in water -> stainless steal needed - In natural Gas, sulphur is extracted before entering the pipeline, so we have gas and

sulphur production (could sell out). This is easy for gases, but very hard for solid fuels.

Clear water low conc. of ions


Cleaner water

dirtier water

Na

Na

Na

Na

Na Na

Na

Na

Na Na

Ca +

Na

Na

Na Na

Na

Na

Na Na

Ca 2Na +

Coming with water

leaving with water


Seite 15

BOILER Stem boilers are the central element in the PP. They convert the energy in the fuel to a useful form: steam. (Steam for electric energy, heating): Its Objective: produce steam flow at desired rate at specified temperature / pressure as efficiently and cost-effectively as possible from the fuel supply. The boiler types ranges from 10 g/s up to 1260 kg/s There are two separate parts separate through: tubes, tube sheets, headers, cylindrical vessel

- Steam/water circuit

- Air/Gas circuit There are two basic boilers: Fire tube boilers: Hot gases pass through the tubes which are immersed in a cylindrical vessel vontaining the water. Cost, size, and technical factors thend to limit firetube boilers to smaller, low-pressure industrial applications Water tube boiler: The water and steam flow through the tubes with the hot gases passing outside. Supply the majority of steam used today in medium and high pressure applications such as thermal power stations. The Fed water (demineralised and prepared) is into steam changed.

History People tried to heat up a tube (J. Watt 1776) First watertube boiler (Teilkammerkessel 1900) Babcock/Willcox, many accidents occurred, Boiler explosions (4-5bar) 1852 they build up a review system many of them was used to get electricity on the streets.Fire tube boiler (to enlarge the surface of heat exchange) 3-pass-boiler used in Locomotives. Major components:

1. Furnace 2. Steam superheater (primary and secondary) 3. Steam reheater 4. Boiler or steam-generating bak (industrial units only) 5. Economizer 6. Steam drum 7. Attemperator or steam temperature control system

Here is a picture of a boiler:

wet steam saturated steam

superheated steam


Seite 16


Seite 17

Here follows a simple flow diagram from the water supply (feedwater) inlet.

The superheater, reheater, boiler bank (if used) and economizer are tube bundles located in the hot gas stream in an area of the boiler feferred to as the convection pass. The fuel is burned in the furnace. The walls are made up of panels of tubes throufh which the water flows. Heat transfer through the tubes heats the water, generating steam. The flowing water also cools the tube wall to prevent overheating and possible failure. Water circulates from a drum at the top of the boiler down through pipes called down comers (fewer bigger tubes outside the boiler) to the bottom of the boiler and then up through the waterwall tubes to the drum again (RISER: many small tubes inside boiler. Sometimes orifice build in, to mature the average temperature flow, depends on the position in wall). Here are natural circulation and forced circulation. Circulation is needed to cool down the tubes! The lines on the walls are tubes, needed for change saturated water to saturated steam. The preheating of the feedwater is in the economizer. We need superheated steam because that increase the efficiency.

Natural circulation Water is heated, its density decrease the watercolumn in (B-C) weigh less than the water in the downcomer (A-B). That causes the water to flow from the steam drum, down to the downcomer (A-B), up through the waterwall tube (B-C) and back into the drum. When the water in the waterwall tubes reaches the saturation temperature, steam bubbles form, futher reducing the density of the waterwall tube leg (B-C) and enhancing the natural circulation.


Seite 18

When steam and water enter the drum, provision must be made to separate and dry the steam for use or further heating and return the water to the downcomer for recirculation and further steam gen. Circulation rate is dependent of 4 factors: 1 height of the boiler

Taller boilers result in a larger total pressure difference between the heated and unheated legs and therefore can produce larger total flow rates

2 operating pressure Higher operating pressure results in higher-density steam water mixtures. This reduce the total weight difference between the heated and unheated segments and tends to reduce the flow rate

3 heat input rate Increase the amount of the steam in the heated segments, increasing the total flow rate 4 free-flow areas of the components

Increase in the cross-sectional areas for the water or steam-water tends to increase the circulation rate.

Natural circulation is depended on a certain difference in Water density. A natural circulation tends to be self-compensating. This can include sudden changes in load, changes in heating surface cleanliness and changes in burner operation.


Seite 19

Forced circulation Mechanical pumps are added to the simple flow loop, usually in the downcomer piping. Orifices or other flow resistance devices are usually profuded in the boiler tube inlets to balance flow in the waterwalls. There is also an additional power loss resulting from the circulating pump.

Once Through Boiler Water enters the bottom of the tubes and is completely transformed to steam by the time it reaches the outlet, passing through the boiler only once. There is no separation of steam and water and no drum. Most once-through boilers operate at p above critical 221 bar where distinct two phase steam water mixtures do not exist and water is smoothly converted into steam. Many supercritical-pressure once-through boiler require special start up systems to provide high flow rates to the furnace walls for cooling while the boiler load is low.

If water is unevapourated everything sticks to the wall.


Seite 20

Once –Trough with part load Recirculation

Benson Boiler

Sulzer Boiler Different types different to control, control with vauls for Feedwater.

Small stoker-fired boiler for e.g. Chemical factori es Mainly for steam production the electrical energy is only a byproduct. Here by the downstream a second drum, MVD-drum because of pollution of the water (rem. Steam is focused)

Furnace wall heat transfer Critical heat flux (CHF) Film boiling: a thin film of superheated steam covers the inside of the tube wall separating the metal from the liquid. Heat transfer in lower and the wall temperatures will climb and likely overheated if it’s a high heat input zone. The change from nucleate or convective boiling to film boiling is the CHF point. Depending on the flow conditions it is the departure from nucleate boiling (DNB) or dryout (DO) DO: large steam%, evaporator in once through boiler ( so not for natural circulation) BWR boilerized water reactor. Burnout:small steam%, in drum boiler PWR Pressurized water reactor What can be done?

- power incr.? No (increase the danger) - pressure drop? - Feedwater flow? Is done but not often - Pressure? Normally controlled over pressure


Seite 21

Artificial factor “distance fot film boiling” in the hot channel DNB departure from nucleate boiling SF safety factor against film boiling DNB = Linear heat release (critical) / Linear hear release (Hot channel)

Superheater, Reheaters and economizers Steam in drum: 180 bar, 360°C Needed: 550°C � Superheater Superheater and reheater consist of in.line tube bundles that increase the temperature in steam. Single phase heat exchanger with steam flowing inside and flue gas passing outside, generally cross flow. They help control steam outlet temp, keep metal temp. below acceptable limits and control steam flow pressure loss. The economizer is a counterflow heat exchanger for energy recovery downstream of the superheater and the reheater. It inceases the emp. Of the feedwater before it enters the steam drum. The tube bundle os typically an arrangement of parallel horizontal serpentine tubes with water flowing inside bur in the opposite direction to the fuel gas. By design, steam is usually no generated inside these tubes.

Steam-Generating or Boiler Bank The heat transfer furnace may not be sufficient to generate the desired saturated steam flow rate. If this is the case, an additional bank of heat exchanger tubea called the boiler bank is added. This is needed an many smaller, low pressured industrial boilers, not often in high pressure boilers. Composed of the steam drum on top, a second drum on the bottom.

Heat transfer in Combustion Loop Thermal radiation and convection. Thermal radiation is proportional to the fourth power of the absolute temperature. Almost all heat transferred to the waterwalls is by thermal radiation. The thermal radiation is important in boiler not in Superheater. The difference: the radiation will not change with load, the flue gas will increase by burning coal. Superheater RADIATION FLUGAS MASSFLOW Rad. SH + Delta Tr Conv. SH - Delta Tc Independent of load Load P incr. = massflow flugas The combustion temperature is constant so the radiation is also constant. If the mass flow of flugas increase with power the delta Tr will decrease!! If the mass flow of flugas increase with power we get better convective heat flow transfer delta Tc Combine both in order 1. Radiation SH than 2. Convective SH.


Seite 22

Convective section is designed to extract the maximum heat from the partially cooled [1000°C] flue gas and must be as compact as possible. The Temperature after SHs is constant. This is needed for the Turbine that the inlet Temperature is constant.

Preheating air is preheated up to 200-250 °C depends strongly on fuel and technical design. In large boilers the air is not coming from natural draft but helped through fans (forced draft FD) Remember: increasing the velocity of the flue gas increase the convective heat transfer. Why?:

- faster ignition - higher fire room temperature

Regenerativ design Ljungström-preheater

Diameter is from 10 up to 20 m. The rotor sealing system contains simple leaf type labyrinth seals compress against plates, again located at both the hot and cold ends of the rotor. This system separates the air stream from the flue gas stream. If there is the possibility of get under the dew point, the plates on the cold end will be emailliert. There exists steam air preheater to heat up the combustion air to 80°C to become no dew point corrosion. Tubular design

Flue gas outlet 180°C

Air

Hot Gas Inlet 240°C Sheet of metal

rotation, slow: 10m/s

Hot Air


Seite 23

By the tubular design are vertical tubes arrange staggered. The air passes over all the sections of the air heater in sequence, the effect of which is to provide counterflow.

coal: preheated from boiler heat, because transport air will move coal Fuel: extern heat Gas: pipeline transport pressure 60 bar first maybe electrical preheating. Water Circuit: The feedwaterpump 100 to 150 bar system pressure, the rest is natural circulation (most times), but during start up you need possible a pump. Its designed for pressure up to 170 bar, 221 bar is for large PP. DRUM BOILER Cannot overheated you have always liquid water to remove steam. The pressure: large vacuum cleaner to suck out flu gas, pressure under atm, so leakages go the other way namely inside.

Design diagram for boilers


Seite 24

At 160°C for flugas you get in trouble. There are problems with sulforic acid condensation, corrosion at sheet. Note that the screen refers to several rows of water-cooled tubes that precede the superheater in order to limit the amount of thermal radiation from the furnace which reaches the superheater surface E.g. Natural Gas requires the smallest furnace because of its low emissivity, which results in a relatively even heat flux at the tube walls. Fuel oil, having a higher emissivity, requires that the walls of the boiler be farther from the burner to reduce the peak heat flux. Coal firing has different problems. The flue gas temperature at the entrance to the tube bundles located in the convective section must be below the ash-softening point to avoid excessive surface fouling. In addition, the furnace cross-sectional area must be adjusted to control furnace slagging while the burner layout and furnace height must be optimized to reduce NOx formation. Because of the highly erosive nature of coal ash at it is carried through the convective section, the fuel gas velocity must be limited. How can you find out how much water do we need?

a) the water drum level b) (Benson) water steam separator c) Water percentage in Separator d) (Sulzer) Balancing in and out


Seite 25

Pressure Parts Pressure parts thicknesses are selected so that the primary stress falls wihin the allowable stress limits for the material at the design temp. [Americal Society of Mechanical Engineers (ASME)]

The Figure summerizes a classification of stress types includes in the ASME code. The following Picture shows the relationship of furnace exit gas temperature to heat-release rate for fuels


Seite 26

Furnace waterwall construction Furnace and convection pass enclosures are fabricated using membrane panels. The membrane panels are composed of a single row of tubes spaced on centers wider than a tube diameter and joined by a membrane bar securely welded to the adjacent tubes.


Seite 27

The panels are prefabricated with the appropriate openings for burners, observation doors, soot blowers, instrument access, and gas injection ports. There are also tengenttube construction constists of a single row of tubes placed next to each other with a narrow gap. Remember: Nuclear PP has not enough heat for superheated steam. The Water has to removed in the turbine, because even after the first step in the turbine there is 10% wet steam. The water can damage the turbine blades. TURBINE Reheater: Upgrading steam, removing water, between high pressure and low pressure turbine

Steam Drum Separation of saturated steam from the steam-water flow and send it to the superheater or directly to an end use. It also serves to mix the feedwater with the water discharged from the steam-water separators, mix the water treatment chemicalts, purify the steam prior to discharge, remove parts of the water (blowdown) to control boiler water chemistry, and provide limited water storage to accommodate rapid boiler load changes. Large diameter can be 1-1.8 m


Seite 28

Continuous Blowdown. Nonvolatile chemicals that are injected directly into the steam drum and other dissolved solids that may enter the boiler do not leave with the steam � the concentration increase � continuous blowdown removes the solids from the water by removing portions of the water containing these solids. Steam outlet. A series of tubes at the top of the drum that carry steam to the superheater A gage glass is mounted on the end of the drum to monitor the water level. Steam water separation equipment. In low steam flow natural steam separation by gravitiy at the steam water surfave in the drum can be sufficient depending on the drum size, steam flow reate and operatin pressure. Natural separation is mostly too inefficient for economical use. Primary centrifugal separator and one ore more secondary scrubbers. Separation is difficult there is not much time! There is no measuring of the steam quality.

Pulverized-Coal Boilders PC- with appropriate back end air emission control equipment to limit sulphur dioxide nitrogen oxides and particulate emissions tend to be more attractive in larger unit sizes burning reasonable quality coal.

Stoker-Fired Boiler Are available für a wide range of coals and solid waste fuels. Can be designed for mulifuel firing. Low Nox designs are available. So2 scrubber at the end is needed. Coal fell through gate some times.

Fluidized Bed Boilers Provide the widest fuel flexibility of the dhree systems. Burning high ash coals and wast fuel because of its potentially long combustion zone residence time and high intensity bed mixing process. The use of Limestone as the bed material to capture SO2 and low bed temperatures to limit Nox emissions can frequently be employed to meet air emission limits with minimal backend cleanup equipment. Its more complex to control high operating and maintenance costs than comparable systems There are unfired Boilers through hot gas but flame direct on tube is far easier


Seite 29

GAS TURBINE Gas and Steam turbine do not have to be synchronous

Control Control system Open loop control (Steuerung, IF, THEN – Logic) Closed loop control

(Regelung, 0-100%; controller has a math characteristic: P(roportional), PI(proportional integral)

Limitation

- find often passive measure Digital Normally helping protection, or increasing availability

Protection - You need poss. Old backup, hardwire, from button to component one wire.

Point to point wired. The computerization began in the mid nineties. Use of Black boxes in which you can directly define what you hand over Repowering for plant renewing some advices, e.g. replace an old reheater -> you need to combine new and old automation systems -> Black box.


Seite 30

Siemens AG, single shaft unit Overview picture, here you see the most important variables at once! As there is…

- active power - speed of turbine, during start up interesting in normal operation nearly constant - start, RDY for start – there is checklist integrated in the program, if everything. OK

it blinks. If not - Signal missing. Then you have to go to detail diagrams to find error - link to detailed diagrams. Alarm blocs are not seen until they are Alarm occurs

(Alarm, Warning, Tolerance, F INC system sensor fault, B automation systems overwrites a signal from operator)

- T after turbine. Necessary is T before turbine (~1250°C) but is not easy to measure. Better: Calculate T after turbine back.

- Interlock for blocking some If you can not start, run shut down program. E.g. power supply missing -> check what is the status locally for instant the might loop oil pump. Plant coding system: Short codes for every component, room, screens and so on. In German: KKS (Kraftwerk-kennzeichnungssystem). There are dictionaries for one system to another.


Seite 31

Siemens AG, airstreams Air Inlet ->poss. heating against icing-> filter -> flap (for stand still) Blow off system: Compressed air can go to the Atmosphere or combustion chamber, open during start up than closed slowly. To avoid too much air in the beginning during start up is cooling not so necessary


Seite 32

Combustion chamber The gas is going to the combustion chamber not in three single lines, but through many inlets arranged in an ellipse around the chamber. The CO2 (could also done by N) is for inertising the tubes should be connected to all lines! In order to prevent ignition situations in the tubes. If you have not enough air in the chamber there will occur high temperature flames, not god for the ceramics.


Seite 33

Loop oil system If shaft starts rotating, layer of oil betw. Rotor/ stator 500rpm, oil film to save the equipment. Large Tanks with level measurement ->

- oil pump (4-5 bar) - lift oil pump (150 bar) to lift the shaft to the bearings. Black out-lift pump out –cause

damage! So do not often. - Emergency pump (DC battery powered)

Loop oil has to be heated during stand still, because you want the right viscosity during a start up. Heated with the heat losses of the pumps. Turning (bearing) gear To turn the shaft (most times done by start up frequency converter)

- for checking by stand still, every 6 h turned to see if everything ok, blades might deformed.

- During shut down, shaft cast down to speed 0, but cooling down turning (24h) is necessary against deforming.

All this is possible with the frequency converter. It will use the gen. as motor but it is not design for much time to operate. Gas Turbine control Tasks of the Gas Turbine Controller

- Prevention of thermal overloading of the gas turbine - Low-stress start up and shutdown of the turbine


Seite 34

- Synchronisation with the grid - Loading/unlading of the turbine - Frequency stabilisation - Reliable load rejection to unit auxillary power requirement / island operation - Ability to ride out a load rejection

Turbine Controller Here we have close loop and open loop control and automatics to care for different situations (e.g. automatic heating of loop oil or the oil of emergency diesel generator has to be able every time to start). We have:

- 100% Binding operations ( If this happens, then do that) - Dosed reactions

Questions: What is a controller, what do we expect. Power controller: How many MW is wanted -> Software says how much fuel is needed.

This is faster then normal loop control because of new software, if you want to change sth then press key before you have to do mechanical things. Cycle time:

- check things – (input (temp, fuel)) - calculate - do sth

takes 2 ms, every 2 ms the measurements are used to recalculating

The blades can get damage

We want to have low thermal stress Speed adjusting for Generator

More / less fuel

Not all PP have it


Seite 35

Normal loop control 20ms to 1s!

- Peak load PP (Gas turbines) Stand-by-power systems - In France NPP some are running in medium load ( low in Weekend or night) - Short term scattering h or days?

If frequency goes from 50 to 49 Hz Automatic reaction, for stabilization -> more output 1-2 % of the total power. In steam PP you avoid the total amount of steam to turbine -> if needed give it to turbine. Or through stopping some preheating. E.g. stop condensate preheating, then use the steam in turbine (that all reduce efficiency) If frequency goes lower than 49 Hz Load rejections 10% of consumer will be switched of ( in the contract signed, they get better prices) some PP go to island production and can help later restabilizing the network Protection by 108% speed Steam turbine: You are in trouble by steam turbine if you cut down the steam inlet -> but if there is still steam in it, it will run! Vakuum braker are not often used, they might damage the blades Gas turbine: Is simple -> overspeed is easier to handle Hydropower: Overspeed can be 3 times higher! ( so 9 times higher forces!)


Seite 36

Load controller:


Seite 37

If you know the needed output. Super channels can operate as load controller but don’t have to. Speed controller: Needed in Island operation they don’t know when PP is needed. (ships) Start up: To 1500 rpm the start up frequency converter runs it as a motor then the combustion runs alone. Shutdown: Reducing the Power output to 0, then opening the braker Trip: You are also close the fuel valve to prepare for start up.

Signals on one bus and in some PP in addition you have 1 wire per signal. Various speed ranges during start-up of Gas-turbine


Seite 38

Compressor washing takes 1min Boiler purge; compressor use as large fan you: might have hot gases with still burnable parts -> blow out (purging time: volume of whole system times 3,4) Speed controller: is normally nearer to 50 1/s! Master controller

Master controller

- Nowadays just software


Seite 39

- Many input signals -> output mainly one: fuel control MIN. is the point where the decision is taken: the lowest one is the taken one! The other signals are ignored!! Minimum Gate control Input signals are in line with the set points If you try to do too much air, the air may lost the contact to the blades of the compressors. So the blades get shock in interval. They have to be certain calculations to get it before it happens. Actual Speed Value Formation

Why speed signal for open loop control? Lift oil pump have to be on, or cleaning During shut down you need the signals Electric-measurement I/U active Power is calculated. Which we are measuring min, max, average? MIN: If one line is broken we get a = and the system will give more!! Average: the same! MAX

Power Plants

Documents