combustion equipment & power cycles
TRANSCRIPT
ME326 COMBUSTION AND HEAT TRANSFER
Combustion Equipment and Power Cycles
Dr. Kihedu, J.
COMBUSTION OF COAL
Combustion Applications
• There are various types of burners, available in twocategories that depend on particle size;
Large particles on fuel beds, and
Small particles in pulverized form.
Coal Burning Equipment
• Over Feed Stokers
• Traveling-grate or Chain-grate Stoker
• Under Feed or Retort Stokers
• Pulverized Coal Burners
• Cyclone Furnace
Coal Burning Equipment
Over Feed Stokers
• The majority of burners operate on over feed principle as fresh coal is dropped onto the fuel bed.
• The fuel bed thickness varies from 10 to 30 cm and can be divided in four zones.
– The topmost zone is formed as the distillation zone.
– Then reduction zone (endothermic), oxidation zone (exothermic) and ash zone.
• Air for combustion enters from below the fuel bed through the grate and gets heated in turn cooling the ash.
• The oxygen reacts with part of the carbon coke bed.
• When the oxygen becomes depleted, the coke reduces the CO2 back to CO in the reduction zone.
Coal Burning Equipment
Over Feed Stokers vs. Gasifiers
Coal Burning Equipment
Traveling-grate or Chain-grate Stoker
• Popular type of mechanical firing device in which coal isfed from a hopper onto a grate moving through thecombustion chamber.
• The grate consists of a number of cast iron barsinterlocked to form a grate. The traveling-grate stoker hasmore closely interlocking bars.
• The coal enters at one end and by the time it reaches theother end, the combustion is complete and the ash fallsinto the ash pit.
Coal Burning Equipment
Traveling-grate or Chain-grate Stoker
Coal Burning Equipment
Under Feed or Retort Stokers
• Coal is forced into the fire from below by a power ram or screw and moves out to the sides as it burns.
• The main advantage - volatile matter passes through the oxidation zone, thus ensuring its complete combustion.
• The grate is usually inclined and thus the burned fuel and the ash move outwards as the fresh fuel is supplied.
• The air for combustion is supplied through tuyeres.
• Disadvantages;
– Some of the unburnt fuel may pass through the grate.
– Fusion of ash or clinker formation may result in an uneven distribution of air.
Coal Burning Equipment
Under Feed or Retort Stokers
Coal Burning Equipment
Pulverized Coal Burners
• In pulverized coal burners, more than 85% of the coalparticles should have diameter less than 0.063 mm.
• These finely ground particles are blown into thecombustion chamber by the hot primary air.
• This cloud of coal then burns inside the combustionchamber in a manner similar to that of a droplet liquid fuel.
• Advantages are: high efficiency, greater flexibility in theircontrol and operation, flexibility in the quality of coal to beused, and easy design of burners.
• Disadvantages are high cost of pulverizing the fuel and thatmost of the ash is carried along with the exhaust.
Coal Burning Equipment
Pulverized Coal Burners
Coal Burning Equipment
Cyclone Furnace
• Small coal articles usually with a diameter less than 6mmare burned in suspension with air.
• The fuel swirls forward into the main chamber where itmeets the high speed tangential stream of secondary air.
• Some tertiary air is supplied at the axis of the chamber toensure the burning of any fine coal particles.
• Temperature is high therefore ash melt away - ashglobules are carried to the wall by centrifugal force as thefurnace is inclined to permit molten ash to flow down.
• Optimization for smaller furnace is to be done.
Coal Burning Equipment
Cyclone Furnace
Fuel Oil Burners
Fuel oils have domestic and industrial use;
• Kerosene is used for illumination and heating
• Types of kerosene lamps
– Yellow flame wick lamp - incomplete combustion,incandescent carbon particles in the flame radiate light
– Wick fed mantle lamp - mantle increases illuminatingpower, kerosene vapours are supplied and burned insidemantle
– Pressure fed mantle lamp – kerosene is supplied underpressure through a nozzle rather than a mere wick.
Fuel Oil Burners
Types of domestic stoves;
• Wick type - kerosene rises through number of wicks inconcentric cylinders. Cylinders are perforated and heatedby the burner flame itself to vaporize the fuel to give aslow smokeless flame.
• Pressure type – kerosene reservoir is pressurized by asmall hand pump to rise the kerosene through a tube tothe burner head where it gets vaporized. Out of a smalljet, vapours mix with air to give a turbulent blue flame.
Fuel Oil Burners• Industrial burners and furnaces normally operate with cheap
heavier oils.
• The combustion of such oils requires vaporization or at leastatomization into droplets.
• These vapours or droplets need to be thoroughly mixed withair to give a stable flame.
• The finer the atomization, the more rapid will be theevaporation, resulting in more rapid and efficient combustion.
• Typical types include; Vaporizing burner, Rotating cup burner,Mechanical or oil-pressure atomizing burner, Steam or high-pressure air atomizing burner and Low-pressure air atomizingburner.
Fuel Oil Burners
Vaporizing Burners
• These are similar to the wick and pressure stoves.
• The oil is fed by gravity to the bottom of a pot by a pipe wherefuel is evaporated by the radiant heat from the flame and thenearby heated surface.
• The vapours rise in the pot and mix with the primary air. Thefuel-air mixture near the bottom of the pot is too rich tosupport combustion.
• Consequently, the flame rises to a position just above the rimwhere enough air is available to give a good burning mixture.
• Some soot formation is inevitable in such burners whichnecessitate periodical cleaning.
Fuel Oil Burners
Rotating cup burner
• Used in steam boilers, capable of using variety of oils without anymajor modifications in their design.
• Oil flows through a tube into the cup rotated at speeds of 3,500to 10,000 rpm therefore centrifugal force spreads oil into a thinfilm on the inside walls of the cup.
• About 10 to 15% of the theoretical air is supplied as primary air.
• The angle at which the air hits the fine oil mist may be adjustedby regulating the relative position of the cup and the air cone.
• Additional atomizing effect is obtained as air blasts the fuel mist.
• The shape of the flame is controlled by the shape of the cup andthe position of the air nozzle.
• Secondary air is usually supplied by a natural draft through airshutters in the furnace wall.
Fuel Oil BurnersRotating cup burner
Fuel Oil BurnersMechanical or oil-pressure atomizing burner
• Oldest and commonly used burners for land and marine boilers.
• Atomization by fluid pressure and released through an orifice.
• Oil is preheated to attain viscosity of 10 to 30 centistokes and fedtangentially under high pressure into a conical swirl chamber.
• Half of the oil pressure is consumed in generating rotationalenergy in the liquid, which then flows out from the orifice at highvelocity in the form of fine droplets forming a cone of oil mist.
• For large capacity boilers, a greater number of, boilers, a greatnumber of burners are employed instead of a single large capacityburner as it gives better atomization at lower pressure.
Fuel Oil BurnersMechanical or oil-pressure atomizing burner
Fuel Oil BurnersSteam or high-pressure air atomizing burner
• Operates like a "scent spray".
• For heavier oils or for boilers, steam is preferred as it alsopreheats the oil and is available at high pressure.
• However, compressed air gives better mixing of air and fuels andcombustion.
• As with most steam atomizers, the steam and oil flow side byside, thereby preheating the oil so that the viscosity of the oil isreduced, resulting in smaller oil droplets.
• The pressure of the air or steam required for such atomizers isusually greater than 1 kg/cm2 and may be as high as 7 kg/cm2,depending upon the viscosity of the oil.
• Air and fuel may either mix inside the burner or totally outside it,i.e., inside the combustion chamber
Fuel Oil BurnersSteam or high-pressure air atomizing burner
Fuel Oil BurnersLow-pressure air atomizing burner.
• The principle is the same, the only difference is that the airpressure is low, about 0.035 to 0.15 kg/cm2.
• Such burners are more suitable for lighter, less viscous oils,such as kerosene.
• The primary air required for atomization is comparativelyhigher, of about 20% or more.
• Design of almost all the burners is as a result of the longexperience gained in the use of fuel oils.
• Lot of research work is in progress on the mechanism of spraycombustion too, but the gap between the work done so far, andthat required for the direct application, is still wide.
Gas Burners• Gas burners are mainly used for purposes of cooking and
heating in homes, and for ovens and furnaces in industries. Three main classes:– Non-aerated burners, – Aerated burners, and – Surface combustion burners.
• Non aerated burners are used where long, lazy flames are required, e.g., in baking furnaces. In these burners, the fuel enters from a pin hole jet or a slot and all the oxygen is supplied by the air around the flame
• The air is supplied by natural convection only in the case of small burners or by forced convection in the large one.
• A stable flame is obtained irrespective of wide pressure or velocity fluctuations.
Gas Burners• Aerated burners are the most widely used type of gas burners
for domestic and industrial uses. These are based on thefamous Bunsen burner principle.
• The fuel enters a tube through a jet. The suction induced bythe jet of gas draws the primary air.
• The primary fuel-air mixture flows through the tube to theburner top or port where the flame is stabilized. Thesecondary air is supplied from the atmosphere byentrainment, through the outer envelope of the flame.
• If the primary air supply is insufficient, the flame becomeslong, slightly smoky, and luminous; and if the primary airsupply is increased, the flame becomes short and non-luminous.
• Depending upon the pressure of the gas admitted to theburner, the aerated burners may be either the atmospheric orhigh pressure type
Gas BurnersBunsen - burner
Gas BurnersAtmospheric gas burner have the following characteristics:
• Controllable over a wide range of turn down without flashback,
• Provide uniform heat distribution over the heated area,
• Capable of completely burning the gas,
• Provide ready ignition with the flame traveling rapidly from port to port,
• Operate quietly during ignition, burning, and extinction
• Withstand severe heating and cooling during the life of the appliance.
Gas Burners
Atmospheric gas burner
Gas BurnersPressure type aerated burners• Concentric primary air and gas jets under pressure are used in
place of the simple gas jet.• Separate jets can be used to induce secondary air, or they can
be first fed into a mixing chamber.• Large furnaces may be heated by multiple gas jets set in a
common head, each jet being surrounded by compressed air,with a concentric orifice for the supply of air.
• The premixed stoichiometric proportions of gas and air can beburned in a tube or a narrow tunnel.
• These burners can support a laminar or turbulent flame,depending upon the heat release rate required. Turbulencehelps in the proper mixing of fuel and air.
Gas Burners• In surface combustion phenomenon gas premixed with
more than 100% primary air is fired tangential to theincandescent porous refractory surface of the furnace.
• The incandescent surface apparently has a catalytic andradiant effect, which promotes very rapid and completecombustion even at high burning rates.
• Most gas burners can be used for different gases byminor adjustment in the fuel jet and/or burner head.
• As a result of the systematic research carried out in thefield of combustion, it has become possible to design agas burner which is efficient and stable in operation
Thermal Power Plants
Thermal Power Plants (Cont.)
In ME 326, we are concerned
with subsystem A
Thermal Power Plants (System A)
1 2tW m h h
Turbine
2 3outQ m h h
Condenser
4 3pW m h h
Pump
1 4inQ m h h
Boiler5
6
w
6 5
6 5
out w
out w p
Q m h h
Q m c T T
Boiler
• Steam is used invapor powercycles– Low cost,– Availability, and– High enthalpy
of vaporization• Heat transfer to
be discussedlater…
Power Cycles
• The model cycle for vapor power cycles is the
Rankine cycle which is composed of four internally
reversible processes:
– Constant-pressure heat addition in a boiler,
– Isentropic expansion in a turbine,
– Constant-pressure heat rejection in a
condenser,
– Isentropic compression in a pump.
• Steam leaves the condenser as a saturated liquid
at the condenser pressure.
Carnot vs Rankine Cycles
• Carnot cycle is the most efficientpower cycle operating between twospecified temperature limits.– NOT suitable for actual power
cycles due to impracticalities which can be eliminated by: • Superheating the steam,• Completely condensing steam.
• The modified Carnot cycle is called theRankine cycle, where the isothermalprocesses are replaced with constantpressure processes.
Simple Rankine Cycle
© The McGraw-Hill Companies,
Inc.,1998
Steady-flow Energy Equation
Thermal Efficiency
Net work output
Vapor Power Deviation and Pump and Turbine Irreversibilities
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.
(b) The effect of pump and turbine irreversibilities on the ideal cycle.
Mollier Diagram (h-s diagram)
Isentropic efficiencies
Pump
Turbine
Increasing thermal efficiency of the
Rankine cycle
• Increasing the average temperature at which heat is added to
the working fluid and/or by
– The average temperature during heat addition can be
increased by raising the boiler pressure, or
– Superheating the fluid to high temperatures.
– There is a limit to the degree of superheating, however,
since the fluid temperature is not allowed to exceed a
metallurgically safe value.
• Decreasing the average temperature at which heat is
rejected to the cooling medium.
– The average temperature during heat rejection can be
decreased by lowering the turbine exit pressure.
– Consequently, the condenser pressure of most vapor
power plants is well below the atmospheric pressure.
Increasing thermal efficiency (Cont.)
Increasing efficiency (Superheating)• Superheating has the added advantage of decreasing the
moisture content of the steam at the turbine exit.
– Lowering the exhaust pressure or raising the boiler
pressure, however, increases the moisture content.
• To take advantage of improved efficiencies at higher boiler
pressures and lower condenser pressures, steam is
reheated after expanding in the high-pressure turbine.
– This is done by extracting the steam after partial
extraction in the high-pressure turbine, sending it back to
the boiler where it is reheated at constant pressure, and
returning it to the low-pressure turbine for complete
expansion to the condenser pressure.
• The average temperature during the reheat process, and
thus the thermal efficiency of the cycle, can be increased by
increasing the number of expansion and reheat stages.
Increasing efficiency (Reheating)
Incorporation of the single reheat improves efficiency by 4 ~ 5%
Increasing efficiency (Re-generation)
• Another way of increasing the thermal efficiency ofthe Rankine cycle is by re-generation.
• During a regeneration process, liquid water(feedwater) leaving the pump is heated by somesteam bled off the turbine at some intermediatepressure in devices called feedwater heaters.
• The two streams are mixed in open feedwaterheaters, and the mixture leaves as a saturatedliquid at the heater pressure.
• In closed feedwater heaters, heat is transferredfrom the steam to the feedwater without mixing.
Increasing efficiency (Cont.)
Increasing efficiency - Co-generation
• The production of more than one useful form ofenergy (such as process heat and electric power)from the same energy source is called co-generation.
• Co-generation plants produce electric power whilemeeting the process heat requirements of certainindustrial processes.
• This way, more of the energy transferred to the fluidin the boiler is utilized for a useful purpose.
• The faction of energy that is used for either processheat or power generation is called the utilizationfactor of the cogeneration plant.
Increasing efficiency (Binary Cycle
and Combined Cycle)
• The overall thermal efficiency of a power plant can be
increased by using binary cycles or combined cycles.
• A binary cycle is composed of two separate cycles, one
at high temperatures (topping cycle) and the other at
relatively low temperatures.
• The most common combined cycle is the gas-steam
combined cycle where a gas-turbine cycle operates at
the high-temperature range and a steam-turbine cycle at
the low-temperature range.
• Steam is heated by the high-temperature exhaust gases
leaving the gas turbine.
• Combined cycles have a higher thermal efficiency than
the steam- or gas-turbine cycles operating alone.
Increasing efficiency (Binary)
Increasing efficiency (Combined)
50
Flow in Steam Power Plant
Path of the steam• Steam is produced at high pressure from a boiler.• The steam from boiler first goes to super-heater• The super heated steam next goes to turbine.
– High pressure steam rushes through the blades ofthe turbine.
– The momentum of steam is transferred to theturbine.
• The turbine is coupled to the generator.• The turbine transfers its momentum to the generator
to produce electricity.
51
Flow in Steam Power Plant (Cont.)
Path of condensed steam/feed water• Low pressure steam is condensed and exhausted to the
condenser.
• Condensed steam I hot water is pumped the LPheater.
• Next, hot water is pumped to the HP heater. (Feedpump supplies additional water for steam andwater leakages).
• Next, hot water from HP heater goes to economizer.• Next, hot water from economizer goes to the boiler.
52
Cooling Water in Steam Power Plant
Open Circuit Systems• Wet steam comes to the condenser.• Wet steam is condensed when it comes in contact with the cool
water tubes of condenser.• The cool water after receiving heat from wet steam is taken to
river, lake etc. and discharged• Fresh water from river or lake is taken back to the condenser.
Closed Circuit Systems• Wet steam comes to the condenser.• Wet steam is condensed when it comes in contact with the cool
water tubes of condenser.• The cool water receiving heat from wet steam is taken to the
cooling tower and cooled again.• Cool water from tower is re-circulated back to the condenser.
53
Gas turbine power plant (GTPP)
Low and high pressure air compressor (LPC & HPC), High and low pressure gas turbine (HPT & LPT), Combustion chamber (CC)
54
Operation of a GTPP
1. Atmospheric air is passed through air filter.
2. Purified air is passed to the low-pressure compressor(LPC) - the air is compressed to a certain extent.
3. Air is passed to the intercooler - temperature of outletair from LPC is reduced in inter cooler.
4. Air is passed to the high-pressure compressor (HPC) -the air is again compressed to high pressure.
5. Air is passed through regenerator where it absorbs heatfrom outgoing exhaust gases.
6. High temperature, high pressure air is mixed with fueland burnt in combustion chamber (CC).
55
Operation of a GTPP
5. Burnt gas mixture first expands through the highpressure gas turbine (HPT) and rotates the turbine shaft.
6. Not all the heat and mechanical energy of burnt gas isutilized in running HPT. Therefore the gas is reheatedagain in combustion chamber (CC).
7. Burnt gas is again passed through low pressure turbine(LPT) where it rotates turbine shaft.
8. Exhaust gas of low-pressure turbine is sent through theregenerator to atmosphere.
9. In the regenerator the burnt gas heats the incoming airfrom HPC.
10.Since the turbines namely HPT and LPT are connectedto load useful work is done.
56
Advantages of Gas Turbine Power Plant
• Natural gas or any poor quality fuel which is widelyavailable can be used as fuel in gas turbine power plant.
• Gas turbines are widely used in aircrafts, ships wherethe weight and size are more important.
• Low initial cost compared to steam power plant.
• Quick starting of the plant.
• Low maintenance cost.
• It does not require heavy foundation and buildings.
• Speed is very high, (40,000 to 100,000 RPM).
57
Disadvantages of Gas Turbine Power Plant
• Net work output is low since a lot of the power is used tothe compressors.
• Special materials are required for the parts of power plant,since high temperature (2000oC) and high speeds (100000RPM) are involved.
• Part load efficiency is poor compared to diesel power plant.
• High pitch noise due to very high speed
• Special high temperature alloys are needed in thecombustion chamber and in the turbine to compensate forthe higher operating temperature.
• Large size exhaust duct due to increased requirement ofair for combustion and also for cooling.
Reciprocating ICE
• Internal combustion engine(ICE) operates on amechanical cycle becausethe piston system goes tothe same initial points.
• But not in thermodynamiccycle because new air andfuel enters the engine inorder to initiate thecombustion process.
• Internal cycle:
– Intake stroke
– Compression stroke
– Power stroke
– Exhaust stroke
Reciprocating ICE (Cont.)
60
Reciprocating ICE (Cont.)
• Bottom-dead center (BDC) – piston position where volume ismaximum
• Top-dead center (TDC) – piston position where volume is minimum
• Clearance volume – minimum cylinder volume (VTDC)
• Compression ratio (r) - is the ratio of volume at bottom dead centerdivided by volume at top dead center
• Displacement volume
• Four-stroke engine - piston executes intake, compression,expansion, and exhaust while crankshaft completes two revolutions
• Two-stroke engine - piston executes intake, compression,expansion, and exhaust while crankshaft completes one revolution
TDC
BDC
V
V
V
Vr
min
max
TDCBDCdisp VVV
Operation of Four Stroke Engine
Operation of Two Stroke Engine
62
Internal Combustion Engine - Otto Cycle
• Conceptualized by Nikolaus August Otto(1832 - 1891)
Actual and Ideal Otto Cycle
Air Standard Otto Cycle
This is an ideal cycle that assumes thatheat addition occurs instantaneouslywhile the piston is at TDC.Process(1-2) Isentropic Compression
Compression from ν1 => v2
↓ ↓BDC(β=180º ) TDC (θ=0º)
(2-3) Constant Volume heat input: QH
•While at TDC: umin
•Ignition of fuel
(3-4) Isentropic Expansion•Power is delivered while s = const.
(4-1) Isentropic Expansion•QL at umax=constant (BDC, θ=180º)
66
Otto Cycle Analysis
• Thermal efficiency
• Heat addition (process 2-3, v = const)
• Heat rejection (process 4-1, v = const)
in
out
in
outin
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netth
Q
Q
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Q
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