Energy and the Environment Fall 2015 Instructor: Xiaodong Chu Email ： [email protected]...

Energy and the Environment

Fall 2015Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127, 13573122659

mailto:[email protected]

Fossil-Fueled Power Plants: Introduction

• Fossil-fueled electric power plants worldwide consume more than half of the annual supply of fossil fuel, of which most is in the form of coal– Fossil-fueled power plants are major contributors to the anthropogenic anthropogenic

emission emission （人为排放）（人为排放） of CO2 and other pollutants, such as SO2, NOx, products of incomplete combustion, and particulate matter

• Fossil-fueled power plants work on the principle of a heat or combustion engine, converting fossil fuel chemical energy first into mechanical energy, then into electrical energy– Most large-scale power plants use the Rankine steam cycle providing the

base load for a regional grid in combination with nuclear plants– Peak loads are sometimes supplied by gas turbine plants that work on the

Brayton cycle

Fossil-Fueled Power Plants: Introduction

• The relatively low thermal efficiency of power plants is due to two factors– The first is a consequence of the second law of thermodynamics,

whereby in a heat engine cycle, after performing useful work, the residual of the fuel heat needs to be rejected to a cold reservoir, usually a surface water or the atmosphere via a cooling tower

– The second factor is due to parasitic heat lossesparasitic heat losses （寄生热损失）（寄生热损失） through walls and pipes, frictional lossesfrictional losses （摩擦损失）（摩擦损失） , and residual residual heat escapingheat escaping （余热逃逸）（余热逃逸） with the flue gas flue gas （烟气）（烟气） into the atmosphere

Fossil-Fueled Power Plants: Components

• Major components of a fossil fueled power plant – Fuel storage and preparationFuel storage and preparation （燃料存储与制备）（燃料存储与制备）– BurnerBurner （燃烧器）（燃烧器）– BoilerBoiler （锅炉）（锅炉）– Steam turbineSteam turbine （汽轮机）（汽轮机）– Gas turbineGas turbine （燃气轮机）（燃气轮机）– CondenserCondenser （冷凝器，凝汽器）（冷凝器，凝汽器）– Cooling towerCooling tower （冷却塔）（冷却塔）– GeneratorGenerator （发电机）（发电机）– Emission controlEmission control （排放控制）（排放控制）


• Fuel storage and preparation– Coal-fired power plants

• Coal is delivered to a power plant by rail or, in the case of coastal or riverine plants, by ship or bargebarge（驳船）（驳船）

• A power plant usually has several weeks of coal supply on site and some power plants are situated right near coal mines, mine-mouth plantsmine-mouth plants （坑口电厂）（坑口电厂）

• When coal arrives, it is usually carried by a train consisting of wagons that are emptied by a rotary dumprotary dump （旋转翻车机）（旋转翻车机） , and the coal is carried by conveyorsconveyors（输送机）（输送机） to a stockpilstockpil （堆场）（堆场） , or directly to the power plant

• Coal is delivered to a plant already sized to meet the feed size of the pulverizing pulverizing millmill （磨煤机）（磨煤机） , in the order of a few to ten centimeters per coal lump

• The pulverizing mill reduces the coal lumps to particles smaller than 1 millimeter and the pulverized coal is stored in vertical silossilos（筒仓）（筒仓） whence it is blown pneumatically into the burnersburners （燃烧器）（燃烧器）


• Fuel storage and preparation– Oil-fired power plants

• Oil is stored in large tanks, to which oil is delivered either by pipeline, by railroad tankers, or by tanker ship or barge

• Power plants like to have at least a 30-day supply of oil in their tanks• The oil is purchased from refineries in the form it is combusted in the burners, with

specified sulfur, nitrogen, and ash content as well as other properties, such as viscosityviscosity （粘度）（粘度） and vapor pressurevapor pressure （蒸气压）（蒸气压）


• Fuel storage and preparation– Natural gas-fired power plants

• Gas is delivered to the power plant by pipeline at high pressure (compressed natural gas, CNG)

• Some gas-fired power plants use liquefied natural gas (LNG)• The LNG is stored in refrigerated tanksrefrigerated tanks （冷冻罐）（冷冻罐） until used


• Burner– The role of the burner is to provide a thorough mixing of the fuel and

air so that the fuel is completely burned– In the combustion chamber a pulverized coal particlepulverized coal particle （煤粉颗粒）（煤粉颗粒）

or atomized oil dropletatomized oil droplet （雾化油滴）（雾化油滴） burns in a fraction of a second, leaving behind incombustible mineral matter that is called ash

• In modern pulverized coal and atomized oil fired power plants, more than 90% of the mineral matter forms the so-called fly ashfly ash （飞灰）（飞灰） , which is blown out of the boiler by forced or natural draft and is later captured in particle collectorsparticle collectors （除尘（除尘器）器）

• About 10% of the mineral matter falls to the bottom of the boiler as bottom ashbottom ash （（底灰）底灰）

– Coal burns relatively slowly, oil burns faster, and gas burns the fastest • For complete combustion, excess air is delivered—that is, more air than is required

by a stoichiometric balance of fuel and the oxygen content of air


• Burner– Pulverized coal burner


• Boiler– Most modern boilers are of the water wall type, in which the boiler

walls are almost entirely constructed of vertical tubes that either carry feed water into the boiler or carry steam out of the boiler

– In a modern water wall boiler the furnace and the various compartments of the boiler are fully integrated


water wall


• Boiler– Water wall boilerWater wall boiler （水冷壁锅炉）（水冷壁锅炉）


• Boiler– Water wall boiler

• Water from the high pressure feed water heaterfeed water heater（给水加热器）（给水加热器） at a temperature of 230–260oC is further heated in the economizereconomizer（省煤器）（省煤器） section of the boiler to 315oC, then flows into the steam drumsteam drum（汽包）（汽包） , which is mounted on top of the boiler

• In the steam drum liquid water is separated from the steam, and liquid water flows down the downcomer tubesdowncomer tubes（下降管）（下降管） into the headerheader（联箱）（联箱） , where the hot pressurized water flows upward through the riser tubesriser tubes（上升管）（上升管） , where the actual boiling of water into steam occurs

• The separated steam passes the superheatersuperheater（过热器）（过热器） , where its temperature is raised to 565 oC at a pressure of 24 Mpa, higher than the critical temperature (Tc = 374 oC) and pressure (pc = 22 MPa) of water

• The supercritical steamsupercritical steam（超临界蒸汽）（超临界蒸汽） drives the high-pressure turbine• The exhaust steamexhaust steam（排汽）（排汽） from the high-pressure turbine flows through the

reheaterreheater（再热器）（再热器） , where the temperature is raised again to about 500 oC at a pressure of 3.7 Mpa, driving the low-pressure turbine


• Boiler– Water wall boiler

• Near the burners, heat is transferred from the combustion gases to the boiler tubes by radiationradiation （辐射）（辐射） and away from the burners, heat is transferred mainly by convectionconvection （对流）（对流）

• Most of the radiative transfer of heat from all flames occurs in the nonvisible infrared portion of the spectrum


steam drum


• Steam turbine– The steam turbine is the most complex piece of machinery in the

power plant– The steam turbine must face problems of the high pressure and

temperature of the steam, the enormous centrifugal stressescentrifugal stresses （离心（离心应力）应力） on the shaft, and the fact that steam condenses to water while expanding in the turbine (creating a two phase fluid flow)


• Steam turbine– Impulse turbineImpulse turbine （冲动式汽轮机）（冲动式汽轮机）

• In an impulse turbine a jet of steam impinges on the blades of a turbine• Steam coming from the superheater, when expanded through a nozzlenozzle（喷嘴）（喷嘴）

will have a linear velocity of about 1650 m/s• To utilize the full kinetic energy of the steam, the blade velocity should be about

820 m/s, generating unsustainable centrifugal stresses in the rotor• Turbines usually employ compounding or staging, where two or more rows of

moving blades (rotors) are separated by rows of stationary blades (stators) and each pair of stator and rotor blades is called a stagestage （级）（级）

• When the steam kinetic energy is divided among n stages, the linear blade velocity of the rotors will be 1/2n that of a single rotor


• Steam turbine– Impulse turbine


• Steam turbine– Impulse turbine

• The force exerted on a rotor

• The power generated by the blade

• The maximum power

)( bs vvmF

bFvP

4/2max svmP


• Steam turbine– Reaction turbineReaction turbine （反动式汽轮机）（反动式汽轮机）

• A reaction turbine consists of rows of moving (rotor) and stationary (stator) blades• Within the converging blades the steam pressure, density and temperature decline

while converting its enthalpy to kinetic energy• The steam pressure drops steadily through all rows of blades, moving and

stationary, but the steam velocity oscillates, depending on location within the blade


• Steam turbine– Reaction turbine


• Steam turbine– Reaction turbine

• The optimum blade velocity

• The maximum power cos, soptb vv

2,max optbvmP


• Turbine Components - Blades

Impulse Reaction


• Turbine Rotor


• Turbine – Multiple Sets


• Steam turbine– Impulse turbines are usually used for high-pressure steam, and

reaction turbines are usually used for intermediate and low-pressure steam

– In both the impulse and reaction turbines, efficiency losses are due to supersaturation, fluid friction, leakage, and heat transfer losses

– The combined efficiency losses and auxiliary power amount to 10–20%; that is, turbines convert only 80–90% of the available steam enthalpy into mechanical energy that drives the generator

Energy and the Environment Fall 2015 Instructor: Xiaodong Chu Email ： [email protected]...

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Transcript of Energy and the Environment Fall 2015 Instructor: Xiaodong Chu Email ： [email protected]...