energy and the environment

Energy and the Environment

Fall 2014Instructor: Xiaodong Chu

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

mailto:[email protected]

Fossil-Fueled Power Plants: Components

• Gas turbine Gas turbine (燃气轮机 )– In a gas turbine plant where oiloil (石油 ), natural gasnatural gas (天然气 ) , or

synthesis gas synthesis gas (合成气 ) maybe used as a fuel, the hot combustion gases are directly used to drive a gas turbine

– Since cycle efficiencies of gas turbines are lower than those of steam plants and the fuel is more expensive, they are mostly used for peak load production or used in combined cyclecombined cycle (联合循环 ) mode

– Gas turbines operate on the principle of the Brayton cycle


• Gas turbine Gas turbine (燃气轮机 )– Single cycle or combined cyclecombined cycle (联合循环 )


• Gas turbine Gas turbine (燃气轮机 )– Gas turbinesGas turbines are of the reaction typereaction type (反力式 ), where bladesblades (叶片 )

form a converging nozzle nozzle (喷嘴 ) in which the combustion gases expand, thus converting enthalpyenthalpy ( 焓 ) to kinetic energykinetic energy (动能 )

– As in steam turbines, staged turbines are employed, consisting of several rows of moving and fixed bladesblades (叶片 )


• Gas turbine Gas turbine (燃气轮机 )– The working fluidworking fluid (工质 ) in gas turbinesgas turbines (燃气轮机 ), composed of

nitrogennitrogen ( 氮 ), excess oxygenexcess oxygen (超量氧气 ) , water vaporwater vapor (( 水气水气 )), and carbon dioxidecarbon dioxide (二氧化碳 ) , is not recycled into the compressor and combustion chamber but is, instead, vented into the atmosphere

– In some systems, a part of the energy still residing in the exhaust gasexhaust gas (排气 ) is recovered in heat exchangers heat exchangers (换热器 ) to heat up the air entering the combustion chamber combustion chamber (燃烧室 ) in order to enhance the overall thermal efficiency of the Brayton cycle, but eventually the exhaust gas is vented


• Gas turbine Gas turbine (燃气轮机 )


• CondenserCondenser (凝汽器，冷凝器 ) – In heat engine cycles, after performing useful work, the working fluid working fluid (工质 ) must reject heat to a cold body• As much 1.5 to 3 times of heat is rejected as the plant produces work in the form of

electricity

– Some of that heat is added to the environment by the condensing condensing system system (冷凝系统 ) in a steam cycle, and the rest is added by the discharge to the atmosphere of the hot flue gas flue gas (烟气 ) vented through the smoke stacksmoke stack (烟囱 )


• CondenserCondenser (冷凝器 ) – In the Rankine steam cycle, after expansion in the turbine, the steam

is first condensed into water in a condensercondenser (冷凝器 ) , then the water is recycled into the boiler by means of a feed pump feed pump (给水泵 )

– The circulating cooling water of the condensercondenser (冷凝器 ) rejects its heat to the atmosphere by means of a cooling tower or to a nearby surface water• The condensecondenser (冷凝器 ) serves not only the purpose of condensing the high-

quality feed water of the boiler, but also to lower the vapor pressure vapor pressure (蒸气压 ) of the condensate water condensate water (冷凝水 )

• By lowering the vapor pressure vapor pressure (蒸气压 ) , a vacuumvacuum (真空 ) is created which increases the power of the turbine


• CondenserCondenser (凝汽器 , 冷凝器 ) – Direct contact condenser Direct contact condenser (直接接触式冷凝器 )

• The turbine exhaustexhaust (排气 ) passes an array of spray nozzlesspray nozzles (喷头 ) through which cooling water is sprayed, condensing the steam by direct contact

• The warm condensate (凝结水 ) is pumped into a cooling tower (冷却塔 ) where air cools the condensate that flows in tubes

• The cooled condensate (凝结水 ) is recycled into spray nozzlesspray nozzles (喷头 )


• CondenserCondenser (冷凝器 ) – Surface contact condenser Surface contact condenser (壳管式冷凝器 )

• The turbine exhaustexhaust (排气 ) passes an array of tubes in which the cooling water flows

• The contact area should be large enough and the tubes are surrounded by finsfins (翅片 ) to increase the heat transfer heat transfer (传热 ) area

• The incoming steam needs to be deaerateddeaerated (除氧 ) of noncondensablesnoncondensables (非冷凝气 ), mainly air that leaked into the system, which would be corrosive and increase the condensercondenser (冷凝器 ) pressure and thereby reduce the steam turbine power


• Cooling tower Cooling tower (冷却塔 )– The bulk of the heat rejection heat rejection (排热 ) from the steam cycle occurs

either to a surface watersurface water (地表水 ) or to the atmosphere– It is mandated that heat rejection occur into the atmosphere via

cooling towers cooling towers (冷却塔 ) to prevent from thermal pollution thermal pollution (热污染 ) by the direct discharge of hot water to the surface watersurface water (地表水 )


• Cooling tower Cooling tower (冷却塔 )– Wet cooling towerWet cooling tower (湿式冷却塔 )

• Hot water from the condensercondenser (冷凝器 ) is sprayed over a latticeworklatticework (网格 ) of closely spaced slatsslats or barsbars (条板 ), called fillfill (填料 )

• Outside air is drawn in through louverslouvers (百叶窗 ) in the bottom of the tower by natural draft

• Heat is transferred from the cooling water to the air, and the water is further cooled by evaporationevaporation (蒸发 ) of part of the water

• Cold water falls by gravity into the collecting basin from whence it is recirculated into the condensercondenser (冷凝器 )

• A mist eliminator mist eliminator (除雾器 ) is placed above the fillfill (填料 )


• Cooling tower Cooling tower (冷却塔 )– Dry cooling tower Dry cooling tower (空冷塔 )

• The recirculating water flows through finned tubes finned tubes (翅片管 ) over which cold air is drawn

• The advantages are that no water evaporates evaporates (蒸发 ), and a dry tower is less expensive to maintain

• The disadvantages are that it is more expensive to build and the back pressureback pressure (背压 ) on the turbines is higher, causing plant efficiency loss


• GeneratorGenerator (发电机 )– GeneratorGenerator (发电机 ) is where electricity is generated– Compared to the boilerboiler (锅炉 ), turbinesturbines (汽轮机 , 燃气轮机 ),

condensercondenser (冷凝器 ) , cooling tower cooling tower (冷却塔 ), and other auxiliary equipment, the generator occupies a small fraction of the total plant outlay


• Emission controlEmission control (排放控制 )– If left uncontrolled, power plants can emit quantities of air pollutants

that cause ambient pollutant ambient pollutant (环境污染 ) levels to exceed standards designed to protect human health and the environment

– In most countries, environmental regulations require that the operator of the power plant install emission control devices for these pollutants• These devices contribute significantly to the capital and operating cost of the plant,

and reduce to some degree the thermal efficiency thermal efficiency (热效率 ), because the devices draw some of the power output of the plant


• Emission controlEmission control (排放控制 )– Control of products of incomplete combustion (PIC)products of incomplete combustion (PIC) (不完全燃烧产

物 ) and carbon monoxide (CO) carbon monoxide (CO) (一氧化碳 )• The control of PIC and CO is relatively easy to accomplish• If the fuel and air are well-mixed, as is the case in modern burners burners (燃烧器 ), and

the fuel is burnt in excess air, the flue gas flue gas (烟气 ) will contain very little PIC and CO• PIC and CO emissions do occur occasionally, especially during start-upsstart-ups (起始 ) and

component breakdowns breakdowns (故障 ), when the flame flame (火焰 ) temperature and fuel–air mixture is not optimal

• These occurrences should be rare and should not contribute significantly to ambient concentrations of these pollutants


• Emission controlEmission control (排放控制 )– Particle controlParticle control (颗粒物控制 )

• Particles, also called particulate matter (PM) particulate matter (PM) (颗粒物 ), would be the predominant pollutant emanating from power plants were it not controlled at the source

• This stems from the fact that coal, and even oil, contains a significant fraction by weight of incombustible mineral matter mineral matter (矿物质 )

• In modern pulverized coal-fired plantspulverized coal-fired plants (燃煤粉发电厂 ) the majority of the mineral matter is blown out from the boiler as fly ashfly ash (飞灰 ), containing (a) a host of toxic toxic metalsmetals (有害金属 ), such as arsenicarsenic ( 砷 ), seleniumselenium ( 硒 ), cadmiumcadmium ( 镉 ), manganesemanganese ( 锰 ), chromiumchromium ( 铬 ), leadlead ( 铅 ), and mercurymercury ( 汞 ), and (b) nonvolatile organic matternonvolatile organic matter (非挥发性有机物 )including polycyclic aromatic polycyclic aromatic hydrocarbons (PAHs) hydrocarbons (PAHs) (多环芳烃 ); these would pose a public health and environmental risk if emitted into the atmosphere.

• Most countries instituted strict regulations on particle emissions from power plants• Control technologies include electrostatic precipitatorselectrostatic precipitators (ESP) (ESP) (静电除尘器 ) and

fabric filtersfabric filters (袋式除尘器 )



• Electrostatic precipitatorsElectrostatic precipitators (静电除尘器 ) works on the principle of charging particles negatively by a corona discharge corona discharge (电晕放电 ) and attracting the charged particles to a grounded plate

• The collection efficiency of an ESP depends on many factors, primarily the particle diameter (the smaller the diameter, the less the efficiency), the plate area, the volumetric flow rate volumetric flow rate (容积流速 ) of the flue gas flue gas (烟气 ) passing between the plates, and the particle migration particle migration (颗粒运移 ) speed toward the plates



• Fabric filtersFabric filters (袋式除尘器 ) works on the principle of a domestic vacuum cleaner vacuum cleaner (真空吸尘器 ), where particle-ladenparticle-laden (带粒 ) gas is sucked into a fabric bag fabric bag (布袋 ), the particles are filtered out, and the clean gas is vented into the atmosphere

• Fabric filters Fabric filters (袋式除尘器 ) with a high collection efficiency for small particles are expected to have a higher capital and operating cost than ESPs


• Emission controlEmission control (排放控制 )– Sulfur control Sulfur control (硫排放控制 )

• As living organisms contain sulfur in their cellular make-up, this sulfur is mostly retained in the fossilized remnants fossilized remnants (化石遗迹 ) of these organisms

• Without sulfur emission control sulfur emission control (硫排放控制 ) devices, the oxidized sulfur, mainly sulfur dioxide SO2, minor quantities of SO3, and sulfuric acid H2SO4, would be emitted through the smoke stack smoke stack (烟囱 ) into the environment

• The oxides of sulfur are precursors to acid depositionacid deposition (酸沉降 ) and visibility visibility impairing haze impairing haze (降低能见度的霾 )

• Because coal-fired power plants emit the majority of all sulfur oxide emissions worldwide, operators of these plants are required to limit the emissions by either switching to low-sulfur containing fuels or installation of sulfur emission control sulfur emission control (硫排放控制 ) devices

• There are basically three approaches to reducing sulfur emissions: before, during, and after combustion of the fossil fuel



• Sulfur control Sulfur control (硫排放控制 ) before combustion can be realized by coal washing coal washing (洗煤 ), coal gasificationcoal gasification (煤碳气化 ) , and oil desulfurizationoil desulfurization (燃油脱硫 )

• Coal washing Coal washing (洗煤 ) is usually performed at the mine mouth mine mouth (坑口 ), where the crushed raw coal is floated in a stream of water with the lighter coal particles floating on top and the heavier mineralsminerals (矿物质 ) sinking to the bottom, and then the wet coal particles are transferred to a dewatering device dewatering device (脱水装置 )

• In the process of coal gasificationcoal gasification (煤碳气化 ) , coal can be converted by a chemical process into a synthesis gas synthesis gas (合成气 ), in which most of the sulfur can be eliminated before the final stage of gasification

• Oil desulfurizationOil desulfurization (燃油脱硫 ) is usually done in a catalytic reduction–oxidation catalytic reduction–oxidation (催化还原与氧化 ) process called the Claus process



• Fluidized bed combustion (FBC) Fluidized bed combustion (FBC) (流化床燃烧 ) is a sulfur control sulfur control (硫排放控制 ) method during combustion, which is the burning of coal (or any other solid fuel) imbedded in a granular material, usually limestonelimestone (石灰石 ) , riding on a stream of air

• The admixed limestonelimestone (石灰石 ) acts as a sorbentsorbent ( 钙 ) , extracting sulfur and other impurities from the fuel

• The primary aim of the development of FBCFBC (流化床燃烧 ) was not specifically to reduce SO2 emissions, but rather to enable the combustion of all sorts of fuel, including nonpulverizable coalnonpulverizable coal (非粉煤 ), municipal solid wastemunicipal solid waste (城市生活垃圾 ), industrial and medical waste, wood, tar tar (焦油 ), and asphaltene asphaltene (沥青质 )

• FBCFBC’s (流化床燃烧 ) sulfur emission reduction capacity is limited and is difficult to be used for large-scale centralized power plants



• The FBC is a cylindrical retort cylindrical retort (圆甑 ) with a grategrate (炉排 ) in the bottom, over which crushed coal (6–20 mm in size) together with limestonelimestone (CaCO3) (石灰石 ) is blown pneumatically blown pneumatically (气吹 ) and beneath which combustion air is blown from

• After ignition, the burning coal–limestonecoal–limestone ( 煤–石灰石 ) mixture floats over the grate, riding on an air cushion air cushion (气垫层 ) and acting dynamically like a fluid, hence the name fluidized bed combustion

• Boiler tubes are immersed into the fluidized bed where there is direct heat transferheat transfer (传热 ) from the bed to the tubes and another bank of boiler tubes is mounted above the bed, to which the heatheat transfer transfer occurs by convectionconvection (对流 ) and radiation radiation (辐射 ) from the flue gases flue gases (烟气 )



• In a FBC FBC, the sulfur is captured by the sorbentsorbent ( 钙 ) to form a mixture of calcium calcium sulfite (CaSOsulfite (CaSO33) and calcium) and calcium sulfate (CaSOsulfate (CaSO44) ) (亚硫酸钙和硫酸钙 ) particles

• The particles together with unreacted CaCOCaCO33 (石灰石 ) and unburnt coal particles are carried with the flue gases flue gases (烟气 ) into a cyclonecyclone (旋风分离器 )

• The larger particles are separated in the cyclonecyclone (旋风分离器 ) and are recycled into the fluidized bedfluidized bed (流化床 ) for reburning, and smaller particles exit the cyclone cyclone (旋风分离器 ) are removed in an electrostatic precipitator electrostatic precipitator (静电除尘器 ) or fabric filter fabric filter (袋式除尘器 )



• The removal of sulfur oxides from the flue gas flue gas (烟气 ) after combustion of the fuel in a furnace or boiler is called flue gas desulfurization (FGD) flue gas desulfurization (FGD) (烟气脱硫 ) including several methods: sorbent injection sorbent injection (喷钙 ) and wet scrubbers wet scrubbers (湿式除尘器 ) and dry dry scrubbersscrubbers (干式除尘器 )

• In sorbent injection (SI)(SI) (喷钙 ) , a sorbentsorbent ( 钙 ), usually dry sinteredsintered (烧结的 ) CaCO3 or CaO, or a slurryslurry ( 浆 ) is injected into the flue gas flue gas (烟气 ) in the upper reaches of the boiler, and the sorptionsorption (吸附 ) of SO2 proceeds similarly as in FBC by forming a mixture of calcium sulfitecalcium sulfite andand sulfatesulfate (亚硫酸钙和硫酸钙 )

• In a wet scrubbers (湿式除尘器 ) the flue gas flue gas (烟气 ) is treated with an aqueous aqueous slurry slurry (浆液料 ) of the sorbent, usually limestonelimestone (CaCO(CaCO33)) (石灰石 ) or calcined lime calcined lime (CaO) (生石灰 ), in a separate tower

• The chemical reaction mechanism in the dry scrubbers (干式除尘器 ) is similar to that in the wet scrubbers wet scrubbers (湿式除尘器 ) and the difference is that in the dry scrubberdry scrubber (干式除尘器 ) the sorbentsorbent ( 钙 )is introduced as a very fine spray of an aqueous slurry aqueous slurry (浆液料 )



• The water molecules that are attached to calcium sulfitecalcium sulfite andand sulfatesulfate (亚硫酸钙和硫酸钙 ) are called water of crystallizationwater of crystallization (结晶水 )

• Hydrated calcium sulfate Hydrated calcium sulfate (水合硫酸钙 ) is similar to natural gypsum natural gypsum (天然石膏 )

3 2 2 3 2 2

1 1CaCO +SO + H O CaSO H O+CO

2 2

3 2 2 2 4 2

1 3 1CaSO H O H O+ O CaSO 2H O

2 2 2

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