Energy and the Environment

Energy and the Environment Fall 2013 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

Flashbacks of Last Lecture • 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 work on the principle of a heat or combustion engine, converting fossil fuel chemical energy first into mechanical energy, then into electrical energy

Flashbacks of Last Lecture • Burner (燃烧器) • The role of the burner is to provide a thorough mixing of the fuel and air so that the fuel is a completely burned • In the combustion chamber (燃烧室) a pulverized coal particle (煤粉颗粒) or atomized oil droplet(雾化油滴) burns in a fraction of a second, leaving behind incombustible mineral matter(不可燃矿物质) that is called ash(灰) • In modern pulverized coal and atomized oil fired power plants, more than 90% of the mineral matter forms the so-called fly ash (飞灰), which is blown out of the boiler by forced or natural draft and is later captured in particle collectors (除尘器) • About 10% of the mineral matter falls to the bottom of the boiler as bottom ash (底灰)

Flashbacks of Last Lecture • Boiler (锅炉) • Most modern boilers (锅炉) are of the water wall (水冷壁) type, in which the boiler walls are almost entirely constructed of vertical tubes that either carry feed water(给水) into the boiler or carry steam out of the boiler • In a modern water wall boiler the furnace (燃炉) and the various compartments of the boiler are fully integrated

Flashbacks of Last Lecture • Steam turbine(蒸汽轮机) • Impulse turbine (冲击式汽轮机) • Reaction turbine (反力式汽轮机)

Fossil-Fueled Power Plants: Components • Gas turbine (燃气轮机) • In a gas turbine plant where oil(石油), natural gas (天然气), or 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 cycle(联合循环) mode • Gas turbines operate on the principle of the Brayton cycle

Fossil-Fueled Power Plants: Components • Gas turbine (燃气轮机) • Single cycle or combined cycle(联合循环)

Fossil-Fueled Power Plants: Components • Gas turbine (燃气轮机) • Gas turbines are of the reaction type(反力式), where blades (叶片)form a converging nozzle (喷嘴)in which the combustion gases expand, thus converting enthalpy (焓)to kinetic energy(动能) • As in steam turbines, staged turbines are employed, consisting of several rows of moving and fixed blades (叶片)

Fossil-Fueled Power Plants: Components • Gas turbine (燃气轮机) • The working fluid(工质)in gas turbines(燃气轮机), composed of nitrogen(氮), excess oxygen(超量氧气), water vapor(水气), and carbon 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 gas(排气)is recovered in heat exchangers (换热器)to heat up the air entering the combustion chamber (燃烧室)in order to enhance the overall thermal efficiency of the Brayton cycle, but eventually the exhaust gas is vented

Fossil-Fueled Power Plants: Components • Gas turbine (燃气轮机)

Fossil-Fueled Power Plants: Components • Condenser (凝汽器，冷凝器) • In heat engine cycles, after performing useful work, the 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 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 (烟气)vented through the smoke stack(烟囱)

Fossil-Fueled Power Plants: Components • Condenser (冷凝器) • In the Rankine steam cycle, after expansion in the turbine, the steam is first condensed into water in a condenser (冷凝器) , then the water is recycled into the boiler by means of a feed pump (给水泵) • The circulating cooling water of the condenser (冷凝器) rejects its heat to the atmosphere by means of a cooling tower or to a nearby surface water • The condenser (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 (蒸气压) of the condensate water (冷凝水) • By lowering the vapor pressure (蒸气压) , a vacuum (真空) is created which increases the power of the turbine

Fossil-Fueled Power Plants: Components • Condenser (凝汽器, 冷凝器) • Direct contact condenser (直接接触式冷凝器) • The turbine exhaust(排气) passes an array of spray 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 nozzles(喷头)

Fossil-Fueled Power Plants: Components • Condenser (冷凝器) • Surface contact condenser (壳管式冷凝器) • The turbine exhaust (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 fins(翅片) to increase the heat transfer (传热)area • The incoming steam needs to be deaerated (除氧) of noncondensables (非冷凝气), mainly air that leaked into the system, which would be corrosive and increase the condenser (冷凝器) pressure and thereby reduce the steam turbine power

Fossil-Fueled Power Plants: Components • Cooling tower (冷却塔) • The bulk of the heat rejection (排热) from the steam cycle occurs either to a surface water(地表水) or to the atmosphere • It is mandated that heat rejection occur into the atmosphere via cooling towers (冷却塔) to prevent from thermal pollution (热污染) by the direct discharge of hot water to the surface water (地表水)

Fossil-Fueled Power Plants: Components • Cooling tower (冷却塔) • Wet cooling tower (湿式冷却塔) • Hot water from the condenser (冷凝器) is sprayed over a latticework (网格) of closely spaced slats or bars (条板), called fill(填料) • Outside air is drawn in through louvers (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 evaporation (蒸发) of part of the water • Cold water falls by gravity into the collecting basin from whence it is recirculated into the condenser (冷凝器) • A mist eliminator (除雾器) is placed above the fill(填料)

Fossil-Fueled Power Plants: Components • Cooling tower (冷却塔) • Dry cooling tower (空冷塔) • The recirculating water flows through finned tubes (翅片管) over which cold air is drawn • The advantages are that no water 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 pressure (背压)on the turbines is higher, causing plant efficiency loss

Fossil-Fueled Power Plants: Components • Generator (发电机) • Generator (发电机) is where electricity is generated • Compared to the boiler (锅炉), turbines (汽轮机, 燃气轮机), condenser (冷凝器) , cooling tower (冷却塔), and other auxiliary equipment, the generator occupies a small fraction of the total plant outlay

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • If left uncontrolled, power plants can emit quantities of air pollutants that cause 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 (热效率), because the devices draw some of the power output of the plant

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Control of products of incomplete combustion (PIC) (不完全燃烧产物) and 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 (燃烧器), and the fuel is burnt in excess air, the flue gas (烟气) will contain very little PIC and CO • PIC and CO emissions do occur occasionally, especially during start-ups (起始) and component breakdowns (故障), when the flame (火焰) temperatureand fuel–air mixture is not optimal • These occurrences should be rare and should not contribute significantly to ambient concentrations of these pollutants

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Particle control (颗粒物控制) • Particles, also called 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 (矿物质) • In modern pulverized coal-fired plants(燃煤粉发电厂) the majority of the mineral matter is blown out from the boiler as fly ash(飞灰), containing (a) a hostof toxic metals(有害金属), such as arsenic(砷), selenium(硒), cadmium(镉), manganese(锰), chromium(铬), lead(铅), and mercury(汞), and (b) nonvolatile organic matter(非挥发性有机物)including polycyclic aromatic 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 precipitators(ESP) (静电除尘器) and fabric filters(袋式除尘器)

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Particle control (颗粒物控制) • Electrostatic precipitators(静电除尘器) works on the principle of charging particles negatively by a 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 (容积流速) of the flue gas (烟气) passing between the plates, and the particle migration (颗粒运移) speed toward the plates

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Particle control (颗粒物控制) • Fabric filters(袋式除尘器) works on the principle of a domestic vacuum cleaner (真空吸尘器), where particle-laden (带粒) gas is sucked into a fabric bag (布袋), the particles are filtered out, and the clean gas is vented into the atmosphere • 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

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • 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 (化石遗迹) of these organisms • Without 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 (烟囱) into the environment • The oxides of sulfur are precursors to acid deposition(酸沉降)and visibility 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 (硫排放控制) devices • There are basically three approaches to reducing sulfur emissions: before, during, and after combustion of the fossil fuel

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Sulfur control (硫排放控制) • Sulfur control (硫排放控制) before combustion can be realized by coal washing (洗煤), coal gasification (煤碳气化), and oil desulfurization(燃油脱硫) • Coal washing (洗煤) is usually performed at the 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 minerals (矿物质) sinking to the bottom, and then the wet coal particles are transferred to a dewatering device (脱水装置) • In the process of coal gasification (煤碳气化), coal can be converted by a chemical process into a synthesis gas (合成气), in which most of the sulfur can be eliminated before the final stage of gasification • Oil desulfurization(燃油脱硫)is usually done in a catalytic reduction–oxidation (催化还原与氧化) process called the Claus process

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Sulfur control (硫排放控制) • Fluidized bed combustion (FBC) (流化床燃烧) is a 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 limestone(石灰石) , riding on a stream of air • The admixed limestone(石灰石) acts as a sorbent(钙) , extracting sulfur and other impurities from the fuel • The primary aim of the development of FBC (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 coal(非粉煤), municipal solid waste(城市生活垃圾), industrial and medical waste, wood, tar (焦油), and asphaltene (沥青质) • FBC’s (流化床燃烧) sulfur emission reduction capacity is limited and is difficult to be used for large-scale centralized power plants

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Sulfur control (硫排放控制) • The FBC is a cylindrical retort (圆甑) with a grate(炉排) in the bottom, over which crushed coal (6–20 mm in size) togetherwith limestone (CaCO3) (石灰石) is blown pneumatically (气吹) and beneath which combustion air is blown from • After ignition, the burning coal–limestone(煤–石灰石) mixture floats over the grate, ridingon an air cushion (气垫层) andacting dynamically like a fluid, hence the name fluidized bed combustion • Boiler tubes are immersed into the fluidized bed where there is direct heat transfer(传热) from the bed to the tubes and another bank of boiler tubes is mounted above the bed, to which the heattransfer occurs by convection (对流) and radiation (辐射) from the flue gases (烟气)

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Particle control (颗粒物控制) • In a FBC, the sulfur is captured by the sorbent(钙) to form a mixture of calcium sulfite (CaSO3) and calciumsulfate (CaSO4) (亚硫酸钙和硫酸钙) particles • The particles together with unreacted CaCO3(石灰石) and unburnt coal particles are carried with the flue gases (烟气) into a cyclone (旋风分离器) • The larger particles are separated in the cyclone (旋风分离器)and are recycled into the fluidized bed (流化床) for reburning, and smaller particles exit the cyclone (旋风分离器) areremoved in an electrostatic precipitator (静电除尘器) or fabric filter (袋式除尘器)

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Sulfur control (硫排放控制) • The removal of sulfur oxides from the flue gas (烟气) after combustion of the fuel in a furnace or boiler is called flue gas desulfurization (FGD) (烟气脱硫) including several methods: sorbent injection (喷钙) and wet scrubbers (湿式除尘器) and dry scrubbers(干式除尘器) • In sorbent injection (SI)(喷钙) , a sorbent(钙), usually dry sintered (烧结的) CaCO3 or CaO, or a slurry (浆) is injected into the flue gas (烟气) in the upper reaches of the boiler, and the sorption (吸附) of SO2 proceeds similarly as in FBC by forming a mixture of calcium sulfiteandsulfate(亚硫酸钙和硫酸钙) • In a wet scrubbers(湿式除尘器) the flue gas (烟气) is treated with an aqueous slurry (浆液料) of the sorbent, usually limestone(CaCO3) (石灰石) or 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 (湿式除尘器) and the difference is that in the dry scrubber (干式除尘器) the sorbent (钙)is introduced as a very fine spray of an aqueous slurry (浆液料)

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Sulfur control (硫排放控制) • The water molecules that are attached to calcium sulfiteandsulfate(亚硫酸钙和硫酸钙) are called water of crystallization(结晶水) • Hydrated calcium sulfate (水合硫酸钙) is similar to natural gypsum (天然石膏)

Energy and the Environment