Energy and the Environment

Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

Flashbacks of Last Lecture • 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 • Gas turbines operate on the principle of the Brayton cycle • 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) • 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

Flashbacks of Last Lecture • Condenser (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)

Flashbacks of Last Lecture • 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)

Flashbacks of Last Lecture • 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

Flashbacks of Last Lecture • 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

Flashbacks of Last Lecture • 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)

Flashbacks of Last Lecture • 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) • Nitrogen oxide control (氮氧化物控制) • The other major category of pollutants from fossil fuel combustion is nitrogen oxides(氮氧化物),called NOx, which includes nitric oxide NO(一氧化氮), nitrogen dioxideNO2 (二氧化氮)(and its dimer (二聚体)N2O4), nitrogentrioxideNO3 (三氧化氮), pentoxideN2O5(五氧化二氮), and nitrous oxide N2O(一氧化二氮) • NOx usually implies the sum of NO and NO2. • NOx is a pernicious pollutant (有害污染物) because it is a respiratory tract irritant (呼吸道刺激源) and it is a precursor (前驱体) tophoto-oxidants (光氧化剂), including ozone(臭氧) and acid deposition (酸沉降) • Coal and oil contain organic nitrogen in their molecular structureand when burnt, these fuelsproduce fuel Nox (燃料型氮氧化物) • All fossil fuels produce thermal Nox(热力型氮氧化物), resulting from the recombination of atmospheric nitrogen and oxygen under conditions of the high temperatures prevailing in the flame of fossil fuel combustion • There are basically two approaches to reducing sulfur emissions: duringand after combustion of the fossil fuel

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Nitrogen oxide control (氮氧化物控制) • During combustion, a low-Nox burner (LNB) employs a process called staged combustion (分级燃烧) • NOx formation is a function of air-to-fuel ratio (by weight) in the flame

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Nitrogen oxide control (氮氧化物控制) • Fuel (e.g., pulverized coal) and air is injected through the central annulus (环)of the burner and the air/fuel ratio is less than stoichiometric (化学计量比), i.e., fuel-rich (富燃), which produces a luminous flame (发光火焰), with some of the pulverized coal left unburnt, but also with low NOx formation • Secondary and tertiary air arrives through outer annuli (环), creating an outer flame envelope that is fuel-lean (贫燃), where all the unburnt carbon burns up and less NOx is formed

Fossil-Fueled Power Plants: Components • Emission control(排放控制) • Nitrogen oxide control (氮氧化物控制) • After combustion, in a selective catalytic reduction (选择性催化还原) process either ammonia (氨) or urea (尿素) is injected into a catalytic reactor (催化反应器) through which the flue gas (烟气) flows • In a selective noncatalytic reduction (选择性非催化还原) , a reduction of NO can be accomplished at a higher temperature without a catalyst, where urea is used instead of ammonia

Fossil-Fueled Power Plants: Components • Emission control (排放控制) • Toxic emissions (毒性排放物) • Coal and oil contain mineral matter that during the combustion process may produce toxic vapors and particles • Particles larger than 1–2 microns (微米) in diameter are almost all captured by the particle removal system, either by an electrostatic precipitator or a fabric filter • Smaller particles and vapors may escape through the smoke stack and pollute the environment • Mercury (汞) , selenium (硒) , cadmium (镉) , and arsenic (砷) are semivolatile(半挥发性) toxic metals that in part may escape through the smoke stack as vapors • Another problem associated with coal-fired power plants is radon (氡) emissions, which is a disintegration product of uranium (铀), the minerals of which may cling to the coal

Fossil-Fueled Power Plants: Components • Waste disposal (废物处理) • Coal-fired power plants produce a significant amount of solid waste • Oil-fired plants produce much less waste, and gas-fired plants produce practically none • For some coals the fly ash (飞灰)and the scrubber sludge (污泥) may contain toxic organic and inorganic compounds, for which the waste needs to be disposed of in a secure landfill and the landfill must be lined, with impenetrable material (不透材料)so that leaching (浸出)into the soil and groundwater is to be prevented

Fossil-Fueled Power Plants: Advanced Cycles • Combined cycle (联合循环)

Fossil-Fueled Power Plants: Advanced Cycles • Coal gasification combined cycle (煤气化联合循环) • A power plant using coal gasificationand combined cycle is called an integrated gasification combined cycle plant (IGCC) • Various coal gasification methods were developed already in the nineteenth century for providingpiped gas for home heating, cooking, and lighting • Coal can be gasified to low-, medium- and high-heating value syngas • For an IGCC plant, high-heating value syngas is preferred

Fossil-Fueled Power Plants: Advanced Cycles • Coal gasification combined cycle (煤气化联合循环) • The crushed coal is fed to a retort, where, in the presence of catalysts, it is exposed to pure oxygen and steam • To increase the heating value, the carbon monoxide and hydrogen is passed over a catalyst at about 400 ◦C to form principally methane (甲烷), a process called methanation (甲烷化) • An alternative is the production of hydrogen in the water gas shift reaction (水煤气反应)

Fossil-Fueled Power Plants: Advanced Cycles • Cogeneration (热电联产) • Cogeneration is applied to systems that provide both electrical power and useful heatfrom the burning of fuel • In industrial or commercial installations the heat may be used for spaceheating or material processing • The incentive for cogeneration is primarily financial in that the costof supplying electricity and heat via a cogeneration scheme might be less than supplying themseparately • Whether or not a cogeneration system reduces the amount of fuel needed tosupply the electricity and heat depends upon the details of the cogeneration system

Fossil-Fueled Power Plants: Advanced Cycles • Cogeneration (热电联产) • When a heat engine drives an electric generator to produce electricity, it also provides a streamof hot exhaust gas • Where the exhaust gas is warm enough to be used for process or space heat,some of the exhaust gas enthalpy may be extracted to satisfy the heat requirement in a cogenerationplant

Fossil-Fueled Power Plants: Advanced Cycles • Fuel cell(燃料电池) • In a fuel cell, some of the chemical energy of the fuel is directly converted into electrical energy, with the rest appearing as heat rejected to the environment • Its theoretical thermal efficiency in terms of electrical energy generated versus fuel chemical energy input can be close to 100% when producing a low output of power • Fuel cell hydrogen is usually generated by the reforming of methane (甲烷重整)

Fossil-Fueled Power Plants

Energy and the Environment