energy and the environment

Energy and the Environment

Fall 2012Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127

mailto:[email protected]

Flashbacks of Last Lecture

• 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

– Gas turbines operate on the principle of the Brayton cycle– 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 )

– 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


• CondenserCondenser (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 )


• 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 )


• 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 )– 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 )


• 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

Fossil-Fueled Power Plants: Components

• Emission controlEmission control (排放控制 )– Nitrogen oxide controlNitrogen oxide control (氮氧化物控制 )

• The other major category of pollutants from fossil fuel combustion is nitrogen oxidesnitrogen oxides (氮氧化物 ), called NOx, which includes nitric oxide NOnitric oxide NO (一氧化氮 ), nitrogen dioxidenitrogen dioxide NONO22 (二氧化氮 ) (and its dimerdimer (二聚体 ) N2O4), nitrogennitrogen trioxidetrioxide NONO33 (三氧化氮 ) , pentoxidepentoxide NN22OO55 (五氧化二氮 ) , and nitrous oxide Nnitrous oxide N22OO (一氧化二氮 )

• NOx usually implies the sum of NO and NO2.

• NOx is a pernicious pollutant pernicious pollutant (有害污染物 ) because it is a respiratory tract irritant respiratory tract irritant (呼吸道刺激源 ) and it is a precursorprecursor (前驱体 ) toto photo-oxidants photo-oxidants (光氧化剂 ), including ozoneozone (臭氧 ) and acid deposition acid deposition (酸沉降 )

• Coal and oil contain organic nitrogen in their molecular structure and when burnt, these fuels produce fuel Nofuel Nox x (燃料型氮氧化物 )

• All fossil fuels produce thermal Nothermal Noxx (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: during and after combustion of the fossil fuel



• During combustion, a low-Nox burner (LNB) employs a process called staged staged combustion combustion (分级燃烧 )

• NOx formation is a function of air-to-fuel ratio (by weight) in the flame



• Fuel (e.g., pulverized coal) and air is injected through the central annulusannulus ( 环 )of the burner and the air/fuel ratio is less than stoichiometric stoichiometric (化学计量比 ) , i.e., fuel-rich fuel-rich (富燃 ) , which produces a luminous flame 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 annuliannuli ( 环 ), creating an outer flame envelope that is fuel-lean fuel-lean (贫燃 ) , where all the unburnt carbon burns up and less NOx is formed



• After combustion, in a selective catalytic reduction selective catalytic reduction (选择性催化还原 ) process either ammonia ammonia ( 氨 ) or urea urea (尿素 ) is injected into a catalytic reactorcatalytic reactor (催化反应器 ) through which the flue gas flue gas (烟气 ) flows

• In a selective noncatalytic reduction 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

3 2 2 24NO+4NH +O 4N +6H O

2 2 2 2 2 24NO+4CO(NH ) (aq)+O 4N +4CO +2H O


• Emission controlEmission 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 micronsmicrons (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 Mercury ( 汞 ) , selenium selenium ( 硒 ) , cadmium cadmium ( 镉 ) , and arseniarsenic ( 砷 ) are semivolatilesemivolatile (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 radonradon ( 氡 ) emissions, which is a disintegration product of uranium uranium ( 铀 ), the minerals of which may cling to the coal


• Waste disposalWaste 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 fly ash (飞灰 ) and the scrubber sludge 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 materialimpenetrable material (不透材料 ) so that leachingleaching (浸出 )into the soil and groundwater is to be prevented

Fossil-Fueled Power Plants: Advanced Cycles

• Combined cycle Combined cycle (联合循环 )


• Coal gasification combined cycle Coal gasification combined cycle (煤气化联合循环 )– A power plant using coal gasification and combined cycle is called an

integrated gasification combined cycle plant (IGCC)– Various coal gasification methods were developed already in the

nineteenth century for providing piped 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


• Coal gasification combined cycle 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 methane (甲烷 ), a process called methanation methanation (甲烷化 )

– An alternative is the production of hydrogen in the water gas shift water gas shift reactionreaction (水煤气反应 )

2 2 23C+O +H O 3CO+H

2 4 23H +CO CH +H O

2 2 2CO+H O CO +H


• Cogeneration Cogeneration (热电联产 )– Cogeneration is applied to systems that provide both electrical power

and useful heat from the burning of fuel– In industrial or commercial installations the heat may be used for

space heating or material processing– The incentive for cogeneration is primarily financial in that the cost of

supplying electricity and heat via a cogeneration scheme might be less than supplying them separately

– Whether or not a cogeneration system reduces the amount of fuel needed to supply the electricity and heat depends upon the details of the cogeneration system


• Cogeneration Cogeneration (热电联产 )– When a heat engine drives an electric generator to produce

electricity, it also provides a stream of 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 cogeneration plant

fuel el exQ P Q

el th fuelP Q

(1 )proc xch ex xch th fuelQ Q Q


• Fuel cellFuel 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 reforming of methane (甲烷重整 )

Fossil-Fueled Power Plants

energy and the environment

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