Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email ： [email protected]...

Energy and the Environment

Fall 2012Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127

mailto:[email protected]

Flashbacks of Last Lecture

• Of all environmental effects of fossil fuel usage, global warming, including its concomitant climate change, is the most perplexing, potentially most threatening, and arguably most intractable

• It is caused by the ever-increasing accumulation in the atmosphere of CO2 and other gases, such as CH4, N2O, chlorofluorocarbons (CFCs), and aerosols, largely due to emissions of these gases from anthropogenic activities, and reaches levels that exceed those that existed for centuries before the beginning of the industrial revolution

• These substances augment the greenhouse effect of the earth’s atmosphere, which provides a warmer climate at the earth’s surface than would exist in an atmosphere-free earth

Global Warming: Greenhouse Gas Emissions

• The increase of GHGGHG (温室气体 )concentrations in the atmosphere is a consequence of rising emissions of these gases from anthropogenic sources anthropogenic sources (人为来源 )

• The most significant of these gases is CO2, but CHCH44(甲烷 ), CFCs CFCs (氟氯化碳 ), and N2O emissions also need to be considered, as well as changes in O3 concentrations


• Because carbon constitutes a major mass fraction of all living matter, on earth there is an enormous reservoir of carbon in the living biosphere biosphere (生物圈 ) and its fossilized remnant fossilized remnant (化石残余 )

• There is a continuous exchange of CO2 between the biosphere and atmosphere– Carbon dioxide is absorbed from the atmosphere during

photosynthesisphotosynthesis (光合作用 ) of land vegetation and phytoplankton (浮游生物 ) living in oceans and other surface waters

– Carbon dioxide is returned to the atmosphere during respirationrespiration (呼吸 ) of animals and decompositiondecomposition (分解 ) of dead plant matter and animals


• The land-based biotabiota (生物群 ) and oceans are net CO2 absorbers

• Currently, fossil fuel combustion contributes most on CO2 emissions


• The atmospheric concentrations before 1958 were estimated from ice coresice cores (冰芯 )– During descentdescent (降落 ) in the atmosphere, snow flakes snow flakes (雪花 )

equilibrate with the atmospheric concentration of CO2

– Knowing the age of the ice layerice layer (冰层 ), it is possible to reconstruct the prevailing atmospheric concentration of CO2 when the snow flakes were falling

• In 1958, Charles Keating established an accurate atmospheric CO2 concentration measurement device using infrared absorption on top of Mauna Loa, Hawaii


• The historic record indicates that before the twentieth century the atmospheric concentration of CO2 hovered around 280 ppmV, with a dip in the sixteenth and seventeenth centuries, corresponding to the “little ice age”

• Starting around 1900, when the use of fossil fuels accelerated, the CO2 concentration began a steady increase of about 0.4%/y

• If no measures are taken to reduce CO2 emissions, then due to the population increase and the concomitant enhancement of fossil fuel use, the rate of growth of CO2 concentrations will increase more than 0.4% per year, and the doubling time will be achieved sooner


• Methane is emitted by natural processes– The anaerobic decomposition anaerobic decomposition (厌氧分解 ) of organic matter in swamps and swamps and

marshesmarshes (沼泽 ) – Enteric fermentation of fodder in guts of animals Enteric fermentation of fodder in guts of animals (动物消化道中饲料发酵 )

• Anthropogenic emissions of CH4 come from leakage of oil and gas wells, gas pipe lines, coal mine shafts, and storage and transportation vessels

• Worldwide concentrations are presently about 1.7 ppmv and are growing by about 0.6% per year

• Considering that CH4 is a stronger absorber of IR radiation IR radiation (红外辐射 ) per molecule than CO2, its contribution to the greenhouse effect is significant


• Nitrous oxide (N2O) is emitted naturally during bacterial bacterial nitrogen fixationnitrogen fixation (细菌固氮 ) in soils and by lightning discharge

• Minor quantities are emitted from fossil fuel combustion and in some chemical manufacturing processes (e.g., nitric acidnitric acid (硝酸 ), chemical fertilizerchemical fertilizer (化肥 ), and nylonnylon (尼龙 ) production)

• Its current concentration in the atmosphere is about 0.3 ppmv and is growing by about 0.25%per year


• Chlorofluorocarbons (CFC) are entirely man-made products, because they are produced in chemical factories for use as refrigerantsrefrigerants (制冷剂 ), propellants in spray canspropellants in spray cans (喷雾罐中的推进剂 ), foam-blowing agentsfoam-blowing agents (发泡剂 ), solventssolvents (溶剂 ), and so on

• By international conventionsinternational conventions (国际公约 ), they are gradually being phased out of production

• Because of their slow ventingslow venting (缓慢释放 ) from existing appliances and foam insulation materialsfoam insulation materials (泡沫保温材料 ), coupled with their long lifetime in the atmosphere (hundreds of years), they will contribute to global warming for a long time after they cease to be produced


• There are two layers of O3 in the atmosphere: one in the stratosphere (the “good” ozone), the other in the troposphere (the “bad” ozone)

• Because the upper troposphere and lower stratosphere is colder than the earth’s surface, ozone molecules residing in those layers absorb part of the outgoing far-infrared far-infrared terrestrial radiation terrestrial radiation (远红外地面辐射 ) and then re-radiate back to the surface, thus adding to global warming


• The stratospheric ozone is being depleted by CFC, which results in a small negative feedback to global warming because less of the outgoing terrestrial infrared radiation is absorbed by stratospheric ozone

• With the phasing out of CFC production worldwide, it is estimated that stratospheric ozone concentrations will return slowly (tens to hundreds of years) to pre-CFC times

• Tropospheric ozone is on the increase because of increased emission of the ozone precursorsprecursors (前驱体 ), NOx and VOCVOC (挥发性有机化合物 )

• On balance, it is may be assumed that in the foreseeable future the stratospheric ozone deficit and the tropospheric ozone surplus will cancel each other, and no appreciable contribution to global warming is to be expected from ozone


• By the year 2100, CH4, N2O, and CFC may contribute about one-third to global warming, with the remaining two-thirds being contributed by CO2

– In regard to methane, efforts can be made to limit its leakage from gas wells, transport pipelines, storage tanks, LNG tankers, coal mines, and other sources of anthropogenic CH4

– The production of anthropogenic N2O is poorly understood; hence it will be difficult to control its emission

– Because CFC production is being phased out worldwide, it will eventually disappear from the atmosphere

– The greatest technical and economic problem will be the reduction of CO2 emissions

Global Warming: Controlling CO2 Emissions

• Emission reductions of CO2 can be accomplished by a combination of several of the following approaches– End-use efficiency improvements and conservation– Supply side efficiency improvements– Capture and sequestration of COCapture and sequestration of CO22 (二氧化碳的捕获与封存 ) in

subterranean reservoirssubterranean reservoirs (地下水库 ) or in the deep ocean– Utilization of CO2 for enhanced oil and natural gas recovery and for

enhanced biomass production (photosynthesis)– Shift to non-fossil energy sources


• The simplest and most cost-effective approach to reducing carbon emissions is by end-use efficiency improvements and conservation

• In the residential–commercial sector, they range from lowering the thermostat in the winter (less heating), raising it in the summer (less air conditioning), better insulation, less hot water use, replacement of incandescentincandescent (白炽灯 ) with fluorescent lightingfluorescent lighting (荧光灯 ), replacement of electric clothes dryers with gas dryers, and so on

• In the industrial sector the largest savings could come from reductions in direct use of fossil fuels (e.g., coal for process heat or smeltingsmelting (冶炼 )), process modification, energy-efficient motors, better heat exchangers, and so on


• In the transportation sector, fossil fuel energy consumption is growing by leaps and bounds leaps and bounds (跨越式发展 ) all over the world

• Increasing population and living standards, coupled with the movement from agricultural to urban-industrial societies, puts more and more people and cargo in automobiles, trucks, trains, airplanes, and ships

• Because it is unrealistic to expect that the number of transportation vehicles, or the distances covered, will diminish, the only chances for reducing carbon emissions in the transportation sector lie in efficiency improvements


• By supply-side efficiency improvements we mean principally electricity supply

• The electricity industry has many options to reduce carbon emissions while supplying an ever-increasing electricity demand– Shift from coal– Replacement of single-cycle gas-fired steam power plants with combined combined

cycle gas turbine plants cycle gas turbine plants (CCGT) (联合循环燃气轮机电厂 )• Because single-cycle power plant thermal efficiencies are in the range 35–40%, whereas

the combined cycle plants can achieve 50–55%, the carbon emission savings are in the range of 10–20%

– Replacement of single-cycle coal-fired power plants with gas-fired CCGT• The carbon emission savings are in the range 60–70%(50%on account of shift from coal

to gas, and 10–20% on account of higher efficiency)


– Replacement of single-cycle coal-fired power plants with coal-derived synthetic gascoal-derived synthetic gas-fired (煤基合成气 ) combined cycle gas turbine plants• The efficiency of such plants is 40–45% based on the

coal input energy• Here, the carbon emission savings would be only on

the order of 5–10% • While a CCGT has a higher efficiency than a single-cycle

plant, coal gasification requires some of the coal energy to be spent on gasification


• The capture of CO2 is only worthwhile in large power plants, especially those burning coal (because coal emits more CO2 than oil or gas)

• A 1000-MW coal-fired power plant emits between 6 and 8 Mt y−1 of CO2

• The capture of CO2 from all the world’s large coal-fired power plants would make a significant dent in the global carbon emissions

• The following technologies for CO2 capture from power plants are being developed– Air separation–COAir separation–CO22 recycling recycling (空气分离—二氧化碳循环 )

– Solvent absorptionSolvent absorption (溶剂吸收 )– Membrane gas separationMembrane gas separation (膜气体分离 )


• Air separation–CO2 recycling– This method is based on combustion of the fossil fuel in pure oxygen,

instead of air– A plant using this method requires an air separation unit air separation unit (ASUASU) (空气分离装置 )


• Air separation–CO2 recycling– The world's first "clean coal" power plant (Schwarze Pumpe power

station) went on-line in September 2008 in Germany– The facility captures CO2 and sulfides, separates them, and

compresses the CO2 into a liquid state

– Plans are to inject the CO2 into depleted natural gas fields or other geological formations


• Solvent absorption– Carbon dioxide is soluble in some solvents, notably ethanolaminesethanolamines,

e.g., monoethanolaminemonoethanolamine (MEAMEA) (乙醇胺，例如，单乙醇胺 )


• Membrane gas separation– Gas separation by membranes relies on the different permeation rates

of gases through the membrane pores– The membrane method could be used for capturing CO2 from a

mixture of CO2 and H2, which is the product of coal gasification and coal gasification and the water shift reactionthe water shift reaction (煤的气化和水转移反应 )


• After capture, the CO2 needs to be sequestered in a reservoir for an indefinite period, so it will not reemerge into the atmosphere

• The following reservoirs are being investigated, and in some instances already employed, for sequestering CO2

– Depleted oil and gas reservoirs– Deep ocean– Deep aquifers ( aquifers ( 蓄水层蓄水层 ))


• Depleted oil and gas reservoirs– Oil and gas reservoirs are usually covered by an impenetrable layer of

rock, so that CO2 deposited into the reservoirs would not reemerge into the atmosphere

– In respect to sequestering CO2, oil and gas reservoirs behave differently

– Whereas CO2 can be injected into oil reservoirs while the oil is being pumped out of it, it can be injected into gas reservoirs only after depletion of the gas

– In fact, injecting CO2 into semi-depleted oil reservoirs is a well-established technology, which is not done for sequestering CO2, but rather for enhanced oil recovery (EOR)


• Deep ocean– The ocean is a natural repository for CO2

– The ocean is vast: It covers about 70% of the earth’s surface, and the average depth is 3800 m

– There is a continuous exchange of CO2 between the atmosphere and the ocean

– The ocean is a net absorber of carbon, which probably is part of the reason that CO2 concentrations in the atmosphere do not increase as fast as expected from anthropogenic emissions


• Deep aquifers– Deep aquifers may underlay vast areas under the continents and

oceans– The deep aquifers themselves consist of permeable, porous rock, such

as sedimentary shale-, lime-, or sandstone, the pores of which are saturated with brine

– The injected CO2 (in liquid or supercritical phase) would dissolve in the brine as carbonic acid

– In the case of limestone formation, some of the carbonate would dissolve into bicarbonate, furthering the absorption capacity of the reservoir and reducing the risk of leakage


• Flue gas CO2 can be used for enhanced oil or gas recovery

• Other uses would be for dry ice manufacturing, for carbonated carbonated drinksdrinks (碳酸饮料 ), and as a raw material for chemical products, such as urea, methanol, or other oxygenated fuelsurea, methanol, or other oxygenated fuels (尿素、甲醇或其他含氧燃料 )

• The problem with such propositions– Most of the carbon in the product would eventually burn up or

decompose back to CO2 and would wind up in the atmosphere

– The reduction of CO2 into the useful product requires virtually the same amount of energy as was given off when carbon oxidized into CO2

– The present market for chemical products that could be based on CO2 is quite limited


• Shift to non-carbon energy sources eliminates CO2 emissions completely, except perhaps for the fossil energy (mainly coal) used in smelting steel and other construction materials used in the non-fossil energy conversion devices

• Because a shift to nuclear energy appears for the foreseeable future unacceptable to the public and body politic in most countries, the only recourse would be a shift to renewables

• Among the renewables, hydro energy also appears to run into public opposition

• All expectations are turned toward solar, wind, geothermal, and ocean energy

Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email ： [email protected]...

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