Environmental Problems, Their Causes, and Sustainability 1

AdvantagesModerate cost(oil sand)Large potentialsupplies, especiallyoil sands in CanadaEasily transportedwithin andbetweencountriesEfficientdistributionsystem in placeTechnology is welldevelopedTrade-OffsHeavy Oils from Oil Shaleand Oil SandDisadvantagesHigh cost (oil shale)Low net energyyieldLarge amount ofwater needed forprocessingSevere landdisruption fromsurface miningWater pollutionfrom miningresiduesAir pollutionwhen burnedCO 2 emissionswhen burnedFigure 17-18 Trade-offs: advantages and disadvantages ofusing heavy oils from oil shale and oil sand as energy resources.Pick the single advantage and disadvantage that youthink are the most important.However, unless a natural gas pipeline has beenbuilt deposits of natural gas found above oil deposits(Figure 17-2) cannot be used. Indeed, the natural gasfound above oil reservoirs in deep sea and remote landareas is often viewed as an unwanted byproduct and isburned off. This wastes a valuable energy resource andreleases carbon dioxide into the atmosphere.Unconventional natural gas is found in other undergroundsources. One is methane hydrate, in which smallbubbles of natural gas are trapped in ice crystals deepunder the arctic permafrost and beneath deep-oceansediments. Globally the amount of energy in methanehydrates is about twice that in the earth’s oil, naturalgas, and coal resources combined.So far it costs too much to get natural gas frommethane hydrates and unconventional sources ofnatural gas, but the extraction technology is beingdeveloped rapidly, especially by Japan that has largedeposits off its coast and few deposits of conventionaloil and natural gas. One problem is that whenmethane hydrate is brought to the surface it warms upand releases methane (a greenhouse gas) into theatmosphere.When a natural gas field is tapped, propane andbutane gases are liquefied and removed as liquefiedpetroleum gas (LPG). LPG is stored in pressurizedtanks for use mostly in rural areas not served by naturalgas pipelines. The rest of the gas (mostlymethane) is dried to remove water vapor. Then it iscleansed of poisonous hydrogen sulfide and other impuritiesand pumped into pressurized pipelines fordistribution.At a very low temperature natural gas can be convertedto liquefied natural gas (LNG). This highlyflammable liquid can then be shipped to other countriesin refrigerated tanker ships.17-3 NATURAL GASWhat Is Natural Gas? Mostly MethaneNatural gas, consisting mostly of methane, is oftenfound above reservoirs of crude oil.In its underground gaseous state, natural gas is a mixtureof 50–90% by volume of methane (CH 4 ), the simplesthydrocarbon. It also contains smaller amounts ofheavier gaseous hydrocarbons such as ethane (C 2 H 6 ),propane (C 3 H 8 ), and butane (C 4 H 10 ), and smallamounts of highly toxic hydrogen sulfide (H 2 S).Conventional natural gas lies above most reservoirsof crude oil (Figure 17-2). Like oil, natural gas wasformed from fossil deposits of phytoplankton and animalsburied on the seafloor for millions of years andsubjected to high temperatures and pressures.How Is Natural Gas Used? A Versatile FuelNatural gas can be burned to heat space and water,generate electricity, and propel vehicles.Natural gas is a versatile fuel that can be burned toheat water and buildings and to generate electricity. Itcan also be used as a fuel for cars and trucks with fairlyinexpensive engine modifications. Natural gas is especiallyuseful for running fleets of taxis and deliveryand work vehicles operating from garages and maintenancefacilities that can be used to supply them withthis fuel.Increasingly, natural gas is used to run mediumsizedturbines that produce electricity. These cleanburningturbines have a much higher energy efficiency(50–60%) than coal-burning power plants (24–35%).They are cheaper to build per kilowatt-hour, requireless time to install, and are easier and cheaper to maintainthan large-scale coal and nuclear power plants.362 CHAPTER 17 Nonrenewable Energy Resources