Flue gas that would normally go out the stack is bubbled through a solution of water and amines in what is called a contactor vessel.
The amines in the water react with the carbon dioxide in the flue gas to form an intermediate chemical called a “rich” amine. Rich amines are water soluble and stay in the water solution.
Some of the flue gas bubbles out of the top of the amine solution and is emitted to the air, but a portion of the carbon dioxide has reacted with the amines and remains in solution.
The rich amines are pumped to a vessel where they are heated to make them decompose back into regular “lean” amines and carbon dioxide gas.
The pure carbon dioxide gas is collected from this vessel and the regular amines are recycled to the flue gas contactor.
Of course, something must then be done with the captured CO2. There are three basic options: Use the carbon dioxide as a value-added commodity; store the carbon dioxide in underground formations; or convert the carbon dioxide to methane, biomass, mineral carbonates, or other substances.
Currently, 98% percent of all coal-fired power plants burn their fuel in air and exhaust flue gas that contain carbon dioxide in moderate concentrations (3-12% by volume). Retrofitting carbon dioxide capture to these facilities is expensive. For pulverized coal plants, the cost of carbon dioxide capture, transport, and storage in an underground formation would add at least 70-100% to the cost of electricity.537
Emerging R&D technologies are attempting to lower the cost to less than a 20% increase in the cost of electricity compared to a non-capture counterpart. A new technology for coal-fired power plants, integrated gasification and combined cycle (IGCC), has much lower cost for carbon dioxide capture and storage because of inherent characteristics of the process. Equipping an IGCC plant for capture and storage would add at least 30% to the cost of electricity.
A different approach is being investigated by researchers from the Ohio Coal Research Center at the University of Ohio. They are among several groups of scientists trying to capture coal's excess CO2 using photosynthesis. This approach feeds algae on carbon dioxide exhaust and sunlight, resulting in biomass. It is estimated that algae could recapture 75% of the CO2 emitted by a coal-fired power plant, at the same time producing 8,000 gallons (30,000 liters) of biodiesel and enough leftover plant material to make (9,000 liters) of ethanol.538 This totally defeats the purpose of carbon sequestration and biofuel use. If the carbon captured is eventually released into the atmosphere by burning the biodiesel and ethanol, it will have the same effect as releasing it directly. This scheme amounts to a carbon dioxide shell game, not solving the emissions problem, but obscuring it with technological slight of hand.
Other concepts for converting carbon dioxide to different chemicals, especially fuels, are in the very early stages of research but disposal of the carbon remains the main stumbling block. To quote from the US DOE report on solar energy:
The challenge for carbon sequestration is finding secure storage for the 25 billion metric tons of CO2 produced annually on Earth. At atmospheric pressure, this yearly global emission of CO2 would occupy 12,500 km3, equal to the volume of Lake Superior; it is 600 times the amount of CO2 injected every year into oil wells to spur production, 100 times the amount of natural gas the industry draws in and out of geologic storage in the United States each year to smooth seasonal demand, and 20,000 times the amount of CO2 stored annually in Norway’s Sleipner reservoir. Beyond finding storage volume, carbon sequestration also must prevent leakage. A 1% leak rate would nullify the sequestration effort in a century, far too short a time to have lasting impact on climate change. Although many scientists are optimistic, the success of carbon sequestration on the required scale for sufficiently long times has not yet been demonstrated.539
Illustration 153: World coal consumption. Source U.S. EIA.
In short, no one knows if carbon sequestration will even work. This has led to suggestions that CO2 emissions be offset by planting new forests. The DOE estimates that about 220,000 acres would be required to offset emissions from an average size power plant. Given competition for land use from food production and possibly biofuels, this option seems untenable.
Meanwhile, worldwide use of coal as a primary energy source continues to grow. With oil and natural gas prices expected to continue rising, coal is an attractive fuel for nations with ample coal resources. Despite government policies aimed at reducing coal use, its share of world energy consumption is projected to increase further (see Illustration 153). China is building a new coal plant every week to ten days and will continue to do so for the foreseeable future. Germany is faced with aging existing coal plants, green opposition to retaining nuclear power stations, and politically unreliable supplies of natural gas from Russia. Their only option is to build new coal generating plants, even though advanced sequestration technology is not yet available. The US, with a 200 to 400 year supply of indigenous coal, is also tempted by the allure of anthracite. But coal has a dark side.
In 1952, London experienced a period of thick smoggy conditions that came to be called the “Killer Fog.” On December 5th, a toxic mix of dense fog and sooty black coal smoke enveloped the city, causing residents to gasp for air. Soon, the roads were littered with abandoned cars as visibility fell to less than a foot. Before long, Londoners started dying. A study in the journal Environmental Health Perspectives, estimated that as many as 12,000 may have been killed before the fog lifted, four days later.540
Since requiring non-smoking fuels be used within the city, London is no longer threatened by killer fog, but other cities around the world are not so fortunate. In developing countries such as Mexico, India and China, rising emissions from trucks and automobiles combined with soot and sulfur from burning wood, dung and coal have created conditions similar to those that afflicted London. In China, the sulfur dioxide produced by burning coal poses an immediate threat to people's health. In China alone, the infamous “brown clouds” contribute to an estimated 400,000 premature deaths a year. It also causes acid rain that poisons lakes, rivers, forests and crops across Asia and around the world.541
Illustration 154: Killer Fog hits London.
With its economy expanding at an annual rate of 10%, China already uses more coal than the United States, the European Union and Japan combined. With China rapidly adding new coal-fired plants, each with an operational lifetime of 75 years, the brown clouds can only be expected to grow worse in the future. And China is not alone—India is right behind China in stepping up its construction of coal-fired power plants.
What is worse, around the world huge underground coal fires are pointlessly burning millions of tons of coal each year. Mega-fires burning in India, China and elsewhere in Asia spew out huge volumes of air pollutants, force local residents to relocate, and ruin the land above them. Coal fires burn throughout the Chinese coal mining region, an area that stretches 3000 miles (5000 km) from east to west and about 450 miles (750 km) from north to south. Chinese fires alone consume 120 million tons of coal each year, equivalent to the annual coal production of Pennsylvania, Ohio, and Illinois combined.542 In India, coal fires also rage. Though naturally-occurring coal fires can be dated back to the Pleistocene, mining activity only aggravates the situation by making new fires more probable.543
Suffocating smog is not the only way that coal kills. Each year, the mining of coal takes a terrible toll among coal miners. In the US, 688 coal miners died between 1990 and 2006. In 2006, US coal mining deaths soared to a 10-year high, reversing an 80-year trend of fatality reduction.544 In China, official government statistics estimate that 250,000 people have died in the mines since 1949. More than 2,000 perished in 2007 alone.545 No one knows how many people coal has killed, but the truth is plain to see—coal is the most harmful and deadly energy source on the planet.
Methane Ice
There is another, new source of hydrocarbon fuel being talked about for the future—gas hydrates. Gas hydrates, also known as clathrates, are actuall
y natural methane-water ices, which form under conditions of high pressure and low temperature in many areas worldwide. A hydrate is a crystalline solid consisting of gas molecules, usually methane, each surrounded by a cage of water molecules. Methane hydrate looks very much like water ice. It is stable in ocean floor sediments at water depths greater than 300 meters and, where it occurs, it is known to cement loose sediments in a surface layer up to several hundred meters thick. Since their discovery, scientists have been investigating seafloor hydrates as a future source of energy.
Illustration 155: Distribution of coal fires across China from satellite data.
As with all fossil fuel resources, it is hard to estimate just how much methane is trapped in clathrates worldwide. Deposits have been identified off the coasts of every continent and several of the larger lakes in Central Asia are cold enough to permit clathrate formation. In theory, this undersea source of methane could greatly expand the world's supply of relatively clean-burning natural gas. Deposits may surpass 100 times proven natural gas reserves and twice all other sources of hydrocarbon-based fuels combined.
A Japanese collaboration has drilled about 30 wells, with a time-line to start production and distribution of methane from hydrates by 2016. China is reported to have recovered the first methane-bearing samples from the South China Sea. In the US, a consortium of government agencies and petroleum companies has been drilling for clathrates in the Gulf of Mexico with some success.
It now appears that these vast stockpiles of frozen seafloor methane are more unstable than previously thought. The sudden release of methane from seafloor clathrates may have been responsible for sudden global warming events in the past. Scientists have blamed tsunamis resulting from sudden sea floor movements for several catastrophic events. Around 8,000 years ago, the Storegga submarine landslide resulted in the movement of 700 cubic miles (3000 km3) of ocean floor sediments from an area rich in gas hydrates off the western coast of Norway. This movement generated a tsunami that inundated the coasts of Scotland and Norway. Greenland ice core records show that, following the event, atmospheric methane concentrations increased by 80-100 parts per billion. This increase corresponds to methane releases from the slide debris at an estimated rate of 20-25 megatons each year for several hundred years following the slide.546
The release of large volumes of methane from sea floor deposits have been suggested as the cause for warming anomalies like the PETM and even the Permian-Triassic Extinction. A sudden release of large volumes of methane would have a dramatic impact on greenhouse warming, since methane is 20 times as potent a GHG as CO2. Methane rapidly decomposes in the presence of oxygen so a methane release would show up in geological records as a sudden increase in CO2 levels. Such a release could deplete oxygen in the ocean and lower atmospheric levels as well. This has led to speculation that a warming trend could trigger a devastating release of methane that would cause a spike in global temperatures or even worse—a worldwide extinction event.
We have to ask ourselves, is developing yet another form of hydrocarbon-based energy worthwhile? Particularly when poking and prodding underseas clathrate deposits may result in devastating tsunamis or catastrophic release of methane that could cause real global warming. We should let this sleeping dog lie.
Renewable Energy Redux
If our goal is to significantly reduce emission of carbon dioxide, use of fossil fuels must be severely curtailed. Illustration 156 provides a breakdown of US energy consumption and CO2 emissions by fuel type and use. Petroleum is the largest source of CO2 emissions, accounting for 43% of the total, while coal is second with 37%. The majority of petroleum fuel use is in the transportation sector. This is of particular concern because, as we have shown, there are no suitable renewable replacements for liquid fossil fuels. Since world automobile use is predicted to continue rising, a successful energy strategy for the future must solve the problem of automotive emissions.
It is no surprise that coal is used in more stationary applications than petroleum. In the US, it accounts for 51% of electrical generation and an astounding 81% of CO2 emissions from power generation. Emanating from fewer locations than petroleum emissions, pollution from coal is a major part of the GHG problem. Because it is inexpensive and abundant, replacing coal as a major energy source will be as difficult as rethinking the automobile. Thus, the challenge for the future is twofold—redesign the automobile and replace coal in electrical generation.
Illustration 156: US energy consumption and CO2 emissions by sector and fuel type. EIA.
While the preceding examination of potential renewable and existing energy sources may seem discouraging, things are not as bad as they appear. Within the bounds of economic and physical possibility, renewables may be able to provide as much as 25% of the world's energy needs using current technology. What is inadvisable is forcing the adoption of renewable sources that would prove unreliable or excessively costly. This will cause consumer backlash that could damage the appropriate use of renewable energy for decades to come.
Diversifying our sources of energy is a good idea for many reasons, not just to reduce GHG emissions and other forms of pollution. Major fossil fuel deposits are located within the territories of many of the world's more unsavory regimes. Oil money often props up repressive governments and bankrolls international terrorism. Lessening the world's dependency on fossil fuels would diminished these bad actors' income stream—a beneficial side-effect of reducing pollution.
Renewable energy from hydroelectric, geothermal and wind power should be pursued. Solar also has a place but, without significant reduction in cost, will remain a promise for the future. But other steps can be taken now. The intelligent use of energy holds a potential as great as that of current renewable technology. Moreover, other sources of non-polluting energy are available. Building on the contributions of renewables and energy conservation, the next chapter will explain why the future is not as bleak as many fear—technologies already available can be expanded, and world CO2 emissions can be greatly diminished.
A Plan for the Future
“When it comes to the future, there are three kinds of people: those who let it happen, those who make it happen, and those who wonder what happened.”
—John M. Richardson, Jr
As we have seen, there is little prospect of solving the problem of greenhouse gas emissions using existing renewable energy sources and technologies. The world demand for energy continues to rise and the sources that are best positioned to fill that demand are the ones that are most damaging to Earth's environment (see Illustration 157).
Notice that the largest fuel source is labeled “Liquids,” meaning oil. The area with the greatest potential for reducing oil consumption is the transportation sector.
Getting from Point A to Point B
Parts of the transportation industry are already moving towards higher fuel efficiency, with commensurate reduction in GHG emissions. Airlines and airplane manufacturers have taken steps with only minimal prodding by governments. For them, it is a matter of simple economics, the cost of jet fuel is rising and accounting for an ever larger portion of airline operating budgets. For most airlines, fuel is the second largest expense category behind labor. Increase in fuel consumption over the past 15 years was 52.2% from domestic operations and 111.8% from international. When these figures are combined the net total increase is 68.7%.547
Illustration 157: Growing dependence on fossil fuels. Source U.S. EIA.
Even with this increased fuel consumption, aviation is the most carbon-efficient form of transportation generating only 2-3% of total current CO2 emissions and 13% of CO2 emissions from all transportation sources. Aircraft account for just 3% of the world's annual petroleum usage and modern aircraft are more than three times as efficient as today's average car, with fuel efficiencies of 67 passenger-miles per gallon. Next generation aircraft, like the Boeing 787 and Airbus's planned A350 XWB, will increase fuel efficiency to 78 passenger-miles to the gallon, much highe
r than any modern compact car.548
Airbus, which participates in the European Commission's Clean Sky initiative, has a program that should speed development of new technology that helps reduce air transport's impact on the environment. The seven year, €1.7 billion initiative started early in 2007 is expected to reduce CO2 emissions by 50% through reduced fuel consumption and cut NOx emissions by 80%. Clean Sky also addresses greener aircraft life cycles, which includes evaluating maintenance and aircraft disposal.
Fuel savings are possible even for older planes. The amount of fuel they burn can be reduced through changes to air traffic control approach patterns and delaying engine start until cleared to an active runway. Airlines operating Boeing 737s, in Europe, have been given approval to use an optimized landing approach that significantly reduces the amount of fuel used during approach operations. Changes in approach patterns reduce CO2 and NOx emissions by roughly 20% compared to previous arrival procedures.549
Illustration 158: CDA keeps aircraft higher longer and has them descend at near-idle power. Credit: NAVERUS and AVTECH
In the US, the use of continuous descent approaches can save airlines up to $30 million in annual fuel costs. All of these savings can be realized without increasing the air traffic controller workload.550 This is an example of how being greener and more efficient benefits industry and the environment.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 37