NETL: What is greenhouse gas, and how will carbon capture and sequestration off-set global climate change?

Site Map

About NETL

Key Issues & Mandates

Research

Technologies

Energy Analyses

Solicitations & Business

Education

NewsRoom

Contact NETL

Carbon Sequestration
FAQ Information Portal

What is greenhouse gas, and how will carbon capture and sequestration off-set
global climate change?

What are greenhouse gases?

Watch this video clip for more information about greenhouse gases in the atmosphere.

Greenhouse gases (GHGs) include carbon dioxide, water vapor, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride. These chemicals have been lumped together as GHGs because of their contribution to the greenhouse effect, which is due to the following phenomena. Visible sunlight travels through the atmosphere to warm the Earth. About half of this radiated heat reaches the Earth’s surface. The heating of the ground causes the Earth’s surface to produce infrared radiation, only a small portion of which makes it back into space because infrared radiation cannot pass as readily through the atmosphere. As some of the infrared radiation is trapped in the atmosphere, this thermal energy is radiated down to the Earth’s surface. While most GHGs occur naturally, and are necessary to keep our atmosphere stable, there are concerns due to elevated levels of GHGs caused by human activities. GHG levels in the atmosphere have significantly increased above the pre-industrial era level – emissions of CO₂ from human activity have increased from an insignificant level two centuries ago to over 33 billion tons worldwide today. This increase of GHGs is considered by many scientists to contribute to the phenomenon of global warming, and could cause unwelcome shifts in regional climates.

How will carbon capture and sequestration off-set global climate change?


	CLICK TO SEE LARGER IMAGE

Carbon dioxide capture and sequestration is a process consisting of separating CO₂ from emission sources, transporting it to a storage location, and ensuring the long-term isolation of CO₂ from the atmosphere. Carbon capture and sequestration is one option of a number of mitigation actions aimed at stabilizing atmospheric greenhouse gas concentrations. Other options include energy efficiency improvements; the shift to less carbon intensive fuels such as nuclear power and renewable energy sources; and reduction of non-CO₂ greenhouse gas emissions. No single technology option will provide all of the emission reductions needed to accomplish emission stabilization. In combination, however, these options could achieve a range of stabilization levels.

What is an avoided emission?

Avoided emissions are those emissions that are not produced (are avoided) by using non-emitting technologies or by capturing and sequestering emissions from an emitting source. For the fossil energy industry, avoided emission measures reduction in CO₂ and other greenhouse gas emissions but also takes into account the reduced capacity of power plants caused by the addition of a CO₂ capture and sequestration system. Avoided emissions are calculated from a baseline that describes what the emissions would have been without carbon capture and sequestration. For capture and sequestration purposes, calculating avoided emissions takes into consideration 1) the reduction in power plant efficiency resulting from the extra energy load of the CO₂ CCS system, and 2) the fact that not all exhaust CO₂ can be captured with current technology. Assessing carbon capture and sequestration systems on an avoided emissions basis is the best way to show results that can be compared to nuclear, renewables, and other GHG reduction options.

Aside from carbon capture and sequestration, what are other options for reducing greenhouse gas (GHG) emissions, and how effective will they be?

Other options for reducing GHG emissions include:

Reducing non-CO₂ GHG emissions
Reducing energy demand through increased energy efficiency
De-carbonizing energy supplies by increasing the use of renewable energy resources

It is unlikely that any single solution will meet this challenge. Reducing non-CO₂ GHG emissions is important, for example, but it doesn’t address the problem of rising atmospheric concentrations of CO₂, which is the largest component of GHGs at more than 84 percent.

Increasing energy efficiency does cut CO₂ emissions, but that option has technological and economic limits. According to the International Energy Agency (IEA), potential energy savings based on proven technologies and best practices could result in CO₂ emissions reductions of 7 to 12 percent of today’s global CO₂ emissions. Most forecasts, including the IEA’s, call for steadily rising CO₂ emissions this century.

Fuel-switching to renewables and nuclear energy isn’t perfect either. Renewables have shortcomings in terms of cost and reliability, and there remains heavy public opposition to siting new nuclear facilities. DOE’s Energy Information Administration (EIA), in its 2007 Annual Energy Outlook, forecasts that the U.S. energy market share of carbon-neutral renewables and nuclear energy combined will remain relatively flat at 14 percent from 2005 through 2030.

Our fossil fuel-based energy system has taken many years and significant investments by the private sector and government to develop. As a result, this mature network delivers energy at a reasonable price. We need to maintain a strong energy sector and a reasonable price to maintain a strong economy as we transition from a fossil fuel economy with significant CO₂ emissions to an energy economy with limited CO₂ emissions. We could put all of our eggs in one basket by focusing on one new potential technology and scrapping the existing infrastructure, or we could follow a strategy of cutting back on CO₂ emissions from existing sources and developing non-CO₂ producing technologies. This approach could include reducing the CO₂ output of our existing facilities by using CO₂ capture and storage technologies, investing in new technologies like hydrogen and wind, building high efficiency fossil fuel facilities, and encouraging energy conservation. The climate change problem is big enough, meaning the needed reductions in greenhouse gas concentrations are large enough, that we will need technological development in all these areas to make a difference.

How can atmospheric CO₂ be stabilized?

Global atmospheric concentrations of carbon dioxide (CO₂) have increased significantly since the dawn of the Industrial Age around 1750, when the world’s atmospheric concentration of CO₂ stood at about 280 parts per million (ppm). Writing in the August 13, 2004, issue of Science magazine, Princeton University professors S. Pacala and R. Socolow noted that proposals to limit atmospheric CO₂ levels to a concentration that would prevent the most severe levels of climate change have focused on a goal of 500 ±50 ppm. As of July 2007, the global CO₂ concentration was about 383 ppm. If we take no action to curb CO₂ emissions, energy demand growth and other factors are likely to cause CO₂ atmospheric concentrations to more than double in the next 50 years — the so-called “business-as-usual” scenario that some believe will usher in catastrophic climate change. According to Pacala and Socolow, stabilization of CO₂ levels at 500 ppm requires that emissions be held near the current level of 7 billion tons of carbon per year (GtC/year). They identified 15 options for reducing CO₂ emissions with current technologies; each option has the potential to come to account for as much as 1 GtC/year of reduced emissions within 50 years, for a cumulative total of 25 GtC each over the 50 years:

Improve fuel economy for 2 billion cars from 30 to 60 miles per gallon.
Reduce travel for 2 billion 30-mpg cars from 10,000 to 5,000 miles per year.
Improve building energy efficiency by 25% versus levels projected in 50 years.
Double the 32% capacity of efficient baseload (60% energy efficient) coal-fired power versus inefficient baseload (40% energy efficient) coal-fired power
Replace 1,400 gigawatts (GW) of 50%-energy-efficient coal-fired power plants with natural gas-fired power plants — quadrupling current gas-fired capacity.
Introduce carbon capture and storage (CCS) at 800 GW of coal baseload power capacity or 1,600 GW of natural gas baseload power capacity.
Introduce CCS at plants producing 250 million tons per year (mty) of hydrogen from coal or 500 mty from natural gas.
Introduce CCS at synthetic fuels plants producing 30 million barrels per day of liquid fuels from coal. (CCS options include a provision for geologic sequestration of CO₂ at a level equivalent to developing 3,500 Sleipner CO₂ injection projects).
Add 700 GW of nuclear power, doubling current capacity.
Add 2 million 1-megawatt (MW)-peak windmills — 50 times current capacity.
Add 2,000 GW of photovoltaic capacity — 700 times current capacity.
Add 4 million 1-MW-peak windmills to produce hydrogen for fuel cells in hybrid cars.
Increase biomass fuel capacity a hundredfold.
Decrease tropical deforestation to zero and establish 300 million hectares of new tree plantations.
Apply conservation tillage to all cropland — a tenfold increase.

Pacala and Socolow point out that some of these options interact with each other, sometimes counteracting their effects. For example, improving the efficiency of coal-fired power plants offsets some of the CO₂ emissions benefits that would otherwise accrue from pursuing a fuel substitution option. The two scholars also didn’t focus on costs, public acceptability, or feasibility (for example, increasing biomass fuel capacity a hundredfold would require an area equal to about one-sixth of the world’s cropland).

Thus stabilizing global concentrations of atmospheric CO₂ over the next 50 years will certainly entail a combination of portions of several options listed above. A wide array of solutions will be needed that, to have broad acceptance, will have to include efficiency improvements and carbon capture and sequestration schemes for conventional fossil fuel-fired power plants.

Why not avoid creating CO₂ in the first place?

Ideally, it would be best to avoid creating CO₂ emissions. But right now our energy systems are based on fossil fuels, and burning fossil fuels generates CO₂ emissions. That means when we drive a car, mow the grass, heat and cool our homes, or use electricity (unless it’s hydropower or nuclear power), we’re generating CO₂. U.S. policies such as the Global Climate Change Initiative are aimed at determining what we can do now to reduce CO₂ through the use of sequestration, energy conservation, and renewables while we assess and develop improved energy systems that will put out less CO₂. The magnitude of greenhouse gas (GHG) emissions that must be offset to achieve atmospheric stabilization is daunting and requires that all options jointly contribute toward this reduction. Sequestration and a host of other GHG mitigation options — as well as adaptive measures — must be aggressively explored in parallel so that the best mix of cost-effective solutions are found.

As a practical matter, there is little prospect for a significant reduction in our need for fossil fuels over the next couple of decades. DOE’s Energy Information Administration (EIA), in its 2007 Annual Energy Outlook, projects that fossil fuels will continue to provide the roughly 86 percent share of U.S. primary energy supply in 2030 that they did in 2005. EIA sees the combined market share of renewable plus nuclear energy in the U.S. energy mix remaining stable at only 14 percent during 2005–2030, while coal actually gains in market share to 26 percent in 2030 from 23 percent in 2005. But that’s for all forms of energy, transportation fuels included. When just electricity generation is considered, coal’s market share is expected to jump to 57 percent in 2030, up from 50 percent in 2005 — and this scenario assumes that carbon capture and storage (CCS) technology will not be a factor, as no policies are in place yet to encourage its use. In this projection, EIA sees the market outlook for renewables in electricity generation as staying relatively flat at about 9 percent through the forecast period, while nuclear power’s market share is expected to fall from 19 percent to 15 percent during 2005–2030.

Despite technology improvements, rising fossil fuel costs, and public support, the contribution of renewable fuels to U.S. electricity supply will remain relatively small for another 25 years, EIA notes in its 2007 outlook. Although conventional hydropower remains the largest source of renewable generation through 2030, environmental concerns and the scarcity of untapped large-scale sites limit its growth, and its share of total generation falls from 6.6 percent in 2005 to 5.3 percent in 2030. Electricity generation from non-hydroelectric renewable fuels is projected to increase, however, bolstered by technology advances and state and Federal support. EIA projects that the share of non-hydropower renewable generation will increase by 60 percent, from 2.3 percent of total generation in 2005 to 3.6 percent in 2030. A projected market share of less than 25 percent for all low- or no-carbon technologies — nuclear and hydro included — does not suggest that rapid and widespread replacement of our fossil fuel infrastructure is technically, economically, or pragmatically feasible.

The fossil fuel-based energy system that underpins our economy is the product of trillions of dollars of investment by the private sector and government over the last century. The result is a mature energy infrastructure that delivers reliable supplies of energy at a reasonable price. Such an infrastructure cannot readily be replaced overnight by a non-fossil fuel-based energy system, without wreaking economic havoc. In an analysis of proposed legislation to require that a 25 percent share of electricity sales be produced from renewable sources by 2025, EIA concluded that reaching such a goal would require an almost 13-fold increase in non-hydropower renewable generation from 2005 levels. Almost 70 percent of the generating capacity added in that period would have to be renewable technology. The cost? EIA estimates that consumers will spend an additional $65 billion for electricity during 2009–2030 if the 25 percent renewables standard is to be achieved. U.S. gross domestic product losses from a 25 percent standard for renewables in electricity and transportation fuels markets would total $296 billion during 2009–2030. Yet even under the 25 percent standard, U.S. energy-related CO₂ emissions — while declining 14 percent to 2030 from EIA’s projected “business-as-usual” reference case — would still be 14 percent above 2005 levels. This further illustrates the need for multiple and varied approaches to reducing GHGs.

There are practical limitations to renewables achieving significantly accelerated penetration of energy markets as well. Reaching a 25 percent renewables standard in electric power producers’ portfolios would require successful development and rapid deployment of new technologies that currently are not commercially available, according to EIA. And deployment of even current technologies, such as wind power, engender significant uncertainties — for example, local opposition to siting of new facilities such as the political roadblocks thrown up against the proposed Cape Wind offshore wind farm development in Massachusetts’ Nantucket Sound. There remains significant and widespread opposition to the installation of new nuclear or hydropower systems as well.

Can we transition from a fossil fuel economy with significant CO₂ emissions to an energy economy with limited CO₂ in a way that does not put all our eggs in one basket by focusing on one area of new potential energy technologies and scrapping existing infrastructure? Remember that our current energy mix has fossil fuels accounting for about 86 percent of our energy needs. So any strategy to reduce CO₂ output will have to include a guarantee of an uninterrupted flow of energy while we reduce our dependence on technologies that yield CO₂ as a combustion product and introduce new technologies that put out little or no CO₂. This means continuing the use of fossil fuel for the near future while we transition to new energy sources. It could be accomplished by following a strategy of cutting back on CO₂ emissions from existing sources and developing non-CO₂-producing energy technologies. Such an approach could include reducing the CO₂ output of our existing facilities by using CCS technologies, investing in new technologies such as hydrogen and wind energy, building high-efficiency fossil fuel facilities, and encouraging energy conservation. The climate change problem is big enough — meaning the needed reductions in GHG concentrations are large enough — that we will need both sequestration and aggressive development of renewable energy technologies to make a difference.

Developing CCS technologies for new and existing fossil energy systems, such as those under investigation by NETL and its partners, could be the ideal bridge to a low-carbon future. That’s because CCS ultimately offers a potentially cost-effective approach versus that of heavily subsidized renewable fuels by focusing on reducing the cost and expanding the applicability of proven technologies instead of “reinventing the wheel” in a way that scraps our existing energy infrastructure. And reducing the cost of CCS is key to NETL sequestration R&D. NETL’s Carbon Sequestration Program’s overarching goal is to develop fossil fuel conversion systems that provide 90 percent CO₂ capture with 99 percent storage permanence at less than a 10 percent increase in the cost of services.

CCS technologies, if commercialized and widely deployed, would allow us to continue using fossil fuels in tandem with the existing energy infrastructure. It would be great if we could flip a switch and become a non-CO₂ emitting society overnight, but that’s not possible — even with massive investments. Even the most optimistic estimates indicate that converting to a renewable energy economy would take many years and would not work as well in many cases as what we get with fossil fuels. Even the most seemingly attractive renewable energies have their drawbacks as well — technical, practical, economic, and even environmental (e.g., the toll on avian wildlife by wind turbines).

As we continue to use current fossil energy sources for years to come, we need ways to control CO₂ emissions for the energy systems we’re using. For example, if we can borrow a technology such as CO₂ flooding from the oil industry and demonstrate it for an application in CO₂ sequestration, we have a tool to apply soon that would reduce CO₂ emissions for a number of facilities. By attacking the problem from several fronts — energy conservation, expanded use of renewables, as well as CO₂ emission controls including CO₂ capture and sequestration, on existing and new systems — we have the best chance at reducing GHGs while maintaining a strong economy, which requires energy at a reasonable cost.

How important are the non-carbon dioxide greenhouse gases?

Non-CO₂ greenhouse gases (GHGs) include water vapor; methane; ozone; nitrous oxide (N₂O); the group of fluorinated gases consisting of hydroflurocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6); and chlorofluorocarbons (CFCs). CO₂, methane, water vapor, N₂O, and ozone are naturally occurring as well as anthropogenic GHGs; the other compounds are strictly anthropogenic.

It is important to recall that even the naturally occurring GHGs, such as water vapor, are major contributors to the so-called greenhouse effect. Water vapor is the most important GHG in terms of the natural “greenhouse” process. As atmospheric temperatures rise, more water is evaporated from rivers, oceans, soil, etc., leading to a higher concentration of water vapor in the atmosphere. That causes more infrared energy radiated from the Earth to be absorbed, further warming the atmosphere, which in turn increases the amount of water vapor in the atmosphere, and so on — a phenomenon known as a positive feedback loop. On the other hand, as water vapor levels increase in the atmosphere, more of it will eventually condense into clouds, which are better able to reflect incoming solar radiation — in turn allowing less thermal energy to reach the Earth’s surface and heat it. There is a great deal of scientific uncertainty in defining the extent and importance of this feedback loop. In the future, monitoring atmospheric processes involving water vapor will be crucial to fully understand the complexity of the feedbacks in the climate system. While we have good atmospheric measurements of key GHGs such as CO₂ and methane, our ability to measure global water vapor is poor, so we can’t be sure how much atmospheric concentrations of this most abundant GHG have changed over the years.

Although it is relatively short-lived in the atmosphere (10–20 years) compared with CO₂ (100 years on average), methane is more than 20 times as effective at trapping heat in the atmosphere. According to the Intergovernmental Panel on Climate Change (IPCC), methane concentration increased by 143 percent over the past 250 years, compared with a 35 percent increase for CO₂ during that period. Because the various GHGs have differing capacities for absorbing radiation and varying lifetimes in the atmosphere, IPCC developed the Global Warming Potential (GWP) concept to compare the ability of each GHG to trap heat in the atmosphere relative to another gas. In 2005, carbon dioxide accounted for 83.9 percent of the total U.S. anthropogenic GHG emissions. Methane accounted for 7.4 percent of total GHG emissions; but because it has a GWP ratio of 20:1 over CO₂, methane has a much more significant impact when measured over a 20-year or even a 1-year GWP basis. Methane is released as part of the natural biological processes in low-oxygen environments, such as in swamplands or rice paddies. During the past 50 years, increased concentrations of methane in the atmosphere can be attributed to such anthropogenic sources as growing rice, raising cattle, burning natural gas, and mining coal. Direct measurements have shown increases in methane concentration of 1 percent per year from 1978 to 1990. But since 1990, there has been little sustained increase, and there is no scientific consensus on why methane concentrations haven’t risen much since then. However, there are several indicators of declining emissions during 1990–2005 from several key methane emission contributors: landfills (capturing and burning more methane), coal mining (mining of less gassy coal and increased use of methane collected from degasification systems), and natural gas systems (better management practices and technology improvements).

N₂O is mainly produced by microbial processes in soil and water, including those reactions occurring in nitrogen-based fertilizers. Increased use of such fertilizers have contributed to growing concentrations of N₂O in the atmosphere since the beginning of the Industrial Revolution, and accounted for 78 percent of N₂O emissions in 2005. Other anthropogenic emission sources of N₂O include fossil fuel-fired power plants, nylon and nitric acid production, waste management, and vehicles. Although total emissions of N₂O are only a small fraction of those of CO₂, N₂O is about 300 times more powerful than CO₂ at trapping heat in the atmosphere. Since 1750, atmospheric concentrations of N₂O have climbed by about 18 percent and accounted for about 6.5 percent of GHG emissions in 2005. In recent years, vehicular emissions of N₂O have declined due to improved control technologies.

IPCC considers anthropogenic ozone to be the third most important GHG, although there is no tracking of ozone emissions per se because anthropogenic ozone is the product of other gases and compounds interacting with each other. Oxygen and ultraviolet light from the sun interact to form ozone in a broad band in the stratosphere known as the ozone layer. A small portion of this ozone naturally falls to Earth’s surface and is termed tropospheric ozone. Anthropogenic emissions of nitrogen oxides, carbon monoxide (CO), and non-methane volatile organic compounds (VOCs) from motor vehicles, factories, and burning vegetation act as ozone precursors in that they form ozone when acted upon by sunlight. Ozone in turn is a major contributor to smog, especially in urban areas. Another complication of ozone is that it also interacts with and is modified by methane concentrations. Ozone concentrations in the atmosphere have increased by about 30 percent in the past 250 years.

CO is not considered significant as a direct GHG because it is a weak absorbent of infrared radiation; but CO does influence the production of methane and tropospheric ozone. CO levels were on the rise until the late 1980s and since have fallen significantly — owing, it is thought, to reductions in vehicular CO emissions following widespread use of catalytic converters.

VOCs also have a small direct impact as GHGs and play a role in the processes that affect ozone production. These include non-methane hydrocarbons (NMHCs) and oxygenated NMHCs such as alcohols and organic acids. Most non-methane VOC emissions are from vegetation, but a growing share of this group’s emissions is accounted for by vehicles, fuel production, and biomass burning.

Fluorinated gases are a mixed bag in terms of potential for contributing to climate change. CFCs and hydrochlorofluorocarbons (HCFCs) have a long lifetime in the atmosphere and thus could be potent GHGs, if it were not for the 1989 Montreal Protocol on ozone-depleting substances outlawing CFCs and HCFCs. Since then, the levels of CFCs in the atmosphere have remained level or declined. However, it is likely that some concentration of CFCs in the atmosphere will persist for over 100 years. Substitutes developed to replace CFCs and HCFCs used as refrigerants, aerosol propellants, and cleaning solvents include HFCs and PFCs. These substitutes don’t harm the ozone layer but do act as GHGs, as does SF₆, which is used as insulation in the electric transmission and distribution industry. Emissions of HFCs, PFs, and SF₆ have almost doubled since 1990 and account for about 2.2 percent of total U.S. anthropogenic GHG emissions.

Back to FAQ Portal

USA.GOV | U.S. DEPARTMENT OF ENERGY | DOE OFFICE OF FOSSIL ENERGY
DOE OFFICE OF ENERGY EFFICIENCY & RENEWABLE ENERGY | OFFICE OF ELECTRICITY DELIVERY & ENERGY RELIABILITY

Disclaimer | Privacy Policy | Web Policies | FOIA/Privacy Act | Employees Only