Carbon Capture & Sequestration: How Hopeful Should We Be?
- Published on Thursday, 20 December 2007 17:00
- 2 Comments
The burning of fossil fuels provides about 85% of the energy consumed in the United States1. One societal cost of this source of energy is the release of carbon dioxide; a potent greenhouse gas2. The dream of capturing carbon before it is released into the atmosphere is capturing the imagination of policy makers. The Energy Policy Act of 2005 includes $1.8B for “clean coal” of which carbon capture & sequestration is a component3. Yet the questions remain: Can it work? Is it cost effective? Are there more effective alternatives?
Carbon Capture & Sequestration (CCS) is the general phrase that describes efforts to “capture” the carbon dioxide (CO2) emissions from the burning of fossil fuels (primarily coal) and “sequestering” them from release into the atmosphere. Although there are a variety of methods of carbon management that legitimately could be labeled as CCS, this article will define the term narrowly to mean capturing the emissions from burning coal at power generation plants and preventing their release into the atmosphere. This article will briefly:
· Explain the technical process of CCS
· Review of the potential sequestration methods
· Introduce the existing CCS pilot projects
· Compare the costs of CCS to some alternative approaches
About 85% of the energy consumed in the United States is provided by the burning of fossil fuels4. The combustion of fossil fuels (natural gas, petroleum, jet fuel, and coal) release significant amounts of CO2 into the atmosphere – globally 5.8GtC/year as of 20035. This appears to be about 1% of the atmospheric carbon cycle, but serves as a relatively unbalanced forcing on the overall system6. This forcing contributes to the observed global increase in CO2 concentrations of about 40% since 1850 or about the onset of the industrial revolution7. This increase in CO2 concentration is of concern since CO2 has been identified as a potent Greenhouse Gas (GHG), contributing significantly to climate change8. (See Figure 1.)
There has been a spectrum of responses to the observation that anthropogenic GHG emissions are changing the climate:
· Removing the CO2 from the atmosphere after release
· Preventing emissions from entering the atmosphere
· Transitioning to less carbon intensive fuels
· Obfuscation and denial
· Managing demand for energy
o Educating and engineering society to use less energy
o Creating economic incentives for reducing emissions
CCS is an example of a method of preventing emissions from entering the atmosphere, thus mitigating coal’s role as an agent of climate change.
First We Capture
The current thinking regarding this strategy depends on a process called Integrated Gasification Combined Cycle (IGCC). Coal is extracted from the Earth and brought to the IGCC plant. Under high pressure the energy content of the coal is gasified9. Most of the impurities, such as chromium, arsenic, and mercury remain in the form of sludge or “slag” (in contrast to conventional power plants where these impurities are released as airborne emissions).10 This gas is burned and used to create steam which turns turbines, generating electricity. Heat from the burning of the gas is also extracted, which does not happen in conventional pulverized coal plants. The utilization of what is normally waste heat contributes to its greater efficiency and is the reason the process is called “Combined Cycle”.
Conventional plants tend to burn coal in air so the emission stream is mostly nitrogen. IGCC plants are designed to burn the gas in pure oxygen resulting in an emission stream more concentrated with CO2. This makes capturing the carbon from an IGCC plant an easier engineering task relative to a conventional coal plant.
The scenario is described in Figure 2.
Then We Sequester
After the CO2 emissions have been captured they are cooled and compressed. They are then pumped via pipeline to a sequestration site. Currently several sequestration strategies are being considered. There are many factors to consider such as storage capacity, geologic stability, leakage, biogeochemical interactions, and cost. Several methods are being considered.11
Presuming a significant amount of CO2 can be successfully captured and sequestered for a geologically significant amount of time, is it worth doing? That would depend on the cost. The cost of CCS should be evaluated versus both the cost of alternative ways of reducing CO2 emissions and the cost of doing nothing. A thorough survey of the array of approaches is beyond the scope of this article, but one can be included. For the sake of comparison, CCS can be compared to Demand Side Management (DSM) as an alternate method of reducing emissions.
DSM is a promising tack being promoted around the country for reducing carbon emissions by major utility companies and public utility commissions.19 This term refers to efforts meant to reduce or modify the demand for electricity by the consumers. Examples of DSM include rebates to customers if they buy energy-efficient appliances or a compact fluorescent light bulb subsidy. It could also involve an education campaign to encourage voluntary energy conservation. There are many techniques which can be classified as DSM, but a common goal is to reduce emissions and waste.
The cost of electricity with CCS applied is estimated to be about 50% higher than conventional power generation (where emissions are “dumped” into the air). Whereas, if emissions were reduced we attempted to reduce emissions by burning less coal it would actually save the public 61% of their current electricity costs. Communities that have made the choice to practice DSM are often highly satisfied.30
Pumping CO2 underground has been used for many years and with demonstrated success as a method of recovering additional fossil fuel reserves. However, the main source of the CO2 has been from geologic sources, and there has been little attempt to permanently store the CO2 underground. Often it is intentionally recovered to be re-used in other wells.
The widespread application of CCS (and hence its effectiveness at mitigating climate change) depends on the introduction of IGCC plants. However, of the approximately 150 coal plants proposed to be built in the United States over the next 25 years, only 34 are expected to be IGCC.31 If these estimates remain valid then even if CCS were cost-effective, its net effect on CO2 reduction would be marginal.
Regardless of the unlikely widespread implementation of CCS in the near future, there are about 18 international pilot projects underway.32 Most of these have begun only in the last year or two as federal subsidies in part due to increased tax incentives for this type of research. Very little information about their aggregate effectiveness is yet available.
The hope that CO2 emissions can be captured and pumped into the ground for eternal storage is alluring. Initial trials indicate that it is possible from an engineering perspective to pump significant quantities of CO2 into geological structures. Will they be able to hold the CO2 indefinitely? What is the seismic stability of these reservoirs? These questions have yet to be definitively answered.
Assuming it’s possible, is it worth pursuing? For the technologies that presently exist, sequestration is an expensive endeavor, raising the price of electricity by an estimated 50%. And unless there are significant advances in the technology, the process will probably not be widely applied in the near future. CCS may be viewed as a long-term strategy for addressing the problem of emissions reductions. However, considered by itself, sequestration can not meet the immediate goals of the United States; i.e., an 18% reduction of carbon intensity by 2012.33 Carbon intensity is defined as the carbon emissions per unit of productivity. The US carbon intensity has been decreasing by about 2% per year for the last few decades despite the increase in actual emissions.34
For every choice, there is an opportunity cost…what could we have done instead? Although CCS may have some promise in the long term, what is its opportunity cost? There are vast regions of the US which can take advantage of renewable resources, such as wind in the Great Plains and solar energy in the southwest. These under-exploited forms of renewable energy require additional research and development to meet our needs and reduce emissions. These technologies are already being chosen in appropriate geographies for the competitive pricing and ability to generate electricity without CO2 emissions. Simultaneously, CCS is being vigorously pursued by the federal government as a serious option for reducing emissions, competing directly against these proven and effective technologies. See Figure 4
It is generally acknowledged that anthropogenic CO2 emissions need to be curtailed. The question remains how this should be done. Some advocate for a transition to clean renewable energy sources (e.g. wind and solar power).37 Others promote less consumptive life-styles.38 CCS and similar technologies attempt to mitigate the pollution of our current sources of energy.39 The influential “wedges” paper describes how a combination of efforts (including CCS and behavioral changes) can reduce our emissions enough to stabilize the climate.40
Will CCS actually play a significant part of the solution? If so, it must be both cost-effective and scaled to address the scope of the problem. At present it meets neither of these criteria, nor is it likely to within the near future. However, several decades from now it might be sufficiently employed as a viable method of impacting CO2 emissions.
There are actions that can be taken today that can shrink the carbon footprint of the North American lifestyle and technologies that can drastically reduce emissions. These should be pursued regardless of whether CCS will ultimately be useful. There is danger in relying on distant technologies without a serious re-evaluation of the social conditions that have created this climate crisis.
12 Zeebe, R. and D. Archer, Feasibility of ocean fertilization and its impact on future atmospheric CO2 levels. Geophys. Res. Letters, doi:10.1029/2005GL022449, 2005.
16 Rychel, D; CO2; Past, Present, and Future for Independents; Presentation to the Tulsa Chapter SPE; Oct 2003; National Petroleum Technology Office
22 assume $10/tCO2
28 Stern, N., S.Peters, V.Bakhshi, A.Bowen, C.Cameron, S.Catovsky, D.Crane, S.Cruickshank, S.Dietz, N.Edmonson, S.-L.Garbett, L.Hamid, G.Hoffman, D.Ingram, B.Jones, N.Patmore, H.Radcliffe, R.Sathiyarajah, M.Stock, C.Taylor, T.Vernon, H.Wanjie, and D.Zenghelis (2006), Stern Review: The Economics of Climate Change, HM Treasury, London.