Serious Black: The Quest for Clean Coal
The search for ways to reduce carbon emissions has led to government grant money for schemes ranging from promising to wacky. Recognizing that there is no currently viable replacement for fossil fuels, with the possible exception of nuclear power, the US and other countries with large coal deposits are desperately looking for ways to continue burning coal without incurring the wrath of nature or the IPCC. Clear evidence of the seriousness of this effort is evident in this week's special edition of Science, dedicated to carbon capture and sequestration (CCS) technology.
If the world's nations have not taken the effort to lower greenhouse gas emissions seriously in the past that has changed. Fossil fuels—coal, oil and natural gas—were once considered national treasures, the keys to a bright and prosperous future. Nowadays they are poisonous pariahs, and the worst among them is coal. There are many who say clean coal is an oxymoron, that the ubiquitous black mineral which has done so much for humankind—powering the Industrial Revolution and providing much of the modern world's energy—should be abandoned as a power source because of the pollution it causes. Yet global coal use is rising and there is new talk of coal as the fuel of the future.
Coal is primarily used as a fuel to produce electricity and heat through combustion. World coal consumption is about 6.2 billion tons annually. China produced 2.38 billion tons in 2006 and India produced about 447.3 million tons in 2006. 68.7% of China's electricity comes from coal. The US consumes about 1.053 billion tons of coal each year, using 90% of it for generation of electricity. Currently, there are 417 coal-fired power plants in the US rated 100 MW and above.
In the US Energy Information Administration's International Energy Outlook 2009 reference case, world coal consumption increases by 49% from 2006 to 2030, and coal’s share of world energy consumption increases from 27% in 2006 to 28% in 2030. Despite the wailing of climate change alarmists coal, responsible for 30% of the humanity's total carbon emissions, is actually increasing it's piece of the energy pie.
Nowhere is this upward trend more evident than in the developing nations. In 2006, non-OECD energy-related emissions of carbon dioxide exceeded OECD emissions by 14%. With China and India leading the way, by 2030 energy-related carbon dioxide emissions from the non-OECD countries are projected to exceed those from the OECD countries by 77%.
The primary reason coal gets so much attention is because a number of countries, including China and the US, have abundant reserves of this fossil fuel. The technology to mine, transport and convert coal's energy content to other forms is also well know and widely available. So how can coal be the fuel of the future? This hope all hinges on an experimental and mostly non-existent technology called “carbon sequestration.” To understand what the hoopla is all about we need to know a few things about atmospheric CO2 levels and the carbon cycle.
The Carbon goes Round and Round
As I detailed in my earlier post, “The Grand View: 4 Billion Years Of Climate Change,” the amount of carbon in the atmosphere has varied widely over geologic time. This fluctuation was caused by the effects of ice ages, volcanism, asteroid impacts, and the spread and retreat of vegetation. About 500 million years ago, atmospheric CO2 density is estimated to have been 20 times as high as it is today. Since the end of the last glacial period, around 10,000 years ago, atmospheric CO2 has remained around 280 parts per million, very low by historical standards. Over the past couple of centuries, increasing use of fossil fuels has increased atmospheric CO2 levels to about 385 ppm.
Atmospheric carbon dioxide over the past 600 million years.
The carbon from the CO2 in Earth's atmosphere, along with all the other forms of carbon contained in the oceans, rocks and living things, is constantly being exchanged in what scientists call the carbon cycle. Dennis Normile has provided a rather nice diagram describing the main points of the carbon cycle in the aforementioned special issue of Science (see “Round and Round: A Guide to the Carbon Cycle”). Quoting from the text of his diagram:
Carbon continuously cycles through living creatures, the atmosphere, the oceans, and Earth itself in one of nature’s more amazing balancing acts. The main building block of life, carbon is fixed into terrestrial and marine other organisms tissue through photosynthesis. Animals eat other organisms and burn carbohydrates for energy, releasing carbon dioxide (CO2) through respiration and through decay after death. For much longer than humans have walked the earth, carbon generation has roughly equaled carbon consumption. But humankind has tipped the scales, adding CO2 to the atmosphere by burning fossil fuels—the products of eons of accumulated plant matter transformed into coal and oil by geologic processes.
When carbon is released into the environment the carbon's point of origin is called a source, when carbon is removed from the enviroment and stored for some period of time the storage place is called a sink. Sinks include vegetation, ocean waters, and rock. Sources include rotting vegetable matter, the oceans, and, of course, burning fossil fuels.
Humans currently pump around 7 gigatons of CO2 into the atmosphere each year, but only about 50% remains there. The rest disappears into what is known in scientific circles as the “missing sink.” We described some of the places this carbon could be going in Chapter 7 of The Resilient Earth, “Changing Atmospheric Gases” (pdf). Among the possibilities are greater-than-expected uptake by northern forests, the oceans, and desert soils. Solving the mystery is more difficult because of uncertainties about how much carbon is going where.
An example of how science is constantly discovering new components of the carbon cycle is seen in the recent discovery of a new carbon sink in the world's oceans. Christened the “Jelly Pump” by its discoverers, it is the result of the actions of jellyfish like creatures called thaliaceans. You can read more about this discovery and how it is causing scientists to re-evaluate their ideas about the carbon cycle in my article “New "Jelly Pump" Rewrites Carbon Cycle.” The amount of carbon released by mankind is small compared with the total amount “in play” in the carbon cycle. But even with the missing sink, carbon dioxide levels in Earth's atmosphere keep increasing, which is causing panic in some quarters.
Can Carbon Black Become Eco Green?
This leads us back to carbon capture and sequestration (CCS). Those rising CO2 levels, which are blamed for causing dreaded global warming, must be reined in. At least that is the official story from the UN and many national governments. To slow the atmospheric buildup of CO2, the US National Research Council has called for building 15 to 20 coal-fired power plants with CCS before 2020. “The urgency of getting started on these demonstrations to clarify future deployment options cannot be overstated,” the report said. Today, a number of projects are under development. Most aim either to bury CO2 separated from natural gas reservoirs or to pump it into oil reservoirs to push out more oil. Science has provided a map that shows some of the major CCS projects around the world (unfortunately, you will need an AAAS membership to view it online).
The commercialization of carbon capture presents many technological and political challanges. The technology to actually capture CO2 must be developed on an industrial scale, as well as transport of liquified carbon dioxide and its storage in exploited oil fields or saline formations. Many hurdles remain to be overcome. According to R. Stuart Haszeldine's review article, “Carbon Capture and Storage: How Green Can Black Be?,” urgent action is required if carbon capture and storage is to play a large role in limiting climate change. Quoting from the review:
CCS strips out, purifies, and concentrates CO2 emissions from fossil fuel combustion at large single sources such as power plants (Fig. 1). Three methods of CO2 capture are currently being investigated. Postcombustion capture separates the CO2 with the use of chemical solvents, precombustion capture chemically strips off the carbon, leaving hydrogen to burn, and oxyfuel combustion burns coal or gas in denitrified air to yield only CO2 and water. After leaving the power plant, the captured CO2 is pressurized to 70 bar, forming a liquid that can be transported to a storage site, where the fluid is injected into rock pores deeper than 800 m below the surface. Good choices of storage sites will retain CO2 without appreciable seepage for tens of thousands of years. Monitoring will be required for decades into the future, combined with techniques to remediate deficient storage.
Haszeldine estimates that fitting all coal and gas power plants with CCS by 2050 would reduce world CO2 emissions from energy by 20%. At the same time, at least in the UK, CCS may cost each household an extra 10% per year for electricity. For reasons given below I find that is a very optimistic cost estimate. The type of closed loop system envisioned for CCS is shown in Figure 1 from the review article, shown below.
Figure 1 from the Science article showing the envisioned CO2 life cycle.
Among the technical hurdles facing CCS is carbon capture itself. CO2 separation, if scaled-up, could consume 25 to 40% of the fuel energy of a power plant. Estimates from experts in the power generation industry are that for every three coal-fired plants generating electricity you would effectively need a fourth plant to power the capture operation. All capture technologies face significant challenges to rapid commercial deployment. Haszeldine estimates that at least two “learning cycles” are needed to demonstrate operation and enable commercial guarantees for construction. “This is a technically possible but politically optimistic pathway,” he states.
Perhaps the biggest challenge is injection and geological storage of the captured CO2. To have an impact on worldwide emissions of fossil fuel–derived CO2, extremely large volumes of geological storage are needed. According to the review article “efforts to scale-up injection face a fundamental problem: The subsurface contains no empty space. Any injection of CO2 into a depleted hydrocarbon field or a saline formation has to displace or compress the existing pore fluid by raising the pressure.” In other words, it isn't as simple as just drilling a well and pumping the gas underground.
For various reasons, depleted oil and gas fields cannot provide sufficient storage space for the volumes of captured gas involved in this scheme. The only possible storage areas that promise to be large enough are saline aquifers. In theory, saline aquifer formations could store the equivalent of hundreds of years of present day power plant emissions, but that estimate is probably too optimistic as well. Additionally, CO2 stored in unconfined formations might migrate tens of kilometers during a 30-year injection period, making leakage monitoring more expensive and less reliable. If the storage sites do leak then CCS is nothing more than a very expensive fools game.
The idea that clean coal will save the world from global warming has become something of an article of faith among policymakers. In the US Energy Information Administration's International Energy Outlook 2009 CCS features prominently in all the main blueprints for reducing greenhouse-gas emissions. The Stern Review, a celebrated report on the economics of climate change, considers it “essential”. It provides one of the seven major areas of emissions cuts proposed by Robert Socolow of Princeton University. So important is this effort to the Obama Administration Energy that Secretary Chu wrote the lead editorial for the special edition of Science, in which he stated:
The scale of CCS needed to make a significant dent in worldwide carbon emissions is staggering. Roughly 6 billion metric tons of coal are used each year, producing 18 billion tons of CO2. In contrast, we now sequester a few million metric tons of CO2 per year. At geological storage densities of CO2 (~0.6 kg/m3), underground sequestration will require a storage volume of 30,000 km3/year. This may be sufficient storage capacity, but more testing is required to demonstrate such capacity and integrity.
I believe that Dr Chu has effectively captured the enormity of the problem. To make this scheme work we must find a way to permanently store 30,000 km3 of captured carbon dioxide each year. For the metric challenged that's 7,200 cubic miles of CO2 each and every year, possibly for hundreds of years. Again quoting from the Haszeldine review article: “Worldwide, the original static estimates of storage capacity are now being substantially downgraded to many decades rather than hundreds of years of emissions.”
All told, CCS could require the drilling of 100,000 wells each year and constructing tens of thousands of miles of pipeline to transport the captured CO2 from power plants to the wells. It could end up costing $1.3 trillion a year for the US alone. This is the equivalent of a Wall Street bail out every year for the foreseeable future. Where would the money to pay for this come from? From the consumers of electricity of course—every last one of us. I have been given estimates from people in the power industry that they expect residential electrical bills in the American Midwest, where 85% of electricity comes from coal, to rise by $1,800 a year.
The energy source of the past is now the energy source of the future?
No one really knows the how pumping 7,200 cubic miles of CO2 underground each year will affect Earth's ecology. But that is probably not an immediate concern. Though some predict efficiencies of 90-95% capture a more realistic level is 70%. It is also probably that only a few pilot plants will be up and running by 2020. If they are successful, these initial demonstrations are only the first step toward building a new CCS based coal industry. Low-cost reliable capture at clusters of CCS power plants must emerge, and national pipe networks must be developed, delivering to aquifer storage capacity that must have been validated. Even on a 20 year time schedule only a small percentage of coal-fired plants will be using CCS two decades from now. CO2 from coal account for around 30% of total human carbon emissions. By 2030, even if an optimistic 20% of the worlds coal-fired power plants are converted, CCS may save 70% of 20% of 30% of total human carbon dioxide emissions, a paltry 4.2% total.
Given the punishing cost, plus the fact that this technology has yet to be proven on an industrial scale, are there no alternatives? Steven Chu, when asked what he would do if faced with a choice of living next to a coal-fired energy plant or a nuclear facility, answered “me personally, I'd rather be living near a nuclear power plant.” In an interview on National Public Radio's Morning Edition show, the Secretary went on to explain that a nuclear plant produces less pollution and US nuke plants have good safety records. Chu also noted that advances in technology will make storage of nuclear waste less of an issue in coming years. Maybe clean coal isn't the answer after all.
Can clean coal become a reality? There are those who claim clean coal is an impossible dream, while others say it is America's “God given right to burn coal.” While clean coal sounds like a great idea, as engineers we must deal with what is available now, not what might work sometime in the future. But we are also realists—king coal is not about to be put out of business anytime soon, even if the greens can overcome their atavistic attitudes toward nuclear power.
Supposedly our world is afflicted by the malady of global warming. If this is the recommended treatment it may well be a case of the cure being worse than the disease. Little wonder that many skeptical governments are willing to take the chance that global warning's impact will not be as devastating as the climate change alarmists claim.
Be safe, enjoy the interglacial and stay skeptical.