Fossil fuels will remain an indispensable part of the global energy supply for at least the next 50 years, so a means must be found to burn them without pumping carbon dioxide into the atmosphere.
According to Martin Wolf, chief economics commentator of the Financial Times: "(Just) as the civilisation of ancient Rome was built on slaves, ours is built on fossil fuels. What happened in the beginning of the 19th century was not an industrial revolution but an energy revolution. Putting carbon into the atmosphere is what we do."
But there is no necessary connection between using fossil fuels and belching CO2 skywards. In future, carbon capture and storage (CCS) projects could sever the link, enabling fossil fuels to be burned safely in power plants while storing the emissions underground.
Deploying CCS is essential if the rise in average global temperatures is to be limited to no more than 2 degrees by the middle of the century, according to the International Energy Agency ("Technology roadmap: carbon capture and storage" 2013).
"As long as fossil fuels and carbon-intensive industries play dominant roles in our economies, carbon capture and storage will remain a critical greenhouse gas reduction solution," the agency warned in 2013. "There is no climate-friendly solution in the long run without CCS."
But progress towards deploying the technology remains achingly slow.
The technology of each of the three components of CCS - capturing the carbon dioxide emissions, transporting them, and pumping them underground - is fairly well understood. Each of them has been applied on a modest scale at various locations around the world for several decades.
But nowhere have they been applied to capture all the emissions from a utility-scale, coal-fired power plant. The first two large-scale power plant CCS projects, in Mississippi and Saskatchewan, will only become operational later this year.
Both projects will inject captured CO2 into depleted oil fields near power plants to enhance crude recovery.
Their operational and financial performance will not be known for several years, but given that both are pioneering, there will almost certainly be teething problems.
Southern Company's integrated gasification and combined cycle project at Kemper County in Mississippi is already a financial disaster.
Kemper's projected cost has spiralled from $1.8 billion to $5.5 billion, making it the most expensive power plant in the world for its output. Construction costs now top of those for a similar-sized nuclear power plant.
Financial and operational problems with first-of-a-kind engineering projects are common. The challenge is to learn from them and apply the lessons in second and subsequent generations of the same type of project.
In 2008 to help the new technology, the leaders of the United States, Japan, Germany, France, the United Kingdom, Italy, Canada and Russia pledged to "support the launching of 20 large-scale CCS demonstration projects by 2010 ... with a view to beginning broad deployment of CCS by 2020".
Since then, however, progress has been disappointingly slow. The target of 20 projects has been missed by a wide margin, and the timeline for deployment has slipped badly.
Cost of capturing CO2
Transportation and storage of carbon dioxide are fairly mature technologies, though no one has ever tried to deploy on the massive scale that would be needed to capture most of the emissions from the world's coal and gas-fired power plants.
For more than 40 years, carbon dioxide has been injected into depleted oil and gas fields in the United States, Norway, Algeria and China to help maintain reservoir pressure and sweep the remaining hydrocarbons towards producing wells.
The United States already has almost 4,000 miles of pipelines dedicated to carrying carbon dioxide from gas fields and industrial facilities to oil fields in Texas and Canada for such enhanced recovery (EOR) projects.
The tricky part of CCS is capturing the carbon dioxide in the first place.
CO2 can be separated from other gases using amine or ammonia scrubbers, which have been around for decades. The problem is how to do it efficiently.
When fossil fuel power plants burn coal or gas, four-fifths of the air that passes through them consists of nitrogen, which plays little part in the combustion process. Just one fifth is oxygen, which reacts with the hydrogen in fossil fuels to produce water and the carbon to produce carbon dioxide.
As a result, the exhaust gases from a typical power plant contain as little as 3 per cent CO2 for a gas-fired plant and 15 per cent for a coal-fired one. The rest is mostly nitrogen with some pollutants. To treat all this gas, scrubbers have to be very large to separate out the small proportion of CO2 from the much larger amount of nitrogen.
Scrubbers require a lot of energy. A typical scrubber will have large fans to blow the gas through the unit; pumps for all the water; a stripping unit to regenerate the chemical solvents; and a compressor. The entire process is energy intensive, especially regenerating the solvent by heating it to between 100 and 140 degrees Celsius in the stripping unit.
Capturing the CO2 from a typical coal-fired power plant would use 25 per cent of the total electrical output from the plant, something known as the "energy penalty". Given that a typical coal-fired power plant is only about 33-40 per cent efficient anyway, the loss of a quarter of its net power output is a major barrier to the commercial application of CCS.
"For a modern (high-efficiency) coal-burning power plant, CO2 capture using an amine-based scrubber increases the cost of electricity generation by approximately 40-70 per cent while reducing emissions per kilowatt-hour by about 85 per cent," the Intergovernmental Panel on Climate Change warned almost a decade ago ("IPCC Special Report on Carbon Capture and Storage" 2005).
Improving the economics
Carbon capture projects are focused on making the process more efficient and less expensive. One set of options centres on reducing the amount of nitrogen being processed and increasing the concentration of CO2
One route is gasifying rather than burning the coal, turning it into hydrogen and carbon monoxide, which are then used to run a combined cycle of gas and steam turbines. The byproduct of this process is a concentrated stream of CO2, which is cheaper to treat.
Kemper is the leading example of an integrated gasification and combined cycle power plant with carbon capture.
A big drawback is that gasifiers are expensive to build and run, as Kemper has illustrated. And integrating the process so that the gasifier, gas turbine, steam turbine and CO2 capture unit all work seamlessly is a major challenge. Kemper's ability to make it all work as planned has yet to be determined.
Another option is to burn the coal or gas in a nearly pure stream of oxygen, rather than ordinary air, a process known as oxyfuel or oxycombustion. But that requires an air separation unit to produce oxygen in the first place, and separation units require a lot of energy to run.
In Britain, a consortium of Alstom, Drax, British Oxygen Company and National Grid plan to build an oxycombustion plant in North Yorkshire equipped with CCS and have secured financial backing from the British government and the European Union.
In the United States, the Department of Energy is backing the FutureGen 2.0 project in Illinois, which would also employ oxycombustion.
A different approach is to try to make the power plant more efficient so that the energy penalty accounts for a smaller fraction of the usable output.
Super-critical and ultra-supercritical coal-fired power plants can achieve thermal efficiencies of up to 46 per cent, compared with just 33-39 per cent for an ordinary plant. If they can ever be made to work, advanced ultra-supercritical plants could push efficiency to 50 per cent or more.
Coupling carbon capture with a supercritical or ultra-supercritical coal-fired power plant would make the costs much less forbidding.
Even so, CCS plants will be expensive to build and run compared with today's coal-fired power plants.
But the only way to cut the cost is to start building many more power plants with CCS and learn how to build and operate them more efficiently.
Originally published by Reuters. Reproduced with permission.