Carbon capture and storage (CCS) technologies show promise in keeping temperature increases below 2 degrees Celsius. But there are currently too few operational CCS projects to determine if that promise of carbon capture can be a reality.
There are a number of reasons for the slow spread of these technologies designed to reduce CO2 emissions from fossil-fuel fired power plants. A major one is the cost associated with the massive amounts of energy required to capture and store CO2 — the cost of electricity can increase by up to 80 percent when applying commercial capture technologies to coal-fired power plant, according to the IEA Clean Coal Center. This means CCS would only be viable with a carbon price as high as $60 per metric ton, the Center says.
Next-generation carbon capture technologies show promise in lowering the cost and energy use associated with CCS, according to new research from the IEA Clean Coal Center.
“Besides its significant cost, which is common to many low-carbon energy sources, I believe the large energy penalty associated with first-generation CCS is a major barrier to public and government acceptance, and the dramatic efficiency improvements possible with some next-generation technologies could go a long way to improving the image of CCS,” report author Toby Lockwood told Environmental Leader. “A broader range of technologies will also help cater for the wide variation in power plant scenarios, such as a lack of cooling water or space.”
Post-Combustion Carbon Capture
In post-combustion capture processes CO2 is separated from standard coal flue gases, traditionally with amine solvents. Recently, novel liquid solvents, dry sorbents, and membranes have been investigated as alternatives.
Last month Carbon Clean Solutions said the results of its pilot project show that its drop-in solvent using the company’ patented “APBS” chemical could cut the cost of carbon capture by at least 50 percent by halving the energy consumption required compared with conventional solvents.
The IEA Clean Coal Center report says the use of engineered forms of the enzyme carbonic anhydrase to accelerate the reaction of CO2 with environmentally safe carbonate solvents shows promise. It means lower-grade heat can be used and brings capture costs down to below $40 per metric ton of CO2.
Meanwhile, harnessing some of the heat released in CO2 adsorption for the desorption step — a traditionally energy-intensive step — has promised capture costs approaching $30 per metric con of CO2. CO2-selective polymer membranes avoid steam extraction or chemical waste, and could be scaled up in a modular fashion.
Additionally, some post-combustion capture concepts allow the capture plant to generate its own power, which mitigates the energy consumption of the gas separation step. Calcium looping is a form of sorbent-based capture where sorbent regeneration takes place in its own oxyfuel-fired boiler, the report says. At a similar scale, gas-fuelled molten carbonate fuel cells can act as CO2 separating devices while generating electrical power. Last month ExxonMobil and Fuel Cell Energy said they are working together to advance a CCS technology that uses fuel cells to reduce emissions while increasing power generation, which the companies say could “substantially reduce costs” associated with carbon capture.
Pre-Combustion Carbon Capture
In pre-combustion capture CO2 is removed from a high-pressure mixture of CO2 and hydrogen obtained from coal-derived syngas, leaving the hydrogen to power a gas turbine. The report says the high partial pressures of CO2 offer greater potential for the efficient use of sorbent- and membrane-based separations at high temperatures.
The CO2 capture step can be combined with the prior water-gas-shift necessary to convert CO to CO2, helping drive the reaction to completion and reduce consumption of steam reagent. The report says these systems remain limited to small-scale trials, but could bring capture costs down to the region of $30 per metric ton.
By substituting combustion air for oxygen, oxyfuel combustion produces a relatively pure stream of CO2 for sequestration. Pressurized reactor concepts with minimal flue gas recycle have been developed that could reduce the efficiency penalty to below 5 percentage points. Even higher efficiencies are possible by firing coal syngas in high-pressure oxyfuel gas turbine cycles such as the Allam Cycle, which has estimated lower power generation costs than unabated coal plant.
Chemical looping combustion instead delivers oxygen to the fuel in the form of a solid oxide ‘carrier’ material, so avoids any gas separation step and enables high efficiencies and low costs. For solid fuel applications in particular, a low-cost carrier material is essential. The report says companies are actively pursuing pilots beyond the current 3 MWth level and costs below $30 per metric ton are expected at full scale.
“In terms of the wider role of CCS in mitigating climate change, I think it’s important that nations start to deploy existing CCS technologies if there is to be any hope of reaching the 2 degrees goal, much less the 1.5 degree target,” Lockwood said. “With some established CO2 infrastructure, we could then transition more easily to the advanced technologies detailed in this report, most of which will require at least another few years of development. However, if adoption of CCS in the power sector continues to stall, some of these more economic solutions should represent more attractive solutions for governments and investors in the future, providing a belated kick-start to the industry.”
It is too soon to say if these next-generation technologies will enable a cost-effective solution that can be deployed at the necessary scale to mitigate climate change. But with government support and deep pockets like ExxonMobil entering the race to develop cheaper, less energy-intensive CCS technologies, they may be the industry’s best shot.
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