Keeping the Lid On at 1.5°C: Part 2

Point-source carbon capture and sequestration: Also not the answer.

In the first blog post of this series, I explored air capture of CO2 and found it unsuitable as a way to address mounting greenhouse gas concentrations due to cost as well as ecological considerations.  In this blog, I explore point-source capture – that CO2 capture where it is created by burning fossil fuels or other industrial processes and then emitted to the atmosphere via a stack. is at coal- or gas-fired power plants, steel plants, or cement plants.

The biggest technical difference between air capture and point-source capture of CO2 is its concentration. Because point source capture is at the source, it is highly concentrated; it is then compressed and transported to a disposal site that could be hundreds of miles away. Whereas air capture, in principle, can be located near the sequestration site. 

Point-source CO2 capture covers a large variety of situations, coal in electric power, natural gas-fueled power plants and expansion to the industrial sector where CO2 emissions are high, notably steel and cement plants. As the climate crisis has intensified, interest in and support for CCS has grown significantly. 

A coal fired power plant retrofitted for CCS illustrates the range of issues with point source carbon capture and sequestration.

CCS makes power production less efficient because it increases the energy requirements for generating a unit of electricity from coal by about 30 percent (PNNL 2011, p. 19; Tang et al. 2014, Table 1). The increased energy input and large investment needed for a carbon capture plant also make electricity more costly.  CCS also makes the power produced more polluting, except for the CO2 captured.  More coal means more toxic fly-ash and more coal ash, which is already polluting water at hundreds of coal-ash pond sites around the United States.  Nitrogen oxide pollution would also increase, both because of the increase coal burning and because, in many cases, a chemical solvent, monoethanolamine (MEA), is generally used to extract the CO2 from flue gases. 

Water pollution would increase.  Eutrophication, which deprives aquatic life of oxygen, would increase significantly because of increased nitrogen oxide and ammonia emissions. Mercury pollution of the air and water would increase. While estimates vary by approach and power plant specifics, one study found that nitrogen discharges to water would increase by roughly 1,000 times and air emissions of nitrogen oxides would double (Tang et al. 2014, Table 2). Some of these added impacts could be reduced with advanced equipment but significant added environmental impacts will remain, including “increase in human toxicity, ozone layer depletion and fresh water ecotoxicity potential” (Koornneef et al. 2008).  In view of these problems, a variety of other approaches, such as absorbing CO2 in solid materials (“sorbents”) and separation of CO2 from other gases using membranes are being explored (National Energy Technology Lab).

CCS would create safety and environmental hazards.  Siting for sequestration is also a major issue with point-source CCS.  Scores or hundreds of miles of pipelines carrying high pressure CO2 would have to be built from the capture location to the sequestration location, creating safety and environmental issues along the route.  The safety issues were illustrated when a high pressure CO2 pipeline burst in Sartaria, Mississippi in February 2020, making more than twenty people ill. The pipeline was above the town on a hill, but the CO2, being heavier than air, flowed downward, poisoning people. The pipeline was carrying CO2 to be injected into an oil field to stimulate more petroleum production.

Finally, CCS is costly, currently estimated at $43 to $65 per metric ton of CO2. Existing coal-fired power plants emit about a metric ton of CO2 per megawatt-hour (about the amount of electricity consumed monthly in a typical medium to large detached house).  More CO2 is generated because of the added energy requirement for CCS and about 90% of this would be captured.  In round numbers, the added cost of delivered electricity would be about $50 to $70 per megawatt-hour, making the total cost of coal-fired power from an existing coal plant in the $80 to $100 range, compared to about $40 for power from a new utility-scale solar or wind power plant.  It the case of a natural gas combined cycle power plant, the added cost of CCS would be $25 to $35 per megawatt-hour, also making the total cost of power more expensive than solar and wind plants, before even taking into account the outsize problem of the warming impact of methane leaks.

Applying CCS to fossil fuel power plants does not make ecological, economic, or climate sense. Power plant CCS would still leave a fifth or more of the CO2 emissions of coal-fired electricity, when the need is for complete elimination of electricity sector emissions. That is, in fact, a key to climate mitigation in other sectors, specially transportation and space heating. Fossil fuel power plant CCS would increase costs; it would increase environmental impacts; and it would increase environmental injustice since fossil fuel plants being disproportionately in communities already overburdened with pollution and create new locations of injustice like the farms and rural communities through which CO2 pipelines would run.  Midwestern communities are already fighting proposed pipelines carrying CO2 from ethanol plants (Leanna First-Arai, 2022).

How about the “difficult sectors” for CO2 mitigation?  First of all CCS is unsuitable for some of them, notably aircraft, shipping, and construction machinery.  The most important sectors it could potentially be applied are cement and steel, each of which represents hundreds of billions of dollars of annual output; together they account for about one-seventh of global CO2 emissions.  Cement and steel are also the foundation of the world’s construction industry.  This is a complex topic.  Suffice it to say here fossil fuels in steel and cement can be replaced with hydrogen derived from renewable energy (known as “green hydrogen”).  In the case of steel, it is also possible to use renewable electricity directly to reduce iron ore instead of using coal in a blast furnace.

The main issue is cost. Low-cost green hydrogen is feasible if it is made with low cost utility-scale solar and wind (DOE 2020, p. 17). Given the ecological and safety issues associated with CCS and the likely difficulty of siting and operating sequestration sites, making zero-emissions steel in the first place is clearly preferable from considerations of climate, ecology, and environmental justice.  The same can be said of cement production so far replacing coal or natural gas with green hydrogen is concerned.  The issue of CO2 emissions from limestone reduction to lime -a process integral to cement production – is perhaps the most stubborn and difficult one, where some combination of alternative materials and carbon capture might still be explored.  

Long term storage of captured CO2 in concrete substitutes may have some promise.  For instance, a start-up Canadian company is piloting a cement-less concrete.  It starts with slag, a waste material from the steel industry, and subjects it to a stream of CO2 over 24 hours to make calcium carbonate. The resulting material, called “carbicrete”  can be cast into concrete blocks and used in construction; the product is shown here in a photograph supplied by the company. Whether such materials can be commercialized with CO2 captured from some industry is still an open question.  The source of the CO2 and the emissions associated with its capture will obviously affect how much net CO2 is captured and stored in the blocks.

CO2 capture and use may help in some cases, but it is no panacea.  On the contrary, most proposed uses are not suitable for climate change mitigation. To date, the most common use of captured CO2 has been to inject it into oil fields to stimulate oil production.  As another example, it is proposed to captured CO2 to make liquid fuels for aircraft; but the CO2 would be re-emitted when the fuel is burned.  Such CO2 capture and use approaches do not address the fundamental problem of an atmosphere overburdened with CO2 and the imperative of eliminating emissions as completely as possible.  Uses that emit CO2 back into the atmosphere in days, months, or even a few years are not climate solutions.

In sum, cost, ecological, safety, and environmental justice considerations indicate that point-source capture of CO2, like air capture, is, overall, damaging, costly, and not a climate solution, though there may be niche uses, such as the case of cement substitutes.  In the next blog post of this series, I will examine approaches to reducing greenhouse gas concentrations that are more promising ecologically and economically so that net negative CO2-equivalent emissions can be achieved as a complement to eliminating CO2 emissions for fossil fuel use.

CarbiCrete construction block made of slag injected with CO2.With permission of CarbiCrete,