Removing Carbon Dioxide from the Atmosphere

In October 2018 the Intergovernmental Panel on Climate Change (IPCC) issued a Special Report updating the occurrence of weather and climate extremes since 2014.  It compared the effects of an increase in the long-term global average temperature of 2°C (3.6°F) with that of a 1.5°C (2.7°F) increase, referenced to the pre-industrial climate, on humanity and the natural world.  Its conclusions emphasized the necessity of limiting warming to the more stringent goal of 1.5°C to avoid severe harms to our planet by later in this century.

In its models the Report used four climate scenarios.  All the scenarios admit that global emission rates, to greater or lesser extents, will not be reduced adequately, or fast enough, to limit the temperature increase without deployment of technologies to remove carbon dioxide (CO₂), a principal greenhouse gas, from the atmosphere.  CO₂ results from humanity’s burning of fossil fuels.  CO₂ removal, also called “negative emissions”, would compensate for any failure to reduce direct emission rates of greenhouse gases from their original sources.  An example of one of the scenarios is shown here:

Net emission rates per year throughout the remainder of the 21^st century (Blue line), representing the result of the following contributions.  Grey shading , annual CO₂ emission rates from burning fossil fuels; Brown shading , annual CO₂ emission rates or reductions from agriculture and forestry;  Gold shading , annual CO₂ reduction rates from bioenergy with carbon capture and storage.

Source: IPCC Special Report, Summary for Policymakers https://www.ipcc.ch/site/assets/uploads/sites/2/2018/07/SR15_SPM_High_Res.pdf

 

Large scale CO₂ removal is considered in an article by Lu and coworkers, entitled “Gasification of coal and biomass as a net carbon-negative power source for environment-friendly electricity generation in China”, released April 9, 2019 in Proceedings of theNational Academy of Sciences.

 

As an aside, the importance of scrutiny of scientific reports by anonymous peer reviewers is brought out in this article, for the authors thank “…the reviewers for valuable and constructive suggestions. We are particularly grateful to one of the reviewers for her/his painstaking efforts to critique several versions of the manuscript and for questions raised that contributed to an important improvement in the final presentation.”  Peer review remains the gold standard for assessing the worthiness of scientific reports.

 

The authors recognize the wide availability of plant waste in China.  In Combination with the widespread use of coal already in place in the country, the authors model using various mixtures of agricultural Biomass with coal in an efficient technology for generating Electricity, coupled with use of process heat to drive CO₂ Capture and Storage (CBECCS).  The model extends over the long-term lifetime of such equipment. 

 

The authors find:

• Crop waste proportions greater than 35% mixed with coal would yield net-zero emission of CO₂ evaluated over the service lifetime, moving toward significant levels of negative emissions as the crop waste proportion increases;
• The cost of generating electricity over the lifetime, including the equipment costs, is US$ 0.092/kwh (kilowatt-hour);
• As China moves toward a national policy of imposing a price on carbon in the near future, a cost of US$52/ton would render CBECCS competitive with China’s current pulverized coal power plants; and
• Conventional pollutants widely acknowledged to arise from coal-fired electricity generation and vehicle exhaust (oxides of sulfur, oxides of nitrogen, black soot and PM_2.5 aerosols (2.5-micrometer or smaller particles detrimental to human health)) are significantly reduced by CBECCS. Severe urban smog in China has been a major driver to curb use of fossil fuels because they produce high levels of these pollutants.  CBECCS would contribute significantly to improving public health in the country.

CBECCS is especially feasible in China at the scale needed because the country is endowed with a very large geological storage capacity for the CO₂ produced in the carbon capture and storage portion of the technology.  The model projects that only 0.036% of known geological formations suitable for storage would be needed each year; this capacity is widely distributed geographically across China.

The article lists barriers to implementing CBECCS technology at scale, including

• Deploying and integrating the many component advanced technologies to ensure smooth operation, and to extend it on a scale needed to make a significant contribution to reducing net emission rates;
• Implementing infrastructure to enable delivery of waste biomass to the CBECCS facilities at the scale and regularity needed; and
• Infrastructure and operating costs evaluated for CBECCS are more than double those for current coal generation. These costs can become competitive as China’s carbon pricing regime becomes operational as planned in 2020.

CO₂ Removal from the atmosphere (ambient air) in a pilot project in Canada.

The New York Times published a report on April 8, 2019 describing new technology for CO₂ removal from ambient air and preparing it for geological storage underground.  The company doing this work, Carbon Engineering, has attracted funding from oil companies Chevron and Occidental Petroleum, and the large Australian mining company BHP, as well as others.  Recently the company raised US$68 million.  The oil companies, sensitive to enterprise risk as renewable energy threatens to displace gasoline for transportation, are interested in carbon removal technologies such as being developed by Carbon Engineering as a way potentially to offset CO₂ emissions due to use of their products.  Fiona Wild, BHP’s vice president for sustainability and climate change states “This is about recognizing that climate change poses significant risk to all economic sectors.  Climate change is … a business risk that requires a business response.”  Similarly, Dieter Helm, professor of energy policy at Oxford University, says “If money is being spent on research and development to develop ways to sequester carbon, that is a good thing.”

A schematic flow diagram of Carbon Engineering’s technology is shown here:

Flow diagram for capturing the dilute CO₂ gas present in ambient air (1, left), and preparing it as pure CO₂ (upper arrow at stage 3, Calciner) for storage underground or for use in chemical processes to synthesize fuels.  The fan units include an alkaline solution that serves to absorb most of the ambient CO₂.  As shown at the right, the alkaline substance needed to absorb the CO₂ is regenerated and ultimately fed back to absorb more CO₂ in the fan units.

Source: New York Times, https://www.nytimes.com/2019/04/07/business/energy-environment/climate-change-carbon-engineering.html.

 

So far, according to the report, the pilot plant has produced the calcium carbonate pellets in stage 2.  Calcium carbonate is the mineral limestone.  If heated to a high temperature in the calciner the limestone would release pure gaseous CO₂ for collection.  

 

Discussion

 

An alarming impression of fossil fuel consumption by humanity since the beginning of the industrial revolution is shown here:

Global annual use of the three fossil fuels (gray, coal; orange, crude oil; teal, natural gas) shown from 1800 (before the industrial revolution) to 2017, in energy units of terawatt-hours.  For 2017 the energy from each fuel translates to approximately 5.4 billion metric tons/yr of coal; 30 billion barrels/yr of oil; and 122 billion cubic feet/yr of natural gas (using conversions provided at https://www.unitjuggler.com/convert-energy-from-kgSKE-to-Wh.html). Source: https://ourworldindata.org/fossil-fuels

 

These fuels, when burned, yield man-made CO₂ in comparably large amounts.  Since planetary warming is determined by the total amount of CO₂ that we have added to our atmosphere, one need only look back to this graphic and in her mind’s eye estimate the total area under the curve shown.

 

This result should impress the reader about the daunting task facing implementation of negative emission technology.  Even achieving fractional depletion of added CO₂, as suggested in the first graphic above, is a huge challenge.  A mitigating factor is that humanity has perhaps 1-2 decades to begin carbon removal at scale; in the first graphic above significant negative emissions (Gold shading) aren’t apparent until about 2040.

 

Carbon capture and storage technology at a less sophisticated level than presented here by Yu and coworkers has been known for more than a decade.  Even so, there are only a handful of such projects, operating as pilots, around the world.

 

Occidental Petroleum, and other petroleum extracting companies, already use CO₂, injecting it into operating oil wells to pressurize the crude oil and enable extracting more.  Thus CO₂ is being injected underground to produce more oil, which when refined and burned produces fresh CO₂!  Occidental believes this cycle could help make its operations carbon-neutral.

 

Carbon Engineering, and Chevron, in contrast, envision using CO2 to synthesize fuels, a process that requires the input of at least as much energy as was released when a fossil fuel was burned and yielded CO₂. Carbon Engineering plans to use renewable energy to generate hydrogen gas needed for making the synthetic fuel.  Thus a competition is implied, wherein a choice must be made between using renewable energy to serve the public directly versus using it in industrial processes to regenerate a carbon-containing fuel. 

 

As with the CBECCS process described by Lu and coworkers, the hurdle to achieve operation of the direct CO₂ removal at scale, as envisioned by Carbon Engineering, is high.  A single Carbon Engineering plant could remove 1 million tons of CO₂ per year.  This is a tiny fraction, about 0.003%, of the CO₂ produced by humanity around the world per year.

 

Both technologies described here are believed by the protagonists to become price competitive as the scale of operations increases and as use of fossil fuels falls due to market pressure if and when a meaningful price on carbon fuels is implemented.  In the meantime governmental resources and the private sector will drive the development of these technologies.

© 2019 Henry Auer