Expert Dialogue: Should we be pursuing strategies to remove non-CO2 greenhouse gases from the air?

Dr. Renaud de Richter from Montpellier, France recently reached out to the Center for Carbon Removal with an interesting question: “would the Center consider activities related to "greenhouse gas (GHG) removal" beyond the main most prevalent GHG, CO2?" An email exchanged ensued, which I have lightly edited and published below:

Noah Deich:  Can you explain the difference between “CO2 removal” and “GHG removal”?

Renaud de Richter: Carbon dioxide (CO2) is the main greenhouse gas (GHG) contributing to climate change. But it is not the only GHG—many other GHGs (such as methane, nitrous oxide, CFCs, HCFCs, etc.) also have a warming effect on Earth, and contribute around a third of anthropogenic warming (see two charts, below).

Source: US EPA

Source: US EPA

Natural processes in the atmosphere slowly destroy these non-CO2 GHGs over time, mainly by oxidation in the troposphere (0-20 km high), and by "photolysis" with UV rays in the stratosphere (>20km). But these natural processes take a long time, making the atmospheric lifetime expectancy of these non-CO2 GHGs quite long (see table below).

The chart above also highlights another key difference between CO2 and non-CO2 GHGs: non-CO2 GHGs are several orders of magnitude less concentrated than CO2. Sherwood’s rule postulates that the cost of separating a mixture into its components increases with dilution, which suggests that capturing non-CO2 GHGs will be even more challenging to capture than CO2. But if we can figure out an economically-viable way around this problem, non-CO2 GHG removal could provide a valuable tool in our negative emissions toolkit.

ND: Do non-CO2 GHGs serve any beneficial purpose in the atmosphere (that is, would destroying these GHGs lead to any adverse consequences to the environment / society)? 

RR: Almost all CFCs and HCFCs are non-natural, so destroying these would provide no adverse consequences (alongside a number of additional environmental benefits as these compounds can lead to dangerous ozone depletion as well). There are natural methane and nitrous oxide emissions, but destroying these GHGs will not lead to any adverse consequences to the environment / society because we can reduce removal efforts once their concentrations return to pre-industrial (i.e. sustainable) levels.

ND: How does your non-CO2 GHG removal idea work?

RR: Our proposal is to enhance the speed of destruction of the non-CO2 GHGs by catalysts (often natural common mineral oxides, like titanium dioxide, zinc oxide or manganese oxide) that are activated by sunlight. Thus the destruction of these GHGs is accelerated and takes only minutes instead of hundreds of years.

The main problem is to put the highly-diluted GHGs from the atmosphere into contact with the photo-catalysts. To solve this problem we propose using "solar updraft towers" which is a novel type of renewable energy power plant (with no-CO2 emissions) that involves massive air flows through a concentrated tower to generate power. Because such a large quantity of air passes through each tower, it is possible to contact and remove the GHGs in this air stream with a much greater efficiency.

Small-scale solar updraft towers have been built throughout the world, but they can be capital-intensive to build at a large scale, and so no such projects have been undertaken to date.

ND: How scalable is this idea? 

RR: In theory, if all our world's electrical energy needs (by 2050) were satisfied by 200 MW solar updraft towers, one atmospheric volume would pass under them every 15 years. If 25% of the non-CO2 GHGs were destroyed passing through these towers, they would destroy hundreds of millions of tons of CO2-equivalent GHG emissions each year, and at the same time produce CO2-free renewable electricity. 

This many solar updraft towers will take a lot of land – equivalent to the size of Montana (but so will many other carbon removal solutions). So even if we only had a small fraction of our energy needs met by such “carbon-negative” solar updraft towers, they still could provide a meaningful contribution to the portfolio of carbon removal solutions

ND: How much is it likely to cost?

RR: The cost of one of these 200 MW solar updraft tower power plants is estimated to be around US$1 billion (less according to some experts). Researchers have estimated that it will cost more or less 0.08 €/kWh to produce electricity from such a plant.  

On a carbon basis, enough air would pass through one 200 MW solar updraft tower to destroy about 0.4 million tons of CO2-equivalent GHGs per year (at a 25% net non-CO2 GHG-removal efficiency).  The marginal cost of adding photocatalysts to the towers for GHG removal would be approximately $20 million—if these photocatalysts remain active 5 years, this costs translates into $10/t CO2-equivalent cost of GHG removal.

Assuming a 25 year book life and small opex for such a 200 MW tower, this translates into roughly a $100/t CO2-equivalent cost of GHG abatement. If you also include the value of avoided fossil fuel use in this calculation, the total GHG abatement cost of the whole system would come out even lower(i.e. around $10/t CO2-equivalent).

As for economic viability of solar updraft towers compared to solar PV systems is very difficult to estimate. There are many different prices for energy production from solar PV systems depending on location, insolation and on year of installation of the plant. While we can expect that PV will likely be cheaper in the future, solar updraft towers could provide an economically-viable alternative to "PV + energy storage" for peak load (as solar updraft towers can store thermal energy in the ground for night electricity production to greatly reduce intermittency at minimal additional cost). (These two articles, here and here, mention possibilities for night capture of these non-CO2 GHGs and then their release and photocatalytic destruction during the day, which can improve the amount of GHG destruction.)

ND: So what is needed to better understand the potential of this non-CO2 GHG removal approach?

RR: A "big" prototype of solar updraft tower has already been built and tested (in Manzanares, Spain, 1982-1989). But a large-scale prototype is less important to test the efficiency of non-CO2 GHG removal. For this purpose, we can start with a greenhouse used in agriculture where sewage sludge or pig manure are drying, and measure the amount of some GHGs inside and outside after passing through a large surface area of photocatalyst exposed to sunlight.  Critically, this will help us understand whether our assumptions on efficiency and cost can be confirmed, which will then help validate the move to a larger-scale prototype.

It will likely only cost around 200 000 € ($250 000) for next steps (half of this amount for the apparatus to analyze the quantity of the GHGs gases before and after removal). Similar experiments have already been made in Italy, which we hope to build upon by testing nano TiO2 doped photocatalysts, active under UV and also under visible light, under a greenhouse and sunlight.

ND: Thanks Renaud!

RR: Thanks Noah! Also thanks to the Center for Carbon Removal for informing your readers about the possibilities of non-CO2 GHG removal from the atmosphere.

What are your thoughts on non-CO2 GHG removal? Share in the comment section below or on Twitter @CarbonRemoval!