FIXING CARBON DIOXIDE
August 9, 2020 § 4 Comments
This discussion is about fixing, as in solving, but it is also, and mostly, about fixing as in rendering immobile. The impact of the greenhouse gas CO2 can be mitigated either by producing less or by capture and storage. A recent paper in the journal Nature triggered this piece. It discusses the feasibility of fixing CO2 in the form of a stable carbonate or bicarbonate by reacting atmospheric CO2 with minerals in the volcanic rock basalt, one of the most ubiquitous rocks on earth. Crushed basalt is to be distributed on farmland. The bicarbonate fraction is water soluble and run offs take it to the ocean, where the alkalinity mitigates ocean acidification. The reaction products are also a desirable soil amendment. This paper is mostly not about the technology. It studies scalability and the associated economics. The authors estimate the process can be accomplished at a cost ranging from USD 80 to 180 per tonne of CO2. Putting that in perspective, the current US regulation has a 45Q Federal Tax Credit of USD 50 per tonne sequestered in this fashion. This lasts for another 12 years. While no business ought to be built on the promise of subsidies, the length of time allows cost reduction to occur. At USD 80, the lower end of the range noted by the authors, the cost is in an acceptable range.
The use of basalt to fix CO2 is a part of the genre referred to as mineralization of CO2. Divalent species, but principally Ca and Mg, are present in rocks. In low pH conditions they react with CO2 to produce a carbonate (or bicarbonate). Olivine, another common mineral, often found in association with basalt, is a mixture of MgO.SiO2 and FeO.SiO2. The reaction product is MgCO3 and SiO2. For CO2 sequestration purposes this may be accomplished in situ or ex situ. The term sequestration most properly includes both capture and storage, but is often used just for the second step, and that is how we will use the term here.
A promising approach for in situ storage of CO2 is injection into oceanic basalt deposits. Basalt is formed when the magma from volcanic eruption cools rapidly. When it cools slowly, it produces species such as granite, with large crystals and high hardness, a rock more suitable for structural applications. Basalt on the other hand is fine grained and weathers easily. This is good for reactivity. In oceanic deposits it is even more so the case when the rapid cooling in water results in “pillows”, which partially disintegrate to be permeable. They are often overlaid with later placements of magma sheets. These impermeable layers act as barriers to injected CO2 escaping, affording time for mineralization. The mineralization is further accelerated if the injected CO2 is in the supercritical state (achieved at greater than 31 oC and 1070 psi). All fluids in this state have properties of both gas and liquid. Here the supercritical CO2 permeates the rock as if it were a gas and reacts with the mineral as if it were a liquid.
Ex situ fixing of CO2 follows the same chemistry as in situ, at least in the aspect that the product is a carbonate. The raw material can be tailored to the need if cost permits. The CO2 capture cost is the same in either case. However, an ex situ process has many advantages over in situ ones. The process kinetics can be advanced using higher rates of reaction using standard process engineering methods such as fluidized beds. Catalysis could also be employed. The products could also be expected to have value, such as in substitution of concrete ingredients. But, as in the case of fly ash from coal combustion, also a simple additive to concrete, the realization of that value can be elusive. Niche uses can be found, but monetization on the massive scales required to make a dent in climate change will require concerted effort.
The cost of production will still dominate the economics and the largest component of that is the acquisition of CO2 from the industrial combustion process or air. Air capture is a relatively recent endeavor and targets production cost of USD 100 per tonne CO2, at which point it becomes extremely interesting. The principal allure of this method is that it can be practiced anywhere. If located near a “sink”, the utilization spot, transport costs and logistics are eliminated. This underlines a key aspect of ex situ sequestration, the availability and cost of CO2 in the form needed.
The original premise for this discussion, mineralization of CO2 from the air, skips the CO2 acquisition constraint. But the focus shifts to the procurement of massive quantities of rock and crushing into small particles. Two pieces of good news. One is that basalt is possibly the most abundant mineral on earth, although a lot of it is at ocean bottoms. The other is that basalt crushes relatively easily, especially if weathered (contrasted to its country cousin granite). But the elephant in that room is that procurement still involves open pit mining, anathema to environmental groups. In recognition of this, the authors of the cited Nature paper encourage a study of availability of tailings from mining operations as basalt substitutes for oxides of divalent ions. They opine there are vast hoards of such tailings from mining operations over the years. They also suggest the use of Ca rich slags from iron making. These are oxides of Ca and Si in the main, with some oxides of Al. Lest this idea be extrapolated to slags from other smelting operations, a caution: the slags from some processes could have heavy metals and other undesirables such as sulfur. On the plus side of that ledger, the processing of certain nickel ores entails a beneficiation step that results in a fine-grained discard rich in Mg silicates, which ought to be very reactive with atmospheric CO2.
While the use of industrial waste for sequestering CO2 is technically accurate, acquisition and use of alkaline earth rich oxides will have hurdles of location, ownership, and acceptability to farmers, to name just a few. I am also reminded of the fact that when “waste” products with no or negative value create value for someone else, the price will often be revised, upwards. But the method in the cited paper certainly is a useful addition to the arsenal of measures to mitigate global warming, provided field operations verify the predictions on rates of reaction. This battle will only be won with many different arrows in the quiver.
August 9, 2020