Can we use the carbonate "rocks" from carbon sequestration?

Photo credit: Construction Consulting and Testing

In my previous post, I discussed the launch of a pilot plant by a group of entities with the goal of sequestering CO₂ in manufactured carbonates that would subsequently be used in construction. These uses may include bricks, aggregates, pavers, etc. Of course, even were the CO₂ not used, it's better to have it in an inert mineral material than contributing to radiative forcing in the atmosphere. I suppose that any unsold inventory could simply be dumped. But what about a market?

The claim I mentioned in the previous post was that 50 plants could sequester a gigatonne of CO₂. The veracity of the claim aside, what would this mean? For a start, how much rock is represented by converting a gigatonne of carbon dioxide to calcium carbonate (CO₂ + CaO → CaCO₃₎, or possibly magnesium carbonate (CO₂ + MgO → MgCO₃)?  As an aside, it should be noted that these are EXTREMELY simplistic versions of the actual production reactions. A paper giving technical details is available here. Anyway, the molar weight of CO₂ is 44 grams, the molar weight of CaCO₃ is 100 grams. Thus, converting a gigatonne of carbon dioxide would produce (100/44)*1 billion tonnes, or 2.27 gigatonnes. The same calculation for MgCO₃ yields 1.92 gigatonnes. Let's call it 2 gigatonnes. Of course, it wouldn't make a lot of sense to convert this to cement (!) so what's the demand for bricks and aggregate worldwide?

What is typically thought of when "brick" is mentioned is the iconic red clay brick. These are usually formed by an extrusion process of pulverized clay materials mixed with water. The most common result is a brick of length 4", height 2 1/4", and depth of 4" weighing 2.7 kg. A similar volume of calcium carbonate weighs 3.20 kg, and of magnesium carbonate, 3.49 kg.  A gigatonne is 1000 kilograms, so 2 gigatonnes is 2 trillion (~2*10^{12}~) kg. I'll use 3.3 kg to determine that 606 billion bricks could be manufactured. The best information I was able to find says that around "seven to nine billion" bricks per year are used.

OK, can't use it all in brick, what about aggregate? This site estimates that demand for construction aggregate worldwide is on the order of 26 gigatonnes. Clearly, this is where the manufactured carbonates are best used. And, it would seem, there is sufficient demand. In fact, working backward, 26 gigatonnes of calcium carbonate aggregate would absorb 13 gigatonnes of CO₂, about 40% of our annual emission. And, one would assume, as emissions rise with a growing developing nations economy, so would aggregate demand. If it works, I like it!

There are two further considerations: energetics and economics. After all, if the energy required to manufacture the carbonates is excessive, particularly if it involves fossil fuel energy, there's a problem. And if the cost is too high, it won't matter about demand because it won't be purchased (unless, of course, carbon is taxed or credited in such a way as to balance the price).

I may or may not be able to get a handle on the economics but I should be able to nail down the energetics. I'll do that in my next post.

A home for CO2?

Photo credit: MIT

A hybrid group comprised of The University of Newcastle (through it's commercial entity, Newcastle Innovation), Orica (a chemical company), and GreenMag Group (appears to be an ecologically oriented innovation facilitator) has launched a pilot plant that converts carbon dioxide into bricks, pavers, aggregate, and other construction products. The idea is to capture CO₂ from power plants and other large industrial emitters and turn it into such products. It's seemingly plausible. After all, cement is made, in large part, by cooking the CO₂ out of limestone, which is CaCO₃, i.e., calcium carbonate, to form CaO, lime. The Mineral Carbonation International facility, in a simplistic view, reverses this.

In the inhabitat.com article that alerted me to this concept, it's stated that "fifty carbon capture plants around the world could potentially sequester over a billion tons of CO₂ annually." Now, our species emits on the order of 31.6 gigatonnes (Gt) annually, so this potential, if it's real, represents maybe 3.2% of our emissions (I'm assuming that metric tons or "tonnes" is the unit, if not, then it's 2.9%). I'm assuming that the sequestration plant would be collocated with the carbon dioxide emitter. I wouldn't think compressing and shipping the CO₂ would be feasible. There are some 50,000 coal fired power plants worldwide. So, if they all had such capability, we could sequester some 1,000 billion tonnes of CO₂, 32 times our actual total annual emissions! Woo hoo, let's get started!

But how do we reconcile these numbers? A "typical coal fired power plant," per the Union of Concerned Scientists, emits 3.5 million tons of carbon dioxide annually. If I assume they're utilizing short tons, it fits fairly well with an estimate calculated from Wikipedia that yields 2.4 megatonnes, using a one gigawatt power plant at 80% capacity factor. OK, so 50,000 plants times 3.5 million tons per plant yields 175 billion tonnes, or over five times worldwide emissions. Nothing sensible there either, clearly none of that makes sense unless the average coal fired plant is much smaller than a gigawatt plant. The Union of Concerned Scientists must be referring to a very large, gigawatt scale coal fired power plant.

OK, finally, looking through the EIA site, I can estimate that about 14 gigatonnes of CO₂ are emitted worldwide via coal combustion for energy. So the average annual emission of a plant would be 14 billion/50,000 or 280,000 tonnes per plant per year.  Does THIS make sense? The estimate in the previous paragraph of 2.4 megatonnes came from my plugging in a 1 gigawatt nameplate capacity plant at 80% capacity factor. That would mean that the "average" plant operates at (280,000/2,400,000)*800 megawatts or 93 megawatts. I don't find this to be unrealistic, there may very well be many thousands of relatively small coal fired power plants around the world.

But back to the claim that 50 pilot plants can sequester a gigatonne. That would be 20 megatonnes per plant, FAR more than the 2.4 megatonnes emitted by a large (gigawatt size) power plant. The only way for the claim to make sense is if, in contradiction to my speculation above, carbon dioxide is compressed and transported to a sequestration facility. Sorry, I doubt it. Next post: how much carbonate will be produced and what can be done with it? After that, energetics and economics.