I've become quite interested in Bana grass as a biochar CDR option. It grows really fast on marginal lands in the subtropics and tropics. It has all kinds of uses, e.g. as fodder for livestock, and it helps loosen up compacted soils, so it can help with land remediation -- as an early step in a sequence of plantings to restore damaged soils. Helpfully, it's a hybrid that doesn't put out viable seeds, so it cannot readily become an invasive weed. Bana grass biomass can be used as input for all kinds of things, from paper-making to biofuels, and even to replace some of the wood chips in particleboard making.
In relation to CDR, Bana grass biomass could be processed into biochar, and ... hmmm. I've been wondering what supply chain pathway makes most sense. (By the way, there's a company in the
Philippines selling Bana grass biomass as "green coal"; it can replace coal in power generators, apparently. But that's CU, not CDR.)
Some researchers have worked out that by using solar power to desalinate seawater, and then precision agroforestry watering systems to water plants in the desert, e.g. in North Africa or Australia, vast quantities of trees could be grown.
Link to report here.
I'm thinking that the same desert land and desalinated seawater could be used to grow Bana grass, and that would likely generate biomass more quickly (more biomass per hectare per year) than any tree plantation. The question is what to do with that biomass, if CDR is the purpose. Processing Bana grass biomass into useful products is one possibility, but that's more elaborate than just pyrolyzing it, separating off the flue gases for whatever chemical synthesis or power generation purposes might be useful, collecting the biochar, and then... what?
One possibility is to use the biochar to improve soils. Make terra preta. And maybe desert soils could be turned into fertile soils by sequencing Bana grass, then leguminous plants, etc., adding biochar to the desert soil as part of the process. Terraforming. That would be interesting.
But I'm wondering about the energy costs of trucking biochar all over the place, working it into the soil, etc., and also about its long-term stability as a carbon store if it's put in or on soils in forests and fields.
What if the goal is just to sequester as much carbon as cheaply and quickly as possible, and forget about soil improvement?
I've been thinking that for CDR purposes, the simplest thing might be to simply put most of the Bana grass biochar in a huge desert-land operation like this into a mechanical compressor, apply serious pressure, and crush it into very dense cubes of black carbonaceous material, and then... I dunno. Stack up those cubes in the Australian desert. Make an artificial mountain range out of them (perhaps so high that it affects air circulation and water vapor condensation conditions and starts raining on one side of it, greening the land below, who knows - I say this half-jokingly, to be clear). Australia is 7.7 million sq km in size. Imagine a vast plantation in northwestern and central Australia 1 million sq km in size, i.e. 100 million hectares, i.e. a little bigger than British Columbia, fed by a grid of pipes carrying desalinated seawater (with some fertiliser added in solution), with half the net revenues from the operation going to the local Aboriginal tribes in the northwestern desert region to get their permission to use the land.
It's apparently possible to harvest 20 to 40 dry tons per ha per year from Bana grass plantations. Let's say 30t. This would mean about 3 billion dry tons of Bana grass biomass per annum could be harvested from 100m ha, of which ca. 40% (by mass) is carbon. Not sure how much of that carbon could be captured in biochar post-pyrolysis, and how much would escape in carbon-containing-gases offgassing during pyrolysis. Any idea?
ChatGPT says: "Carbon Retention in Biochar: Biochar typically retains a significant portion of the initial carbon present in the biomass. Depending on the conditions of pyrolysis, the carbon content in the resulting biochar can be 50-80% or even more of its weight. This means, if you started with biomass that was 50% carbon, and 50% of the initial biomass weight remains as biochar after pyrolysis (this is a simplifying assumption), then the carbon retained in the biochar could be anywhere from 25-40% of the initial carbon."
So perhaps we'd get around 10 tons of carbon locked away in biochar per hectare per year from a Bana grass biochar operation?
Anyway, pounding that biochar into tightly compressed black cubes and stacking them up in the desert, that would build a big black mountain, over time. I wonder how chemically stable those cubes would be. Quite stable, I should think.
And yet this insanely vast 100m ha operation would only generate around a Gigaton of C sequestration (give or take) per year, which is not nothing, but reminds us again of the folly of pouring 50 GtCO2eq (= 13.5 GtC) into the atmosphere every year.
Similar vast Bana grass operations should be feasible, in principle, in other mostly-desert countries like Libya, Egypt, Mauritania, etc. Ingredients: Cheap, otherwise relatively useless land, sunlight, piped-in desalinated seawater, some added fertilizer made via renewable energy powered pathways. How much would such an operation cost per ton of C sequestered, on a net basis? How much value could be captured from the energy-rich flue gases?
The scale of a 1 GtC per annum operation, if I'm right about the 100m hectare estimate, is mind-boggling. (And that doesn't even include the land required for the solar farms needed to power the seawater desalination operation, which would also have to be biiiig.)
If Bana grass is among the most fast-growing, high-productivity-per-hectare options for biochar production, and it would take a hundred million hectares per GtC CDR operation, that makes a nice baseline that one can visualize, to compare with other possible CDR approaches... and it underlines that we'd best look around for some less land-hungry options, n'est-ce pas?
For example, I have no clear sense of how much land and energy would be required for OAE operations of various kinds to sequester one GtC, and whether it would be less land- and energy-hungry or more so, when the whole supply chain is considered, than an operation to grow enough Bana grass on desert land with piped-in solar PV seawater to sequester one GtC via biochar.
In general, I find it helpful to try to get a sense of how much land and how much energy will be required as inputs to achieve one GtC for each different CDR approach. I wonder if there have been studies explicitly comparing CDR approaches on this basis (total land and total energy inputs per GtC sequestered). I imagine there must be some attempts at this kind of calculation already out there. If you know of any, please comment.
