First, a nit about terminology: there seems to be an evolving tendency to
blur "torrefaction" and "pyrolysis". My original understanding of
"torrefaction" was as a very mild form of pyrolysis, specific to biomass,
which drove off all moisture but only just began thermal decomposition that
leads to evolution of H2, CH4, CH3OH, and traces of light aromatics. The
product was "darkened" but was not charcoal. (Think roasted coffee beans).
But i've seen the term applied to production of 'biochar" or, as in Rod's
article, "bio-coal". Both of those would be products of harsher forms of
pyrolysis. Couldn't we use the terms "low temperature pyrolysis" for
bio-char and "high temperature pyrolysis" for bio-coal, and reserve
"torrefaction" for the mild roasting process?
As I said, that's a nit. A more substantive issue is that the limitations
on bio-char as a vehicle for CDR lie with the availability of input biomass
and the cost of distribution and application; thermal energy for production
is not a significant issue, as the volatiles given off in production of
bio-char are more than enough to drive the process. Or at least it is if
one starts with bone-dry biomass. Admittedly, having thermal energy to
waste and the high heat transfer capacity of molten salts to drive the
process would avoid the need for an extended air-drying step. That would
help with the economics, but it wouldn't do anything to address the input
If small modular molten salt reactors were to become as common and as cheap
as Rod's article envisions, I would think that there are better ways to use
them. First priority would be replacement of all existing fossil-fueled
power generation. Second would be expansion of electricity supply to
support electrification of transportation as much as feasible. Third would
be production of synthetic fuels for transportation and heating
applications that can't readily be electrified. That would get us close to
zero fossil carbon emissions.
To go beyond that for CDR, use high temperature thermal energy from MSRs to
calcine limestone, capturing and sequestering the CO2 output stream. The
resulting CaO will support DAC of CO2 equal to what was given off in
calcining the limestone. That's the "brute force approach" to CDR that I
wrote about in part 2 of "The Carbonate Solution".
None of this means that production of bio-char and use of it to enhance
soil fertility isn't desirable. I think it is. I just don't see it as
having much to do with development of small modular MSRs.
*Addendum*: it's worth noting that neither calcining of limestone nor quick
pyrolysis of green biomass via molten salt need to wait for the advent of
small modular MSRs. Ordinary CSP is capable of producing molten salt of the
requisite temperature. But the land footprint for the solar mirrors would
be hundreds of times larger than that of a small MSR.
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