Industrial scale microwave ovens could be used to produce biochar on a large scale. Professor Chris Turner from Exeter University, has built a 5 meter long prototype device that can lock-up a tonne of CO2 by pyrolysing wood into biochar, at a cost of $65. Each crop of trees absorbs CO2 from the atmosphere through photosysnthesis while they grow, and much of that carbon can be stored in a form of charcoal that is stable in soil for hundreds to thousands of years. In addition to storing carbon in this way, and remediating its presence in the atmosphere, the strategy could also produce synthetic terra preta soil, which is particularly fertile and less demanding in the amounts of water and nutrients that need to be added to it to grow plants in it.
Biochar has received regular mention for the past several months, and I have outlined its nature and advantages on various postings here. The wood or other biomass does need to be heated in some way, ideally such that liquid and gaseous decomposition products can be used as fuels to drive the process itself, making it self-supporting, and needing only biomass as a fuel and a reactant.
Frankly, I am sceptical as to how many microwaves can be built, and quickly at that, but the same question mark follows many proposed methods of environmental engineering. Allegedly, Turney's idea stems from a piece of serendipity that arose when he was a teenager. He microwaved a potato for 40 minutes by accident and found it had been converted into charcoal. He stresses that the kind of carbon capture and sequestration projects that are being promoted around the world only deal with emissions e.g. from power plants as they arise and do not address the carbon that is already in the atmosphere. If this can be pulled down through photosynthesis into biomass and the latter pyrolysed into biochar, in principle it is possible to decrease the atmospheric CO2 concentration.
As noted, however, building enough microwave ovens to produce biochar on a scale of the 8.5 billion tonnes of carbon per year released from fossil fuels will be no mean task, and probably impossible.