I was referencing the feed concentrations, and this ties into the use of CO2 extraction from the flue gas. There is no economic incentive, to do that concentration since the higher levels of CO2 in the feed would not be productive. Remember also that the off-gas from the process is oxygen.

I assumed too much. In my mind, there is only one long term source of CO2, the atmosphere. Gas wells for CO2 suffer from the same depletion problems that plague methane gas wells. Their flow slows and eventually stops. On the other hand we badly need to develop some technology that depletes CO2 from the atmosphere. CO2 atmospheric concentration is already to high.

In flue gas the push is to keep the CO2 level high because it is easier to capture at high concentration, and it needs to be captured in order to address global warming problems. In an extreme, some proposals envision burning fuel in oxygen in order to eliminate dilution by nitrogen and pollution by nitrogen oxides.

I expect that there will come a time when the only source of flue gas will be from power plants that are burning biofuels, like wood or something cellulosic like dead algae. What then? The recovery of CO2 surely cannot be 100% efficient. CO2 will gradually leak into the atmosphere. The Earth will continue to warm.

The earth will continue to warm for the next 150 years, even if we halt all fossil fuel burning tomorrow:

In fact, the world's leading scientists agree that it's already too late to halt global warming entirely. "We can't prevent some damage," says Stephen Schneider, co-director of the Center for Environmental Science and Policy at Stanford University. Even if we were to magically end CO2 emissions tomorrow, the gases that we've already unleashed will continue to raise temperatures for another 150 years. "That's unpreventable," Schneider says.Source

The solution (combined with other measures of course) is geoengineering. Many leading climate scientists now support geoengineering, including Paul Crutzen, Tom Wigley and Ken Caldeira. For example, see ALBEDO ENHANCEMENT BY STRATOSPHERIC SULFUR INJECTIONS: A CONTRIBUTION TO RESOLVE A POLICY DILEMMA?(pdf).

Sure! Because the climate system is so resilient we can just mess with it at will!

This is a very dangerous and, imho, foolish way to go. Let's just live differently and get the same results without the unknown feedbacks.

Cheers

"The (emphasis added) solution (combined with other measures of course) is geoengineering."

I strongly disagree. Geoengineering is not the solution, but a class of proposed solutions, none of which have been validated by any means excepting hype. I think nature will heal itself. It's not clear what size population of homo will remain after the healing. Geoengineering likely will have unintended consequences that will make problems much worse, and healing much slower.

The solution is geoengineering.

Sure it is 'john denver'.

Odds are your 'vision' of what geoengineering is is far different than mine - but hey - you won't be around to see the results of your plan, so why should you care?

I expect that there will come a time when the only source of flue gas will be from power plants that are burning biofuels, like wood or something cellulosic like dead algae. What then? The recovery of CO2 surely cannot be 100% efficient. CO2 will gradually leak into the atmosphere.

But if only recently grown biomass is used as feedstock, all that CO2 came from the atmosphere.  You only increase atmospheric CO2 if you "mine" carbon from long-term stores, such as fossil fuels, old-growth forests, peat bogs etc.

we badly need to develop some technology that depletes CO2 from the atmosphere.

If you look at the annual cycle of the Keeling curve, you'll see it already exists:  plants deplete CO2 from the atmosphere to the tune of 5-6 ppm every growing season.  All we need to do is direct some of that captured CO2 to a destination other than the atmosphere, and we will create a net reduction in CO2.  Easier said than done, but perhaps not all that difficult to do either.

Apparently nitrogen might be a problem. Guess we need to plant more nitrogen-fixing plants. I believe this is a feature of most sustainable farming methods, so an increase in organic farming - perhaps excepting rice due to methane production - and the use of nitrogen-fixing cover crops should help with CO2.

Being the non-scientist I am, I may well be wrong.

Cheers

"... Easier said than done, but ..."

Yes! It will be a big job to harvest a significant fraction of natural annual plant growth and sequester it somehow.

The great bulk of the biomass in a old growth forest is less than 200yr old. Growing forest and letting it reach climax and decay is not really a good idea, but it is a beginning. If at some time in the near future there is a great deal of near climax forest, the problem of scheduling the harvest and sequestering would not be so great as it would be otherwise.

Also note that the 5-6ppm annual fluctuation is the difference between northern and southern hemisphere growth/decay. The total rate of carbon capture must be somewhat larger than 5-6ppm/y.

A method for sequestration that I find appealing is turning the biomass to charcoal and burying it. (But not as soil amendment, which has recently been shown to have environmental problems. Wardle, et al. Science v320, p 629)