Burning natural gas with no emissions?

Energy startup NetPower, led by MIT alums, is about to fire up a 25 MW natural gas-burning power plant that produces no net carbon emissions.The key is producing a waste stream of pure carbon dioxide, which can be directly stored in rock formations ("carbon capture and sequestration") without the need for the costly purification required by traditional power plants. If scaled up, such plants could reduce the carbon emissions of the electricity sector, but it still requires the extraction of fossil fuels. Could this be a component of the transition to a renewable energy economy?



Rick Shankman's picture

"Burning natural gas with no

"Burning natural gas with no emissions?"... Well no, not really.

The Allam cycle (Nobel Peace Prize winning Sir Rodney John Allam of Air Products in Allentown, PA -  got that Nobel along with Al Gore), requires an air separation unit (ASU - not the Sun Devils' ASU) to supply the combustor with pure oxygen.  As you know, ambient outside air is 20.95% oxygen.  Two problems: One, the ASU must itself be powered and that power requirement must be included in any calculation of net efficiency; Two, the ASU is constantly sucking more oxygen out of the atmosphere than the regular combustion cycle would use.

Further, the Allam cycle loves natural gas.  Sir Rodney's home state of Pennsylvania loves natural gas too; well, not the residents of Pennsylvania, but the fracker natural gas conglomerates.  That natural gas extraction causes emissions itself, especially in Pennsylvania towns besieged by the industry, like Dimock... https://m.youtube.com/watch?v=DRkEmyofXwM. These emissions need to be factored into realistic calculations of efficiency and releases.

As for the Allam cycle's use of supercritical CO₂ and water to drive the power turbine, this is clever and more efficient than regular steam; as the supercriticality of the CO₂ avoids energy losses inherent in the usual physical phase changes of the driving liquid/steam.  The captured CO₂ is recycled back to the combustor (much like automobile catalytic converters) and mixed into the pure oxygen stream from the ASU, where a portion is finally used to replenish lost (from the fluid) CO₂ emissions destined for sequestration.

The moral of the story... this is more diversionary tactics, as it's still a process requiring fossil fuels whose extraction produces copious emissions (both water and air) upstream of the Allam cycle process.

For the polluted (now, radioactive too) water side of the equation, see... https://m.youtube.com/watch?v=iiLQqykoWb4

Britta Voss's picture

That's my (also non-expert)

That's my (also non-expert) understanding of it - you need a concentrated enough stream of CO2 for it to be economical to store in geologic formations.
When the CO2 reacts with minerals and becomes part of the solid rock, it's no longer at risk of escaping to the surface, and the higher the proportion of CO2 relative to other waste components the more efficient this reaction will be. It's also partly for transportation, because the CO2 has to be condensed to a liquid to put in a tanker or pipeline, which is much easier at higher purity. This article gives a nice primer on CCS technology (and policy, pre-Paris): https://theconversation.com/pumping-co2-underground-can-help-fight-climate-change-why-is-it-stuck-in-second-gear-37572
Rajesh Kasturirangan's picture

I am not an expert by any

I am not an expert by any means, but the article gives a couple of clues as to why carbon capture isn't happening with traditional coal fired plants:
"But CCS systems bolted to conventional fossil fuel plants have struggled to take off because CO2 makes up only a small fraction of their exhaust. Capturing it saps up to 30% of a power plant's energy and drives up the cost of electricity." I understand that as saying that the problem isn't in storing the carbon but in the economies of capturing it in the first place. In contrast, this new scheme uses supercritical CO2 directly instead of steam so that it's more than 14% of the exhaust and, even better, capturing it makes sense because it's the active ingredient rather than the byproduct. That's my take - an expert will probably have a better answer.
Sarah Simon's picture

Curious about why the direct

Curious about why the direct exhaust couldn't be stored in rock formations.  

is it the chemistry of CO2 and the rocks?

Or is it the volume of power plant exhaust? (you need ~9x the volume of air to oxidize 1 unit of methane to CO2, + NOx + water)

or i