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Carbon Capture and Sequestration: Where Does it Fit? (Part 3: Big Picture Issues)

Stacks with Nice Sky
Roger Arnold's picture
Owner Silverthorn Engineering

Roger Arnold is a systems architect and engineer, recently focusing on energy systems and controls. His consulting company, Silverthorn Engineering, is developing architectures and software for...

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  • Aug 7, 2012
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Carbon capture and sequestration (CCS) is a surprisingly controversial topic. Coal interests and climate change deniers insist, predictably, that it is unnecessary and would create an intolerable drag on the economy. But CCS is also opposed by some militant environmentalists. Their position is that CCS would be a band-aid to justify continued reliance on fossil fuels and prop up an unsustainable "business as usual".

The only way to avoid disaster, as these militant greens see it, is social transformation. We must learn to use much less energy overall, and phase out fossil fuels as quickly as possible in favor of low-carbon alternatives. CCS requires energy and would increase our total energy consumption. It would thus consume resources that these individuals feel would be better spent elsewhere. We must focus, they say, on efficiency, on consuming less, and on replacement of fossil fuels with low-carbon alternatives.

The Problem of Scale

The anti-CCS school has its points. The sheer scale of investment required is daunting if we intend to rely mainly on carbon sequestration to stabilize atmospheric CO2. Consider:

  • Based on EIA data, pulverized coal (PC) power plants in the US currently consume about one gigaton of coal annually, produce some three million GWh of electricity, and emit about three gigatons of CO2;
  • Equipping an existing PC plant for carbon capture and sequestration (CCS) using available technology is estimated to cost about $1000 / kW rating. It reduces the plant's net power output by about 24%;
  • For the country as a whole, that translates to about $425 billion in capital for CC equipment, plus about $100 billion in new generation (assuming NGCC) to replace the power lost to CCS operations;
  • Add to that a few $100 billion for the cost of the pipelines to transport CO2 to sequestration sites and the cost of the injection wells and pumps for pushing compressed CO2 into deep geological formations.

All told, we can figure on the order of $1 trillion in capital spending. That would be spread over a period of decades, so it's not impossible. It's very much less, for example, than our recent military misadventures have been running. However, it only deals with the ~33% of our CO2 emissions that are associated with coal-fired power generation. Most of the other two thirds is from vehicles, and that portion is harder to deal with.

The bottom line is that, yes, to stabilize atmospheric CO2 levels, it would be a very challenging and possibly futile task to rely only on sequestration of carbon from our current mix of energy resources. We must reduce our consumption of fossil fuels themselves, and shift toward non-fossil alternatives.

The Policy Issue

Well, duh! But that's not really the issue, is it? There's no question that if we decide to burden fossil fuels with something near their true external costs, it will make them a lot more expensive. That will make efficiency and non-fossil alternatives correspondingly more attractive. And even if we decline to tax CO2 and leave future generations to deal with the consequences of our fossil fuel addiction, rising global demand and the increasing difficulty of replacing depleted reserves will eventually get us to the same place. So one way or another, we will be shifting away from fossil fuels.

The issue is just what our policy toward sequestration should be in the meantime. Would the resources we might apply toward sequestration be better allocated to promote conservation and further accelerate the introduction of non-fossil alternatives? Should support for carbon sequestration even be a part of our national energy policy?

If one looks at the issue in terms of return on energy investment -- where "return" means avoided CO2 emissions into the atmosphere -- then sequestration clearly does make sense. The energy cost for CCS using available technology, as noted above, is 24% of a PC plant's power output. New technology might drop that to 15%, or even less. But that technology is unproven. So let's say that 240 kWh will sequester the CO2 from 1000 kWh of PC plant output. That means that if we have 240 kWh of energy from a zero-carbon source, we can either supply it to end users and avoid the CO2 from 240 kWh of coal-fired power generation, or, we can use it to replace the energy lost in capturing and sequestering the CO2 from 1000 kWh of coal-fired power generation. CCS, in this example, is over four times more effective than substitution for reducing CO2 emissions into the atmosphere!

It isn't quite that simple, of course. CCS involves additional capital expenditure that isn't needed for substitution. One could perhaps argue that the value of keeping 760 kWh worth of coal-generated CO2 out of the atmosphere isn't enough to justify the additional capital cost of CCS equipment over and above that of the 240 kWh of zero-carbon energy. Climate change deniers would certainly have no problem with that position. But from an anti-carbon environmental faction, it makes no sense. With the much higher capital cost per annual megawatt hour for renewable vs. coal, the 4:1 advantage of CCS in return on energy investment translates to at least a 10:1 advantage in return on capital. If the avoided CO2 emissions can't justify the capital required for CCS, how can they possibly justify the capital for renewables?

Anthropogenic CO2 either is or isn't a serious threat. If it isn't, then it blows most of the case for renewables. There may still be a "peak oil" case for reducing our dependence on oil; cheap and easily accessed oil is in decline, while economic development in emerging nations is driving demand through the roof. However, that's an oil problem and specific to the transportation sector. It's not an across-the-board fossil fuel problem. Rising oil prices should provide plenty of incentive to improve fuel efficiency and to electrify our transportation without the further incentive of a price on carbon emissions.

If, on the other hand, rapidly mounting levels of CO2 in the atmosphere are the threat that climate science is telling us, then our response logically must consider sequestration as an option. If we can reduce carbon emissions to the atmosphere more quickly by investing capital in CCS than we can via subsidized renewable energy, what rationale is there for not doing so?

I suspect that a big part of what drives the militant green position is concern that, once the investment in CCS for coal-fired power generation has been made, the climate change argument for abandoning coal goes away. If one's real objection to coal is the local environmental damage from mining it rather than global climate change, then opposition to CCS makes tactical sense. It's dishonest, but there's certainly ample precedent in politics for dishonesty. Lobbies expose positions using arguments they think will sell, however little those arguments have to do with their backers actual motives for those positions.

Of course it's not a simple either / or choice that we face. We can and should be supporting both ultra-low carbon energy production and CCS. We can't reduce the cost of ultra-low carbon energy if we don't continue to ramp or at the very least maintain production. At the moment, the double whammy of unnaturally cheap natural gas and withdrawal of subsidies threatens the entire clean energy sector. At the same time, we will never learn what the real economic costs of CCS will be, or discover which forms are the most practical, if we don't start moving beyond lab experiments and small pilot projects.

The Geo-Engineering Hangup

In addition to feelings against coal mining, some of the sentiment against CCS within the environmental sector seems to stem from an ideological opposition to anything that smacks of "geo-engineering". The arguments are couched in terms of rational concerns about the potential for "unintended consequences" and our limited understanding of the complexities of natural systems. But read very much material from that camp, and it becomes pretty clear that it's an emotional issue. The rational arguments are mustered in defense of a position that would still be held, independent of the rational arguments. Geo-engineering is simply evil. To its opponents, it represents the pinnacle of technological hubris, and a "man over nature" philosophy. It runs counter to everything that green militants wish for the world to be.

While I sympathize with their outrage over what humankind has been doing to the natural world, I think that unconditional opposition to all geo-engineering is misguided. There is every reason to be cautious in our approach to it; the systems we would be attempting to modify or control truly are complex, and the potential for adverse consequences is real. But the problem with making opposition ideological and unconditional is that it ignores the fact that humans have been doing "blind geo-engineering" for quite a long time now. Even in prehistoric times, we were using grass fires to control brush and create better habitat for hunting herd animals. Later, we cleared land, killed off predators, depleted fertile soils, and raised cattle, goats, and sheep in numbers that stripped the land and spread deserts. All that was well before we developed industrial technology, when our numbers were much less than today.

Today, we burn fossil fuels at a rate that is changing the atmosphere of the planet. Loss of wildlife habitat and human-caused changes in ecosystems are driving what is shaping up to be the largest mass extinction of species for the last 250 million years. Abandoning technology and quitting fossil fuels "cold turkey" is not an option. It would mean the prompt die off of 99% of human population. But to avoid disaster, we may need to employ some benign geo-engineering. We'll need it to manage our impact and minimize damages.

Geo-Engineering example: OTEC+

An example of the sort of geo-engineering we might be driven to consider is large-scale ocean thermal energy conversion (OTEC) coupled with boosting the alkalinity of ocean surface waters (what I label "OTEC+"). I mentioned that approach previously in part 2. It has a number of effects that work together to mitigate global warming and related environmental problems:

  • Generates carbon-free base load power. This reduces the need for coal and natural gas for power generation;
  • Brings deep water with enhanced alkalinity to the surface. This counters acidification of ocean waters due to higher atmospheric CO2 levels. It also enables the dissolution of more CO2 in surface waters;
  • Reduces average temperature of surface waters. Among other effects, this directly reduces average global temperature;
  • Lowers humidity downwind of cooler surface waters. Water vapor is a much stronger greenhouse gas than CO2, and dryer air allows more thermal radiation to escape from the surface, indirectly lowering average global temperature;
  • Enhances biological productivity of the surface waters around the OTEC installation. OTEC plants would create local "islands" of cooler, nutrient-rich water around each installation.

As a power generation technology alone, OTEC suffers from capital costs that are likely too high for economic viability -- despite an unlimited supply of free "fuel". But it does lend itself to ocean pH adjustment via dissolution of carbonates in cold, deep waters. It brings those more alkaline waters immediately to the surface, vs. the several hundred to a thousand years it would take for natural ocean circulation to do the job. This system supports ocean sequestration of CO2 from diffuse sources. It would provide a means, for example, to sequester CO2 released from melting tundra, should that develop into a dominant source of emissions.

The long term results of the immediate reductions in water temperatures and atmospheric water vapor are hard to estimate. What can be said with certainty is that changes in ocean surface temperatures have a strong effect on both weather patterns and general climate. Indeed, recent evidence indicates that in past epochs, ocean currents were a stronger driver of global climate change than CO2 levels2. For evidence of the impact of relatively small changes in ocean surface temperatures, one need look no further than the Pacific El Niño / La Niña Southern Oscillation (ENSO). ENSO affects an ocean surface area several orders of magnitude larger than would be affected by any single OTEC plant; however, an El Niño episode is defined by an increase in average surface temperature in the tropical east-central Pacific of as little as 0.5 °C. At that, its ability to affect global weather patterns is infamous. The temperature change associated with OTEC operation -- at least in the immediate vicinity of the plant -- would be 10x greater.

The good news / bad news for this system is that the impact of individual OTEC installations, when measured against the natural thermohaline circulation, would be minute. OTEC requires a cold water flow of 175 -- 200 cubic meters per second per gigawatt. That may seem a lot, but it's a bare trickle against the 17 million cubic meters per second of the thermohaline circulation. As a result, we could build and operate quite a number of such systems without fear of adverse consequences. That would allow us to safely study them, and to develop and validate models of their overall effects. It may turn out that OTEC+ systems would not be a cost-effective way to mitigate climate change, but it's the sort of thing we'll need to explore seriously.

Getting on With the Job

The unfortunate fact is that we cannot avoid an immense environmental footprint. There are simply far too many of us for that. The issue then is simply stated. Do we continue to bury our heads in the sand and disown responsibility for our blind impact on the terrestrial environment and ecosystems? Or do we buckle down and get serious about making the impact as benign as possible?

Taking responsibility is admittedly uncomfortable. But I have no sympathy for those who cling to childhood, and dismiss the consequences of our blind actions as "acts of god". I don't think that's what a creator god would expect of us.

References

  1. Part 1 - Carbon Capture And Sequestration: Where Does It Fit? (Part 1: Carbon Capture) and Part 2  - Carbon Capture And Sequestration: Where Does It Fit? (Part 2: Sequestration)
  2. Nature preprint posted at Skeptical Science web site (Today's Climate More Sensitive to Carbon Dioxide than in Past 12 Million Years) -- http://www.skepticalscience.com/Todays-Climate-More-Sensitive_NSF.html

     

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