Direct Injection

source: Herzog, Caldeira, and Adams p.4

The process of direct injection is basically human interaction to speed up the natural process of carbon dioxide entering the atmosphere.  Natural ocean-atmosphere interaction is limited by ocean transport (time scale ~300 years).  Direct injection would bypass the slow natural ocean mixing. [1]   The theory behind direct injection is to avoid biological surface layer and the climate effects of first emitting the carbon dioxide to the atmosphere.  The methodology is driven by the state of the carbon dioxide at the injection depth with the residence times and the behavior of the carbon dioxide varying directly with the injection depth.  The deeper you inject the CO₂, the slower equilibrium with the atmosphere is reached.  It is generally, agreed, however, that for sequestration to be most effective injection must occur below the thermocline layer to prevent mixing with the surface waters.  Because there haven't been any large-scale experiments yet, we are relying completely on models, which can often prove to be inaccurate, for residence time and other information about the carbon dioxides behavior in the ocean. [2]

 

The success of carbon dioxide direct injection directly depends on where and how deep the CO₂ is injected. Sequestering in the Pacific Ocean is generally more effective than sequestering in the Atlantic Ocean. [3]  

source: Herzog 5/24/2001 p.17

There are currently trade-offs between cost and effectiveness of the sequestration.  "Sequestration effectiveness will depend on the exact depth and location of the injection.  In general, the deeper the CO₂ is injected, the more effectively it is sequestered: but injecting deeper requires more advanced technologies and may increase costs." [4]   "Our analysis suggests that that CO₂ injections into mid-depth waters may be an effective sequestration strategy, and injection into deepest waters may not be economically justifiable. This conclusion could change if carbon emissions costs were to rise rapidly in the future. We find that ~98 % of global warming costs, GWC, can be avoided by CO₂ injection at ~1400 m with a 4 % discount rate, if the costs of carbon emission are taken to be constant in time. Under these same assumptions, it would take injection at ~1600 m to avoid ~99 % of the cost. The question of whether it makes sense to inject ~200 m deeper to avoid an additional 1 % of the global warming cost is largely an economic one." [5]

source: Herzog 5/24/2001 p.10

 

"The fraction of injected carbon that is permanently sequestered depends on the atmospheric CO₂ concentration, through the effect of atmospheric CO₂ on surface-ocean chemistry.  The concentration of CO₂ in the atmosphere today is about 370 ppm, meaning that over 80% of any carbon sequestered in the ocean today would be permanent.  Even at an atmospheric concentration of 550 ppm (double pre-industrial levels), just under 80% of CO₂ injected into the ocean would be permanently isolated from the atmosphere.  The amount of time over which the reaming 20% of the injected CO₂ would leak depends on the location and depth of the injection.It can be seen that the deeper the injection, the longer it takes for the 20% of the CO₂ to return to the atmosphere.  Also, to make sure the leakage does not significantly exceed the long term value of 20% in the shorter-term, injection depths should be greater than 1000 m.  This is because the 1000 m depth is roughly the bottom of the thermocline, which is the layer of the ocean that is stably stratified by large temperature and density gradients, thus inhibiting vertical mixing and slowing the leakage of CO₂. Beyond injecting CO₂ deeper, the amount of leakage could potentially be minimized by injecting the CO₂ in a way that would maximize interaction with carbonate sediments or by purposefully enhancing the dissolution of carbonate minerals." [6]

 

 

One of the disadvantages of direct injection is the cost associated with the pre-sequestration processing.  When choosing to process carbon dioxide for ocean sequestration one can use a solid, liquid, or gas, but processing liquid CO₂ is the easiest. [7]

 

 

Because direct injection requires a relatively pure stream of carbon dioxide, costs for separation can be expensive. [8]   Also, unless the source of the carbon dioxide is near the ocean or a navigable waterway, transportation costs for such a pure liquid stream of CO₂ can be large.  By far the most economical method for near-shore deposits is pipeline technology.

 

 

Herzog summarizes the pros and cons of direct injection: pros - effective sequestration for 100's of year, based on proven technologies, strategies can be developed to enhance effectiveness and diminish adverse environmental results; cons - it consumes energy and is expensive, suitable only for large sources near a waterway, and there are large potential environment risks. [9]   As we see it our knowledge of ocean sequestration or lack there of is the biggest disadvantage to the method. "We have the technology to proceed with this option.  However, we do not have the knowledge to adequately optimize the costs, determine the effectiveness of the sequestration (i.e. its impact in mitigating climate change), and understand the resulting changes in the biogeochemical cycles of the oceans." [10]   There are several areas research that be understood before direct injection can be used: biological impacts must be better characterized, larger scale release experiments, modeling on the scale of 100's of meters to 100's of kilometers, and the public must be well educated. [11]

source p.5

 

 

There are 6 proposed methods for direct injection:

1. injecting liquid CO2 at a depth of ~1000 m from a manifold lying near the ocean bottom and forming a rising droplet plume
2. injecting liquid CO2 at a depth of about 1000 m from a pipe towed by a moving ship and forming a rising droplet plume
3. creating a dense CO2-seawater mixture at a depth of between 500 and 1000 m forming a sinking bottom gravity current
4. releasing dry ice at the ocean surface from a ship
5. injecting liquid CO2 at a depth of >3000 m from a manifold near the ocean bottom and forming a sinking droplet plume
6. introducing liquid CO2 to a sea floor depression forming a stable "deep lake" at a depth of about 4000 m [12]

 

Method 1 - Rising Droplet Plume from a Manifold

 

CO₂ is discharged through a diffuser in what as known as an unconfined release.  The release CO₂ has a positive buoyancy and it has been shown that the plume will rise less than a hundred meters making injection at any depth below the well-mixed surface layer permissible. [13]

 

 

source: Herzog 5/24/2001 p.37

Method 2 - Rising Droplet Plume from a Towed Pipe

 

In this method liquid CO₂ is injected from a pipe about 1000m down towed by a moving ship.  The principle is pretty much the same as method one except the carbon dioxide is discharged from a moving pipe rather than a stationary platform.  Method 1 and 2 are probably the most viable at least in the short-term. Both methods rely on available technology and injection occurs below the thermocline.  Also, designing the plumes to have high dilution can minimalize the environmental impact [14]  

 

 

Method 3- Sinking Bottom Gravity Current

 

The advantage of this method is that it has a long residence time for a shallow injection.  The principle of it is that a dense sinking plume of CO­₂-enriched seawater, which is about 1% denser than unsaturated seawater, is injected at a depth of 200-400m, where it then proceeds to move along the sloping ocean floor to greater depths.  The problem with the method is that as the plume moves the CO₂ dissociates and the small density difference is quickly lost.  This may be overcome, however, if the release is at the top of a submarine canyon, where confinement will allow the plume chemistry to dominate. [15]   Because the plume is highly concentrated many questions surround the its environmental impact. [16]

 

 

Method 4 - Dry Ice

 

The concept of sequestering carbon in the form of dry ice is based on the fact that dry ice is 50% denser than seawater, so presumably when dropped from the surface of the ocean it will sink to the ocean floor.  In theory it is a good idea, however, untreated blocks will usually "melt" or sublime as they sink, unless, perhaps, they have a high enough volume to surface area ratio.  In an effort to combat this problem, research is being done on coatings for the dry ice blocks and on creating aerodynamic shapes like torpedoes to speed the blocks decent to the ocean floor.  Even if the research is successful it is unlikely that dry ice will ever be a significant contributor to sequestering carbon dioxide because the costs of handling solids are more than liquids and the cost of forming dry ice is twice that of liquefying carbon dioxide. [17]

 

 

Method 5 - Injecting liquid CO₂ at a depth of >3000 m

 

At these large depths the properties of the liquid CO₂ would be governed by hydrates, which will slow dissolution into the surrounding seawater.  At a depth of greater than 3000 m CO₂ sinks as it dissolves and it forms hydrates. [18]   "Hydrates are members of the class of compounds called clathrates from the Latin word 'clathratus' meaning 'encaged'.  A guest molecule, in this case CO₂ is held in a 'cage' of hydrogen-bonded water molecules by van der Waals forces.  CO₂ hydrates contain between 6-8 water molecules per molecule of CO₂ and are about 5% denser than seawater." [19] Hydrate formation is governed by the equation CO₂ + nH₂O → Co₂^.nH₂O + ∆H where n=5.75 and H=60.4 kJ/mole at 277K. [20] "Formation of CO₂ hydrate will occur in most ocean CO₂ disposal, because CO₂ hydrate is stable at the conditions of pressures greater than 4.45 MPa and temperatures less than 10.2˚C.  Formation of natural CO₂ hydrate was observed at submarine gas vents on ocean floor, and CO₂ hydrate was experimentally formed." [21] The CO₂ would be stable at this depth for greater than 500 years. [22]

 

Method 6 - Deep Lake Formation

 

At a depth of more than 3700 m "there will be a CO₂ saturated seawater layer on the hydrate film, because at this depth the density of CO₂ saturated seawater is smaller than that of liquid CO₂.  This CO₂ saturated seawater will protect the CO₂ hydrate film from decay for a long time.  The decay rate of hydrate depends on the molecular diffusion-rate of CO₂ into the seawater from CO₂ saturated layer.  Therefore the thin film of CO₂ hydrate will prevent CO₂ diffusion in the ocean and will control greatly the change of pH." [23]