Why bother heating up the limestone?

Posted on July 24th, 2008 by Tim Kruger in General

It seems like heating up limestone until it breaks down into lime is a lot of hard work to get alkalinity into the ocean. Why not put the limestone directly in the water?

Well, the problem is that limestone is very insoluble, which explains the continuing existence of the White Cliffs of Dover. It is possible to get limestone to dissolve in water, but you have to grind it to an extremely small size and even then it dissolves very slowly. To enhance the ability of the oceans to act as a carbon sink, you need the reaction to take place in the top 100m of the ocean, as this is the layer where most of the mixing with the atmosphere takes place. Even small particles of limestone sink like stones (which is after all what they are) and little of the required reaction will occur in the right place. To grind limestone down into a fine enough powder is both expensive both in terms of energy and in terms of the equipment required. Also the energy that is required is mechanical energy which is much more expensive than heat energy.

There are other sources of alkalinity that can be added. Kheshgi identified mineral deposits of soda ash (Na2CO3) as one such source, but noted that using the world’s entire recoverable resources of soda ash would offset just a single year of CO2 emissions. Caldera and Rau have put forward an idea that reacts CO2 from the flue gases of power stations with limestone and then puts the resulting calcium bicarbonate solution into the sea – this has a lot of merit – it addresses CO2 before it is emitted, but does not remove CO2 from the air after it has been emitted, as the process described on this website does.

All these methods can be used – the questions are how can they be done in an environmentally beneficial way and how can they be done economically? These are the key questions we are trying to answer – and with your help we will

 

3 people have left a comment

shwetha s says: October 12th, 2009

why not mix up two research together… evensky spraying can be used as one of the technique to reduce the amount of co2 in atmosphere and also to some extent organic farmaing which wil benefit the mankind economically and also helps us the scientific manner..i think merging of more techniques is one of the useful way to come out of this problem….

Kiko says: December 8th, 2009

I’m puzzled: why not use slag? It is full of CaO, meaning it would act just as lime, without any need for burning limestone (or I am mistaken?). True it contains also other oxides like Al2O3 but I don’t think it would be such a problem?
For the moment, it is a by-product of iron production and many uses had to be found as it is strictly illegal to dump it, due to the basification of soils when you do that, but it would be an elegant way to reduce the impact of a necessary industry (and there are sources of slag all other the world, mainly near the see as it is less expensive to have a blast furnace near the supply arrival point).
Perhaps you could discuss with Ulcos project leaders? (NB: Ulcos is a European initiative uniting iron producers to reduce the CO2 emissions of the iron processing)

Jem Cooper says: February 9th, 2010

The problem with using powdered calcium carbonate (limestone) is not the cost of grinding but its insolubility. The ocean is supersaturated with calcium carbonate at the surface by a factor of about 4 on the ion concentration multiple ([CO3--] times [Ca++]) because photosynthesis removes carbon dioxide from the water thus increasing the ratio of carbonate [CO3--] to bicarbonate [HCO3-] ions as the chemistry adjusts to keep an equal number of negative and positive ions. At greater depths usually at least 1000 metres the decomposition of sinking organic matter releases carbon dioxide thus reducing making calcium carbonate soluble. Even then the rate of dissolving is very low but below the calcite compensation depth (typically 400 metres) no calcium carbonate sediment accumulates because the rate of dissolving equals the rate of arrival in sinking skeletal remains. Water from the ocean depths takes a very long time to return to the surface where the dissolved calcium carbonate can intereact with atmospheric carbon dioxide.

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