Why CO2 conversion by chemical method is not feasible on board???

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Global shipping is a major contributor of CO2 to the atmosphere.It is approximately responsible for 3.3% of total global CO2 emission. In this context, it is clear that there is a pressing need to consider a long term strategy for the cost effective mitigation of the CO2 emission originating from international shipping.
   In principle there are four fundamental categories of options for reducing emissions from shipping. It can be reduced by

  1. Increasing efficiency
  2. Using less carbon intensive fuel
  3. Using different power sources including renewables   and finally
  4. Using emission reduction technologies such as chemical conversion
We will discuss in the below paragraphs how chemical conversion works and what practical difficulties will be faced on board while using chemical conversion.
  Exhaust gas emission fro ship typically have following composition:--
  • 5% vol CO2
  • 13% vol O2
  • 1500 ppm NOx 
  • 600 PPM SOx
  • 60 ppm CO
Normally CO2 intensity of shipping is between 3 to 4 kg/sec. 
    An attractive land based option for the gas clean up is flue gas scrubbing with amine solvents. This processes operate by scrubbing the flue gas with an aqueous soln of Amine---typically a 30 wt% soln of MEA is used. CO2 is absorbed into liquid phase via an exothermic  reaction.
     Not much going into the chemical reaction, it can be summarized that when the soln comes at equilibrium, after absorbing CO2, the soln needs to be recycled. Then the rich solvent is regenerated by boiling off CO2 allowing the solvent to be recycled. This is a significant energy cost. The CO2 is then dehydrated and compressed  to approx 110 bar in preparation for transport and subsequently for storage.One could envisage a scenario where in the captured CO2 is compressed and stored on board ship. This would entail significant extra cost both in form of energy required to compress the CO2 and space required to store it.Further one should consider the hazard associated with storing large quantities of a pressurized asphyxiant gas.
        More over Amina solvents like MEA are degraded by acid gases such as SOx and NOx in addition to reacting irreversibly with O2. The relative high concentration of O2 in exhaust gases thus poses an interesting challenge. The degradation byproducts can be hazardous to huan health in addition to reducing the efficiency of whole system. A further issue is volatility of MEA leading to solvent losses and thus creation of fire hazard.

Thus the principal challenges associated with CO2 capture fro ship can be summarized as :--
  • Energy penalty associated with solvent regeneration
  • Solvent degradation by SOx, NOx and O2
  • Solvent and CO2 storage.

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