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1.1 Scope of the Current Study

IEA GHG has sponsored investigations of impurities in CO2 streams. A previous report was issued in 2004 (PH/4–32), where the impacts on storage in saline aquifers were not a focus. The scope of the current study specified by IEA GHG is as follows:

“Review of existing information and published research on the potential impact of CO2 waste stream purity on storage engineering and associated costs; aiming to provide a ‘high level’ overview of available knowledge. A range of storage scenarios may be considered including deep saline formations, depleted gas fields and CO2-EOR schemes, although the study should focus primarily on deep saline storage as this scenario has the largest theoretical storage capacity. Consideration of other geological storage scenarios, such as coal beds and basalts, is not required by this study.

Particular aspects that should be considered include:

  • The potential effects of impurities on phase behaviour and storage capacity calculations
  • Effects on the rates of geochemical reactions with both formation and caprock and associated buoyancy forces and trapping mechanisms
  • Potential effects on injectivity, reservoir permeability and caprock integrity both near the well-bore and deeper in the formation
  • Potential for corrosion of well components and estimated impact on system reliability if not mitigated

The findings of the literature and research review, combined with use of engineering judgment should identify key issues, uncertainties and knowledge gaps. The results of the study should be able to contribute to development of risk assessment methodologies for CO2 storage and relevant sections of ‘best practice’ manuals.”

In this study the data on impurity species and their concentration levels have been provided by IEA GHG, based on CO2 quality recommended for evaluation under the COORETEC study for fossil-fueled power plants (Kather, 2009), as shown in Table 1.1. It should be noted that the effects of impurities studied in this work are also expected to be applicable to other CCS scenarios, such as those for oil refineries, iron and steel making, and cement production. Whereas the composition spectra may vary, the types of impurities in the other scenarios would be largely the same, as long as the CO2 is from burning fossil fuels.

A number of relevant physical properties of the major impurities are given in Appendix A. From Table 1.1 it can be seen that the impurities vary with the sources of CO2 streams. Reductive impurities H2, H2S and CH4 are present in pre-combustion streams. Oxide impurities, including nitrogen oxides (NOx) and sulphur oxides (SOx), are present in oxyfuel and post-combustion streams. Hg is not shown in the table but is expected to be present in trace amounts in pre-combustion and post-combustion streams. Air-derived impurities N2, O2 and Ar exist in high levels in oxyfuel combustion streams, which have the highest total impurity levels among the three types of CO2 streams. IEA GHG has considered three scenarios for CO2 purity from oxyfuel combustion streams:

  • Scenario 1 – low-purity (CO2 purity between 85 - 90%)
  • Scenario 2 – medium-purity (CO2 purity between 95 - 97%)
  • Scenario 3 – high-purity (CO2 purity greater than 99%)

where N2, O2 and Ar comprise the major part of the impurities. Clearly, increasing impurity concentrations will have more significant effects on storage, thus the oxyfuel streams are most important for the current study because of their high impurity concentrations.

Table 1.1 Compositions of CO2 streams

Component Pre-combustion Post-combustion Oxyfuel
Selexol Rectisol Comp.1 Comp.2 Comp.3 Comp.1 Comp.2 Comp.3
CO2 (vol %) 97.95 99.7 99.93 99.92 99.81 85.0 98.0 99.94
O2 (vol %) - - 0.015 0.015 0.03 4.70 0.67 0.01
N2 (vol %) 0.9 0.21 0.045* 0.045* 0.09* 5.80 0.71 0.01
Ar (vol %) 0.03 0.15 4.47 0.59 0.01
H2O (ppmv) 600 10 100 100 600 100 100 100
NOx (ppmv) - - 20 20 20 100 100 100
SO2 (ppmv) - - 10 10 20 50 50 50
SO3 (ppmv) - - 20 20 20
CO (ppmv) 400 400 10 10 20 50 50 50
H2S + COS(ppmv) 100 100 - - - - - -
H2 1 vol% 20 ppm - - - - - -
CH4 (ppmv) 100 100 - - - - - -
NH3 (ppmv) - - - 50 - - - -
CH3OH (ppmv) - 200 - - - - - -

* Total concentration of N2 + Ar

Total concentration of SO2 + SO3