E.6 Summary of CO2 Stream Composition

Based on theoretical calculations, indicative compositions of CO2 streams generated from the three main capture technologies, as well as a cement plant and process heaters feeding a combined stack at a refinery, are summarised in Table E.2. It should be noted that the real behaviour of heavy metals and other trace elements cannot be predicted in a laboratory or from calculations, because coal combustion is conditioned by highly complex processes, such as combustion temperatures, halogen species concentrations, redox conditions, and interaction between different species. The capture process used to produce the CO2 stream is listed in the second row of the Table. In these processes, sulfur has been removed as needed to extend the life of the process step which removes CO2 from the stream; and water has been removed as needed from the stream to meet a CO2 pipeline specification of 0,064 % by volume (30 lbs/MMscf). Oxy-fuel combustion has the highest level of contamination for many of these constituents, as the oxy-fuel combustion has no stack emissions. Furthermore, the oxy-fuel system modelled in Table E.2 does not have a flue gas desulfurisation (FGD) in order to consider a worst case scenario for SOx in the CO2 stream. If FGD is included in the oxy-fuel plant, or if the CO2 were treated, the SOx concentration in the stream would be reduced.

The CO2 streams captured from coal combustion by all three basic processes can have significant heavy metals content, although most of the heavy metals from coal combustion are typically collected in the fly ash and other waste streams. In post-combustion capture, the remainder travels up the stack, although the proportion of the heavy metal content may vary.

In pre-combustion capture (IGCC process), the heavy metals will mostly be collected in the ash and slag and waste water filtrand, with the remainder going up the stack. In oxy-fuel processes, particulate filters will also remove most of the metals, and, in combination with an FGD plant, and other processes required to remove mercury, it is again expected that the concentration of heavy metals in the CO2 stream will be nearly zero.

Table E.2 Illustrative calculated examples of composition of CO2 streams (after dehydration, but before compression)

Source: ICF International

Notes:

- These estimates are based on engineering calculations performed by ICF based on a typical US bituminous coal (Illinois #6) with 2,5 % sulfur by weight. The actual amount of substances in a CO2 stream could vary widely depending on flue gas pre-treatment and capture processes.

- The calculations for the pre-combustion IGCC plant are based on Case 2 scenario analysis in the DOE/NETL-2007/1281 report.

- The calculations for the oxyfuel combustion plant are based on Case 5 scenario analysis in the DOE/NETL-2007/1291 (revision 2) report.

- The concentrations of mercury, arsenic, and selenium are based on stack gas measurements at a coal-fired power plant in Spain burning a 'mixture of two types of coal' (Otero-Rey et al., 2003). However, the concentrations of heavy metals in the CO2 stream from an IGCC plant may be different from the assumptions in Otero-Rey et al., 2003, particularly if a GAC filter is fitted before the AGR plant. This would also reduce the ash levels.

- For post-combustion capture, sulfur was removed as needed for economic operation of the capture step (e.g. to limit amine degradation) from the stream.

- Water was removed from the CO2 streams to meet US CO2 pipeline specification of 640 ppm (see penultimate point), using glycol-based dehydration. Note, that during the compression stage, water, as well as SOx, NOx, and Hg, can be removed from the CO2 stream141.

- Heavy metals are typically removed with the particulate matter (fly ash), and therefore more stringent particulate emission standards would further reduce heavy metal content in the CO2 stream. Furthermore, Hg can be removed during the compression stage along with nitric acid.

- Oxy-fuel combustion has the highest level of contamination for many of these constituents in these examples, because ICF assumed for these calculations that the flue gas is not treated except for particle removal by electrostatic precipitation and for water removal to 640 ppm.

- The lack of an FGD allows the sulfur concentration to be high in this calculation; however, this high sulfur content would adversely impact boilers and heat exchangers due to corrosion, and hence a low sulfur coal (<1 %) would need to be used to prevent corrosion. If an FGD is included in the oxy-fuel plant, or if the CO2 were treated after capture, SOx, HCl, and heavy metal content (e.g. mercury) in the CO2 stream would be reduced.

The concentrations of all incidental substances can be decreased by adding additional stages of purification. This will result in higher costs of the capture process and affect overall plant efficiency. Furthermore, given that different capture plants will have different compositions of CO2 streams, it will be important to consider the impact of mixing these streams into the CO2 pipeline networks, especially when combining CO2 streams with reducing and oxidising properties.

141 CO2 Processing Unit - Challenges in Meeting the Required CO2 Quality, Stanley Santos and Jinying Yan, Presentation at the Oxy-Fuel Combustion Network - 2nd Working Group Meeting on CO2 Quality and other Relevant Issues, 7 September 2009, Cottbus, Germany.