A Review of Hazardous Chemical Species Associated with CO2 Capturefrom Coal-Fired Power Plants and Their Potential Fate in CO2 GeologicStorage

Abstract

Conventional coal-burning power plants are major contributors of excess CO2 to the atmospheric inventory. Because such plants are stationary, they are particularly amenable to CO2 capture and disposal by deep injection into confined geologic formations. However, the energy penalty for CO2 separation and compression is steep, and could lead to a 30-40 percent reduction in useable power output. Integrated gas combined cycle (IGCC) plants are thermodynamically more efficient, i.e.,produce less CO2 for a given power output, and are more suitable for CO2 capture. Therefore, if CO2 capture and deep subsurface disposal were to be considered seriously, the preferred approach would be to build replacement IGCC plants with integrated CO2 capture, rather than retrofit existing conventional plants. Coal contains minor quantities of sulfur and nitrogen compounds, which are of concern, as their release into the atmosphere leads to the formation of urban ozone and acid rain, the destruction of stratospheric ozone, and global warming. Coal also contains many trace elements that are potentially hazardous to human health and the environment. During CO2 separation and capture, these constituents could inadvertently contaminate the separated CO2 and be co-injected. The concentrations and speciation of the co-injected contaminants would differ markedly, depending on whether CO2 is captured during the operation of a conventional or an IGCC plant, and the specific nature of the plant design and CO2 separation technology. However, regardless of plant design or separation procedures, most of the hazardous constituents effectively partition into the solid waste residue. This would lead to an approximately two order of magnitude reduction in contaminant concentration compared with that present in the coal. Potential exceptions are Hg in conventional plants, and Hg and possibly Cd, Mo and Pb in IGCC plants. CO2 capture and injection disposal could afford an opportunity to deliberately capture environmental pollutants in the gaseous state and co-inject them with the CO2, in order to mitigate problems associated with solid waste disposal in surface impoundments. Under such conditions, the injected pollutant concentrations could be roughly equivalent to their concentrations in the coal feed. The fate of the injected contaminants can only be determined through further testing and geochemical modeling. However, the concentrations of inadvertent contaminants in the injected CO2 would probably be comparable to their ambient concentrations in confining shales of the injection zone. In general, the aqueous concentrations of hazardous constituents in distal parts of the injection zone, regardless of source, are likely to be limited by equilibrium with respect to coexisting solid phases under the acid conditions induced by the dissolved high pressure CO2, rather than by the initial concentrations of injected contaminants. Therefore, even if a deliberate policy of contaminant recovery and injection were to be pursued, water quality in USDWs would more likely depend on thermodynamic controls governing aqueous contaminant concentrations in the presence of high pressure CO2 rather than in the injected CO2. The conclusions reached in this report are preliminary, and should be confirmed through more comprehensive data evaluation and supporting geochemical modeling.