Developing effective technology for surface monitoring of CO2 leakage is an instrumental part of ensuring the wide-scale deployment of CCS technology (Madsen et al. 2009). The primary methods used to monitor surface CO2 flux as the gas is released from the soil into the atmosphere include the micrometeorological method and the chamber method. Although there are many methods that fall under these two broad categories, this paper focuses specifically on the closed-chamber method (a chamber method) and the Eddy Covariance method (a micrometeorological method). Madsen et al. argue that two new designs for monitoring equipment have made surface monitoring far more effective and reliable. These equipment models are examined; both the LI–8100 Automated Soil CO2 Flux System (closed-chamber method) and the LI–7500 Open Path CO2/H2O Analyzer (Eddy Covariance method). The study details the advantages of these models and outlines the importance of employing their use in CCS projects so as to build public confidence in the safety of this technology.— Shanna Hoversten
Madsen, R., Xu, L., Claassen, B., McDermitt, D., 2009. Surface Monitoring Method for Carbon Capture and Storage Projects. Energy Procedia 1, 2161
R. Madsen and colleagues at LI–COR Biosciences, estimate the capacity of this surface monitoring equipment to pick up CO2 fluxes using mathematical models combining elements of the new technology with environmental variables that effect flux measurements. The paper also provides details on new design features that allow for additional accuracy in flux measurements, such as the LI–8100’s vent design to minimize the effect of wind on the flux data. The efficacy of this new technology is assessed by looking at its use in the monitoring and verification regimes at several pilot CCS plants around the world.
In discussions about the closed-chamber method, Madsen et al. identify several key difficulties with getting accurate flux readings. Maintaining pressure equilibrium between inside the chamber and the ambient air is one of the greatest challenges, and it can be especially problematic under windy conditions. Ensuring good mixing within the chamber is also necessary, but if a fan is installed to do this it can oftentimes alter the pressure equilibrium. The LI–8100 factors in these concerns by having a unique chamber geometry that allows mixing without a fan, thereby maintaining the pressure equilibrium. It also has a new feature in which the chambers all open and close automatically and slowly so that fresh ambient air is not pushed into the soil or removed from the soil and thereby distorting the data.
The Eddy Covariance method measures fluxes by taking measurements of the deviation of vertical wind velocity and of an associated scalar from their mean values. The sensors for Eddy Covariance are usually mounted on a tower and they then measure the average CO2 flux over an integrated area that can extend to an area about 100 times the height of the sensors to the up-wind direction. For this monitoring technique to be effective, it is necessary that the model can correct for density perturbations caused by sensible heat and latent heat flux, and it must be set up over a relatively large and flat field site. Both the LI–7500 and the LI–8100 have been used in CCS projects all over the world, and have so far been effective, however there is still a great need to see how these technologies will perform over the long lifespan of a project.