“Quo Vadis Anthropogenic CO2?” Part 2: Carbon capture and storage techniques
In yesterday’s blog, I lay out the problem at hand, but now, what do we do to fight it?
Carbon capture and storage (CCS) is one option in the portfolio of mitigation methods for the stabilization of man-made GHGs. Where with CCS, CO2 is captured at large point sources, transported to a storage site, and injected into a geological formation including depleted oil and gas fields, unmineable coal seams, and saline formations [IPCC2005]. Even with a growing share of renewable energies in the power generation sector, CCS from power plants is still expected to be necessary to meet increasing energy demand along with reducing emissions.
There are three basic approaches for CO2 capture from fossil fuel-based power processes, as shown in Figure 1. In oxy-combustion power plants, fuel is burnt in a nearly stoichiometric atmosphere. The main combustion products are then water vapor and CO2 (provided that the fuel comprises no high content of impurities like nitrogen). Oxygen can be produced either by external or integrated air separation. However, combustion of fuel in pure oxygen would lead to excessive temperatures. Exhaust gas recirculation is therefore used to moderate the temperature. In pre-combustion power plants, fuel reacts with oxygen in an under-stoichiometric atmosphere to produce a synthetic gas (syngas) having a high concentration of carbon monoxide, hydrogen, water vapor and CO2. Additional reactors are used to further increase the hydrogen content and to separate it from the gas stream so that it can be fed to a gas turbine. In post-combustion power plants, fuel is burned in air where the CO2 is captured afterwards from the exhaust gas.
Natural gas and coal represent the main fuels that can be used for power generation (Figure 1). At this point it is important to mention that combustion of coal results in more than twice as high carbon emissions per kWh than that of natural gas. Hence, switching from coal- to natural gas-based power plants represents an effective method for the reduction of CO2 emissions in the short-term. In view of the huge amounts of recoverable natural gas resources (4.6 trillion cubic meters, i.e. the amount corresponding to 150 times current annual global gas consumption), GE and other companies focus on natural gas applications for power generation.
One can imagine that the separation of CO2 from a gas stream does not come for free. In fact, the capture process can be very energy intensive. That means more fuel is needed to generate the same amount of power than for processes without additional CO2 capture unit. This surplus of energy can be in the range of 10-40% of the total energy requirement. Moreover, the capture process accounts for more than 70% of the overall cost. Ultimately, it is expected that 85-95% of the CO2 produced will be captured. So there are not only technical questions that need to be addressed but also economics to evaluate the feasibility of such measures.
GE is actively involved in the development of novel technologies for CO2 capture which have the potential to substantially reduce energy requirements and consequently the time till implementation. Gonzalez-Salazar et al. [Gonzalez-Salazar2011] recently investigated the performance of a novel post-combustion capture method, i.e. phase-changing absorbents, and compared it with state-of-the-art monoethanolamine (MEA). MEA is often used as a reference technology to which other technologies must be compared because of its maturity level. These absorbents are based on an aminosilicone material with a high boiling point and low viscosity, which readily forms a solid when brought in contact with CO2. Absorbent recovery can be carried out by heating. Literature reveals that many chemical absorbents for CO2 capture have emerged in the past few decades. So what is special about phase-changing absorbents? Two very important factors that determine the feasibility of processes in research and even more in industry are its long-term stability and energy requirement. This aminosilicone was found to have a higher thermal stability than MEA and, depending on the exhaust gas conditions, a lower energy requirement than MEA. However, one should consider the individual technology maturity level. As you can imagine, novel technologies often have an inherently higher uncertainty in performance and economic risk than mature technologies.
References
[Gonzalez-Salazar2011] Gonzalez-Salazar et al.; Comparison of current and advanced post-combustion CO2-capture technologies for power plan applications; Energy Procedia; 2011
[IPCC2005] IPCC; 2005
