In oxyfuel combustion, the combustion air is replaced by oxygen. Thus, the concentration of CO2 in flue gas is increased by using pure or enriched oxygen (O2) instead of air for combustion, either in a boiler or in the gas turbine. In this approach, oxygen will be produced by various methods such as cryogenic air separation, which is currently utilized on a large scale in the industry. Part of the CO2-rich flue gas is recycled to the combustor to avoid the excessively high flame temperature associated with combustion in pure O2. The advantage of this approach is that the flue gas contains only CO2 and water vapor.  Thus, the CO2 separation is a simplified process which is achieved by a cooling process in a heat exchanger for condensing the water vapor. The main disadvantages of this approach are the high capital cost of equipment needed for the cryogenic O2 separation and the high-energy consumption. Oxyfuel combustion for power generation has been demonstrated, so far, on a small scale (up to about 30 MW). In order to improve the system efficiency new technologies such as ion transport membranes are utilized where on site O2 separation and combustion are achieved.  The work of Carbon Sequestration Leadership Forum (CSLF, 2009) has identified the gaps between the present technologies and the required progress.

Oxy-fuel combustion refers to the ignition of pulverized coal or other carbon-based fuels in a nearly pure O2 environment and represents a relatively new process for mitigating CO2 emissions compared with pre-combustion and post-combustion CO2 capture. The significant advantages of this process stem from the fact that the flue gas (following removal of particulates, water, and trace impurity gases) is almost entirely CO2, which greatly simplifies the capture step, and that most existing power plants could be readily retrofitted with an oxy-fuel combustion system.

Conventional Oxy-fuel Combustion Set up

In a conventional set up, O2 (purity >95%) is fed into the plant from a cryogenic separation unit, which separates O2 from the other components of dry air by a distillation process. The O2 inlet gas is diluted with CO2 from the flue stream to a partial pressure of 0.21 bar in order to control the temperature of fuel combustion and to reduce the formation of NOx impurities that frequently form when coal is burned in an O2-enriched atmosphere. The exhaust gas, which is essentially pure CO2, can then be directly subjected to sequestration. Indeed, in addition to CO2 (55-65 wt %), the other major component of the gas stream is water vapor (25-35 wt %), which is easily condensed and removed.  In fact, CO2 capture rates higher than 95% have been achieved by this method, a level not currently possible with precombustion and post-combustion separations. An additional advantage related to combustion in an O2/CO2 mixture lies in the fact that compared with a process utilizing air, which is rich in N2, the formation of NOx is largely inhibited, allowing for a smaller, cheaper NOx removal step than required in current power plants.

Challenges for Oxy-fuel Combustion

 A significant challenge for the implementation of oxy-fuel combustion methods is in the large-scale generation of pure O2 from air. This separation is currently carried out on a scale of over 100 Mt/year, but the large energy requirement for this process creates an urgent need for alternative separation methods if oxy-fuel combustion is to be widely used in mitigating carbon emissions. Micro-porous solids that selectively adsorb O2 from the air could potentially significantly reduce this energy cost. Indeed, zeolites have been used in this separation on an industrial scale and in portable medical devices, although the separation performance and energy efficiency is considered to be insufficient for use in oxy-fuel combustion applications. In this regard, metal-organic frameworks offer tremendous promise for delivering high-performance materials that are specifically optimized for the removal of O2 from the air. This separation is essentially an O2/N2 separation, for which metal-organic frameworks exhibiting high selectivity, and O2 adsorption capacities have been recently reported.

 

 

 


 

 

Copyright 2013  KACST-TIC CCS