The Oxy-Fuel Process - Towards high efficient operation

Licentiate thesis, 2008

The power generation system is facing large changes to meet the demands on reduced emissions of carbon dioxide (CO2). Oxy-fuel combustion with capture of CO2 is a promising option to meet those demands. To make the transition from current technology to oxy-fuel technology easier, much work on the oxy-fuel process is focused on achieving combustion conditions similar to air-fired power plants. However, in doing so, opportunities to optimize the plant performance with respect to the characteristics of the oxy-fuel process are lost. This work maps and assesses the characteristics of the oxy-fuel process in order to find new optimum operation conditions and process designs. Two possibilities are investigated: High-temperature combustion as a primary measure for reduction of nitrogen oxides (NOx), and oxygen lean combustion with co-production of electricity and synthesis gas. Experimental and modeling work shows that the emissions of NOx (mg/MJ) are significantly lower in oxy-fuel combustion than in air-firing. This reduction is caused by reburning mechanisms when NOx in the recycled flue gas is reintroduced to the flame. However, the reduction achieved by reburning mechanisms are small compared to the possible reduction by the Zeldovich (thermal) mechanism, and the optimal operational conditions for NOx reduction in oxy-fuel combustion is at high combustion temperatures. An increased combustion temperature also promotes combustion efficiency and reduced furnace volume, both through intensified combustion and higher concentration of oxygen. A process simulation of an integration between an oxy-fuel power plant and a synthesis of DME shows that production of synthesis gas will benefit from oxygen lean combustion (stoichiometric ratio ≈ 0.6) compared with conventional gasification. Besides increased efficiency, the oxygen lean combustion with integrated DME production is favorable due to combined use of equipment and the large scale and infrastructure of a power plant.