Oxyfuel boiler design in a lignite-fired power plant E. Kakaras, A. Koumanakos, A. Doukelis * , D. Giannakopoulos, I. Vorrias Laboratory of Steam Boilers and Thermal Plants, National Technical University of Athens, Heroon Polytechniou 9, 15780 Athens, Greece Received 6 October 2006; received in revised form 13 March 2007; accepted 14 March 2007 Available online 25 April 2007 Abstract In the context of CO 2 capture and storage, the oxyfuel technology provides a promising option applicable in centralised power pro- duction schemes. This technology is based on combustion with pure oxygen instead of air and the flue gas mainly consists of CO 2 and H 2 O. The work presented in this paper is focused in the application of the oxyfuel technology in a lignite-fired power plant. Significant design issues are the required extended flue gas recirculation in order to provide the ballasting effect of the absent N 2 and moderate the furnace temperatures. Therefore, a modified design of heat exchange surfaces of the oxyfuel steam boiler was formulated and was com- pared to a conventional air-fired boiler. A typical modern Greek air-fired power plant has been used as reference. The dominating factors that affect the dimensioning of the oxyfuel boiler are the higher radiative heat transfer – due to the high concentrations of CO 2 and H 2 O in the flue gas – and the different flue gas mass flow, compared to a conventional air-fired boiler. For the determination of the thermo- dynamic cycle characteristics, simulations were made with the use of a thermodynamic cycle calculation software [Stamatelopoulos GN. Calculation and optimisation of power plant thermodynamic cycles, VDI-Regulations, Series 6, Nr. 340. Braunchweig, Mechanical Engi- neering Department; 1996 [in German]]. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Lignite-fired power plants; CO 2 sequestration; Oxyfuel; Boiler design 1. Introduction Carbon sequestration can be defined as the capture and secure storage of carbon (CCS) that would otherwise be emitted to, or remain, in the atmosphere. The rationale for carbon capture and storage is to enable the use of fossil fuels while reducing the emissions of CO 2 into the atmo- sphere, and thereby mitigating global climate change. At present, fossil fuels are the dominant source of the global primary energy demand, and will likely remain so for the rest of the century supplying over 85% of all primary energy [2]. The commercial or under development CO 2 sequestration technologies for coal-fired power plants can be divided into three broad categories [3,4]: – Separation of CO 2 from waste gas. – Combustion in O 2 instead of air. – Production of a carbon free fuel. The oxyfuel concept is based on combustion with pure oxygen instead of air, which is produced by an air separa- tion unit (ASU). Cryogenic air separation is the most suit- able technique for nitrogen separation from air [5]. Up to 15% of the power plant’s electrical output is consumed by the process [6]. The purity of the oxygen produced in the ASU plays an important role regarding power con- sumption for air separation. Taking into account the energy required for the air separation, the production of O 2 with purity of 95% is a promising option [7]. Fuel com- bustion with pure oxygen results in very high combustion temperatures. In order to lower the temperature, part of the flue gas is recycled in the combustion chamber. Several studies propose that when the oxyfuel combustion CO 2 separation technique is applied to a boiler, approximately 0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.03.037 * Corresponding author. Tel.: +30 210 7722720; fax: +30 210 7723663. E-mail address: adoukel@central.ntua.gr (A. Doukelis). www.fuelfirst.com Fuel 86 (2007) 2144–2150