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type     June, 2005

Vol 1 Chapter 28: Zero or Low Recycle In-Duct Burner Oxyfuel Boiler Feasibility Study

Mark Simmonds and Graeme Walker

The CO2 Capture Project (CCP) has been established by eight leading energy companies to develop novel technologies that significantly reduce the cost of capturing CO2 for long-term storage. One area considered by the CCP is the use of oxygen in combustion systems (oxyfuel combustion). This is attractive to the CCP as it produces a flue gas essentially containing only CO2 and water, from which CO2 can be easily captured. This study evaluates the potential benefits of a novel oxyfuel boiler design that splits the fuel gas between a number of in-line burners. The adiabatic flame temperature is limited to a maximum of 850 8C by cooling the flue gas between each successive burner and thereby permitting conventional stainless steel construction. Steam is raised in these inter-stage coolers and superheated in the exhaust stream exiting the boiler. The design intent is to use this inter-stage cooling to control the combustion temperature rather than the more conventional alternative of recycling flue gas. Therefore, the objective is to eliminate, or at least minimise, flue gas recycle. The study concludes that a zero recycle case is technically feasible. However, in order to deliver the required amount of steam to the specified superheated conditions, either a large number of burner stages are required (.14), or the oxygen stream needs to be over-supplied to help suppress the flame temperature. Both of these factors will add to the cost and complexity of the system considerably and the zero recycle case is not pursued further in this study on the grounds that it is not considered to be the most economic configuration. A second case incorporating flue gas recycle is then considered. In order to limit the number of burner stages required, a substantial flue gas recycle is required. This study shows that by recycling 75% of the flue gas, a 3-stage burner design will deliver the required steam production to the required superheated conditions. Even though this case has a large recycle, it is considered to offer the lowest cost option of incorporating the in-duct oxyfuel boiler concept for the steam generation design basis. The installed cost of the in-duct oxyfuel boiler design with flue gas recycle, including the associated air separation and CO2 capture/compression units, is estimated to be £30 million ($52.5million), equating to a CO2 capture cost of £90.80($158.90) per tonne of CO2 captured per year. The installed capital expense is roughly 10% cheaper than an alternative oxyfuel boiler design based on conventional boiler technology and incorporating flue gas recycle. The footprint required by the in-duct oxyfuel boiler is also assessed and is estimated to be about twice the size of a conventional oxygen-fired boiler. Based on the cost and footprint evaluation, it is considered that there is insufficient justification to develop the in-duct oxyfuel boiler concept within the CCP framework. Although the installed cost is slightly lower than a more conventional boiler design, it still represents a high cost of CO2 capture and does not offer a sufficiently large enough prize to warrant further development.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project Capture and Separation of Carbon Dioxide from Combustion Sources - Volume 1
Edited by:
David C. Thomas, Senior Technical Advisor, Advanced Resources International Inc, USA

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