Oxy-coal combustion for retrofit: challenges and opportunities

This paper is concerned with Oxy-Fuel Combustion, a process for the control of carbon dioxide from a range of solid fuel combustion processes. The focus is on the applicability of this technology for retrofit and hence on entrained flow combustors rather than on circulating fluidized bed systems. First, the importance of retrofit technology for CO2 control from coal fired units in the US is outlined. The current state of the technology (applied mainly to pulverized coal combustors), and critical research needs are identified. There are three over-arching issues that must be resolved in order for oxy-coal retrofit to be attractive: the first is related to the O2 supply energy penalty since current cryogenic technology can consume 15-20% of energy produced; the second is related to the purity of CO2 in the flue gas for sequestration; and the third is related to air ingress through leaks that are present in most existing systems. Resolution of these over-arching issues requires efforts by industrial gas engineers, politicians, and combustion engineers, respectively. One objective of current research in oxy-coal combustion for retrofit is to create enabling technology to be used for: 1) extrapolation of boiler performance from conventional air firing to oxygen firing with sufficiently large amounts of flue gas recycle to match heat transfer to existing heat exchange surfaces. 2) extrapolation to substantially modified existing units that "still look like boilers" but that optimize and reduce the amount of flue gas to be recycled and therefore may require some relocation of heat transfer surfaces. This enabling technology will be comprised of simulations employing validated heat transfer sub-models (radiant and convection zones), and validated chemistry sub-models (coal jet ignition, chemistry, char burnout, ash partitioning, trace metals, and combustion by-products - NOx, SOx, Hg), where validation must be under oxy-fuel combustion conditions, with varying amounts and compositions of flue gas recycle streams. Recent research results in each of these areas are presente