A Technical Analysis of Oil Shale Firing Power Units Retrofitting for Carbon Capture and Storage (CCS)

Abstract

In Estonia the significant part of power generation is still based on firing oil shale fuel in thermal power plants despite of the growth of the share of the green energy in recent years. Electricity generation is based on power units utilizing different combustion technologies. A significant part still comprises old units with capacity 185-195 MWe commissioned in early 70s of last century. These pulverized combustion (PC) based units utilize oil shale as a fuel and are operating with subcritical steam parameters, characterizing by relatively low efficiency 26 - 30% (net, LHV based). Due to their low performance, operational issues (intensive fouling of heating surfaces), and remarkable environmental impact, these units were decommissioned or retrofitted with boilers implementing circulating fluidized bed combustion (CFBC) technology in 2004-2005. Implementation of CFBC technology for oil shale combustion significantly reduced both operational issues and environmental impact. With some additional modifications in turbine island allowing to increase rated capacity up to 215 MWe and after increasing steam parameters, the net efficiency of up to 37% was achieved. Consequently, the specific CO2 emissions in the range of 0.99 – 1.05 t/MWhe depending on quality of oil-shale used, was achieved. Recently, the new 300 MWe CFBC based power unit was commissioned. It is operating with subcritical steam parameters: live steam temperature 565 °C and pressure 17 MPa. Net efficiency (LHV based) of that unit is 40 % and specific CO2 emissions up to 0.9 t/MWhe in case of firing only oil shale. In this study the technical viability of retrofitting oil shale based power generation units with CC is described. In the analysis two CC methods were considered: post-combustion (assuming conventional amine process) and oxy-fuel combustion. Among other CC technologies these processes are at the present time characterized by higher technical readiness level, can be applied for existing units with minimal modifications and are applicable for oil shale fuel with it specifics. Besides power generation, Estonia has for almost 100 years of experience in production of shale oil through thermal decomposition of oil shale kerogen in retorts. Currently, solid heat carrier (SHC) method is the main method used for producing shale oil. In this method, direct CO2 emission comes from the preparation of SHC by combustion of solid retorting residue (semi-coke). Mainly two combustion methods are presently applied: lift pipe combustion and CFBC which was recently successfully integrated into shale oil production process. The possibility of utilizing CC technologies in shale oil production units and their technical analysis is described in this study.