Carbon dioxide emission factors for oxy-fuel CFBC and aqueous carbonation of the Ca-rich oil shale ash

Abstract

Oil shale (OS) is a low-calorific-value fossil fuel. Today, Estonia’s OS usage is the largest in the world. Approximately 76 % of the electricity is produced from Ca-rich OS. Yearly, approximately 12 million tons of OS is used for power generation utilizing pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies that produce nearly 6 million tons of Ca-rich ash.Estonian kukersite OS consists about one third of carbonate minerals, mainly calcite. Therefore, in addition to the combustion of organic carbon, the carbonaceous minerals decompose and release additional CO2. The extent of decomposition of carbonaceous minerals depends on combustion technology. Using oxy-fuel CFBC technology alters ash properties, including decomposition of carbonate minerals. By means of aqueous carbonation of Ca-rich ash, CO2 can be stored safely and leakage-free for very long time.In order to understand the changing CO2 sequestration potential, oil shale ashes were produced at a 60 kWth CFBC facility in oxy-fuel mode. The ash was treated with water and CO2 in order to mimic the ash treatment technology currently in use in the industry. The ash bound 81 kgCO2/tash it means that 6 % of the CO2 emitted would be bound at the ash fields. Due to decreased decomposition of carbonates, when using oxy-fuel CFBC, the CO2 specific emission of combustion would decrease by 5 % compared to regular CFBC and 19 % compared to PF. Decreased CO2 production would result in reduced CO2 transportation and further utilization or storage cost.