Oil shale (OS) is an unconventional low-calorific-value fossil fuel, the usage of which is increasing due to increasing energy demand. Today, Estonia's OS usage is the largest in the world. Approximately 90% of the electricity consumed is produced from Ca-rich OS. Most of the OS, approximately 12 million tons yearly, is used for power generation utilizing pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies that produce nearly 6 million tons of ash. As a result, Estonia has one of the world's highest CO2 emission rates per capita. The current study is focused on determination of the amount of CO2 bound by CFBC and PC boiler OS ashes in an ash field. The binding ability toward CO2 of the ash stored in an ash field is observed. Based on the in situ experimental data, when looking the binding abilities of ashes from both the technologies separately, it is found that approximately 9.7% of the CO2 emitted is bound by the CFBC ash and about 3.4% by the PC boiler ash in an ash field. On the basis of experimental data it is found that approximately 5-6% of the CO2 emitted is bound back by oil shale power plant ash fields and sediment ponds.