Long-term diagenetic transformation and carbon sequestration potential of Ca-rich oil shale ash waste deposit sediment

The oil shale industry has granted the energy independence of Estonia over decades. But, in addition to producing electricity and shale oil, the industry is also responsible for producing millions of tonnes of solid waste and emitting vast amounts of greenhouse gases. Over the last few years, the use of oil shale and therefore the production of oil shale ash and CO2 emissions have nearly halved. To achieve Paris climate goals, this trend is set to continue. Still, the industry will continue working in the foreseeable future and it is important to study the ash transformation and diagenesis in order to assess its environmental impact and the structural integrity of legacy ash deposits. The aim of this dissertation is to study long-term changes in mineral composition, the capacity of ash sediment to bind atmospheric CO2 and recommend actions to increase mineral carbonation in the open conditions of the ash deposits. The ash formed by oil shale combustion and retorting is deposited with water on ash deposits, where it can bind atmospheric CO2 in carbonate minerals due to its chemical reactivity. The high content of free lime in the ash that quickly hydrates makes the water circulating at the deposits highly alkaline and in addition to the estimated fast process of carbonation, various slow recrystallization processes take place over decades. The results of the thesis show that CO2 transport in the ash sediment is extremely limited due to fast deposition and formation of largely impenetrable sediment layers. This inhibits carbonation reactions and deposited ash sediment will only bind an estimated 2% of the CO2 emitted during combustion of oil shale. This is several times less than previous estimates based on laboratory experiments. Therefore, wide-spread ash sediment carbonation as previously believed to be the case inside ash deposit is strongly subdued and instead the most prominent transformation is alkali activation of the material and progressive formation of polymeric minerals over time. These minerals will increase the cementitious properties of the ash sediments over time but will also reduce the potential of mineral carbonation. Therefore, by using active carbonation processes, the potential CO2 binding capacity of the ash sediments can likely be increased fourfold.