Tõnu Pihu

The global demand for resources is escalating in today’s rapidly evolving world. As traditional raw materials become scarcer and more expensive, alternative sources have to be found. One such emerging resource is oil shale ash. This article provides a comprehensive overview of the various fractions of oil shale ash generated in the oil shale industry in Estonia. The ash results from the direct combustion of oil shale with pulverised combustion (PC) and circulating fluidised bed combustion (CFBC) technologies, as well as from shale oil production processes. It offers detailed information about the proportions of ash derived from different technological processes and a thorough analysis of their mineralogical and chemical compositions, trace element content, and leaching characteristics. By examining these diverse characteristics, the study enhances understanding of the ash’s potential implications and applications.

Low-temperature circulating fluidised bed combustion (CFBC) of oil shale results in Ca-rich ashes with low pozzolanic properties, raising concerns regarding the long-term stability of ash depositories. This paper presents findings from a long-term field study investigating the mineral and chemical transformations of total CFBC ash sediments over a 15-year period. The study reveals that the pozzolanic properties of CFBC ashes are primarily influenced by the formation of Ca-Al sulphate mineral ettringite and the compactness of sediment. The compaction of ashes during deposition contributes to the development of a dense microstructure, providing uniaxial compressive strength that exceeds 20 MPa. Our findings suggest that compacted and hydrated CFBC ashes are sufficiently stable in ash deposits, holding promise as a sustainable alternative for cement-free construction materials.

A co-pyrolysis of woodchips and oil shale was conducted in a continuous reactor at 520 °C in a CO₂ atmosphere. Experimental product yields were derived and an analysis of the liquid products was conducted, using gas chromatography, infrared spectroscopy, and physicochemical analysis. A maximum yield of liquids and gases was obtained as the share of biomass increased (43.9 and 35.1 wt%, respectively). Product characterization confirmed additive behavior in co-pyrolysis. The liquid products from co-pyrolysis blends exhibited fewer oxygenated compounds, derived from biomass, and fewer aromatic compounds, derived from oil shale. Co-pyrolysis liquids contained abundant aliphatic hydrocarbons (C₆ to C₁₁).

In order to reach future goals of net carbon neutrality and climate change mitigation, various carbon capture and sequestration techniques must be implemented. Industrial waste rich in chemically active alkaline metal oxides is considered as a potential material for CO₂ sequestration. The authors studied the long-term CO₂ binding capacity of Ca-rich oil shale ash (OSA) deposits at oil shale(OS)-fired power plants of Estonia and estimated the remaining sequestration potential for in-situ carbonation. Providing energy security, the Estonian oil shale industry is the biggest national producer of solid waste and the leading greenhouse gas emitter, making the country one of the largest per capita producers of CO₂ in Europe. The study shows that ash deposits are currently only partially carbonated, with an average CO₂ binding rate of 51 kg per tonne of hydrated sediment, most of which being bound in such carbonate minerals as calcite and vaterite, as well as in Ca-silicate thaumasite. It is estimated that at full carbonation of reactive Ca and Mg phases in ash (portlandite, ettringite and semicrystalline C-S-H), the projected average total CO₂ binding potential could rise to ca 200 kg per tonne of ash.

The Estonian energy industry relies on local calcareous oil shale that is used to produce the majority of the country’s electricity, but the shale is also used for shale oil retorting. The solid residues remaining after combustion are upon open air deposition potentially capable of binding part of the released CO₂ by carbonation of reactive Ca-phases. We studied the isotopic composition of authigenic carbonate phases in an alkaline Ca-rich oil shale ash waste deposit accumulated over nearly 50 years to reveal the carbonation mechanisms and the sources of CO₂ required for carbonation. The secondary carbonate phases forming in oil shale waste deposits have low δ¹³C and δ¹⁸O values characterized by δ¹³C_V-PDB values between -12‰ and -24‰ and δ¹⁸O_V-PDB between -8‰ and -15‰. The negative isotopic composition of carbonate carbon seemingly points to a contribution of CO₂ derived from the degradation of residual organic material, potentially present in shale retorting ashes. The low δ¹³C value of carbonate is rather caused by the non-equilibrium fractionation effects during diffusion and hydroxylation reactions of CO₂ in hyperalkaline conditions under a limited CO₂ diffusion rate.

Coal co-firing experiments were conducted in a 250 MW oil shale fired circulating fluidized bed combustion (CFBC) boiler. The objective of the experiments was to test whether adding coal to oil shale would allow the use of the latter with lower heating value. Bituminous coal was mixed with oil shale and fed into the boiler via existing fuel feeding ports. Two test series were accomplished: 11–29% thermal input of coal mixed with 8.4 MJ/kg oil shale (standard fuel), and 12–32% thermal input of coal mixed with 7.5 MJ/kg oil shale. During the experiments, which lasted in total for 15 days, ash samples were collected and flue gas analysis was performed. The boilers were able to continue work with all the fuel mixtures, but a significant increase of nitrogen oxides (NO_x) emissions and heat losses due to unburnt carbon in the bottom and fly ashes were observed. The heat losses can be reduced by upgrading the fuel preparation system, but NO_x emissions limit can be reached only with installation of an additional DeNO_x system. The ash chemical composition remained similar. Sulphur emissions stayed minimal, but a slight increase of carbon monoxide concentration was noticed. Coal co-firing is possible in oil shale CFBC boilers, but the coal must have low fuel nitrogen content and extra attention to the fuel preparation system has to be paid.

The solid heat carrier (SHC) retorting method, so-called Galoter process, was developed for oil shale processing at the end of the 1940s. Since then the method has undergone several improvements. Nowadays there are different modifications of Galoter process in use – Petroter, Enefit-140 and TSK-500 technologies. The major differences between these technologies are in sizing (throughput), technical solutions and layouts. Recently a shale oil plant based on a new technology, Enefit-280, was commissioned. Enefit-280 is a technology successor of Enefit-140 where the heating of solid heat carrier is accomplished using the circulating fluidized bed (CFB) combustion technology as opposed to the conventional heat carrier combustion technology in Enefit-140. The CFB technology in Enefit-280 was integrated into the process to improve the performance of SHC heating process and reduce the emissions. Operational experience has demonstrated that the modified technology of SHC oil shale retorting has a potential to play a key role in shale oil production with reduced environmental impact.

Estonia’s basic power supply is covered mainly by oil shale-fired thermal power plants. The pulverized combustion (PC) and circulating fluidized bed combustion (CFBC) technologies are used. The power plant exploitation has revealed the emission of gaseous pollutants, as well as ash handling problems. The hydro ash removal is used at large power plants in Estonia. An overview of the formation and properties of oil shale ash is given. The polluting impact of ash in contact with water is analyzed. Taking into account precipitation and evaporation conditions the amount of water bound by ash as well as ash field water balance is given. The leaching behaviour of oil shale ash is analyzed. The analysis of the ash field structure shows that the degree of water penetration of the ash field body meets the requirements for hazardous waste landfills. The water permeability through dense layers ranges from 0.15 × 10^–9 to 16.1 × 10^–9 m/s.

Carbon dioxide emissions are considered a major environmental threat. To enable power production from carbon-containing fuels, carbon capture is required. Oxy-fuel combustion technology facilitates carbon capture by increasing the carbon dioxide concentration in flue gas. This study reports the results of calcium rich oil shale combustion in a 60 kW_th circulating fluidized bed (CFB) combustor. The focus was on the composition of the formed flue gas and ash during air and oxy-fuel combustion. The fuel was typical Estonian oil shale characterized by high volatile and ash contents. No additional bed material was used in the CFB; the formed ash was enough for the purpose. Two modes of oxy-fuel combustion were investigated and compared with combustion in air. When N₂ in the oxidizer was replaced with CO₂, the CFB temperatures decreased by up to 100 °C. When oil shale was fired in the CFB with increased O₂ content in CO₂, the temperatures in the furnace were similar to combustion in air. In air mode, the emissions of SO₂ and NO_x were low (3 @ 6% O₂, respectively). Pollutant concentrations in the flue gas during oxy-fuel operations remained low (for OXY30 SO₂ < 14 and NO_x 130 mg/Nm³ @ 6% O₂ and for OXY21 SO₂ 23 and NO_x 156 mg/Nm³ @ 6% O₂). Analyses of the collected ash samples showed a decreased extent of carbonate minerals decomposition during both oxy-fuel experiments. This results in decreased carbon dioxide emissions. The outcomes show that oxy-fuel CFB combustion of the oil shale ensures sulfur binding and decreases CO₂ production.