ENERGETIKA. 2007. T. 53. Nr. 2. P. 8–18 © Lietuvos mokslų akademija, 2007 © Lietuvos mokslų akademijos leidykla, 2007 Estonian oil shale properties and utilization in power plants Arvo Ots Oil shale is a sedimentary rock containing organic matter and belongs to the sapropel fuel group. Estonia has significant oil shale recourses. Estonia is the only country in the world that uses oil Thermal Engineering Department, shale fired power plants to supply most of its electricity to domestic customers and can export Tallinn University of Technology, it to neighboring countries. Estonian oil shale is a carbonaceous fuel which contains a compli- Kopli 116, 11712 Tallinn, Estonia cated composition of organic and mineral matter. The complicated compositio offers numerous E-mail: [email protected] sophisticated problems that can influence power plant equipment operation and reliability.We have examined oil shale as a power fuel and its utilization problems. Key words: organic matter, mineral matter, combustion characteristics, pulverized firing, fluid- ized bed combustion, environmental protection 1. INTRODUCTION Various aspects of Estonian oil shale and its characteristics as well as energy use are presented in [1–4]. Oil shale is a sedimentary rock containing organic matter, ker- Below, we present Estonian oil shale as a power fuel and its ogen, and belongs to the group of sapropel fuels. A significant utilization problems in power plants. feature of oil shale organic matter is its low solubility in strong solvents. 2. ESTONIAN OIL SHALE GEOLOGY AND Oil shale differs from humus fuels by its high content of or- RESERVES ganic matter. The atomic ratio of hydrogen to carbon (H/C) is about 1.5, which is approximately the same as for crude oil, for The oil shale body lies at a depth of between 30 to 60 m, but in coals being only about 0.3 to 0.4. The high hydrogen-to-carbon the center of the Pandivere upland its depth of occurrence is 100 ratio is the main reason for the high yield of volatile matter and to 120 meters. The total area is about 30002 km . The mined area condensable oil during the thermal decomposition of kerogen. covers a total of 425 km2. Oil shale deposits have been discovered on all continents. Oil shale beds have a stratified structure,with oil shale seams More than 600 oil shale basins are known. The world oil shale alternating with mineral interbeds. resources are estimated to about 11.5 Tt. Figure 1 shows a cross-section of an Estonian oil shale deposit. The largest oil shale resources belong to the Middle In an oil shale seam, organic matter is tightly bound with Cambrian, Early and Middle Ordovician, Later Devonian, sandy-clay minerals and forms a uniform mixture. Oil shale is Late Jurassic, and Paleogene periods. The formation oil shale a yellow-brownish, relatively soft material. The main substances took place mostly in marine conditions and less often in lakes. in the interbeds are carbonate minerals – mainly calcium car- Therefore, the geotectonic structure of oil shale deposits is pre- bonate, to a lesser extent dolomite. dominantly of the platform type. A commercial bed consists of seven indexed oil shale seams Estonia has significant oil shale resources; they are esti- counted from bottom to top. The majority of oil shale seams mated to more than 7 Gt. Estonia is the only country in the contain lens-like concretions of kerogenous limestone. They are world that operates oil shale fired power plants to supply most present in different layers and in various proportions. The thick- of its electricity to domestic customers and can export power ness of oil shale seams varies from 0.05 to 0.6 m. to neighboring countries. In addition to thermal power plants, There are four oil shale seams in the Leningrad oil shale basin, Estonia has also oil shale thermal processing plants for shale oil and they are marked with Roman numbers from top to bottom. production. Power plants and processing factories in Estonia are Considering oil shale as a fuel, its dry matter consists of supplied with oil shale from two underground mines and two three parts: organic – R, sandy-clay (terrigenous) – T, and car- opencast mines. bonate – K. Consequently, Estonian oil shale as a power fuel undoubtedly belongs to Rd + Td + Kd = 100%. the fuel class with the most complicated organic and mineral The values of R,T,and K differ for different oil shale seams. matter composition in the world. The composition of oil shale Figure 2 shows the dynamics of Estonian oil shale produc- offers numerous complicated and interrelated problems that tion over time and also the operation time of opencast and un- can influence power plant equipment operation and reliability. derground mines. Estonian oil shale properties and utilization in power plants 9 Fig. 1. Cross-section of oil shale deposit: 1 – limestone, 2 – kerogenous limestone, 3 – oil shale The largest consumers of oil shale in Estonia are thermal power plants. Today power plants consume about 12 Mt/y. The shale oil production industry consumes the second highest amount – about 2 Mt/y. 3. OIL SHALE PROPERTIES 3.1. Organic and combustible matter The elemental composition of the organic matter of oil shale is given in Table 1. The main characteristics of the organic matter of oil shale are a high hydrogen and oxygen content and a low nitrogen percent- age. Oil shale organic matter contains on average about 1.8% of organic sulfur. An important characteristic of the organic matter is a high chlorine content. One of the combustible components of oil shale is marcasite (pyrite) with the general chemical formula FeS2 (Tables 2 and 3). The sum of organic and pyretic sulfur is called combustible or volatile sulfur: SV = So + Sp. Organic matter together with pyretic sulfur is considered a combustible matter of oil shale. Figure 3 presents the thermal decomposition characteristics of oil shale as the release of different components during devola- tilization. The first signs of the thermal decomposition of organic mat- Fig. 2. Estonian oil shale production and operation time of mines ter appear at 150 °C. Visible decomposition starts at 200–250 °C. Table 1. Elemental composition of organic matter of Estonian oil shale, % Element C H O N S Cl Range 77.11–77.80 9.49–9.82 9.68–10.22 0.30–0.44 1.68–1.95 0.60–0.96 Average 77.45 9.70 10.01 0.33 1.76 0.75 10 Arvo Ots Their ratio corresponds to the average oil shale carbonate part CaO/MgO = 7.3. The average ratio of oxides and 2CO in the oil shale carbonate part is MOc/CO2 = 1.217, where MOc and CO2 denote the con- tent of oxides in carbonate minerals and mineral CO2 content in mineral matter, respectively. Table 3. Mineralogical composition of Estonian oil shale mineral matter, % Group of Mineral Formula Content minerals Calcite CaCO3 69.1 Carbonate Dolomite CaMg(CO ) 30.6 minerals 3 2 Siderite FeCO3 0.3 Total 100.0 Quartz SiO2 23.9 Rutile TiO2 0.7 Fig. 3. Material balance of Estonian oil shale organic matter as a function of tem- Orthoclase K O·Al O ·6SiO 29.0 perature 2 2 3 2 Albite Na2O·Al2O3·6SiO2 6.0 Sandy-clay Anorthite CaO·Al2O3·2SiO2 1.4 minerals One feature in the decomposition process of oil shale organic Muskovite [K1-x(H2O)x·Al3Si3O10(OH)2]2 23.7 matter is thermobitumen formation at temperatures between Amphipole [NaCa2Mg4)Fe,Al)Si8O22(OH)2] 2.1 250–425 °C. It is also notable that the proportion of the condens- Markasite FeS2 9.3 ing phase in volatile matter decreases with temperature and the Limonite Fe2O3·H2O 2.9 decrease is accompanied by an increase in the gaseous phase. Gypsum CaSO4·2H2O 1.0 Due to the high H/C ratio, oil shale has a very high volatile Total 100.0 matter content. Its standard value is 85–90%. The higher heating value (HHV) of oil shale organic matter is The main components in the sandy-clay part are SiO2, Al2O3 36.68 MJ/kg and the lower heating value (LHV) is 34.56 MJ/kg. and K2O. The content of potassium (given as2 K O) exceeds that The LHV of volatile matter is approximately 34 MJ/kg. of Na2O by about 8 to 12 times. Table 3 gives the mineralogical composition of Estonian oil 3.2. Mineral matter shale mineral matter. The mineral matter of oil shale can be divided into two large The main minerals in the carbonate part are calcite and dol- groups: sandy-clay or terrigenous part and carbonate matter. omite. The sandy-clay part is densely intertwined with organic matter. Carbonate minerals in the Estonian oil shale deposit occur as separate layers or are distributed as limestone concretions in oil shale seams. Table 2 presents the chemical composition of the sandy-clay and carbonate parts of oil shale. The main component of the oil shale carbonate matter is calcium oxide (48.1%), followed by magnesium oxide (6.6%). Table 2. Chemical composition of Estonian oil shale mineral matter, % Sandy-clay part Carbonate part Component Content Component Content SiO2 59.8 CaO 48.1 CaO 0.7 MgO 6.6 Al2O3 16.1 FeO 0.2 Fe2O3 2.8 CO2 45.1 TiO2 0.7 MgO 0.4 Na2O 0.8 K2O 6.3 FeS2 9.3 SO3 0.5 H2O 2.6 Total 100.0 Total 100.0 Fig. 4. Thermal effects occurring during combustion of oil shale in a calorimetric bomb Estonian oil shale properties and utilization in power plants 11 The main minerals in the oil shale sandy-clay part are quartz, ∆QC – thermal effect due to incomplete combustion of carbon, feldspars (mainly as orthoclase), and micas (mainly as musco- ∆Qc – thermal effect due to incomplete decomposition of car- vite).