Experimental analysis of the combustion characteristics of Estonian oil shale in air and oxy-fuel atmospheres Lauri Loo a, ⁎, Birgit Maaten a , Andres Siirde a , Tõnu Pihu a , Alar Konist a,b a Department of Thermal Engineering, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia b School of Engineering, Brown University, 182 Hope St., Providence, RI 02912, USA abstract article info Article history: Received 19 August 2014 Received in revised form 10 December 2014 Accepted 11 December 2014 Available online 27 February 2015 Keywords: Oil shale Oxy-fuel combustion Carbonate mineral decomposition DSC TGA O 2 /CO 2 combustion A non-isothermal experimental study using thermogravimetric analysis and differential scanning calorimetry coupled with a quadrupole mass spectrometer was conducted to investigate Estonian oil shale combustion characteristics. The analyses were performed in air (N 2 /O 2 ) and oxy-fuel (CO 2 /O 2 ) atmospheres with various oxygen ratios (10, 20 and 30 vol.%). Our experimental results in TGA show that combustion in the CO 2 /O 2 atmosphere is delayed compared to that in the N 2 /O 2 atmosphere. Carbonate minerals in oil shale decompose in air in one step and in the oxy-fuel atmosphere in two separate steps: the decomposition of dolomite (CaMg(CO 3 ) 2 ) and the subsequent decomposition of calcite (CaCO 3 ). An increased oxygen ratio in combustion in the oxy-fuel atmosphere increases the overall combustion rate, whereas the CO 2 emission volumes decrease because of the lower decomposition extent of carbonates. The quadrupole mass spectrometer measurements indicate several combustion products. A higher CO reading is registered in the CO 2 /O 2 atmosphere, but there is no other significant difference. Based on the measurement results, a combustion model for Estonian oil shale is proposed. Combustion in the oxy-fuel atmosphere is similar to combustion in air, which eases the design of oxy-fuel combustors. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The world's growing demand for energy enforces the usage of low- grade fuels. In Estonia, oil shale (OS) power and oil production sectors are the largest CO 2 emitters. Estonia depends on OS because over 90% of its electricity is produced from OS [1,2]. Therefore, improving the OS combustion process has large environmental and economic benefits. OS is a low-grade fossil fuel that consists of kerogen (organic) and mineral components. The organic matter contains a relatively large amount of hydrogen, and the H/C atomic ratio is 1:4–1:5 [3]. Estonian OS has a high content of mineral matter, which consists of carbona- ceous, sandy-clay-carbonaceous and sandy-clay parts [4]. A higher CO 2 concentration can alter the decomposition of carbonates and decrease CO 2 emission [5]. Therefore, implementation of oxy-fuel technology offers many benefits. The most energy- and cost-effective CCS technology is considered to be the oxy-fuel technology [6]. Implementing the carbon capture and storage (CCS) systems on oxy-fuel combustion technology significantly reduces CO 2 emission. The concept of the oxy-fuel technology is the removal of nitrogen from the combustion process, that is, combustion occurs in an oxygen and carbon dioxide atmosphere. As a result, the formed flue gas mainly consists of carbon dioxide and water vapor, and the volume of flue gas considerably decreases [7,8]. Previous studies [5,9–14] concluded that a higher carbon dioxide concentration in the combustion atmosphere decreases the decomposition of carbonate minerals in the OS, but to our best knowledge, there is no data about higher CO 2 atmosphere effects on the combustion of OS kerogen. Thermogravimetric analysis (TGA) of coal oxy-fuel combustion has proven that the oxy-fuel atmosphere considerably changes the combustion process [15–18]. Nevertheless, Niu and co-workers considered that the investigation of the reaction mechanism and kinetic parameters of the oxy-fuel combustion of coal are not sufficiently reviewed [15]. The same applies to OS oxy-fuel combustion. Simultaneous use of TGA, DSC and QMS allow one to obtain more data with higher precision from different processes that occur in the OS during the thermal treatment. TGA is a common technique to rapidly investigate and compare thermal events and kinetics during the combustion and pyrolysis of a material [19]. TGA measures the mass loss of the sample as a function of time and temperature. A quadrupole mass spectrometer (QMS) improves the analyses of the processes by adding information about the evolving gases. The temperatures at which the mass changes occur can be viewed using TGA, and quantita- tive methods can be applied to the data to obtain the kinetic parameters. DSC adds thermal information about the processes that occur during the measurement, and QMS detects the evaporating ions. Fuel Processing Technology 134 (2015) 317–324 ⁎ Corresponding author. http://dx.doi.org/10.1016/j.fuproc.2014.12.051 0378-3820/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc