Oxy-fuel combustion of Estonian oil shale: Kinetics and modeling

Abstract

In this work, non-isothermal thermal analysis (TA) methods combined with Fourier transform infrared (FTIR) spectroscopy were applied to investigate specifics of different stages of oxy-fuel (OF) combustion of Estonian oil shale (EOS) and its char. Kinetics of EOS and its char oxidation were analyzed in different atmospheres in order to understand the complex mechanism of oil shale (OS) OF combustion. Additionally, the OF combustion of OS in circulating fluidized bed (CFB) was simulated with the recently released Aspen Plus fluidized bed (FB) reactor which treats bottom zone and freeboard hydrodynamics. Particular attention was given to the determination of the required elutriated mass flows to maintain the heat balance of the system for OF combustion cases. Four case studies were simulated including: Case 1: Air combustion, Case 2: 21%O2/flue gas, Case 3: 23%O2/flue gas and Case 4: 30%O2/flue gas. The results of TA experiments show that the pyrolysis behavior is very similar in Ar and CO2 until 500°C and there is no visible char carbon and CO2 reaction under OF conditions. The emissions of CO2 from mineral part of OS can be diminished as decomposition of calcite takes place at higher temperatures in OF combustion. Activation energies calculated for oxidation of OS and its char in CO2/O2 are notably less than activation energies calculated for Ar/O2 atmosphere. Modeling results show that higher fuel mass flow rate and higher O2 concentration in the oxidizer have to be considered in the set of conditions for OF in order to extract the same amount of heat as in air combustion. The Case 3 with 23% inlet O2 concentration has a similar behavior as compared to air combustion in terms of temperature of the boiler and recirculation rate of the particles. The data obtained from experimental measurements and models is valuable for the possible implementation of OF combustion of EOS in CFB boilers.