Thermodynamic and kinetic analyses of the solar thermal gasification of petroleum coke

Abstract

The objectives of this thesis are the theoretical and experimental analyses of the chemical kinetics and thermodynamics of the solar thermal gasification of petroleum coke. Equilibrium computation of the stoichiometric system of petcoke and steam at 1 bar and 1300 K result in an equimolar mixture of H2 and CO. A 2nd-law analysis for electricity generation using the gasification products indicates the potential of doubling the specific electrical output and halving the specific CO2 emissions visa-vis conventional petcoke-fired power plants. Rate laws for pyrolysis and reactive gasification are derived: Pyrolysis is modeled as a linear combination of first order decomposition reactions. The model for the reactive gasification bases upon a Langmuir-Hinshelwood type reaction mechanism considering reversible sorption of gaseous species and irreversible reactions among adsorbed species and with molecules from the gas phase. Further, the grain model is used to account for mass transfer limitations in the solid. The rate constants are determined experimentally by thermogravimetry in the 900-1300 K temperature interval using H2O-CO2-Ar mixtures and gaseous reaction products are detected by GC. The TG experiments show a different reactivity for delayed coke samples if they are gasified as received, after partial gasification in an entrained flow reactor, or after temperature-treatment above 1300 K. Experiments with laboratory scale fluidized bed reactors featuring two different modes of heat transfer were performed. The particles in the bed were heated by direct and indirect thermal radiation from the ETH’s high-flux solar simulator using transparent and opaque fluidization tubes. An effect of the heat transfer mode on the reaction kinetics was not observed and rate data complies with data from the TG experiments. The design of a 5 kW solar thermal cavity reactor with entrained flow of gas and solids operated with a fine ground coke slurry is presented. Experiments at PSI’s solar furnace yielded chemical conversion of carbon and steam up to 87% and 69%, respectively, at a solar power input in the range 3.3-6.6 kW and a coke mass flow rate in the range 1.85-4.45 g/min. Operation with a stoichiometric feed produced a gas with H2/CO ≈ 2 and CO2/CO ≈ 0.3. A mathematical reactor model is developed based on the axial dispersion model. The axial dispersion number is obtained from residence time distribution measurements. The model is extended by means of radial temperature profiles extracted from a heat transfer simulation. The kinetic model is used to calculate the reactor performance and the modeled data is compared with the experimental campaign. Mass transfer in the gas phase was analyzed and found to have no effect for the used particle size and temperatures. The research presented in this work serves as a fundamental reaction kinetics study that can be applied for the design of solar thermal gasification reactors.