Browsing by Author "Simell, Pekka, Dr., VTT Technical Research Centre of Finland Ltd, Finland"

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  • Reactor intensification for CO2 utilization and related C1 chemistry
    (2018) Vidal Vázquez, Francisco
     School of Chemical Engineering | Doctoral dissertation (article-based)
    Carbon capture and utilization and Power-to-X technologies arise as synergetic solution for storing renewable energy and reducing CO2 emissions. Fuels and chemicals can be produced from CO2 as a carbon source instead of fossil-based raw materials. Synthesis reactions using CO2 and H2 are generally highly exothermic, which complicates a good control of reaction temperature. Reactor and process intensification can be used for good control of the reaction conditions and reducing the size and required equipment of a chemical plant. Reactor intensification was applied to the methanol steam reforming process. Control of the temperature in the catalyst bed proved to be a crucial aspect for obtaining reliable kinetic models. A heat exchanger reformer was designed and manufactured for material and thermal integration with a polymeric electrolyte membrane fuel cell stack. This reformer enabled good control of the catalyst temperature and homogeneous flow distribution of the heat transfer fluid. Production of CO from CO2 was investigated by studying the reverse Water-Gas shift (rWGS) reaction at high pressure and temperature. Ni-based catalysts showed the highest activity compared to a Rh-based catalyst. A kinetic model was obtained for the catalyst with 2 w-% of Ni, which displayed high selectivity towards CO formation. The two-step synthesis of linear hydrocarbons from CO2 and H2 was demonstrated in a container-sized unit. This process was formed by a rWGS reactor as a first step and a Fischer-Tropsch synthesis (FT) reactor as a second step. The experimental results revealed the limitations of this process concept for achieving high overall CO2 conversions. For this reason, an enhanced two-step synthesis concept was developed. This concept achieves almost complete CO2 conversion by combining rWGS and catalytic partial oxidation as a first step, high pressure operation and recirculation of the gaseous effluent from the FT reactor. Reactor intensification of CO2 methanation was investigated using Ni-based hydrotalcite (HT) catalyst coated on heat exchanger reactors. The lab-scale heat exchanger reactor allowed excellent control of reaction temperature in the catalyst layer. Kinetic modeling of coated catalyst using this reactor proved to be a reliable method. This was validated by comparison between experimental results and modeling results. A simulated plate type heat exchanger reactor with catalytically coated corrugated plates displayed good performance thanks to the high activity of the Ni-HT coated catalyst, homogeneous flow distribution and high surface area of the reactor. This proved that corrugated plates are a suitable alternative to microchannel plates.
  • Tar reforming in biomass gasification gas cleaning
    (2017) Kaisalo, Noora
     School of Chemical Engineering | Doctoral dissertation (article-based)
    Thermochemical conversion of biomass can be used to produce synthesis gas via gasification. This synthesis gas can be further upgraded to renewable fuels and chemicals provided that the gas is ultra clean. To achieve this, impurities, such as light hydrocarbons and tar compounds present in the gasification gas can be converted to syngas by reforming. The amount of tar in gasification gas can be reduced already in the gasifier by using catalytically active bed materials. Typical bed materials in fluidized bed gasification are sand, olivine, dolomite and MgO. The tar conversion activity of dolomite and MgO were found to be high at atmospheric pressure. However, the activity was lost when the pressure was increased to 10 bar. Gasification gas contains, in addition to tar, ethene, which may contribute to further tar formation in high temperature zones of the process, especially at elevated pressures. Ethene forms tar compounds by radical chain reactions. The tar formed by thermal reactions of ethene resembles the tar from high temperature fluidized bed gasification, which contains mainly secondary and tertiary tar compounds. Carbon formation on the reformer catalysts presents a challenge in biomass gasification gas cleaning. The presence of sulfur in the gas, mainly in the form of H2S, also complicates reforming. Typical catalysts used in the reformer after the gasifier are precious metal and nickel catalysts. The heat for reforming can be brought either indirectly in the case of steam reforming or by adding oxygen to the feed for autothermal reforming. Nickel and precious metal catalyst activities were analysed in experiments of around 500 hours with several different gas compositions. Catalyst deactivation was higher with steam than autothermal reforming. The use of catalytically active bed materials to reduce tar concentration already in the gasifier is especially favourable for steam reforming as the catalyst deactivation rate was decreased by the lower hydrocarbon content of the gas. Benzene, a highly stable compound, is a typical residual compound in the gas after the reformer. Thus, the reformer could be designed based on the reforming kinetics of benzene, for example in the production of synthetic natural gas. For this purpose, qualitative analysis of the effect of the main gasification gas compounds (H2, CO, CO2, H2O) on reforming kinetics were studied with a nickel catalyst. Benzene reforming can be described by first order kinetics if the parameters are estimated for the specific gas composition.