• Energy Research

Abstract Chemical looping combustion (CLC) is an emerging CO 2 capture technology utilizing two interconnected circulating fluidized beds (CFB). An oxygen carrier is oxidized in the air reactor and reduced in the fuel reactor. In comparison to a classic oxyfuel process, no air separation unit is required to provide the oxygen needed to burn the fuel. The solid fuel, such as coal or biomass, is gasified in the fuel reactor, and the products from gasification are oxidized by the oxygen carrier. This work presents results of 110 h autothermal and partial autothermal CLC operation with ilmenite and iron ore as oxygen carriers. Hard coal and a fuel blend of hard coal and torrefied biomass were used as fuels. The results show that the oxygen demand of the fuel reactor required for a complete conversion of unconverted gases was in the range of 27%. At the same time, the carbon capture efficiency was low in the present configuration of the 1 MW th pilot. This means that unconverted char left the fuel reactor and burned in the air reactor. The reason for this is that no carbon stripper unit was used during these investigations. Some possible modifications like an oxygen carrier with oxygen uncoupling, a lower fuel feeding point or a secondary fluidization of the fuel reactor could significantly enhance the carbon capture efficiency.

This work focuses on a comparison between the Euler-Euler Two Fluid Model (TFM) approach and the coupled coarse grain discrete element CFD-DEM numerical model for the simulation of a 1 MWth CFB carbonator reactor located at TU Darmstadt (TUD). The effect of the drag force formulation and its associated application in the numerical model for both approaches in terms of their numerical accuracy, compared to experimental data is investigated, by implementing either the conventional Gidaspow model or the advanced EMMS one. Moreover, for the coarse grain CFD – DEM model, the range of values for important numerical parameters as the particle per parcel and cell to parcel size ratios are investigated to shed light on the necessary resolution such a model should have in order to reproduce valid and not parameter dependent numerical results. An adequate cell length to parcel diameter ratio is found to be around 2.6 while as concerns the parcel to particle diameter ratio a value around 58.5 proved to be sufficient, at least for the range of parameters investigated in this paper (size of riser, flow rates and particles average diameter). The EMMS model improved the accuracy of results derived by the coarse grain CFD – DEM model, while further research on the appropriate drag models for the coarse grain CFD – DEM is a sine qua non for its successful implementation in similar studies. For instance it is of interest to answer whether the individual particles slip velocity instead of the particles cell averaged slip, should be used for the calculation of the momentum interexchange coefficient (β) as well as the treatment of different particle diameters in the EMMS equation scheme.

Abstract The rapid expansion of renewables due massive use of subsidies in the last years changes the operation of fossil fired power plants in Germany dramatically. Since the large-scale storage of electricity is still difficult and forecasts for wind and solar energy have unavoidable uncertainties, the conventional power plants have to compensate the demand in the grid. Several new measures are introduced by power plants operators in order to respond to these new market conditions like the reduction of the low load operation of the nominal load and the continuous operation at an off-design operation point. In this work, a full-scale dynamic model of large scale coal-fired power plant has been developed to investigate the operation flexibility. The dynamic model, developed in the process simulation program APROS, includes all power plant components and its associated control schemas. The only boundary conditions are the coal composition, the ambient temperature and the temperature and amount of cooling water in condensers. The developed model is validated against operational data from the real power plant during part load transients, showing a very good agreement. The validated model is of high relevance for further investigations regarding flexibility increase in conventional large coal-fired power plants.

A comprehensive process model is proposed to simulate the steam gasification of biomass in a bubbling fluidized bed reactor using the Aspen Plus simulator. The reactor models are implemented using external FORTRAN codes for hydrodynamic and reaction kinetic calculations. Governing hydrodynamic equations and kinetic reaction rates for char gasification and water-gas shift reactions are obtained from experimental investigations and the literature. Experimental results at different operating conditions from steam gasification of torrefied biomass in a pilot-scale gasifier are used to validate the process model. Gasification temperature and steam-to-biomass ratio promote hydrogen production and improve process efficiencies. The steam-to-biomass ratio is directly proportional to an increase in the content of hydrogen and carbon monoxide, while gas yield and carbon conversion efficiency enhance significantly with increasing temperature. The model predictions are in good agreement with experimental data. The mean error of CO2 shows the highest value of 0.329 for the steam-to-biomass ratio and the lowest deviation is at 0.033 of carbon conversion efficiency, respectively. The validated model is capable of simulating biomass gasification under various operating conditions.

Abstract The oxy-fuel process is a promising technology for capturing carbon dioxide emitted by coal-fired power plants. Due to the combustion of coal with pure oxygen and recycled flue gas, the concentrations of carbon dioxide and water vapor in the flue gas of the power plant boiler are higher than in the air-fired process. Both species show strong absorption and emission characteristics in the infrared wavenumber region providing enhanced radiative heat transfer to the evaporator tubes along the furnace walls. To accurately predict thermal radiation, the non-gray behavior of gas radiation has to be accounted for. Efficient non-gray radiation models have been developed to be applied to numerical simulations of oxy-fuel furnaces. One approach is the application of new weighted sum of grey gases model (WSGGM) to 3D-CFD simulations. In this study, the simulation of a demonstration scale oxy-fuel power plant furnace is presented. Gas radiation is modelled with the WSGGM with a state of the art parameter set. The WSGGM formulation includes the molar ratio of H2O-CO2 mixtures. Therefore, it is suitable for the modeling of radiation under oxy-fuel conditions. The simulation results deliver information on the combustion process and the heat transfer inside the boiler, especially in the evaporator section.

The particle-laden flow in the separator of a laboratory beater wheel mill is computationally and experimentally investigated. The main aim of the present study has been the identification of convenient modelling approaches and the assessment of the modelling predictive capability for this class of problems. For the particulate phase a Lagrangian formulation is adopted. Particle computations are carried out as post-processing, assuming different size class distributions. A modulation of the gas turbulence by the particulate phase is neglected. Different turbulent models are used. The turbulent particle dispersion is modelled by a stochastic approach. Different geometric configurations of the separator are investigated. With the results of the simulation study it became possible to predict the trends correctly.

Efficient and flexible operation is essential for competitiveness in the energy market. However, the CO2 emissions of conventional power plants have become an increasingly significant environmental dilemma. In this study, the optimization of a steam power process of an IGCC was carried out, which improved the overall performance of the plant. CCPP with a subcritical HRSG was modelled using EBSILON Professional. The numerical results of the model were validated by measurements for three different load cases (100, 80, and 60%). The results are in agreement with the measured data, with deviations of less than 5% for each case. Based on the model validation, the model was modified for the use of syngas as feed and the integration of heat into an IGCC process. The integration was optimized with respect to the performance of the CCPP by varying the extraction points, adjusting the steam parameters of the extractions and modifying the steam cycle. For the 100% load case, a steam turbine power achieved increase of +34.2%. Finally, the optimized model was subjected to a sensitivity analysis to investigate the effects of varying the extraction mass flows on the output.

Solar-assisted combined cycle power plants (CCPPs) feature the advantages of renewable clean energy with efficient CCPPs. These power plants integrate a solar field with a CCPP. This integration increases the efficiency of solar power plants while decreasing the CO2 emissions of the CCPPs. In this paper, energy and exergy analyses were performed for an existing solar-assisted CCPP. The overall thermal efficiency and the exergetic efficiency of each component in the power plant were calculated for different solar field capacities. Also, a parametric study of the power plant was performed. The analysis indicated that the exergetic efficiency of the power plant components has its lowest value in the solar field while the condenser has the lowest exergetic efficiency in the combined cycle regime of operation. Further, a parametric study revealed that the thermal efficiency and the exergetic efficiency of the power plant as a whole decrease with increasing ambient temperature and have their highest values in the combined cycle regime of operation. Owing to these results, an investigation into the sources of exergy destruction in the solar field was conducted.