Pebble-bed reactor

The pebble-bed reactor (PBR) is a design for a graphite-moderated, gas-cooled nuclear reactor. It is a type of very-high-temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative. The basic design of pebble-bed reactors features spherical fuel elements called pebbles. These tennis ball-sized pebbles (approx. in diameter) are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro-fuel particles called tristructural-isotropic (TRISO) particles. These TRISO fuel particles consist of a fissile material (such as 235U) surrounded by a ceramic layer coating of silicon carbide for structural integrity and fission product containment. In the PBR, thousands of pebbles are amassed to create a reactor core, and are cooled by a gas, such as helium, nitrogen or carbon dioxide, that does not react chemically with the fuel elements. Other coolants such as FLiBe (molten fluoride, lithium, beryllium salt)) have a

Analysis and Removal of Product Gas Contaminants Present in Gasification Processes

Sven Philip Edinger

Energy demand and economic development are intrinsically linked. Earlier economic development came at the price of serious environmental pollution, posing a risk to life on earth. Thus, a transition from fossil to renewable energy is needed. Among the renewable energy technologies, gasification of biomass promises high process efficiency and flexibility concerning the feedstock and products. In such a process, gas contaminants have to be removed to protect downstream equipment and the environment. Finding an optimal configuration of process steps in gas cleanup chains, offering reliability, availability and high efficiency at low operating and capital costs is still a challenge and one of the major hurdles for commercial deployment of biomass gasification processes. This thesis aimed to investigate the gas cleanup chain as part of gasification processes. Two aspects, the analysis of gas contaminants and their elimination, were examined. At first, three analytical methods, aimed at the detection of sulphur containing hydrocarbons, tars and trace elements, were developed. In the next step, these analytical tools were employed to investigate the performance of gas cleanup units, intended for the removal of the sulphur containing hydrocarbon thiophene, tars and the trace element Se. The removal of thiophene by activated carbons (AC) was explored experimentally and theoretically. Experiments were performed in a packed bed column between 100-200°C. The adsorption process was described by a 1D approach, including mass transfer phenomena and axial dispersion. Experimental validation showed good agreement between measured and predicted breakthrough times and capacities. A global sensitivity analysis (GSA) was performed to rank the importance of model input factors and gain further process insight. The work showed that ACs are a promising option for thiophene removal and provided a generic approach for the application of GSA to adsorption models. In this context, the analysis demonstrated that the selection of common model assumptions deserves as much attention as the model parameters. Further work chapter aimed at understanding the effect of H2O, H2 and H2S on the decomposition of thiophene over CaO. Experiments were performed in a fixed bed reactor at 810°C. The factors were varied as part of a full factorial design at two levels. Results were fitted to a linear model with interactions. Analysis showed that H2O and H2S had a major effect on thiophene conversion, while that of H2 was less pronounced. Interactions between varied factors were found to be negligible. A UV-Vis method was developed to obtain online information on tar compound concentrations in real process gases in the low ppmv region. Spectra were analysed for their tar composition by two chemometric approaches. Applied to two case studies, the developed tool proved to be a rapid, sensitive tool, applicable to qualitative process monitoring with the added benefit of quantification in gases with a limited number of tar compounds. An ICP-MS was employed as a tool for the online detection of trace elements. The low detection limits (< 2 ppbv for H2Se, PH3; < 0.1 ppbv for AsH3; < 0.01 ppbv for Hg) and the high temporal resolution render this method a quick and robust tool for the performance evaluation of sorbent materials. A case study investigated the capture of H2Se by a Zn/Al2O3 sorbent between 150-350°C.

EPFL2017

Statistical Burnup Distribution Of Moving Pebbles In Htr-Pm Reactor

Jiri Krepel, Andreas Pautz, Horst-Michael Prasser, Fanny Vitullo

In the pebble-bed high temperature reactor under construction in China, called HTR-PM, the spherical fuel elements continuously flow downward in the cylindrical core. After the discharge, the burnup of each pebble is checked at the core outlet and, according to the achieved burnup level, the pebble might be disposed or reinserted into the upper section of the core, distributing randomly in the radial direction and defining a variable number of passes necessary to achieve the average maximum burnup of 90 MWd/kgU. Discrete Element Method (DEM) simulations have been carried out to achieve a clear understanding of the movement of 420,000 fuel pebbles in the HTR-PM core. At the same time, neutronic properties have been investigated for a single pebble and for the full core with Serpent 2 Monte Carlo code in order to perform a parametrization of the one-group microscopic cross sections at the core-level. The pebble movement has been coupled with the neutronic behavior of a single pebble in a dedicated burnup script called Moving Pebble Burnup (MPB), developed in Matlab. 3,000 single pebble burnup histories were simulated to obtain sufficient statistics and insight on the burnup process in the HTR-PM. The decrease of the average burnup gained per single pass implies that a miss-handling of recirculated fuel elements is unlikely to lead to exceedance of the maximum allowed burnup of 100 MWd/kgU. Furthermore, the core demonstrates a self-compensation effect of burnup, meaning that it always compensates burnup under- or over-runs in the successive passes. Finally, it is possible to conclude that the fuel cycle of the HTR-PM, as it has been laid out, is well-designed and feasible.

AMER SOC MECHANICAL ENGINEERS2018

Analysis and Removal of Product Gas Contaminants Present in Gasification Processes

Sven Philip Edinger

EPFL2017

Analysis and Removal of Product Gas Contaminants Present in Gasification Processes

Growth and productivity of Beta vulgaris cell culture in fluidized bed reactors

Statistical Burnup Distribution Of Moving Pebbles In Htr-Pm Reactor

Growth and productivity of Beta vulgaris cell culture in fluidized bed reactors

Analysis and Removal of Product Gas Contaminants Present in Gasification Processes

Statistical Burnup Distribution Of Moving Pebbles In Htr-Pm Reactor