Gazéification | EPFL Graph Search

La gazéification est un processus à la frontière entre la pyrolyse et la combustion. Celui-ci permet de convertir des matières carbonées ou organiques en un gaz de synthèse combustible (souvent appelé « syngas » ou « syngaz »), composé majoritairement de monoxyde de carbone (CO) et de dihydrogène (H2), contrairement à la combustion dont les produits majoritaires sont le dioxyde de carbone () et l'eau (H2O). Principes Par réaction thermochimique à des températures élevées et avec un déficit en oxygène (ou en injectant une quantité contrôlée d'oxygène/ou de la vapeur) à haute température (), la matière carbonée est pyrolysée. L'énergie apportée par la combustion des produits de pyrolyse permet ensuite la gazéification et/ou la réaction des gaz de combustions avec la matière carbonée. L'équilibre entre la pyrolyse et la combustion reste délicat et le gaz obtenu par gazéification contient, comme pour tous dispositifs de combustion, de nombreux composés indésirables tels que

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

Evaluation of the State-of-the-art of Biomass gasification for heat and electricity production

In the order to produce energy for the large amount of people still currently out of the national girds, different possibilities exist. For practical and economic reasons, the most popular one is the use of Diesel engines coupled with generator sets. Petrol reserves being not infinite, renewable solutions have to be found to insure a sustainable development of the poorest areas in the world. One of those could be the gasification of biomass. The latter has two advantages: Biomass is a renewable energy source and is present almost everywhere in the world: the gas produced can have various applications among them fuel for unmodified Diesel engines. Unfortunately, things are not so simple: the gas produced has a low heating value, its use in an internal combustion engine requires cleaning, biomass is not uniform through the world and technology has to be adapted for each properties, biomass supplying can product to environmental hazards if it is not strictly controlled, etc. Moreover, no standardization exists currently for the development of the technology. However, positive results have been obtained and it seems that interesting opportunities exist for the future. It requires further investments and global communication between the protagonists. The present study tries to give a complete overview of the current biomass gasification field. Some important results obtained on existing gasification plants, as well as problems encountered on technical, social, environmental and economic point of view, are also presented. Finally, some promising alternatives are proposed, as well as the main required research.

2001

Supported ruthenium nanoparticles for supercritical water gasification - synthesis, activity and stability

Christopher Marc Hunston

Supercritical water gasification (SCWG) is a promising and versatile technology for the conversion of a variety of wet biomass streams into renewable natural gas. In this work, the focus was set on methane production with the help of an active and stable methanation catalyst. The supercritical water (SCW) environment can lead to rapid catalyst deactivation through different mechanisms, namely poisoning, coking, sintering or leaching. Although addressed, it remains challenging to disentangle them in SCWG. The goal of this thesis is to get a better understanding of the aforementioned deactivations pathways by trying to study them individually, in order to tailor an active and robust SCWG catalyst.The intrinsic gasification activity of carbon supports exhibiting different properties was assessed by feeding glycerol at 400°C and 30MPa. The negative effect of micropores was shown by significant coke deposition and surface area loss around 90%, compared to mesoporous carbons that only lost 20%. Carbon nanofibers (CNF) proved to be an interesting support showing very good stability and inertness. The synthesis of 5%Ru catalysts on selected supports confirmed these trends. Highly microporous Ru/C catalysts were less stable than the ones supported on carbon frameworks of more open pore structure. Ru/CNF seemed to be a promising SCWG catalyst, with an enhanced resistance towards coke formation. The completely open pore structure of CNF was found to be a crucial parameter.Ruthenium loss was investigated by measuring the its content in the process waters with ICP-MS. Ru was successfully quantified at levels close to thermodynamic models for Ru/AC and Ru/CNF of different loadings (0.01-0.2 µg L-1). Furthermore, the process parameters had no effect on the quantified Ru, showing that the steady-state loss represents a thermodynamically-driven leaching. Fluctuations in feed rate and pressure damaged the catalyst through friction and led higher Ru losses, mainly through the release of carbon domains containing Ru. Metal oxides were compared to AC by synthesising 2%Ru catalysts on alpha-Al2O3, ZrO2 and TiO2. They yielded 20-40 times higher Ru loss rates than for 2%Ru/AC and a significantly lower activity, confirming once again the superiority of Ru/C as SCWG catalyst. Ru losses were similar on larger rigs during glycerol and sewage sludge gasification, with values around 0.14-0.29 µg gRu-1 h-1.With Ru/CNF proving to be an interesting SCWG catalyst, more in-depth studies were performed on this system. Catalysts of different loadings (1-30%) were successfullysynthesised yielding very small Ru NPs (0.9-2.2 nm) with an easy synthesis method. Ru/CNF catalysts exhibited high gasification activities and improved stability compared to Ru/AC. The initial activity correlated with the Ru dispersion. However, 1%Ru/CNF catalysts underwent rapid deactivation compared to 5%Ru/CNF, showing the importance of site density to avoid rapid deactivation. The effect of surface Ru atom density was shown with a optimum in turnover frequency in the range 0.4-0.7 atomRu,sfc nm-2. For the first time, a particle size effect and more importantly a surface atom density effect was shown during SCWG of glycerol. Coke deposition was also investigated and observed at very high space velocities. The coke deposits could partially be removed by extraction in a mix of solvents, opening a potential new route for the regeneration of Ru/C catalysts.

EPFL2021