hydrogen production from biomass wastes by reforming in hot compressed water - studies with model oxygenates in the quest for finding an optimal catalyst
Anna Vikla is a PhD student in the Catalytic Processes and Materials group. Her supervisor is prof.dr. K. Seshan from the Faculty of Science and Technology.
For the past three decades increasing concerns for climate pollution and depletion of fossil fuel sources have driven scientists to seek for alternative, sustainable sources of energy. Today, the concerns are higher than ever and solutions are not just scientifically challenged, but also politically and morally challenged. It is paramount that societies switch to more sustainable usage of energy and other resources to decrease their impact on the environment in the coming years. The quest for renewable energy production is still ongoing and it is crucial to find ways to produce them feasibly in order to provide replacement for fossil energy sources.
Biomass is a promising renewable source and it can partly provide for the need of sustainable energy. There are multiple ways to convert biomass to energy. For production of liquid fuels, e.g. for transportation sector, pyrolysis is the most promising option. In this process the biomass based solid feedstock is converted at a high temperature (> 300 °C) in an inert atmosphere in a matter of seconds to a liquid. Due to the nature of biomass, this liquid contains components that form an oil phase and a water-based phase, an aqueous phase. The oil phase as such is not stable and it needs to be upgraded to be used as conventional oil and it needs hydrogen. Hydrogen at the moment is mainly produced by fossil sources. However, the aqueous phase that is left from pyrolysis contains organic molecules, such as sugars, acids, alcohols, ketones, that can be converted to hydrogen and carbon dioxide. As water content of this phase is high, utilization of these molecules for steam reforming/gasification by evaporation of the water would be energy intensive. Aqueous phase reforming (APR) has been developed to process these types of diluted streams to hydrogen while keeping the water in liquid phase. APR is typically operated at 200-275 °C, at elevated pressures and a catalyst is required for efficient conversion of the feedstock to hydrogen. It has been shown earlier that design of stable and active catalyst for this process is challenging due to the drastic conditions used in APR. However, this step has to be overcome for commercialization of APR. In this thesis development of stable and active catalyst for APR is studied.
In Chapter 1 the objective and scope for this study is elaborated. The challenges for commercialization of APR in terms of catalyst development are discussed. These facts are used to set the scope for the research described in the later chapters.
Background for APR is explained in Chapter 2 with the current status. Catalytical hydrogen production from biomass based wastes is discussed in detail.
In Chapter 3 the experimental techniques are explained. Techniques for catalyst preparation is briefly discussed along with characterization techniques used to determine the properties of the materials studied in APR. The setup for APR studies is also described along the methods used for determining product distributions.
Important quality for a catalyst is its stability under the conditions studied. Due to drastic APR conditions some catalyst supports suffer from phase transformation. In Chapter 4, we prepared a Platinum catalyst on a support (AlO(OH), boehmite) that had already gone through phase transformation from g-alumina. The performance of this catalyst was then compared to Pt/g-Al2O3, a catalyst initially active in APR. Under APR of 5wt% of ethylene glycol (EG) at 270 °C/90 bar Pt/AlO(OH) showed stable performance and surprisingly, a higher hydrogen formation rate than Pt/Al2O3 which deactivated due to phase change of the support. The higher hydrogen rate of Pt/AlO(OH) was suggested to be due to its hydroxylated form that enhances reforming of carbonaceous species on the catalysts surface or due to enhanced oxidation of Pt. Due to the stability and selectivity to hydrogen Pt/AlO(OH) shows great promise as an APR catalyst.
In Chapter 5 the APR studies are continued with a more challenging model component, hydroxyacetone (HYDA). HYDA is a major ketone component in aqueous phase of pyrolysis oil. Three catalysts were prepared on supports known to be hydrothermally stable, Pt/AlO(OH), Pt/ZrO2 and Pt/C. The catalysts were studied in APR of 2.5wt% HYDA under 225 °C/ 35 bar. All the catalyst showed initially high activity for HYDA conversion, however, the oxide supported catalyst deactivated rapidly due to formation of coke on the catalysts caused by liquid phase conversion of the feed. Pt/C catalyst remained stable and active during time on stream. The catalyst converted most of the feed to gas phase products and preventing formation of liquid by-products that could lead formation of coke/char. The gas phase products were mostly H2, CO, CO2 and CH4. The stability of the carbon support and its resilience towards coking showed promise for further study in APR. In addition, carbon supported catalysts can be easier handled after use as the carbon can be burned and metal harvested for recycling. This set a good starting point for a further development for APR application.
In APR activity and selectivity of the catalyst for hydrogen is essential for feasible operation. Selectivity can be altered by changing the properties of the catalyst or by changing the reaction conditions. In Chapter 6 we studied further Pt/C catalyst, by preparing them by different methods in order to alter the particle size of Pt and its distribution on the carbon support. Catalysts named as Pt-IM, Pt-OX, Pt-PR and Pt-CL, were characterized with multiple methods to learn in detail the metal size and distribution. Transmission electron microscopy (TEM) was applied with X-ray photo electron spectroscopy to view the cross-sections of the catalysts grains. These confirmed that that Pt-PR and Pt-CL had more of an egg-shell type structure compared to Pt-IM and Pt-OX which had more uniform distribution of Pt on the carbon supports. The catalysts were tested in APR of 2.5wt% of EG at 225 °C and 35 bar. All catalysts were stable under these conditions during 420 minutes on stream. The results showed that the egg-shell type structure was more favorable for hydrogen turnover rate. Further, larger Pt particles seemed to enhance C-C bond cleavage and improve hydrogen production. Findings support that Pt/C is an good choice for a catalysts in APR.
In Chapter 7 effect of mass transfer and reaction temperature for product selectivity was studied for APR of HYDA over Pt/C catalysts. Two types of mass transfer can influence the catalysts performance, 1) external mass transfer that lowers the conversion rate and 2) internal mass transfer that affects the product selectivity. For this study 2 catalysts were prepared 1.4wt% Pt/C with grain size of 100-250 µm, named as I-Pt/C and 1.7wt% Pt/C with grain size of 70-100 µm, named as II-Pt/C. Stability of these catalysts were studied at 2.5wt% of HYDA at 225 C / 35 bar to confirm that stability was not an issue for the rest of the study. External mass transfer was studied for both catalysts by keeping ratio of catalyst weight and feed flowrate similar and internal mass transfer was studied by comparing product selectivities under the studied conditions between I-Pt/C (100-250 µm) and II-Pt/C (70-100 µm). Results in comparison with transport criteria showed that external mass transfer was not an issue in our studied conditions. Internal mass transfer could not be explicitly ruled out by the results, however, the calculated transport criteria showed showed that it is unlikely. The effect of reaction temperature for product selectivity was also measured at 250 °C/50 bar and attempts were made to calculate activation energy. Unfortunately, almost full conversion was obtained at 250 °C and therefore it was not possible to calculate activation energy with this test data. Increase in temperature from 225 °C to 250 °C did improve Water-Gas Shift (WGS) reaction and therefore hydrogen selectivity. This confirms that Pt/C has potential to be developed for APR.
In Chapter 8 the work in this thesis is concluded and remarks for further research are given. This thesis focused on finding a stable and active catalyst for APR of biomass based feedstocks. The studied Pt/C catalyst was highly promising due to its stable mesoporous structure and activity towards gas phase products to prevent coking. The usage of carbon as a support material also enables to recycle the metal after use, which will add to more sustainable process. Further development of Pt/C catalysts should be focused to improve the WGS activity of the catalyst and to study the ways to prevent methane formation in order to take steps closer for commercialization of APR application.