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  • 1.
    Sadegh-Vaziri, Ramiar
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    Biomass to Biofuel: Syngas Cleaning and Biomass Feedstock2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis builds around the idea of a biofuel production process that is comprised of biomass production, biomass gasification, gas cleaning, and fuel production. In this work, we specifically looked into H2S removal as a part of cleaning the producer gas and flocculation of microalgae which is involved in the harvesting of microalgae after biomass production. One of the impurities to remove from the producer gas is hydrogen sulfide which can be removed by using a packed bed of zinc oxide. Despite the regular use, it was only recently shown that during reaction with H2S, nano-size particles of ZnO exhibit void formation and outward growth. In this work, a micro-scale model was introduced to describe the void formation and outward growth. On the macro-scale, the simulations captured pore clogging of pellets due to the outward growth. The pore clogging prevents the full conversion of pellets and consequently leads to shorter breakthrough times of beds. The second problem investigated here deals with the flocculation of microalgae. Microalgae is produced in relatively low concentrations in the incubator liquid medium and during the harvesting, the concentration is increased to an acceptable level. The harvesting process includes a flocculation followed by a filtration or centrifuge unit. During flocculation, microalgae are stimulated to aggregate and form clusters. The experiments showed that the mean size of clusters formed during flocculation increases with time to a maximum and then starts decreasing, resulting in an overshoot in the mean size profile. The size of clusters influence the efficiency of the afterward filtration or centrifuge, thus it is of interest to carefully track the size evolution of clusters, making the studying of overshoot a crucial research topic. In this work, the possible mechanisms behind this overshoot were investigated.

  • 2.
    Sadegh-Vaziri, Ramiar
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Energy Processes.
    Modeling in Biomass Harvesting, Biomass Pyrolysis and Producer Gas Cleaning2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Biomass is a viable alternative in order to mitigate the environmental effectscaused by the aggressive use of fossil feedstock during the last century.This thesis builds around the idea of a biofuel production process that iscomprised of biomass production, biomass gasication, gas cleaning andfuel production. Biomass production includes microalgae synthesis followedby harvesting to supply microalgae to the gasication process. In the gasi-cation process, the prepared microalgae is dried, pyrolyzed and gasiedto obtain a producer gas containing syngas and impurities. The producergas is cleaned from the impurities and fed to a fuel production unit, e.g. amethanation process.This thesis investigates three distinct aspects related to this processscheme, namely the occulation of microalgae as part of harvesting, biomasspyrolysis as a part of gasication, and sulfur removal from the producer gasas a part of gas cleaning.The investigation of occulation focuses on secondary phenomena thataccompany the aggregation and breakup of the suspended particles, namelyrestructuring, decay of oc strength and settling. For the study of theconsidered phenomena, a population balance model is developed.Slow pyrolysis of biomass is studied on both the reactor scale and thepellet scale. A model for a rotary drum reactor, using principles of isoconversionalanalysis, is developed for the study of dierent biomass feedstock.The proposed model allows for deriving a preliminary reactor design withminimal experimental input data. A one-dimensional nite volume schemeis developed for the investigation of pyrolysis on the pellet scale. The proposedscheme accounts for convective and diusive heat and mass transfer,and is tested against analytical solutions and commercial software packages.Sulfur removal by metal oxides in a packed bed is studied on both thesystem level and the process level. Criteria for the selection of metal oxidesand the design of packed bed units are derived. A detailed analysis isundertaken to study the reaction of H2S with ZnO in a packed bed, wherethe nano-particles of ZnO experience void formation and outward growth.

  • 3.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    Amovic, Marko
    Ljunggren, Rolf
    Engvall, Klas
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
    A Medium-Scale 50 MWfuel Biomass Gasification Based Bio-SNG Plant: A Developed Gas Cleaning Process2015In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 8, no 6, p. 5287-5302Article in journal (Refereed)
    Abstract [en]

    Natural gas is becoming increasingly important as a primary energy source. A suitable replacement for fossil natural gas is bio-SNG, produced by biomass gasification, followed by methanation. A major challenge is efficient gas cleaning processes for removal of sulfur compounds and other impurities. The present study focuses on development of a gas cleaning step for a product gas produced in a 50 MWfuel gasification system. The developed gas cleaning washing process is basically a modification of the Rectisol process. Several different process configurations were evaluated using Aspen plus, including PC-SAFT for the thermodynamic modeling. The developed configuration takes advantage of only one methanol wash column, compared to two columns in a conventional Rectisol process. Results from modeling show the ability of the proposed configuration to remove impurities to a sufficiently low concentrations - almost zero concentration for H2S, CS2, HCl, NH3 and HCN, and approximately 0.01 mg/Nm(3) for COS. These levels are acceptable for further upgrading of the gas in a methanation process. Simultaneously, up to 92% of the original CO2 is preserved in the final cleaned syngas stream. No process integration or economic consideration was performed within the scope of the present study, but will be investigated in future projects to improve the overall process.

  • 4.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Bäbler, Matthaus U.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Numerical investigation of the outward growth of ZnS in the removal of H2S in a packed bed of ZnO2017In: Chemical Engineering Science, ISSN 0009-2509, E-ISSN 1873-4405, Vol. 158, p. 328-339Article in journal (Refereed)
    Abstract [en]

    Sulfidation of zinc oxide is a viable option for the removal of hydrogen sulfide from raw syngas and biogas. Recent experiments showed that sulfidation of nanoscopic zinc oxide particles leads to an outward growth and the formation of voids inside the particles. In this work, we derive a micro-scale model to describe this phenomenon. The model accounts for nucleation and growth of voids inside the particles, diffusion of Zn and O through the product layer, and deposition of the solid product at the particle surface as a result of the reaction between ZnO and H2S. The model is thus opposite to the well known shrinking core model where an inward growth of the product layer is assumed. To explore the effect of the outward growth on the dynamics of a packed bed adsorber the micro-scale model is combined with a macro-scale model that accounts for intra-pellet diffusion and convection along the packed bed. In the limit of fast nucleation and growth of voids inside the zinc oxide particles, the micro-scale model shows a qualitatively similar conversion profile to the shrinking core model, while when nucleation controls an inflection point in conversion profile is found. On the macro-scale, the outward growth can cause the clogging of pores inside the pellets which prevents the pellets from reaching full conversion. This leads to shorter breakthrough times of the packed bed due to the sealing of unreacted zinc oxide. Our results thus provide a possible explanation of the incomplete conversion of zinc oxide in packed beds.

  • 5.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Bäbler, Matthäus
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    A hollow core shell model for describing chemisorption of H2S from raw syngas in a packed bed reactor of ZnO2016In: 22nd International Congress of Chemical and Process Engineering, CHISA 2016 and 19th Conference on Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction, PRES 2016, Czech Society of Chemical Engineering , 2016, p. 66-67Conference paper (Refereed)
  • 6.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Bäbler, Matthäus
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Modeling of slow pyrolysis of various biomass feedstock in a rotary drum using TGA data2018In: Chemical Engineering and Processing, ISSN 0255-2701, E-ISSN 1873-3204, Vol. 129, p. 95-102Article in journal (Refereed)
    Abstract [en]

    Design and optimization of biomass gasification faces the challenge of feedstock variation. Specifically, design calculations require kinetic rate expressions for the given feedstock, whose rigorous determination is demanding and often exceeds available recourses in an early development stage. In this work, we model the slow pyrolysis of biomass for the production of biochar. The aim is to predict the conversion of raw biomass to biochar as a function of the process conditions. Here, we will show that TGA data processed with an isoconversional method is enough to obtain an effective rate expression which allows for predicting the behavior of the biomass at an arbitrary temperature evolution. Such rate expressions can then be used in the process model to simulate conversion of raw biomass to biochar. To illustrate the feasibility of this approach we consider four vastly different biomass, namely spruce wood, pulp, lignin and xylan–lignin, undergoing slow pyrolysis in an indirectly heated rotary kiln reactor. The results of our modeling are compared to experimental data obtained from a 500 kW pilot plant pyrolyzer and to a more detailed process model.

  • 7.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    Bäbler, Matthäus
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Energy Processes.
    PBE Modeling of Flocculation of Microalgae: Investigating the Overshoot in Mean Size Profiles2017In: Proceedings of the 9th International Conference on Applied Energy, Elsevier, 2017, Vol. 142, p. 507-512Conference paper (Refereed)
    Abstract [en]

    Microalgae is considered as a viable feedstock to biomass gasification. After synthesis in water medium, microalgae are separated and dried to a suitable degree to be fed to the gasification process. In order to achieve an efficient separation, a flocculation process is employed, in which microalgae primary particles aggregate and form larger clusters. Although flocculation is a well-established process, there are still some unknown issues related to it, that are worth further research. Experiments show that the mean size of clusters during flocculation goes through a maximum and then decreases with time. We refer to this pattern in the mean size profile as the overshoot. Studying this phenomenon is crucial since the size of clusters has a significant effect on the overall efficiency of the separation of microalgae from water. In this work, we aim at investigating the mechanisms behind the overshoot. The flocculation process is modeled as an aggregation-breakup system by using population balance equations (PBEs). The primary results show that the aggregation and breakup alone cannot lead to the overshoot in the mean size profile. Thus, we suggested three mechanisms that can lead to the overshoot: deposition of large clusters (DLC), restructuring of clusters (RC), and primary particle aggregation (PPA). These mechanisms were examined with numerical simulations and it was revealed that all three lead to the overshoot.

  • 8.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    Bäbler, Matthäus
    KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology.
    PBE Modeling of Overshoot in Mean Cluster Size Proles in Aggregation-Breakup ProcessesManuscript (preprint) (Other academic)
    Abstract [en]

    In monitoring the mean size of clusters during an aggregation-breakup process, it has been repeatedlyobserved that the mean size increases with time to a maximum and then decreases, resulting in an overshootin the means size prole. The decrease in the mean size prole has often explained as an eect of breakupof aggregates. In this work, we investigate three possible mechanisms that lead to an overshoot by usingnumerical analysis of population balance equations describing aggregation-breakup systems. The consideredmechanisms are deposition of large clusters, restructuring of clusters, and primary particle aggregation, wheretwo clusters aggregate only if one of them is a primary particle. The results show that all the suggestedmechanisms lead to an overshoot. It is only in the case of primary particle aggregation that the decrease inthe mean size is due to the dominance of breakup. Also, restructuring of fractal aggregates to form morecompact clusters explains the overshoot during aggregation-breakup with restructuring of clusters, while inthe case of deposition of large clusters, mass loss due to deposition of large aggregates leads to an overshoot.

  • 9.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
    Bäbler, Matthäus
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Energy Processes.
    Providing sulfur free syngas to a fuel cell system2019In: Energy Procedia, Elsevier Ltd , 2019, p. 448-453Conference paper (Refereed)
    Abstract [en]

    Fuel cells are viable alternatives as power backup systems for mini-grids. In this work a case is considered, where the hydrogen fuel to the fuel cells is supplied from biomass gasification. However, the producer gas obtained from biomass gasification needs to be cleaned of impurities and contaminants. In this work we examined the superiority of the hot producer gas cleaning, which results in a better thermal efficiency since the heat loss from the system is reduced. In order to have a viable hot cleaning process, sulfur should be removed at 800°C and this was shown possible by promising primary data from the experiments where H 2 S was removed down to an acceptable level. 

  • 10.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Bäbler, Matthäus
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Energy Processes.
    Removal of hydrogen sulfide with metal oxides inpacked bed reactors - a review from modelingperspective with practical implicationsManuscript (preprint) (Other academic)
  • 11.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Ludwig, Kristin
    Max Planck Inst Dynam Complex Tech Syst, Proc Syst Engn, Sandtorstr 1, D-39106 Magdeburg, Germany..
    Sundmacher, Kai
    Max Planck Inst Dynam Complex Tech Syst, Proc Syst Engn, Sandtorstr 1, D-39106 Magdeburg, Germany.;Otto von Guericke Univ, Proc Syst Engn, Univ Pl 2, D-39106 Magdeburg, Germany..
    Bäbler, Matthäus
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Mechanisms behind overshoots in mean cluster size profiles in aggregation-breakup processes2018In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 528, p. 336-348Article in journal (Refereed)
    Abstract [en]

    Aggregation and breakup of small particles in stirred suspensions often shows an overshoot in the time evolution of the mean cluster size: Starting from a suspension of primary particles the mean cluster size first increases before going through a maximum beyond which a slow relaxation sets in. Such behavior was observed in various systems, including polymeric latices, inorganic colloids, asphaltenes, proteins, and, as shown by independent experiments in this work, in the flocculation of microalgae. This work aims at investigating possible mechanism to explain this phenomenon using detailed population balance modeling that incorporates refined rate models for aggregation and breakup of small particles in turbulence. Four mechanisms are considered: (1) restructuring, (2) decay of aggregate strength, (3) deposition of large clusters, and (4) primary particle aggregation where only aggregation events between clusters and primary particles are permitted. We show that all four mechanisms can lead to an overshoot in the mean size profile, while in contrast, aggregation and breakup alone lead to a monotonic, "S" shaped size evolution profile. In order to distinguish between the different mechanisms simple protocols based on variations of the shear rate during the aggregation-breakup process are proposed.

  • 12.
    Sadegh-Vaziri, Ramiar
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Winberg-Wang, Helen
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering.
    Bäbler, Matthäus
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemical Engineering, Energy Processes.
    1D finite volume scheme for simulating gas-solid reactions in a porous spherical particleManuscript (preprint) (Other academic)
1 - 12 of 12
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