| 1. |
- Barza, Sorina, 1967-, et al.
(författare)
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A new variational characterization of Sobolev spaces
- 2015
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Ingår i: Journal of Geometric Analysis. - : Springer Science and Business Media LLC. - 1050-6926 .- 1559-002X. ; 25:4, s. 2185-2195
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Tidskriftsartikel (refereegranskat)abstract
- We obtain a new variational characterization of the Sobolev space $W_p^1(\Omega)$ (where $\Omega\subseteq\R^n$ and $p>n$). This is a generalization of a classical result of F. Riesz. We also consider some related results.
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| 2. |
- Berguerand, Nicolas, 1978, et al.
(författare)
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Use of Nickel Oxide as a Catalyst for Tar Elimination in a Chemical-Looping Reforming Reactor Operated with Biomass Producer Gas
- 2012
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 1520-5045 .- 0888-5885. ; 51:51, s. 16610-16616
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Tidskriftsartikel (refereegranskat)abstract
- A secondary tar-cleaning process based on Chemical-Looping Reforming (CLR) was investigated for upgrading biomass producer gas, derived from the Chalmers University of Technology 2-4 MW indirect gasifier. The experiments were conducted in a bench-scale CLR reactor using a manufactured nickel oxide (NiO) catalyst. Although Ni is a well-documented and efficient steam-reforming catalyst, it is susceptible to rapid deactivation under tar-rich conditions. The aim of this study was to explore the advantages of CLR as a gas-cleaning application, a process which offers continuous regeneration of the carbon deposits on catalysts. The tar-reforming performance of this Ni material and its influence on the gas composition and in particular its potential to increase the H2/CO ratio, were studied. The system was tested at reforming temperatures that ranged from 700°C to 880°C and at oxygen concentrations of 1.0% and 2.2% in the inlet feed to regenerator section. The results confirm the strong ability of the catalyst to reform tars. Higher process temperatures clearly promoted tar conversion, with 96% overall conversion at 880°C (99% if benzene is excluded), as compared with 45% conversion at 700°C. The hydrogen production was favored when temperature was raised. Though, a maximum ratio H2/CO of 2.2 was observed at 750°C. Finally, no time-on-stream deactivation of the catalyst in the CLR was observed during the test, which lasted almost 7 hours.
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| 3. |
- Esslamzadeh, G. H., et al.
(författare)
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The Modulus of p-Variation and Its Applications
- 2021
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Ingår i: Journal of Fourier Analysis and Applications. - : Springer. - 1069-5869 .- 1531-5851. ; 28:1
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Tidskriftsartikel (refereegranskat)abstract
- Let nu be a nondecreasing concave sequence of positive real numbers and 1 <= p < infinity. In this note, we introduce the notion of modulus of p-variation for a function of a real variable, and show that it serves in at least two important problems, namely, the uniform convergence of Fourier series and computation of certain K-functionals. Using this new tool, we first define a Banach space, denoted V-p[nu], that is a natural unification of the Wiener class BVp and the Chanturiya class V[nu]. Then we prove that V-p[nu] satisfies a Helly-type selection principle which enables us to characterize continuous functions in V-p[nu] in terms of their Fejer means. We also prove that a certain K-functional for the couple (C, B V-p) can be expressed in terms of the modulus of p-variation, where C denotes the space of continuous functions. Next, we obtain equivalent optimal conditions for the uniform convergence of the Fourier series of all functions in each of the classes C boolean AND V-p[nu] and H-omega boolean AND V-p[nu], where omega is a modulus of continuity and H-omega denotes its associated Lipschitz class. Finally, we establish sharp embeddings into V-p[nu] of various spaces of functions of generalized bounded variation. As a by-product of these latter results, we infer embedding results for certain symmetric sequence spaces.
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| 4. |
- Kolyada, Viktor, 1947-, et al.
(författare)
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On functions of bounded p-variation
- 2009
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Ingår i: Journal of Mathematical Analysis and Applications. - : Elsevier BV. - 0022-247X .- 1096-0813. ; 356:2, s. 582-604
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Tidskriftsartikel (refereegranskat)abstract
- We obtain estimates of the total p-variation (1<p<∞) and other related functionals for a periodic function f∈Lp[0,1] in terms of its Lp-modulus of continuity ωp(f;δ). These estimates are sharp for any rate of the decay of ωp(f;δ). Moreover, the constant coefficients in them depend on parameters in an optimal way.
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| 5. |
- Kolyada, Viktor, 1947-, et al.
(författare)
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On moduli of p-continuity
- 2012
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Ingår i: Acta Mathematica Hungarica. - : Springer. - 0236-5294 .- 1588-2632. ; 137:3, s. 191-213
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Tidskriftsartikel (refereegranskat)abstract
- Moduli of p-continuity provide a measure of fractional smoothness of functions via p-variation. We prove sharp estimates of the modulus of p-continuty in terms of the modulus of q-continuity (1
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| 6. |
- Kruschwitz, Jens, et al.
(författare)
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Modelling, simulation and parameter identification of active pollution reduction with photocatalytic asphalt
- 2019
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Ingår i: Acta Polytechnica. - Prague : Czech Technical University in Prague. - 1210-2709 .- 1805-2363. ; 59:1, s. 51-58
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Tidskriftsartikel (refereegranskat)abstract
- We develop and implement a numerical model to simulate the effect of photocatalytic asphalt on reducing the concentration of nitrogen monoxide (NO) due to the presence of heavy traffic in an urban environment. The contributions in this paper are threefold: we model and simulate the spread and breakdown of pollution in an urban environment, we provide a parameter estimation process that can be used to find missing parameters, and finally, we train and compare this simulation with different data sets. We analyse the results and provide an outlook on further research.
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| 7. |
- Larsson, Anton, 1984, et al.
(författare)
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Using Ilmenite To Reduce the Tar Yield in a Dual Fluidized Bed Gasification System
- 2014
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 28:4, s. 2632-2644
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Tidskriftsartikel (refereegranskat)abstract
- Biomass gasification plays an important role in the emerging production of second-generation biofuels. One of the major challenges facing biomass gasification is to find simple and efficient ways to reform tar components. While the tar causes operational problems, it can be reformed to increase the chemical efficiency of the gasification process. With respect to tar reforming, catalytic materials are of special interest. Many of the materials that have been proposed as promising catalysts are metal oxide-based materials. However, metal oxides also have the ability to transport oxygen when subjected to alternating oxidizing and reducing atmospheres, similar to that which occurs in a dual fluidized bed gasification system. In this work, ilmenite was used as the catalytic material in the Chalmers 2-4 MWth dual fluidized bed gasifier to decrease the yield of tar. The ilmenite was mixed with the silica sand, which was used as the bed material, to investigate how the level of ilmenite affected chemical efficiency and tar yield. Furthermore, energy balance calculations were established to elucidate the general aspects of oxygen transport in dual fluidized bed gasification systems. The results presented in this paper reveal that adding low levels of ilmenite reduces the tar yield by similar to 50%(mass). However, the oxygen transport induced by ilmenite caused a reduction in the chemical efficiency of the gasifier and the heating value of the gas, compared to using 100% silica sand as the bed material. The impact of adding ilmenite was found to be dependent upon the operational conditions of the gasifier; a low fluidization velocity gave the highest reduction of the tar yield, whereas higher fluidization velocities led to increased levels of heavy components. Overall, the use of ilmenite as a catalyst for reduction of the yield of tar appears promising, provided that the level of oxygen transport can be restricted.
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| 8. |
- Lind, Fredrik, 1978, et al.
(författare)
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Manganese oxide as catalyst for tar cleaning of biomass-derived gas
- 2012
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Ingår i: Biomass Conversion and Biorefinery. - : Springer Science and Business Media LLC. - 2190-6815 .- 2190-6823. ; 2:2, s. 133 - 140
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Tidskriftsartikel (refereegranskat)abstract
- The possibilities to upgrade raw gas with the use of a manganese oxide have been investigated in an application for secondary tar cleaning of biomass derived gas. Experiments were conducted in a reactor system where a novel technique that combines tar cleaning with catalyst regeneration is applied. Raw gas from the Chalmers non-catalytic steam biomass gasifier - containing roughly 32 gtar/Nm3gas - was fed to the tar cleaning reactor. The tar reforming qualities of the manganese oxide were evaluated in the reactor system using a mixture of 23 wt% catalysts in silica sand at the temperatures 700 and 800°C. Experiments showed that the catalyst was continuously regenerated from carbon deposits and that the total amount of tars was decreased by as much as 44,5% at a gas residence time of 0.4 s in the bed. The catalyst showed activity in water-gas shift reaction and the H2/CO-ratio increased from 0.6 in the raw gas to a peak value of 1 in the reformed gas at 800°C. Only a slight decrease in methane and acetylene content was observed for both operating temperatures.
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| 9. |
- Lind, Martin, 1985-, et al.
(författare)
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A Priori Feedback Estimates for Multiscale Reaction-Diffusion Systems
- 2018
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Ingår i: Numerical Functional Analysis and Optimization. - : Taylor & Francis. - 0163-0563 .- 1532-2467. ; 39:4, s. 413-437
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Tidskriftsartikel (refereegranskat)abstract
- We study the approximation of a multiscale reaction–diusion system posed on both macroscopic and microscopic space scales. The coupling between the scales is done through micro– macro ux conditions. Our target system has a typical structure for reaction–diusion ow problems in media with distributed microstructures (also called, double porosity materials). Besides ensuring basic estimates for the convergence of two-scale semidiscrete Galerkin approximations, we provide a set of a priori feedback estimates and a local feedback error estimator that help in designing a distributed-high-errors strategy to allow for a computationally ecient zooming in and out from microscopic structures. The error control on the feedback estimates relies on two-scale-energy, regularity, and interpolation estimates as well as on a ne bookeeping of the sources responsible with the propagation of the (multiscale) approximation errors. The working technique based on a priori feedback estimates is in principle applicable to a large class of systems of PDEs with dual structure admitting strong solutions. A
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| 10. |
- Lind, Martin, 1985-, et al.
(författare)
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A semidiscrete Galerkin scheme for a two-scale coupled elliptic-parabolic system : Well-posedness and convergence approximation rates
- 2020
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Ingår i: BIT Numerical Mathematics. - : Springer. - 0006-3835 .- 1572-9125. ; 60:4, s. 999-1031
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we study the numerical approximation of a coupled system of elliptic–parabolic equations posed on two separated spatial scales. The model equations describe the interplay between macroscopic and microscopic pressures in an unsaturated heterogeneous medium with distributed microstructures as they often arise in modeling reactive flow in cementitious-based materials. Besides ensuring the well-posedness of our two-scale model, we design two-scale convergent numerical approximations and prove a priori error estimates for the semidiscrete case. We complement our analysis with simulation results illustrating the expected behaviour of the system.
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