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ABSTRACT
In most industrial applications, combustion and gasification of char progresses under regime II conditions. Unlike in other regimes, both particle size and density change simultaneously in regime II due to non-uniform consumption of carbon inside the particles. In this work, mathematical predictions of diameter changes in regime II were made by a one-dimensional simulation tool, where transient species balances are resolved locally inside the particle. This simulation is computationally expensive and usually not appropriate for the implementation in comprehensive CFD simulations of combustion or gasification processes. To overcome this restraint, an alternative shortcut method with affordable computation time has been developed and validated against the detailed model. This method allows the calculation of diameter changes during combustion and gasification from precalculated effectiveness factors. Additionally, the change of particle size has been investigated experimentally in a single particle converter setup. Therein, particles are fixed on a sample holder placed in the hot flue gas of a flat flame burner. Size and temperature trends are optically assessed by a 3CCD camera.
Acknowledgments
The authors would like to thank the German Research Foundation (DFG) for funding this work within the SFB/Transregio 129 “Oxyflame” (project number 215035359). The authors also would like to thank Bio4Energy, Swedish Center for Biomass Gasification, Swedish Research Council and Kempe Foundation for the financial support. Nils Erland L. Haugen acknowledges the research project “Gaspro”, financed by the Research Council of Norway (267916).
Nomenclature
| = | dimensionless parameter | |
| = | molar concentration | |
| = | heat capacity | |
| = | diameter | |
| = | diffusion coefficient | |
| = | activation energy | |
| = | heat or mass transfer coefficient | |
| = | Heat of reaction | |
| = | volumetric flux | |
| = | rate constant | |
| = | Length of particle | |
| = | mass | |
| = | mass flow per unit area | |
| = | molar mass | |
| = | Avogadro constant | |
| = | Nusselt number | |
| = | pressure | |
| = | Prandtl number | |
| = | Reynolds number | |
| = | radius | |
| = | Stefan-Boltzmann constant | |
| = | average collision diameter | |
| = | tortuosity | |
| = | Thiele modulus | |
| = | reaction rate | |
| = | universal gas constant | |
| = | surface area | |
| = | Sherwood number | |
| = | temperature | |
| = | time | |
| = | local carbon conversion | |
| = | global carbon conversion | |
| = | molar fraction | |
| = | dimensionless parameter | |
| = | dimensionless parameter | |
| = | emissivity | |
| = | particle porosity | |
| = | effectiveness factor | |
| = | fractional surface coverage of oxygen | |
| = | fractional free carbon sites | |
| = | thermal conductivity | |
| = | dynamic viscosity | |
| = | surface concentration of C sites | |
| = | density | |
| = | volumetric reaction rate | |
| = | structural parameter | |
| = | collision integral |
Subscripts
| 0 | = | initial |
| c | = | carbon |
| comp | = | completed reaction |
| conv | = | convection |
| crit | = | critical value |
| eff | = | effective |
| exp | = | experimental |
| g | = | gravimetric |
| g | = | gas |
| i | = | index variable: gases |
| j | = | index variable: reactions / shells |
| m | = | mass |
| m | = | exponent in differential equation |
| p | = | particle |
| q | = | heat |
| rad | = | radiation |
| reac | = | reaction |
| s | = | surface |
| t | = | true |
| TC | = | Thermocouple |
| tot | = | total |
| w | = | wall |
Correction Statement
This article has been republished with minor changes. These changes do not impact the academic content of the article.