During the thermogravimetric measurement of reaction rates involving mass change in a solid

such as coal char gasification reactivity

the sample is contained in a cup-like crucible

and CO2 has to access through the top surface of the particle bed.CO2 mass transfer limitations control their reaction in such situations.Different types of crucibles and coal char initial mass loadings were employed to study the effect of gas mass transfer on coal char gasification rate.The reaction temperature was 1140 ℃

which was in the chemical controlled reaction regime for the coal char gasification.The conclusion indicated that the coal char gasification rate strongly dependent on the crucible type and char initial mass loading.However

the gas mass transfer around and inside a crucible could be simulated by the computational fluid dynamics PHOENICS package.By simulating the space above the bed

it was confirmed that diffusion could supply most of the CO2 to the bed surface and molecular diffusion governed the mass transfer.Also

a mono-dimensional model was developed.Two parameters were used to indicate whether mass transfer was constraining the reaction either inside or to the bed according to the model.And

the CO2 concentration calculated by PHOENICS software was in good agreement with what was predicted by the model developed in this work so that the mono-dimensional model could play an important role on studying the gas-solid reaction kinetics when a cup-like crucible was employed during the thermogravimetric measurement.