(297e) Heat and Mass Transfer Analysis of Solar-Driven Steam-Based Gasification of Biochar

Steam-based gasification of biomass using solar process heat has the potential of becoming a low-carbon footprint route to transportation fuels from a renewable resource. At high temperatures and in the presence of steam, biomass is thermochemically converted to syngas (a mixture of H₂ + CO), which can be processed into diesel or kerosene (Fischer-Tropsch process) or first into methanol and then into gasoline (MTG, Mobil). Efficient delivery of the concentrated solar heat to the reaction site is crucial for yield and favorable selectivity of the highly endothermic gasification reactions. The most commonly suggested method for accomplishing such a task is to indirectly irradiate biomass particles entrained through optically opaque tubes with a residence time of less than a second. This, however, requires impractically high operating temperatures as well as energy-intensive and costly preprocessing of the biomass into particles that are small enough to react within such a short residence time. Moreover, due to the low density of the gas-particle suspension the heat transfer to the gas phase is generally insufficient resulting in low gas temperatures, and hence an adverse effect on the selectivity of gas-phase reactions. Alternative concepts such as packed or moving beds allow the use of coarser biomass particles but suffer from significantly reduced overall heat transfer due to high extinction of the radiation by the densely packed opaque solids.

This work investigates a method for increasing the residence time of solids and enhancing heat transfer to the gas phase by incorporating a porous structure into a drop-tube reactor that provides a resistance to the flow of solids. As the particles are gasified while trickling through the porous structure, the mean residence time is longer compared to unimpeded entrained particle flow. Thermal radiation penetrating into the porous structure gets absorbed, thereby enhancing the heat transfer to the gas phase. Because the structure is less optically dense than a packed or moving bed, the penetration depth of the radiation is significantly larger. Based on these enhancements and due to the additional heat transfer by conduction through the structure, a more homogeneous temperature distribution and therefore increased reaction yields are expected.

To explore this concept, a 2-D finite-volume heat- and mass-transfer model was developed. In this model an externally heated tubular reactor is considered, in which activated carbon particles trickle through a porous structure while undergoing steam gasification. The model also includes chemical reaction coupled with conduction, convection, and radiation within the porous structure. The effects of tube diameter, steam concentration, particle loading in the porous structure, and the inner tube-wall temperature were investigated.

The model was validated using steam-gasification experiments with activated carbon particles carried out in an electrically heated tubular reactor. The particles and steam were introduced from the top into the reactor that was filled with the porous structure. The temperature within the reactor and the carbon conversion determined via gas composition measurements were compared to the model predictions.

A more uniform temperature distribution over the tube cross-section for the reactor filled with the porous structure is obtained compared to a packed or moving bed. This is mainly due to the increased solid conductivity but also due to the radiation penetrating further towards the center of the tube. Furthermore, under certain conditions (temperature, tube diameter, particle size, and particle loading) the conversion rate per tube surface area of the proposed configuration is higher than in the packed or moving bed and that higher gas temperatures are achieved than in the entrained flow configuration.