Chemical Looping Steam Gasification of Biomass in the Presence of Co3O4 in a Bubbling Fluidized Bed, Kinetic Modeling

Chemical looping is a relatively new technology for the combustion and gasification of fossil fuels. In this technique, to avoid the dilution of the product by nitrogen, the oxygen is provided indirectly by an oxygen carrier often in the form of a metal oxide (MeO). The effluent gas is nitrogen free and there is no need for further separation. Chemical looping gasification consists of two vessels (reduction and oxidation) working in parallel at 800 ^οC. In the reduction vessel, the biomass reacts with steam and oxygen and recycled back to the oxidation zone where it regenerates to full oxidation state by air.

Chemical looping steam gasification of biomass takes place in four steps as shown in Figure1. In the first step MeO releases oxygen into steam. Pyrolysis as second step decomposes the biomass to permanent gases and char. The char reacts with steam and oxygen and ultimately the produced gases from pyrolysis and gasification react together.

Figure 1: Gasifier process block diagram

A transient kinetic model for biomass gasification is developed based on the mechanism of surface reactions. The apparent rate constants are computed by minimizing the differences between experimental data and theoretical results for different temperatures (700, 750, 800 ^οC), steam content (0, 7, 10, 18%) as well as MeO/sand ratio (0, 10, 30, 50 %). Experiments were carried out in a 7.8 cm ID by 167 cm length fluidized bed reactor in the presence of Co₃O₄ as MeO. 10 g of biomass was injected to the reactor and after completion of reaction the fluidizing gas was switched to air to regenerate the cobalt oxide. The product gas was sampled and analysed online with a Pfeiffer mass spectrometer.