(374am) Numerical Evaluation of the Low-Temperature Biomass Pyrolysis and Gasification Process Intensification Potential of Rotating- and Circulating Rotating Fluidized Beds in a Static Fluidization Chamber

The process intensification potential of rotating- and circulating rotating fluidized beds in a static fluidization chamber (Figure 1) when used for the low-temperature pyrolysis and gasification of biomass is numerically evaluated. The species continuity equations and energy balance equations are based on complete mixing for the particles within given zones of the particle bed and plug flow for the gas. The reaction mechanism accounts for pyrolysis of biomass and tar, gas-phase combustion, the water gas shift reaction, and combustion and gasification of char. A comparison with current circulating fluidized bed riser technology is made. The circulation of inert solid with a high heat capacity and the separation of the flue gas from the production gas are also studied.

Rotating fluidized beds in a static fluidization chamber are shown to allow extremely high specific biomass conversion rates, that is, biomass converted per unit reactor volume and per unit time. Even when operating at lower temperatures, process intensification by one order of magnitude is easily possible. This opens perspectives for working at temperatures below the ash melting temperature and for optimizing the liquids production. A typical simulation result is shown in Figure 2.

Circulating rotating fluidized beds allow multi-zone operation, i.e. separate combustion and pyrolysis zones, and as such a further increase of the liquids production. Lower temperatures in the combustion zone can be achieved, in particular by the circulation of an inert, high heat capacity solid (sand). A typical simulation is summarized in Figure 3. Circulating rotating fluidized beds also allow separating the flue gas and the production gas, significantly reducing the nitrogen and CO₂ content of the production gas.

(a)

(b)

Figure 1: (a) Rotating fluidized bed in a static geometry (top view). (b) Circulating rotating fluidized bed (side view). The left part of the reactor is presented without particles for a clearer view. Two different reaction zones are introduced: a char combustion zone and an anaerobic biomass gasification zone. Char is circulating longitudinally between the two reaction zones.

(a)

(b)

Figure 2: (a) Simulation results for the pyrolysis / gasification of biomass in a RFB-SG at a temperature below the ash melting temperature and atmospheric pressure and at high particle bed biomass volume fraction (= 0.30). (b) Corresponding calculated radial concentration and temperature profiles.

(a)

(b)

Figure 3: (a) Simulation results for the pyrolysis / gasification of biomass in a CRFB at temperatures below the ash melting temperature and atmospheric pressure and using a two-zone particle bed model. Combustion zone height = 0.08 m_r; Gasification zone height = 0.92 m_r; Char recycled = 1.6 kg / (m²_reactor s); Sand recycled = 64.7 kg / (m²_reactor s). (b) Corresponding calculated radial concentration and temperature profiles.