A mathematical model has been developed for analyzing the steady-state performances of vortex pyrolysis reactors used to convert particles of raw biomass materials (usually small wood chips) into char, tar, and gas. Optimal designs are usually considered to be those that maximize the production of tar. This model and its submodels can be regarded as more highly evolved versions of the model and submodels described in "Mathematical Model of a Direct-Contact Pyrolysis Reactor" (NPO-20069), "Mathematical Model of Pyrolysis of Biomass Particles (NPO-20070), "Generalized Mathematical Model of Pyrolysis of Plant Biomass" (NPO-20068), and "Production of Tar in Pyrolysis of Large Biomass Particles" (NPO-20067), NASA Tech Briefs, Vol. 22, No. 2 (February 1998).

Vortex reactors have been investigated for commercial production of tar from biomass because they are able of rapid heating of biomass particles through direct-contact ablation, and thereby offer the potential to achieve relatively high efficiencies. In a vortex reactor (see figure), the biomass particles are injected, along with a flow of a hot feed gas (usually, superheated steam), tangentially into a vertical cylindrical chamber with a heated wall. In the resulting strongly swirling flow of particles and gas, the particles are held against the wall by the centrifugal force. Thus, the particles are heated primarily by direct conduction from the wall.

A Vortex Reactor features a strong swirling flow of gas and particles, with resultant direct-contact ablation and rapid heating.

Pyrolysis causes layers of char to form on the particles. The char layers could retard pyrolysis because they are partially thermally insulating, but, as the particles continue to slide along the wall, they are scraped off (ablated). This ablation brings the unpyrolyzed remainders of the particles closer to the wall, thereby increasing the effective rates of heating and pyrolysis. Incompletely pyrolyzed particles that reach the outlet at the bottom of the reactor are collected and reinjected at the inlet along with the hot gas and the raw feedstock.

Like the previously reported model, the present mathematical model for the steady-state performance of the vortex reactor includes submodels of pyrolysis of particles, turbulent flow, and particle trajectories. The pyrolysis submodel is based on the one reported previously, with a modification to account for ablation by providing for fragmentation of char when the char attains a critical porosity. The flow submodel is one of compressible flow with a transport-equation sub-submodel of each component of the Reynolds stress tensor. In the particle-trajectory submodel, each particle is represented as moving under the combined influences of the flow (with drag forces represented by a simplified sub-submodel of flow in the immediate vicinity of the particle) and friction with the wall.

These submodels are coupled through boundary conditions and conservation laws, and the resulting equations of the overall model are solved numerically. The rates of injection of feedstocks and distribution of initial particle sizes are specified for steady-state operation. The distribution of particle sizes is altered as particles make repeated passes through the reactor, so that steady-state operation is characterized by, among other things, multiple particle-size distributions, each representing particles at a different stage of pyrolysis.

Numerical simulations that have been performed thus far with this model have yielded information pertinent to designing vortex pyrolysis reactors. In particular, a wall temperature of about 900 K was found to maximize tar yield; this temperature is practically independent of initial particle sizes. Analysis of the numerical results also revealed that a small reactor could not be scaled up successfully, so that it is recommended that pyrolysis at industrial scales (large mass feed rates) should be envisaged by using multiple small reactors operating in parallel rather than a single large reactor.

This work was done by Josette Bellan and Richard Miller of Caltech for NASA's Jet Propulsion Laboratory. NPO-20258


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This article first appeared in the March, 1999 issue of NASA Tech Briefs Magazine.

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