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.
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
This Brief includes a Technical Support Package (TSP).
Mathematical model of vortex pyrolysis of boimass
(reference NPO20258) is currently available for download from the TSP library.
Don't have an account?
Overview
The document is a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing a mathematical model for vortex pyrolysis of biomass, aimed at optimizing the conversion of raw biomass into valuable products such as char, tar, and gas. The work was conducted under a contract with the National Aeronautics and Space Administration (NASA) and is part of a broader effort to enhance biomass utilization for energy production.
The model focuses on the dynamics of vortex reactors, which are designed to achieve rapid heating of biomass materials. This rapid heating is crucial for effective pyrolysis, a thermal decomposition process that occurs in the absence of oxygen. The document emphasizes the importance of understanding the behavior of individual pure components and their particle sizes, as this knowledge can be applied to polydisperse mixtures, which are common in biomass feedstocks.
Key findings suggest that time-averaged values for free stream temperature and pressure boundary conditions do not result in a loss of information during the pyrolysis process. This indicates that the pyrolysis can be effectively decoupled from disturbances in the free stream, allowing for more accurate modeling and predictions of the pyrolysis outcomes.
The document also includes disclaimers regarding the endorsement of specific commercial products or processes, clarifying that references to any trade names or manufacturers do not imply government endorsement. It highlights the collaborative nature of the research, acknowledging the contributions of the JPL Technology Reporting Office and the broader NASA community.
Overall, the document serves as a comprehensive resource for researchers and engineers interested in biomass conversion technologies, providing insights into the mathematical modeling of vortex pyrolysis and its potential applications in sustainable energy production. The findings could have significant implications for improving the efficiency and effectiveness of biomass reactors, ultimately contributing to advancements in renewable energy technologies.
