Effects of Particle Shape and Size on Biomass Combustion

1. Collection Probe Reactor body Preheater Dry air Feeding probe Fig.1 Schematic Diagram of Entrained Flow Reactor Effects of Particle Shape and Size on Biomass Combustion Hong Lu, Justin Scott, Tom Fletcher, Larry Baxter Chemical Engineering Department, Brigham Young University, Provo, UT ACERC Introduction Over the last two decades, the increasing concern about the environmental impact and longevity of fossil fuels (e.g., coal) has generated a growing interest in renewable energy sources. Understanding the combustion process of these new fuel sources is key to their effective use. The biomass combustion process is different from that of pulverized coal because biomass usually has higher volatile content and lower density. In addition, the irregular shape and relatively large size of biomass particles strongly influence the heat and mass transfer processes during the combustion process. • At different gas velocities, the mass loss rate of the sawdust particles are measured. The results are shown in Fig.3. • Char Particle Oxidation Analysis • The shape of the sawdust char particle can be approximated by an ellipsoid. Holding the particle volume constant, a spherical particle has less surface area than an ellipsoidal particle. Heat and mass transfer analysis in ellipsoidal coordinates shows that the ratio of overall burning rate of an ellipsoidal particle to that of a spherical particle increases as the particle’s aspect ratio increases. Detailed modeling results are illustrated in Fig.4. • Mathematical analysis also shows that for a ellipsoidal particle with aspect ratio b/a, the ratio of mass transfer coefficients at points 1 and 2, as well as the aspect ratio are constant during the oxidation process; as illustrated in Fig.5. • Objectives • Build a new entrained flow reactor for biomass particle combustion, which can provide up to three seconds residence time (good for particle sizes up to 1.5 mm) and a standard operation temperature up to 1700K ; • Develop an imaging system (including the hardware and software) to measure the particle shape, size, surface area, volume and surface temperature during the combustion process; • Experimentally investigate the effects of particle shape and size on biomass combustion, i.e., how they influence the mass loss rate of biomass particles during the pyrolysis and oxidation processes at different temperatures; • Develop an integrated particle combustion model considering the effects of particle shape and size on biomass combustion kinetics. Fig.3 Mass loss rate of maple wood sawdust particles • Methods and Results • Currently, the new entrained flow reactor and the imaging system are being built. Using an existing entrained flow reactor, some results have been obtained about the mass loss rate of sawdust during combustion. A preliminary analysis of char particle oxidation is also shown here. • Mass Loss Rate • Schematic diagram of the existing entrained flow reactor is shown in Fig.1. • Materials: Maple wood sawdust; collected between sieves having mesh sizes of 0.317 mm and 0.363 mm • Operation temperature: shown in Fig. 2. • Procedures: • Room temperature air (oil free) is preheated up to 1250K and then injected into the reactor as secondary gas • A small portion of room temperature air, the primary gas, carries the sawdust particles into the reactor • Room temperature N2 is introduced into the collection probe to quench down the hot exhaust gas • Mass loss data is collected at multiple residence times/feed gas velocities; the residence time is varied by changing the distance between the collection and feeding probes. The mass transfer coefficient on the ellipsoid surface is: 1 b At point 1 and 2, the ratio of mass transfer coefficients are constant: a 2 Based on equation (2) and diffusion control model, the following result is derived: Fig.4 Effects of aspect ratio on char oxidation rate Fig.5 Constant aspect ratio during char oxidation • Future Work • Finish building the entrained flow reactor and the imaging system; • Develop the sub-model for biomass particle pyrolysis; • Conduct pyrolysis and oxidation experiments on the new entrained reactor, compare the experimental data and modeling results, and improve the integrated particle combustion model. Acknowledgement US DOE/EE Office of Industrial Technologies & Sandia National Laboratories