Ignition behavior and air delivery requirements observed during the carbonization of pressurized packed beds of biomass

Abstract

Flash Carbonization ™ (FC) is a novel procedure by which biomass is converted to bio-carbon (i.e. charcoal) quickly and efficiently. To begin this process, a canister containing a packed bed of feedstock is placed within a pressure vessel, compressed air is used to charge the system to an initial pressure of ~1-2 MPa, and a flash fire is ignited at the bottom of the bed. After approximately two minutes, air is delivered to the top of the bed and the biomass is converted to a high-yield bio-carbon. This technology has progressed to the point that a commercial-scale demonstration reactor is currently being assembled on the campus of the University of Hawaii, Manoa. The adaptation of the technology from the lab-scale reactor to the larger pilot plant is the underlying theme of this thesis. Specifically, the subjects of ignition behavior and air delivery are explored in detail. In lab-scale tests of the FC process, some fuel have been observed to ignite violently, resulting in a sudden drastic pressure rise (ΔP~1 MPa within 2-3 seconds). Because this event could be potentially hazardous, a study of this phenomenon was undertaken in preparation for the scale-up of the FC process. The influence of initial pressure, feed moisture content, and ignition heater power were examined following a 23 full factorial experimental design approach. It was determined that the violence of the ignition is related to the moisture content of the feedstock and the operating pressure of the process. The information gained from this study was used to size the emergency venting components of the pilot plant. Additionally, this work details the impact of compressibility effects on previously published air-to-biomass ratios (ABRs)-a value used to indicate the optimum level of air delivery for an individual biomass feedstock. As a result of this study, the accuracy of the ABR values were improved for all feedstock types.