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Constant-Volume Carbonization of Biomass.

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Title:Constant-Volume Carbonization of Biomass.
Authors:Legarra Arizaleta, Maider
Contributors:Mechanical Engineering (department)
Date Issued:Aug 2018
Publisher:University of Hawaiʻi at Mānoa
Abstract:Carbonization in constant-volume reactors has received little attention in current biomass pyrolysis research. In this conversion process, volatiles linger in close proximity to the carbonaceous solid material resulting in long vapor residence times and high partial pressures. The formation of additional secondary charcoal through heterogeneous reactions between the pyrolyzing charcoal and the tarry vapors is therefore greatly enhanced minimizing carbon losses in the form of gases and liquids. The result is the relatively quick formation of a charcoal product with a higher fixed-carbon yield and a lower content of volatiles compared to charcoals derived from conventional, hydrothermal carbonization or flash carbonization processes.
This work presents the effect of processing conditions (pressure, temperature, heating rate, reaction time, biomass loading) and fuel properties (biomass type and particle size) on product yields and char properties in constant-volume carbonization processes. Raising the pre-test system pressure with an inert gas from 0 to 2.17 MPa did not significantly affected product yields or char proximate analysis. It seems that the volatiles partial pressures, rather than the total system pressure, accounts for the dominant effect on the high yields and fixed-carbon contents reported for constant-volume carbonization processes. Raising the reaction time from 30 to 190 minutes and the temperature in a 300-550°C range improved fixed-carbon contents and reduced volatiles while maintaining fixed-carbon yields near theoretical limiting values. In contrast with flash-carbonization or traditional carbonization observations which showed a beneficial effect of the use of larger particles, constant-volume carbonization manifested higher fixed-carbon contents and yields (or similar under certain conditions) when using smaller biomass particles, offering possibilities for smaller, lower-grade biomass to produce a charcoal high in fixed-carbon.
A fascinating phenomenon has been reported from certain constant-volume carbonization experiments. Under specific heating rate, pressure and temperature conditions, the particulate biomass seems to exhibit a transient plastic phase that converts it into a single solid piece of char. The roles of pressure, temperature, heating rate, particle size and mass loading in the formation of this transient liquid phase are briefly summarized.
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2018.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Mechanical Engineering

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