STUDIES ON ADVANCED MEANS OF BIOMASS TORREFACTION
Nhuchhen, Daya Ram
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Biomass is a carbon neutral primary energy source that could be used as a sustainable energy resource in solid, liquid, and gaseous forms. Despite all its potential benefits, because of several shortcomings, biomass faces some challenges in its wide scale use in energy conversion systems. Torrefaction process can help biomass to overcome its limitations to a great extent. This is a mild roasting process that produces an upgraded biomass fuel, improving the technical viability of biomass use for co-firing with coal in power plants. This thesis examines two new novel techniques of torrefaction using (i) pressurized batch reactor and (ii) continuous rotary reactor. These techniques could eliminate the need for expensive nitrogen gas for torrefaction while producing more uniform and better quality products. The first part of this work investigated a mild pressure torrefaction in air and nitrogen medium. The energy density enhancement factor of its torrefaction products was 1.42 in air medium while it was only 1.34 in N2. Minimum solid mass yield was 49% in pressurized air torrefaction while it was 56% in N2. Torrefaction in this reactor also gave higher fuel ratio, similar energy yield, but a reduced mass yield compared to those in pressurized N2. This confirms that N2 can indeed be eliminated in the mild pressure reactor. The second part of the work studied continuous two-stage torrefaction in an indirectly heated inclined rotary torrefier (TIR). This reactor was studied in details examining a) solids motion in TIR reactor, b) heat transfer from wall to solids in TIR reactor, and c) torrefaction of wood chips in TIR torrefier. A cascaded motion of solid was deployed to study the kinematics of solids in an inclined rotating reactor to develop a semi-empirical model for mean residence time. The model was validated against experimental data and those from published literature. Another expression for filling factor was also derived and validated with experimental data. A mechanistic model for predicting overall heat transfer coefficient in the externally heated TIR reactor was developed and then verified experimentally. The model for the effective wall to solids contact heat transfer was validated against published data. To study the torrefaction capability of TIR torrefier, yellow poplar wood was torrefied in a volatile gas medium. Energy density in products is comparatively low, but it gives higher mass yield. Thus, this new torrefier retains greater part of the energy in the raw biomass while replacing nitrogen with its own volatile. This reactor also resulted in up to 26% improvement in energy yield over that from fixed bed reactor, and this improvement depended on torrefaction temperature and time.