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ISSN Online: 2377-424X

ISBN Print: 978-1-56700-421-2

International Heat Transfer Conference 15
August, 10-15, 2014, Kyoto, Japan

Numerical Determination of Autothermal Operation Limits for Beechwood Torrefaction Processes as a Function of Different Operating Parameters

Get access (open in a dialog) DOI: 10.1615/IHTC15.tdy.008349
pages 7797-7814

Sinopsis

Torrefaction is a process for the production of "bio coal" from lignocellulosic biomass through thermal treatment in an oxygen-free atmosphere. For economic reasons, the auxiliary fuel usage has to be minimized by supplying the heat demand of the process by combustion of the volatiles released from the biomass. A process running without any auxiliary fuel is called autothermal and whether such a process is possible or not depends on three parameters:
1. Amount of energy required: Depends mainly on the moisture content of the raw biomass.
2. Amount of energy available: Depends mainly on the torrefaction degree.
3. Amount of energy usable: This depends on the required temperature differences for heat transfer, the air-fuel equivalence ratio of combustion, the degree of predrying, the reactor temperature and the application of combustion air preheating.
Numerical investigations of energy balances with various boundary conditions for a generic torrefaction process are presented within this paper. Calculations are based on previously published experimental results for beechwood torrefaction [12]. Despite considerable differences regarding the minimum air requirement, lower heating value (LHV) and composition, the use of methane or raw beechwood as auxiliary fuel does not show a significant difference in the energy balance of the process. Considering a torrefaction degree corresponding to 21% mass reduction of the dry solid, depending on process parameters an autothermal operation seems theoretically feasible up to 78.7% moisture content (dry basis) of the raw wood. An increase of the torrefaction degree is found to be a reasonable approach for meeting the energy requirements of the process.