Page 146 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 123

list of torrefaction technologies currently available in the market. Figure 4.9
shows the schematic of some of the available technologies.

4.7.1 Classification of Torrefaction Reactors

A wide range of torrefier or torrefaction reactors is in use or in development.
Such designs often evolve from other biomass processing units like dryer,
pyrolyzer, and carbonizer. These reactors may, however, be divided into
some specific generic groups based on two aspects of the torrefaction pro-
cess: heat transfer and solid contacting.
4.7.1.1 Classification on Mode of Heating
Heating is an important part of the torrefaction process. A medium carries
heat and transfers it to the biomass particles. The transfer of heat to biomass
particles could take place through one of the following means:
Gas particle convection
Wall particle conduction
Electromagnetic heating of biomass
Particle particle heat transfer
Liquid particle heat transfer.
Based on the mode of heating, torrefaction reactors may be grouped into
two basic types:
1. Directly heated type
2. Indirectly heated type.
Some reactors may, however, have a combination of these basic modes
of heating. The following sections describe some of the common types of
directly and indirectly heated reactors.

Directly Heated Reactors
In directly heated reactors, biomass is heated directly by a heat-carrying
medium, and the heat is exchanged through direct contact between the bio-
mass and the heat carrier. The heat carrier could be either a hot gas without
oxygen or one with limited amount of oxygen. It could also be hot nonreac-
tive solids or hot fluid-like pressurized water, steam, or waste oil.

Convective Reactor (Moving/Fixed/Entrained Bed) This is the most com-
mon type of reactor used for torrefaction. Here, the heat carrier is a hot gas per-
colating through or flowing past biomass particles that are either stationary or
moving (Figure 4.9B). The hot gas may be completely inert (Energy research
centre of the Netherlands (ECN) moving bed) or with a small amount (2 3%)
of oxygen (Thermya moving bed) (Ryall, 2012). If the biomass particles remain
stationary with respect to reactor wall, it is called fixed bed. Such beds are used