Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 131

Energy required in each step is to be estimated for the overall design of
the plant. The following is an order of magnitude estimate of energy require-
ment for these stages:

Chipping of wood: 180 2360 kJ/kg wood (Cielkosz and Wallace, 2011).

Grinding: 270 450 kJ/kg of feedstock (Cielkosz and Wallace, 2011).

Drying of raw wood: 3000 9000 kJ/kg water removed (Cielkosz and

Wallace, 2011).

Torrefaction of dried wood: 130 350 kJ/kg torrefied wood (estimated).

4.8.1.1 Choice of Reactor Type
Choice of a reactor depends on several factors or considerations. For exam-
ple, a choice made from capital cost consideration may not give the best
operating cost or highest yield, while one with the highest yield may not suit
the available feedstock or may require high capital investment. Applying
proper weightage to a selection criterion, one could make a final selection.

4.8.1.2 Design Approach
The following section discusses an approach to the design of a continuous-
type torrefaction plant. The whole process of torrefaction can be divided into
five stages ( Figure 4.3 ). For the sake of convenience, we combine them into
three functional units:

1. Drying of raw feed in dryer
2. Torrefaction of dried feed in torrefier
3. Cooling of torrefied product in cooler

As shown in Figure 4.3 , the energy demand for each of the above func-
tional units or zones of a torrefaction plant is different. Hence, a reactor
needs to provide the right amount of heat to the specific zone. Torrefaction
is a slow conversion process. Hence, average-sized feed would need a rela-
tively long residence time in the torrefaction reactor. This has to be provided
by an adequate volume of the torrefaction zone. The product leaves the torre-
fier at the torrefaction temperature, which happens to be the highest tempera-
ture of the biomass. So, the hot product needs to be cooled before it is taken
to storage. This is accomplished in the cooler or cooling zone of an inte-
grated system.
For a desired set of product properties like HHV, grindability, hydropho-
bicity, and the type of torrefier reactor, the following two important para-
meters must be known in advance:

1. Torrefaction temperature
2. Torrefier residence time
Such information can be gathered from a bench/pilot scale test, published
research, or experience from similar plants. To illustrate the process, we take