Torrefaction temperature and torrefaction time (reaction time)

Difficulties in interpreting the torrefaction process may arise from the definition of the torrefaction time. The term residence time is frequently used, but it only expresses the hold-up time of biomass in a torrefaction reactor. It does not tell how long actual torrefaction takes place, since part of the residence time is 'lost' due to heating of the biomass possibly in combination with drying. Misunderstanding about the torrefaction time automatically leads to inaccuracies in relating product quality to torrefaction operating conditions. To overcome this problem, the use of (reactor) residence time has been abandoned and instead the definition of reaction time is introduced.

When biomass is processed in a torrefaction reactor, it passes several stages with each having its own time-temperature characteristics. This is illustrated in Figure 2.4 for a typical batch operation. When moist biomass of ambient temperature is fed into a batch torrefaction reactor, the biomass is first heated to a temperature at which the biomass is dried. Then the temperature further increases until the desired torrefaction temperature is reached. This temperature is maintained until the reactor is cooled again.

The temperature window of torrefaction is generally considered to range from about 200 ?°C to 300 ?°C. (illustrated by the green horizontal lines in Figure 2.4). Only in this temperature window the torrefaction decomposition reactions occur. In this temperature window three time-temperature phases are recognised. First the biomass is heated from 200 ?°C to the desired torrefaction temperature (Ttor) in period ttor,h. Then the temperature is hold for period ttor at the torrefaction temperature, until cooling during period ttor,c- During ttor the decomposition reactions will contribute predominantly, but this will depend on the time contribution of the heating and cooling period.

The reaction time has been defined as the sum of ttor,h+ttor and thus leaving out the cooling time ttor,c- The heating period is important, as during this period the most thermally labile parts of the feed biomass will rapidly start to decompose. In contrast to the cooling period when the solid product is much more thermally stable as the highest reactive parts already reacted. It is therefore expected that the decomposition reactions will stop as soon as the temperature is decreased. Hence the cooling period hardly contributes to the decomposition of the biomass.



Figure 2.4 Stages in the heating of moist biomass from 'ambient' temperature to the desired torrefaction temperature and the subsequent cooling of the torrefied product. Temperature-time profile is considered typical for a torrefaction batch process. Explanation: ty, = heating time to drying, tdry = drying time, th,int = intermediate heating time from drying to torrefaction, ttor = reaction time at desired torrefaction temperature, ttor,h = heating time torrefaction from 200?° C to desired torrefaction temperature (Ttor), ttor,c = cooling time from the desired Ttor to 200 ?°C,tc = cooling time to ambient temperature

By using this approach, it is also possible to define the heating rate of the biomass that is really relevant to the process. On average this is (Ttor-200 ?°C)/ttor,h, which will normally not exceed 50 ?°C/min. Hence the particle heating rate is considered to be a less relevant parameter in torrefaction, as the torrefaction reactions are relatively slow. Note that Figure 2.4 is quite illustrative in what kind of error could be faced when the reactor residence time is mistaken with the reaction time of torrefaction.

In presenting and discussing the results (coming Chapter), use is made of a coding system by which is referred to a particular experiment. The coding represents the reaction coordinates of the pertinent experiment:
Reaction co-ordinate = (Biomass type, temperature [?°C], reaction time [min], particle size [mm]) Typical examples of usage:

(W,230,30,0-10) means: willow torrefied at 230 ?°C and 30 min reaction time using 0-10 mm sized particles.

Willow (230,30,0-10): similar as first example.

Willow (230,30): again similar as first example. Particle size is only mentioned when it differs from the reference size of 0-10 mm.