Environmental Engineering Reference
In-Depth Information
11.7 FAST PYROLYSIS PROCESSES
Figure 11.10 gives the flow sheets of four selected fast pyrolysis processes: Ensyn,
Dynamotive, BTG-BTL, and Metso
Fortum. The core elements of a pyrolysis
plant have already been introduced. In this section, these elements will be dis-
cussed in more detail based on the flow sheets of the four (proposed) demonstra-
tion units.
Before doing this, first, the overall mass and energy balances of pyrolysis of
woody biomass will be discussed at input
-
-
output level starting from the reactor
feed, which consists of biomass with
10 wt% moisture (see Table 11.5). The
energy required to run the pyrolysis reactor (heating of the feed, the reaction
enthalpy, plus the heat loss of the reactor to the surrounding) is not very well known;
however, it can be estimated to be in the range of 1.5
kg
-1
feed for relatively
-
2MJ
dry biomass (
10 wt% moisture). Based on these numbers, it can be calculated that
combustion of the produced char yields 4.8 MJ
kg
-1
(0.16 × 30), which is more than
enough to run the pyrolysis reactor. Combustion of only the product gases does not
result in enough energy for running the reactor. Combusting the product gases
together with the light part of the oil (vapors), however, can also deliver enough
energy.
The core elements of a pyrolysis process are discussed below:
Pretreatment
: Usually consists of (i) size reduction and homogenization to
simplify feeding into the reactor and to obtain a high heating rate of the solids
and (ii) drying to
20 wt% moisture.
Feeding system
: Screw conveyers are most often used for larger-scale installations.
Reactor
: Many concepts for pyrolysis reactors (auger, moving bed, rotary drum,
Herreshoff furnace, fluidized bed, circulating fluidized bed, ablative, micro-
wave) have been proposed and reviewed (see Bridgwater, 2012 and Garcia-
Perez and Kruger, 2011 for more information). Most operational or planned
larger-scale units are of the fluid bed type. These reactors can be scaled,
are fuel flexible, and show good heat transfer characteristics. The fluidized
beds recycle the gases produced within the process for fluidization in order
to prevent the need for additional gas. Characteristics of the proposed reactors
are discussed in more detail in Chapter 9. Important design parameters for
pyrolysis reactors are:
10
-
550
C.
Operating temperature: 450
-
Residence time of the biomass: Should be long enough to achieve a high
conversion of the biomass particles. The required conversion can be calculated
by using single-particle models. For small particles of several millimeters,
typically, tens of seconds are required, while for large particles (centimeters),
minutes are required.
Residence time and temperature of the vapors: The residence time should be
minimized to ca. 2 s inside the reactor. Downstream, in the hot zone, cracking
reactions are minimized below 400
C, allowing longer residence times.
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