Environmental Engineering Reference
In-Depth Information
6.
Corrosion and Fouling.
Corrosion is a serious problem at high temperatures,
especially if oxygen and sulfur are present. This is one of the reasons for devel-
oping catalysts that can reduce the required gasification temperatures. With
respect to the selection of the construction material for a commercial plant, a
two-barrier solution could be adapted from the technology of SCW oxidation
that has been developed earlier. Up to now, it is recommended to construct the
essential parts from a custom-made alloy like Inconel 625.
A problem of general nature in SCWG is the required heat exchange
between the reactor outlet and inlet streams. Heating of the biomass slurry in
a heat exchanger is likely to cause fouling/plugging problems because the
thermal decomposition starts already at approximately 250
C producing oily
products (tars) and, more seriously, polymers (char). Another likely cause of
fouling is the production and accumulation of minerals from the biomass
ash. Ash removal from the reactor as deposits on inert or catalyst particles
circulating through the reactor system seems a better option than entraining
the ash with the reactor effluent. For feedstock with higher than 10 wt% dry
matter, there is evidence that deposition of salts inside the reactor occurs.
Removal of these deposits together with circulating particles may prevent
blockage and corrosion/erosion problems.
7.
Feeding.
A special problem in the process development of SCWG is the feed-
ing. The nature of biomass feedstocks for SCWG varies from dilute waste
streams of organics dissolved in water to heavy slurries of biomass in water.
In case the starting material is coarse or fibrous, the original biomass should
be ground and mixed with water to make a pumpable slurry. For instance, verge
grass, wine-grape residues, and municipal waste fractions must be treated in that
way. Although pumps for light slurries of fines are commercially available, they
have been hardly tested for biomass feedstocks. High-pressure pumping is
required for heavy viscous streams, and sometimes, the cement pump, known
from building with concrete, is proposed as a possible solution.
10.5 GAS CLEANING FOR BIOMASS GASIFICATION PROCESSES
10.5.1
Introduction: Requirements for Gas Cleaning
The product gas generated by the different biomass gasification processes is usually
far from being applicable in downstream conversion processes. Different downstream
product gas utilization processes impose a variety of requirements for removal of
impurities from the gas. This involves a multistep approach. The most relevant classes
of impurity species are:
Particulate matter
Tars (polyaromatic hydrocarbons (PAHs), two-ring and higher ring PAHs,
such as naphthalene, phenanthrene, and chrysene)
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