Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 322

9.2.3 Advantages of SCW Gasification over Conventional
Thermal Gasification
The following are two broad routes for the production of energy or chemical
feedstock from biomass:
1. Biological: Direct biophotolysis, indirect biophotolysis, biological reac-
tions, photofermentation, and dark fermentation are the five major biolog-
ical processes.
2. Thermochemical: Combustion, pyrolysis, liquefaction, and gasification
are the four main thermochemical processes.
Thermal conversion processes are relatively fast, taking minutes or sec-
onds to complete, while biological processes, which rely on enzymatic
reactions, take much longer, on the order of hours or even days. Thus, for
commercial use, thermochemical conversion is preferred.
Gasification may be carried out in air, oxygen, subcritical steam, or water
near or above its critical point. This chapter concerns hydrothermal gasifica-
tion of biomass above or very close to the water's critical point to produce
energy and/or chemicals.
Conventional thermal gasification faces major problems from the forma-
tion of undesired tar and char. The tar can condense on downstream equip-
ment, causing serious operational problems, or it may polymerize to a more
complex structure, which is undesirable for hydrogen production. Char resi-
dues contribute to energy loss and operational difficulties. Furthermore, very
wet biomass can be a major challenge to conventional thermal gasification
because it is difficult to economically convert if it contains more than 70%
moisture. The energy used in evaporating fuel moisture (2257 kJ/kg), which
effectively remains unrecovered, consumes a large part of the energy in the
product gas.
Supercritical water gasification (SCWG) can largely overcome these
shortcomings, especially for very wet biomass or organic waste. For exam-
ple, the efficiency of thermal gasification of a biomass containing 80% water
in conventional steam reforming is only 10%, while that of hydrothermal
gasification in SCW can be as high as 70% (Dinjus and Kruse, 2004).
Gasification in near or SCW therefore offers the following benefits:
Tar production is low. The tar precursors, such as phenol molecules, are
completely soluble in SCW and so can be efficiently reformed in SCW
gasification.
SCWG achieves higher thermal efficiency for very wet biomass.
SCWG can produce in one step a hydrogen-rich gas with low CO, obviat-
ing the need for an additional shift reactor downstream.
Hydrogen is produced at high pressure, making it ready for downstream
commercial use.