Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 342

hydrocarbon or carbon in the feed to combust in the gasifier, but then a part
of the energy in the feedstock is lost.
An SCW gasifier operates at a much lower (450 650 C) temperature and

thus requires a much lower but finite amount of heat. Thermodynamically, the
heat recovered from the gasification product is inadequate to raise the feed to
the gasification temperature (450 600 C) and provide the required reaction

heat. This shortfall is made up either by an external source or by combustion
of part of the product gas in a heater.
Both options are expensive. For example, a study of an SCWG design for
gasification of 120 t/day (5000 kg/h) of sewage sludge with 80% water
showed that 122 kg/h of natural gas is required to provide the gasification
heat. This, along with an electricity consumption of 541 kW, constitutes 23%
of the total revenue requirement for the plant (Gasafi et al., 2008). A better
alternative would be controlled combustion of the unconverted char upstream
of the gasifier, which would make SCWG energy self-sufficient.
Although SCWG is known for its low char and tar production, in practice
we expect some char formation. A low gasification temperature is thermody-
namically more efficient, but raises the char yield as (Figure 9.7) , gasifica-
tion efficiency is low at lower temperatures. If this char can be combusted in
SCW, it can provide the extra heat needed for preheating the feed, thereby
improving the efficiency of the overall system.
Combustion of char offers an additional benefit for an SCWG that some-
times uses solid catalysts which are deactivated after being coated with
unconverted char in the gasifier. A combustor can burn the deposited carbon
and regenerate the catalyst. The generated heat is carried to the gasifier by
both solid catalysts and the gasifying medium (SCW and CO 2 ).
Recycling of solid catalysts is an issue for plug-flow reactors. Special
devices such as fluidized beds may be used for these, as shown in Figure 9.12 .
Here, the catalysts or their supports are granular solids, which are separated
from the product fluid leaving the reactor in a hydrocyclone operating in an
SCW state. The separated solids drop into a bubbling fluidized bed combustor,
where oxygen or air is injected to facilitate burning of the deposited carbon.
The bed is fluidized by pressurized water already heated above its critical tem-
perature in a heat-recovery heat exchanger.
Under supercritical conditions, oxidation or combustion reactions occur
in a homogeneous phase where carbon is converted to carbon dioxide.
C mol (9.16)

Because these reactions are exothermic, the process can become ther-
mally self-sustaining with the appropriate concentration of oxygen. Heated
water from the combustor carries the regenerated catalysts to the gasification
reactor, into which the biomass is fed directly.
Under supercritical conditions, water acts as a nonpolar solvent. As a
result, the SCW fully dissolves oxygen gas. The mass transfer barrier between

O 2 5 CO 2 2 393 : 8kJ =

1