THERMOCHEMICAL CONVERSION: (CO)GASIFICATION AND HYDROTHERMAL GASIFICATION - Biomass as a Sustainable Energy Source for the Future - page 328

TABLE 10.5 Comparison of different large-scale gasifier types
Advantages

Disadvantages

(Bubbling) fluidized bed
Flexible feed rate and composition Operating temperature limited by ash melting

temperature

High-ash fuels acceptable High tar and fine-particle content in gas

High volumetric capacity Possibility of high-C content in fly ash

Temperature distribution

CFB
Easy to scale up to medium/large scale Corrosion and attrition problems

Medium operating temperature
(about 850 C)

Potential for agglomeration

Double (indirect) fluidized bed

Oxygen not required More tar due to lower bed temperature

Temperature distribution Difficult to operate under pressure

Scalability more limited than common BFB/CFB

EF

Very low tar and CO 2 Severe feedstock size reduction required

Flexible feedstock Complex operational control

Exit gas temperature Carbon loss with ash

Higher exergy loss due to high gasifier temperature
Ash slagging

from a combustion zone to a pyrolysis zone and a reforming zone. The transportation
is mechanical and the heat carriers, alumina (Al 2 O 3 ) granules, are transported to its
coolest stage before being heated by flue gases that originate from char combustion.
The char is reformed by steam into a product rich in hydrogen and carbon dioxide. The
steam amount used ensures a high hydrogen concentration. The gasifier is able to
operate on various fuels.
Another way to split the drying/pyrolysis, combustion, and reforming zones has
been realized in the heat-pipe reformer. Here, the heat is not transferred mechanically,
but using heat pipes. These are tubes with a medium (e.g., liquid Na) inside that takes
up heat on one side via evaporation and gives off heat via condensation at the end of
the pipe (Karellas et al., 2008).
Novel ways of intensification of the gasification process that result in cleaner prod-
uct gases are nowadays coming up. In this context, plasmas are used to convert bio-
mass or waste into clean synthesis gas. Plasma is considered as the fourth aggregation
state next to solid, liquid, and gas; an example is lightning. Organic material is decom-
posed to combustible gas and (partially) molten mineral matter at extreme temperature
and radical and charged reactive species concentrations in the plasma. There are
basically two types of plasma torches: transferred and nontransferred torches. In
the transferred torch type, an electric arc is established between torch-tip and metal
or slag phase in the reactor bottom part, or the conductive reactor wall lining. In