Environmental Engineering Reference
In-Depth Information
given. In the updraft gasifier, the biomass is dried in the drying zone by the downward
generated hot producer gas. Below this zone, the biomass is pyrolyzed, and the char
generated there moves further down the reactor. On the other hand, higher molecular
weight tar vapors and gases follow the updraft flow pattern within the reactor. Only a
fraction of the tars condenses on the drying biomass particles; a substantial amount of
the tars stays in the gas and leaves the reactor as product gas. The generated char
slowly moves down toward the combustion zone in which it is converted by hetero-
geneous gasification reactions. As a result of this process configuration, the gas pro-
duced in an updraft gasifier is characterized by a relatively high tar and hydrocarbon
content, which leads to a comparatively high heating value of the gas. However, this
producer gas requires substantial cleaning before it can be processed further in, e.g.,
prime movers for power generation. The updraft gasifier has found application in, e.g.,
the Lurgi dry ash gasifier, developed in the 1930s, with subsequent installation of
about 150 gasifiers since. It is a pressurized gasifier, and the oxidizer introduced in
the bottom is a mixture of steam and oxygen. Temperatures in the bottom combustion
part are moderated by steam supply (
1100
C) so that ash is still removed in non-
molten form using a rotating grate and lock hopper in series. The produced raw syn-
thesis gas is quenched with recycle water to remove tars. A water jacket surrounds the
reactor, generating part of the steam used in the gasifier.
Finally, in the category of fixed/moving bed gasifier configurations, the
cross-draft
gasifier
is described. In this reactor configuration, the fuel and gasification agent flow
in a direction mainly perpendicular to each other, which is shown in Figure 10.8
(right). The cross-draft gasifier is only suitable for the gasification of charcoal. The
temperatures in the combustion zone can rise to 1500
C, and a point of concern is
that the reactor wall material has to endure the high temperature in the reactor for
long-running durations. Like the updraft gasifier, the cross-draft gasifier has a low
tar conversion. Therefore, a high-quality charcoal has to be used.
For large-scale biosyngas generation, these gasifiers are not attractive due to their
relatively small scale. Two other classes of reactors, the fluidized bed and EF gasifiers,
can be scaled up to the large sizes needed for biosyngas production in view of trans-
portation fuel production.
10.3.2 Large-Scale Gasifiers: Entrained Flow Gasifiers
EF gasifiers are designed for the large-scale operation (>> 100 MW
th
) on varying
fuels. Such scales allow more economical production of biofuels for transportation
or for power production using advanced, efficient cycles such as the integrated gas-
ification combined cycle (IGCC: gas turbine with downstream steam turbine operat-
ing on the hot flue gas of the turbine).
The operating temperatures in these EF gasifiers range from approximately 1200 to
1500
C. They are mostly operated using varying coal types. EF gasifiers generally
use fuel in the form of a powder or slurry. The fuel is then mixed with steam, or
steam and oxygen, and gasified in a flame. When using biomass as fuel, it must
either be ground to a powder or in some cases pyrolyzed to gas, oil, and coke; the latter
Search WWH ::
Custom Search