2. The gasification of biomass materials was initially
used commercialy in the 1940’s as a source of
gaseous fuels for internal combustion engines,
over the next 30-40 years, a number of small
and medium sized biomass gasification units
were introduced for heat and power generation
applications.
With the increasing interest in the development
of renewable energy technologies over the past
20 years or so, there has been a significant
increase in the technical interest in biomass
gasification processes, particularly in North
America and in Europe.
3. Based on scale of operation and the product gas quality
requirements, which operate under very different
condition;
• Fixed bed and moving bed systems, which tend, for
biomass, to be at the smaller end of the scale, from a few
kWth up to around 10 MWth, principally for the production of
heat, but also for small scale power generation.
•Fluidised bed gasifier, generally operate in the MW range,
and are generally of two types,i.e. bubbling fluidised bed
(BFB) gasifiers and circulating fluidised bed (CFB) gasifiers. In
the main, the CFB units tend to be towards the larger end of
the scale, i.e. above around 15 MWth
•Entrained flow gasifier are generally larger units for power
generation and operate at very high temperatures, in excess
of 1200 C and for very short residence times. These are
normally fired with fossil fuels, and are co-fired with
biomass materials at relatively low co-firing ratios.
7. The fusion or partial fusion of the ash
components can result in the formation of ash
agglomerates in fixed bed and fluidised bed
reactors, leading to poor air distribution and
difficulties with ash removal from the system.
The formation of ash deposits on the furnace and
heat exchanger surfaces,
The carryover of particulate material and of
nitrogen, sulphur and chlorine-containing
species, along with condensable organic species,
into the heat exchangers and gas clean-up
systems, and
The utilisation/disposal of the solid residues
from the plant.
8. A. Fixed Bed Gasifier
Because the temperatures in the oxidation zone
can be up to around 1200 C or so,
particularly in updraft gasifiers, there’s
potential for significant ash fusion to occur
in these system. The succesful operation of
fixed bed reactors depends to a large extent
on maintenance of a good bed structure,
good air and gas distribution through the
bed and the effective operation of the ash
take-off system
9. Fluidised Bed Gasifier
The fouling of furnace and heat exchanger surfaces,
and the formation of ash agglomerates and the
subsequent defluidisation of the bed, are key
processes issues, and these have been fairly
extensively studied at laboratory, pilot plant and
industrial scale.
-For the biomass ashes rich in silica or using a silica
bed material, the risks of bed agglomeration were
high, with significant melting of the ash
temperatures at around 800 C, i.e. at normal bed
temperatures for fluidised bed gasification system
for biomass materials,
-For the biomass ashes rich in CaO or when
processes using a CaO-rich bed material, the risks
of bed agglomeration at normal bed temperatures
were significantly lower.
10. In practice, the key to avoiding bed agglomeration is the
establishment of good fuel and air distributors, and operation at
a bed temperature that is appropriate for the chemistry and
fusion behaviour of the fuel ash, and bed material. It is also
apparent that circulating fluidised beds, which operate at
significantly higher fluidisation velocities, are generally less
prone to bed agglomeration problems than are bubbling beds.
Entrained flow gasifier.
More commonly,biomass and waste material have been co-fired in
large coal-fired integrated gasification combined cycle (IGCC)
plants and significant technical work has been done on the ash-
related impacts. The majority of the entrained flow gasification
systems for coal fire dry milled fuel or aqueous slurries, and are
designed to operate at very high temperatures and for short
residence times. The pre-milled biomass materials are normally
mixed with the fossil fuel feed material. A portion of the ash is
retained within the reactor and is drawn off at a slag tap at the
bottom. The fly ash carried over from the reactor normally
comprises small molten ash particles entrained in the syngas.
11. The fouling of heat exchangers and other plant components has
been one of the most common problem areas with
gasification systems for biomass at industrial scale. The
syngas produced from the gasification of biomass materials
can have significant levels of both inorganic and organic
volatile species, which can condense on cooled surfaces in
the syngas coolers and gas cleaning system. This has been a
significant issue in industrial plants, both in dedicated
biomass gasification system and in the co-gasification of
biomass with coal in large gasifier, and has been the subject
of intensive study over the past few years. The deposition
mechanisms and the design of the syngas coolers had been
seriously underestimated by the plant designers and that
these issues should be the subjects of significant further
study.
12. The Major undesirable constituents of the syngas are :
-Particulate materials entrained in the syngas
-Low and high molecular weight organic compounds
(tars),
-H2S, COS and other sulphur gas,
-NH3, HCN, and ther nitrogen-containing species,
-Other impurities, including HCl, and volatilised alkali
metals and heavy metals.
Fabric and ceramic filters, and electrostatic
precipitator, are capable of particulate collection
efficiencies are capable of particulate collection
efficiencies appropriate to the utilisation of the
syngases in gas engine and gas turbine applications.
13. -As with the ash residues from combustion
processes, the preferred route for utilisation
of the gasifier residues is to recycle them
directly to the land on which the fuel was
produced, or utilise the residues as low grade
fertiliser.
-Biomass gasifier and boiler system generates
two major solid discard streams:
a. The biomass/waste gasifier bottom ash
b. The boiler filter ash
