Page 26 - Impiantistica Italiana Maggio Giugno 2014

ated, which implies high cost for the waste water However, it has to be noted that this technology
treatment. has a lot of potential and some of the major mar-
ket suppliers and R&D centers, like Haldor Topsoe,
3.2. Oil scrubbing Süd-Chemie, Basf, VTT, are working intensively
and are now ready to commercially offer different
The oil scrubbing is carried out by means of a multi- catalysts for operation at commercial scale.
stage cleaning system, followed by a regeneration
stripper. This process is supplied by Dahlman and
is named “Olga”. 4. Syngas purifcation process
The main advantages associated with this process The purifcation of synthesis gas essentially con-
are: expected high system reliability, the recycling sists of acid gas removal (H S, COS, HCN, NH ,
2
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of tar separated in the gasifer (the chemical energy optionally CO etc.) which can be obtained through
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remains in the gas), and high removal effciency. On either physical or chemical absorption.
the other hand, the complexity of the system is in- Additionally, residual components like benzene, to-
creased with the associated cost. luene or naphtalene that have not been fully remo-
ved or decomposed in the previous steps, should
3.3. Thermal cracking also be reduced down to acceptable levels for the
downstream catalytic methanation (typically less
The process of thermal tar removal is based on than 5 ppm vol).
heating the syngas produced in the gasifer at a
temperature as high as 1200 °C. Tar is then ther-
mally decomposed and tends to form additional 4.1. Physical absorption systems
syngas and, under specifc circumstances, solid The solvents’ absorption capacity is tied to Henry’s
carbon residues. law and, therefore, proportional to the partial pres-
In order to reach the operating temperature, it be- sure of the component to remove. At a certain total
comes necessary to oxidize part of the product pressure, the solvent’s circulation is then linked to
(methane) with oxygen and steam, which repre- the volume of gas processed.
sents the most important loss for this tar removal The physical absorption process allows reaching a
process. hydrogen sulfde concentration in the exiting stre-
The very high tar removal effciency is somehow am lower than one part per million on a volumetric
counterbalanced by methane loss, soot formation, basis. This process is characterized by high selec-
and the need to employ high-temperature-resistant tivity, low thermal load at the regenerator, and an
materials. effcient removal of COS, benzene, toluene, HCN
and NH . On the other hand, a higher investment
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3.4. Catalytic cracking cost compared to chemical absorption (due to a
higher plant confguration complexity) and the pos-
Similarly to results in the thermal cracking opera- sible co-adsorption of methane are to be taken into
tion, during the catalytic cracking process, tar is account.
decomposed to form additional syngas. The pre- The selective removal of hydrogen sulfde at high
sence of a specifc catalyst allows the decompo- rates typically forces the use of a refrigerating cycle
sition reaction to occur at much lower operating in order to cool down the solvent sent back to the
temperatures, i.e. in the range between 750 °C and absorber. Selexol and Rectisol are examples of
900 °C, which is very close to the gasifer operating such technologies.
temperature.
This process presents several advantages:
• the possibility of thermal integration with the 4.2. Chemical absorption systems
gasifer; The absorption capacity of a chemical scrubbing
• the possibility to adjust the composition of the process is related to the quantity of the active com-
gaseous product by catalysis; ponent in the solution. The solvent’s circulation is
• the conservation of thermal energy associated approximately proportional to the volume of acid
with the tar in the product; gas to be removed and the regeneration is often
• low investment cost if compared to other achieved through a combination of depressuriza-
technologies. tion and stripping.
On the other hand, the disadvantages include: The chemical absorption, which typically employs
• possible catalyst deactivation due to sulfur, amine-based solvents, is able to lower the concen-
chlorine and alkali metals; tration of sulfur in the exiting stream down to few
• the formation of coke (for high levels of tar); parts per million on a volumetric basis. Moreover,
• the need to reduce the dust content of the input; this process is characterized by good selectivity to-
• the costs associated with the consumption of wards the acidic components and very low losses
catalyst; of hydrocarbons, carbon monoxide and hydrogen.
• the need for disposal of the spent catalyst. The plant layout is consequently simple and it is

22 Impiantistica Italiana - Maggio-Giugno 2014