Page 27 - Impiantistica Italiana Maggio Giugno 2014

Fig. 7 - Conceptual design
of VESTA process

possible to have two separate units for the removal stream of the methanation reactors rather than be-
of hydrogen sulfde and carbon dioxide (possibly fore, as in conventional processes (fgure 7).
with a common regeneration section). On the other This choice permits:
hand, the chemical scrubbing does not allow COS • to avoid the installation of a recycle gas com-
removal and leads to possible foam formation, due pressor to mitigate the temperature increase
to the presence of benzene, toluene, and heavier due to methanation (with subsequent higher
tars. investment and operating costs), as in con-
ventional processes;
4.3. Deep desulphurization • to avoid reactors with internal lining;
•
to use a simple CO removal system (amine,
2
The downstream catalytic processes are easily hot carbonates etc.) because at that point the
poisoned by sulphur, and therefore it should be re- gas does not contain any “contaminant”;
duced to ppb (parts per billion) level. The sulphur • to easily produce high purity CO without
2
reduction from ppm to ppb is performed in a fxed- sulphur and other contamination.
bed absorption process, acting as a non-regenera-
ble guard bed before the SNG process. The VESTA process is extremely fexible and com-
patible with any gasifcation technology present in
5. Methanation: the VESTA the market.
No matter what gasifer feedstock, from biomass
process to coal, the VESTA process can produce up to
250,000 Nm /h of SNG in a single train without
3
The most important stage of SNG production is recycle compressor and without risk of temperatu-
represented by the methanation process, during re runaway or metal dusting. The highest tempera-
which the carbon oxides, mainly carbon monoxide ture in the VESTA process does not overcome 550
and hydrogen, are converted into methane in line °C: internal lining reactors are not required.
with the following equilibrium reaction: The novelty of the VESTA process does not reside
in the chemical reactions or catalysts, which are
CO + 3H ? CH + H O well known, but rather in the combination of such
2 4 2
reactions in an effcient and competitive way.
This reaction is strongly exothermic and takes pla- The fnal stage of SNG production is strictly con-
ce in a series of adiabatic fxed-bed catalytic reac- nected to the specifc characteristics that the tran-
tors, typically 3 or 4 with intermediate cooling. sportation grid dictates. Therefore, the process
As mentioned, the catalysts are extremely sensiti- might include one or more of the following steps:
ve to the syngas composition and especially to the • methane dehydration with technologies em-
sulfur, benzene and toluene content. ploying TEG (triethylene glycol) or molecular
In cooperation with the German catalysts manu- sieves;
facturer Clariant, Foster Wheeler has developed a • compression of the product to the grid pres-
new technology for SNG production: the VESTA sure;
process. In this process, the true methanation re- • possible injection of LPG (Liquefed Petroleum
action section previously described is preceded by Gas or similar in order to control the Wobbe
the water shift reaction (clean shift): index and heating value.

CO + H O ? CO + H
2 2 2 6. Biomass to SNG case study
In the conventional SNG processes the risk of ru- Foster Wheeler has recently made a technical and
naway of the methanation reaction is extremely economical assessment of a large-scale biomass-
high. In the VESTA process, however, such risk is toSNG plant, using woody materials as main
avoided due to the presence of the CO acting as feedstock for the gasifer and producing approxi-
2
a temperature damper. This implies that the CO mately 200 MW (or 21,000 Nm /h) of SNG.
3
2
th
removal in the VESTA process is performed down- The plant confguration chosen for the assessment
Impiantistica Italiana - Maggio-Giugno 2014 23