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PROCESS & TECHNOLOGY FOR METHANOL AND DME

CO , H O, CH , CxHy aliphatic hydrocarbons, benzene, toluene, tars and inorganic impurities
2
4
2
(mainly H S, NH , HCl) etc. A simplified flow sheet for syngas cleaning is shown below in Fig.6
2 3
FIG. 6

Raw Tar
Syngas Removal Scrubber ZnO Bed

Amine Clean
Cyclones Cooling
Unit Syngas

2.1.3.1

TAR REMOVAL FROM SYNGAS
Apart from particulate removal the two critical issues are sulfur and tar removal. Maximum
efforts are taken to reduce tar formation in the gasification step itself. The high temperature
gasifiers such as entrained bed gasifier; the syngas is almost tar free. This is one of the
main advantages of operating the gasifier at higher temperature. While the low temperature
gasifiers, this becomes important since most of the subsequent process units or turbines are
having stringent limits of tar concentration. Tar formation is also a major issue particularly for
MSW processing. In addition to control in the gasifier itself and through physical separation
with cyclones etc, the chemical treatment is necessary. The chemical routes are: by increasing
reaction rates of tar decomposition by thermal cracking, catalytic cracking or by plasma
cracking. [137].
Tar reforming is carried out in a tar reformer to remove tars, light hydrocarbons, and ammonia
before any additional gas treating or cooling. Reforming must occur prior to cooling the syngas
to prevent tar condensation and deposition on downstream equipment. In the tar reformer, tars
(mono and polyaromatic compounds) and light hydrocarbons such as methane, ethylene, and
ethane are converted to H and CO. Ammonia is converted to N and H .
2 2 2

2.1.3.2

SULFUR REMOVAL FROM SYNGAS
For sulfur removal, the amine is mostly used. An amine unit with a high circulation rate can
reduce the syngas sulfur concentration to below 10 ppm. The liquid phase oxidation systems
such as LO-CAT for sulfur removal from gases uses iron based catalyst for removing H S,
TM
2
but for very large scale syngas with high carbon dioxide concentration, this is not economical.
In amine unit, both carbon dioxide and sulfur are simultaneously removed by amine absorption.

38 Methanol and DME Production: Survey and Roadmap | 2017