Page 163 - AWSAR 2.0
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sulphur in the coal is converted to several acid gas compounds, such as hydrogen sulphide (H2S), carbonyl sulphide (COS), and thiophenes. The nitrogen in the coal is primarily converted to gaseous ammonia and hydrogen cyanide. The biomass matrices in the coal may lead to formation of tar and wax that may block pipes in a methanol production plant. In addition, the ash component in coal consists of toxic compounds, such as arsenic, mercury, cadmium, selenium, lead, which get converted to toxic gaseous compounds during the gasification process. All these contaminants are entrained in the syngas during the coal-gasification process and no single process is capable of removing all of these contaminants in one vessel. The gas- cleanup process hence has to be composed of several steps and sub-processes specifically tailored for removal of each contaminant, thus making it quite a tedious exercise to design an integrated process for syngas
clean-up without affecting the economic viability of the methanol production process.
In order to build such a
technology, the goals of our
research involve screening,
design, arrangement, and
integration of several processes
to form a unified process for
clean-up of syngas derived
from Indian coal. Hence in
the first stage of the research
project, we identified all the
potential contaminants and
their allowable concentrations
in the Indian coal-derived syngas to control the poisoning of the methanol production catalyst. Accordingly, the final process is supposed to cater the removal of particulate matter, heavy and light tars, hydrogen sulphide (H2S), carbonyl sulphide (COS), carbon dioxide (CO2), ammonia (NH3), thiophenes,
Ms. Dyana Joseline || 139
hydrogen cyanide (HCN) and heavy metals (Hg,As)fromthesyngas.Asthenextstep,we studied the feasibility of different processes commercially used for removal of each of these contaminants. For example, different processes that employ liquid solvents to absorb gaseous compounds were studied for removal of hydrogen sulphide (H2S). These processes were screened on basis of technical feasibility and economic viability. Various combinations of the best processes are currently being studied and the next step would be to finalize the overall process flow for syngas clean-up after exploring various options.
The problem with selecting the best industrial process is that it is often incomplete and misleading to screen the processes merely on the basis of literature and rules of thumb. Thankfully, process simulation is a field where computer programmes can be developed so as to approximately predict the performance
of various processes. We have been using Aspen Plus, a commercial process simulation tool developed specifically for design of chemical plants, to screen the various syngas clean-up processes and combinations thereof. Different absorption columns, reactors, separators, heat exchangers have been simulated along with their combinations. However, powerful simulation capabilities still need validation by practical experiments. Further, simulated models may require tuning by
experimentation. Thus, for acquiring robust and specific data, the future process design exercise would consist of bench-scale experiments to be carried out on miniaturized process vessels.
To conclude, the methanol production technology from Indian coal is in its nascent
   A poisoned catalyst no more speeds up the methanol production reaction, becomes useless quickly and needs to be replaced. That is why, it is essential to identify and remove any contaminants present in the syngas before feeding it to the methanol production reactor.
  












































































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