Fuels: Syngas Conversion to Methanol
Methanol is an important primary chemical product, used as a chemical feedstock for production of a range of important industrial chemicals, primarily acetic acid, formaldehyde, methyl methacrylate and methyl tertiary-butyl ether (MTBE). Methanol is also used directly as a fuel or fuel supplement. As fuel, MeOH can be used to fire rapid-start utility peak-shaving combustion turbines; to substitute for or blend with gasoline to power vehicles; to be converted to gasoline via the ExxonMobil methanol-to-gasoline (MTG) process; or to be converted to dimethyl ether (DME) to power diesel engines.
Most methanol is made from syngas. Although the majority of methanol synthesis is based on natural gas as feedstock, coal-derived syngas is also used; coal/solid feedstocks are used to make 9% of the worldwide output of methanol (Gasification, Higman C., Van der Burgt M., 2003).
Catalytic conversion of hydrogen (H2) and carbon monoxide (CO) from coal-derived syngas into MeOH can be done with conventional gas-phase processes, or with a liquid phase methanol (LPMEOH™) process developed by Air Products and Chemicals. The reactions of interest are:
|2 H2 + CO → CH3OH
|CO2 + 3 H2 → CH3OH + H2O
|CO + H2O → CO2 + H2
All three reactions are highly exothermic. The conventional commercial gas-phase process carries out the conversion in fixed-bed reactors at high pressure. Depending on the catalyst supplier, the synthesis reaction is normally carried out at about 600 to 1,700 psig and 400 to 600°F. Substantial process gas recycle of H2 rich gas moderates the temperature rise across the adiabatic reactor. CO concentration at the reactor inlet is normally limited to about 10-to-15%, after dilution with recycled H2.
Catalyst systems used for methanol synthesis are typically mixtures of copper, zinc oxide, alumina and magnesia. Recent advances have also yielded a possible new catalyst composed of carbon, nitrogen, and platinum. This catalyst is based on an earlier catalyst created by Dr. Roy Periana of the Scripps Research Institute. This newer catalyst is a solid material that is suspended in sulfuric acid to aid in the catalysis. The material is easily recyclable as it can be filtered from the acid.
Of the three methanol synthesis reactions, the latter is the well-known water-gas-shift (WGS) reaction. Since the H2/CO ratio in syngas from today’s slagging gasifiers typically ranges from 0.3 to 1, extensive water gas shift is required to meet the stoichiometric H2/CO ratio of 2 for full conversion to MeOH.
Examples of Technology and Plant:
Methanol production from syngas is a commercially demonstrated technology, using both natural gas and coal as feedstock. The current world-class methanol plants are typically in the order of 2,000 to 2,500 metric tons per day (t/d). Larger-scale (5,000 t/d) single train methanol process technologies are being offered. Major technology providers include:
- Toyo Engineering Corporation
- Lurgi Chemie GmbH
- Foster Wheeler/Starchem
DOE NETL's Gasification World Database contains a list of coal gasification-based methanol plants, most of which are found either in Germany or China.