Methanol (MeOH) can be produced from coal-derived syngas to supplement liquid fuel supplies or to be used as a chemical feedstock. 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 EXXON/Mobil methanol-to-gasoline (MTG) process; or to be converted to dimethyl ether (DME) to power diesel engines. As chemical feedstock, MeOH is used to produce acetic acid, formaldehyde, methyl methacrylate and methyl tertiary-butyl ether (MTBE).
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
Of the above, the latter is the well-known water-gas-shift (WGS) reaction. All three reactions are highly exothermic. Catalyst systems used are typically a mixture of copper, zinc oxide and alumina. 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. Large amounts of recycled H2 are used to control 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.
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.
The LPMEOH™ process from Air Products and Chemicals has great promise as an emerging methanol synthesis technology. It offers superior reaction temperature control and higher conversion. At the heart of the process is its bubble slurry reactor, shown in Figure 1. The process uses an inert mineral oil/powdered catalyst slurry as a reaction medium and heat sink. As the feed gas bubbles through the catalyst slurry forming MeOH, the mineral oil transfers the reaction heat to an internal tubular boiler where the heat is removed by generating steam. The ability to remove heat and the large oil slurry inventory allows the LPMEOH™ reactor to operate at isothermal (constant temperature) conditions by dampening large and rapid process changes, and when handling CO-rich (in excess of 50%) syngas with wide compositional variations. Having the ability to handle CO-rich syngas, an upstream water-gas-shift (WGS) unit to increase the syngas H2/CO ratio is not needed, for partial MeOH production up to full utlilization of feed H2. In this manner, the LPMEOH™ process can be designed for either baseload or IGCC co-production operation.
Figure 2 shows a simplified process flow diagram depicting the use of LPMEOH™ with IGCC for MeOH and power co-production. Part or all of the treated syngas from gasification is routed through the once-through LPMEOH™ reactor to make MeOH. The syngas feed passes through a carbonyl guard bed, COS Hydrolysis reactor and sulfur guard bed to remove trace contaminants and residual COS. The reactor gaseous effluent is cooled, entrained oil removed and cooled to condense-out crude MeOH product, before the offgas is burned in the gas turbine for power generation. The crude MeOH product is separated and purified by distillation before being exported.
The amount of MeOH conversion through the LPMEOH™ reactor can be increased with internal recycle, CO2 removal, and water addition options. Feed compression and product expander options may also be added to increase system operating pressure for higher MeOH conversion.
The U.S. Department of Energy (DOE) helped with theLPMEOH™process development, first by housing its initial pilot plant testing at the DOE LaPorte Alternative Fuels Development facility in Houston, Texas, and later by funding the Demonstration Plant at Eastman Chemical Company’s chemicals-from-coal complex in Kingsport, Tennessee. To date, the LPMEOH™ process has not been fully commercialized.
LPMEOH™ Reactor and Reaction Schematics
Simplified LPMEOH™ Process Flow Diagram
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.