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PROCESS & TECHNOLOGY FOR METHANOL AND DME

The major gasifier configurations are:
• Moving Bed and Fixed Bed
• Fluidized Bed
• Entrained Bed
• Transport Bed

The names themselves imply the modes of operation. For the moving bed, there could be
co-current or counter-current flow of gas and carbon source. The main characteristics for the
different configurations are summarized in table:

TABLE 8

Characteristics of Different Gasifiers
Parameters Moving Bed Fluid Bed Entrained bed
Energy Usage low high low
Carbon Conversion poor medium high
Heat and Mass Transfer poor good excellent
Feed Particle Size 50mm (approx) small Small below 100 mesh
(not suitable for highly coking coal)
Oxygen requirement low moderate high
Operating Temperature low moderate high
Operating Pressure low moderate high

Since both fluidized bed and entrained bed reactors can operate at higher temperature and
pressure, they are suitable for gas turbines. Entrained bed gasifiers are more convenient
because of the feed flexibility, including capability to handle high ash coal. The operating
temperature is higher than the fusion point of the ash, so it flows easily out of the gasifier.
Since the recent focus is towards power and chemical production from different types of solid
fuels, entrained bed gasifiers are popular. A simplified flow sheet for gasification with power
and chemical production is shown in Figure 2.
The advantage here is that the plant could produce either methanol or power depending upon
market demand. Due to the combined cycle the energy efficiency is also high, leading to lower
carbon emission.
In case of moving bed gasifier, the bed moves freely, therefore this is suitable only for non-
coking coals. Also the operating temperature is lower, and large amount of tar is produced.
The reactor size is larger than modern fluidized bed or entrained bed gasifiers. Almost all
the technology licensors have their own technology with some innovative component built
in. However, at present, most of the new gasifiers are based on fluidization, which could be
circulating or entrained bed, or even high speed transportation.
The different types of gasifiers have certain common features, such as: handling of slurry or
dry powders, though the temperature, pressure and contact times differ widely. As a result,
they widely differ in their carbon conversion efficiency. Interestingly, the carbon conversion
in fluidized and entrained bed gasifiers is higher, particularly for low grade coals. Due to their
ability to operate at higher pressure and temperature, and ability to handle different types of
feed including biomass, these reactors are particularly suitable for chemical production from
syngas, which requires higher pressure. Most of the new installations coming up are based on
fluidized bed gasification, preferably with oxygen firing.

30 Methanol and DME Production: Survey and Roadmap | 2017