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INDIAN SCENARIO

4.2.1.2
PROCESSING HIGH ASH COAL FOR METHANOL PRODUCTION

The earlier section discussed about different types of gasifiers. The recent trend for power as
well as liquid fuel production is to use the entrained bed gasifies, although high pressure fluid
bed reactors are also used. There are several reasons for the preference towards entrained
bed gasifiers, particularly for chemical production as it produces tar free syngas. Due to its high
temperature operation, the combustion takes place in few seconds resulting smaller reactor
size. Since these reactors generally operate at the temperature higher than the gas melting
temperature, the ash is easily removed from the reactor as liquid form. Many of the modern
gasifiers are operating at above 2000 C. and higher pressure than 40 bar. As for example, Udhe
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Prenflow Reactor can operate under this process condition with 99% carbon conversion. They
reported to have licensed this gasifier technology to Mozambique for a 9,500 Barrels/day plant
processing 37% ash coal [119]. While most of the high ash coals will melt at this temperature,
due to the non-coking nature of Indian coal there could be some concern. However, the ash
melting point could be manipulated with addition of lime, etc. The main issue here is the loss
of energy due to the large amount of molten ash going out of the reactor when high ash coal
is processed.
In a recent presentation L & T has reported that they are working on a patent taken by Indian
Oil where a two stage gasifier is to be used to process high ash Indian coal. Here the first stage
will operate at lower temperature and the second gasifier at higher temperature. Most of the
ash and tar will be removed in the first stage itself and the clean syngas will be produced in
the second stage at higher temperature and pressure [154]. Since the gasifier is the one of the
major cost centre, its overall economic benefits to be evaluated.
To reduce the overall concentration of ash, high ash coal could be co processed with either
pet coke or biomass. There are several reports of successful gasification with oxygen for
processing pet coke with coal to reduce the impact of high ash content of coal and almost
negligible ash in the pet coke. Although some of them are operating for power generation only
but the quality of the tar free syngas is suitable for methanol production. The ease of carbon
emission control in these oxygen fired units is also very important for future up gradation [153].

With our current interest to use Indian coal for methanol production, the high pressure operation
is desirable so that the cost of compression of syngas could be avoided. With the entrained
bed gasifiers operating at above 40 bar, the low pressure liquid phase methanol production
process is worth considering. A detailed study is reported by Air Products where the reactor is
operated at 500 psig only. It has been shown that methanol production cost is about 70% lower
than that of conventional gas phase reactor [93].
The low calorific value, high ash content and non-coking nature bring restriction on the choice of
suitable gasifiers for good quality syngas production. Also makes the economics unfavourable
to some extent. The ash content adds up to the cost for almost every step in the process.
Therefore, it is often debated if the washing of coal is desirable or low grade coal should be
used as such. The cost of washing vs. the additional penalty for using non washed high ash
coal must be evaluated carefully before taking the decision.

4.2.1.3

MANAGING RESIDUES AND SLAG FROM GASIFIERS
While handling of Indian coal with high ash content, its disposal becomes an important issue.
Almost half of the coal processed returns as bottom ash, fly ash or as slag. Unless it is disposed
efficiently, the operation cannot sustain for long.

62 Methanol and DME Production: Survey and Roadmap | 2017