I don’t claim to know as much as a hard core Power Engineer, nevertheless having spent a lifetime in the oil & energy sector, this is something thing that I know a little about. After reading the posts in this thread; I would take the liberty of clarifying certain aspects.
Thar Coal is “Lignite” or brown coal, therefore in this post I would refer to it as Lignite.
Lignite as it is dug out from the ground (mostly open pit mines) has up to 45% moisture because geologically it is much younger than bituminous coal. Therefore, unlike bituminous or semi bituminous coal; it cannot be used as it is. It needs to be processed; essentially dried; before it can be used. In fact lignite is only good for large scale power production; burnt after it is dried or after gasification in the IGCC (Integrated gasification combined cycle) & in Carbon Capture technologies. FYI 25% of German power is from lignite.
There are 3 main considerations in the choice of fuel in the power production. These being Technical, Economic & Environmental.
When it comes to large scale power generation “Economics” is everything. Environmental aspects are tied to the economics as well. Since coal has a lot more sulphur & ash; for clean emissions; initial investment in a coal plant is nearly twice as much as a gas based power plant. Only a year ago oil was at $100 per bbl. Everyone was going back to coal, now it is at $40 per bbl. and this makes oil & gas based power plants more economic. However the situation can easily change. New investment in ‘Fracking’ has almost died out and it is inconceivable to think that oil prices will remain so low. Forecast is that we would see $60 bbl. oil within next two years if not sooner.
On the technical side; two processes for external lignite drying are currently being used. The first is a mechanical-thermal drying process that dries raw lignite coal that has been heated to around 140 to 200 °C.
The second process, steam fluidised-bed drying, is already at an advanced stage of development. In this process, raw lignite is fluidised in a steam atmosphere in a stationary fluidised bed and is heated at the same time. The process can be operated at atmospheric pressure or under higher pressure. In the atmospheric variant, the steam released can be used within the process to heat the dryer. I am aware of two technologies using fluidized beds.
Heyl & Patterson custom-engineered a series of Fluid Bed Dryers for the using waste heat from the power plant rather than employing a separate drying plant to gently dry the lignite and cause it to burn cleaner and more efficiently. The drying process also reduces gases such as carbon dioxide (CO2) and removes compounds containing Sulphur and mercury, making the fuel less volatile and of higher quality. Since this is fuel enhancement technology which simultaneously dries and refines lignite, it is called Dryfining.
RWE Germany developed WTA technology to pre-dry lignite at the Niederaussem power station. This is a system that employs fluidized bed drying process with internal waste heat utilization (WTA). The extent to which these processes are suitable for lignite drying first has to be demonstrated in plants of an appropriate size. In addition, issues regarding combustion and pollution behaviour have to be investigated for dried lignite.
In Pakistan’s case it is also a question of foreign dependence. It is always better to have an indigenous source of fuel than depending upon the imports. Power plant has 30 year life cycle and it probable that withing a few years higher oil prices would make Thar coal use as fuel very economical. It is therefore not wise to ‘Pooh pooh’ development of Thar coal. Prudence demands that we should welcome it as mining operation alone would generate hundreds of jobs indirectly helping in economic upliftment of the Thar region.
Finally. the following site has lot of information about Lignite for anyone who wants to know more.
https://www.lignite.com/
