1. Pakistan is claiming reserves from a 1992 survey. The Thar coal reserves were discovered by geological surveys in early 90s.The Pakistani surveyors estimated that the Thar reserves could (hypothetically) contain upto 175.5bn tons of lignite coal based on their exploration.
2. US govt estimates potential Thar reserves to be at 1.9bn(1900mn) tonnes. It also points out that the reserves are at great depth,have a high moisture content and there is a dearth of infrastructure required for its mining.
3. Now lignite is a brown coal, young in age, with a low calorific value (amongst the lowest quality of coal). It cannot achieve high temperatures though can be used to drive steam/thermo powerplants. Fortunately for Pakistan is supposed to be relatively impurity-free for a lignite, hence I am assuming minimal washing and blending with imported coal will be necessary.
4. Recoverable/proven reserves are calculated by their economic viability. For eg. "The Shenhua Group Corp of China is quitting the US$1.5 billion Thar coal project in Pakistan. The Chinese firm has decided to roll back its plan for setting up coal-fired power plants at Thar in the Pakistani province of Sindh. The decision is considered a major setback for the South Asian country, which is facing a serious power shortage......The main reason for the company's withdrawal is the power tariff rate offered by Pakistan, which the Chinese side deemed insufficient to continue power generation."
5. BP and World Energy Council in their 2008 report have actually downgraded Pakistan's coal reserves from 3bn tons to 1.9bn tons. A far cry from 180bn tons claimed by you.
6. Another worrying trend discovered by scientists is that coal reserves may be hugely inflated - a possibility that has profound implications for global energy supply and climate change...... "As scientists, we were surprised to find that so-called proven reserves were anything but proven," says the report's lead author Werner Zittel. "It is a clear sign that something is seriously wrong."
1. prr.hec.gov.pk/Chapters/1870-1.pdf
2.
http://www.eia.doe.gov/emeu/cabs/Pakistan/Coal.html
3.
Lignite - Wikipedia, the free encyclopedia
4.
Asia Times Online :: South Asia news - China quits $1.5bn Pakistan coal project
5.
BP Global | BP & attached image
6.
Lump sums: Global reserves of coal may be rapidly diminishing | Environment | The Guardian