It varies depending on the type and location of the coal etc. A general idea can be found by searching for information on "energy return on energy invested" or EROEI (or EROI). For coal you will generally find numbers that rank very high compared to other fuels. The actually numbers you will find also vary depending on what was included in the calculation - for example does it go all the way to electricity or just to heat or the HHV. In other words, it is important that you know and take into account what the assumptions and system boundaries were if you compare different people's numbers. This scientific american article suggests that the number for coal to electricity is 18. http://www.nature.com/scientificamerican/journal/v308/n4/box/scientificamerican0413-58_BX2.html but you can find other values.

I meant to add that EROI is an imperfect measure - it doesn't include any insight for example into relative environmental impacts and other externalities. It is a good place to start however.

World coal production is expected to increase over the next ten to fifteen years by about 30 %, mainly driven by Australia, China, Russia, Ukraine, Kazakhstan, and South Africa. Production will then reach a plateau and will eventually decline thereafter. The production of coal is expected to increase until 2020 and after that year the production of coal will start to decline until 2100. In 2010, world’s coal confirmed reserves were around 909.4 billion short tons, enough to satisfy the increase in world energy demands, at least for the next 120-130 years at present level of consumption . It is important to highlight that between 85 % and 90 % of global coal reserves are concentrated in six countries: USA, Russia, India, China, Australia, and South Africa. The USA alone holds 30 % of the world’s coal reserves and is the second largest producer. In the European region, only one country, Russia, is among the six top countries with the highest coal reserves.

However, according to the EWG (EWG Series No 1, 2007), the historical world’s coal reserve assessments raise doubts regarding the quality of this information. The reasons are the following:

a) Proven coal reserves of China have not changed since 1992; those of some other countries not even since 1965;

b) Proven recoverable coal reserves (as reported by the WEC) for other countries – e.g. Botswana, Germany and the UK – have been downgraded over the past years by more than 90 %. Even the coal reserves of Poland were 50 % smaller now than twenty years ago. This downgrading cannot be explained by volumes produced in that period. The revisions are probably due to better data;

c) Since 1987, the proven recoverable coal reserves (as reported by WEC) of India were continuously revised upward from about 21 billion tons to more than 90 billion tons in 2002. However, India is the only country with such huge upward revisions.

Estimated world’s coal resources have declined from 10 billion tons coal equivalent to about 4.5 billion tons coal equivalent, a decline of 55 % within the past twenty five years. Moreover, this downgrading of estimated coal resources shows a trend supported by each new assessment. Therefore, it is possible that resource estimates will be further reduced in the future (EWG Series No 1, 2007).

China is by far the largest coal producer, but possesses only half the coal reserves of the USA. Other important coal producers are: India, Australia, South Africa, Russia, Indonesia, Poland, Kazakhstan and Colombia. Therefore, the outlook for coal production in these countries, particularly China and the USA, will dominate the future of global coal production. In addition, it is important to highlight that coal consumption mainly takes place in the country of origin; only 15 % of coal production is exported. Largest net coal exporters are: Australia, Indonesia, Russia, Colombia, South Africa and China. These countries account for 85 % of all exports with Australia providing almost 40 % of all exports (EWG Series No. 1, 2007). In the absence of national policies and/or binding international agreements that would limit or reduce greenhouse gas emissions, world coal consumption is projected to increase from 139 quadrillion Btu in 2008 to 209 quadrillion Btu in 2035, at an average annual rate of 1.5 % . Regional growth rates are uneven, with little growth in coal consumption in OECD nations, but robust growth in non-OECD nations, particularly among the Asian economies (IEO, 2011). Much of the projected increase in coal use will occurs in non-OECD Asia, which accounts for 95 % of the total net increase foreseeable in world’s coal use from 2007 to 2035. Increasing demand for energy to fuel electricity generation and industrial production in the non-OECD Asia is expected to be met in large part by coal (IEO, 2010). In 2007, coal-fired power generation accounted for 42 % of world electricity supply; in 2035, it is expected that the coal’s share increases slightly to 43 %; this represents a very modest increase of only 1 % in the next twenty eight years. Sustained high prices for oil and natural gas make coal-fired power generation more attractive economically, particularly in nations that are rich in coal resources. World net coal-fired power generation is expected to nearly double over the projection period from 7.9 trillion kWh in 2007 to 15 trillion kWh in 2035. In 2020, the participation of coal in electricity generation is expected to be 40 %; this represents 2 % lower than the level reached in 2007. In 2035, the participation of coal in electricity generation is expected to be 43 %; this represents 3 % higher than the level reached in 2020, and 1 % higher than the level reached in 2007. In general, the use of coal in the European region (OECD countries ) will drop from around 19 quadrillion Btu in 1980 to a little bit higher of 10 quadrillion Btu in 2035; this represents a reduction of 9 quadrillion Btu or 47.4 %.

According to the IEO (2010) report, coal is in decline for electricity generation in many countries. It is expected a reduction of 0.3 % per year during the coming years. Nevertheless, coal-fired power plants are expected to supply the second biggest amount of electricity after renewable sources by 2035 and it is expected also to be the second fastest growing sources of energy for electricity generation in the projection period, except for the Latin American and the Caribbean region. Undoubtedly, coal is the predominant fuel used for electricity generation worldwide and it is expected that continue to play this role during the coming decades. In 2010, coal-fired generation accounted for 40 % percent of overall worldwide electricity generation. Coal-fired electricity generation is expected to grow at a 1.8 % annual rate from 2010 to 2040. In 2040, total world electricity generation from coal is expected to be 73 % higher than the 2010 level, although coal’s share of the electricity market falls to 36 % in 2040. China and India alone account for 89 % of the projected growth in coal-fired generation. In contrast, OECD nations reduce their reliance on coal-fired electricity generation, with environmental factors, particularly in OECD Europe, playing a sizable role in the reduction (IEO, 2013). However, this projection could be altered substantially by any future national policies or international agreements that aim to reduce or limit the growth of greenhouse gas emissions. Finally, it is important to highlight that coal will continue to be, during the coming years, the fossil fuel with the largest share of worldwide electric power production by a wide margin (see Figure 5).

Total coal consumption in the countries of OECD Europe is expected to decline from 12.2 quadrillion Btu in 2010 (27 % of the OECD total) to 10.7 quadrillion Btu in 2040 (25 % of the worldwide total). Although all nations in the region consume coal, 65 % of OECD Europe’s 2010 total coal consumption were concentrated in Germany, Poland, Turkey, and the UK, with Germany alone consuming 26 % of the regional total. The electric power sector accounted for 67 % of the region’s total coal consumption in 2010, and most of the rest was consumed in the industrial sector.

Electric power demand for coal declines steadily in the region and drives the downward trend in the region’s overall coal consumption (IEO, 2013).

The Industrial Emissions Directive (IED), agreed to by the European Council of Ministers and the European Parliament in 2010, as well as regional climate change policy goals, drive the decline. The IED requires the use of best available technology for reduction of sulfur dioxide and nitrogen oxides, among other pollutants, beginning in 2016, and is likely to trigger retirements of some coal-fired power plants, especially in the four leading coal-consuming countries (EC, 2010). The scale of the retirements outweighs the scale of new coal-fired capacity additions in Germany, Turkey, Poland, the Netherlands, and Italy, where new coal-fired capacity is needed to fill the supply gaps left by a nuclear power phase-out (in Germany), to replace less competitive power plants (such as oil-to-coal conversions and replacements in Italy), and to supply more power to meet demand growth (especially in Turkey) (World Resource Institute, 2012). Total installed coal-fired generating capacity in OECD Europe is expected to decline from 204 GW in 2010 to 169 GW in 2040 (a reduction of 17.2 %) , and coal’s share of total electricity generation is expected to decline from 24 % in 2010 to 15 % percent in 2040; this represents a reduction of 9 %.

Coal consumption in the OECD Europe industrial sector remains largely flat. The effects of energy efficiency measures in OECD European countries, such as moving away from less efficient processes like open-hearth steelmaking, are more than offset by the effect of the increase in industrial output. For example, the gross output of OECD Europe’s iron and steel plants is expected to increase by 26 % from 2010 to 2040 (IEO, 2013).

The main European coal producers in 2012 were Russia with 4.5 % of the world total, followed by Germany with 2.5 % and Poland with 1.8 %. The main European coal exporter in 2012 was Russia with 8.8 % of the world total. The main European coal importers in 2012 were Germany with 3.8 % of the world total, followed by the UK with 3.7 %, Turkey with 2.4 %, and Italy with 2 %. The world’s coal reserves were estimated at 946.1 billion short tons. The countries with the major coal reserves are the following: USA (258.6 billion short tons or 27.3 % of the total); Russia (173.1 billion short tons or 18.3 % of the total) and China (126.2 billion short tons or 13.3 % of the total). The reserves-to-production ratio was estimated in 2009 to be 119 years. In 2012, the world coal reserves were estimated at 865.99 billion short tons, this represents a decrease of 8.5 % respect to 2009.

It is very big Question.

It can't be answered here in few sentences. 

https://www.belfercenter.org/sites/default/files/legacy/files/ETIP-DP-2010-15-final-4.pdf