Regulations have certainly has an impact on coal mining but the economics of energy have had even more effect on decreasing coal use.
In 2019 Renewable energy surpassed coal for the first time in 130 years.
Here's some guesses from the US Energy Information Administration.
https://www.eia.govThe U.S. Energy Information Administration estimates in the Annual Energy Outlook 2020 that as of January 1, 2018, there were about 2,828.8 trillion cubic feet (Tcf) of technically recoverable resources (TRR) of dry natural gas in the United States. Assuming the same annual rate of U.S. dry natural gas production in 2018 of about 30.6 Tcf, the United States has enough dry natural gas to last about 92 years. The actual number of years the TRR will last depends on the actual amount of dry natural gas produced and on changes in natural gas TRR in future years.
The amount of coal that exists in the United States is difficult to estimate because it is buried underground. In 1975, the U.S. Geological Survey (USGS) published the most comprehensive national assessment of U.S. coal resources, which indicated that as of January 1, 1974, coal resources in the United States totaled 4 trillion short tons. Although the USGS has conducted more recent regional assessments of U.S. coal resources, a new national-level assessment of U.S. coal resources has not been conducted.
The U.S. Energy Information Administration (EIA) publishes three measures of how much coal is left in the United States. The measures are based on various degrees of geologic certainty and on the economic feasibility of mining the coal.
EIA's estimates for the amount of coal reserves as of January 1, 2020, by type of reserve
Demonstrated Reserve Base (DRB) is the sum of coal in both measured and indicated resource categories of reliability. The DRB represents 100% of the in-place coal that could be mined commercially at a given time. EIA estimates the DRB at about 473 billion short tons, of which about 69% is underground mineable coal.
Estimated recoverable reserves include only the coal that can be mined with today's mining technology after considering accessibility constraints and recovery factors. EIA estimates U.S. recoverable coal reserves at about 252 billion short tons, of which about 58% is underground mineable coal.
Recoverable reserves at producing mines are the amount of recoverable reserves that coal mining companies report to EIA for their U.S. coal mines that produced more than 25,000 short tons of coal in a year. EIA estimates these reserves at about 14 billion short tons of recoverable reserves, of which 60% is surface mineable coal.
Bar chart with 2 data series.
The chart has 1 X axis displaying categories. Range: 3 categories.
The chart has 1 Y axis displaying billion short tons. Range: 0 to 500.
Chart graphic.
billion short tons
U.S. reserves of coal by type and mining method as of January 1, 2020
underground
surface
demonstrated reserve base
estimated recoverable reserves
recoverable reserves at producing mines
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Source: U.S. Energy Information Administration, U.S. Coal Reserves, Table 15, October 2020
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Based on U.S. coal production in 2019, of about 0.706 billion short tons, the recoverable coal reserves would last about 357 years, and recoverable reserves at producing mines would last about 20 years. The actual number of years that those reserves will last depends on changes in production and reserves estimates.