RELATED-PARTY TRANSACTIONS - Related Party Purchases (Details) - Affiliated Entity - Waste Coal Agreement (the “WCA”) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transaction [Line Items]
Coal purchases from related party	$ 252,917	$ 0	$ 631,416	$ 0
Coal Valley Sales, LLC
Related Party Transaction [Line Items]
Coal purchases from related party	$ 252,917	$ 0	$ 631,416	$ 0

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