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Business Segment Information - Segment Assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Dec. 31, 2019
Segment Reporting Information [Line Items]          
Assets $ 181,773 $ 146,671      
Equity method investments 2,884 7,692      
Deferred tax assets, net 1,558 10,604 $ 1,600    
Cash, cash equivalents and restricted cash 82,139 35,932 $ 25,029   $ 17,080
Advanced Purification Technologies          
Segment Reporting Information [Line Items]          
Assets       $ 58,300  
Operating Segments          
Segment Reporting Information [Line Items]          
Assets 91,044 92,393      
Operating Segments | Refined Coal          
Segment Reporting Information [Line Items]          
Assets 7,182 11,516      
Operating Segments | Advanced Purification Technologies          
Segment Reporting Information [Line Items]          
Assets 83,862 80,877      
Long-lived assets 37,000 34,600      
All Other and Corporate          
Segment Reporting Information [Line Items]          
Assets 90,729 54,278      
Cash, cash equivalents and restricted cash $ 82,100 $ 17,200