Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total Revenues	$ 4,401	$ 8,948	$ 8,179	$ 19,103
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	1,937	4,803	3,133	10,592
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	1,469	3,795	2,198	8,536
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	214	384	413	533
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	254	624	522	1,523
FUEL CHEM
Disaggregation of Revenue [Line Items]
Total Revenues	2,464	4,145	5,046	8,511
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 2,464	$ 4,145	$ 5,046	$ 8,511