Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenues	$ 8,948	$ 11,847	$ 19,103	$ 24,638
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	4,803	8,407	10,592	16,990
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	3,795	7,592	8,536	15,314
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	384	229	533	529
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	624	586	1,523	1,147
FUEL CHEM
Disaggregation of Revenue [Line Items]
Total Revenues	4,145	3,440	8,511	7,648
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 4,145	$ 3,440	$ 8,511	$ 7,648