Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenues	$ 6,452	$ 16,070	$ 25,555	$ 40,708
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	1,816	10,882	12,408	27,872
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	1,191	10,202	9,727	25,516
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	299	334	832	863
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	326	346	1,849	1,493
FUEL CHEM
Disaggregation of Revenue [Line Items]
Total Revenues	4,636	5,188	13,147	12,836
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 4,636	$ 5,188	$ 13,147	$ 12,836