Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total Revenues	$ 16,070	$ 13,548	$ 40,708	$ 31,780
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	10,882	8,799	27,872	18,346
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	10,202	7,365	25,516	15,466
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	334	312	863	848
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	346	1,122	1,493	2,032
FUEL CHEM
Disaggregation of Revenue [Line Items]
Total Revenues	5,188	4,749	12,836	13,434
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 5,188	$ 4,749	$ 12,836	$ 13,434