Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total Revenues	$ 10,155	$ 12,791
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	5,789	8,583
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	4,741	7,722
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	149	300
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	899	561
FUEL CHEM
Disaggregation of Revenue [Line Items]
Total Revenues	4,366	4,208
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 4,366	$ 4,208