Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total Revenues	$ 3,778	$ 10,155
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	1,196	5,789
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	729	4,741
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	199	149
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	268	899
FUEL CHEM
Disaggregation of Revenue [Line Items]
Total Revenues	2,582	4,366
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 2,582	$ 4,366