Revenue Recognition - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total Revenues	$ 56,535	$ 45,166	$ 55,161
Air Pollution Control
Disaggregation of Revenue [Line Items]
Total Revenues	38,417	27,808	34,052
Air Pollution Control | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	35,176	24,422	30,082
Air Pollution Control | Spare parts
Disaggregation of Revenue [Line Items]
Total Revenues	1,083	1,022	1,328
Air Pollution Control | Ancillary revenue
Disaggregation of Revenue [Line Items]
Total Revenues	2,158	2,364	2,642
FUEL CHEM | Technology solutions
Disaggregation of Revenue [Line Items]
Total Revenues	$ 18,118	$ 17,358	$ 21,109