Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues from external customers	$ 8,155	$ 6,452	$ 16,334	$ 25,555
Air Pollution Control [Member]
Revenues from external customers	2,886	1,816	6,019	12,408
Air Pollution Control [Member] | Technology Solutions [Member]
Revenues from external customers	2,326	1,191	4,524	9,727
Air Pollution Control [Member] | Spare Parts [Member]
Revenues from external customers	254	299	667	832
Air Pollution Control [Member] | Ancillary Revenue [Member]
Revenues from external customers	306	326	828	1,849
FUEL CHEM [Member]
Revenues from external customers	5,269	4,636	10,315	13,147
FUEL CHEM [Member] | Technology Solutions [Member]
Revenues from external customers	$ 5,269	$ 4,636	$ 10,315	$ 13,147