Note 3 - Revenue Recognition - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Revenues	$ 6,216	$ 8,155	$ 4,401	$ 3,778	$ 4,912	$ 6,452	$ 8,948	$ 10,155	$ 22,550	$ 30,467
Air Pollution Control [Member]
Revenues									8,557	14,082
Air Pollution Control [Member] | Technology Solutions [Member]
Revenues									5,668	10,640
Air Pollution Control [Member] | Spare Parts [Member]
Revenues									906	1,031
Air Pollution Control [Member] | Ancillary Revenue [Member]
Revenues									1,983	2,411
FUEL CHEM [Member]
Revenues									13,993	16,385
FUEL CHEM [Member] | Technology Solutions [Member]
Revenues									$ 13,993	$ 16,385