Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues	$ 7,988	$ 8,017	$ 20,736	$ 19,920
Air Pollution Control [Member]
Revenues	3,711	2,728	10,692	7,670
Air Pollution Control [Member] | Technology Solutions [Member]
Revenues	1,910	1,904	7,449	5,711
Air Pollution Control [Member] | Spare Parts [Member]
Revenues	1,339	384	1,938	742
Air Pollution Control [Member] | Ancillary Revenue [Member]
Revenues	462	440	1,305	1,217
FUEL CHEM [Member] | Technology Solutions [Member]
Revenues	$ 4,277	$ 5,289	$ 10,044	$ 12,250