Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenues	$ 5,461	$ 6,368	$ 12,748	$ 11,903
Air Pollution Control [Member]
Revenues	3,422	2,738	6,981	4,942
Air Pollution Control [Member] | Technology Solutions [Member]
Revenues	2,557	1,966	5,539	3,807
Air Pollution Control [Member] | Spare Parts [Member]
Revenues	425	262	599	358
Air Pollution Control [Member] | Ancillary Revenue [Member]
Revenues	440	510	843	777
FUEL CHEM [Member] | Technology Solutions [Member]
Revenues	$ 2,039	$ 3,630	$ 5,767	$ 6,961