Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Revenues	$ 5,558	$ 7,042	$ 11,940	$ 11,999
Air Pollution Control [Member]
Revenues	2,505	3,949	3,808	6,267
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	1,644	2,619	2,216	4,283
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	425	805	675	1,004
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	436	525	917	980
FUEL CHEM [Member]
Revenues	3,053	3,093	8,132	5,732
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 3,053	$ 3,093	$ 8,132	$ 5,732

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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+ Details