Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenues	$ 7,490	$ 7,851	$ 19,430	$ 19,850
Air Pollution Control [Member]
Revenues	2,707	3,224	6,515	9,491
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	1,227	2,554	3,443	6,837
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	840	424	1,516	1,428
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	640	246	1,556	1,226
FUEL CHEM [Member]
Revenues	4,783	4,627	12,915	10,359
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 4,783	$ 4,627	$ 12,915	$ 10,359

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