Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Revenues	$ 4,957	$ 7,287
Air Pollution Control [Member]
Revenues	2,318	3,559
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	1,664	2,982
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	199	174
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	455	403
FUEL CHEM [Member]
Revenues	2,639	3,728
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 2,639	$ 3,728

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