Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues	$ 6,368	$ 5,218	$ 11,903	$ 10,251
Air Pollution Control [Member]
Revenues	2,738	986	4,942	1,893
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	1,966	223	3,807	596
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	262	236	358	416
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	510	527	777	881
FUEL CHEM [Member]
Revenues	3,630	4,232	6,961	8,358
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 3,630	$ 4,232	$ 6,961	$ 8,358

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