Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Revenues	$ 8,017	$ 7,559	$ 19,920	$ 17,810
Air Pollution Control [Member]
Revenues	2,728	1,944	7,670	3,837
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	1,904	1,172	5,711	1,768
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	384	407	742	823
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	440	365	1,217	1,246
FUEL CHEM [Member]
Revenues	5,289	5,615	12,250	13,973
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 5,289	$ 5,615	$ 12,250	$ 13,973

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