Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues	$ 7,559	$ 8,155	$ 17,810	$ 16,334
Air Pollution Control [Member]
Revenues	1,944	2,886	3,837	6,019
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	1,172	2,326	1,768	4,524
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	407	254	823	667
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	365	306	1,246	828
FUEL CHEM [Member]
Revenues	5,615	5,269	13,973	10,315
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 5,615	$ 5,269	$ 13,973	$ 10,315

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