Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues	$ 5,218	$ 4,401	$ 10,251	$ 8,179
Air Pollution Control [Member]
Revenues	986	1,937	1,893	3,133
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	223	1,469	596	2,198
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	236	214	416	413
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	527	254	881	522
FUEL CHEM [Member]
Revenues	4,232	2,464	8,358	5,046
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 4,232	$ 2,464	$ 8,358	$ 5,046

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