Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Revenues	$ 7,287	$ 5,535
Air Pollution Control [Member]
Revenues	3,559	2,204
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	2,982	1,841
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	174	96
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	403	267
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 3,728	$ 3,331

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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+ Details