Note 2 - Revenue Recognition - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Revenues	$ 27,081	$ 26,941
Air Pollution Control [Member]
Revenues	13,483	10,597
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	9,497	7,935
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	2,239	911
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	1,747	1,751
FUEL CHEM [Member]
Revenues	13,598	16,344
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 13,598	$ 16,344

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
-Topic 924
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-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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-Name Accounting Standards Codification
-Section 50
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-URI https://asc.fasb.org//1943274/2147479806/606-10-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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-Name Accounting Standards Codification
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-Publisher FASB
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
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-Section 50
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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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
-Topic 280
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-4

+ Details