Note 2 - Revenue Recognition - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Revenues	$ 26,941	$ 24,261
Air Pollution Control [Member]
Revenues	10,597	6,896
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	7,935	4,189
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	911	1,138
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	1,751	1,569
FUEL CHEM [Member]
Revenues	16,344	17,365
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 16,344	$ 17,365

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Section 50
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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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
-Publisher FASB
-Name Accounting Standards Codification
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
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-Section S99
-Paragraph 1
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-URI https://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details