Note 2 - Revenue Recognition - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Revenues	$ 24,261	$ 22,550
Air Pollution Control [Member]
Revenues	6,896	8,557
Air Pollution Control [Member]
Revenues	6,896	8,557
Air Pollution Control [Member] \| Technology Solutions [Member]
Revenues	4,189	5,668
Air Pollution Control [Member] \| Spare Parts [Member]
Revenues	1,138	906
Air Pollution Control [Member] \| Ancillary Revenue [Member]
Revenues	1,569	1,983
FUEL CHEM [Member]
Revenues	17,365	13,993
FUEL CHEM [Member] \| Technology Solutions [Member]
Revenues	$ 17,365	$ 13,993

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
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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
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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
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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 http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details