Note 16 - Goodwill and Other Intangibles (Details Textual) $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Number of Reporting Units	2
Goodwill, Ending Balance	$ 2,116		$ 2,116
Goodwill, Impairment Loss	0	$ 0
Impairment of Intangible Assets, Finite-lived	0	$ 0
Air Pollution Control [Member]
Goodwill, Ending Balance	0
FUEL CHEM [Member]
Goodwill, Ending Balance	$ 2,116		$ 2,116

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-SubTopic 20
-Section 45
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- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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-Name Accounting Standards Codification
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-SubTopic 30
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-Paragraph 3
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Number of reporting units tested for impairment of goodwill. A reporting unit is an operating segment or one level below an operating segment.

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