Note 16 - Goodwill and Other Intangibles (Details Textual) $ in Thousands	6 Months Ended
Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	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.

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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.

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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

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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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