Note 2 - Revenue Recognition 1 (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Contract with Customer, Liability, Current	$ 390	$ 850
Contract with Customer, Liability, Revenue Recognized	$ 776	$ 565
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0	1
Provision for Loss on Contracts	$ 0	$ 176
Revenue, Remaining Performance Obligation, Amount	9,119
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss, Total	1,277	2,079
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss, Total	$ 0	$ 269