Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Contract with Customer, Liability, Current	$ 578		$ 578		$ 390	$ 850
Contract with Customer, Liability, Revenue Recognized	$ 20	$ 49	$ 250	$ 157
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0		0
Revenue, Remaining Performance Obligation, Amount	$ 10,547		$ 10,547
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss, Total	1,106		1,106		$ 1,277	2,079
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss, Total	$ 0		$ 0		$ 0	$ 269