Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Contract with Customer, Liability, Current	$ 938		$ 938		$ 372	$ 390
Contract with Customer, Liability, Revenue Recognized	$ 9	$ 20	$ 368	$ 250
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0		0
Revenue, Remaining Performance Obligation, Amount	$ 6,613		$ 6,613
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	2,946		2,946		$ 3,082	1,277
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 0		$ 0		$ 0	$ 0