Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Contract with Customer, Liability, Current	$ 1,365		$ 1,365		$ 372	$ 390
Contract with Customer, Liability, Revenue Recognized	$ 0	$ 131	$ 368	$ 381
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0		0
Revenue, Remaining Performance Obligation, Amount	$ 5,624		$ 5,624
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	1,944		1,944		$ 3,082	1,277
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 0		$ 0		$ 0	$ 0