Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Contract with Customer, Liability, Current	$ 761		$ 1,279	$ 372
Contract with Customer, Liability, Revenue Recognized	$ 1,121	$ 359
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0
Revenue, Remaining Performance Obligation, Amount	$ 6,181
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	1,507		$ 2,285	3,082
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 0		$ 0	$ 0