Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Contract with Customer, Liability, Current	$ 276		$ 276		$ 1,279	$ 372
Contract with Customer, Liability, Revenue Recognized	$ 438	$ 9	$ 1,221	$ 368
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0		0
Revenue, Remaining Performance Obligation, Amount	$ 4,250		$ 4,250
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	1,543		1,543		$ 2,285	3,082
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 0		$ 0		$ 0	$ 0