Note 2 - Revenue Recognition 1 (Details Textual) Pure in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Contract with Customer, Liability, Current	$ 1,279	$ 372	$ 390
Contract with Customer, Liability, Revenue Recognized	$ 368	384
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0
Revenue, Remaining Performance Obligation, Amount	$ 7,458
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	2,285	3,082	1,277
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 0	$ 0	$ 0