Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2023 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Contract with Customer, Liability, Current	$ 427		$ 372	$ 390
Contract with Customer, Liability, Revenue Recognized	$ 359	$ 230
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0
Revenue, Remaining Performance Obligation, Amount	$ 7,588
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	3,444		$ 3,082	1,277
FUEL CHEM [Member]
Contract with Customer, Asset, after Allowance for Credit Loss	$ 0		$ 0	$ 0