Note 3 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2018
Deferred Tax Assets, Capitalized Research and Development	$ 2,233	$ 1,053
Stock Issued During Period, Value, Stock Options Exercised	0	42
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	0	0
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	470	326
Fuel Tech S.p.A [Member]
Deferred Federal Income Tax Expense (Benefit), Net of Adjustment	136	$ 149
Increase (Decrease) in Deferred Income Taxes	(13)
Deferred Income Tax Expense, Valuation Allowance	(136)
Domestic Tax Jurisdiction [Member]
Operating Loss Carryforwards	27,054		$ 8,378
Foreign Tax Jurisdiction [Member] | Ministry of Economic Affairs and Finance, Italy [Member]
Operating Loss Carryforwards	6,459,000
Foreign Tax Jurisdiction [Member] | State Administration of Taxation, China [Member]
Operating Loss Carryforwards	$ 1,098,000