Note 3 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2018
Stock Issued During Period, Value, Stock Options Exercised	$ 42	$ 0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	0
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	326	293
Fuel Tech S.p.A [Member]
Deferred Federal Income Tax Expense (Benefit), Net of Adjustment	149	$ 151
Increase (Decrease) in Deferred Income Taxes	(2)
Deferred Income Tax Expense, Valuation Allowance	(149)
Domestic Tax Authority [Member]
Operating Loss Carryforwards	27,953		$ 10,733
Foreign Tax Authority [Member] | Ministry of Economic Affairs and Finance, Italy [Member]
Operating Loss Carryforwards	6,373
Foreign Tax Authority [Member] | State Administration of Taxation, China [Member]
Operating Loss Carryforwards	$ 13,735