Note 3 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2018
Deferred Tax Assets, Capitalized Research and Development	$ 1,920	$ 2,233
Stock Issued During Period, Value, Stock Options Exercised	0	0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	0	0
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	555	470
Fuel Tech S.p.A [Member]
Deferred Federal Income Tax Expense (Benefit), Net of Adjustment	159	$ 136
Increase (Decrease) in Deferred Income Taxes	23
Deferred Income Tax Expense, Valuation Allowance	$ (159)
Domestic Tax Jurisdiction [Member]
Open Tax Year	2022 2023 2024
Operating Loss Carryforwards	$ 30,357		$ 8,040
Foreign Tax Jurisdiction [Member]
Open Tax Year	2017 2018 2019 2020 2021 2022 2023 2024
Foreign Tax Jurisdiction [Member] | Ministry of Economic Affairs and Finance, Italy [Member]
Operating Loss Carryforwards	$ 6,361
Foreign Tax Jurisdiction [Member] | State Administration of Taxation, China [Member]
Operating Loss Carryforwards	$ 1,087