Note 3 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2017
Stock Issued During Period, Value, Stock Options Exercised	$ 0	$ 296
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	0	0
Unrecognized Tax Benefits, Ending Balance	0	0
Deferred Federal Income Tax Expense (Benefit)	(0)	(22)
Fuel Tech S.p.A [Member]
Deferred Federal Income Tax Expense (Benefit)		$ 155
Increase (Decrease) in Deferred Income Taxes	(19)
Deferred Federal Income Tax Expense (Benefit), Net of Adjustment	136
Deferred Income Tax Expense, Valuation Allowance	136
Domestic Tax Authority [Member]
Operating Loss Carryforwards, Total	27,701		$ 10,733
Foreign Tax Authority [Member] | Ministry of Economic Affairs and Finance, Italy [Member]
Operating Loss Carryforwards, Total	6,847
Foreign Tax Authority [Member] | State Administration of Taxation, China [Member]
Operating Loss Carryforwards, Total	$ 13,476