Note 4 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Stock Issued During Period, Value, Stock Options Exercised	$ 296	$ 0
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)	22
Fuel Tech S.p.A [Member]
Deferred Federal Income Tax Expense (Benefit)		$ 155
Increase (Decrease) in Deferred Income Taxes	15
Deferred Federal Income Tax Expense (Benefit), Net of Adjustment	170
Deferred Income Tax Expense, Valuation Allowance	170
Domestic Tax Authority [Member]
Operating Loss Carryforwards, Total	25,486		$ 10,733
Foreign Tax Authority [Member] | Ministry of Economic Affairs and Finance, Italy [Member]
Operating Loss Carryforwards, Total	6,246
Foreign Tax Authority [Member] | State Administration of Taxation, China [Member]
Operating Loss Carryforwards, Total	$ 12,763