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Note 3 - Income Taxes (Details Textual) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2017
Stock Issued During Period, Value, Stock Options Exercised $ 0 $ 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) 19 (0)  
Fuel Tech S.p.A [Member]      
Deferred Federal Income Tax Expense (Benefit)   $ 136  
Increase (Decrease) in Deferred Income Taxes (15)    
Deferred Federal Income Tax Expense (Benefit), Net of Adjustment 151    
Deferred Income Tax Expense, Valuation Allowance 151    
Domestic Tax Authority [Member]      
Operating Loss Carryforwards 28,227   $ 10,733
Foreign Tax Authority [Member] | Ministry of Economic Affairs and Finance, Italy [Member]      
Operating Loss Carryforwards 6,760    
Foreign Tax Authority [Member] | State Administration of Taxation, China [Member]      
Operating Loss Carryforwards $ 13,684