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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 |  |

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