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Note 3 - Revenue (Tables) | 6 Months Ended
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Jun. 30, 2026
Notes Tables |
Disaggregation of Revenue [Table Text Block] |
|  | Three Months Ended |  |  | Six Months Ended |
|  | June 30, |  |  | June 30, |
|  | 2026 |  |  | 2025 |  |  | 2026 |  |  | 2025 |
Air Pollution Control |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Technology solutions |  | $ | 2,078 |  |  | $ | 1,644 |  |  | $ | 2,833 |  |  | $ | 2,216 |
Spare parts |  |  | 417 |  |  |  | 425 |  |  |  | 811 |  |  |  | 675 |
Ancillary revenue |  |  | 290 |  |  |  | 436 |  |  |  | 745 |  |  |  | 917 |
Total Air Pollution Control technology revenues |  |  | 2,785 |  |  |  | 2,505 |  |  |  | 4,389 |  |  |  | 3,808 |
FUEL CHEM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
FUEL CHEM technology solutions |  |  | 3,700 |  |  |  | 3,053 |  |  |  | 8,176 |  |  |  | 8,132 |
Total Revenues |  | $ | 6,485 |  |  | $ | 5,558 |  |  | $ | 12,565 |  |  | $ | 11,940 |
Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Geographical Area, Revenue [Table Text Block] |
|  | Three Months Ended |  |  | Six Months Ended |
|  | June 30, |  |  | June 30, |
|  | 2026 |  |  | 2025 |  |  | 2026 |  |  | 2025 |
United States |  | $ | 5,901 |  |  | $ | 4,442 |  |  | $ | 11,148 |  |  | $ | 9,801 |
Foreign Revenues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Latin America |  |  | 189 |  |  |  | 65 |  |  |  | 375 |  |  |  | 413 |
Europe |  |  | 305 |  |  |  | 707 |  |  |  | 698 |  |  |  | 1,184 |
Africa |  |  | 14 |  |  |  | 156 |  |  |  | 14 |  |  |  | 194 |
Asia |  |  | 76 |  |  |  | 188 |  |  |  | 330 |  |  |  | 348 |
Total Foreign Revenues |  |  | 584 |  |  |  | 1,116 |  |  |  | 1,417 |  |  |  | 2,139 |
Total Revenues |  | $ | 6,485 |  |  | $ | 5,558 |  |  | $ | 12,565 |  |  | $ | 11,940 |
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table Text Block] |
|  | Three Months Ended |  |  | Six Months Ended |
|  | June 30, |  |  | June 30, |
|  | 2026 |  |  | 2025 |  |  | 2026 |  |  | 2025 |
Products transferred at a point in time |  | $ | 4,407 |  |  | $ | 3,914 |  |  | $ | 9,732 |  |  | $ | 9,724 |
Products and services transferred over time |  |  | 2,078 |  |  |  | 1,644 |  |  |  | 2,833 |  |  |  | 2,216 |
Total Revenues |  | $ | 6,485 |  |  | $ | 5,558 |  |  | $ | 12,565 |  |  | $ | 11,940 |
Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] |
|  | As of |
|  | June 30, 2026 |  |  | December 31, 2025 |
Trade receivables |  | $ | 2,690 |  |  | $ | 4,494 |
Unbilled receivables |  |  | 1,223 |  |  |  | 887 |
Other short-term receivables |  |  | 98 |  |  |  | 82 |
Allowance for credit losses |  |  | (107 | ) |  |  | (108 | )
Total accounts receivable |  | $ | 3,904 |  |  | $ | 5,355 |

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