| Factors | Names | Min level (−1) | Max level (+1) |
| U1 | Initial concentration | 1.00 | 2.00 |
| U2 | Impregnation time | 24.00 | 48.00 |
| U3 | Carbonization temperature | 400.00 | 500.00 |
| U4 | Activating agent | NaOH | H2SO4 |