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Top twenty keywords regarding the frequency of usage.
| Ranking | Keyword | Frequency |
|---|---|---|
| 1 | biomass | 4780 |
| 2 | performance | 3974 |
| 3 | anaerobic digestion | 3537 |
| 4 | transesterification | 3486 |
| 5 | optimization | 3323 |
| 6 | energy | 3142 |
| 7 | oil | 2890 |
| 8 | fuel | 2554 |
| 9 | ethanol | 2436 |
| 10 | conversion | 2387 |
| 11 | lignocellulosic biomass | 2351 |
| 12 | combustion | 2029 |
| 13 | fermentation | 1950 |
| 14 | growth | 1875 |
| 15 | pretreatment | 1844 |
| 16 | soybean oil | 1826 |
| 17 | biodiesel production | 1813 |
| 18 | biofuels | 1720 |
| 19 | temperature | 1715 |
| 20 | life cycle assessment | 1678 |
