Based on the economic analysis, the profits of the three different modes in 5,000 tons sugarcane pressed plants are showed in table 4. There are high costs of fermentation and distillation for sugarcane directly for ethanol fermentation due to low concentration of sugarcane juice, and about 15 tons waste water need treatment. It is also uneconomic to produce fuel ethanol using concentrated juice because of high energy costs. Therefore, that sugarcane is used directly for fuel ethanol production does not reflect its best economic benefits and flexible market response capacity.

In traditional opinion, people prefer to produce sugar as possible as they can rather than use more molasses to produce ethanol. They think that it is uneconomic to produce 1 ton ethanol with nearly 2 tons sugar consumption. In fact, we can achieve the maximized economic benefits applying "the simultaneous production of sugar and ethanol " mode, in which we boil the A-syrup that have the good characteristic of low energy consumption, to produce the top-grade white sugar production. B-green syrup and second pressed juice are mixed to produce the fuel ethanol. Costs of the ethanol production can be greatly reduced.

According to the calculations, it will bring more economic benefits while employ "the simultaneous production of sugar and ethanol" mode.

6.
Conclusions

Various technologies have been identified for immediate increases in the efficiency and sustainability of current and future sugarcane ethanol. In conclusion, recycle utilization design are seems to be suitable for sugarcane bioethanol development, for example, recycling of byproducts of sugarcane in the fields reduces chemical fertilizers application rates, reducing water consumption with closure of water-processing circuits and the use of bagasse to generate electricity or to manufacture bagasse polymer composites (Xu et al., 2010), improving the energy balance of ethanol production; as well as in production and

harvesting processes. At present, we think bagasse is not preferable for directly bioethanol production due to their high bioconversion costs. Adequate developed technology is available to achieve sustainable sugarcane production and bioethanol. However, the adoption of new technologies requires a favorable economic and political environment that facilitates investments in clean technologies. Pollution problems require strict enforcement of legislation and inspection of agricultural and industrial activities.

Developing the sugarcane ethanol provides a novel option for utilization of the sugar industry, and it will be also helpful to the fuel ethanol development in China.

7. Acknowledgements

This research work was supported by Ministry of Science and Technology of China, (NCSTE-2006-JKZX-023), Guangdong Science and Technology Department (2010A010500005) and the Natural Science Foundation of Guangdong Province (10451031601006220). We specially thank Guangdong Key Laboratory of Sugarcane Improvement and Biorefinery, Guangzhou Sugarcane Industry Research Institute for supporting this research.