Research On Optimal Control Methods Of Current In Sludge Electro-deatering

With the development of economy and society,the output of sludge in China is increasing year by year.The huge volume of sludge brings great difficulties and costs to the follow-up treatment and disposal.Sludge electro-dewatering has attracted more and more attention because of its advantages such as fast dewatering rate,no need to add chemicals and significantly lower energy consumption than thermal drying technology.Aiming at the problem of excessive current attenuation in the process of the sludge electro-dewatering under the constant voltage mode,the current optimization control method of segmented voltage was put forward.Combining the segmented voltage mode with the constant current mode,the constant current switching constant voltage mode was proposed and adopted to optimize the current in the sludge electro-dewatering.The dewatering effect,current,temperature and unit energy consumption under each electric control mode were explored,and the relationship and differences between the constant current switching constant voltage mode and other electric control modes were deeply discussed and analyzed.In the sludge electro-dewatering under the constant current mode,too high voltage led to excessive drying of anode,while too low voltage led to insufficient electric power.The dewatering effect was the best when the voltage was 40 V.The moisture content of the sludge decreased from 86.64%to 53.51%in 528 s,and the unit energy consumption was 0.185 k Wh/kg_water.In the sludge electro-dewatering under constant current mode,limited by the final stop voltage,the larger the constant current,the higher the final moisture content of the sludge.The sludge under all constant current conditions did not reach the dewatering limit when the power was stopped.In the sludge electro-dewatering under the segmented voltage mode,affected by the dewatering rate and the temperature,the voltage switching in the two-segment voltage mode too early or too late increased the final moisture content and unit energy consumption.Based on the two-segment voltage mode,the three-segment voltage mode was proposed.The"low-medium-high"combination form in the three-segment voltage mode had the best dewatering effect and the lower unit energy consumption because the current transition was the most stable.The experimental results of the effect of different voltage switching time on sludge electro-dewatering under the combination form of 20-40-60 V further proved that the stable transition of current was the necessary condition for better dewatering effect and lower unit energy consumption.In the sludge electro-dewatering under the constant current switching constant voltage mode,too large or too small constant current in constant current stage led to poor dewatering effect and high unit energy consumption.Under this mode,the best combination form of dewatering effect was 2.2 A-60 V.The moisture content of the sludge was reduced from the initial value to 47.96%within 525 s,and the unit energy consumption was 0.175 k Wh/kg_water.Constant current switching constant voltage mode limits the current in the early stage of the constant voltage mode to a small range,which can effectively alleviate the problem of excessive drying of anode caused by too fast dewatering rate in the early stage of the constant voltage mode;The constant current stage of the constant current switching constant voltage mode can be regarded as a case where the voltage switching reaches the limit in the segmented voltage mode,and the current transition is more stable,so as to reduce the final moisture content and unit energy consumption.Therefore,the constant current switching constant voltage mode not only ensures the dewatering efficiency,but also optimizes the constant voltage mode and segmented voltage mode in terms of dewatering effect and energy consumption.