Replacement of Steam Driven Mill Drives with Electric DC Motors
Background
Conventionally, steam turbines are used as the prime movers for the mills, in a sugar industry. These steam turbines are typically, single stage impulse type turbines having about 25 - 30% efficiency.
The recent installation of commercial cogeneration system, with provision for selling the excess power to the grid, has made the generation of excess power in a sugar mill, very attractive. One of the methods of increasing the cogeneration power in a sugar mill is to replace the smaller low efficiency mill turbines, with better efficiency drives, such as, DC motors or hydraulic drives.
The power turbines (multi-stage steam turbines) can operate at efficiencies of about 65 - 70%. Hence, the equivalent quantity of steam saved by the installation of DC motors or hydraulic drives can be passed through the power turbine, to generate additional power.
This replacement can aid in increase of net saleable power to the grid, resulting in additional revenue for the sugar plant. This case study highlights the details of one such project, implemented in a 5000 TCD sugar plant.
Previous Status
A 5000 TCD sugar mill had six numbers of 750 HP mill turbines and one number of 900 HP shredder turbine. The average steam consumption per mill (average load of 300 kW) was about 7.5 TPH steam @ 15 Ata. The steam driven mill drives had an efficiency of about 35%, in the case of single-stage turbine and about 50%, in the case of two stage turbines.
The plant team was planning to commission a commercial cogeneration plant. This offered an excellent opportunity for the plant team to replace the low efficiency steam turbine driven mills, with DC motors or hydraulic drives and maximise the cogeneration potential.
Energy saving project
The plant team contemplated the replacement of the steam driven mills with electric DC motors, along with the commissioning of the cogeneration plant.
Concept of the project
The conventional single stage impulse type steam turbines have very low efficiencies of 35%. Hence, the steam consumption per unit of power output is very high. A single high capacity steam turbine is more efficient as compared to multiple numbers of smaller capacity steam turbines. Hence, the steam can be passed through the larger capacity steam turbine to generate more saleable power.
The latest drives, such as, DC drives and hydraulic drives have very high efficiencies of 90%. The steam saved by the installation of DC drives, can be passed through the larger capacity power turbines of higher efficiency (about 65 - 70%), to generate additional saleable power.
Implementation methodology, problems faced and time frame
The steam turbine mill drives were replaced with DC drives, once the cogeneration plant was commissioned. The modifications carried were as follows:
Four numbers of 900 HP and two numbers of 750
Two numbers of 1100 kW AC motors were installed for the fibrizer, in place of the single 900 HP shredder turbine
There were no major problems faced during the implementation of this project. The implementation of the project was completed in 24 months.
Benefits achieved
The comparative analysis of the operational parameters before and after the modification is as follows:
The equivalent power saved (850 kW/mill) by the implementation of this project, could be exported to the grid, to realize maximum savings.
The annual energy saving achieved was Rs.62.37 million. This required an investment of Rs.42.00 million, which had an attractive simple payback period of 9 months.
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