Installation of 30 MW Commercial Co-generation Plant

Background

The Indian sugar industry by its inherent nature can generate surplus power, in contrast to the other industries, which are only consumers of energy. This is mainly possible because of the 30 % fibre content in the sugar cane used by the sugar mills. This fibre, referred to as bagasse, has good fuel value and is used for generation of the energy required, for the operation of the sugar mill.

The bagasse is fired in the boiler, for producing steam at high pressures, which is extracted through various back-pressure turbines and used in the process. This simultaneous generation of steam and power, commonly referred to as Co-generation. Conventionally, the cogeneration system was designed to cater to the in-house requirements of the sugar mill only. The excess bagasse generated, was sold to the outside market.

In the recent years, with the increasing power Demand-Supply gap, the generation of power from the excess bagasse, has been found to be attractive. This also offers an excellent opportunity for the sugar mills to generate additional revenue. Co-generation option has been adopted in many of the sugar mills, with substantial additional revenue for the mills. This also contributes to serve the national cause in a small way, by bridging the Demand - Supply gap.

This case study describes the installation of a commercial co-generation plant in a 5000 TCD mill.

Previous status

A 5000 TCD sugar mill in Tamilnadu operating for about 200 days in a year had the following equipment:

Boilers - 2 numbers of 18 TPH, 12 ATA

2 numbers of 29 TPH, 15 ATA

1 number of 50 TPH, 15 ATA

Turbines - 1 number 2.5 MW

1 number 2.0 MW

1 number 1.5 MW

Mill drives - 6 numbers 750 BHP steam turbines

1 number 900 BHP shredder turbine

The plant had an average steam consumption of 52%. The power requirement of the plant during the sugar-season was met by the internal generation and during the nonseason from the grid.

Energy saving project

The plant went in for a commercial co-generation plant. The old boilers and turbine were replaced with high pressure boilers and a single high capacity turbine. The new turbine installed was an extraction-cum condensing turbine.

A provision was also made, for exporting (transmitting) the excess power generated, to the state grid. The mill steam turbines were replaced with DC drives. The details of the new boilers, turbines and the steam distribution are as indicated below:

Boilers - 2 numbers of 70 TPH, 67 ATA

Multi-fuel fired boilers

Turbines - 1 number of 30 MW turbo-alternator set

(Extraction-cum-condensing type)

Mill drives - 4 numbers of 900 HP DC motors for mills

2 numbers of 750 HP DC motors for mills

2 numbers of 1100 kW AC motors for fibrizer

Implementation methodology, problems faced and time frame

Two high capacity, high-pressure boilers and a 30 MW turbine was installed in place of the old boilers and smaller turbine. While selecting the turbo-generator, it was decided to have the provision for operation of the co-generation plant, during the off-season also. This could be achieved, by utilising the surplus bagasse generated during the season, as well as by purchasing surplus bagasse, from other sugar mills and biomass fuels, such as, groundnut shell, paddy husk, cane trash etc.

The shortfall of bagasse during the off-season was a problem initially. The purchase of biomass fuels from the nearby areas and the use of lignite solved this problem. The entire project was completed and commissioned in 30 months time.

Benefits

The installation of high-pressure boilers and high-pressure turbo-generators has enhanced the power generation from 9 MW to 23 MW. Thus, surplus power of 14 MW is available for exporting to the grid.

The following operating parameters were achieved:

Typical (average) crushing rate = 5003 TCD

Typical power generation

During season = 5,18,321 units/day

During off-season = 2,49,929 units/day

Typical power exported to grid

During season = 3,18,892 units/day

(13.29 MW/day)

During off-season = 1,97,625 units/day

(8.23 MW/day)

Typical no. of days of operation = 219 days (season)

= 52 (off-season)

The summary of the benefits achieved (expressed as value addition per ton of bagasse fired) is as follows:

Financial analysis

The annual monetary benefits achieved are Rs.204.13 million (based on cost of power sold to the grid @ Rs.2.548/unit, sugar season of 219 days and off-season of 52 days). This required an investment of Rs.820.6 million. The investment had an attractive simple payback period of 48 months.

Note :

Critical factors affecting power generation

The efficient operation of a co-generation system depends on various factors. This has a direct bearing on the loss in power generation and the power exported to the grid. Some of these critical factors affecting the power generation (quantified as loss in generation per day) are as follows:

1% drop in bagasse % in cane : 18300 units

1% increase in moisture content of bagasse : 6800 - 10200 units

1% increase in process steam consumption : 4200 units

1% drop in crushing rate : 5000 - 7400 units

1 hour downtime : 20600 units

Drop in 1 ton of cane availability : 60 units

The above figures are based on the following operational parameters:

Crushing rate : 5000 TCD

Steam . bagasse ratio : 1 : 2.2

NCV of bagasse (50% moisture) : 1804 kCal/kg

Bagasse content, in % cane : 27%