How to improve the cogeneration cycle for India's sugar industry

India’s sugar industry is a major user of steam turbine cogeneration technology – and that technology is improving to maximize performance both during the production season and during the off season, when power exports are the priority. Triveni Turbine’s Engineering & Product Development Team explain.

Depleting conventional energy resources and mounting pressure to reduce carbon emissions together make the conservation of energy and the identification of new and/or renewable energy sources the prime challenge for industry and the public at large today. Industrial consumers of energy resources must implement energy saving techniques as a means for achieving cost reductions and competitive advantage.

The cogeneration of power and steam (combined heat and power – CHP) is a proven method of energy cycle optimization. The main process industry segments that benefit from CHP are textiles, paper, food, district heating and sugar production.

The last decade witnessed a huge momentum in sugar cogeneration in India, with many customers opting for CHP plants. The primary model envisioned power as a by-product to sugar production and a shield against the cyclical nature of sugar price between the production season and the off season. In India. Potential power generation from bagasse as fuel is estimated to be about 5000 MW, and more than 1000 MW has already been achieved.

Steam turbine manufacturer Triveni Turbine has more than four decades of experience and more than 2500 installations. The company is the market leader in India, with a market share of more than 50%, and has a significant presence in international market with installations in more than 30 countries. Triveni delivers steam turbine generators of up to 30 MW from its manufacturing facility in Bangalore and, with its 24 x 7 service network, helps customer to achieve 99% steam turbine island uptime.

The evolution of sugar cogeneration has been marked by many milestones in plant specification, such as:

* increases in MW rating of captive power plants;
* increases in pressure/temperature rating;
* season/off-season operation;
* maximized plant heat rates;
* flexible fuel firing options.

The cost benefits for customers include:

* savings in energy cost through CHP plants;
* a revenue stream through sale of excess power to grid;
* revenue through sale of carbon credits.

In this article, we will discuss two bagasse-fuelled cogeneration plants supplied in the last three years – one in 2007 and another in 2010 – to illustrate the increasing awareness towards energy optimization.

Steam turbine technology from Triveni – as employed in India’s sugar industry

FIRST STEAM TURBINE APPLICATION

The 27 MWe steam turbine generator unit that was supplied in 2007 is installed in a sugar plant in Northern India and the boundary conditions of the steam turbine are summarized in Figure 1.

The design of the steam turbine takes into account both production-season and off-season conditions. The goal was to maximize the cogeneration of power and heat during production-season conditions, and to maximize power output under off-season conditions.

The low pressure (LP) section of the turbine was put through a careful analysis of aero and structural aspects to operate under a wide range of mass flow rates – the most critical being low flow analysis for flutter condition. The season and off-season operating points are at opposite ends of the mass flow range with a gap of 60–70% between them.

Boundary conditions for a 27 MWe steam turbine installation in Northern India
Figure 1. Boundary conditions for a 27 MWe steam turbine installation in Northern India

Under sugar cogeneration season conditions, as extraction is of primary importance, the design of the extraction valve enables it to control large extractions of about 40–70% of the inlet steam flow rate. The LP zone of the turbine experiences very low flow, leading to flow reversal, which is manifested in rapid exhaust temperature increases. A robust LP module is provided that can cater for these very low flow conditions.

The turbine control system is designed to withstand varying frequency and other disturbances resulting from the plant’s connection to the final leg of the state grid. It is also designed to manage the frequent load throw-off conditions and bring the set safely to home load. This cogeneration plant marks a milestone for the Indian sugar industry and the steam turbine generator set has run successfully for the last three sugar seasons.

Over the last three to four years, the Indian sugar industry has evolved further towards maximizing cycle efficiency. One such development in the industry was to increase the inlet pressure and temperature of the steam cycle. Inlet conditions evolved first to 85 bar and 515°C, and further to 105 bar and 535°C.

INCREASED TEMPERATURE AND PRESSURE

In 2009, another sugar mill in the western part of India, decided to set up a 2 x 20 MW cogeneration plant, which is one of the largest generation capacity sizes for the sugar industry segment in India. The customer specified steam pressure of 105 ata and steam temperature of 535°C at the inlet of the turbine. This is commonly specified for captive power plants and independent power plants; however, the sugar industry segment was taking initial steps towards this direction.

Conditions for a two x 20 MWe cogeneration plant with higher operating temperatures and pressures

Figure 2. Conditions for a two x 20 MWe cogeneration plant with higher operating temperatures and pressures

The boundary conditions for this turbo-generator are shown in Figure 2. The steam turbine for this application was designed with an array of special features:

* The extraction section is designed to allow about 60–70% of the inlet steam flow for extraction.
* The LP flow conditions vary from low flow to high flow to suit off-season/season conditions respectively, and these conditions are separated by about 60–70%.
* The high pressure (HP) section is designed with inner and outer casing sections to make the turbine thermally agile during transient conditions.
* The emergency stop valve body, throttle valve chest and outer casing upper portion are moulded in a single casting, keeping in view the high pressure conditions. The integral casting design eliminates high pressure joints and potential leakage areas.
* The nozzle chest is integrated with inner casing to minimize internal leakage and also allow steam admission in fuller arc including lower half.
* A flexible joint between inner and outer casing allows for faster start-up cycle and takes care of the transient conditions.
* The gland leak off is piped to downstream sections to recover energy. Also bleed steam is used to cool the high temperature HP glands.
* The turbine is provided with quick-closing stop valve with closure time of 110 milliseconds.
* A servo system is provided with high speed actuator which will take care of load and speed control for all possible operating conditions.
* The turbine is designed for full auto operation from remote.
* The turbine is fitted with instrumentation to monitor critical health parameters such as rotor-to-stator differential expansion and casing differential temperatures.

In this project, there is a good reduction in the heat rate of the CHP plant due to cycle efficiency improvement, on account of higher steam inlet conditions and also due to high extraction flow capability.

This article was written by the Engineering & Product Development Team at Triveni Turbine, Noida, India. trivenigroup

Cogeneration for the sugar industry

Bagasse is a by-product of manufacturing sugar from sugar cane. Bagasse is a useful source of energy with a gross calorific value of about 9200 kJ/kg but it contains a very high level of moisture (around 50% by weight) and needs specially designed handling, feeding and combustion systems. The commonly used combustion systems are dumping grate, pinhole grate and travelling grate. Over the years, as the sugar mills have tended towards cogeneration, combustion technology has also advanced. Today supplementary fuels are used along with bagasse, namely coal, biomass and biogas (a by-product in an integrated sugar mill).

An integrated sugar mill/distillery process is heat-intensive. Many stages – such as diffusion, evaporation, crystallization and distillation – need heat in the form of steam. Electrical energy is consumed by material handling equipment, shredders, mill drives, clarifiers/centrifuges and other process pumps.

Using a by-product – bagasse – as an energy source and producing combined heat and power (CHP) to meet their sugar mills’ demand makes immense sense for customers. In addition, surplus electrical power from the cogeneration process can be exported to the state grid, providing the customer with an extra revenue stream.

source: cospp