Water/Steam Closed Cycle

The boiler consists of the following water and steam heating areas: Economiser, boiler drum, furnace wall, primary and final superheater stages. Water passes through these stages in this order, absorbing heat energy along its path, and eventually reaching the superheated steam phase needed by the turbo-generator to generate electricity.

Economiser

The process of feedwater entering the boiler drum as part of the closed system starts at the Economiser, in the coldest section of the flue gas pass. This is a finned type tube heat exchanger, heated by a crossflow of flue gasses. The water in the economiser is heated to just before boiling point at about 290 °C. Boiling point for demineralised water is about 300 °C at 8 MPa. There are other heat exchangers external to the boiler before this phase that already heat the feedwater to a temperature of around 190 °C. This is not discussed further as this heating is done primarily using some steam that is bled off from various turbine stages.

Boiler Drum

The main purpose of the drum is to act as a steam separation vessel [6, p. 82]. The boiler drum has 4 entry and exit points: A feedwater inlet, an inlet and outlet point used for recirculation of water, and a steam outlet. For steam to be generated the water in the drum is recirculated through “down-comers” that are not exposed to the furnace, and back upward through furnace wall tubes (sometimes called risers).

Benson type boilers do not have a boiler drum since they operate past the critical pressure on demineralised water. No boiling occurs in the vessel as water turns directly to steam. This higher pressure allows boilers to be designed with a much higher nominal load rating. The Benson type boilers also suffer from significantly less lag since no boiling occurs, and has no need for a steam separation vessel. The boiler drum in subcritical boilers therefore introduces lag into the system as well as acts as an energy store due to the water capacity in the drum that can store heat energy. The boiling occurring in subcritical boilers are also subject to specific disturbances. Sudden

22

changes in pressure and heat energy transfer can severely affect boiling and also the water level in the drum.

Some boiler types are able to harness this heat store during a loss of fire in the boiler for short periods of time. The heat energy is usually only enough to sustain the turbine and generator to maintain in-house electrical demand while returning the boiler to full operation. This type of operation is not available at Komati due to the relatively small sizing of the boiler drum.

Furnace Wall Natural Circulation

The water in the furnace wall tubes, or risers, is continuously boiling and therefore at a lower density compared to water in the “down-comers”. They are connected with a water header at the bottom of the boiler. The density gradient causes natural circulation to occur with the boiling water flowing upwards in the furnace wall tubes and back into the boiler drum [6, p. 87]. The steam is separated from the water in the boiler drum. The remaining water along with feedwater pumped into the drum, continues to recirculate. The steam exits the drum to the Saturated Steam Header after which superheating of steam begins.

Primary Superheater

From the saturated steam header, the steam goes through the primary superheater where it is heated to well in excess of 450 °C. After this point the spray water system introduces atomised feedwater directly mixed in with the superheated steam, in order to control the final steam temperature. As it might be guessed, this will affect thermal efficiency, but the loss is designed to be small for the required control at this point of the process. At this point the steam legs also cross to opposing sides in the boiler to counteract possible heat imbalances. This is a common arrangement [6, p. 134], In this thesis each leg will be treated as a straight section with no distinction made between left- and right-hand sides, or where they cross.

Final Superheater

After the final superheater stage, the temperature should be at 510 °C and goes to the turbo- generator to produce power. Some steam is also tapped off along the line, and at various turbine stages, for heating and deaeration of feedwater amongst other things. After most of the usable heat has been extracted from the steam in the turbine, the steam is condensed and will eventually

23

make its way through the condenser and pre-heating stages, back to feedwater supplied to the economiser. This completes the closed cycle of demineralised water.

As per the codification convention used on site, the term condensate is used while being preheated at a low pressure and until entering the deaeration storage vessel. From the deaeration vessel up to the economiser the term feedwater is used. The feedwater pressure can be as high as 12 MPa as it is pumped through several more pre-heating stages and up to the economiser inlet.

Efficiency

There are two major contributors to energy losses. Condensing the steam wastes a large fraction of the added heat in the steam. The vacuum this consequently creates in the condenser, due to the steam contracting as it turns to water, this adds to the overall cycle efficiency. The other large contributor to wasted heat energy is in the form of exhaust flue gas. Only about a 1/3 of the total added heat energy is left as a very rough estimate. There have been several improvements to the Rankine cycle, over the years, that attempt to recover some waste energy.