Chapter twosteam power plant

2.02.3 – Feed-Water HeaterOFWH & CFWH

Direct contact FWH

This is more commonly known as open feed-water heater (OFWH), which allows mixing of the extracted steam with feedwater to be heated.This is done by an arrangement of spray nozzle and trays to distribute the water evenly and in fine droplet forms throughout the steam space.The water is heated by the steam which condenses and both leaves together as a single flow towards the boiler.in order to facilitate the mixing, and since the extracted steam is normally at higher pressure than feed-water from condenser, then a pump is required in the feed-water line before the FWH to raise the feed water pressure up to that of the extracted steam.

Surface FWH

this is a closed type FWH (CFWH), which is simply a shell & tube heat exchanger.The two streams (extracted steam & feed water) are kept separate in a separate flow lines, where the feed water passes through multiple tubes , while steams condenses on the outside them.The condensed steam is drained back to the condenser as water.

Large steam power plants may have a series of FWH.The calculation of the cycle eff. on such plant is no more different than that for one with a single heater, but rather tedious.Each heater has a different mass flow rate, which must be calculated by doing mass balance on each one. Usually condensed steam drain flow is cascaded down to the next lower pressure FWH, and finally to the condenser.The heat balance between heaters are therefore linked through a series of simultaneous equations.The cycle efficiency is the work from each section of the turbine minus the pump work all divided by the heat input from the boiler.

FWH heat balance

It is very important for plant cycle analysis to specify the extracted steam mass flow rate.To calculate the steam mass flow required to heat up the feed-water, a heat and mass balance is carried out on the heater

Such mass rate is generally handled as fraction values.If unit mass rate of steam flow from boiler, and the extracted steam flow to heater is (m), then the balance leaving the turbine is (1-m), and so the heat balance will be :

Power & efficiency

• If steam is drawn for example from the turbine extraction port which is at part way through it’s expansion, the work done in the turbine is as follows :

• The heat input to the boiler is given by:

• And the cycle efficiency then becomes :

CHAPTER TWOSTEAM POWER PLANT2.02.5 – cogeneration

introduction

Cogeneration (also known as combined heat and power, CHP) is the use of a power station to simultaneously generate both1 - work ( generate electrical power) plus2 - heat as high-quality or saturated steam at a moderately high (T & P) for some other use elsewhere .

Conventional power plants emit the heat created as a by-product (waste) of electricity generation into the natural environment through cooling towers, flue gas, or by other means, which in actual sense is a loss in percentage of fuel energyBy contrast CHP captures the by-product heat for domestic or industrial heating purposes, either very close to the plant, or as hot water for district heating with temperatures ranging from approximately 80 to 130 °C. This is also called Combined Heat and Power District Heating or CHPDH. Small CHP plants are an example of decentralized energy

. It is considered as one of the most common forms of energy recycling

Cogeneration is a thermodynamically efficient use of fuel. In separate production of electricity some energy must be rejected as waste heat, but in cogeneration this thermal energy is put to good use.Thermal power plants (using fissile elements, coal, petroleum, or natural gas), and heat engines in general, do not convert all of their thermal energy into electricity. In most heat engines, a bit more than half is lost as excess heat . By capturing the excess heat, CHP uses heat that would be wasted in a conventional power plant, potentially reaching an efficiency of up to 89%, compared with 55% for the best conventional plants. This means that less fuel needs to be consumed to produce the same amount of useful energy.

In the process flow diagram shown here, the high-quality effluent from the high pressure turbine, stream 3, is used as a heat source by the external system. When stream 8 returns from the external system, it generally has a low quality or it can be a saturated liquid.Stream 8 is mixed with the subcooledliquid effluent from Pump #1 and then pumped up to the pressure of the boiler to complete the cycle.

1-2: Boiler - Heat added at constant pressure. 2-3: High-Pressure Turbine (HP) - Isentropic expansion. 3-4,5: Stream Splitter - A tee in a pipe. 4-9: External System - Uses heat. 5-6: Low-Pressure Turbine (LP) - Isentropic expansion. 6-7: Condenser - Heat rejected at constant pressure. 7-8: Pump #1- Isentropic compression 8,9-10: Mixing Chamber 10-1: Pump #2- Isentropic compression

•In power plants and chemical plants, there are many uses for steam :* It can be used to provide heat for processes * it can be used to drive pumps. In the process diagram above, we have lumped all

of these external energy requirements into the circle labeled “System”.•Most of the uses for steam in this external system do not require valuable, expensive high-pressure steam.•So, instead of sending the superheated steam in stream 2 to the external system, it is a more efficient use of energy to use stream 2 to produce work in the high-pressure turbine.•Then, the needs of the external system can be met by diverting a portion of the cooler less valuable steam in stream 4 to the external system.•When the steam returns from the external system it is usually a saturated liquid, but it is still at the discharge pressure of the high-pressure turbine.•So, the condenser effluent must be pumped up to this pressure before the streams are combined. This helps avoid irreversibility's due to mixing streams at very different temperatures and pressures.•The cycle is completed by pumping stream 10 back up to the pressure in the boiler.•This cycle will be easier to understand when you see it on a TS Diagram.

USEFUL NOTES

However, it is often not efficient to use the very valuable heat from the high temperature reservoir in the external process.

It is often more efficient to use the heat from the high temperature reservoir to generate power and then use less valuable heat that is available at a lower temperature (stream 4) as the heat source for the external system.

Cogeneration does not improve the efficiency of the ideal Rankine Cycle

TS Diagram for Cogeneration

Notice that streams 8 & 9 are mixed at the same pressure to form stream 10.•This provides the fringe benefit of increasing the temperature of the boiler feed.•This reduces the irreversibility of the heat transfer in the boiler just as in the regenerative Rankine Cycle.• cogeneration does not improve the efficiency of the power cycle, but it can improve the efficiency of an entire plant by reducing the lost work in the external system.•The key to this improvement is providing steam that is just hot enough to do the job in the external system and not wasting hotter more valuable high-pressure steam.

Application examples

A car engine becomes a CHP plant in winter, when the reject heat is used for warming the interior of the vehicle.district heating systems of cities,

hospitals, oil refineries, paper mills, wastewater treatment plants, thermal enhanced oil recovery wells and industrial plants with large heating needs.Thermally enhanced oil recovery(TEOR) plants often produce a substantial amount of excess electricity. After generating electricity, these plants pump leftover steam into heavy oil wells so that the oil will flow more easily, increasing production.

In the United States, Con Edison company distributes 30 billion pounds of 180 °C steam each year through its seven cogeneration plants to 100,000 buildings in Manhattan—the biggest steam district in the world. The peak delivery is 10 million pounds per hour (corresponding to approx. 2.5 GW) This steam distribution system is the reason for the steaming manholes often seen in "gritty" New York movies.