Power Basics

Power Basics Rosemount Inc.Power Basics Rosemount Inc.

Power Industry Sales Training Page 1

© 2000 Rosemount Inc. For Internal Use Only

Power Basics

INTRODUCTION

The electrical power industry is changing rapidly. Deregulation and economic and environmental

pressures are creating significant sales opportunities for process control instrumentation in the power

industry.

Large utility power companies are upgrading and modernizing to meet environmental guidelines.

Small power plants are being built to serve local markets. Many process industries (e.g., chemical, pulp

and paper, refining, and food and beverage) are generating their own power and using the surplus heat

for other purposes such as heating office buildings or sterilizing equipment.

All of these changes require the purchase of process control instruments and systems. To help you take

advantage of these new sales opportunities, this training provides you with a basic understanding of the

power industry.

PERFORMANCE OBJECTIVE

After completing this training, you will be able to recognize key measurement systems in the power

generating process and identify opportunities to bring Rosemount technical specialists into the sales

process to recommend ways to apply Rosemount® products to help customers achieve their operational

goals.

Rosemount Inc. Power Basics

Page 2 Power Industry Sales Training

For Internal Use Only © 2000 Rosemount Inc.

POWER BASICS

Steam power generation involves four basic steps:

1. Heat is produced.

2. The heat transfers to liquid water to produce steam.

3. Steam drives a turbine that turns a shaft connected to a generator.

4. The generator converts mechanical energy to electricity.

From a process measurement and control perspective, these four steps are parts of two interrelated

processes:

q Steam and Water Process

q Fuel, Air, and Flue Gas Process

The Steam and Water Process

A boiler is basically a set of tubes through which water passes. The tubes surround a burner that

converts the water to steam. Converting water to steam is called evaporation.

As evaporation begins, water and steam exist together. The steam and water are separated in the boiler

steam drum. The water is recirculated into the water tubes, and the steam, called saturated steam, is

routed to another set of tubes that is suspended over the burner. This set of steam tubes is called a

superheater.

The energy added to the water and steam in the system is measured in units of energy per unit of

weight of water or steam. The energy in the water or steam is called enthalpy and is represented by the

letter h. Enthalpy is expressed in British thermal units per pound (Btu/lb) or kilojoules per kilogram

(kJ/kg). The temperature and the enthalpy of the steam are greatly increased in the superheater.

Industry Driver:

Engineers regularly schedule times to shut parts of the power plant down for

maintenance. Since these maintenance shutdowns are very expensive, engineers can

save money by lengthening the time between shutdowns. A two-year interval between

shutdowns is common; but, because many Rosemount transmitters operate accurately

without recalibration for longer periods of time (up to 5 years), engineers can save

money by upgrading to Rosemount devices. Ask plant managers when the next

shutdown is scheduled so that you can help engineers plan, design, and budget for

instrument upgrades (note that designing and budgeting may require a lead-time of up

to 2 years).




The finished product of the boiler is called dry superheated steam. The dry superheated steam exits the

boiler through a pipe called the main steam header. In a power plant, the main steam header leads to a

series of turbines.

Some of the steam is extracted from the turbines at particular stages. The extraction steam is sent to

several places, including:

q To a reheater, which is another type of superheater in the boiler

q To heaters used to preheat water coming to the boiler

q To a pump that pumps water to the boiler

Reheated steam has a lower pressure than steam in the main steam header. As a result, the reheated

steam is routed to specially designed intermediate- or low-pressure turbines. Usually, all of the

turbines are aligned and drive the same generator rotor.

As the steam passes the final stage of the low-pressure turbine, the steam is cooled in a condenser and

converted back into liquid water. Because the liquid water occupies less space than it did as steam, the

condensation creates a vacuum. The vacuum helps provide energy to pull steam through the turbine

system.

The condensed water collects in the hotwell of the condenser. The water is then pumped back into the

boiler as feedwater. Because some steam and water are lost in the system, additional water, called

make-up water, is added as needed.

Ultimately, the efficiency of a power generation system is measured by the amount of fuel burned

versus the amount of electricity (measured in megawatts [MW]) produced. The balance between the

energy used and the electricity generated is called the heat rate. Improvements in process control

instrumentation can provide more accurate information for use in heat-rate monitoring and

improvement.

Industry Driver:

Power plant equipment budgets are driven by five main goals:

1.Improve the efficiency (heat rate) of the boiler or turbine cycle.

2.Reduce or predict maintenance and extend the life of the equipment.

3.Prevent outages, trips, or unit restrictions.

4.Comply with environmental regulations.

5. Provide marketable ancillary services (e.g., voltage support, load following, quick

peaking).

Rosemount Inc. Power Basics

Page 4 Power Industry Sales Training

For Internal Use Only © 2000 Rosemount Inc.

The Fuel, Air, and Flue Gas Process

Fuel is mixed with air and ignited in the combustion chamber of a boiler. The resulting flame and the

hot exhaust gases are used to transfer heat to the water in the boiler tubes, turning the water to steam.

The exhaust, or flue gas, is then cleaned and processed before it is released through a chimney stack to

the atmosphere.

The amount of electricity a power generation plant must produce is called the load. As the load

increases, more steam is required to drive the generator to meet the load demand. Because more steam

is required, more fuel and air must be fed into the boiler to heat the water and create steam.

The mixture of fuel and air must be precisely controlled to maintain safe and optimum combustion. If

the mixture contains too much air, the heat created by the burner will be absorbed by the excess air

instead of being available to heat water, making the system wasteful and expensive to operate. If the

mixture contains too much fuel, the unburned fuel may accumulate in the boiler and eventually ignite

all at once, causing a dangerous explosion.

Fuel and air are controlled variables that are manipulated to meet steam demand. Steam demand is

monitored by a pressure measurement in the main steam header. When steam demand rises, more

steam is used and pressure drops. When steam demand falls, less steam is used and pressure increases.

Fuel and air flows are increased or decreased to meet demand.

In many cases, a fossil fuel such as oil or powdered coal is carried into the combustion chamber in a

stream of air. The air that is initially mixed with the fuel is called primary air. The primary air passes

through a heat exchanger before it mixes with the fuel. In the heat exchanger, which is called an air

heater, hot flue gases from the furnace transfer heat to the primary air. The air-heating process greatly

increases the heat efficiency of the system by capturing heat that would otherwise be lost to the

atmosphere.

Efficient combustion in a boiler requires more air than is supplied by the primary air. Additional air,

called secondary air, is blown into the combustion chamber to help the fuel burn completely.

Secondary air also passes through the air heater and is warmed before being blown into the combustion

chamber.

Combustion produces hot gas. The hot gas produced by combustion in a boiler is not simply sent up a

chimney. Every effort is made to extract heat energy from the gas before it leaves the system as

exhaust. To minimize damage to the environment, the gas—called flue gas—is processed extensively.

The flue gas is extracted from the boiler by a fan that produces suction. The fan is called an induced

draft fan. To extract heat from the flue gas, the gas is typically routed upward, past the waterwall riser

tubes of the boiler. Superheater tubes are suspended directly above the burner flame, and the hot gas

heats the superheater. Often, the flue gas is then routed downward and past the set of tubes, or platen,

of the boiler’_ reheater. The flue gas continues downward and provides heat for feedwater entering the

boiler through a heat exchanger, called the economizer. The flue gas is then drawn to the air heater to

heat the primary and secondary air. In some systems, part of the flue gas may be mixed with the

secondary air to reduce the proportion of oxygen in the secondary air supply. Mixing flue gas with

secondary air is a combustion control technique and also a method of cooling the burner flame. A

cooler flame produces less nitrogen oxide and nitrous oxide (NOx), an air pollutant.




By the time the flue gas leaves the air heater, the gas has given up a significant amount of its heat

energy. However, the flue gas may contain a great amount of soot, particularly if the power plant uses

coal or oil as fuel. The soot is called fly ash. Power plants draw the flue gas into a fly ash removal

system to remove the fly ash from the flue gas. Fly ash removal systems use one of several

technologies to accomplish their purpose, including:

q Electrostatic precipitators—use static charges to capture fly ash (most commonly used in power

utility plants)

q Baghouse systems—draw the flue gas over and around fabric baffles to capture fly ash

q Cyclone systems—cause the flue gas to swirl, removing the fly ash by centrifugal force

After removing fly ash, power plants must reduce the level of sulfur dioxide (SO2), another air

pollutant. To reduce the level of SO2, power plants process flue gas in a system called a scrubber.

Typically, the scrubber sprays an alkaline solution through the flue gas. The droplets of solution

chemically bind the SO2 in the flue gas and carry it to the bottom of the scrubber. The product

produced from the chemical binding of SO2 and the alkaline slurry can be used in certain

manufacturing processes.

After leaving the scrubber, the flue gas is routed to the chimney stack to be released into the

atmosphere. Power plants monitor the flue gas inside the stack to ensure that environmental standards

are met and that the combustion process is operating efficiently.

Industry Driver:

NOx reduction is an important goal of power plants, both to preserve the environment

and to comply with environmental regulations.

Industry Driver:

The cost of replacing old transmitters (more than 10 years old) with newer technology is

paid back in cost savings after approximately one year. New transmitters allow

engineers to control processes in the power plant more closely within setpoints than

engineers can using older technology because today’_ transmitters detect process

variable changes far more quickly than older technology.

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Power Basics