141

What are the four essential factors for ensuring complete combustion in boiler?

v Adequate quantity of air (oxygen) supplied to the fuel,

v Oxygen and fuel thoroughly mixed (turbulence),

v Fuel-air mixture maintained at or above the ignition temperature (temperature), and

v Furnace volume large enough to give the mixture time for complete combustion (time).

142

What is combustion?

Combustion can be defined as the rapid chemical reaction of oxygen with the combustible elements of a fuel.

Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat.

The original substance is called the fuel, and the source of oxygen is called the oxidizer.

The fuel can be a solid, liquid, or gas.

143

What is a Stoichiometric air-fuel ratio?

Stoichiometric ratio describes the chemically correct air-fuel ratio (theoretical) necessary to achieve complete combustion of the fuel.

144

What is Stoichiometry?

Stoichiometry is a section of chemistry, which involves using relationships between reactants and/or products in a chemical reaction, to determine the desired quantitative data.

145

Write short notes on combustion and excess air?

In theory, to have the most efficient combustion in any combustion process, the quantity of fuel and air would be in a perfect ratio to provide perfect combustion with no unused fuel or air. This type of theoretical perfect combustion is called stoichiometric combustion.

In practice, however, for safety and maintenance needs, additional air beyond the theoretical “perfect ratio” needs to be added to the combustion process which is referred to as “excess air.”

With boiler combustion, if some excess air is not added to the combustion process, unburned fuel, soot, smoke, and carbon monoxide exhaust will create additional emissions and surface fouling.

From a safety standpoint, properly controlling excess air reduces flame instability and other boiler hazards.

Even though excess air is needed from a practical standpoint, too much excess air can lower boiler efficiency. So a balance must be found between providing the optimal amount of excess air to achieve ideal combustion and prevent combustion problems associated with too little excess air, while not providing too much excess air to reduce boiler efficiency.

Generally, the excess air supplied to a utility boiler would be 15-25% of the stoichiometric air requirement.

146

List some possible hazards during steam turbine operation?

Abnormal turbine operating conditions will cause damage to the plant and possibly to personnel if allowed to persist.

The possible hazards during steam turbine operation includes:

v  Over speeding

v  Lubricating oil failure

v  High turbine exhaust pressure (low condenser vacuum)

v  Governor failure

v  Water ingress to blading

v  Thrust bearing failure

v  Excessive vibration

v  Excessive temperature differentials

v  Excessive eccentricity

147

What would happen if the generator gets disconnected from the grid first before the steam valves closes, and vice versa?

If the steam valves fail to close and the generator gets disconnected from the grid, the unit will suffer disastrous consequences due to over speeding of the turbine.

However, if the generator remains connected to the grid when the steam valves fail to close, the turbine will not over speed. The unit can be shut down safely by closing the boiler stop valves.

148

What are Category B trips for a Steam turbine?

v  Governing system failure

v  Lubricating oil failure

v  Over speed

v  Water ingress

v  Manual emergency stop

149

What are the consequences of over speeding of steam turbine?

The consequences of over speed are very serious for the plant and personnel.

Therefore, the protective systems have been designed to prevent over speed.

The turbine governing system protects the unit from over speed.

However, if it malfunctions, a separate over speed tripping system will become activated.

When the generator is connected to the grid, the turbine cannot over speed (the generator is coupled magnetically with the grid).

The possibility of over speed occurs during run-up and when the unit is disconnected suddenly from the load (during a load rejection).

The possibility of over speed is normally reduced by coordinating the opening of the circuit breaker and the closure of the steam valves.

150

Fill in the blanks:

When the steam flow drops below the one required to overcome the friction losses [bearing and windage losses] of a generator, the generator starts to act as a motor. It starts to pull current from the grid to continue running at the operating speed. The circuit breaker now opens on reverse power. This sequence of activities prevents the chance of over speed.