Combustion

Question 1

What are the three T’s of combustion

Answer 1

Time- contact between fuel and air
Temperature- Elevated temperature
Turbulence- thorough mixing of air and fuel

Question 2

What is incomplete combustion?

Answer 2

Not burning all of the fuel

Question 3

Why is sulfur in fuel undesirable

Answer 3

Sulfur has a low heating value (4050 btu/lb) and contaminates atmosphere with sulfur dioxide. It causes corrosion in flues, economizers, air heaters and ID fans

Question 4

Define perfect, complete, and incomplete combustion

Answer 4

Perfect- burning all fuel using theoretical amount of air
Complete- burning all fuel using excess air
Incomplete- not all the fuel is burned

Question 5

What is meant by excess air

Answer 5

Air supplied to the combustion process above that of theoretical air needed

Question 6

What two methods are used to analyze flue gas

Answer 6

Using an orsat analyzer or bacharach fyrite, or CEM system (continuous emission monitoring)

Question 7

Excess air is important to ensure combustion. Why then should it be minimized

Answer 7

To much excess air will result in diluted carbon dioxide meaning the burner is not burning all the fuel completely

Question 8

In a boiler design, what is draft and what is its purpose

Answer 8

Draft is a difference in pressure between 2 points that causes air or gas to flow. Provides air for combustion and carries the products of combustion over boiler heating surfaces

Question 9

What is draft loss? In what units is it normally defined

Answer 9

Draft loss is when the stack damper opens to allow more air for makeup of air loss when you have increased flow of gases. This is measured in inches of water column (“w.c).

Question 10

Define boilers that are balanced draft as compared with those that are pressurized. Provide the advantages and disadvantages of each

Answer 10

Balanced draft uses both forced and induced draft to control combustion and pressure in a boiler. A fan is located on the outside of burner along with inside between boiler and stack. Forced draft is used for pressurization and located on the outside of the furnance. Balanaced maintains constant pressure and results in greater flow (most commonly used today). Pressurized units are used in large pulverized coal boilers. Problems that occur are flange gas leaks, need for cleaning equipment and increase draft loss.

Question 11

Define the various classifications of fuel oils. Provide examples of those oils which are used as a fuel oil

Answer 11

Residual Oils (ie No. 6 Oil)- Products that remain after more violatile hydrocarbons have been extracted. 
Crude Petroleum (ie gas lube oil/ kerosene)- comes from oil well without subsequent processing. 
Distillate Oils (ie Grade 1 &2 fuel Oil)- obtained by fractional distillation and of a consistency  between kerosene and lubrication oil. 
Blended Oils- Mix of two or all of the above in proportions to meet the desired specifications

Question 12

What is a continuous-emission monitoring (CEMS) system

Answer 12

CEMS systems monitor up to 6 gases at the same time, records history, gas flow, opacity and dust loading. They are used for environmental compliance monitoring

Question 13

List and discuss the objection to sulfur in fuels

Answer 13

Sulfur compounds formed during combustion process raise the dew point of the flue gases, mix with the moisture and corrode metal parts. This results in plugging, tube deterioration of casings, economizers and air heaters. Sulfur compounds also contaminate the atmosphere. The corrosion is reduced by having a higher flue gas which means efficiency loss. If emission limits are exceed a need for scrubbers will be needed. This can be combating by using burner and accessing equipment that lowers excess air

Question 14

Discuss the four most important losses which occur in the operation of a power boiler

Answer 14

Heat out the stack, Fuel Consumption, Water consumption and

Question 15

What losses might be taking place even when the stack is clear

Answer 15

Excess air in combustion process.Improper draft causing incomplete combustion of fuel.Ash build up on tubes which reduces efficiency. Moisture or sulfur in fuel source which has low heat value. High temp out stack with no heat recovery

Question 16

Maximum amount of excess air when burning gas

Answer 16

10%

Question 17

What are combustibles in fuel

Answer 17

Carbon, hydrogen and Sulphur

Question 18

Induced and forced draft fans are examples of

Answer 18

Mechanical draft

Question 19

At what temperature is #6 oil atomized

Answer 19

180F

Question 20

6 oil should be heated prior to_____?

Answer 20

Recirculating, pumping and atomizing

Question 21

Flame impingement occurs when the flame

Answer 21

Comes in contact with boiler surfaces

Question 22

Stack temperature has sudden increase. What could cause this

Answer 22

Broken baffle

Question 23

Spalling

Answer 23

is the erosion of internal refractory

Question 24

SO2, CO2, H20, NO2

Answer 24

Sulphur Dioxide, Carbon Dioxide, Water, Nitrogen Dioxide

Question 25

When can you exceed the flash point of oil

Answer 25

Flash point should only be exceeded at the burner

Question 26

Furnace heat release for fire tube and water tube boilers

Answer 26

Fire tube 15000 - 25000 BTUS

Water tube 50000 - 125000 BTUS

Question 27

Name 3 ways to determine the heating value of fuel

Answer 27

Bomb Calorimeter
Ultimate analysis (lab) + dulongs formula
Proximate analysis + graph

Question 28

How do we determine heating value of fuel using ultimate analysis

Answer 28

Send sample of fuel to lab, the fuel is tested for NOCASH in laboratory and results are plugged into dulongs formula

Question 29

How do we determine heating value of fuel using proximate analysis

Answer 29

Moisture content - A 1 gram sample is placed in an oven for one hour at 200degf to dry (remove moisture) and weighed, difference in weight is the moisture content
Volatile Matter - Sample is placed in a furnace in a covered crucible where temp is maintained at 1700 degf for 7 minutes. Gaseous volatile matter is driven off and sample is weighed again.
Fixed Carbon - The lid is removed from the crucible and the furnace temperature is increased until the sample is completely burned, what remains is the incombustible material or ash. The fixed carbon is determined by subtracting the moisture, Volatile Matter, and Ash from the original sample.
The fixed carbon (%) is plotted on a graph to determine the approximate heating value

Question 30

How is efficiency in boilers determined

Answer 30

• The efficiency of anything is simply output / input
  E=OUTPUT/INPUT
• In boilers the steam output in lbs per hour converted to BTU’S / The input, fuel burned per hour also expressed in BTUS
  Efficiency = BTUS per hr (output) / BTUS per hr (input)
• A more accurate efficiency calculation will take into account that heat is usually added to the feedwater prior to entering the boiler
  Efficiency = Ws(BTUS per lb steam - BTU per lb feedwater / Wf x BTU per unit of fuel

Boiler Heat Balance., Measures losses in a boiler

• Heat loss out the stack (10%)
• Water vapor from burning Hydrogen\ - Unburned fuel in the ash
• Incomplete combustion
• Moisture in the fuel
• Moisture in the combustion air
• Radiation

Flue gas analysis
- What comes out the stack is a direct indication of how efficient the combustion process is we use electronic analyzers and hand held devices (Orsat, Bacharach Fyrite)

Question 31

Name 3 ways to determine efficiency in boilers

Answer 31

1 A formula
2 Boiler heat balance
3 Flue gas analysis