Combustion Flashcards
What are the three T’s of combustion
Time- contact between fuel and air
Temperature- Elevated temperature
Turbulence- thorough mixing of air and fuel
What is incomplete combustion?
Not burning all of the fuel
Why is sulfur in fuel undesirable
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
Define perfect, complete, and incomplete combustion
Perfect- burning all fuel using theoretical amount of air
Complete- burning all fuel using excess air
Incomplete- not all the fuel is burned
What is meant by excess air
Air supplied to the combustion process above that of theoretical air needed
What two methods are used to analyze flue gas
Using an orsat analyzer or bacharach fyrite, or CEM system (continuous emission monitoring)
Excess air is important to ensure combustion. Why then should it be minimized
To much excess air will result in diluted carbon dioxide meaning the burner is not burning all the fuel completely
In a boiler design, what is draft and what is its purpose
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
What is draft loss? In what units is it normally defined
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).
Define boilers that are balanced draft as compared with those that are pressurized. Provide the advantages and disadvantages of each
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.
Define the various classifications of fuel oils. Provide examples of those oils which are used as a fuel oil
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
What is a continuous-emission monitoring (CEMS) system
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
List and discuss the objection to sulfur in fuels
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
Discuss the four most important losses which occur in the operation of a power boiler
Heat out the stack, Fuel Consumption, Water consumption and
What losses might be taking place even when the stack is clear
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
Maximum amount of excess air when burning gas
10%
What are combustibles in fuel
Carbon, hydrogen and Sulphur
Induced and forced draft fans are examples of
Mechanical draft
At what temperature is #6 oil atomized
180F
6 oil should be heated prior to_____?
Recirculating, pumping and atomizing
Flame impingement occurs when the flame
Comes in contact with boiler surfaces
Stack temperature has sudden increase. What could cause this
Broken baffle
Spalling
is the erosion of internal refractory
SO2, CO2, H20, NO2
Sulphur Dioxide, Carbon Dioxide, Water, Nitrogen Dioxide
When can you exceed the flash point of oil
Flash point should only be exceeded at the burner
Furnace heat release for fire tube and water tube boilers
Fire tube 15000 - 25000 BTUS
Water tube 50000 - 125000 BTUS
Name 3 ways to determine the heating value of fuel
Bomb Calorimeter
Ultimate analysis (lab) + dulongs formula
Proximate analysis + graph
How do we determine heating value of fuel using ultimate analysis
Send sample of fuel to lab, the fuel is tested for NOCASH in laboratory and results are plugged into dulongs formula
How do we determine heating value of fuel using proximate analysis
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
How is efficiency in boilers determined
- 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)
Name 3 ways to determine efficiency in boilers
1 A formula
2 Boiler heat balance
3 Flue gas analysis
