Combustion Processes

Question 1

What are the fundamentals of combustion?

Answer 1

Combustion is a chemical process.
Combustion fundamentals can be grouped into two main categories
1. Combustion technology: burners,flames,boilers,oxy-fuel technology,combined heat and power ,turbines and generators, fluidized beds ,domestic :stoves etc, transport:engines.

2. Emissions: CO,SO2,NOx,soot,particles,organic compounds.

Question 2

Is this statement true or false:75% of the energy used in the world is from combustion.

Answer 2

True

Question 3

What are the principles of combustion ?

Answer 3

Energy transition : chemical -heat-mechanical-electrical.

Total reaction: CxHy+O2=CO2+H2O+Heat.

Fuels: gas, liquids , solids
Combustibles : C,H,S.

Question 4

What is the combustion process?

Answer 4

Air and fuel are introduced into a combustion chamber and the products from that:
1. Loss of heat ,
2. ehxaust: ( CO2,NOx,SO2,HCL (hydrochloric acid gas), inorganic compounds, hydrocarbons (methane),and soot.
3. Ash/slag : silicate slag, Al, K, Ca, phosphates.

Question 5

What the types of fuels that are common?

Answer 5

Fuels can be grouped into three main categories:
Solids: coal,coke,wood, peat, waste…
Liquids: diesel, gasoline, ethanol,methanol.…
Gas: methane, propane, hydrogen,natural gas …

Question 6

What does too much water in combustion cause?

Answer 6

Too much water in combustion disturbs combustion by making it slower because water has a very high specific heat capacity.

Question 7

What are the two sure products of combustion?

Answer 7

CO2 and H2O

Question 8

Why K,Ca, Al, phosphorus etc. be found as a byproduct of combustion?

Answer 8

This is because they can be found in gasoline, coal and biomass. They can be found here because these are fuels that are found in the earth and were formed from the fusion of many other elements. Eg biomass needs nutrients in order to grow and some of these include the elements stated above.

Question 9

What are non combustible fuel components?

Answer 9

Water (can be evaporated with heat )
Ash

Question 10

What is the ash content in various fuels?

Answer 10

Oil = 0.01- 0.3
Coal = 5- 15
Wood = 1-5
Peat =0.5 - 6
Waste = 12 - 30
Straw = 3-6

Straw has a higher ash content than wood because it grows much quicker and hence it requires a lot of elements in other to grow. Any thing that grows or has a lot of elements has ash content.

Question 11

State some characteristics about ash

Answer 11

It’s important for furnace conditions because of heat retention .

Negatives
Combustible material may get trapped in ash reducing efficiency.
Molten ash (abrasive and causes damage to surfaces).
May form deposits on heat exchange surfaces.

Question 12

What are some of the characteristics of a good combustion?

Answer 12

Good mixing of fuel and oxygen (allows for completely combustion )
Good fuel / air ratio (allows for optimum combustion)
Stable ignition and operating conditions(consistent and reliable combustion)
Reasonable regulation range
Low emissions and low noise.
Long enough residence time in combustion to ensure combustion has ended before exhaust gas leaves the chamber.

Question 13

What is choice combustion chamber dependent on?

Answer 13

This is dependent on fuel characteristics
I.e if it is a solid , liquid or gas fuel.
The volatility of the fuel
The water content
The ash content
Heating value
Solid particle size (for solid fuels)

You must also consider the effective transformation of heat to mechanical work ( to maximize energy conversion efficiency) and corrosion(addressing potential damage to equipment).

Question 14

How solid fuels burnt?

Answer 14

In the case of coal

Ignition-release of volatiles-burning-cooling after burning-formation of coke particles .
The temperature increases as you keep going down the line.

The volatile are released to ensure a controlled and efficient burning process and to also get get rid of some emissions .

You can also devolution before burning separating volatile which burn in a homogeneous combustion and char which burns in a heterogeneous combustion

Question 15

Which fuels easily meet good combustion requirements ?

Answer 15

Gases is the best fuel followed by liquids.

Question 16

Why are solids not considered good fuels ?

Answer 16

Because they have to undergo drying and gasification before combustion begins

Question 17

What are combustion efficiencies of oil (liquid), gas and coal (solid).

Answer 17

Oil and gas =99%
Coal = 95%

Question 18

TECHNOLOGY 1
Explain how coal is pulverized and converted into electricity in the pulverized coal fired boiler.

Answer 18

The coal is mashed into powder and moved into the combustion chamber , and heat is applied. The heat then heats up a steam boiler containing hot water which then releases hot steam onto turbines that convert mechanical energy into electricity.

Question 19

What is necessary for any steam boiler to function ?

Answer 19

For any steam boiler to function properly, there must be hot and cold water. The temperature difference is what ensure efficiency in converting mechanical energy into electricity

Question 20

Why is a repeater usually located in boiler steamer combustion technology?

Answer 20

It removes moisture and reheats steam that escapes from the 1st
Level of turbines

Question 21

What is the function of a steam turbine?

Answer 21

It converts heat energy to mechanical energy then the mechanical energy from the turbine is converted to electricity with the help of generator connected to them

Question 22

Do steam turbines have cooling mechanisms?

Answer 22

Yes they do they have cooling chambers /condensers that condense and cool the vapor the has lost its energy back into liquid, the cooled water is then move back into the system to be reused

Question 23

TECHNOLOGY 2
combustion of solid particles (coal , waste or biomass) in a grate fired boiler.

Answer 23

Solid fuels are placed on a grate dried and it is injected from the bottom with oxygen, then the fuel is ignited(causes pyrolysis )releasing volatile substances into a burnout chamber. As the temperature increases hot gases transfer energy to steam which turns turbines to produce electricity. 25% less efficient that combustion of pulverized coal because of ash content and drying.

Question 24

TECHNOLOGY 3

Fluidized bed combustion

Answer 24

This is the combustion of fuels in a fluidized bed of solid material (eg. Sand). When mixed properly with enough air the solid fuel becomes and moves like a fluid . Used for a much more diverse range of fuels like sewage and sludge.

Question 25

TECHNOLOGY 4
Combined generation of heat and power (cogeneration)

Answer 25

Where a electricity and heat are produced simultaneously. Efficiency here is really high about 90% because it requires less fuel , has minimal energy losses because it captures the heat lost during electricity generation and passes it on for heating of homes.
, etc.

Question 26

TECHNOLOGY 5
Oxyfuel combustion

Answer 26

This combustion using pure oxygen instead of air. Here nitrogen is removed leaving oxygen to be used in the process with a steam boiler. Sulphuric is removed This causes the by products to be only CO2and H2O. CO2 can be captured , water can be condensed and reused.
Disadvantage :uses a bit more energy to separate oxygen from nitrogen.

Question 27

Domestic combustion for cooking and heating

Answer 27

Has very low efficiency because the smaller the scale of combustion the less efficient
Has higher emissions

Question 28

Transport : Gasoline and Diesel engines.

Answer 28

Diesels have les CO than gasoline but mor NOx and particulate matter. Fuel is converted into mechanical energy through a series of explosions. The are cylinders that move up and down take up fuel mix it with oxygen ignite it with a spark plug producing mechanical energy that move the pistons and inturn move the wheel.
Mechanical efficiency is usually around 30-35% with gas engines and up to 45% for large gas engines.