4 Combustion

Total reaction

CxHy + O2 —→ CO2 + H2O + heat

Fuels:

Gas: methane, propane, hydrogen, natural gas

Liquid: gasoline, diesel, ethanol, methanol

Solid: coal, peat, wood, waste

Non-combustile fule components:

Water and ash

Ash

- highest in waste and coal, low in oil.

- Important for furnace conditions

- May form deposits on heat Exchange surfaces

- Combustible material may become trapped in the ash --> decreased efficiency

Characteristics of an efficient combustion process

• Good mixing between oxygen and fuel

• Carefull control of fuel/air ratio

• Stable ignition and operating conditions

• Resonable regulation range

• Low emmisions and low noice

• Long enough recidence time in the combustion chamber so that the combustion has terminated before exhuast gas leaves the chamber

Choise of combustion depends on?

• Choise of fuel: Gas, liquid or solid

• Volatility of the fuel

• Ash content

• Water content

• Heating value

• Solid particle size

• Effective transformation of heat to mechanical work

• Corrosion

Technology in practice:

• The combustion requirements are most easy met with gaseous fuels, second liquid

• Solid fuels are more difficult since they have to undergo drying and gasification before combustion takes place

• Gas, oil 99%, coal 95%

• Excess air is needed to ensure that complete combustion takes place. Depends on the mixing of air and fuel

• Excess air decreases the combustion temperature - cooling effect

Plant efficensy

• The efficency is limited by the high heat of vaporization of the working fluid Unless the preassure and temperature reach supercritical levels in the broiler, the temperature range over which the cycle can operate is quite small

• Mot supercritical power plants adopt a steam inlet preassure of 24 Mpa and inlet temperature between 538C and 566C which results in plant efficency of 40%

• If pressure is further increased the power plant is refered to as a ultra-supercritical, and the steam inlett can be incrased to 600C, thus acheiving thermal efficency of 42%

Cogeneration efficency

Combined generation heat and power: efficency up to 90%, compared to 45% for best conversion plants

Oxyfuel combustion advantage and dissadvantage

Advantage: high concentration of CO2 in the exhaust

Dissadvantage: cost energy to sepatate oxygen from nitrogen

Examples of domestic combustion

Open fires

Iron stoves

Tiled stoves

Biomass and pellet burners

• Lower efficency

• Higher emissions

Enviromental effects of diesel engines

less CO2, more pm and NOx

Carbon monoxide

C + O2 --> 2CO

• Formed due to incomplete combustion

• Minimized by control of air/fule ratio, residence time, temperature, or turbulence

Sulfur dioxide

Typically 90-95% of the fuel sulfur realeased to the gas phase during combustion

• Organic sulfur: Chemically bonded to the hydrocarbon matix

• Inorganic sulfur: Imbedded in the coal as loose pyrite och marcasite

• Sources in coal and oil: original living material, seweed sulfates, limestone

Thermal Nox

Only reqires nitrogen in the air to combine with O and OH radicals which are abundant suply in flame

N2 + O --> NO +N

N + O2 --> NO + O

N + OH --> NO + H

• The reactions are highly temperature dependent; the hotter the combustion the more Nox is formed

• The Nox formation rate is also pressure and recidense time dependent

• The exponential dependande on temperature makes reducing the combustion temperature the key stategy to lower Nox

Fuel NOx

• When nitrogen is chemicaly bonded to the fuel essentially all of it converts to Nox in the exhaust!!!

• When gaseous fuels such as natural gas are free of the fuel bound nitrogen, it is often found in liquid and solid fuels

• Fortunatley, refinery processes thet remove sulfur also remove fuel bound nitrogen

Prompt Nox

When thermal and fuel Nox are eliminated, some Nox formation is still observed. Attributed to the raction of atmospheric nitrogen with combustion radicals occurring in hte earilest stages of combustion

CH +N2 --> CHN + N

N + OH --> NO + H

N + O" --> NO + O

CHN + O2 --> NO + CO + H

• The mechanism does not have SIGNIFICANT TEMPERATURE DEPENDANCE. Becomes impotant when other Nox formation mechanisims have been supressed

• Promt Nox cannot be practically reduced, and focus is normally on supressing the two main other Nox mechanisims

Organic compounds

Include volatile, semivolatile and condensable orgaic compounds either pressent in the fuel or formed as a product of incomplete combustion

Particularly common during start-up of a cold combustion unit - ex car starting

Formation of particulate matter

Combustion particles formed from gas precursors:

• At high temperature

- Soot

- Inorganic particles

• At exhaust temperatures:

- Xondensation of sulfuric acid

- Condensation of organic material

Combustion particles formed from solid or liquid precursors:

- Fly ash particles

Soot formation

• Foorms under fuel-rich conditions

• A critical step is the fomation of the first aromatic ring

• Growth proceeds with formation of polycyclic aromatic hydrocarbons

Combustion of Biomas:

• Usually a lager water content than coal

• Lower heating value compared to fossil fuels

• Transportation and handling complicated by the large volumes requred

• May be combusted with coal to reduce CO2 emissions

Combustion of peat

• Freash peat has higher water content

• Has to be dried before combustion

Combusstion of waste

• Very hetrogenous fuel

• Ash with low melting point

• Corrosion

• Large chamber volume to ensure combustion

• Large exess air suply

• Large requirements on filterns to minimize emissions

• Potentially dangerous emissions and expensive cleaning of exhaust