Combustion

Combustion

Heat released by the process of combustion is the main source of heat for most boilers. Some boilers heat water using other means, such as electric boilers, but these types of boilers are not as common as fossil fired boilers.

The combustion process involves burning of a fuel’s hydrogen atoms in order to release heat. As fossil fired boilers and steam are directly related, a look at the combustion process is necessary to better understand how steam is generated. This chapter covers the combustion process and its relevance to steam generation.

Combustion Chemical Reaction

Combustion is the rapid oxidation of fuel, which results in an energy transfer in the form of heat and light. For this lesson, it is necessary to be aware of some common chemical elements and their symbols.

Chemical Symbols Table

Note: Although Oxygen has the chemical symbol ‘O’, it is most common in its chemical composition of ‘O2’ and is often indicated as such.

Fuels are hydrocarbon based and represented by the chemical symbols of CxHy; the values of x and y vary depending upon the fuel. Other substances can be formed when elements bond together, such as when two parts hydrogen (H) bond with one-part oxygen (O) to form water (H20).

The below image shows the chemical reaction that occurs when fuel (CxHy) is burnt with oxygen (O). The reactants of the chemical reaction are fuel (hydrocarbons) and oxygen. The products of the chemical reaction are carbon dioxide (CO2), water (H20) and ΔH.

The symbol ‘ΔH’ (delta H) is the heat of reaction (a.k.a. enthalpy of reaction). ΔH represents the amount of energy released/transferred by the chemical reaction. ΔH varies depending upon how complete combustion is.

Combustion Chemical Reaction

Combustion Requirements

Combustion can only occur if three factors are present:

• Oxygen
• Heat
• Fuel

In the case of a boiler, oxygen is provided by ambient air, which contains approximately 21% oxygen. Predominant boiler fuels are natural gas, fuel oil and coal. Heat can be supplied during ignition, such as when creating an electrical arc to ignite the air/fuel mixture.

A fire triangle (a.k.a. combustion triangle) indicates the requirements of combustion; a fire tetrahedron (shown below) performs a similar purpose.

Fire Tetrahedron

The three combustion factors must also be provided in the proportionally correct manner, otherwise combustion cannot occur. Too much air and too little fuel may prevent combustion, as will too much fuel and too little air.

Perfect, Complete and Incomplete

Boiler combustion is termed perfect, complete or incomplete.

• Perfect Combustion – the exact amount of air is provided in order to achieve complete combustion of the fuel. Perfect combustion is only achievable in laboratory conditions.
• Complete Combustion – the minimum amount of air is provided in order to achieve complete combustion. Boiler operators should always try to achieve complete combustion.
• Incomplete Combustion – too little air is provided and incomplete combustion occurs. 

Complete combustion is essential to avoid a reduction in boiler efficiency and potential safety hazards. When fuel combusts completely, common products of combustion include carbon dioxide (CO2), water vapour (H2O), Nitrogen (N2), sulphur oxide (SOx) and nitric oxide (NOx). If fuel does not fully combust, products of combustion may include carbon monoxide (CO), soot and smoke.

Carbon monoxide is a toxic flammable gas that is less dense than air. Incomplete combustion may lead to generation of carbon monoxide, which is not desired, as it poses a health and safety risk. Other by-products of incomplete combustion include soot and smoke, both of which can reduce heat transfer within the boiler leading to an overall reduction in efficiency.

Primary, Secondary and Excess Air

The amount of air delivered to a boiler for combustion varies depending upon if combustion is complete or incomplete. Its possible to measure the gases of combustion to determine if combustion was complete or incomplete. Too much oxygen in the gases of combustion indicates that too much air for combustion was provided. High levels of carbon monoxide in the gases of combustion indicates too little air was provided and combustion was incomplete.

If fuel is unburnt during combustion, carbon monoxide levels will be high, and more air will need to be supplied by the boiler burner in order to achieve complete combustion. If too much air is supplied during combustion, the levels of oxygen in the gases of combustion will be high, and the amount of air supplied to the boiler burner will need to be reduced.

The total air supplied to the boiler is further categorised as either primary, secondary or excess.

• Primary Air – is mixed with the fuel prior to it reaching the combustion space. Primary air controls the amount of fuel burnt.
• Secondary Air – is added to the combustion space during combustion. Secondary air controls how efficiently fuel is burnt.
• Excess Air – is air added to the combustion process that was not required for combustion. Excess air is secondary air that was not part of the combustion process.
• Total Air – the sum of the air supplied for the combustion process i.e. primary air + secondary air + excess air = total air.

Note: The term dilution air describes the air added to the breeching (space between the boiler and stack) to aid gases of combustion flow from the boiler to atmosphere

Additional Resources

https://en.wikipedia.org/wiki/Combustion

https://www.bbc.co.uk/bitesize/topics/zypsgk7/articles/zcwxcj6#

http://www.auburn.edu/academic/forestry_wildlife/fire/combustion.htm