Combustion

What is Combustion?

Combustion takes place when fuel, most commonly a fossil fuel, reacts with the oxygen in air to produce heat. The heat created by the burning of a fossil fuel is used in the operation of equipment such as boilers, furnaces, kilns, and engines. Along with heat, CO2 (carbon dioxide) and H2O (water) are created as byproducts of the exothermic reaction.

By monitoring and regulating some of the gases in the stack or exhaust, it is easy to improve combustion efficiency, which conserves fuel and lowers expenses. Combustion efficiency is the calculation of how effectively the combustion process runs. To achieve the highest levels of combustion efficiency, complete combustion should take place. Complete combustion occurs when all of the energy in the fuel being burned is extracted and none of the Carbon and Hydrogen compounds are left unburned. Complete combustion will occur when the proper amounts of fuel and air (fuel/air ratio) are mixed for the correct amount of time under the appropriate conditions of turbulence and temperature.

Although theoretically stoichiometric combustion provides the perfect fuel to air ratio, which thus lowers losses and extracts all of the energy from the fuel; in reality, stoichiometric combustion is unattainable due to many varying factors. Heat losses are inevitable thus making 100% efficiency impossible.

In practice, in order to achieve complete combustion, it is necessary to increase the amounts of air to the combustion process to ensure the burning of all of the fuel. The amount of air that must be added to make certain all energy is retrieved is known as excess air.

In most combustion processes, some additional chemicals are formed during the combustion reactions. Some of the products created such as CO (carbon monoxide), NO (nitric oxide), NO2 (nitrogen dioxide), SO2 (sulfur dioxide), soot, and ash should be minimized and accurately measured. The EPA has set specific standards and regulations for emissions of some of these products, as they are harmful to the environment.

Combustion analysis is a vital step to properly operate and control any combustion process in order to obtain the highest combustion efficiency with the lowest emissions of pollutants.


Objective of Combustion

The objective of combustion is to retrieve energy from the burning of fuels in the most efficient way possible. To maximize combustion efficiency, it is necessary to burn all fuel material with the least amount of losses. The more efficiently fuels are burned and energy is gathered, the cheaper the combustion process becomes.


Complete Combustion

Complete combustion occurs when 100% of the energy in the fuel is extracted. It is important to strive for complete combustion to preserve fuel and improve the cost efficiency of the combustion process. There must be enough air in the combustion chamber for complete combustion to occur. The addition of excess air greatly lowers the formation of CO (carbon monoxide) by allowing CO to react with O2. The less CO remaining in the flue gas, the closer to complete combustion the reaction becomes. This is because the toxic gas carbon monoxide (CO) still contains a very significant amount of energy that should be completely burned.


Stoichiometric Combustion

Stoichiometric combustion is the theoretical point at which the fuel to air ratio is ideal so that there is complete combustion with perfect efficiency. Although stoichiometric combustion is not possible, it is striven for in all combustion processes to maximize profits.

Fuels

There are many fuels currently used in combustion processes throughout the world, the most common are: Coal, Oils (#2, # 4, and # 6), Diesel Oil, Gasoline, Natural Gas, Propane, Coke Oven Gas, and Wood. Each fuel has different chemical characteristics including, a unique C/H2 ratio, and calorific value, among others. The amount of combustion air required to completely burn a specific fuel will depend on those characteristics especially the C/H2ratio. The higher the carbon in the fuel the more air is required to achieve complete combustion. When monitoring the efficiency of a combustion process, it is important to know the fuel being burned since this information will help not only determine a boiler’s optimal working conditions but also maximize the boiler’s efficiency.

 

Effect of burning different fuels

Coal

There are many varieties of coal being used in combustion processes around the world; the most widely used are anthracite, bituminous, sub-bituminous, and lignite. When burning coal a considerable amount of carbon dioxide is generated given the extremely high levels of carbon in coal; since carbon requires more oxygen to burn, more combustion air is needed to burn coal that other fossil fuels.

In addition to the carbon dioxide emissions, coal burning creates some other pollutants including NOx, sulfur dioxide (SO2), sulfur trioxide (SO3), and particle emissions. Sulfur dioxide chemically combines with water vapor in the air to produce a weak form of sulfuric acid, one of the main causes of acid rain.

Oil

Oil fuels are mostly a mixture of very heavy hydrocarbons, which have higher levels of hydrogen than those found in coal. At the same time, oil contains less carbon than coal and therefore requires less combustion air to achieve complete combustion. Therefore, burning oil releases less carbon dioxide than burning coal, but more carbon dioxide than burning natural gas. Most of the pollutants produced when burning coal are also a byproduct of burning oil.

Natural Gas

Natural gas requires much less air in combustion because of its relatively low amounts of carbon and high amounts of hydrogen. The burning of natural gas is cleaner than the burning of oil and coal. When gas is burned with insufficient combustion air some volatile hydrocarbons can be created, which could become a safety hazard; care should be taken to avoid dangerous conditions.

The burning of natural gas produces less greenhouse gases, which are believed to be one of the main sources for global warming. In equivalent amounts, burning natural gas produces about 30% less carbon dioxide than burning oil and 45% less carbon dioxide than burning coal.

In addition to the carbon dioxide emissions, gas burning creates NOx emissions, while the emissions of sulfur dioxide (SO2) and Particles are negligible.

Other fuels including wood, diesel, gasoline, propane, butane, bio fuels such as ethanol, etc. have there own combustion properties that will affect the combustion efficiency and emissions of the process.

 

 

Air Flow


Maintaining appropriate airflow during combustion is fundamental to ensure safe and complete combustion. The total airflow includes combustion air, infiltration air, and dilution air.

Combustion Air
Combustion air is the air that is used to actually burn the fuel. Without combustion air, which is normally forced into the furnace, combustion is impossible. Infiltration Air Infiltration air is the outdoor air that is not deliberately in the boiler. Sources of infiltration air maybe cracks or leaks. Dilution Air Dilution air is the air that combines with the flue gases and lowers the oncentration of the emissions. There are two types of dilution air, natural and induced (artificially created).

Time, Temperature and Turbulence

The combustion process is extremely dependent on time, temperature, and turbulence. Time is important to combustion because if a fuel is not given a sufficient amount of time to burn, a significant amount of energy will be left in the fuel. Too much time to burn on the other hand will produce very long flames, which can be a function of bad mixing. The correct balance of time and mixing will achieve complete combustion, minimize flame impingement (boiler maintenance hazard), and improve combustion safety. In addition, a properly controlled combustion process strives to provide the highest combustion efficiency while maintaining low emissions of harmful gases.

Excess Air

In order to ensure complete combustion, combustion chambers are fired with excess air. Excess air increases the amount of oxygen and nitrogen entering the flame increasing the probability that oxygen will find and react with the fuel. The addition of excess air also increases turbulence, which increases mixing in the combustion chamber. Increased mixing of the air and fuel will further improve combustion efficiency by giving these components a better chance to react. As more excess air enters the combustion chamber, more of the fuel is burned until it finally reaches complete combustion. Greater amounts of excess air create lower amounts of CO but also cause more heat losses. Because the levels of both CO and heat losses affect the combustion efficiency, it is important to control and monitor excess air and the CO levels to ensure the highest combustion efficiency possible.

air

Calculating Excess Air

As discussed earlier, under stoichiometric (theoretical) conditions, the amount of oxygen in the air used for combustion is completely depleted in the combustion process. Therefore, by measuring the amount of oxygen in the exhaust gases leaving the stack we should be able to calculate the percentage of excess air being supplied to the process.

The following formula is normally used to calculate the excess air:

air


FUEL TYPE OF FURNACE EXCESS AIR %
Pulverized Coal Partially Water Cooled Furnace 15-40%
Coal Spreader stoker 30-60%
Coal Underfeed Stoker 20-50%
Fuel Oil Oil Burners, register type 5-10%
Fuel Oil Multifuel burners & flat-flame 10-20%
Natural Gas Register type Burners 5-10%

What is Draft?

The pressure of the gases in the stack must be carefully controlled to insure that all the gases of combustion are removed from the combustion zone at the correct rate. This draft pressure can be positive or negative depending of the boiler design; natural draft, balance draft, and forced draft boilers are the most commonly used in the industry.

Monitoring draft is important not only to increase combustion efficiency, but also to maintain safe conditions. Low draft pressures create build-ups of highly toxic gases such as carbon monoxide and highly explosive gases. These build ups may take place in the combustion chamber or may even be ventilated indoors creating the risk of injury and death. Conversely, extremely high draft pressures can cause unwanted turbulences in the system preventing complete combustion. Unwanted high draft pressures tend to damage the combustion chamber and heat exchanger material by causing flame impingemen

 


What is a Boiler?

A boiler is an enclosed vessel in which water is heated and circulated, either as hot water, steam, or superheated steam for the purpose of heating, powering, and/or producing electricity. The furnace of the boiler is where the fuel and air are introduced to combust; fuel/ air mixtures are normally introduced into the furnace by using burners, where the flames are formed. The resulting hot gases travel through a series of heat exchangers, where heat is transferred to the water flowing though them. The combustion gases are finally released to the atmosphere via the stack of exhaust section of the boiler.

Utility Boilers
Photograph of a Utility Boiler
Industrial Boilers
Diagram of an Industrial Boiler
Commercial Boilers
Diagram of a Commercial Boiler
 

 

Condensing BoilersDiagram of a Condensing Boiler

A condensing boiler preserves energy by using heat exchangers designed to remove additional energy from the gases of combustion before leaving the stack. The flue gases produced from condensing boilers are at a much lower temperatures than those of non condensing boilers to the extent that the water vapor in the flue gases condenses, thus releasing their latent heat and increasing efficiency of the boiler. Condensing boilers have efficiencies of 95% or greater as compared to the normal 70%-80% for non-condensing boilers.

 

 

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