Efficiency is an undeniably important factor while choosing a boiler manufacturer. Many other components contribute to the overall efficiency of the boiler. One of them is the combustion efficiency of a system. It is a well-known fact that the efficiency rated by the boiler manufacturer is more than the actual boiler efficiency. However, combustion efficiency plays a major part in the overall efficiency and can be calculated to ensure optimal and proper combustion in process operations. Combustion efficiency depends on various factors such as the amount of air, heat losses, burners used, etc.
What is Combustion Efficiency?
Combustion efficiency is measured and calculated in percentage to determine the capability of the steam boiler in the burning of fuels. Naturally, every industry aims to achieve 100% combustion efficiency by the complete burning of fuels by the system. But, such a feat is impossible to achieve, and the maximum combustion efficiency found in most cases is between 90% and 95%
Combustion efficiency depends on three major factors:
- The stack gas net temperature
- Percent of carbon dioxide (CO2) and oxygen in volume after the combustion
- The chemical properties of the fuel
Heat Output by Fuel:
The calorific value is further classified, into two terms i.e. Gross Calorific Value and Net Calorific Value
1. Gross Calorific Value:
Gross Calorific Value is the net total energy in the fuel. Most of the commonly used fuel consists of hydrogen that, after burning with oxygen, produces water that passes as steam. Since the flue gases are not condensing in nature, the amount of heat in steam boilers is reduced than actually available. The gross calorific value determines the amount of energy utilized in evaporating the water.
2. Net Calorific Value:
Net calorific value is used in calculating the efficiency of the boiler. Net calorific value is calculated by excluding the energy that gets released in steam from the gross calorific value. In other words,
Net Calorific Value = Gross Calorific Value – Energy Released in Steam
Amount of Air Required for Optimal Combustion:
For proper and complete combustion, a certain amount of excess air is required. However, it is not an easy task to achieve optimal combustion due to the given reasons,
The burner in the furnace may not always operate as required, and the mixing of carbon, oxygen, and hydrogen is not always precise
Sometimes, the oxygen blends with nitrogen and forms nitrogen oxide (NOX)
Precision in the amount of air is important to ensure proper combustion. Failing which might result in,
Incomplete combustion and unburned fuels with carryover and smoke due to less than required air
Cooling of furnace and carryover of useful heat due to excessive heat
1. Heat Losses:
Although the air amount is important, it is not the only factor to ensure the complete combustion in the steam boilers. Heat loss, too, is an essential component that decides combustion efficiency. Let us have a look at the various sources of heat losses.
2. Flue Gases Heat Losses:
Heat loss by flue gases is one of the major losses affecting boiler efficiency. The contributor to the flue gas heat loss is the flue gas temperature that exits the furnace. Simply put, as the temperature of the flue gases increases, the efficiency of the boiler decreases. However, if the flue gas temperature is less than optimal or ‘dew point,’ it may lead to corrosion in the steam boilers.
3. Radiation Losses:
Poor installation or insulation of the steam boilers results in radiation loss. Even a well-insulated boiler loses about 0.3% to 0.5% of its energy. Although it may not seem like a great loss, it is noteworthy that the loss is excluded from the overall efficiency rating ensured by the boiler manufacturer. Radiation loss tends to remain constant even in case of the boiler not being operational.
Burners in Combustion:
Burner Turndown:
Turndown is imperative in burners. In general terms, burner turndown is a ratio calculated by the maximum ignition rate divided by the minimum controllable ignition rate. Burner turndown rate provides important results for efficient and complete combustion, controlling emission that affects the economic decisions of the process plant.
Types of Burners:
Some of the majorly used burners for the combustion process are:
1. Oil Burners:
One of the important factors of liquid fuels is viscosity. In other words, liquid fuels of high temperatures flow easily. It is interesting to note that due to changing temperature, the viscosity of the fuels affects the size of the oil particles formed at nozzles. Effective combustion of oil fuels requires a high area-to-volume ratio. Boiler manufacturers suggest that the ideal particle size is between 20 to 40 μm for effective combustion. Less than 20 μm travel quickly without burning properly, and more than 40 μm might get carried without complete combustion.
2. Gas Burners:
With gas fuels, one has to ensure atomization and proper blending of gas and air to achieve optimal combustion. Gas burners are classified as low-pressure burners and high-pressure burners in which low-pressure burners operate between 2.5 to 10 mbar and the high-pressure burners operate between 12 to 175 mbar.
3. Rotary Cup Burners:
In a rotary cup burner, the fuel is transported through a central tube and released inside a rotating cone. With fuel rotating in the cup, it gets thinner and gets released as a spray. The turndown ratio is higher due to atomization by the rotary cup.
4. Pressure Jet Burners:
Pressure jet burners use pressure to tear droplets from the oil stream and propel it for combustion requiring simple and spill-back nozzles. However, the nozzles chock frequently due to sticky and dirty fuels.
Rakhoh Boilers are a leading name as boiler manufacturers in India and overseas. With more than 38 years of expertise in thermal solutions, Rakhoh manufactures a wide range of highly efficient and reliable industrial steam boilers that ensure effective and complete combustion with different types of fuels.