In the case of any burner system, the total air is divided into primary air, secondary, and tertiary air. Each of this air plays a very vital role in combustion. The primary air gives the initial ignition oxygen, the secondary air allows the higher hydrocarbon or char to burn, and the tertiary ensures the delayed combustion process purposely introduced by designers to reduce NOx level. While this distribution of total air will be normally 30% primary, 55 to 60% secondary, and about 10 to 15% tertiary air. The burners are designed to ensure the intended ratio by actual design of the resistance introduced in each section for the required flow area. All these will be for a set register pressure or windbox pressure depending upon wall firing or tangential firing.
Airflow equalization in multi burners – Coal fired boiler
There are three important factors to be kept in mind when airflow equalization is done among burners.
- The primary air has to be kept at the minimum required level with designed excess air level
- The secondary and tertiary air combined together enter the individual burner, and this needs to be equal between all burners
- The proportioning of secondary and tertiary air is done within the burner and also needs to be equal in all burners but the setting can change slightly depending upon the variation in burners due to manufacturing tolerances.
All burner and boiler designers provide air equalizing dampers at inlet of each burner. This damper or shroud is used to vary the resistance offered by individual burners so as to achieve equal flow of air at all loads. Many burner manufactures nowadays prefer to factory-set the secondary air swirl position and in low NOx burners the most preferred setting is 30 degree open for creating a moderate swirl. This keeps the flame long and reduces the flame temperature. The secondary air flow damper is normally kept full open. The damper or the shroud provided in the burner is all first set to full open to create the required furnace to windbox differential pressure. Some manufactures of burner- provide individual air flow measurement in each burner. This can be used to set equal air flow between burners. In absence of this it is the practice to keep the static pressure equal in all burners after the shroud or damper provided for flow equalization.
After setting equal airflow when the burner is lighted up the flame colour, flame temperature, the overall CO level and NOx levels are checked at all loads (Normally 50%, 60%, 80% and 100%) for proper performance. In case required a minor adjustment can be done after identifying which burner need to be adjusted.
Airflow equalization in multi burners – Oil / Gas fired boiler
Oil and Gas fired boilers are more sensitive to air flow equalization between burners for few reasons.
- These burners are designed for much lower excess air levels
- The primary, secondary and tertiary airs are all proportioned inside the burner unlike coal burner.
- Any minor deviation in proportioning can increase the CO level very high. It can also cause lot of soot formation which has its own effect in boiler performance.
The procedure for equalizing air flow between all burners is the same as coal fired boiler.