How to solve the slagging of biomass boilers?
Biomass boilers adopts biomass wastes or biomass pellets as fuel. In the burning of pellet fuels, the volatiles turn into gas and have complicated reaction with smoke and dust, coagulate and adhere to the heating pipe surface. The slagging and ash deposit will affect the operation and safety of boilers. In this article, I will analyze the features of slagging, find out the reason of biomass boiler slagging, and put forward effective solutions.
Reasons for biomass boiler slagging
The slagging of biomass boilers refers to the ash generated in the burning of biomass fuel. At high temperature, the ash is in liquid or soft solid status. Once they touch the heating surface, they would adhere to it and form into slag.
Biomass fuel composition and environmental medium
The ash melting point is affected by the composition of biomass fuel and environmental medium. If there are incompletely combusted fuels, they will make the environmental medium weakly reductive, and lower the melting point. Under this condition, the ash are easy to attach to the hearth and the wall of superheater. If the biomass fuel has high moisture content, the steam will soften potassium, which is the major content in ash.
Impurities in biomass fuels cause slagging
A large part of biomass fuels are made by more than 1 raw material. Sometimes they are mixed with dirt and sand. These impurities changed the composition and melting temperature of biomass fuels. With high ash and alkali metal content, these impure biomass fuels are easier to cause slagging.
Insufficient air in boilers for burning
Sometimes the biomass fuels are distributed unevenly in boilers. They become high temperature combustion mass and burn with insufficient oxygen, which will cause slagging. Insufficient ventilation quantity will increase the slagging degree. Therefore, to avoid biomass boiler slagging, you should control the proportion of fuel and air.
You can adjust the air proportion by removing some fuels. Besides, slagging may arise from boiler problems. The feeding device can cause fuel stacking and aggravate slagging.
Uneven heat distribution of boiler surface
The boiler inside temperature and heat distribution have great impact on the slagging. As experience shows, slagging usually appears in flue and superheater surface. When the liquid or soft solid ash move toward the heating surface, it don’t have enough time to cool down, so it is easy to adhere to the heating surface and accumulate. As researches proved, the higher the boiler temperature, the more slagging. Slagging affect the heat exchange of boiler surface, block the flue, increase the smoke flow speed, and aggravate the heating surface wear.
How to solve the slagging of biomass boilers?
Now we know the slagging of biomass boilers arises from the biomass fuel properties and boiler design. To reduce the slagging, we should choose high quality biomass pellets and high-efficiency boilers, and control the feeding speed. But you can’t eradicate slagging completely. You need to clean the biomass boilers regularly.
Steam soot blower
Traditionally, they use steam soot blower to remove slagging in boilers, but the cleaning effect is barely satisfactory. They have to wash the boiler with high-pressure water.
The potassium in biomass fuel generate silicon in burning and lower the softening temperature to 800℃, which is in the range of superheater. So the soften ash is easy to attach to the heat wall and hard to be removed by steam soot blower.
Besides, steam soot blowers has following disadvantages. The boilers cleaned by steam soot blower have to wait for 15 days for next use. The purging area is limited, with some dead angles. As the soot blowing interval is long, there are too much ash deposited in the heating surface. They may sintered and harden, and increase the cleaning difficulties.
defocusing inhibitor and pulse gas ash blower
Nowadays, many biomass boilers adopt defocusing inhibitor and pulse gas ash blower to solve biomass boiler slagging and achieved remarkable effect.
Defocusing inhibitor is a fuel additive with high melting point and combustion improver. When it is sprayed into the hearth, it will mix with fly ash and stick to the half-melt ash. It will change the melting point of ash and destroy slagging. It can also form metal film on the pipe surface to prevent acid dew point corrosion.
Pulse gas ash blower can clear the ash deposit on the boiler heating surface by purging, acoustic fatigue, thermal cleaning and local vibration. At last, the gas flow take away the ash.
Choose the best way to solve slagging problem
To use high quality biomass fuels in boilers can reduce slagging, but the cost will increase. If you have to use low quality biomass fuels, which has high moisture and impurities content, slagging is inevitable. To ensure the stable running of boilers, you can combine defocusing inhibitor and pulse gas ash blower to effectively solve the slagging problem.
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