How to solve the problem of easy coking of burning biomass stalks in biomass boilers

Since the ash melting point of the biomass fuel is low, the ash deposit easily adheres to the wall of the furnace and the superheater, which not only affects the heat transfer of the boiler heating surface (according to the data obtained from the test: the thermal conductivity of the ash layer is 0.0581~0.116w/ m2.cC, and the thermal conductivity of the metal wall of the boiler heating surface is 46.5~58.lw/m2·°C, the thermal conductivity difference is 500-800 times), and the coke block and ash block the flue gas passage and increase the flue gas flow rate. The formation of flue gas corridors will aggravate the wear of the heated surface and affect the normal production. Futong New Energy produces and sells biomass boilers . Biomass boilers mainly burn wood pellets and biomass pellets pressed by straw pellets.

1. Analysis of the cause of coking of biomass boiler

Due to the wide variety of fuels in biomass power plants, fuels have high water content (generally above 45%), more impurities (mixed with soil, fine sand), high ash content, high alkali metal content (Table 1), and fuel in the furnace. After internal combustion, it is easy to coke and accumulate on the heating surface of the boiler.

The main factor of coking. Biomass boiler coking mainly refers to the ash produced after the combustion of the fuel. At high temperatures, it is mostly melted into a liquid state or in a softened state. If the ash still remains soft and hits the heated surface, it is bonded to the heated surface due to cooling. On, the formation of coking. There are many factors affecting boiler coking, and it is generally believed that there are mainly:

(l) The ash of the fuel itself and the coking formed after the doping. The main factor affecting the ash melting point is the chemical composition of the ash and the surrounding high-temperature environmental medium. The two interact with each other. Once the boiler is adjusted and not in place, incomplete combustion products will appear, making the surrounding medium weak. Reducibility, reducing ash fusibility and causing coking in the furnace.

At the same time, biomass fuels generally enter the furnace in the situation of blending into mixed fuels, while fuel brokers incorporate a large amount of soil and fine sand into the fuel. The presence of these impurities changes the composition, form and melting temperature of the fuel. Increased coking on the heated surface.

(2) The temperature level of the surface of the heated surface in the furnace. In the case where the ash melting point is constant, the temperature level in the furnace and its distribution become an important factor in whether or not coking occurs. Experience has shown that the coking of the boiler is mostly on the surface of the flue and superheater. The liquid or soft ash particles are subjected to inertia and move to the heated surface. Because of the fast moving speed of the ash particles, the cooling effect is poor, and the molten ash particles are easy. Adhesion, so that the slag layer quickly accumulates and grows up. Temperature has a very important influence on coking in the furnace. Studies have shown that the temperature will increase and the degree of coking will increase exponentially.

2, ash and coke treatment

2.1 Conventional coking treatment method. Early biomass power plants generally used steam soot blowers to coke and clean the heated surface, but from the practical point of view, the decoking requirements were not met. It can only be treated by rinsing with high pressure water after stopping the furnace. Mainly because of the high content of potassium in biomass fuel, its presence reduces the ash melting point, while the silicon element forms a low melting point compound with potassium during combustion, resulting in lower softening temperature of ash, according to experimental data. The deformation temperature of the plant ash is about 800 °C, and the temperature of the furnace superheater of the boiler is mostly within this range. Therefore, under high temperature conditions, the softened ash is easily attached to the outer wall of the heating surface pipe, and it is difficult to use the steam soot blower. The accumulated coke block is processed. According to the past experience, the general boiler using steam soot blower after 15 days of cleaning, the main steam temperature control does not need to use the desuperheating water regulation, the temperature is normally maintained at about 510 0C, after a month of operation, it is necessary to stop the furnace for water washing. Otherwise, the main steam temperature will deviate more and more from the rated value (540 ° C), the efficiency of the boiler will drop, and the exhaust gas temperature will rise by 5-10 ° C left. Moreover, there are the following problems in the use of steam soot blowing: (1) the medium purging area is limited, some dead angles exist, and the flue gas corridor is easily formed, which aggravates local wear; (2) the soot blowing period is long, so that the heated area is excessively gray. Even the ash sintering and hardening increases the difficulty of soot blowing; (3) If the pressure is too high or long-term use, it will accelerate the wear of the metal pipe wall, and the pressure will be too low and affect the soot blowing effect; (4) increase the smoke inside the furnace Gas humidity, low temperature condensation formed at the air preheater, causing serious corrosion of the air preheater tube; (5) high mechanical failure rate and high maintenance cost.

2.2 Discussion on the new method of removing coking. At present, our factory has achieved remarkable results when using the combination of decoking inhibitor and pulse gas soot blowing device on the boiler to treat boiler coking ash.

The decoking inhibitor (SlagTr011508) is a high melting point fuel additive containing a combustion improver that reduces the problem of fly ash deposition on the flue gas side. When it is sprayed, it mixes with the fly ash leaving the furnace and adheres to the semi-melted ash, destroying the coking by changing the melting point of the ash and forming cracks inside the coke, while passing through the surface of the pipe. The formed metal film helps to reduce the acid dew point problem.

In combination with the pulse gas soot blowing device, the dust on the heating surface of the boiler is removed by purging, acoustic fatigue, hot cleaning and local rapping. After the dust is removed, the dust is taken up by the flue gas stream, thereby improving the thermal efficiency of the boiler.

The specific operation method is: during the operation of the boiler, every time the operation value is added, the Skg decoking inhibitor is put into the furnace every time, and is added from both sides of the furnace, and after 30 minutes of dosing, the pulse gas soot is started. Using the method of decoking inhibitor and pulse gas soot blowing device, the main steam temperature of the boiler can be maintained for 2 months, and the ash accumulation and coking phenomenon of the heated surface almost disappears. The exhaust gas temperature can be compared with the previous steam soot blowing. When the device is used, it will be reduced by 3-5 °C. It is estimated that the indirect economic benefits will be about 1 million yuan per year.

3, the conclusion

Due to the large amount of fuel, high impurities and poor quality of biomass power plants, it is impossible to improve in a short period of time. The coking phenomenon of boiler coke is difficult to avoid. In order to improve the operation hours of the boiler and ensure the normal production, the combination of the decoking inhibitor (SlagTr011508) and the pulse gas soot blowing device can effectively solve the problem of boiler ash and coking, which is worthy of reference for the biomass power plant. .