1. Air distribution ratio of biomass boiler:

Under the condition of certain biomass fuel, the uneven distribution of blast in the furnace of the burner and the formation of local high temperature is also the cause of coking in the furnace of the burner. Lowering the blast pressure, adding or strengthening boiler exhaust will also reduce the degree of coking. Therefore, it is very important to choose a suitable air distribution ratio. In addition to the biomass fuel itself and the air distribution ratio of the biomass boiler, the design of the furnace chamber and the feeding speed of the biomass boiler can also cause coking. Therefore, if you encounter coking problems, you need to investigate them step by step. Don't blindly think that it is the cause of pellet raw materials or the problem of biomass boiler. Improper operation can also be an important factor of coking.

2. The ash content of the biomass fuel itself and the coking formed by the doped substances.

Biomass boiler coking mainly refers to the ash produced after the fuel is burned, which is mostly melted into liquid or softened state at high temperature. If the ash is still softened and touches the heated surface, it will stick due to cooling On the heated surface, coking is formed.

The main factor affecting the melting point of ash is the chemical composition of the ash and the surrounding high-temperature environment media. The two influence each other. Once the boiler combustion adjustment is not in place, incomplete combustion products will appear, making the surrounding media weak. Reducibility, reduce the ash meltability and cause coking in the furnace. Because the ash melting point of the biomass fuel burned by the biomass boiler is low, the ash deposits are easy to adhere to the tube walls of the furnace and the superheater. If the fuel moisture is too large, the water vapor generated during combustion will soften the potassium (because of the The main component is potassium). Potassium causes coking over time after being heated.

The temperature level of the heating surface in the furnace. In the case of a certain ash melting point, the temperature level and distribution in the furnace become an important factor in whether coking occurs. Experience has shown that: boiler coking is mostly on the surface of the flue and superheater. When the liquid or soft ash particles are moved to the heating surface by inertia, because the ash particles move fast, the cooling effect is poor, and the molten ash particles are easy Adhesion, so that the slag layer quickly accumulates and grows. Studies have shown that as the temperature increases, the degree of coking will increase exponentially. Coking not only affects the heat transfer of the heating surface of the boiler, but also the coke block and ash block the flue gas passage, increase the flue gas flow rate, form a flue gas corridor, aggravate the wear of the heated surface, and affect the normal production.

Coking formed after fuel doping. After the fuel is burned in the furnace, it is easy to coke and deposit on the heating surface of the boiler.
A. Since it is impossible to guarantee that biomass fuel is processed from a raw material during the manufacturing process, it has a wide variety of types, many impurities (mixed with soil, fine sand), high ash content, and high alkali metal content, so it is not allowed in the production process. Avoid mixing soil and fine sand into the fuel. The presence of these impurities changes the composition, form and melting temperature of the fuel, and aggravates the coking on the heated surface.
B. When we purchase fuel pellets, we cannot control that biomass pellet manufacturers mix a large amount of soil and fine sand into the fuel. The presence of these impurities changes the composition, existence form, and melting temperature of the fuel, and aggravates the heating surface. Coking.
Through the analysis, I believe everyone has already understood that the reason for the coking of the biomass pellet machine is mostly the problem of the biomass boiler itself. Therefore, as long as we control the raw materials for the biomass pellets, we will have nothing to do, but if we meet my friend’s In this case, competitors are clearly grabbing market share, so we have to take measures. First, we must declare that the production of biomass particles does not need to add any binders. However, to prevent coking, you can add additives (such as quartz sand). , Gypsum, bentonite or fly ash, etc.) can effectively prevent biomass ash from slagging. Relatively speaking, gypsum and calcium hydrogen phosphate have poor slagging resistance, and bentonite has better slagging resistance, but it is more expensive. Additives are generally added during the conveying process before prepressing to facilitate uniform mixing. When adding, pay attention to uniformity to prevent uneven coalescence due to different specific gravity.

1. Analysis of the cause of coking of biomass particles

Due to the wide variety of fuels in biomass power plants, the fuel has the characteristics of high water content (generally more than 45%), more impurities (mixed with soil and fine sand), high ash content, and high alkali metal content (Table 1). The fuel is in the furnace. After internal combustion, it is easy to coke and accumulate dust on the heating surface of the boiler. The main factor of coking. Biomass boiler coking mainly refers to the ash produced after the fuel is burned, which is mostly melted into liquid or softened state at high temperature. If the ash still remains softened and touches the heated surface, it will stick to the heated surface due to cooling. On top, coking is formed. There are many factors that affect boiler coking, and it is generally believed that the main ones are:

The ash content of the fuel itself and the coking formed after doping. The main factor that affects the melting point of ash is the chemical composition of ash and the surrounding high-temperature environment medium. The two influence each other. Once the boiler combustion adjustment is not done properly, incomplete combustion products will appear, making the surrounding medium weak. Reducibility, reduce the ash meltability and cause coking in the furnace.
At the same time, biomass fuel generally enters the furnace in the form of blending into a mixed fuel, and the fuel broker mixes a large amount of soil and fine sand into the fuel. The presence of these impurities changes the composition, existence form, and melting temperature of the fuel. Intensified coking on the heated surface.

The temperature level of the heating surface in the furnace. In the case of a certain ash melting point, the temperature level and distribution in the furnace become an important factor in whether coking occurs. Experience has shown that: boiler coking is mostly on the surface of the flue and superheater. When the liquid or soft ash particles are moved to the heating surface by inertia, because the ash particles move fast, the cooling effect is poor, and the molten ash particles are easy Adhesion, so that the slag layer quickly accumulates and grows. Temperature has a very important effect on coking in the furnace. Research shows that the higher the temperature, the degree of coking will increase exponentially.

2. Treatment methods for ash accumulation and coking

2.1 Conventional coking treatment methods. Early biomass power plants generally used steam soot blowers for coking and ash removal treatment on the heating surface, but from the actual effect point of view, it did not meet the requirements for decoking. It can only be processed by washing with high-pressure water after stopping the furnace. The main reason is that the content of potassium in biomass fuel is relatively high. Its presence reduces the melting point of ash, while silicon forms a low-melting compound with potassium during the combustion process, resulting in a lower softening temperature of ash. According to the experimental data The deformation temperature of plant ash is about 800℃, and the temperature of the furnace superheater of the boiler is mostly within this range. Therefore, under high temperature conditions, the softened ash can easily adhere to the outer wall of the pipe on the heating surface, and it is difficult to use a steam soot blower. Process the accumulated focal block. According to past experience, the general boiler using steam sootblower is cleaned and put into use for 15 days, the main steam temperature does not need to be adjusted by desuperheating water, the temperature is normally maintained at about 5100C, and the boiler needs to be shut down for water flushing after one month of operation. Otherwise, the main steam temperature will deviate more and more from the rated value (540℃), the efficiency of the boiler will decrease, and the exhaust gas temperature will increase by 5-10℃. In addition, the use of steam blowing soot will have the following problems: (1) The medium purging area is limited, and there are some dead corners, which are easy to form flue gas corridors and aggravate local wear; (2) The soot blowing cycle is long and the heated area is too ash. It even makes the soot sintered and hardened, increasing the difficulty of soot blowing; (3) If the pressure of steam soot blowing is too high or long-term use, it will accelerate the wear of the metal pipe wall, and the pressure will affect the effect of soot blowing; (4) increase the smoke in the furnace The air humidity causes low temperature condensation at the air preheater, causing serious corrosion of the air preheater tube; (5) The failure rate of mechanical parts is high, and the maintenance cost is high.

2.2 Discussion on a new method of removing coking. At present, when our plant uses a combination of decoking inhibitor and pulse gas soot blowing device on the boiler to deal with boiler coking and ash deposits, significant results have been achieved.

Decoking Inhibitor (SlagTr011508) is a high melting point fuel additive containing a combustion enhancer, which can reduce the problem of fly ash deposition on the flue gas side. When it is sprayed into the furnace, it will mix with the fly ash leaving the furnace and adhere to the semi-melted ash. By changing the melting point of the ash, cracks are formed inside the coking to destroy the coking, and pass on the surface of the pipe. The formed metal film helps reduce acid dew point corrosion problems. With the pulse gas soot blowing device, the dust on the heating surface of the boiler is removed through purging, acoustic fatigue, thermal cleaning and local vibration. Finally, the dust is engulfed by the flue gas flow, thereby improving the thermal efficiency of the boiler.

The specific operation method is: during the operation of the boiler, at each operating value every day, put Skg of decoking inhibitor into the furnace each time, add it from both sides of the furnace, and start pulse gas soot blowing after adding chemicals for 30 minutes. Using the two-pronged method of decoking inhibitor and pulse gas soot blowing device, the main steam temperature of the boiler can be maintained for about 2 months. At the same time, the fouling and coking phenomenon on the heating surface almost no longer exists, and the exhaust gas temperature can be higher than that of the previous steam soot blowing. When the device is used, the temperature is lowered by 3-5°C, and it is preliminary estimated that it can bring about 1 million yuan of indirect economic benefits each year.

Due to a series of problems such as high water content, high impurities, and poor quality of biomass power plant fuels, it is impossible to improve in the short term, and the phenomenon of boiler coking and ash accumulation is unavoidable. In order to increase the operating hours of the boiler and ensure the normal production, the use of a decoking inhibitor (SlagTr011508) combined with a pulsed gas soot blowing device can effectively solve the problem of boiler fouling and coking, which is worthy of reference for the majority of biomass power plants.