1. Analysis of the cause of coking of biomass boiler
Due to the wide variety of biomass fuels, fuels have high water content (generally above 45%), more impurities (mixed with soil, fine sand), high ash content, high alkali metal content (Table 1), fuel in the furnace After burning, 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 influence 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. When the liquid or soft ash particles are subjected to inertia and move toward the heated surface, the ash particles are easy to move due to the fast moving speed of the ash particles, 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 as the temperature increases, the degree of coking will increase exponentially.
2, ash and coke treatment
2.1 Conventional coking treatment method. In the early stage, the steamed ash cleaner was used to coke and clean the heated surface, but from the practical point of view, the decoking requirement was not achieved. 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 is put into use, the control of the main steam temperature does not need to use the desuperheating water regulation, the temperature is normally maintained at about 510 0C, and the furnace needs to be shut down for water washing after one month of operation. 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 is too low, which will affect the soot blowing effect; (4) increase the smoke in 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 method of removing coking. When the combination of the decoking inhibitor and the pulse gas soot blowing device is used in the boiler to treat the boiler coking ash, a remarkable effect is obtained.
DAA-A Biomass Fuel Decoking Agent is a high melting point fuel additive containing combustion improver. When this product is fed into a burner and a high temperature furnace, it can melt immediately and quickly evaporate. Most of it With the flow of smoke, it chemically reacts with the scale on the heated surface. Since the alkaline slag has a lower melting point than the acidic slag, the product promotes the highly corrosive metal salts and oxide slag from weak acid to weak alkali through chemical action, so that the effect of cleaning and decoking is exerted, thereby coking and accumulating Ash and scale can fall off.
The specific operation method is as follows: the decoking agent is uniformly mixed with the biomass fuel according to the prescribed amount; the burner and the boiler must be under normal combustion when being placed. Use ratio: The ratio of each use is 1‰ of biomass fuel. If the coking condition is serious, the DAA-A addition ratio can be adjusted.