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Discussion on circulating fluidized bed boiler and refractory and wear-resisting material

Words:[Big][Medium][Small] Mobile Page Two-Dimensional Code 2018-10-15     

1. Characteristics of refractory and wear-resistant materials

Refractories are the basic materials for high temperature technology. As the auxiliary materials in the high temperature technology industry, refractories are often ignored by people.Because of this, the quality of fire-resistant and wear-resistant materials often can not meet the requirements of the standard, which often causes the main equipment outage accident.For example, due to the short service cycle of refractory materials, the metallurgical system often stops the furnace, which will affect the production.Power generation, pharmaceutical, papermaking, chemical and other industries as a result of the fire resistant and wear-resistant materials not pass the standard often caused by the furnace, not only to the use of the unit caused direct economic losses, but also to the lives of the broad masses of people bring a lot of inconvenience, and sometimes even caused joint and several economic losses.So, although the refractory wear-resistant materials in the field of high technology is the auxiliary material properties, but absolutely not for the material, must according to the features of furnace type carefully choose suitable refractory wear-resistant materials, but also needs to formulate the corresponding standards, specifications, requirements and reasonable formula, and organize the strong technical force team to complete the construction task. 

Refractory material is a kind of mineral resources. The quality of mineral resources is related to the temperature, pressure and age of synthesis.Refractory is a kind of heterogeneous body, there are main and minor components, we usually call its basic component as the main component, and the other components as the minor component, and the minor component is intentionally added to improve the performance of some aspects of the product and the material itself carried impurities.The main component of refractories is the main part of the material and is the only standard we choose for the material to be used.It is an important link to ensure the quality of refractories to choose the proportion with higher main components and reduce the proportion with less harmful components or impurities.Or through some effective ways and means, for example: using some additives, binder, sintering agent and water reducing agent, stabilizer, water repellent, antioxidants, coagulant, and so on, these are all in order to improve the compressive strength of refractory itself, heat resistance, abrasion resistance, scouring resistance, corrosion resistance, the ultimate goal is to make refractory wear-resistant materials can meet the operation requirements under the condition of high temperature, take good service for the high temperature in the field of technology. 

As we know, the main function of refractory materials is to reach high temperature, so it must be composed of higher melting point compounds.Only when the content of compounds with higher melting point is higher in the raw materials can the refractory temperature be higher.Only the higher refractory temperature can be used to withstand the high temperature of the case of all kinds of destruction of refractory materials, this is the principle of our choice of refractory wear-resistant materials.If we choose the refractory wear-resistant material can not be in high temperature by physical, chemical, mechanical and other aspects of the acting force, can not withstand the corrosion of slag, smoke, corrosive gas and other harmful impurities, it is bound to produce the melting, softening, melting corrosion, wear and bursting damage of refractory materials.In order to avoid this kind of situation, we should choose some fire-resistant and wear-resistant materials which are suitable for circulating fluidized bed boiler itself.If the selection of materials can not meet the requirements of this type of furnace operation, economic refractory materials can not be used in the boiler equipment, so what kind of material is a strong technical, knowledge is very wide, need to consider many factors of meticulous work, is not a general work, more can not be careless.

2 wear mechanism

Boiler operation adjustment, put it bluntly, is the adjustment of combustion, and combustion itself is a chemical reaction process.After the fuel is put into the furnace, it is completely burned under the conditions of proper temperature, sufficient oxygen, uniform stirring and sufficient time.Circulating fluidized bed boiler is no exception, but the combustion in the furnace is the use of low temperature combustion technology, the combustion temperature in the furnace than pulverized coal furnace, chain furnace, cyclone furnace and other furnace type lower.However, uniform stirring and sufficient burning time are superior to any furnace type currently put into operation, which is one of the biggest advantages of high combustion efficiency of CFB boilers. 

The combustion principle of circulating fluidized bed boiler is: after the new fuel is put into the furnace, it is immediately involved in a large number of materials.This series of combustion processes are all carried out in a violent fluidization cycle.The new fuel of the circulating fluidized bed boiler with medium multiplier accounts for only 6 ~ 8% or even less of the whole circulating material. It can be said that all the heat released by the new fuel in the combustion process is first transferred to the circulating material and maintained the temperature in the furnace.In the normal operation of CFB boiler, the so-called circulating material is a kind of heat carrier.Because the whole combustion process is carried out in a violent fluidization cycle, so in the whole boiler operation, there is a circulation material and each heating surface, circulation material and lining material wear problem, whether the wear is serious everywhere, how much wear.After many times of experiments and the actual running situation of all countries the results are as the following formula: A = V3. P. ф Q/K, A????Wear and tear, V??Movement speed of circulating material, P??Furnace pressure φ??Circulation material diameter, Q??Circulating material concentration, K??The grindability coefficient of burned coal.That is, wear and material movement speed third power, and pressure in the furnace, the material diameter and the material concentration in the furnace is proportional, and the coal is inversely proportional to the grindability coefficient.Therefore, in the dense phase area of the furnace, due to the fast flow rate of gas and solid, relatively high pressure, relatively large diameter and concentration of materials, resulting in serious wear.Although the furnace outlet pressure, circulation material diameter and concentration are small, but the flow rate is accelerating, the same wear is very serious, dense phase area and furnace outlet are on the heating surface a layer of wear resistant material, its purpose is here.  

Analysis we know that the movement velocity of wear is due to the material, material concentration, the diameter of the material and the pressure in the furnace caused by, and all of these factors is the inherent characteristics of circulating fluidized bed boiler, it is not by the will of the people for transfer, is can't change and resist, only the material movement velocity loop is sent in with the combustion adjustment, the ventilation of how much the change at any time.No matter from the Angle of reducing wear, or from the perspective of economic operation, we must adjust the optimal air volume.Because of the excess air supply, not only can cause rapid gas velocity accelerated wear.Moreover, it will cause the rise of power consumption, increase the heat loss of smoke exhaust, and decrease the boiler efficiency, thus wasting power consumption and heat energy.So too much supply is not only a matter of economic loss, but also a matter of safety indicators.The principle of operation adjustment must be to ensure safety, to improve the economic operation of the boiler as the core, to create the maximum economic benefit for the purpose of the enterprise, to obtain a longer full load of the furnace operation cycle as the starting point, really do a good job in the operation of the adjustment. 

Within the furnace, the movement of the gas and solid two substances is intricate, tell from the macro is carried out in accordance with the design flow, but from the microcosmic point of view, the direction of material movement since beginning of dense-phase zone is chaotic, the material and charging of raw coal in the primary and secondary under the action of the wind was strong fluidization and upward movement, in all material particles upward movement forces from the three aspects at the same time;The force of gravity on the particles themselves, the force of force on the particles flowing in the wind direction of the smoke, and the force of friction between the particles as they move upward.Through analysis that: when the particles themselves gravity and the friction between particles upward movement of the smoke is greater than the sum of the wind flow upward impetus of the particles, the particles will be down to land or to smoke around the wind force smaller drift along the wall and fins after falling, falling to a certain height when the smoke particles are less than the sum of the gravity and the friction wind was held when high power, large particle materials until the return to the dense phase zone.Coal particles in the movement of the combustion of their own into fly ash, the impurities in the coal into materials, the original material in the intense movement of friction so that their own quality decline, rising height is constantly increasing until finally left the furnace, the furnace is always in this cycle at all times.So said the circulating fluidized bed boiler chamber from the microscopic analysis, there are thousands of small cycles.And in the height of no matter which section, and section and section between the material concentration, diameter, pressure in the furnace are not the same, these parameters and the height of the furnace is inversely proportional to the relationship, so the amount of wear in the furnace is more serious than the above relationship changes. 

Wear inside the furnace, whether on the heating surface or lining material, starts from the surface and gradually extends inward.Due to the high temperature in the furnace and some corrosive gases, the material wear is only a part of it, but also to withstand high temperature corrosion, oxidation corrosion, sulfur dioxide and triatomic gas corrosion and so on.The heating surface is made of high quality metal materials, which are less corroded by various gases, mainly by the wear of circulating materials, but the wear and corrosion of lining materials is really all-directional.We choose the lining material is made of various minerals according to the different chemical formula, in the process of preparation, selection of the ratio of various level of aggregate and a considerable amount of powder and ultrafine powder plus additives and coagulant, and obtain a relatively ideal high strength, wear resistance, scouring resistance, spalling resistance, good thermal stability of refractory materials.Whether the performance of fire-resistant and wear-resistant material can meet the needs of circulating fluidized bed boiler is the urgent desire of various fire-resistant material manufacturers, and also the hope of the users of circulating fluidized bed boiler, so it is very important to choose the quality of fire-resistant and wear-resistant material for the safe and economic operation of the boiler the day after tomorrow. 

According to the running condition of several circulating fluidized bed boilers after several boiler factories put on the market, there are a considerable number of furnaces whose refractory and wear-resistant lining materials cannot meet the running requirements.After thousands of hours of running, the surface will have obvious wear, various grades of aggregate exposed on the surface, there is a considerable part of the movement will immediately drop.From the actual wear left traces of analysis;It is the choice of wear-resisting material adhesive or binder can not be all the various grades of aggregate and powder together, so as to achieve the cycle fluidized bed boiler operation needs high strength, wear resistance, anti-erosion, anti-peeling, good thermal stability requirements.Because of the wear resistant castables of aggregates and the powder is evenly distributed, the compressive strength is not good cause powder after wear, aggregate is exposed on the surface becomes a non-existent, naturally fall off or long were all polished, protected by heating surface and ACTS as a sealing function will be useless, the situation is good.There is also a situation, used in the refractory on the equipment, not only on the surface of the wear serious, internal variable intensity of degeneration and fell sharply, the wear-resisting material often caused extensive loss, the loss of heating surface and seal protection as early as possible, the vast majority of boiler will cause accidents, seriously affect the safe operation of the thermal equipment.The root cause of this kind of thing is not the poor quality of refractory and wear-resistant aggregate and powder materials we choose, but the poor preparation technology and inappropriate ratio.The wear-resisting material produced by advanced and developed countries is only 20-30 mm thick in the dense phase area. It can be used for more than 50,000 hours and the surface is very smooth. The quality is very good.From the refractory wear-resistant materials we use wear traces left by the analysis of the refractory aggregate and all kinds of powder wear-resisting performance is possible, by special smooth surface after construction, when running after a period of time, the surface is very rough, rough or when more rough surface will fall off a layer, the more serious a break it down to a large, with the hand with a hammer knock a degeneration of popping sound, chased the fundamental reason is the use of wear-resistant materials in the binder or binder is bad, can't be all sorts of gradation of refractory organic combination together, cannot obtain high strength and wear resistance,Can't make the concrete strength to achieve the strength of the aggregate, so it is hard to resist corrosion and damage, caused by high temperature is difficult to ensure the normal operation of the furnace, the furnace often cause accidents is the inevitable result, has become an irresistible fact, so choose what kind of refractory wear-resistant material particularly profound and significant to the safe operation of the significance of the day after tomorrow.If you choose an unsuitable refractory and wear resistant material, it will take several times of efforts to completely replace it in the future. There are too many lessons in this regard.


3 conclusion

Human survival, life and production are inseparable from the use of fire, in order to prevent damage caused by high temperature, it is necessary to study and make good use of the refractory technology.Oxidation of heat-resistant material distribution and content of the earth's basic same, synthesis and matching technology, and the day after tomorrow is along with the technical conditions vary widely from country to country, compared with the developed countries of refractory materials in behind a lot, both compressive strength, flexural strength, bond strength, volume density, real share all have a great gap with developed countries, such as in this case, we should attach great importance to the quality of the high temperature technical service of wear-resisting material.Carefully select and prepare materials according to the situation before use;Careful construction and maintenance in strict accordance with the requirements;In the operation to avoid as far as possible to fire resistant, wear-resistant materials caused by rapid cooling and heat expansion caused by uneven damage.Let refractory materials play their due role in thermal power equipment, metallurgical industry, human life and other aspects, and serve the high temperature technology and human production and life.At the same time we also called for a national scientific research units of early developed several kinds of high strength, wear resistance, resistance to erosion wear resistant material, thermal stability, good heat resistance, to our country and advanced countries in the field of refractory gap, so as to catch up with or exceed the developed countries, so that the heat resistance, wear-resisting, the auxiliary materials in the process of service for thermal equipment does not influence the safe and economic operation.


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