Cause analysis and treatment of material collapse in preheater

The collapse of preheater is a common problem in the actual production of kiln system. It not only affects the thermal efficiency of the firing system and the operation rate of the kiln, but also results in the blockage of the precalciner system or the tempering of the kiln head, resulting in personal injury accidents.

 

  1. Cause analysis of material collapse

Collapse refers to the uneven flow or accumulation of raw material in the preheater, resulting in a large amount of raw material concentrated from the cyclone cone into the outlet pipe of the next stage cyclone through the ash discharge valve and discharge pipe. However, the wind speed of the pipe is not enough to disperse and hold up the large amount of raw material, and the raw material directly collapses into the kiln through the cyclone or decomposition furnace. The main causes of material collapse in precalciner system are as follows:

1.1 uneven feed rate

The raw material is easy to collapse due to long time of pressing, large moisture content and poor looseness; the arch of feeding bin makes the raw material sometimes absent in the feeder; the rotation speed of screw feeder is unstable, resulting in uneven feeding, which is easy to cause material collapse.

1.2 the structure of cyclone is unreasonable

The inclination angle of cyclone inlet and vortex shell bottom is too small, and the horizontal section is too long. At the initial feeding stage, due to the small air volume of the system and low cross-section wind speed, a large amount of raw meal is easy to settle and accumulate in the small inclined angle and horizontal section. When the system air flow disturbance or pressure changes, a large number of materials suddenly slide down, resulting in material collapse

1.3 air leakage of cyclone cone outlet, ash discharge valve and discharge pipe

There are two types of air leakage in the pre decomposition system: external air leakage and internal air leakage. External air leakage refers to the cold air leakage from inspection hole, measuring hole, ash discharge valve, connecting pipe flange and cyclone hole door under the effect of system negative pressure. Internal air leakage refers to that when the ash discharge valve is burnt out and deformed or the counterweight is too light, the hot gas flow at the outlet of the next stage cyclone directly enters the cyclone through the discharge pipe and the cone outlet through the ash discharge valve.

 

  1. Several measures to reduce collapse

2.1 strictly control the moisture content of raw material out of mill and stabilize the feed quantity of raw material into kiln

The uniformity of raw meal composition is an important factor to determine the quality of cement clinker.

2.2 change the horizontal section of cyclone to the inclination angle to avoid material accumulation

2.3 a conical expansion bin is set below the cone outlet of the cyclone

The diameter of the cone outlet can be enlarged by setting a conical expansion bin under the cone of the cyclone, thus effectively reducing the possibility of material accumulation and arching at the bottom of the cone.

2.4 reasonably adjust the angle of ash discharge valve rod and its counterweight

2.5 skip the collapse danger area with low yield as soon as possible

The feeding range can be increased at the initial stage and slowed down after 80% feed rate.

 

At this time, the amount of material in the pre decomposition system has reached a certain extent, and the material flow is smooth. There are a lot of materials passing through the cone outlet of cyclone, ash discharge valve and discharge pipe at any time, which can inhibit the external and internal air leakage of the above parts. Therefore, there is little collapse, even if there is one, which has no impact on the operation. Therefore, it is said that the higher the NSP kiln output, the easier the operation is.

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The major changes can be brought to cement industry by intelligent laboratory

At present, the global new round of scientific and technological revolution and industrial transformation are accelerating the evolution, and the manufacturing industry is increasingly showing the trend of digitization, networking and intelligence. As a traditional manufacturing industry, the intelligent innovation and integration of cement industry has opened a green transformation road from resource consumption to environmental protection.

 

Especially in recent years, the cement industry has been trying to catch up with intelligence and has made breakthrough progress. It is worth noting that while cement enterprises continue to explore intelligence, as the backbone of China’s industrialization, equipment manufacturing enterprises are also transforming from traditional industrialization to modern industrialization. Among them, the intelligent laboratory equipment system which is expected by most cement enterprises has been booming, which will bring great changes to the production and operation mode of enterprises. What does intelligent laboratory mean for cement production? What changes can it bring to the industry?

 

The intelligent laboratory plays a coordinating and central role in the whole cement production process, which is of great significance. Two aspects of reducing human factors and automatic adjustment of production ingredients are elaborated

 

1、 Reducing human error in laboratory

 

Reducing human factors in laboratory can improve sampling quality and provide more accurate data for production. 1. The intelligent cement laboratory can improve the quality of sampling. In the past, if it was in a manual laboratory, sampling often had a lot of randomness, that is, human factors, such as no sampling, less sampling or missing samples, etc., which will definitely affect the whole system production. The sampling frequency of intelligent laboratory is controllable and adjustable, which can solve the problem of discontinuous and uncontrollable sampling. 2. It can provide more accurate and representative data for the quality control system. After eliminating the human error in the laboratory, the intelligent laboratory can achieve stable and efficient data acquisition and provide more accurate and representative data for production. However, due to the quality control has many links, the laboratory can not completely replace the quality control, but as a link in the whole quality system, it has a long-term guiding significance for the cement quality control and cement production operation in the production process.

 

2、 Automatic adjustment of production ingredients

 

The integrated batching software and kiln grinding expert system are combined together to provide more timely and accurate data analysis for batching production and improve the intelligent level of the whole plant through detection and analysis with neutron cross band online system. At present, the industry is subject to the limitation of capacity expansion and the arrival of the era of “quantity to quality”. In the future, China’s cement industry will be fully intelligent, and the automation laboratory will soon exceed the total number of automation laboratories in the industry in the world.

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Quality requirements of active lime

1. Particle size

According to the particle size requirements of active lime, for the calcination process of rotary kiln, it is to ensure that the calcination process of rotary kiln can avoid under burning or over burning due to uneven particle size of limestone, excessive grade difference and uneven heating. To prevent the limestone from accumulating and staying in the container due to uneven particle size, resulting in uneven air permeability or poor gas flow.

For converter steelmaking, the requirement of particle size of active lime is to ensure slagging speed and effect in steelmaking process with time requirement. If the particle size of lime is too large, the reaction time between lime particles and molten steel will be prolonged, which will slow down the slagging speed and affect the slagging effect. On the contrary, if the lime particle is too small, it will easily cause particles or dust splashing during steelmaking, which will worsen the operating environment.

2. Activity

The so-called activity refers to the reaction ability of lime with water.

The activity refers to the speed of dissolution reaction between lime and water after mixing a certain amount of lime with a certain temperature and a certain amount of water. It represents the ability of lime to react with other substances (impurities) in molten steel. Because it is very difficult to measure the reaction rate of lime with molten steel during slagging.

At the same time, it can judge the quality of lime calcination and guide the production by detecting the activity. Therefore, it is necessary to test the activity of calcined lime products.

There are many methods to detect the quality or activity of active lime. Among them, hydrochloric acid titration is the main method. In the process of calcination, hydration comparison method, hydration weighing method and sampling method are used to judge the calcination quality of lime. For example:

1) Titration method

Take out a number of lime samples from the kiln, crush them, sieve them with a 1 mm pore size sieve, and then sift through a 5 mm aperture sieve. Select 50 g of lime with a particle size of 1-5 mm, dissolve it in 2000 ml water at 40 ± 1 ℃ and stir it. Add phenolphthalein as indicator in the solution, and titrate with 4N HCl (4 g equivalent hydrochloric acid) for 5-10 minutes.

At this time, the volume consumption of HCl (ML) reaching the end of titration is the activity of the tested lime sample. According to the theoretical calculation, the highest activity index of pure active Cao is 446ml.

The theoretical activity of pure active Cao is calculated as follows:

Molecular weight: Ca = 40.08  O = 16.00  H = 1.008  Cl = 35.45

Solution: it can be seen from the activity test method of active lime Cao coarse particle titration method,

CaO + H2O = Ca（OH）2                                 （1）

Ca（OH）2 + 2HCl = CaCl 2 + 2 H2O            （2）

（1）+（2）: CaO + 2 HCl = CaCl 2 + H2O   （3）

56.08   72.92

50     X

X = 50×72.92 / 56.08 = 65.01

Because: a liter of 4 N HCl solution contains 145.84 grams of HCl

Therefore, 4N HCl solution can be prepared from 65.01g HCl

65.01÷145.84×1000 = 445.79 ≈ 446 ml

2) Hydration weighing method

In the absence of chemical reagents:

a. Take some lime samples and weigh them, and the weight is g 1.

b. Dry the weighed sample in water and let it be fully digested.

c. Filter lime water, collect insoluble residue, dry and weigh it as G 2.

d. The digestion part of the reaction was calculated: G 1-g 2 = G 3.

e. The lime decomposition rate (G 3 △ G 1) × 100% can reflect the calcination quality of lime.

3) Hydration comparison method

Take out some lime clinker from kiln, cool it, put it in a container, add water to dissolve it, pour the lime solution and residue into the screen, wash away the residual lime with water, observe the size of the residue particles and compare with the amount of lime clinker taken to judge the calcination quality.

4) Sampling and tapping method

When the kiln is cooled, some lime particles are red in appearance. The surface texture of lime particles is clean and the color is white. The particle weight is light. When the lime particles are knocked with a hammer, the texture is loose and easy to be broken, with obvious nucleation and small volume.

3. SiO2 (silica)

High Cao and low SiO2 are the basic requirements and guarantee for slag forming in steelmaking process. The purpose of slagging is to remove s and P from molten steel, especially S. the basicity of slag is expressed by the ratio of Cao to SiO2. Higher SiO2 will destroy the surface structure of lime and affect the slagging speed and effect.

During the calcination of lime, the melting point of pure SiO2 can be as high as 1713 ℃. However, when the temperature is 700-800 ℃, the secondary reaction will occur between SiO2 and Cao in solid state. With the reaction going on, Cao · SiO2 (calcium metasilicate), 3CaO · 2SiO2 (wollastonite), 2CaO · SiO2 (dicalcium silicate) and 3 Cao · SiO2 can be produced successively The effect of these products on lime is to reduce the activity.

4. S (s) P (P)

In converter steelmaking, the main purpose of making high basicity slag with active lime is to remove sulfur and phosphorus from molten steel.

When the content of P and P in steel products is too high, the cold brittleness of steel will increase at room temperature (i.e. P > 0.13). That is to say, the cracking of steel is caused.

When the content of sulfur in steel products is too high, it can obviously damage the weldability of steel, reduce the impact toughness of steel, especially make the steel produce cracks during heating, rolling or casting, i.e. “hot brittleness”. The corrosion resistance (corrosion resistance) and wear resistance of the steel are obviously reduced. Therefore, the harm of sulfur to steel products has the name of “termite”.

Because lime has the characteristics of combining with sulfur, especially when lime is at high temperature, the ability of lime to absorb sulfur is very strong. Therefore, the effect of lime on the removal of sulfur from steel is very large.

However, due to the influence of sulfur content of raw materials, fuel and high-temperature calcination factors on limestone itself, the lime generated from limestone will also contain sulfur and phosphorus in different degrees. Therefore, the sulfur and phosphorus content of lime itself is required to be low. For the former, the low sulfur and phosphorus content of limestone (raw material) and fuel is required to be low.

5. Residual CO2 (CO2)

The so-called residual CO2 actually means that there is no burnt out core or sandwich in the lime particles, and there is no residual lime inner layer completely decomposed. The content of CO2 in lime is mainly controlled by calcination. It has a great influence on the quality of lime and the effect of steelmaking.

a. Small or no nucleation: the surface of lime particles is easy to sinter and produce over burning, and the characteristics of activity will be destroyed.

b. Excessive hearting: it will undoubtedly affect the effective decomposition of lime, resulting in insufficient formation of lime characteristics and reduction of activity.

In the process of C and steelmaking, if the residual CO2 is too high, the amount of scrap will be affected, the heat consumption will be increased, the utilization rate of lime will be reduced, and it is also difficult to control the slag and spatter.

Therefore, while strictly controlling the degree of lime calcination, attention should also be paid to the waterproof work during the storage and transportation of the calcined lime products, so as to reduce the pulverization rate.

d. Conversion of CO2 content:

The decomposition reaction of limestone by heating is the reaction of removing CO2. According to the decomposition equation of CaCO3, the results show that the decomposition of limestone is a reaction of removing CO2. When CaCO3 = 100, Cao = 56, CO2 = 44.

100÷44 = 2.272

When CaCO3 of 1.785 kg is needed to produce Cao per kg unit,

1.785÷2.272 = 0.79 m3

0.79÷1.97 = 0.4 Nm3 / kg

It can be concluded that when CaCO3 is 1.785 kg to produce kg of Cao, the amount of CO2 produced is 0.4 Nm3.

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Resource utilization technology of municipal solid waste incineration slag

In order to continuously increase the quantity of municipal solid waste, incineration technology has been widely used in domestic waste treatment. China’s relevant documents clearly stipulate that the incineration slag is treated with general solid waste, and the comprehensive utilization standard of such waste is clarified. Various resource utilization technologies are increasingly mature, which brings considerable economic benefits.

1. Present situation of resource utilization of incineration slag

As early as several decades ago, foreign developed countries began to study the utilization of domestic waste incineration resources, mainly from the aspects of environmental impact and resource utilization, collected and treated the fly ash and slag produced in the incineration plant, so as to make them into secondary resources for use. Slag is the product of MSW incineration, accounting for 20% – 30% of the total weight of MSW, and fly ash accounts for about 0.5%. According to the survey, in 2019, 5932t domestic waste will be incinerated, which will produce 5.2-7.8 million tons of slag, and the amount of fly ash will be about 13 tons, which will virtually increase the difficulty of slag treatment, and the land resources of landfills will be increasingly tense. In order to save land for landfill and reduce treatment cost, it can be realized by reusing slag.

In developed countries, the slag is mainly treated by sanitary landfill and resource utilization. The utilization rate of slag in EU countries ranks first, more than 50% of the total. China’s relevant regulations point out that the slag can be directly put into the sanitary landfill site for treatment, but because the fly ash contains dioxin and other harmful substances, it needs special treatment to reduce the content of harmful substances to the allowable range before sanitary landfill, otherwise, it will be treated as hazardous waste. From the physical and chemical properties, slag has aggregate characteristics, low content of heavy metal and dissolved salt, no radioactive hazard, less interference to the environment in resource utilization, and relatively less organic matter content in slag, high strength, so it can be made into building materials and put into civil engineering. At present, slag resources are mainly used to make aggregate and covering materials, which are applied to road engineering and landfill.

2. Resource utilization technology of municipal solid waste incineration slag

2.1 slag pretreatment and metal recovery

In order to promote the optimal utilization of slag resources, it is necessary to carry out pretreatment. The particles with diameter less than 100 mm are selected into the subsequent links by using the screening device, and the metal with the volume of more than 100 mm is crushed. The incomplete burning garbage is manually picked out and sent back to the incinerator for re incineration. The whole process does not need external drainage, which can achieve zero discharge of sewage. According to the investigation, there are some scrap metals in the slag, such as iron, copper, aluminum, etc. generally, the recoverable metal content is 5% – 8%, which has certain recycling value and can be reused. The recovery rate of iron, copper and aluminum in slag is 14.8%, 52.7% and 73.1%, respectively. It can be seen that the recovery rate of valuable metals in slag is relatively high, and the recovery treatment has certain feasibility.

2.2 making environmental protection bricks

After pretreatment, the moisture content of the slag is about 2%, the density is about 1250kg / m3, and the water absorption rate is about 9%. After five times of soaking and drying with saturated sodium acid solution, the slag mass loss is about 4.31%, and the soundness meets the relevant requirements of non fired brick. At present, in the United States and the Netherlands and other places, the slag from domestic waste incineration has been made into building materials and applied to engineering construction. The more frequently used manufacturing method is to mix the slag and aggregate in a specific proportion, and then mix with cement and water to form concrete bricks. The U.S.A. has carried out an experimental evaluation on the environmental impact of such unburned bricks. Two artificial reefs have been built on the seabed by using the unburned bricks, and the environmental changes around them have been followed up for six years. The results show that there are no toxic components in the unburned bricks, which can meet the requirements of concrete replacing aggregate. Rothel et al. Used these non fired bricks to build the shipyard and monitored the air quality inside the building for two and a half years. The results showed that the particulate pc-dd, TSP and volatile organic matter in the air of the building did not change significantly. Therefore, the pollutants in the slag were locked in the cement bricks. On the whole, it is feasible to make environmental protection brick with slag in engineering practice and environmental protection.

2.3 making subgrade filling material

In terms of physicochemical properties and engineering properties, slag is similar to lightweight natural aggregate. After treatment, the slag can be paved with asphalt, cement and other materials. According to the leakage of metal elements in such mixture, it can be seen that the release of cadmium, lead and zinc is relatively low. Through the evaluation of environmental, human health, life cycle and other factors, it is found that there is a close relationship between management technology and risk. If the management technology is scientific and reasonable, it can effectively reduce the probability of risk occurrence and avoid adverse interference to the environment. In the United States, Europe and other developed countries, slag has been successfully applied to the foundation filling of parking lots and roads, and it is also more common in China. For example, Shanghai Nursery Road is a typical successful case. According to the monitoring of road surface deflection, road condition, base strength and other indicators, it is found that the application of slag in base materials is very feasible, and the effect is still good after long-term application.

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Cement kiln collaborative waste disposal: which factors will affect the burn out rate of waste?

The burning out rate of waste in the calciner will affect its final effect on reducing coal consumption. What factors will affect the burn out rate of waste in the calciner?

The burnout rate of waste in the calciner can be simplified as the reaction degree of a solid substance in a certain volume container

Burnout rate = chemical reaction rate × reaction time

Therefore, the factors affecting the chemical reaction rate and reaction time will affect the burnout rate of waste in the calciner.

1. Factors affecting the rate of chemical reaction

To identify the factors that affect the rate of chemical reaction, the type of chemical reaction must be identified first. Compared with the combustion of pulverized coal, the waste has higher moisture content, larger particle size and more types, and its combustion process is more complex.

The combustion process of different kinds of waste is summarized in the table below. It can be seen from the table below that due to the difference of components, the combustion process of wastes is quite different. The specific performance is that the moisture drying of wastes containing fixed carbon starts from the inside of particles after inert heating, then decomposes at high temperature when the shape of particles remains unchanged, and finally the combustion of fixed carbon; the waste without fixed carbon starts moisture drying from the surface of particles after inert heating, and then It’s a melting and decomposition process.

Generally speaking, when the shape of waste particles remains unchanged (mass combustion), the reactant O2 needs to enter the particle for reaction, and the product CO2 needs to diffuse out of the particle, so the particle size (or size) and porosity (internal channel) of the waste are very important; if it is said that the shape of waste particles will change with the combustion process (surface combustion), such as splitting, splitting, etc The diffusion rate of O2 to the surface of waste, the specific surface area of particles and the rate of chemical reaction are very important. In addition, inert heating is required before combustion, that is, the temperature of the waste is heated to the ignition point. At this time, the moisture content of the waste will be greatly affected, because the moisture content of the waste must be heated and evaporated first. Therefore, the following three points are important factors affecting the combustion rate of waste

(1) Waste disposal capacity

With the increase of waste disposal capacity, more waste will be surrounded around a waste. Unlike pulverized coal, these wastes can not be well dispersed, so it will affect the diffusion of O2 to the surface of waste and the diffusion of evaporated water and combustion products. At the same time, more waste means that under the premise of the same flue gas enthalpy, the waste will burn The initial temperature increases slowly before sintering, which will affect the overall reaction rate when the reaction time is fixed. In addition, with the increase of waste disposal capacity, when the waste can not replace the pulverized coal well, the amount of oxygen needed for combustion and the amount of flue gas generated will inevitably increase, which requires the increase of high temperature fan, and the residence time of waste in the calciner will also be shortened.

(2) Moisture content of waste

It is unnecessary to say much about the effect of moisture content. Other reactions, such as volatilization analysis and fixed carbon combustion reaction, can continue only after the water is evaporated or almost evaporated. Therefore, the higher the moisture content, the longer the time required for water evaporation. The evaporation rate of water is affected by particle temperature, moisture content in flue gas and specific surface area of particles.

(3) Size of waste

It can be said that the size of waste is the key of the key! Different from pulverized coal, the size of waste is usually mm or even cm. For mass combustion, the increase of waste size will seriously affect the diffusion of O2 to the interior of waste and the diffusion of product CO2; for surface combustion, the increase of waste size will reduce the specific surface area of particle reaction, thus reducing the impact on its combustion rate. The results show that the main factors affecting the burning out process of polypropylene (PP) are the diameter of the material, the burning rate of the particle and the burning out time of the tire.

2. Factors affecting reaction time

For the waste fed into the calciner, if the small-sized waste is carried away with the flue gas, the residence time is basically equivalent to that of the flue gas, usually within 5S; if the large-scale waste falls into the cone of the calciner and circulates in the calciner, the reaction time is affected by the wind speed of the cone and the structure of the calciner, and the reaction time is usually within 30s; If the large-scale waste directly falls into the smoke chamber, the reaction time will be affected by the rotary kiln speed and other factors (this scenario will be discussed separately later, which will cause greater harm). For the first two scenarios, the residence time of 5-7s or 30s is obviously not enough for the waste with high moisture content and large size.

3. Summary

(1) The burn out rate of waste in calciner is affected by chemical reaction rate and reaction time

(2) In terms of chemical reaction rate, the reaction path of different wastes is quite different, but for the disposal of waste in calciner, the amount of waste disposal, the moisture content of waste and the size of waste are the main factors affecting the reaction rate

(3) For the reaction time, if the waste is carried away with the flue gas, the residence time is within 5-7s; if the recycle occurs in the calciner, it is usually within 30s, which is far from enough for the waste with high moisture content, large size and low specific surface area

(4) As far as the actual situation is concerned, we hope to increase the amount of waste disposal. If other conditions remain unchanged, it will be at the cost of sacrificing the burn out rate of waste. It is very difficult to further reduce the moisture and size of waste (higher economic cost is required). Therefore, how to extend the reaction time of waste is the key to improve the burn out rate of waste!

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Cement kiln collaborative waste disposal: waste feeding mode and influence

It is an important direction for the development of cement industry in China in recent years and in the future. At present, there are more than 160 cement kiln waste disposal production lines in China, including 60 domestic waste disposal lines with an annual disposal capacity of 7 million tons, more than 40 sludge collaborative disposal lines and more than 60 hazardous waste disposal lines. How to identify and reduce the impact of collaborative disposal of waste on cement kiln, and optimize and improve the ability of cement kiln to dispose waste, is a concern of many cement enterprises. The official account is to introduce a series of articles on cement kiln co disposal of waste, and explain the problems related to cement kiln disposal.

The way of feeding waste into cement kiln and its influence

In short, all wastes entering the cement kiln can be divided into three categories: gas entering (such as domestic waste gasifier), liquid injection (domestic waste leachate injection, etc.) and solid addition (almost all). In this paper, three kinds of waste adding into the kiln are described, and the influence on the kiln operation is discussed from the perspective of technology.

1. Gas in

The gas produced by the waste mainly includes the gas produced when the gasifier is used to treat the domestic waste, the odor generated during the pretreatment of the domestic waste, the gas generated during the sludge drying, and the gas generated during the desorption of the contaminated soil.

These gases usually need to be sent to the calciner for further incineration. Its impact on the system includes the following points:

(1) Balance between resistance of gasifier and pipeline system and resistance of kiln system (2) influence of gas inlet temperature, flow rate and composition fluctuation on stable operation of calciner (general) (3) influence of flue gas generated by gas combustion on preheater system resistance and high temperature fan (general) (4) influence on temperature distribution of kiln tail system (general) (5) gas (combustible gas) The problem of mixed combustion with tertiary air after entering the calciner will definitely produce a large amount of CO gas if it can not be burned. (6) the influence of harmful components carried by the gas on the crust blockage (general) (7) other problems, such as pollutant emission and impact on denitration system, etc

2. Liquid injection

The liquid in waste disposal mainly includes landfill leachate and some waste liquid (such as acid liquor and alkali liquor).

This part of the liquid is usually sprayed into the high temperature part of the furnace for disposal. As the liquid disposal capacity is generally small, its impact is relatively small, mainly including the following points:

(1) The influence of some components in the liquid on SNCR denitration system (such as ammonia nitrogen), the influence on the stable operation of the kiln (such as alkali, chlorine, etc.) (2) the heat consumed by the gasification of a large amount of water in the liquid and the resulting large amount of flue gas on the pressure distribution, temperature distribution and high temperature fan of the kiln tail system; (3) the influence of the fluctuation of liquid flow and composition on the kiln (4) Other effects: such as corrosion of equipment.

3. Solid addition

Almost all cement enterprises dealing with waste involve solid addition, including domestic waste, dried sludge, solid hazardous waste and so on. For solid waste (part or all) into the furnace, it is usually added to a part of the calciner. The details are as follows

Among them, 1) after being added into the calciner, the heavy waste will fall into the flue gas chamber due to the unstoppable flue gas; 2) part of the waste with higher calorific value will be mainly burned at the kiln head (less in China at present); 3) after being added into the calciner, the lighter weight of the waste will move up with the flue gas of the calciner, some of which will be burnt out in the calciner, and some may continue to enter the kiln with the raw material; 4) some of them are larger in size The waste is directly fed into the tobacco room for disposal (rarely in China at present)

Therefore, 1) and 3) are the focus of the discussion. For 1), the key is how to reduce the amount of particles falling into the smoke chamber! The influence of heavy particles falling into the smoke chamber on the kiln is mainly as follows:

The carbon containing components in the waste are burned here, causing local reduction atmosphere, leading to massive decomposition of sulfate and aggravating sulfur cycle, which affects the normal operation of the kiln.

The ash size of large-scale waste is also large, which may affect the quality of clinker if it is mixed into clinker;

The combustible components in the waste are burned here, and the high temperature makes the liquid phase of clinker appear ahead of time, and the kiln end forms a circle;

It may affect the ventilation in the kiln.

For 3), the key point is to improve the burn out rate of waste at the outlet of calciner! Its influence on kilns mainly includes the following aspects:

If part of the waste goes into the kiln with hot raw material, the effect is as described in 1)

Part of the waste is not burned out at the outlet of the calciner, which indicates that the heat enthalpy of the waste is not effectively utilized, and the fuel substitution value brought by the waste disposal will be reduced;

If it is not burned out, the CO concentration at the outlet of calciner will increase significantly, which will make the overall temperature of preheater rise, and the temperature of C1 outlet will increase more significantly;

If it is not burned out, the excessive CO concentration in the flue gas will affect the SNCR denitrification efficiency;

If it is not burned out, it may affect the emission of other pollutants;

In addition, the addition of solid waste will also produce some common problems, including: the impact of solid flow and composition fluctuation on the stable operation of calciner (general); the influence of flue gas generated by combustion on the resistance of preheater system and high-temperature fan (general); the influence on the temperature distribution of kiln tail system (general); the influence of harmful components carried on the crust blockage Ring (Universal).

In general, from the perspective of technology, no matter what form of waste is added into the kiln, it will inevitably affect the air consumption, temperature distribution and stable operation of the kiln tail system. However, for the gas form, the key is to balance the resistance of the pipeline system and ensure the full mixing and combustion of combustible gas and tertiary air; for the liquid form, when its disposal capacity is large, it is necessary to pay attention to the calculation of heat budget and large amount of steam; for the solid form, try to reduce the large-scale waste directly into the smoke chamber, and improve the waste in the decomposition furnace Burn out rate is the key!

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Why is the “waist” of the cooling tower so thin

Why is the shape of the cooling tower so designed and what are the advantages? This question is actually very easy to confuse people, because various factors are intertwined, in fact, there are several different problems in the middle:

First. The earliest cooling towers had various shapes, such as straight cylinders and octagonal cylinders;

Second. Hyperboloid tower
After Iterson first invented the hyperboloid tower in 1915, this configuration quickly became popular in thermal power stations. And with the emergence of large thermal nuclear power plants, there is this kind of natural ventilation hyperboloid cooling tower.

This is a chain of relationships: 1. Increase in installed capacity of power stations — 2. Need to build a larger-scale cooling tower — 3. Cooling capacity is directly affected by area and height, so cooling towers are taller and larger — 4. The tall cylindrical structure is very unstable, and the cost is very high even if it is built-5. It is necessary to use economic means to build a large cooling tower-6. The hyperboloid tower is the most economical;

All thin-shell curved structures have the characteristics of high strength and material saving. There are also cooling towers of other shapes and materials. The exploration of the structure is endless. At present, a typical large cooling tower is about 150m high, and its bottom diameter is about 150m, that is, its bottom can accommodate a football field. However, its thickness is very thin, only 20cm at its thinnest point. If the cooling tower is scaled down to the diameter of the egg shell, it will be thinner than the egg shell, only 1/5 of the thickness of the egg shell.

Third. Why is the hyperboloid structure the most economical?
First of all, according to the structure of the cooling tower, it can be seen that the narrowed design in the middle makes the area of ​​the air inlet larger under the same water spray area, which helps increase the air volume. Therefore this surface should be curved inward.
The reason for the economy of hyperboloids is not because of the most saving of materials, but because of the way it is constructed. Hyperboloids are a kind of ruled curved surfaces that are composed of a straight line through continuous motion, which is its most important geometric property. The straight line rotates around an axis to form a hyperboloid. Therefore, the steel bars do not need to be bent when arranged, that is, they are parallel to the oblique straight line in space. Of course, nowadays, with the increase in size, the construction method of the hyperbolic cooling tower is to cast the concrete in sections.

After years of engineering practice, the mechanical properties and wind-proof performance of this structure have been well tested, and it has become the most common form of cooling tower. Therefore, the use of hyperboloids is also a historical inertia. In fact, in engineering practice, the construction is not completely based on the geometric shape of the curved surface. In actual construction, the curved surface is mostly constructed in sections. Given the radius and thickness of the cylinder wall generatrix, the multi-section flat steel template is used to approximate it. Therefore, strictly speaking, its final shape is different from the hyperbolic busbar. Today’s tower shape is the result of the mutual influence of optimized design, engineering practice and construction habits, and it is different from the geometrical hyperboloid.

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Great challenges for waste incineration companies

For a long time, the income from power generation has been the main income of enterprises in the waste incineration field. Among them, the payers of power generation revenue are mainly the two power grid companies. Among the fees paid, part of the cost is paid by the power grid company with reference to the local benchmark power generation price. In addition, the state will generate 280 kWh per ton of garbage and implement local thermal power per kWh. Benchmark electricity price + 0.25 yuan/kWh subsidy renewable energy subsidy price, that is, the standard subsidy of 0.65 yuan per kilowatt-hour of electricity, through the power grid company, on behalf of the waste incineration enterprises; electricity exceeding 280 kWh is normally sold at the local benchmark electricity price. At the same time, the value-added tax of waste incineration power plants will be refunded immediately, and the income tax will be subject to a preferential policy of three exemptions and three halves. Starting from the tax year in which the project obtains the first production and operation income, the corporate income tax will be exempt from the first to the third year, and the corporate income tax will be reduced by half from the fourth to the sixth year.

The benefits of waste incineration are a very cost-effective thing for local governments. Therefore, since 2016, waste incineration projects have been rapidly launched in various places. According to incomplete statistics, there were only more than two hundred waste incineration plants nationwide in 2016. Today, four years later, there are 486 waste incineration enterprises nationwide. However, with the accompanying increase in public anxiety, and the increase in the amount of waste incineration on the atmospheric environment, the emission risks and growth potential of persistent organic pollutants, dioxins, have become increasingly prominent.

Our country’s thinking on waste treatment has been relatively clear from single incineration to comprehensive waste classification and full-stage classification treatment, reducing the amount of waste incineration treatment, and increasing non-incineration treatment and resource utilization methods. Because of this, it is possible to reduce the persistent organic matter emissions caused by waste incineration, such as dioxins and polycyclic aromatic hydrocarbons, to a greater extent. As a result, the relevant subsidy thinking has also undergone a huge adjustment. The weight ratio of harmlessness such as regulatory emission reduction has increased, and the weight ratio of resource utilization such as power generation has decreased, which has become the main direction of subsidies. In the new plan, three important subsidy policy orientations are reflected:

First, state subsidies will gradually withdraw, and local finances must fill the gap;
Second, the subsidies that waste incineration companies can receive for new projects in the future are no longer the previous quotas. They must compete with other peer companies in price competition and corporate operating technology strength, with strong operating capabilities and low electricity sales prices. Companies can receive electricity price subsidies, and this subsidy should be subject to different local policy conditions, and the amount of subsidy received by each company may be different;
Third, in the future, national and local financial subsidies will be more inclined to the front, middle and back ends of waste sorting, and more emphasis will be given to non-electric and other energy supply fields that are more market-oriented or require incineration companies to develop themselves.

To sum up, these adjustments will bring huge challenges to the profit model and operation model of waste incineration companies!

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Future mainstream technology route for sludge disposal

To solve the sludge problem, it is the consensus of the industry to “dispose” and decide “treatment”. Drawing on international experience, there are four main paths for the technical development of sludge treatment and disposal in the future:

(1) Anaerobic digestion technology route based on biogas energy recovery and land utilization
It is generally believed that the cost of anaerobic digestion is low, and sludge reduction and stabilization can be achieved. According to the statistics of “China Environment News”, the investment cost of simple anaerobic digestion is about 200,000 to 400,000 yuan/(ton/day). The cost is saved due to the lack of blast aeration. The operating cost of simple anaerobic digestion is about 60- 120 yuan/ton (80% moisture content, excluding concentration and dehydration), and the operating cost of aerobic fermentation is 120-160 yuan/ton.
More than 50% of the sludge in Europe and the United States is treated by anaerobic digestion, and the generated biogas can be converted into electricity to meet 33% to 100% of the electricity required by the sewage plant. However, the application of sludge anaerobic digestion in my country is not smooth. Of the approximately 50 sludge anaerobic digestion facilities built in China, only more than 20 can operate stably. The main reasons are the poor quality of sludge and the low level of operation and management of treatment plants in my country.
In recent years, research and practice have shown that pretreatment of sludge through alkaline hydrolysis, heat treatment, ultrasonic treatment, microwave treatment and other methods can increase the hydrolysis rate of sludge and improve the anaerobic digestion performance of sludge. Sludge anaerobic digestion technology will be a mainstream direction in the future.

(2) Aerobic fermentation technology route based on land use
Aerobic fermentation has high efficiency, relatively short stabilization time, reduced moisture content and sterilization. At the same time, the finished sludge is mainly used to repair saline-alkali land, urban greening, garbage dump coverage, and construction, so as to realize the organic matter in the sludge and Efficient use of nutrient elements, less equipment investment and convenient operation and management.

However, at present, aerobic composting still has problems such as net energy expenditure. At the same time, there is a lack of theoretical research on control factors such as reasonable ventilation and C/N in different stages. Traditional slab or trough composting covers a large area and the fermented products are polluted by heavy metals, making it difficult to develop aerobic fermentation technology in my country.
In the future, the sludge aerobic fermentation project can adopt an efficient, fast, stable, and intensive design and operation mode, which can achieve a significant reduction in the area; research shows that the proportion of heavy metals in urban domestic sludge exceeds the standard by about 5%, which is a pollution risk Smaller. This technology has great application prospects in relatively underdeveloped areas.

(3) Sludge drying-incineration technical route
Sludge drying and incineration are the most harmless, but the equipment investment and operating costs are high, and the flue gas pollution produced by incineration is serious. A complete flue gas treatment system needs to be established, which also increases the cost of sludge treatment. Therefore, the dry incineration process is generally suitable for areas with tight land use and developed economies.
At this stage, when the technology of anaerobic digestion and aerobic fermentation of sludge in our country is not mature, the drying and incineration of sludge may increase in a certain period, especially the method of co-incineration in industrial kilns.

(4) Technical route of sludge drying treatment based on building materials utilization
The high-dry dehydration process can reduce the sludge moisture content to 10%. And most building materials companies have sufficient waste heat and waste heat, such as kiln waste heat and flue gas in brick and tile factories. Using waste heat to dry sludge is a popular research direction in recent years. Treat waste with waste and dry sludge at the lowest cost to achieve the goal of substantial reduction. The dried sludge is mixed into building materials in a certain proportion as raw materials to realize resource utilization.

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Sludge disposal

Sludge disposal refers to the disposal of treated sludge in the natural environment (ground, underground, water) or reuse, which can achieve long-term stability and the ultimate consumption method without adverse effects on the ecological environment. The main sludge disposal methods in my country currently include sanitary landfill, incineration, land utilization, and building materials utilization.

In 2009-2011, under the leadership of the Ministry of Housing and Urban-Rural Development, a batch of urban sewage treatment plant sludge treatment and disposal standards was compiled and issued, of which the “GB/T23484-2009 Urban Sewage Treatment Plant Sludge Disposal Classification” clearly provided four categories and a total of 11 items Disposal routes, corresponding to these disposal routes, issued (extended) a series of mud quality standards.

Among the 11 disposal routes in the above four categories, the three routes of separate landfill, production of lightweight aggregates, and fuel utilization (combined burning in power plants) do not yet have specific corresponding mud quality standards. There are objective policy risks and should be considered technically There are obstacles or lack of conditions for large-scale use in China. In practical applications, the two routes of brick making and separate incineration have basically not been adopted due to economic and environmental reasons. Mixed landfill (including the use of covering soil) is frequently eliminated as the resources of the landfill are gradually depleted. Some insiders suggest that the remaining five routes (landscaping, soil improvement, agricultural use, cement production, and mixed burning of garbage) should be selected in the following order according to the level of environmental protection requirements:

S.N. Item Range Details Standards
1 Sludge land use Landscaping Substrate material or fertilizer raw material for construction and maintenance of urban green space system or suburban forest land GB/T23486-2009、CJ/T362-2011
Land improvement Soil improvement materials for saline land, sandy land and abandoned mines GB/T24600-2009
Agricultural Agricultural fertilizer or farmland soil improvement materials GB4284-1984、CJ/T309-2009
2 Sludge landfill Separate landfill Landfill disposal in a landfill dedicated to landfill sludge There is no corresponding mud quality standard
Mixed landfill Mixed landfill in municipal solid waste landfills (including utilization of landfill covering materials) GB/T23485-2009
3 Utilization of sludge building materials Cement Some raw materials or additives for cement CJ/T314-2009
Brick making Some raw materials for making bricks GB/T25031-2021
Lightweight Aggregate Some raw materials for making lightweight aggregates (ceramsite, etc.) There is no corresponding mud quality standard
4 Sludge incineration Burn separately Special sludge incinerator for incineration GB/T24602-2009
Incineration mixed with garbage Incinerate with domestic waste There is no corresponding mud quality standard
Sludge fuel utilization Used as fuel in industrial incinerators or incinerators in thermal power plants There is no corresponding mud quality standard

(1) When the mud quality meets the requirements of GB 4284-1984 and CJ/T309-2009, and there is enough farmland for consumption, the agricultural disposal method of sludge is preferred;
(2) When the mud quality does not meet the above conditions (1) but meets the requirements of GB/T 23486-2009 and CJ/T362-2011, and there is enough green land (or woodland) for consumption, use sludge landscaping disposal method;
(3) When the mud quality does not meet the above conditions (1) and (2) but meets the requirements of the GB/T 24600-2009 indicators, and there are sufficient saline-alkali land, sandy land and abandoned mines waiting to be repaired for land use Disposal method of sludge soil improvement;
(4) When the mud quality does not meet the above conditions (1), (2), (3), or meets the requirements of various indicators in GB/T 24600-2009 but does not have enough land for consumption to be repaired and has a waste incineration plant At the same time, when the mixed burning can meet the requirements of GB18485-2014, the mixed burning with garbage disposal method shall be adopted;
(5) When the mud quality does not meet the above conditions (1) and (2), and the cement kiln resources are not available for the land to be repaired and the waste incineration plant, the cement kiln co-processing method is adopted; because the cement kiln Resources are extremely limited, and many central cities reserve them as hazardous waste and emergency solid waste disposal resources, so it is difficult to use them as a long-term means of large-scale sludge disposal.

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