Currently, among the CFB boilers put into operation domestically, a large portion is affected in normal and stable operation due to fuel particle characteristics not meeting boiler design requirements, which to some extent restricts the promotion and development of the main CFB boiler technology. This article analyzes the current status of domestic CFB fuel preparation system process schemes and its main auxiliary equipment (screening and crushing machinery), proposing technical measures such as fuel drying and classification pretreatment, wear resistance life of main functional components, and equipment selection. Keywords: fuel preparation drying classification pretreatment

　　0 Preface

　　Circulating Fluidized Bed Boiler (CFB) technology has developed considerably domestically, with large 450t/h class CFB boilers being successively built and put into operation in recent years. CFB boilers, with their **, low pollution, adaptability to various fuels, good load regulation performance, and comprehensive utilization characteristics of ash and slag, occupy a dominant position in the domestic combined heat and power field.

　　The many superior characteristics of circulating fluidized bed boilers are determined by combustion characteristics completely different from bubbling fluidized beds and pulverized coal boilers. This special boiler combustion characteristic imposes strict requirements on the particle characteristics—particle size and particle size distribution—of the fuel entering the furnace. Appropriate fuel particle size and distribution are prerequisites and guarantees for the normal operation of circulating fluidized bed boilers. According to our understanding and related information, currently, a large portion of domestically operated circulating fluidized bed boilers do not meet design requirements for boiler output, combustion efficiency, and other operational technical indicators due to fuel particle characteristics not conforming to boiler design requirements, even seriously endangering normal and stable boiler operation. Fuel particle characteristics are crucial to the safe and stable operation of circulating fluidized bed boilers. Considerable and effective work has been done domestically to prepare fuel that meets the particle characteristics specified by circulating fluidized bed boiler design. However, compared to the rapid development of the main circulating fluidized bed boiler technology, there is still a significant gap, which to some extent restricts the promotion and development of the main technology. This article provides a preliminary analysis and discussion on the current status of domestic circulating fluidized bed boiler fuel preparation systems, focusing on preparation process design and process equipment (mainly screening and crushing machinery), aiming to synchronize fuel preparation technology and process with the main technology development.

　　1 Requirements of circulating fluidized bed boilers for fuel particle characteristics

　　1.1 Influence of fuel particle characteristics on circulating fluidized bed boiler operation

　　Fuel particles entering the combustion chamber are suspended and flow within the bed under the action of flue gas flow at a certain velocity. When the fluidization gas velocity increases, particles in the high-speed gas-solid suspension are elutriated by fluidization gas with a velocity greater than the terminal velocity of individual particles, and a high proportion returns to the furnace bottom, forming a certain degree of particle recirculation. Meanwhile, due to fluid dynamic interactions, the non-uniform suspension of fine particles gathered together moves up and down in a very dilute upward flowing gas-solid continuous medium (fluidized flue gas carrying sparse dispersed particles moves upward in piston flow). The movement and mixing of particles in the bed form internal and external circulation, and fuel burns in the upper, middle, and lower parts of the bed according to certain combustion shares, ensuring uniform temperature inside the furnace and greatly increasing the combustion time of recirculated particles, thereby improving the utilization rate of desulfurizing agents.

　　Coal particles entering the combustion chamber undergo a continuous process of heating and drying, volatile matter release and combustion, expansion and initial fragmentation (for some coals), coke combustion and secondary fragmentation, and abrasion during combustion. Therefore, the combustion efficiency of circulating fluidized beds is closely related to the combustion rate of particles at each combustion stage.

　　In summary, fuel particle characteristics greatly affect the combustion share inside the circulating fluidized bed boiler, circulation rate, and wear of heat transfer surfaces. Therefore, ensuring the combustion shares and circulation material quantities in various parts of the bed as determined by boiler design is a necessary condition for achieving heat balance in all parts during boiler operation, guaranteeing boiler output, and preventing local overheating and material coking, thus ensuring safe and stable boiler operation. To meet these conditions and fully utilize the characteristics of circulating fluidized bed boilers, each circulating fluidized bed boiler imposes special requirements on fuel particle characteristics—particle size and particle size distribution.

　　1.2 Requirements of circulating fluidized bed boilers for fuel particle characteristics

　　The determination and selection of fuel particle size distribution entering the circulating fluidized bed boiler are related to the choice of fluidization velocity. Circulating fluidized bed boilers require a large proportion of fine particles in the fuel with terminal velocities less than the fluidization velocity, so that these fine particles, once entering the furnace, have enough fine coal particles blown into the suspension section space to burn, ensuring the combustion share in the upper part and increasing the circulation material quantity. Coarser fuel particles burn in the lower high-temperature dense phase zone of the circulating fluidized bed to improve the heating rate of newly added fuel, which helps shorten the burnout time of fuel particles.

　　(1) The particle size distribution of the fuel should ensure that under the determined fluidization velocity conditions, enough fine particles are blown into the suspension section to guarantee the combustion share in the upper (dilute phase) zone and form sufficient bed material to maintain material balance. That is, the particle size distribution of the fuel entering the furnace should meet the requirements of a wide screening distribution.

　　(2) The particle size of furnace fuel should be adjustable: that is, adjusted according to different coal types and circulation rates. Generally, circulating fluidized bed boilers with high circulation rates use finer coal particles, while those with low circulation rates use coarser fuel particles; coals with low volatile content generally require finer particles, while high volatile, easily combustible coals can have coarser particles.

　　(3) The particle size range and particle size distribution of fuel entering the furnace: due to differences in furnace type or parameter selection, each boiler company has its own adopted particle size range. In China, circulating fluidized beds generally use particle sizes of 0-13mm, 0-10mm, or 0-8mm, and fuel particle size distribution requirements are proposed.

　　2 Analysis of the current status of domestic circulating fluidized bed boiler fuel preparation technology

　　2.1 Fuel preparation system process forms

　　Currently, the process form of domestic circulating fluidized bed boiler fuel preparation systems is basically "coarse crushing + screening + fine crushing," with other forms appropriately changed or recombined according to the specific situation and conditions of each project. Common forms include:

　　(1) "Coarse crushing + screening + fine crushing": This form can basically meet the requirements of coal entering circulating fluidized bed boilers. Its characteristics are: ① reasonable distribution of the total crushing ratio of the system; ② reduction of excessive pulverization of coal, with fuel particle size distribution basically conforming to the wide screening distribution law; ③ selection of small-sized fine crushers.

　　This form is suitable for systems where the raw coal contains many particles exceeding the specified size, and particles larger than 50mm account for a certain proportion.

　　(2) "Screening + Fine Crushing": This form is suitable for raw coal where the vast majority is less than 50mm, with very few large particles over 50mm and not exceeding 80mm in the system.

　　(3) "Coarse Crushing + Fine Crushing": This form is more suitable for raw coal with larger particle size, more impurities, relatively higher moisture content, and systems prone to screen clogging. Its disadvantage is the serious over-crushing of fuel.

　　According to information, foreign CFB boiler fuel preparation process systems have various other forms, such as

　　(1) Grinding Preparation System: Rod mills are widely used in the building materials industry and non-ferrous metal grinding processes. Based on the characteristics of circulating fluidized bed boiler fuel preparation, rod mills are used as fuel grinding equipment. The finished coal particle size prepared by rod mills is 0-10mm, with 66% ≤1.1mm and 60% ≤0.84mm.

　　The rod mill preparation system is simple, reliable in operation, generally unaffected by moisture and "four blocks" in raw coal, and the coal particle size can be adjusted within a certain range. Therefore, it is more suitable for circulating fluidized bed boilers that burn anthracite, stone coal, or require finer coal particle sizes. Its disadvantage is relatively high investment.

　　(2) Shaft Hammer Mill System: This system is an improvement based on the traditional vertical shaft hammer mill pulverizing system. It uses a new type of hammer crusher and adds a sorting and drying system.

　　This coal preparation system can adjust the finished coal particle size by adjusting the sorting and drying system and the hot air speed inside the hammer mill chamber, and is not affected by raw coal moisture. The finished coal particle size can be adjusted according to coal type changes. Since the system operates under negative pressure, the environment is cleaner and operation is reliable. Its disadvantage is the relatively high cost of the hammer mill because hot drying air passes through the grinding chamber, requiring special cooling for the main shaft and parts; investment is large and the system is complex.

　　(3) Drying and Sorting Coal Preparation System: The coal preparation system with drying and sorting (flue gas drying sorting or steam drying sorting) is an ideal coal preparation solution for circulating fluidized bed boiler systems. Because it uses drying and sorting devices, the crusher is not affected by raw coal moisture, and the system operates reliably. This system's investment is less than rod mills and vertical shaft hammer mill systems, slightly higher than ordinary screening and crushing systems, but overall economic benefits are higher.

　　In addition, there are designs as closed-loop systems, such as circulating screening and circulating crushing and other process design forms.

　　2 Fuel Preparation Process Equipment

　　3 Analysis of Problems in Fuel Preparation Systems

　　Currently, many problems exist in domestic circulating fluidized bed boiler fuel preparation systems, directly affecting the safe and stable operation of boilers.

　　3.1 The particle size and particle size distribution of fuel entering the furnace do not meet boiler operation requirements

　　Circulating fluidized bed boilers in operation and under construction in China generally require fuel particle sizes within 0-8mm or 0-10mm, with particle size distribution following a wide screening distribution law. Some boilers require particles of 0.1-1mm to account for about 40-60%. However, in actual operation, some systems produce fuel that is too fine, while others have too many large particles exceeding the required range. The main reasons are as follows:

　　(1) The crushers commonly used domestically are ring hammer crushers or toothed plate impact crushers, mainly using impact crushing. Domestic crusher manufacturers design crushers considering the complexity of raw coal varieties and impurities, especially hard impurities like gangue that are difficult to finely crush, by increasing rotor linear speed and hammer mass to enhance crushing capacity. This results in relatively excessive crushing energy, causing over-crushing of coal.

　　(2) In system renovation projects, due to funding or existing system structure, and mainly because no dry screening equipment suitable for difficult-to-screen coal under domestic conditions exists so far, a two-stage full-pass crushing process without screening and classification measures must be used. Raw coal already contains a considerable amount of small particles, and all pass through two-stage crushing without intermediate screening separation, resulting in particle size distribution deviating from normal wide screening distribution and over-crushing. This two-stage full-pass crushing process without screening and classification measures can only be used when raw coal contains many large blocks over 100mm and few small particles, or when raw coal contains much gangue, or when the average particle size of raw coal is above 80mm.

　　(3) After wear of the main working parts of crushers, especially fine crushers, such as hammers and crushing plates, firstly, hammer mass decreases causing reduced crushing capacity; secondly, the impact surface changes from flat to curved, altering the impact angle (see Figure 1). To some extent, the original impact crushing becomes compression crushing, and the gap between hammer and crushing plate — an important factor controlling crushing size — widens. As a result, product particle size becomes too large, and the hammer's material throwing function decreases sharply, causing a significant drop in crusher output.

　　Figure 1 Working condition before and after hammer wear

　　(4) The particle size of raw coal entering the crusher cannot be effectively controlled, and oversized raw coal beyond the crusher's capacity directly enters the crusher, causing product particle size to exceed standards. This situation is common in single-stage crushing systems. Hammer crushers mainly using impact crushing should control the crushing ratio around 10. Exceeding the allowable crushing ratio will cause product particle size to exceed standards or output to decrease.

　　3.2 Screening and Classification Efficiency is Low

　　Screening and classification of materials before crushing - coarse and fine crushing - is very important for fully utilizing crusher efficiency and ensuring product coal meets the particle characteristics required by circulating fluidized bed boilers. However, most domestic power plants receive coal or raw coal supplied to coal preparation systems with many non-mineral impurities and high moisture. Conventional screening equipment cannot perform normal screening functions and efficiency, resulting in very low screening and classification efficiency.

　　Currently, domestic circulating fluidized bed boiler fuel preparation systems commonly use vibrating screens, string screens, mechanical spiral screens, and fine-hole roller screens. However, these screening machines generally have low screening efficiency or even clog screen holes and lose or weaken screening function when screening raw coal without drying and removal of non-mineral impurities, sometimes only functioning as low-efficiency feeders.

　　According to the information, the UK has developed a method to assess the difficulty of dry screening of final coal, which is to evaluate the screening difficulty based on the coal's treatability index. The treatability index of coal is related to the raw coal particle size distribution, moisture content (especially surface moisture), mud shale content, and its mudification characteristics. When the index exceeds 5 seconds, vibration screening becomes very difficult. Generally, raw coal with 4-14% moisture content screened dry below 13 mm is considered difficult to screen material. The fuel required for circulating fluidized bed boilers falls within the range of difficult-to-screen materials. Therefore, without changing the raw coal condition and blindly pursuing screening and crushing equipment adaptable to various working conditions, it is impossible to achieve ideal results under current technical conditions.

　　(1) The main functional parts of crushing machinery have a short wear-resistant life and cannot operate continuously and stably within one maintenance cycle. Frequent shutdowns for replacement and maintenance are required, which affects system stability and increases maintenance workload.

　　(2) In the process design of the circulating fluidized bed boiler fuel preparation system, due to various constraints such as limited project funding or existing system structure limitations, or mostly because there are no more suitable screening and crushing equipment options for the working conditions, unreasonable system structure designs or less ideal equipment have to be used, resulting in the system operation failing to meet the fuel characteristics requirements of the circulating fluidized bed boiler. For example, secondary crushing systems without screening and classification measures and single-stage crushing systems often fail to achieve the expected results. At the same time, some projects have unreasonable selection of screening and crushing equipment.

　　(3) The issue of large-scale fuel preparation equipment for circulating fluidized bed boilers.

　　Domestic fuel preparation equipment is currently basically in the trial operation stage, and its specifications are only below 200t/h, which can only meet the needs of small and medium-sized fluidized bed boiler systems. Screening and crushing equipment for large circulating fluidized bed boiler systems above 410t/h basically use imported equipment. Therefore, the design technology of domestic screening and crushing equipment urgently needs to be improved to quickly design and produce large screening and crushing machinery to meet the needs of large fluidized bed systems.

　　4 Discussion on measures to improve circulating fluidized bed boiler fuel preparation technology.

　　4.1 Pre-treatment of raw coal supplied to the fuel preparation system by drying and impurity removal.

　　During system process design, effective measures should be taken as much as possible to create favorable working conditions for screening and crushing equipment to improve their working efficiency. That is, raw coal should be dried and non-mineral impurities removed before entering the preparation system, such as drying and sorting. Drying and sorting devices for coal have been introduced in relevant materials, such as flue gas drying and sorting devices (Fig. 2), steam drying and sorting devices (Fig. 3), etc. Some foreign circulating fluidized bed boiler systems also use raw coal drying for coal preparation. After raw coal drying, common problems in existing fuel preparation systems such as sticky coal, blockage, low efficiency of screening and crushing equipment, reduced output, and difficulty in ensuring product particle size are largely alleviated. Moreover, the commonly used screening and crushing equipment, especially screening equipment, can perform their designed functions and efficiency normally.

　　Dust pollution problems occurring during crushing and screening after raw coal drying treatment are solved by strengthening the sealing of screening and crushing equipment and adding dust collectors.

　　Fig. 2 Flue gas drying and sorting device Fig. 3 Steam drying and sorting device.

　　4.2 Wear-resistant material technology is the key to the current development of screening and crushing technology.

　　Wear-resistant materials, mainly the wear resistance of the main functional parts of crushing equipment, are the main factors affecting the performance of domestic crushing equipment.

　　In conventional power plant coal conveying systems, the hammer heads of coal crushers have long used cast steel Mn13 material. The national standard stipulates that its wear-resistant life should reach 3000 hours. This can basically be achieved in systems with pre-crushing screening separation, but in full-through systems without pre-crushing screening, if the raw coal contains more fine coal and less large lumps, the wear-resistant life index is basically not met. This is due to the special properties of Mn13 material.

　　In the circulating fluidized bed boiler fuel preparation system, whether single-stage or two-stage crushing, the coal entering the fine crusher is medium and fine coal. Although the crusher uses impact crushing mechanism, fine coal particles do not cause impact hardening on the hammer head material (Mn13). Therefore, the hammer head wear life of early developed fine crushers (generally using Mn13) is only a few hundred hours. So far, the wear life of hammer heads produced domestically can only reach about 1000 hours. In the latter half of use, operation can only be maintained under severe wear with relatively low product particle size and output. In recent years, some manufacturers have been continuously researching and testing various new materials, but their wear life is only within 1500-2000 hours. Severe hammer head wear not only causes serious fuel preparation over-standard emissions but also significantly reduces crusher output. To ensure system indicators, frequent replacement of new hammer heads is required, increasing operating costs and labor intensity of maintenance personnel. This also lowers the market reputation of domestic products and restricts the development of domestic auxiliary machinery technology. It is said that American or German products have wear lives of 8000-10000 hours, which is 5-10 times longer than domestic products.

　　Currently, domestic manufacturers are actively cooperating with relevant material research departments and wear-resistant material experts for research and breakthroughs. However, our funding, experimental research methods, and industrial test conditions are extremely limited, and research progress is very slow. This requires policy guidance and support in funding and industrial testing from national or industry-related departments.

　　4.3 Selection of fuel preparation process schemes.

　　The selection of fuel preparation process schemes should comprehensively consider the comprehensive functions and economy of process equipment:

　　(1) The aforementioned process scheme of drying and sorting raw coal, although increasing investment and occupying space, is extremely important because it improves the overall function of the fuel preparation system and enhances the combustion efficiency and safe, stable, and reliable operation of the circulating fluidized bed boiler.

　　(2) Raw coal is screened before entering the coarse crusher and fine crusher respectively to avoid repeated crushing of finished coal that meets particle characteristic requirements, which would destroy the original particle characteristics. That is, the system adopts a two-stage screening and two-stage crushing process scheme of drying-screening-coarse crushing-screening-fine crushing. The advantages of this process scheme are:

　　1) The screening particle size can be adjusted according to the fuel particle characteristic requirements of different boilers, effectively ensuring the product's particle characteristics;

　　2) Small-sized crushing equipment can be selected based on the proportion of material on the screen;

　　3) Reduce wear on the main functional parts of crushing equipment and extend their service life;

　　4) Basically eliminate the sticking and blocking phenomenon of screening and crushing equipment, maintaining stable operation.

　　(3) Reasonably allocate the total crushing ratio: When selecting the crushing process based on the particle size of the raw coal and the required product particle size, the coarse crushing ratio is generally around 10 (hammer impact crushing), and the fine crushing ratio should be chosen as small as possible, generally between 3-6.

　　(4) The crushing ratio of the fine crusher should be chosen as small as possible (between 3-6), while the power of its matching drive motor should be selected larger.

　　When crushing materials, the crushing machinery applies a huge force to the material particles, and the force exceeds the bonding force between particles causing crushing. During the crushing process, especially during grinding, the specific surface area of the product greatly increases, producing so-called mechanochemical effects, increasing deformation work, surface energy, crystal structure change energy, etc., and the crushing energy consumption sharply increases. According to P.P. Riottinger's theory, crushing energy consumption is proportional to the new surface area generated during crushing. Therefore, at different crushing stages, although the crushing ratio is the same, the power consumption used is quite different.

　　Currently, the Bond (F.C. Bond) formula suitable for the coarse grinding stage is used to calculate and compare the crushing energy consumption at different crushing stages:

　　A=10Wi(1/√d80 - 1/√D80)

　　In the formula: A is the energy consumption for crushing one ton of material (kWh/short ton)

　　Wi is the proportional constant, also called the work index, (kWh/short ton)

　　For coal, Wi = 11.37 kWh/short ton = 12.53 kWh/ton.

　　D80, d80 are the particle sizes (microns) corresponding to 80% cumulative content of the material before and after crushing.

　　The crushing energy consumption calculated by the above formula for different crushing stages with different feed and product particle sizes but the same crushing ratio is listed in the table below:

　　From the data analysis in the table, it can be seen:

　　(1) Although the crushing ratio is the same, the power consumption differs when the feed and product particle sizes differ. For example, crushing coal from 100mm to 10mm and from 10mm to 1mm consumes very different power; the former is one-third of the latter;

　　(2) For coal with different gradations, the energy consumption required to crush to the particle size composition required by the circulating fluidized bed boiler also varies greatly.

　　4.4 Introduce and digest international ** technology to improve auxiliary machinery technical level

　　There is a considerable gap with international ** countries.

　　Structural design of fine crushers: Currently, the widely used hammer-type fine crusher domestically is represented by the American and German tooth plate impact fine crushers, which we call the American type (Figure 5) and the European type (Figure 6).

　　In the circulating fluidized bed boiler fuel preparation system, the main coal preparation equipment includes screening separators, machinery, and crushing, mainly fine crushing machinery. Domestically, a considerable scale of development, design, and manufacturing has been formed, and the structural design and manufacturing technology level has approached international similar product levels, but some technical fields still lag behind international ** countries.

　　Figure 5 American type Figure 6 European type

　　The American type features a relatively small crushing chamber volume, hammer heads are integrally cast, with short hammer handles and large hammer heads, arranged densely along the main shaft axis (see Figure 7). The crushing plate adjusts the crushing gap by changing the angle around the top shaft; the European type features a large crushing chamber volume, small hammer heads with long handles, and the hammer head and handle are made of two different materials using an assembled structure, arranged with large axial spacing (see Figure 8). The crushing gap adjustment can be done by angle change or translation.

　　According to our analysis, the advantage of the American type is the large hammer head and strong crushing capacity, more suitable for crushing hard coal with a high amount of gangue; however, for large output systems above 300t/h, its axial dimension is too large, making uniform feeding difficult. The European type crusher has a larger chamber volume, a wide range of output adjustment, and stronger adaptability to various working conditions.

　　The domestic auxiliary machinery manufacturing industry basically develops and designs based on international ** equipment technology. However, so far, the domestically designed and produced fine crushers only reach about 200t/h. Shenyang Electric Power Machinery General Factory is designing and manufacturing 300t/h and 400t/h class tooth plate impact fine crushers, but there are no trial operation examples yet.

　　Therefore, we must introduce and digest international ** technology, research, design, and produce auxiliary machinery equipment suitable for the domestic market and working conditions, while adapting to the development and construction scale and speed of circulating fluidized bed boiler technology, and develop large-scale screening and crushing equipment.

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