【正文】 煤在其中較長的停留時間可以實現(xiàn)有效的磨制。同時，筒式鋼球磨難于控制且有較高的磨損速度。 Ballandtube mills have largely been supplanted by vertical airswept pulverizers for new boilers. They typically require larger building volume and higher specific power consumption than the vertical airswept pulverizres. They are also more difficult to control and have higher metal wear rates. They are, however, well suited for grinding extremely abrasive, low moisture and difficult material such as petroleum coke. Their long time makes them effective for fine grinding. 2917 教材 32 頁 對新建鍋爐來說，中速磨已經大量的取代了筒式鋼球磨。筒式鋼球磨不具有類似立式磨的流化床特點，同時由于空氣和煤粉的混合不均勻限制了干燥能力。雙進雙出型磨煤機是空氣和原煤從兩端進入，磨好的干燥的煤粉從兩端流出。 Ballandtube mills may be either single or double ended. In the former, air and coal enter through one end and exit the opposite. Double ended mills are fed coal and air at each end and grounddried coal is extracted from each end. In both types, classifiers are external to the mill and oversize material is injected back to the mill with the raw feed. Ballandtube mills do not develop the fluidized bed which is characteristic of vertical mills and the poor mixing of air and coal limits the drying capability. When coal with moisture over 20% must be ground in ballandtube mills, auxiliary equipment, usually crusher dryers, must be used. 筒式鋼球磨有單進單出和雙進雙出兩種。轉速取離心力可以克服重力時速度的 80%，這樣可以使鋼球貼在筒體的內壁上。它是一個臥式的筒體，里面裝有小直徑的鋼球。風粉混合物中的細煤粉顆粒保持懸浮狀態(tài)，并最終上升進入煤粉管。風粉混合物以一定角度進入，從而發(fā)生旋轉并產生離心力。 As the airsolids mixture flows upward, the flow area increase and velocity decreases returning larger particles directly to the grinding zone. The final stage of size separation is 教材 31 頁 provided by the classifier located at the top of the pulverizer. This device is a centrifugal separator. The coalair mixture flows through openings angled to impart spin and induce centrifugal force. The coarser particles impact the perimeter, e out of suspension and fall back into the grinding zone. The finer particles remain suspended in the air mixture and exit to the fuel conduits. 空氣煤粉向上流動時，由于流動面積增大使流動速度降低，大粒徑的煤粒就會回落到磨盤上。即使煤中水分到 40%也能在中速磨中很好地得到干燥，干燥水分再高些的煤粉也是可能的，但是需要的一次風溫度則要求使用特殊材料，并且增加了磨煤機著火的可能。 Vertical pulverizers are effective drying devices. Coal with moisture content up to 40% have been successfully handled in vertical mills. Higher moisture levels are possible, but the primary air temperature needed would required special structure materials and would increase the chance of pulverizer fires. A practical moisture limit is 40%, by weight, requiring air temperature to 750186?？諝夂兔毫ｋx開流化床形成了第一步的分離。上升的空氣流與煤?；旌显谶M風環(huán)上面產生流化的顆粒床。離心力加上磨輥對煤層的沉降力共同作用，將部分磨好的煤粉擠出磨盤邊緣，由上升的空氣流流化并攜帶這些煤粉。在磨煤機下部有一個轉動的臺面，稱為輥胎的輥子在臺面上滾動。 As grinding proceeds, fine particles are removed from the process to prevent excessive grinding, power consumption and wear. presents a simplified MPS vertical pulverizer, showing the essential elements of a vertical airswept design. A table is turned from below and rollers, called tires, rotate against the table. Raw coal is fed into the mill from above and passes between the rollers and the rotating table. Each passage of the particles under the rollers reduces the size of coal. The bined effects of centrifugal force and displacement of the coal layer by the rollers spills partly ground coal off the outside edge of the table. An upward flow of air fluidizes 2915 and entrains this coal. 隨著磨煤的進行，為了防止過度磨制和降低能耗及磨損，磨好的煤粉從磨煤機中排出。當磨煤區(qū)的工作面間距離很近時，比如到了一個顆粒大小，三個部件（磨輥，顆粒，磨盤）間的磨損就會大大增加，磨損速率會是正常磨煤機 的 100 倍。一定壓力下在煤粒層上的運動引起摩擦（煤粒依靠摩擦力破碎），這就是磨煤機的工作原理。 Vertical airswept pulverizers The roller passes over a layer of granular material, pressing it against a moving table. The movement of the roller causes motion between particles, while the roller pressure 教材 30 頁 creates pressive loads between particles. Motion under applied pressure within the particle layer cause attrition (particle breakup by friction) which is the dominant size reduction mechanism. The pressed granular layer has a cushioning influence which reduces grinding effectiveness but also reduces the rate of roller wear dramatically. When working surfaces in a grinding zone are close together, near the dimensions of single product particles, wear is increased by three body contact (roller, particle and table). Wear rates can be three body contact has also been observed in operating mills when significant amounts of quarts bearing rock are present in sizes equal to or greater than the grinding layer thickness. 中速磨 磨輥在一層耐磨層上滾動，通過移動的磨盤把煤壓碎。相對來說，負壓磨煤機的密封標準并不需要這樣高，但也不允許漏入過多空氣，因為冷空氣難以干燥濕煤。然而，磨煤機需要由單獨風機提供高于磨煤機內部壓力的密封空氣。 On pressurized pulverizing systems the primary air fan which provides the pulverized fuel transport medium is situated before the pulverizer and there handle clean air and is not subject to erosive wear as is an exhaust fan. This is the chief advantage of the pulverizing system, however the pulverizer does need sealing air which is usually provided by a separate fan at a pressure higher than that of the pulverizer interior. 正壓制粉系統(tǒng)中，提供煤粉輸送介質的一次風機位于磨煤機前，因而它運送的是清潔空氣，不會像排粉風機一樣受到侵蝕磨損。較早的系統(tǒng)使用筒式球磨機磨煤粉，并且在燃燒前利用儲倉暫時儲存煤粉。為了使煤在爐膛中有效燃燒，煤在離開燃燒器時必須被粉碎到一定的大 小，這樣才能迅速燃燒，這就意味著煤必須被加工成小顆粒，才能被迅速加熱到著火溫度并和空氣良好混合。在中國最大的流化床鍋爐是 330MWe，設計最大的鍋爐是 600 MWe，但是還沒有投建。這些為煙氣凈化節(jié)省了大筆的投資，但是低的 SOx 排放需要燃燒低硫分煤，并且 NOx 的排放受燃燒反 應的限制。通過低燃燒2913 溫度和空氣分級燃燒來控制 NOx 的生成。另一部分蒸汽是由流化床中的高溫固體在返回爐膛前放出熱量產生的。煤和煤焦燃燒的同時，空氣攜帶煤、煤焦、煤灰和脫硫劑通過爐膛。循環(huán)流化床鍋爐適合與生物質共燃，最近就新建了幾臺燃燒褐煤的循環(huán)流化床機組。對于 煤 燃燒，蒸汽循環(huán)可以是亞臨界，也可能是超臨界，它們具有相近的發(fā)電效率。流化床中主要是石灰和少量的煤，煤焦在其中循環(huán)。循環(huán)流化床最適合于燃燒低成本廢棄燃料、 低品質或低熱量煤。 Fluidizedbed bustion A variation on PC bustion is fluidized bed bustion in which coal is burned with air in a fluid bed, typically a circulating fluidized bed (CFB). CFBs are best suited to lowcost waste fuels and lowquality or low heating value coals. Crushed coal and lime stone are fed into the be