【正文】 er walls in boilers for subcritical steam conditions are generally configured as increasing steam temperatures and pressures, the fraction of evaporator heating surfaces decreases, with the result that parts of the water walls must also be configured as superheaters, of the separator. In the highly loaded furnace area, spiralwound evaporator tubing is usually used with smooth tubes and high mass fluxes– approx. 2022– 2500 kg/㎡ s. As spiralwound furnace tubing of this type is not selfsupporting, it is reinforced with support straps which are welded to the tube wall with support blocks. High steam parameters also lead to higher material loading in the evaporator. The previously existing design reserves are no longer available, with the result that a detailed stress analysis is required for the design of the evaporator tubing in each case. As a result of the requisite 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 31 頁(yè) 共 40 頁(yè) large wall thicknesses, the design of highly loaded heating surface areas is in part no longer determined by the primary stresses due to internal pressure but rather by the secondary stresses due to restrained thermal expansion. The higher evaporator temperatures also result in increasing temperature differences between the tubes and support straps on startup and shutdown. This in turn leads to longer startup times, especially on cold start. For steam temperatures up to approx 550℃ , all heating surfaces can be constructed of ferritic or martensitic materials, while at 600℃ austenitic materials are necessary for the final superheater heating surfaces for both the HP section of the boiler as well as the reheater. In addition to the strength parameters, corrosion behavior on the fluegas and oxidation behavior on the steam sides is especially determinative for material selection. With regard to strength parameters, construction of superheater heating surfaces for steam temperatures up to 650℃ is currently already feasible with austenitic steel materials. The corrosion resistance of the available materials however reduces the design limits to about 630℃ . Superheaters and reheaters are specially designed inline tube bundles that increase the temperature of saturated steam. In general terms, they are simplephase heat exchangers with steam flowing inside the tubes and the flue gas passing outside, general in crossflow. These critical ponents are manufactured from steel alloy material because of higher operating temperature. They are typically configured to help control steam outlet 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 32 頁(yè) 共 40 頁(yè) temperature, keep metal temperature below acceptable limits and control steam flow pressure loss. The main difference between superheaters and reheaters is the steam pressure. The physical design and location of the surfaces depend upon the desired outlet temperatures, heat absorption, fuel ash characteristics and cleaning equipment. These surfaces can be either horizontal or vertical. The superheater and sometimes reheater are often divided into multiple sections to help control steam temperature and optimise heat recovery. Economizers and air heaters perform a key function in providing high overall boiler thermal efficiency by recovering the low level, . low temperature, energy from the flue gas before it is exhausted to the atmosphere. Economizers recover the energy by heating the boiler feedwater while air heaters heat the bustion air. Air heating also enhances the bustion of many fuels and ensuring ignition. The economizer is a counterflow heat exchanger for recovering energy from the gas beyond the superheater and, if used, the reheater. It increases the temperature of the feedwater。 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 25 頁(yè) 共 40 頁(yè) （ 10） 鍋爐低負(fù)荷運(yùn)行如發(fā)生爐膛負(fù)壓波動(dòng)大時(shí)應(yīng)及時(shí)查明原因，采取相應(yīng)的調(diào)整措施，保證鍋爐負(fù)壓運(yùn)行，嚴(yán)禁冒正壓。提高運(yùn)行氧量一方面可以降低爐膛煙溫，另一方面富氧燃燒可以防止產(chǎn)生還原性氣氛，而還原性氣氛會(huì)降低灰熔點(diǎn)導(dǎo)致結(jié)焦發(fā)生。 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 24 頁(yè) 共 40 頁(yè) 52 防止結(jié)焦的對(duì)策 （ 1） 有停爐機(jī)會(huì)應(yīng)檢查爐膛水冷壁和屏式過(guò)熱器粘污程度，進(jìn)一步確認(rèn)鍋爐結(jié)焦部位。 7 月份夜間降負(fù)荷后經(jīng)常能夠聽(tīng)到 7 號(hào)鍋爐爐膛有掉焦的聲音。 通過(guò)對(duì)華能德州電廠的 660 MW 機(jī)組定滑壓運(yùn)行數(shù)據(jù)分析， 得到以下結(jié)論： （ 1） 確定在現(xiàn)有調(diào)節(jié)系統(tǒng)的情況下，機(jī)組經(jīng)濟(jì)的運(yùn)行方式應(yīng)按照定 滑 定的運(yùn)行曲線運(yùn)行，即在額定負(fù)荷下采用定壓運(yùn)行方式，在 90%額定負(fù)荷和 50%額定負(fù)荷之間運(yùn)行 時(shí)按照綜合得出的滑壓曲線運(yùn)行，低于 50%額定負(fù)荷時(shí)采用定壓運(yùn)行，以使機(jī)組獲得較高的運(yùn)行經(jīng)濟(jì)性 。 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 21 頁(yè) 共 40 頁(yè) 第 四 章 機(jī)組 變壓運(yùn)行的經(jīng)濟(jì)性 分析 41 理論經(jīng)濟(jì)性分析 從熱力循環(huán)的理論角度考慮，機(jī)組在變壓運(yùn)行時(shí)，一方面進(jìn)汽閥的節(jié)流損失降低，漏汽損失也減少，高壓缸的效率有所增加，高壓缸的排汽溫度增高使機(jī)組更容易達(dá)到其設(shè)計(jì)的再熱蒸汽溫度，機(jī)組的汽動(dòng)給水泵耗功也會(huì)因鍋爐的給水壓力降低而降低，小機(jī)耗汽量減少使得機(jī)組的經(jīng)濟(jì)性得以提高；另一 方 面由于汽輪機(jī)高壓缸的焓降減少循環(huán)熱效率要減少，而機(jī)組的絕對(duì)內(nèi)效率就是相對(duì)內(nèi)效率和循環(huán)熱效率的乘積，所以機(jī)組變壓運(yùn)行的負(fù)荷范圍和參數(shù)選擇必須綜合考慮這兩個(gè)方面的因素，分析比較相對(duì)內(nèi)效率的增加幅度和循環(huán)熱效率的下降幅度 ，才能尋找出合理的滑壓運(yùn)行和定壓運(yùn)行點(diǎn) 。 缺點(diǎn)就是：?jiǎn)螐膯?dòng)時(shí)間看，配置高低壓汽機(jī)旁路與不帶鍋爐循環(huán)泵的啟動(dòng)系統(tǒng)，其啟動(dòng)時(shí)間相對(duì)于帶 BCP 循環(huán)泵的啟動(dòng)系統(tǒng)來(lái)說(shuō)要長(zhǎng)一些；從鍋爐熱效率看，不帶 BCP 循環(huán)泵的啟動(dòng)系統(tǒng)由于在啟動(dòng)時(shí)高、低壓加熱器及爐內(nèi)的熱量，要通過(guò)冷凝器來(lái)冷卻，在啟停時(shí)熱量有一定的損失，因此相比較帶 BCP 循環(huán)泵的系統(tǒng)而言，其熱效率要低些。小循環(huán)回路由汽水分離器、分離器儲(chǔ)水罐、 BCP循環(huán)泵 (包括其輔助系統(tǒng) )和 360 閥組成。 （ 2） 高溫?zé)煔獠糠侄搪分量諝忸A(yù)熱器進(jìn)口處。 （ 5） 加強(qiáng)火 焰監(jiān)視，一旦出現(xiàn)燃燒不穩(wěn)定，要及時(shí)采取措施穩(wěn)定燃燒。 （ 3） Bamp。 鍋爐燃燒穩(wěn)定性與爐膛形式、燃燒器性能、爐膛熱強(qiáng)度、煤質(zhì)等因素有關(guān)，很多時(shí)候取決于燃燒器的性能。上述問(wèn)題導(dǎo)致節(jié)流圈給水冷壁的安全運(yùn)行帶來(lái)很大的隱患。與垂直管圈水冷壁相比，螺旋管圈水冷壁的管子數(shù)量大大減少，而且不需加大管子之間的節(jié)距，使管子和肋片的金屬壁溫在任何工況下都安全。另外，為確保水冷壁管子安全、可靠，在爐膛高負(fù)荷區(qū)，螺旋管圈可使用內(nèi)螺紋管。對(duì)于螺旋管圈，由于各管工質(zhì)在爐膛中的吸熱量相差較小，其熱偏差較小。 產(chǎn)生脈動(dòng)的根本原因是蒸發(fā)受熱面中蒸汽與水的質(zhì)量體積存超超臨界直流鍋爐變壓運(yùn)行特性分析 第 10 頁(yè) 共 40 頁(yè) 在差異，引發(fā)脈動(dòng)的外因主要是爐內(nèi)熱負(fù)荷突變，特別是蒸發(fā) 開(kāi)始區(qū)段熱負(fù)荷的突變，造成局部壓力突增，從而引發(fā)脈動(dòng)。對(duì)于螺旋管圈，管子進(jìn)、出口壓力差與質(zhì)量流量的關(guān)系主要由摩擦阻力與質(zhì)量流量之間的關(guān)系決定，摩擦阻力與管子內(nèi)工質(zhì)的平均質(zhì)量體積和質(zhì)量流量的平方有關(guān)，由于不同管子工質(zhì)的平均質(zhì)量體積有可能不同，因而可能出現(xiàn)多值性。這種變壓運(yùn)行方式使水冷壁的工作條件變得極為復(fù)雜，從啟動(dòng)至額定負(fù)荷運(yùn)行，鍋爐運(yùn)行壓力從高壓、超高壓、亞臨界壓力逐漸增加到超臨界壓力，水冷壁的工質(zhì)由雙相流體轉(zhuǎn)變?yōu)閱蜗嗔黧w，工質(zhì)溫度也發(fā)生很大變化。 變壓運(yùn)行也有一定的如下缺點(diǎn)： （ 1） 變壓運(yùn)行時(shí)，隨著負(fù)荷降低機(jī)組循環(huán)效率明顯下降，這主要是由于初壓減低使得機(jī)組蒸汽可用焓減少的緣故。 （ 2）變壓運(yùn)行時(shí)高壓缸排汽溫幾乎不變，從而可在 很寬的負(fù)荷范圍內(nèi)維持再熱汽溫不變，提高了低負(fù)荷的經(jīng)濟(jì)性。第一級(jí)后汽溫基本不變，代表了其后各級(jí)汽溫也基本不變。但由于正常運(yùn)行時(shí)，調(diào)節(jié)汽門(mén)不能全開(kāi)，有一定的節(jié)流損失，也會(huì)降低機(jī)組運(yùn)行的經(jīng)濟(jì)性。 221 純變壓運(yùn)行 在整個(gè)負(fù)荷變化范圍內(nèi)，汽輪機(jī)的進(jìn)汽調(diào)節(jié)汽門(mén)全開(kāi)，由鍋爐改變主 蒸汽壓力來(lái)適應(yīng)機(jī)組負(fù)荷變化。 超超臨界直流鍋爐變壓運(yùn)行特性分析 第 4 頁(yè) 共 40 頁(yè) 第 二 章 超超臨界直流鍋爐變壓運(yùn)行的方式 21 兩種運(yùn)行方式的 介紹 單元機(jī)組的運(yùn)行目前有兩種基本形式，即定 壓運(yùn)行（或稱(chēng)等壓運(yùn)行）和變壓運(yùn)行（或稱(chēng)滑壓運(yùn)行）。