韓小渠， 劉 明， 嚴(yán)俊杰， 種道彤， 劉繼平， 肖 峰

(1． 西安交通大學(xué) 動(dòng)力工程多相流國(guó)家重點(diǎn)實(shí)驗(yàn)室， 西安 710049； 2． 東北電力設(shè)計(jì)院， 長(zhǎng)春 130021)

韓小渠1， 劉 明1， 嚴(yán)俊杰1， 種道彤1， 劉繼平1， 肖 峰2

(1． 西安交通大學(xué) 動(dòng)力工程多相流國(guó)家重點(diǎn)實(shí)驗(yàn)室， 西安 710049； 2． 東北電力設(shè)計(jì)院， 長(zhǎng)春 130021)

基于風(fēng)扇磨倉(cāng)儲(chǔ)式制粉系統(tǒng)的褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)建立了全廠分析模型，從熱力學(xué)第二定律角度探究該系統(tǒng)的節(jié)能原理.以某600 MW超臨界機(jī)組為例，對(duì)各級(jí)子系統(tǒng)及主要設(shè)備展開(kāi)分析.結(jié)果表明：褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)的全廠效率相對(duì)常規(guī)褐煤發(fā)電系統(tǒng)可提高3.42%；采用倉(cāng)儲(chǔ)式制粉系統(tǒng)將干燥乏氣與煤粉分離，杜絕了水分在爐內(nèi)的循環(huán)，提高了燃燒溫度，有效地降低了燃燒過(guò)程的損失，提高了鍋爐效率；褐煤煙氣預(yù)干燥系統(tǒng)的效率只有20.20%，不完善程度較大，通過(guò)優(yōu)化低溫爐煙抽取點(diǎn)至省煤器出口，可減小干燥介質(zhì)混合過(guò)程的損失，從而使全廠效率提高0.33%.

褐煤； 煙氣預(yù)干燥； 節(jié)能；分析； 系統(tǒng)優(yōu)化

我國(guó)褐煤礦藏資源豐富，目前已探明儲(chǔ)量達(dá)1 300 億t，占全國(guó)煤炭總儲(chǔ)量的13%左右.隨著我國(guó)動(dòng)力用煤供需矛盾的加劇，褐煤將逐漸成為我國(guó)火電機(jī)組的主要燃料之一.但是褐煤中的水分含量高、熱值低，導(dǎo)致直接燃燒褐煤的鍋爐排煙熱損失大，輔機(jī)耗電量高，發(fā)電效率相比煙煤發(fā)電系統(tǒng)普遍偏低.對(duì)褐煤進(jìn)行預(yù)干燥，降低入爐煤中的水分，提升其熱值，可以顯著提高機(jī)組的發(fā)電效率.因此，國(guó)內(nèi)外學(xué)者針對(duì)褐煤干燥技術(shù)進(jìn)行了廣泛研究[1]，為褐煤預(yù)干燥發(fā)電系統(tǒng)的實(shí)現(xiàn)奠定了基礎(chǔ)，同時(shí)對(duì)褐煤預(yù)干燥發(fā)電系統(tǒng)的節(jié)能潛力進(jìn)行了深入論證[2-4]，認(rèn)為褐煤預(yù)干燥發(fā)電系統(tǒng)將成為褐煤高效發(fā)電的有效手段.基于風(fēng)扇磨倉(cāng)儲(chǔ)式制粉系統(tǒng)的褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)(Flue Gas Pre-dried Lignite-fired Power System，F(xiàn)PLPS)，既發(fā)揮了風(fēng)扇磨煙氣干燥技術(shù)較為成熟的優(yōu)勢(shì)，又將開(kāi)式制粉系統(tǒng)與高效水回收技術(shù)相結(jié)合，通過(guò)乏氣深度冷卻，回收水分和相應(yīng)的汽化潛熱，這不僅提高了褐煤鍋爐效率，更實(shí)現(xiàn)了“煤中取水”.對(duì)于FPLPS的熱經(jīng)濟(jì)性和水回收效益，Ma等[5-6]進(jìn)行了理論分析和討論，筆者對(duì)其系統(tǒng)耦合變工況能耗特性展開(kāi)了仿真研究[7-9].

但是，以前的研究都是基于熱力學(xué)第一定律，僅從“量”的角度分析了褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)在設(shè)計(jì)工況和變工況條件下的節(jié)能潛力，尚沒(méi)有從“質(zhì)”的層次揭示其節(jié)能本質(zhì)和系統(tǒng)優(yōu)化方向.而分析方法[10]是定量剖析系統(tǒng)節(jié)能本質(zhì)、深入發(fā)掘系統(tǒng)節(jié)能潛力的有力工具，已有學(xué)者采用該方法進(jìn)行了大量的熱力系統(tǒng)優(yōu)化研究.如Ganapathy等[11]對(duì)褐煤發(fā)電機(jī)組進(jìn)行了分析，發(fā)現(xiàn)系統(tǒng)的損失中60%發(fā)生于燃燒過(guò)程.Xiong等[12]對(duì)富氧燃燒鍋爐進(jìn)行了分析，發(fā)現(xiàn)鍋爐效率提高的原因在于爐膛中不可逆損失的下降.Butcher等[13]對(duì)余熱鍋爐乏氣熱能回收系統(tǒng)進(jìn)行了分析，討論了乏氣參數(shù)變化對(duì)系統(tǒng)效率的影響.孟翔宇等[14]對(duì)太陽(yáng)能熱泵干燥系統(tǒng)進(jìn)行了分析，給出了提高系統(tǒng)效率的措施.陸萬(wàn)鵬等[15]對(duì)電站鍋爐排煙余熱能級(jí)提升系統(tǒng)進(jìn)行了分析.呂國(guó)強(qiáng)等[16]對(duì)300 MW亞臨界燃煤機(jī)組、劉強(qiáng)等[17]對(duì)600 MW超臨界機(jī)組、蔡小燕等[18]對(duì)1 000 MW超超臨界機(jī)組分別進(jìn)行了分析，指出了熱力系統(tǒng)節(jié)能優(yōu)化的方向.

1 褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)

褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)的原理如圖1所示.其主要特點(diǎn)有：(1) 在風(fēng)扇磨煤機(jī)中利用高溫?zé)煔獾臒崃棵摮褐械乃?，從而提高褐煤的能量密度?2) 通過(guò)風(fēng)扇磨煤機(jī)與倉(cāng)儲(chǔ)式制粉系統(tǒng)的有機(jī)結(jié)合，將干燥乏氣與煤粉分離，使干燥乏氣不再進(jìn)入爐膛從而降低爐內(nèi)煙氣量和水分含量，降低鍋爐排煙溫度，提高鍋爐效率；(3) 干燥乏氣用于預(yù)熱鍋爐送風(fēng)，彌補(bǔ)抽取爐煙造成的空氣加熱不足.該系統(tǒng)鍋爐為塔式鍋爐，爐煙抽取點(diǎn)位于末級(jí)過(guò)熱器上方；省煤器分兩級(jí)，中間布置脫硝裝置以保證其入口煙溫.

2 褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)的分析模型

(1)

式中：ew為機(jī)械，kJ/kg；w為機(jī)械功，kJ/kg.

(2)

式中：eq為熱量，kJ/kg；q為熱量，kJ/kg；T為系統(tǒng)溫度，K；T0為基準(zhǔn)溫度，K.

(3)

式中：ex為比，kJ/kg；h為焓值，kJ/kg；h0為基準(zhǔn)狀態(tài)焓值，kJ/kg；s為熵值，kJ/(kg·K)；s0為基準(zhǔn)狀態(tài)熵值，kJ/(kg·K).

相應(yīng)能質(zhì)系數(shù)λ的定義為：

(4)

取基準(zhǔn)狀態(tài)p0=101.325 kPa，T0=298.15 K.

(5)

式中：ef為燃料化學(xué)，kJ/kg；Qgr,ar為燃料收到基高位發(fā)熱量，kJ/kg.

圖1 基于風(fēng)扇磨倉(cāng)儲(chǔ)式制粉系統(tǒng)的褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)示意圖

(6)

(7)

(8)

(9)

(10)

式中：qm,f為原煤質(zhì)量流量，kg/s.

(11)

圖2 鍋爐系統(tǒng)各項(xiàng)損失

圖3 風(fēng)扇磨干燥系統(tǒng)分析模型

(12)

全廠系統(tǒng)的熱效率ηtot為

(13)

式中：Qnet,ar為燃料收到基低位發(fā)熱量，kJ/kg.

(14)

3 結(jié)果與分析

3.1 設(shè)計(jì)工況熱力參數(shù)

3.1.1 褐煤煤質(zhì)參數(shù)

伊敏褐煤全水分達(dá)39.5%，經(jīng)過(guò)煙氣干燥后水分降至9.82%.原煤和干燥煤煤質(zhì)參數(shù)如表1所示.由表1可知，干燥前后發(fā)熱量均有較大幅度提升.另外，由于水分的蒸發(fā)在煙氣預(yù)干燥系統(tǒng)而不在爐膛內(nèi)進(jìn)行，因此文中以高位發(fā)熱量為基準(zhǔn)進(jìn)行燃料計(jì)算(公式5).

表1 原煤和干燥煤煤質(zhì)參數(shù)

3.1.2 FPLPS節(jié)能效果

前文所述FPLPS與CLPS均按照600 MW超臨界濕冷機(jī)組設(shè)計(jì)，采用超臨界凝汽式汽輪機(jī)(N600-24.2/566/566)，燃用伊敏褐煤.通過(guò)GSE仿真計(jì)算，得到2種系統(tǒng)主要經(jīng)濟(jì)性參數(shù)的對(duì)比結(jié)果，參見(jiàn)文獻(xiàn)[9].其中，表2所列為FPLPS主要工作點(diǎn)熱力參數(shù)的仿真結(jié)果.由表2可知，與CLPS相比，F(xiàn)PLPS具有顯著的節(jié)能優(yōu)勢(shì)：在干燥乏氣廢熱僅用于預(yù)熱鍋爐送風(fēng)時(shí)，系統(tǒng)熱效率和效率可提高3.42%(相對(duì)值，如圖4所示)，對(duì)應(yīng)節(jié)煤量為9.41 g/(kW·h).

圖5給出了將系統(tǒng)劃分為鍋爐和汽輪機(jī)2個(gè)主要子系統(tǒng)后的熱損失和損失分布的對(duì)比.可見(jiàn)，熱量法和方法的全廠結(jié)果一致，但是損失的部位不同.熱量法分析表明2種系統(tǒng)主要損失在于汽輪機(jī)側(cè)(凝汽器冷源損失)；分析方法則顯示2種系統(tǒng)主要損失在于鍋爐中(由于燃燒過(guò)程和傳熱過(guò)程的不可逆性導(dǎo)致).FPLPS的節(jié)能效果主要取決于鍋爐效率的提升，其中鍋爐熱損失從7.43%下降至4.27%，損失從54.51%下降至52.96%.

表2 FPLPS主要熱力參數(shù)仿真結(jié)果

圖4 FPLPS與CLPS效率對(duì)比

(a) 熱損失

(b) 損失

圖6 FPLPS與CLPS各項(xiàng)損系數(shù)的對(duì)比

汽輪機(jī)系統(tǒng)的主要參數(shù)如表3所示.

表3 汽輪機(jī)系統(tǒng)主要參數(shù)

表4 汽輪機(jī)系統(tǒng)各項(xiàng)損失計(jì)算結(jié)果1)

Tab.4 Calculation results of various exergy losses in different turbine subsystems

單元E·D，κ/MWσκ/%τκ/%εκ/%汽缸47．6043．086．7092．65加熱器11．9310．801．6892．84凝汽器25．5123．093．593．28發(fā)電機(jī)9．118．241．2898．50給水泵4．413．990．6280．76除氧器3．192．890．4587．55小汽輪機(jī)2．662．400．3785．64管道6．085．510．8699．14總計(jì)110．49100．0015．5584．45

注：1)系統(tǒng)輸入值W.

圖7 煙氣預(yù)干燥系統(tǒng)中各項(xiàng)質(zhì)量、能量和流

圖8 煙氣預(yù)干燥系統(tǒng)的各項(xiàng)損率

3.3.1 褐煤預(yù)干燥過(guò)程能質(zhì)匹配分析

褐煤預(yù)干燥過(guò)程的能質(zhì)匹配是系統(tǒng)優(yōu)化的方向.不同介質(zhì)能質(zhì)系數(shù)對(duì)比如表5所示.

褐煤直接燃燒時(shí)水分在爐內(nèi)蒸發(fā)，消耗了高品位的燃料化學(xué)能(λ=1).而FPLPS中水分在干燥系統(tǒng)內(nèi)蒸發(fā)，消耗的是品位相對(duì)較低的煙氣熱能(λ=0.56).因此，雖然單位水分蒸發(fā)消耗的熱量相同，但是能量品位卻是截然不同的.且高溫?zé)煔獾钠肺蝗匀惠^高，如能采用能質(zhì)系數(shù)更低的介質(zhì)作為干燥熱源則可以獲得更高的發(fā)電效率，如采用汽輪機(jī)低壓抽汽作為干燥熱源(λ=0.27)[21].

表5 不同介質(zhì)的能質(zhì)系數(shù)對(duì)比

3.3.2 褐煤預(yù)干燥過(guò)程優(yōu)化分析

圖9 低溫爐煙抽取點(diǎn)變化對(duì)系統(tǒng)效率的影響

4 結(jié) 論

圖10 干燥介質(zhì)混合過(guò)程的圖像化損分析

(1) 與常規(guī)褐煤發(fā)電系統(tǒng)相比，基于風(fēng)扇磨倉(cāng)儲(chǔ)式制粉系統(tǒng)的褐煤煙氣預(yù)干燥發(fā)電系統(tǒng)具有明顯的節(jié)能優(yōu)勢(shì)：設(shè)計(jì)工況下系統(tǒng)效率相對(duì)提高了3.42%，對(duì)應(yīng)節(jié)煤量為9.41 g/(kW·h).FPLPS燃燒過(guò)程損下降是其性能提升的主要原因.入爐水分降低使得燃燒溫度提高是燃燒過(guò)程損失下降的根本原因，而采用直吹式系統(tǒng)將乏氣引入爐膛不利于鍋爐效率的提高.

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Exergy Analysis and System Optimization of a Flue Gas Pre-dried Lignite-fired Power System Based on Fan Mill Dryer and Open Pulverizing System

HANXiaoqu1,LIUMing1,YANJunjie1,CHONGDaotong1,LIUJiping1,XIAOFeng2

(1. State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University,Xi'an 710049, China; 2. Northeast Electric Power Design Institute, Changchun 130021, China)

An exergy analysis model was developed for the flue gas pre-dried lignite-fired power system (FPLPS) based on fan mill dryer and open pulverizing system, so as to explore its energy-saving mechanism from the perspective of the second law of thermodynamics, and to perform exergy analysis for the subsystems and main components of a 600 MW supercritical unit. Results indicate that the efficiency of the plant with FPLPS is relatively 3.42% higher than that with conventional lignite-fired power system (CLPS). The improvement of plant efficiency originates from the replacement of high-grade chemical energy by lower-grade flue gas as the heat source for moisture evaporation, during which the exergy destruction is reduced accordingly. Moreover, the dryer exhaust gas is separated from the coal powder in the open pulverizing system, which prevents the evaporated moisture from recycling in the furnace, thus raising the combustion temperature, reducing the inefficiency of the combustion process and increasing the boiler efficiency. However, the dryer exergy efficiency is only 20.20% due to high irreversibility. The retrofitting option by extracting cold flue gas from economizer outlet is estimated to increase the plant efficiency relatively by 0.33%, benefiting from a reduction of the exergy destruction in the mixing of drying agents.

lignite; flue gas pre-drying; energy-saving; exergy analysis; system optimization

2016-04-21

2016-05-31

國(guó)家自然科學(xué)基金資助項(xiàng)目(51406152，51436006)；國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展規(guī)劃資助項(xiàng)目(2015CB251504)；國(guó)家留學(xué)基金委建設(shè)高水平大學(xué)公派研究生資助項(xiàng)目(201506280074)

韓小渠(1989-)，男，江蘇南京人，博士研究生，研究方向?yàn)楹置侯A(yù)干燥發(fā)電系統(tǒng)仿真及優(yōu)化研究.電話(Tel.)：029-82668072；E-mail:hanxiaoqu1989@stu.xjtu.edu.cn.

1674-7607(2017)02-0148-08

TM621

A 學(xué)科分類(lèi)號(hào)：470.10