余 萬,劉驍緯,廖 雷,王 崗,高振軍,ISHNAZAROV Oybek
稻殼與煤混合燃燒特性試驗(yàn)
余 萬1,2,劉驍緯1,2,廖 雷1,2,王 崗1,2※,高振軍1,2,ISHNAZAROV Oybek3
(1. 水電機(jī)械設(shè)備設(shè)計(jì)與維護(hù)湖北省重點(diǎn)實(shí)驗(yàn)室(三峽大學(xué)),宜昌 443002;2. 三峽大學(xué)機(jī)械與動(dòng)力學(xué)院,宜昌 443002;3. Institute of Energy Problems of the Academy of Sciences of the Republic of Uzbekistan, Tashkent 100125, Uzbekistan)
采用非等溫?zé)嶂胤▽?duì)稻殼、煤及其混合燃料進(jìn)行燃燒試驗(yàn),使用Flynn-Wall-Ozawa(FWO)和Kissinger- Akahira-Sunose(KAS)2種方法分析了樣品的燃燒反應(yīng)動(dòng)力學(xué)參數(shù),分析了摻混比、升溫速率對(duì)燃燒特性參數(shù)的影響,探討了燃燒過程中混合燃料成分間的協(xié)同效應(yīng)。結(jié)果表明:混合燃料的點(diǎn)火溫度與純煤相比有明顯降低,燃燒特性有明顯改善,隨著稻殼摻混比由10%升至70%,燃盡溫度從730℃降低至650℃,燃盡程度也逐漸變大,失質(zhì)量從86.1%增至91.5%,綜合燃燒特性指數(shù)從2.153×10-8增加到1.183×10-7;隨著升溫速率的增加,混合燃料的燃盡溫度和綜合燃燒指數(shù)均會(huì)增大,但點(diǎn)火溫度和燃盡程度幾乎不變;稻殼與煤混合燃燒時(shí)組分間會(huì)發(fā)生協(xié)同作用,且隨著稻殼摻混比的增加,協(xié)同效應(yīng)參數(shù)逐漸增大,協(xié)同效應(yīng)越來越明顯;FWO和KAS法得到的煤的活化能分別為37.66和31.00 kJ/mol,稻殼的活化能分別為20.96和16.30 kJ/mol;混合燃燒過程中,活化能隨著溫度和轉(zhuǎn)化比而改變,隨著稻殼摻混比的增加呈現(xiàn)出先增大后減小的趨勢(shì)。
動(dòng)力學(xué);燃料;稻殼;燃燒特性;摻混比
生物質(zhì)是唯一可再生的含碳資源,發(fā)展生物質(zhì)能是實(shí)現(xiàn)“雙碳”目標(biāo)的重要途徑[1]。作為農(nóng)業(yè)大國(guó),中國(guó)每年生產(chǎn)大量的生物質(zhì)資源,然而,稻殼和秸稈等生物質(zhì)通常被視為農(nóng)業(yè)廢棄物,被留在耕地或直接焚燒,這可能造成空氣和水的污染以及能源損失[2-3]。若將這些生物質(zhì)廢料作為能源利用起來,預(yù)計(jì)有4.02億t標(biāo)準(zhǔn)煤當(dāng)量,相當(dāng)于全國(guó)煤炭消費(fèi)量的13%[4]。但是生物質(zhì)熱值低,密度低,含水量高等缺點(diǎn)也限制了生物質(zhì)能源的大規(guī)模應(yīng)用[5]。所以將煤和生物質(zhì)混合燃燒被認(rèn)為是促進(jìn)生物質(zhì)利用和減輕碳排放最有效的方法之一,研究煤和生物質(zhì)的混合燃燒特性對(duì)于實(shí)現(xiàn)“雙碳”目標(biāo)具有重要意義[6]。
對(duì)于生物質(zhì)與煤的混合燃燒特性研究包括分析燃料的特征溫度、反應(yīng)性、動(dòng)力學(xué)參數(shù),并分析混合燃料成分對(duì)所列特性的影響。梅艷陽(yáng)等[7-8]發(fā)現(xiàn)生物質(zhì)經(jīng)過初步熱處理(炭化)后能使其特性更接近于煤的特性,從而能在不需要對(duì)鍋爐進(jìn)行重大改造的情況下實(shí)現(xiàn)生物質(zhì)燃料在燃煤鍋爐中的混燃,也不會(huì)對(duì)鍋爐效率有負(fù)面影響[9-10]。陳繼輝等[11]認(rèn)為生物質(zhì)與煤的混合可以更好地控制整個(gè)燃燒過程,主要有以下兩點(diǎn):在混燃中,生物質(zhì)的加入不僅能降低點(diǎn)火溫度還能提供更穩(wěn)定的火焰,這是由于生物質(zhì)內(nèi)含有較高比例的揮發(fā)分[12]。其次,煤和生物質(zhì)的混合燃燒可以減少純生物質(zhì)燃燒時(shí)灰燼沉積和結(jié)垢的問題[13]。劉翔等[14]分析了草本類生物質(zhì)的質(zhì)量對(duì)煙煤混燒特性的影響,羅娟等[15]研究了生物質(zhì)顆粒的種類對(duì)燃燒特性及燃燒產(chǎn)物影響,楊洺溦[16]分析了煤與生物質(zhì)混合燃燒對(duì)于NOX排放的影響。對(duì)于生物質(zhì)與煤混合燃料特性的研究主要集中在燃料的熱解、燃燒及動(dòng)力學(xué)分析[17-19]。
本文選用農(nóng)業(yè)生物質(zhì)中產(chǎn)量較大的稻殼為對(duì)象,采用熱重法對(duì)稻殼與煤的混合燃料在不同摻混比和不同升溫速率下的燃燒特性進(jìn)行試驗(yàn)分析,并與純稻殼和純煤的燃燒特性進(jìn)行比較。在此基礎(chǔ)上,利用熱重分析曲線結(jié)合Flynn-Wall-Ozawa(FWO)和Kissinger-Akahira-Sunose(KAS)動(dòng)力學(xué)方法計(jì)算混合燃料的活化能,分析影響因素和反應(yīng)機(jī)理,旨在為稻殼的高效燃燒提供的數(shù)據(jù)參考。
試驗(yàn)所用煤為華能平?jīng)雒海?jiǎn)稱煤(Huaneng coal, HC),所用稻殼(rice husk, RH)為重慶地區(qū)2021年稻谷所產(chǎn),使用小型粉碎機(jī)分別對(duì)其進(jìn)行粉碎,經(jīng)篩分后進(jìn)行工業(yè)分析和元素分析,結(jié)果如表1所示。然后通過物理混合得到稻殼質(zhì)量摻混比10%、30%、50%、70%的四種混合燃料樣品。
表1 稻殼與煤的工業(yè)分析和元素分析
注:ad為空氣干燥基樣品。
Note: ad is an air-dried base sample.
熱重試驗(yàn)采用的儀器是北京恒久科學(xué)儀器廠生產(chǎn)的綜合熱分析儀HCT-4型。單次樣品質(zhì)量為(10±0.5)mg,陶瓷坩堝采用Al2O3材料,氣體為干燥空氣,流量100 mL/min,試驗(yàn)溫度范圍在25~1 500 ℃。在10、20、30 ℃/min三種不同升溫速率下,對(duì)煤(HC)、稻殼(RH)及混合燃料(10%RH90%HC、30%RH70%HC、50%RH50%HC、70%RH30%HC)分別進(jìn)行燃燒試驗(yàn)。
本文采用點(diǎn)火溫度、燃盡溫度和綜合燃燒特性指數(shù)作為燃燒特性的評(píng)價(jià)指標(biāo)。
點(diǎn)火溫度由TG-DTG(thermogravimetric-derivative thermogravimetry)曲線作圖法得到,DTG曲線最大峰值處作垂線與TG曲線相交,過交點(diǎn)作TG曲線的切線,然后在TG曲線上失質(zhì)量開始處作水平線交切線于一點(diǎn),此點(diǎn)對(duì)應(yīng)的溫度即為點(diǎn)火溫度(T,℃)。
燃盡溫度(T,℃)為樣品質(zhì)量損失達(dá)到總失質(zhì)量的98%時(shí)所對(duì)應(yīng)的溫度。
點(diǎn)火溫度高意味著燃料的熱穩(wěn)定性好,難以點(diǎn)燃。燃盡溫度值高是由于燃料的固定碳含量較高,需要更長(zhǎng)的加熱時(shí)間和更高的溫度才能完全轉(zhuǎn)化。
綜合燃燒特性指數(shù)是綜合表征燃料的著火和燃盡特性的參數(shù),其值越高意味著燃料擁有更好的燃燒性能。綜合燃燒特性指數(shù)由式(1)計(jì)算[20]:
式中max是樣品燃燒過程中的最大失質(zhì)量速率,%/min;mean是樣品點(diǎn)火溫度和燃盡溫度之間的平均失質(zhì)量速率DTGmean,%/min;由式(2)計(jì)算[20]:
為探討混合燃料燃燒過程中各成分間的協(xié)同效應(yīng),可對(duì)混合燃料熱重曲線的試驗(yàn)值、理論值及協(xié)同效應(yīng)參數(shù)進(jìn)行分析。
首先基于組分的加權(quán)平均值計(jì)算每種混合燃料的理論DTG曲線[21]:
理論值=1稻殼+2煤(3)
式中1和2分別是混合燃料中稻殼與煤的質(zhì)量分?jǐn)?shù),%;為各組分失質(zhì)量速率。
將稻殼與煤燃燒過程中的最大失質(zhì)量速率點(diǎn)對(duì)應(yīng)的溫度、燃盡溫度及點(diǎn)火溫度到最大失質(zhì)量速率點(diǎn)溫度所用的時(shí)間結(jié)合起來,得到協(xié)同效應(yīng)參數(shù),通過參數(shù)數(shù)值大小來判斷燃燒過程中是否存在協(xié)同作用。協(xié)同效應(yīng)參數(shù)S可由式(4)計(jì)算[22]:
式中blend為混合燃料的協(xié)同效應(yīng)指數(shù),HC為HC的協(xié)同效應(yīng)指數(shù),℃-3·min-1/2。協(xié)同效應(yīng)指數(shù)I可通過下式求解
式中t為點(diǎn)火溫度到最大失質(zhì)量速率點(diǎn)溫度所用的時(shí)間,min;T為最大失質(zhì)量速率點(diǎn)溫度,℃。
為了保證參數(shù)的準(zhǔn)確性,基于混合燃料的理論結(jié)果創(chuàng)建基線,通過比較確定燃燒過程中是否會(huì)發(fā)生協(xié)同效應(yīng);當(dāng)S>1.15時(shí),燃燒過程會(huì)發(fā)生明顯的協(xié)同效應(yīng);當(dāng)0.8≤S≤1.15時(shí),燃燒過程的協(xié)同效應(yīng)不明顯;當(dāng)S小于0.8時(shí),說明混合燃料的燃燒性能變差[22]。
動(dòng)力學(xué)分析是研究燃燒反應(yīng)機(jī)理的基礎(chǔ)。對(duì)熱重試驗(yàn)結(jié)果進(jìn)行定量分析,可以得到熱分解過程的動(dòng)力學(xué)參數(shù)[23]。在試驗(yàn)測(cè)量中,參數(shù)r為質(zhì)量百分?jǐn)?shù),%;為測(cè)試時(shí)的質(zhì)量,0為樣品初始質(zhì)量,g。
式(7)和(8)這兩個(gè)通用動(dòng)力學(xué)表達(dá)式通常用來計(jì)算在氣-固反應(yīng)中的整個(gè)反應(yīng)速率,其中()是作為時(shí)間函數(shù)的無量綱質(zhì)量轉(zhuǎn)化比[24]:
式中()為時(shí)刻樣品的質(zhì)量,g;m為樣品燃燒后的最終質(zhì)量,g。
的變化率由下式給出:
式中為溫度,℃;為指前因子,為理想氣體常數(shù)(8.314 J/mol·K),為活化能,kJ/mol;()為微分反應(yīng)機(jī)理函數(shù)。()表示反應(yīng)過程中材料的化學(xué)或物理特性的變化[25]。
函數(shù)()表示為等式
式中是非等溫試驗(yàn)中的反應(yīng)級(jí)數(shù)。
式(8)可以轉(zhuǎn)換為非等溫的反應(yīng)速率,將其描述在恒定加熱速率(=d/d)下的溫度函數(shù):
積分上式得到非等溫速率定律的積分形式:
式中()是轉(zhuǎn)化的積分函數(shù),0是燃燒反應(yīng)的初始溫度, ℃。
根據(jù)式(6)可以使用各種方法來獲得動(dòng)力學(xué)參數(shù)和,kJ/mol。
1.5.1 Flynn–Wall–Ozawa(FWO)法
FWO法基于Doyle近似[26]。將反應(yīng)速率定律變?yōu)閷?duì)數(shù)形式,得到式(12):
對(duì)于給定的轉(zhuǎn)化比,從lg與1/的線性相關(guān)關(guān)系中可以得到不同加熱速率的活化能。在這種方法中,右側(cè)的對(duì)數(shù)也是常數(shù)。即通過繪制lg與1/的關(guān)系,可以從所得直線的斜率獲得活化能,kJ/mol。
1.5.2 Kissinger–Akahira–Sunose(KAS)法
KAS方法的原理與FWO方法類似。該模型基于式(13):
所以,對(duì)于KAS來說,根據(jù)ln(2)和1/,可以從試驗(yàn)數(shù)據(jù)中獲得一條直線來計(jì)算不同轉(zhuǎn)化比對(duì)應(yīng)的活化能,kJ/mol。
表1顯示了稻殼與煤的物理化學(xué)特性。工業(yè)分析表明,稻殼相比于煤具有更高的揮發(fā)分含量,這意味著稻殼的加入有利于提高混合燃料在溫度較低時(shí)的反應(yīng)性,從而提高著火性能;而煤中更高的固定碳含量會(huì)使混合燃料的燃燒過程更穩(wěn)定。同樣,稻殼較低的灰分含量不會(huì)影響混合燃料的燃燒速度并造成結(jié)垢。從元素分析來看,稻殼更高的氫含量和氧含量,使得其具有較強(qiáng)的熱反應(yīng)性,而稻殼中較低的硫含量使得其在熱分解過程中僅有少量硫氧化物的釋放。對(duì)比稻殼與煤的基本特性發(fā)現(xiàn)兩種燃料在燃燒方面具有互補(bǔ)性,因此稻殼與煤的混合燃料具有一定的燃燒潛力。
圖1和圖2分別為煤(HC)和稻殼(RH)及混合燃料在20 ℃/min的升溫速率下燃燒獲得的失質(zhì)量(TG)和失質(zhì)量速率(DTG)曲線。
從圖1可以發(fā)現(xiàn),不同摻混比下,樣品的燃燒特性存在著顯著差異。當(dāng)溫度升至580 ℃時(shí),稻殼(RH)幾乎完全燃燒完畢,而此時(shí)煤(HC)的失質(zhì)量?jī)H為62%。煤(HC)和稻殼(RH)在燃燒結(jié)束后的失質(zhì)量分別為84.5%和92.2%,殘余質(zhì)量與其灰分含量近乎一致。而4種混合燃料的TG曲線位于煤(HC)和稻殼(RH)曲線之間。
注:圖例中百分?jǐn)?shù)為質(zhì)量分?jǐn)?shù),下同。
圖2 稻殼/煤混合燃料的微商熱重曲線
結(jié)合圖1和圖2曲線分析,稻殼(RH)的燃燒過程分為3個(gè)階段。在溫度從25 ℃到140 ℃的區(qū)間為第一個(gè)階段,這與其固有水分的蒸發(fā)有關(guān),失質(zhì)量為總質(zhì)量的7%左右,與工業(yè)分析的水分含量6.46%相近。第二階段約從140 ℃到370 ℃,這個(gè)階段主要為揮發(fā)分的釋放,失質(zhì)量為67%左右。文獻(xiàn)研究表明,稻殼主要由纖維素(34%~42%)、半纖維(16%~22%)、木質(zhì)素(21%~26%)、二氧化硅(10%~21%)以及少量粗蛋白和粗脂肪組成[27-29]。由于半纖維素、纖維素和木質(zhì)素的分解分別發(fā)生在225~325、305~375和250~500 ℃的范圍內(nèi)[30]。因此,第二階段有大量半纖維素、纖維素和木質(zhì)素等有機(jī)物熱解生成揮發(fā)性物質(zhì)和燃燒形成生物質(zhì)炭,在此階段TG曲線急劇下降,DTG曲線在297.9 ℃時(shí)出現(xiàn)顯著峰值,失質(zhì)量為總質(zhì)量的38.5%。第三個(gè)階段為370到580 ℃的區(qū)間,此階段的失質(zhì)量為總質(zhì)量的28%。這是木質(zhì)素分解最多的階段,木質(zhì)素是生物質(zhì)受熱分解中最穩(wěn)定的成分,也是形成生物質(zhì)炭的主要成分[31]。由于反應(yīng)性與DTG曲線峰值對(duì)應(yīng)的溫度值成反比并且與峰高成正比,失質(zhì)量率的小峰值也表明木質(zhì)素的低反應(yīng)性。
由于水分的失去,煤(HC)在25到235 ℃的溫度范圍內(nèi)出現(xiàn)了初始失質(zhì)量階段。煤的熱反應(yīng)在240到780 ℃的溫度區(qū)間內(nèi)僅有一個(gè)主峰,其最大失質(zhì)量率對(duì)應(yīng)的溫度約為505 ℃。這是由于煤(HC)的揮發(fā)分和固定碳的含量占總成分的75%左右,且這兩種成分受熱反應(yīng)的溫度區(qū)間重合,所以整個(gè)失質(zhì)量過程中的唯一主峰即煤質(zhì)量損失基本取決于揮發(fā)分釋放和固定碳燃燒,這與他人的研究結(jié)論相吻合[32]。由表1可知,因?yàn)槊海℉C)的揮發(fā)分含量低于稻殼(RH),其燃燒的放熱量對(duì)固定碳燃燒的促進(jìn)作用不明顯,使得煤(HC)的最大失質(zhì)量速率遠(yuǎn)小于稻殼(RH)。
由圖1和圖2中混合燃料樣品燃燒的TG曲線和DTG曲線與煤(HC)對(duì)比得出,混合燃料具有2個(gè)氧化過程,第一個(gè)過程在180到370 ℃的溫度區(qū)間,這個(gè)過程主要為半纖維素和纖維素降解生成揮發(fā)性物質(zhì),使得DTG曲線出現(xiàn)峰值;在370到700 ℃這個(gè)較大的溫度區(qū)間內(nèi)為第二個(gè)氧化過程,這是由于煤中揮發(fā)物的釋放,連同生物質(zhì)炭在整個(gè)溫度范圍內(nèi)緩慢燃燒。由圖可知,隨著稻殼摻混比的增加:混合燃料的曲線向稻殼(RH)靠攏,失質(zhì)量占樣品質(zhì)量的比例隨之增大,分別為86.1%、86.2%、89.2%、91.5%,且燃盡溫度也隨之減小,表明燃燒反應(yīng)逐漸提前完成且燃盡程度也逐漸變大,這是由于稻殼的摻混減少了混合燃料中灰分的含量,降低了灰分阻燃的影響,使得反應(yīng)可以提前完成;混合燃料的DTG曲線向低溫區(qū)移動(dòng)即低溫時(shí)燃燒失質(zhì)量速率增大,說明燃燒過程的反應(yīng)性越來越好,這是由于稻殼的摻混增加了混合燃料中的揮發(fā)分含量,揮發(fā)分析出后增大了樣品的孔隙率,增大了與反應(yīng)氣體的接觸面積,且揮發(fā)分燃燒所產(chǎn)生的熱量對(duì)混合物中固定碳起到了預(yù)加熱和結(jié)構(gòu)疏松的作用,使得混合物燃燒向低溫區(qū)移動(dòng)。
圖3顯示了煤(HC)、稻殼(RH)及混合燃料的燃燒特性參數(shù)。從圖中看出,隨著稻殼(RH)的加入,混合燃料的點(diǎn)火溫度和燃盡溫度相比于煤(HC)都出現(xiàn)大幅降低。隨著稻殼摻混比由10%升至70%,點(diǎn)火溫度維持在純稻殼點(diǎn)火溫度250 ℃左右,遠(yuǎn)低于煤的點(diǎn)火溫度360 ℃,混合燃料的燃盡溫度從730降低至650 ℃。值得注意的是,在混合燃料僅有10%稻殼摻混比例時(shí),其點(diǎn)火溫度與稻殼(RH)相差甚微,這是由于在較低溫度時(shí),高揮發(fā)分的稻殼(RH)點(diǎn)火燃燒引起了混合燃料整體點(diǎn)火;而少量的稻殼摻混對(duì)燃盡溫度和綜合燃燒指數(shù)的影響不及較高摻混比例的混合燃料。在較高摻混比時(shí),混合燃料的燃盡溫度趨于定值,這是由于其中較少量的煤固定碳對(duì)燃燒過程延遲影響有限。70%摻混比混合燃料的燃燒指數(shù)相比于30%和50%時(shí)具有更大增幅,同時(shí)70%摻混比混合燃料的點(diǎn)火溫度和純稻殼幾乎一致,而其燃盡溫度相較純稻殼又有較大增幅,這意味著較高摻混比混合燃料擁有更寬泛的燃燒溫度區(qū)間,明顯改善了純稻殼燃燒過快的特性。同樣較高的摻混比混合燃料提高了純煤的燃燒特性,綜合燃燒指數(shù)由2.153×10-8增加到1.183×10-7。綜上,較高反應(yīng)性稻殼(RH)的加入有助于啟動(dòng)煤的燃燒過程。
圖3 稻殼/煤混合燃料的燃燒特性參數(shù)
升溫速率是影響燃料燃燒過程十分重要的因素。70%RH30%HC混合燃料在10、20和30 ℃/min升溫速率下的TG和DTG曲線如圖4、圖5所示。由圖可知,樣品在不同升溫速率下的TG、DTG曲線變化趨勢(shì)相似。由TG曲線可知,樣品在不同升溫速率下的最大失質(zhì)量幾乎一致,說明升溫速率對(duì)燃料的燃盡程度影響不大,最大失質(zhì)量主要取決于燃料的灰分含量。由DTG曲線可知,混合燃料的失質(zhì)量過程中2個(gè)峰值點(diǎn)都出現(xiàn)增大,第一個(gè)峰值點(diǎn)增大是由于提高升溫速率使得半纖維素和纖維素?zé)峤到饧涌欤诙€(gè)峰值點(diǎn)是由于提高升溫速率對(duì)固定碳燃燒具有促進(jìn)作用。
圖4 不同升溫速率下混合燃料的熱重曲線
圖5 不同升溫速率下混合燃料的微商熱重曲線
圖6為不同升溫速率下混合燃料的燃燒特性參數(shù),由圖可知,隨著升溫速率從10增加至30 ℃/min,混合燃料的點(diǎn)火溫度(T)變化甚微,仍在250 ℃左右,而燃盡溫度(T)從562.3升至700 ℃。這是由于生物質(zhì)炭燃燒是一個(gè)緩慢氧化過程,需要較長(zhǎng)的時(shí)間,因此高升溫速率也具有更高的燃盡溫度,意味著樣品的燃燒過程的溫度區(qū)間更寬泛。升溫速率的提高對(duì)混合燃料的綜合燃燒指數(shù)()有著增益作用,綜合燃燒指數(shù)從3.8×10-8增加到1.923×10-7。這是由于樣品的非等溫行為,較高的升溫速率意味著達(dá)到特定環(huán)境溫度的時(shí)間更短,這會(huì)導(dǎo)致表面與顆粒核心之間的溫差相對(duì)較大。因此,加熱速率肯定會(huì)增強(qiáng)從顆粒表面到核心的熱傳遞。然而隨著升溫速率的增加,樣品的內(nèi)部溫差隨之變大,燃燒過程的產(chǎn)物不能夠及時(shí)逸出,從而限制了樣品內(nèi)部的燃燒進(jìn)程,造成了燃燒熱滯后的現(xiàn)象,與ZHUO等[33]的研究相吻合。
4種混合燃料在20 ℃/min的升溫速率下的DTG理論和試驗(yàn)曲線如圖7所示,試驗(yàn)與理論曲線的變化趨勢(shì)比較相似。
相比于理論曲線,試驗(yàn)曲線主要偏差為燃燒過程的DTG峰值更大且燃燒溫度范圍更小,這是由于半纖維素和纖維素?zé)岱纸猱a(chǎn)生的揮發(fā)物質(zhì)加速了煤的脫揮發(fā)分過程,其次稻殼中存在的一些堿金屬或堿土金屬對(duì)煤的燃燒起到了促進(jìn)作用。
煤(HC)和不同摻混比的混合燃料的協(xié)同效應(yīng)參數(shù)如表2所列。由表2可知,不同摻混比例的混合燃料協(xié)同效應(yīng)參數(shù)都在比較基線以上,說明了不同摻混比的混合燃料在燃燒過程中都發(fā)生了明顯的協(xié)同作用,而且隨著稻殼摻混比的增加,協(xié)同效應(yīng)參數(shù)逐漸增大,協(xié)同效應(yīng)越來越明顯,其中70%RH30%HC混合燃料的協(xié)同效應(yīng)最為明顯。
圖6 不同升溫速率下混合燃料的燃燒特性參數(shù)
圖7 混合燃料的試驗(yàn)和理論DTG曲線比較
表2 煤和不同摻混比的混合燃料的協(xié)同效應(yīng)參數(shù)
綜上,理論與試驗(yàn)偏差表明混合燃料的燃燒行為不能通過純?nèi)剂咸匦缘暮?jiǎn)單線性相加來預(yù)測(cè)??梢哉J(rèn)為,在稻殼與煤的混燃過程中確實(shí)存在協(xié)同促進(jìn)作用。
通過FWO和KAS方法評(píng)估了摻混比對(duì)燃燒反應(yīng)動(dòng)力學(xué)的影響。其中不同混合燃料活化能的求解由同一轉(zhuǎn)化比時(shí),對(duì)應(yīng)10、20和30 ℃/min升溫速率的溫度,求出三個(gè)數(shù)據(jù)點(diǎn),進(jìn)行線性擬合得到線性方程式解析解,將所得方程式與方程(12)、(13)分別對(duì)應(yīng)比較,根據(jù)斜率求解得到兩種方法的活化能值。在求解中僅使用轉(zhuǎn)化比在0.2至0.8范圍的對(duì)應(yīng)值,這是因?yàn)樵摲秶鈹?shù)據(jù)的相關(guān)系數(shù)較差,且該范圍之外過程為水分損失和燃盡后過程,不具有分析意義。表3顯示了每個(gè)轉(zhuǎn)換比相應(yīng)的活化能值和其決定系數(shù)(2)?;罨転閷?duì)應(yīng)于不同轉(zhuǎn)化比的算術(shù)平均值。從表3可發(fā)現(xiàn)在轉(zhuǎn)化比()0.2~0.8范圍內(nèi)對(duì)樣品表觀活化能進(jìn)行計(jì)算時(shí)具有良好的線性擬合結(jié)果,所有決定系數(shù)(2)大于0.93,并且大于BURATTI等[34-35]研究中的相關(guān)系數(shù),說明本文中FWO和KAS兩種方法計(jì)算的結(jié)果具有可靠性。
需要注意的是,上述兩種方法計(jì)算的活化能值為聚合值,與任何單個(gè)反應(yīng)步驟無關(guān),其反映了不同轉(zhuǎn)化比時(shí)平行反應(yīng)過程對(duì)總反應(yīng)速率的影響。
由表3得知,F(xiàn)WO法得到的活化能均高于KAS法,但整體趨勢(shì)相似,這是由于樣品和試驗(yàn)條件的不同,不同的動(dòng)力學(xué)方法在計(jì)算活化能方面會(huì)有不同的值,這與K?K等[36]的結(jié)論一致。表中華能平?jīng)雒海℉C)燃燒過程中通過FWO和KAS方法計(jì)算的活化能值分別為37.66和31.00 kJ/mol,稻殼(RH)的活化能值分別為20.96和16.30 kJ/mol。
表3 稻殼/煤混合燃料的活化能
在固體燃燒過程中,樣品活化能隨著溫度和轉(zhuǎn)化比而改變。所有樣品的活化能都呈現(xiàn)出先增大后減小的趨勢(shì)。在轉(zhuǎn)化比0.2~0.8的范圍內(nèi),稻殼的活化能始終小于煤,混合燃料與煤的活化能變化有相似趨勢(shì)。煤(HC)在轉(zhuǎn)化比0.4~0.5時(shí)其活化能減小最多,溫度范圍在480~525 ℃間,這是由于揮發(fā)分釋放和固體碳燃燒;稻殼(RH)在轉(zhuǎn)化比0.5~0.6時(shí)其活化能減小最多,這是由于揮發(fā)性物質(zhì)生成和燃燒形成生物質(zhì)炭,溫度為300 ℃與上文中所述最大失質(zhì)量速率點(diǎn)溫度一致,因此活化能隨轉(zhuǎn)化比的關(guān)系證實(shí)了DTG曲線顯示樣品燃燒中的存在多個(gè)熱分解過程的特性。不同樣品活化能隨轉(zhuǎn)化比的不同趨勢(shì)反應(yīng)了混合燃燒的復(fù)雜性。
值得注意的是,對(duì)10%RH90%HC混合燃料,活化能呈增大減小再增大減小的趨勢(shì),這是因?yàn)樵?.2~0.4轉(zhuǎn)化比時(shí)即溫度較低時(shí),稻殼已進(jìn)行到燃燒階段,使其活化能出現(xiàn)第一次增大減小趨勢(shì);而由于含量較少無法引起整體混合燃料的燃燒反應(yīng),所以在0.5~0.7轉(zhuǎn)化比時(shí)即煤的燃燒階段時(shí),活化能呈現(xiàn)再次增大再減小的趨勢(shì)。
為進(jìn)一步探討混合燃料成分間的協(xié)同作用,從表3中可以發(fā)現(xiàn),各樣品的活化能平均值呈現(xiàn)出先增大后減少趨勢(shì),這表明在加入較少量稻殼時(shí),無法促進(jìn)混合燃料反應(yīng),且會(huì)出現(xiàn)相比純煤更難引起燃燒反應(yīng)的現(xiàn)象,這是因?yàn)樯倭康練な沟闷浞磻?yīng)過程分為兩個(gè)階段,即低溫時(shí)少量稻殼已結(jié)束熱分解,高溫時(shí)僅有剩余純煤燃燒,如圖7所示,10%RH90%HC混合燃料的試驗(yàn)曲線第二個(gè)氧化過程和理論曲線的高度重合也證明了上述觀點(diǎn)。其中70%RH30%HC相比幾種混合燃料的活化能值最低,同樣此種混合燃料相比純稻殼的活化能差量較小,這意味著此混合燃料燃燒反應(yīng)是幾種混合燃料中最容易的。而在上文分析中,此種混合燃料的綜合燃燒指數(shù)也是相對(duì)最優(yōu)的,活化能分析也證實(shí)了這一點(diǎn)。
1)稻殼的點(diǎn)火溫度和燃盡溫度均比煤要低。與純煤相比,稻殼/煤混合燃料的燃燒特性有明顯改善,點(diǎn)火溫度維持在純稻殼點(diǎn)火溫度250 ℃左右,遠(yuǎn)低于煤的點(diǎn)火溫度360 ℃,但隨著稻殼摻混比由10%升至70%,混合燃料的燃盡溫度從730 ℃降低至650 ℃,燃盡程度也逐漸變大,失質(zhì)量從86.1%增至91.5%,綜合燃燒特性指數(shù)從2.153×10-8增加到1.183×10-7;混合燃料擁有更寬泛的燃燒過程,明顯能改善純稻殼燃燒過快的特性,同時(shí)也能提升純煤的燃燒特性
2)不同的升溫速率下,混合燃料的點(diǎn)火溫度變化不大,仍維持在純稻殼點(diǎn)火溫度250 ℃左右,燃盡程度也幾乎不變,但隨著升溫速率從10增加至30 ℃/min,混合燃料的燃盡溫度從562.3升至700 ℃,綜合燃燒指數(shù)從3.8×10-8增加到1.923×10-7。
3)混合燃料的燃燒失質(zhì)量情況不能簡(jiǎn)單由組分疊加而成,因?yàn)榻M分間會(huì)發(fā)生協(xié)同作用,且隨著稻殼摻混比的增加,協(xié)同效應(yīng)參數(shù)逐漸增大,協(xié)同效應(yīng)越來越明顯。
4)FWO法和KAS法用來計(jì)算燃燒過程活化能時(shí),均具有較好的可靠性,兩種方法得到的煤的活化能分別為37.66和31 kJ/mol,稻殼的活化能分別為20.96和16.3 kJ/mol;隨著燃料轉(zhuǎn)化比的增大,活化能呈現(xiàn)出先增大后減小的趨勢(shì),混合燃料的活化能隨著稻殼摻混比的增加先增大后減小,在10%摻混比時(shí)達(dá)到最大。
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Experimental investigation on the co-combustion characteristics of rice husk and coal
YU Wan1,2, LIU Xiaowei1,2, LIAO Lei1,2, WANG Gang1,2※, GAO Zhenjun1,2, ISHNAZAROV Oybek3
(1.,,443002,; 2.,443002,;3.,100125,)
Rice husk is one of the major biomass sources in the boilers and furnaces in modern agriculture. Particularly, the combustion characteristics can be in the appropriate state. It is very essential to the combustion characteristics of rice husk and coal blend fuels, in order to effectively promote the biomass utilization and mitigate carbon emissions. Fortunately, the thermogravimetric analysis can serve as one of the most significant approaches for the combustion characteristics. In this study, a systematic investigation was made to determine the combustion characteristics of rice husk, coal, and their blended fuel using thermogravimetric analysis. The samples of blended fuel were prepared with the rice husk mass ratio of 10%, 30%, 50%, and 70%. A series of combustion experiments were was carried out in the temperature zone of 25℃ to 1200℃ with three heating rates of 10, 20, and 30℃·min-1. The evaluation indexes were selected as the ignition temperature, burnout temperature, and the integrated combustion characteristic index. Furthermore, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods were used to calculate the combustion kinetic characteristic parameters. The results show that the ignition temperature and burnout temperature of rice husk were lower than those of coal. There were the significantly improved combustion characteristics of rice husk/coal blended fuel, compared with the pure coal. The addition of rice husk with the high volatile matter content was improved the reactivity of the blended fuel at the lower temperatures, and thus improve the ignition performance. The higher content of fixed carbon in the coal was greatly contributed to the more stable combustion process of the blended fuel. There was no influence in of the lower ash content of rice husk on the combustion rate of the blended fuel and fouling. The optimal ignition temperature of rice husk and coal were 250℃ and 360℃, respectively. The ignition temperature of blended fuel was around 250℃, which was independent of the rice husk mass ratio. However, the burnout temperature of the blended fuel decreased from 730℃ to 578℃ with the increase of rice husk blending ratio from 10% to 70%. The burnout degree also gradually increased, with the weight loss increasing from 86.1% to 91.5%. The comprehensive combustion characteristic index also increased from 2.153×10-8to 1.183×10-7. The blended fuel presented a wider combustion process. At the same time, the combustion characteristics of blended fuel were significantly improved, compared with the pure coal and rice husk. There was little effect of the heating rate on the ignition temperature of blended fuel, which was still around 250℃ at different heating rates. The maximum weight loss rate was also unrelated to the heating rate. However, the burnout temperature of the blended fuel increased from 562.3℃ to 700℃, as the heating rate increased from 10℃·min-1to 30℃·min-1. The comprehensive combustion index increased from 3.8×10-8to 1.923×10-7. The experimental curves of blended fuel presented the a larger peak weight loss rate and smaller temperature range, compared with the theoretical TG curves. The combustion weight loss of blended fuel was failed to simply calculate using linear stacking of the components. There was the a synergistic effect between the rice husk and coal during the co-combustion process. Specifically, the synergistic effect parameters gradually increased with the increasing increase of the rice husk mass ratio, indicating the a more outstanding synergistic effect. The combustion activation energy of all samples was calculated by the FWO and KAS, indicating the an excellent correlation coefficient. The activation energy of coal obtained by FWO and KAS were 37.66, and 31 kJ/mol, respectively. The activation energy of rice husk were 20.96, and 16.3 kJ/mol, respectively. The activation energy of blended fuel increased first and then decreased with the increase of combustion temperature, and rice husk mass ratio, respectively. The maximum activation energy was obtained at a 10% blending ratio.
kinetics; fuel; rice husk; combustion characteristics; mass ratio
10.11975/j.issn.1002-6819.202210113
S216.2
A
1002-6819(2023)-01-0203-09
余萬,劉驍緯,廖雷,等. 稻殼與煤混合燃燒特性試驗(yàn)[J]. 農(nóng)業(yè)工程學(xué)報(bào),2023,39(1):203-211. doi:10.11975/j.issn.1002-6819.202210113 http://www.tcsae.org
YU Wan, LIU Xiaowei, LIAO Lei, et al. Experimental investigation on the co-combustion characteristics of rice husk and coal[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2023, 39(1): 203-211. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.202210113 http://www.tcsae.org
2022-10-17
2022-12-31
湖北省引進(jìn)外國(guó)人才和智力項(xiàng)目(2022EJD023)
余萬,博士,副教授,研究方向?yàn)樾履茉蠢眉皬?qiáng)化傳熱。Email:yuwan@ctgu.edu.cn
王崗,博士,講師,研究方向?yàn)樾履茉蠢眉皬?qiáng)化傳熱。Email:gwang2019@126.com