顆粒密度對垃圾衍生燃料燃燒特性的影響

黎 濤，熊祖鴻，房科靖，魯 敏，謝 森，熊培培

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顆粒密度對垃圾衍生燃料燃燒特性的影響

黎 濤，熊祖鴻，房科靖，魯 敏，謝 森，熊培培

（中國科學院廣州能源研究所，廣州 510640）

垃圾衍生燃料（RDF，refuse derived fuel）是垃圾焚燒發(fā)電的原料，該文研究RDF預加工的形態(tài)對其燃燒釋放能量過程的影響。將RDF制備成不同密度等級的成型顆粒，通過熱重、熱值、灰分分析，同時參比與非成型的RDF和生物質秸稈燃料，觀察RDF成型顆粒燃燒特性。研究結果表明，該組非成型RDF的著火點為234.0 ℃，3個最大失質量速率分別為6.30，2.21，0.53% min；高密度RDF的著火點為238.2 ℃，3個最大失質量速率分別為5.70，3.11，0.61%/min，表明較高密度RDF顆粒在燃燒過程中，著火點較高，燃燒速率較為均衡。該組高密度RDF比非成型RDF燃燒后的灰渣少6.7%，表明高密度RDF燃燒較為充分。在熱值方面，方差顯著性分析表明，RDF顆粒密度值對其燃燒產(chǎn)生的熱值無明顯影響。

垃圾；燃燒；生物質；RDF；密度；灰分

0 引 言

隨著社會經(jīng)濟的發(fā)展、人口增長、農(nóng)村城鎮(zhèn)化加大、居民生活水平提高等，城市垃圾產(chǎn)量逐年增加，同時也必然導致全球范圍的能源需求提高[1-8]。據(jù)統(tǒng)計，2014年，中國246個大、中城市生活垃圾產(chǎn)生量16 816.1萬t，2016年增加到18 564.0萬t，年增長率達5.20%。垃圾發(fā)電（waste to energy，WTE）是處理城市垃圾問題、緩解能源需求壓力的優(yōu)選方案。焚燒可使垃圾質量減少80%，體積減少70%；有機物中大約65%～80%的能量可轉化為熱能，25%～30%可通過焚燒轉化電能[9-13]。政府通過WTE方案妥善處理垃圾和降低垃圾容量，解決垃圾的污染、填埋問題，還貢獻出可再生能源，緩解能源需求壓力。目前，全球的垃圾產(chǎn)量為2.4萬億t/a，2025年將達到2.6萬億t/a[14]。中國也投入建設大量垃圾焚燒發(fā)電廠。通過焚燒垃圾產(chǎn)生熱量發(fā)電，減輕日益增長的能耗負擔，同時也大大減少垃圾的填埋場地的壓力[15]。

垃圾經(jīng)過分選、除鐵、破碎、高壓成型等過程，將垃圾中的可燃燒部分制備成垃圾衍生物顆粒燃料，簡稱RDF[16]。Hirunpraditkoon等[18-20]認為每千克RDF燃燒的有效熱約為15.3 MJ；每18 kg干的RDF在燃燒完成后，產(chǎn)生1 kg CO[17]。Bosmans等[20]從動力學角度分析，RDF熱解單方面的熱能轉換，也包括氣化和燃燒過程。Younan等[21-22]認為RDF顆粒尺寸對其內部和外部傳熱有著很大的影響，小顆粒溫度均勻，在反應器中的反應也是均勻的；但對于大顆粒，傳熱速度變慢，顆粒加大到一定的限度，就會導致顆粒內溫度非均勻分布，降低反應的熱效能。本文針對RDF成型顆粒，在不同擠壓強度下制備不同密度等級的成型顆粒的燃燒特性進行探討，探索RDF顆粒密度對其燃燒特性的影響。

1 試驗部分

1.1 樣品制備

本試驗使用的生活垃圾來自佛山南海綠電再生能源有限公司廠區(qū)分選的可燃燒垃圾樣品，包括廢紙(18.17%)、草木（8.84%）、布碎（10.28%）、塑料和皮革（39.32%）、廚余（23.39%）等，經(jīng)干燥、粉碎、混合、添加微量的黏合劑，通過擠壓設備制備成4個密度等級的成型顆粒，同時取一未成型樣品作為對照。樣品原料的工業(yè)分析、元素分析含量見表1，樣品密度分別為0.51、0.42、0.31 g/cm3，對照的非成型RDF密度為0.10 g/cm3。

表1 RDF樣品的工業(yè)分析和元素分析

1.2 試驗儀器與裝置

試驗儀器主要包括TGAQ50熱重分析儀（美國TA公司）、Wvario EL cube元素分析（德國elementar儀器公司）、ZR-1T-CII微電腦自動熱量計（長沙奔特儀器有限公司）、AL204電子天平（梅特勒-托利多儀器公司）、SX2-4-13N箱式電阻爐（上海一恒科技有限公司）、GZX- 9240 MBE數(shù)顯鼓風干燥箱（上海博訊實業(yè)有限公司）。

1.3 試驗方法

熱重分析每次樣品10 mg左右，天平凈化流氮氣30 mL/min，樣品凈化流氮氣40 mL/min，空氣氣氛，升溫速率10 ℃/min，從室溫升至900 ℃。工業(yè)分析揮發(fā)分、灰分、固定碳項目參照GB/T212-2008，熱值項目參照GB/T213-2008。

2 結果與分析

2.1 RDF各階段燃燒趨勢分析

圖1為樣品1～4的熱重分析的TG曲線圖。由圖1可以看到，顆粒樣品1～3與非成型樣品4的燃燒趨勢大致相同，分為4個燃燒階段，即脫水、揮發(fā)物熱解、揮發(fā)物燃燒與固定碳燃燒并存、固體碳的進一步燃燒[23]，分別在165、165～335、335～498、498～672 ℃。在水分揮發(fā)階段，1號樣在最下端，4號樣在最上方，表明這個階段高密度顆粒水分溢出較慢，但差別并不太大，0.51 g/cm3比0.10 g/cm3的樣品在同一溫度點只有0.01%減幅。

樣品在完成水分揮發(fā)后，進入揮發(fā)物熱解，各樣的TG曲線逐漸重疊，表明這個階段的燃燒側重熱解，需要的氧氣量不多，同物料不同顆粒密度的RDF燃燒速率在此階段接近。值得注意的是，樣品完成揮發(fā)物熱解，質量已經(jīng)減少了40%，如果除去RDF所含的不可燃燒的無機成分（灰分），該燃燒部分實際占樣品原始質量的50%，說明RDF有一半的易燃物質在165～335 ℃溫度段燃燒，為熱值做出重大貢獻。

注：樣品1～4分別為成型RDF 0.51, 0.42, 0.32 g·cm-3, 非成型RDF。

在335 ℃后，RDF進入揮發(fā)物及固定碳燃燒。由圖1的TG圖可以看到，未成型的非顆粒樣品4號RDF的失質量速度以絕對優(yōu)勢超過成型顆粒RDF1～3，結合DTG圖更清晰地表現(xiàn)，４號樣比1～3號更加劇烈地燃燒?？梢姡琑DF在這個階段的燃燒需要氧氣的助燃，而密度大的RDF間隙少，接觸空氣面積小，氧氣少，使其燃燒速度降低[24]。

2.2 RDF燃燒特征參數(shù)分析

系列樣品1～4的TG和DTG整理出著火點T和最大失質量溫度Max和最大失質量速率DTGMax，見表2。

表2 不同樣品的著火點Ti，最大失質量溫度TMax和最大失質量速率DTGMax

由表2看出，隨著RDF顆粒密度的增加，著火點逐漸提高，從234.0 ℃升至238.2 ℃，表明著火點與物料接觸到的空氣面積密切相關。從另一角度看，物料密度直接影響著火點，擠壓成型物料單元體積也會影響著火點。

為突出RDF的特點，本試驗同時做了一個非成型的秸稈樣品作為對照樣。從圖2看到，對照樣的第一個失質量峰后的溫度段，已經(jīng)沒有多少物質可以燃燒了，這就是生物質燃料與RDF的重大區(qū)別。由于RDF是混合物，有塑料、紙纖維、布纖維、植物纖維等組成，不同的物質具有不同的燃燒特征，使得本試驗的RDF在DTG出現(xiàn)3組失質量峰。在第1組是失質量峰里，未成型RDF，密度為0.10 g/cm3，最大失質量溫度TMax1最高，最大失質量速率DTGMax1也最高，隨著RDF密度提高，Max1降低，DTGMax1也有所降低。有趣的是，在第2組的失質量峰里，未成型的RDF的TMax2降至第3，DTGMax2降至最末。2號成型RDF，密度為0.42 g/cm3，在第2組排第1。在第3組失質量峰里，未成型RDF的Max3、DTGMax3都降至最末；1號成型RDF，密度為0.51 g/cm3排位第1?？梢?，較高密度的RDF成型顆粒，有利于燃燒的持久性和均勻性。

2.3 RDF殘渣量分析

在實際生產(chǎn)中，RDF的燃燒產(chǎn)生大量的灰渣所涉及的廢棄物處理是非常嚴峻的工作，盡量減少RDF的殘渣量也是垃圾處理的解決方案之一[25-26]。儀器分析與實際焚燒的情況可能有出入，但系列條件樣品平行比對的結果仍然具有參考價值。從圖1的TG組圖可以看到，RDF在672 ℃以后，基本上燃盡，密度較大的1號和2號樣灰渣量幾乎一致，比4號未成型樣品低6.7%，而密度較小的3號樣的灰渣量比4號樣高3.1%。這組數(shù)據(jù)表明，增加RDF的成型顆粒密度，可燃物質分子之間的距離減小，燃燒過程原子活化能較高的獲得能量釋放離子；在距離較短的情況下，離子克服較少的位能即可與相鄰的離子發(fā)生反應，釋放反應能[27]，因此，較大的密度有利于促進RDF在燃燒的后期進一步燃燒，燃燒后部分物質轉化氣體離開，相應減少灰渣量。

圖2 樣品1～4與秸稈的DTG曲線

2.4 RDF熱值分析

本試驗的計算得為0.25，從分布表[28]查得0.05（3，8）=4.07，0.01（3，8）=7.59，<0.05<0.01，故RDF顆粒密度對熱值無顯著影響。

表3 樣品1～4的熱值

表4 顆粒密度對熱值影響的方差分析表

Note：=0.25<0.05<0.01.

3 結 論

垃圾的易燃部分經(jīng)破碎混合后再經(jīng)過擠壓制備RDF顆粒。對一系列不同密度的RDF顆粒的熱性能分析，得出以下結論：

1）高密度RDF顆粒在燃燒過程中，著火點較高，燃燒劇烈程度小于低密度顆粒。但高密度顆粒在燃燒過程的幾個階段，其燃燒速率較為均衡，有利于熱量吸收。

2）高密度RDF顆粒在后期的進一步燃燒較為充分，燃燒后的灰渣含量比低密度和非顆粒的RDF少7%左右，有利于減少灰渣含量，對減少固體廢棄物排放有著重要的意義。

3）試驗數(shù)據(jù)分析，RDF顆粒密度大小對燃燒過程釋放的熱量值無直接影響。

除了技術上的因素，RDF在實際生產(chǎn)中的工藝受到諸多因素的制約，如場地、設備、運輸，以及生產(chǎn)過程中能耗；另外，垃圾焚燒爐空間大，溫度分布不均勻，導致灰渣量增加、灰渣成分復雜等等情況。RDF的研究需要從每一個環(huán)節(jié)入手，在細節(jié)上考慮優(yōu)化生產(chǎn)工藝；同時，開辟其他能提高其RDF附加值的產(chǎn)品，如對灰渣進行資源化利用也值得深入研究。

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黎 濤，熊祖鴻，房科靖，魯 敏，謝 森，熊培培.顆粒密度對垃圾衍生燃料燃燒特性的影響[J]. 農(nóng)業(yè)工程學報，2017，33(23)：241－245. doi：10.11975/j.issn.1002-6819.2017.23.031 http://www.tcsae.org

Li Tao, Xiong Zuhong, Fang Kejing, Lu Min, Xie Sen, Xiong Peipei.Effect of particle density on combustion characteristics of refuse derived fuel [J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2017, 33(23): 241－245. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2017.23.031 http://www.tcsae.org

Effect of particle density on combustion characteristics of refuse derived fuel

Li Tao, Xiong Zuhong, Fang Kejing, Lu Min, Xie Sen, Xiong Peipei

(510640,)

Waste to energy (WTE) plants have been built more and more worldwide nowadays. It is one of the most successful types to resolve the impact of city rubbish on environment. Refuse derived fuel (RDF) is the fuel of WTE plant, which is made of combustible refuse such us cardboard, paper, various plastics, textile and wood. According to literature, the incineration process has the advantage of reducing waste weight by 80% and volume by 70%, and around 25%-30% energy within RDF can be obtained through conversion of power facilities. Combustible refuse can be processed into pellets with physical density of more than 0.3 g/cm3through extrusion forming after separating, crushing, and drying procedure. In this paper, pelleted RDF with different densities of 0.51, 0.42, and 0.31 g/cm3was adopted in this experiment, and their combustion characteristics from 30 to 900 ℃were investigated by means of thermogravimetric (TG), calorific value, and ash content analysis. Meanwhile, pelleted RDF was compared with non-pelleted RDF and straw which represented biomass fuel. By comparing TG of pelleted RDF and non-pelleted RDF, the results indicated that non-pelleted RDF was faster in burning, showing pelleted RDF can release energy slower, which is good for boiler equipment to absorb energy more efficiently. By comparing DTG of pelleted RDF and non-pelleted RDF, the ignition point of the non-pelleted RDF (0.10 g/cm3) was 234.0 ℃, and 3 maximum mass loss rates were 6.30, 2.21 and 0.53 percentage points per minute, respectively; While for the highest density of pelleted RDF (0.51 g/cm3), the ignition point was 238.2 ℃, and 3 maximum mass loss rates were 5.70, 3.11, and 0.61 percentage points per minute, respectively, showing that the higher density, the higher ignition point, and the higher burning rate got more balance. DTG data also show that pelleted RDF is more suitable for the boiler to absorb heat efficiently. After finishing burning, the content of combustion ash coming from the highest density pelleted RDF sample in this experiment was 6.7% less than that of the non-pelleted one, showing that procession of extrusion forming will help to reduce weight and volume of RDF ash, thus relieving the pressure of landfill, which is the most serious problem to our environment. For the burning calorific value, and the data from pelleted RDF and non-pelleted RDF checked by calorimeter, each sample was checked 3 times. By calculating all sample values with the method of variance analysis,value was 0.25, whilevalues between groups and within group were 3 and 8, respectively, the final0.05(3, 8) value was 4.07, and0.01(3, 8) value was 7.59, thus<0.05<0.01.The results of variance analysis show that calorific value difference between pelleted RDF and non-pelleted RDF is not obvious. Though pelleted RDF has some advantage for equipment to absorb energy, its procession is limited by many factors, such us space, equipment structure, transportation, moreover, investment cost and energy cost are also included in the consideration. On other hand, we also can make use of RDF ash as product material, so there are still many researching works to be done.

wastes; combustion; biomass; refuse derived fuel; RDF density; ash

10.11975/j.issn.1002-6819.2017.23.031

TK16

A

1002-6819(2017)-23-0241-05

2017-07-27

2017-10-28

廣州市民生科技攻關計劃項目（705272504057）

黎 濤，廣東湛江人，高級工程師，從事固體廢棄物資源化利用。Email：litao@ms.giec.ac.cn