H2O(g)对富氧燃烧超细颗粒物生成特性影响

雷 雨,牛艳青,王亨通,温丽萍,王光耀,惠世恩

(西安交通大学 能源与动力工程学院 动力工程多相流国家重点实验室,陕西 西安 710049)

摘 要:为实现富氧燃烧技术的广泛推广,对煤粉燃烧在富氧气氛下的颗粒物排放特性进行了研究。在1 800 K 管式炉内进行煤焦燃烧试验,研究了富氧气氛下H2O(g)体积分数(0、5%、10%、20%、30%)对煤焦燃烧超细颗粒物的影响;采用荷电低压撞击器(ELPI+)获得超细颗粒物质量和数量浓度粒径分布并进行分析。结果表明,H2O(g)对超细颗粒物质量浓度和数量浓度粒径分布无影响,但会导致超细颗粒物的峰值波动。超细颗粒物总数量由最小粒径超细颗粒物决定,5种水蒸气浓度下ELPI+第1级撞击器收集到的超细颗粒物数量占比均超过65%。超细颗粒物总质量由最大粒径超细颗粒物决定,5个水蒸气浓度下ELPI+第7级撞击器收集到的超细颗粒物质量占比均超过94%。低H2O(g)浓度会抑制超细颗粒物生成,H2O(g)体积分数为5%时的抑制作用最显著;高H2O(g)浓度会促进超细颗粒物生成。这是因为一方面H2O(g)与煤焦发生气化反应,使煤焦颗粒周围产生还原性气氛,促进矿物质还原为单质,进一步促进矿物质蒸发;另一方面气化反应是吸热反应,会降低煤焦颗粒燃烧温度,同时H2O(g)加入也导致烟气热容增加进一步降低,煤焦燃烧温度抑制煤中矿物质的蒸发,导致超细颗粒物生成减少,是2种作用相互竞争的结果。此外,H2O(g)的加入使超细颗粒物平均粒径增大,0~5% H2O(g)时超细颗粒物平均粒径增大最迅速。

关键词:富氧燃烧;H2O(g);超细颗粒物;平均粒径

中图分类号:X701

文献标志码:A

文章编号:1006-6772(2021)02-0198-06

收稿日期:2020-10-09;责任编辑:张晓宁

DOI:10.13226/j.issn.1006-6772.CCUS20100906

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基金项目:国家自然科学基金资助项目(51776161)

作者简介:雷 雨(1991—),男,山东泰安人,博士研究生,从事煤清洁燃烧及颗粒物排放研究。E-mail:lei19920303@mail.xjtu.edu.cn 。

通讯作者:牛艳青,副教授,博士生导师。E-mail:yqniu85@mail.xjtu.edu.cn

引用格式:雷雨,牛艳青,王亨通,等.H2O(g)对富氧燃烧超细颗粒物生成特性影响[J].洁净煤技术,2021,27(2):198-203.LEI Yu,NIU Yanqing,WANG Hengtong,et al.Effect of H2O(g) on the formation of ultrafine PM under oxy-fuel atmosphere[J].Clean Coal Technology,2021,27(2):198-203.

Effect of H2O(g) on the formation of ultrafine PM under oxy-fuel atmosphere

LEI Yu,NIU Yanqing,WANG Hengtong,WEN Liping,WANG Guangyao,HUI Shien

(State Key Laboratory of Multiphase Flow in Power Engineering,School of Energy and Power Engineering,Xian Jiaotong University,Xian 710049,China)

Abstract:To extend the oxygen enriched combustion technology in a large scale,it is necessary to study the emission characteristics of particulate matter in oxy-fuel combustion of pulverized coal. In this paper,the effect of H2O(g) volume fraction(0,5%,10%,20%,30%) on the formation of ultrafine particulate matter(PM) under oxy-fuel combustion atmosphere was studied in a 1800 K drop tube furnace(DTF). Furthermore,the mass-based and number-based particle size distribution(PSD) were obtained and analyzed by a 14-stage electrical low pressure impactor(ELPI+). The results indicate that the mass-based and number-based PSD of ultrafine PMs remains almost unchanged in various H2O(g) contents(0,5%,10%,20%,30%),while the peak of ultrafine particles fluctuates. The total number of ultrafine PM is determined by the number of ultrafine PM with the smallest particle size. The number fractions of the ultrafine PMs collected by the first impactor of ELPI+ are all higher than 65% under all H2O(g) contents. The total mass of ultrafine PM is determined by the mass of ultrafine with the largest particle size and the mass fractions of the ultrafine PMs collected by the seventh impactor of ELPI+ are all higher than 94%. Low concentration of H2O(g) can inhibit the formation of ultrafine particles,and the inhibition effect is the most significant when the volume fraction of H2O(g) is 5%;high concentration of H2O(g) can promote the formation of ultrafine particles. Because the gasification reaction of H2O(g) with coal char produces a reducing atmosphere around the coal char particles,which promotes the reduction of minerals to elemental matter and further promotes the evaporation of minerals. On the other hand,the gasification reaction is endothermic reaction,which will reduce the combustion temperature of coal char particles. At the same time,the addition of H2O(g) also leads to the increase of heat capacity of flue gas. The combustion temperature of coal char inhibits the evaporation of minerals in coal,resulting in the reduction of ultrafine particles,which is the result of the competition between the two kinds of interaction. In addition,the addition of H2O(g) makes the average particle size of ultrafine particles increase,and the average particle size of ultrafine particles increases the most rapidly when 0-5% H2O(g) is added.

Key words:oxy-fuel combustion;H2O(g);ultrafine PM;average particle size

0 引 言

2019年我国煤炭能源消费占比57.7%,原煤产量同比增加4%,达38.5亿t,“富煤、贫油、少气”的资源禀赋决定了我国的能源结构长期仍以煤炭为主[1]。煤炭燃烧会产生大量SOx、NOx和颗粒物[2-3]等,严重污染环境,对人类健康造成威胁[4]。相比成熟的脱硫脱硝技术,颗粒物尤其是超细颗粒物(空气动力学粒径<0.3 μm[5])的控制面临很多困难[5-6]。超细颗粒物粒径小、比表面积大的特性使其无法被传统除尘设备有效捕集,且极易富集痕量重金属元素[6-7];超细颗粒物大量排放还会导致雾霾[8]。我国制定了严格的控制标准,规定颗粒物排放质量浓度需低于30 mg/m3,重点地区颗粒物排放质量浓度低于20 mg/m3,超细颗粒物排放低于5 mg/m3(GB 3095—2012),使燃煤电厂在超细颗粒物排放控制方面面临更加严峻的挑战[9-10]

富氧燃烧因在碳捕集和储存方面的优势而受到广泛关注,但其用CO2替换N2可能引入大量H2O(g),影响煤焦燃烧特性[11-13]。超细颗粒物由煤中矿物质通过蒸发-冷凝-成核-凝并机理形成,燃烧气氛和燃烧温度对其形成影响显著[14]。水分会导致炉内气氛和燃烧温度改变,进而影响超细颗粒物的生成。一般来说,煤燃烧过程中的H2O(g)主要来自煤中的氢和水分,煤中水分一般不超过10%,但褐煤中的水分可能超过70%[15-16]。此外,烟气再循环作为工业锅炉常用的降低NOx的方法,也会增加炉内H2O(g)量,部分新型燃烧技术中H2O(g)被用于调节火焰温度以降低NOx和SOx排放[17],因此富氧条件下H2O(g)对超细颗粒物生成特性的研究非常必要。文献[18]研究表明,超细颗粒物形成初期对H2O(g)非常敏感,H2O(g)会促进颗粒物的生成。Xu等[19]通过研究湿富氧气氛下超细颗粒物生成和SiO2的气化特性,证明了H2O(g)促进SiO2的气化和超细颗粒物的生成。文献[20]研究表明,H2O(g)能促进微粒的成核、团聚和凝并,还能增加粒径小于0.1 μm的超细颗粒物的排放量和平均粒径。但目前H2O(g)在超细颗粒物形成过程中的影响机理尚不明确,仍需要进一步研究。

本文模拟富氧气氛(27% O2/73% CO2),在1 800 K 高温管式炉内进行黄陵煤焦燃烧试验,通过荷电低压撞击器收集0~10 μm颗粒物,并实时获取颗粒物质量/数量粒径分布,研究H2O(g)体积分数为0、5%、10%、20%、30%时超细PM的生成特性,以获得H2O(g)浓度对超细颗粒物粒径分布、不同粒径超细颗粒物占比、超细颗粒物平均粒径的影响,为进一步研究超细颗粒物生成机理提供基础数据。

1 试 验

1.1 燃料特性

本文选取黄陵烟煤为试验煤样,其工业分析、元素分析及灰分组成见表1,煤中挥发分为30.34%,灰分为13.64%。

制备黄陵煤焦:选取75~120 μm黄陵煤颗粒在98% N2、2% O2富氧气氛、热解温度1 200 K、停留时间1.5 s条件下热解,并将热解后收集的煤焦颗粒重新筛分至75~90 μm,在马弗炉378 K干燥备用。

表1 黄陵煤工业分析、元素分析及灰组分分析[21]

Table 1 Proximate,ultimate analysis and ash component of Huangling coal[21]

元素分析/%CdHdOdNdSd工业分析/%MadAadFCadVad65.873.968.630.850.546.5113.6449.5130.34灰组分分析/%Na2OK2OSiO2Al2O3CaOMgOFe2O3SO3其他0.9311.0236.220.419.01.7311.17.781.839

1.2 试验系统

试验系统主要由给粉系统、配气系统、H2O(g)发生系统、高温管式炉、取样系统和荷电低压撞击器(Dekati ELPI+)等组成,如图1所示。煤粉颗粒经CO2气流携带由电动给粉机的储粉试管通过水冷给样探针送入高温管式炉内恒温区,给粉速率为100 mg/min,携带气流量200 mL/min。高温管式炉中心位置为内径52 mm、长900 mm的刚玉管,其周围由硅钼棒加热,保证管式炉恒温区长度不小于300 mm。试验过程中炉膛恒温区温度保持1 800 K,上、下水冷枪距离为100 mm。

图1 高温一维炉试验系统示意

Fig.1 Schematic diagram of the drop tube furnace system

炉内煤焦燃烧产生的超细颗粒物经过水冷取样探针稀释取样和两级稀释器后,进入包含14级撞击器ELPI+(实时显示超细颗粒物的质量浓度和数量浓度)。14级撞击器上收集颗粒物对应的中位径分别为0.009 26、0.016 66、0.025 61、0.041 48、0.070 63、0.129 07、0.231 41、0.431 47、0.733 76、1.218 58、2.010 57、3.012 77、4.437 43和7.298 08 μm,ELPI+前7级收集到的颗粒物为超细颗粒物。水冷取样探针出口到ELPI+入口沿程均设有伴热装置,防止颗粒物冷凝。煤焦燃烧所需H2O(g)由蒸汽发生系统提供,去离子水通过蠕动泵(iPump 2L+YZ)定量送入蒸汽发生装置,电阻丝炉加热后产生的H2O(g)由CO2气流携带送入炉膛,携带气流量为150 mL/min。通过质量流量计控制通入炉膛内的O2和CO2量以模拟富氧燃烧气氛,控制气体总量为1.74 L/min,确保煤焦在炉内停留时间为1.2 s。

2 结果与讨论

2.1 H2O(g)对超细颗粒物粒径分布的影响

图2 不同H2O(g)浓度下超细颗粒物质量/数量浓度粒径分布

Fig.2 Mass-based and number-based particle size distribution of ultrafine PMs under various water vapour contents

图2为5个水蒸气浓度下煤焦燃烧产生粒径小于1 μm颗粒物的质量浓度粒径分布和数量浓度粒径分布。煤焦燃烧试验过程中,生成的超细颗粒物被ELPI+前7级撞击器收集。5个H2O(g)浓度下超细颗粒物的质量/数量浓度粒径分布不是简单的单峰分布,主峰峰值出现在0.2 μm附近,副峰峰值出现在0.01~0.10 μm。同时,发现沉积在ELPI+第1级撞击器表面的超细颗粒物质量浓度很低,但数量浓度最高;相反,沉积在ELPI+第7级撞击器表面的超细颗粒物质量浓度最高,但是数量浓度相对较低。

图3为ELPI第1级、第7级撞击器(峰值处)收集的超细颗粒物在超细颗粒物总数量和中质量中的占比。第1级撞击器收集到的超细颗粒物对超细颗粒物总数量贡献最大,5个H2O(g)浓度下均超过65%;第7级撞击器收集到的超细颗粒物对超细颗粒物总质量贡献最大,5个H2O(g)浓度下均超过94%,说明超细颗粒物总数量由小粒径超细颗粒物数量决定,超细颗粒物总质量由大粒径超细颗粒物质量决定。随着H2O(g)体积分数增加,第1级撞击器上超细颗粒物的数量占比和第7级撞击器上超细颗粒物的质量占比均呈先减后增的趋势,并在5% H2O(g)下达到最小值,说明H2O(g)的加入影响不同粒径超细颗粒物的生成。

图3 第1级和第7级超细颗粒物数量/质量占比

Fig.3 Number/ mass ratio of ultrafine PMs on the first and seventh impactor of ELPI

2.2 H2O(g)对超细颗粒物生成率的影响

H2O(g)不仅导致不同粒径超细颗粒物生成占比发生变化,还会影响超细颗粒物生成量和生成效率。图4为不同H2O(g)浓度下超细颗粒物及粒径小于1 μm的颗粒物生成总量。图4(a)中煤焦燃烧生成的超细颗粒物总数量浓度与粒径小于1 μm的颗粒物总数量浓度几乎相同,这与2.1节颗粒物总数量由小粒径颗粒颗粒物决定的结论一致。图4(b)中超细颗粒物总质量与粒径小于1 μm颗粒物总质量有明显差距,因为大粒径颗粒对颗粒物质量贡献更大。随着H2O(g)浓度增加,超细颗粒物的总数量浓度和总质量浓度均呈先减后增的趋势,并在5% H2O(g)时达到最小值。

图4 H2O(g)对超细颗粒物及粒径小于1 μm的颗粒物生成量的影响

Fig.4 Effect of H2O(g) on the formation of ultrafine particles and particles with particle size less than 1 μm

造成该变化的主要原因是H2O(g)加入导致煤焦颗粒燃烧温度降低而对超细颗粒物生成产生的抑制作用,与H2O(g)与煤焦反应时在煤焦颗粒周围形成的还原性气氛对超细颗粒物生成的促进作用,相互竞争[22]。在低H2O(g)浓度时,温度降低对超细颗粒物生成的抑制作用大于还原性气氛对超细颗粒物生成的促进作用,且两者之间在5% H2O(g)浓度时差距最大;高H2O(g)浓度时,由于气化反应加剧,还原性气氛对超细颗粒物生成的促进作用大于水分加入煤焦颗粒燃烧温度降低对超细颗粒物生成产生的抑制作用,整体表现出促进超细颗粒物生成。

2.3 H2O(g)对单个超细颗粒物粒径的影响

不同H2O(g)浓度下超细颗粒物质量浓度和数量浓度的变化不同步,说明不同H2O(g)浓度下单个超细颗粒物质量发生变化,可通过超细颗粒物总质量浓度和数量浓度的比值确定单个超细颗粒平均质量。假设所有颗粒物的密度一致,不同H2O(g)浓度下超细颗粒物粒径发生变化,超细颗粒物粒径相对值可通过单个超细颗粒质量计算。图5(a)为超细颗粒物平均质量随H2O(g)浓度变化曲线,随着H2O(g)浓度提高,单个超细颗粒物质量先增大后减小再增大,0~5% H2O(g)下超细颗粒物平均质量增加最快,5%~10%时超细颗粒物平均质量轻微下降,10%~30%时超细颗粒物平均质量持续稳定上升。

图5 超细颗粒物及粒径小于1 μm的颗粒物平均质量及相对粒径

Fig.5 Average mass and particle size of ultrafine PMs and PM1

与无 H2O(g)时相比,5% H2O(g)下超细颗粒物平均粒径增加约7%,30% H2O(g)时单个超细颗粒物平均粒径增加约13%。粒径小于1 μm的颗粒物平均质量先增大后减小,在5% H2O(g)时最大,5% H2O(g)下单个粒径小于1 μm的颗粒物平均粒径增加约14%。有H2O(g)参与时,超细颗粒物平均粒径均大于无H2O(g)时。H2O(g)的加入增加了水分子和颗粒物之间的撞击,促进了煤中矿物质蒸发成核和颗粒物的凝并[20,23]。同时,超细颗粒物平均粒径的增大使部分大粒径超细颗粒物粒径进一步增大,超出超细颗粒物粒径上限,导致超细颗粒物生成数量减少,这可能是导致5% H2O(g)时超细颗粒物生成数量和质量减少的一个原因。

3 结 论

1)H2O(g)加入对超细颗粒物的质量/数量粒径分布影响不大,但会导致超细颗粒物峰值大小波动。超细颗粒物生成的数量由最小粒径超细颗粒物决定,第1级超细颗粒物占超细颗粒物总数量超过65%;超细颗粒物生成的质量由最大粒径超细颗粒物决定,第7级超细颗粒物占超细颗粒物总质量超94%。

2)H2O(g)的加入导致煤焦颗粒燃烧温度降低,抑制超细颗粒物的生成,H2O(g)与煤焦颗粒发生气化反应生成的还原性气氛会促进超细颗粒物的生成,两者之间的竞争作用导致低H2O(g)浓度抑制超细颗粒物生成,高水蒸气浓度促进超细颗粒物生成。

3)H2O(g)的加入会促进超细颗粒物平均粒径增大,且在5% H2O(g)前,超细颗粒物平均粒径增加最快,在5% H2O(g)时超细颗粒物平均粒径增加了7%。

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