煤种性质对煤粉工业锅炉结焦的影响

王 志 强1,2,3

(1.煤科院节能技术有限公司,北京 100013;2.煤炭资源开采与环境保护国家重点实验室,北京 100013;3.国家能源煤炭高效利用与节能减排技术装备重点实验室,北京 100013)

摘 要:燃煤锅炉内结焦会对锅炉运行的安全性和经济性造成极大损害,因而分析影响燃煤锅炉结焦的因素,进而有效预防燃煤锅炉结焦至关重要。在实际应用中,针对影响燃煤锅炉结焦的不同因素,可采取不同的预防措施。研究发现煤的灰熔融性温度、煤粉颗粒大小、锅炉燃烧气氛、一二次风动力场、锅炉截面热负荷和锅炉热负荷等都会影响燃煤锅炉结焦。为了解决某地区煤粉工业锅炉预燃室、炉膛、对流受热面大面积燃烧结焦问题,笔者结合燃煤锅炉燃烧结焦的机理,先后采取调整燃烧气氛、增大二次风刚性、减小煤粉颗粒粒径、更换孙家岔煤粉等措施对不同条件下的结焦现象进行对比分析,发现煤种、煤粉粒径大小是影响某地区煤粉工业锅炉燃烧结焦的因素。通过SEM-EDS(扫描电镜和能谱分析)对锅炉焦块进行微观形貌与元素组成分析,现场取样锅炉现用煤粉和孙家岔煤粉进行煤质及灰成分对比分析,并根据灰成分进行结渣性判别指标计算,结果表明锅炉燃烧现用煤种灰熔融性温度较低,煤灰软化温度Ts为1 170 ℃,小于1 200 ℃,为易熔煤,容易结渣,属于典型的易结焦煤种;结渣性判别指标计算结果显示,4项指标评价为“严重”,1项指标评价为“中等”,结渣性严重。综合分析认为:锅炉燃烧煤种发生改变,煤的灰熔融温度较低是影响某地区煤粉工业锅炉燃烧结焦的最本质因素。为进一步解决现场实际问题,采取破坏煤灰中酸碱平衡,提升煤的灰熔融温度,配合调节煤粉粒径等措施,如对锅炉现用煤种掺混5%的石英,提高煤灰中SiO2含量,掺混后煤粉的灰熔融温度达到1 280 ℃,提高了110 ℃;调大煤粉磨机频率,从19 Hz增大到22 Hz,煤粉粒度(200目,0.075 mm)过筛率从70%增大到85%。经过上述调整后,锅炉运行平稳,结焦状况显著改善,燃烧调整措施取得了较好的效果。

关键词:煤粉炉;结焦;结焦机理;结渣指数;燃烧调整

中图分类号:TK16

文献标志码:A

文章编号:1006-6772(2020)02-0137-08

收稿日期:2020-03-01;责任编辑:白娅娜

DOI:10.13226/j.issn.1006-6772.20030101

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基金项目:天地科技股份有限公司科技创新创业资金专项项目重点项目(2018-TD-ZD001)

作者简介:王志强(1990—),男,河南周口人,助理研究员,研究方向为煤炭清洁高效燃烧。E-mail:782714533@qq.com

引用格式:王志强.煤种性质对煤粉工业锅炉结焦的影响[J].洁净煤技术,2020,26(2):137-144.

WANG Zhiqiang.Influence of coal properties on coking of pulverized coal industrial boilers[J].Clean Coal Technology,2020,26(2):137-144.

Influence of coal properties on coking of pulverized coal industrial boilers

WANG Zhiqiang1,2,3

(1.China Coal Research Institute Company of Energy Conservation,Beijing 100013,China;2.State Key Laboratory of Mining and Environmental Protection,Beijing 100013,China;3.National Energy Technology and Equipment Laboratory of Coal Utilization and Emission Control,Beijing 100013,China)

Abstract:Coking in coal-fired boilers will cause great harm to the safety and economy of boiler operation.Therefore,it is important to analyze the factors affecting coking of coal-fired boilers and effectively prevent coking in coal-fired boiler.In practical application,different preventive measures can be taken for different factors affecting coking of coal-fired boiler.It is found that the melting temperature of coal ash,particle size of coal,combustion atmosphere of boilers,primary and secondary air force field,thermal loads of boiler section and boilers are the factors that affect the coking of coal-fired boilers.In order to solve the problem of large area combustion coking in precombustion chamber,furnace and convection heating surface in a specific pulverized coal boiler,based on combustion coking mechanism of coal-fired boilers,the measures of by adjusting combustion atmosphere,increasing secondary air rigidity,reducing the pulverized coal particle size,replacing the pulverized coal were taken to analyze coking phenomenon under different conditions.The conclusion is that the types of coal and the particle size are the factors affecting the combustion coking of this specific pulverized coal boiler.The micro morphology and element composition of the coking block as well as the composition of different coals were analyzed by SEM-EDS,the coal quality and ash composition between the coal powder currently used in the on-site sampling boiler and Sunjiacha coal powder were compared and analyzed,and the slagging discrimination index was calculated according to the ash composition.The melting temperature of coal ash used in the boiler is relatively low.The softening temperature Ts is 1 170 ℃,less than 1 200 ℃.It is called fusible coal and easy to slag and belongs to a typical kind of easily coking coal.The calculation results of slagging discrimination index show that 4 indexes are evaluated as "serious",1 index is evaluated as "medium",meaning high slagging tendency.In general,the change of coal type and the low melting temperature of the coal are the essential factors affecting the combustion coking of the boiler.To further solve practical problems,the acid-base equilibrium of coal ash was destroyed to increase the melting temperature and the particle size was refined by the following methods,5% quartz was blended into the coal to increase the content of SiO2 and thus increase the melting temperature of the coal to 1 280 ℃,and the temperature would be increased by 110 ℃.The frequency of pulverized coal mill was turned up from 19 Hz to 22 Hz to increase the sieving rate of pulverized coal particle size(200 mesh) from 70% to 85%.Eventually,the boiler ran smoothly and the coking condition is improved significantly.Overall,the combustion adjustment measures achieve good results and relevant experience are available for reference.

Key words:pulverized coal furnace;coking;coking mechanism;slagging index;combustion adjustment

0 引 言

锅炉作为煤炭提供动力的设备,极易发生结焦现象,影响燃煤锅炉的安全性及经济性。由于结焦造成的锅炉非正常停机达到10%以上。关于燃煤锅炉结焦问题,国内外进行了大量研究,主要集中在煤质因素和外部条件方面。煤质因素方面,姚星一[1]、王泉清等[2]对煤灰中化学成分进行分析,发现煤灰中的酸碱氧化物在高温下会相互反应,生成低熔点的矿物,且一些氧化物自生熔融性温度较高,含量高低对煤灰熔融性起决定性作用;张德祥等[3]、马永静[4]、Unuma等[5]对煤中矿石进行分析,发现煤的灰熔融性取决于石英、高岭石和长石的含量,随着高岭石含量的增加,灰熔融性升高,高岭石含量相同时,长石含量高的煤,灰熔融性低;Vassilev等[6]分析了煤化学组成和矿物组成,认为煤灰中硅酸盐含量高时,灰熔融性较高;而含有较多氧化物和长石时,灰熔融性较低,进而影响结焦情况;Nicholas等[7]将英国国内煤种分别与美国和南非的煤种混合燃烧,研究混煤成分对结渣特性变化的影响,发现含量高的Fe2O3英国国内煤种与含量高的CaO煤种混合后结渣性能显著提高。外部条件方面,吴英[8]等针对600 MW四墙切圆燃烧超临界锅炉燃烧器区域易结焦问题进行了研究,发现燃烧器区域截面热负荷设计较大,造成其内火焰燃烧温度较高,引起结焦;林树亮等[9]分析350 MW超临界直流锅炉结焦原因,发现煤粉偏细,导致着火提前,燃烧器区域热负荷骤增,导致结焦;韩廷秀[10]针对670 t/h锅炉燃烧器进行防结焦改造,通过研究切圆燃烧锅炉带侧二次风的一次风射流刚性,发现侧二次风对一次风的刚性能有显著影响,侧二次风与一次风的动量比应维持在0.73~0.86,此时气流沿水冷壁的速度有所减少,气流贴壁减少,氧含量升高,受热面管壁温度下降,能有效防止结焦发生;赖禄斌[11]针对600 MW锅炉结焦原因进行分析,发现适当提高锅炉内部过量空气系数,可以抑制还原性状态的出现,防止结焦。以上成果为解决燃煤锅炉结焦问题提供了参考,但鲜见解决现场锅炉结焦问题,特别是煤粉工业锅炉结焦问题的相关研究。

某地区煤粉工业锅炉在冬季供暖期间,出现大规模结焦现象,且结焦频繁,导致频繁停炉清焦,增加了运营成本和安全隐患。为了解决燃烧结焦问题,本文结合燃煤锅炉燃烧结焦的机理,采取不同措施对不同条件下的结焦现象进行对比分析,并通过SEM-EDS对锅炉焦块进行微观形貌与元素组成分析,以发现影响某地区煤粉工业锅炉结焦的因素,继而进行针对性调整,以期达到抑制锅炉结焦的影响。

1 问题分析

1.1 锅炉概况

某地区煤粉工业锅炉选用煤炭科学技术研究院(简称煤科院)自主设计、泰山锅炉厂加工制造的型号为SZS20-1.6-AIII的煤粉饱和蒸汽锅炉,炉型整体布局如图1所示。

图1 锅炉整体布局
Fig.1 Overall layout of boiler

锅炉采用煤科院自主设计的浓相中心逆喷燃烧技术与双锥煤粉燃烧器和强旋流、浓相煤粉中心逆喷及双锥强制回流技术,具有着火容易、燃烧稳定,可实现煤粉在线气化等特点,与分级配风炉膛相结合,形成独特的低NOx煤粉燃烧体系[12-13]

双锥燃烧器由二次风仓、导流叶片、双锥燃烧室、风粉喷管及回流帽等部件构成。双锥燃烧器在工业应用时,可完成60%以上的煤粉燃烧进程,导致温度高、易结焦。

1.2 结焦情况

某地区煤粉工业锅炉采用现有煤种运行期间,经点火运行平稳后,随着运行时间的增加,炉膛膜式壁、二回程对流管入口、预燃室出口结焦、挂壁现象严重。受热面逐渐减小,导致炉膛温度从700 ℃逐渐增加到950 ℃左右(位于炉膛尾部测点温度);二回程对流管入口结焦、积灰越来越多,导致炉膛负压降低,逐渐出现正值;预燃室出口封堵,破坏燃烧流场。某地区煤粉工业炉结焦情况如图2所示。

图2 某地区煤粉工业锅炉结焦情况
Fig.2 Coking situation of pulverized coal industrial
boiler in a certain area

2 原因分析

2.1 结焦机理及影响因素

锅炉运行时,煤粉在炉内燃烧,熔化或部分熔化的颗粒被夹带在烟气中,与炉内水冷壁、对流管和燃烧器进行热量交换,随同烟气共同放热降温。在靠近水冷壁、燃烧器和炉膛内壁前,如果软化的灰渣颗粒温度降低凝结固态,会在受热面上形成一层疏松的灰层,吹灰操作即可清除。当燃烧区域高温过热时,一部分灰渣在接触水冷壁前,未经过足够的降温放热处于熔融和半熔融状态,此时的灰渣有相对高的黏性,易吸附在水冷壁、燃烧器和水冷壁上,已黏附的颗粒物极易黏附其他灰渣颗粒物,使结渣面继续扩大,形成结焦[14-18]

通常煤的灰熔融温度为1 000~1 600 ℃,软化温度>1 350 ℃称为难熔煤,软化温度在1 200~1 350 ℃为中熔煤,软化温度<1 200 ℃为易熔煤,灰熔融温度越低越容易结渣,灰中化学成分和矿物质对熔融性有较大影响,化学成分主要是无机氧化物[19-23],氧化物含量决定了灰熔融性的高低,一般来说,酸性氧化物(SiO2、Al2O3、TiO2)含量较多,灰熔融性较高;碱性氧化物(Fe2O3、CaO、MgO、Na2O、K2O)含量较多,灰熔融性较低;矿物分为助熔矿物(长石、硅酸钙、赤铁矿、硬石膏、重晶石)和耐熔矿物(石英、高岭石、莫来石、钛氧化物),助熔矿物的总量占矿物总量的比例,决定了煤灰熔融温度的高低[3-5,24]

锅炉燃烧结焦与煤的灰熔融性、炉膛温度、反应时间等因素相关[25]。通常情况下,灰熔融温度越低,炉膛温度越高,还原性气氛越强,反应时间越长,锅炉结焦越严重。

2.2 焦块形貌与成分分析

停炉后,在结焦部位取得典型焦块——预燃室出口焦块、炉膛膜式壁与对流换热面入口焦块,利用SEM-EDS对样品断面进行分析,具体如图3所示。

图3 焦块外观
Fig.3 Appearance of coke

由图3可知,焦块1(预燃室出口焦块)有鼓泡、颜色黑亮,质地坚硬、光滑,呈现出釉质光泽,应为高温、还原性气氛下煤灰中矿物成分形成的非晶相玻璃体;焦块2(炉膛膜式壁与对流换热面入口焦块)颜色灰白、质地疏松多孔、表面粗糙,应为低温、氧化气氛下形成的焦块。

为探究焦块1和焦块2的形貌、组成成分差异,对焦块1和焦块2的横断面取小颗粒样品进行SEM-EDS分析,微观形貌分别如图4所示。

图4 焦块微观形貌
Fig.4 Micromorphology of coke

由图4(a)可知,焦块1呈规则的层状多边形结构,结构致密,推测预燃室温度较高,煤灰中的无机物在高温条件下团聚熔融,煤灰中的矿物质本身及相互之间发生复杂的反应[26],形成了大量非晶相玻璃体;燃烧器为低氮燃烧设计,预燃室处于还原性气氛,导致煤粉燃烧不充分,显现黑色。研究表明[27],煤灰中的晶相物质在高温条件下向非晶相物质的转化反应剧烈,结合焦块1的外观,说明预燃室温度较高,此处煤灰中的石英和黏土矿物等大量转化为非晶相的玻璃体[28],所以表面光滑,呈现亮黑色,且带有釉质光泽。由图4(b)可知,焦块2疏松多孔,呈现不规则结构,布满褶皱,结合焦块2的外观,推测煤粉在低温、氧化气氛(充分燃烧和控制NOx生成)下,燃烧后的煤灰夹杂在熔融颗粒里面,软化的灰渣颗粒附着在受热面上,形成焦状结构。

对焦块1和焦块2进行EDS分析,如图5所示,发现焦块中O、Fe、C、Ca、Si、Al元素含量较高,其中焦块1中的C含量高于焦块2,与黑色外观一致;焦块1中的O含量高于焦块2,说明预燃室高温导致煤粉燃烧的过程中生成了大量氧化物,使得焦块1中的氧含量较高;焦块1中的Fe含量较高,推测煤中赤铁矿在高温条件下,与SiO2和Al2O3发生低温共熔反应[29],生成了大量非晶相玻璃体,使得焦块1呈现出釉质光泽。

2.3 煤质对比分析

为改善燃烧结焦状况,保持煤种不变,减小煤粉粒度,结焦情况和清焦频率有轻微改善;调成弱还原性燃烧气氛[30]和增大二次风压[10]等工况参数,结焦情况及清焦频率无变化;在其他工况条件不变的情况下,使用孙家岔煤粉,燃烧结焦状况大为改善,炉内结焦情况如图6所示。

图5 焦块元素含量分析
Fig.5 Analysis of element content in coke

图6 孙家岔煤粉燃烧结焦情况
Fig.6 Coking situation of Sunjiacha pulverized coal combustion

对比图2和图6,发现煤粉是影响燃烧结焦的主要因素,煤矿近期采煤层已不适用坑口煤粉工业锅炉,对比化验孙家岔和锅炉现用煤粉(补连塔煤粉),煤质分析见表1和表2。

表1 煤质分析对比

Table 1 Comparative analysis of coal quality

煤粉工业分析/%MtMadAarVdaf元素分析/%CarHarOarNarSarQnet,ar/(MJ·kg-1)孙家岔4.301.337.6036.3957.783.4011.710.580.3026.89补连塔16.705.576.2637.6754.433.8112.560.850.3227.08

表2 灰成分及灰熔融温度对比

Table 2 Comparation between components and melting point of ash

煤粉灰成分/%SiO2Fe2O3MgOTiO2P2O5Na2OAl2O3CaOSO3K2OMnO2灰熔融温度/℃DTSTFT孙家岔60.253.850.532.571.021.6519.504.832.792.560.451 2501 3101 390补连塔28.0718.061.040.410.009 42.4210.2530.786.990.630.411 1501 1701 200

由表1可知,锅炉现用煤粉水分高,低位发热量相当,燃烧时具有较低的炉膛温度,对煤种的结焦有一定的抑制作用,但锅炉现用煤粉挂壁严重,受热面减小,炉膛温度会逐渐升高,增大结焦可能性。

由表2可知,SiO2、Al2O3和CaO是煤灰的主要成分,自身熔点比较高,分别为1 700、2 050、2 580 ℃。高温下CaO易与灰中Al2O3和SiO2作用,生成低熔点的矿物,降低了煤的灰熔融性。孙家岔煤粉中19.50%的Al2O3和60.25%的SiO2含量远高于4.83%的CaO含量,酸碱中和后,多余的Al2O3和SiO2能大大提高煤的灰熔融性;补连塔煤粉中10.25%的Al2O3和28.07%的SiO2在高温下与30.78%的CaO反应后,只剩少许的酸性氧化物,煤的灰熔融性较低,增大结焦的可能性。

Fe2O3、MgO为助溶剂[31-32],在高温下与灰中硅铝酸盐相互作用,生成低熔点长石,降低灰熔融温度;TiO2为无机酸性氧化物,会提高灰熔融温度;SO3在高温下会生成硫酸蒸汽,遇到灰中的钙氧化物就会产生硫酸钙,硫酸钙会附着在设备上,形成结焦[33]。由表2可知,补连塔煤粉Fe2O3、MgO含量较高,TiO2含量较低,SO3含量较高,Fe2O3为孙家岔煤粉的5倍左右,MgO为孙家岔煤粉的2倍左右,TiO2仅为孙家岔煤粉的1/6,SO3为孙家岔煤粉的2.5倍,这些含量差异造成补连塔煤粉灰熔融温度较低,加大结焦的概率。

煤灰软化温度常用来评估煤种结焦性[26]。通常情况下,ST<1 200 ℃称为易熔煤,容易结渣,补连塔煤粉灰熔融温度为1 170 ℃,属于典型的易结焦煤种;孙家岔煤粉灰熔融温度为1 310 ℃,不易结焦。综合分析可知,补连塔煤质改变,灰熔融温度较低是锅炉结焦的根本原因。

2.4 结渣性分析

为定量判别锅炉现用煤种的结渣特性,引入经验判别指数进行对比分析,具体见表3。

表3中,酸碱比即煤灰中碱性氧化物含量与酸性氧化物含量之比,计算式为

表3 结渣指数判别标准

Table 3 Rating standard of slagging index

参数轻微结渣中等结渣严重结渣酸碱比B/A<0.2060.206~0.400>0.400硅比G>0.7880.661~0.788<0.661硅铝比C<1.871.87~2.65>2.65铁钙比D<0.3或>3.00.3~3.0接近1时综合结渣指数R<1.51.5~2.5>2.5

(1)

硅比为

(2)

硅铝比为

C=w(SiO2)/w(Al2O3)

(3)

铁钙比为

D=w(Fe2O3)/w(CaO)

(4)

综合结渣指数为

(5)

式中,w(i)为各物质的质量分数,%。

补连塔煤粉和孙家岔煤粉各项结渣指标判断结果计算见表4。

表4 煤粉结渣指标计算结果

Table 4 Calculation results of slagging index of coal powder

项目孙家岔B/AGCDR补连塔B/AGCDR数值0.160.873.090.802.221.370.362.740.594.85结渣性轻微轻微严重中等中等严重严重严重中等严重

由表4可知,锅炉现用补连塔煤粉中,4项评价指标为“严重”,1项评价指标为“中等”,结渣性强烈;孙家岔煤粉中,1项评价指标为“严重”,2项评价指标为“中等”,2项评价指标为“轻微”,结渣性中等偏下。

综合分析可知,锅炉现用煤粉灰熔融温度较低,结渣性强烈,是造成锅炉结渣的最本质因素,在现用煤种无法改变的情况下,通过掺入添加剂,提高现用煤粉的灰熔融温度,是解决某地区煤粉工业锅炉结焦的主要办法;同时配合调节煤粉粒度。

3 调整措施

煤种灰熔融温度较低是锅炉结焦的最本质原因,结焦挂壁,受热面减小,炉温升高,进一步促进结焦的发生。因此如何提高煤的灰熔融温度,配合调节煤粉粒度是解决燃烧结焦的主要思路。

3.1 5%石英的掺入

由表2可知,SiO2、Al2O3和CaO是煤灰的主要成分,且熔点较高,对煤质灰熔融温度起主要作用,如何扩大灰中酸碱氧化物的差异是提升灰熔融温度的关键。由表3可知,提升灰中SiO2、Al2O3和CaO含量可以得到较低的结焦评价指数。提升灰中SiO2和Al2O3含量或CaO含量,都可以提高灰熔融温度,降低结焦评价指数,但部分电厂要求灰成分中CaO含量低于12%,所以掺入富含SiO2或Al2O3的物质,提升灰熔融温度,改变结焦的本质因素。部分矿物质的化学成分[34]见表5。

表5 矿物质化学成分

Table 5 Mineral chemical composition

矿物烧失率/%化学成分/%SiO2Al2O3Fe2O3CaOMgOSO3TiO2MnO2K2OP2O5石英1.1293.060.663.870.0500.030.230.0200.01高岭土3.3257.8836.321.390.180.190.060.990.010.520.03围顶夹矸33.2992.830.813.440.1700.160.010.0300.01

综合考虑成本和运输问题,选用石英作为添加剂[35],掺入量占混成煤的5%,掺混后灰中酸性氧化物SiO2含量为56.75%、Al2O3含量为6.02%,碱性氧化物CaO含量为17.22%,灰中主要酸碱氧化物含量相差45.55%,掺混后锅炉现用煤粉的灰熔融温度达到1 280 ℃,相比未掺混的煤粉灰熔融温度提高110 ℃,结焦倾向得到较大抑制。

3.2 煤粉粒度调整

粗煤粉颗粒燃烧时间长,导致火焰拉长,对流管入口的灰处于温度较高的环境下,易造成对流管入口处结焦;煤粉过细,着火过早,极易破坏燃烧器,在燃烧器周围结焦。某地区煤粉工业锅炉对流管入口处结焦严重。调整煤粉磨机频率由19 Hz增大到22 Hz,煤粉粒度(200目,0.075 mm)过筛率从70%增大到85%,煤粉粒径变小,对流管入口结焦状况改善。改造后某地区煤粉工业锅炉连续稳定运行,供料频率10 Hz,负荷15.4 t/h下,测得前炉膛温度、后炉膛温度、炉膛出口温度,并与调整前进行对比,结果如图7所示。

图7 调整前后温度对比
Fig.7 Temperature comparison before and after adjustment

由图7可知,掺混5%石英、减小煤粉粒度后,前后炉膛温度降低了100 ℃左右,炉膛出口温度降低了50 ℃左右,且能长时间保持稳定运行,这是由于改造后炉膛挂壁结焦减少,受热面增加。

连续运行5 d,停炉后观察炉膛内部情况,如图8所示。可知,结焦现象明显减小,改造效果良好。

图8 调整后炉膛内部情况
Fig.8 Internal condition of furnace after adjustment

4 结 论

1)预燃室出口焦块与炉膛膜式壁、对流换热面焦块明显不同,预燃室焦块为颜色黑亮、表面光滑、质地坚硬的非晶相玻璃体;炉膛膜式壁和对流换热面焦块为颜色灰白、表面粗糙、质地疏松的初始结焦面;这主要是由于预燃室温度高,处于还原性气氛,炉膛膜式壁和对流换热面温度低,处于氧化性气氛。

2)灰中酸性氧化物Al2O3(10.25%)、SiO2(28.07%)含量之和与灰中碱性氧化物CaO(30.78%)含量相差仅为7.54%,相差不大,高温下会相互反应,生成低熔融性温度的矿物,导致现用煤种的灰熔融性温度仅为1 170 ℃,容易结焦;结渣指数评价指标表明,锅炉现用煤种结渣性趋于严重,属于强结渣性煤种。

3)通过掺混5%的石英,灰中酸性氧化物SiO2(56.75%)、Al2O3(6.02%)含量之和与灰中碱性氧化物CaO(17.22%)含量相差45.55%,相差较大,掺混后现用煤种灰熔融性温度为1 280 ℃,提高了110 ℃;配合减小煤粉粒度措施,煤粉粒度(200目,0.075 mm)过筛率从70%增大到85%,缩短了燃烧时间,有效抑制了结焦状况。

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