煤粉大比例掺混不同生物质的混燃特性研究

可再生能源生物质清洁低碳、易于获取、利于着火,含硫、氮量少且属于碳中性物质,但其能量密度低。在煤粉中大比例掺混生物质(生物质/煤粉质量比大于5∶5)可有效改善煤粉着火特性,碳排放水平接近燃烧天然气,且污染物排放显著降低,进而达到节能减排目的。目前研究主要集中在低掺混比例(小于5∶5)下生物质与煤粉的混燃特性,针对北方常见的玉米秸秆、稻杆和玉米芯等生物质与煤粉在大掺混比例下的燃烧特性,尚有待深入。笔者利用热重分析技术分别研究了煤粉与不同生物质种类(玉米秸秆、稻杆及玉米芯)在不同掺混比例下(5∶5、6∶4、7∶3和8∶2)的混燃特性,分析生物质种类和掺混比例对混合燃料的着火温度、燃尽温度、交互反应以及燃烧特性指数等的影响,确定了不同生物质的最佳掺混比例。结果表明:掺混比例对混合样品失重曲线的影响从大到小依次为玉米秸秆、玉米芯和稻杆。随掺混比例增加,第1阶段最大质量变化速率逐渐增大且燃烧进程前移,第2阶段则逐渐减小,这是由于挥发分相对增加且焦炭相对减少的原因。混合样品的着火温度和燃尽温度比纯煤粉分别下降约100和60℃。随掺混比例的增加,玉米芯着火温度逐渐减小,玉米秸秆和稻杆则先减小后增大,且均在7∶3时达到最小;燃尽温度均呈现下降趋势,下降幅度由大到小分别为玉米芯、稻杆和玉米秸秆。玉米秸秆和稻杆在8∶2时燃尽性能较差。混合样品发生不同程度的交互作用,该交互作用正是生物质的促进和抑制的协同作用,使3种生物质均在5∶5时对煤粉燃烧抑制作用大;玉米秸秆和稻杆在7∶3时、玉米芯在6∶4、8∶2时促进作用大。同时,3种生物质的燃烧特性指数远大于煤粉,随掺混比例的增大,玉米芯的燃烧特性指数变化最大并在8∶2时达到最大值,6∶4和7∶3时几乎相同;稻杆的变化最小且在7∶3时达到最大值;玉米秸秆在7∶3和8∶2时几乎相同并达到最大值。小范围改变掺混比例时,燃烧特性指数变化不大。这可能是由于燃烧特性指数不仅与着火温度和燃尽温度有关,还与样品在其主要燃烧过程的反应速率有关,而煤粉在焦炭燃烧阶段的反应剧烈程度高于生物质挥发分析出阶段,使不同掺混比例的混合样品出现以上现象。

Study on co-combustion characteristics of pulverized coal and different biomass at large mixing ratios

As a renewable energy,biomass is clean,low-carbon,easy to obtain and ignite,which has a higher volatile content,lower sulfur and nitrogen content,and belongs to carbon neutral substance,but it has a lower energy density.Blending biomass in a large proportion in pulverized coal( biomass/pulverized coal mass ratio is greater than 5/5) can effectively improve the ignition characteristics of pulverized coal,make the carbon emission level close to burning natural gas,and significantly decrease the pollutant emissions,finally can achieve the goal of energy conservation and emission reduction.At present,the study of the co-combustion characteristics of biomass and pulverized coal at low mixing ratios( less than 5/5) is mainly focused on.It is necessary to study the co-combustion characteristics of biomass and pulverized coal at the large mixing ratio,such as corn stalks,rice stalks and corn cobs,which is common in the north of China.In this paper,thermogravimetric analysis techniques were adopted to study the co-combustion characteristics of the pulverized coal and different biomass( corn straw,rice straw,and corn cob) at different mixing ratios( 5/5,6/4,7/3 and 8/2). The effects of biomass species and mixing ratios on the ignition temperature,burn-out temperature,interaction,and combustion characteristic index of the mixed fuel were analyzed and the optimal blending ratio for different biomass was determined.The results show that the effect of the blending ratio on the weight loss curve of the mixed fuel is corn stalk,corn cob,and rice straw,in order from large to small. As the blending ratio increases,the maximum weight loss rate gradually increases in the first stage and the combustion process moves forward,and gradually decreases in the second stage,which is due to the relative increase of volatile matter and the relative decrease of coke.Compared to pure coal powder,the ignition and burn-out temperatures of the mixed samples are reduced by approximately 100 ° C and 60 ° C,respectively.With the increase of the blending ratio,the ignition temperature of the corn cob gradually decreases,and the corn stalks and rice stalks decrease and then increase,and both reach the minimum at 7/3. The burn-out temperature shows a downward trend,and the decline range is corn stalk,corn cob,and rice straw,in order from large to small.Corn straw and rice stalks have poor burnout performance at 8/2.The mixed fuels interact to varying degrees,and the interaction is the synergy effect between the promotion and inhibition of biomass.So the three kinds of biomass have a large inhibitory effect on pulverized coal combustion at 5/5;if the corn straw and rice stalks are at 7/3,and corn cob is at 6/4 and 8/2,the promotion effect is great.At the same time,the combustion characteristics index of the three biomass is much larger than that of pulverized coal.As the blending ratio increases,the combustion characteristics index of corn cob changes the most and reaches a maximum at 8/2,and is almost the same at 6/4 and 7/3.The change of rice stalk is the smallest and reaches the maximum at 7/3. The corn stalk is almost the same and reaches the maximum at 7/3 and 8/2.When the blending ratio changes in a small range,the combustion characteristic index does not change largely.This may be because the combustion characteristic index is not only related to the ignition temperature and burn-out temperature,but also to the reaction rate of the sample during its main combustion process.However,the reaction intensity of pulverized coal in coke combustion stage is higher than that in biomass volatilization stage,which makes the mixed samples with different mixing proportion appear the above phenomenon.