生物质流化床气化与结渣特性中试试验

Pilot-scale test on gasification and slagging characteristics of biomass in fluidized bed

为优化生物质流化床气化工艺,该研究在中试规模流化床实验台上进行了成型树皮和成型秸秆的空气气化试验,研究了空气当量比、气化温度与送风温度对成型树皮和成型玉米秸秆气化特性的影响,同时采用电子探针显微分析仪与X射线光谱分析仪对气化过程中结渣的微观结构与成分进行了分析。结果表明:气化效果随着空气当量比增大先升后降,空气当量比较佳值在0.24左右,此工况下树皮与秸秆合成气热值分别为5.66和3.92 MJ/m^(3),气化效率分别为59.62%与33.92%;气化温度增加促进气化效果提升,气化温度从700℃升高至800℃,树皮合成气热值与气化效率分别提升了1.01 MJ/m^(3)与14.28%;一次风温度的提升对气化效果无显著影响(P>0.05),但明显提升了炉膛底部温度,容易导致结渣,不利于设备稳定运行。2种生物质都有明显结渣现象,其表面呈现熔融玻璃状。结渣主要由KAl(SiO_(3))、K_(2)MgSi_(5)O_(12)等复杂化合物与SiO_(2)组成。导致结渣的原因主要是生物质中K、Mg等碱金属元素在床料中富集,与石英砂床料反应形成低熔点熔融盐;树皮含有较多Ca,气化中形成高熔点的CaSO_(4)进而抑制结渣,而秸秆成灰率高,含有较多的K,导致结渣更为严重。

Biomass gasification is an efficient utilization way to produce high calorific value syngas via the chemical conversion of biomass in renewable energy.Most previous studies focused mainly on biomass gasification in small-scale fixed beds and fluidized beds.Therefore,it is necessary to explore the slagging characteristics in the large and medium-sized gasification equipment in recent years.In this study,a pilot-scale fluidized bed was selected to investigate biomass gasification using an infrared gasboard-3100 p syngas analyzer.Taking the molded bark and corn stover as raw materials,the amount of feed and air supply was used to determine the effects of air equivalent ratio,gasification temperature,and air supply temperature on the gasification characteristics.The results are as follows.1)A better effect was obtained in the fluidized-bed gasification of formed bark,compared with the corn stover.In formed bark syngas,the calorific value and gasification efficiency reached 5.66 MJ/m^(3) and 59.62%,respectively,when the feedstock was 40 kg and the air equivalent ratio was 0.24.The hydrogen content of 11.2%was high in the syngas,due mainly to the high moisture content of formed bark(11.6%).In formed corn stover,the caloric value and gasification efficiency were 3.92 MJ/m^(3) and 33.92%,respectively,due possibly to low volatiles,the low fuel calorific value,and high ash content.The air equivalent ratio had an important influence on the gasification characteristics.The gasification effect of molded bark increased first and then decreased,with the increase of air equivalent ratio.Specifically,the low air equivalent ratio inhibited the gasification reaction,and the high air equivalent ratio promoted the oxidation reaction,where part of combustible gas was burned by oxygen.The optimum air equivalent ratio was around 0.24.2)The high gasification temperature was a benefit to the gasification effect.In formed bark syngas,the heat value and gasification efficiency increased by 20.07%and 28.76%,respectively,at the gasification temperature from 700℃to 800℃.The gasification effect was not significantly improved when the primary air temperature increased from 100℃to 300℃.But the bottom temperature of the furnace increased by about 200℃,which affected the safety of the equipment.3)Both kinds of biomass had obvious slagging phenomenon.Quartz sand particles were bonded together,and then the slag surface formed a molten glass state.The slagging compositions were mainly complex compounds,such as KAl(SiO_(3)),K_(2) MgSi_(5)O_(12),and SiO_(2).The reason for the formation of slagging was that alkali metal elements,such as K and Mg,in biomass were enriched in quartz sand particles during the gasification process,and the molten salt with a low melting point was formed at high temperature to bond quartz sand particles into slag blocks.The ash rate of tree bark was three times higher than that of corn stover,although the content of K in corn stover ash was lower than that of tree bark.If the content of K was higher in the same amount of feed,it was easier to form compounds with lower melting points.Bark ash contained more Ca.CaSO_(4) with a high melting point was produced by Ca and S during gasification,which was not easy to melt to bond quartz sand particles,thus inhibiting slagging.These two reasons can lead to the more serious slagging of corn stover than that of tree bark during biomass gasification in a pilot-scale fluidized bed.