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

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-3100p 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/m3 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/m3 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(SiO3), K2MgSi5O12, and SiO2. 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. CaSO4 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. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.