Agglomeration during Fluidized Bed Combustion and Gasification of Biomass

Biomass is considered as a renewable and carbon-neutral energy source, and has been utilized as a substitute for fossil fuels. Fluidized bed combustion and gasification are promising technologies for biomass utilization. However, agglomeration in fluidized bed combustion and gasification of biomass, caused by the presence of molten phase originated from biomass, affects the operation of the reactors, and may eventually lead to defluidization and unscheduled shut down of the plant. Therefore, an in-depth understanding of agglomeration in fluidized bed combustion and gasification of biomass and the development of countermeasures for minimizing the operational problems caused by agglomeration are important.

Systematic experiments on agglomeration during combustion and gasification of biomass were carried in a lab-scale bubbling fluidized bed reactor. The experiments focused on the influence of equivalence ratio (ER) in air blown, air/steam and air/carbon dioxide gasification. The influence of biomass type on agglomeration was studied in the same way. Different techniques, such as scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), were applied for characterization of agglomerates, biomass ash, and char. The fusion behaviors of the ash were investigated a fixed bed reactor under combustion and gasification conditions to further understand the influence of gas atmosphere on agglomeration. The thermodynamic equilibrium calculations were carried out to shed light upon the distribution of potassium in the ash under the experimental conditions. A pulsed flow was applied to explore the possibility of mitigating the agglomeration in fluidized bed combustion/gasification of biomass and its effect on the conversion process.

Combustion and gasification of wheat straw at different ERs in the fluidized bed reveal a trend that the agglomeration tendency first increases gradually with decreasing ER. When ER reaches a critical value, the agglomeration tendency decreases with further decreasing ER. It has been observed that the amount of unconverted carbon in the bed increases as ER decreases. By adding a high concentration of steam to the fluidized bed, the agglomeration is promoted. However, when carbon dioxide is added, slightly lower agglomeration tendency is observed. The visual examination of the fusion behaviors of wheat straw ash in the fixed bed indicates that the ash obtained from gasification with steam has the highest fusion tendency followed by the ash from combustion with presence of steam and lastly the ash from normal combustion. The experimental results indicate that an enhanced reducing atmosphere may result in a slightly higher agglomeration tendency, probably due to an increased melting of ash. The inhibition effect of residual carbon on the agglomeration is probably attributed to a reduced fusion tendency and melting flow behavior of ash and a retarded interaction between ash and bed material. The competition between an enhanced reducing atmosphere and an increased amount of unconverted carbon by lowering ER results in the occurrence of a maximal agglomeration tendency at a critical ER. The presence of high concentration of steam accelerates the agglomeration probably due to an increased carbon conversion and a lowered ash viscosity, which also leads to a lower critical ER value.

Combustion and gasification of sunflower husk in the fluidized bed indicate that a reducing atmosphere, a high concentration of steam, and the residual carbon show consistent influences on the agglomeration of sunflower husk and wheat straw, although the agglomeration tendency of sunflower husk ash is higher than that of wheat straw ash in both combustion and gasification. In the air blown system, the critical ERs of sunflower husk and wheat straw are 0.04 and 0.36, respectively. The differences in agglomeration characteristics of these two types of biomass may be attributed to their different char reactivity and different molar ratios of silicon to potassium, which are 1.56 and 0.04, respectively, for wheat straw and sunflower husk. Different with wheat straw, the accelerated agglomeration of sunflower husk under a reducing atmosphere and a high concentration of steam may be attributed to their promotive effect on the interaction between sunflower husk ash and bed material. The results of TGA experiments indicate that sunflower husk has a higher char reactivity than wheat straw, leading to a less amount of residual carbon during gasification of biomass, and thereby causing a reduced inhibition effect of residual carbon. Compared to wheat straw, the lower critical ER of sunflower husk may be attributed to a higher char reactivity and a different mechanism of reducing atmosphere on agglomeration results from a lower molar ratio of silicon to potassium in sunflower husk. Gasification of two batches of wheat straw show that the straw with a lower char reactivity has a lower agglomeration tendency in gasification at low ERs due to a more pronounced inhibition effect caused by a higher amount of residual carbon.

The results of pulsed fluidized bed experiments suggest that the introducing of a pulsed flow in the primary gas mitigates the agglomeration in fluidized bed combustor and gasifier, and the pulsed flow shows a minor effect on the combustion behaviors. Among the parameters that have been studied, the pulsation duty cycle shows the most significant effect on the agglomeration tendency, while the pulsation frequency shows the least impact. Under the combination of pulsation flow rate ratio of 0.4, pulsation frequency of 1.5 Hz and pulsation duty cycle of 25/75, the amounts of straw fed for inducing defluidization are 151.7% and 137.5% of that at the condition with the continuous flow for combustion and gasification, respectively.

The results of this work reveal that the agglomeration in fluidized bed combustion and gasification of biomass can be promoted by a reducing atmosphere and mitigated by the residual carbon. A decrease in ER results in an enhanced reducing atmosphere and an increased amount of residual carbon, and the competition between two factors results in the occurrence of a maximal agglomeration tendency at a critical ER. The presence of high concentration of steam accelerates the agglomeration tendency of biomass. A higher char reactivity leads to a reduced inhibition effect of residual carbon, thus causing a promoted agglomeration. An increase in steam concentration and char reactivity will V
result in a decreased critical ER. The different molar ratios of silicon to potassium for different types of biomass may result in different agglomeration tendency and mechanisms. The observed results indicate that the application of pulsed fluidized bed of promising to mitigate the agglomeration during combustion and gasification of biomass and shows insignificant effect on conversion behaviors.