Elsevier

Fuel Processing Technology

Volume 180, November 2018, Pages 130-139
Fuel Processing Technology

Research article
Biomass ash induced agglomeration in fluidized bed. Part 2: Effect of potassium salts in different gas composition

https://doi.org/10.1016/j.fuproc.2018.08.004Get rights and content

Highlights

  • The mechanisms of agglomeration induced by potassium salts in various gas composition were investigated.

  • Defluidization temperature in H2– or H2O-containing gas are in general, lower than those in air.

  • KCl melting is responsible for defluidization in air; KCl reacts with sand to form silicates in H2O containing gas.

  • K2SO4 decomposed and reacted with SiO2 to form low-melting-temperature potassium silicates in H2-containing gas.

  • Equilibrium calculations agree with the experimental results qualitatively.

Abstract

Agglomeration is one of the main challenges for combustion and gasification of biomass in fluidized beds. Bed agglomeration is related to K species present in biomass. Understanding the role of different types of K species on formation of agglomerates at different conditions can reveal the mechanisms of biomass ash induced agglomeration. Extensive experiments are conducted in a laboratory scale fluidized bed reactor, using mixtures of quartz sand and K species, including KCl, K2SO4 and K2CO3, to study the agglomeration mechanisms. The effects of gas composition, including air, H2– and H2O– containing gas are investigated. The morphology and elemental analyses of the agglomerate samples are examined by SEM/EDS analysis. Thermodynamic equilibrium calculations are performed for verifying the proposed mechanisms. The results show that the role of various forms of potassium salts on agglomeration in fluidized beds is different. Gas composition also has strong impact on the agglomeration tendency. In the air and H2-containing gas, defluidization of KCl in a sand bed is caused by the melt of KCl. However, KCl reacts with SiO2 to form K-silicates in the H2O-containing gas, which results in a lower defluidization temperature. No defluidization is observed for K2SO4 in the presence of oxygen or water. However, K2SO4 decomposes and reacts with SiO2 to form K-silicates, causing defluidization at 850 °C in the H2-containing gas. In the air, H2– or H2O-containing gas, K2CO3 would react with SiO2 to form potassium silicates and KOH may be additionally formed in the H2– and H2O-containing gas. The possible mechanisms of agglomeration of various potassium salts at different conditions are discussed.

Introduction

In our previous work [1], biomass ash induced agglomeration and defluidization phenomena were investigated in different gas composition in order to reveal the mechanisms of agglomeration in fluidized bed combustion and gasification of biomass. It was revealed that gas composition has a significant effect on the agglomeration mechanisms. In high concentration of H2O, the agglomeration induced by corn straw and rice straw ash appears to follow the coating-induced mechanism. However, at oxidizing (air) or reducing (H2-containing gas) conditions, the melting-induced mechanism seems dominant. Furthermore, defluidization temperature at reducing condition is significantly lower than that at oxidizing condition. It is also observed that the release of potassium to gas phase is enhanced in the H2O-containing gas, while the amount of melted potassium is significant in the H2-containing gas. It is speculated that the formation of coating in the H2O-containing gas is related to the transformation of KCl and the higher liquidus potassium is caused by the transformation of K2SO4. However, the speculations need further confirmation.

Gas composition may influence the transformation of different form of K-compounds. The type of K-compounds may play an important role in the formation of agglomerates [2,3]. During fluidized bed combustion or gasification, K-species in biomass ash may react with SiO2 to form low melting point potassium-silicates, resulting in the formation of agglomerates [[4], [5], [6], [7], [8], [9], [10], [11]]. The gaseous K-species released from biomass may react with bed material, forming coating layers on the particle surfaces and increasing the agglomeration tendency [12,13].

KCl, K2CO3 and K2SO4 are the typical potassium compounds present in biomass ash [14]. Char-bonded K may be oxidized to form K2CO3 during combustion [15]. Most chlorine in biomass ash exists likely in the form of KCl [16,17]. K2SO4 is formed during the combustion of biomass with relatively high content of sulfur [10]. The slag phase of the biomass ash may exist in the form of K2O·nSiO2 [7,18], which is formed by the reaction between K-containing compounds and SiO2 [2,16,19,20]. At high temperatures, potassium in biomass may be released to the gas phase in form of KOH when the chlorine content is low and the presence of H2O may enhance the formation of KOH [16,[21], [22], [23], [24]].

Many investigations have been carried out to study the mechanisms of K-induced agglomeration in fluidized bed combustion/gasification using biomass [4,[25], [26], [27]]. The existing form of K-compounds may have large influence on the agglomeration. It has been observed that the agglomeration tendency induced by biomass with similar ash composition have huge differences, which may be related to the existing form of K-compounds [6]. Defluidization experiments of quartz bed with potassium salts were reported [19]. The influence of type of potassium salts, bed temperature, the amount of stepwise added salts and the air with and without steam on defluidization was studied. It is shown that KCl and quartz sand do not react and defluidization was caused by the melting of KCl as glue between sand particles. K2CO3 reacts with sand forming potassium silicates, resulting in defluidization. No defluidization occurs for K2SO4 at temperatures up to 900 °C. The similar conclusions with respect to the KCl-sand and K2CO3-sand interactions were reported by heating up of a fluidized bed until defluidization occurs [28]. However, systematic study on the contribution of individual K-compound to agglomeration at various gas composition is scarce, though the effect of steam is reported [19].

This paper focuses on the characteristics of agglomeration induced by different potassium compounds in different gas composition. The same procedure as out previous work [1] is applied, e.g. the mixture of potassium compounds and quartz sand was heated in fluidized bed until defluidization occuring or the temperature reaching 1020 °C. Defluidization temperature is used to indicate the tendency of agglomeration. The surface morphology and elemental distribution of agglomerates are further analyzed to reveal agglomeration mechanisms. The transformation of potassium salts in the presence of SiO2 in the different gas composition is indicated by thermodynamic equilibrium calculations.

Section snippets

Apparatus

The experiments were carried out in a bench scale bubbling fluidized bed (BFB). The details of the setup are described elsewhere [1]. Here, only a brief summary of the setup is given. The setup consists of a fluidized bed reactor, a gas dosing system and a control/data acquisition system. The reactor is made of quartz with a height of 900 mm. A sintered quartz distributor is located in the middle of the reactor, below which is a preheating section, and above is the dense bed section. A

Results and discussion

A comparison of the defluidization temperature with different potassium salts in various gas compositions is summarized in Fig. 2.

The results show that the defluidization temperature varies significantly when using different form of potassium salts in the different gas composition. The values of Td for KCl in the air and H2-containing gas are the same and close to the melting point of KCl, listed in Table 1, which indicates that the defluidization may be mainly caused by the melting of KCl at

Conclusions

The mechanisms of agglomeration induced by KCl, K2CO3, K2SO4 in various gas composition containing air, H2 and H2O have been investigated. The results show that different potassium salts interact differently with the typical bed material, quartz sand. Gas composition has a significant effect on the agglomeration tendency and influence the defluidization temperature.

In the air and H2-containing gas, the melting of KCl is the main reason for agglomeration. The defluidization temperature is close

Acknowledgements

This study is supported by International S&T Cooperation Program of China (2013DFG62640) funded by MOST, Sino-Danish collaboration project (DANCNGAS) funded by Innovation Fund Denmark, and The National Natural Science Foundation of China (51104137).

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