Experimental study on calcination and fragmentation characteristics of limestone in fluidized bed

doi:10.1016/j.joei.2021.01.014

Journal of the Energy Institute

Volume 95, April 2021, Pages 206-218

https://doi.org/10.1016/j.joei.2021.01.014 Get rights and content

Highlights

•
The size evolution of limestone particles was measured.
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SEM analysis was used to observe the surface morphology.
•
The effects of temperature and heating rate were studied.
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The effect of SO₂ and CO₂ atmosphere was observed.
•
The effect of firing exhaust gas atmosphere was studied.

Abstract

The fragmentation degree of limestone affects seriously the particle size distribution of sorbent in the desulfurization process in fluidized bed combustion. In this paper, the variation of particle size and differences in microstructure of calcination and fragmentation were measured under different fluidizing medium in the same equipment. The effects of temperature, reaction rate, SO₂ and CO₂ atmosphere, firing exhaust gas atmosphere on the calcination and fragmentation of limestone with different properties in the fluidized bed were studied. It was found that the size evolution of larger limestone was more obvious than that of finer ones. Under the condition of fragmentation, the particle size evolution of limestone was more intense than that in calcination. The particle size was smaller under the condition of a higher reaction rate. Higher temperature can promote the calcination reaction and reduce the particle size to a certain extent. The coarser fragmentation products at high temperature may be related to the enhancement of agglomeration between particles. SO₂ and CO₂ can inhibit the fragmentation of limestone and lead to coarsening of products, which further confirms the main contribution of calcination reaction to limestone size evolution. The main chemical reaction in a given atmosphere is direct sulphation reaction and the primary fragmentation is restrained by the high concentration of CO₂ in firing exhaust gas, which is more obvious in the oxygen-enriched exhaust gas.

Introduction

Fluidized bed combustion technology is an economic and effective clean combustion technology [[1], [2], [3], [4], [5]]. Due to the low cost and availability, limestone is usually used as a desulfurization sorbent in the fluidized bed (FB) and circulating fluidized bed (CFB) boilers to reduce SO₂ emissions from coal-fired flue gas [[6], [7], [8], [9]].

The desulfurization reaction of limestone can be carried out in two different ways, which depends on the degree of calcination [[10], [11], [12]]. The calcination of limestone is determined by the partial pressure of CO₂ in the reactor. Indirect desulfurization occurs when the concentration of CO₂ is low or the temperature is high [[13], [14], [15]]: ${CaCO}_{3} \to CaO + {CO}_{2}$ $CaO + {SO}_{2} + 0.5 O_{2} \to {CaSO}_{4}$

If the CO₂ concentration is very high or the reaction temperature is lower than the calcination temperature of limestone, the calcination reaction will not occur. Limestone will not be calcined, and sulfation will occur between CaCO₃ and SO₂, which is called direct desulfurization [16,17]: ${CaCO}_{3} + {SO}_{2} + 0.5 O_{2} \to {CaSO}_{4} + {CO}_{2}$

Calcination temperature and partial pressure of CO₂ in fluidization medium have a very important influence on the fragmentation of limestone. When the partial pressure of CO₂ is greater than the equilibrium pressure of limestone decomposition, the calcination reaction will be inhibited and the crushing degree of limestone is small; when the partial pressure of CO₂ is less than the equilibrium pressure of limestone decomposition, the generation of CO₂ from calcination reaction causes the internal pressure to increase and then the crushing degree will increase.

It can be seen that the capture of SO₂ by calcium-containing materials depends on these parameters: temperature, particle size, and its properties (age, porosity, and composition), total pressure and partial pressure of carbon dioxide, and the concentration of SO₂ [18].

When the limestone particles stay in the bed, they are impacted by internal stress, chemical reaction, and collision from other particles and the inner wall of the reactor. All of these phenomena may cause fragmentation and attrition, which causes the size to change. Due to the thermal stress and internal overpressure caused by CO₂ emission, primary fragmentation will occur [19]. The development of chemical reactions involved in desulfurization (calcination, sulfation) interferes with the fragmentation process, making the phenomena more complex [20].

Hu et al. [21] found that limestone did not suffer an important thermal shock and show a high fragmentation level after calcination under high-temperature conditions. The results of Scala et al. [22] showed that attrition and fragmentation patterns under oxy-firing conditions were much different from those occurring under air-firing combustion conditions. Previous studies have investigated the effects of limestone type and size, fluidizing gas velocity, and temperature on fragmentation [23]. However, they focused more on the chemical reactivity of limestone in the SO₂ atmosphere, rarely involving the effect of SO₂ on limestone fragmentation behavior. Meanwhile, the effect of sulfation on limestone fragmentation is a problem of practical significance. In most studies, the calcination and sulfidation reactions in the desulfurization process are treated separately, but in the actual fluidized bed, the calcination and sulfidation reactions are carried out simultaneously. There are obvious differences in particle composition, reaction rate and final conversion rate of limestone under calcination and sulfidation conditions, which will also affect the fragmentation. Due to the non decoupling characteristics of sulfidation and calcination, it is necessary to explore the interaction between sulfidation and calcination, so as to provide a reference for the calculation of limestone desulfurization efficiency in fluidized bed boiler. Also, the influence of particle size evolution, temperature, and reaction rate on fragmentation is still a controversial topic [24]. Because the behavior of limestone is very different between air combustion and oxygen-enriched combustion, it is also necessary to explore the characteristics under oxygen-enriched conditions.

CaCO₃ exists in nature as calcite and aragonite. Calcite and aragonite are polymorphs of CaCO₃. The main difference between calcite and aragonite is that the crystal system of calcite is triangular, while the crystal system of aragonite is orthogonal. In addition, calcite is more stable than aragonite. In this paper, the calcination and fragmentation of the above two kinds of limestone in a fluidized bed reactor were systematically studied. The effects of temperature, reaction rate, SO₂ and CO₂ atmosphere, and firing exhaust gas atmosphere were studied. Besides, scanning electron microscopy (SEM) analysis has been used to observe the surface morphology of the original and reaction samples. The experimental results and analysis will provide more information for understanding the size evolution of limestone particles during the desulfurization process in the fluidized bed.

Section snippets

Experimental apparatuses and parameters

The experiment was carried out in a lab-scale fluidized bed reactor as shown in Fig. 1. The reactor (54 mm ID, 800 mm height) is made of quartz glass and heated by the silica carbon tube located outside the reactor. The heating system is divided into two sections. The lower section is mainly used to control the temperature of the hot air, while the higher section can maintain the bed temperature. The bed temperature is controlled by PID (Proportional, integral and derivative control) that

Size evolution in the calcination and fragmentation process

The limestone sample was calcined in the furnace at 850 °C. The calcination reaction produced fresh lime and CO₂ during the process, therefore, the particles would suffer from size evolution. As listed in Table 3, the Sauter diameter of particles decreased about 5–20% than original samples and larger limestone with 600–800 μm has more notable size evolution than finer ones. It can be seen from Table 3 and Fig. 3 that the particle size of the fragmentation product is much finer than that of the

Conclusions

The particle size distribution of sorbent is seriously affected by limestone calcination and fragmentation. In this paper, the effects of temperature, reaction rate, SO₂ and CO₂ atmosphere, firing exhaust atmosphere on the calcination and fragmentation of limestone in fluidized bed were studied by using calcite and aragonite with different particle sizes. The main conclusions are as follows:

Compared with limestone 1, limestone 2 is easier to detect particle size reduction and has higher

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This research was supported by the National Key Research Plan (2019YFE0102100) and C9 University science and technology project (201903D421009).

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Experimental study on calcination and fragmentation characteristics of limestone in fluidized bed

Highlights

Abstract

Introduction

Section snippets

Experimental apparatuses and parameters

Size evolution in the calcination and fragmentation process

Conclusions

Declaration of competing interest

Acknowledgment

Energy

Fuel Process. Technol.

Chem. Eng. Sci.

Appl. Therm. Eng.

Fuel

Fuel Process. Technol.

Fuel

Powder Technol.

Fuel

Fuel

Exp. Therm. Fluid Sci.

Prog. Energy Combust. Sci.

Attrition during steam gasification of lignite char in a fluidized bed reactor

Fuel Process. Technol.