Original Research Paper
Structure optimization of granular bed filter for industrial flue gas filtration containing coagulative particles: An experimental and numerical study

https://doi.org/10.1016/j.apt.2020.03.017Get rights and content

Highlights

  • A numerical model for the filtration of industrial flue gas in GBF was established.

  • The local filtration efficiency based on different granule layers was investigated.

  • An optimized GBF structure excellent performance was designed.

  • The underlying mechanism of GBF performance optimization was revealed.

  • The filtration of coagulative particles was experimentally investigated.

Abstract

A numerical model for the flow and filtration characteristics of industrial flue gas in granular bed filter (GBF) was established and the local filtration efficiency for different granule layers was investigated. Numerical validation results show that the GBF structure with large size granules at the inlet region and small size granules at the outlet region can effectively improve the filtration performance of GBF and the underlying mechanism was revealed. Then an experimental system was built to validate the suitability of the optimized GBF structure for the filtration of industrial flue gas with coagulative particles. The experimental results show that the optimized GBF structure is also suitable and its superiority is more significant with the increase of filtration time. The results show that the pressure drop and filtration efficiency of the experimental system increase with the increase of dust particles concentration. The existing of coagulative particles is conducive to the growth of smaller size dust particles, the pressure drop and filtration efficiency increase significantly. In addition, the pressure drop and filtration efficiency decrease with the increase of cooling rate. The results of this study are expected to be useful for the design and optimization of industrial flue gas purification and waste heat recovery.

Introduction

Recently, the problems of energy waste and environmental pollution caused by the emission of a large amount of flue gas in the industrial fields of chemical engineering, building materials and metallurgy have aroused widespread concern, which thus needs to be solved urgently [1], [2], [3]. In most cases, some kinds of industrial flue gas have the characteristics of high temperature and high waste heat grade, so it is possible to reduce industrial energy consumption by recovering heat from industrial flue gas through waste heat recovery equipment. However, the composition of industrial flue gas is usually complex, it contains high content of dust particles, coagulative particles, corrosive components and toxic components and so on [4], [5], [6], [7]. The deposition of dust particles accompanied by coagulative particles makes the surface of heat exchanger tubes easily clogged, fouled and corroded, which significantly reduces the heat transfer performance and threatens the operation stability of the waste heat recovery system. At the same time, due to the failure of heat exchanger tubes and soot blowing process, additional economic losses are also caused [8], [9], [10], [11]. In addition, dust particles contained in the industrial flue gas can easily form inhalable particles in the atmosphere, which leads to a variety of respiratory diseases and thus threatens the survival of human beings and animals [12]. So the purification and recovery of waste heat of industrial flue gas have been widely concerned.

In the past few years, many dust removal technologies, such as dust catcher, pocket dust collector, ceramic dust collector, centrifugal dust separator and wet dust collector, have been widely used for the purification of industrial flue gas under low temperature conditions [13], [14], [15]. However, these dust removal technologies are not applicable to the high temperature environment, and it is difficult to achieve the high efficiency and low cost purification target for fine particles. As an example, ceramic filters usually have high filtration efficiency; nevertheless, at high temperatures, the filter element may suffer a lot due to thermal shock, fracture and mechanical fatigue, which lead to the formation of micro cracks [16]. The working principle of the wet dust collector is to cool down the high temperature flue gas, and a large amount of heat energy will be wasted [17].

Granular bed filter (GBF) is considered as an excellent candidate technology for industrial flue gas purification owing to the numerous advantages, such as high temperature resistance, high pressure and corrosion resistance and low cost. As a promising technology, it has been widely used in the integrated gasification combined cycle (IGCC) and advanced pressurized fluidized bed combustion (PFBC) technologies, which has made GBF become a much more attractive filtration technology [18], [19], [20].

Many researchers have studied the filtration and resistance characteristics of GBF experimentally and numerically. Xiao et al. [21] summarized and discussed the characteristics and performances of GBF which can be designed as fixed beds, fluidized beds and moving granular beds and the results showed that the fixed beds had the highest filtration efficiency under the same conditions, and filtration efficiency can even have reached greater than 99%. Guan et al. [22] investigated the influences of granular bed depth, gas velocity and granule diameter on the grade filtration efficiency in a three-dimensional randomly GBF model. The simulation results and experimental results showed the effect of GBF depth on the overall filtration efficiency and pressure drop, the pressure drop approximately linearly correlates with the GBF depth. Chen et al. [23] investigated the characteristics of filtration and resistance of the moving GBF designed to filter out coal-fired dust particles. The results showed that the overall porosity of the filter granules decreased, but the filter resistance and the collection efficiency increased, with an increase in the amount of smaller-sized filter granules in the bed. Brown et al. [24] found that a moving GBF can operate with a high filtration efficiency, typically exceeding 99%, and low pressure drop without the need for periodic regeneration through the use of a continuous flow of a fresh granular filter medium in the filter. Fine particles are mainly filtered through the formation of a dust cake on the surface of the granular layer. Chen et al. [25] experimentally investigated the efficiency and stability of moving GBF in high-temperature environment with various operation conditions. The influence of various parameters, such as flue gas temperature, mass flow rate of filter granules and filtration superficial velocity were studied. The experimental results show that an average increase at 100 °C resulted in a decrease in average filtration efficiency of 1.74%. In our previous work [26], the structural optimization was performed for the fixed GBF and an optimized GBF structure with different granules sizes was designed. The validation results indicate that the designed GBF structure has excellent filtration and fluid flow performance compared with the traditional structure with single granules size. Under the investigated filtration superficial velocity region, the average filtration efficiency is enhanced 3.23% and the pressure drop is reduced 49.94%. However, the above literatures do not consider coagulative particles contained in the industrial flue gas, which means that the above conclusions are not suitable for the flue gas coagulative particles, and it is clearly divorced from industrial reality.

For the industrial flue gas with coagulative particles, available research has been carried out mainly to investigate the flow and heat transfer characteristics of flue gas in GBF. Wen et al. [27] experimentally studied the heat transfer of gas mixed with particles flowing through a packed GBF under the constant wall temperature conditions. Chen et al. [28] carried out an experimental study on the overall heat transfer coefficient of gas with coagulative particles flowing through a packed GBF. It was found that the heat quantity released by the concretion of coagulative particles has an improved influence on the heat transfer when the inlet gas temperature of GBF is above the melting point of the coagulative particles and outlet gas temperature is below the melting point. However, the heat quantity absorbed by the melting of coagulative particles can weaken the heat transfer when both the inlet and outlet gas temperature of GBF are above the melting point of coagulative particles.

From the above literature review, it can be found that although many researchers have studied the influence factors of GBF to improve its filtration performance, most of them are only focused on the industrial flue gas without coagulative particles. Little attention has been paid to the influence of coagulative particles on flow and filtration performance of GBF. Unfortunately, the composition of the industrial flue gas is very complex with both coagulative particles and non-coagulative dust particles. The filtration process of industrial flue gas in GBF is a multiphase transport process, which involves complex heat and mass transfer process including phase transition, agglomeration and adhesion of coagulative particles. That means the existing filtration theory of GBF is far from practical engineering application, especially for the industrial flue gas with coagulative particles.

Therefore, in this paper, the structure optimization was performed for GBF to suit the filtration of industrial flue gas with coagulative particles. An optimized GBF structure was obtained by adjusting the arrangement of granules size, its performance was validated by numerical and experimental methods, and the underlying mechanism of performance optimization is revealed by numerical simulations. The effects of geometric and operating parameters on the performance of the designed GBF were also investigated.

Section snippets

Physical model

Generally, the granules are placed in a GBF in a free stacking manner in industrial applications. The complex and irregular structures are difficult to describe by conventional numerical methods. Previous studies have shown that the disordered stacking structure can be abstracted into a regular and orderly packing [29], such as simple cubic (SC), face centered cubic (FCC) and body centered cubic (BCC) structures respectively. In order to get closer to the porosity of GBF in the engineering

Local filtration efficiency based on different granule layers

The local filtration efficiency reflects the number of dust particles deposited on each layer of granules. It is defined as:ϕ=miiNmi

where mi is the deposited mass of dust particles on the granules at layer i, and N represents the total number of layers. It is a key parameter for the optimization of GBF. In the numerical simulation, before cake formation, the pressure drop of each layer in the GBF can be calculated by the Ergun formula, which can be written as [35]:ΔP=i=1nΔPiΔPiLi=1501-ε2ε3μUDi

Experimental description

In the purification process of flue gas with coagulative particles using a GBF, the coagulative particles will condensate or solidify on granule surface due to the heat transfer between coagulative particles and the packed granules. Therefore, the coagulative particles are easier to be trapped and congealed on the surface of the first several layer granules compared to the traditional dust particles. In order to validate the suitability of the optimized GBF structure to the filtration of flue

Optimization result validation

In the filtration experiments of flue gas containing coagulative particles and dust particles, the inlet temperature of the flue gas is 340 °C, the concentration of dust particles is 3.6 g/m3, the concentration of the coagulative particles is 2.4 g/m3, and the cooling water flow rate is 240 L/h. Firstly, the variations of filtration efficiency and pressure drop of the system with time were studied when the filtration superficial velocity is 0.98 m/s. The experimental results are shown in Fig. 10

Conclusions

In this paper, experimental and numerical methods were adopted to study the filtration and resistance characteristics of a GBF used for the purification of industrial flue gas with dust particles and coagulative particles. The local filtration efficiency of GBF for different layers was investigated, based on that the optimized GBF structure was designed by adjusting the granules size along flue gas flow direction. The numerical and experimental validation results show that the designed GBF

Acknowledgements

The present work is supported by the National Key R&D Program of China (2016YFB0601100).

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