Elsevier

Powder Technology

Volume 363, 1 March 2020, Pages 7-22
Powder Technology

Conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure

https://doi.org/10.1016/j.powtec.2020.01.010Get rights and content

Highlights

  • Interparticle cohesion/cohesive forces of pulverized lignite were quantified.

  • Flowability and fluidization of pulverized lignite were evaluated.

  • The conveying mechanisms were revealed clearly.

Abstract

The pressurized entrained-flow gasification is the most efficient and promising technology for utilization of lignite, where dense-phase pneumatic conveying under high pressure is one of the key techniques. The study aims at clarifying the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure. For this purpose, bulk density of pulverized lignite with different moisture content was measured and analyzed, and the quantitative relationship between interparticle cohesion/cohesive forces and the moisture content was obtained. On the basis of this quantitative relationship, Mohr-Coulomb yield criterion was introduced to quantify particle flowability, and Castellanos's model was introduced to evaluate the fluidization characteristics. Combined with particle flowability and the fluidization quality, the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure were explored. Results indicated that when the supplementary gas flow rate rises, the conveying capacity becomes weak, and the conveying resistance shows a decreasing firstly and then increasing tendency. While when the moisture content rises, the conveying capacity becomes weak, the conveying resistance strengthens, and the conveying stability weakens.

Graphical abstract

Interparticle cohesion/cohesive forces of pulverized lignite with different moisture content were quantified to evaluate the flowability and fluidization. And the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure were explored based on the flowability and fluidization.

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Introduction

Energy is an important material foundation for human existence and development. It greatly pushes the economic development and social progress. A recent survey from the World Energy Outlook (WEO) demonstrates that fossil fuels account for >80% of the world primary energy consumption and will remain dominant the world energy supplies at least in the coming decades [1]. Coal, as one of the most important and cheapest fossil fuels in the world, still plays a crucial role in global primary energy [2]. Especially in China, coal will continue to dominate primary energy source in the foreseeable future [3]. With the excessive consumption of high-quality coal and the gradual depletion of its reserves, lignite, a low-rank coal, attracts increasing attention due to its advantages over high-rank coal such as lower mining cost, abundant reserves and high reactivity. However, on the one hand, some of its features such as high moisture content (25–65%), high ash content, abundant oxygen-functional groups and low carbon content cause low calorific value and thermal efficiency via direct combustion or other conventional utilizations of lignite [4]. Furthermore, lignite has high spontaneous combustion tendency, which increases its storage and transportation costs [5]. All of these result in a very limited utilization of lignite. On the other hand, lignite can also offer some other features such as high reactivity, high volatile matter content, low mining cost, and low content of pollution forming impurities. This indicates that it is highly suitable for coal gasification [5,6].

Coal gasification is one of the most potential clean coal technologies, and it has been developed to reduce environmental problems and realize the highly efficient and clean coal utilization [7,8]. The pressurized entrained-flow gasification is a typical coal gasification technology characterized with large-scale, high efficiency, and cleanness. For this reason, it has been considered to be the most efficient and promising technology for utilization of lignite so far [8]. This technology involves feed, transportation and gasification of pulverized lignite, where dense-phase pneumatic conveying under high pressure is one of the key techniques for the transportation of pulverized lignite [9].

In fact, dense-phase pneumatic conveying under high pressure is a typical dense-phase gas-solid two-phase flow featured with low velocity, high pressure and high solid volume concentration. Therefore, dense-phase pneumatic conveying under high pressure is usually complex and non-linear dynamical, and thus the conveying system is always easy to become unsteady. Especially when the conveying feedstock is pulverized lignite, its moisture content has a negative impact on its flowability and fluidization which are the two crucial influence factors for dense-phase pneumatic conveying under high pressure [10,11]. In other words, the moisture content easily causes the degradation of the conveying capacity and stability in dense-phase pneumatic conveying of pulverized lignite under high pressure. Meanwhile, the conveying system will become more complicated and unsteady. Nevertheless, the moisture content of pulverized lignite can also reduce high-pressure steam consumption and promote lignite gasification reaction in the pressurized entrained-flow gasification system [12]. In a word, the moisture content, as one of the most important physical properties of pulverized lignite, plays a vital role in dense-phase pneumatic conveying under high pressure or even the pressurized entrained-flow gasification system. Therefore, in order to ensure the reliable operation and stable production of the pressurized entrained-flow gasification system, the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite under high pressure should be clarified.

However, the conveying mechanisms for dense-phase pneumatic conveying under high pressure have been not fully understood. And there is also no well-developed theory worth applying to reference at present. What is more, to the best of the author's knowledge, the related research is still very limited. Several studies from Guo's research group at East China University of Technology were made to elucidate the conveying characteristics of dense-phase pneumatic conveying, but their experiments were conducted at atmospheric pressure [[13], [14], [15], [16]]. In addition, a small amount of research had been carried out to probe into the conveying mechanisms of dense-phase pneumatic conveying under high pressure by Chen's research group of Southeast University in China [8,12,[17], [18], [19], [20]]. For example, Liang et al. [18,19] developed Wavelet transform and Shannon entropy analysis of pressure drop to reveal the conveying characteristics and stability of dense-phase pneumatic conveying under high pressure. Lu et al. [8] introduced the Flow Function (FF) to evaluate the flowability of pulverized lignite and applied the flowability to examine the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite under high pressure. However, both Shannon entropy and Flow Function (FF) above are the macroscopic parameters for characterizing flowing or conveying characteristics of pulverized lignite, so they are difficult to reveal the underlying mechanisms accurately on the micro level in the flowing or conveying process of pulverized lignite.

In order to overcome the limitation, interparticle cohesion/cohesive forces were introduced to explore the conveying characteristics of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure in the previous research of our group [7]. This results demonstrated that the transformation of interparticle cohesion/cohesive forces due to the moisture content of pulverized lignite has a significant impact on dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure. However, considering that it is exceedingly challenging to reveal the relationship between interparticle cohesion/cohesive forces and the moisture content, only the qualitative relationship between interparticle cohesion/cohesive forces and the moisture content was predicted. Meanwhile, according to this qualitative relationship, a preliminary investigation on the conveying characteristics of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure was carried out. In fact, only the influence of the moisture content on the solids mass flow rate at different differential pressure was analyzed in our previous research. Therefore, in this paper, we will try to further explore the relationship between interparticle cohesion/cohesive forces and the moisture content and seek to make a more detailed and in-depth research on dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure. Meanwhile, it is hoped that the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure can be clearly understood.

For this purpose, bulk density and angle of repose of pulverized lignite with different moisture content will be determined and analyzed. Meanwhile, a series of dense-phase pneumatic conveying experiments of pulverized lignite with different moisture contents using nitrogen as carrier gas will also be performed at different supplementary gas under high pressure (up to 4 MPa) in this study. The principal purposes of the present work can be outlined as follows: 1) Trying to quantify the relationship between interparticle cohesion/cohesive forces and the moisture content of pulverized lignite. 2) Further exploring the influence mechanisms of the moisture content on the flowability and fluidization of pulverized lignite. 3) Providing a more in-depth understanding for the conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure.

Section snippets

The preparation and measurement of the conveying feedstock

A typical Chinese lignite from Xilin Gol in Inner Mongolia was selected as the conveying feedstock. At first, the raw lignite should be prepared into the pulverized lignite feedstock. The preparation process of the feedstock consists of the following steps: 1) Crushing and sieving the raw lignite into pulverized lignite with the mean particle size of about 200 μm. 2) Drying pulverized lignite to a very low moisture content. 3) Spraying proper water uniformly into pulverized lignite to prepare

Forms of water in lignite

Lignite is a porous gel-like low-rank coal, its developed pore structures (see Fig. 2 and Table 2) and abundant polar oxygen-containing functional groups are the main reasons for its high moisture holding capacity and strong hydrophilicity [22]. As a result, its forms of water are very complex. According to the forms of water in lignite, there are many classification methods. For example, freezable and non-freezable water; external water, internal water and combined water; free water, pore

Interparticle cohesion/cohesive forces of pulverized lignite

According to the degree of interparticle cohesion, particles can be categorized into two large groups: non-cohesive particles (Bog < 1) and cohesive particles (Bog > 1) [39]. Generally, the larger Bog, the stronger interparticle cohesion. Interparticle cohesion/cohesive forces depend mainly on the environmental conditions and the physical-chemical properties of particles [40]. For this reason, both fine particles (dp < 100 μm) and wet particles are usually considered as cohesive particles. For

Conclusions

In this paper, combined with the correlation theories on interparticle cohesion/cohesive forces (van der Waals forces and capillary forces), the relationship between bulk density and the moisture content of pulverized lignite was further analyzed. The “dry” regime (MC < 8.20 wt%), roughness regime (8.20 wt% ≤ MC < 14.50 wt%), macroscopic regime (14.50 wt% ≤ MC < 18.10 wt%) and filling regime (MC ≥ 18.10 wt%)—four regimes for capillary forces corresponding to the transform of interparticle

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.

Acknowledgements

The authors are grateful for the financial support provided by the National High-tech R&D Program of China (863 Program) (2011AA05A201) and the National Basic Research Program of China (973 Program) (2010CB227002).

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