Conveying mechanisms of dense-phase pneumatic conveying of pulverized lignite in horizontal pipe under high pressure
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.
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).
References (66)
- et al.
Projection of world fossil fuels by country
Fuel
(2015) - et al.
Study on the moisture adsorption isotherms and different forms of water for lignite after hydrothermal and thermal upgrading
Fuel
(2019) - et al.
Kinetic analysis on the microwave drying of different forms of water in lignite
Fuel Process. Technol.
(2018) - et al.
A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization
Fuel Process. Technol.
(2013) - et al.
An assessment on co-combustion characteristics of Chinese lignite and eucalyptus bark with TG-MS technique
Powder Technol.
(2016) - et al.
Effect of moisture content on dense-phase pneumatic conveying of pulverized lignite under high pressure
Powder Technol.
(2016) - et al.
Experimental investigation of pressure letdown flow characteristics in dense-phase pneumatic conveying at high pressure
Powder Technol.
(2015) - et al.
Design optimization of a venturi tube geometry in dense-phase pneumatic conveying of pulverized coal for entrained-flow gasification
Chem. Eng. Res. Des.
(2017) - et al.
Pressure drop prediction for horizontal dense-phase pneumatic conveying of pulverized coal associated with feeding to gasifier
Chem. Eng. Res. Des.
(2013) - et al.
Flow patterns of pulverized coal pneumatic conveying and time-series analysis of pressure fluctuations
Chem. Eng. Sci.
(2013)
Investigations of pulverized coal pneumatic conveying using CO2 and air
Powder Technol.
Experimental research and HHT analysis on the flow characteristics of pneumatic conveying under high pressure
Appl. Therm. Eng.
Flow characteristics and stability of dense-phase pneumatic conveying of pulverized coal under high pressure
Exp. Thermal Fluid Sci.
Conveying characteristics and resistance characteristics in dense phase pneumatic conveying of rice husk and blendings of rice husk and coal at high pressure
Powder Technol.
Study on the pore structure and oxygen-containing functional groups devoting to the hydrophilic force of dewatered lignite
Appl. Surf. Sci.
Quantitative model for predicting the desorption energy of water contained in lignite
Fuel
Water occurrence in lignite and its interaction with coal structure
Fuel
Discrete characterization tools for cohesive granular material
Powder Technol.
A review of underlying fundamentals in a wet dispersion size analysis of powders
Powder Technol.
Role of relative size of asperities and adhering particles on the adhesion force
J. Colloid Interface Sci.
Characterization of particle and bulk level cohesion reduction of surface modified fine aluminum powders
Colloids Surf. A Physicochem. Eng. Asp.
Adhesion and aerodynamic forces for the resuspension of non-spherical particles in outdoor environments
J. Aerosol Sci.
Prediction of porosity from particle scale interactions: surface modification of fine cohesive powders
Powder Technol.
An overview of the role of capillary condensation in wet caking of powders
Chem. Eng. Res. Des.
The capillary bridge between two spheres: new closed-form equations in a two century old problem
Adv. Colloid Interf. Sci.
Physics of humid granular media
C. R. Phys.
Liquid bridge between two moving spheres: An experimental study of viscosity effects
J. Colloid Interface Sci.
Discrete particle simulation of particulate systems: a review of major applications and findings
Chem. Eng. Sci.
Simulation of the packing of cohesive particles
Comput. Phys. Commun.
Coal agglomeration and its effect on bulk density
Powder Technol.
Effect of liquid addition on the packing of mono-sized coarse spheres
Powder Technol.
Flowability and handling characteristics of bulk solids and powders - a review with implications for DDGS
Biosyst. Eng.
Fluidization and separation characteristics of gas–solid separation fluidized bed with wet coal
Fuel
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