Study On The Mechanism And Technology Of Lignite Hot-pressing Dehydration

Lignite is one kind of low rank coal, and has the characteristics of high innerwater and low heat value. With the worldwide increase in demand for energyresources, and gradual lack of oil, high rank coal, the abundant lignite resources haveattracted ever-increasing attentions. The Hot-pressing Dehydration (HPD) technologyhas the advantages that the water in lignite can be removed in liquid form, and theproduct from HPD has higher heat value, and an improved performance for utilizationwith the stability of not easily reabsorb moisture and spontaneous combustion. XiaoLongtan lignite (XLT) Adopted as raw material, the mechanism of dewatering of HPD,and the process parameters and the mechanism of the stability of briquette from HPDwere studied. The paper includes the followings:Some properties of raw lignite were analyzed. The results show that XLT ligniteis rich in pore structure and oxygen-containing functional group structure, and hashigher oxygen element content, and is easy to reabsorb moisture and prone tospontaneous combustion, and the volatile initial escaping temperature is about220℃.The mechanisms of dewatering of lignite by HPD were explored. The basicprinciple of water removal in PHD is thermal effect, compacting consolidation, andflash evaporation. The underlying dewatering mechanism is that the changes inphysical＆chemical properties owing to the organic groups decomposition caused bythermal effect, which weakening the force between coal and water system, creatingthe conditions for the separation of water from lignite; then, the moisture is removedcompletely from coal by the volume contraction of lignite and the drainage effect ofgas from organic decomposition and the effect of compacting consolidation caused bymechanical press. On the basis of the HPD process feature, the model of HPD processwas created on the theory of consolidation, in which the factors of temperature,pressure and coal property are included.The mechanisms of the stability of lignite briquette from HPD were studied. Thestability of lignite briquettes were analyzed, and the mechanisms of the stability wasrevealed through the studies on the apparent morphological, and the relations betweentotal pore volume and stability, and the changes in physical＆chemical properties.Which the significant decreases of oxygen content and the increase of hydrophobicityof lignite, and the significant reduction of porosity and total pore volume, and thesome degree carbonizing of briquette surface are the underlying reasons. At the conditions of test, the reabsorbed moisture difference between dried lignite from dryerand dewatered lignite from HPD is5percent, respectively are9percent and4percent.The grade of spontaneous combustion of the coal briquetted changes from Ⅰgrade toⅡgrade.Dewatering of lignite by HPD technology in a100kg/h scale was studied. Itshows that the significant factors effecting on dewatering of lignite are temperature,pressure and pressing time, and the factors of temperature and pressure and theirinteraction are the key to the water removal; suitable conditions parameters aredepended on the demand for the water content in lignite, and can be in the ranges oftemperature150-200℃, pressure5-10MPa and pressing time1-5min. The productsproperties were analyzed, and it shows that the heat density can be significantlyincreased by above37percent; and the carbon element content has been increased by9percent, and oxygen and sulfur elements content decreased significantly in theprocess; the process has some purification on some soluble metal ions with the ion Na,Mg, Ca content significantly reduction and Cl, Ni, As some degree reduction; the coalrank can be upgrade by HPD reflecting in the notable reduction of MHC and volatilecontent, and the fixed carbon content increased significantly; and the volatile initialescaping temperature, burning temperature and the fastest burning temperature can beraised, while burning out temperature changes little, which illustrates the rise ofstability and the improvement of burning performance.Based on obtained product from the HPD process conditions of temperature200℃, pressure10MPa, the theory energy consumption were calculated and discussed,and it gives the1195.7kJ theory energy consumption per kilogram product, and itequals to17.52Yuan per ton(base on the price of raw lignite price). That the cost ofenergy to process1ton raw lignite is17.52Yuan.75figs.,31tabs.,165refs.