Experimental Study On Quantification Law Of Low-rank Coal Spontaneous Combustion Characteristics In High Temperature Environment

The spontaneous combustion of coal has caused incalculable economic losses and numerous hidden safety hazards in all aspects of mining,storage,and transportation process.According to the test method for the solid combustible materials by the United Nations International Occupational Safety and Health Information Center,the paper proposes the definition of critical self-ignition temperature based on the Frank-Kamenetskii parameter.Critical self-ignition temperature is a minimum ambient temperature which can maintain its heat production capacity always greater than the heat dissipation capacity(ignition).Meanwhile,the coal sample temperature and the thermal environment temperature are the same.In this paper,the critical self-ignition temperature and heating law of coal(mainly lignite)under various conditions were determined.Fundamental research was conducted on the low rank coal spontaneous combustion characteristics under natural accumulation conditionsBased on the Frank-Kamenetskii’s boundary theory,an experimental determination method of open type constant temperature heating system(high Biot number and low air flow resistance contact boundary conditions)was established to carry out the research.The temperature of coal pile and index gases produced during the self-heating process was continuously monitored.The critical self-ignition temperature values of coal samples under different conditions were obtained.Then,the theoretical method for evaluating the spontaneous combustion risk of coal based on the concept of critical self-ignition temperature was proposed.Then,the study determined the internal heating patterns and heat production characteristics of the natural coal samples under multi-field coupling conditions.Seven types’ coal samples piled in cubic shape with length of 2.5 cm,5 cm,10 cm,15 cm,25 cm,50 cm,and 100 cm were designed.And a large-scale outdoor constant temperature test system was established for the extremely large volume of coal samples(more than 25 cm in length)experiments.Based on the test results of different coal samples temperature rise characteristics,a four-stage theory of heating process under natural accumulation conditions is proposed(Initial Stage,Drying Stage,Heat Generation Stage and Accelerate/Decline Stage).Then,the relationship between various heating stages and various factors such as heat conduction,natural convection,and water evaporation were clarified according to the rate of temperature rise and the change in gas concentration.Experiments were carried out with different particle sizes(0.5mm,1mm,3mm,5mm,7.5mm and 10mm)in six different coal samples.The relationship between particle size,coal volume and critical self-ignition temperature were obtained by analyzing the effect of particle size during in the temperature rising process.The effects of water contents on heating characteristics of coal samples were discussed.It is concluded that water effection is mainly appearied in the initial stage.Meanwhile,the external water of the coal sample plays a leading role.The larger the volume of coal pile,the more obvious of the water effection on its temperature rise.There is no relationship between water content and critical self-ignition temperature.From a macro perspective,water only extends the temperature rise time of the coal pile without affecting it’s spontaneously ignites.Through the above-mentioned large number of indoor experiments,the experimental method and numerical prediction formula for estimating the critical self-ignition temperature of large-volume coal pile is proposed.The feasibility of predicting the critical self-ignition temperature of coal piles is preliminarily verified by outdoor large volume experiments.Currently,it can be used for the prediction of critical self-ignition temperature of coal piles in a limited volume range.The dissertation contains 91 figures,13 tables and 180 references.