Thermal forming method and device for secondary carbonized briquette coal

Based on the low strength and high permeability of briquette coal(BC)used in coal and gas outburst physi⁃ cal simulations for coal mine disasters，a self⁃developed thermal forming method and device for secondary carbonized briquette coal under vacuum conditions was introduced. The thermal forming device is composed of a servo⁃controlled loading system， heating furnace， vacuum and atmosphere protection system， electric control system， water⁃cooling system，graphite molds，and carbon steel molds. The maximum pressure of this device is 200 kN and the highest sintering temperature is 1 200 °C . The vacuum working temperature uniformity，heating rate，and vacuum are ±6 °C ，0.01 ~ 10 °C / min，and ≤ 10 Pa， respectively. The internal dimensions of the furnace body is 240 mm × 260 mm (diameter × height)，respectively. Cylindrical BC specimens with sizes of φ25 mm×50 mm and φ50 mm×100 mm(di⁃ ameter×height)can be formed by using graphite and carbon steel molds. The thermal forming device has the follow⁃ ing characteristics:1 The closed⁃loop control of force and displacement is achieved by automatic loading with a com⁃ puter.2 The furnace size design is compact，the temperature is uniform in the furnace，and the furnace can be adjus⁃ ted to achieve hot⁃pressing sintering，non⁃pressing sintering，vacuum sintering，and atmosphere protection sintering. 3 The mold has high⁃temperature resistance，corrosion resistance，good thermal stability，low thermal expansion coeffi⁃ cient，so many kinds of briquette coals can be formed by the device. Using the thermal forming device，several groups of briquette coal specimens with different heating temperatures were made. Based on the packing density theory，the optimal ratio of the 0.250-0.425，0.125-0.150，0.075-0.083 mm particle sizes is 91.77‥5.87‥2.35. Then a series of uniaxial compression tests and triaxial compression seepage tests on these briquette coal specimens were carried out. The density，uniaxial compressive strength，Poisson’ s ratio，elastic modulus，the effect of the initial permeability and minimum permeability were analyzed，and the mechanism of changes in the strength and permeability of these specimens was revealed. The rate of the change of and density of the BC first increases，then decreases，and then in⁃ creases again. The mechanical parameters(strength and elastic modulus)first increase and then decrease，whereas the Poisson’s ratio first decreases and then increases. Both the initial and minimum permeability increase. Considering the density，strength，and permeability of BC，the optimal heating temperature，at which the similarity be⁃ tween the BC and raw coal(RC)is the highest，was determined as 300 °C. It strengthens the bonds between raw coal and briquette coal and helpful for the similar simulation study of coal and gas outburst.