Study On Heat Fragmentation Characteristics Of Baori Lignite During Upgrading

Heat upgrading technology was the basis for the use of lignite to the depth. However, the severe fragmentation and pulverization of lignite, which occurred in the process, had been a key problem to prevent normal running of process. The main problems include that a large number dust are produced during lignite fragmentation and pulverization; dust and tar mix with each other and block the ducts; tar and dust are difficult to separate. According to the common problems of different drying / pyrolysis processes, the key solving direction, which included strengthen the basis law in research and develop inhibitory method on lignite pulverization, was proposed.Rotary kiln is widely used with the characteristics of simple structure, large dispose capacity and easy industrialization. However, the technology of rotary kiln is severely restricted by its large dust and severe fragmentation of lignite particles. In this paper, the fragmentation and pulverization characteristics of lignite from Inner Mongolia were studied by a kilo rotary kiln experimental system. Its affecting factors and transformation of inorganic mineral in lignite were revealed by multiple characterization methods with ash test, XRF, XRD, SEM, EDS, MLA in typical temperature, as with element and mineral composition in heat fragmentation produces. The transformation effect of mineral on heat fragmentation was studied as well. Based on multi-factors affecting heat fragmentation, a inhibiting method to the pulverization of lignite was explored. The main conclusions of the study are as follows:I Lignite fragmentation characteristics during upgrading1) The influence of multiple process factors affecting Baori lignite size distribution and degree of fragmentation/pulverization was studied in kilo rotary kiln during upgrading. The results show that with the temperature increasing from 120℃ to 700℃, the fraction of retention particles(25~13mm) is decreased from 86.62% to 4.32%, and the group of particles formed by fragmentation(13~1mm) and pulverization(-1mm) are increased from 9.51% / 3.87% to 80.05% / 15.63%, respectively. The fractions of particles at 13~6mm and-0.075 mm are significantly changed. Fragmentation and pulverization are increased with rotary rate, holding time and feed size as well. Impact of four factors on fragmentation is revealed using gray relational method as follows: temperature > size > rotary rate > time, and the impact on pulverization is: temperature > rotary rate > time > size. The descriptive model of lignite fragmentation-pulverization in rotary kiln during upgrading is built using these factors, combined with the internal incentives affecting heat fragmentation, including vapor-volatile arising, mechanical force, indirect reinforcement, particle material resistance, and internal air resistance.2) The impacts of fast and slow heating rates on the heat fragmentation characteristics of lignite were investigated using online temperature measurement device. It can be observed from the results that as the preset temperature increased(400~600℃), with fast heating method(programmed to be a two-step heating where the average heating rates for the first and the second steps were 45~156 and 3.70~8.66℃/min, respectively), total fragmentation rate increased from 88.81% to 95.74% and the pulverization rate increased from 15.28% to 25.76%. However, with slow heating method(average heating rate at 1.05~1.56℃/min with only one-step heating), the total fragmentation rate increased from 80.58% to 95.68% and the pulverization rate increased from 7.82% to 15.63%. Lignite tended to eventually reach approximately the same fragmentation degree under both heating methods while the fast heating modeled to a higher degree of pulverization. From the analysis of the lignite porous structure and surface morphology under both fast and slow heating processes, the stimulation effects of heating rate on the internal factors for heat fragmentation of lignite(e.g., behavior of water vapor, volatilization of volatile matter, and damage of pore structure) were clarified. Based on these findings, a descriptive model was established to characterize lignite heat fragmentation during upgrading under fast heating in rotary kiln.II Transformation of inorganic mineral in lignite and its effect on heat fragmentation1) Ash content is decreased with fragmentation products sizes reducing from(25~13)mm to(3~1)mm, but increased after(3~1) mm to(-0.075)mm. Heterogeneity of ash was caused by different composition of minerals. XRF analysis showed that the elements transformation degree of Si and Al was decreased with solid product size, but Fe was increased. No obvious changes are occurred in Ca element.2) XRD analysis showed that SiO2 characteristic peak intensity in lignite was weaken with size reduction. It is indicated that SiO2 was hard to migrate to the fine-size particles, which was occurred passive migration process. Fe2O3 characteristic peak intensity was increased with size reduction and indicated active migration was occurred.3) The MLA analysis indicated, quartz and aluminum silicates were dominant among the 25~13mm particles; however, more iron oxide minerals were appeared among-0.075 mm particles. Size of siderite occurring in the raw lignite was drastically smaller after pyrolysis. Due to a decrease with the particle size of the siderite and weakness of its structure, some center of fracture development could potentially form, meanwhile, some cracks and holes also formed in situ. All of these weakened the particle structure of lignite, and then induced the pulverization of lignite particle, which became the other inducement of the heat fragmentation of lignite. Aluminum silicates and quartz were migrated into fine particles along the fragmentation of organic matter in the pyrolysis, which made them in different size fractions to be not homogeneous.III Inhibitory effect of coal direct liquefaction residue on lignite pulverization1) Coal direct liquefaction residue(CDLR) inhibits lignite pulverization during co-pyrolysis in the rotary kiln. CDLR significantly reduced the pulverization rate(β) of CDLR-Baori lignite co-pyrolysis products compared to Baori lignite without CDLR. The reduction of the pulverization rate ranged from 3.14% to 5.67%; Granulation was occurred through CDLR and Baori lignite co-pyrolysis with granulation rate(λ) ranging from 0.38% to 4.23% at 450~550°C, and saturated at around 4% after 550°C; Co-pyrolysis char bonding on the furnace wall surface was appearred at 450~500°C and greatly reduced after 550°C. Nonetheless, slight carbonation occurred in the remaining products on the furnace surface at 600°C and 650°C. Through the analysis of pore structure and surface appearance of lignite pulverization products, the major reasons of CDLR inhibiting pulverization were capturing fine particles with sticky surface and reinforcing pore structure of particles. Furthermore, rotary kiln generally provides better lignite pulverization compared to fixed bed, with more thoroughly mechanical mixing. Lignite particles were coverred more area by CDLR.2) Inhibition effect gradually obvious with the quality ratio of CDLR increasing from 10% to 40%, pulverization rate β of co-pyrolysis products was decreased from 7.55% to 1.98%, and granulation rate λ was increased from 2.73% to 4.90%, respectively. However the proportion reached 40% of CDLR would be occurred bonding phenomenon on the furnace wall surface, thus excessively large proportion of CDLR need to be avoided.3) Different inhibitory effect would be occured by different sizes of lignite with 3~1mm CDLR. Co-pyrolysis products pulverization rate β was significantly reduced to 2.82% compared to Baori lignite(30.69%); β is decreased from 3.02% to 7.89% with feed size increasing from 6~3mm to 25~13mm. The reduction in the pulverization rate ranged from 3.02% to 9.06%. No granulation phenomenon was occured when lignite feed size larger than 13~6mm, which indicated granulated particle strength was weaker than the material strength of the larger particles.