The Study On Lignite Upgrading And Its Effect On Combustion Characteristics

The development of upgrading technology is the key to the clean and effective use of lignites because the high moisture content of lignites leads to high transportation cost, the low thermal efficiency of boilers firing lignites and high carbon dioxide emissions. This thesis studies the behavior of lignite during upgrading and the effects on pyrolysis and combustion reactivity by means of various experimental approaches.The drying experiments focusing on the characteristics of an Indonesian lignite coal (assigned as YN sample) and two Chinese Shenhua lignite coals (assigned as SH1 and SH2 samples) were carried out and the factors influencing the drying rates were carefully investigated, including drying temperature and particle size, et al. The drying kinetic parameters and mechanistic functions in drying process were obtained by correlative judging method. The physical and chemical structure changes during drying process were qualitatively analyzed using SEM and FTIR. The results showed that drying rate was not limited by heat transfer when the coal particle size was smaller than 10mm, and the drying process may be described with random nucleation and growth mechanism function. As water was progressively removed, the macropores tended to collapse and shrinkage took place while hydroxyl, carbonyl and carboxyl groups decomposed to release H2O and CO2.The high-strength binderless briquettes were produced by a specially designed double-ring roller briquetting machine. Factors influencing the compressive strength were investigated, including briquette moisture content and compression ratio, et al. The results showed that moisture was a critical factor in the binderless briquetting of dried lignites. The compressive strength could be improved by increasing the compressing pressure, reducing particle size and higher drying temperature. The surface of the YN briquettes was visually compact, smooth and lustrous. The YN briquettes had good molding characteristics, and had the highest compressive strength and the strength after water immersion among the three coals.The kinetics of moisture re-adsorption was studied experimentally and theoretically. The results showed that the moisture desorption and readsorption isotherms of the coal showed irreversibility in the desorption-readsorption cycles, indicative of irreversible coal structure changes during drying, which was dependent on drying temperature and independent of coal particle size. The moisture readsorption rate and equilibrium moisture content decreased with increasing drying temperature. The water vapor was physically adsorbed on the particle surface and capillary pore surfaces. The formation of hydrogen bond between the water molecules and the surface functional groups during the moisture re-adsorption process was not evident. Pyrolysis and combustion characteristics of YN lignite were studied under air (O2/N2) and oxy-fuel (O2/CO2) conditions using a drop tube furnace and a thermogravimetric analyzer. Raw coal, dried coal and binderless briquette samples of the same coal were used in the experiments and the effects of drying and binderless briquetting on the reactivity of the coal under different conditions were investigated. The results showed that the briquetted coal had a decreased volatile matter yield, flammability index, burnout index and combustion synthesis index.The industry application potential of YN briquettes was investigated through coking tests and burnout experiments on a pilot scale boiler. The results showed that the YN briquettes could be used for blending for coking. Upgrading of YN lignite has improved its calorific value and achieved favourable results in milling. Laboratory tests for spontaneous combustion indicated that YN briquettes had low to moderate propensity to spontaneous combustion. The burnout was high and the briquetted coal belonged to moderate slight-slaging coal. The emission of SO2 and NOX was low compared to reference coals.