Study On The Formation And Release Of Nitrogen-containing Compounds During Coal Pyrolysis And Gasification

The combined pressures to establish energy security, improve pollution control and reduce global warming have encouraged the development of cleaner, more efficient technology to utilize coal. The emission of NOX is the inevitable result of coal large-scale combustion.Studies have shown that the nitrogen in coal can be converted into NOX precursors, N2 as well as some NOX during coal pyrolysis and gasification. Some of these NOX precursors in the gasification gas may then be converted into NOX during combustion. Obviously, the best option to reduce the NOX emission is to minimize the formation of these NOX precursors during coal pyrolysis and gasification. The formation and destruction of these NOX precursors during coal pyrolysis and gasification are affected by many factors, the type of coal and the gasification conditions are particularly important. The purpose of this study was to experimentally investigate the release trend and reaction mechanism of the NOX and its precursors (NHj and HCN) formation during the pyrolysis and gasification of coal.A set of rank-ordered Chinese (the Northern Hemisphere) and Australian (the Southern Hemisphere) coal samples were chosen. In particular, Australian coal B has identical carbon content with the Chinese coal A and Australian coal C has similar volatile yield to Chinese coal A. Coals in the Southern Hemisphere and those in the Northern Hemisphere were generally formed under very different geological conditions. Some of their physical and chemical properties are very different. The samples used in experiment were fully characterized inclusing proximate, ultimate, petrographic analyses and XPS, XRD, surface determination, FT-IR, TG-DTA analyses. The behaviour of nitrogen has been compared during coal pyrolysis and gasification.A novel fluidized-bed/fixed-bed reactor, a temperature-programmed fixed-bed reactor and a drop-in rapid heating reactor were used for coal pyrolysis in Ar and gasification in COa, O2/Ar(4 %V/V) and H2O/Ar(15%V/V). The configuration of the reactors allows the evaluation of the effect of factors on the formation of HCN and NHs from volatile and char/coal on the same basis for a wide range of coals. HCN and NH3 yields were quantified with an ion chromatograph following their absorption in basic and acidic solution, respectively. NO and NO2 were quantified with a chemiluminescence NOX analyzer.The relations of average formation rates and yields of HCN, NH3 and NOX with temperature, heating rate, gas flow rate, coal feeding rate, coal size, coal type and so on were studied. The experimental results show that the formation profiles of NH3 and HCN during coal pyrolysis and gasification are obviously affected by reaction conditions and coal properties. A significant amout of NH3 and HCN in feeding period and only a tail in not-feeding periods are observed during coal pyrolysis. HCN originates mainly from the direct thermal cracking of N-containing heteroaromatic ring systems by thermally less stable and NH3 from the reactions within the solid particle involving adsorbed H radicals generated in situ.To further confirm the action of the availability of H radicals and the stability of N-containing heteroaromatic ring systems, CO2, O2 and steam (H2O) were used as gasifying agents for coal gasification experiments. The results indicate: CO2 tendes to slow down the formation of NH3 and HCN by absorption on solid particle at lower temperature and speeds up the formation of NH3 and HCN by H radicals generated slowly at higher temperature. In O2, significant amount of HCN can be formed as a result of the direct rupture of N-containing heteroaromatic ring systems associated with the high reactivity of O2, and NH3 formation is also improved by H radicals formed from the ring rupture during slow reaction of O2 and nascent char. In H2O, NH3 is the dominant N-containing species from the conversion of coal-N due to the addition H radicals formed during the reaction between H2O and nascent char and HCN yield slightly increases by the direct rupture of thermally less s...