Characterization of early stage coal oxidation and effects of gas residence time on nitrogen oxides reduction in oxy-coal combustion.

摘要

This first part of this work reports our first study on the deactivation of young chars in flame conditions. The quantity and strength of surface oxides on young chars are monitored in situ by temperature-programmed desorption (TPD) up to 1700 °C. Search of the oxygen source for the huge amount of CO production at 1700 °C reveals that commonly adopted alumina tubes and support materials decompose to Al2O(g) and emit a notable amount of O2 at temperatures above 1300 °C. Thus, different reactor materials are examined Alumina appears suitable for the oxidation part of the experiments while SiC appears acceptable for TPD.;Oxygen from the gas phase, organic portions of the coal and minerals in the coal has been found to have profound influence on the formation and desorption of stable surface oxides in the early stages of coal combustion. In an attempt to isolate the effects of minerals, demineralized coals (DMC) are oxidized in O2 with a contact time less than 1 second. Young chars derived from both demineralized lignite and bituminous coals show low and flat TPD profiles over a wide temperature range, signifying the minerals' catalytic activities in forming stable surface oxides for both coals.;In the third part of this work, the dispersion of CO and CO2 from temperature-programmed desorption (TPD) of char before they reach the mass spectrometer (MS) is sequentially examined by a set of methods. The Taylor-Aris criterion assures that an axially dispersed, plug-flow model is sufficient to determine the dispersion effects. A novel tracer experiments are performed to determine the residence time distribution (RTD) of the product. Finally, fast Fourier transform (FFT) is also conducted to reconstruct the actual product evolution. This analysis suggests that the dispersion does not severely distort the measured desorption and oxidation rate constants reported previously.;A gravity-driven particle feeder has been modified to achieve sustained operation at steady rates. A solenoid with high power output and a secondary reservoir are used. The secondary reservoir can be refilled during the operation without disturbing the feed rate. Test results showed both good long-term and short-term stability.;We evaluate the potential benefits of long residence time of gas in a recycle system on NO reduction in oxy-coal combustion. In both single-pass and recycle tests, residence time within a range of 0.36 to 2.0 seconds shows only minimal benefits on NO reduction. The reverse Zeldovich mechanism appears not a governing factor up to 1700 °C, which is close to the peak temperature in practical boilers. The equilibrium concentrations of NO are much lower than those experimentally observed. Thus, NO reduction in oxy-coal combustion is kinetically controlled, not thermodynamically controlled.