500 / 2018-09-11 10:34:16

CONVERSION OF CHAR-N TO NO AND N2O DURING COMBUSTION OF BIOMASS CHAR

全体主题 > 生物质发电

NOx and N2O, which can lead to environmental pollution and are harmful to human health, are the main gaseous pollutants during biomass combustion. Pyrolysis is the first step of combustion, during which, part of the fuel bound nitrogen (fuel-N) is released with volatile (volatile-N) and the rest is retained in the char (char-N). Both of the volatile-N and char-N can be converted into NOx and N2O via subsequently combustion. It means that char-N is a major contributor to the emission of NO and N2O during biomass combustion. Therefore, it is necessary to well understand the conversion of char-N to NO and N2O.
In this study, biomass char was produced by isothermal pyrolysis of pine sawdust (PS) and rice straw (RS) in a horizontal tube furnace reactor under Argon atmosphere. First, pyrolysis char was obtained at temperatures ranging from 500 to 900 °C with a heating time of 1 h. Besides, pyrolysis was conducted at 800 °C with a heating time of 0.5 and1.5 h respectively to produce the char of different reactivity and properties. The carbon, hydrogen and nitrogen content in the char was determined by an elemental analyzer. The BET surface area of the chars was measured by means of N2 adsorption at -196 °C on Brunauer–Emmeet–Teller. Besides, the nitrogen functionalities in selected biomass samples and the derived chars were characterized using XPS. Then combustion experiments were conducted in Ar/O2 atmosphere to research the conversion of char-N to NOx and N2O as well as the contribution of char-NO to fuel-N. The effects of temperature, heating time, O2 concentration, the amount of the char, the nitrogen functionalities aa well as the surface area and the pore volume were investigated. The concentrations of NO, NO2 and N2O were on-line measured using a Fourier transform infrared spectroscopy (FTIR) gas analyzer (Gasmet DX 4000, Finland). The convert ratios of char-N to NO and N2O and the proportion of char-NO to fuel-N were calculated.
The results showed that N-A was confirmed to be the dominated nitrogen functionality in PS and RS, while it completely vanished in the chars. N-5, N-6, N-Q and N-X were all identified in the N1s spectra of the chars. NO was the main pollutant and the convert ratio of char-N to NO was considerably higher than that of char-N to N2O. NO2 was not detected. As temperature increased from 500 °C to 900 °C, the concentration of NO increased while the convert ratio of char-N to NO was confused and was irrelevant to the nitrogen functionalities, the surface area and the pore volume. However, the proportion of char-NO to fuel-N decreased. This should be attributed to the increasingly reduction reaction of NO and CO over the char. The heating time showed little effect on the NO concentration. Both the conversion of char-N to NO and the proportion of char-NO to fuel-N decreased with increasing heating time. The decrease of NO was attributed to the deactivation of the char through thermal annealing at high pyrolysis temperatures as well as long heating time. Furthermore, the BET surface area of the char increased with the increasing residence time and temperature, due to which, the opportunity for the contact of NO with char increased and then inspired the reduction reaction of NO over the char. Therefore, the conversion of char-N was affected by surface area when the chars that produced with different heating time were combusted at the same temperature although the influence could be negligible compared to that of temperature. For O2 concentration ranging from 6% to 51%, the convert ratios of char-N to NO firstly increased and then decreased, with a peak value at 36% of the O2 concentration. Hence, although higher O2 concentration to some extent could promote the oxidation reaction of char-N + O2, this mechanism was less important at O2 concentration higher than 36%. In addition, the convert ratios of char-N to NO and N2O rapidly decreased with increasing amount of char due to a longer residence time for NO in the pores, and consequently a larger reduction of NO over char as well as surface catalyzed reduction of NO by CO. This study provided a better understanding on the conversion mechanism of char-N to NO and N2O and was helpful for NOx control during biomass combustion.