Temperature and oxygen partial pressure dependencies of the coal-bound nitrogen to NOx conversion in O2/CO2 environments

This is an experimental and numerical study aiming to assess the emissions of nitrogen oxides (NOx) from combustion of bituminous and lignite coals in O-2/CO2 environments, simulating one-pass dry oxy-combustion conditions, and to investigate the influences of temperature and oxygen partial pressure on such emissions. Combustion of coal particles and, separately, coal char particles, both in the size range of 75-90 mu m, were conducted under globally fuel-lean conditions in a laboratory electrically-heated drop-tube furnace (DTF). The emissions of nitrogen oxides were correlated with separately-observed single particle volatile flame and char temperatures, deduced with multi-color optical pyrometry. As atmospheric nitrogen was absent in these experiments, the entirety of the monitored NOx was attributed to the oxidation of fuel-bound nitrogen. Experiments were conducted by separately increasing the furnace temperature or the oxygen mole fraction in the gas. Results showed that nitrogen oxide fractions in the combustion effluents increased both with increasing partial pressure of oxygen in the background gas (from 21% to 40%) and with increasing furnace temperature (from 1300 to 1500K). As increasing either of these two parameters increases the particle temperatures (both volatile flame and char) it was hypothesized that the NOx evolution is a function of these particle temperatures. To further investigate such hypothesis a published kinetic model was used, which assumes that the coal-bound nitrogen (a) converts mostly to hydrogen cyanide and then to NO during the volatile combustion phase, and (b) converts directly to NO during the char combustion phase. Flow tube and flame simulations were performed using Cantera to investigate the relative impacts of temperature and oxygen mole fraction, and to understand the causes of the observed trends. Results support the hypothesis that the evolution of NO from fuel-bound nitrogen is a strong function of the particle volatile matter temperature. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.