| This work studied the ignition and combustion of burning pulverized coals and biomasses particles under either conventional combustion in air or oxy-fuel combustion conditions. Oxy-fuel combustion is a 'clean-coal' process that takes place in O2/CO2 environments, which are achieved by removing nitrogen from the intake gases and recirculating large amounts of flue gases to the boiler. Removal of nitrogen from the combustion gases generates a high CO2-content, sequestration-ready gas at the boiler effluent. Flue gas recirculation moderates the high temperatures caused by the elevated oxygen partial pressure in the boiler. In this study, combustion of the fuels took place in a laboratory laminar-flow drop-tube furnace (DTF), electrically-heated to 1400 K, in environments containing various mole fractions of oxygen in either nitrogen or carbon-dioxide background gases.;The experiments were conducted at two different gas conditions inside the furnace: (a) quiescent gas condition (i.e., no flow or inactive flow) and, (b) an active gas flow condition in both the injector and furnace. Eight coals from different ranks (anthracite, semi-snthracite, three bituminous, subbituminous and two lignites) and four biomasses from different sources were utilized in this work to study the ignition and combustion characteristics of solid fuels in O2/N2 or O2/CO2 environments. The main objective is to study the effect of replacing background N2 with CO2, increasing O2 mole fraction and fuel type and rank on a number of qualitative and quantitative parameters such as ignition/combustion mode, ignition temperature, ignition delay time, combustion temperatures, burnout times and envelope flame soot volume fractions.;Regarding ignition, in the quiescent gas condition, bituminous and sub-bituminous coal particles experienced homogeneous ignition in both O2/N 2 and O2/CO2 atmospheres, while in the active gas flow condition, heterogeneous ignition was evident in O2/CO 2. Anthracite, semi-anthracite and lignites mostly experienced heterogeneous ignition in either O2/N2 or O2/CO2 atmospheres in both flow conditions. Replacing the N2 by CO 2 slightly increased the ignition temperature (30--40K). Ignition temperatures increased with the enhancement of coal rank in either air or oxy-fuel combustion conditions. However, increasing oxygen mole fraction decreased the ignition temperature for all coals. The ignition delay of coal particles was prolonged in the slow-heating O2/CO2 atmospheres, relative to the faster-heating O2/N2 atmospheres, particularly at high-diluent mole fractions. At higher O2 mole fractions, ignition delays decreased in both environments. Higher rank fuels such as anthracite and semi-anthracite experienced higher ignition delays while lower rank fuels such as lignite and biomasses experienced lower igniton delay times.;In combustion, fuel particles were observed to burn in different modes, such as two-mode, or in one-mode combustion, depending on their rank and the furnace conditions. Strong tendencies were observed for all fuels to burn in one-mode when N2 was replaced by CO2, and when O 2 mole fraction increased in both environments. Moreover, increasing the coal rank, from lignite to bituminous, enhanced the tendency of coal particles to exhibit a two-mode combustion behavior. Particle luminosity, fragmentation and deduced temperatures were higher in O2/N2 than in O2/CO2 atmospheres, and corresponding burnout times were shorter, at the same O2 mole fractions. Particle luminosity and temperatures increased with increasing O2 mole fractions in both N2 and in CO2 background gases, and corresponding burnout times decreased with increasing O2 mole fractions. Bituminous coal particles swelled, whereas sub-bituminous coal particles exhibited limited fragmentation prior to and during the early stages of combustion. Lignite coal particles fragmented extensively and burned in one-mode regardless of the O2 mole fraction and the background gas. The timing of fragmentation (prior or after ignition) and the number of fragments depended on the type of the lignite and on the particle shape. Temperatures and burnout times of particles were also affected by the combustion mode.;In nearly all bituminous and biomass particles combustion, sooty envelope flames were formed around the particles. Replacement of background N 2 by CO2 gas decreased the average soot volume fraction, fv, whereas increasing O2 from 20% to 30--40% increased the fv and then further increasing O2 to 100% decreased the soot volume fraction drastically. bituminous coal particle flames generated lower soot volume fractions in the range 2x10 -5--9x10-5, depending on O2 mole fraction. Moreover, biomass particle flames were optically thin and of equal-sized at all O2 mole fractions. (Abstract shortened by UMI.). |
