The Combustion Characteristics Of Pulverized Coal And The NO Formation Characteristics In Oxy-steam Atmospheres

The global climate change caused by CO2-based greenhouse gases has become a focus of attention around the world. Oxy-fuel combustion technology is considered to be the effective measures to control CO2 emissions from power plants and the O2/CO2 cycle combustion is a representative of oxy-fuel combustion technology. Oxy-steam combustion technology is a new oxy-fuel combustion technology with great potential. Oxy-steam combustion technology has technical predominance of compact structure, small equipment size, low NOx level, low oxygen, high radiation efficiency and convective heat transfer compared with O2/CO2 cycle combustion. Presently, there are little studies on pulverized coal combustion in O2/H2O atmospheres. Based on oxy-steam combustion technology, this paper makes a primary exploration on the combustion characteristics and reaction kinetics of coal/char combustion in O2/H2O atmospheres through thermogravimetric analysis, the coal ignition mechanisms under O2/CO2 and O2/H2O atmospheres are studied by CFD software and thermogravimetric analyzer, and NO formation characteristic of coal combustion in O2/H2O atmospheres is also studied in a drop tube furnace.Firstly, the combustion tests of PDS bituminous coal and SH meagre coal in O2/H2O and O2/N2 mixtures are carried out using a thermogravimetric analyzer in this paper. The effect of combustion atmospheres, oxygen concentration, heating rate and particle size on the combustion characteristics and kinetic behaviors of SH and PDS pulverized coal in O2/H2O mixtures are analyzed. The results show that, based on thermogravimetric analysis, replacing the inert N2 gas in the oxidizer with steam delayed the ignition of SH and PDS pulverized coal and the comprehensive combustibility index S of SH and PDS pulverized coal in O2/H2O atmospheres are lower than that in O2/N2 atmospheres. With the increasing of the oxygen concentration, the comprehensive combustibility index S increases both in O2/H2O and O2/N2 atmosphere. The effect of oxygen concentration on the combustion performances decreases when oxygen concentration exceeds 40%. In O2/H2O atmosphere, the combustion of SH and PDS coal occur in a higher temperature region as the heating rate increases, but the increase in heating rate can enhance the coal combustion property. Particle size has certain effect on the combustion characteristics. The combustion performances are improved as the particle size decreases. The results of kinetic analysis indicate that the combustion reactions of SH and PDS pulverized coal in O2/H2O mixtures follow the first-order kinetics, and compensation effect exists between apparent activation energy and pre-exponential factor in different oxygen concentrations during coal combustion in O2/H2O mixtures.Then, the oxy-fuel ignition mechanism is determined by CFD simulation and the detail effects of physicochemical properties on both coal homogeneous and heterogeneous ignitions in O2/CO2 atmosphere are analyzed, meanwhile, the combustion characteristics and combustion ignition mechanism of SF fine and micro-pulverized coal in O2/H2O atmospheres are studied. The effects of atmosphere, oxygen concentration and particle size on the combustion characteristics are analyzed based on the TG-DTG curves. The simulation results indicate the in the homogeneous ignition mechanism of coal in O2/CO2 atmospheres, chemical properties and high specific heat are the main reasons for the ignition delay compared to the corresponding O2/N2 atmosphere and the physical properties of CO2 played important roles in the ignition delay in the heterogeneous ignition of pulverized coal in O2/CO2 atmospheres compared to O2/N2 atmospheres. The thermogravimetric experiment results show that the substitute N2 with H2O has obvious influence on the comsbution characteristics of SF fine and micro-pulverized coal. Under the same oxygen concentration, the comsbution performances of three different particle size pulverized coals in O2/H2O atmospheres are significantly promoted compared with that in O2/N2 atmospheres. Under O2/H2O atmospheres, the comprehensive combustion characteristic index of SF pulverized coal increases and the comsbution performance is improved with the increase of oxygen concentration. The influences of the change of oxygen concentration on the comsbution characteristics are different. The change of oxygen concentration has a great influence on the comsbution characteristics when the oxygen concentration is lower than 30%. and the influence becomes weak when the oxygen concentration is higher than 30%. The decrease of the particle size helps to improve the combustion performances of pulverized coal. According to TG-DTG-DTA method, the combustion ignition mechanism of SF fine and micro-pulverized coal in O2/H2O atmospheres gradually change from heterogeneous ignition mechanism to hetero-homogeneous and homogeneous ignition mechanism with increasing oxygen concentration. The hetero-homogeneous and homogeneous ignition mechanism are most likely to occur under lower oxygen concentration in O2/H2O atmospheres.Followly, the combustion characteristics and reaction kinetic mechanism of coal chars in O2/H2O atmospheres are studied. The results show that the combustion reactivity of FG and XS char is higher in O2/H2O mixtures compared to O2/N2 atmospheres with the identical oxygen concentration and using H2O instead of N2 is conducive to improve the combustion performances. The activation energies of FG char and XS char obtained by the FWO, Starinks and KAS methods decrease with the increasing conversion level. According to the combination of Popescu method and Coast-Redfern integral method, the combustion reaction of FG char in O2/H2O atmospheres is the first-order chemical reaction kinetics and the mechanism function is G (?)=-ln (1-?).Finally, the NO formation characteristics of SF bituminous coal under different atmospheres (O2/N2 and O2/H2O) are studied in a drop tube furnace and the effects of oxygen concentration, temperature, excess oxygen coefficient and primary stream ratio on the NO formation characteristics are analyzed. The experimental results indicate the NO emission level of pulverized coal combustion in O2/H2O atmospheres is much lower than that in O2/N2 atmospheres. In O2/H2O atmospheres, the NO emissions increase with increasing oxygen concentration. Temperature has a great influence on the NO emission, the NO emissions increase as the temperature increases. NO emissions increase as the excess oxygen coefficient increases. The primary stream ratio has less effect on the NO emissions when the oxygen remains unchanged.