Experimental Study On Ignition Characteristics Of Reburning Coal And NO Reduction Process

With the global continually-increasing demand of fossil fuel consumption, the problem of pollutant emission has caused widespread concern of the world. China is the biggest coal producing and consuming country in the world. Coal occupies about 75% of the energy requirement annually and this situation will last for a long period. Coal burning has brought serious pollution and threat to environment and human health, which becomes an urgent problem to be solved. Coal combustion in power plant is a major source of NOx.Many control methods have emerged as the times require for solving the NOx emission problem. Among these methods coal reburning is the most promising technology. Especially due to its high NOx reducing efficiency (50-70%), micronized coal reburning technology has drawn much attention of many researcher and a series of exploring research work is carried out. Micronized coal is not only in favor of coal burnout but also has more transnormal reducing activity than ordinary coal particles. Thus the NOx reduction efficiency (RE) transfers noncontinuously with the particle size decreasing as well as oxygen concentration. In this paper, the noncontinuous change of RE is thought to be related with the ignition status of coal particles. When the particles ignite, the reducing matters are consumed and can not react with NOx.Experiment study and theoretical analysis are employed to investigate the mechanism of coal reburning process and the influence of some key factors. The experiments are performed in a 1-D reactor system. Combined with coal ignition analysis the impact mechanism of various factors on the ignition status as well as RE are explained reasonably.Firstly, the fast pyrolysis characteristics of three kinds of micro-pulverized coal were investigated by using curie-point pyrolyzer and gas chromatography. The experiment results show that the weight loss of coal mainly occurs in the temperature rising stage. The mass fraction of tar is ranked first in volatiles for both bituminous coal and anthracite, which exceeds 50% for bituminous coal. Besides in the volatile matters the mass fraction of carbon oxides in bituminous coal is higher than anthracite. Bituminous coals are better as reburning fuel. According to pyrolysis experiment, the single-equation model is employed to calculate the heating rates, the frequency factors and the activation energy, which provides foundation for the further research on the simplified ignition model and reburning work.A 1-D coal reburning furnace is built up in this paper. The ignition characteristics of shenmu bituminite particles are experimentally studied under reburning condition and the effect of oxygen concentration and particle sizes on the homogeneous ignition temperature (HIT) are studied. The experiment indicated that the homogeneous ignition process of pulverized coal under low oxygen condition was week and discontinuous without strong flash. O2 concentration is a key factor for homogeneous ignition of coal cloud. With O2 concentration decreasing, the HIT increases exponentially. The lower O2 concentration is, the higher HIT is measured and the longer ignition delay time is obtained. The influence of particle sizes suggests that HIT increases with the sizes decreasing. This investigation shows some guidance to coal ignition simulation work and to optimize coal reburning process.Finally the experimental investigation on reburning of NOx is carried out on the furnace system. The influence of some factors such as particle sizes, reburning temperatures, the stoichiometric ratio of O2 to reburning fuel (SR) and resident time are studied. Base on the analysis of coal homogeneous ignition, heterogeneous ignition, NO homogeneous reduction and heterogeneous reduction, the results indicate that NO reduction efficiency (RE) varies non-monotonically with SR values. The higher reburning temperature is helpful for NO reduction on the whole. The effect of partice size on the ignition status and NO reduction is very complicated. The reduction efficiency increases as particle size decreases above 40μm. But the RE of particles under 40μm is lower than the situation above 40μm.