Combustion Of Single Coal Particles Research Articles

Combustion of Coal Particles in H2O/O2 Supercritical Fluid

Combustion of single coal particles of 1−5 mm diameter in H2O/O2 supercritical fluid with a mass share of oxygen of 0−6.6%, pressure of 30 MPa, and temperature of 400−750 °C was studied in the semi-batch reactor. A decrease in coal mass was observed in two parallel processes: gasification by water and oxidation by oxygen. With an assumption of zero order in H2O concentration and Arrhenius dependence for the rate of gasification by water, the activation energy was estimated as 19 kJ/mol and the pre-exponential factor was determined as 1.02 × 10-2 s-1. It was found out that oxidation by oxygen under the temperature of above 500 °C is limited by the rate of O2 diffusion to combustible coal residue. Below 500 °C the rate of heterogeneous oxidation by oxygen is described by the first-order reaction in concentration of O2 and zero-order reaction in concentration of H2O with activation energy of 166 kJ/mol and pre-exponent of 1.5 × 108 cm3/(g·s).

Experimental and modelling investigations of single coal particle combustion

Experimental and theoretical investigation of combustion of a single coal particle is presented. As model of coal particle combustion the shell progressive mechanism was considered. The pore structure of an original, devolatilized and partially combusted particle was analysed. Due to the bimodal character of the pore structure, the pore size distribution was determined by a sorption method and mercury porosimetry. Combustion experiments were carried out in an equipment by applying the TGA method. This equipment allowed to study the behaviour of the coal particle during the process of drying, degassing and combustion. During the combustion process, the particle appeared to consist of two regions — the non-combusted core and the ash layer. Time dependences of the particle mass, temperature in the particle and gas phase, and combustion products were measured. The obtained experimental results confirmed that the shell progressive mechanism can be applied for the mathematical description of combustion of a single coal particle.

96/00681 Direct observation of single coal-particle combustion in a laminar-flow furnace

96/00681 Direct observation of single coal-particle combustion in a laminar-flow furnace

95/06295 Combustion of single coal particles

A coflow burner has been developed to react sodium vapor with gaseous halides in a flame configuration similar to the hydrocarbon coflow flame, and produce metals, composites, and nonoxide ceramics. Nanometer-sized particles of elemental titanium and titanium diboride were successfully produced with this burner as indicated by XRD. In a novel approach, the flame is operated under conditions that lead to condensation of the NaCl byproduct onto the particles. The NaCl coating acts to control size and eliminate agglomeration of the particles in the flame because the particles are encapsulated in NaCl. The coating was also found to be highly effective at limiting oxidation during postflame handling of Ti powders. TEM analysis indicates that the as-received products are composed of individual nanoparticles in an NaCl matrix. The NaCl coating has been efficiently removed from the TiB2 particles by both water washing and sublimation at 800°C under vacuum. It is proposed that this method of encapsulation, with a removable coating-material, is a general method of obtaining unagglomerated nanoparticles in a flame. Other applications and coating materials need to be explored.

Study on N 2O formation/destruction characteristics in coal combustion under a wide temperature range

Characteristics of N 2 O formation/destruction in coal combustion are studied experimentally and theoretically over a wide range of combustion temperature. The combustion experiments are carried out in pulverized (1200–1700 K) and bubbling fluidized bed (1000–1200 K) coal combustors to find the influence of combustion temperature, combustion air ratio and coal type on N 2 O formation. The effect of catalytic reactions by char, CaO and stainless steel particles on the N 2 O destruction reaction are also tested by using a packed bed reactor. In order to get a deeper understanding of the mechanisms of N 2 O formation/destruction, numerical simulations are carried out for homogeneous volatile combustion and for combustion of single coal particles. The controlling reactions of N 2 O formation/destruction are analyzed. The experimental and numerical study found that N 2 O was mainly formed from the volatiles combustion. The low temperature and oxidizing atmosphere favor N 2 O formation. N 2 O concentration increases with decreasing temperature and with increasing air ratio. HCN and NH 3 , evolved as volatile-N species, make important contributions to the formation of N 2 O as well as NO. Especially, HCN contributes more to the formation of N 2 O than NH 3 in the low temperature range. On the other hand, H radicals produced by the oxidation reactions of CO and H 2 promote the destruction of N 2 O. Char, CaO and stainless steel particles also act as N 2 O destruction catalysts.

A Review of Combustion of Single Coal Particles in Fluidized Beds

The state of the art of the theory of the process of combustion of single coal particles in fluidized beds is reviewed. The process of combustion of coal is divided into two stages: devolatilisation, and burning of the char. The devolatilisation and the subsequent combustion of the volatile matter takes an order of magnitude less time than that for burning of the char. Therefore, the burning of char dominates the characteristics and performance of the combustor. Generally, the combustion of char is found to be controlled by the mass transfer to and reaction on the external surface if the area of pores in the char is less than 0.3 × 106m2/m3. Existing information suggests that in a fluidized bed oxygen diffuses to the carbon surface and it enters into heterogeneous reaction with carbon producing mainly CO. The carbon monoxide burns to CO2 in a diffusion flame surrounding the char particle. The surface temperature of the char, which is always higher than the bed temperature, is found to increase with decreasing size of the char, but it may start decreasing again if the site of CO oxidation moves away from the char surface for very small char particles. The mass transfer rate to the char particles may be calculated by considering the radial variation of local voidages, but this effect is significant only in limited cases.

Laser-initiated combustion of single coal particles in quiesent oxygen environments

Laser-initiated combustion of single coal particles in quiesent oxygen environments

Combustion of single coal particles in a jet

Fluidized bed combustion has attracted much interest in recent years, but there is very little data on the behavior of coal particles at these new conditions. Coal of much larger diameter (1–10 mm), much lower furnace temperatures (~850 °C), and different fluid mechanical conditions exist compared to pulverized coal furnaces. This paper presents experimental data on the behavior and combustion rates of individual coal particles aerodynamically suspended in a heated jet, to stimulate flow conditions in a fluidized bed. Tests of bituminous, sub-bituminous and lignite coals from 2 to 12 mm at jet temperatures of 705 and 816 °C in air and air diluted with equal parts of nitrogen were conducted. The ignition delay time varied from 2 to 44 sec. The devolatilization time extended up to 80 sec and was dependent mainly on particle size. The total burn time was independent of coal type and temperature, and varied as the square of the size and inversally with the oxygen concentration. The total turn time varied from 25 to 740 sec independently of coal type. The square law for the char burning rate was investigated.