N2O formation during fluidized bed combustion of chars

Conference is much lower with iron catalyst. Further, the tar analysis shows that the amounts of some N-containing groups are considerably decreased in the pyrolysis with iron. The following mechanism for the iron-catalysed nitrogen removal is considered from these observations. In the absence of iron, the N-containing aromatic tar evolved on devolatilization may be deposited as carbon with a significant nitrogen content on the char surface. On the other hand, fine particles of metallic iron may promote the secondary cracking of tar, which is selectively decomposed into N, gas and N-free carbon. In conclusion, the iron catalyst finely dispersed on coal realizes not only the efficient conversion of coal nitrogen into non-pollutant N, gas but also the drastic reduction of residual nitrogen in char during atmospheric pyrolysis.

N,O formation during fluidized bed combustion of chars

Abstracts

possible to monitor the progress of agglomeration over time. Samples of the final product were collected, examined with a microscope, and analysed for ash. Given sufficient ‘oil’, nearly spherical agglomerates were produced with either heptane or hexadecane regardless of whether air was present or not. However, the rate of agglomeration was considerably greater with air present for any given oil dosage. Also when air was present the agglomerates were larger than when air was absent. The size of the agglomerates also increased with increasing oil dosage. When agglomeration was carried out with large amounts of heptane in the presence of air, the resulting large agglomerates were oblong rather than spherical. Larger amounts of hexadecane seemed to promote the aggregation of agglomerates into clusters. With a given, limited amount of oil, more coal was recovered by agglomeration in the presence of air than in its absence. On the other hand, when a large amount of oil was used, the recovery was not affected noticeably by the presence or absence of air.

I. Gulyurtlu, H. Esparteiro and I. Cabrita Departamento de Energias Convencionais Laboratbrio National de Engenharia e Tecnologia Industrial, Edificio J, 1699 Lisboa Codex, Portugal Experimental studies were carried out in a laboratory scale 0.08 m i.d. fluidized bed combustor over a temperature range of 70&95o”C to determine the amounts of N,O and NO, formed during the combustion of different chars. It is not yet well established whether char-N or volatile-N is the major contributor of N,O although it is now an accepted fact that NO, mainly originates from char-N. The present study aims at a better understanding of the oxidation reactions of char-N resulting in either NO, or N,O. It should be noted that the oxidation of char-N involves heterogeneous reactions involving both physical and chemical adsorption of oxygen on the carbon surface. On one hand, char-N forms a significant source for the formation of both NO and N,O whilst char also acts as a reducing medium for these gases. It is, therefore, highly important that the nature of the heterogeneous reactions ofN0 and N,O with char are well understood. Further studies need to be carried out to verify: if the same surface sites are responsible for the N,O from fuel-N; if NO and N,O are reduced on the same surface the nature of adsorption of both NO and N,O whether being physical or chemical; the nature of the products of the adsorption; the reaction between the products of adsorption the rate of desorption of surface products.

formation

of NO and

complexes ofchar; on the carbon surface

H. Ban, J. Schaefer and J. D. Stencel University of Kentucky, Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, KY 40511, USA The tribocharging characteristics of coal, representative mineral matter constituents, and mineral/carbon mixtures were experimentally studied under laminar flow conditions to understand fundamental aspects of charge and particle motion under the influence of electrostatic fields. Spherical silica and glassy carbon particles, with an approximate average diameter of 45-60pm, and finely ground coals, with an average size diameter of 70 pm, were characterized by scanning electron microscopy (SEM) and petrographic analyses. They were then subjected to triboelectrically charging and separation under a high intensity electric field (100_300kVm-‘). The velocity and number density of charged particles were measured using a two-dimensional, laser phase Doppler particle analyser (PDPA). The size distribution of the spherical silica and glassy carbon were also measured as they passed through the electric field. By using theoretical modelling that analyses the force balance in the particle-gas system, particle charge distributions and separation histograms were calculated. The effects of gas composition on the charge and particle separation were also investigated. These results are discussed in relation to the application of tribocharging/electrostatic separation for dry coal cleaning.

and NO and N,O;

The mechanism involving N,O and char reaction is still not well established although the reduction of NO by char has been studied extensively. However, the NO-char reaction has to be studied again taking into account the effect of the presence of N,O. This paper reports the experimental results from the fluidized bed combustion studies of chars of different origin.

The role of air in oil agglomeration shear rate

Particle tribocharging characteristics relating to electrostatic dry coal cleaning

of coal at a moderate

7: D. Wheelock, G. Milana* and A. Vettor* Department of Chemical Engineering, Iowa State University, Iowa State Mining and Minerals Resources Research Institute, Ames, IA 50011, USA * Eniricerche SpA, Via F. Maritano 26, 20097 San Donato, Milanese, Italy The oil agglomeration characteristics of a highly hydrophobic coal were investigated with a specially designed laboratory mixing unit equipped with a turbine agitator which produced a moderate shear rate. The sample of coal was from the No. 2 Gas Seam in Raleigh County, West Virginia. The coal had an ash content of 17% and was classified as highly volatile A bituminous coal. It was wet ground in a stirred ball mill to produce particles with a median size of 8 pm. The particles were suspended in water and agglomerated either with heptane or with hexadecane in a closed system which made it possible to have air either present or absent. The system was instrumented so as to provide a continuous indication of the turbidity of the suspension making it

Session

7: Modelling

Characteristics

of coal char combustion

Reginald E. Mitchell Mechanical USA

Engineering

Department,

Stanford

University,

An overview is given of the results of an experimental project that was undertaken to characterize the physical and chemical processes that govern the combustion of pulverized coal chars. The experimental endeavour established a data base on the reactivities of coal chars as a function of coal type, particle size and temperature, and gas temperature and composition. The project provided a better understanding of the mechanism of char combustion under technologically relevant conditions and yielded an accurate predictive engineering model of the overall char combustion process. In the experimental programme, use was made of an optically accessible laminar flow reactor that allows particles to be observed in environments simulating those found in the upper furnace region of pulverized coal-fired boilers. The reactor is equipped with a particlesizing pyrometer that permits in situ measurement of the sizes, temperatures and velocities of individual particles at various residence times and with an isokinetic solids-sampling probe that permits the extraction of particles from the reactor for off-line physical and chemical analyses. Overall extents of conversion, particle densities, and specific surface areas are determined from the bulk extracted samples. Chemical analysis of the extracted samples yields, in addition, C, H, 0, N and S contents, total ash content, and an elemental analysis of the ash (Ti,

Fuel 1993

Volume

72 Number

5

701