Analysis of role of erosion process of coal particles in generation of loss of incomplete combustion in fluidized bed boilers

T. Sobota,J. Taler,B. Węglowski,Piotr Pelka

doi:10.1051/e3sconf/20171304005

Abstract

In the herein paper the results of research carried out in the Institute of Thermal Machinery of Technical University of Czestochowa have been presented. They concern the process of combustion of coal particles in a fluidized bed combustor. The tests were carried out on a test stand which allows modelling of conditions around the fuel particle in a real object. On the basis of the registered mass loss of the particles of Polish hard and brown coal chosen, the impact of the mass flow rate of inert material with different particle size distribution and different collision velocity with burning coal particle on the rate and level of its burning out was evaluated. The obtained results enables the estimation of the influence of both parameters on the intensity of erosion and determination of its role in the generation of fine particles elutriated out of the combustion chamber. The results also facilitated the verification of the methodology [1] of calculation described in literature relating to the total mass loss of coal particles as a result of combustion and erosion. According to this methodology, the slip velocity between the char particle and the fine bed solid is the main parameter of the erosion process alongside the attrition rate constant k a .

Highlights

The loss of incomplete combustion depends on many factors
In the experiment the inert material with different particle size distribution is presented in Fig.2 and the characteristic parameters shown in Table 4 were used
The rate of mass loss for the parameters taken in the tests presented that match the conditions in the middle and upper zone of CFB boilers may be even more than 100% in relation to combustion without the influence of inert material

Summary

Introduction

The loss of incomplete combustion depends on many factors. Alongside the coal type and the associated physical and chemical properties, both the construction type and operating factors of the fluidization process have an impact on its level. Where Xc is the fractional carbon in the solid waste, Wa is the ash per unit mass of the fuel feed and HHV is the heating value of coal This loss has essential contribution to the heat balance especially in CFB boilers. Many factors influence the unburned carbon loss and the amount of carbon in the flue ash in CFB boilers, of which the following are first and foremost distinguished: coal type, particle size distribution of coal, temperature in the bulk of particle, oxygen concentration, furnace design, type of fluidization. The basic condition of the complete burning out of solid fuel is that the residence time of char particle τp must be only a little longer than the burnout time τs Both of these time periods depend on the magnitude of the afore-mentioned parameters which hinders the precise determination of their values, the fact that. In the case of conditions in CFB, the hydrodynamic force is proportional to the slip velocity between the char and the abrading bed material [9] Ug=(Uc - Gs/ρb).This means that the slip velocity is the decisive parameter of the erosion process for coal particles of the same type and diameter

Experimental Verification

Effect of erosion in char combustion model

Findings

Conclusions