Volatility and chemistry of trace elements in a coal combustor

doi:10.1016/S0016-2361(01)00105-3

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Volume 80, Issue 15, December 2001, Pages 2217-2226

https://doi.org/10.1016/S0016-2361(01)00105-3 Get rights and content

Abstract

The volatility of 16 trace elements (TEs) (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, Te, Tl, V, Zn) during coal combustion has been studied depending on the combustion conditions (reducing or oxidizing) and type of coal (high- or low-ash coal), together with their affinities for several active gaseous atoms: Cl, F, H, O, and S.

The elements can be divided into three groups according to their tendencies to appear either in the flue gases or in the fly ashes from a coal combustor:

Group 1: Hg and Tl, which are volatile and emitted almost totally in the vapor phase.
Group 2: As, Cd, Cu, Pb and Zn, which are vaporized at intermediate temperature and are emitted mostly in fly ashes.
Group 3: Co, Cr, Mn and V, which are hardly vaporized and so are equally distributed between bottom ashes and fly ashes. In addition, Sb, Sn, Se and Te may be located between Groups 1 and 2, and Ni between 2 and 3.

At 400 and 1200 K, the 16 TEs behave differently in competitive reactions with Cl, F, H, O and S in a coal combustor.

Introduction

The trace elements (TEs) which are released during coal combustion may cause an environmental and human health risk. Extensive studies have been developed with respect to their abundance, their physico-chemical form, their toxicity and their partitioning behavior in the combustion/environmental control systems. Many papers [1], [2], [3], [4], [5], [6], [7], [8], [9] show that in a combustion process, the speciation of a TE and its content in the flue gas or in the ash varies a lot depending on the coal composition and the combustion conditions. It is thus of great interest to study their behavior in the various processes involved in combustion in order to understand their fate, especially in flue gases.

During coal combustion, several parameters related to the TE influence their transformation, they are as follows: their volatilization tendency, the kinetics of their release from particles, possible interactions, their competitive affinity to active atoms (like halogens, H, O and S), etc. Almost each TE has its own particular behavior in these processes. Therefore, in a multicomponent and multiphase combustion system involving many TEs, their chemical transformations are complex. It is especially true when considering all co-existing coal major (C, H, O, N, S) and minor elements (Si, Al, Ca, Mg, K, Na, Fe, Mn, Ti, P, and halogens), which are expected to have a great influence on the TE behavior. Tendencies related to the TE behavior can hopefully be drawn from thermodynamic calculations, although great difficulties exist in dealing with such a complex system.

Thermodynamic analysis [1], [10], [11], [12], [13], [14], [15], [16], [17], using the principle of ‘minimizing the total Gibbs free energy of the system’, is a powerful tool to predict the elemental chemistry in a multicomponent and multiphase system, with respect to the elements interactions, dominant species and competitive affinity. Although the kinetics and mass transfer limitations associated with metal/particle reactions cannot be explained by a thermodynamic equilibrium analysis, this method can provide useful information on the most favored and stable product distributions at various temperature ranges [14].

In this study, one of the most complex thermodynamic systems considered so far concerning coal combustion is computed by gemini software [18] and involves 54 elements and 3200 species. Among these 54 elements, 16 TEs (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sb, Se, Sn, Te, Tl, V, Zn) are considered, which are all limited by the French Draft Regulation. The system considered here differs from previous ones since it considers simultaneously all possible interactions.

In a complex system where many elements co-exist, the TE behaviors are greatly influenced not only by their interactions, but also by their competitive affinities for the active gaseous atoms (i.e. Cl, F, H, O, S) in the system. This influences the formation of stable TE oxides, sulfides, hydrides and halides; it may also explain the existence of some particular dominant species under given conditions, especially the differences found when solving a complex thermodynamic system or a simple one. Moreover, the TE volatilization properties will influence their behavior during coal combustion; volatile elements may leave the combustion system totally (or partly) in the vapor phase, causing a serious environmental harm; other hardly vaporized elements tend to distribute in bottom ashes and/or fly ashes. The various distributions of the 16 TEs in gas, fly ash and bottom ash can be predicted by comparing their volatilization tendencies. Moreover, the affinities to several active atoms will be a useful tool for simulating the combustion and the condensation zones.

Section snippets

Equilibrium calculation methodology

The computation method and the system considered have been described before [19], including the software (gemini) and database (coach), the system (54 elements and about 3200 species), etc. The method for determination of the air excess number (λ) was described previously when dealing with simple thermodynamic systems [20], [21]. λ values for combustion under reducing and oxidizing atmospheres are set, respectively, to 0.6 and 1.2 in this work.

For the study of the volatilization tendency, two

Results and discussion

The 16 TE behaviors are described and compared in this section, with respect to their volatilization tendencies versus temperature, and their affinities to Cl, F, H, O and S versus the increase of these active atoms in the system. Several changing tendencies concerning the various behaviors of the 16 TEs were obtained, the results are discussed hereafter and compared with previous ones.

Summary and conclusion

Fig. 7, Fig. 8, Fig. 9 summarize the calculation results concerning a complex thermodynamic system containing 54 elements and 3200 species, with respect to the TE volatilization tendencies, and their affinities to Cl, F, H, O, S. Results at low temperature (400 K) describe the TE behavior in the condensing zones, whereas results at high temperature (1200 K) describe the volatilization tendencies.

Acknowledgements

This work has been supported by European Community of Steel and Coal (ECSC) through a subcontract between CERCHAR and CNRS. The authors are grateful to Dr B. Cheynet (THERMODATA) for help with gemini software.

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Volatility and chemistry of trace elements in a coal combustor

Abstract

Introduction

Section snippets

Equilibrium calculation methodology

Results and discussion

Summary and conclusion

Acknowledgements

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