Thermodynamic equilibrium calculation studies

There are ongoing research activities in the fields of biomass thermo-chemical conversions technologies using thermodynamic equilibrium calculations (e.g. modelling of the behaviour of alkali metals, the formation of low melting compounds as well as the behaviour of heavy metals), but they are very limited and almost no previous study has considered the interactions between major and multiple minor coal (or biomass) elements.

Coals contain: major elements (C, H, N, O, S, Cl), with a concentration range usually of >1000 ppmw; minor elements (Si, Al, Fe, Ca, Mg, K, Na, Ti, Ba, Mn, P), with a concentration range usually of 100-1000 ppmw; and trace elements (almost all the other elements of the Periodic Table), with a concentration range of < 100 ppmw [Miller et al., 2002; Ratafia-Brown 1994]. Frandsen et al., [1994] reported the interest in studying the behavior of minor coal elements during coal combustion is due to their effects on the ash formation, particle emission and on the partitioning of trace elements between flue gases and condensed phases. Yan et al., [1999] stated that most previous studies neglected the interactions between minor coal elements during combustion because of insufficient thermochemical data. They reported that in order to simulate the

coexistence of any element in the combustion zone and obtain its most reasonable speciation, it is necessary to consider a complex computation system involving all the important elements in coal and their major species, as well as to investigate the interactions between these elements. They used ALEX software [Baronet, 1983] (which includes thermochemical data for about 600 pure condensed, liquid, solid and gaseous components of 30 minor and trace elements) to study the behavior of 10 minor elements during coal combustion under oxidizing conditions in the temperature range 400-2000 K. It was found that the equilibrium calculation was strongly affected by the species initially considered in the system and so it was concluded that all the important chemical species present must be considered; otherwise the output of the equilibrium analysis might be misleading. Daniel, [1991] indicated an equilibrium flue-gas composition and molecular composition of condensate deposits on boiler tubes, for incinerating a refused-derived fuel (RDF). The thermodynamic calculation showed that sodium sulphate and sodium chloride were the predominant compounds to deposit on superheater tubes at 527 °C.

Bradshaw et al., [2008] used MTDATA to identify the volatile trace species produced by gasification of biomass and waste fuels and whether these species could pass through the hot fuel gas path of a gasification system to form surface deposits on hot components within gas turbines. These thermodynamic studies showed that some trace species could pass through the gasifier fuel gas path to produce deposits in the gas turbine. However, the locations of these deposits in the gas turbines were limited by their dewpoints at that point in this power system. Along with alkali metal sulphates/chlorides, they identified trace species of CdSO4, PbSO4 and SbO2 at metal

temperatures of 640-800 °C, 730-1020 °C, 700-910 °C respectively, as having the potential to initiate the hot corrosion at species locations in the gas turbines.

Goni et al., [2003] used the MTDATA program combined with the NPLOX database for coal ash oxides composition (SiO2-Al2O3-CaO-FeO-MgO and K2O) during work on

CHAPTER 2 LITERATURE REVIEW

morphologically by scanning electron microscopy (SEM). According to their conclusion, the dominant crystalline phases detected by XRD of fly ashes and bottom ashes were similar (containing mullite, quartz, plagioclase, hematite and anhydrite) which agreed with results obtained in MTDATA modelling. Also MTDATA evaluations of the proportion of liquid phase generated by each type of coal (depending on the oxide composition) at 1250 °C gave the same results as obtained from SEM observations.

Gibbs et al., [2008] used MTDATA combined with the MTOX database to predict the formation of an oxide melt from the combustion of as-supplied Binungan coal, and as- supplied and cleaned Gascoigne Wood, El Cerrejon and Harworth coals. They studied the partitioning of the elements Ba, Be, Cd, Co, Mo, Nb, Sb, V, and W during combustion by model predictions and compared these with bottom and fly ash analysis results obtained from a pilot scale combustor (by PowerGen, in the UK). It was predicted that the amount of melt fell more rapidly with falling temperature for cleaned fuels than for as-supplied fuels. They concluded that the mobilities of the elements concerned agreed with those implied by the ratio of bottom ash and fly ash concentrations found in the experimental industrial pilot-scale combustor.

Miller et al., [2002] used MTDATA to predict the speciation of individual trace elements during the co-combustion of coal with biomass fuels and compared the predictions to experimental data. They used coal (Polish and Colombian) and biomass (wood-bark, straw, pulp sludge, paper sludge, agricultural waste, sewage sludge, and plastic waste) in bench-scale fluidised co-firing experiments. It was found that the emissions of trace elements associated with the ash constituents could increase or decrease from co-firing fuel mixtures based on the blends and it was concluded that MTDATA was useful for interpreting the experimental results. It was found that the most volatile trace elements were Hg and Se, followed by Cd, Ti, Pb, and As. For example, complete Se volatilisation (as SeO and SeO2 in the gas phase) was predicted

by the MTDATA model for all the fuels and blends used in the study. In another example, MTDATA predicted Pb would be partially volatilised (due to PbSO4

coal/sewage sludge at 800 °C, but Pb would be completely volatilised (as Pb, PbO and PbCl2in the gas phase) for other fuel mixtures.

Otsuka, [2002] used the thermodynamic database MALT 2 software package developed by National Chemical Laboratory (in Japan) [Kagaku Gijutsu-Sha, 1992], to understand the fireside corrosion of tube materials for boilers firing dirty fuels. The flue-gas composition and deposition/condensation of fuel impurities, such as Na, K, Cl and S, on tube surfaces were estimated by thermodynamic equilibrium calculations. The results showed that for boilers firing municipal solid waste fuel, the flue-gases at 650-1000 °C and condensate deposit species formed on metal surface at 600 °C, contained NaCl and KCl along with ZnO The equilibrium calculations indicated sodium and potassium sulphates as deposit condensates for boilers firing high-Cl, high-S coal as with flue- gases at 1100-1200 °C and metal surfaces at 600 °C. Also for boilers firing black liquor fuel, mixtures of potassium and sodium chlorides, sulphates and carbonates were predicted for 600 °C metal surface temperatures and flue-gases at 1000 °C. It was concluded that the amount of Cl, S, K, Na, Pb and Zn in fuels together with other metals such as Ca, Mg must be examined by equilibrium calculation to interpret the deposit chemistry on boiler tubes surfaces. However, the programme used allows the calculation of thermodynamic equilibria containing up to 10 elements.