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A mathematical model, based on the assumption of isothermal and isobaric plug flow, has been developed, to interpret the data produced at a laboratory scale on SCWO/G of the TOC content of tar-water mixtures. Gasification and oxidation of TOC have been described by an irreversible, first-order, Arrhenius rate reaction. The kinetic constant has been evaluated for the different tests together with the experimental residence time.
Then the construction of the Arrhenius plot has allowed the two kinetic parameters, A and E, to be estimated as: I) E=75.7kJ/mol, A=897 s-1 (gasification), II) E=76.3kJ/mol, A=7740 s-1 (oxidation).
The effects of catalysts have been extensively examined only at a pilot scale and thus an engineering evaluation of the reaction kinetics has been pursued. The chief assumption made is that the activation energy of the catalytic reactions is the same as previously evaluated for non-catalytic conversion.
Then the pre-exponential factor has been estimated, by solving the mass conservation equations, assuming that the temperature coincides with that experimentally measured, and requiring that the deviation between simulated and the measured conversion of the tar TOC is minimum. Evaluations have been obtained for the separate effects of active carbon, potash and ammonia on SCWG and of active carbon on SCWO of tar water mixtures.
As the gasification of activated carbon catalyst also plays a role in determining the catalyst lifetime, the kinetics of this process have been determined. It is based on the gas composition measured in the experiments (twenty tests in the temperature range 392-600C) carried out with the PDU, assuming that the gasification rate is proportional to the water mass fraction and the specific surface of char, the variations in the specific surface of the activated carbon are negligible, and the temperature, for the reactor section packed with catalyst particles, is constant. The usual Arrhenius plot allows the activation energy and the product between the specific surface and the pre-exponential factor to be determined as: E=144.32kJ/mol and A=3.4902m/s. It should be noted that the activation energy determined in this study is very close to that reported by other authors.
Following the results on the quantification of tar compounds carried out for the SCWG tests, an analysis has been carried out aimed at individuating the chief reaction paths. Sugars, contributing in large amount to the initial tar composition in waste water, are promptly decomposed into furfurals (2-and 3-furaldehyde, (5H)-furan-2-one, 5-hydroxymethyl-2-furaldehyde, 5-methyl-(5H)-furan-1-one, x-butyrolactone, 5-methyl-2-furaldehyde).
The majority of these compounds present a high reactivity even at relatively low temperatures, thus giving rise to gases and short-chain aldehydes and acids. Complex phenols, such as 4-methylguaiacol and guaiacol, also present a high reactivity, producing simpler phenols (phenol, cresols, 2,4-dimethylphenol), which add to those initially present in the waste water sample.
These species appear to be the most difficult to be destroyed, although their concentrations are significantly dependent on both temperature and residence times. Acetic acid, which is the major contributor among the organic components polluting the waste water and also a rection intermediate, is scarcely reactive and not very sensitive to the residence time. Low reactivity is also shown by propanoic acid and 1,2-ethanediol. On the contrary 1-hydroxy-2-propanone, which is the third species in the quantitative composition of the tar sample considered in this study, undergoes complete conversion for temperatures slightly above 800K.
Given the complexity of the chemical composition of the tar-water mixture and the numerous reactions taking place, the interpretation of the decomposition kinetics by means of first-order Arrhenius kinetics for a set of species ((5H)furan-2-one, 2-hydroxy-3-methyl-2-cyclopentene-1-one 2-furaldehyde, guaicol,1-hydroxy-2-butanone, acetol) with mass fraction below one and limited to the temperature range where decreasing values should be considered empirical. The results report activation energies in the range 32 and 193 kJ/mol.
A method has been developed for the qualitative and quantitative analysis of tar waters generated from gas cleaners from biomass gasification units. The method comprises pre-concentration and pre-cleaning steps by solid phase extraction followed by gas chromatographic separation and mass spectroscopic identification of tar components.
Simulations have been made of the pilot scale reactor using input data, which reproduce the conditions established in the experimental tests. Validation of the reactor model has been carried out by using the TOC conversion and the temperatures as measured and simulated for all the SCWO/G tests carried out with the PDU, obtaining good or acceptable agreement. Extensive parametric and sensitivity analyses have been carried out to investigate the effects of operating variables (for instance, the inlet temperature in the HE section, the total electric power supplied, the inlet mass flow rate, the TOC mass fraction in the feed, the effects of catalyst) and the applicability of simplifications in the mathematical description of the problem.
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