• Doctoral thesis – Ash Formation

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Ash Formation, Deposition and Corrosion When Utilizing Straw for Heat and Power Production 

Summary of the Doctoral Thesis by Flemming Frandsen, Technical University of Denmark

This Doctoral Thesis contains an outline of Danish experiences on ash and deposit formation, and corrosion, when utilizing straw for heat and power production, seen mainly in relation to research activities within the CHEC Research Centre at the Department of Chemical and Biochemical Engineering at the Technical University of Denmark, in the period [1991 – 2009].

Major findings on ash deposition and corrosion in grate-fired units, during co-firing in PF-fired suspension-boilers, and, finally, in pilot-scale facilities, have been outlined. The main conclusion is that in the grate-fired units, utilization of straw will cause problems if the steam temperature is kept too high, i.e. hotter than 540 °C. On the other hand, straw may be co-fired with coal, which is fully manageable from a deposition and corrosion point of view, provided that the coal has a high quality, i.e. a certain amount of Al-silicates in its ash, which will react with the K and release Cl and HCl. These species are not nearly as harmful at high temperatures as KCl(s) being deposited directly on the heat transfer surfaces. A change in the regulation on the quality of the fly ash to be allowed for cement and concrete production, moved the main obstacle for coal-straw co-firing as a mean of utilizing straw for energy production in suspensions-fired boilers with a high electrical efficiency. Now, only the potential problem of poisoning of SCR-catalysts by K-species from the straw, remain to be solved.

An attempt has been made to connect the release of K, S, and Cl, via gas phase chemical kinetics and physical transformations to aerosol formation. In this context, the Si/K and the Cl/K ratios play a major role for the release, although working in opposite directions. A high Si/K-ratio indicates a chance of forming non-volatile K-silicates, while a high Cl/K-ration pulls in the opposite direction, indicating increased volatilization of K, through interactions with Cl. For the deposits, the Cl/S-ratio in the fuel is very important, since the data presented in this thesis indicates that a high Cl/S in the fuel will cause a high Cl-level in the deposits, while an excess of S will cause the deposit chemistry to shift toward the less harmless K-sulfate. Also, the secondary capture of K, either by char or by active Ca-Si-rich ashes in the char structure, was addressed. Furthermore, a detailed study of the link between fuel and aerosol chemistry in grate-fired units, indicated that the fuel chemistry is controlling for the aerosol mass loading and chemistry, at least when compared for different fuels in the same plant.

Concerning corrosion and material aspects in plants (co-)fired with straw, the data generated in Denmark during the last two decades are in line with the deposition investigations, indicating potential severe corrosion in the dedicated straw-fired plants (grates) but somewhat lower corrosion when co-firing straw with coal in suspension boilers. This is outlined in Chapter 8.

Application of additives for minimizing aerosol formation and/or deposition in straw-fired boilers has been outlined. A clear effect of additives on the aerosol mass loading and chemistry was shown at the AVV-2, while test-firing in the CHEC-EFR (entrained flow reactor) indicated that also the deposit chemistry may be affected seriously by a proper choice of an additive. Thus, additives may affect the ash and deposit chemistry and thereby the corrosive potential in straw-fired boilers. The best choice for additive is and Al-Si – based material which will act in basically the same way as the coal ash in coal-straw co-fired systems.

Finally, shedding of ash deposits, has been described. We have been able to quantify the mass gain of deposit as well as the heat-uptake in the probe via an intelligent new shedding probe design. The data from the shedding probe has been applied to take the modelling of deposit build-up and shedding to a whole new level of understanding and quantification.

A copy of the related presentation can be viewed online here.

To request a copy of the thesis document, please email Flemming Frandsen on ff@kt.dtu.dk