Ash behavior and de-fluidization in low temperature circulating fluidized bed biomass gasifier

Vikas Narayan

Research output: Book/ReportPh.D. thesisResearch

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Biomass is increasingly used as a fuel for power generation. Herbaceous fuels however, contain high amounts of alkali metals which get volatilized at high temperatures and forms salts with low melting points and thus condense on pipelines, reactor surfaces and may cause de-fluidization. A Low-Temperature Circulating Fluidized Bed System (LTCFB) gasifier allows pyrolysis and gasification of biomass to occur at low temperatures thereby improving the retention of alkali and other ash species within the system and minimizing the amount of ash species in the product gas. In addition, the low reactor temperature ensures that high-alkali biomass fuels can be used without risks of bed de-fluidization. This thesis aims to understand the behavior of alkali metals and ash in the LTCFB system. The thesis work involved measurements made on bed material and product gas dust samples on a 100kW LTCFB gasifier placed at Risø and a 6 MW LTCFB gasifier owned by DONG ENERGY and placed in Kalundborg. In addition to the analysis of the inorganic elemental composition of the collected samples, SEM and TGA analysis of the samples were made to improve understanding on the behavior of the ash forming species within the system. It was observed that of the total fuel ash entering the system, a large fraction (40-50%) of the ash was retained in the secondary cyclone bottoms and a lower amount (8-10%) was released as dust in the exit gas; the residual ash was accumulated within the fluidized bed system. A dominant fraction of alkali and alkaline earth metals were retained in condensed state along with Si and some Cl, while a large fraction of Cl and S appeared in gaseous form and was released with the product gas. Measurements on the product gas from the 100 kW LTCFB gasifier showed the presence of Cl in the form of gaseous methyl chlorides (90-100 ppm). Release of K and other inorganic species with the tar in the product gas from the LTCFB gasifier were found to be low. The major forms in which K and Si could exist in the LTCFB gasifier are K-salts (KCl and K2CO3), organically bound K (K bound to ion exchange sites of the char matrix and intercalated K), and K-silicates. At the temperature in the pyrolysis chamber (6500C) of the LTCFB gasifier, the above K species are expected to be mostly present in the solid state. In the char reactor (where the char from the pyrolysis chamber is gasified and combusted at temperatures around 7300C), KCl(s) will partially vaporize and the released K could react with silica to form silicates. When the flue gas enters the pyrolysis reactor, the temperature is reduced and KCl aerosols are formed. The release and retention of the condensed ash species from the system was seen to be controlled by the ash particle size and the cut size of the primary and secondary cyclones. A model accounting for the ash collection by the plant cyclones was developed which predicted the product gas ash particle release reasonably well.
The present work also aims to understand the effect of biomass fuel ash composition and fluid bed operation conditions especially temperature on agglomeration and de-fluidization of alkali-rich bed material under gasification conditions. The de-fluidization studies involved measurements with mixtures of sand and pure potassium salts (KCl and K2CO3) as well as bed material samples obtained from a 6 MW Low Temperature Circulating Fluidized bed (LTCFB) gasifier on a bench-scale fluidized bed reactor set up. The mechanism of agglomeration in the bed particles was seen to vary with the speciation of K. It was seen that in sand and KCl agglomerates, the sand particles were bound by KCl melts. There was very limited chemical reaction observed between KCl and the sand particles with no presence of silicate melts in the agglomerates. In sand and K2CO3 mixtures and the LT-CFB bed material samples the agglomeration was seen to occur due to a coating of viscous silicate melts formed from reaction of alkaline and alkali earth species with silica from the bed particles (coating induced agglomeration). It was also seen that the composition of the bed particles affected the de-fluidization temperatures. The de-fluidization took place at higher temperatures in the case of LTCFB bed material particles (780-7850C) as compared to the sand and K2CO3 mixtures (7300C) with similar K contents (4.2-4.5%), though both showed that the de-fluidization occurred by the mechanism of coating induced agglomeration. This is attributed to the presence of Ca and Mg in the bed particles; these elements shift the formation of the eutectic melts to higher temperatures increasing the viscosity levels of the coatings. A mathematical model for de-fluidization of alkali rich bed material was developed to predict the de-fluidization temperatures as a function of parameters such as initial alkali concentrations within the bed particle diameters and the fraction of K entrained from the system. The model was also applied to study the de-fluidization behavior of alkali-rich samples in a large scale LTCFB gasifier. The model was used to predict the variations in de-fluidization time on a full scale LTCFB plant with respect to parameters such as temperature, fuel alkali concentrations and bed particle diameter.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages83
Publication statusPublished - 2016


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