Layer Formation on Feldspar Bed Particles during Indirect Gasification of Wood. 1. K-Feldspar

Abstract

The choice of bed material for biomass gasification plays a crucial role for the overall efficiency of the process. Olivine is the material conventionally used for biomass gasification due to the observed activity of olivine toward cracking of unwanted tars. Despite its catalytic activity, olivine contains high levels of chromium, which complicates the deposition of used bed material. Feldspar has shown the same activity as olivine when used as a bed material in biomass gasification. As opposed to olivine, feldspar does not contain environmentally hazardous compounds, which makes it a preferred alternative for further applications. The interaction of bed material and ash heavily influences the properties of the bed material. In the present study interactions between feldspar and main ash compounds of woody biomass in an indirect gasification system were investigated. Bed material samples were collected at different time intervals and analyzed with SEM-EDS and XRD. The obtained analysis results were then compared to thermodynamic models. The performed study was divided in two parts: in part 1 (the present paper), K-rich feldspar was investigated, whereas Na-rich feldspar is presented in part 2 of the study (DOI: 10.1021/acs.energyfuels.9b01291). From the material analysis performed, it can be seen that, as a result of the bed materials’ interactions with the formed ash compounds, the latter were first deposited on the surface of the K-feldspar particles and later resulted in the formation of Ca- and Mg-rich layers. The Ca enriched in the layers further reacted with the feldspar, which led to its diffusion into the particles and the formation of CaSiO3 and KAlSiO4. Contrary to Ca, Mg did not react with the feldspar and remained on the surface of the particles, where it was found as Mg- or Ca-Mg-silicates. As a result of the described interactions, layer separation was noted after 51 h with an outer Mg-rich layer and an inner Ca-rich layer. Due to the development of the Ca- and Mg-rich layers and the bed material–ash interactions, crack formation was observed on the particles’ surfaces.

Impact of residual fuel ash layers on the catalytic activation of K-feldspar regarding the water–gas shift reaction

“…Experimental campaigns using the same alkali-feldspar as bed material in DFB steam gasification further supported the catalytic activity observed before [37]. Faust et al [38] and Hannl et al [39] extensively studied the layer formation of alkali-feldspars obtained from DFB steam gasification experiments in a two-part study focusing on two separate feldspars, namely part 1, K-feldspar [38] and part 2, Na-feldspar [39]. While both feldspars formed a calcium-rich outer layer, an additional potassium-rich inner layer formed only on Na-feldspar particles.…”

Comparison of Ash Layer Formation Mechanisms on Si-Containing Bed Material during Dual Fluidized Bed Gasification of Woody Biomass

“…Recent research has focused on the application of alkali feldspar ((Na,K)­AlSi 3 O 8 ) as an alternative to olivine. − Feldspar has been found to show activity similar to that of olivine toward tar reduction. , Berguerand et al reported increased catalytic activity during prolonged ash interaction while at the same time the material withheld its resistance toward agglomeration, similar to the case of olivine. The interaction of woody biomass ash and alkali feldspar was described by Hannl et al and Faust et al , The authors found a transition of the Na feldspar to K feldspar and the formation of an ash layer rich in CaSiO 3 and (Na,K)­AlSiO 4 .…”

“…Recent research has focused on the application of alkali feldspar ((Na,K)­AlSi 3 O 8 ) as an alternative to olivine. − Feldspar has been found to show activity similar to that of olivine toward tar reduction. , Berguerand et al reported increased catalytic activity during prolonged ash interaction while at the same time the material withheld its resistance toward agglomeration, similar to the case of olivine. The interaction of woody biomass ash and alkali feldspar was described by Hannl et al and Faust et al , The authors found a transition of the Na feldspar to K feldspar and the formation of an ash layer rich in CaSiO 3 and (Na,K)­AlSiO 4 . Wagner et al investigated the interaction of K feldspar with fuel rich in phosphorus and found a change in ash chemistry toward phosphates instead of silicates which improved the melting resistance of K feldspar.…”

Dual Fluidized Bed Gasification Configurations for Carbon Recovery from Biomass

“…The choice of the operating temperature to minimize the deleterious impact of tar on the operation will, therefore, be a balance between the amount of tar and the tar composition. The reactions that involve tar and precursors thereof can be affected by the use of catalytic materials − or by the interaction of the bed material with the fuel ash, leading to the development of catalytic activity. − Nevertheless, the materials that catalyze the tar reactions will also likely affect the reforming and cracking reactions of light hydrocarbons, and they can also catalyze the WGS reaction. As noted in the previous paragraph, this will result in an increase in the heat demand of the process.…”