Thermal Stability of Bed Particle Layers on Naturally Occurring Minerals from Dual Fluid Bed Gasification of Woody Biomass

Abstract

The use of biomass as feedstock for gasification is a promising way of producing not only electricity and heat but also fuels for transportation and synthetic chemicals. Dual fluid bed steam gasification has proven to be suitable for this purpose. Olivine is currently the most commonly used bed material in this process due to its good agglomeration performance and its catalytic effectiveness in the reduction of biomass tars. However, as olivine contains heavy metals such as nickel and chromium, no further usage of the nutrient-rich ash is possible, and additional operational costs arise due to necessary disposal of the ash fractions. This paper investigates possible alternative bed materials and their suitability for dual fluid bed gasification systems focusing on the behavior of the naturally occurring minerals olivine, quartz, and K-feldspar in terms of agglomeration and fracturing at typical temperatures. To this end, samples of bed materials with layer formation on their particles were collected at the industrial biomass combined heat and power (CHP) plant in Senden, Germany, which uses olivine as the bed material and woody biomass as feedstock. The low cost logging residue feedstock contains mineral impurities such as quartz and K-feldspar, which become mixed into the fluidized bed during operation. Using experimental and thermochemical analysis, it was found that the layers on olivine and K-feldspar showed a significantly lower agglomeration tendency than quartz. Significant fracturing of particles or their layers could be detected for olivine and quartz, whereas K-feldspar layers were characterized by a higher stability. High catalytic activity is predicted for all three minerals once Ca-rich particle layers are fully developed. However, quartz may be less active during the buildup of the layers due to lower amounts of Ca in the initial layer formation.

Layer formation on K-feldspar in fluidized bed combustion and gasification of bark and chicken manure

“…The studied K-feldspar particles are "impurities" introduced by the feedstock. The data presented in this work is also described in an earlier work [8].…”

“…Interactions between feedstock ash and bed material particles have been investigated in the past. Ash layer formation has been described on a mechanistic level for combustion of various feedstocks and for gasification of bark-rich logging residues [2,[6][7][8]. Furthermore, positive catalytic effects have been assigned to Ca-rich ash layers in gasification leading to an improvement of the product gas quality compared to fresh olivine [9,10].…”

“…The influence of P on the layer formation has been studied in combustion [2] and gasification [17] atmosphere, but to the authors' knowledge no study focused on a comparison of P-rich feedstocks in both combustion and gasification atmosphere. Several groups currently focus on the substitution of olivine as bed material in DFB steam gasification [17][18][19][20][21][22][23][24][25][26] due to its heavy metal content [8]. K-feldspar was found as impurity in CHP in Senden, where layers rich in Ca developed and thermochemical analysis showed that these layers show a lower tendency towards agglomeration compared to layers formed on quartz particles [8].…”

“…Several groups currently focus on the substitution of olivine as bed material in DFB steam gasification [17][18][19][20][21][22][23][24][25][26] due to its heavy metal content [8]. K-feldspar was found as impurity in CHP in Senden, where layers rich in Ca developed and thermochemical analysis showed that these layers show a lower tendency towards agglomeration compared to layers formed on quartz particles [8]. Alkali-feldspar (a mixture of K-feldspar and Na-feldspar, with traces of Ca-feldspar) was shown to have a catalytic activity towards tar reforming [19] and was tested as bed material in the Chalmers 2-MW gasifier [18].…”

Combustion of seaweed — A fuel design strategy

“…The fluidized bed combustion technique allows well-controlled bed temperatures which facilitate studies investigating changes in the potential for silicate melt formation. Olivine was selected as bed material since it is known to be less susceptible to reactions with gaseous alkali compounds than quartz [20,21], the most commonly used bed material today. For all experiments, both the…”