High-Temperature Interactions between Molten Miscanthus Ashes and Bed Materials in a Fluidized-Bed GasifierEnergy & Fuels

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Authors
Judit Kaknics, Rudy Michel, Annie Richard, Jacques Poirier
Year
2015
DOI
10.1021/ef502750t
Subject
Fuel Technology / Energy Engineering and Power Technology / Chemical Engineering (all)

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Article

High temperature interactions between molten miscanthus ashes and bed materials

Judit Kaknics, Rudy Michel, Annie Richard, and Jacques Poirier

Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502750t • Publication Date (Web): 15 Feb 2015

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High temperature interactions between molten miscanthus ashes and bed materials in a fluidized bed gasifier

Judit Kaknics † , Rudy Michel † , Annie Richard § , Jacques Poirier* † † CNRS, CEMHTI UPR3079, Univ. Orléans, F-45071 Orléans, France § UFR Sciences - Centre de Microscopie Electronique, 1 Rue de Chartres - BP 6759 45067 ORLEANS CEDEX 2

ABSTRACT

One of the main concerns about biomass fluidized bed gasification and combustion is the risk of bed particle agglomeration due to ash melting. Although many studies have been conducted about the agglomeration mechanism using silica sand, olivine is mostly mentioned as an alternative bed material for tar decomposition, and its interaction with biomass ash has not been yet fully understood. The aim of this work is to investigate the agglomeration of miscanthus ashes, focusing on thermophysical and thermochemical aspects. Three different bed materials (silica sand (SiO2), raw and calcined olivine ((Mg,Fe)2SiO4)) and an additive to prevent agglomeration (dolomite (CaMg(CO3)2) were tested. The effects of atmosphere and miscanthus

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 harvest time were also investigated. It was found that the key parameter of agglomeration is the wettability of bed particles by molten ashes. In contact with ashes all three bed material showed good wetting tendencies, while dolomite had non-wetting properties. The adhesion between bed materials and molten ashes increases in the order of silica, olivine and calcined olivine. While in the case of silica sand only physical adhesion occurred, the diffusion of iron oxide into the molten ash was observed using olivine. Calcined olivine has a roughened surface which further increased the adhesion. The atmosphere did not influence the mechanism of ash/bed material interaction. On the other hand, miscanthus harvest time had a significant effect on ash reactivity and interaction with raw and calcined olivine. 1. INTRODUCTION

Miscanthus is a perennial energy crop originating from Eastern-Asia. It has been in the spotlight as an alternative energy source for the last 30 years both in Europe and the United States due to its high yield, good adaptability to different climates and soils and low requirement of pesticide and fertilizer.1–3 However, as with all herbaceous biomass, its ash contains a high amount of potassium and silicon, which can cause slagging, fouling or agglomeration at high temperature during gasification and combustion. Although harvesting before the winter would give 30 to 60% higher yield, miscanthus is usually harvested from February to April to improve the fuel quality.2

Over the winter the ash content decreases and the concentration of potassium declines as a result of leaf loss and mineral wash out. The moisture content decreases as well due to the drying effect of the wind.2

Fluidized beds are well adapted to biomass gasification and combustion due to good fuel mixing, relatively low operating temperature (750-900ºC) and medium scale application (5-50 MWth). In fluidized beds, silica sand is the most typical bed material as it is easily available and it has good

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 mechanical properties. Olivine is also often used as it has catalytic effect in tar decomposition due to its iron content.4,5 Olivine is a natural mineral with a general formulae of (Mg,Fe)2SiO4. It consists of two main crystalline phases, forsterite (Mg2SiO4) and fayalite (Fe2SiO4), whose ratio varies with the locality of the mineral. Olivine is often calcined at elevated temperatures (900 ºC to 1600 ºC) to increase its catalytic effect by promoting the migration of iron oxide to the surface.5 Michel et al. studied the phase transformation of olivine at high temperature in detail. 6,7