ECN publication
Conversion of biomass, prediction and solution methods for ash agglomeration and related problems: contract JOR3-95-0079, Final report 1 March 1996 to 1 March 1999
Published by: Publication date:
ECN 1-11-1999
ECN report number: Document type:
ECN-C--99-090 ECN publication
Number of pages: Full text:
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When biomass is used as fuel for thermal conversion plants, minerals fromthe fuel can be responsible for major problems. Generally, these problems are associated with the existence and development of low melting compounds or eutectics, which form sticky layers. In a fluidised bed, this can result in bed-agglomeration and defluidisation. This causes local high temperature, which often accelerates the process. It ultimately can lead to a completely sintered bed content with a glassy phase gluing the bed particles together and shut-down of the plant. The main objective of the title project is to develop a methodology to predict ash/bed agglomeration and sintering problems, to indicate related problems and, furthermore, to identify solution methods to make different types of biomass streams more viable for energy production. Within the present study, selected fuels are subjected to different existing methods together with some new ones, in order to determine the agglomeration temperature. The selected fuels are verge grass, Danish wheat straw (both stored dry and partly leached due to rainfall), sewage sludge, cacao shells and willow as a reference. The methods used within the study are chemical analysis of fuel and ashes, determination of standard ash melting temperatures, compression strength measurements of the ash, DTA/TG analysis of the ash, SEM and ESEM (high temperature environmental scanning microscopy), two different lab-scale bubbling fluidised bed combustion facilities, a lab-scale bubbling fluidised bed gasifier and a circulating fluidised bed gasifier. The lab-scale facilities have been used to test potential measures to reduce the problem of agglomeration. These measures are the use of additives (kaolin, magnesite, dolomite, gibbsite and sewage sludge) and non-quartz bed materials (alumina and mullite). The work performed within the project has lead to the following results. Chemical analysis of the fuel can give a first indication of whether there might be an agglomeration problem during thermal conversion. In general a high K-content means an increased risk for agglomeration. However, the K-content alone is not a good indicator. Also chlorine proved to be very important. From the methods used in the project, laboratory fluid bed agglomeration experiments seem to give the most reliable information about conditions and temperatures where agglomeration takes place. Contrary to methods like DTA, compression strength and ash melting temperatures, all processes that. might be relevant for agglomeration actually can occur during fluid bed experiments: fuel-bed material interactions, volatilization and condensation, shear forces, temperature homogeneity and accumulation. Standard tests have been developed where process temperature is gradually increasing until agglomeration. These tests have been applied in the project at three different laboratories: ECN (Netherlands), ETC (Sweden) and VTT (Finland). They have proved to be accurate and reproducible. It has been shown that the addition of kaolin, magnesite, dolomite and sewage sludge significantly reduce the risk of agglomeration. The agglomeration temperatures increased with at least 60C in these cases. During combustion experiments, measured particle temperatures appeared to be up to 100C higher than the bed temperature. This might have a large influence on agglomeration. Because gasification, contrary to combustion, is a process where peak temperatures are lower or even absent, one might expect that agglomeration during gasification will occur at higher temperatures than during combustion. This however can not be concluded from the experiments, illustrating the importance of other factors like the design of fluid beds (the gas distribution, the type of nozzles, etc.). It can be concluded that not only the type of fuel and other chemical 'input' is determining the agglomeration temperature, also other factors like gas distribution, size of bed material, type of nozzles, cyclone efficiency in CFBs, etc. can have an important role. This means that results from lab-scale facilities can be interpreted in a relative way (comparing fuels and evaluate possible solution) but should always be used with great care when trying to draw conclusions for full-scale plants. Nevertheless, standardised lab-scale bubbling fluidized bed experiments, as developed and used in this project, seem to be the most reliable tools for the prediction of agglomeration. 119 refs.

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