Elsevier

Biomass and Bioenergy

Volume 61, February 2014, Pages 211-226
Biomass and Bioenergy

Conversion and leaching characteristics of biomass ashes during outdoor storage – Results of laboratory tests

https://doi.org/10.1016/j.biombioe.2013.12.014Get rights and content

Highlights

  • The main reaction during storage is the formation of Ca(OH)2 from CaO and water.

  • Water is also attached to amorphous or poorly crystallized phases during storage.

  • Ca and Mg leaching ratios decrease while K and Na leaching ratios increase over time.

  • The optimum water mixing rate depends on the Ca mass fraction in the ash.

Abstract

The laboratory storage tests were performed with different ash fractions from fixed and fluidized combustion plants utilizing wood chips and bark. There, the ash fractions were stored under varying conditions (dry, wet, open, airtight) over 16 weeks to investigate the changes in physical and chemical properties of the ashes. The results show that the main chemical reactions during storage are the formation of Ca(OH)2 from CaO and H2O and a water uptake by poorly crystallized or amorphous phases. The ash samples showed only a rather small increase in the TIC mass fraction so the carbonatation of Ca(OH)2 with CO2 to form CaCO3 played only a minor role during the time period investigated. Other reactions like the formation of ettringite or gypsum (due to the small amount of S contained in the ashes investigated) were not observed. Regarding the leaching characteristics, the Ca leaching ratio decreases over time while the K and Na leaching ratios increase. The results regarding the water demand during storage indicate that the mass fraction of Ca in the dry material in the ashes is a suitable parameter to determine the optimum water admixing rate to facilitate aging of the ashes.

Section snippets

Introduction and objectives

In recent years, the promotion of energy production from biomass in Austria and the European Union has led to a strong increase in the amount of combustion residues, i.e. ashes. Finding ways to utilize these ashes in an environmentally and economically efficient manner is thus an important goal throughout Europe.

The utilization of the nutrient rich and rather heavy metal poor wood ash fractions (bottom and coarse fly ash) for fertilizing and soil improvement purposes is already implemented in

General

In the laboratory tests four different wood ash fractions from two combustion technologies were stored under different storage conditions (dry/wet, open or in absence of air). The ash fractions investigated were bottom ash as well as a mixture of bottom ash and coarse fly ash from a grate furnace/fixed bed furnace (GF) with a fuel power input of 10 MW, fired with bark and wood chips, boiler fly ash from a bubbling fluidized bed furnace (BFBF) with a fuel power input of 43 MW, fired with wood

Particle size distribution

The particle size distribution of the original untreated ash fractions is shown in Fig. 2.

The particle size distribution of the untreated ash fractions varies significantly between the ashes from a grate furnace and the ashes from the fluidized bed boilers. The ashes from a grate furnace show, apart from the expected higher mass fraction of particles smaller than 400 μm in the mixture of bottom ash and coarse fly ash, a rather similar particle size distribution with still 30% dry mass fraction

Conclusions

The results of the laboratory storage test of different biomass ash fractions show that the main reactions under the conditions given during the laboratory storage test, at least over the first 16 weeks, are the formation of Ca(OH)2 from CaO and water and the attachment of water to amorphous or poorly crystallized phases by hydration or hydroxylation. The overall increase of the TIC mass fraction in the dry material of all ash samples was rather low, so the carbonatation of Ca(OH)2 with CO2 to

Acknowledgments

The authors are grateful to the Austrian Research Promotion Agency (FFG) for funding and the FHP (Association of the Austrian Wood Industries), namely Rainer Handl, for funding and supporting this project.

The authors are grateful to Tanja Gollinger (Bioenergy2020+ GmbH, Austria) and Tereza Rodriguez (MSc. Student, Graz University of Technology, Austria) for the good cooperation and their support regarding the organization and performance of the laboratory tests. The authors are grateful to

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