Angelo Saraber, Marian Cuperus, and Jan Pels

Abstract A case study has been formulated concerning the use of ashes from combustion of cacao residues (shells) for electricity production and for nutrient recycling to the original soil. The effect in terms of kilograms of fertilizer per hectare and the environmental impact of closing the nutrient and mineral cycle are quantified. If the ashes are used as fertilizer, this fertilizer will only replace about 2% m/m of phosphorus and potassium that is necessary to fulfill the nutrient demand. This means that the contribution of the ashes is small. Furthermore, nitrogen has to be added as fertilizer. There is also a small advantage of reduction of CO2 emissions by nutrient recycling; this reduction is negligible from the point of view of the plantation, but from the point of view of the filter ash, the potential emission reduction is significant. This study shows that ashes from stand-alone combustion of certain agricultural residues are an potential valuable mineral source for elements such as phosphorus and potassium.

8.1 Introduction

Biomass is one of the sustainable sources of energy that is used for today’s production of electricity and heat. Sustainable use of biomass for energy production encompasses many aspects. They range from social aspects such as security of food supply and workers’ health to environmental aspects such as clean emission and protection of nature. Although interesting and relevant, this study is limited to only one of those aspects: the role of ash management in nutrient recycling and emission reduction. In Finland and Sweden, for instance, ashes from peat and wood combus­tion are utilized for fertilization in forestry (Emillson 2006). In 2004, about 27,0001

A. Saraber (H) and M. Cuperus,

KEMA, P. O. Box 9035, 6800 ET Arnhem, The Netherlands e-mail: asaraber@vliegasunie. nl

J. Pels

ECN, P. O. Box 1, 1755 ZG Petten, The Netherlands

H. Insam and B. A. Knapp (eds.), Recycling of Biomass Ashes,

DOI 10.1007/978-3-642-19354-5_8, © Springer-Verlag Berlin Heidelberg 2011 was used for forest fertilization in Finland. Certain wood ashes contain high amounts of calcium compounds, which make them suitable as an alternative liming agent. The calcium carbonate equivalent may be 26-59% depending on the source (Ohno and Erich 1990). Other biomass ashes may also be interesting for fertiliza­tion, such as ash from cacao residues as suggested by Simpson et al. (1985), which contain high amounts of potassium.

The Netherlands is one of the biggest producers of cacao products in the world, but all cacao beans are imported from other countries such as Cote d’Ivoire, one of the largest (40%) producers of cacao beans. Some of the process residues (shells) is currently used for direct co-combustion for power generation and some is used in gardening as soil cover. Cacao residues (shells) contain about 8-10% m/m ash-forming matter, which mainly consists of potassium and phosphorous, which are interesting nutrients for agriculture. The caloric value of cacao shells is about 19-22 MJ/kg (higher heating value), whereas the water content is 7-13% m/m (ECN 2010).

An interesting step forward would be to use the ashes from cacao residues as a source for nutrients by recycling the ashes back to the plantations where the cacao was grown or to use them as raw material for fertilizer production. In this case study, the impact on the nutrient balance and reduction of CO2, NOx, and SO2 emissions has been assessed. Disposal of the ashes in a mine is used as a reference.