Rogelio Nogales, Gabriel Delgado, Mar Quirantes, Manuel Romero, Esperanza Romero, and Eduarda Molina-Alcaide

Abstract Wet and dry olive cakes are the most important wastes generated when olive oil is produced. In recent years, both olive wastes have been incinerated to produce electricity, and thereby large amounts of fly and bottom ash are generated. In this study, physical, physicochemical, and chemical characteristics of olive waste ashes produced in Andalusian biomass power plants were analyzed to evaluate their suitability for agriculture. High variability among fly and bottom ashes may be ascribed to the origin of the olive waste and the combustion temperature. Waste olive ashes, which contained all particle sizes, showed high values of pH, salinity, water holding capacity, calcium carbonate equivalent, and P, K, Cu, and B contents. In contrast, moderate values were recorded for Ca, Mg, Zn, and Ni. Nitrogen is scarce in olive waste ashes; they thus can only be part of any fertilization strategy.

5.1 Introduction

The renewable energy consumption in the EU will increase from 11.6% in 2009 to 20% by 2020. Biomass is the main (61%) resource for the renewable energy consumed (10th EurObserv’ER 2010). In Spain, the contribution of renewable energy tp total gross domestic consumption in 2008 was 7.6%, of which biomass up 5.1 Mt (47% of total renewable energy, INE 2010).

Olive oil production is one of the most important industries in Mediterranean countries. In Spain, olive tree cultivation mainly occurs in Andalusia. In 2008, 600 x 103-900 x 103 t olive oil was produced in this region, using the two-phase centrifugation systems as common (90%) extraction technology (Fig. 5.1). This system generates huge amounts (between 2.5 and 3.5 Mt/year in Andalusia) of a

R. Nogales (H), M. Quirantes, M. Romero, E. Romero, and E. Molina-Alcaide Estacion Experimental del Zaidin (CSIC), Profesor Albareda, 1, 18008 Granada, Spain e-mail: rogelio. nogales@eez. csic. es

G. Delgado

Department of Edaphology, University of Granada, Campus Universitario de la Cartuja, 18071 Granada, Spain

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

DOI 10.1007/978-3-642-19354-5_5, © Springer-Verlag Berlin Heidelberg 2011

waste called crude wet olive cake, or alperujo, which is composed of olive pulp, stones, and skins together with residual oil, and water added during the oil extrac­tion process. Alperujo is an acidic, semihumid waste, rich in organic matter and potassium. After drying (less than 1% moisture), its low heating values ranges from 15 to18 MJ kg-1. Part of this waste is dried and treated with solvents to obtain olive-cake oil and a waste called dry olive cake, or orujillo. The orujillo (0.6-0.9 Mt/year in Andalusia) has recalcitrant organic matter, high potassium content, and low heating values between 14.5 and 20 MJ kg-1 (Agencia Andaluza de Energia 1999; Alburquerque et al. 2004; Caputo et al. 2003; Nogales et al. 1998).

In recent years, both olive wastes are being used as fuel for electrical energy production. In Andalusia, ten biomass power plants have been established and produced 0.81 TWh (80% of the total renewable electricity generation in this region) in 2009. In general, the previously mentioned olive wastes, alone or mixed with other wastes, are burned in conventional boilers at 450°C. The heat released is used to heat water to turn a steam turbine, which generates green electric energy. In some biomass power plants, fluidized-bed combustors at 850°C are used as boilers. The combustion of olive wastes for energy production generates great amounts of fly ash and bottom ash (between 4 and 8% of the total burned olive wastes) as end waste. In general, this end waste is landfilled in sites adjacent to the biomass power plants. However, the use of landfills for ash disposal is expensive and is being discouraged by more stringent regulations and public opposition.

Numerous studies focused on chemical characteristics of ash produced by coal combustion or gasification (Ahmaruzzarman 2010; Hytonen 1998; Jala and Goyal 2006). In contrast, ash generated from biomass has received less attention and most of the research has focused on ash from woody biomass combustion (Demeyer et al. 2001; Kuba et al. 2008; Someshwar 1996; Vance 1996; see Chap. 6, Omil et al. 2011). Other ashes have been characterized, such as those from the pulp and paper industry (Naylor and Schmidt 1989; Muse and Mitchell 1995) and those produced
by the incineration of municipal solid waste (Zhang et al. 2002) and biosolids (Benitez et al. 2001; Merino et al. 2005) or other agricultural wastes (Mozaffari et al. 2000). However, information is not available concerning the characteristics of ashes from olive waste combustion.

The aim of this study was to analyze the physical, physicochemical, and chemi­cal characteristics of fly ash and bottom ash produced in Andalusian biomass power plants, which use olive wastes (wet and dry olive cakes) as fuel, to evaluate their suitability for use in agriculture.