Thermochemical Conversion

The thermochemical conversion of biomass is based on processes of combustion, gasification and pyrolysis, which involve temperatures higher than 400°C in oxidizing environments. These operating conditions cause severe changes in the molecular structures of the raw material and the generation of various gaseous products; the more oxidizing the reaction medium is and higher the temperature is, the simpler the gaseous compounds.

Combustion

The combustion involves the oxidation of carbon to carbon dioxide and hydrogen to water. The combustion of biomass takes place under very oxidizing atmosphere. The reaction products are primarily gases, leaving a solid residue whose composition depends on the mineral material content of the fuel. The main technical characteristics that must be taken into account for the combustion of a biomass are the calorific value, ignition temperature, combustion rate, content and characteristics of ash, and moisture content. In regard to olive stones:

• Calorific value.

Olive stones have a relatively high calorific value when compared with other lignocellulosic materials. Some data of higher heating value (HHV), in descending order, are provided by Jenkin et al. (1998)-5200 kcal/kg [56], Mata-Sanchez et al. (2013)-4895 kca/kg [57], Yanes Duran (1985)-4700 kcal/kg [58], Doymaz et al. (2004)-4679 kcal/kg [59] Gonzalez et al. (2004)-4600 kcal/kg [60] and Garcia et al. (2012)-4279 kcal/kg [14]. That is, it can be considered a HHV value close to 4800 kcal/kg. Mata-Sanchez et al. (2013) calculated an average lower calorific value of 4581 kcal/kg from 30 different olive stones [57]. The calorific value is affected by the presence of pulp, whose HHV is 5307 kcal/kg [59].

• Ignition temperature and combustion rate

Olive stones have a relatively low ignition point (about 215°C) while the maximum combustion rate (0.341 1/min) is achieved at 284°C [12].

• Ash content and melting temperature

Olive stones not only contain little mineral matter (Table 1) but also have a melting temperature ash exceeding 1400°C [5], which helps to reduce costs associated with cleaning the burners.

• Moisture percentage

This parameter is very important, since an excess of water in fuel causes a decrease in combustion efficiency [61] and problems during transport and storage of the material. Gomez de la Cruz et al. (2014) reported initial and equilibrium moisture of 23% and 8%, respectively, in olive stones provided by olive mills [62]. These authors also analyzed the drying kinetics of olive stones at four temperatures (100, 150, 200 and 250°C) and three thicknesses of the layer of solids (10, 20 and 30 mm).

At present, most olive stones end up being burned to generate heat energy because besides presenting high calorific values and low percentages of sulfur, ash and chlorides, the bulk density is relatively high (between 573 and 709 kg/m3 [5, 13, 63]) without applying particle size reduction. According to Manya et al. (2007) solids milled to 0.3-0.5 mm size would have a bulk density of 749 kg/m3 [17].

Furthermore, the fuel supply to the burners is simpler than when softer and more fibrous wastes are used, thus reducing clogging metering devices. The average price of fragmented endocarps derived from olive mills in Spain was between 150 and 180 euros per ton in the past two years.

 
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