Quality of olive stone as a fuel: Influence of oil content on combustion process

Olive stones are being widely used as a fuel in Southern Europe, especially in Spain, where a standard that grades their quality for combustion has been developed. Oil content is one of the parameters considered, although no studies about its influence on the combustion process have been found in literature. In order to determine the quality of the olive stones marketed in Spain, a sampling plan was designed and the collected samples were characterised. Most of the samples showed good characteristics for being used as a fuel. It can be noticed that higher fractions of impurities (pulp, skin and stalks) caused lower bulk densities and higher net calorific values, ash, fines below 1 mm, oil and nitrogen contents. With the aim of studying the influence of oil content in the combustion of olive stones, samples with four different oil contents (between 0.16 and 2.0%) were used in combustion tests in two domestic devices (boiler and stove). Oil contents above 1% resulted in an increase in CO, TOC, TSP, PM1 and PM0.1 emissions, which was noticeable when using olive stones with 2% oil content. NOx emissions increased as the oil content raised, due to the higher N content in the fuel.