The effect of particle size, temperature and residence time on the yields and reactivity of olive stones from torrefaction

Olive stones obtained as a by-product from olive oil extraction in combination with the favourable climate in Mediterranean countries are value-added feedstocks for the energy sector due to low moisture content ( 20 wt. %), suitable calorific value ( 18.7 MJ kg−1 as received) and high bulk density (about 750 kg m−3). The torrefaction process at Arigna Fuels with high energy efficiency of (above 90%) improves biomass properties for conversion to a high-value fuel for use in solid fuel stoves. This study reports the effect of moisture content, organic composition, inorganic matter, particle size, heat treatment temperature and residence time on product yields, O2/CO2 reactivity, calorific value, composition and thermal conductivity value of torrefied olive stones. Results showed that both lignocellulosic content and ash composition equally influenced the reactivity of torrefied material. For the first time, time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that the structure of torrefied material from small olive stone particles contains more cellulose than lignin when compared to large grains. Importantly from a technological standpoint, the lower heating values of torrefied olive stones (21.8 MJ kg−1) [1] from a small scale reactor were within the range of values for torrefied woodchip briquettes containing high starch binder content which was an energy increase of 15% when compared to the raw feedstock. The results showed that olive stones of particle size ≤ 2 mm produced during torrefaction at 270 °C for 30 min are the most suitable material and conditions for briquetting due to high solid yield, low reactivity and low thermal conductivity values. These conditions are recommended for the pilot plant operation using olive stones from the Mediterranean region.