Pilot-Plant Gasification of Olive Stone: a Technical Assessment

Abstract

A 150 kW th bubbling-fluidized-bed gasifier pilot plant was designed to study the gasification performance of different biomasses and blends. This paper presents the results of pilot-plant gasification tests carried out at atmospheric pressure and temperatures within the range of 700−820 °C in order to assess the technical viability of gasifying untreated olive stone, also called “orujillo”, a byproduct of the olive oil industry that comprises both olive stone and pulp. Different air flow rates and air-to-biomass ratios were used, giving an equivalence ratio (ER) within the range of 0.17−0.31. A series of parameters, such as gasification efficiency, thermal efficiency, gas yield, overall carbon conversion, gas quality, and composition, were measured as a function of ER. The results indicate that the operation temperature attained in the gasifier (controlled by the air-to-biomass ratio) has a strong impact on these parameters. These results reveal that the gasification process at higher temperatures is better as far as the carbon conversion and gas yield are concerned. However, the low heating value of the gas decreased with increasing ER and, thus, a maximum of the gasification efficiency was found around ER = 0.26. In addition, the high potassium content and low ash fusion temperature of the orujillo ashes were found to be responsible for the bed agglomeration problems. Thus, two inert materials (sand and ofite) were used, and their impact on fluidization quality was tested. The use of ofite turned out to be critical for a properly fluidized bed operation and for achieving long-term tests.