Evaluation of equilibrium moisture content in ligno-cellulosic residues of olive culture

The use of ligno-cellulosic residuals from agriculture as means for renewable energy production is well known; nonetheless significant problems still exist on development of bioenergy value chains. Moisture content and hygroscopicity are among these problems. Higher moisture content of the biomass means lower calorific value and higher perishability, hence storage difficulties. For this reason it is important to evaluate how the moisture content of the material varies while it is stored and how the calorific value of the feedstock is affected by it. The purpose of this study is to determine the variation of moisture content and its influence on lower calorific value (LCV) of four types of solid residual biomass: oven dried chipped olive tree trimmings, milled olive pomace; oven dried olive tree trimmings and fresh olive tree trimmings. Moisture content of the samples is calculated by means of thermogravimetric analysis (TGA) according to standard CEN/TS 14774-2:2004, while the LCVs are calculated by means of a calorimeter, following standard UNI EN 14918:2010. All the biomass samples were left to reach equilibrium moisture content (EMC) in a temperature and humidity controlled environment. Two different phases were analyzed with respect to moisture uptake rates: 1) fast moisture uptake rate phase (first four hours of exposure) and 2) slow moisture uptake rate, (the days following the first four hours of exposure). Samples experiencing fast moisture uptake rate, during the first four hours of exposure, were kept in a monitored ambient at T=22 ± 1oC and RH=59 ± 2%, while samples exposed to slow moisture uptake rate were kept in a climatic chamber at three different set of temperature and humidity controlled environment simulating the climatic conditions in different periods of the year in Enna province: 10oC - 80% RH (winter), 15oC - 70% RH (spring/autumn) and 20oC - 55% RH, (summer). The results obtained show that the olive pruning chips (0.425 mm to 1.00 mm and 1.00 mm to 2.00 mm particle size ranges) stored in a heap in a controlled climatic chamber require approximately 20 days to reach EMC. Depending on the particle size range EMCs reach the values of 6.2 and 7.5% by weight in the "summer" condition, 14.3% and 16.9% in "spring/autumn" condition, 24.1% and 28.2% in "winter" condition. Moisture is absorbed gradually over time and results show that in the first four hours, the dry sample exposed to ambient condition (T=22 ± 1 oC and RH=59 ± 2%) reaches a moisture content between 0.75 and 0.96% of its weight; the LCV at equilibrium is evaluated between 18, 576 J•odg-1 and 18, 793 J•odg-1, the higher value related to the bigger particle size range examined. Pomace heap under the same experimental set up and time period, reaches an equilibrium moisture content of 8.5% and 9.7% (summer conditions), 19.2% and 22.0% (spring/autumn condition) and between 30.9% and 34.1% (winter condition). Olive pomace accumulates moisture relatively faster than olive trimmings as dry sample recovers about 1.61% to 1.97% of moisture in the first four hours of exposure. The LCV at equilibrium is determined between 20, 145 J•odg-1 and 20, 436 J•odg-1. Pruning dried samples reach an EMC equal to 6.1% in approximately 25 days, with a LCV equal to18, 921 J•odg-1 ± 1.9%. Heaps of fresh prunings reach an EMC of 8.2% in approximately 20 days; LCV of the feedstock is 19, 356 J•odg-1± 1.6%.©2006-2014 Asian Research Publishing Network (ARPN). All rights reserved.