Evaluation of Pellets Produced with Undergrowth to be Used as Biofuel

Abstract

Many European countries are developing the use of pellets or wood briquettes for the supplying of local energy. Many industries are being forced to replace fossil fuels with biomass because of its neutrality in relation to CO2 emissions, thus creating an interesting market for pellets. In Portugal, the most common undergrowth includes: esteva (Cistus ladanifer), tojo (Ulex europaeus), giesta (Sarothamus scoparius), feto (Pteridium aquilium) and silva (Rubus ulmifolius). These undergrowth specimen are attractive due to the large volume available, but are not usually used for making pellets. It is possible to prevent large fires cleaning the forests, but this is an expensive task that could be compensated if the undergrowth can have some commercial value. The use of the common undergrowth for the manufacture of pellets and briquettes will allow the undergrowth to be considered not as a problem to be removed from the forest but an opportunity as renewable energy. The aim of this work is to obtain pellets with standard properties and evaluate the parameters that can qualify the performance of these pellets: moisture content, particle size distribution, high heating value, height of flame and burning time, pressure and temperature of the pelletizing process and mechanical durability. The pelletizing process was first simulated on laboratorial scale using small amounts of each vegetal specimen which was pressed in a heated mould. Some samples were submitted to a torrefaction treatment. After the laboratorial screening, the selected material was processed using a pellet mill to prepare pellets with 6 mm of diameter. The durability test was performed using a tumbling device following the ASAE S 269.4 standard. Burning tests of pellets with the same weight were performed and video images analysed, where height of flame and ignition time were measured. It was found that undergrowth pellets had good power of agglomeration and allow an easy ignition. The apparent density varies around 1,260 kg/m3. The moisture content of different pellet varied between 2.79 and 7.46 %, ash content was less than 3 %, the calorific value is between 16 and 18 kJ/kg, the durability results respect the values recommended by standards. After ignition the flame increases to an average height of 25 mm. The total burning time was in all cases dominated by the cinder combustion without visible flame and the total complete burning time varied between 400 and 600 s. The results showed that it is possible to produce pellets of undergrowth with physical and thermochemical characteristics similar to the existing in the market at the present time.