Study of Lignocellulosic Biomass Pyrolysis : State of the Art and Modelling

Abstract: Fast pyrolysis is a technology that is emerging as an alternative to transform organic and fossil materials into a new form of energy, mainly the so-called bio-oil. Until nowadays, examples of raw materials that have been experimented are wood and forest residues, bagasses as well as straw and agro-residues. After pyrolysis conversion, the final products are solids (char), liquids (tar) and gases. Through several experimental studies, authors have been found that there are parameters which influence final products quality. They are feedstock composition, type of reactor, final temperature, heating rate, sweeping gas flow rate, vapors residence time and particle sizes. In feedstock composition, the quantities of cellulose, hemicellulose, lignin, ash and water have received special attention. Usually, feedstock is pre-heated to reduce water content and grinded to particle sizes smaller than 1.8 mm; before it enters into the reactor. Among different reactor configurations, the most common is the fluidized bed reactor at industrial and laboratory scale. Generally, the reactor is designed and constructed for laboratory experiments. It has been reported that final temperature and heating rate greatly affect final products. Authors coincide that higher temperature increases overall conversion and liquid yields present a maximum. However, for higher heating rates there is a controversy in liquid yields. During pyrolysis, an inert gas is used to sweep the gases and to diminish vapors residence time. It is believed that at longer vapors residence time, secondary reactions between the gases and the char may occur. The most common sweeping gas is nitrogen followed by helium and argon. In order to study experimentally these parameters, a test bench will be built at the “Center for Energy and Processes” at the Ecole des Mines de Paris. Wood samples will be used as a feedstock and will pyrolized under nitrogen in a horizontal quartz reactor. The sample will be heated by radiation, emitted at a constant flux by a radiant. During the experiments, temperature, heating rate, sweeping gas, and particle sizes will be varied in order to observe their effect in final yields. Based on the kinetics equations proposed by Radmanesh et al., a computational model was created with the experimental conditions. The sample was modeled as a set of five layers, considering each one as a porous media. For each layer, a heat transfer coefficient was calculated as the sum of conduction and radiation coefficients. The results illustrate the evolution of temperature, heating rates, total gases, condensable and noncondensable gases at different radiations and particle sizes. They show agreement with the results obtained by Radmanesh et al. as well with other experimental studies. In addition, evolution of  ellulose, hemicellulose and lignin were observed through the time and according to the layer temperature, demonstrating concordance with literature and previous studies.