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Pulverized biomass may self-heat and spontaneously ignite when stored or processed at intermediate or even low temperatures. In this work, reaction kinetic parameters for biomass oxidation and pyrolysis were determined for the temperature range 423–523 K. Thermogravimetric analysis was used to determine mass loss kinetics in a stepwise-isothermal heating program. Two wood species (pine and beech), two agricultural residues (wheat straw and sunflower husks), and two commercial wood pellet samples were investigated. Atmospheres with 0, 20, and 80% oxygen were used in the experiments. A pyrolysis model of four parallel reactions for extractives, hemicellulose, cellulose, and lignin fit the experimental data for 0% O2 well. Oxidation kinetics could be modeled by additional reactions in parallel to the pyrolysis mechanism. Two mechanisms were tested: (1) considering oxidation of a lumped “volatilizable” component plus oxidation of char; and (2) separate oxidation reactions for volatilizable extractives, hemicellulose, cellulose, and lignin, plus char. The more complex mechanism did not give a clear advantage over the simpler mechanism. It was further found that pyrolysis and oxidation reactions for the components could be modeled with the same activation energy, regardless of which biomass they appear in. For the lumped component oxidation model, an apparent activation energy of 130 kJ/mol was found. The observed reaction order in oxygen was in the range 0.4–0.5. The models also compared favorably to additional experimental data between 373 and 773 K for a heating rate of 5 K/min. The kinetic models presented here are intended mainly to describe low-temperature reactions, such as self-heating of biomass and the onset of smoldering combustion.

Replacing coal with pelletized biomass on pulverized-fuel CHP-plants has led to an increased fire risk in storages and processing facilities. Accurate models to predict this behavior are scarce, mainly owing to a lack of reaction kinetic data for low temperatures.

Self-ignition temperatures determined in the framework of conventional thermal ignition theory does not explain why biomass is much more susceptible to spontaneous ignition in power plant mills or storages. Examining the onset of reactions at low temperatures may provide a better understanding of the process, which can then be incorporated into refined models of self-ignition for biomass and other organic solids. In the present study, the slow, transient heating of several lignocellulosic biomasses and a bituminous coal from ambient temperature to around 300° C were investigated in a lab scale tube oven, with sample sizes between 11–40 g. Tests were carried out under oxidizing (20 % O2) and inert atmospheres. Judged by off-gas measurements of CO and CO2, a reaction onset could be seen at temperatures below 100° C. Under oxidizing atmosphere, reactions were more intense and set off earlier, suggesting that a heterogeneous oxidation is the dominating mechanism in self-ignition. It could also be shown that both mechanisms compete for reactive material. While oxidation was exothermic, pyrolysis was largely thermally neutral in these experiments. Reaction behavior was seen to depend highly on the material, and the results indicate that higher ash contents may promote reaction onset. However, further work is needed to arrive at a comprehensive model of self-ignition.

Pelletization of biomass residues increases the energy density, reduces storage and transportation costs and results in a homogeneous product with well-defined physical properties. However, raw materials for fuel pellet production consist of ligno-cellulosic biomass from various resources and are therefore different in their composition and structural properties. This has the consequence that different types of biomass require different processing conditions such as press channel length and moisture content. Nowadays the process optimization is mainly based on expensive and time consuming “trial and error” experiments and personal experience. However in recent years the utilization of single pellet press units for testing the biomass pelletizing properties has attracted more attention. The present study outlines an approach where single pellet press testing is combined with modeling to mimic the pelletizing behavior of new types of biomass in a large scale pellet mill. This enables a fast estimation of key process parameters such as optimal press channel length and moisture content. Secondly, the study addresses the question of the origin of the observed relationship between pelletizing pressure and temperature.

peer-reviewed, This study aims to provide a geometrical description of biomass particles that can be used in combustion models. The particle size of wood and herbaceous biomass was compared using light microscope, 2D dynamic imaging, laser diffraction, sieve analysis and focused beam reflectance measurement. The results from light microscope and 2D dynamic imaging analysis were compared and it showed that the data on particle width, measured by these two techniques, were identical. Indeed, 2D dynamic imaging was found to be the most convenient particle characterization method, providing information on both the shape and the external surface area. Importantly, a way to quantify all three dimensions of biomass particles has been established. It was recommended to represent a biomass particle in combustion models as an infinite cylinder with the volume-to-surface ratio (V/A) measured using 2D dynamic imaging.