Influence of Biomass Chemical Properties on Torrefaction Characteristics

Abstract

Different biomass types may differ with respect to torrefaction characteristics, and an improved understanding and ability to predict the torrefaction performance is, therefore, desired. In this study, the influence of the chemical properties (lignocellulose composition and alkali content) on the torrefaction behavior with respect to mass loss and grindability is investigated by simultaneous thermal analysis (STA) and by using a combined torrefaction and grinding reactor. The torrefaction behavior of six raw biomass samples and selected pretreated samples (washed and impregnated with KCl and K2CO3) has been studied. The investigated biomasses have reasonably similar carbohydrate compositions (hemicelluloses 18–25 wt % db; cellulose 38–48 wt % db; lignin 17–29 wt % db) with the exception of spruce bark, which is lower in hemicellulose content (12.9 wt % db) and cellulose content (24 wt % db), and higher in lignin content (36.8 wt % db). An increasing biomass potassium (K) content decreases the temperature of maximal conversion for both raw and alkali-impregnated biomass samples, thus decreasing the solid product yield at 270 and 300 °C. This was especially pronounced when the biomass potassium content increased from 0 to 0.2 wt %. However, the higher lignin content in bark causes a higher solid yield than what would be expected from the alkali content, illustrating that both potassium content and lignocellulose composition affect the solid yield obtained by torrefaction. The grindability of the torrefied products was evaluated by determining the d50 value of the particle size distribution of the biomass before and after torrefaction in the combined torrefaction and mill reactor. A significant decrease in d50 value was observed when the alkali content increased from 0 to 0.2 wt % db, whereas no additional effect is seen for higher potassium contents.

Influence of fast pyrolysis conditions on yield and structural transformation of biomass chars

“…The demineralization of wheat straw decreases the required holding time for complete pyrolysis from 2 to 1 s at 1000 ∘ C. The leached wheat straw of size N 0.425 mm obtained a higher char yield at 600 ∘ C than untreated straw within 1 s. The present results show that at lower temperatures a high alkali content induces lower char yield, similarly to results of Saleh et al [2]. Moreover, the holding time effect on the weight loss is not particularly dependent on the straw particle size.…”

“…Fine biomass milling is energy-consuming, and fibrous particles with low bulk densities may cause feeding problems. Large biomass particles may create challenges with flame stability, burnout and ignition [1,2]. The challenge of achieving high fuel conversion at short residence times depends on biomass origin, morphological changes of fuel particle of different sizes, char yield, fuel reactivity, heating rate, local oxygen concentration and temperature in the boiler [3,4].…”

Effects of water washing and torrefaction pretreatments on rice husk pyrolysis by microwave heating

“…Some researchers (Saddawi et al, 2012;Saleh et al, 2013) have reported that potassium had the effect on reducing the degradation temperature and promoting volatile release during the process of torrefaction. Although the mass yield of W-RH during the process of torrefaction pretreatment was significantly higher than that of RH, the higher heating values between them had no obvious difference.…”

One way of representing the size and shape of biomass particles in combustion modeling

“…Thus, CO 2 released with biofuels can be reconsumed faster by plants via photosynthesis than the time needed to regenerate coal. The milling process is a necessary step in suspension firing [1]. Size reduction improves fuel conversion processes because of the creation of larger reactive surface areas [2,3].…”

“…The accuracy of particle models depends on both correct size distribution and characterization of fuel inhomogeneity in terms of shape and structure. The objective of this study is twofold: (1) to provide a geometrical description of biomass particles that can be used in combustion model; (2) to make suggestions for the size and shape of biomass particles. In this work, the biomass particles' size and shape are characterized by using both 2D dynamic imaging analysis and microscopy.…”