5 research outputs found
Release of Chlorine and Sulfur during Biomass Torrefaction and Pyrolysis
The release of chlorine (Cl) and
sulfur (S) during biomass torrefaction
and pyrolysis has been investigated via experiments in two laboratory-scale
reactors: a rotating reactor and a fixed bed reactor. Six biomasses
with different chemical compositions covering a wide range of ash
content and ash-forming elements were torrefied/pyrolyzed in the temperature
range of 150–500 °C. The relative release of chlorine
and sulfur was calculated based on mass balance and analysis of the
biomass before and after torrefaction. In selected cases, measurement
of methyl chloride (CH<sub>3</sub>Cl) in the gas from straw torrefaction
has furthermore been conducted. The release of chlorine from straw
was first observed at 250 °C and peaked with about 60–70%
at 350 °C. Analysis of the released gas showed that most of the
chlorine was released as methyl chloride. Increasing the straw content
in the reactor resulted in a lower fractional release of Cl, probably
due to more reactive sites in contact with gas phase Cl species leading
to secondary binding of Cl to the solid product. Almost complete release
of chlorine was observed for woody biomass at 350 °C. This result
is in agreement with previous studies reporting that biomasses with
a lower chlorine content release a higher fraction of chlorine during
the pyrolysis process. A significant sulfur release (about 60%) was
observed from the six biomasses investigated at 350 °C. The initial
sulfur content in the biomass did not influence the fraction of sulfur
release during torrefaction
Influence of Biomass Chemical Properties on Torrefaction Characteristics
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
Efficient Fuel Pretreatment: Simultaneous Torrefaction and Grinding of Biomass
Combining torrefaction and grinding of biomass in one reactor may be an attractive fuel pretreatment process. A
combined laboratory torrefaction and ball mill reactor has been constructed for studies of the influence of temperature and residence time on the product yields and particle size reductions of Danish wheat straw, spruce chips, and pine chips. On the basis of initial experiments, which evaluated the influence of reactor mass loading, gas flow, and grinding ball size and material, a standard experimental procedure was developed. The particle size reduction capability of the torrefaction process has been
evaluated by the relative change in d50, and this method was compared to the Hardgrove grindability index (HGI), showing reasonably similar results. Significant differences in torrefaction behavior have been observed for straw and spruce chips torrefied at 270−330 °C. Torrefaction of straw for 90 min yielded a higher mass loss (27−60 wt %) and relative size reduction (59−95%)compared with spruce (mass loss of 10−56 wt % and size reduction of 20−60%). The two types of biomass investigated differ with respect to hemicellulose type, lignocellulosic composition, particle morphology, and ash composition, where straw has a higher alkali content. This and other studies indicate that the large difference in the alkali contents of the biomasses is the main cause for the observed difference in torrefaction characteristics. Experiments with separate particle heating and grinding showed a swift grinding of the torrefied biomass. This implies that the rate-limiting step in the laboratory reactor is the heat transfer and not the grinding process. Large pine particles (8−16 mm) showed a slightly higher mass loss than 4−8 and <4 mm particles. This
could be the consequence of exothermic reactions in the particle core, which locally increase the temperature and conversion