Source ? T R Miles Technical Consultants Inc. 503-780-8185 [email protected] Sent from mobile.
On Feb 25, 2012, at 4:19 PM, Paul Olivier <[email protected]> wrote: > Crispin, > > I think that the following sums things up quite well. > > A Review on Biomass Torrefaction Process and Product Properties > > Grindability > > Biomass is highly fibrous and tenacious in nature, because fibers form links > between > particles and make the handling of raw ground samples difficult. During the > torrefaction > process the biomass loses its tenacious nature, which is mainly coupled to the > breakdown of the hemicellulose matrix and depolymerization of the cellulose, > resulting > in the decrease of fiber length (Bergman et al., 2005; Bergman and Kiel, > 2005). The > decrease in particle length, but not in diameter per se, results in better > grindability, > handling characteristics, and flowability through processing and > transportation systems. > During the torrefaction process the biomass tends to shrink; become > lightweight, flaky, > and fragile; and lose its mechanical strength, making it easier to grind and > pulverize > (Arias et al., 2008). Bergman and Kiel (2005) conducted studies on the energy > requirements for grinding raw and torrefied biomass like willow, woodcuttings, > demolition wood, and coal using a heavy duty cutting mill. They concluded > that power > consumption reduces dramatically, from 70–90%, based on the conditions under > which > the material is torrefied. They have also found that the capacity of the mill > increases by > a factor 7.5–15. The most important phenomenon they observed was that the size > reduction characteristics of torrefied biomass resulted in a similar product > as coal. > > Particle size distribution, sphericity, and particle surface area > > Particle size distribution curves, sphericity, and surface area are important > parameters > for understanding flowability and combustion behavior during cofiring. Many > researchers > observed that ground, torrefied biomass produced narrower, more uniform > particle sizes > compared to untreated biomass due to its brittle nature, which is similar to > coal. > Phanphanich and Mani (2011) study on torrefied pine chips and logging > residues found > that smaller particle sizes are produced compared to untreated biomass. They > have > also observed that the particle distribution curve was skewed towards smaller > particle > sizes with increased torrefaction temperatures. > Torrefaction also significantly influences the sphericity and particle > surface area. > Phanphanich and Mani (2011) results also indicated that sphericity and > particle surface > area increases as the torrefaction temperature was increased to 300°C. For > ground, > torrefied chips, they found that the sphericity increased from 0.48–0.62%, > concluding > that an increase in particle surface area or decrease in particle size of > torrefied biomass > can be desirable properties for efficient cofiring and combustion > applications. Also, the > bulk and particle densities of ground torrefied biomass increases as it > reduces the inter > and intra particle voids generated after milling (Esteban and Carrasco, > 2006). Studies > have indicated that ground torrefied material results in a powder with a > favorable size > distribution and sphericity, allowing it to meet the smooth fluidization > regime required for > feeding it to entrained-flow processes (gasifier and pulverized coal). > > Pelletability > > Torrefying the biomass before pelletization produces uniform feedstock with > consistent > quality. Densification following torrefaction is considered by several > researchers > (Lipinsky et al., 2002; Reed and Bryant, 1978 and Bergman et al., 2005). > These studies > indicated that the pressure required for densification can be reduced by a > factor of two > when material is densified at a temperature of 225°C and the energy > consumption > during densification is reduced by a factor of two compared to raw biomass > pelletization > using a pellet mill. Densification experiments were carried out on untreated > and torrefied > biomass using a piston press (Pronto-Press), which can be operated at > different > pressures and temperatures, to understand the densification behavior of > different types > of torrefied biomass. The pellets produced based on the TOP process had > higher bulk > densities, in the range of 750–850 kg/m3, with relatively high-calorific > value (LHV basis), > generally 19–22 MJ/kg. The energy density of TOP pellets ranged from 15–18.5 > GJ/m3 > and is comparable to subbituminous coal, which typically has a value of 21–22 > GJ/m3. > The pellets produced had a higher mechanical strength, typically 1.5–2 times > greater, > than the conventional pellets. The higher mechanical strength of these > pellets is due to > densification of the biomass at high temperature, which causes the biomass > polymers to > be in a weakened state (less fibrous, more plastic). Higher durable pellets > from torrefied > biomass can be due to chemical modifications, occurring during torrefaction, > that lead to > more fatty structures that act as binding agent. In addition, the lignin > content increases > by 10–15%, as the devolatilization process predominantly concerns > hemicellulose > (Bergman, 2005). > > Chemical composition of the torrefied biomass > > Besides improving physical attributes, torrefaction also results in > significant changes in > proximate and ultimate composition of biomass and makes it more suitable for > fuel > applications. Sadaka and Negi’s (2009) study on torrefaction of wheat straw, > rice straw, > and cotton gin waste at 200, 260, and 315°C for 60, 120, and 180 minutes > concluded > that moisture content was reduced at the extreme conditions (315°C for 180) > for all > three feedstock’s by 70.5, 49.4, and 48.6%, and the heating value increased > by 15.3, > 16.9, and 6.3%, respectively. Zanzi et al. (2002), in their study on > miscanthus > torrefaction made similar observations, where increasing temperature from > 230–280°C > and time from 1–3 hours increased the carbon content and decreased the > hydrogen, > nitrogen, and oxygen content. At 280°C, the carbon content increased to about > 52% > from an initial value of 43.5% while hydrogen and nitrogen content decreased > from > 6.49–5.54% and 0.90–0.65% for 2 hours of torrefaction. In general, increased > torrefaction temperatures result in increased carbon content and decreased > hydrogen > and oxygen content due to the formation of water, CO, and CO2. This process > also > causes the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios to > decrease > with increasing torrefaction temperature and time, which results in less > smoke and > water-vapor formation and reduced energy loss during combustion and > gasification > processes. In torrefaction studies of reed canary grass and wheat straw > torrefaction at > 230, 250, 270, and 290°C for 30-minute residence times, Bridgeman et al. > (2008) found > that the moisture content decreases from an initial value of 4.7%–0.8%. They > found that > carbon increased 48.6–54.3%, and hydrogen and nitrogen content decreased from > 6.8– > 6.1% and 0.3–0.1%, respectively. Bridgeman et al. (2010) in their studies on > torrefaction > of willow and miscanthus indicated that at higher temperatures and residence > times, the > atomic O: C and H: C ratios are closer to that of lignite coal. Table 6 shows > the effect of > different torrefaction temperatures on ultimate compositional changes in > woody and > herbaceous biomass. Table 2 and 3 indicates the elemental composition of the > torrefied > biomass at different temperatures and times. > > Off-gassing > > Storage issues like off-gassing and self-heating may also be insignificant in > torrefied > biomass as most of the solid, liquid, and gaseous products that are > chemically and > microbiologically active are removed during the torrefaction process. Kuang > et al. (2009) > and Tumuluru et al (2010) studies on wood pellets concluded that high storage > temperatures of 50°C can result in high CO and CO2 emissions, and the > concentrations > of these off-gases can reach up to 6% for a 60-day storage period. These > emissions > were also found to be sensitive to relative humidity and product moisture > content. The > same researchers at University of British Columbia conducted studies on > off-gassing > from torrefied wood chips and indicated that CO and CO2 emissions were very > low; > nearly one third’s of the emissions from regular wood chips at room > temperature (20°C). > The reason could be due to low moisture content and reduced volatile content > which > could result in less reactivity with the storage environment. > > Biomass is porous, often moist, and prone to off-gassing and self heating due > to > chemical oxidation and microbiological activity. In general, the biomass > moisture > content plays an important role in initiating chemical and microbial > reactions. Moisture > content coupled with high storage temperatures can cause severe off-gassing > and selfheating > from biomass-based fuels. Another important storage issue of ground torrefied > biomass is its reactivity in powder form, which can result in fire during > storage. It is > preferred to store the torrefied biomass in an inert environment to avoid > accidents due > spontaneous combustion. Kiel (2007) in his laboratory-scale combustion > studies of > torrefied wood found that it is highly reactive, similar to coal. > > Hydrophobicity > > An advantage of torrefied pellets over regular raw pellets is that they are > hydrophobic > (moisture uptake is almost negligible) even under severe storage conditions. > In general, > the uptake of water by raw biomass is due to the presence of OH groups. > Torrefaction > produces a hydrophobic product by destroying OH groups and causing the > biomass to > lose the capacity to form hydrogen bonds (Pastorova et al., 1993). Due to > these > chemical rearrangement reactions, non-polar unsaturated structures are > formed, which > preserve the biomass for a long time without biological degradation, similar > to coal > (Bergman and Kiel, 2005; Wooten et al., 2000). > > Bergman (2005) determined the hydrophobicity of torrefied pellets by > immersing them in > water for 15 hours. The hydrophobic nature was evaluated based on the state > of the > pellet after this period and by gravimetric measurement to determine the > degree of > water uptake. Bergman (2005) study indicated that raw pellets swelled rapidly > and > disintegrated into original particles. Torrefied pellets produced under > optimal conditions, > however, did not disintegrate and showed little water uptake (7–20% on mass > basis). > He also concluded that torrefaction conditions play a vital role in the > hydrophobic nature > of biomass. Sokhansanj et al. (2010) compared the moisture uptake of the > torrefied > biomass to the untreated biomass and found that there is about a 25% decrease > in the > water uptake when compared to the control (Figure 6). > > It is clear that the product characteristics of torrefied material like > handling, > milling, and transport requirements are similar to coal. In cofiring > operations torrefied > pellets allow for higher co-firing percentages up to 40% due to matching fuel > properties > with coal, and they can use the existing equipment setup for coal. > > Crispin, in gasifying rice hulls, we speak of a specific rate of gasification > which is measured in terms of kg's/m2/hour. > To have high gasifications temperatures (roughly from 800 to 1,000 C), the > rate of gasification has to be above 100 kg's. > If the rate is too low, only a small amount of gas is produced, and this gas > is of a very poor quality. > > But what would happen if the rate were turned down to only 20 kg/m2/hour? > Would this lower the temperature to less than 250 C? > Would the "biochar" from this low-temperature pyrolysis look like torrefied > biomass? > Of course the gas coming off this process would have to be cooled down and > processed. > > I could easily imagine a TLUD reactor of a diameter of 0.5 meters and a > height of a meter or two. > This reactor would be stuffed with rice straw and pyrolyzed at a very low > specific rate. > The gas would be cooled to condense out the water, > and it would be further processed to recover acetic acid and other compounds. > Would this not give a torrefied straw that could then be pelleted? > > Thanks. > Paul > > Thanks. > Paul > > On Sat, Feb 25, 2012 at 9:04 PM, Crispin Pemberton-Pigott > <[email protected]> wrote: > Dear Paul > > > > Thanks for the concise (distillation?) of facts about torrefaction. Just one > question: > > > > Torrefaction greatly reduces the amount of power needed for pelletizing. > > > > Can you give us a reference on that, or if not, can you suggest a general > rule about the reduction in energy requirement? That would be a valuable > number to remember. > > > > The point about processing of fuels is very reasonable. In the South Africa > they make paraffin out of coal. Zero sulphur… > > > > Regards > Crispin > > > > > _______________________________________________ > Stoves mailing list > > to Send a Message to the list, use the email address > [email protected] > > to UNSUBSCRIBE or Change your List Settings use the web page > http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org > > for more Biomass Cooking Stoves, News and Information see our web site: > http://www.bioenergylists.org/ > > > > > > -- > Paul A. Olivier PhD > 27C Pham Hong Thai Street > Dalat > Vietnam > > Louisiana telephone: 1-337-447-4124 (rings Vietnam) > Mobile: 090-694-1573 (in Vietnam) > Skype address: Xpolivier > http://www.esrla.com/ > _______________________________________________ > Stoves mailing list > > to Send a Message to the list, use the email address > [email protected] > > to UNSUBSCRIBE or Change your List Settings use the web page > http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org > > for more Biomass Cooking Stoves, News and Information see our web site: > http://www.bioenergylists.org/ >
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