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/
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