[Stoves] Is there a role for combining torrefaction and char-making stoves?

[Tom Miles Easystreet]

Source ?

T R Miles Technical Consultants Inc. 503-780-8185
tmiles at trmiles.com
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On Feb 25, 2012, at 4:19 PM, Paul Olivier <paul.olivier at esrla.com> 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 <crispinpigott at gmail.com> 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
> 
>  
> 
> 
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> 
> 
> 
> -- 
> Paul A. Olivier PhD
> 27C Pham Hong Thai Street
> Dalat
> Vietnam
> 
> Louisiana telephone: 1-337-447-4124 (rings Vietnam)
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