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

Sat Feb 25 20:41:46 CST 2012

A search on the title gives:

www.inl.gov/technicalpublications/Documents/5094547.pdf

On Sun, Feb 26, 2012 at 8:09 AM, Tom Miles Easystreet <tmiles at trmiles.com>wrote:

> Source ?
>
> T R Miles Technical Consultants Inc. 503-780-8185
> tmiles at trmiles.com
> Sent from mobile.
>
> 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)
> Mobile: 090-694-1573 (in Vietnam)
> Skype address: Xpolivier
> http://www.esrla.com/
>
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-- 
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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