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

Sat Feb 25 18:19:58 CST 2012

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

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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