(21c) Influence of Process Conditions on Properties of Ash From Biomass Gasification

Influence of process conditions on
properties of ash from biomass gasification

Naomi Klinghoffer¹, Marco J. Castaldi¹,
Ange Nzihou²

¹Columbia
University Department of Earth and Environmental Engineering

²Ecole
des Mines d'Albi Carmaux, Department of Chemical and Environmental Engineering

Abstract

Biomass
is likely to be a significant energy resource in the future. A common way to
recover energy from biomass is through gasification where synthesis gas is
produced; a by-product of this process is ash. This research investigates the
potential to use the ash as a catalyst by understanding the properties of ash
generated under different gasification conditions. Specifically, it is desired
to produce a porous ash which could be used as a catalyst or as a support for
more catalytically active metals.

In this work, poplar
wood was gasified under CO₂, steam, and air at different reaction
temperatures. Experiments were done in a fluidized bed reactor at temperatures
of 750^oC and 920^oC and ash was recovered. BET-surface
area measurements showed that ash from gasification under CO₂ had a
higher surface area than ash produced from steam gasification. TGA experiments
showed that gasification under CO₂ resulted in a higher mass loss
compared to gasification with steam. Gasification with steam/CO₂
mixtures yielded a mass loss similar to that of steam only which could be
indicative of competitive reactions between steam and CO₂.

Introduction

Gasification
is a promising technology to yield a usable gaseous product from biomass or
waste with high efficiency.  There are three main components that are produced
from gasification: light gases (such as H₂, CO, and CO₂)
that can be used for synthesis of fuels or combustion applications, tars
(heavier hydrocarbons), which need to be removed as they can cause problems
with downstream equipment, and ash, which is made up of minerals and metals
that do not enter into the gas phase. It has been shown that the
characteristics of the ash (for example, composition and porosity) can be
affected by the reactive gases that are used for gasification⁽¹⁾.
The porosity of the ash, combined with high content of metals and minerals,
makes ash a good candidate to be used as a catalyst⁽²⁾. It is common
that ash generated from gasification of biomass or waste is either sent to a
landfill or used in concrete. Use of ash in catalytic applications would
increase its value as well as avoid the need for more expensive catalysts. This
research investigates the potential to use the ash as a catalyst by
understanding the properties of ash generated under different gasification
conditions. Specifically, it is desired to produce a porous ash which could be
used as a catalyst or as a support for more catalytically active metals.

Experimental

In
this work poplar wood was gasified and analyzed at three different scales.
Gasification was done in an in-house built fluidized bed reactor; various
flow-through gases were used and results are presented here from experiments
done under 10% CO₂ in N₂ and 90% H₂O in N₂.
The reactor was heated up to a maximum temperature of 750^oC or 920^oC
and held at the maximum temperature for 1 hour or 30 minutes. The top of the
reactor was equipped with a porous frit therefore all ash was trapped inside
the reactor and recovered after experiments. BET surface area of the ash
samples was measured (Micrometrics Gemini). TGA/DSC (Setaram 111) experiments
were done where the biomass samples were heated to 800^oC at 20^oC/min
Experiments were done at different concentrations of CO₂ and H₂O
in N₂. Gasification was also done in an environmental scanning electron
microscope (FEI XL30) to observe the changes in physical properties of the
sample during gasification. Gasification was done under air, CO₂ and
H₂O and images were recorded throughout the experiments.

Results
and Discussion

After
fluidized bed experiments, the ash was recovered and BET-surface area was
measured. Results are shown in Figure 1. When comparing surface area of ash
generated under steam at 750^oC, it is clear that longer gasification
time leads to a higher surface area ash.  However, mass recovery data shows
that the mass of ash recovered is similar in both cases; for 30 minutes 5.60%
of the initial mass is recovered as ash and at 1 hour 4.95% of the initial mass
is recovered.  TGA tests were also done with steam at 800^oC (with
9%H₂O in N₂); after 30 minutes at 800^oC the
mass loss is 80.5% and after 1 hours mass loss is 82.7%. Therefore, both TGA
tests at low H₂O concentrations and fluidized bed experiments at
high concentrations of H₂O both show that mass loss is very gradual
at temperatures in the range of 750-800^oC. However, the BET surface
area of the ash from fluidized bed experiments showed very different surface
areas based on the time spent at maximum temperature. At 1 hour the surface
area of the ash sample is 621m²/g and at 30 minutes the surface area
is 429m²/g. This indicates that it is possible to produce ash with
significantly different properties without necessarily influencing overall
production of gas/tars. This is important because if it desired to produce ash
with specific properties (for example, higher porosity), one would want to do
this without impacting overall gas yield.

 A.

B.

Figure
1
Ash recovered from fluidized bed experiments: A. surface area B. mass recovery

Gasification
was done in the fluidized bed at 750^oC for 30 minutes under CO₂
(10% CO₂ in N₂) and H₂O (90% H₂O in
N₂) and BET-surface area was measured.  The mass recovered was
15.44% under CO₂ and 5.6% under H₂O; it is expected that
the mass loss would be higher under steam since more reactant was introduced.
However, the surface area of the two samples was the same. This indicates that
the CO₂ produces a more porous ash per gram of biomass reacted.
Therefore, if it desired to produce a more porous ash, CO₂ is likely
a better candidate than H₂O.

TGA
tests were done on biomass under 100% CO₂, 9% CO₂ in N₂,
9% H₂O in N₂, and H₂O/CO₂/N₂
mix (9%, 8%, balance), as shown in Table 1. The highest mass loss was 98.25% with
100% CO₂.  It is reported in the PHYLLIS database that poplar wood
has an ash content of 1.5% (dry) therefore the results with pure CO₂
demonstrate almost complete reaction.  In the temperature range of 100-200^oC,
there is dehydration of the sample and a higher mass loss is observed for the
dry samples where there is a stronger driving force for water to leave the
sample. When 200^oC is reached the mass loss under pure CO₂
is 7.11% while the mass loss under 9% CO₂ in N₂ is 5.85%.
If dehydration is the only process taking place then one would expect equal
mass loss in both cases, since both use dry gases. For example, TGA experiments
were done at with air and N₂ at identical conditions (heating rate
of 2^oC/min to 500^oC); both gases were dry. While the
final mass loss was very different for the two gases (98.4% for air and 76.6%
for N₂), the mass loss was similar until temperatures of 200^oC.
 Therefore, it is possible that under CO₂ reaction begins to take
place at temperatures as low as 150^oC. ESEM images also showed more
physical changes at low temperatures under CO₂ compared to H₂O.
For example, at 400^oC the pores in the biomass sample have expanded
under CO₂ whereas the sample under steam has very similar physical
properties to the raw biomass, as shown in Figure 2.

Table
1
Mass loss in TGA gasification experiments

Figure
2
ESEM images A. CO_2, raw biomass B. CO₂, 430^oC
C. H₂O, raw biomass D. H₂O,  425^oC

Mass
loss was 86.66% under 9% CO₂ in N₂ and 81.95% under 9% H₂O
in N₂.  The higher mass loss under CO₂ could be partly
due to the Boudouard reaction, shown in Equation 1. It was also interesting to
note that mass loss under the mixture of CO₂ and H₂O was
almost identical than mass loss under H₂O only (81.27% with CO₂
and H₂O and 81.95% with H₂O only).  This could be
indicative of a competitive reaction between H₂O and CO₂.

 Equation
1  CO₂ + C ßà 2CO    Boudouard Reaction                    

Conclusion

If
the ash generated from biomass gasification is to be used in catalytic
applications it is important to understand the properties of the ash as well as
the ash yield that one can expect from different reaction conditions. Overall,
it is desirable to produce an ash with catalytic properties without
significantly impacting overall gas recovery.  Gasification with CO₂
produces a more porous ash compared to gasification under steam. A higher mass
loss is observed under CO₂ compared to steam and reactions under CO₂
begin at lower temperatures compared to steam.

References: 
(1)
Butterman, H.C.;  Castaldi, M. J. Environ. Sci. Technol. 2009,
43, 9030-9037

(2)   El-Rub,
Z. A.; Bramer, E. A.;  Brem, G. Fuel. 2008, 87, 2243-2252