Production of calcium carbide from fine biochars

Article abstract of DOI:10.1002/anie.201004169

Rising from the ashes: Fine chars produced from biomass can be used as a feedstock for the production of CaC₂ in an autothermal process that operates at a temperature 500 °C lower than the traditional electric arc process and with greatly reduced reaction time (see scheme for the reaction of CaO and C forming CaC₂).

Full text of DOI:10.1002/anie.201004169

Communications
DOI: 10.1002/anie.201004169
Biomass
Production of Calcium Carbide from Fine Biochars**
Guodong Li, Qingya Liu, Zhenyu Liu,* Z. Conrad Zhang,* Chengyue Li, and Weize Wu
Carbon is the most abundant source of energy and chemicals
on the earth. Biomass produced from photon-activated
operations, typically less than 40 kt CaC2 per year. This
process requires granular char and CaO of 5–30 mm in size
and with sufficient mechanical strength, such as coal char, to
allow unrestricted release of byproduct CO. Because of the
low reaction rate resulting from the low surface area and poor
contact between the large feed particles, high temperatures
(about 22008C) and long reaction times (1–2 h) are usually
conversion of atmospheric CO , and biomass fossils such as
2
coal and petroleum are all carbon-rich sources. In around only
one century of heavy industrial use of petroleum, this
hydrocarbon source has already depleted to a point of a
widespread concern over its scarcity in the decades to
[
1]
[6]
follow. Biocarbon, also known as biochar, can be readily
produced from a vast sustainable supply of lignocellulosic
required. These constraints inevitably result in high energy
ꢀ1
consumption (4000 kWhtCaC2 ), high production cost, and
[2]
[7]
biomass through pyrolysis. It is often in fine form and
characterized by low mechanical strength and high activity in
comparison to coal-derived chars. The ability to use biochar
for the production of chemicals with high energy efficiency
will largely alleviate our dependence on shrinking petroleum
feedstock. Herein, we show reaction of fine biochars with fine
high CO emissions in electricity generation.
2
Autothermal heating by combustion of chars has been
[8–10]
studied as an alternative process for CaC preparation.
2
However, the use of large feed particles, as with the electric
arc process, requires long production times and high reaction
temperatures to obtain sufficient yields; therefore, the
process is not economically attractive for commercial devel-
opment.
CaO for the production of CaC , an important starting
2
material for production of many commodity chemicals. The
process offers the potential to redirect the carbon conversion
We report a considerably reduced temperature (by
5008C), shortened reaction time, and increased thermal
pathway.
1
CaC is produced by the reaction 3C + CaO + E !
efficiency in converting fine biochar into CaC . More
2
2
1
CaC + CO, where E is the energy required for the process,
importantly an autothermal reactor similar to the modern
high throughput gasification reactors could be used. Figure 1
2
ꢀ1
[3]
about 445.6 kJmol at above 20008C. CaC can be readily
2
converted into acetylene by treatment with water: CaC2
+
2
H O!C H
+
Ca(OH) . Acetylene is an oxygen-free
2
2
2
2
platform chemical for production of chemicals, for example,
polyvinylchloride (PVC), vinyl acetate, and 1,4-butanediol. In
this carbon conversion process, the main products CaC and
2
then C H are readily separated from other components.
2
2
The current CaC production technology dates back to
2
[
4,5]
1
892 and has not changed much since then.
It uses an
electric arc furnace, which is limited only to small-scale
[
*] G. Li, Q. Liu, Prof. Dr. Z. Liu, C. Li, W. Wu
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Beijing, 100029 (PR China)
Fax: (+86)10-6442-1073
E-mail: liuzy@mail.buct.edu.cn
Dr. Z. C. Zhang
KiOR Inc., 13001 Bay Park Road, Pasadena, TX 77507 (USA)
Fax: (+1)713-299-4165
E-mail: conrad.zhang@kior.com
zczhang@yahoo.com
Figure 1. TG/DTG curves and MS signal for release of CO from the
reaction of CaO and a pine wood char of size 0.022 mm at a C/CaO
molar ratio of 3.6.
[**] We thank the Nature Science Foundation of China (20821004 and
20976011) for financial support. Z.L. conceived the research and
shows thermogravimetric/differential thermogravimetric
developed the interpretation of the results. Z.C.Z. proposed and
designed some of the experiments and structure of the paper. G.L.
performed all the experiments and Aspen Plus simulation. Q.L.
designed some of the experiments and contributed to the structure
of the paper. C.L. proposed interpretation of the solid-solid reaction.
W.W. contributed to calculation of thermal efficiency. All the authors
discussed the results and commented on the manuscript.
(
TG/DTG) data and mass spectra for released CO for the
reaction of pine wood char and CaO at a nominal particle size
of 0.022 mm with a C/CaO molar ratio of 3.6. The total mass
loss at temperatures higher than 15108C corresponds to a
CaC yield of 99% based on CaO. XRD analysis (Figure S1 in
the Supporting Information) confirms CaC2 as the main
2
product. Gas chromatograph analysis of the C H released
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004169.
2
2
from CaC hydrolysis shows a CaC yield of 97%.
2
2
8480
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Figure 2 shows initial and DTG peak temperatures of the
reaction using chars of different particle sizes derived from
pine wood and bamboo. The initial temperature, defined in
Figure 1 as the temperature when DTG signal reaches
the reactants and CaC except for the release of CO. The high
2
CaC yield indicates constant mass transfer between the chars
2
and CaO at the conditions used. Since the melting points of
char and CaO are much higher (36258C for graphite and
2
5808C for CaO), the mass transfer may be attributed to
nanocrystal migration. The temperatures in Figure 2 are all
higher than the Tammann temperature of CaO, one half of its
melting point in Kelvin (11548C), and above which nano-
[
11]
crystal migration becomes significant. This hypothesis is
supported by the experiments shown in Figure 4. Figure 4a,b
Figure 2. The initial and DTG peak temperatures of CaC formation
2
through CaO and biochars (from pine wood and bamboo) of different
particle sizes (d ).
p
ꢀ
1
ꢀ
0.5 mgmin (corresponding to a CaO conversion rate of
ꢀ
1
about 1.2% min ), decreases from about 17008C for a size of
.675 mm to 15108C for a size of 0.022 mm; correspondingly,
Figure 4. Photos of fine reactants of size 0.022 mm and the products
0
ꢀ1
obtained after heating to 20008C at 208Cmin . Top: at a C/CaO
molar ratio of 3.5, a) before and b) after the reaction; bottom: at a C/
CaO ratio of 1.8 c) before and d) after the reaction.
the peak temperature decreases from about 1850 to 17108C.
Figure 3 shows mass-loss rates (DTG) at 16508C for
reactions of CaO with various biochars as well as other carbon
materials. Clearly the biochars have higher d002 values (from
XRD) and are more active than the coal chars, petroleum
coke, and graphite.
The results indicate that CaC2 can be produced at
temperatures much lower than that required by the current
electric arc process. The process involves little vaporization of
shows the appearance of a sample before and after the
reaction when a C/CaO mixture with a molar ratio of 3.5 was
ꢀ1
heated to 20008C at a rate of 208Cmin . The carbidization
between 1510 and 19008C yielded a final mixture containing
86.5% CaC and 13.5% char (Figure S2 in the Supporting
2
Information). The product (Figure 4b) is loose with dense
particles. When the C/CaO molar ratio (1.8) was below the
reaction stoichiometry (Figure 4c,d), the carbidization tem-
perature was unchanged but additional mass loss after the full
conversion of the char at 18288C was observed (Figure S3 in
the Supporting Information). Figure 4d indicates the forma-
tion of eutectics consisting of CaC and CaO.
2
When a larger particle size (e.g. 5 mm in diameter) is used
(Figure 5), the initial temperature is shifted to 20208C and the
DTG peak temperature to 20908C (Figure S4 in the Support-
ing Information). The core (char) and shell (CaC ) structure
2
in the products indicate migration of CaO into the char during
the process.
Contacting the core–shell structure (Figure 5b) with H O
2
transformed the CaC shell into loose Ca(OH) that peels off
2
2
from the core, yielding a CaO conversion of 77.8% to CaC2
and a mass loss of 4.8% in addition to the release of CO. The
visual spillage on the mouth of the crucible indicates
formation of foaming slurry during the experiment and
explains the additional weight loss at temperatures higher
than 21288C (Figure S4 in the Supporting Information). The
Figure 3. Conversion rates of CaO (R) at 16508C for reaction of CaO
with chars. The d002 values are the interplanar distances from XRD
analysis of the chars treated at 16508C. ~: biochars (from apricot
shell, willow wood, bamboo, corncob, and pine wood, in the order of
increasing d002); *
: coal chars (from anthracite, bituminous coal, and
lignite, in the order of increasing d002); &: petroleum coke; ^: graphite.
formation of a eutectic mixture by CaO and CaC under the
2
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Communications
Figure 5. Appearance of the reactants and the product. a) Two CaO
and two char granules of size 5 mm at a C/CaO molar ratio of 3.15 in
ꢀ
1
a TG crucible; b) after heating at a rate of 208Cmin to 21508C in Ar;
c) cut product particles (top: black char core and gray CaC shell;
2
bottom: black char core and white Ca(OH) shell, upon contact with
2
moisture).
Figure 7. Conversion rate of CaO (R) at different particle sizes (d ) at a
p
pine wood char/CaO molar ratio of 3.6.
experimental conditions agrees with published results, which
record the lowest eutectic temperature of 16508C for a CaO/
[12]
CaC molar ratio of 0.62.
2
The experimental observations suggest that CaC produc-
2
tion at temperatures between 11548C (the Tammann temper-
ature of CaO) and 25808C (the melting point of CaO) is
controlled initially by CaO nanocrystal migration. Eutectic
mixtures of different compositions may be formed at the
interface of CaO and CaC when temperature is higher than
2
1
6508C. The eutectic phase accelerates CaO migration
towards the char. This mechanism is schematically shown in
Figure 6.
Figure 8. Carbon yields to CaC (C% CaC ), CO (C% CO), and CO
2
2
2
(
C% CO ), and thermal efficiency (h) of autothermal and electric arc
2
processes determined by Aspen Plus simulation. The autothermal
process includes preheating by the high-temperature side-product CO.
*
: C% CaC , &: C% CO, !: C% CO , ~: h.
Figure 6. Reaction process of CaO and C for the production of CaC2.
2
2
thermal efficiency than the electric arc process especially
when the reactants are preheated by the high-temperature
Clearly CaC production is controlled by mass transfer of
2
CaO toward the char phase. A smaller particle size results in a
greater contact surface area and a reduced mass transfer
barrier for the reaction; thus, the reaction rate is enhanced
even at lower temperature (Figure 7). From the data in
Figure 7, a reaction time of 4.6 min for 95% conversion of
CaO at 17508C was determined when the particle size was
CO generated from the CaC production (see the Supporting
Information).
2
In summary, fine biochars as feed for CaC production
2
make the autothermal process feasible at a temperature about
5008C lower than that of the current electric arc process. The
reaction time is also reduced from 1–2 h to less than 5 min.
The reaction mechanism may involve eutectic mixtures at the
CaO/char interphase. Coupling this process with biomass
pyrolysis forms a new route for biomass conversion, which is
extendable to coal-based processes.
0
.022 mm (assuming a first-order reaction to CaO; see the
Supporting Information). The dramatic decrease in temper-
ature and reaction time, compared to 1–2 h at about 22008C
for the electric arc process, point to a tremendous potential
for energy saving. This result also implies that CaC produc-
2
tion can be carried out in autothermal mode, since the
temperature becomes comparable to many autothermal Experimental Section
processes such as pulverized coal gasification (1400–
A thermoanalyzer from Setaram (Setsys Evolution 24) and a mass
spectrometer from Balzers (Omnistar 200) were used. CaO had a
purity of 98.3%. The carbon sources include five biochars (derived
from apricot shell, willow wood, bamboo, pine wood, and corncob),
three coal chars (derived from anthracite, bituminous coal, and
lignite), petroleum coke, and graphite. The preparation procedure
2
0008C) and the BASF process of oxidative dehydrogenation
[
6,13,14]
of methane for acetylene production (15408C).
Computer simulation using Aspen Plus (Figure 8) con-
firms that autothermal CaC2 production is achievable by
oxidizing a fraction of char to CO to meet the energy
requirement of the reaction, and the process is of higher
and conditions of all the chars include pyrolysis in N at 9008C for 3 h,
2
acid washing with a HF + HCl solution at 608C for 24 h, and drying at
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Angewandte
Chemie
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