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Short Communication

Solid fuel production by hydrothermal carbonization of black liquor

Shimin Kang, Xianglan Li, Juan Fan, Jie Chang

State Key Laboratory of Pulp and Paper Engineering, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

a r t i c l e i n f o

Article history:

Received 3 October 2011

Received in revised form 14 January 2012

Accepted 18 January 2012Available online 28 January 2012

Keywords:

Hydrothermal

Black liquor

Solid fuel

Lignin

Formaldehyde

a b s t r a c t

Formaldehyde was used as a polymerization agent to perform hydrothermal carbonization of black liquor

for solid fuel production from 220 to 285 C. Compared to hydrochar prepared without formaldehyde,

hydrochar produced in the presence of a 2.8 wt.% formaldehyde solution (hydrochar-F) had 1.272.13times higher yield, 1.021.36 times higher heating value (HHV), 1.202.31 times higher C recovery effi-

ciency, 1.202.44 times higher total energy recovery efficiency, 0.510.64 times lower sulfur content, and

0.480.89 times lower ash content. The HHV of hydrochar-Fs ranged from 2.2 104 to 3.0 104 kJ/kg,

while the HHV of hydrochar-F produced at 285 C was 1.90 times greater than that of the raw material

(black liquor solid). These considerable improvements indicated that formaldehyde was an effective addi-

tive in hydrothermal carbonization of black liquor.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Black liquor is a major waste product of the pulp and paper

making industries, as well as a biomass fuel containing inorganic

pulping salts and organic compounds. Black liquor is traditionally

burned in a recovery boiler to recover cooking chemicals and to re-

lease combustion heat for energy generation. Although widely

used, the recovery boiler technology is facing problems with the

deposition of inorganic alkalies salt in the upper furnace resulting

in corrosion of material and reduction of effectiveness of heat

transfer (Wessel and Baxter, 2003). Therefore other treatments

such as steam gasification (Naqvi et al., 2010; Snchez et al.,

2005), supercritical water gasification (Sricharoenchaikul, 2009;

Rnnlund et al., 2011), and pyrolysis (Whitty et al., 2008a,b) have

been studied. Lignin is a polymeric phenolic compound and the

main organic component in black liquor. Most thermo-chemical

conversion methods have used black liquor as a carbon-rich organ-

ic source, and the objectives were mainly to get liquids and/or gas

fuels, while few special considerations were used to take advan-tage of the existence of lignin. Direct thermo-chemical depolymer-

izations of lignin to liquids have been conducted, and phenolic

compounds were obtained as the main products (Amen-Chen

et al., 2001; Kleinert and Barth, 2008; Jin et al., 2011; Kang et al.,

2011). Formaldehyde is a well known polymerization agent, and

polymerization also occurs between formaldehyde and lignin (Jin

et al., 2010). Hydrothermal carbonization of biomass is a method

to obtain solid carbon materials, and it was reported that carbon-

ization of some lignocellulosic biomass under hydrothermal

conditions produced high-energy chars (Lynam et al., 2011;

Hoekman et al., 2011). Hydrochar has been considered as a coal

substitute in gasification and combustion applications (Hoekman

et al., 2011), and hydrothermal carbonization of black liquor would

be attractive because: (1) drying would not be necessary, (2) depo-

sition of inorganic alkalies salt in reactor would be inhibited,

because alkalies salt should be dissolved in water under hydrother-

mal condition, (3) the nitrogen and sulfur oxides formed would be

dissolved in the alkaline water solution, and further treatment

would be unnecessary, and (4) the required temperature would

be lower than that required for other thermo-chemical methods,

e.g. gasification. Therefore, in the present study, formaldehyde

was used as a polymerization agent in hydrothermal carbonization

of black liquor. The object was to explore a new application of

black liquor for energy-dense hydrochar production.

2. Methods

2.1. Materials

Spray process powders of black liquor (black liquor solids, BLS)

were obtained from Wuhan East China Chemical Co. Ltd., with a

crude lignin content of 30.0 wt.%. The BLS were dried in a vacuum

drying oven at 105 C and 100 mbar to constant weight. Formalde-

hyde was obtained from Hubei Osen Xincailiao Technology Co. Ltd.,

as a 37 wt.%, with about 8 wt.% of methanol.

2.2. Reaction and separation processes

Hydrothermal conversion experiments were conducted in a

250 mL stainless steel 316 autoclave. The autoclave was loaded

0960-8524/$ - see front matter 2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2012.01.093

Corresponding author. Tel./fax: +86 20 87112448.

E-mail address:changjie@scut.edu.cn(J. Chang).

Bioresource Technology 110 (2012) 715718

Contents lists available atSciVerse ScienceDirect

Bioresource Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / b i o r t e c h

http://dx.doi.org/10.1016/j.biortech.2012.01.093mailto:changjie@scut.edu.cnhttp://dx.doi.org/10.1016/j.biortech.2012.01.093http://www.sciencedirect.com/science/journal/09608524http://www.elsevier.com/locate/biortechhttp://www.elsevier.com/locate/biortechhttp://www.sciencedirect.com/science/journal/09608524http://dx.doi.org/10.1016/j.biortech.2012.01.093mailto:changjie@scut.edu.cnhttp://dx.doi.org/10.1016/j.biortech.2012.01.093

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with 25 g of dried BLS and 100 mL of water, or 25 g of BLS and

100 mL of a 2.8 wt.% formaldehyde solution (prepared with

93 mL of water and 7 mL of 37 wt.% formaldehyde). The autoclave

was purged five times with nitrogen to remove air. The tempera-

ture was raised to the set values (220, 245, 265, 285 C), and the

autoclave was kept at the reaction temperature for 8 h. Control

experiments of only 100 mL of 2.8 wt.% formaldehyde solution

and 100 mL of 2.8 wt.% formaldehyde solution with 5 g NaOH wereperformed under the same conditions.

The separation process of the hydrothermal products is shown

in Fig. S1. The products were filtered under negative pressure

through a preweighed Shuangquan brand quantitative filter paper.

The solid products (hydrochars) and filter paper were dried at

105C to constant weight in a vacuum drying oven. The aqueous

phase 1 was acidified with 1 mol L1 HCl solution until the pH va-

lue reached 12, and filtrated through filter paper to obtain residue

and aqueous phase 2, and the residue was maintained as uncon-

verted lignin and lignin derived oligomers. The acidic aqueous

phase 2 was extracted with ethyl acetate (EA) to form an EA phase

with aqueous phase 3. Yields of hydrochar and residues derived

from unconverted lignin were measured two or three times

through weighing, respectively. The data were averaged, and the

relative error was 6%.

The hydrochars produced without and with formaldehyde were

labeled as hydrochar-WF and hydrochar-F, respectively. While the

hydrochar-WF and hydrochar-F produced in X C were labeled as

hydrochar-WFX and hydrochar-FX, respectively. Similarly, the EA

phases produced without and with formaldehyde at a temperature

of YC were labeled as EA-WFY and EA-FY, respectively.

2.3. Calculations

The yield, lignin conversion, total energy recovery efficiency,

and C recovery efficiency were calculated by Eqs. (1)(6), respec-

tively. m in the equation meant the weight of the sample in

the parentheses:

hydrochar-WF orhydrochar-F yield wt:%

mhydrochar-WF or mhydrochar-F

mBLS 100 1

Total energy recovery efficiency of hydrochar-WF

Total HHV of hydrochar-WF

Total HHV of BLS 100

3

Total energy recovery efficiency of hydrochar-F

Total HHV of hydrochar-F

Total HHV of BLSformaldehyde 100 4

C recovery efficiency of hydrochar-WF

Total C content of hydrochar-WF

Total C content of BLS 100 5

C recovery efficiency of hydrochar-F

Total C content of hydrochar-F

Total C content of BLSformaldehyde 100 6

2.4. Analysis and characterization

The EA phase products were analyzed by Gas Chromatography

Mass Spectrometry. Analysis and HHVs, ash and elemental (C, H, N,

S) contents, functional groups distribution of BLS and hydrochars

were determined as described in supplementary material.

3. Results and discussion

3.1. Yields and C recovery efficiencies of hydrochar-WF and

hydrochar-F

As shown in Fig. 1, the yield of hydrochar-WF increased and

then decreased with increasing temperature from 220 to 285 C,

and a maximum yield of 14.4 wt.% was obtained at 265 C. This

result was similar to that obtained by lignin liquefaction with

hot-compressed water (Tymchyshyn and Xu, 2010). These authors

had demonstrated that condensation reactions of the lignin-

degradation intermediates resulted in more insoluble polymers

and chars at a higher temperature. The decreased yield of hydro-

char-WF at 285 C was likely due to the increased cracking of the

BLS hydrothermal products to form gases and/or liquids. The yieldsof hydrochar-Fs were much higher than those of hydrochar-WFs at

all these corresponding temperatures. In particular, the yield of

hydrothchar-F220 reached a maximum of 19.6 wt.%, which was

more than twice the yield of hydrochar-WF220. Negligible

amounts of hydrochar content were detected in the hydrothermal

conversion of formaldehyde only, no matter alkalies was added or

not. This result indicated that the additional hydrochar content

was generated by polymerization of formaldehyde with BLS and/

or its hydrothermal products. As shown in Fig. 1, the conversion

of lignin with formaldehyde was higher than that without formal-

dehyde at 220 and 245 C. Moreover, as shown inTable S1, the

phenolic peak areas for EA-Fs were much lower than those for

EA-WFs at all temperatures, which indicated that formaldehyde

Fig. 1. Yields of hydrochar-WF and hydrochar-F, and conversion of lignin with or

without formaldehyde addition to the reactions. Values are averages of two or three

measurements.

Lignin conversionwt:% mlignin content in BLS-munconverted lignin and it derived oligomer residue

mlignin content in BLS 100 2

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had also reacted with part of the BLS-derived phenolics to form

hydrochars. In contrast to the yield trend of hydrochar-WF, the

yield of hydrochar-F decreased slightly with elevated temperatures

within the range from 220 to 285 C. Furthermore, phenol was not

found in EA-F220 and EA-F245 but found in all the EA-WFs,

EA-F265, and EA-F285. These observations indicated that at low

temperatures (220, 245 C), polymerization of lignin and phenolicswith formaldehyde played a major role in obtaining higher poly-

mer production. Formaldehyde was a C source in the polymeriza-

tion reaction as the C content and C recovery efficiency in

hydrochar-Fs were much higher than those in hydrochar-WFs at

all temperatures (Table 1). At higher temperatures, these formalde-

hyde-lignin/phenolics high polymers depolymerized to produce

phenol and other low molecular compounds.

3.2. Properties of hydrochar-WF and hydrochar-F

As shown inTable 1, the HHVs of hydrochar-WFs and hydro-

char-Fs increased as reaction temperatures increased. The HHVs

of hydrochar-WF285 and hydrochar-F285 were 1.58 and 1.90

times larger than that of BLS, respectively. The HHVs of hydro-char-Fs were in the range of 2.2 1043.0 104 kJ/kg, equivalent

to the heating value of medium-rank and high-rank coals accord-

ing to Chinese National Standard (GB/T15224.3-2004). The in-

creased HHV is likely related to the increased C content, and the

correlation between HHV and C content was very high

(R2 = 0.9949). The ash content was also an important factor affect-

ing the HHV, as the basic trend of ash content was BLS > hydro-

char-WF > hydrochar-F. A possible reason for the decrease in ash

content in hydrochars is the removal of soluble inorganic salts in

BLS. Moreover, the increased H content at high temperatures

(265, 285C) resulted in high HHVs. As shown in Fig. S2, the

relative intensity change in OH groups was not obvious, while

the relative intensity of CO groups at a low temperature

(220 C) was distinctly higher than that at high temperatures(265, 285 C). The FTIR spectrum indicated that the O content de-

creased at high temperatures due to the decrease in CO groups,

which should result in higher HHVs. The CO groups are methoxy

group and/ or ether linkages. Methoxy groups and ether linkages

are easy to fracture at high temperatures (Kang et al., 2011). As

shown inTable S1, the relative contents of total methoxy group-

containing phenolics in EA phases decreased with increasing tem-

peratures. Interestingly, the S content decreased as the tempera-

ture increased, and the S content in hydrochar-F was much lower

than that in hydrochar-WF. The reasons for the decrease in S con-

tent is probably due to the dissolution of sulfides and conversion of

organic sulfur compounds under hydrothermal conditions, and in-

creased desulfurization at higher temperatures (Li and Guo, 1996).

Low S content in fuel is beneficial to the environment. As shown inTable 1, the total energy recovery efficiency of hydrochar-F was

higher than that of hydrochar-WF at each corresponding tempera-

ture, and the hydrochar-F265 obtained a maximum value of total

energy recovery efficiency (44.4%).

4. Conclusion

Formaldehyde improved the solid fuel production by hydro-

thermal carbonization of black liquor with maximum total energy

recovery efficiency at 265 C. The hydrochars produced with form-

aldehyde showed preferable properties, including yield, HHV, C

recovery efficiency, total energy recovery efficiency, and sulfur

and ash content. Considering the additional cost of formaldehyde

and possible problems of hydrochar application as a solid fuel, an

overall economic study of this new technique is required before

the process may be considered on an industrial scale.

Acknowledgements

We acknowledge financial support from the National Basic Re-

search Program of China (973 Program) (No. 2010CB73220).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.biortech.2012.01.093.

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Table 1

The higher heating value (HHV), ash, total energy recovery efficiency, C recovery efficiency, elemental composition of BLS and hydrochars.

Sample HHV (kJ/kg) Ca (wt.%) N (wt.%) S (wt.%) H (wt.%) Ash (wt.%) C recovery efficiency (wt.%) Total energy recovery efficiency (%)

BLS 10266.6 29.84 1.20 5.937 8.00 40.5

BLS + formaldehydeb 11000.0 30.86

Hydrochar-WF220 16634.3 43.79 1.44 4.89 5.26 30.0 13.5 14.9

Hydrochar-WF245 24272.5 55.67 1.96 4.57 6.26 23.1 23.4 29.6

Hydrochar-WF265 26303.5 61.74 2.01 3.53 7.54 18.9 29.8 36.9

Hydrochar-WF285 26356.8 61.86 1.84 3.35 7.06 20.0 25.7 31.8Hydrochar-F220 22648.7 54.53 1.94 3.12 6.93 14.5 31.2 36.4

Hydrochar-F245 24782.5 57.39 1.91 2.33 8.23 16.5 32.4 39.3

Hydrochar-F265 29694.4 67.30 2.02 1.96 10.59 16.8 35.8 44.4

Hydrochar-F285 29783.6 67.52 1.90 1.80 9.76 15.0 35.1 42.6

a The relevance (R2) between HHV and C content was 0.9949.b The calculations of HHV and C content were shown in the supplementary material.

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