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dioxide. This practice not only generates potentially toxic chlorinated chemicals but also prevents the combustion of the bleaching efluent because it contains chloride ions. As a result, bleaching is
by far the main contributor to the water pollution of a kraft pulp mill.
This paper summarizes the research efforts which address these problems and will contribute to the development of a sustainable cellulose industry.
The Kraft Bioreinery Concept
Converting a kraft pulp mill to a bioreinery represents the most realistic mean to develop a sustainable production of chemicals from lignocellulosic biomass. In theory, many other processes may be used to
this purpose. They are not described here. For many reasons, it makes more sense and is technically and economically more attractive to take proit of existing cellulose production mills to develop such
a chemical platform. The challenges are then to extract the hemicelluloses prior to the deligniication and to recover some of the lignin dissolved in the cooking liquor, which are today industrially feasible.
Therefore, many people consider that pulp mills are going to be the future large scale bioreineries. One example will be the start up in 2017 of the new Metsa mill at Aanekoski in Finland which should
produce both 1.3 million tons of cellulose per year and a series of bioproducts and biofuels, including sulfuric acid, methanol, textile ibres, lignin derivatives, fertilizers, biogas [1].
Figure 1 gives a general scheme of a kraft bioreinery. In this process the wood is treated at high temperature with vapor prior to kraft cooking. During this step named autohydrolysis, the hemicelluloses
are depolymerized and made soluble in water. Part of them is recovered as simple sugars or oligomers which may be the raw material for sugar chemistry [2]. Some of the lignin present in the liquor after
cooking is precipitated and recovered as a source of phenolic compounds. However the drawbacks of the kraft process are not addressed. Moreover, the presence of sulfur in the recovered lignin may be a
problem for subsequent applications. Our recent work has been devoted to the understanding of the reactions taking place during the autohydrolysis step.
Figure 1. Scheme of the kraft bioreinery mill
2.1 Impact of Autohydrolysis on Lignin and Lignin-Carbohydrates Complexes
In wood, lignin and carbohydrates are covalently linked. Several types of linkages have been described. Some of them will not be cleaved during the kraft process which means that even though
lignin is depolymerized, it may not go into solution. This hinders lignin removal and contributes to the well known fact that residual deligniication has a very slow rate. The quantity of lignin linked to
ISBN : 978-602-17761-4-8
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Proceedings of 2
nd
REPTech Crowne Plaza Hotel, Bandung, November 15-17, 2016
© 2016 Published by Center for Pulp and Paper through 2
nd
REPTech
carbohydrates LCC has been investigated for both hardwood mixed and softwood. The procedure for the measurement of LCC was adopted from Due et al 2013 [3].
Table 1 shows that most of the lignin in wood is linked to carbohydrates. After hydrolysis, it is clear that in softwood some lignin carbohydrates linkages are cleaved, since some lignin is left free of
carbohydrates. This demonstrates that autohydrolysis has the capability of detaching some lignin from the carbohydrates. In hardwood lignin remains linked to carbohydrates. However it does not mean
that no lignin carbohydrates linkages have been cleaved since one lignin molecule may be originally linked to carbohydrates at many locations. The lower proportion of carbohydrates engaged in LCC after
autohydrolysis can be consistent with the cleavage of lignin carbohydrates bonds [4]. This should help
deligniication. The effect of autohydrolysis on lignin itself is dificult to study. The reason is that, due to the lack
of in situ analytical techniques, lignin is usually extracted by acidolysis for analysis. This extraction procedure is known to introduce some modiication to the lignin, which weakens the validity of the
conclusions. We have developed an in situ method to measure the phenolic OH groups [5]. The method is based on the fact that at low temperature chlorine dioxide reacts exclusively with the free phenolic
groups in lignin. The consumption of chlorine dioxide is then correlated to the content in these groups the higher the ClO
2
consumption, the higher is the phenolic OH content. Some secondary reactions may happen and consume further ClO
2
. However at low temperature 0°C here and appropriate pH phosphate buffer pH 6.7 the extent of these reactions is minimized. Figure 2 compares the consumption
of ClO
2
for hardwood chips before and after autohydrolysis. It appears that autohydrolysis introduces new free phenolic groups. Since these groups originate from the cleavage of aryl ether linkages, one
may conclude that partial depolymerisation of lignin occurs during autohydrolysis, at least in a irst step, since the possibility of recondensation of lignin fragments cannot be totally excluded.
Table 1. Proportion of lignin and carbohydrates engaged in LCC before and after prehydrolysis of softwood and hardwood chips
Ratio LCCwood Lignin in LCCs
lignin in wood GGM in LCCs
GGM in wood Xylans in LCC
xylans in wood Cellulose in LCCs
cellulose in wood Control softwood
0.98 0.61
0.63 0.83
Prehydrolysed softwood
0.79 0.66
0.41 0.81
Control hardwood 0.92
0.67 1.03
0.83 Prehydrolysed
hardwood 0.90
0.47 0.86
0.62
Note: The values do not take into consideration the acetyl and methylglucuronic acid groups in carbohydrates.
Figure 2. Consumption of ClO
2
by milled hardwood chips before and after
autohydrolysis 0°C, pH 6.7
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As a consequence, cooking should be much facilitated after autohydrolysis. Moreover the removal of part of the hemicelluloses which are the main responsible for caustic soda consumption should allow
for a substantial decrease in the alkali requirement. Finally, the departure of hemicelluloses must have resulted in a more porous and accessible lignocellulose matrix. This has not been investigated so far but
appears logical. Many trials have conirmed that kraft cooking is much easier after autohydrolysis.
2.2 Replacing The Kraft Cook by A Caustic Soda Cook
