Ethanol from Nordic wood raw material by simplified alkaline soda cooking pre-treatment
Highlights
► Ethanol prod via alkaline pretreatment enables a coproduction of valuable lignin. ► Lignin content of 2.5% on pulp was found optimum for aspen and 5% on pulp for pine. ► Aspen required lower alkali charge and lower temperature compared to pine. ► Yield gain of fermentable sugars is 1–2% charging AQ(0.1% on wood) in pretreatment. ► Ethanol yield between 81.6% and 87.8% on theoretical max was obtained.
Introduction
The demand for liquid biofuels in Europe is heavily increasing. At the same time the pulp and paper industry undergoes a change that leads to an increased need of finding new product niches. A major barrier for the deployment of wood-based fuel-ethanol is its high production cost. In particular, the pre-treatment is one of the most expensive processing steps in cellulosic biomass to fermentable sugars conversion. Thus, the wood pre-treatment step, preceding the hydrolysis and fermentation steps, has a great potential for improvement. In an alkaline pre-treatment process, wood can be fractionated to relatively pure carbohydrate and lignin fractions by the well-known soda cooking and kraft processes. The solid fibrous carbohydrate fraction can be further subjected to enzymatic hydrolysis, fermentation and distillation for the production of ethanol. “Black liquor” i.e. spent pre-treatment liquor, may be processed to sulphur-free lignin (if soda cooking is used) and energy, see Fig. 1 [1].
Besides the ethanol a nearly sulphur-free lignin with low ash content (1% or lower) and high dryness (65–70%) can be recovered as a potentially valuable by-product that can be used as a fuel or as an intermediate for chemicals production. The kraft cooking process that is the most used method in pulp mills today gives a lignin containing sulphur, but if the separation of the wood components is performed using the soda cooking process there is a possibility to obtain a lignin that is sulphur free. This is an advantage in some future possible applications of the lignin as a high-value added product, e.g. carbon fibres.
The process will also give the possibility to separate the hemicelluloses of the wood as a third product besides the sugars and lignin. The hemicelluloses can be used as fibre additive, gas barriers, hydrogels and thermoplastics.
Harmsen et al. [2] prepared a literature review on physical and chemical pre-treatment processes for lignocellulosic biomass as part of the BioSynergy project. This review concluded that organosolv and alkaline pre-treatment, are the most suitable pre-treatment processes if one wants to produce lignin in sufficient quality for the production of chemicals. In most other processes the lignin generally remains with the cellulose fraction, giving an impure lignin product containing un-hydrolysed sugar polymers as well as other organics. However, the organosolv process has a high cost of organic solvents, necessitating high solvent recovery, which causes increased energy consumption. Mosier et al. [3] reviewed process parameters and their fundamental modes of action for different pretreatment methods such as steam explosion, liquid hot water pre-treatment, acid pre-treatment and alkaline pre-treatment. He concludes that pre-treatment processing conditions must be tailored to the specific chemical and structural composition of the various, and variable sources of lignocellulosic biomass.
The objectives of this study were to define the conditions in the alkaline pre-treatment stage for the separation of wood to a carbohydrate fraction for the ethanol production and to a lignin fraction. Also combinations with ethanol or anthraquinone addition and with acidic pre-treatment were studied, and a comparison between hardwood and softwood raw material was made.
Section snippets
Materials and methods
Wood of aspen (Populus tremula) and pine (Pinus sylvestris) from Swedish mills were used. Aspen and pine logs were laboratory chipped, screened, dried and hand sorted at Innventia. The chips were analyzed with respect to lignin content, carbohydrate composition and extractives, see Table 1.
Results – alkaline pre-treatment of hardwood
The hardwood raw material studied was aspen wood. The chips quality used was equal to that utilized in Nordic pulp mills. The different pre-treatment methods were evaluated regarding (i) delignification, (ii) the yield of carbohydrate fraction (iii) the alkali charge and consumption and (iv) cooking time. The range of studied lignin content (carbohydrate fraction/pulp) was between 1.5% (kappa number ca. 10) and 5% (kappa number ca. 30) lignin on pulp.
Results II – alkaline pretreatment of softwood
The softwood studied was pine wood. The chips quality used was equal to that utilised in Nordic pulp mills.
Pulps for hydrolysis and fermentation
To study the ability to enzymatically hydrolyse and produce ethanol by fermentation from alkaline pre-treated aspen three differently alkaline pre-treated materials were tested by using commercial enzyme mixtures and Saccharomyces cerevisiae yeast. In addition, the effect of lignin content (1.6–4.1% Klason-lignin) in the material after pretreatment was investigated in ethanol production. All materials tested could be efficiently hydrolysed and fermented in SSF studies using 5%, 10% and 15% dry
Conclusions
The most attractive pre-treatment conditions and results are summarized in Table 10 for both raw materials. Both wood species were relatively easy to fractionate with alkali. As expected, aspen wood required lower alkali charge and lower temperature in the alkaline pre-treatment stage compared to that of pine wood. The lignin content of ca. 2.5% on pulp seems to be optimum for aspen wood in alkaline pre-treatment and ca. 5% on pulp for pine wood. The “optimum” lignin content was defined as
Acknowledgement
The Nordic Energy Research is acknowledged for the financial support of this work. Leelo Olm is gratefully acknowledged for her work within this project.
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