(285a) Production of Bio-Oil from Lignin Extracted from Kraft Black Liquor By the Sunmembrane Process

It is essential for the pulp and paper industry to utilize as much of the forest raw material as possible to achieve the sustainability goals and to increase the profit. The fibres that are liberated during the kraft pulping process are used for pulp production, while the rest of the components from the wood are mainly incinerated. Lignin is one major by-product from the kraft pulping process. It has a high heating value, and has not a large market outside internal boiler fuels.

Today, many researchers are focused on finding alternative bio-based and sustainable resources that can replace the fossil oil for production of liquid fuels used for vehicles. Lignin, which is already extracted from the wood in the kraft process, is an appropriate raw material for this purpose since it is available in large quantities. Using this lignin for production of renewable fuels does not only have an environmental advantage, but it also increases the income for the kraft pulp mills.

Scientists have proposed several methods for the conversion of kraft lignin into a liquid fuel, e.g. pyrolysis [1], palladium-catalysed depolymerisation [2] and base-catalysed depolymerisation [3]. However, these methods are all dependent on an efficient extraction process for lignin from the kraft black liquor.

Lignin can be extracted from the kraft black liquor by either acid precipitation [4], or by membrane filtration [5]. Membrane filtration has the advantage, compared to the acid precipitation technology used today, that the negative impact on the pulp mill operation is negligible, that no chemicals are added to the black liquor, and that the sodium sulphur balance in the pulp mill is not impaired. We have developed two processes for the production of a bio-oil from kraft lignin. Both processes are based on a membrane process, called the SunMembrane process. The first configuration allows production of a solid lignin. This solid lignin can then be fed into a pyrolysis unit for production of pyrolysis oil. By the use of the second process, a precursor for green gasoline is directly produced in the process with integrated base-catalysis, without production of an intermediate solid product.

  2. Separation of Lignin from the Black Liquor

The process is based on membrane separation of the components in the black liquor. The first part of the process comprises several steps, and separates the lignin from the rest of the components in the black liquor. These steps can include one or more of the following processes: ultrafiltration (UF), fine ultrafiltration (FUF), nanofiltration (NF) and diafiltration (DF).

The UF step is an optional pre-filtration step. This step removes larger compounds in the black liquor, e.g. polymeric hemicelluloses and larger lignin molecules. In some applications these compounds have to be removed. However, in most applications, this step is not necessary.

The FUF/NF step isolates the lignin from the water and cooking chemicals. This is achieved by retaining the lignin and other organic compounds of relatively high molecular weight while the water, the cooking chemicals and other low molecular weight compounds can pass through the membrane without any significant retention. The stream that has passed through the membrane is sent to the recovery cycle for the cooking chemicals, while the stream with high lignin concentration is further processed with DF.

Water is continuously added to the stream with high lignin concentration during DF. By continuously adding water at the same rate as the permeate is withdrawn from the process, washout of the cooking chemicals from the lignin-rich solution is achieved. The DF permeate is directed to the recovery cycle while the stream with high lignin concentration is either concentrated further, or sent directly to next step.

  3. Production of a Lignin Powder  

If the aim of the process is to produce a lignin powder, the lignin in the solution with high lignin concentration is precipitated. In the precipitation, acid is added to the lignin-rich solution in order to decrease the pH. By decreasing the pH, the lignin starts to precipitate and form particles. Due to the previous recovery of the major part of the cooking chemicals, only a small amount of acid is needed compared to if the precipitation would be performed on the black liquor directly.

After the precipitation, the lignin slurry is filtered through a filter cloth in order to separate the lignin particles from the liquor. The filter cake of lignin is then washed before it is dried. The dried lignin powder can finally be fed into different downstream processes, e.g. a pyrolysis unit for production of a bio-oil.

  4. In-situ Conversion of Lignin by Base Catalysis

Depolymerisation of the lignin molecules is needed to convert the lignin into a bio-oil. By smart integration into the membrane-based separation process, this can easily be done at a pulp mill. This process comprises, in addition to the previously described membrane process, a depolymerisation reactor and a separation step.

The depolymerisation is performed as a base-catalysed reaction, and is placed directly after the NF step. During the depolymerisation, partial deoxygenation and desulphurization also occurs. The main catalyst in this reactor is the sodium hydroxide that is already present in the black liquor. However, to improve the conversion of lignin into an oil fraction, a heterogeneous catalyst is also added.

The lignin molecules that are fully depolymerised are extracted from the liquor afterwards, and can be sold as a precursor for production of gasoline and diesel, or be used internally at the pulp mill to replace fossil fuel, e.g. in the lime kiln. The part of the lignin that is not converted into a bio-oil is recovered in the recovery boiler at the pulp mill.

  5. Summary

Two different routes for production of a bio-oil have been presented. Both processes (patent pending) are currently under evaluation in lab-scale. The plan is to build a demonstration plant next year for evaluation of both alternatives for the production of bio-oil in a continuously operating plant.

  Acknowledgments

The Swedish Foundation for Strategic Environmental Research, MISTRA, and Sweden’s Innovation Agency, VINNOVA, are gratefully acknowledged for their financial support.

  References

1. Ben H, Ragauskas A J, NMR characterization of pyrolysis oils from kraft lignin, Energy Fuels, 25 (2011) 2322-2332.

2. Liguori L, Barth T, Palladium-nafion sac-13 catalysed depolymerisation of lignin to phenols in formic acid and water, J. Anal. Appl. Pyrolysis, 92:2 (2011) 477-484.

3. Vigneault A, Johanson D K, Chornet E, Base-catalysed depolymerization of lignin: separation of monomers, Canadian J. Chem. Eng., 85:6 (2007) 906-916.

4. Wallmo H, Lignin extraction from black liquor: Precipitation, filtration and washing, Chalmers University of Technology (2007) doctoral thesis.

5. Arkell A, Olsson J, Wallberg O, Process performance in lignin separation from softwood black liquor by membrane filtration, Chem. Eng. Res. Des., (2014) In press.