The kraft pulp mill biorefinery platform

Transcription

1 The kraft pulp mill biorefinery platform Peter Axegård, INNVENTIA AB, Sweden, Niklas berglin, INNVENTIA, Sweden, Karin Lindgren, INNVENTIA, Sweden, Per Tomani, INNVENTIA, Sweden, Fredrik Öhman, Innevntia, Sweden, Abstract This paper summarizes Innventias approach to pulp mill biorefining. The focus is on the development of value chains for the wood polymers cellulose, xylan and lignin including efficient separation processes, upgrading and final applications. Xylan can be extracted from several locations in the mill and can be used as a fibre surface modifier, in gas barriers and thermoplastics. Lignin can be produced from black liquor and can be used as fuel, additive to wood pellets, fedd-stock for carbon fibers and other value added applications. Specialty cellulose with high alfa-cellulose content can be produced in a modified alkaline process is another interesting option. Forestry residues can be co-processed in a kraft pulp mill as feed-stock for ethanol, lactic acid or specialty cellulose and sulfur-free lignin production. Keywords: biorefining, kraft pulping, lignin, xylan, cellulose Introduction Wood biorefining can be defined as full utilization of wood into a wide range of products. There are many process alternatives currently under development. Compared to most process alternatives alkaline treatment of wood has a unique capability to separate wood components into relatively pure streams. Innventias strategy is to develop efficient separation technologies, which efficiently can be integrated in kraft and soda pulp mills. Also new raw material streams like forestry residues can be co-processed in the pulp mill and be converted to upgraded solid fuels and transportation fuels. Wood chips Pretreatment Pulping Fibres Products Paper pulp Special cellulose Forestry residues Separation /upgrading LignoBoost Energy and Recovery Lignin Polymeric carbohydrates Energy Other Fuel Carbon fibres Activated carbon Binder Fibre additive Barriers Co-polymers Lactic acid Power, heat Motor fuels, pellets Tall oil, turpentine Figure 1. Products from the near term kraft pulp mill bio-refinery. Innventia has during several years developed processes for production of polymeric hemicelluloses and lignin as well on basic changes in cellulose morphology during pulping. This presentation will focus on processes for production of polymeric xylan, lignin and cellulose in the kraft pulp mill and on promising applications for these products. Lignin extraction from black liquor One way of exploiting the huge energy surplus that is possible to obtain in a modern kraft pulp mill is to extract lignin from the black liquor. This gives the pulp mill an opportunity to develop new revenues or to reduce production costs. Lignin extraction has the additional advantage of providing incremental capacity in

2 the chemical recovery area when thermal capacity of the recovery boiler is the bottleneck limiting pulp production increases. Lignin removal from kraft/soda black liquors has been developed to a commercial process (the LignoBoost process) in cooperation between Innventia and Chalmers. The process concept has been verified since 2007 at about 4,000 tonnes per year in a demonstration plant located in Bäckhammar, Sweden, owned and operated by Innventia. The LignoBoost technology was sold to Metso Power 2008 but the demonstration plant is kept by Innventia and used as a biorefinery platform. There is significant industrial interest for LignoBoost installations. Södra Cell has been given financial support for demonstration in commercial scale from the Swedish Energy Aagency. Other companies have made advanced pre-studies. A principal scheme for the LignoBoost process is shown in Figure 2. One advantage of the LignoBoost process is the possibility to produce very pure lignin ( %-w ash). Lignin with higher purity ( %-w ash) can be produced using a modified washing procedure, see Table 2. Lignin production with the LignoBoost technology Source: Metso Figure 2. Layout of the LignoBoost process integrated into a kraft pulp mill. Table 1. Lignin composition from normal washing and modified washing in the LignoBoost Demo plant. Normal washing Modified washing Ash content (%) Acid insoluble lignin (%) Acid soluble lignin (%) Hemicelluloses (%) There are three major variable cost items; chemicals (CO 2, H 2 SO 4 and NaOH), loss of electricity production and in some cases loss of green certificates. The total variable cost depends on local conditions and for most cases ranges between per tonne lignin. Credits for increased pulp capacity and lignin sales are not included. The lowest costs are reached without green certificates for electricity and when CO 2 from the lime kiln flue gases is used for precipitation. The cost can be further lowered by using ClO 2 spent acid instead of fresh H 2 SO 4. Lignin energy applications

3 Lignin has high energy content and is thus a good fuel. We have performed trials where lignin has tested in different fuel applications, e.g. co-firing with biomass, co-firing with coal, and firing in a lime kiln. The lignin burned well and showed no dramatic effects on important combustion parameters such as bed temperatures, flame stability, CO emissions, etc., in the various types of combustion equipment tested. The major challenges have been in the handling and feeding of the lignin fuel. The lignin is more easily handled when the moisture content is low, below 10 %. On the other hand, dusting is a problem that increases with increasing dry content, and the risk for dust explosions must be considered. Sulfur emissions can be controlled with normal in-bed desulfurization or by firing the lignin in applications where there is an inherent excess of calcium, such as in a lime kiln. From the trials, it can also be concluded that lignin can be used as an additive to reduce the stickiness of deposits in biomass boilers. The most economically attractive alternative in many cases is to replace fuel oil in the lime kiln where up to 100 % of the fuel oil can be replaced with lignin in powder form. It is also possible to mix solid lignin with liquids such as fuel oil, tall oil, tall oil pitch and glycerine from bio-diesel production. The advantages of liquid lignin are that powder handling is avoided and the ease to store and pump the liquid lignin. These trials are in progress. Lignin non-energy applications To fully utilize the economic potential of the lignin focus should be on a combination of fuel applications together with high value added lignin products such as carbon fibres, activated carbon, lignin as a binder or dispersant etcetera. This double focus enables a combination of economy of scale with bulk production of lignin for fuel applications at the same time as value added lignin products could be produced. The purity requirement on lignin for different products varies from high purity demands for lignin as carbon fibre precursor to medium requirements on lignin as binder and dispersant. However, a stable and uniform lignin product quality is always required. Binder Recent studies at Innventia have shown that kraft lignin can successfully be used as a binder in the manufacturing of MDF and HDF (Medium and High Density Boards). Other experiments have shown that molten kraft lignin applied onto kraft pulp fibres under controlled conditions gives fibres with very high degree of water resistivity. Some samples were completely hydrophobic as shown in Figure 3. Figure 3. Unbleached kraft pulp fibers impregnated with LignoBoost lignin were totally unaffected by the water droplet after several weeks. Pellets from sawdust and forestry residues can be improved by addition of lignin as a binder making the pellets less sensitive to degradation into finer particles. Also the moisture sensitivity is improved as lignin from the kraft process is hydrophobic. The same effects have been seen for pellets from wheat straw.

4 Activated carbon Experiments have been initiated regarding the effect of ash in kraft lignin on the yield, surface and micropore characteristics of final activated carbon. Three levels of ash levels of lignin were used. The samples were chemically activated with phosphoric acid. A trend with higher specific surface area in activated carbon made from lignin with low ash content could be seen. However, for the specific micropore area no significant difference could be observed for the standard and gently washed lignin. For the sample with highest ash content (unwashed lignin) no porosity at all could be obtained with phosphorous acid. Carbon fibers Recent work at Innventia has shown that LignoBoost lignin from both hardwood can be extruded by melt spinning to fibers without using any additives. Low ash content and low levels of carbohydrates seem to be important parameters of the lignin as well as defined thermal properties all which can be controlled in the LignoBoos process. The resulting lignin fibers can be carbonized to over 90 % carbon. The ambitions are high and we hope to be able to produce a lignin based carbon fibre that ultimately can replace structural steel in automotives. Figure 4. Scanning electron of a carbonized lignin based lignin fibre. To the left a straight carbon fibre and to the right a cross-section of a carbon fibre. Xylan production All hardwoods contain xylan which a polymer based on xylose monomers. The amount typically varies between % on weight in the wood. About 50 % of the xylan is dissolved into the black liquor during kraft pulping. The xylan can be used directly as fibre surface modifier for increased surface charge or chemically modified before addition for an even higher fibre surface charge. It can be used as gas barrier in packaging. It also possible to graft lactic acid with xylan. This xylan-pla co-polymer has interesting thermal properties and can be extruded. A possible process for separation of xylan from black liquors or from or from pre/post-treatment of wood and cellulose product) has been proposed by Innventia and is patent pending, Figure 5. The xylan is concentrated by ultrafiltration, optionally followed by diafiltration to further purify the retentate. The resulting retentate can be further purified by anti-solvent precipitation (e.g. by using methanol or ethanol) to further increase the purity.

5 Xylan rich Process liquors Ultrafiltration / diafiltration Permeate Retentat MeOH/EtOH Precipitation MeOH/EtOH recovery Precipitation Solid/liquid sep. & washing - Impact of Figure 5. Separation process for technical xylan from xylan-rich process liquors. Xylan purity varies between % depending on extraction point and upgrading conditions. Xylan extraction points in the pulp mill There are several possible extraction points for xylan in the kraft pulp mill, summarized in Table 2 together with important implications for the separation process. Several research groups are currently exploring the possibility to extract xylan from the wood chips before pulping. The extraction can be performed either under alkaline or acidic conditions and the extracted amount of xylan as well as the product properties are strongly dependent on the conditions applied. If the pre-extracted wood chips are used for kraft pulping, both pulp yield and pulp properties can be affected. Another option is removal of xylan from early cooking liquor, where the xylan is less degraded and lignin content is low. This is of particular interest for kraft pulping of hardwood where differences in xylan and lignin dissolution kinetics can be used favourably to extract black liquor with high xylan and low lignin content. An important aspect is the severe process conditions (pressurized system and highly alkaline) which requires special ultrafiltration equipment (ceramic membranes). Also in this case pulp yield and quality can be negatively affected. In the weak black liquor the temperature is lower, but the ph is still strongly alkaline. The xylan after the cook is more degraded and considerable amounts of lignin are dissolved in the black liquor. One possibility is to extract xylan from black liquor after lignin separation ( LignoBoost filtrate ), where the process conditions are mild in terms of temperature and ph, allowing for more freedom in choice of membranes for the ultrafiltration process. An important advantage is that xylan extraction from weak black liquor and LignoBoost filtrate are both disconnected from the fibre line and thus has no impact on pulp yield and quality. Finally, xylan can be extracted from the pulp, which is currently studied by e.g. VTT in Finland. This directly affects the pulp yield and removing one tonne of xylan corresponds to one tonne less pulp. The removal of hemicelluloses either by pre-extraction of wood chips before pulping (e.g. pre-hydrolysis kraft) or from the resulting pulp is desired for certain qualities (dissolving pulp),

6 Table 2. Summary of different xylan extraction points in the pulp mill and implications for important parameters for the separation process. Extraction positions Before cooking Early cooking liquor Weak black liquor LignoBoost filtrate Bleached pulp Impurities Very low lignin Low lignin High lignin Medium lignin Very low lignin Process conditions Can be varied C, Strongly alkaline 100 C Strongly alkaline C, ph 10 Can be varied Xylan properties Can be varied High Mw Low Mw Low Mw High Mw Few sidegroups Effects on fibreline Yield loss Strength loss Some yield loss No effects No effects Yield loss Strength loss Alternate cellulose products Paper pulp will most likely be the dominating use for kraft pulps many years to come. The interest in other cellulose products like dissolving pulps for cotton replacement and special cellulose is increasing. In these products fibre strength is not important so conventional kraft cooking conditions are not needed and non pulpwood wood can be used. Innventia is currently working with two approaches for alternate cellulose products using modified pulping conditions. Akaline processing of other bio-mass streams Compared to other technologies for second generation ethanol alkaline treatment has a unique capability to separate wood components into relatively pure streams. The ideal concept would be to add a side process to a kraft pulp mill where e.g. forestry residues are treated with alkali. The resulting cellulose is hydrolyzed and fermented to ethanol. An even more attractive option would be to produce higher value added products like lactic acid or specialty cellulose instead of ethanol. The concept has been studied by Innventia and partners with "pure" cellulose in the hydrolysis stage, which makes it unique from other processes that aim to produce ethanol from lignocellulose. A sulfur-free lignin with low ash content and high dryness can be recovered as a valuable by-product. Laboratory trials with spruce, birch, aspen and annual plants indicate a yield of litres of ethanol per tonne of dry biomass, which corresponds to % of the theoretical value. The concept was demonstrated in a large scale mill trial where we produced about 70 tonnes of cellulose using the alkaline fractionation concept, and a corresponding amount of nearly sulfurfree spent liquor. Part of the cellulose was hydrolyzed and fermented to ethanol in a pilot plant. Results from the large-scale trial agreed well with those from the laboratory tests.

7 Figure 6. Alkaline co-processing of forestry residues (or bagasse) in a kraft pulp mill where the resulting fibres are converted to ethanol/biogass (or lactic acid) and sulfur free lignin is produced with the LignoBoost-process. Specialty cellulose Basic cellulose properties like porosity, specific surface area, chemical composition and degree of polymerisation are important factors governing chemical properties in kraft pulps especially in advanced applications like chemical reactivity and solubility. In these applications, high cellulose content combined with high fibre wall accessibility and a controlled degree of polymerisation is the aim. Normal kraft pulping and further processing results in a significant cellulose aggregation and corresponding low cellulose surface areas. Controlling the degree of cellulose aggregation and fibre wall porosity is crucial for maintaining a high accessibility and reactivity of the cellulose. Post-production de-aggregation of cellulose fibril aggregates is one option preventing aggregation. A more attractive alternative is to choose pulping conditions that prevents cellulose aggregation to start with. This is schematically illustrated in Figure 7. As an example of what can be achieved, we have been performed cooking trials where conditions were harsh compared to producing paper pulp. The idea was to produce a pulp with high cellulose content and high cellulose surface area by pre-hydrolysis, alkaline cooking and oxygen delignification. The resulting pulps had high specific surface areas (94-96 m 2 /g) and a high cellulose content (94-97 % glucose) which are attractive properties for dissolving pulps or a speciality grade cellulose. The resulting pulp also showed a good solubility in alkali which is important for dissolving pulp applications. Figure 7. Illustration of the relationship between fibril aggregation and specific cellulose surface area during kraft pulping. High surface area is normally occurring when the cellulose is less aggregated which is the case in wood. During kraft pulping aggregation occurs.

8 Conclusions There are many promising alternatives for converting a kraft pulp mill to a true biorefinery where all cellulose, hemicelluloses and lignin are converted to significantly higher potential value than today. In all cases, the value chains starting with separation/fractionation is the first step followed by upgrading/modification, conversion before the final applications. Xylan can be extracted from several locations in the pulp mill and can be used as fibre surface modifier, gas barriers and in thermoplastics. Lignin can be produced from black liquor and can be used as fuel, additive to wood pellets, activated carbon fibers and in other value added applications. Specialty cellulose can be produced in a modified alkaline process with high alfa-cellulose content. Specialty cellulose can be used as dissolving pulp for conversion to fibers that can replace cotton fibers or more advanced cellulose derivatives. Forestry residues can be coprocessed in a kraft pulp mill as feed-stock for ethanol, lactic acid or special cellulose and sulfur free lignin production. The future kraft pulp mill can increase its future revenues from production of value added products from lignin and hemicelluloses, alternate cellulose products and from several energy products. However it has to be understood that the economical attractiveness in production of fuels and energy can only achieved if the energy production is combined with more value added products such fibres, fibre polymers, materials and chemicals.