Source: UNIV OF MINNESOTA submitted to
ENVIRONMENTALLY FRIENDLY PULPING, BLEACHING AND BIOMASS CONVERSION PROCESSES
Sponsoring Institution
State Agricultural Experiment Station
Project Status
REVISED
Funding Source
Reporting Frequency
Annual
Accession No.
0175217
Grant No.
(N/A)
Project No.
MIN-12-065
Proposal No.
(N/A)
Multistate No.
(N/A)
Program Code
(N/A)
Project Start Date
Jul 1, 2012
Project End Date
Jun 30, 2017
Grant Year
(N/A)
Project Director
Tschirner, U. W.
Recipient Organization
UNIV OF MINNESOTA
(N/A)
ST PAUL,MN 55108
Performing Department
Bioproducts & Biosystems Engineering
Non Technical Summary
Due to growing human population and limitation in readily available fossil fuel we are facing unprecedented challenges that require the development of new technologies for a continuous increase in non-food based, sustainable use of agriculture, forest and other renewable bio-resources for bio-energy and bio-products. In this connection the concept of a "bio-refinery" has been discussed for some time now, promoting the separation of biomass into several streams, including pulp, paper, energy, chemicals and other Bioproducts. For example, during traditional pulping hemicelluloses are degraded into low molecular weight isosaccharinic acids and end up in the so called black liquor. Black liquor currently is concentrated and burned. As the heating value of hemicellulose sugars are considerably lower than that of lignin, extracting the hemicelluloses before the pulping for generation of high value products or bioenergy will improve the overall economics. Hemicelluloses can either be utilized directly in polymeric form for novel industrial applications (biopolymers, hydrogels) or, once hydrolyzed, they can serve as a source of sugars for fermentation. Other streams in pulp mills also offer themselves for fractionation. For example, all pulp fibers contain some short fibers that result in slower dewatering on the papermachine and additional energy consumption during drying. A fiber fractionation, separating the fiber streams into a high quality longer fibers (for papermaking) and short fiber fractions (to be converted to biofuels) has the potential to provide a clean glucose stream suitable for production of biofuels. Another pulp mill stream currently not utilized at its highest potential is lignin waste material. Lignin accounts for 15-35% of dry biomass, depending on the species and is composed of phenyl propane units. Lignin is biodegradable and due to its aromatic character clearly has the potential to be the source of high value biofuels, phenolic compounds, or other bioproducts . Combustion of lignin in black liquor is currently the most common use of this material. The value of lignin when used for combustion (based on dry basis, replacing coal) is estimated to be only $ 0.04/lb. Currently several new uses of lignin are being considered, varying from direct conversion to liquid fuels, to chemicals, fillers and polymers. One of the most effective use of lignin in an integrated biorefinery would be the direct conversion of lignin to liquid fuels. While this approach offers a high potential profit margin as well as a large potential market volume this approach also is very challenging. Gasification of lignin followed by Fisher Tropsch conversion to biofuels is technically feasible but costly. Another option being considered is pyrolysis, often combined with a subsequent hydrocracking process. The lignin macro-molecules, though less oxidized than carbohydrates, still have significant amounts of oxygen. Fast pyrolysis of biomass, including lignin, results in liquid products known as bio-oils and some carbon and gaseous products. The goal is to adjust the conditions in a way that the majority of material is in the form of a stable liquid bio-oil.
Animal Health Component
70%
Research Effort Categories
Basic
20%
Applied
70%
Developmental
10%
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
4030650200030%
4031799200020%
4020660200030%
5111799200020%
Goals / Objectives
Objective 1: To develop commercially viable modifications of existing pulping and bleaching processes with the goal to optimize biomass fractionation for high value fiber, biofuel, and other potential byproducts. Objective 2: To decrease environmental impact of existing or developing biomass conversion processes.
Project Methods
Methods Objective 1: Direct conversion of lignin to liquid fuel through pyrolysis of modified waste lignin. Develop fundamental understanding of changes introduced into the lignin molecule through biomass pre-treatment methods (for example: pre-steaming at different temperatures. Develop effective reductive treatment of lignin before pyrolysis. Study pyrolysis behavior and products in fast pyrolysis experiment with pretreated lignin. Determine pyrolysis products, C/O ratios, stability and fractionation behavior. Develop a high value filler/adhesive product from lignin recovered after hydrolysis of polysaccharides. Molecular weight reduction through oxidative and /or reductive treatments. Bleaching through oxidative or reductive treatments. Stabilization of brightness through propylene oxide. Incorporation of modified lignin into compostable plastics (i.e. PLA, PHA).Characterization of plastic composites (strength, degradability, water absorption). Harvest of Hemicellulose before subsequent commercial pulping procedures. Optimization of hemicelluloses removal to reduce pulping time (energy reduction) and bleaching chemical addition. Objective 2: Reduced energy and chemical application in commercial pulping procedures through process modifications (hemicelluloses removal) or additives.Utilization of agricultural residues as sustainable source of papermaking fiber.

Progress 01/01/12 to 12/31/12

Outputs
OUTPUTS: The overall objective this year was to develop value-added bio-based products capable of increasing overall economical feasibility of biomass converting industry (paper industry or biofuels industry). Specifically, we focused on developing novel bio-plastics from waste lignin. The objectives of this work included the development of effective methods to remove and modify lignin from biomass while releasing polysaccharides or mono sugars for other applications (e.g cellulose for paper, sugars for ethanol, butanol). We utilized Kraft lignin (lignin removed during Kraft pulping) as well as organosolv lignin (lignin removed with ethanol/water extraction) and cellulase enzyme lignin (lignin isolated by removing the carbohydrate fraction of biomass enzymatically). The isolated lignin, without and with varying degrees of modification, were used to develop novel lignin-lactide co-polymers. Poly Lactic Acid (PLA) was selected as the co-polymer component for our lignin based bioplastic based on the fact that PLA is a bio-based polymer that is gaining increasing market acceptance, is currently made from corn and is biodegradable and compostable. The lignin biopolymer is also naturally biodegradable and hence a co-polymer of lignin-PLA will continue to remain biodegradable and compostable. Low impact strength and low moisture barrier properties (higher water sorption and permeation) are two of the major still unresolved technical challenges with Poly Lactic Acid impeding their more widespread use. Lignin on the other hand is stronger, and relatively more hydrophobic. So far most lignin plastics formulations use polyurethanes, phenol-formaldehyde resins, epoxies, acrylics, or other petro-chemical based materials. We expect that a lignin-PLA copolymer can be tailored to address the above shortcomings and thus enabling wide spread use and product applications while avoiding use of petrochemicals. We decided to modify the lignin to make it better compatible with the very hydrophilic PLA, in addition the original dark brown lignin color needed to be removed or made lighter to allow wide spread use of this material as filler or as part of a co-polymer. The hydrophilic lactide molecules polymerize through ring opening reactions with -OH or COOH groups. Introduction of additional -OH and -COOH groups into lignin through oxidative treatment allowed better incorporation of lignin into a true lignin-PLA co-polymer. In addition, oxidative treatments also lightened the color of lignin. We were able to show that switchgrass lignin pretreated with a strong oxidizing agent (ClO2) reacted to a larger degree with Lactide to a lignin-PLA co-polymer than the untreated lignin; in addition the resulting co-polymer was considerably lighter in color. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: Biomass converting industry, including pulp and paper companies or companies interested in cellulosic ethanol or other biomass based biofuels PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
To accomplish our objectives of developing a PLA/lignin co-polymer we developed a novel oxidative pretreatment method using in situ peracetic acid generation utilizing glucose pentaacetate. The glucose pentaacetate can be synthesized in a very simple one step process using glucose and acetic anhydride. As glucose pentaacetate is reacted with hydrogen peroxide at high pH (11.4-11.6) peracetic acid and glucose are generated. Our lab studies indicate that after optimization of the process we can release 98% of the theoretically possible peracetic acid from glucose pentaacetate. This is significantly larger than current conventional pretreatment methods. We were also able to demonstrate that in situ generated peracetic acid effectively oxidizes and degrades lignin from biomass. In our experiments approximately 40% of the original lignin was removed from aspen biomass increasing enzymatic sugar release from 25% (for untreated biomass) to 85% (with peracetic acid pretreatment). Also, the lignin removed has been depolymerized resulting in smaller molecular weights. This approach combines both effective biomass pretreatment producing higher sugar yields in subsequent enzymatic hydrolysis and lignin modification for production of high performance bio-plastics into one single process step, thereby potentially reducing capital and operating costs. The modified lignin from the peracetic acid treatment can be separated and characterized before being utilized for our filler and co-polymer experiments. We were able to show that lignin fragments incorporated into a PLA structure delay biodegradation and water absorbency as compared to PLA, but will not prevent it. Early degradation is considerably slower for these co-polymers than degradation for PLA without lignin. Nevertheless, at longer reaction times the delayed co-polymer reaches the same level of degradation than the original PLA. The development of a bio-based PLA/lignin co-polymer holds the promise of making an integrated bio-refinery more economically feasible.

Publications

  • Schilling, J.S., Ai, J., Blanchette, R.A., Duncan, S.M., Filley, T.R., and Tschirner, U.W. (2012) Lignocellulose modifications by brown rot fungi and their effects, as pretreatments, on cellulolysis. Bioresource Technology 116: 147-154.


Progress 01/01/11 to 12/31/11

Outputs
OUTPUTS: One of the main goals in 2011 was to complete a more detailed study and optimize the process using kraft mill white liquor to pre-extract hemicelluloses from commercial aspen chips before subsequent kraft pulping. This approach allows recovery of hemicelluloses for high value applications, such as fermentation to biofuels. Another major factor is the option of reducing pulping time after pre-extraction and reduced the load going to the kraft recovery boiler. Our study was performed using lab size digesters and commercial aspen chips. After pre-extraction at varying temperatures and times all chip residues were analyzed to determine overall composition of the pre-extracted chips as compared to the starting material. Xylan extracted under these strongly alkaline conditions is in polymeric form (higher than 21,000 Da). The higher molecular weight allowed the use of ultra filtration with fairly large membrane pores (10,000 Da) and low pressure (310 kPa to 345 kPa). Since concentration of hemicelluloses clearly will be a requirement to obtain a useful starting material for biofuels fermentation processes or other applications this improvement will have a significant positive impact on the economics of the proposed process. After pretreatment the chips were directly moved to the digester and pulped using the permeate of the ultra filtration obtained from the same chip batch. Adjustments were made to take into consideration that during the pretreatment process the chips absorbed white liquor which stayed with the chips since we did not wash or dry the chips. In addition, the overall chip solid mass changed due to material loss in the pretreatment stage and the permeate addition rate was adjusted to account for this material loss. The permeate had a pH of 12.89 and was used as is, no additional chemicals were added to the system. Several different H-factor conditions were chose for the pulping. H-factor is a pulping variable combining time and temperature into one variable. In our studies we did not vary pulping temperature; all cooks were performed at 170 degree C. This means H-factor variation simply expresses variations in cooking time. To demonstrate that this process does not impact paper quality pulps from this study were converted to standard handsheets, using TAPPI test methods T205-88 and were tested for optical and physical properties. PARTICIPANTS: PI: Ulrike W. Tschirner, overall supervision of project. The wood chips used in this project were donated by SAPPI, Cloquet mill. TARGET AUDIENCES: Target audience is Pulp and paper Industry as well as biofuels industry (use of pre-extracted hemicellulose for biofuels) or industry interested in converting hemicellulose into other value added products (e.g. adhesives) PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
This study demonstrated that the use of typical kraft white liquor to pre-extract wood chips before commercial kraft pulping is a viable option to obtain a stream of fermentable sugars. Our process showed acceptable yields of polymeric hemicelluloses (mass fraction up to 4.8 percent based on o.d chips) that could effectively be concentrated in an ultra filtration stage to concentrations of up to 97 g per L using high pore sizes and low pressure. We also were able to demonstrate that no significant amounts of sulfur are retained with the xylan. Sulfur compounds have the potential to impact fermentation performance in subsequent biological conversions to biofuels. Considering the need for highest possible hemicelluloses yield pre-extraction at 90 degree C had a clear advantage as compared to the lower 70 degree C conditions. The permeate from the ultrafitration can effectively be used as pulping liquor without any additional treatments and/or additional chemical addition. The pretreatment has the potential to reduce pulping time, thereby increasing overall mill through put, or alternatively reduce the final lignin content, thereby reducing needs for bleaching chemicals. Reduction of chemicals such as Chlorine Dioxide commonly used for chemical pulp bleaching will reduce overall cost as well as discharge of harmful AOX to the environment. It was shown that paper properties after subsequent kraft pulping actually benefited from the pretreatment. If compared at the same lignin content pulps originating from runs with pre-extraction required shorter cooking times and resulted in paper with the same or slightly improved strength properties. This effect is likely due to the reduced exposure to high temperature at high pH in the cooking stage. No change in overall pulp yield was observed.

Publications

  • Ai, Jun; Tschirner Ulrike, 2012, Hemicellulose Extraction From Aspen Chips Prior To Kraft Pulping Utilizing Kraft White Liquor, Biomass and Bioenergy, in press


Progress 01/01/10 to 12/31/10

Outputs
OUTPUTS: As reported by several researchers mild alkali extraction of hardwood with green liquor or 3% NaOH or Na2CO3 removed about 6-8% of the starting wood material and resulted in a final pH close to neutral. In a subsequent Kraft pulping step the pulp yield was approximately 1% lower than the control. Unfortunately, the hemicelluloses yield for this mild alkali process is very low (around ~ 2-3 % xylan yield based on wood). The majority of the 6-8% of wood material removed are acetyl groups since hardwoods contain approximately 4% acetyl groups (on wood) which rapidly released under alkaline conditions. The mild alkali pretreatment process uses high temperatures and hence requires costly pressurized vessels and significantly increased capital costs. In addition, the final pH of the mild alkali process is around 7.0 and the extracted xylans are recovered in the form of less desirable low molecular weight oligosugars. Our own high alkali process extracted 50 to70 kg of hemicellulose per ton of wood and due to the higher pH the hemicelluloses remain in the polymeric form and are easier to separate. Subsequent Kraft pulping of the extracted wood chips demonstrated that the pulp yield could be maintained or even increased by 1-2% as compared to the control (not pre-extracted wood chips), alleviating one of the key concerns for pulp and paper manufacturers. In addition to maintaining quality and yield, we also demonstrated decreased residence times in the Kraft process thus resulting in potentially increased pulping capacity or the option of using lower cooking temperatures. In order to make the high alkaline pre-extraction approach commercially viable and economically feasible, all the chemicals from the extraction stage have to be recovered and reused. We concluded that a logical step forward for high alkaline pre-extraction would be to replace the NaOH used in our earlier studies with Kraft white liquor (containing NaOH and Na2S). Kraft white liquor has a pH value of above 12 and was shown to be suitable as pretreatment liquor for hemicelluloses pre-extraction of aspen chips. Several reaction times and temperatures were explored. Pre-extracted hemicelluloses was removed using membrane filtration and the permeate was used for subsequent pulping processes without any additional chemical charges. This process offers a very cost effective option to pre-extract hemicelluloses in existing Kraft pulp mills; it does not require additional chemicals and works under atmospheric conditions, using simple open reaction vessels. PARTICIPANTS: Ulrike W. Tschirner, PI Responsible for planning, evaluation and reporting of research. TARGET AUDIENCES: Scientific community in the area of pulping, bleaching and biomass conversion. Pulp and Paper Industry, other industry focused on conversion of biomass PROJECT MODIFICATIONS: Not relevant to this project.

Impacts
The overall goal of our research was the pre-extraction of hemicelluloses using existing white liquor before subsequent Kraft pulping. We explored different reaction times and temperatures. The maximum reaction temperature was selected to be 90 degrees Celsius, eliminating the use of costly pressurized vessels. The pre-treatment using white liquor for hemicelluloses did not show any significant effect on the cellulose content, but as expected strongly influenced the hemicelluloses content. At a pretreatment temperature of 90 degrees Celsius xylan content in the pre-treated chips decreases steadily with increased extraction time. The lower reaction temperature of 70 degrees Celsius show the same xylan content at 4 hour extraction time than the 90 degree extraction at only 1 hour. About 1 % of lignin is removed in all of the pre-extraction stages. After the extraction we used membrane filtration to separate out the polymeric hemicelluloses. The retentate (material retained after membrane filtration) contained 26%~28% of xylan which amounts to a yield of 3.8 %- 5.7 % based on the original aspen wood chips. As mentioned before, cellulose was resistant to the chemicals used under the studied pretreatment conditions and very little glucose was found in the retentate. The chips after pretreatment were directly moved to the digester and pulped with the permeate (liquor after hemicelluloses removal) obtained from the same chip batch. Several different H-factor conditions were chose for the pulping. The control samples (without pretreatment) showed the highest Kappa number for all designated H factors. The pretreatment led to less rejects and reduction of kappa number. For example, at an H-factor of 558, the Kappa number for the control chips was 25. It decreased to 24.3 with a 1 hour pretreatment at 90 degrees Celsius and to 23.8 and 23.1 with 2 and 4 hours pretreatment at 90 degrees Celsius respectively. It is clearly demonstrated, that the longer the pretreatment time, the higher the reduction of Kappa number at given cooking conditions. Another way of considering these finding are the potential for reduced cooking time to given lignin content. For example, Kappa number at 671 H-factor and 4 h 90 degree pretreatment is 19.8. To reach the same Kappa number with the control chips the cook has to be extended to an 824 H-factor. For a pulping temperature at 170 degrees Celsius this would mean a 16 minutes longer cooking time for untreated chips. The physical properties of handsheets made from pulp with different pretreatments and H factors were tested. The tensile index, burst and tear index were all improved with the pretreatment, clearly demonstrating that this approach is a viable alternative.

Publications

  • Huang, Hua-Jiang; Ramaswamy, Shri; Al-Dajani, Waleed Wafa; Tschirner, Ulrike. Process modeling and analysis of pulp mill-based integrated biorefinery with hemicellulose pre-extraction for ethanol production: A comparative study. Bioresource Technology (2009), Volume Date 2010, 101(2), 624-631.


Progress 01/01/09 to 12/31/09

Outputs
OUTPUTS: Encouraged by our findings related to alkali pre-extraction of hemicelluloses of wood chips before subsequent pulping procedures we continued our attempts in optimizing hemicelluloses yield as well as pulp yield and pulp quality. Besides Kraft pulping, we also explored other pulping technologies, specifically Alkaline Sulfite Anthraquinone (ASA) and Alkaline Sulfite Anthraquinone Methanol (ASAM). We selected these specific two processes because it has been shown that ASA and ASAM processes produce pulps with higher yields and with better strength properties than the Kraft process. The resulting pulps were analyzed for composition and tested for optical and physical properties. In addition, we examined several options for hemicelluloses concentration, including solvent precipitation and membrane filtration. This work resulted in several poster presentations (Bioproducts and Biosystems Engineering Advisory Council Meeting, poster session October 2009) and papers. PARTICIPANTS: Ulrike Tschirner, Waleed Al Dajani TARGET AUDIENCES: Scientific Community in the area of pulping, bleaching and biomass conversion, Pulp and Paper Industry, Ethanol Industry, other Bioproducts Industry. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
In line with current discussions on integrated biorefineries, pulp mills are considering pre-extraction of hemicelluloses prior to the traditional pulping processes. During the commercial chemical pulping process, part of the wood components, namely hemicelluloses, are degraded and subsequently combusted in the recovery furnace of the pulp mill. These hemicelluloses provide less than 20% of total energy resources for a recovery furnace as the bulk of the energy is derived from the combustion of lignin. Pre-extraction of the hemicellulose prior to the pulping process would make these polysaccharides available for other applications, such as fermentation to bioenergy (ethanol) or conversion to other bioproducts. We demonstrated that pre-extraction of aspen wood chips with alkaline media resulted in an improved Kraft pulping process. Since it has been shown that ASA and ASAM processes produce pulps with higher yields and better strength properties than the Kraft process we decided to focus on these processes. In addition, efficient recovery of hemicellulose after the extraction stage is a key step for a successful conversion to ethanol or other products. This includes separation, concentration and purification of the hemicellulose without any dilution or pH adjustment as well as recovering the inorganic cooking chemicals. Nanofiltration is currently being tested as a viable option for separating hemicelluloses from alkaline hydrolysates. Compared with ASA pulping of original aspen chips, the most obvious advantages of the ASA-pulping of the extracted residue are the considerably lower chemicals (27.5 vs. 15% total NaOH and 0.2 vs. 0.1% AQ) and the shorter cooking time at 170 oC (165 vs. 60 minutes) required to give pulps with the same delignification (9.6 vs. 10.9 kappa number) and similar yield (539 vs. 528 kg). In addition, the pulp produced after the extraction of hemicelluloses retained its advantage in all physical characteristics except for a somewhat lower tensile index, probably due to a higher cellulose/hemicellulose ratio. Similarly, more chemicals were consumed during the ASAM control cook (25 vs. 15% total NaOH, 0.2 vs. 0.1% AQ and 20 vs. 10% CH3OH) and 90-minute-longer cooking time at 170 oC to get pulp with similar kappa number as the one produced from the extracted chips. Unlike the control ASAM cook, not all the NaOH was consumed during the production of the ASAM pulp from the extracted chips. The milder pulping conditions after the hemicellulose extraction resulted in a similar pulping yield as the control cook. These results make ASA and ASAM pulping after the extraction of hemicellulose a contender with the corresponding conventional processes, provided that all the chemicals after the extraction step are efficiently separated and recovered. This work demonstrated advantages of using the residue after alkaline extraction of aspen chips over unextracted chips for ASA and ASAM processes; 50-60% less charges of chemicals and 1.0-1.5 hr less pulping time without compromising the yield or physical properties of the final pulp. It can be applied in sulfite mills with sodium base recovery systems without major modifications.

Publications

  • Wafa Al-Dajani, Waleed; Tschirner, Ulrike W.; Jensen, Tryg. Pre-extraction of hemicelluloses and subsequent kraft pulping. Part II: acid- and autohydrolysis. Tappi Journal (2009), 8(9), 30-37.
  • Ai, Jun; Tschirner, Ulrike. Fiber length and pulping characteristics of switchgrass, alfalfa stems, hybrid poplar and willow biomasses. Bioresource Technology (2009), Volume Date 2010, 101(1), 215-221


Progress 01/01/08 to 12/31/08

Outputs
OUTPUTS: We continued our research on pre-extraction of hemicellulose before subsequent pulping processes. Acid, alkali as well as water extraction were explored. Different time and temperature sets as well as different dilution factors were explored. Pulping processes explored included Kraft pulping, sulfite pulping as well as ASAM pulping. One of the main challenges for these processes is the concentration of hemicellulose after the extraction. We explored several options such as precipitation using ethanol followed by several washing stages and filtration using ultra- or nano-filtration set-ups. Hemicellulose generated this way was used for subsequent fermentation processes. The wood species examined was aspen. We also continued our research on alternative fiber sources for papermaking. Hybrid poplar, willow, false indigo, alfalfa stems, and switchgrass were defibered and fiber length, fiber length distribution as well as width of the fiber were determined. Selected samples were pulped using the Soda AQ or the Kraft process, paper handsheets were produced in the lab and tested for strength and optical properties. The results were disseminated in several posters and as a presentation in a international conference. Several papers gave been submitted. PARTICIPANTS: Ulrike W. Tschirner, Waleed Al-Dajani, Ai Jun TARGET AUDIENCES: Scientific community in the area of pulping, bleaching and biomass conversion. Pulp and Paper Industry, other industry focused on conversion of biomass PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.

Impacts
Hemicellulose pre-extraction significantly improved pulping conditions, resulting in reduced reaction time and chemical addition rate. Unfortunately acid pre-extraction and hot water extraction, which also is acid due to the naturally present acetyl groups in wood, resulted in significant yield loss in all pulping processes examined. Alkali pre-eaxtraction while resulting in lower hemicellulose yield had a significantly better response during pulping. Our study has demonstrated that it is possible to extract hemicelluloses from aspen chips without detrimentally affecting the quality of the final fiber product during subsequent pulping. When compared to a control Kraft pulp produced conventionally without any extraction, the benefits obtained from extracting some of the hemicelluloses from aspen chips prior to Kraft pulping include the isolation of 40-50 kg hemicellulose per tonne of wood chips while maintaining the same pulp yield. By modifying the pulping conditions for the extracted material, a 10-minute-shorter residence time at maximum cooking temperature is made possible as well as a saving of ~ 5.5-8.0 kg of NaSH per tonne of chips. NaOH charge is slightly higher for the modified process, but total NaOH consumption is similar or slightly lower as compared to a control cook. Although hemicellulose extraction can have some impact on pulp properties, optimizing the extraction and pulping conditions allows full control over the cellulose/hemicellulose ratio and, consequently, provides a pulp with retained fiber properties. Results fro the alternative fiber pulping experiments showed different behavior for switchgrass, alfalfa stems and second year willow and hybrid poplar biomass. Alfalfa stems and switchgrass show average fiber length close to values found for aspen wood chips, but have a much more narrow fiber distribution and significantly more fines. The second year willow and hybrid poplar have lower fines content than alfalfa or switchgrass but a very low average fiber length. These fiber properties translate in good tensile and tear strength for alfalfa and switchgrass but poor tear properties for the two fast growing tree species. Switchgrass could be pulped successfully using Soda and Soda AQ pulping conditions. Anthraquinone addition, as expected, increased overall pulp yield and reduced residual lignin content. Both fast growing tree species pulped well using the Kraft process. Interestingly alfalfa stems showed very poor response to soda and soda AQ pulping but responded well to Kraft pulping and Kraft AQ pulping. One big drawback for all four biomass samples examined is the very low overall pulp yield. While traditional wood based pulp mills expect around 50% pulp yield our material showed considerably lower values. The highest yields were obtained with poplar and switchgrass (both around 43%). The low yield is caused by the overall composition or the material, while mature aspen wood has a total of 65.7% polysaccharide content (cellulose and hemicellulose) this number is considerably smaller for switchgrass (52.4%), alfalfa stems (39.4%), willow (50.6%) and hybrid poplar ( 45.3)

Publications

  • Al-Dajani, W., Tschirner, U. Pre-extraction of hemicelluloses and subsequent Kraft pulping. Part I. Alkaline Extraction, TAPPI Journal June 2008
  • Hua-Jiang Huang, Shri Ramaswamy, U. W. Tschirner and B.V. Ramarao; A review of separation technologies in current and future biorefineries, Separations and Purification Technology 62 (2008)1-21, 2008


Progress 01/01/07 to 12/31/07

Outputs
OUTPUTS: Planned and executed lab research focused on improved pulping processes and chlorine free bleaching procedures. This included experiments using phosphonates as additives into the pulping process and lignin activation using peracetic acid followed with subsequent peroxide bleaching stages. A novel approach included pre-extraction of hemicellulose from aspen wood chips before traditional pulping processes, thereby considerably reducing pulping time and chemical addition level. Alternative fiber sources (wheat straw, hybrid poplar, willow, false indigo, alfalfa) were evaluated with respect to their behavior in traditional pulping processes. Dissemination attempts included a presentation at the TAPPI pulping conference, October 2007. PARTICIPANTS: Ulrike W. Tschirner (PI) William Kusch, Graduate Research Assistant TARGET AUDIENCES: Scientific community in the area of pulping, bleaching and biomass conversion. Pulp and Paper Industry, other industry focused on conversion of biomass. PROJECT MODIFICATIONS: NA

Impacts
Hemicellulose preextraction significantly improved pulping conditions, resulting in reduced reaction time and chemical addition rate. In addition the pulps show a higher brightness at given lignin content. These findings have the potential to significantly improve the economics of existing pulp mills. Not only does this process provide a new source of fermentable sugars, it also reduces cost by reducing cooking time and chemical addition levels. In also has the potential to increase mill throughput. In addition, alternative fibers have the potential to partially replace commercial wood chips.

Publications

  • Tschirner et. al. (2007) RECYCLING OF CHEMICAL PULP FROM WHEAT STRAW AND CORN STOVER, Bioresources, 2(4) 536-543


Progress 01/01/06 to 12/31/06

Outputs
We continued our study using HEDP as an additive in the Kraft pulping process. The main focus was on characterization of changes in black liquor lignin with the addition of phosphonates. Lignin chemical reactivity in various modification processes is strongly influenced by its phenolic hydroxyl content. During lignin degradation new phenolic groups are generated. Thus, the quantitative measurement of phenolic hydroxyl groups supplies relevant information concerning the lignin structure and reactivity as well as the extent of lignin degradation. Phenolic groups in lignin were determined using the UV spectroscopy method. The cleaving of alkyl-aryl ether linkages during the Kraft cook leads to formation of new phenolic end groups. Thus, the phenolic hydroxyl content in the residual lignin increases during the cook. We observed lower amounts of phenolic groups in the black liquor lignin of HEDP cooks compared to control cooks. This could indicate that the HEDP lignin have been less degraded. This indicates that HEDP is promoting lignin removal from the cell wall before it has reacted to the same extent than the lignin in the control runs. The discrepancy of free phenolic group between control and HEDP cooks appears to be more visible when the H-factor increases. Phenolic groups vs. kappa number indicate that at given kappa number, the cooks treated by HEDP have lower free phenolic groups compared to the control ones. For kappa number below 20, there is a significant difference between control and HEDP cooks in terms of amount of phenolic units. However, when the kappa number is above 20 the difference becomes negligible. In an earlier study it was observed that softwoods showed no response to the addition of phosphonates in the Kraft process. Therefore it was speculated that the amount of Syringyl present or the Syringyl/Guaiacyl ratio could be the determining factor for response to phosphonates addition. S/G ratio for different wood species and pulps cooked to different lignin content with and without HEDP was determined. No relationship between S/G ratio and response of wood species to HEDP could be established. Comparing S/G ratio at a given degree of delignification (kappa number) makes it apparent that the pulps from the control run has significantly lower S/G ratios. Molecular weight and its distribution are an essential aspect to distinguish the polymeric properties of lignin as well as its condensed structures. This study is using size exclusion chromatography using NaOH as a solvent. PEG (Poly Ethylene Glycol) standard is used to determine the molecular weight of lignin. The molecular weights of black liquor lignin from runs treated with HEDP are higher compared to the control pulp. Addition of phosphonates to the Kraft process causes faster lignin removal and therfore results in shorter cooking times and/or lower temperatures. This effect is caused by a improved removal of lignin from the wood cell walls.

Impacts
Use of additives in commercial pulping processes is expected to improve pulp yield and response of these pulps to environmentally friendly bleaching chemicals (such as oxygen, hydrogen peroxide or ozone). The impact of this approach is twofold. Increased yield will reduce wood use and better response to bleaching chemicals will reduce production cost as well as the environmental impact of bleaching stages (less chlorine or chlorine dioxide will be used).

Publications

  • Prasetio, Ruth M. Impact of HEDP addition in aspen Kraft pulping, Master Thesis, University of Minnesota, October 2006


Progress 01/01/05 to 12/31/05

Outputs
Pulping processes using additives in the digester were examined. One of the distinct differences observed during Kraft cooks with and without the Phosphonate additives appears to be molecular size of the lignin fragment in the black liquor. Molecular weigh measurements indicate a slightly higher molecular weight for residual lignin in cooks using phosphonates as additives. This indicates that either the lignin is removed faster or reprecipitation of lignin onto fiber is prevented by the additive. In a continuous effort to understand the fundamental reactions involved, we determined free phenolic groups present in lignin fragments isolated from black liquor. While the data are slightly scattered there is some indication that lignin fragments from Kraft cooks using the phosphonate additives show lower free phenolic content. This finding agrees with the fact that these fragments also have slightly higher molecular weight.In addition to the above mentioned study we continued examining response of different wood species to addition of high levels of phosphonates. For example, birch, which was not responding well at lower levels clearly demonstrated improved pulping properties at very high addition levels. At this time we are running experiments using different addition levels on number of different wood species. In addition to our effort focused on different wood species we also stared examining number of non-wood material. Using non-woods such as corn-stover, wheat straw, flax straw etc, also has the potential to reduce use of wood material. Interestingly non-wood material showed a very positive response to begnin bleaching chemicals such as hydrogen peroxide. It was possible to obtain fully bleached chemical pulps using several hydrogen peroxide stages.

Impacts
Use of additives in commercial pulping processes is expected to improve pulp yiled and response of these pulps to environmentally friendly bleaching chemicals (such as oxygen, hydrogen peroxide or ozone). The impact of this approach is twofold. Increased yield will reduce wood use and better response to bleaching chemicals will reduce production cost as well as the environmental impact of bleaching stages (less chlorine or chlorine dioxide will be used).

Publications

  • "Natural Fibers Application and Composites - Potentials for Alternative Non-Wood Fibers", Shri Ramaswamy, Ulrike Tschirner Bio-based Industry Outlook Conference, Ames, IA, August 2005


Progress 01/01/04 to 12/31/04

Outputs
Kraft cooking using phosphonates as additives was continued. It could be shown that yield improvement found with HEDP (1-Hydroxyethylene-1,1-di-phosphonic acid) addition to the Kraft digester is caused by increased cellulose retention. Hemicellulose content at given yield is not significantly affected. At given cooking conditions HEDP addition to the digester results in improved lignin removal and cellulose retention. Examination of physical properties of aspen pulps showed that tensile strength, burst and tear properties as well as freeness were either unchanged or slightly improved. Zero-span tensile on the other hand showed up to 8 % improvement. In attempt to quantify the different behavior of hardwoods and softwoods detailed mass balances of aspen and spruce Kraft cooks with and without addition of HEDP were performed. In both cases the majority of HEDP is removed from the Black Liquor and is carried out of the system with the unwashed pulp; approximately 90 % for both wood species stayed with the pulp). This material is largely removed in the following washing stage. In addition to the above mentioned experiments we are continuing to explore numerous methods to determine fundamental reaction mechanism involved in yield improvement and enhanced lignin removal through phosphonates. It could be confirmed that molecular weight of lignin fragments in black liquor at the end of a cook are larger in cooks with HEDP added than in the control. This could indicate that the HEDP is helpful in removing larger lignin fragments, speeding up the overall removal of lignin from the wood fiber. One other hypothesis tested was suggestion that softwoods and hardwoods show a distinctly different behavior due to the fact that their lignin structure is different. Softwood has considerably lower methoxygroups (conifer-type lignin) than hardwood (syringyl-type lignin). Preliminary studies attempting to correlate methoxy content to response of the additives in the Kraft digester did not confirm this theory. There appears to be no correlation between methoxy group content and response in the digester.

Impacts
Use of additives in commercial pulping processes is expected to improve pulp yield and response of these pulps to environmentally benign bleaching chemicals (such as oxygen, hydrogen peroxide or ozone). This approach will help reduce wood use and will improve bleach plant effluent (reduced use of chlorine containing chemicals).

Publications

  • Tschirner, U.W. ; Smith T. Phosphonates as additives in Kraft Pulping. Paper Summit, Altanta GA, May 2004


Progress 01/01/03 to 12/31/03

Outputs
Focus of study this year was the use of additives to Kraft cooks. The additives used included several phosphonates and surfactants. The goal was to achieve faster lignin removal and improved pulp yield. Comparison between several species confirmed that aspen shows the most favorable response. Commercial red maple chips showed some improvement with phosphonate addition at lower cooking times, the effect was less pronounced at longer processing. This indicates that the addition of phosphonates might be useful to reduce cooking time in commercial Kraft processes, thereby reducing energy consumption or cost of bleaching chemicals. It was discovered that increasing carbonate levels present in the cooking liquor have an increasingly detrimental impact on the response of the phosphonates. Bleaching of phosphonate treated pulps showed a distinct improvement in final brightness and chemical usage. Especially chlorine free bleaching stages such as oxygen or hydrogen peroxide showed a significant improvement. An extensive study focused on the fate of metals in the pulping process using phosphonates as additives. As expected transitions metals such as Fe, Cu Mn were reduced considerable. This explains the improved response of bleaching chemicals. Interestingly the fate of phosphonates was different for different compounds. Smaller molecules such as HEDP remained with the pulp, even after extensive washing. Larger phosphonates such as DTMPA obviously do not have the same capability to attach to pulp and are mainly found in the effluent.

Impacts
Use of additives in commercial pulping processes is expected to improve pulp yield and response of these pulps to environmentally benign bleaching chemicals (such as oxygen, hydrogen peroxide or ozone). This approach will help reduce wood use and will improve bleach plant effluent (reduced use of chlorine containing chemicals).

Publications

  • Effect of cereal straw fibre addition ot papermaking furnish, U. Tschirner, S.Ramaswamy, A. Goel, Pulp and Paper Canada, 104:10 (2003)
  • Use of Phosphonates in Kraft pulping, U. Tschirner, L. Villalba, G. Sutton, Poster presentation TAPPI pulping conference, October 2003 Chicago
  • Fate of Metal Cations in the Kraft Pulping Process Modified by Phosphonates, Wei Li, U. Tschirner, TAPPI pulping conference proceedings, October 2003 Chicago


Progress 01/01/02 to 12/31/02

Outputs
This year the focus of this project has been yield improvement for the Kraft pulping process. The Kraft process is used to produce approximately 51 million metric tons of pulp per year in the US. It generally has yields of 45-50%. We examined the potential use of phosphonates during Kraft cooking, considering the fact that phosphonates have dispersant properties they are expected to facilitate penetration of chemicals into chips as well as removal of dissolved lignin from chips. This will mean shorter cooking times (energy savings and yield improvements). In addition their chelating properties favor brightness improvements in subsequent bleaching processes, reducing the overall chemical application rates. A wide variety of phosphonates was used for a preliminary screening study. The two most promising were determined to be HEDP and DTMPA.Results demonstrate that 0.1-0.2% addition of either HEDP or DTMPA give rise to significant reduction in kappa number varying from 4.2 to 7.9 points in the entire H-factor range studied, i.e. 558 to 1000, indicating that delignification is dramatically improved by phosphonate addition. The lower the applied H-factor, the greater is the reduction. Pulps cooked with 0.1-0.2% phosphonates at H-factor 600 are comparable in kappa number to pulps processed without additive at H-factor 1000. In other words, for a target kappa number (e.g. 21) at the fixed H-factor 1000, application of 0.1-0.2% phosphonates would mean 27 minutes savings in total cooking time. This would certainly result in significant savings in operating costs while increasing productivity.In industrial practice, at desired lignin removal levels pulp yield is of great interest to pulp-producers as it is directly associated with production and thus profitability. 0.2% HEDP improve screened pulp yield by 1-5% at H-factors over 600. In addition, the reject level is reduced by 0.5-5% at H-factors below 800. The outcome of low rejects was most likely due to improved delignification. Measuring the degree of cellulose polymerization (DP), pulp viscosity is considered an important parameter monitoring the extent of carbohydrate degradation during pulping; it is generally considered an indicator of pulp strength properties. Absolute pulp CED viscosity at equivalent degree of delignification reveal that HEDP treated pulps show considerable increases with respect to pulp viscosity. Use of AQ resulted in similar viscosity improvement, but the difference to the control is significantly less.Phosphonate treated pulps display improved responses to subsequent bleaching sequences such as DEDED and OPD, allowing ClO2 minimization without compromising high target brightness. Beyond extended lignin removal, a measurable decrease in metal ions of pulps including Mn and Cu is likely to be responsible for the enhanced bleaching performance.

Impacts
Energy savings of around 10 % of the digestion process are expected. Assuming energy consumption of 4.7 million BTU/metric ton this could mean as much as 169.2 billion BTU or approximately $ 1 million in energy savings per year for a 1000t/day mill (350 days). In addition this process has the potential for yield increases of 4-6%, which means a mill keeping the throughput at the same level will have a decrease in wood cost of 8-12 %. Depending on wood cost for a particular mill this can amount to an additional $ 3 million/year for the same type of mill. Besides the two benefits listed above this process also can be used to achieve an "extended delignification" effect without investment costs. This in turn means reduced bleaching chemical use, reduced overall effluent discharge and reduced dischare of chlorinated organic material.

Publications

  • Li, W. ; Tschirner Ulrike, Phosphonates as additives in Kraft pulping- a preliminary study" , November 2002 TAPPI Journal


Progress 01/01/01 to 12/31/01

Outputs
The project continued to place strong focus on oxygen bleaching of chemical pulps. The purpose of our research is to investigate different additives as to their impact on the selectivity of oxygen bleaching. A series of potential additives was screened and two were selected as promising candidates.The two selected additives are HEDP (1-Hydroxyethylidene-1,1-diphosphoric acid) and magnesium D-gluconate. The two additives showed different responses. Gluconate addition slightly slowed down lignin removal in the oxygen bleaching stage, while HEDP promoted lignin removal. In addition the HEDP treated pulps showed the highest brightness values. To characterize the selectivity of the oxygen delignification, pulp viscosity and strength of pulps were examined. At a given degree of delignification viscosity is highest for the HEDP samples. No positive effect on viscosity could be observed for gluconate. Examination of burst and tensile index demonstrated that HEDP pulps have a significantly higher bonding strength than the control. The gluconate samples behave significantly different. At higher KAPPA numbers no improvement in burst strength is visible.At lower KAPPA number values gluconate is as effective as the 1% HEDP in protecting bonding strength of the pulp. Since the hydroxyl radical is considered the most detrimental radical present during oxygen bleaching an attempt was made to demonstrate the concentration of hydroxyl radicals generated under different conditions. Interestingly, there is no significant difference between the runs using HEDP or gluconate. Both show the same increase in effectiveness with increasing additive levels. Also, for both additives the effect levels off near 3% addition levels.Analysis of metal content confirm that HEDP is a better chelant. Most noticeable is the impact on Mn and Ca ions.Based on the viscosity measurements is was suspected that the gluconate treated samples might have seen an increased amount of random chain cleavage reactions. Since random chain cleavage gives rise to new peeling and stopping reactions this effect should be reflected in the carboxyl content of pulps. Analysis of carboxyl groups content appears to confirm this hypothesis. The carboxyl groups content in the gluconate treated pulps was 6-20 % higher than the control. The HEDP treated pulps are slightly lower in carboxyl groups content than the control.It can be concluded that HEDP has a good potential as additive to oxygen bleaching. Unfortunately, so far the more significant strength improvements could only be observed at application levels that are not commercially applicable due to cost issues. Therefore, part of the continuation of this project will focus on optimization. In addition, comparative runs using "superchelated" pulps will be performed. "Superchelation" will result in pulps with essentially no residual metal content. Exposing these pulps to the same oxygen bleaching procedures will allow us to draw conclusion on the actual impact metal have in this particular pulp. A possible reaction path for the protective capabilities of HEDP will be proposed.

Impacts
Oxygen bleaching has been widely accepted in paper industry as an environmental friendly bleaching stage, mainly replacing chlorine dioxide. Improvement in selectivity will allow even more wide spread use of this technique. One presentation at the TAPPI pulping conference resulted in considerable interest in this project by industry representatives

Publications

  • Jewell, M.; Tschirner, U. " Improved Selectivity of Oxygen Bleaching through use of Chemical Additives", 2001 TAPPI Pulping Conference Proceedings, November 4-7, Seattle, WA


Progress 01/01/00 to 12/31/00

Outputs
The project was started using peracid as pretreatment to improve bleachability. As planned it was shifted to include oxygen bleaching as another one of the environmental friendly bleaching methods.Presently, the extent of delignification during commercial oxygen bleaching is restricted by poor selectivity (pulp strength loss) and by mass transfer limitations (poor solubility and diffusion rates of oxygen in water).Oxygen is a free radical, which tends to react with appropriate substrates in regions of high electron density. This results in a four-step process by which oxygen is reduced to water and the substrate is oxidized. During this pathway several types of radicals (for example superoxide radical, hydroperoxyd radical, hydroxyl radicals) are generated. Several of these species show poor selectivity, which means they not only attack lignin, but also react with the polysaccharide part of the pulp. Our present research efforts are focussed on improving the selectivity of oxygen bleaching under medium consistency condition. Medium consistency conditions were selected since medium consistency operations are more abundant due to their lower capital costs, and greater ease of stock handling.A wide variety of additives was selected and screened under oxygen bleaching conditions. Several additives showed a slight improvement in lignin removal while maintaining slightly higher pulp viscosity. At the present time a detailed statistical analysis of the screening results is being performed to determine the most promising additives. Two additives will be selected and used for more detailed examination. This examination will include the use of different reaction conditions and will propose a possible reaction pathway.At the present time one graduate research associate ( PhD level) is assigned to this project.

Impacts
Paper Industry has a strong interest in oxygen bleaching and in this particular project. For example our raw material for this study was obtained from a commercial mill and the mill personal was very supportive and interested. If successful this project could have a tremendous impact on bleaching practices in paper industry. Our earlier work using peracid treatment to improve bleachability of pulp was puplished and received considerable attention.

Publications

  • U. Tschirner, R. Segelstrom "Peracid pretreatment for improved ink removal and enhanced bleachability of recycled fiber" Progress in Paper Recycling, February 2000, p 15-23


Progress 01/01/99 to 12/31/99

Outputs
Evaluation of feasibility of wheat straw fiber as raw material for papermaking was performed. The two pulping processes used were a.) the traditional Soda AQ process followed by chlorine dioxide bleaching and b.) the more environmentally friendly organosolv process (ethanol based). The pulps produced with the organosolv process were bleached without a TCF (Totally Chlorine Free) process using hydrogen peroxide. Physical and optical properties for both pulp types were tested. The pulps showed substantially different properties. The traditional pulps were stronger and slightly brighter, but the organosolv/peroxide pulping process is considerably more environmentally friendly. Presently mechanical treatment after the pulping process for the organosolv/peroxide pulps is being explored. Preliminary studies indicate considerable strength improvements. Other fiber sources such as flax, hemp and recycled fiber were evaluated in addition to the cereal straw. Flax and hemp are long fiber material, resulting in very high tear strength properties. A recycled fiber process capable of effectively remove ink particles was developed.

Impacts
Organosolv/peroxide process presently evaluated for potential use for small scale mill in North West Minnesota. Funding for feasibility study and pilot plant trial provided by LCMR grant ( $200,000).

Publications

  • Tschirner, U. Jagannadh S. "Use of Canadian Oils Seed Flax Fiber in Traditional Kraft Pulping" TAPPI pulping Conference proceedings, October 1999.
  • Wagner, N. ;Ramaswamy S. ;Tschirner, U.; Feasibility of Cereal Straw for Industrial Utilization in Minnesota" American Journal of Alternative Agriculture , Spring issue 2000.
  • Tschirner, U. Segelstrom R. "Peracid Pretreatment for improved Ink removal and enhanced bleachability of recycled fiber" Progress on paper Recycling, February 2000.
  • Cao, B. Tschirner, U., Ramaswamy S. "A study of changes in Wet Fiber Fexibility and Surface Condition of Recycled Fibers". Paperi ja Puu, Finnish Pulp and Paper Journal, p. 117 February 1999.


Progress 01/01/98 to 12/31/98

Outputs
Peracid treatment evaluated as deinking help for low grade recycled fiber. Brightness and ink removal could be improved considerably. Also continued evaluation of possible use of agricultural residues for papermaking fiber. Chlorine free bleaching sequences appear very suited for straw based fiber.

Impacts
(N/A)

Publications

  • Tschirner, U., Wang, D. "Hydrogen Peroxide Bleaching of Mixed Recycled Fiber." Progress in Paper Recycling, November 1998, p. 15.
  • Cao, B., Tschirner, U., Ramaswamy, S. "Effect of Chemical Composition on Recycling." TAPPI Journal, Vol. 81, No. 12, December 1998.


Progress 01/01/97 to 12/31/97

Outputs
Continued evaluation of peracids as agents for improved deinking and bleachability of reycled fiber. Several peracids were evaluated. It was found that a mixture of peracidic acid and caro's acid gave the best response. Dirt count of treated material is significantly below untreated material. Bleaching response was improved by up to 5 points. Project will be continued by including response of physical properties such as tensile index, burst and tear in the evaluation.

Impacts
(N/A)

Publications

  • Cao, Bangji, Ulrike Tschirner, Shri Ramaswamy, and Arthur Webb. 1997. A Rapid Modified Gas Chromatographic Method for Carbohydrate Analysis of Wood Pulps. TAPPI Journal, Vol. 80, No. 9, pp. 193-197.
  • Cao, Bangji, Ulrike Tschirner, and Shri Ramaswamy. Impact of Pulp Chemical Composition on Recycling. 1997 TAPPI Pulping Conference, pp. 1081-1096.
  • Orth, Leslie and Ulrike Tschirner. Evaluation of Native Grasses for Pulp and Papermaking. 1997 TAPPI Engineering & Papermakers Conference, Nashville, TN. Proceedings TAPPI-PPERA Student Conference Poster Session, pp. 91-93.
  • Cao, Bangji, Ulrike Tschirner, and Shri Ramaswamy. Effect of Chemical Composition on Recycling. 1997 TAPPI Engineering & Papermakers Conference, Nashville, TN. Proceedings TAPPI-PPERA Student Conference Poster Session, pp. 73-79.