Case studies on sugar production from underutilized woody biomass using sulfite chemistry

Abstract

We examined two case studies to demonstrate the advantages of sulfite chemistry for pretreating underutilized woody biomass to produce sugars through enzymatic saccharification. In the first case study, we evaluated knot rejects from a magnesium-basedsulfite mill for direct enzymatic sugar production.We found that the sulfite mill rejects are an excellent feedstock for sugar production. In the second study, we presented SPORL (sulfite pretreatment to overcome the recalcitrance of lignocelluloses),a sulfite pretreatment process based on modified sulfite pulping for robust bioconversion of softwood forest residues. Sulfite pulping technology is well developed, with proven commercial scalability, and sulfite pretreatment is a strong contender for commercial adoption. woody biomass through enzymatic saccharification. Application: Mills can consider sulfite chemistry, which has the advantage of high-yield sugar production from roducing sugars from underutilized woody biomass for pretreating woody biomass for sugar production using Pcan be a potential revenue stream for pulp mills enzymes. Unlike pulping, where the goal is to achieve without competing with feedstock for pulp production. as much as delignification as possible while preserving To efficiently release sugar from woody biomass through hemicelluloses, pretreating biomass for sugar production enzymatic saccharification, a pretreatment step is does not need to achieve complete delignification required to remove the strong recalcitrance of wood but requires significant dissolution of hemicelluloses polymer matrix to biological deconstruction [1]. Several [8] to produce a porous substrate to improve cellulose chemical-including pulping processes have been studied accessibility to cellulase. The dissolution of hemicelluloses for pretreating woody biomass [2-6]. However, limited can also fractionate hemicelluloses into the form of successes were achieved in terms of good sugar yield. monomeric sugars, which is very desirable for biomass Sulfite chemistry has several unique characteristics that biorefining. The ability of delignification by sulfite under are considered disadvantages for pulping; for example, acidic conditions can facilitate hemicellulose dissolution at deploymerization of hemicelluloses often results in pulps high temperatures to reduce reaction time while partially with low strength and yield [7]. Furthermore, acidic or solubilizing and sulfonating lignin. Table I lists the utility bisulfite pulping requires low temperature and prolonged of the characteristics of sulfite chemistry for enzymatic time for delignification to avoid lignin condensation at saccharification of woody biomass by comparing with low pH. However, these disadvantages can be beneficial their effects on wood pulping [9-13]. SEPTEMBER 2015 I VOL. 14 NO. 9 I TAPPI JOURNAL 577 9 We have demonstrated the robust performance of sulfite pretreatment to overcome the recalcitrance of lignocelluloses (SPORL), based on modified sulfite pulping for ethanol production from a variety of woods including hybrid poplar and softwoods [9,14-17]. All these studies used pulp mill wood chips (i.e., competing feedstock with lumber and fiber productions). In this study we will demonstrate sulfite chemistry for high yield sugar production from two underutilized feedstocks, sulfite mill rejects and Douglas-fir harvest forest residue. Case study 1 was a study of glucose production from magnesium sulfite pulp mill rejects, and case study 2 was a study of high titer sugar production from Douglas-fir harvest forest residue by SPORL. A few studies have demonstrated that sulfite mill rejects are highly digestible for sugar production [18-20]. The main char­ acteristic of the present sulfite mill rejects was from magne­ sium sulfite pulping of softwood, different from ammonia sul­ fite pulping in previous studies. The metal base may affect enzyme activities for sugar production, which warrants the present study. Softwood forest residues are available in large quantities in the United States, but are highly recalcitrant to enzymatic saccharification due to high lignin content. Few studies reported sugar production from softwood forest resi­ due. Our previous study was conducted at a laboratory scale of 150 g ovendry (o.d.) forest residue [21]. We will demonstrate sulfite pretreatment at a pilot scale and using a sulfite solution prepared according to pulp mill practice; that is, bubbling sul­ fur dioxide (SO2) into a hydroxide solution instead of using commercial sodium bisulfite with sulfuric acids to adjust pH reported in all our previous studies [9,14-17,21]. In view of the mature technology for sulfite pulping, this study has practical importance, especially considering colo­ cating sugar production on kraft pulp mills for recovery chem­ icals as well as making use of underutilized woody biomass at pulp mills. MATERIALS AND METHODS Case study 1: Sulfite mill rejects Sulfite mill rejects were obtained from Cosmo Specialty Fibers Inc. (Cosmopolis, WA, USA). The mill produces high-grade dissolving pulp from softwood using magnesium sulfite with magnesium recovery. The rejects were unbleached reject knots with a typical particle size of 2 in. The collected rejects had a moisture content of approximately 70% and were shipped to the USDA Forest Service, Forest Products Labora­ tory (FPL), in Madison, WI, USA. Burning these rejects at the mill did not produce much heat due to the high moisture con­ tent (private communication with two sulfite mills). The asreceived rejects were then directly disk milled in a 12-in. laboratory disk refiner (Andritz Sprout-Bauer Atmospheric Refiner; Springfield, OH, USA) using two disks with plate pat­ tern DB2-505 at a disk plate gap of 1 mm, approximately 10 times larger than that used for typical mechanical pulping. The energy consumption for refining was minimal at approx­ imately 100 W-h/kg because of the large plate gap used. Case study 2: Douglas-fir harvest forest residue The Douglas-fir forest residue was from a regeneration harvest Douglas-fir stand in Lane County, OR, USA, and owned by Weyerhaeuser Co. Forest residue was chosen because of its lower cost than wood, and competing for feedstock with pulp and lumber production can be avoided. A horizontal drum fixed-hammer grinder (Model 4710B, Peterson Pacific Corp.; Eugene, OR, USA) equipped with a combination of 76and 102-mm grates was used to grind road piles of the residue (Fig. 1). The ground residue was shipped to Weyerhaeuser Co. at Federal Way, WA, USA, by truck. The moisture content of the residue measured at arrival was 43.9%. A gyratory screen (Black-Clawson; Middleton, OH, USA) equipped with a 44.5-mm (1.75-in.) diameter round-hole punched-plate top deck was used to remove oversized particles and a 3.2-mm (1/8-in.) clear-opening woven wire bottom screen (6 wires/ in. mesh) to remove fines. The oversize fraction was further hammer milled, which resulted in near zero oversized parti­ cles and 14.9% fines from the 9.8% original screen oversize fractions. The total rejection of fines was 9.0%. Fractionation through screening was found to selectively remove bark and ash [22,23]. The accept forest residue labeled as FS-10 was then air-dried to a moisture content of 15% before being shipped to the FPL. A sulfite pretreatment (SPORL) was applied to 61.75 kg FS-10 of 81.4% moisture using a pilot-scale rotating digester of 390 L [24]. A dilute sulfite solution was prepared by bubbling 3.3 kg SO2 at a gauge pressure of 34.5 kPa into a 139-L solution containing 1.25 kg (95% purity) calcium hydroxide. The resultant total SO, and calcium bisulfite charge on o.d. weight FS-10 was 6.6 wt% and 6.46 wt%, respectively. The FS-10 was steamed after loading into the digester to result in a final pretreatment liquor-to-o.d. wood ratio of 3.55:1 (L/kg). This gave an equivalent true combined SO, concentration in the cooking liquor of 1.15 wt% and true free SO, concentration of 0.68 wt%. These SO, loadings are significantly lower than the approximately 8 wt% total SO, (at liquor-to-wood ratio of 4:1) typically used in sulfite pulp mills, or a reduction of 80%. To accommodate facility limitations at sulfite mills, the pretreatment temperature was conducted to 145°C, slightly higher than typical sulfite pulping temperature. It took ap­ proximately 37 min for the 390-L digester to be heated to T = 145°C using a steam jacket. The temperature was main­ tained for another 240 min to result in an effective pretreat­ ment duration, tT145, approximately within the calculated time of 225-270 min based on optimal pretreatment condition of T = 180°C for tT180 = 25-30 min [9], as in Eq. (1):