Linoj Kumar

The lignin present in steam pretreated softwood binds enzymes and limits cellulose accessibility.

The influence of cellulose accessibility and protein loading on the efficiency of enzymatic hydrolysis of steam pretreated Douglas-fir was assessed. It was apparent that the lignin component significantly influences the swelling/accessibility of cellulose as at low protein loadings (5FPU/g cellulose), only 16% of the cellulose present in the steam pretreated softwood was hydrolyzed while almost complete hydrolysis was achieved with the delignified substrate. When lignin (isolated from steam pretreated Douglas-fir) was added back in the same proportions it was originally found to the highly accessible and swollen, delignified steam pretreated softwood and to a cellulose control such as Avicel, the hydrolysis yields decreased by 9 and 46%, respectively. However, when higher enzyme loadings were employed, the greater availability of the enzyme could overcome the limitations imposed by both the lignins restrictions on cellulose accessibility and direct binding of the enzymes, resulting in a near complete hydrolysis of the cellulose.

Can the same steam pretreatment conditions be used for most softwoods to achieve good, enzymatic hydrolysis and sugar yields?

Wood chips from six different Douglas-fir trees and a representative Lodgepole pine were steam pretreated at a single pretreatment condition (200 °C 4% SO₂ 5 min) which had previously been shown to be effective for Spruce and Lodgepole pine chips. All of the softwood samples responded in a similar fashion with more than 60% of the cellulose hydrolysed after 72 h, at an enzyme loading of 20 FPU/g cellulose. However, when the enzyme loading was reduced to 5FPU, less than 27% of the cellulose was hydrolysed. When the steam pretreated substrates were subsequently delignified they were almost completely hydrolysed, at both high, 20 FPU/g cellulose (less than 12 h) and low, 5 FPU/g (within 72 h) enzyme loadings. Although optimized steam pretreatment could result in greater than 90% glucose recovery, in order to obtain complete hydrolysis of the cellulosic component at reduced enzyme loadings a delignification step will likely be required.

Does densification influence the steam pretreatment and enzymatic hydrolysis of softwoods to sugars

The global trade in wood pellets continues to grow. However, their potential as a feedstock for large scale cellulosic ethanol production has not been evaluated. We anticipated that the reduced moisture content and pressure exerted on the wood biomass during the pelletisation process would result in some carbohydrate loss as well as making the biomass more recalcitrant to pretreatment and subsequent hydrolysis. However, when softwood chips and pellets were steam pretreated at medium severity, little hemicellulose loss occurred while more than two-thirds of the cellulose present in the cellulose rich water insoluble fractions were hydrolysed (at 20 FPU cellulase/g cellulose). In addition, prior steaming substantially reduced the particle size of the wood chips enabling direct pelletisation without the need for grinding. Surprisingly, it was also possible to apply a single steam pretreatment to facilitate both pelletisation and subsequent enzymatic hydrolysis without the need for a further pretreatment step.

How effective are traditional methods of compositional analysis in providing an accurate material balance for a range of softwood derived residues

BackgroundForest residues represent an abundant and sustainable source of biomass which could be used as a biorefinery feedstock. Due to the heterogeneity of forest residues, such as hog fuel and bark, one of the expected challenges is to obtain an accurate material balance of these feedstocks. Current compositional analytical methods have been standardised for more homogenous feedstocks such as white wood and agricultural residues. The described work assessed the accuracy of existing and modified methods on a variety of forest residues both before and after a typical pretreatment process.ResultsWhen “traditional” pulp and paper methods were used, the total amount of material that could be quantified in each of the six softwood-derived residues ranged from 88% to 96%. It was apparent that the extractives present in the substrate were most influential in limiting the accuracy of a more representative material balance. This was particularly evident when trying to determine the lignin content, due to the incomplete removal of the extractives, even after a two stage water-ethanol extraction. Residual extractives likely precipitated with the acid insoluble lignin during analysis, contributing to an overestimation of the lignin content. Despite the minor dissolution of hemicellulosic sugars, extraction with mild alkali removed most of the extractives from the bark and improved the raw material mass closure to 95% in comparison to the 88% value obtained after water-ethanol extraction. After pretreatment, the extent of extractive removal and their reaction/precipitation with lignin was heavily dependent on the pretreatment conditions used. The selective removal of extractives and their quantification after a pretreatment proved to be even more challenging. Regardless of the amount of extractives that were originally present, the analytical methods could be refined to provide reproducible quantification of the carbohydrates present in both the starting material and after pretreatment.ConclusionDespite the challenges resulting from the heterogeneity of the initial biomass substrates a reasonable summative mass closure could be obtained before and after steam pretreatment. However, method revision and optimisation was required, particularly the effective removal of extractives, to ensure that representative and reproducible values for the major lignin and carbohydrate components.

Evaluation of hemicellulose removal by xylanase and delignification on SHF and SSF for bioethanol production with steam-pretreated substrates.

Steam-pretreated sweet sorghum bagasse (SSB) and Douglas-fir (DF) were employed for SHF and SSF to evaluate the effects of xylanase supplementation and delignification on ethanol production. Results indicated final ethanol concentration in SHF could reach 28.4 g/L (SSB) and 20.4 g/L (DF) by xylanase supplementation with the increase of 46% and 61% in comparison with controls. The delignification could significantly enhance final ethanol concentration to 31.2g/L (SSB) and 30.2 g/L (DF) with the increase of 61% and 138%. In SSF, final ethanol concentration in the delignified SSB and DF arrived at 27.6 g/L and 34.3 g/L with the increase of 26% and 157% compared with controls. However, only 2.2 g/L (SSB) and 6.9 g/L (DF) ethanol were obtained with xylanase supplementation. According to these results, it could be concluded that delignification was beneficial to improve ethanol production of SHF and SSF. The xylanase supplementation (0.12 g protein/g glucan) was only positive to SHF while retarded SSF seriously.

SO2-catalyzed steam pretreatment enhances the strength and stability of softwood pellets

Densification can partially resolve the logistical challenges encountered when large volumes of biomass are required for bioconversion processes to benefit from economies-of-scale. Despite the higher bulk density of pellets, their lower mechanical strength and sensitivity to moisture are still recurring issues hindering long term transportation and storage. In this study, we have evaluated the potential benefits of SO(2)-catalyzed steam treatment to achieve both the needed size reduction prior to pelletization while improving the stability of the produced pellets. This pretreatment substantially reduced the particle size of the woodchips eliminating any further grinding. The treated pellets had a higher density and exhibited a two-time higher mechanical strength compared to untreated pellets. Despite a higher moisture adsorption capacity, treated pellets remained intact even under highly humid conditions. The high heating values, low ash content and good overall carbohydrate recovery of treated pellets indicated their potential suitability for both biochemical and thermochemical applications.

Effect of Steam Treatment on Pellet Strength and the Energy Input in Pelleting of Softwood Particles

Three whitewood species (spruce, Douglas fir, and pine) and one sample of bark (Douglas fir) were treated with high-pressure steam at 220°C for 5 min. The steam treatment resulted in a reduction in average particle size by as much as 25%. Pine particles showed the largest reduction in size, while bark showed the least. Despite a slightly lower density, pellets made from treated particles had a higher mechanical strength (hardness) than untreated pellets. The mechanical energy required to compact steam-treated material was higher than the energy required to make pellets from untreated wood. Douglas fir required the least energy input among debarked samples. Spruce was the stickiest pellet to be pushed out of the cylindrical die. Bark pellets required the lowest energy to be compacted and pushed out of the cylindrical die. The overall conclusion is that steam treatment reduces particle size, reduces pellet density slightly, but increases the mechanical strength of the produced pellets. Steam treatment increases the energy input required to make pellets, and more energy is required to push pellets out of the die compared to pellets made from untreated biomass.

Special Issue from the NSERC Bioconversion network workshop: pretreatment and fractionation of biomass for biorefinery/biofuels.

World demand for energy and commodity chemicals continues to increase at a rapid pace in parallel with global industrialization and economic development. The current annual global energy demand of 13 billion tonnes of oil equivalent (btoe) is predicted to increase by 35% in 2035 [1]. The convenience, infrastructure investment and high energy content of liquid hydrocarbons makes them the preferred energy source for all modes of transportation and the dominant feedstock for the majority of today’s commodity chemicals. However, we face significant challenges with the continued use of oil, from concerns about carbon emissions contributing to climate change to ongoing depletion of finite oil reserves affecting the lifestyles of future generations. It is inevitable that we will have to evolve from a finite, hydrocarbon driven global industry to a more sustainable, carbohydrate based society. If managed sustainably, biomass will be the major, alternative, renewable, source of many of the chemicals and fuels that we currently derive from hydrocarbons such as coal, oil and natural gas [2].