How chip size impacts steam pretreatment effectiveness for biological conversion of poplar wood into fermentable sugars

BACKGROUND: Woody biomass is highly recalcitrant to enzymatic sugar release and often requires significant size reduction and severe pretreatments to achieve economically viable sugar yields in biological production of sustainable fuels and chemicals. However, because mechanical size reduction of woody biomass can consume significant amounts of energy, it is desirable to minimize size reduction and instead pretreat larger wood chips prior to biological conversion. To date, however, most laboratory research has been performed on materials that are significantly smaller than applicable in a commercial setting. As a result, there is a limited understanding of the effects that larger biomass particle size has on the effectiveness of steam explosion pretreatment and subsequent enzymatic hydrolysis of wood chips. RESULTS: To address these concerns, novel downscaled analysis and high throughput pretreatment and hydrolysis (HTPH) were applied to examine whether differences exist in the composition and digestibility within a single pretreated wood chip due to heterogeneous pretreatment across its thickness. Heat transfer modeling, Simons’ stain testing, magnetic resonance imaging (MRI), and scanning electron microscopy (SEM) were applied to probe the effects of pretreatment within and between pretreated wood samples to shed light on potential causes of variation, pointing to enzyme accessibility (i.e., pore size) distribution being a key factor dictating enzyme digestibility in these samples. Application of these techniques demonstrated that the effectiveness of pretreatment of Populus tremuloides can vary substantially over the chip thickness at short pretreatment times, resulting in spatial digestibility effects and overall lower sugar yields in subsequent enzymatic hydrolysis. CONCLUSIONS: These results indicate that rapid decompression pretreatments (e.g., steam explosion) that specifically alter accessibility at lower temperature conditions are well suited for larger wood chips due to the non-uniformity in temperature and digestibility profiles that can result from high temperature and short pretreatment times. Furthermore, this study also demonstrated that wood chips were hydrated primarily through the natural pore structure during pretreatment, suggesting that preserving the natural grain and transport systems in wood during storage and chipping processes could likely promote pretreatment efficacy and uniformity

The effect of shaking regime on the rate and extent of enzymatic hydrolysis of cellulose

In an attempt to elucidate the effect of mixing on the rate and extent of enzymatic hydrolysis of cellulosic substrates, alpha -cellulose was hydrolysed using a commercial cellulase preparation at varying levels of substrate concentration (2.5,5 and 7.5% (w/v)) and by using three shaking regimes: continuous at low-speed (25 rpm), continuous at high-speed (150 rpm) and an intermittent regime comprised of high and low-speed shaking intervals. The continuous, high-speed shaking produced the highest conversion yields, whereas the intermittent and low-speed shaking regimes resulted in lower conversions. After 72 h, at all shaking regimes (150 rpm, 25 rpm and intermittent), using a low substrate concentration (2.5%) produced conversion yields (82, 79 and 80%) higher than those obtained at high (7.5%) substrate concentration (68, 63 and 68%). As the substrate concentration increased, the conversion yields at intermittent shaking gradually approached those resulting from high-speed shaking. Thus, it appears that intermittent shaking could be a beneficial process option as it can reduce the mixing energy requirements while producing reasonably high conversion yields. (C) 2001 Elsevier Science B.V. All rights reserved

Fibre porosity development of dissolving pulp during mechanical and enzymatic processing

Dissolving grade pulps are used as raw material for manufacture of regenerated cellulose fibres and their use is constantly growing. Despite intensive research, there is still a need to develop cellulose dissolution-regeneration processes that would be economically viable, fulfil the pre-conditions of sustainability and would be able to meet the strict product quality requirements. The basis for creation of such a process is in deep understanding of the biomass structure and factors affecting the cellulose modification and dissolution. In this paper, the effects of the mechanical and enzymatic pre-treatments on the pore structure and alkaline solubility of dissolving grade pulp are discussed. Formation of micro- and macropores in the pulp fibres during mechanical shredding was found to correlate with the susceptibility of the fibres to enzymatic hydrolysis. The fibre porosity development during the processing was studied by a modified solute exclusion approach, which revealed differences between the effect of mild enzyme or acid hydrolysis on the pore structure of fibres. The dissolution of the modified fibres in NaOH/ZnO was evaluated and found to correlate with overall pore volume and accessible surface area analysed by the modified solute exclusion method

Assessing dilute acid pretreatment of different lignocellulosic biomasses for enhanced sugar production

In this study, dilute acid pretreatment of five biomass feedstocks viz., sugarcane trash, sugarcane bagasse, rice straw, corn stover and palm empty fruit bunch were compared at a given combined severity factor (CSF) range of 1.4–3.2 and were characterised using an alternative Simons’ staining dye—Direct Yellow 11 fraction (DY 11, molecular weight >100,000) to better understand the correlations of pretreatment effectiveness with biomass physicochemical properties and pretreatment conditions. Good polynomial correlations (n = 2) of CSF were obtained with hemicellulose removal, cellulose digestibility and glucose yield resulting in R2 > 0.95. The results show that the total contents of extractives and ash have negative impacts on dilute acid pretreatment. Simons’ staining results show that DY 11 can also be used to estimate cellulose accessibility to cellulase enzymes. Good linear correlations of maximum adsorption capacity of DY 11 with CSF (R2 = 0.87–0.99) and cellulose digestibility (R2 = 0.91–0.99) were observed for most of the pretreated biomass samples