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Agricultural residues (e.g. corn stover (CS)) representing a huge lignocellulosic biomass waste are regarded as a promising renewable resource that can be converted to fuels and chemicals via biochemical route. Nevertheless, the unfavorable properties, such as low bulk density, contamination by microorganisms, fluffy and thereby difficult to handle, cause huge problems for biomass logistics and biomass conversion. Furthermore, traditional biomass pretreatment methods often use severe conditions, consume much energy, difficult to scale up and generate a feedstock with many toxic degradation products that inhibit fermentation. In this study, we developed a novel, low-cost and easy-to-implement pretreatment method: “Densifying Lignocellulosic biomass with acidic Chemicals (DLC)” on CS. The DLC-CS owning a uniform shape showed a bulk density 4 times higher compared to loose CS and was highly resistant to microbial contamination, which greatly facilitates biomass handling, transportation and storage. DLC-CS after regular steam autoclave treatment at 121 ○C exhibited high enzymatic digestibility and much higher fermentability compared to traditional dilute acid pretreatment. An ethanol titer as high as 68.1 g/L was achieved without washing or detoxification of the pretreated biomass. The superior performances of DLC for biomass logistics and biomass conversion render it very promising for industrial use.

Cellulosic ethanol has attracted much attention as it is of great benefit to social and environmental sustainability. However, the adverse properties of lignocellulosic biomass (i.e. low bulk density, fluffy/difficult to handle, and being easily contaminated by microbes), and issues related to biomass pretreatment (i.e. high energy consumption, difficulty in scale-up and low fermentability of pretreated biomass), largely impede the commercialization of cellulosic ethanol. To address these issues, we invented a novel pretreatment method, Densifying Lignocellulosic biomass with alkaline Chemicals (DLC), and studied DLC on corn stover (CS). DLC-CS has high density and high durability, with contamination prevented and sugar fully preserved, which greatly facilitates biomass logistics. DLC-CS showed high enzymatic digestibility and high fermentability after storage. DLC-CS containing calcium hydroxide yielded 21.4 g of ethanol per 100 g of CS, which was further enhanced to 25.3 g of ethanol per 100 g of CS using a regular steam autoclave. An ethanol titer as high as 70.6 g L−1 was achieved, for the first time, without washing or detoxification of the pretreated biomass. These promising results demonstrated the great potential of DLC pretreatment for lignocellulosic biofuel production.

A novel pretreatment, Densifying Lignocellulosic biomass with acidic/alkali Chemicals (DLC), was recently invented and owns unique advantages for biomass logistics and fermentation. The pretreatment was largely completed during biomass storage, which renders the storage conditions critical. In this study, the effects of storage temperature (-80 °C to 60 °C) and storage time (up to half a year) on the enzymatic digestibility and fermentability of DLC corn stover (CS) were investigated. DLC-CS containing calcium hydroxide(ch) showed increased enzymatic digestibility with increased storage temperature and time. High glucan conversions (>90%) and ethanol titers (e.g. 73.1 g/L) were achieved after regular steam autoclave of DLC(ch)-CS, without washing or detoxification. DLC-CS containing sulfuric acid(sa) was sensitive to storage conditions, and autoclaved DLC(sa)-CS reached the highest ethanol titer (66.6 g/L) when DLC(sa)-CS was stored at room temperature for 14 days. Results indicated that different ambient temperatures in different regions and seasons have a far-reaching impact on DLC-CS for bioconversion.

Background For bioethanol production from lignocellulosic biomass, phenolics derived from pretreatment have been generally considered as highly inhibitory towards enzymatic hydrolysis and fermentation. As phenolics are produced from lignin degradation during pretreatment, it is likely that the pretreatment will exert a strong impact on the structure of phenolics, resulting in varied levels of inhibition of the bioconversion process. Despite the extensive studies on pretreatment, it remains unclear how pretreatment process affects the properties of generated phenolics and how the inhibitory effect of phenolics from different pretreatment varies on enzymatic hydrolysis and fermentation. Results In this study, the structural properties of phenolic compounds derived from four typical pretreatment [dilute acid (DA), liquid hot water pretreatment (LHW), ammonia fiber expansion (AFEX) and alkaline pretreatment (AL)] were characterized, and their effect on both enzymatic hydrolysis and fermentation were evaluated. The inhibitory effect of phenolics on enzymatic hydrolysis followed the order: AFEX > LHW > DA > AL, while the inhibitory effect of phenolics on Zymomonas mobilis 8b strain fermentation followed the order: AL > LHW > DA > AFEX. Interestingly, this study revealed that phenolics derived from AFEX showed more severe inhibitory effect on enzymatic hydrolysis than those from the other pretreatments at the same phenolics concentrations (note: AFEX produced much less amount of phenolics compared to AL and DA), while they exhibited the lowest inhibitory effect on fermentation. The composition of phenolics from different pretreatments was analyzed and model phenolics were applied to explore the reason for this difference. The results suggested that the amide group in phenolics might account for this difference. Conclusions Pretreatment process greatly affects the properties of generated phenolics and the inhibitory effects of phenolics on enzymatic hydrolysis and fermentation. This study provides new insight for further pretreatment modification and hydrolysate detoxification to minimize phenolics-caused inhibition and enhance the efficiency of enzymatic hydrolysis and fermentation.

Additional file 1. Additional figures and tables.

Microbial lipid production from lignocellulosic biomass is largely impeded by unsatisfactory fermentation performance due to the presence of toxic degradation products generated by pretreatment. DLC (Densifying Lignocellulosic biomass with Chemicals) pretreatment completed at room temperature, generating fewer degradation products, showed great potentials for lignocellulosic biorefineries. This work developed DLC pretreatment with sodium hydroxide and calcium hydroxide on rice straw (RS) for microbial lipid production. Conditions for steam autoclave on DLC-RS (i.e. DLCA treatment) were optimized using response surface methodology. High solid loading enzymatic hydrolysis and lipid fermentations by Trichosporon dermatis 32,903 together with mass balances were performed. Lipid fermentations were successfully conducted, for the first time, on pretreated biomass at a solid loading as high as 35 wt.% without washing or detoxification. Lipid titers as high as 28.3 g/L and 29.5 g/L were achieved on DLC-RS and DLCA-RS, respectively, with lipid yields of 0.24 and 0.23 g/g consumed sugar.

The presence of toxic degradation products in lignocellulosic hydrolysate typically reduced fermentation rates and xylose consumption rate, resulting in a decreased ethanol productivity. In the present study, Zymomonas mobilis 8b was investigated for high cell density fermentation with cell recycling to improve the ethanol productivity in lignocellulosic hydrolysate. The fermentation performances of Z. mobilis 8b at various conditions were first studied in yeast extract-tryptone medium. It was found that nutrient level was essential for glucose and xylose co-fermentation by Z. mobilis 8b and high cell density fermentation with cell recycling worked well in yeast extract-tryptone medium for 6 rounds fermentation. Z. mobilis 8b was then studied in enzymatic hydrolysates derived from dilute acid (DA) pretreated corn stover (CS) and ammonia pretreated CS for high cell density fermentation with cell recycling. Ethanol productivity obtained was around three times higher compared to traditional fermentation. Ethanol titer and metabolic yield were also enhanced with high cell density fermentation. Z. mobilis 8b cells showed high recyclability in ammonia pretreated CS hydrolysate.