Potential of ozonolysis as a pretreatment for energy grasses

Oxidative pretreatment of lignocellulosic biomass for enzymatic hydrolysis: Progress and challenges

Deconstruction of cell wall structure is important for biorefining of lignocellulose to produce various biofuels and chemicals. Oxidative delignification is an effective way to increase the enzymatic digestibility of cellulose. In this work, the current research progress on conventional oxidative pretreatment including wet oxidation, alkaline hydrogen peroxide, organic peracids, Fenton oxidation, and ozone oxidation were reviewed. Some recently developed novel technologies for coupling pretreatment and direct biomass-to-electricity conversion with recyclable oxidants were also introduced. The primary mechanism of oxidative pretreatment to enhance cellulose digestibility is delignification, especially in alkaline medium, thus eliminating the physical blocking and non-productive adsorption of enzymes by lignin. However, the cost of oxidative delignification as a pretreatment is still too expensive to be applied at large scale at present. Efforts should be made particularly to reduce the cost of oxidants, or explore valuable products to obtain more revenue.

Pretreatment of Switchgrass for Production of Glucose via Sulfonic Acid-Impregnated Activated Carbon

In the present research, activated carbon-supported sulfonic acid catalysts were synthesized and tested as pretreatment agents for the conversion of switchgrass into glucose. The catalysts were synthesized by reacting sulfuric acid, methanesulfonic acid, and p-toluenesulfonic acid with activated carbon. The characterization of catalysts suggested an increase in surface acidities, while surface area and pore volumes decreased because of sulfonation. Batch experiments were performed in 125 mL serum bottles to investigate the effects of temperature (30, 60, and 90 °C), reaction time (90 and 120 min) on the yields of glucose. Enzymatic hydrolysis of pretreated switchgrass using Ctec2 yielded up to 57.13% glucose. Durability tests indicated that sulfonic solid-impregnated carbon catalysts were able to maintain activity even after three cycles. From the results obtained, the solid acid catalysts appear to serve as effective pretreatment agents and can potentially reduce the use of conventional liquid acids and bases in biomass-into-biofuel production.

Effects and Mechanisms of Alkali Recycling and Ozone Recycling on Enzymatic Conversion in Alkali Combined with Ozone Pretreatment of Corn Stover

In order to minimize waste liquor, save resources, and reduce costs, the effects of alkali recycling and ozone recycling on enzymatic conversion in alkali combined with ozone pretreatment of corn stover and the mechanism were studied. The results showed that as the number of cycles of alkali/ozone filtrate increased, the enzymatic conversion and the loss of reducing sugars showed a downward trend. It was indicated that the ability of alkali to damage lignocellulosic decreased with an increasing number of alkali circulation and the accumulation of lignin degradation products generated during ozonolysis inhibited enzymatic conversion. When the ozone filtrate was recovered and used for hydrolysis directly, the enzymatic conversion rates were basically the same compared with the first self-circulation of ozone filtrate, and no sewage was discharged. In conclusion, the optimal circulating pretreatment was four times alkali circulation and ozone filtrate was used as an enzymolysis liquid directly, and the conversion rates of cellulose and hemicellulose were 85.96% and 34.26%, respectively, saving 44% alkali consumption at the same time. This paper provided the theoretical basis for the development of lignocellulose pretreatment technology with low cost, high efficiency, and high conversion rate.

Effect of enzymolysis time on biohydrogen production from photo-fermentation by using various energy grasses as substrates

Energy grass is an ideal raw material of biomass energy, and hydrogen energy is one of the ideal renewable energy. In order to study the feasibility of different energy grasses for hydrogen production from photosynthetic organisms, the enzymatic hydrolysis of energy grasses is a very necessary process. Therefore, biohydrogen production from photo-fermentation by using energy grasses as substrates was investgated by changing enzymolysis time. The hydrogen production results were evaluated by the experimental results of hydrogen yield, hydrogen production rate and hydrogen production efficiency. The experimental results showed that Medicago sativa L. with enzymolysis time of 60 h had the highest hydrogen yield, which was 147.64 mL. The highest hydrogen production rate was 5.53 mL/(h·g TS), which was obtained from Arundo donax with enzymolysis time of 36 h, and the highest hydrogen production efficiency was 1.15 mL/(h·g TS), which was obtained from Miscanthus with enzymolysis time of 0 h.

Nanocellulose production from recycled paper mill sludge using ozonation pretreatment followed by recyclable maleic acid hydrolysis

Nanocellulose (NC) have garnered much interest worldwide due to its physical and chemical properties. Nanocellulose is produced from biomass materials by bleaching pretreatment, followed by acid hydrolysis. This work demonstrated the production of NC from recycled paper sludge (RPS), a crystalline cellulose rich waste, by ozonation pretreatment, followed by maleic acid hydrolysis. Ozonation resulted in removal of lignin (as evident by TGA analysis), negative zeta potential of RPS and enhanced NC production, from 60 mg/L to over 80 mg/L after 60-120 min of ozone treatment. Maleic acid was successfully recovered, although longer ozonation times reduced the amount of acid available for recovery. These results demonstrate that ozonation can be used as an effective pretreatment for NC production.

Dielectric barrier discharge plasma microbubble reactor for pretreatment of lignocellulosic biomass

A novel lignocellulosic biomass pretreatment reactor has been designed and tested to investigate pretreatment efficacy of miscanthus grass. The reactor was designed to optimize the transfer of highly oxidative species produced by dielectric barrier discharge plasma to the liquid phase immediately after generation, by arranging close proximity of the plasma to the gas-liquid interface of microbubbles. The reactor produced a range of reactive oxygen species and reactive nitrogen species, and the rate of production depended on the power source duty cycle and the temperature of the plasma. Ozone and other oxidative species were dispersed efficiently using energy efficient microbubbles produced by fluidic oscillations. A 5% (w/w) miscanthus suspension pretreated for 3 h at 10% duty cycle yielded 0.5% acid soluble lignin release and 26% sugar release post hydrolysis with accelerated pretreatment toward the latter stages of the treatment demonstrating the potential of this approach as an alternative pretreatment method. © 2018 The Authors. AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers. © 2018 The Authors. AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers. AIChE J, 64: 3803-3816, 2018.

Evaluation of the Effects of Isolated Lignin on Cellulose Enzymatic Hydrolysis of Corn Stover Pretreatment by NaOH Combined with Ozone

In this experiment, corn stover was treated with optimal combined pretreatment conditions: 2% NaOH at 80 °C treated 2 h combined with initial pH 9 at the ozone concentration of 78 mg/mL treated 25 min. The effect of lignin removal rate on the enzymatic hydrolysis degree of cellulose during the treatment process was studied. At the same time, the lignin in the optimal pretreated corn stover was separated and extracted by enzymatic acidolysis, and its structure and connection were characterized. The results showed that the alkali combined with ozone pretreatment improved the enzymatic hydrolysis degree of the cellulose while exfoliating and degrading the macromolecular lignin into small molecules. The stable crosslink structure of the lignin-cellulose-hemicellulose was destroyed, and the lignocellulosic structure changed in favor of the enzymatic hydrolysis of the cellulose.

Effect of ozonolysis pretreatment parameters on the sugar release, ozone consumption and ethanol production from sugarcane bagasse

A L9(3)(4) orthogonal array (OA) experimental design was applied to study the four parameters considered most important in the ozonolysis pretreatment (moisture content, ozone concentration, ozone/oxygen flow and particle size) on ethanol production from sugarcane bagasse (SCB). Statistical analysis highlighted ozone concentration as the highest influence parameter on reaction time and sugars release after enzymatic hydrolysis. The increase on reaction time when decreasing the ozone/oxygen flow resulted in small differences of ozone consumptions. Design optimization for sugars release provided a parameters combination close to the best experimental run, where 77.55% and 56.95% of glucose and xylose yields were obtained, respectively. When optimizing the grams of sugar released by gram of ozone, the highest influence parameter was moisture content, with a maximum yield of 2.98gSUGARS/gO3. In experiments on hydrolysates fermentation, Saccharomyces cerevisiae provided ethanol yields around 80%, while Pichia stipitis was completely inhibited.

Ozonolysis: An advantageous pretreatment for lignocellulosic biomass revisited

Ozonolysis, as a lignocellulosic biomass pretreatment, goes back to 80s; however, in the last years it is becoming widespread again owing to its efficiency and mild operation conditions. Ozone reacts preferably with lignin than carbohydrates, promoting biomass destructuration and delignification, and so the sugar release by enzymatic hydrolysis. The hydrolysate from pretreated biomass has being used as sugars source for second-generation fuels production, mainly ethanol, methane and hydrogen. Short-chain carboxylic acids are the main inhibitory compounds generated, being properly removed by water washing. The most common inhibitory compounds reported for other pretreatments, furfural and HMF (5-hydroxymethylfurfural), are not found in ozone-pretreated hydrolysates. Composition of pretreated biomass and ozone consumption depends on several process parameters: reactor design, moisture content, particle size, pH, reaction time, ozone/air flow and ozone concentration. Additional studies are necessary to clarify process parameters effect and to optimize the process to achieve high yields with economic feasibility.

Simulation of the ozone pretreatment of wheat straw

Wheat straw is a potential feedstock in biorefinery for sugar production. However, the cellulose, which is the major source of sugar, is protected by lignin. Ozonolysis deconstructs the lignin and makes cellulose accessible to enzymatic digestion. In this study, the change in lignin concentration with different ozonolysis times (0, 1, 2, 3, 5, 7, 10, 15, 20, 30, 60min) was fit to two different kinetic models: one using the model developed by Garcia-Cubero et al. (2012) and another including an outer mass transfer barrier or "cuticle" region where ozone mass transport is reduced in proportion to the mass of unreacted insoluble lignin in the cuticle. The kinetic parameters of two mathematical models for predicting the soluble and insoluble lignin at different pretreatment time were determined. The results showed that parameters derived from the cuticle-based model provided a better fit to experimental results compared to a model without a cuticle layer.

Effects of pre-treatment technologies on dark fermentative biohydrogen production: A review

Biohydrogen production from dark fermentation of lignocellulosic materials represents a huge potential in terms of renewable energy exploitation. However, the low hydrogen yield is currently hindering its development on industrial scale. This study reviewed various technologies that have been investigated for enhancing dark fermentative biohydrogen production. The pre-treatment technologies can be classified based on their applications as inoculum or substrates pre-treatment or they can be categorised into physical, chemical, physicochemical and biological based on the techniques used. From the different technologies reviewed, heat and acid pre-treatments are the most commonly studied technologies for both substrates and inoculum pre-treatment. Nevertheless, these two technologies need not necessarily be the most suitable since across different studies, a wide array of other emerging techniques as well as combined technologies have yielded positive findings. To date, there exists no perfect technology for either inoculum or substrate pre-treatment. Although the aim of inoculum pre-treatment is to suppress H2-consumers and enrich H2-producers, many sporulating H2-consumers survive the pre-treatment while some non-spore H2-producers are inhibited. Besides, several inoculum pre-treatment techniques are not effective in the long run and repeated pre-treatment may be required for continuous suppression of H2-consumers and sustained biohydrogen production. Furthermore, many technologies employed for substrates pre-treatment may yield inhibitory compounds that can eventually decrease biohydrogen production. Consequently, much research needs to be done to find out the best technology for both substrates and inoculum pre-treatment while also taking into consideration the energetic, economic and technical feasibility of implementing such a process on an industrial scale.

Miscanthus as cellulosic biomass for bioethanol production

The members of the genus Miscanthus are potential feedstocks for biofuels because of the promising high yields of biomass per unit of planted area. This review addresses species, cultivation, and lignocellulose composition of Miscanthus, as well as pretreatment and enzyme saccharification of Miscanthus biomass for ethanol fermentation. The average cellulose contents in dried biomass of Miscanthus floridulus, Miscanthus sinensis, Miscanthus sacchariflorus, and Miscanthus × giganteus (M × G) are 37.2, 37.6, 38.9, and 41.1% wt/wt, respectively. A number of pretreatment methods have been applied in order to enhance digestibility of Miscanthus biomass for enzymatic saccharification. Pretreatment of Miscanthus using liquid hot water or alkaline results in a significant release of glucose; while glucose yields can be 90% or higher if a pretreatment like AFEX that combines both chemical and physical processes is used. As ethanol is produced by yeast fermentation of the hydrolysate from enzymatic hydrolysis of residual solids (pulp) after pretreatment, theoretical ethanol yields are 0.211-0.233 g/g-raw biomass if only cellulose is taken into account. Simultaneous saccharification and fermentation of pretreated M × G and M. lutarioriparius results in experimental ethanol yields of 0.13 and 0.15 g/g-raw biomass, respectively. Co-production of value-added products can reduce the overall production cost of bioethanol.

Ozonolysis pretreatment of maize stover: the interactive effect of sample particle size and moisture on ozonolysis process

Maize stover was ozonolyzed to improve the enzymatic digestibility. The interactive effect of sample particle size and moisture content on ozonolysis was studied. After ozonolysis, both lignin and xylan decreased while cellulose was only slightly affected in all experiments. It was also found that the smaller particle size is better for ozonolysis. The similar water activity of the different optimum moisture contents for ozonolysis reveals that the free and bound water ratio is a key factor of ozonolysis. The best result of ozonolysis was obtained at the mesh of -300 and the moisture of 60%, where up to 75% lignin was removed. The glucose yield after enzymatic hydrolysis increased from 18.5% to 80%. Water washing had low impact on glucose yield (less than 10% increases), but significantly reduced xylose yield (up to 42% decreases). The result indicates that ozonolysis leads to xylan solubilization.

Combined pretreatment using ozonolysis and ball milling to improve enzymatic saccharification of corn straw

Two clean pretreatments, ozonolysis (OZ) and planetary ball milling (BM) were applied separately and in combination to improve the enzymatic hydrolysis of corn straw. Pretreatment of corn straw by OZ and BM alone improved the enzymatic hydrolysis significantly, primarily through delignification and decrystallization of cellulose, respectively. When combined, OZ-BM and BM-OZ pretreatments made the enzymatic hydrolysis more efficient. The glucose and xylose yield of corn straw treated with OZ for 90 min followed by BM for 8 min (OZ90-BM8) reached to 407.8 and 101.9 mg/g-straw, respectively under cellulase loading of 15 FPU/g-straw, which was fivefold more than that of untreated straw. Under much lower cellulase loading of 1.5 FPU/g-straw, the glucose and xylose yield of treated straw OZ90-BM8 remained at 416.0 and 108.4 mg/g-straw, respectively, while the yield of untreated straw decreased. These findings indicate that the combined OZ-BM can be used as a promising pretreatment for corn straw.

Acetone-butanol-ethanol (ABE) production by Clostridium beijerinckii from wheat straw hydrolysates: efficient use of penta and hexa carbohydrates

ABE fermentation by Clostridium beijerinckii of steam-exploded and ozonated wheat straw hydrolysates was investigated. In steam-exploded hydrolysates, highest yields of 0.40 g/g ABE yield and 127.71 g ABE/kg wheat straw were achieved when the whole slurry from the pretreatment was used. In ozonated hydrolysates, 0.32 g/g ABE yield and 79.65 g ABE/kg wheat straw were obtained from washed ozonated wheat straw. Diverse effects were observed in steam explosion and ozonolysis of wheat straw which resulted in hemicellulose removal and acid insoluble lignin solubilization, respectively. SEM analysis showed structural differences in untreated and pretreated biomass. Depending on the operational strategy, after pretreatment and enzymatic hydrolysis, the glucose recovery ranged between 65.73-66.49% and 63.22-65.23% and the xylose recovery ranged between 45.19-61.00% and 34.54-40.91% in steam-exploded and ozonated hydrolysates, respectively. The effect of the main inhibitory compounds found in hydrolysates (oxalic acid, acetic acid, 5-hydroxymethylfurfural and furfural) was studied through ABE fermentation in model media.

Hydrolysis of ozone pretreated energy grasses for optimal fermentable sugar production

Ozonated energy grass varieties were enzymatically hydrolyzed to establish process parameters for maximum fermentable sugar production. Conditions for ozonolysis were selected on the basis of maximum delignification and glucan retention after pretreatment. To study the effect of lignin degradation products generated during ozonolysis on cellulolytic enzymes, hydrolysis was carried out for washed and unwashed pretreated solids. Washing the solids significantly (p<0.05) enhanced glucan conversion from 34.3% to 100% while delivering glucose yields of 146.2-431.9 mg/g biomass. Highest fermentable sugars were produced when grasses were ozonated for maximum delignification and washed solids were hydrolyzed using 0.1g/g Cellic® CTec2. In a comparative study on alkaline pretreatment with 1% NaOH for 60 min, Saccharum arundinaceum exhibited the highest glucan conversion with maximum sugar production of 467.9 mg/g. Although ozonolysis is an effective and environmentally friendly technique for cellulosic sugar production, process optimization is needed to ascertain economic feasibility of the process.