http://umu.diva-portal.org/smash/get/diva2:882304/FULLTEXT01

Pretreatment of lignocellulose: Formation of inhibitory by-products and strategies for minimizing their effects

Trends in bioconversion of lignocellulose: Biofuels, platform chemicals & biorefinery concept

https://backend.orbit.dtu.dk/ws/files/190081841/BiogasUpgrade_Review.pdf

Biogas upgrading and utilization: Current status and perspectives

One of the major challenges faced in commercial production of lignocellulosic bioethanol is the inhibitory compounds generated during the thermo-chemical pre-treatment step of biomass. These inhibitory compounds are toxic to fermenting micro-organisms. The ethanol yield and productivity obtained during fermentation of lignocellulosic hydrolysates is decreased due to the presence of inhibiting compounds, such as weak acids, furans and phenolic compounds formed or released during thermo-chemical pre-treatment step such as acid and steam explosion. This review describes the application and/or effect of biological detoxification (removal of inhibitors before fermentation) or use of bioreduction capability of fermenting yeasts on the fermentability of the hydrolysates. Inhibition of yeast fermentation by the inhibitor compounds in the lignocellulosic hydrolysates can be reduced by treatment with enzymes such as the lignolytic enzymes, for example, laccase and micro-organisms such as Trichoderma reesei, Coniochaeta ligniaria NRRL30616, Trametes versicolor, Pseudomonas putida Fu1, Candida guilliermondii, and Ureibacillus thermosphaericus. Microbial and enzymatic detoxifications of lignocellulosic hydrolysate are mild and more specific in their action. The efficiency of enzymatic process is quite comparable to other physical and chemical methods. Adaptation of the fermentation yeasts to the lignocellulosic hydrolysate prior to fermentation is suggested as an alternative approach to detoxification. Increases in fermentation rate and ethanol yield by adapted micro-organisms to acid pre-treated lignocellulosic hydrolysates have been reported in some studies. Another approach to alleviate the inhibition problem is to use genetic engineering to introduce increased tolerance by Saccharomyces cerevisiae, for example, by overexpressing genes encoding enzymes for resistance against specific inhibitors and altering co-factor balance. Cloning of the laccase gene followed by heterologous expression in yeasts was shown to provide higher enzyme yields and permit production of laccases with desired properties for detoxification of lignocellulose hydrolysates. A combination of more inhibitor-tolerant yeast strains with efficient feed strategies such as fed-batch will likely improve lignocellulose-to-ethanol process robustness.

Biotechnological strategies to overcome inhibitors in lignocellulose hydrolysates for ethanol production: review.

https://www.cell.com/trends/biotechnology/pdf/S0167-7799(13)00220-5.pdf

Lignocellulosic ethanol production at high-gravity: challenges and perspectives

BACKGROUND: The production of bio-ethanol from softwood is considered a promising alternative to fossil fuels in Sweden. In order to make fuel ethanol economically competitive with fossil fuels, it is important to reduce the production cost, which can be done by increasing the dry matter content of the fermentation medium, thus reducing the energy demand in the final distillation of the fermentation broth. Running simultaneous saccharification and fermentation l at higher dry matter content has, however, been found to decrease the ethanol yield. RESULTS: The use of different stirrer types and stirring speeds in the present study has shown to have an influence on the final ethanol yield in SSF with 10% water-insoluble solids (WIS). Also, higher concentration of pretreatment hydrolysate, i.e., with increased inhibitor concentration, at the same WIS resulted in a decreased ethanol yield. However, despite stirring problems and high inhibitor concentration, ethanol was produced at 12% WIS with an ethanol yield in the SSF step of 81% of the theoretical based on the content of fermentable sugars in the fermentor. CONCLUSION: The decrease in ethanol yield in SSF at high dry matter content has been shown to be a combined effect of increased mass transfer resistance and increased inhibitor concentration in the fermentation broth. (c) 2008 Society of Chemical Industry (Less)

Production of fuel ethanol from softwood by simultaneous saccharification and fermentation at high dry matter content.

The effect of prehydrolysis and improved mixing on high-solids batch simultaneous saccharification and fermentation of spruce to ethanol

Background
To make lignocellulosic fuel ethanol economically competitive with fossil fuels, it is necessary to reduce the production cost. One way to achieve this is by increasing the substrate concentration in the production process, and thus reduce the energy demand in the final distillation of the fermentation broth. However, increased substrate concentration in simultaneous saccharification and fermentation (SSF) processes has been shown to result in reduced ethanol yields and severe stirring problems. Because the SSF medium is being continuously hydrolyzed, running the process in fed-batch mode could potentially reduce the stirring problems and lead to increased ethanol yields in high-solids SSF. Different enzyme feeding strategies, with the enzymes either present in the reactor from start-up or fed into the reactor together with the substrate, have been studied, along with the influence of the enzyme feeding strategy on the final ethanol yield and productivity.

/pdf/effects-of-enzyme-feeding-strategy-on-ethanol-yield-in-fed-cgjisvzyic.pdf

Effects of enzyme feeding strategy on ethanol yield in fed-batch simultaneous saccharification and fermentation of spruce at high dry matter

Biogas produced by anaerobic digestion is often used in gas turbines to produceelectricity. In order to increase the value of the gas and to enable utilization of the gas in other applications, it may be advantageous to upgrade the biogas. In this way, the carbon dioxide as well as various impurities are removed and biomethane is produced. Biomethane is similar to natural gas and can be used in similar applications, e.g. fed into the natural gas grid, or as vehicle fuel. Several different biogas upgrading techniques are on the market today. Some of themmake use of the fact that carbon dioxide and methane have different solubility indifferent solvents. By choosing a solvent which has a high solubility for carbon dioxide but lets methane pass through unchanged, the carbon dioxide can be separated from the methane in biogas efficiently. Common solvents used for biogas upgrading are water, amines as well as organic solvents such as Genosorb®. The difference in adsorption behavior of carbon dioxide and methane on a surface at different pressures is used in pressure swing adsorption (PSA), which can be used to effectively separate carbon dioxide from methane. Another common biogas upgrading technique uses the fact that carbon dioxide is more likely to pass through a semi permeable barrier, e.g. a membrane, than methane. By letting biogas pass through such a membrane, the carbon dioxide can thus be removed from the gas, leaving concentrated methane in the product stream. Finally, the difference in boiling point between methane and carbondioxide may be used to separate the gases in cryogenic distillation. For this report, data on the specific investment cost was collected from companies supplying biogas upgrading plants using the above described processes. The data shows a span of investment costs, but it also shows that there is no significant general difference in investment cost between the different techniques when considering a given standard project. Also the energy consumption is rather similar for the different upgrading techniques. When deciding on a suitable biogas upgrading process, it is therefore important to rather consider other aspects. These may include the ability of the different processes to handle specific impurities present in the actual project orspecific requirements in product gas quality. Also the need for consumables such as anti-foam, chemicals for pH regulation as well as operational costs for any needed pretreatment differs between biogas upgrading processes but is of course also dependent on the pretreatment needed in a project. It is important to remember that the conclusion regarding specific investment cost in this report is related to a standard case. In a real project, where more or less pre- and posttreatment will be needed depending on the choice of upgrading technique, the investment cost for different biogas upgrading techniques will most likely differ. Biogas produced from various substrates such as agricultural residues, biological waste or sewage sludge contains low concentrations of unwanted substances, e.g. impurities, such as H2S, siloxanes, ammonia, oxygen and volatile organic carbons (VOC). H2S is separated from the methane in most biogas upgrading techniques. How efficient this removal is and thus whether it is enough to meet product gas requirements differs between the different techniques. Scrubbers using absorption in water, amines or organic solvent usually remove most of the H2S, while polishing filters are needed for membrane upgrading and PSA. When separated from the methane gas, H2S, however, ends up in a CO2 rich side stream such as stripper air where it usually needs to beremoved due to environmental legislation. If the CO2 stream is utilized, the necessity to remove H2S depends on what the gas is used for. H2S thus needs to be removed from the gas at some point in most cases, but depending on the biogas upgrading technique used, this may need to be done in the raw biogas or there may be a choice regarding where in the process to remove H2S. Siloxanes may be harmful to process equipment when present at too high concentrations. In scrubber systems the produced biomethane usually needs further drying and the main part of siloxanes are removed in the dryers. Ammonia is soluble in water and the concentrations commonly found in biogas are usually removed in the condensation which is usually part of a biogas upgrading system in order to protect the upgrading system from liquid water. Ammonia is not usually a problem in biogas upgrading systems. However, when H2S and ammonia arepresent simultaneously, it is important to prevent precipitation of compounds formed when these two react with each other. Since anaerobic digestion occurs under anaerobic conditions, e.g. with no oxygen present, the concentration of oxygen in biogas is usually low. Improper adjustment of oxygen injection systems used in order to biologically remove larger concentrations of H2S may increase the oxygen levels of the raw biogas. However, the oxygen concentration is commonly monitored carefully in biogas systems in order to minimize the explosion risk. Biomethane quality requirements when the gas is fed into a natural gas grid are currently limiting the oxygen content in biomethane to almost zero, especially in gas grids which include gas storage systems. It may therefore be necessary to remove oxygen from the product gas, or raw biogas if preferred, if the oxygen present in the raw biogas is passed to the produced biomethane. This is valid for scrubber techniques except membrane and PSAsystems which remove a significant amount of the oxygen. The product gas leaving the plant must uphold certain gas quality criteria, either set as a bilateral agreement with the transporter and/or buyer of the biomethane, which currently are based on national specifications. A new CEN standard on biogas injection of H gas quality has recently been sent to formal vote, regulating levels of minor impurities such as siloxanes and ammonia, and major ones such as hydrogen and oxygen. The minimum calorific content is specified, but the wobbe index is not. Allowed sulfur levels are still relatively high, and not including the contribution of odorization, which is still an issue handled nationally in Europe. Biomethane and compressed natural gas (CNG) delivered at the point of retail is also under standardization. There are efforts to introduce a second dedicated non-grid based grade, which will be beneficial to the sales of biomethane, since most of the parameters will easily be upheld by normal upgrading, with the exception of raw biogas containing larger amounts of siloxanes. In biogas upgrading with membrane separation, amine scrubbers and PSA, very pureCO2 can be produced. In biogas upgrading using these techniques, besides biomethane, CO2 can be produced and utilized. The most common ways to use CO2 are for the use in greenhouses, in the food and cooling industry or to utilize access electricity to let the CO2 react with H2 to produce methane, co-called power-to-gas. Power to gas constitutes a way to store access electricity in the form of gas which is gaining increased interest during recent years. (Less)

Biogas upgrading - Technical Review

Background
Saccharification and fermentation of pretreated lignocellulosic materials, such as spruce, should be performed at high solids contents in order to reduce the cost of the produced bioethanol. However, this has been shown to result in reduced ethanol yields or a complete lack of ethanol production. Previous studies have shown inconsistent results when prehydrolysis is performed at a higher temperature prior to the simultaneous saccharification and fermentation (SSF) of steam-pretreated lignocellulosic materials. In some cases, a significant increase in overall ethanol yield was reported, while in others, a slight decrease in ethanol yield was observed. In order to investigate the influence of prehydrolysis on high-solids SSF of steam-pretreated spruce slurry, in the present study, the presence of fibers and inhibitors, degree of fiber degradation and initial fermentable sugar concentration has been studied.

/pdf/influence-of-fiber-degradation-and-concentration-of-3qo5y904l2.pdf

Influence of fiber degradation and concentration of fermentable sugars on simultaneous saccharification and fermentation of high-solids spruce slurry to ethanol

Kerstin Hoyer

Papers

Production of fuel ethanol from softwood by simultaneous saccharification and fermentation at high dry matter content.

The effect of prehydrolysis and improved mixing on high-solids batch simultaneous saccharification and fermentation of spruce to ethanol

Effects of enzyme feeding strategy on ethanol yield in fed-batch simultaneous saccharification and fermentation of spruce at high dry matter

Biogas upgrading - Technical Review

Influence of fiber degradation and concentration of fermentable sugars on simultaneous saccharification and fermentation of high-solids spruce slurry to ethanol