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  • ISSN: 2333-7117
    Special Issue on
    Industrial Biotechnology-Made in Germany: The path from policies to sustainable energy, commodity and specialty products
    JSM Biotechnol Bioeng 2(1): 1023.
    Submitted: 14 April 2014; Accepted: 12 May 2014; Published: 14 May 2014
    Short Communication
    Lignocellulose to Biogas and other Products
    Streffer, F.,
    Maxbiogas GmbH, Germany
    *Corresponding author: Dr. Friedrich Streffer, maxbiogas GmbH, Alte Dorfstr. 14A, 16348 Marienwerder, Germany, Tel: +49 3337 3774140 ; Fax: +49 3337 3774189; Email: @
    Abstract
    Great efforts are made to realize concepts for replacing oil and using renewable resources as starting material in biorefineries. Currently, biorefineries produce chemical base materials on an industrial scale from readily available sugar-or starch-containing plant components. However, thesefeedstocks only account for about 1% of the available plant biomass. The majority of available plant biomass, constitutes lignocellulose, which is currently inaccessible to conventional biorefineries and biogas processes.However, in future generating higher economic efficiency for biorefineries and biogas plants is important to ensure these operations can compete with the efficiency of oil refineries even in the absence of government subsidies. Further, it is desirable to increase the ecological efficiency of these operations in order to reduce the required agricultural land use and to improve the CO2 balance. All these claims could be achieved if hitherto waste products such as digestates, agricultural, food and municipal wastestreams could be used as feedstock. Physico-chemical and biotechnological pretreatment technologies, such as the LX process are being established, which would allow utilization of these feedstocks particularly for biogas plants. This review summarized the technical and economic framework to establish these enabling technologies with a particular focus on development of second generation biogas process.
    Keywords: Pretreatment process; Lignocellulose; Biogas; Efficiency
    Abbreviations
    atm: atmosphere; CBP: Combined Bio Processing; CO2: Carbon dioxide; C5 sugar: Pentoses; C6 sugar: hexoses; °C: Degree Celsius; KTBL: Kuratorium für Technik und Bauwesen in der Landwirtschaft e.V.; min: minute(s); SHF: Saccharification followed by Fermentation; SSF: Simultaneous Saccharification and Fermentation
    Introduction
    Oil is the basis of modern life, ranging from energy production to packaging material, from synthetic fibre to the production of basic chemicals [1]. Highly efficient processes for extracting and refining crude oil, which have been optimized over the last 100 years allows its economical use today. However, our oil reserves are finite. Therefore, great efforts are made to realize concepts for replacing oil by renewable biomass based resources as feedstock in biorefineries.
    Currently, first generation biorefineries produce chemical base materials on an industrial scale from readily available sugar-or starch-containing plant components. However, sugar and/or starch based feedstocks are also the basis for food production and account only for about 1% of the available plant biomass [2]. By contrast the majority of available plant biomass constitutes lignocellulose, which is not accessible by first generation bioprocesses such as biogas production. In the future, energy and raw material production in biorefineries can preserve our current standard of living only if the efficiency and cost of production can compete with today's efficiency and economy of petroleum refineries [3,4].
    Significant cost determining factors of a biorefinery are commodity prices, costs and expenses of the fermentation process and for product workup in the downstream process. Only efficient and cost effective solutions in all three unit operations will secure the economic energy and raw material production.This is possible using raw materials which are hitherto waste products. In biorefineries these are e.g. plant residues that can only be of commercial use after pretreatment. Therefore, pretreatment processes are key enabling technologies, which allow utilization of a cheap and available feedstock base for design of mass- and economically efficient, second generation biomanufacturing processes [5]. The focus of these efforts is to achieve the process more cost- and energy-efficient [6].
    Efficiency of biorefineries
    Plants used as a carbon source for industrial purposes in biorefineries are a reality today in different markets of renewable materials and renewable energy. Examples include production of lactic acid, 1,3-propandiol, ethanol etc. (see Table 1) [7]. All these processes have in common that their production is based on the utilization of energy-rich and easily accessible plant parts (the sugar or starch based depot substances). This in turn requires the cultivation of specialized plants for these systems such as sugar beet or wheat grains for bioethanol production or so-called energy crops like corn for biogas production [8]. For these production processes to be economical plant sizes of more than 10.000 tons are required [8,9]. As consequence investment costs of such systems as well as their operation costs are high, this limits the number economic project realizations. However in the biogas market the situation is different due to government subsidies. Due to these subsidies biogas plants already operate efficiently in this market with a throughput in the lower four digit range of tons per year [10]
    Table 1 White biotechnology products [7].

    Product

    Markets

    Volume p.a.

    Companies

    Resources

    Ethanol

    Fuels

    Solvents

    Polyethylene

    65.000.000 tons

    (biobased)

    Solvay

    Dow

    Braskem

    Starch, sugar

    Methane

    Energy

    Fuels

    4.300.000 tons

    (biobased, Germany)

    Many

    Starch, sugar

    cellulose, others

    Lactic acid

    Polylacticacid (PLA)

    Food additiv

    400.000 tons (biobased)

    Dow

    Cargill

    Starch

    1,3-Propandiol (PDO)

    Polyurethane

    Personal care

    PTT

    50.000 tons

    (biobased)

    DuPont

    Starch

    Succinic acid

    1,4 Butanediol

    Pharmaceuticals

    Fibers

    50.000 tons

    (biobased)

    DSM

    Starch

    Table 1 White biotechnology products [7].

    ×
    Now the market is expected to mature. Among other things the objectives include:
    • economic utilization of plants as a carbon supplier without subsidies.
    • ensuring sustainable production processes,
    Reduction of substrate costs important
    One of the largest cost positions in the operation of biorefineries are substrate costs [11]. Thus it is one of the major tasks to reduce these costs to ensure the world-wide success of sustainable production of materials and energy in an economically viable way. But how can this be achieved? Only approximately only 1 % of the available plant biomass represents the depot substances starch and sugar. If the "energy crops" used today could be replaced by plant parts which accumulate as waste in agriculture, food production or in communities, the amount of cheaply available input material would increase immensely. In the current processes however, it is not possible to use the waste as substrate, mainly because of the contained lignocellulose that cannot be fermented in biorefineries efficiently due to the chemical structure.
    Lignocellulose - hard to crack
    There are several excellent reviews on the structure of lignocellulose [12,13]. Therefore we keep the description brief. Lignocellulose is the most abundant source of unutilized biomass and its availability does not necessarily impact land use. Lignocellulose in general consists of three biopolymers:
    • Cellulose (40%-50%)
    • Hemicellulose (25%-30%)
    • Lignin(10%-30%)
    Additionally it contains other extractable components [6]. The relative content of each polymer depends on the origin, but in general the lignin content will increase with the age of the plant. In nature cellulose fibers are embedded in a matrix of other structural biopolymers, mainly hemicellulose and lignin with cotton balls being the only exception. Lignin is composed of the three major phenolic components p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. Lignin is synthesized by polymerization of these three components and their ratio varies between different plants wood tissues and cell wall layers. Lignin is a complex hydrophobic, cross-linked aromatic polymer that interferes with the carbohydrate hydrolysis process [14]. A representative diagrammatic framework of lignocellulosic biomass is illustrated in figure 1. The cellulose chains are organized as bundles which are stabilized by hydrogen bonds. Embedded in hemicellulose and covered by lignin these bundles are called microfibrils and have diameters in the range of 10 to 20 nm [15]. These micro fibrils are tightly packed. Neither enzymes nor small molecules like water can enter the complex framework [14]. The microfibrils are usually associated to macrofibrils and also higher structure (see figure 1).
    Figure 1 Schmatic structure of lignocellulose. The hexagons denote the lignin subunits p-coumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S).

    Figure 1 Schmatic structure of lignocellulose. The hexagons denote the lignin subunits p-coumaryl alcohol (H), coniferyl alcohol (G) and sinapyl alcohol (S).

    ×
    The major impediment towards development of an economic viable technology for degradation of cellulose is its association with lignin, its crystallinity and the small surface area for an attack [14]. However, the efficient utilization of the three components cellulose, hemicellulose and lignin will be the key to economic viability of lignocellulose biorefineries.
    The biorefinery marktet
    The German chemical industry already covers more than 10% of their raw material requirements from renewable raw materials [16]. This proportion will increase in the coming years and the utilization of lignocellulose for microbiological processes will be a prerequisite for the economic success of such biorefineries.
    As described above pretreatment processes should ensure a separation of lignocellulose at the molecular level into the individual components.This ideally takes place under mild conditions to ensure that no toxins arise. Such protocols are currently under development. For their success in the market, the economic viability of this process will play a very crucial role.
    Typical processing in biorefineries
    Biorefining is described as the transfer of the efficiency and logic of fossil-based chemistry and substantial converting industry as well as the production of energy onto the biomass industry [16]. Usually the microorganisms of a biorefinery utilize either carbohydrates like starch, cellulose, or hemicellulose. Crop residues often consist of lignocellulose, the tight composite of cellulose, hemicellulose and lignin. Great efforts are necessary to convert lignocellulose in biorefineries to enable their recovery.Currently, the following steps are applied to gain accessibility to the components for downstream microbial conversion processes [17]:
    • Step 1: Separating lignincellulose into hemicellulose, cellulose and lignin (often referred to as a pretreatment of the substrate) to make the (hemi-)cellulose accessible for microorganisms and thus to make the (hemi-)cellulose degradable.
    • Step 2: Hydrolysis of the hemicellulose and cellulose, in order to divide them into oligomeric or monomeric sugars molecules (C5 and C6 sugar).
    • Step 3: Product generation, to prepare the desired product from the monomeric sugars by microorganisms, for example, ethanol, butanol, lactic acid, biogas, etc.
    • Step 4: Product recovery.
    Step 1: Pretreatment methods [6]
    To be able to utilize crop residues efficiently it is imperative to break down the lignocellulosic structure. This is implemented in so-called pretreatment processes, which differ substantially in their type of treatment [14]. Among them are:
    • Biological pretreatment processes [6]
    These methods use microorganisms like fungi which are able to degrade lignin. The lignin degradation always requires oxygen, and the pure lignin degradation process cannot serve as the sole energy and carbon source for the microorganism [18]. It is important to note that most of white biotechnology processes are anaerobic fermentation processes and therefore inhibited by oxygen. Furthermore, the microorganisms mainly destroy the lignin in order to use the cellulose or hemicellulose as carbon and/or energy source [18].
    • Physical / mechanicalpretreatment processes [6]
    These methods aim to make the lignocellulose components accessible by mechanical treatment or by high pressure and/or high temperature. This approach is usually very energy-intensive but does not lead to a separation of cellulose, hemicellulose and lignin, on a molecular level.
    • Chemical pretreatment processes [6]
    Using solvents cellulose, hemicellulose and lignin can be separated in the molecular components. Subsequently, the individual components have to be recovered by separate chemical processes from the solution. The chemical pretreatment nevertheless has some disadvantages as the process itself is energy-intensive, much water is necessary and undesirable degradation products of biopolymers occur at the required high temperatures, which are toxic to microorganisms.
    Steps 2 and 3: Hydrolysis and product formation [6]
    The fermentation of the hemicellulose and cellulose carbohydrate polymers requires the hydrolysis of the polymer into oligomers, or even monomers. Currently, three approaches are implemented mainly [14,19]:
    • Saccharification followed by fermentation (SHF)
    Initially oligomers and/or monomers will be produced with the help of additionally employed enzymes. Then these oligomers or monomers are subjected to a fermentation process.
    • Simultaneous saccharification and fermentation (SSF)
    In this method oligomers and/or monomers are produced with the help of additionally employed enzymes and these oligomers and/or monomers are simultaneously subjected to a fermentation process.
    • Combined bio processing (CBP))
    A microorganism (or consortium of microorganisms) is responsible for the formation of the oligomers and/or monomers and their fermentation at the same time.
    From an economic perspective CBP is preferable, among other things, because there are no additional enzymes used and consumed in this process. For an application of this method, it is imperative that no toxins (e.g. furfural) are contained in the pretreated cellulose or hemicellulose which inhibit the growth of the organisms [15].
    Step 4: The products generated in step 3 are initially in aqueous fermentation media. Before they can be sold or used for further applications they need to be cleaned and/or isolated from unwanted by-products.
    Production of biogas
    Among the most commonly used fermentation processes and thus one of the largest markets for fermentation in Germany is the production of biogas. As a consequence of missing or insufficient pretreatment processes [17] the depot substances of plants are well decomposed, while lignocellulose is, however, scarcely degraded at all in current biogas digesters [20].Hence, lignocellulose remains practically unused as fibrous content in the digestate, which has hardly any economic value.
    The biogas market is currently considered to consist of about 7.600 plants that do not have good efficiency for the utilization of lignocellulosic residues. Government policies however, are in favor of support the usage of residues. In addition, the pressure on plant operators to improve the efficiency of their investments is steadily increasing because of rising feedstock prizes.
    How to improve biogas plant efficiency
    The economic upgrading of conventional biogas plants can be achieved with a pretreatment process implemented that can be integrated into the existing biogas process, recovering the huge biogas potential of lignocellulosic materials [21]. However, current biorefinery pretreatment process concepts are not compatible with the biogas market. A successful biogas pretreatment process must be able to be operated economically at processing approximately 1000 ton dry matter a year. In this scenario it is mandatory to use the waste heat of a biogas plant to satisfy the energy demand. However, a pretreatment processes that meets this particular technical framework the biogas market offers excellent conditions for market entry.
    Discussion
    The Lignin extraction process (LX-process)
    The newly developed LX-process is a chemical pretreatment process to break up lignocellulose in its components. In a first step, the biomass is dissolved. In the second step the cellulose, hemicellulose and lignin are precipitated as solids. The precipitation is optionally carried out in a fractionated fashion, so that the individual components cellulose, hemicellulose and lignin can be obtained separately (see Figure 2).
    Figure 2 Scheme of the lignin extraction process (LX-process).

    Figure 2 Scheme of the lignin extraction process (LX-process).

    ×
    In the special case of the biogas market a fractionated precipitation is not absolutely necessary. The microorganisms of a biogas plant convert cellulose and hemicellulose even in the presence of lignin. The prerequisite is that the structure of the molecular composite between the individual components has been broken up beforehand.
    A first implementation of the LX-process in a pilot-LX plant will reveal that the method is very interesting for the biogas market. But the LX-process also offers other advantages that make its use attractive. These benefits include, in particular:
    1. The LX system is highly compatible.
    The process conditions are that mild that toxic degradation products of cellulose and hemicellulose are avoided and thus the obtained cellulose and hemicellulose can be degraded directly by microorganisms into biogas.
    2. The available waste heat of biogas plants can be used.
    The LX-process conditions can be selected in a form that the available waste heat of a biogas plant is sufficient with its temperature range to operate the LX-process.
    3. Nutrient and carbon cycles are closed.
    After the fermentation processes the digestate still containing lignin and minerals may be supplied to the soil again.
    4. The Greenhouse Gas Balances (GHG-balance) of a standard biogas plant can be significantly improved
    Enhancing the biogas yield significantly reduces in consequence the CO2 emission. Furthermore, the emission contribution to the GHG-balance can be significantly reduced, as drying of the solid digestate and the associated pasteurization of the digestate brings the nitrification process to a halt. Thus, the particularly harmful nitrous oxide (N2O) emissions are almost completely suppressed. The extensive discussion on (indirect) land use change [4,23,24,25] can be mitigated switching from a 1. Generation to a 2. Generation production of biogas employing residues [4,25] and the (partial) return of carbon to the soil [4].
    Special features of the LX-process
    LX-process is operated at temperaturesbelow 100 °C, both the process it self as well as the work-up of the operating supplies. This allows suppressing the degradation reactions mainly of hemicellulose, an important prerequisite for subsequent utilization by microorganisms. Most pretreatment processes have still a considerable energy demand even after heat integration. This can be significantly reduced, by keeping water away from the process and thus also heating or evaporating as little water as possible.The integration of this knowledgeinto the LX-process means that the LX-process without heat integration according to present estimates can gain the required amount of heat energy from the processed residues in the LX-plant.
    With the successful launch and the implementation of the recovery of the individual components cellulose, hemicellulose and lignin LX-plants are becoming increasingly interesting for other markets where, for example, chemical raw materials obtained with the help of microbiological processes (see Table 1). Furthermore, the process can also be transferred to the production of other products than biogas from the rich palette of white biotechnology.
    Increasing product yield
    In all these processes that operate successfully on the basis of starch or sugar today, the efficiency of these processes can be increased by utilizing the residual materials with the LX-process as additional input for the particular process. This is the potential of the LX-process.
    The LX-celluloses are successfully converted by commercially available cellulase preparations into monomeric sugars and by microorganisms into biogas without inhibiting the growth of the microorganisms. In addition to the carbohydrate stream (the LX-celluloses) also the lignin can be recovered. Initial results show that the lignin of the LX process is similarly well soluble in various solvents, such as in DMSO, ethanol or acetone as organosolv lignins, while the carbohydrate content is very low with substantially less than 1%.
    In the past two years, maxbiogas GmbH realized the LX-process in a mini plant capable of converting up to 10 dry kilograms of plant residues per day to LX-celluloses and/or LX-lignin. First results show that the properties for LX-cellulose from the batch LX-process and from the continuous LX-process of the mini plant are comparable, e.g. in their biogas yield indicating the near total conversion of the carbohydrate stream. Currently maxbiogas GmbH scales up the LX-process, expanding from its 10 kg per day mini plant line to a pilot facility capable of converting about three dry tons of plant residues to LX-cellulose per day.
    In addition the combination of the LX-process with other fermentation processes is investigated in the laboratory. Current results in the process and product development of the LX-process show the great potential of using LX-plants not only for efficiency improvements in the biogas sector but also to increase the efficiency of other production processes in biorefineries.
    Conclusion
    It is obvious and the political will that biorefining will play an important role for energy production as well as the production of numerous products in the future. In order to become economically viable which means to be able to operate without subsidies and to improve sustainability in the production processes the efficiency of biorefineries has to improve significantly. In recent years, great efforts have been made in the field of biorefinery and first successes are achieved (see for example Table 1). Biorefinery plants focused in particular on developments in steps 2 and 3 of product generation For example, certain methods have been developed, as well as enzyme preparations or genetically modified microorganisms that can produce a variety of products from carbohydrate-based substances.
    New developments improve the system
    However, there is still huge potential within the system, especially in the increased use of residues. Therefore, a number of companies in different parts of the world are currently working on an economic realization of step 1, the break-down of lignocellulose [26-30]. One promising approach is the LX-process. The focus of the process lies especially on the application of mild process conditions, so that toxic degradation products of the cellulose are avoided and microorganisms will degrade the celluloses obtained from the LX-process directly. In addition, the use of water in the LX-process has been limited to a minimum. The relevance of this particular point in the development of economic biorefinery plants was, inter alia, described in the final report of the cluster BIOREFINERY2021 [26]. The authors of the module "Development and evaluation of integrated biorefinery 'New Concepts'" summarize their findings as follows: "The combination of the pre-treatment process and the subsequent process steps, however, turned out to be extremely important. Especially the low water loading in all stages of the process should be a top priority. Otherwise laborious processes (e.g. evaporation) are necessary in order to subsequently separate the desired products." [26].
    Entering a new era
    Furthermore the method has the potential to lift biorefining to a new level since more or less all disadvantages of the processing today will be reduced. The digestate which is today treated as end of the process still contains large amounts of substrate bound within the lignocellulose. This potential can be used by the LX-process. Even the carbon- and mineral-cycle is closed, since the granular LX-digestate can still be used for treatment of soil afterwards. As a consequence the need for freshly produced substrates decreases and agricultural land can be used for the production of food instead of substrate for biorefineries. Additionally the possibility to ferment also other waste products from agricultural or food production further reduces the amount of especially grown substrates. These factors help to save natural resources on the one side and to reduce substrate costs, an important factor for efficiency on the other side. This increases the land efficiency of biorefinieries and economic viability of the process will increase automatically.
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    Cite this article: Streffer F (2014) Lignocellulose to Biogas and other Products. JSM Biotechnol Bioeng 2(1): 1023.
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