Integration of Lignocellulosic Biomass into Renewable Energy

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ProEnvironment

ProEnvironment 2 (2009) 32 - 37

Review

Integration of Lignocellulosic Biomass into Renewable Energy Generation Concepts KUSCH Sigrida*, Maria V. MORARb a

University of Stuttgart, Institute for Sanitary Engineering, Water Quality and Solid Waste Management, Bandtaele 2, 70569 Stuttgart, Germany University of Agricultural Sciences and Veterinary Medicine Cluj – Napoca, Faculty of Agriculture, Mănăştur 3 – 5, 400372 Cluj – Napoca, Romania Received 17 May 2009; received and revised form 28 May 2009; accepted 5 June 2009 Available online 15 August 2009

Abstract In all European countries various lignocellulosic biomasses such as agricultural residues (straw, straw containing dung) or fractions from municipal solid waste are available in large amounts, but currently hardly any of this potential is being used for energy generation. This paper reviews the different options for including lignocellulosic biomass into renewable energy generation schemes. Not all wastes are suitable to be treated by principally available techniques such as anaerobic digestion, ethanol production or thermal valorisation. The present paper gives an overview of utilisation options for lignocellulosic biomass to either produce biofuels or to integrate such biomass into anaerobic digestion. Biorefinery concepts are discussed as well. Keywords: lignocelluloses, bioenergy, biofuels, biogas, biorefinery

1.Introduction Awareness of climate change and the need to reduce human impacts on the environment has increased significantly over the past years worldwide. Consideration of greenhouse gas emissions closely links to the context of energy. Among potential alternative bioenergy resources, lignocellulosics have been identified as the prime source of biofuels and other value-added products [13]. Corresponding author. Tel.: ++49 711 68565409; Fax: ++49 711 68563729 e-mail: [email protected]

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Lignocelluloses comprise a large fraction of municipal solid waste, crop residues, animal manures, woodlot arisings, forest residues or dedicated energy crops [26]. Focusing on residuals, it can be stated that lignocelluloses such as agricultural, industrial and forest residuals account for the majority of the total biomass present in the world [12]. Global crop residues alone were estimated at about 4 billion Mg for all crops and 3 billion Mg per annum for lignocellulosic residues of cereals 14]. Various conversion technologies exist for lignocellulosic biomass; main technologies are depicted with table 1. Classical and in a European perspective commercially available pathways for

KUSCH Sigrid et al./ProEnvironment 2 (2009) 32 - 37

energy generation from lignocellulosic sources are in particular the incineration of woody substrates or of straw and the production of biogas with anaerobically degradable material. Bioethanol is

already to be found on the European market, but it is mainly ethanol production from plants rich in sugar which is state-of the-art, while utilisation of lignocelluloses is subject of ongoing research and development.

Table 1. Main alternatives for utilisation of lignocellulosic biomass as energy carrier Energy carrier Conversion Main Utilisation Wood pellets, straw pellets Thermal Heat, electricity Biogas Biochemical Heat, electricity Ethanol, ethyl tertiary butyl Biochemical Fuel, additive ether (ETBE) Fischer-Tropsch fuels (FTF); Biomass-to-Liquid fuels (BTL) Methanol; Methanol-toSynfuels (MTS) Hydrogen

Thermochemical

Fuel

Thermochemical

Fuel, possibly fuels cells

Thermochemical Biochemical

Fuel, possibly fuel cells Fuel, electricity, heat

2. Biofuels from Lignocellulosics Bioethanol is much discussed as future fuel for combustion engines. Production costs are still high. When using cultivated crops, currently up to 80% of total production cost is the cost of feedstock [2], utilisation of waste materials therefore can have decisive influence on cutting down overall costs. Typically, bioethanol generation is envisaged from cellulose feedstocks such as corn stalks, straw, sugar cane bagasse, pulpwood, switchgrass, and municipal solid waste [2]. When utilising lignocellulosic biomass, pretreatment is an important factor to achieve high ethanol yields [6]. Processing of lignocellulosics to ethanol consists of four major steps: pretreatment, hydrolysis, fermentation, and product purification. Hydrolysis converts carbohydrate polymers into monomeric sugars which are then fermented to ethanol. The goal of all pretreatment is to break the lignin seal and to disrupt the crystalline structure of cellulose in order to make cellulose more accessible to enzymes that convert the carbohydrate polymers into fermentable sugars [20]. An effective and economical pre-treatment should meet the following requirements: production of reactive cellulosic fibre for enzymatic attack, avoidance of destruction of hemicelluloses and cellulose, avoidance of formation of possible inhibitors for hydrolytic enzymes and fermenting microorganisms, minimisation of energy demand, reduction of costs for size reduction of feedstocks, reduction of costs of material for construction of pre-treatment reactors, production of less residues, consumption of little or no chemicals, and utilisation of cheap chemicals [27].

Level of development State-of-the-art State-of-the art (State-of-the art with cultivated crops), with lignocellulosics: pilot scale Pilot scale

Pilot scale Research level

Pre-treatment methods are subject to ongoing and intense research worldwide. Possible pre-treatment methods can be classified as follows, although not all of them have yet developed enough to be feasible for applications in large-scale processes [27]: • Physical pre-treatments: milling (ball milling, two-roll milling, hammer milling, colloid milling, vibro energy milling), irradiation (gamma-ray, electron-beam, microwave), others (hydrothermal, high pressure steaming, expansion, extrusion, pyrolysis) • Chemical and physicochemical pre-treatment methods: explosion (steam explosion, ammonia fibre explosion, CO2 explosion, SO2 explosion), alkali treatment (treatment with sodium hydroxide, ammonia or ammonium sulfite), acid treatment (sulphuric acid, hydrochloric acid, phosphoric acid), gas treatment (chlorine dioxide, nitrogen dioxide, sulphur dioxide), addition of oxidizing agents (hydrogen peroxide, wet oxidation, ozone), solvent extraction of lignin (ethanol-water extraction, benzene-water extraction, ethylene glycol extraction, butanol-water extraction, swelling agents) • Biological pre-treatments (fungi and actinomycetes) Altough lignocellulosic biomass was assessed to be the most promising feedstock considering its great availability and low costs, large-scale commercial production of fuel bioethanol from lignocellulosic materials has not yet been implemented [3]. Fischer-Tropsch fuels are generated by special synthesis and provide the opportunity to use especially composed fuels of well-defined 33

KUSCH Sigrid et al./ProEnvironment 2 (2009) 32 - 37

composition, thus being able to meet stringent exhaust air limits. C1 to C35 hydrocarbons are produced using syngas (H2 and CO). Syngas can be created from natural gas, but also from a wide variety of solid biomass. However, technologies are quite complex, investment is rather high and economic viability needs to be carefully assessed. Several pilot plants exist in Europe and worldwide [21]. Fischer-Tropsch fuels could be integrated in the existing infrastructure and could fuel combustion engines. Methanol and hydrogen are regarded to be possible fuels for the future. However, they require expensive new infrastructure. Their availability in the future will highly depend upon creation of such infrastructure. Factors influencing such development are not only of technical character. General policy, economical viability and regulatory frameworks will be decisive.

3. Anaerobic Digestion to Biogas and Biohydrogen Anaerobic digestion with production of biogas is state-of-the art in processing agricultural materials, municipal substrates and industrial wastewaters. On farm it can be a source of additional income, thus making a farmer to an energy producer [24]. At present, trends can be observed to use energy crops such as maize silage or whole plant crop silage for the production of biogas. Since neither monocultures nor food competition are desirable, the feedstock should be expanded to lignocellulosic raw material such as straw, agricultural residues and landscape maintenance material [8]. In addition, such materials are more reasonably priced than cultivated biomass, a factor which can influence economic viability of a biogas facility. Biogas production from lignocellulosic biomass is a slow (without pre-treatment having been applied to the substrate prior to digestion) but steady process. Methane originates mostly from hemicellulose and cellulose, but not from lignin which cannot be degraded by anaerobic microorganisms. As in other biochemical conversion pathways, in the anaerobic digestion of this substrate type, enzymes must first break the lignin barrier in order to gain access to the degradable components. The reaction rate is directly related to the surface to which hydrolyzing bacteria can attach [28]. Particle size reduction is an appropriate method to increase methane generation from lignocellulosic substrates such as straw and dung containing these components when used as 34

litter, but the achievable exploitation degree does not reach the level of the determined total methane potential [13]. In order to make these biomasses better available to anaerobic degradation, appropriate and more sophisticated pre-treatment techniques are a prerequisite. Various physical or chemical pretreatment technologies are known, including thermochemical or ultrasonic pre-treatment, use of different additives or steam pressure disruption, all aiming to enhance anaerobic degradability of the biomass [17, 23, 31]. However, it may be difficult to achieve economic viability in smaller scale. Implementation of additional pre-treatment steps to increase the biogas yield therefore needs to be carefully assessed in particular on farm, where in general smaller digestion facilities are of interest. Anaerobic digestion to biohydrogen is an alternative which is currently under intense research worldwide. Hydrogen is considered as the main energy carrier with high potential to replace conventional fossil fuels in the future, and it is in particular hydrogen from biomass including various organic wastes as a versatile and renewable energy source which is of special attractiveness [7, 25]. Biohydrogen production by dark fermentation is a process that resembles much to the conventional biogas production, but in which specific process conditions are maintained to produce hydrogen by metabolic activity of microorganisms. In general pH values between 5.0 and 5.5 were found to be optimum [11, 16, 22]. Hydrogen producing bacteria need to be enriched in the microbial population, which can be done by different techniques such as treatment of inoculum or recyclate by acid, base, chloroform, heat-shock or aeration [30]. Lignocellulosic biomass contains 70 to 80% carbohydrates and could serve as the ideal feedstock for fermentive hydrogen production [5]. As with other biochemical conversion pathways, identification and optimisation of pre-treatment methods to make carbohydrates available for microbial consumption is one of the key engineering challenges. 4. Biorefinery and Integrated Concepts In addition to being a resource for energy generation, lignocellulosic biomass has potential to serve for multiple purposes. There is not necessarily a concurrence of various options. Highest value can be achieved by diverting individual components to optimum routes, thus aiming to achieve complete valorisation of the material. Among possible target utilisation options are to be mentioned in particular:

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Electricity and fuel generation Production of chemicals Precursors for industrial products such as biodegradable plastics • Utilisation as soil amendment The idea of so-called biorefineries is to process bioresources such as agricultural or forest biomass to produce energy and a wide variety of precursor chemicals and bio-based materials. Industrial platform chemicals such as acetic acid, liquid fuels such as bioethanol and biodegradable plastics such as polyhydroxyalkanoates can be produced from wood and other lignocellulosic biomass in such a concept [10]. Currently, despite its availability in large amounts, lignocellulosic biomass is not much used for the production of chemicals, heterogeneity of the substrates was mentioned as negative factor [4]. Fractionation of woody biomass into major components was discussed within a biorefinery concept [1]. Delignified wood was found to be a suitable feedstock for biohydrogen production by dark fermentation [15]. As an example, this could be taken as starting point in a biorefinery concept. Lignin is known to have the potential to improve mechanical stability of biodegradable plastic produced from agricultural resources. Delignified material could then be used to produce biohydrogen by dark fermentation. Biohydrogen and biogas production can be very efficiently coupled in two subsequent reactors, with digestate from the hydrogen reactor serving as ideal feedstock for the methanogenic step [9, 29]. Identification of further or alternative components in the process chain could thus lead to a versatile and efficient valorisation of substrates. A holistic approach to valorisation of lignocellulosic biomass needs to take into account sustainability of chosen options. If concepts are too heavily orientated towards energy production or industrial use, this can even be at the expense of environmental protection. If crop residues such as straw are no longer left on field, this will result in depletion of soil organic matter. While anaerobic digestion results in a digestate which can be brought back to field to supply not only nutrients but also organic matter, thermal valorisation and production of second generation biofuels result in complete consumption of the biomass and consequently a lack of nutrients and organic matter. Soil requirements vary within a wide range and need to be assessed locally. Only lignocellulosic biomass which is in surplus of soil demand for organic matter should be considered for treatment options with complete consumption of the substrate. In regions with concern about declining organic content of soils,

• • •

anaerobic digestion should be given special attention even if the net energy recovery is lower compared to that of alternative technologies with total consumption of the biomass. With regard to the importance of crop residues for soil organic matter content, it has been recommended to give higher priority to utilisation of animal waste and of municipal solid waste for worldwide biofuels production [14]. Establishing biofuel plantations on agriculturally marginal or degraded lands was also found to be sustainable and of net benefit for the environment [148], as was afforestation of degraded areas [18]. Composting needs to be mentioned as treatment option for organic wastes. It provides a product which is suitable for use as soil amendment. Biomass composting also releases energy in the form of heat, but this energy cannot be recuperated economically [19]. A two-step process consisting of anaerobic digestion followed by composting is a particularly advantageous concept. In Germany, since the year 2009 the Renewable Energy Law guarantees a higher feed-in tariff for electricity originating from biogas generated in such facilities.

5. Conclusions Lignocellulosic biomass has potential to be a key element in further increasing the amount of generated bioenergy. Its global availability in large amounts and the fact that hardly any of the potentially available biomass is actually being used today, are reasons why lignocellulosic biomass is considered as one of the most promising resources for future bioenergy generation. Biochemical conversion pathways, in particular biogas generation and ethanol fermentation, are regarded to be economically attractive alternatives to thermochemical procedures. Biochemical pathways incorporate a hydrolysis step. Cellulose crystallinity, accessible surface area, protection by lignin, and cellulose sheathing by hemicellulose all contribute to the resistance of this biomass to enzymatic hydrolysis [20]. Pre-treatment can significantly improve energy yields and needs to be seen a key element in biochemical conversion of lignocelluloses. Attention should be focused on integration of lignocellulic biomass into biorefinery concepts. It should be kept in mind that lignocellulosic biomass such as crop residues or compost has an important role to play in stabilising soil organic matter content. Utilisation of lignocelluloses for energy generation should not be at the expense of long-term soil productivity. 35


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