The pretreatment of lignocellulosic biomass (LC biomass) prior to the anaerobic digestion (AD) process is a mandatory step to improve feedstock biodegradability and biogas production. An important potential is provided by lignocellulosic materials since lignocellulose represents a major source for biogas production, thus contributing to the environmental sustainability. The main limitation of LC biomass for use is its resistant structure. Lately, biological pretreatment (BP) gained popularity because they are eco-friendly methods that do not require chemical or energy input. A large number of bacteria and fungi possess great ability to convert high molecular weight compounds from the substrate into lower mass compounds due to the synthesis of microbial extracellular enzymes. Microbial strains isolated from various sources are used singly or in combination to break down the recalcitrant polymeric structures and thus increase biogasgeneration. Enzymatic treatment of LC biomass depends mainly on enzymes like hemicellulases and cellulases generated by microorganisms. The articles main purpose is to provide an overview regarding the enzymatic/biological pretreatment as one of the most potent techniques for enhancing biogas production.

Seeking to become more climate-friendly and less energy-consuming, the European Union has pledged to cut its greenhouse gas emissions and milestones to achieve this are set to 20 % by 2020, 40 % by 2030, 60 % by 2040 and 80 % by 2050. Due to its abundance, biomass is gaining more and more importance both for the production of thermal energy by direct combustion or gasification of vegetal materials, for electricity and for the production of biofuels. Direct combustion of biomass generates CO 2 , but the process is neutral in terms of greenhouse gas emissions, because the same amount of CO 2 was absorbed by plants from the air during their life cycle. Ecological solid fuels such as pellets have become rapidly a viable alternative to fossil fuels, due to their high energy content, which makes them suitable for use by both small households and industrial consumers. Pellets are obtained from a variety of raw materials such as: agricultural residues, energy crops, forestry and wood residues, used exclusively or mixed and having different physical-chemical properties. This paper presents a summary of literature on the effect of the moisture content on the properties of pellets obtained from various types of biomass. Moisture content of raw material is one of the most important factors that influence negatively the properties of pellets, such as bulk density or mechanical durability during storage and transportation. Energy consumption increases during pelletizing of high moisture biomass, as moisture is a lubricant that lowers friction in the die. Other studies found a positive correlation between pellets durability and optimal moisture (10 %), because water together with the die temperature, pressure and chemical composition of raw material acts like a binding agent that increases pellet quality. Pellets with 5 % moisture have low strength, become brittle, and large amounts of dust are produced during their storage and transportation. Moisture higher than 15 % damages pellets during storage.

In order to obtain bioenergy (biogas, biofuel) or pellets, different types of lignocellulosic biomass are subjected to a mechanical pretreatment, first by size reduction, then by separating, and ultimately by fracturing or bio-refining. Biomass processing mainly refers to a grinding process that occurs until reaching certain limits. The size reduction process, such as grinding, is an operation that is executed with different levels of energy consumption, considering biomass mechanical characteristics and the necessary grinding level. This paper, illustrates a comparative analysis of experimental results obtained by grinding multiple types of vegetal biomass (Miscanthus, corn stalks, alfalfa, willow) used in the process of bio-refining and bio-fracturing. Experiments were realized using both a laboratory knife mill Grindomix GM200 (Retsch GmbH, Haan, Germany), and a 22 kW articulated hammer mill, using different grinding system speeds and different hammer mill sieves. Results have shown that biomass mechanical pre-processing grinding leads to supplementary costs in the overall process through bio-refining or bio-fracturing in order to obtain bio-products or bio-energy. So, specific energy consumption for grinding using a hammer mill can reach 50–65 kJ/kg for harvested Miscanthus biomass, and 35–50 kJ/kg for dried energetic willow, using a 10 mm orifice sieve, values which increase processing costs.

Abstract. The interest in bioenergy, as a renewable energy source, has increased tremendously in the recent years. Biomass is a renewable source of energy that has an important contribution in reducing the use of conventional fuels and in reducing greenhouse gas emissions to the atmosphere. In this context, an increasing attention is given to the production and use of environmentally friendly biofuels: pellets and briquettes, biogas, biodiesel and bioethanol. Pelleting and briquetting processes have been used for many years to produce densified biomass for fuel applications. Densified biomass fuels such as pellets are preferred as they provide better economic viability for transport, storage and handling. The aim of this review is to examine the main factors that influence the durability of the pellets, (such as: biomass characteristics, moisture content, size reduction, and also the pelleting conditions, including the use of binders, feedstock mixes and operating temperature), because the durability is considered a measure of the quality of the pellets. Pellet durability can also be affected by the storage conditions. The mechanical durability of pellets is the measure of the resistance of densified fuels towards shocks or abrasion in consequence of transport and handling processes. In the paper there are analyzed all the parameters mentioned above for the pellets produced from different types of biomass (cereal residues, wood chips and energy crops), highlighting the influence of each on the process and their sustainability.Keywords: biomass, durability, pellets, energy plants. IntroductionInterest in renewable energy resources has increased in recent years because of the rising energy demand and fuel costs, environmental and national security concerns and finite supplies of fossil fuels. As power generators continue their search for alternative energy sources to coal, biomass remains a promising carbon-based renewable fuel [1]. Biomass is a renewable source of energy that has an important contribution in reducing the use of conventional fuels and in reducing greenhouse gas emissions to the atmosphere. In this context, an increasing attention is given to the production and use of environmentally friendly biofuels: pellets and briquettes, biogas, biodiesel and bioethanol. Biomass is the totality of organic substances occurring in a natural habitat, a distinction being made between phytological and zoological mass [2]. Biomass pellets (wood, wheat straw, rape straw, and peat) as fuels are of great interest for district heating because they can contribute to greenhouse gas emission reductions and provide a more efficient use of local energy resources [3]. Pellets are a suitable biomass feedstock for both heat and power applications, with co-firing in coal-fired power stations currently being their main large-scale application. The industrial trade for wood pellets involves the international bulk transport of more than 10 million tons annually.Production of pellets and the demand have been rapidly increased in Can...

In the context of increasing pressure regarding the sustainable utilization of food waste in a circular economy, one of the trends is their biological transformation, through anaerobic digestion, into biogas as a renewable source of energy. We presented the physical-chemical properties of the main categories of food waste from different sources: dairy, meat, and poultry, fish, fruit and vegetable, cereal and bakery, brewing and winery industries, and others. Due to the high organic load, the presence of a multitude of nutrients, and an insignificant amount of inhibitors, food waste can be successfully used in the biogas production process in co-digestion with other materials. Physical (mechanical and thermal), chemical (alkali, acid, and oxidative), and biological (enzymatic, bacterial, and fungal) techniques have been widely used for pretreatment of different substrate types, including food waste. These pretreatments facilitate the degradation of pretreated food waste during anaerobic digestion and thus lead to an enhancement in biogas production. The purpose of this study is to review the situation of food waste generated in the food industry and to formulate the main trends of progress in the use of this waste in the anaerobic digestion process.