Jin M. Triolo, Alastair J. Ward, Lene Pedersen and Sven G. Sommer

Additional information is available at the end of the chapter http://dx. doi. org/10.5772/54424

1. Introduction

Climate change has become an important global issue and animal manure has been pointed out as a major source of greenhouse gas (GHG) emissions. The Danish government targets animal manure as a key biomass with the aim of producing renewable fuels and reducing GHG emissions. Animal manure is a mixture of excreta and materials added during management. Apart from the major part of animal slurry which is feces and urine, animal slurry is composed of many materials, i. e., sand, water from cleaning, small branches and straw from the bedding materials. Thereby a wide variation of characteristics can be found depending on different management systems, animal type and diet, etc. which make for difficulties in the estimation of manure quality for biogas production.

There is no doubt that in the future the world’s energy supply market will be dominated by renewable energy, since there is no alternative. While combustion is the most common method to gain energy from plant biomass such as wood and wood chip, the high content of water in animal slurry suits wet fermentation for conversion to energy, since direct combustion is not appropriate for most animal manures. Direct combustion dry matters (DM) content must be at least 45% [1]. Animal slurry is typically in a liquid form where DM typically contains 1-10% [2]. The production of energy through combustion can be made by enriching fiber fractions by separation technology. Fiber rich animal slurry through separation technology can potentially replace 3.6 PJ of coal energy, which corresponds to 4.3 % of the yearly Danish energy consumption, if one third of the Danish manure is separated [3,4]. The European Commission made a considerable effort by making mandatory national targets for renewable energy shares of final energy consumption in 2020 with the goal: Increasing energy efficiency by 20% by 2020 and reducing GHG emissions at least 20% within the same period [5]. To commit to the targets of the European Commission, the Danish government targets animal slurry as a key element, setting an ambitious goal of increasing the utilization of animal manure for energy production from current levels (5%) up to 40% by 2020 [6].

© 2013 Triolo et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons. org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Facing an "aggressive growth" of biogas production using animal slurry as prime feedstock, it is of great importance to understand critical barriers of characteristics of animal slurry on economic viability. Further, it is of current interest to find solid organic residues as co­substrate, in order to bring the best synergy by overcoming barriers of animal slurry. Biomass is the term given to all organic matter. Its production worldwide is estimated at 146 billion metric tons per year, composing mostly of wild plants [7-9]. The energy of biomass originates from solar energy through photosynthesis, which converts water and CO2 into organic materials in plant biomass. It comprises i. e., plant, wood, energy crop, aquatic plants. Whereas plants store energy in the form of organic materials from solar energy directly, animals generate excreta through metabolizing and digesting. Hence, animal slurry has unique characteristics compared to other biomasses, since during digestion the relatively easily degradable organic matter is utilized while recalcitrant carbon concentrations are increased by animal digestion [10], which limits subsequent anaerobic degradability (BD) and biogas potential. Moreover, the quantity of organic polls in liquid slurry is often too small to perform economically viable operations [10,11].

Hence, the aim of this study is intensive investigation and identification of critical barriers in characteristics of animal slurry. The study was carried out using diverse animal slurry collected from 20 different farms in Denmark, firstly focusing on the Biochemical Methane Potential (BMP) of animal slurry with respect to the total feedstock fresh weight, organic fractions (VS) and DM. Physicochemical characteristics were determined to qualify animal slurries as prime substrates for biogas reactors, and the results were applied to construct algorithms to assess potential methane yield. This study finally highlights the characteristic digestibility of animal slurry compared to plant lignocellulosic biomass. The study further aims to improve our suggested model to predict BMP [10]. In accordance with the objective of the study, quantification of nutrients and characterization of indigestible organic pools of a wide range of animal slurry will be carried out.