The Biogas Handbook

Science, Production and Applications

Arthur Wellinger, Jerry D Murphy, David Baxter

• Publisher: Woodhead Publishing
• Year: 2013
• ISBN:
  9780857097415
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

With pressure increasing to utilise wastes and residues effectively and sustainably, the production of biogas represents one of the most important routes towards reaching national and international renewable energy targets. The biogas handbook: Science, production and applications provides a comprehensive and systematic guide to the development and deployment of biogas supply chains and technology.Following a concise overview of biogas as an energy option, part one explores biomass resources and fundamental science and engineering of biogas production, including feedstock characterisation, storage and pre-treatment, and yield optimisation. Plant design, engineering, process optimisation and digestate utilisation are the focus of part two. Topics considered include the engineering and process control of biogas plants, methane emissions in biogas production, and biogas digestate quality, utilisation and land application. Finally, part three discusses international experience and best practice in biogas utilisation. Biogas cleaning and upgrading to biomethane, biomethane use as transport fuel and the generation of heat and power from biogas for stationery applications are all discussed. The book concludes with a review of market development and biomethane certification schemes.With its distinguished editors and international team of expert contributors, The biogas handbook: Science, production and applications is a practical reference to biogas technology for process engineers, manufacturers, industrial chemists and biochemists, scientists, researchers and academics working in this field.

• Provides a concise overview of biogas as an energy option
• Explores biomass resources for production
• Examines plant design and engineering and process optimisation

Part 1

Biomass resources, feedstock treatment and biogas production

1

Biogas as an energy option: an overview

Claudius Da Costa Gomez, Fachverband Biogas, Germany

Abstract:

This chapter describes the potential, requirements, risks, required general conditions and the prospects of biogas production. The necessity of effective government support is emphasized. The pressing question of the ecological importance of biogas production is discussed and it is made clear that state-of-the-art biogas plants are no risk either to humans or the environment. One conclusion at which the author arrives is that biogas has a special role to play in the scenario of the fundamental change of today’s energy supply patterns because electricity and heat can be produced as and when needed.

Key words

biofuel

biogas

biomass

biomethane

electricity

energy balance

fixed-price support system

transport biofuel

life cycle assessment

1.1 Introduction

Biogas is produced in biogas plants by the bacterial degradation of biomass under anaerobic conditions. There are three categories of biomass: (1) substrate of farm origin such as liquid manure, feed waste, harvest waste and energy crops; (2) waste from private households and municipalities such as separately collected organic waste (in organic waste containers), market waste, expired food or food waste; (3) industrial by-products such as glycerine, by-products of food processing or waste from fat separators. The organic substance is converted to biogas by bacteria in several steps in airtight digesters. The bacteria are similar to those found in the pre-stomachs of ruminants.

As with fossil natural gas, the main component of biogas that determines the energy content of the gas is flammable methane (CH₄). Depending on the substrate digested in the biogas plant, the methane content of the biogas fluctuates between 50% and 75%. The second main component of biogas is carbon dioxide (CO₂) with a share between 25% and 50%. Other components of biogas are water (H₂O), oxygen (O₂) and traces of sulfur (S₂) and hydrogen sulfide (H₂S). If biogas is upgraded to biomethane with approximately 98% methane in a biogas treatment plant, the biomethane has the same properties as natural gas.

After simple desulfurization and drying, biogas can be converted to electricity and heat in cogeneration units (combined heat and power (CHP)) or the biogas is burnt to produce heat. After treatment to natural gas grade, the so-called biomethane can be used in all applications commonly known for natural gas. Thus, biogas and biomethane produced from biogas are flexible renewable fuels that can be stored. Motor fuel, electricity and heat can be produced from them, which makes them important functions in the context of sustainable energy supply. Besides, biogas can also replace carbon compounds in plastic products.

Experts are not agreed as far as the importance of biogas for the sustainable supply of energy is concerned. Basically, a difference should be made between two different origins of the substrate on which biogas plants feed: waste and energy plants. Whereas the untapped reserve of digestible organic waste is enormous on a world scale, large unused areas of land on which energy crops can be cultivated are also available. For Europe, including the European succession states of the Soviet Union, it has been calculated (Thrän et al. 2007) that by the year 2020, 250 billion standard cubic meters (m³_N) of biomethane from digested feedstock could be produced, which would be enough to meet 50% of the present gas consumption in the 28 European Union (EU) member states. These figures illustrate that biogas can make a sizable contribution to the energy supply. Besides, biogas is a versatile fuel: biogas produced from substrates by digestion, is the only renewable fuel at present that is a viable alternative to fossil natural gas and can be used for all purposes for which natural gas is used and also by the same infrastructure. Thus, biogas technology can contribute to solving the pressing questions of safe and sustainable energy supply for electricity, heat and transport fuel.

1.2 Biogas technologies and environmental efficiency

Biogas is produced by anaerobic bacteria that degrade organic material to biogas in four steps: hydrolysis, acidification, production of acetic acid and production of methane. The product of the digestive process, raw biogas, consists of 50–75% methane, 25–50% carbon dioxide and 2–8% other gases such as nitrogen, oxygen and trace gases (e.g. hydrogen sulfide (H₂S), ammonia (NH₃) and hydrogen). Before the biogas can be converted into electricity in engines at the place at which it is produced, the raw biogas must be cleaned in a first process in which the water vapor saturated biogas is desulfurized and dried by cooling.

Certain basic conditions must be met to enable the bacteria to degrade the substrate efficiently. These are: (1) absence of air (anaerobic atmosphere); (2) uniform temperature; (3) optimum nutrient supply; (4) optimum and uniform pH. The equipment of a biogas plant should be able to meet these basic requirements. Therefore, a biogas plant designer should know from the beginning what kind of substrate the plant will feed on so that the right equipment for efficient biogas production can be selected.

The methods of biogas production can be characterized by the number of process steps, the process temperature, the dry matter content and the way in which the substrate is fed. Biogas plants feeding on agricultural byproducts such as liquid manure, harvest residue and energy crops often employ a single-step process in the mesophilic (32-42°C) temperature range with wet fermentation and quasi-continuous feeding. The method can be varied depending on the requirements the process must meet in terms of speed, the degree of digestion and the hygienizing action. For example, hydrolysis as the first step usually accelerates the process and may also result in a higher degree of degradation. Increasing the process temperature from the mesophilic (32-42°C) to the thermophilic (45-57°C) level also speeds up degradation and improves the health status of the substrate (Eder and Schulz 2006).

Better health results are also reported for the plug flow fermentation method in which the substrate is mixed by the slow rotation of an agitator and moved through a long horizontal digester. Because the substrate in the digester is not mixed in one pass, quick passage from the feed point to the delivery point is prevented and a minimum dwell time of the substrate in the digester is obtained. This enforced dwell time of the substrates improves the hygienizing action of this method. Unlike the full-mix wet digestion process, a plug flow digester can normally carry a higher volume load of organic material per cubic meter of digester volume.

If the substrate digested in the biogas plant contains more than 20% dry matter, so-called dry digestion methods will normally be applied. In these methods, the digester is charged with stackable substrates. The substrates are not mixed, but a liquid called percolate runs through them. After a sufficiently long dwell time, the digester is opened and the digested product removed. In addition to this batch process, several other methods for digesting solid substrates have been developed that like the batch processes, are now primarily used for digesting municipal waste. The different processes will be dealt with in another chapter of this handbook and so are not described in detail here.

The purpose of biogas technology is the conversion of organic substances to methane as fuel and valuable fertilizer from available resources that otherwise would go unused. This is particularly the case when exclusively by-products and waste are used as substrates for digestion. But even if energy crops serve as the substrate for biogas production, the energy balance is positive as Effenberger et al. (2010) were able to demonstrate with scientific support from the example of ten biogas plants.

In addition to the energy balance, the carbon balance is an important indicator assessing the environmental efficiency of a technology providin...