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Production of biogas via anaerobic digestion
Abstract
Anaerobic digestion (AD) is a biochemical process that converts the organic matter present in various types of wastes (sludge, agro-industrial wastes, energy crops) into: (1) biogas (rich in methane, used for heat and/or electricity generation); (2) biosolids (microorganisms grown on the organic matter and fibers, used as soil conditioner); and (3) liquor (dissolved organic matter, recalcitrant to AD, used as liquid fertilizer). AD has been described as one of the most energy-efficient and environmentally friendly technologies for energy production. It can significantly decrease greenhouse gas emissions compared to fossil fuels by the utilization of readily available waste biomass. The improvement in various aspects in AD (pretreatments, codigestion, reactor engineering, process modeling, and control) helped to gain a better insight into the process and to improve process efficiency. In addition, the policy to promote biogas utilization contributed to boost the application of the AD to achieve energy production and waste minimization.
Keywords
Anaerobic digestion; Biogas; Biomass; Codigestion; Modeling; Pretreatment; Process control; Waste management10.1. Introduction
AD is a biochemical process accomplished by the combined action of a group of several types of microorganisms, which metabolize the organic compounds into a gaseous mixture consisting of mainly methane and carbon dioxide (biogas) in anaerobic conditions. It is a widely used technology in order to cost-effectively reduce the volume of the biomass such as sewage sludge, organic fraction of municipal solid waste, manure, and lignocellulosic biomass, and to capture energy in the form of methane (Weiland, 2010). For instance, wastewater treatment plants generate a large amount of sludge, which is costly to incinerate. However, AD significantly decreases the costs of sludge treatment while effectively reducing its volume and recovering renewable energy in the form of biogas.
Methane in the biogas is generally upgraded and then sent to a natural gas network. It can also be used directly for the production of electricity, heat, and transportation fuel. The final digestate is used as soil conditioner and fertilizer. AD also provides significant environmental benefits such as controlling the pathogens through sanitation, preventing biomass putrefaction and acidification, reducing air and water pollution, and reducing greenhouse gas emissions by stabilizing and controlling the waste biomass (Cuéllar and Webber, 2008). Therefore, the interest in AD applications increased in many countries and it is supported by legislation, particularly in the European Union. It is estimated that at least 25% of the bioenergy will be derived from biogas in the European Union by 2020 (Holm Nielsen and Oleskowicz-Popiel, 2008). For example, the electricity production from biogas would increase from 25.2 TWh in 2009 to 63.3 TWh in 2020 in the European Union. In China, the construction of biogas plants is supported by the government and the production of biogas dramatically increased from 10.5 billion m3 to 248 billion m3 (annually) from 2007 to 2010 (Deng et al., 2014; Wellinger, 2011).
There is also an increasing interest in AD research to further improve the efficiency of the process. Most of the researchers investigate the complex anaerobic digestion process in order to optimize the process parameters, digester design, and the substrate types (Morita and Sasaki, 2012; Palatsi et al., 2009; Parawira et al., 2005). Unfortunately, the microorganisms in AD are sensitive to variations in process parameters, such as temperature, pH, substrate and inoculum types, organic loading ratio, etc. Therefore, the AD process should be monitored and controlled properly to prevent any process instability. Online monitoring of the process parameters, particularly the metabolic intermediates, such as VFA and free ammonia, is very crucial to understand what is going on in the digester and to detect any upcoming instabilities in a biogas plant and prevent it (Bernhard, 2013). The feedstock's characteristics are also very crucial, some feedstocks are difficult to handle while some others inhibit the AD. The pretreatment of the feedstock to make it more amenable and/or its codigestion with other feedstocks to have a well-balanced medium are the main strategies to overcome any feedstock-related challenges (El-Mashad and Zhang, 2010; Li et al., 2009; Zheng et al., 2011, 2014).
This chapter reviews the principles of the AD process, discusses the factors affecting the AD process, the advantages and disadvantages of biogas production, different reactor configurations, current state and perspectives of applications used for enhancing the efficiency of AD including pretreatments and co-digestions, the process control and monitoring tools, and also summarizes the current existing biogas installations.
10.1.1. The principles of the anaerobic digestion process
AD is a complex biochemical process consisting of consecutive and interactive reactions carried out by several types of anaerobic microorganisms, which have different growth rates and sensitivity to environmental conditions, such as pH, partial pressure of hydrogen, etc. The AD process is composed of the following steps (Fig. 10.1):
Disintegration: The complex biomass is disintegrated into organic polymers such as carbohydrates, proteins, and lipids. Disintegration includes several steps such as lysis, nonenzymatic decay, phase separation, and physical breakdown (Batstone et al., 2002).
Hydrolysis: The organic polymers (carbohydrates, proteins, and fats) are hydrolyzed to their respective monomers (sugars, amino acids, lipids) by extracellular enzymes to facilitate the nutrient transport through the cell membrane. In order to improve the biomass hydrolysis, the biomass is generally pretreated. Pretreatments make the complex substrate matrix more amenable to microorganisms and enzymatic attacks.
Acidogenesis: Different kinds of acidogenic microorganisms are able to hydrolyze these simple monomers and oligomers into dicarboxylic acids, short-chain fatty acids, carbon dioxide, hydrogen, and other simpler organic compounds. At this acidogenesis step, large amounts of carbon dioxide and hydrogen are produced. If the carbohydrate content of the substrate is high, hydrogen production will be high and it can be harvested directly to use it as biofuel. The acidogenic bacteria have high growth rate and can tolerate low pHs (5–6). As a result of their rapid growth, the whole AD process might be inhibited due to the decrease in medium pH, if the acidic products cannot be metabolized by acetogenic bacteria eventually.
Acetogenesis: Fatty acids and other organic molecules are metabolized to acetate, carbon dioxide, and hydrogen by acetogenic microorganisms during the acidogenesis step. The inhibition of methanogenesis can occur if the hydrogen molecules are not metabolized subsequently. Acetogenesis takes a long time as the growth of acetogenic bacteria are slow and sluggish, generally they have long doubling times, ie, days.
Figure 10.1 COD flux during the AD for a particulate biomass consisting of 10% inert and 30% each of the main organic polymers (Batstone et al., 2002).
Methanogenesis: There are two different microbial groups that produce methane and carbon dioxide from different substrates. The first group is the acetoclastic methanogens which grow on acetic acid and produce methane. Acetoclastic methanogens are slow-growing microorganisms (doubling time is in the order of days) and are particularly sensitive to a number of parameters such as pH, nutrient, and trace element concentrations, etc. The second group is the hydrogen-utilizing methanogens which utilize hydrogen and carbon dioxide to produce methane. The methane content of biogas depends on the substrate composition, particularly the oxidation state of the organic carbon found in the substrate; the more reduced the initial substrate is, the more methane will be generated.
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