A review of biogas and an assessment of its economic impact and future role as a renewable energy source

Anaerobic digestion (AD) has gained popularity as an effective way to treat organic materials, produce clean energy, and reduce greenhouse gas emissions. There is a significant number of large-scale AD facilities operating world-wide, largely treating livestock wastes, and used primarily for electricity production in industrialized countries. At the same time, there are millions of small, household-scale ADs deployed in developing countries, mostly to provide biogas resources for heating and cooking. Decentralized low-volume AD systems could provide a local, renewable energy source (for electricity, heating, or both), reduce or eliminate waste disposal costs, and limit discharges of high strength wastes. The purpose of this study was to evaluate the feasibility of deploying low-volume anaerobic digestion (LVAD) systems at institutions generating significant food waste, using Rochester Institute of Technology (RIT) as a case study. Mass flows and energy balance, net present value (NPV), and discounted payback period (DPP) were used to assess the feasibility of implementing an anaerobic digestion system utilizing the campus organic waste resources. Our study showed that a positive NPV can be achieved if subsidies and incentives were applied to offset the initial capital investment. However, the economics can be improved by driving down equipment cost and accepting food waste from other establishments to generate revenue from tipping fees.

Anaerobic digestion is an established technology for the treatment of wastewater and its sludge and has been used by humans for centuries. Anaerobic digestion is considered to be a useful tool that can generate renewable energy, and significant research interest has arisen recently. The final product of the anaerobic digestion is biogas: a mixture of methane (55–75 vol%) and carbon dioxide (25–45 vol%) that can be used for heating, upgrading to natural gas quality or cogeneration of electricity and heat. Digestion installations are technologically simple with low energy and space requirements. Anaerobic treatment systems are divided into ‘high-rate’ systems involving biomass retention and ‘low-rate’ systems without biomass retention. High-rate systems are characterized by a relatively short hydraulic retention time but long sludge retention time and can be used to treat many types of wastewater and the sludge. Low-rate systems are generally used to digest slurries and are characterized by a long hydraulic retention time, equal to the sludge retention time. The biogas yield varies with the type and concentration of the feedstock and process conditions. The aim of the chapter is to discuss the basic principles of the anaerobic process, the affecting factors, advantages and disadvantages, various treatment methods and energy recovery in the form of biogas. The energy recovery from the anaerobic digestion would open the doors in the conservation of energy resources and also a sustainable method for management of wastewater and its sludge.

Anaerobic digestion is a natural process in which different microorganisms of the biological kingdoms of Bacteria and Archaea work together to convert organic compounds through a variety of intermediates into biogas, a mixture of methane and carbon dioxide and small amounts of hydrogen sulfide and hydrogen. This ancient process, brought about by living species long before the presence of oxygen in the atmosphere, is presently gaining an increased interest because of its potential in the treatment of solid organic waste, sludge, and wastewater.

Biogas is increasingly being investigated as a source of renewable energy, however its utilisation requires an improvement of its quality. Certain biogases contain compounds called siloxanes; these compounds are the origin of some problems when using biogas. At high temperatures siloxanes are transformed to silicate oxides which can damage equipment (i.e. engines corrosion, the clogging of fuel cell membranes.). This paper provides details of the research done in the treatment of siloxane by adsorption process of adsorption. Different porous materials were studied in order to evaluate adsorption capacities. Influences of humidity and temperature on the mass transfer were evaluated. Moreover, reactors were filled with a mixture of methane and carbon dioxide (principal compounds on the biogas) in order to evaluate the impact of the gas composition in the adsorption capacities. Finally the capacity of adsorption is also evaluated in the presence of an organic volatile compound (VOC). Activated carbon cloth was chosen to continue with the experiences realised for the adsorption – desorption process. This porous material has shown good adsorption capacities. Furthermore, desorption process can be performed in situ and their implementation in industrial process is more easy. Adsorption-desorption cycles were performed and the possibilities of the material regeneration evaluated. Joule effect was employed in the desorption process in order to reduce the duration of regeneration. First cycles have been accomplished. Results are promising, even if the operational conditions of the process must be optimised.

Organic waste are materials that, in balanced natural environments, spontaneously degrade and recycle nutrients in nature's processes. The adoption of appropriate methods for the management of organic waste is essential for urban, peri-urban and rural development. The use of Small-scale biodigestion systems in urban areas and mainly in rural areas is a real alternative. Biogas is a renewable source of energy used as a substitute for natural gas and liquefied petroleum gas. From the anaerobic decomposition of different types of biomass and substrates, we have the metabolic product called biogas, a mixture of methane, carbon dioxide and small amounts of other gases such as hydrogen sulfide. The quality of the biogas generated has a direct effect on its use as an energy source, requiring a minimum fraction of CH4 for its combustion and for other processes such as the transformation of the biogas generated into biofuel, its purification to remove the portions of other gases present its necessary. This work evaluated the quality of the biogas generated in a small-scale biodigester, based on the decomposition of food waste from the university environment. A mobile laboratory analyzed the composition of the biogas generated. The results showed that this substrate and its heterogeneity has enormous potential for generating biogas with excellent amounts of methane CH4 composition minimum value was 58.46%, the maximum reached 68.41% and the average was 65.44%. Regarding the presence of CO2 within the biogas evaluated, the minimum value of CO2 was 38.02%; the maximum value was 44.39% and the average stayed in 38.55%. Hydrogen in the composition minimum value was 7.41 ppm, 13.42 ppm maximum and average of 9.63 ppm. H2S minimum value obtained was 0.0 ppm, reaching the maximum value of 249.76 ppm, with average of 138.75 ppm.

Agricultural and agro-industrial activities generate large quantities of waste which are harmful to the environment. This waste, rich in organic matter, can be recycled and transformed by biotechnological processes which constitute a solution of choice for remedying pollution problems. The aim of this study is the treatment of the organic fraction of waste by anaerobic digestion, which consists of degradation in the absence of oxygen of the organic matter into a mixture of methane (CH4) and carbon dioxide (CO2) called biogas. For this we have chosen the following samples (manure ; manure mixed in bananas, oranges, lemons, potatoes and tomatoes ; manure mixed in breads, zucchinis, carrots, cucumbers and strawberry). We used a biogas plants based on simple tools. Among these samples, the T container (manure) the most productive mixture of biogas and the T (manure) and F2 mixture (manure + breads + zucchinis + carrots + cucumbers + strawberry) degraded faster than the F1 manure + mixture (bananas, oranges, lemons, potatoes and tomatoes). The micromycetes that these samples contain are Penicillium italicum, Penicillium digitatum, Rhizopus sp, Mucor, Aspergillus sp, Cladosporium. Micromycetes give good biogas yield. The genus Aspergillus accelerates the degradation of organic matter. Anaerobic digestion not only prevents pollution, but also produces energy, compost and replenishes nutrients. Anaerobic digestion can turn a waste problem into a source of wealth. This technology is becoming essential in the process of reducing waste and producing biogas, a source of renewable energy.

Anaerobic digestion (AD) is a biochemical process that uses the microbiological conversion of organic materials to generate biogas. Biogas is typically composed of 50% to 70% methane (CH4) and 30% to 40% carbon dioxide (CO2). The most simplistic design of an AD system is the solid state digester (SSD), which is able to process very high solids content materials. A SSD has the potential to be utilized in a manure management system of a beef cattle feedlot to integrate seamlessly into an ethanol, feedlot operation to capitalize on the eco-cluster concept for bioenergy production.

Anaerobic digestion (AD) of organic substrates can produce biogas, which consists mainly of methane and carbon dioxide. The objective of this study is to perform, for a given wastewater, useful numerical simulations following model number 1 by integrating an optimization sequence to show biokinetic constants effects on the biogas production rate. Therefore, we obtain via an accurate and simple simulation with less time of calculations a reliable estimation of biokinetic values. Indeed, we evaluate the effect of certain biokinetic constants on the response of 3-steps AD model to select the most significant ones. Thus, the constants in question were varied in specific ranges by setting the others. We conclude that main parameters were saturation constant for acidogenic bacteria (KS1), the solublization rate per unit of acidogenic biomass (ᵝ) and the yield coefficient for the yield of volatile acids from soluble organics (Yb) they have an important effect on the biogas production rate Q max and period to reaches its plateau value.

Anaerobic digestion (AD) is a widely used technology for the treatment of organic wastes and by-products. Through AD, the organic matter is degraded producing a gaseous stream (biogas, which is a mixture of mainly methane and carbon dioxide) and a liquid/slurry stream (digestates) that contains most of the mineralized elements originating in the feedstock. Digestates are a very interesting source of nutrients for growing microalgae to produce valuable biomass with a simultaneous further treatment of the digestates. The performance of microalgae grown using digestates is influenced by various cultivation parameters, such as the physicochemical characteristics of the digestates (nutrient profile, content of inhibitory compounds, etc.), light penetration (turbidity and colored dissolved compounds), mixing regime, and hydraulic retention time (HRT). Digestates are characterized by their high content in ammoniacal nitrogen, suspended solids, and several inhibitors that might limit growth, and therefore pretreatment of digestates is likely to have a positive effect on biomass production. Microalgal cultivation is proven as an efficient technology for the removal of nitrogen, phosphorus, organic load, and other contaminants (heavy metals, pathogens). The produced biomass could be used as feedstock for the production of various commodities (biofuels, feed, etc.); however there are some concerns about the potential contamination of microalgal biomass with unwanted hazardous pollutants. This book chapter aims to give an overview on the cultivation of microalgae utilizing digestates derived from agro-industrial wastes and by-products, discussing the potentials and the drawbacks of such an approach.

Anaerobic digestion is an environmentally sound treatment option for the organic fraction of municipal solid waste (OFMSW) and simultaneously producing methane gas which is a source of renewable energy. The COD reduction of the substrate was 72% for the mesophilic condition and 72.3% for the thermophilic condition. 1200 cm³ of methane was produced in the mesophilic condition and 1260 cm³ in the thermophilic condition. The methane content, by volume, in the biogas was found to be 60% and 63% in the mesophilic and thermophilic condition respectively. It was also found that 0.723 cm³ of methane per unit weight of substrate was produced at the mesophilic condition and 0.760 cm³ of methane per unit weight of substrate at the thermophilic condition. The gas production rate for mesophilic and thermophilic condition were 0.0214 m³ (m³d)−1 and 0.046 m³ (m³d)−1 respectively and the methane yield was more or less the same in both cases. The methane yield in mesophilic condition was 0.0614 m³/kg COD and that at thermophilic condition was 0.0612 m³/kg COD.

Due to rapid increase in population and explosive evolution of life standards, there is tremendous increase in solid waste generation in the last few decades. Furthermore, most of the countries are going to be industrialized; hence more amount of energy will be required in upcoming decades. Today’s more than 85% of the world demanded energy is supplied by fossil fuels. Fossil fuels are finite source of energy and therefore it is necessary to find out other alternatives for energy generation. Improper management of solid waste (MSW, waste biomass, etc.) is responsible for climate change, water and soil and local air pollution. These wastes have a high value with respect to energy recovery. The energy generation from the biological waste materials has been identified as alternative to the fossil fuels due to it’s dual benefit of resource generation and waste minimization. Anaerobic conversion of solid waste biomass is a matured technology for environmental protection and waste management. The end products are biogas (a mixture of methane and carbon dioxide), which is a useful, renewable energy source and organic manure slurry which can be used as fertilizer for agricultural purposes. Anaerobic digestion is a simple process, used to convert organic material (from a wide range of solid waste) into methane. This paper is mainly focused on the anaerobic digestion of solid waste biomass to produce methane, technologies related to pre-treatment of feed materials and post treatment of product gas to enrich the methane composition and the value addition of product fractions are also discussed.

Anaerobic digestion is an established technology for treating different kinds of wastes and to simultaneously produce biogas (mixture of methane and carbon dioxide), which is useful and renewable energy source. In this work, the influence of temperature on the biogas production from cassava peels was studied in batch type anaerobic digesters of volume 500 ml at digestion temperatures of 35±5 0C, 45±5 0C and 55±5 0C for a retention period of 30 days. The quantity of the biogas produced daily were measured with a pressure gauge for the 30 days retention period and converted to volumetric gas yield. It was observed that the cumulative biogas yield increases by 25 % when the digestion temperature was raised from the mesophilic temperature range to thermophilic temperature range. The highest cumulative biogas yield of 289.33×10-5 m 3/kg-TS was obtained with the highest digestion temperature of 55±5 0C. It is therefore concluded that biogas production increases with the increase in digestion temperature and it is recommended that anaerobic digestion should be carried out at the thermophilic temperature range for better gas yield.

Anaerobic digestion using cigarette butts, one of most littered items, was studied not only as a waste treatment, but also as an energy production method. Methane production from cigarette butts was measured through the biochemical methane potential (BMP) test and it was evaluated whether it is possible to produce electrical energy. Intact cigarettes or individual components (filter, paper, and leaf) were supplied as the sole carbon source (substrate) for the BMP test. The tendency of methane production indicated biodegradation in the order of paper, filter, and leaves; however, the filter of cigarettes was the substrate produced the highest amount of methane per total solid. The microbial community was also analyzed in each anaerobic digestion reactor, and substrate-specific microorganisms were identified, such as Proteiniphilum strain (filter) and Methanobacterium formicicum (paper). In intact cigarettes, the related microbial community became dominant over time in the order of paper, filter, and leaf. The conversion of cigarette butts to methane, a renewable energy source, can be proposed as a sustainable route for energy demand, for example, in a smoking room.

Homogeneous nucleation rates and droplet growth rates of water in pure methane and mixtures of methane and carbon dioxide were measured in an expansion wave tube at 235 K and 10 bar. The nucleation rate in pure methane is three orders of magnitude higher than literature nucleation rates of water in low-pressure helium or argon. Addition of carbon dioxide to the carrier gas mixture increases the rates even more. Specifically, rates in a mixture of methane and 3% carbon dioxide are a factor of 10 higher than the rates in pure methane. With 25% carbon dioxide, the rates are four orders of magnitude higher than the rates in pure methane. An application of the nucleation theorem shows that the critical cluster consists of 22 water molecules and 5 methane molecules, for nucleation in pure methane. Growth rates of water droplets were measured in methane and in methane-carbon dioxide mixtures at 243 K and 11.5 bar. At equal temperature, pressure and water vapor fraction, the growth rate of the squared droplet radius is about 20% lower in the mixture with 25% carbon dioxide than in pure methane. The lower growth rate is caused by a smaller diffusion coefficient of water in the mixture with carbon dioxide; the difference of the diffusion coefficients is qualitatively reproduced by the empirical Fuller correlation combined with Blanc's law.

BACKGROUND: The selection of refrigerants for modern air conditioning systems (ACS) in ground facilities is a multidisciplinary task. Particularly, meeting the required energy efficiency of the refrigeration cycle as well as ensuring ecological safety of production, operation, and utilization of the refrigeration system. Herein, the working pressure levels of the refrigeration cycle considerably affect the availability, cost, and safety of the refrigeration equipment. The fire safety of the working substance is also important.
 AIM: To investigate the feasibility of a mixture of dimethyl ether and carbon dioxide as refrigerant for energy efficient and safe application of ACS in ground facilities.
 METHODS: Comparative analysis of a simple one-stage vapor–compression cycle using traditional working substances (R22 and R410A) and the proposed working substance, which is in the form of a mixture of dimethyl ether and carbon dioxide, using packages, such as Mathcad, HYSYS, CoolPack, and REFPROP, was performed.
 Results: An ecofriendly mixture of dimethyl ether and carbon dioxide with low global warming potential and zero ozone depletion potential was proposed as refrigerant. Increasing the percentage of dimethyl ether in the blend reduces the temperature glide in the gas cooler, a property of CO2, and pressures at which the blend operates. The mixture has limited operational properties due to the flammability of dimethyl ether, but its environmental performance makes the material of some practical interest.
 CONCLUSION: Fire safety of the proposed working substance was calculated. The concentration of dimethyl ether in the mixture at which it becomes flammable and unsafe for ACS was determined to be 8.3%.
 With an increase in the dimethyl ether content in the mixture with CO2 from 4% to 8%, the refrigeration coefficient of the cycle increases from 2.53 to 2.88, but it is 1.57 times less than that of R410A.
 The difference in operating pressures between the used non-ecological refrigerants and proposed mixture was determined. Results indicate that the mixture of dimethyl ether and carbon dioxide is currently inapplicable to mass production compressors, which use R410A as refrigerant. The condensation pressure of the most effective and nonflammable mixture of dimethyl ether and CO2 (with dimethyl ether concentration of 8%) is 101 bar against 30 bar for R410A.
 Therefore, we intend to evaluate test mixtures of dimethyl ether with other substances in the future.