Optimization of biogas production by anaerobic digestion for sustainable energy development in Zimbabwe

There is increasing international interest in developing low carbon renewable energy technologies. Biomass is increasingly being utilized as an energy source throughout the world. Several modern technologies have been developed that convert biomass to bioenergy. Anaerobic digestion is a mature energy technology for converting biomass to biogas, which is a renewable primary energy source. Biogas is a robust fuel that can be used to supply heat, electricity, process steam and methanol. There are vast biomass resources in Zimbabwe that have good potential for biogas production by anaerobic digestion. However, anaerobic digestion is not being optimally used as a biomass conversion technology in the country. This paper presents an overview of biogas production in Zimbabwe and outlines technical options that can be utilized to optimize biogas production by anaerobic digestion in the country.     

A re-appraisal on intensification of biogas production

Biogas is said to be a clean and renewable form of energy. It can replace fossil fuel, thereby eliminating environmental concerns caused by them. Due to several constraints in the process of anaerobic digestion, the potential of this technology is not fully utilized. This paper reviews various techniques like co-digestion, pre-treatments, use of additives, variation in control parameters etc, which could be used to intensify the production of biogas.     

The efficacy of an anaerobic digester-based power production from various feedstocks

Biogas is one of the most promising renewable energy fuel sources obtained from various organic matters and used for energy requirements. This work is focused on the usage of biogas in power production. The main aim of this study was to evaluate the different factors that affect the power production during anaerobic digestion. This study examines the usage of Organic Fraction Municipal Solid Waste (OF-MSW), wastewater sludge, and co-digestion of both OF-MSW and wastewater sludge for power production. The digestions are performed under mesophilic conditions at an optimum temperature of 36°C. The maximum power produced is 128 kW when using OF-MSW alone and 48.9 kW for co-digestion (50% OF-MSW + 50% wastewater sludge) and 35.5 kW for wastewater sludge. The paper also focuses on various factors such as pH, temperature, and nitrogen concentration and its influence on power production.     

从废水中回收能源--微生物燃料电池和发酵生物制氢技术

利用微生物在废水处理的同时获得能量主要有3种方式,分别是厌氧产沼气,发酵生物制氢和微生物燃料电池技术.利用厌氧微生物处理污水,在去除COD的同时获得沼气是比较成熟的技术,而后两种的研究开展较晚,仍处于实验室研究阶段.但微生物燃料电池和发酵生物制氢技术发展迅速,而且可以有机结合.文中对两者的基本原理和最新研究进展进行了介绍,并对其发展前景进行了展望.     

Can slurry biogas systems be cost effective without subsidy in Mexico?

Biogas from pig slurry in Mexico has potential to produce 21 PJ per year, equivalent to 3.5% of natural gas consumption in 2013. In this paper, three different scenarios are analysed: mono-digestion of pig slurry in a finisher farm (scenario 1); co-digestion of pig slurry and elephant grass in a finisher farm in situ (scenario 2) and co-digestion of pig slurry and elephant grass in centralised biogas plants (scenario 3). The digesters proposed are anaerobic high density polyurethane (HDPE) covered lagoons. HDPE centralised plants can have capital costs 5 times cheaper than European biogas plants. The economics of utilisation of biogas for electricity generation and as biomethane (a natural gas substitute) were investigated. Economic evaluations for on-site slurry digestion (Scenario 1) and on-site co-digestion of elephant grass and pig slurry (Scenario 2) showed potential for profitability with tariffs less than $US 0.12/kWh_e. For centralised systems (Scenario 3) tariffs of $US 0.161/kWh_e to $US 0.195/kWh_e are required. Slurry transportation, energy use and harvest and ensiling account for 65% of the operational costs in centralised plants (Scenario 3). Biomethane production could compete with natural gas if a subsidy of 4.5 c/L diesel (1 m³ of biomethane) equivalent was available.     

The prospects for an expansion of biogas systems in Sweden—Incentives,barriers and potentials

Biogas is a renewable, high-quality fuel, currently produced at more than 200 locations in Sweden. The present production is some 5 PJ/year but the potential is approximately 10 times higher. Biogas can be produced from a wide range of raw materials, from organic waste to dedicated energy crops, and can be utilised for various energy services such as heat, combined heat and power or as a vehicle fuel. Biogas systems are therefore affected by a number of different incentives and barriers, including energy-, waste treatment- and agricultural policies. In this paper, different policies and policy instruments, as well as other factors, which influence a potential expansion of Swedish biogas systems, are identified and evaluated. Existing incentives and barriers can be divided into those affecting the production of biogas, and those affecting the utilisation of the biogas. Only a few types of biogas systems are competitive in Sweden today, while the majority needs increased incentives of different kinds to reach profitability. Such incentives are often motivated from an energy and environmental point of view. Due to the complexity of the biogas systems and the many actors involved, all with different interests, the process of implementing adequate policy instruments will require concerted efforts.     

Biogas as a potential renewable energy source: A Ghanaian case study

The associated harmful environmental, health and social effects with the use of traditional biomass and fossil fuel has enhanced the growing interest in the search for alternate cleaner source of energy globally. Ghana, a developing country depends heavy on woodfuel as a source of fuel contributing about 72% of the primary energy supply with crude oil and hydro making up the rest. Biogas generation has simply been seen as a by-product of anaerobic digestion of organic waste. Having proven to be a practicable and promising technology, it has been very successful and a very reliable and clean source of energy when proper management programmes are followed. There are vast biomass resources including organic waste in Ghana that have the potential for use as feedstock for biogas production to reduce the over reliance of woodfuel and fossil fuel, and to help reduce the it would reduce greenhouse gas emissions which may be affecting climate change. Ghana having the technical potential of constructing about 278,000 biogas plants, only a little over 100 biogas plants has so far been established. This paper presents the energy situation and the status of the biogas technology and utilization in Ghana. It also presents the potential benefits, prospects and challenges of the biogas technology.     

Anaerobic co-digestion of primary sludge and the fruit and vegetable fraction of the municipal solid wastes: Conditions for mixing and evaluation of the organic loading rate

This paper presents the results obtained for the digestion of primary sludge (PS) and co-digestion of this sludge with the fruit and vegetable fraction of municipal solid wastes (FVFMSW) under mesophilic conditions. This mixture was prepared with a PS content of 22%. The anaerobic digestion process was evaluated under static conditions and with different mixing conditions, with good results being found for the digesters with limited mixing, this representing an energy saving. The results for co-digestion of mixtures of PS+FVFMSW are better than those obtained from digestion of PS on its own. Biogas production for co-digestion is much greater thanks to the larger volatile-solid (VS) content of this feedstock. Nevertheless, biogas yield and specific gas production for the two digestion processes are similar, with values in the range 0.6–0.8 l g⁻¹ VS destroyed for the first parameter and in the range 0.4–0.6 l g⁻¹ VS fed for the second. The co-digestion process was also evaluated at different organic loading rates (OLR) under low mixing conditions, with stable performance being obtained even when the systems were overloaded.     

Anaerobic fermentation technology increases biomass energy use efficiency in crop residue utilization and biogas production

A biomass energy utilization project (Corn stalk→Cattle→Cattle dung→Biogas digester→Biogas/Digester residues→Soil) was conducted in a typical temperate agro-village of China from 2005 to 2010. The present study focused on two key approaches of the ecological loop: (1) increasing corn stalk use efficiency by improving anaerobic fermentation technology; and (2) enhancing biogas productivity by optimizing fermentation conditions. Our results showed that crude protein and fat of corn stalks significantly increased, while crude fiber content and pH decreased considerably during anaerobic fermentation. The cattle digestion rate, forage consumption and increases in cattle weight were higher in cattle fed fermented corn stalks than in those fed non-fermented corn stalks. The rate of biogas production was higher (78.4%) by using cattle dung as a substrate than using crop residues. Heat preservation measures effectively enhanced the biogas production rate (12.3%). In 2005, only two cattle were fed in this village, with only 1.1% corn stalk utilized as forage. No more than three biogas digesters existed, and the proportion of biogas energy used in total household fuel was only 1.7%. At the end of the 5-year experiment, the number of cattle capita reached 169 with 78.9% corn stalk used as forage. Biogas digesters increased to 130, and the proportion of biogas energy used in total household fuel was up to 42.3%. A significant positive correlation was noted between the increasing rate of farmers’ incomes and the proportion of corn stalks used as forage. Available nutrients were higher in fermented cattle dung than in fresh cattle dung. Our findings clearly suggest that anaerobic fermentation technology is important in enhancing crop residue use efficiency, biogas productivity and soil fertility. Fermentation technology may help reduce the use of fossil fuels and improve the environment in rural areas.     

Economics and greenhouse gas balance of biogas use systems in the Finnish transportation sector

Energy decisions play an essential role in reducing greenhouse gas (GHG) emissions in the transportation sector. Biogas is a renewable energy source and can be used as an energy source for gas-operated cars or for electric cars. This paper compares different ways to use biogas, which is produced on a medium scale anaerobic digestion plants, as an energy source for transportation. The research is conducted from an economic and environmental point of view, and the option to deliver upgraded biogas via a natural gas grid is taken into account. Different processes for the use of biogas for transportation purposes are compared using life cycle assessment (LCA) methods in the Finnish operational environment. It seems that the most economical way is to use biogas in gas-operated cars due to the high price of methane for vehicle fuel use. A new feed-in tariff for electricity produced with biogas will, however, have highly positive economic effects on electricity production from biogas. From the environmental point of view, the highest CO₂ reductions are gained when biogas is used in gas-operated cars or in CHP plants for power and heat production. During the transition stage, it might be reasonable to use biogas in gas-operated cars and most importantly in heavy vehicles to reduce GHG and local pollutants rapidly. If biogas production is located near a natural gas grid, the biogas can be delivered effectively via the natural gas grid. The use of biogas in gas-operated cars is an effective way to reduce carbon dioxide significantly in the transportation sector.     

Anaerobic co-digestion of sewage sludge with switchgrass: Experimental and kinetic evaluation

Sewage sludge removal via anaerobic digestion provides energy production in addition to waste minimization. Several strategies, such as anaerobic co-digestion, were developed to increase energy production from sewage sludge by improving C/N balance. In this study, anaerobic co-digestion of sewage sludge with an energy crop, namely switchgrass, was evaluated. As a result of studies implemented at different mixing ratios, maximum methane production was measured as 272.06 mLCH₄/gVS at the mixing ratio of 0.4:0.6 (sewage sludge:switchgrass). According to modified kinetic models used for interpretation of synergetic and/or antagonistic effects, anaerobic co-digestion has a synergetic effect on biogas production from both biomass.     

Biogas production from municipal sewage sludge (MSS)

Biogas is produced by anaerobic (oxygen free) digestion of organic materials such as sewage sludge, animal waste, and municipal solid wastes (MSW). As sustainable clean energy carrier biogas is an important source of energy in heat and electricity generation, it is one of the most promising renewable energy sources in the world. Biogas is produced from the anaerobic digestion (AD) of organic matter, such as manure, MSW, sewage sludge, biodegradable wastes, and agricultural slurry, under anaerobic conditions with the help of microorganism. Biogas is composed of methane (55–75%), carbon dioxide (25–45%), nitrogen (0–5%), hydrogen (0–1%), hydrogen sulfide (0–1%), and oxygen (0–2%). The sewage sludge contains mainly proteins, sugars, detergents, phenols, and lipids. Sewage sludge also includes toxic and hazardous organic and inorganic pollutants sources. The digestion of municipal sewage sludge (MSS) occurs in three basic steps: acidogen, methanogens, and methanogens. During a 30-day digestion period, 80–85% of the biogas is produced in the first 15–18 days. Higher yields were observed within the temperature range of 30–60°C and pH range of 5.5–8.5. The MSS contains low nitrogen and has carbon-to-nitrogen (C/N) ratios of around 40–70. The optimal C/N ratio for the AD should be between 25 and 35. C/N ratio of sludge in small-scale sewage plants is often low, so nitrogen can be added in an inorganic form (ammonia or in organic form) such as livestock manure, urea, or food wastes. Potential production capacity of a biogas plant with a digestion chamber size of 500 m³ was estimated as 20–36 × 10³ Nm³ biogas production per year.     

Evaluation of energy efficiency of various biogas production and utilization pathways

The energy efficiency of different biogas systems, including single and co-digestion of multiple feedstock, different biogas utilization pathways, and waste-stream management strategies was evaluated. The input data were derived from assessment of existing biogas systems, present knowledge on anaerobic digestion process management and technologies for biogas system operating conditions in Germany. The energy balance was evaluated as Primary Energy Input to Output (PEIO) ratio, to assess the process energy efficiency, hence, the potential sustainability. Results indicate that the PEIO correspond to 10.5–64.0% and 34.1–55.0% for single feedstock digestion and feedstock co-digestion, respectively. Energy balance was assessed to be negative for feedstock transportation distances in excess of 22 km and 425 km for cattle manure and for Municipal Solid Waste, respectively, which defines the operational limits for respective feedstock transportation. Energy input was highly influenced by the characteristics of feedstock used. For example, agricultural waste, in most part, did not require pre-treatment. Energy crop feedstock required the respect cultivation energy inputs, and processing of industrial waste streams included energy-demanding pre-treatment processes to meet stipulated hygiene standards. Energy balance depended on biogas yield, the utilization efficiency, and energy value of intended fossil fuel substitution. For example, obtained results suggests that, whereas the upgrading of biogas to biomethane for injection into natural gas network potentially increased the primary energy input for biogas utilization by up to 100%; the energy efficiency of the biogas system improved by up to 65% when natural gas was substituted instead of electricity. It was also found that, system energy efficiency could be further enhanced by 5.1–6.1% through recovery of residual biogas from enclosed digestate storage units. Overall, this study provides bases for more detailed assessment of environmental compatibility of energy efficiency pathways in biogas production and utilization, including management of spent digestate.     

Hydrogen production and End-Uses from combined heat,hydrogen and power system by using local resources

To address the problem of fossil fuel usage at the Missouri University of Science and Technology campus, using of alternative fuels and renewable energy sources can lower energy consumption and hydrogen use. Biogas, produced by anaerobic digestion of wastewater, organic waste, agricultural waste, industrial waste, and animal by-products is a potential source of renewable energy. In this work, we have discussed Hydrogen production and End-Uses from CHHP system for the campus using local resources. Following the resource assessment study, the team selects FuelCell Energy DFC1500™ unit as a molten carbonate fuel cell to study of combined heat, hydrogen and power (CHHP) system based on a molten carbonate fuel cell fed by biogas produced by anaerobic digestion. The CHHP system provides approximately 650 kg/day. The total hydrogen usage 123 kg/day on the university campus including personal transportation applications, backup power applications, portable power applications, and other mobility applications are 56, 16, 29, 17, and 5 respectively. The excess hydrogen could be sold to a gas retailer. In conclusion, the CHHP system will be able to reduce fossil fuel usage, greenhouse gas emissions and hydrogen generated is used to power different applications on the university campus.     

Energy and environmental balance of biogas for dual-fuel mobile applications

Considerable research is currently being devoted to seeking alternative fuels to comply with transportation needs while reducing the environmental impact of this sector. Within the transport activity sector, on road vehicles and agricultural machinery require around 2 Mtoe energy in France. The anaerobic digestion of farm waste could roughly cover these needs. This paper aims to study the environmental and energy interest of this short power supply path. An ideal biogas production system has been built up from the average characteristics of current rural biogas plants in France. Pollutant emissions, energy demands and production are assessed for various scenarios in order to produce methane for dual fuel engines. Life cycle assessment (LCA) is used to evaluate the environmental impact of dual fuel agricultural machines, compared to diesel engines. The energy balance is always in disfavour of biogas fuel, whereas LCA energy indicators indicate a benefit for biogas production. This gap is related to the way in which the input of biomass energy is handled: in conventional biofuel LCA, this energy is not taken into account. A carbon balance is then presented to discuss the impact of biogas on climate change. Dual fuel engines were found to be interesting for their small impact. We also show, however, how the biogenic carbon assumption and the choice of allocation for the avoided methane emissions of anaerobic digestion are crucial in quantifying CO₂ savings. Other environmental issues of biogas fuel were examined. Results indicate that are management and green electricity are the key points for a sustainable biogas fuel. It is concluded that biofuel environmental damage is reduced if energy needs during biofuel production are covered by the production process itself. As agricultural equipment is used during the biofuel production process, this implies that a high substitution rate should be used for this equipment.     

Techno‐economic assessment for energy generation using bagasse: case study

Bagasse is selected as the biomass source that is studied because of its annual significant rate production in Iran and potential for energy generation. Bagasse has been as an energy source for the production of energy required to run the sugar factory. The energy needed by factories was supplied by burning bagasse directly inside furnaces, which had an exceptionally low output. To this end, today, a secondary use for this waste product is in combined heat and power plants where its use as a fuel source provides both heat and power. In addition, low efficiency of traditional methods was caused to increase the use of modern methods such as anaerobic digestion, gasification and pyrolysis for the production of bio‐fuels. In this paper, the energy conversion technologies are compared and ranked for the first time in Iran. Therefore, the most fundamental innovation of this research is the choice of the best energy conversion technology for the fuel production with a higher efficiency. To assess the feasibility application and economic benefit of biogas CHP plant, a design for a typical biogas unit is programmed. The results show the acceptable payback period; therefore, economically and technically, biogas CHP plant appears to be an attractive proposition in Iran. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental comparison of MCFC performance using three different biogas types and methane

Biogas recovery is an environmentally friendly and cost-effective practice that is getting consensus in both the scientific and industrial community, as the growing number of projects demonstrate.The use of fuel cells as energy conversion systems increases the conversion efficiency, as well as the environmental benefits. Molten carbonate fuel cells (MCFC) operate at a temperature of about 650 °C, thus presenting a high fuel flexibility, compared to low temperature fuel cells.Aim of the present study is to compare the performance of an MCFC single cell, fuelled with different biogas types as well as methane. The biogases considered are derived from the following processes: (1) steam gasification in an entrained flow gasifier; (2) steam gasification in a duel interconnect fluidized bed gasifier; (3) biogas from an anaerobic digestion process.The performances are evaluated for different fuel utilization and current densities.The results are an essential starting point for a complete system design and demonstration.     

Which is the preferable biogas utilisation technology for anaerobic digestion of agricultural crops in Ireland: Biogas to CHP or biomethane as a transport fuel?

The utilisation of anaerobic digestion to produce biogas as an energy source is a mature technology in many European countries but is yet to be developed in Ireland. In 2009, the EU issued the Renewable Energy Source Directive 2009/28/EC which requires a 20% share of renewable energy sources (heat and electricity) in final energy consumption for all member states, respectively, including a 10% share of biofuels in the transport sector by 2020. The introduction of biogas to produce power and electricity in the form of CHP technology and biomethane as a transport fuel can help Ireland achieve the mandatory targets set by the directive. The key focus of the paper is to determine the optimum small to medium scale biogas technology and the impact the introduction of that technology infrastructure will have on renewable energy targets for Ireland. In terms of feedstock, agricultural sources such as energy crops and slurry offer a sustainable input to the anaerobic digestion process. The crop rotations under consideration consist of different arrangements of grass silage, maize silage and barley. Grass silage is found to be the most suitable crop for biogas energy production while biogas upgrading to biomethane as a transport fuel has the optimum technology potential in Ireland. To fuel a car operating on biomethane, 0.22 ha of grass land is required annually. Full scale national development of 5% of the area under grass in Ireland will contribute 11.4% of renewable energy to the total final transport energy demand by 2020, surpassing the target set by the Renewable Energy Source Directive 2009/28/EC.     

Economics of anaerobic digestion in organic agriculture: Between system constraints and policy regulations

During its pioneer-stage in Germany, the generation of power and heat from anaerobic digestion (AD) was predominantly developed on organic farms. However, biogas production in organic agriculture (OR) never expanded to the same extent as in conventional farming (CV). Besides various other aspects, this appears to be mainly due to economic reasons related to system-specific production requirements. Therefore, this article analyses the framework conditions of organic biogas generation and assesses its monetary implications on production economics. The structural and economic comparison of organic and conventional generation of power from biogas displays systematic constraints for AD in OR and identifies advantages of conventional biogas plants, particularly concerning lower capital and biomass input costs. Moreover, frequently changing policy regulations, further aggravating the economic situation for biogas production in both farming systems, are reflected. Our study shows that the recent developments of political frameworks will inhibit biogas investments for nearly all types of biogas plants in Germany. Finally, an alternative evaluation approach for organic AD systems, considering monetary benefits from agronomic effects of an integrated biogas generation in organic agriculture is discussed.     

生物质气化技术面临的挑战及技术选择

生物质气化可实现低品位生物质能的深层次利用,不同地区、不同行业有不同的能源需求和产业结构,应合理选择生物质气化技术。固定床气化技术针对的是中小规模应用,该技术存在的问题包括焦油含量高、规模小、机械化和自动化程度较低、发电效率低等。流化床气化技术针对的是中等及以上规模应用,目前需要解决的问题是热效率低,发电效率低,需要开发高气化效率和无焦油的燃气型气化炉、低热值燃气轮机、高效燃气净化系统,以便采用BIGCC技术。沼气技术是一项生物质综合高效清洁利用的多联产工艺,目前急需开发高效高浓度厌氧消化的沼气发酵工艺和配套的集成设备,培育和筛选高效沼气发酵微生物菌群,简化沼气净化工序,解决沼液、沼渣的利用难题等。生物质快速热解技术是一种高温处理过程,其最大的优点是产物生物油易于储存运输,不存在产品规模和消费的地域限制问题。从工艺特点、经济效益和规模化生产来看,沼气技术更适合处理高含水的养殖业粪便,快速热解技术更适合农作物秸秆的规模化转化,燃气型气化技术更适合社区生活垃圾和农林产品加工废弃物的处理。