When does decentralized production of biogas and centralized upgrading and injection into the natural gas grid make sense?

The production of biogas through anaerobic digestion is one of the technological solutions to convert biomass into a readily usable fuel. Biogas can replace natural gas, if the biogas is upgraded to green gas. To contribute to the EU-target to reduce Green House Gases emissions, the installed biogas production capacity and the amount of farm-based biomass, as a feedstock, has to be increased. A model was developed to describe a green gas production chain that consists of several digesters connected by a biogas grid to an upgrading and injection facility. The model calculates costs and energy use for 1 m³ of green gas. The number of digesters in the chain can be varied to find results for different configurations. Results are presented for a chain with decentralized production of biogas, i.e. a configuration with several digesters, and a centralized green gas production chain using a single digester. The model showed that no energy advantage per produced m³ green gas can be created using a biogas grid and decentralized digesters instead of one large-scale digester. Production costs using a centralized digester are lower, in the range of 5 €ct to 13 €ct per m³, than in a configuration of decentralized digesters. The model calculations also showed the financial benefit for an operator of a small-scale digester wishing to produce green gas in the cooperation with nearby other producers. E.g. subsidies and legislation based on environmental arguments could encourage the use of decentralized digesters in a biogas grid.     

Biogas production in low-cost household digesters at the Peruvian Andes

Low-cost tubular digesters originally developed in tropical regions have been adapted to the extreme weather conditions of the Andean Plateau (3000-4000 m.a.s.l.). The aim of this study was to characterise biogas production in household digesters located at high altitude, operating under psychrophilic conditions. To this end, two pilot digesters were monitored and field campaigns were carried out in two representative digesters of rural communities. Digesters’ useful volume ranged between 2.4 and 7.5 m³, and hydraulic residence time (HRT) between 60 and 90 days. The temperature inside the digester’s greenhouse ranged between 20 and 25 °C. Treating cow manure, a specific biogas production around 0.35 m³ kg_VS⁻¹ was obtained, with some 65% CH₄ in biogas. In order to fulfil daily requirements for cooking and lighting, biogas production should be enhanced without increasing implementation costs as not to impede the expansion of this technology at household scale. In this sense, HRT below 60 days and OLR above 1 kg_VS m⁻³ day⁻¹ should be investigated to decrease digesters’ volume (i.e. costs) and increase biogas production rate. The adaptation of conventional gas burners to biogas characteristics can also contribute in improving the efficiency of the system.     

陕西户用沼气池越冬使用技术研究

为了有效解决陕西省农村户用沼气池使用时间短的难题,结合陕西农村的实际情况,对传统沼气池的结构形式进行了改进,增加预处理池为沼气池提供热量,并对沼气池的池墙、进(出)料口进行保温,以达到增加冬季沼气池池温的目的。以榆林市为代表,通过理论计算、试验等手段,证明冬季最低室外温度条件下,沼气池产气率可以达到0.1 m3/(m3.d)以上,一口6～8 m3的沼气池就可以满足一个普通农户一天的炊事用气要求。     

Effect of particle size on biogas yield from sisal fibre waste

The degradation and biogas production potential of sisal fibre waste could be significantly increased by pre-treatment for reduction of particle size. Batch-wise anaerobic digestion of sisal fibre waste was carried out in 1-l digesters with fibre sizes ranging from 2 to 100 mm, at an ambient temperature of 33 °C. Sediment from a stabilisation pond at a sisal production plant was used as starter seed. Total fibre degradation increased from 31% to 70% for the 2 mm fibres, compared to untreated sisal fibres. Furthermore, the results confirmed that methane yield was inversely proportional to particle size. Methane yield increased by 23% when the fibres were cut to 2 mm size and was 0.22 m³ CH₄/kg volatile solids, compared to 0.18 m³ CH₄/kg volatile solids for untreated fibres. By anaerobic digestion and biogas production, the 148,000 tonne of waste sisal fibres generated annually in Tanzania could yield 22 million m³ of methane, and an additional 5 million m³ of methane if pre-treatment by size reduction to 2 mm was applied.     

The effect of waste paper on the kinetics of biogas yield from the co-digestion of cow dung and water hyacinth

The effect of waste paper on biogas yield produced by co-digesting fixed amount of cow dung and water hyacinth in five digesters A-E was studied at room temperature. Waste paper was observed to improve biogas yield in digesters B-E with digester A acting as the control. However, as the amount of waste paper increased the biogas yield was observed to decrease. Kinetic model based on first order kinetic was derived to estimate the maximum, ultimate, biogas yield and also the ultimate methane yield from these biomass mixtures. The maximum biogas yield estimated using this model for digesters B-E were 0.282, 0.262, 0.233, and 0.217 lg⁻¹ VS fed with goodness of fit (R²) of 0.995, 0.99, 0.889, and 0.925 respectively, which were obtained by fitting the experimental biogas yield (y_t) against (exp(kt)−1)/exp(kt). The ultimate biogas and methane yield at very low batch solid load were extrapolated to be 0.34 and 0.204 lg⁻¹ VS fed respectively. In essence, the addition of waste paper in the co-digestion of cow dung and water hyacinth can be a feasible means of improving biogas yield and also alternative means of recycling waste paper. Furthermore, the kinetic model developed can compliment other models used in anaerobic digestion of agricultural and solid waste.     

户用沼气池对农村家庭能源消费的影响--以江苏省涟水县为例

户用沼气池建设不但改变了农村家庭能源消费结构，还带动了农村养殖业、种植业的发展。通过对江苏省涟水县农村家庭能源消费和沼气池使用等相关问题的入户调查，定量研究了户用沼气池建设所产生的影响。证明户用沼气池的使用能提高家庭能源利用效率，减少能源消费量和化肥的使用量，有利于农作物秸秆的还田，提高农户的生活质量。     

Influence of the temperature and hydraulic retention time in bioelectrochemical anaerobic digestion of sewage sludge

At ambient temperature (25 °C), bioelectrochemical anaerobic digestion of sewage sludge was investigated with a hydraulic retention time (HRT) of 10 days and compared to that at a mesophilic condition (35 °C). The methane production and methane content in the biogas at ambient temperature were 612.8 mL/L·d and 73.3%, respectively, which were not significantly lower than that of the mesophilic condition. Additionally, the VS removal was 54.5% which was similar for both temperature conditions. However, for a HRT of 20 days, the bioelectrochemical anaerobic digestion at ambient temperature became more stable, and the VS removal improved up to 65.0%. For the HRT of 10 days, the net energy production at ambient temperature was about 168 kJ/L·d, which was similar to the mesophilic condition; however, the apparent energy efficiency at ambient temperature was 249.2% which was significantly higher than 197.7% at the mesophilic condition. The bioelectrochemical anaerobic digestion that can save the thermal energy input at ambient temperature is recommended for the treatment of organic waste including sewage sludge in moderate and cold climate regions.     

Inline NH3 removal from biogas digesters

During biogas production from various types of substrates such as animal manure, fats and proteins, bacterial growth and biogas production can be inhibited by excessive ammonia (NH₃) concentrations. If NH₃ is removed from the biogas digester without damaging the digestion process, inhibition of the methane (CH₄) producing bacteria will diminish. This study shows that it is possible to remove a significant quantity of NH₃ from the biogas digester headspace and liquid phase by a simple gas circulation method where gas bubbles free of NH₃ is forced through the upper 30 cm of the liquid phase in the biogas digester, into the headspace and out of the digester. The suggested method improves conditions for anaerobic bacteria exposed to high concentrations of NH₃ by simply removing NH₃ from the digester.In full-scale biogas production the system presented in this study can be improved by circulating headspace gas through an ammonia absorber and returning the NH₃ depleted biogas into the biogas digester. This method can also replace the need for mixing in biogas digesters.     

Effect of thermal, chemical and thermo-chemical pre-treatments to enhance methane production

The rise in oil price triggered the exploration and enhancement of various renewable energy sources. Producing biogas from organic waste is not only providing a clean sustainable indigenous fuel to the number of on-farm digesters in Europe, but also reducing the ecological and environmental deterioration. The lignocellulosic substrates are not completely biodegraded in anaerobic digesters operating at commercial scale due to their complex physical and chemical structure, which result in meager energy recovery in terms of methane yield. The focus of this study is to investigate the effect of pre-treatments: thermal, thermo-chemical and chemical pre-treatments on the biogas and methane potential of dewatered pig manure. A laboratory scale batch digester is used for these pre-treatments at different temperature range (25 °C-150 °C). Results showed that thermo-chemical pretreatment has high effect on biogas and methane potential in the temperature range (25–100 °C). Maximum enhancement is observed at 70 °C with increase of 78% biogas and 60% methane production. Thermal pretreatment also showed enhancement in the temperature range (50–10 °C), with maximum enhancement at 100 °C having 28% biogas and 25% methane increase.     

Trace element requirements of agricultural biogas digesters during biological conversion of renewable biomass to methane

The availability of trace metals as micro-nutrients plays a very significant role on the performance and stability of agricultural biogas digesters, which are operated with energy crops, animal excreta, crop residues, organic fraction of municipal solid wastes or any other type of organic waste. The unavailability of these elements in biogas digesters is probably the first reason of poor process efficiency without any other obvious reason, despite proper management and control of other operational and environmental parameters. However, trace metal requirements of biogas digesters operated with solid biomass are not often reported in literature. Therefore, the aim of this article is to review the previous and current literature about the trace metal requirements of anaerobic biogas digesters operated with solid organic substrates for production of methane.     

Production and use of biogas in agriculture

Anaerobic digestion, a process used in municipal waste water treatment since the late 19th century, was originally suggested as a method of controlling pollution from large, intensive, farms. With changes in prices of conventional energy sources and possible future shortages of these, the production of biogas for farm use has become an economic possibility, with the bonus of pollution control.The process uses naturally-occurring bacteria which form a complex, but self-stabilising, mixed flora and this flora and its reactions are briefly described. Laboratory, pilot-plant and full scale tests have shown that agricultural wastes can be digested with production of useful amounts of biogas and that this biogas can be used in boilers or engines. Full-scale plants are, however, only in the developmental stage, and while problems in building and running large-scale farm digesters can be discussed, there is little, if any, information on long-term running of digesters and ancillary equipment.Economics dictate that use should be made of all products of digestion, but, again, while the results of small-scale tests can be discussed there is little information on large-scale uses of digested sludges.Large-farm, automated digesters are still developmental and test plants; the next two years or so should see existing problems overcome, as well as more plants being installed.     

Popularizing household-scale biogas digesters for rural sustainable energy development and greenhouse gas mitigation

Biogas utilization has undergone great development in rural China since the government systematically popularized household-scale biogas digesters for meeting the rural energy needs in the 1970s. In order to comprehensively estimate the significance of biogas utilization on rural energy development and greenhouse gas emission reduction, all types of energy sources, including straw, fuelwood, coal, refined oil, electricity, LPG, natural gas, and coal gas, which were substituted by biogas, were analyzed based on the amount of consumption for the years from 1991 to 2005. It was found that biogas provided 832749.13 TJ of energy for millions of households. By the employment of biogas digesters, reduction of greenhouse gases (GHG) was estimated to be 73157.59 Gg CO₂ equivalents (CO₂-eq), and the emission by the biogas combustion was only 36372.75 Gg CO₂-eq of GHG. Energy substitution and manure management, working in combination, had reduced the GHG emission efficiently. The majority of the emission reduction was achieved by energy substitution that reduced 84243.94 Gg CO₂, 3560.01 Gg CO₂-eq of CH₄ and 260.08 Gg CO₂-eq of N₂O emission. It was also predicted that the total production of biogas would reach to 15.6 billion m³ in the year 2010 and 38.5 billion m³ in the year 2020, respectively. As a result, the GHG emission reductions are expected to reach 28991.04 and 46794.90 Gg CO₂-eq, respectively.     

Biogas production from brown grease using a pilot-scale high-rate anaerobic digester

Food wastes are typically disposed of in landfills for convenience and economic reasons. However, landfilling food wastes increases the organic content of leachate and the risk of soil contamination. A sound alternative for managing food wastes is anaerobic digestion, which reduces organic pollution and produces biogas for energy recovery. In this study, anaerobic digestion of a common food waste, brown grease, was investigated using a pilot-scale, high-rate, completely-mixed digester (5.8 m³). The digestibility, biogas production and the impact of blending of liquid waste streams from a nearby pulp and paper mill were assessed. The 343-day evaluation was divided into 5 intensive evaluation stages. The organic removal efficiency was found to be 58 ± 9% in terms of COD and 55 ± 8% in terms of VS at a hydraulic retention time (HRT) of 11.6 ± 3.8 days. The removal was comparable to those found in organic solid digesters (45–60%), but at a much shorter HRT. Methane yield was estimated to be 0.40–0.77 m³-CH_4@STP kg-VS_removed⁻¹, higher than the typical range of other food wastes (0.11–0.42 m³-CH_4@STP kg-VS_removed⁻¹), with a mean methane content of 75% and <200 ppm of hydrogen sulfide in the biogas. The blending of selected liquid wastes from a paper mill at 10 vol% of brown grease slurry did not cause significant reduction in digester performance. Using a pseudo-first-order rate law, the observed degradation constant was estimated to be 0.10–0.19 d⁻¹ compared to 0.03–0.40 d⁻¹ for other organic solids. These results demonstrate that brown grease is a readily digestible substrate that has excellent potential for energy recovery through anaerobic digestion.     

Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure

Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm³) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH₄ and H₂S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH₄ potential than dairy manure on a volatile solids basis. While no synergistic effect on CH₄ production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH₄ production. Initial H₂S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H₂S concentrations (0.10–0.14%) and higher CH₄ content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH₄ yield (372 ± 12 L kg⁻¹ VS). The co-digestion mixture containing 40% radish had a lower specific CH₄ yield (345 ± 2 L kg⁻¹ VS) but also showed significantly less H₂S production at start-up and high quality biogas (58% CH₄). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH₄ production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH₄ production of manure digesters in the fall/winter.     

Laboratory investigations on continuous bio-methanization of energy crops as mono-substrate without supplementation

Continuous bio-methanization of an energy crop, namely the beet silage, was investigated in this laboratory-scale work as mono-substrate, using a mesophilic biogas digester controlled by a fuzzy logic control (FLC) technique and without using any supplementing or buffering agent, despite the low pH of the substrate around 3.80. The temperature, pH, redox potential (ORP), daily biogas production and composition of digester biogas were continuously measured online. During the operation, the hydraulic retention time (HRT) varied between 24.8 and 9 days, as the organic loading rate (OLR) ranged from 2.6 to 4.7 g L^?1 d^?1. The average pH, specific gas production rate (spec. GPR) and volumetric gas production rate (vol. GPR) were determined to be 7.12, 0.31 L g VS^?1 d^?1 and 1.084 L L^?1 d^?1, respectively. The average methane (CH₄) content of digester biogas was about 56%. The FLC technique, which was developed at HAW Hamburg for anaerobic conversion of acidic energy crops to methane, determined the daily feeding volume (～ OLR/HRT) for the biogas digester, depending on the feedback from online pH and methane measurements, and on the calculation of the spec. GPR. The spec. GPR was calculated by the corrected daily biogas production. Through online monitoring of pH, biogas production rate and composition, and by use of the FLC technique, the acidic beet silage could continuously be converted to biogas, without using manure or any other kind of buffering or supplementing agent(s). The lab-scale anaerobic biogas digester performed stable and safe, without encountering any problems of instability, as indicated by an adequate amount of buffering capacity, a VFA content below 0.5 g L^?1 and a neutral pH range throughout the study.     

Biogas production from sewage sludge and microalgae co-digestion under mesophilic and thermophilic conditions

Isochrysis galbana and Selenastrum capricornutum, marine and freshwater microalgae species respectively, were co-digested with sewage sludge under mesophilic and thermophilic conditions. The substrates and the temperatures significantly influenced biogas production.Under mesophilic conditions, the sewage sludge digestion produced 451 ± 12 mL_Biogas/g_SV. Furthermore, all digesters were fed with I. galbana, or mixed with sludge, resulting in an average of 440 ± 25 mL_Biogas/g_SV. On the contrary, S. capricornutum produced 271 ± 6 mL_Biogas/g_SV and in the mixtures containing sludge produced intermediate values between sludge and microalgae production.Under thermophilic conditions, the sewage sludge digestion achieved yet the highest biogas yield, 566 ± 5 mL_Biogas/g_SV. During co-digestion, biogas production decreased when the microalgae content increased, and for I. galbana and for S. capricornutum it reached minimum values, 261 ± 11 and 185 ± 7 mL_Biogas/g_SV, respectively. However, no evidence of inhibition was found and the low yields were attributed to microalgae species characteristics.The methane content in biogas showed similar values, independently from the digested substrate, although this increased by approximately 5% under thermophilic condition.     

CDM促进江西省养猪业沼气利用及减排潜力研究

清洁发展机制(CDM)给江西省养猪业沼气利用带来新的契机.阐述了江西省养猪业沼气利用现状以及甲烷回收利用CDM项目发展状况,对江西省养猪业大中型沼气工程和户用沼气池的减排温室气体的潜力作了定量的分析,研究结果显示:江西省养猪业产沼气潜力约3.65亿m3/a,可减少温室气体排放108万t CO2当量.     

Simulation of steam reforming of biogas in an industrial reformer for hydrogen production

The paper aims to investigate the steam reforming of biogas in an industrial-scale reformer for hydrogen production. A non-isothermal one dimensional reactor model has been constituted by using mass, momentum and energy balances. The model equations have been solved using MATLAB software. The developed model has been validated with the available modeling studies on industrial steam reforming of methane as well as with the those on lab-scale steam reforming of biogas. It demonstrates excellent agreement with them. Effect of change in biogas compositions on the performance of industrial steam reformer has been investigated in terms of methane conversion, yields of hydrogen and carbon monoxide, product gas compositions, reactor temperature and total pressure. For this, compositions of biogas (CH₄/CO₂ = 40/60 to 80/20), S/C ratio, reformer feed temperature and heat flux have been varied. Preferable feed conditions to the reformer are total molar feed rate of 21 kmol/h, steam to methane ratio of 4.0, temperature of 973 K and pressure of 25 bar. Under these conditions, industrial reformer fed with biogas, provides methane conversion (93.08–85.65%) and hydrogen yield (1.02–2.28), that are close to thermodynamic equilibrium condition.     

The influence of using biogas digesters on family energy consumption and its economic benefit in rural areas—comparative study between Lianshui and Guichi in China

The construction of biogas digesters has improved the family energy consumption structure, promoted the development of livestock breeding and farm production in the countryside. Through the comparative study of the questionnaires filled out by individual families in Lianshui, Jiangsu Province and Guichi, Anhui Province, we figured out the different effects of biogas digesters in different areas on household energy consumption and their benefits to economy and environment.     

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.