Pretreatment of lignocellulosic biomass for enhanced biogas production

Lignocellulosic biomass is an abundant organic material that can be used for sustainable production of bioenergy and biofuels such as biogas (about 50–75% CH₄ and 25–50% CO₂). Out of all bioconversion technologies for biofuel and bioenergy production, anaerobic digestion (AD) is a most cost-effective bioconversion technology that has been implemented worldwide for commercial production of electricity, heat, and compressed natural gas (CNG) from organic materials. However, the utilization of lignocellulosic biomass for biogas production via anaerobic digestion has not been widely adopted because the complicated structure of the plant cell wall makes it resistant to microbial attack. Pretreatment of recalcitrant lignocellulosic biomass is essential to achieve high biogas yield in the AD process. A number of different pretreatment techniques involving physical, chemical, and biological approaches have been investigated over the past few decades, but there is no report that systematically compares the performance of these pretreatment methods for application on lignocellulosic biomass for biogas production. This paper reviews the methods that have been studied for pretreatment of lignocellulosic biomass for conversion to biogas. It describes the AD process, structural and compositional properties of lignocellulosic biomass, and various pretreatment techniques, including the pretreatment process, parameters, performance, and advantages vs. drawbacks. This paper concludes with the current status and future research perspectives of pretreatment.     

Bio-methanization of energy crops through mono-digestion for continuous production of renewable biogas

The aim of this laboratory-scale study was to investigate the long-term anaerobic fermentation of an extremely sour substrate, an energy crop, for continuous production of methane (CH₄) as a source of renewable energy. The sugar beet silage was used as the mono-substrate, which had a low pH of around 3.3–3.4, without the addition of manure. The mesophilic biogas digester was operated in a hydraulic retention time (HRT) range between 15 and 9.5 days, and an organic loading rate (OLR) range of between 6.33 and 10 g VS l⁻¹ d⁻¹. The highest specific gas production rate (spec. GPR) and CH₄ content were 0.67 l g VS⁻¹ d⁻¹ and 74%, respectively, obtained at an HRT of 9.5 days and OLR of 6.35 g VS l⁻¹ d⁻¹. The digester worked within the neutral pH range as well. Since this substrate lacked the availability of macro and micro nutrients, and the buffering capacity as well, external supplementation was definitely required to provide a stable and efficient operation, as provided using NH₄Cl and KHCO₃ in this case. The findings of this ongoing long-term fermentation of an extremely acidic biomass substrate without manure addition have reflected crucial information about how to appropriately maintain the operational and particularly the environmental parameters in an agricultural biogas plant.     

Enhancement of paddy straw digestibility and biogas production by sodium hydroxide-microwave pretreatment

Biogas has become a promising energy substitute to fossil fuels. Lignocellulosics, being cheap and renewable resource, could be very well used as a feedstock for biogas generation. Paddy straw is an abundant lignocellulose which is rich in organic matter like cellulose, hemi-cellulose and lignin. It is generally disposed off by burning causing environmental pollution whilst it can be used for biofuel generation. The digestibility of paddy straw is low due to high lignin and silica content. The present study was carried out to enhance paddy straw digestibility and biogas production through sodium hydroxide (NaOH) pretreatment. The paddy straw was pretreated with NaOH by soaking (24 h) in different concentrations of NaOH (2, 4, 6, 8 and 10%) and supplementing with microwave irradiations (30 min, 720 W, 180 °C). 4% NaOH-30 min microwave was found to be the best pretreatment which resulted in 65.0% decrease in lignin content and 88.7% reduction in silica content. This increased digestibility due to reduced lignin and silica content resulted in 54.7% increase in biogas production. Scanning Electron Microscopy of pretreated paddy straw revealed breakdown of lignocellulose structure resulted from the tearing of different layers of cell wall of paddy straw.     

Challenges in biogas production from anaerobic membrane bioreactors

Spectacular applications of anaerobic membrane bioreactors (AnMBRs) are emerging due to the membrane enhanced biogas production in the form of renewable bioresources. They produce similar energy derived from the world's depleting natural fossil energy sources while minimizing greenhouse gas (GHG) emissions. During the last decade, many types of AnMBRs have been developed and applied so as to make biogas technology practical and economically viable. Referring to both conventional and advanced configurations, this review presents a comprehensive summary of AnMBRs for biogas production in recent years. The potential of biogas production from AnMBRs cannot be fully exploited, since certain constraints still remain and these cause low methane yield. This paper addresses a detailed assessment on the potential challenges that AnMBRs are encountering, with a major focus on many inhibitory substances and operational dilemmas. The aim is to provide a solid platform for advances in novel AnMBRs applications for optimized biogas production.     

How to report biogas production when monitoring small-scale digesters in field

The aim of this research was to evaluate the error originated when biogas production from field monitoring digesters, influenced by the diurnal temperature cycle, was normalized to standard conditions for pressure and temperature (273.15 K and 100 kPa) from local conditions. The biogas production data is often reported without indicating if done under local conditions, whether these conditions have been standardized and, if they have actually been standardized, the standard temperature and pressure is not indicated. In this research ambient and biogas temperature, as well as biogas production were monitored with a 30 min frequency during three consecutive days, in three different tubular digesters. Normalization was realized using the high frequency data collected as reference values, and also using daily biogas production with mean daily biogas, ambient and nearby meteorological station temperatures. The outcome of this research shows that normalization of biogas production can be obtained using daily biogas production and the daily mean ambient temperature with an overestimation by no more than 1.5%, in comparison to the normalization achieved by using high frequency data from biogas temperature and production. Using mean daily ambient temperature or mean daily biogas temperature results in the same overestimation, while using mean daily ambient temperature from a nearby airport weather station pushes the overestimation up to 2.7%. So, if ambient temperature and altitude is identified, biogas production reported in local conditions can be normalized.     

Optimization of the production and quality of biogas in the anaerobic digestion of different types of biomass in a batch laboratory biodigester and pilot plant: Numerical modeling,kinetic study and hydrogen potential

The study aims to evaluate the biogas production and quality from four biomasses (microalgae (MB), sorghum (S), corn stubble (CS), rapeseed oil (RO)) in a digestion process carried out in two batch reactor (6 L) and pilot plant (1.5 m³) agitated mechanically.The substrates were characterized and anaerobic digestion was carried out as batch tests in mesophilic conditions for 30–35 days. Inoculum/substrate ratio was 1:1–2:1. Gas composition and total gas volume produced were monitored. Methane yields of 306, 345, 419, and 740 NL kg VS^?1 were obtained for MB, CS, S, and RO, respectively, in laboratory tests, while in pilot plant tests were 182, 151, 397 and 655 NL kg VS^?1. CH₄ percentage in biogas was 49–60%. The yield of H₂ generated for the four biomasses in the two types of biodigesters has been estimated, obtaining values between 16 and 39 mL g VS^?1.First-order, Modified Gompertz, and Cone models have been applied to evaluate the kinetic parameters on the methane produced in the batch and pilot plant tests, obtaining an excellent fit. ADM1 model with 19 biological processes (disintegration of biomass composite, enzymatic hydrolysis, and digestion of soluble materials mediated by organisms), acid-base equilibria, kinetic study, and liquid-gas transference has been used to fit the cumulative methane volume.     

Enhancement of biogas production from sunnhemp using alkaline pretreatment

This study investigates enhancing the biogas production of sunnhemp by pretreatment, before the anaerobic digestion and co-digestion processes, to address the complex and recalcitrant structure of the plant. Fresh sunnhemp harvested at a cutting interval of 50 days is used in the study. Five systems (each with a 5 litre useable volume) are operated semi-continuously with five different ratios of the feedstock by feeding separate feedstocks every five days with a hydraulic retention time (HRT) of 40 days. The system operates at room temperature (30 °C). The study uses sunnhemp as 20% of the feedstock and also considers sunnhemp mixed with cow manure at different ratios, with the weighed sunnhemp being pretreated with dilute sodium hydroxide. Pretreatment of sunnhemp before digestion produces a methane (CH₄) yield 89% greater than that of the untreated sunnhemp. It requires 3.597 kg of dry sunnhemp to produce 1 m³ of CH₄ and the annual CH₄ yield per hectare is 19,015 m³. In the pretreatment of sunnhemp before co-digestion, the increased CH₄ yield depends on the amount of pretreated sunnhemp in the feedstocks. However, the %CH₄, the CH₄ production level and the system stability depend on the optimal ratio of the sunnhemp to cow manure. The initially prepared sunnhemp to cow manure ratio is recommended at 10 g:10 g in 80 mL of water. At this ratio, the %CH₄ and the CH₄ yield are 53.84% and 313 kg chemical oxygen demand (COD) removed, respectively, and the COD removal efficiency is 56.4%. Sunnhemp has high potential and it is worth pretreating before producing biogas. Using sunnhemp to produce biogas is recommended to decrease greenhouse gas emissions and mitigate global warming.     

Validation of a simple gravimetric method for measuring biogas production in laboratory experiments

This work presents a gravimetric method for measuring biogas or methane production from anaerobic reactors, based on measurement of reactor mass loss. Results are most sensitive to error in biogas methane content, and less so to temperature and pressure. To evaluate the method, we applied it and volumetric methods to 133 laboratory-scale batch and semi-continuous reactors, ranging in size from 37 g to 8.0 kg of reacting mass. For most observations, the relative difference between the two methods was <10% when the “true” biogas composition was used in calculations. Small systematic differences observed in some cases were probably due to error in estimates of biogas pressure, temperature, and composition, as well as biogas leakage. Based on theory and observation, it is reasonable to expect relative accuracy better than 15% of the true value.     

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.     

Modeling the performance of the anaerobic phased solids digester system for biogas energy production

A process model was developed to predict the mass and energy balance for a full-scale (115 t d⁻¹) high-solids anaerobic digester using research data from lab and pilot scale (1-3000 kg d⁻¹ wet waste) systems. Costs and revenues were estimated in consultation with industry partners and the 20-year project cash flow, net present worth (NPW), simple payback, internal rate of return, and revenue requirements were calculated. The NPW was used to compare scenarios in order to determine the financial viability of using a generator for heat and electricity or a pressure swing adsorption unit for converting biogas to compressed natural gas (CNG).The full-scale digester consisted of five 786 m³ reactors (one biogasification reactor and four hydrolysis reactors) treating a 50:50 mix (volatile solids basis) of food and green waste, of which 17% became biogas, 32% residual solids, and 51% wastewater. The NPW of the projects were similar whether producing electricity or CNG, as long as the parasitic energy demand was satisfied with the biogas produced. When producing electricity only, the power output was 1.2 MW, 7% of which was consumed parasitically. When producing CNG, the system produced 2 hm³ y⁻¹ natural gas after converting 22% of the biogas to heat and electricity which supplied the parasitic energy demand. The digester system was financially viable whether producing electricity or CNG for discount rates of up to 13% y⁻¹ without considering debt (all capital was considered equity), heat sales, feed-in tariffs or tax credits.     

Grass as a high potential by-product: Buffalo grass to biogas and the increase of system performance and stability

Buffalo grass and alkaline-pretreated buffalo grass samples were co-digested with cow manure separately to generate biogas in anaerobic reactors. The study considered a solid content of 20% (10% buffalo grass and 10% cow manure). The methane (CH₄) content and CH₄ yield of the distinct experiments were compared. For the untreated buffalo grass, the weighed buffalo grass was mixed with cow manure and water. For the alkaline-pretreated buffalo grass, the weighed buffalo grass was soaked in 1% sodium hydroxide for 1 day prior to being mixed with cow manure and water. The untreated and pretreated buffalo grass-manure were fed semi-continuously at the rate of 125 mL/day for five days feeding in a 5 L reactor, with 40 days hydraulic retention time. The experiments were conducted for approximately 100 days. Results were reported when the systems were in steady-state conditions. The chemical oxygen demand (COD) conversion efficiency of co-digestion of the untreated and pretreated buffalo grass-manure were 46.21 and 62.76%, respectively, and for the total volatile solids (TVS) were 68.50 and 71.80%, respectively. The CH₄ contents generated from co-digestion of the untreated and pretreated buffalo grass-manure were 48.32% and 50.36%, respectively. The CH₄ yields generated from co-digestion of the untreated and pretreated buffalo grass-manure were 328 and 385 L/kgTVS conversion, respectively. It was observed from the experiments that pretreatment of the buffalo grass prior to co-digestion provided system stability during biogas production.     

Anaerobic digestion of molasses by means of a vibrating and non-vibrating submerged anaerobic membrane bioreactor

Bio-refineries produce large volumes of waste streams with high organic content, which are potentially interesting for further processing. Anaerobic digestion (AD) can be a key technology for treatment of these sidestreams, such as molasses. However, the high concentration of salts in molasses can cause inhibition of methanogenesis. In this research, concentrated and diluted molasses were subjected to biomethanation in two types of submerged anaerobic membrane bioreactors (AnMBRs): one with biogas recirculation and one with a vibrating membrane. Both reactors were compared in terms of methane production and membrane fouling. Biogas recirculation seemed to be a good way to avoid membrane fouling, while the trans membrane pressures in the vibrating MBR increased over time, due to cake layer formation and the absence of a mixing system. Stable methane production, up to 2.05 L L⁻¹ d⁻¹ and a concomitant COD removal of 94.4%, was obtained only when diluted molasses were used, since concentrated molasses caused a decrease in methane production and an increase in volatile fatty acids (VFA), indicating an inhibiting effect of concentrated molasses on AD. Real-time PCR results revealed a clear dominance of Methanosaetaceae over Methanosarcinaceae as the main acetoclastic methanogens in both AnMBRs.     

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.     

Enhanced biogas production from anaerobic co-digestion of municipal wastewater treatment sludge and fat,oil and grease (FOG) by a modified two-stage thermophilic digester system with selected thermo-chemical pre-treatment

Anaerobic co-digestions with fat, oil and grease (FOG) were investigated in two-stage thermophilic (55 °C) semi-continuous flow co-digestion systems. One two-stage co-digestion system (System I) was modified to incorporate a thermo-chemical pre-treatment of pH = 10 at 55 °C, which was the best pre-treatment condition for FOG co-digestion identified during laboratory-scale biochemical methane potential (BMP) testing. The other two-stage co-digestion system (System II) was operated without a pre-treatment process. The anaerobic digester of each digestion system had a hydraulic retention time (HRT) of 24 days. An organic loading rate (OLR) of 1.83 ± 0.09 g TVS/L·d was applied to each digestion system. It was found that System I effectively enhanced biogas production as the thermo-chemical pre-treatment improved the substrate hydrolysis including increased COD solubilization and VFA concentrations. Overall, the modified System I yielded a 25.14 ± 2.14 L/d biogas production rate, which was substantially higher than the 18.73 ± 1.11 L/d obtained in the System II.     

牛粪厌氧消化过程中温度对产气特性影响的研究

本文采用新鲜的奶牛粪便为原料,利用实验室自制的小型厌氧发酵装置进行不同温度条件下牛粪厌氧消化的实验研究。结果表明,温度为55±2℃的试验组可增加沼气总产气量,同时有助于提高气体品质。     

Effects on the biogas and methane production of cattle manure treated with urease inhibitor

Urease inhibitors are in general known as potential measure for reducing ammonia emissions in dairy and cattle housing systems. Due to the application of the urease inhibitor on the exercise areas within a housing system the inhibitor is “mixed” with cattle manure and this “mixture” remains unchanged during manure storage. In Germany, a large part of the total stored cattle manure is utilized as a substrate in biogas plants. Therefore, the aim of the current study was to test if different concentrations of urease inhibitor mixed with typical cattle slurry will have any (negative) effects on the biogas and methane yield. The Hohenheim Biogas Yield Test (HBT) was used to determine if the biogas and methane production of cattle manure is influenced by the admixture with urease inhibitor. Altogether, four urease inhibitor concentrations (0%, 0.1%, 1% und 10% of total Kjeldahl nitrogen) were tested in the HBT experiments with two different substrates, cattle manure and cellulose, as a reference, in repetitions each. The average biogas and methane production of cellulose was 740 L_N/kg_ODM and 403 L_N/kg _ODM and of cattle manure 471 L_N/kg_ODM and 295 L_N/kg_ODM. Both substrates treated with urease inhibitor showed no significant change in the biogas and methane production compared to the untreated ones. The use of urease inhibitors to reduce ammonia is harmless from the view of biogas plants.     

Electrical energy production and operational strategies from a farm-scale anaerobic batch reactor loaded with rice straw and piggery wastewater

A farm-scale biogas plant loaded with untreated rice straw and co-digested with raw pig wastewater was operated and monitored during a complete digestion cycle. One active anaerobic digester cell (6600 m³) containing 727 tons of rice straw, 285 tons of pig wastewater and approximately 1300 tons of water was operated for a total of 422 days. Cumulative energy production of 295 MWh and an estimated specific methane yield of 181 LCH₄/kgVS added was achieved. A direct correlation between daily power production and digester temperature was observed, with a maximum power production of 2.74 MWh/d. Mesophilic conditions were reached inside the digester during the summer months by recovering waste heat from the engine and recycling it through the leachate recirculation process.A slow start-up period of approximately 200 days was observed, but increased leachate recirculation rates (from 0.04 to >0.14 m³/m³straw-d) resulted in increased gas production that initiated the microbial growth phase in the digestion cycle. Although sufficient buffering capacity as well as macro- and micronutrients were supplied to the system by the pig wastewater, an overall straw (dry wt.) to wastewater ratio (wet wt.) of 1 to 1.4 is recommended to improve gas production and decrease the acclimation period. A raw economic assessment of the system shows an investment recovery time of 8.3 years. Improvements such as continuous leachate recirculation, a more efficient heat exchange system to maintain mesophilic conditions year round, and periodic addition of fresh wastewater and sludge acclimated to lignocellulosic material are recommended to achieve a more sustainable and profitable system.     

Comprehensive monitoring of a biogas process during pulse loads with ammonia

A thermophilic pilot scale anaerobic digester treating chicken litter was subjected to pulse loads of ammonia of increasing concentration. A micro gas chromatograph (μ-GC) measured CO₂, CH₄, N₂, H₂ and H₂S in the biogas. In the liquid, NH₄-N was measured manually, volatile fatty acids (VFA) were measured manually and with Near Infrared Spectroscopy (NIRS). Within the first 7–24 h after the pulse, the concentrations of iso-butyric and iso-valeric acid increased rapidly and were the best indicators of process stress due to ammonia pulses, confirmed by multivariate analysis. NIRS was not capable of accurate prediction of VFA iso-forms most likely due to low concentrations. Propionic acid was persistent. The ammonia pulses caused an overall decrease of biogas production. The biogas composition was not a good indicator of imbalances; little correlation with VFA measurements was observed.     

Comparative performance of a UASB reactor and an anaerobic packed-bed reactor when treating potato waste leachate

The results presented in this paper are from studies on a laboratory-scale upflow anaerobic sludge blanket (UASB) reactor and an anaerobic packed-bed (APB) reactor treating potato leachate at increasing organic loading rates from 1.5 to 7.0 g COD/1/day. The hydraulic retention times ranged from 13.2 to 2.8 days for both reactors during the 100 days of the experiment. The maximum organic loading rates possible in the laboratory-scale UASB and APB reactors for stable operation were approximately 6.1 and 4.7 g COD/l day, respectively. The COD removal efficiencies of both reactors were greater than 90% based on the total COD of the effluent. The methane yield increased with increasing organic loading rate up to 0.23 l CH₄/g COD_degraded in the UASB reactor and 0.161 CH₄/g COD_degraded in the APB reactor. The UASB could be run at a higher organic loading rate than the APB reactor and achieved a higher methane yield. Signs of reactor instability were decreasing partial alkalinity and pH and increasing amounts of volatile fatty acids. The study demonstrated the suitability of the UASB and a packed-bed reactor for treating leachate from potato waste.