Real Biogas Research Articles

A study of nano-micro-bubble process for CO2 absorption in water for biogas upgrading application

The work proposed a novel micro-bubble system using swirling jet flow method for improving the physical absorption of CO2 in water. The effects of operating conditions; e.g. gas flow rate (from 500 to 2000 mL/min), inlet pressure (2 and 3.5 bar) and inlet CO2 concentration (from 5 to 60 % v/v in CO2/N2 gas mixture) on the physical absorption of CO2 were studied. It was found that the CO2 absorption system reached to steady state after 30 min with the outlet CO2 concentrations of 2.9 % and 8.8 % at 40 % and 60 % of inlet CO2 concentrations, respectively. These outlet CO2 concentrations demonstrated the standard of bio-methane for replacing natural gas in vehicle use, from which <10 % CO2 is required. Moreover, the increase of gas flow rate resulted in increase of outlet CO2 concentration. The rising of inlet pressure from 2 to 3.5 bars let to reduction of the outlet CO2 concentration together with enhancing the dissolved CO2 concentration in water from 49.1 to 51.7 mg/L at gas flow rate of 2000 mL/min and 60 % of inlet CO2 concentration. The highest efficiency of CO2 removal was 98.41 % under gas flow rate at 500 mL/min with 60 % of inlet CO2 concentration. Under the optimized conditions, this system was tested with real biogas as raw material containing mainly 57.7 % of CH4 and 40.3 % of CO2 which achieved to remove the 95.1 % of CO2 removal and no significant loss of CH4. This highlight demonstrated the potential application of this technology for biogas upgrading or purification further.

Ammonia recovery from real biogas slurry in the up-scaled Donnan Dialysis-Osmotic Distillation membrane-based system: Performance and potentials

Recovering ammonia from wastewater is crucial for building a circular economy and achieving carbon neutrality. Some innovative ammonia recovery technologies have shown great potential yet lacking theoretical guidance for up-scaling from laboratory to practical applications. Herein, upon our previously-proposed Donnan dialysis-osmotic distillation (DD-OD) hybrid process for selective ammonia recovery at nearly zero energy input, two challenges as process modelling/prediction and product application were solved in this study. Under the guide of the tailor-made mass transfer model, a reasonable design of the reactor and operating parameters was fixed, whose performance in recovering ammonia from real biogas slurry and the feasibility of the recovered product as irrigation water was then investigated. The up-scaled DD-OD reactor achieved efficient and stable ammonia removal and recovery (∼75 % and ∼ 65 %, respectively) and excellent impurity retention efficiency (∼95 %) in 360 h continuous run. Impurities deposited on the cation-exchange membrane did not affect ammonium transfer and were effectively removed by water washing, and no fouling was found for the hydrophobic membrane, highlighting its long-term capability for ammonia recovery. Moreover, the cost-effectiveness of the process was analyzed, which could be greatly improved given the future improved membrane materials. A proper dilution of the recovered product for irrigating cabbage and radish promoted plant growth, and recycled product containing a moderate ammonia concentration (136 mg/L) was highlighted, with a well-rounded, multi-element nutrient profile recommended. The above results provide theoretical guidance for the scaling-up of ammonia recovery module and a potential pathway for the application of recycled products in agricultural irrigation.

H2S-laden biogas triggers sulfur amino acids production in microbial protein synthesized during mixed culture fermentation by methane and sulfur oxidizing bacteria

Biogas can be converted into high-value bioproducts through fermentation processes. This study proposes a conversion platform where sulfide-rich biogas is utilized as the substrate for the growth of a mixed culture of methane- and sulfur-oxidizing bacteria for the production of high-value and protein-rich microbial biomass. The role of several process variables such as biomethane vs biogas, synthetic vs real biogas, biogas composition and H2S concentration during the fermentation process performed in bubble column reactors was evaluated in terms of biomass growth, yield and volumetric productivity, protein content and amino acids profile as well as microbial community composition and evolution. Despite the presence of 1500 ppm of equivalent H2S in the synthetic biomethane or biogas (CH4:CO2 ratio of 70:30), good performances in terms of biomass growth (559.9 ± 13.3–677.8 ± 46.8 mg of volatile suspended solids (VSS) per liter), biomass yield (0.042 ± 0.004–0.050 ± 0.08 mg VSS per mg of CH4-equivalent chemical oxygen demand (CH4-COD)) as well as protein content (51.9 ± 3.6% − 59.2 ± 3.6% g protein/g VSS) were achieved. The presence of increasing sulfide concentrations until 4000 ppm of equivalent H2S was well tolerated by the mixed culture and increased the production of valuable S-amino acids, i.e., cysteine and methionine (61.3 mg/g biomass and 58.3 mg/g biomass, respectively), which was up to 2.50 and 2.57 times higher than in the absence of sulfide, respectively.

Comparison at laboratory scale of different types and configurations of commercial molecular sieves for one-step direct upgrading of biogas produced by anaerobic digestion

In the present study we investigated, at laboratory scale, the possibility of upgrading biogas with a single step, without involving any pre- or post-treatment or multiple stage recirculation. This would allow small-scale AD plants to achieve convenient in-situ biomethane production entirely in accordance with REPowerEU objectives. In this respect, three commercial molecular sieves, i.Honeywell 13X, 4A, and 5A, were tested in two configurations in series (4A/13X and 4A/5A) in 25/75 v/v relative amounts. A control trap containing only the 4A molecular sieve using the same amount as the in series configuration was also used. Real biogas fed directly from the production reactors at atmospheric pressure was used in the experiments. The ability of the sieves to adsorb CO2 in the proposed operational conditions was tested and proved. In particular, sample 4A was highly efficient in biogas upgrading (CO2 retention rate of 42.4 gCO2/kg, equivalent to 0.96 mmolCO2/g) but its activity was limited to 18 days, while increasing the amount of material and combining different types of molecular sieves in series improved the performance. The possibility to regenerate and reuse the sieves was also proven.

Renewable gases production coupled to synthetic wastewater treatment through a microbial electrolysis cell

This study describes the use of a microbial electrolysis cells for the production of gaseous biofuels sustained by the oxidation of a synthetic wastewater. During the overall experimental investigation, the MEC’s bioanode removed on average 855 ± 57 mgCOD/d producing an average electric current of 66 ± 7 mA which was diverted into gaseous biofuels like biomethane, biohydrogen and biohythane. Three different MEC cathodic configurations were investigated selecting the electrodic materials (graphite granules GG, and mixed metal oxide MMO) and operating conditions (pH of the catholyte, additional sorption chamber). Biomethane production increased from 26 ± 4–102 ± 8 meq/d when the MMO electrode was used with respect to GG electrodic material. In contrast, the MMO electrode in combination with a CO2 sorption chamber was successfully utilized for simultaneous H2 production and CO2 sorption from a N2/CO2 mixture which simulates an anaerobic digestion biogas. The combination of H2 production and CO2 sorption allowed to obtain a gaseous mixture composed of 9% H2, 5% CO2, and 80% N2 that according to the assumption of replacing the N2/CO2 mixture with real biogas corresponded to a commercial-grade biohythane. Overall, the results highlight the potential of MECs as an efficient approach for biogas upgrading, allowing biohythane production increasing CH4 content and lowering CO2 concentration.

Unravelling the kinetics, isotherms, thermodynamics, and mass transfer behaviours of Zeolite Socony Mobil - 5 in removing hydrogen sulphide resulting from a dark fermentative biohydrogen production process.

Research into the speciation of sulfur and hydrogen molecules produced through the complex process of thermophilic dark fermentation has been conducted. Detailed surface studies of solid-gas systems using real biogas (biohydrogen) streams have unveiled the mechanisms and specific interactions between these gases and the physicochemical properties of a zeolite as an adsorbent. These findings highlight the potential of zeolites to effectively capture and interact with these molecules. In this study, the hydrogen sulphide removal analysis was conducted using 0.8 g of the adsorbent and at various reaction temperatures (25-125 °C), a flow rate of 100 mL min-1, and an initial concentration of approximately 5000 ppm hydrogen sulphide. The reaction temperature has been observed to be an essential parameter of Zeolite Socony Mobil - 5 adsorption capacity. The optimum adsorption capacity attains a maximum value of 0.00890 mg g-1 at an optimal temperature of 25 °C. The formation of sulphur species resulting from the hydrogen sulphide adsorption on the zeolite determines the kinetics, thermodynamics, and mass transfer behaviours of Zeolite Socony Mobil - 5 in hydrogen sulphide removal and Zeolite Socony Mobil - 5 is found to improve the quality of biohydrogen produced in thermophilic environments. Biohydrogen (raw gas) yield was enhanced from 2.48 mol H2 mol-1 hexose consumed before adsorption to 2.59 mol H2 mol-1 hexose consumed after adsorption at a temperature of 25 °C. The Avrami kinetic model was fitted for hydrogen sulphide removal on Zeolite Socony Mobil - 5. The process is explained well and fitted using the Temkin isotherm model and the investigation into thermodynamics reveals that the adsorption behaviour is exothermic and non-spontaneous. Furthermore, the gas molecule's freedom of movement becomes random. The adsorption phase is restricted by intra-particle diffusion followed by film diffusion during the transfer of hydrogen sulphide into the pores of Zeolite Socony Mobil - 5 prior to adsorption on its active sites. The utilisation of Zeolite Socony Mobil - 5 for hydrogen sulphide removal offers the benefit of reducing environmental contamination and exhibiting significant applications in industrial operations.

Pilot-scale fabrication, separation performance and field test of tubular SSZ-13 membranes for efficient CO2 separation

Large-area SSZ-13 membranes were reproducibly prepared on 50-cm-long commercial and alumina tubes by secondary growth. Separation performance of the long membranes were higher than that of most small-area membranes. The effects of operation parameters such as pressure, temperature, feed flow rate and CO2 concentration on the separation performance of membranes were investigated. A mathematic model for concentration polarization index was established to evaluate the extent of concentration polarization in the single-tube and the muti-module test. A separation system with four single-tube modules connected in series was applied to field test of real biogas. The biogas could be purified to pipeline-grade natural gas (>98 mol%) by this small system at feed flow rate of 12 standard liter per minute at 1.4 MPa pressure drop. The membrane displayed long-term stability in the wet and dry biogas. This work has made an important progress toward the industrialization of zeolite membranes for gas separation.

Adsorption of Hydrogen Sulfide on Activated Carbon Materials Derived from the Solid Fibrous Digestate.

The goal of this work is to develop a sustainable value chain of carbonaceous adsorbents that can be produced from the solid fibrous digestate (SFD) of biogas plants and further applied in integrated desulfurization-upgrading (CO2/CH4 separation) processes of biogas to yield high-purity biomethane. For this purpose, physical and chemical activation of the SFD-derived BC was optimized to afford micro-mesoporous activated carbons (ACs) of high BET surface area (590-2300 m2g-1) and enhanced pore volume (0.57-1.0 cm3g-1). Gas breakthrough experiments from fixed bed columns of the obtained ACs, using real biogas mixture as feedstock, unveiled that the physical and chemical activation led to different types of ACs, which were sufficient for biogas upgrade and biogas desulfurization, respectively. Performing breakthrough experiments at three temperatures close to ambient, it was possible to define the optimum conditions for enhanced H2S/CO2 separation. It was also concluded that the H2S adsorption capacity was significantly affected by the restriction to gas diffusion. Hence, the best performance was obtained at 50 °C, and the maximum observed in the H2S adsorption capacity vs. the temperature was attributed to the counterbalance between adsorption and diffusion processes.

Mobile pilot plant for CO2 capture in biogas upgrading using 30 wt% MEA

This study utilizes a new mobile pilot plant to capture CO2 from real biogas with 30 wt% monoethanolamine (MEA). The pilot plant is described in detail and validated by data reconciliation showing that the pilot plant produces reliable steady state data. It is demonstrated that the pilot plant can upgrade 32 kg biogas per hour by capturing 20 kg CO2 per hour. Further, the pilot plant can achieve below 2.5 mol% CO2 in the methane product at a specific reboiler duty of 3.74 GJ per ton of CO2. The absorber column was found to be operated close to pinch conditions as temperatures reached above 90 °C giving a potential for process optimization and heat integration.

Unraveling prevalence of homoacetogenesis and methanogenesis pathways due to inhibitors addition

Three inhibitors targeting different microorganisms, both from Archaea and Bacteria domains, were evaluated for their effect on CO2 biomethanation: sodium ionophore III (ETH2120), carbon monoxide (CO), and sodium 2-bromoethanesulfonate (BES). This study examines how these compounds affect the anaerobic digestion microbiome in a biogas upgrading process. While archaea were observed in all experiments, methane was produced only when adding ETH2120 or CO, not when adding BES, suggesting archaea were in an inactivated state. Methane was produced mainly via methylotrophic methanogenesis from methylamines. Acetate was produced at all conditions, but a slight reduction on acetate production (along with an enhancement on CH4 production) was observed when applying 20 kPa of CO. Effects on CO2 biomethanation were difficult to observe since the inoculum used was from a real biogas upgrading reactor, being this a complex environmental sample. Nevertheless, it must be mentioned that all compounds had effects on the microbial community composition.

Implementation of a Pilot-Scale Biotrickling Filtration Process for Biogas Desulfurization under Anoxic Conditions Using Agricultural Digestate as Trickling Liquid.

A pilot-scale biotrickling filter (BTF) was operated in counter-current flow mode under anoxic conditions, using diluted agricultural digestate as inoculum and as the recirculation medium for the nutrient source. The process was tested on-site at an agricultural fermentation plant, where real biogas was used. The pilot plant was therefore exposed to real process-related fluctuations. The purpose of this research was to attest the validity of the filtration process for use at an industrial-scale by operating the pilot plant under realistic conditions. Neither the use of agricultural digestate as trickling liquid and nor a BTF of this scale have previously been reported in the literature. The pilot plant was operated for 149 days. The highest inlet load was 8.5 gS-H2Sm-3h-1 with a corresponding removal efficiency of 99.2%. The pH remained between 7.5 and 4.6 without any regulation throughout the complete experimental phase. The analysis of the microbial community showed that both anaerobic and anoxic bacteria can adapt to the fluctuating operating conditions and coexist simultaneously, thus contributing to the robustness of the process. The operation of an anoxic BTF with agricultural digestate as the trickling liquid proved to be viable for industrial-scale use.

Landfill gas upgrading using amine-functionalized silica sorbents

ABSTRACT Separation of acid gases such as CO2 from biogas is a key step for its purification before biomethane is used in various applications such as injection into a pipeline or use as a vehicle fuel. In this study, the single step purification of real biogas (locally collected landfill gas) was demonstrated using (3-Aminopropyl) triethoxysilane (APTES)-grafted SBA-15. The sorbent was optimized between amine amount and accessibility to adsorption sites. In studies using a surrogate mixture of CH4/CO2 the sorbent recorded an adsorption capacity of 0.85 mmolCO2/gads under dry conditions and this decreased to 0.72 mmolCO2/gads when the surrogate biogas was saturated with steam. Even though the capacity was further decreased to 0.55 mmolCO2/gads using real landfill gas, it remained fairly stable over 5 cycles despite the presence of impurities, namely hydrogen sulfide (68 ppm). In all cases, the capacity was regenerated with a thermal treatment in inert gas at 100°C. The results suggested possibilities for a single step purification unit to remove acid gasses and water and provide potential for a streamlined operation for the separation of carbon dioxide from biogas. The enhanced economics should propel the move toward a green energy future.

Challenges in part load operation of biogas-based power-to-gas processes (part I)

Direct methanation of biogas enables the storage of electrical energy in carbon-neutral (bio-)methane and subsequent injection into the national gas distribution grid. The intermittent availability of renewable electricity, as well as the varying nature of biogas production by e.g. anaerobic digestion, necessitate enhanced operational flexibility of such a Power-to-Gas process and intermediate buffer storage. This work provides experimental data of dynamic part load operation of a TRL 5 methanation plant with gas upgrading to grid-ready biomethane. Full recycling of unreacted H2 could be achieved by a commercial biogas upgrading membrane (Evonik SEPURAN® Green). Using real biogas, a sequence of load levels down to 45% of the full load capacity were tested. Stable operation of the plant could be demonstrated and grid injection limitations could be fulfilled after equilibration of the system. Additionally, an idealised PtG process chain was simulated with the focus on a sensitivity analysis of the H2 storage capacity on the methanation operation hours. It was shown that increasing the hydrogen storage capacity decreases the number of start-up and shut down procedures of the methanation plant. It could be shown that by using an equally sized electrolyser and methanation unit, allowing part load operation of the methanation only during the emptying of the H2 tank leads to longer activity phases of the methanation unit, while allowing part load operation of the methanation reactor during filling of the H2 storage tank had a negative effect.

Evaluation and analysis of biogas potential from agricultural waste in Hubei Province, China

CONTEXTBiogas, a clean and environmentally sustainable energy source, reduces agricultural waste pollution and contributes to solving the problem of rural energy shortages. Therefore, biogas development and use have increased in numerous nations, including China. OBJECTIVEThis study estimates the biogas potential of agricultural waste in Hubei Province, China. It also identifies the gap between the available biogas potential and actual total biogas production to find the cause of the low utilization rate. METHODSThe agricultural waste, including its biogas potential in Hubei Province, was estimated and analyzed using crop yields and livestock production through geographical approach. Based on that, the environmental and economic benefits of carbon emission reduction were also discussed. Then, a comparison was made between the estimated biogas potential and the actual biogas production via biogas production difference rate that could visualize the gap between them. RESULTS AND CONCLUSIONSThe results revealed that the estimated biogas potential of agricultural waste could reach 4.67 × 109 m3. This high biogas potential could lead to a carbon emission reduction effect of 2.33 × 106 tCO2. According to Clean Development Mechanism (CDM), these carbon emission savings could have an economic benefit of 2.33 × 107 USD. In addition, results also showed a significant difference between the estimated potential and actual biogas production, and the average production difference rate was 96.83%. The discrepancy between Hubei regions was also evident; areas with the lowest biogas production density per unit area accounted for only 0.5% of the highest areas. SIGNIFICANCETo sum up, this study provides a visual tool to compare theoretical biogas potential to real biogas output and offers referable optimization strategies.

Fouling-free membrane stripping for ammonia recovery from real biogas slurry

Hydrophobic gas permeable membranes (GPMs) exhibit great potential in stripping or recovering ammonia from wastewater, but they also suffer from severe fouling issues due to the complex water matrix, since the related process is often operated under highly alkaline conditions (pH > 11). In this study, we proposed a novel membrane stripping process by integrating a cation exchange membrane (CEM) in alkali-driven Donnan dialysis prior to GPM for efficient and robust ammonia recovery from real biogas slurry. During the conventional stripping for diluted biogas slurry, the ammonia removal across GPM finally decreased by 15% over 6 consecutive batches, likely due to the obvious deposition of inorganic species and penetration of organic compounds (rejection of 90% only). In contrast, a constant ammonia removal of 80% and organic matter rejection of more than 99%, as well as negligible fouling of both membranes, were found for the proposed novel stripping process operated over 120 h. Our results demonstrated that additional divalent cations clearly aggravated the fouling of GPM in conventional stripping, where only weak competition across CEM was found in the CEM-GPM hybrid mode. Then, for raw biogas slurry, the new stripping achieved a stable ammonia removal up to 65%, and no fouling occurrence was found, superior to that in the control (declined removal from 87% to 55%). The antifouling mechanism by integrating CEM prior to GPM involves size exclusion and charge repulsion towards varying foulants. This work highlighted that the novel membrane stripping process of hybrid CEM-GPM significantly mitigated membrane fouling and can be regarded as a potential alternative for ammonia recovery from high-strength complex streams.

Purification and upgrading biogas from anaerobic digestion using chemical asborption of CO2 with amines in order to produce biomethane as biofuel for vehicles: a pilot-scale study

ABSTRACT Methane from biogas is a promising option for the transition to a circular economic system based on renewable and sustainable energy. Currently, there is a wide variety of technologies capable of efficiently extracting the impurities contained in biogas in order to produce biomethane. In the present study, work was carried out in a pilot plant for cleaning and upgrading real biogas from anaerobic digestion by means of the chemical absorption method. In contrast to other technologies, chemical absorption stands out for having the lowest CH4 losses, which was crucial for this study. The objective was to purify and upgrade the biogas from anaerobic digestion of a WWTP to achieve a high concentration of methane and use it as biofuel in vehicles. The two aqueous solutions used as absorbents were mono-ethanolamine (MEA) and N-methyldiethanolamine (MDEA), both at 20% and 40% concentrations. It was found that the most effective solution was the MEA at 20% concentration, since it has the highest CO2 removal efficiency (94%) and, therefore, CH4 richness (96%). Finally, for every 100 m3 of biomethane generated after the chemical absorption process, 62 Nm3 of biofuel were obtained.

Fouling behavior of anion-exchange membrane during electrodialysis of biogas slurry: Correlation to applied current

Electrodialysis draws recent attention toward valorization of biogas slurry, especially in terms of inorganic nutrient selectivity. Nevertheless, wide application of this process is impeded by organic fouling of anion-exchange membrane (AEM) dealing with such a complex stream, which lacks of a systematic investigation so far. Herein, AEM fouling by real biogas slurry under varying current density was delicately evaluated in terms of transmembrane electrical potential and membrane bulk resistance, beyond the conventional monitoring on dilute conductivity and solution pH for this desalination process. Our results showed that TMEP of the target AEM was very sensitive to the applied current, but a negligible variation of desalination rate and solution pH indicated a limited occurrence of water splitting. Then, the existence of apolar and large sized foulant in biogas slurry (probably the detected humic-like substances) and few salt crystals from the membranes together suggested that organic fouling was mainly responsible for the fouling issue. Finally, EIS data revealed Rm as the main component of membrane resistance for the fouled membrane, indicating a non-negligible internal fouling, which was also positively related to the applied current. Our result implied operating electrodialysis at a lower current density for biogas slurry is recommended from the perspective of membrane fouling mitigation and operation cost optimization in pursuit of long-term efficacy for nutrient recovery and valorization.

Efficient Methane Production from Anaerobic Digestion of Cow Dung: An Optimization Approach

In the context of addressing the global challenge of facilitating a decision-making process based on methane production using a predictive model, the study seeks to evaluate the performance of a biogas digester in varying operating conditions for optimization purposes. One of the techniques for doing this is the application of constrained linear least-square optimization. This has been employed to optimize the input parameter with the corresponding measured desired response. The developed model was built from 430 measured data set points of all the predictors over an 18-day monitoring period with an interval of 30 min. The result showed that the difference between the optimized model and the general model output for methane production in the biogas digester was less than 4%. Hence, the performance of the model demonstrated a strong validity as the determination coefficient (R2) between the modeled, and optimized output was 0.968 for the volume of methane produced in the biogas digester. The obtained determination coefficient of the developed and optimized model suggests that the modeled value of the methane fits well with the measured value of methane for validation. Thus, from the test dataset, the optimized and modeled methane volume was reported as 28%. In this scenario, under the various operational parameters, an increase of 26.5% in methane was obtained when comparing the maximum volume of methane from the optimization process with the maximum methane volume (54.5%) produced in the real biogas digester. Interestingly, the biogas digester produced a maximum methane yield of 0.24 m3 and a methane composition of 60%. Evidently, methane yield was influenced by temperature as well as other meteorological factors in the developed model hence, these factors should be widely considered for sustainable biogas production.

Anoxic desulphurisation of biogas from full-scale anaerobic digesters in suspended biomass bioreactors valorising previously nitrified digestate centrate

Nitrification of centrate from anaerobic sewage sludge digestion presents a major opportunity as an electron acceptor in anoxic biogas biodesulphurisation. Nitritation and nitrification inhibition by free ammonia was detected at laboratory scale, but was avoided during the scale-up operation in a 4 m3 reactor treating ammonium loads up to 19 gN m−3 h−1. This nitrate-rich stream was fed to two pilot-scale suspended biomass bioreactors (SBBs) treating real biogas for 220 days. After an adaptation period of 21 days, nitrate and alkalinity concentrations in the liquid medium below 10 mgN L−1 and 100 mgCaCO3 L−1 were found to limit hydrogen sulphide (H2S) oxidation. Once controlled, 95% of the H2S was removed in SBB1 and 90% in SBB2, at a gas residence time (GRT) of 5.9 min, treating average values of 321 ± 205 ppmv and 457 ± 205 ppmv, respectively. Outlet H2S concentrations of 16 ± 24 ppmv in SBB1 and 46 ± 39 ppmv in SBB2 were obtained, which are below the requirements of biogas combustion heat and power engines. Unlike H2S, siloxanes were not removed with these GRTs. The results demonstrate the feasibility of the combined process for H2S treatment, potential valorisation of precipitated elemental sulphur and a reduction in the reagents currently used to control H2S.

An experimental study of NO reduction by real biogas reburning at high temperature under oxy-fuel complex atmosphere

In order to systematically obtain the denitrification capacity of biogas in advanced oxy-fuel reburning system, the NO reduction by real biogas was studied in pyrolysis-reduction two-stage fixed bed reactor. The mechanism of NO reduction by real biogas at CO2 atmosphere, such as reduction temperature (900 ∼ 1500 °C), complex atmosphere (O2/CO/H2O/NH3) and pyrolysis temperature (600 °C, 750 °C, 900 °C) was revealed. The experimental results show that the NO reduction rate of different kinds of biogas increases first and then decreases with the reduction temperature increasing. The free radical pool in CO2 atmosphere is mainly composed of OH radicals. The effect of O2 on NO reduction shows an opposite trend in different temperature ranges. CO can promote the NO reduction at high temperature. Water vapor can promote the formation of OH radical and inhibit the NO reduction. C4H8 in biogas plays a decisive role in NO reduction. In the absence of oxygen, the combination of biogas and NH3 has a high reduction rate (>50 %) at 1100 ∼ 1400 °C, and the optimal reduction temperature shifts to the low temperature in the presence of oxygen. The combination of biogas and NH3 ensures a high reduction rate (>50 %) at 900 ∼ 1300 °C under complex atmosphere. Finally, the reaction paths of NO reduction by biogas at CO2 atmosphere are summarized.