Influence of Leachate Recirculation on Landfill Degradation and Biogas Production

Population growth combined with the rising standard of living of people around the world is the reason for the ever-increasing production of waste which management is costing states a lot of money for its disposal. Among available waste treatment techniques, landfill is one of the most promoted waste management techniques with the emergence of the bioreactor concept. However, the control of biodegradation parameters in order to accelerate waste stabilization is an important issue. For environmental and economic reasons, the technique of leachate recirculation by injection into the waste is increasingly used to improve the degradation of landfilled waste. The injection of leachate is possible using vertical boreholes, horizontal pipes, infiltration ponds or a combination of these. Indeed, moisture is the main factor in waste degradation and biogas production. The migration of leachate to the bottom of the landfill creates low moisture in the upper areas of the landfill reducing the growth of microbial populations. This results in low or no biogas production. The main objective of the present work is to develop a numerical model of leachate recirculation by injection into the waste to rewet the waste and restart biological activity. The analysis of the results shows that the diffusion of the wet front increases with time and depth. The lateral widening of the wet front is slow in relation to the progression of the wet front towards the bottom of the waste cell. This indicates the predominance of gravity effects over diffusion phenomena. The results reveal that the distributed re-injection is the best mode of leachate recirculation because the moisture distribution on the whole waste mass is totally satisfactory and the biogas generation is more important. Leachate recirculation campaigns should be done periodically to rewet the waste, boost microbial activity and hope for a quicker stabilization of the landfill.

Performance of the Full-scale Loop Hybrid Reactor Treating Coal Gasification Wastewater under Different Recirculation Modes

This paper aims to investigate the simultaneous removal efficiencies of both COD and nitrogen in a single reactor treating coal gasification wastewater( CGW). A novel loop hybrid reactor was developed and operated under different recirculation modes in order to achieve simultaneous removal of refractory compounds and total nitrogen( TN) in a full-scale CGW treatment plant. Mid-ditch recirculation was superior to other operational modes in terms of the NH3-N and TN removal,resulting in a TN removal efficiency of 52. 3%.Although the system achieved equal COD removal rates under different recirculation modes,hydrophobic acid( HPO-A) fraction of effluent dissolved organic matter( DOMef) in mid-ditch recirculation mode accounted for35.7%,compared to the proportions of 59. 2%,45. 3% and 39. 4% for the other modes. The ultraviolet absorbance to dissolved organic carbon ratio test revealed that effluent under mid-ditch recirculation mode contained more non-aromatic hydrophilic components. Furthermore,appropriate recirculation and anoxic / oxic( A/O) partitions were also demonstrated to remove some refractory metabolites( phenols,alkanes,aniline,etc.),which reduced the chromaticity and improved the biodegradability.

Effects of Flue Gas Internal Recirculation on NOx and SOx Emissions in a Co-Firing Boiler

Volumetric combustion has been developed to realize a high substitution ratio of biomass in co-firing boilers, which features an intensive flue gas internal recirculation inside furnace. However, the characteristics of NOx and SOx emissions in large-scale boilers with volumetric combustion were not fully clear. In this paper, an Aspen Plus model of volumetric combustion system was built up based on a co-firing boiler. In order to characterize the reductions of NOx and SOx, three biomass substitution ratios were involved, namely, 100% biomass, 45% biomass with 55% coal, and 100% coal. The effects of flue gas recirculation ratio, air preheating temperature, oxygen concentration, and fuel types on pollutants emission in the volumetric combustion system were investigated. According to the results, it was concluded the higher substitution ratio of biomass in a co-firing boiler, the lower emissions of NOx and SOx. Moreover, flue gas internal recirculation is an effective pathway for NOx reduction and an increased recirculation ratio resulted in a significant decreasing of NOx emission;however, the SOx increased slightly. The influences of air preheating temperature and O2 concentration on NOx emission were getting weak with increasing of recirculation ratio. When 10% or even higher of flue gas was recycled, it was observed that almost no NOx formed thermodynamically under all studied conditions. Finally, to reach a low emission level of NOx, less energy would be consumed during biomass combustion than coal combustion process for internal recirculation of flue gas.

Process Simulation of a 620 Mw-Natural Gas Combined Cycle Power Plant with Optimum Flue Gas Recirculation

The main objective of this investigation is to obtain an optimum value for the flue gas recirculation ratio in a 620 MW-Natural Gas Combined Cycle (NGCC) power plant with a 100% excess air in order to have a composition of the exhaust gas suitable for an effective absorption by amine solutions. To reach this goal, the recirculated flue gas is added to the secondary air (dilution air) used for cooling the turbine. The originality of this work is that the optimum value of a Flue Gas Recirculation (FGR) ratio of 0.42 is obtained from the change of the slope related to the effects of flue gas recirculation ratio on the molar percentage of oxygen in the exhaust gas. Compared to the NGCC power plant without flue gas recirculation, the molar percentage of carbon dioxide in the flue gas increases from 5% to 9.2% and the molar percentage of oxygen decreases from 10.9% to 3.5%. Since energy efficiency is the key parameter of energy conversion systems, the impact of the flue gas recirculation on the different energy inputs and outputs and the overall efficiency of the power plant are also investigated. It is found the positive effects of the flue gas recirculation on the electricity produced by the steam turbine generator (STG) are more important than its cooling effects on the power output of the combustion turbine generator (CTG). The flue gas recirculation has no effects on the water pump of the steam cycle and the increase of energy consumed by the compressor of flue gas is compensated by the decrease of energy consumed by the compressor of fresh air. Based on the Low heating value (LHV) of the natural gas, the flue gas recirculation increases the overall efficiency of the power plant by 1.1% from 57.5% from to 58.2%.

Research and Application of Airflow Recirculation System in Working Faces with Leaking Airflow

In order to develop the technology of the controlled recircuIation of airflow in the world, some formulas about the airflow recirculation system in the working face with leaking airflow are deduced,which reduces the error between calculating and real values. on the base of the application of the formulas mentioned above, the problem about lack of airflow in the working face 2712 was solved successfully in Xiandewang Coal Mine.

Effects of Biogas Slurry Recirculation on Anaerobic Digestion Performance of Maize Straw Silage

In order to investigate the effects of slurry recirculation technology on anaerobic digestion performance of maize straw silage,maize straw silage was fermented with recirculated biogas slurry,and the gas production,p H value,methane content,volatile organic acids( VFAs)contents,chemical oxygen demand( COD) removal rate and other indicators were studied. The results showed that the fermentation time was positively correlated with daily gas production,methane content,cumulative gas production,VFAs and COD removal rate. Although the p H value fluctuated,it was still in the normal reaction range. The daily gas production was about 1. 26 L. The acetic acid content increased first,then decreased,then increased,and finally stabilized. The biogas slurry recirculation technology saves water resources by 40 m L/d without affecting the normal gas production of anaerobic fermentation,and reduces the consumption of environmental resources. It has important development significance for the sustainable use of biomass resources.

The Experimental Investigation of Recirculation of Air-Cooled System for a Large Power Plant

The paper introduces thermal buoyancy effects to experimental investigation of wind tunnel simulation on direct air-cooled condenser for a large power plant. In order to get thermal flow field of air-cooled tower, PIV experiments are carried out and recirculation ratio of each condition is calculated. Results show that the thermal flow field of the cooling tower has great influence on the recirculation under the cooling tower. Ameliorating the thermal flow field of the cooling tower can reduce the recirculation under the cooling tower and improve the efficiency of air-cooled condenser also.

Sexual Reversal with 17α-Methyltestosterone in Oreochromis sp.:Comparison between Recirculation Aquaculture System(RAS)and Biofloc Technology(BFT)

Precocity in tilapia implies the use of several methods of obtaining monosex seed;the most common tends to use masculinizing hormone 17α-methyltestosterone(17αMT),with variable results.Thus the objective of this study was to compare the efficiency of the sexual reversion process using 17αMT,in a recirculation system and in biofloc.In a totally randomized design,three tanks for recirculation(T-RAS)and three tanks for biofloc(T-BIO)with a capacity of 200 L effective volume were taken and filled with 1,056 larvae of Oreochromis sp.,without reversing and with an initial weight of 0.02 g and an initial total length of 1.4 mm.The study was carried out during 65 d,the fish were fed(10%biomass,adjusted every 15 d)by a commercial diet at 45%of crude protein that included 17αMT(60 mg/kg).Water quality,microbiology,zootechnical and gonadal analysis were monitored.Consequently the water quality results showed that just dissolved oxygen(DO),temperature(T-°C)and alkalinity did not show significant differences.Additionally,in the productive parameters there were significant differences in the final length,the gain in length and in K which were better in T-BIO.The microbiological ones did not present significant differences between the treatments.Lastly,the percentage of reversion was significantly better in T-RAS.Then,this study suggests that settleable solids concentrations above 35 cm decrease the efficiency of the sexual reversion for this species.

Reverse Osmosis Performance in MBR-RO Process with Recirculation of RO Concentrate to MBR for Water Reclamation

An integrated membrane system, membrane bioreactor-reverse osmosis (MBR-RO), has become highly efficient in producing high-quality water for municipal wastewater reclamation. However, disposal of a highly concentrated waste stream (RO concentrate or RO retentate) generated in this combination is an important issue. This work investigated RO behaviour in an integrated pilot scale MBR-RO system for municipal wastewater reuse with the continuous recycling of RO retentate to the MBR influent. RO membrane retention and the fouling propensity were studied. RO concentrate, produced by the RO process at a fixed concentration factor (CF) of 3, was recycled continuously to the MBR, leading to water recovery of the entire process around 92%. Osmotic pressure model, saturation index method, high performance liquid chromatography equipped with size exclusion column (HPLC-SEC) and specific filtration test were used to analyse the fouling potential of the RO membrane. The results obtained showed that even though RO concentrate recycling changed remarkably, the compositions of both MBR permeate and RO concentrate, the quality of RO permeate remained almost constant in terms of organic matters, conductivity, and ionic salts. However, these high concentrations of organic or inorganic substances in RO concentrate were major factors leading to the RO membrane fouling. Before RO concentrate recycling, a decline of approximately 30% of the initial RO permeate flux was observed in the period when the CF was increasing to 3, mainly due to the osmotic pressure effect of retained ions and the deposits of organic matters at the RO membrane surface. After RO concentrate addition to the MBR, due to the continuous accumulation of ionic salts on the RO membrane surface, a gradual reduction in RO permeate flux (additional 19%) was also mainly attributed to the osmotic pressure effect of the retained ions. These observations showed that the continuous addition of RO concentrate to the MBR was successful in a combined MBR and RO process in terms of the excellent quality of RO permeate.

Analysis of Non-Selective Catalyst Reduction Performance with Dedicated Exhaust Gas Recirculation

Rich burn industrial natural gas engines offer best in class post catalyst emissions by using a non-selective catalyst reduction aftertreatment technology. However, they operate with reduced power density when compared to lean burn engines. Dedicated exhaust gas recirculation (EGR) offers a possible pathway for rich burn engines to use non-selective catalyst reduction aftertreatment technology without sacrificing power density. In order to achieve best in class post catalyst emissions, the precious metals and washcoat of a non-selective catalyst must be designed according to the expected exhaust composition of an engine. In this work, a rich burn industrial natural gas engine operating with dedicated EGR was paired with a commercially available non-selective catalyst. At rated brake mean effective pressure (BMEP) the air-fuel ratio was swept between rich and lean conditions to compare the catalyst reduction efficiency and post catalyst emissions of rich burn and dedicated EGR combustion. It was found that due to low oxides of nitrogen (NOx) emissions across the entire air-fuel ratio range, dedicated EGR offers a much larger range of air-fuel ratios where low regulated emissions can be met. Low engine out NOx also points towards a possibility of using an oxidation catalyst rather than a non-selective catalyst for dedicated EGR applications. The location of the NOx-CO tradeoff was shifted to more rich conditions using dedicated EGR.

The recirculation flow after different cross-section shaped high-rise buildings with applications to ventilation assessment and drag parameterization

The building cross-section shape significantly affects the flow characteristics around buildings,especially the recirculation region behind the high-rise building.Eight generic building shapes including square,triangle,octagon,T-shaped,cross-shaped,#-shaped,H-shaped and L-shaped are examined to elucidate their effects on the flow patterns,recirculation length L and areas A using computational fluid dynamics(CFD)simulations with Reynolds-averaged Navier-Stokes(RANS)approach.The sizes and positions of the vortexes behind the buildings are found to be substantially affected by the building shapes and subsequently changing the recirculation flows.The recirculation length L is in the range of 1.6b-2.6b with an average of 2b.The maximum L is found for L-shaped building(2.6b)while the shortest behind octagon building(1.6b).The vertical recirculation area Av is in the range of 1.5b^(2)-3.2b^(2)and horizontal area Ah in 0.9b^(2)-2.2b^(2).The L,Av and Ah generally increase with increasing approaching frontal area when the wind direction changes but subject to the dent structures of the#-shaped and cross-shaped buildings.The area-averaged wind velocity ratio(AVR),which is proposed to assess the ventilation performance,is in the range of 0.05 and 0.14,which is around a three-fold difference among the different building shapes.The drag coefficient parameterized by Ah varies significantly,suggesting that previous models without accounting for building shape effect could result in large uncertainty in the drag predictions.These findings provide important reference for improving pedestrian wind environment and shed some light on refining the urban canopy parameterization by considering the building shape effect.

Thermodynamic Performance Analysis of E/F/H-Class Gas Turbine Combined Cycle with Exhaust Gas Recirculation and Inlet/Variable Guide Vane Adjustment under Part-Load Conditions

Exhaust gas recirculation control(EGRC),an inlet air heating technology,can be utilized in combination with inlet/variable guide vane control(IGV/VGVC) and fuel flow control(FFC) to regulate the load,thereby effectively improving the part-load(i.e.,off-design) performance of the gas turbine combined cycle(GTCC).In this study,the E-,F-,and H-Class EGR-GTCC design and off-design system models were established and validated to perform a comparative analysis of the part-load performance under the EGR-IGV-FFC and conventional IGV-FFC strategies in the E/F/H-Class GTCC.Results show that EGR-IGV-FFC has considerable potential for the part-load performance enhancement and can show a higher combined cycle efficiency than IGV-FFC in the E-,F-,and H-Class GTCCs.However,the part-load performance improvement in the corresponding GTCC was weakened for the higher class of the gas turbine because of the narrower load range of EGR action and the deterioration of the gas turbine performance.Furthermore,EGR-IGV-FFC was inferior to IGV-FFC in improving the performance at loads below 50% for the H-Class GTCC.The results obtained in this paper could help guide the application of EGR-IGV-FFC to enhance the part-load performance of various classes of GTCC systems.

Effect of flue gas recirculation technology on soot and NO formation in the biomass pyrolysis-combustion system

Pyrolysis of biomass followed by combustion of pyrolytic vapors to replace fossil fuels is an economic low-carbon solution.However,the polycyclic aromatic hydrocarbons and N-containing species in biomass pyrolysis vapors result in the soot and NO emissions.The flue gas recirculation(FGR)technology,having the potential to reduce the soot and NO emissions,was introduced to the biomass pyrolysis-combustion system.In addition,it was numerically studied by simulating the biomass pyrolysis vapors based co-flow diffusion flames with CO_(2)addition.Both the experimental and simulated results showed that the FGR had significant suppression effects on the soot formation.When the FGR ratio(i.e.,CO_(2)addition ratio)increased from 0%to 15%,the experimental and simulated soot volume fraction respectively decreased by 32%and 21%,which verified the models used in this study.The decrease in OH concentration caused by the CO_(2)addition was responsible for the decrease in the decomposition rate of A2(A2+OH=A2–+H_(2)O).Hence,more benzo(ghi)fluoranthene(BGHIF)was generated through A1C_(2)H–+A2→BGHIF+H_(2)+H,leading to the increase in inception rate.The decrease in benzo(a)pyrene(BAPYR)concentration was the major factor in the decrease in soot condensation rate.Moreover,the decrease in the C_(2)H_(2) and OH concentrations was responsible for the decrease in the HACA surface growth rate.Furthermore,the simulated results showed that the NO concentration decreased by 0.4%when the content of CO_(2)was increased by 1 vol.%.The decrease in OH concentration suppressed the NO formation via decreasing reaction rates of N+OH=NO+H and HNO+OH=NO+H_(2)O and enhanced the NO consumption via increasing reaction rate of HO_(2)+NO=NO_(2)+OH.

Characteristics of the recirculation zone in a cylindrical confined space and its suppression by an annular orifice plate

The accumulation of pollutants in the recirculation zone can worsen ventilation.It is critical to reduce recirculation zones to improve the ventilation efficiency of buildings.However,the variation rule of the recirculation zone in a cylindrical confined space(CCS)is unclear,and there are few solutions to suppress or eliminate the recirculation zone at present.In this paper,an annular deflector orifice plate for suppressing the recirculation zone was developed based on the structural characteristics of the CCS.This device is simple in structure and can be used flexibly.Through experiments and numerical simulations,the variation rule of the recirculation zone length and the influence of structural parameters of the device on the vortex suppression were explored.Firstly,empirical formulas for calculating the length of the recirculation zone in the CCS were obtained.In addition,it was proved that placing the annular orifice plate inside the CCS effectively reduced the recirculation zone and improved the ventilation efficiency.Compared to the system without the annular orifice plate,the dimensionless length of the recirculation zone was decreased by 76.3%,and the time to completely discharge the pollutants from the CCS was decreased by 16.7%.Finally,parameters of the annular orifice plate that form the best vortex suppression effect were proposed:the porosity range was 40%–50%,uniform in shape with equal ring spacing,and placed more than one inlet diameter away from the inlet.The results help guide the ventilation design of CCS.

CYCLONIC RECIRCULATION ON THE LEFT SIDE OF THE KUROSHIO IN THE EAST CHINA SEA

Based on Princeton Ocean model,circulation in the Yellow Sea and East China Sea and its annual cycle are simulated. The model results show a weak countercurrent on the left side of the Kuroshio. The countercurrent separates from the main stream of the Kuroshio around 30°N, 128°E, where the Kuroshio turns right to the Tokara Strait; it flows southwestward along isobaths, meets with the offshore branch of Taiwan Warm Current around 28°N, 124°E, then returns back and joins the main stream, so that a cyclonic circulation is formed. The center and strength of the countercurrent and cyclonic circulation exhibits seasonal variation. The lower layer cold water observed on PN section of the East China Sea in summer and autumn is considered to be associated with the modeled cyclonic circulation. According to Rossby’s wake stream theory, lateral friction may be the main mechanism of the countercurrent on the left side of the Kuroshio.

Experimental Investigation for NO_x Emission Reduction in Hydrogen Fueled Spark Ignition Engine Using Spark Timing Retardation, Exhaust Gas Recirculation and Water Injection Techniques

The experimental tests were carried out on a single cylinder hydrogen fueled spark ignition(SI)generator set with different spark timings(4-20℃A bTDC),exhaust gas recirculation(EGR)up to 28% by volume and water injection up to 1.95 kg/h(maximum water to fuel mass ratio of 8:1).The engine speed was kept constant of 3000 r/min.The NOx emission and thermal efficiency of engine with gasoline and hydrogen fuel operation at 1.4 kW power output are 5 g/kWh and 12.1 g/kWh,and 15% and 20.9% respectively.In order to reduce the NOx emission at source level,retarding spark timing,exhaust gas recirculation(EGR),and water injection techniques were studied.Nox emission decreased with spark timing retardation,EGR,and water injection.NOx emission with hydrogen at 1.4 kW power output decreased from 12.1 g/kWh with maximum brake torque(MBT)spark timing(10℃A bTDC)to 8.1 g/kWh with retarded spark timing(4℃A bTDC)due to decrease in the in-cylinder peak pressure and temperature.The Nox emission decreased to 6.1 g/kWh with 20% EGR due to thermal and chemical dilution effect.However,thermal efficiency decreased about 33% and 17% with spark timing retardation and 20EGR respectively as compared to that of MBT spark timing.But,in the case of water injection,the NOx emission decreased significantly without affecting the thermal efficiency of the engine and it is 5.6 g/kWh with water-hydrogen ratio of 4:1(water flow rate of 0.92 kg/h).Water injection is the best suitable method to reduce the NOx emission in a hydrogen fueled engine compared with the spark timing retardation and EGR technique.

Combustion Wave Propagation of a Modular Porous Burner with Annular Heat Recirculation

A numerical investigation of the different arrangements of porous media in a combustor with annular heat recirculation is conducted.The effect of annular heat recirculation and porous block arrangement on the characteristics of combustion wave propagation is numerically studied.Results show that power input,heat capacity of porous matrix,arrangement of porous blocks,and annular heat recirculation are major factors that influence the propagation of combustion wave.The overall temperature of ceramic porous burner is higher than that of ceramic-metal type burner due to the lower heat storage capacity of the former,especially for the temperature downstream.The flame temperature is higher upstream and lower downstream with metal foams in the annulus than that without metal foams.The flame temperature of uniformity type burner is more uniform than that of gradually-varied and modular type burners.The flame front moves more slowly with metal foams in the annulus than that without metal foams due to the better preheating effect of metal foams.The flame position moves downstream,and the flame temperature gradually decreases and is eventually extinguished due to the low preheating temperature.

Modelling the HIV persistence through the network of lymphocyte recirculation in vivo

Human Immunodeficiency Virus(HIV)is able to persist in cellular and/or anatomical viral reservoirs,despite the effective inhibition of virus replication by the antiretroviral therapy(ART).Here we develop a mathematical model to gain some insights of HIV persistence relevant to the lymphocyte recirculation network of immune system and the central nervous system(CNS).Our simulations and analyses illustrate the role of the CNS as a virus reservoir to prevent antiretroviral drugs from penetrating the blood-brain(or blood-testis)barrier,and we examine the long-term impact of this reservoir on the transmissibility of an infected individual.We observe numerically that level of HIV in peripheral blood may not accurately reflect the true mechanisms occurring within other organs.

The dissolution of total suspended solids and treatment strategy of tailwater in a Litopenaeus vannamei recirculating aquaculture system

In recirculating aquaculture systems(RASs),the effective treatment of aquaculture tailwater is essential to maintain the health of the RAS.This study investigated the optimal time and method for tailwater treatment during three periods of the aquaculture of the Litopenaeus vannamei:nursery(0–26 d),middle(27–57 d),and later(57–104 d).The variation of several water parameters during the dissolution of total suspended solid(TSS)in tailwater,applied with the effects of ozone on the microorganism and water quality parameters were investigated.Results showed that the TSS concentrations in tailwater decreased with time,although not significantly(P>0.05),whereas total ammonia nitrogen(TAN),nitrite(NO-2-N),and nitrate(NO_(3)^(-)-N)increased significantly(P<0.05).Therefore,TSS should be removed from the tailwater as early as possible,being most optimal within 4 h.Ozone removed 38.24%–48.95%of TSS,17.78%–90.14%of TAN,and 87.50%–98.90%of NO-2-N after 4 h of treatment.However,it resulted in the significant accumulation of NO_(3)^(-)-N.Moreover,the total number of Vibrio and bacterial counts in aquaculture tailwater was reduced completely by ozone within 4 h.Thus,these results provided technical details and data support for the effective treatment of tailwater from shrimp RAS.

EGR的热效应和稀释效应对柴油机燃烧和排放的影响

以改进的KIVA-3V程序为计算平台,在转速为1,600,r/min,每循环喷油量为52.8,mg工况下,分别进行了EGR的热效应和稀释效应对柴油机燃烧和排放影响的数值模拟.结果表明,EGR的热效应对滞燃期影响较大,而稀释效应成为制约最高爆发压力和缸内温度的主要因素,且随着氧气体积分数的降低,热效应对最高爆发压力的影响增强;EGR导致碳烟排放增加是由于EGR的稀释效应,低温燃烧时碳烟排放的降低是EGR的稀释效应和热效应相互作用的结果;相比稀释效应,EGR的热效应是实现低温燃烧的主要因素,高进气热容更易改善NOx和碳烟排放的折衷关系.