Biomass Residence Time Research Articles

Parametric analysis of pyrolysis process on the product yields in a bubbling fluidized bed reactor

This paper presents a numerical study of operating factors on the product yields of a fast pyrolysis process in a 2-D standard lab-scale bubbling fluidized bed reactor. In a fast pyrolysis process, oxygen-free thermal decomposition of biomass occurs to produce solid biochar, condensable vapours and non-condensable gases. This process also involves complex transport phenomena and therefore the Euler-Euler approach with a multi-fluid model is applied. The eleven species taking part in the process are grouped into a solid reacting phase, condensable/non-condensable phase, and non-reacting solid phase (the heat carrier). The biomass decomposition is simplified to ten reaction mechanisms based on the thermal decomposition of lignocellulosic biomass. For coupling of multi-fluid model and reaction rates, the time-splitting method is used. The developed model is validated first using available experimental data and is then employed to conduct the parametric study. Based on the simulation results, the impact of different operating factors on the product yields are presented. The results for operating temperature (both sidewall and carrier gas temperature) show that the optimum temperature for the production of bio-oil is in the range of 500–525 °C. The higher the nitrogen velocity, the lower the residence time and less chance for the secondary crack of condensable vapours to non-condensable gases and consequently higher bio-oil yield. Similarly, when the height of the biomass injector was raised, the yields of condensable increased and non-condensable decreased due to the lower residence time of biomass. Biomass flow rate of 1.3 kg/h can produce favourable results. When larger biomass particle sizes are used, the intraparticle temperature gradient increases and leads to more accumulated unreacted biomass inside the reactor and the products’ yield decreases accordingly. The simulation indicated that the larger sand particles accompanied by higher carrier gas velocity are favourable for bio-oil production. Providing a net heat equivalent of 6.52 W to the virgin biomass prior to entering the reactor bed leads to 7.5% higher bio-oil yields whereas other products’ yields stay steady. Results from different feedstock material show that the sum of cellulose and hemicellulose content is favourable for the production of bio-oil whereas the biochar yield is directly related to the lignin content.

A comparison of the torrefaction behavior of wood, miscanthus and palm kernel shells: Measurements on single particles with geometries of technical relevance

A torrefaction test rig was designed to investigate large single biomass particles up to characteristic sizes of 25 mm, typical for industrial reactors. Time-resolved mass loss for such particles is measured with a magnetic suspension balance at well-defined torrefaction conditions (temperature, residence time, gas atmosphere). This paper comprises the results of woody and non-woody biomass: pine, a coniferous, and beech, a deciduous, wood, palm kernel shells and miscanthus. Influence of process temperature (240 to 320 °C), residence time (up to 1 h) and type of solid biomass on time-resolved mass loss is presented. Additional tests with oxygen in the process gas (0–15 vol%), typical for industrial torrefaction systems, are carried out for selected samples of beech wood. The differences in torrefaction behaviour of bark, sap- and heartwood of pine are evaluated. Finally, it is shown that the torrefaction reactor developed allows to derive kinetic parameters for mass loss. At temperatures up to 300 °C the mass loss for palm kern shells is highest followed by miscanthus, and pine. By examining pine, as an example, it is shown that heartwood is significantly more reactive than sapwood and bark. Finally, it is demonstrated, that for the particle sizes considered here heat and mass transfer limitations can be neglected for the determination of torrefaction kinetics. Kinetic data agree well with data from literature.

Resolving Variables Influencing the Residence Time of Biomass in the Old-Age Forest across Climate Gradients

Standing biomass stocks represent a balance between a number of processes that lead to biomass accumulation or to biomass loss. The average time of biomass residence (ATr) of an ecosystem is the average period of time that carbon is locked up in the living biomass before it is transferred to the litter pool and is an important variable influencing the process leading to biomass loss. Variation in terrestrial ATr with climate is thought to originate from a direct influence of temperature and precipitation on plant mortality. However, variation in ATr may also result from an indirect influence of climate by means of plant age and growing season length. To identify the relative importance of direct and indirect climate effects, we analyzed published data of ecosystem woody biomass and productivity from forest plots across climate gradients, using three approaches: bayesian linear regression, multiple regression, and structural equation modeling. The three approaches provided special insights and they converged in supporting climate as an indirect driver of ATr across climate gradients. Notably, age and growing season length explained most of the variation in ATr, whereas mean annual temperature and precipitation explained almost none, suggesting that climate indirectly influenced ATr. Our analyses provide novel evidence not only supporting that the old-age forests could be a carbon sink with a longer time of biomass residence at a large scale, but also modifying the key drivers of ecosystem processes for vegetation dynamic models.

Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

Environmental conditions and silviculture fundamentally alter the metabolism of individual trees and, therefore, need to be studied at that scale. However, changes in forest biomass density (Mg C ha−1) may be decoupled from changes in growth (kg C year−1) when the latter also accelerates the life cycle of trees and strains access to light, nutrients, and water. In this study, we refer to an individual-based model of forest biomass dynamics to constrain the magnitude of system feedbacks associated with ontogeny and competition and estimate the scaling relationship between changes in tree growth and forest biomass density. The model was driven by fitted equations of annual aboveground biomass growth (Gag), probability of recruitment (Pr), and probability of mortality (Pm) parameterized against field observations of black spruce (Picea mariana (Mill.) BSP), interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco), and western hemlock (Tsuga heterophylla (Raf.) Sarg.). A hypothetical positive step-change in mean tree growth was imposed half way through the simulations and landscape-scale responses were then evaluated by comparing pre- and post-stimulus periods. Imposing a 100% increase in tree growth above calibrated predictions (i.e., contemporary rates) only translated into 36% to 41% increases in forest biomass density. This corresponded with a tree-growth elasticity of forest biomass (εG,SB) ranging from 0.33 to 0.55. The inelastic nature of stand biomass density was attributed to the dependence of mortality on intensity of competition and tree size, which decreased stand density by 353 to 495 trees ha−1, and decreased biomass residence time by 10 to 23 years. Values of εG,SB depended on the magnitude of the stimulus. For example, a retrospective scenario in which tree growth increased from 50% below contemporary rates up to contemporary rates indicated values of εG,SB ranging from 0.66 to 0.75. We conclude that: (1) effects of warming and increasing atmospheric concentrations of carbon dioxide and reactive nitrogen on biomass production are greatly diminished, but not entirely precluded, scaling up from individual trees to forest landscapes; (2) the magnitude of decoupling is greater for a contemporary baseline than it is for a pre-industrial baseline; and (3) differences in the magnitude of decoupling among species were relatively small. To advance beyond these estimates, studies must test the unverified assumptions that effects of tree size and stand competition on rates of recruitment, mortality, and growth are independent of climate change and atmospheric concentrations of carbon dioxide and nitrogen.

Kinetic, volatile release modeling and optimization of torrefaction

Torrefaction is a thermal pretreatment of biomass process carried out in the temperature range of 473–573K under inert atmosphere to enhance fuel properties. In the present study, different biomass are analyzed through numerical simulations and optimized parameters are reported. Kinetic, volatile release and solid composition models are coupled & simulated. Results of the kinetic model are verified by comparing with experimental results. Then, compositional coefficients for volatiles of different biomass are found by volatile release model. Further, volatile release model used to calculate composition of releasing volatiles. By means of mass conservation, composition of remaining solid product is estimated. Effect of torrefaction temperature, residence time and nature of biomass on moisture content of biomass, volatiles release, hydrogen to carbon (H/C) & oxygen to carbon (O/C) molar ratios, energy densification and elemental compositions are given. It is found that during mild torrefaction regime 508–548K; water removal and energy densification increases more rapidly as compared to light 473–508K and severe 548–573K regimes. Further, H/C and O/C molar ratios are found to be decreasing linearly in mild regime. In light and severe torrefaction; moisture removal and energy densification are increasing slowly. So, mild torrefaction was found to be optimum regime of torrefaction for practical applications. In mild torrefaction regime energy densification ratio was 1.05-1.52 for different biomasses studied.

Microalgal cell disruption via extrusion for the production of intracellular valuables

Abstract The objective of this study was to evaluate the effectiveness of extrusion on cell disruption of a marine alga Nannochloropsis oceanica with a single-screw extruder for the production of intracellular valuables. The effect of operating parameters including feed moisture (15–24%) and screw rotation speed (300–450 rpm) was also determined. Experimental results demonstrated that extrusion was effective in algal cell disruption indicated by direct observation of cell break-up through scanning electron microscope (SEM) and significant increases up to 94.3% and 68.7% in lipid and sugars yield, respectively between treatments and the control (no treatment). It was found that feed moisture (15%) generally improved cell disruption efficiency due to longer residence time of biomass in the extruder. Increasing screw rotation speed from 300 to 450 rpm tended to enhance cell disruption because of stronger shear forces on algae. However, the effectiveness was restricted by residence time of biomass in the extruder because a faster screw rotation speed resulted in a higher biomass flow rate. Thus, the optimal cell disruption was not always achieved at the fastest screw rotation speed; instead, it appeared at the levels depending on specific evaluation indicators (around 350 rpm), therefore the optimal conditions for algal cell disruption were obtained at about 15% of feed moisture and 350 rpm of screw rotation speed. This study also confirmed that extrusion was beneficial for the production of more valuables including polyunsaturated fatty acids (PUFA) and essential amino acids (EAA) from microalgae, for example up to 74.3% increase of PUFA and 20.5% increase of EAA after extrusion compared to the control.

Global change in the trophic functioning of marine food webs.

The development of fisheries in the oceans, and other human drivers such as climate warming, have led to changes in species abundance, assemblages, trophic interactions, and ultimately in the functioning of marine food webs. Here, using a trophodynamic approach and global databases of catches and life history traits of marine species, we tested the hypothesis that anthropogenic ecological impacts may have led to changes in the global parameters defining the transfers of biomass within the food web. First, we developed two indicators to assess such changes: the Time Cumulated Indicator (TCI) measuring the residence time of biomass within the food web, and the Efficiency Cumulated Indicator (ECI) quantifying the fraction of secondary production reaching the top of the trophic chain. Then, we assessed, at the large marine ecosystem scale, the worldwide change of these two indicators over the 1950–2010 time-periods. Global trends were identified and cluster analyses were used to characterize the variability of trends between ecosystems. Results showed that the most common pattern over the study period is a global decrease in TCI, while the ECI indicator tends to increase. Thus, changes in species assemblages would induce faster and apparently more efficient biomass transfers in marine food webs. Results also suggested that the main driver of change over that period had been the large increase in fishing pressure. The largest changes occurred in ecosystems where ‘fishing down the marine food web’ are most intensive.

Fast pyrolysis of locally available green waste at different residence time and temperatures

ABSTRACTIn an effort to promote the sustainable utilization of waste biomass resources, the effects of biomass residence time and temperature on the fast pyrolysis yield, higher heating value (HHV), and gaseous composition were explored. Rice straw, corn stover, and its pyrolysis yields were analyzed using proximate analysis, ultimate analysis, and gas chromatography. Bio-oil yield of different biomass was characterized. Increase in biomass residence time favored bio-oil yield, gas yield, and HHV of char. Results showed that increase in fast pyrolysis temperature up to 500°C increases the bio-oil and gas yields while the char content decreases. Above 450-500°C, a sudden increase in gas yield was observed. HHVs of bio-oil and char were favored by increase in temperature up to an optimum value. Pyrolysis temperature had a pronounced effect on gas composition. Increase in temperature favored H2 and CH4 concentration.

Evaluating the tropical forest carbon sink

Evaluating the tropical forest carbon sink

Modeling residence time with a three-dimensional hydrodynamic model: Linkage with chlorophyll a in a subtropical estuary

Residence time calculated with a three-dimensional hydrodynamic model was analyzed together with field measurements of chlorophyll a collected through long-term monitoring programs and synoptic-scale water quality mapping in the Caloosahatchee Estuary located in Florida, USA. Freshwater inflows to the estuary are highly managed at the head water control structure. A total of 14 freshwater inflow rates ranging from 0 to 283m3s−1 were simulated to represent the whole spectrum of hydrologic and water management conditions. Residence time, reported as the e-folding time, ranged from a few days to more than 60 days, depending on the magnitude of freshwater discharge and location in the estuary. The spatial heterogeneity of residence time indicated that there existed a zone in the estuary where water parcels reside for a longer period of time than in other areas of the estuary. The location of this “residence time maximum” zone progressed further toward the mouth of the estuary with increasing freshwater discharge. The location of peak chlorophyll a concentration along the longitudinal axis of the estuary as measured through long-term monitoring and synoptic water quality mapping, also fluctuated with freshwater discharge and coincided with the zone of maximum residence time. The results confirmed the fundamental role of freshwater inflow in the control of residence time and accumulation of phytoplankton biomass. The study helps elucidate how and where phytoplankton respond to freshwater inflows at the head of the estuary.

Improving simulated Amazon forest biomass and productivity by including spatial variation in biophysical parameters

Abstract. Dynamic vegetation models forced with spatially homogeneous biophysical parameters are capable of producing average productivity and biomass values for the Amazon basin forest biome that are close to the observed estimates, but these models are unable to reproduce observed spatial variability. Recent observational studies have shown substantial regional spatial variability of above-ground productivity and biomass across the Amazon basin, which is believed to be primarily driven by a combination of soil physical and chemical properties. In this study, spatial heterogeneity of vegetation properties is added to the Integrated Biosphere Simulator (IBIS) land surface model, and the simulated productivity and biomass of the Amazon basin are compared to observations from undisturbed forest. The maximum RuBiCo carboxylation capacity (Vcmax) and the woody biomass residence time (τw) were found to be the most important properties determining the modeled spatial variation of above-ground woody net primary productivity and biomass, respectively. Spatial heterogeneity of these properties may lead to simulated spatial variability of 1.8 times in the woody net primary productivity (NPPw) and 2.8 times in the woody above-ground biomass (AGBw). The coefficient of correlation between the modeled and observed woody productivity improved from 0.10 with homogeneous parameters to 0.73 with spatially heterogeneous parameters, while the coefficient of correlation between the simulated and observed woody above-ground biomass improved from 0.33 to 0.88. The results from our analyses with the IBIS dynamic vegetation model demonstrated that using single values for key ecological parameters in the tropical forest biome severely limits simulation accuracy. Clearer understanding of the biophysical mechanisms that drive the spatial variability of carbon allocation, τw and Vcmax is necessary to achieve further improvements to simulation accuracy.

Residence times of woody biomass in tropical forests

Background: The woody biomass residence time (τw) of an ecosystem is an important variable for accurately simulating its biomass stocks. Methods and results: We reviewed published data from 177 forest plots across the tropics and found a six-fold variation (23–129 years) in τw across our dataset, with a median τw of ca. 50 years. This value is similar to the median default value across 21 vegetation models for tropical forests, although the range of values used in models is large (20 to 200 years). Conclusions: The notion of a constant τw across all tropical forests may be of limited utility, given the large observed variation in τw. We found that while there was little relationship between climate variables and τw, there was evidence that edaphic factors exerted a strong influence on τw. In both the Neotropics and the Paleotropics, τw was highest in heavily weathered soils, suggesting that low soil fertility and/or non-limiting soil physical conditions exert a critical influence on τw. There is considerable uncertainty in how τw will be affected by global environmental change, especially by increased atmospheric CO2. Even small changes in τw could significantly reduce the future tropical forest carbon sink predicted by many vegetation models.

Microbial community engineering for biopolymer production from glycerol

In this work, the potential of using microbial community engineering for production of polyhydroxyalkanoates (PHA) from glycerol was explored. Crude glycerol is a by-product of the biofuel (biodiesel and bioethanol) industry and potentially a good substrate for bioplastic production. A PHA-producing microbial community was enriched based on cultivation in a feast-famine regime as successfully applied before for fatty acids-based biopolymer production. A glycerol-fed sequencing batch reactor operated at a 2-day liquid and biomass residence time and with feast-famine cycles of 24 h was used to enrich a mixed community of PHA producers. In a subsequent fed-batch PHA production step under growth-limiting conditions, the enriched mixed community produced PHA up to a dry weight content of 80 wt.%. The conversion efficiency of substrate to PHA on electron basis was 53%. Since glycerol is entering the metabolic pathways of the cell in the glycolytic pathway, it was anticipated that besides PHA, polyglucose could be formed as storage polymer as well. Indeed, polyglucose was produced in low amounts (∼ 10 wt.%). The results indicated that the feast-famine-based enrichment strategy was comparably successful to obtain a microbial community compared to fatty acids-based enrichment described before.

Diagnosing and assessing uncertainties of terrestrial ecosystem models in a multimodel ensemble experiment: 2. Carbon balance

This paper examines carbon stocks and their relative balance in terrestrial ecosystems simulated by Biome-BGC, LPJ, and CASA in an ensemble model experiment conducted using the Terrestrial Observation and Prediction System. We developed the Hierarchical Framework for Diagnosing Ecosystem Models to separate the simulated biogeochemistry into a cascade of functional tiers and examine their characteristics sequentially. The analyses indicate that the simulated biomass is usually two to three times higher in Biome-BGC than LPJ or CASA. Such a discrepancy is mainly induced by differences in model parameters and algorithms that regulate the rates of biomass turnover. The mean residence time of biomass in Biome-BGC is estimated to be 40–80 years in temperate/moist climate regions, while it mostly varies between 5 and 30 years in CASA and LPJ. A large range of values is also found in the simulated soil carbon. The mean residence time of soil carbon in Biome-BGC and LPJ is � 200 years in cold regions, which decreases rapidly with increases of temperature at a rate of � 10 yr1C � 1 . Because long-term soil carbon pool is not simulated in CASA, its corresponding mean residence time is only about 10–20 years and less sensitive to temperature. Another key factor that influences the carbon balance of the simulated ecosystem is disturbance caused by wildfire, for which the algorithms vary among the models. Because fire emissions are balanced by net ecosystem production (NEP) at steady states, magnitudes, and spatial patterns of NEP vary significantly as well. Slight carbon imbalance may be left by the spin-up algorithm of the models, which adds uncertainty to the estimated carbon sources or sinks. Although these results are only drawn on the tested model versions, the developed methodology has potential for other model exercises.

Design and Optimization of a two-stage bioreactor with cellular recycling for intensive ethanol production

This paper focuses on ethanol production optimization in the steady-state when the bioprocess involves a two-stage bioreactor with a cell recycling loop to reach high biomass concentrations. This bioprocess is described by a nonlinear model; several nonlinear optimizations for different configurations of the bioprocess are made with a Sequential Quadratic Programming (SQP) algorithm. Biomass residence times in the two bioreactors (TB1 and TB2) are the main parameters of the optimization. These parameters are connected to the volume ratio between the two bioreactors (V1/V2) and the substrate feed concentration (Sf1 and Sf2). The optimal configuration has been found for a volume ratio in the range 3.5 to 4.5 with the volume of the second bioreactor remaining constant. This configuration allows ethanol production higher than 2 kg/h, with a cellular viability greater than 80 %, a null glucose residual concentration and an ethanol titer of 11.6° GL.

Enrichment of a Mixed Bacterial Culture with a High Polyhydroxyalkanoate Storage Capacity

Polyhydroxyalkanoates (PHAs) are microbial storage polymers that attract interest as bioplastics. PHAs can be produced with open mixed cultures if a suitable enrichment step based on the ecological role of PHA is used. An acetate-fed sequencing batch reactor operated with 1 day biomass residence time and with feast-famine cycles of 12 h was used to enrich a mixed culture of PHA producers. In subsequent fed-batch experiments under growth limiting conditions, the enriched mixed culture produced PHA up to a cellular content of 89 wt % within 7.6 h (average rate of 1.2 g/g/h). The PHA produced from acetate was the homopolymer polyhydroxybutyrate. The culture was dominated by a Gammaproteobacterium that showed little similarity on 16S rRNA level with known bacteria (<90% sequence similarity). The mixed culture process for PHA production does not require aseptic conditions. Waste streams rather than pure substrates could be used as raw materials.

Process Improvement by Application of Membrane Bioreactors

Membrane bioreactors (MBR) – combinations of common bioreactors and membrane separation units for biomass retention – offer new possibilities for bioprocesses and wastewater treatment according to the particular goal and demands of the process. MBRs combine the benefits of high biomass concentrations to increase removal rates or volumetric productivity with the possibility to run a continuous process at controlled biomass retention e.g., by individual control of hydraulic and biomass residence times, production or degradation kinetics as well as biocoenosis composition can be optimized beyond chemostat or common activated sludge process performance. Although several practical experiences and data are already available for design and operation of MBR processes there is still considerable optimization potential. In MBRs, bioreactor and membrane filtration cannot be regarded as individual unit operations as they interact in a number of ways. Therefore, MBRs need to be considered as hybrid reactors. In this paper, some of the biology–membrane interactions are exemplarily discussed and data from different research and technical aspects including maintenance kinetics, membrane fouling, and rheology are presented.

The SBR and its biofilm application potentials

Twenty plus years of experience, innovation, and research in the field of biological wastewater treatment and biofilm applications lead to the conclusion that biofilms are in many cases more desirable in reactors than suspended activated sludge. Biofilm reactors can provide very long biomass residence times even when the hydraulic influent loading is low. This makes them particularly suitable when treatment requires slow growing organisms with poor biomass yield or when the wastewater concentration is too low to support growth of activated sludge flocs. Regardless of the settling characteristics of biological aggregates or the hydraulic influent loading the metabolic activity in the reactor can be maintained at a high level. This paper reviews the application of biofilms in sequencing batch reactor (SBR) systems to treat non-readily biodegradable substrates, volatile organic waste constituents, complex waste streams requiring co-metabolism, and particulate wastewaters. Recent research using the SBR to form aerobic granular sludge as a special application of biofilms is also discussed.

Leaf life span differs from retention time of biomass and nutrients in the crowns of evergreen species

Summary A long leaf life span has been interpreted as an adaptation to low nutrient availability. It is commonly assumed that a long leaf life span permits a longer utilization of nutrients in the leaf biomass and that this contributes to improving nutrient use efficiency. However, a long leaf life span is also associated with other leaf traits that reduce instantaneous productivity, which might reduce the growth rate of the leaf biomass and shorten the functional life of the leaf. We studied the relationships between leaf life span and the retention time of biomass and nutrient pools in several woody species with different leaf life spans. We measured the monthly variations in the total number of leaves per annual shoot, mass per leaf, and N and P contents per leaf. With these data, the leaf life span and the mean residence time (MRT) of leaf biomass and nutrient pools were estimated. The increase in the total number of leaves was fast in all species studied at the start of the growth season. In contrast, in evergreen species mass per leaf and total N and P contents per leaf increased gradually after the first year of life of the leaves, and the maximum mass and nutrient contents per leaf were attained only towards the end of the lifetime of the leaves. Owing to the delay in the development of mass and nutrient pools with respect to leaf number dynamics, in evergreen species leaf life span was longer than the MRT of leaf biomass and nutrient pools because part of the leaf biomass and nutrient pool remained in the crown for a shorter time than the mean duration of the individual leaves. The differences between leaf life span and the MRT of the biomass and nutrients increased with leaf life span. The slow growth of evergreen leaves therefore contributes to reduce the adaptive value of a long leaf life span as a mechanism to increase the duration of the photosynthetic machinery.

Hydrodynamics of Biological Aggregates of Different Sludge Ages: An Insight into the Mass Transport Mechanisms of Bioaggregates

Internal permeation and its role in mass transport to biological aggregates are investigated through detailed settling experiments. Three groups of bacterial aggregates (0.8-3.2 mm), which were different in fractal dimension and porosity, were generated in batch activated sludge reactors with biomass residence times (BRTs) of 5, 10, and 20 days. An apparatus of vertically connected double settling columns, which were filled respectively with water and an EDTA solution of a higher density, was used to characterize the settling and permeability features of individual aggregates. The settling velocities observed in water were in good agreement with those predicted by Stokes' law for porous but impermeable aggregates. The BRT10 and BRT20 aggregates, with porosities that were generally less than 0.92, had fluid collection efficiencies ranging from 0 to 0.1, while the BRT5 aggregates with porosities that were generally higher than 0.97 were almost completely impermeable. It is suggested that bioaggregates with either a tightly or a loosely packed structure cannot be as highly permeable as characterized for nonbiological fractal aggregates. While falling from water into the denser EDTA liquid, many BRT10 and BRT20 aggregates stopped and stayed below the interface of the two liquids for a period that was more than an order of magnitude shorter than the prediction based on the assumption of molecular diffusion as the predominant mass transport mechanism to the aggregates. The results indicate that limited intra-aggregate convection, which may not be important to the hydrodynamic properties of bioaggregates, could significantly enhance mass transport to suspended aggregates in biological wastewater treatment reactors.