Freeboard Zone Research Articles

Performance Evaluation of a 40-kWth Prototype Counterflow Particle-sCO2 Fluidized Bed Heat Exchanger

Particle-sCO2 heat exchangers (HXs) can couple particle-based thermal energy storage for concentrating solar power (CSP) plants with recompression closed Brayton power cycles (RCBC) to enable continuous dispatchable renewable electricity. RCBC firing temperatures >700°C require HXs with expensive Ni-based alloys, requiring HX designs to reduce mass with high overall heat transfer coefficients UHX to meet CSP primary HX cost. To enhance UHX, mild bubbling fluidization with downward particle flows and upward fluidizing gas flows can achieve particle-wall heat transfer coefficients hT,w >800 W m-2 K-1 with CARBOBEAD HSP 40/70. To assess how high hT,w with mild particle fluidization impacts UHX, a 40-kWth particle-sCO2 HX with 12-parallel narrow-channel fluidized beds was assembled and tested to particle inlet temperatures up to 530 °C. Tests show reliable steady-state HX operation by maintaining fluidized particles in a freeboard zone above the parallel channels, but axial dispersion mixes partially cooled particles up from the fluidized channels with particles fed into the freeboard zone from the feed hopper. This mixing lowers the effective bed temperatures and the driving force for heat transfer to counterflowing sCO2 in microchannelled walls. Measured UHX based on particle inlet temperature never exceeded 205 W m-2 K-1. The trade-off between increased hT,w, and increased vertical dispersion with gas flows resulted in only small improvements in UHX after the onset of fluidization. Dispersion was incorporated into HX design and performance models that used hT,w correlations fitted to single-channel heat transfer tests. Model results show that reducing dispersion leads to higher particle wall heat transfer.

Demonstration of a multi-channel fluidized bed particle–supercritical carbon dioxide heat exchanger for concentrating solar applications

High-temperature thermal energy storage in oxide particles at temperatures above 600°C can couple concentrated solar energy with high-efficiency thermal power cycles to provide dispatchable solar-driven electricity. Challenges remain in developing cost-effective primary heat exchangers, which require expensive alloys, to extract the high-temperature thermal energy from the particles to power cycle fluids, such as supercritical CO2 (sCO2) in recuperated Brayton cycles. To explore one pathway for cost-effective, high-temperature particle heat exchangers, the current study demonstrates a shell-and-plate, particle–sCO2 heat exchanger with narrow-channel fluidized beds coupled with micro-channel sCO2 flows in the heat exchanger walls. This study evaluates the feasibility of multiple parallel, narrow-channel fluidized beds in shell-and-plate particle–sCO2 HXs, to achieve high bed-wall heat fluxes at elevated temperatures. A reduced-order model simulates the narrow-channel, fluidized-bed particle–sCO2 heat exchanger to design the fluidized bed geometry, in terms of depth, height, and number of channels,for a nominal 40-kWth heat exchanger at particle and sCO2 inlet temperatures up to 600°C and 400°C respectively. The resulting shell-and-plate heat exchanger design operates with bubbling fluidization of the downward-flowing oxide particles to enhance bed-wall heat transfer. The heat exchanger core is fabricated with etched sCO2 micro-channels in thin wall plates that are diffusion bonded to spacer frames to form the shell-and-plate structure with 12 parallel, fluidized bed channels, 10.4 mm deep. The heat exchanger is tested at the National Solar Thermal Test Facility at Sandia National Laboratories with CARBOBEAD HSP particles at design particle flow rates of 0.20 kg s−1 and inlet temperatures up to 525°C. Results show that fluidization across multiple parallel channel beds can maintain uniform particle inventory with a common freeboard zone above the heat exchanger core. Bubbling fluidization improves particle–wall heat transfer coefficients but also increases axial dispersion of particle thermal energy, which lowers the log-mean temperature difference such that total heat transfer remains relatively constant to within ±10% over a broad range of fluidization gas velocities. The axial dispersion required particle and sCO2 flow rates to be increased by 25% over model-designed conditions to achieve the targeted 40 kWth, which indicates the importance of incorporating axial dispersion into heat exchanger design models and of deploying bed structures to suppress it. This study demonstrates the feasibility and preferred fluidizing gas conditions for particle heat exchangers for releasing high-temperature thermal energy storage systems.

Testing of a 40-kWth Counterflow Particle-Supercritical Carbon Dioxide Narrow-Channel, Fluidized Bed Heat Exchanger

Particle-based primary heat exchangers (HXs) must deliver sCO2 fluid temperatures above 700°C to couple particle-based concentrating solar receivers and thermal energy storage (TES) sub-systems with efficient sCO2 power cycles. Particle-sCO2 HX designs have struggled to meet DOE cost targets (≤ $150/kWth) due to the amount of expensive nickel alloys necessary for manufacturing full-scale, particle-sCO2 HXs. Our team has demonstrated that mild bubbling fluidization of falling particles in a counterflow narrow-channel fluidized bed can reduce required HX surface area and thus, costs by increasing particle-wall heat transfer coefficients hT,w > 800 W m-2 K-1. This paper reports on the fabrication and testing of a stainless steel, particle-sCO2 HX with 12 fluidized-bed channels approximately 10.5 mm deep spaced between diffusion-bonded, micro-channel sCO2 plates. The HX with a core length of ≈0.56 m is fed with CARBOBEAD HSP particles through a short, fluidized freeboard zone just above the core. Testing to date in the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories has shown that parallel bed fluidization maintains uniform particle inventory across the instrumented channels. Heat transfer thermal duty between the particle and sCO2 flows exceeds 30 kWth with sCO2 inlet temperatures of 200ºC and particle inlet temperatures up to 440ºC and mass flow rates of 0.2 kg s-1 fluidized by counterflowing gas flow rates of 0.005 kg s-1. Tests at higher particle and sCO2 inlet temperatures (600ºC and 400ºC respectively) are targeted to achieve > 40 kWth with model-predicted overall heat transfer coefficients U > 400 W m-2 K-1.

Reactive simulation of gas-catalyst thermochemical properties in a pilot-plant catalytic MTO fluidized bed

The methanol-to-olefin (MTO) process enables economical production of light olefins thus decreases the reliance on petroleum resources. The reactive multiphase flow patterns and reaction behaviors inside pilot-plant MTO fluidized reactor are numerically simulated with multiphase particle-in-cell model. Based on the model validation, the thermochemical properties of catalytic particle (e.g., heat transfer coefficient (HTC), temperature) and gas phase thermal characteristics are comprehensively investigated with the discussion on several crucial operating parameters. The results reveal that large HTC of catalyst particle is located in the crucial reaction zone and freeboard zone. The HTC of catalyst material covers a range of 50 to 150 W/(m2·K). Gas-solid hydrodynamics and gas concentrations exhibit spatial non-uniform distribution. Moreover, the reactant gas flow rate is more positively sensitive to the product distributions and gas thermal properties. Enlarging the reaction pressure significantly enhances the heat transfer. The vertical dispersion coefficient of catalyst is two orders of magnitude larger than the horizontal ones.

Prediction and minimization of NOx emission in a circulating fluidized bed combustor: Improvement of bed quality by optimizing cyclone performance and coal particle size

• Improvement of cyclone performance contributes to reducing the NO x emission. • Reducing coal particle size can decrease the NO x emission. • NO x emission may be further reduced by reducing the height of the coal inlet. • Effects of cyclone efficiency and coal size on combustion efficiency are discussed. • A comprehensive CFB mathematical model is applied to further analyze the mechanism. The NO x emission of circulating fluidized bed (CFB) boilers can be evidently reduced via optimization of the cyclone performance and the coal particle size, which has been proved in industrial practices, while there are few reasonable interpretations for these phenomena so far. In this paper, a comprehensive 1-dimensional/1.5-dimensional CFB mathematical model was applied to investigate the effect of these two important factors on NO x emission. Simulation results on two commercial CFB boilers indicate that increasing the cyclone efficiency and reducing the coal particle size can significantly decrease the average size of bed materials and increase the solid suspension density in the freeboard zone; namely, the bed quality is improved. The local gas–solid flow behaviors inside the CFB furnace will be accordingly changed, such as the increasing volume fraction of bubble phase in dense bed, shortening of secondary air penetration depth, enrichment of particles in the downward-flow annulus or clusters, etc. Consequently, the reducing atmosphere will be formed or enhanced in local areas, leading to the effective inhibition of NO x formation. Meanwhile, the increase in the bed inventory of fine char particles also facilitates the reduction of NO x . Moreover, if appropriately reducing the height of the coal inlet or feeding fine coal particles from the solid recycle system, the NO x emission may be further reduced due to the adjustment of the volatiles release pattern. In addition, the reduction of coal particle size can contribute to the increase in fuel combustion efficiency, while the unburned carbon content in fly/bottom ash may increase or decrease with the improvement of cyclone efficiency. This study further analyzes the influence of gas–solid flow characteristics on the chemical reaction process and provides a new solution to the low-cost pollutant emission control for CFB boilers.

CFD multiphase combustion modelling of oleic by-products pellets in a counter-current fixed bed combustor

A transient two-dimensional multiphase model was built to study the combustion of pellets of oleic by-products (Olive Pits (OPi)) in a cylindrical counter-current 40 kW fixed bed combustor. The fixed bed is modelled as a porous medium, which is randomly packed with spherical particles of equal size. A κ-ε model for low Reynolds number flows was used for turbulence Modelling. Primary and secondary air injections were supplied at the bed (solid phase combustion) and at the freeboard zone (gas phase combustion), respectively. The mass loss history, the temperature distribution at different heights inside the reactor and the gas emissions of CO, CO 2 , O 2 , H 2 , CH 4 and C org were computed. Key parameters related to the reaction front velocity, the mass conversion rate and the progress of ignition were also computed. We show that computational results are in good agreement with experimental measurements obtained using a similar reactor fed with the same pellet types. These results also motivate the implementation of the present formulation and its extension to industrial scale furnaces, having established the results for the comparison with pilot-scale experiments.

Development of a Partitioning Kinetic Model of Biomass Gasification in Supercritical Water with a Fluidized Bed Reactor

Biomass supercritical water gasification technology is considered an effective way to realize the utilization of biomass, and further development of this technology depends on an accurate kinetic model. In this work, the reactor was distributed into a nozzle affected zone, a dense phase zone, and a freeboard zone according to the characteristic of the physical and chemical field, and a reaction kinetic model suitable for each zone was developed, which could be used for dynamic and scale-up studying of the reactor. First, three groups of gasification experiments (feedstock and biochar with low and high concentrations) were conducted to obtain the gas yield for calculation of the kinetic parameter, and obtain the evolution of the specific area of biochar for the calculation of the porous structure constant ψ. Then, detailed reaction pathways for biomass and biochar were proposed and a random pore kinetic model taking specific surface area into account was developed based on the first order of each reactant.

Catalytic O2-steam gasification of biomass over Fe2-xMnxO3 oxides supported on ceramic foam filters

In the present study, catalytic gasification of biomass was investigated by incorporation of a supported Fe2-xMnxO3 catalyst into the freeboard zone of a bubbling fluidized bed gasifier. The material was processed by combining incipient wetness impregnation with microwave-assisted firing methods. These catalysts were characterized by elemental, structural and microstructural analyses, and redox testing by thermogravimetry. Catalytic performance was assessed by comparison with blank tests and used to study the influence of the catalyst temperature, equivalence ratio and gas hourly space velocity on tar decomposition. The catalyst revealed higher activity in converting tar compounds with increasing temperature and equivalence ratio, while increasing the gas hourly space velocity showed a negative impact on catalyst performance. Under optimal operating conditions, the catalyst exhibited a significant impact on tar conversion (83 %) and gasification parameters, such as the gas yield (0.81 to 0.93 Nmdry,gas3∙kgdry,fuel-1), carbon conversion efficiency (55.3 to 65.1 %) and cold gas efficiency (50.7 to 61.6 %). Post-mortem analysis of the tested catalyst provided further information on redox changes and their dependence on temperature, which were co-related with catalytic performance. Elemental mapping showed traces of sulphur, which increased with temperature and did not correlate with the catalytic performance; this was interpreted by thermodynamic modelling, based on the wide redox stability range of divalent manganese oxide and its enhanced tolerance to hydrogen sulphide.

Siderite/Concrete catalysts for H2-enriched gas production from biomass steam gasification

In this study, granulated siderite/concrete-based catalysts are proposed for in-situ applications in biomass steam gasification, seeking H2-enrichement by the contribution of water gas shift and CO2 adsorption. A cost-effective procedure has been applied for the preparation of granulated catalysts. Water Gas Shift reaction was studied in a fixed bed reactor and Taguchi experimental design was used to elucidate the relevance of experimental parameters on catalyst performance. The results show that the impact of reaction temperature, concrete/siderite ratio and steam to carbon molar ratio on H2 promotion had the following order: reaction temperature (43.1%), concrete/siderite ratio (30.2%) and steam to carbon molar ratio (26.7%). Additionally, the catalyst exhibited high activity when integrated into the freeboard zone of a bubbling fluidized bed gasifier. Compared to the reference condition, one observed significant impact at 700 °C on the H2:CO ratio (1.9 to 3.3), H2 production (33.8 to 48.6 gH2∙kgdry,fuel-1), carbon conversion efficiency (54.1 to 58.6%) and cold gas efficiency (55.0 to 60.9%). Though the catalyst exhibited resistance to sintering, post-mortem analysis suggests loss of active species after repeated cycles of regeneration due to thermal-induced stresses, which caused gradual decrease in activity.

Heat transfer of a coil-tube heat exchanger in the freeboard zone of a rice husk fluidized-bed combustor

This research presents an investigation on heat recovery from a fluidized-bed combustor by using coil-tube heat exchangers positioned inside the freeboard zone. Rice husk was used as a fuel at a constant mass flow rate of 8 kg/h. Heat recovery experiments were performed at four different air mass flow rates (6.0248, 6.8077, 7.8163, and 8.8646 kg/h) using coil-tube heat exchangers wrapped around with steel wires in three different configurations (2_Coil wires, 4_Coil wires and Full length). Air was supplied to the coil-tube heat exchanger with two different air flows corresponding to flue gas flow: co-flow and counter-flow. The results showed that the outlet temperature tended to decrease with an increase in air flow rate in the coil-tube heat exchanger. The outlet temperatures obtained from the system with co-flow were greater than the one with counter-flow by 7–17 °C (2–15%) depending on the coil-tube heat exchanger configuration. The heat transfer rates in heat recovery process were in the range of 279–425 J/s. In addition, it was found that the heat transfer rate and the outlet temperature of the air from the coil-tube heat exchanger increased with an increase in the number of coil wires wrapped around the outer surface of the coil-tube heat exchanger.

Modeling Combustion of Straw-Bitumen-Pellets in a Fluidized Bed.(Dept.M)

Recently straw-bitumen-pellets have been proposed as an alternative fuel. This work presents a mathematical model for steady state combustion of straw-bitumen-pellets in a bubbling fluidized bed. The combustor is divided into three zones; dense bed, splashing zone and freeboard. Important processes including volatile segregation, char comminution and elutriation, bed particles ejection; and post-combustion in splashing zone and freeboard have been considered and simplified. Submodels for hydrodynamic, volatile release and combustion, and char consumption have been implemented. Energy balance for splashing zone and freeboard has been set to predict axial temperature profile. Model results demonstrate that about 53% of volatiles combustion and about 62% of the total heat release take place within the bed. The fraction of heat released in the splashing zone is about 33% whereas the remainder portion (7%) releases in freeboard beyond the splashing zone. The ejected sand particles; however, recover back to the bed about 83% of heat released in the two latter zones. The model yields the axial profiles of different species concentrations in the two bed phases (bubble and emulsion), in the splashing zone and in the freeboard. Moreover, the model predicts the axial temperature profile in the splashing and the freeboard zones that characterizes by two maxima. A small peak is built in the splashing zone with a small overheating as the ejected sand particles recover the great part of released heat. Another maximum arises in the freeboard where the flux of the ejected particles turns out to be very few and its impact on gas temperature becomes insignificant. The second maximum has relatively much higher overheating temperature. The influences of operating variables on the combustion performance have been evaluated as well. In particular, the maximum overheating in freeboard gets higher with decreasing excess air factor, with lowering bed temperature and with increasing fluidization velocity. Bed temperature among the others has the highest impact on combustion performance. An acceptable agreement are found between the predicted and the measured concentrations and temperature profiles.

Combustion of raw and densified Tunisian oleic by-products in a fixed bed reactor

Combustion of biofuels prepared by blending or not olive mill solid residues (olive pomace and olive pits) with pine sawdust (S) and wood chips (WC) was investigated. The mass loss, the bed temperature and the front reaction position were determined. Combustion characteristic numbers such as heating release (HR) and ignition rate (IR), mass conversion rate (MCR) and excess air coefficient (λ) were also determined. Gaseous emissions of main species (O2, CO2, CO, H2O, and Corganic were analyzed and measured in the freeboard zone of the reactor. Moreover, the NOx (N2O + NO) and SO2 emission profiles were drawn in the post-combustion chamber. All results were mainly compared to our previous works that dealt with other blends and pellets of olive wastes, but also with biofuels tested by other researchers in similar conditions. Main results exhibit a substantial decrease of the ash content when blending with pine sawdust. Temperature profiles show an increase from the ambient value to a maximum value ranging between 800 and 1075 °C in accordance with what is usually found in literature when using biomass solid biofuels. In all samples, concentrations of pollutants such as NOx were also notably reduced and were found to be acceptable regarding the European standards. Furthermore, the combustion of pellets made of 50%–50% mixture of pits and of pomace with sawdust leads to a more efficient and homogenous combustion.

Numerical simulations of fluidized bed fast pyrolysis of biomass through computational fluid dynamics

In this study, computational fluid dynamics (CFD) was applied for simulating the hydrodynamics and chemical kinetics for the fluidized bed biomass fast pyrolysis. Based on the Euler-Euler multiphase framework, standard K-ε model and Finite-Rate/Eddy-Dissipation model were selected for the viscous and the species transport model, respectively. Syamlal O’brien model and Arrhenius kinetic model were chosen as the drag and reaction kinetics model, respectively. The volume fractions as well as the temperature distributions of the fluidizing gas, biomass and fluidizing sand at the fluidization velocity of 0.6 m/s were numerically observed. The simulation of the reaction temperature influence on product yield agreed well with the lab-scale experimental results. The distributions of the gas products show that CO and H2 are mostly at the lower part of the reactor, CH4 is in the freeboard region and CO2 is at both the reaction and freeboard zone. The proposed CFD model was expected to make contributions for improving the internal process and reactor optimization for biomass fluidized bed fast pyrolysis.

Biomass Gasification in a Bubbling Fluidized Bed Reactor

The effect of secondary airflow rate and particle size on the combustion temperature inside a bubbling fluidized bed reactor were reviewed. The experiments were carried out for four-particle sizes with different secondary airflow rates. From the results, it was evident that the secondary airflow rate has a more substantial impact on the temperature of the freeboard zone. The temperature profiles show that the maximum temperature is achievable when the magnitude of secondary airflow rate is lower than that of the primary airflow rate. It was concluded that the desired maximum temperature could be achieved regardless of the particle size through the optimum secondary airflow rates.

Activator-assisted pyrolysis of polypropylene

One of the most promising ways to recycle plastic is pyrolysis. Valuable fuels and chemicals can be obtained by pyrolysis of waste plastic. In this study, pyrolysis of polypropylene was conducted using a novel continuous two-stage process equipped with auger and fluidized bed reactors connected in series. The auger reactor played an important role as an activator to elevate the vibrational states of molecules fed into the fluidized bed reactor. Hence, the two-stage pyrolysis conducted herein was called activator-assisted pyrolysis. In this study, the effects of the temperatures of the auger reactor (or activator) and bubbling and freeboard zones of the fluidized bed reactor, the type of the fluidizing medium (product gas and N2), and the residence time of pyrolysis vapor on the product distribution and composition were investigated. In the experiments, the activator facilitated decomposition of polypropylene molecules, resulting in a high yield of the product gases (81 wt%) consisting mainly of H2 and CH4. The maximum olefin (ethene, propene, 1,3-butadiene, and butenes) yield was 52 wt%. The pyrolysis oil mainly consisted of aromatics (up to 92 wt%), particularly mono-aromatics. The activation energies of the activated and unactivated molecules were calculated using the Friedman and Flynn–Ozawa–Wall methods. The activation energy of the activated molecules was approximately 17 kJ/mol lower than that of unactivated molecules. Hence, it was clear that the degradation mechanism of polypropylene was different when activator-assisted pyrolysis was applied. The new activator-assisted pyrolysis proposed in the current work could be helpful to produce valuable chemicals from the pyrolysis of polyolefin waste.

파일럿 규모의 기포 유동층 반응기를 이용한 석탄과 하수 슬러지 고형 연료의 혼소에 대한 실험적 연구

In this study, the co-combustion characteristics of coal and sewage sludge were investigated using 100 kWSUBth/SUB bubbling fluidized bed (BFB) combustion system. Sub-bituminous coal (Coal), dried sewage sludge solid fuel (DSS), and hydrothermal carbonization sewage sludge solid fuel (HTCSS) were considered as fuel blends. The higher heating value of HTCSS was about 1,500 kcal/kg higher than that of DSS since the fuel ratio, H/C and O/C ratio were improved by the hydrothermal carbonization. The hardgrove grindability index (HGI) was performed to understand the feasibility of industrial applications. In the pilot-scale BFB experiment, the heat input, as 72 kWSUBth/SUB, was fixed in all cases. The mixing ratios of HTCSS and DSS were 2, 5 and 10% respectively based on the higher heating value. Under the co-combustion conditions, the temperature in the reactor was increased in the freeboard zone due to the higher volatile matter of HTCSS and DSS than Coal. The trend of CO and NOSUBx/SUB emissions was similar in all cases as mixing ratio was increased. However, SO₂ emission was decreased in HTCSS co-combustion case due to the lower sulfur content compared to than that of DSS. Ash acquired after BFB combustion was analyzed by XRF to evaluate the slagging/fouling tendency. The variation of slagging/fouling tendency was minor in all cases.

Simulation of combustion of sesame and broad bean stalks in the freeboard zone inside a pilot-scale bubbling fluidized bed combustor using CFD modeling

A two-dimensional numerical model has been developed to simulate combustion of sesame and broad bean stalks in the freeboard zone of a pilot-scale Bubbling Fluidized Bed Combustor (BFBC). Eulerian–Lagrangian approach is used so, the gas phase is treated as a continuum by solving the time-averaged Navier-Stokes equations and a Discrete Phase Model (DPM) is used to model the solid phase. The combustion in the freeboard was simulated to be consistent with the experimental conditions and the real combustor work. Prediction of composition for gas and solid phases at the freeboard inlet was performed. A series of simulations have been performed to examine the effect of chemical kinetic parameters, solid phase parameters, gas phase parameters and heat flux through walls on the numerical results. The model results were validated by comparing with experimental results obtained from the BFBC. The maximum absolute deviations corresponding to the numerical results of the mole fraction of flue gas species at the exit were 3.07% for the sesame stalks and 10.89% for the broad bean stalks. The numerical results of the axial temperature distribution have maximum absolute deviations 14.77% and 14.69% for the sesame stalks and broad bean stalks, respectively. It was found that the model simulation results are in a good agreement with the experimental data obtained from the experiments. In the present model, the deviations of the flue gas species and axial temperature distribution along the freeboard height are found within the literature limits.

Experimental investigation and prediction of heat transfer in a swirling fluidized-bed combustor

In this experimental study, the heat transfer coefficient in a twin-cyclonic swirling fluidized-bed combustor (T-FBC) with a conical shape bed was investigated to verify the effects of combustor design feature and operating parameters. The combustor was operated at a ratio of secondary and tertiary air to primary air ranging from 0 to 0.5 at primary air velocity (up) near minimum swirling fluidization condition (ums), to about 3ums by using silica sand with particle size diameters (dp) of 300–500, 600–710, and 710–1000 µm as bed material, for swirl number of an annular spiral air distributor of 2.76, and 2.98. The local heat transfer coefficients were measured in radial positions at the levels of 60, 125, 223, and 288 cm above the primary air distributor and the axial profiles of average heat transfer coefficient were characterized. The experimental results revealed that the radial profile of heat transfer coefficient was maximum at the center of the combustor for most of the test trials. Within the range of the tests, the heat transfer coefficient noticeably increased with the increase of the primary air flow rate and swirl number of the air distributor, and reached the almost constant value at 2.5–3ums. With design features of this combustor, the heat transfer coefficient in the bed region was comparable to the value in freeboard region though about 5–12% higher. Based on the test results, semi-empirical models for prediction of average Nusselt numbers were developed in the dense bed and freeboard zones for the different swirl numbers of 2.76 and 2.98, and the proposed models were in close agreement with the experimental data within ±20% error for both regions.

Efficient diagnosis of grate-fired biomass boilers by a simplified CFD-based approach

This paper describes the development and validation of a numerical tool able to simulate biomass combustion in grate-fired systems and support operation and design improvements of these devices. The modeling method is conceived as a compromise between the demand of computing time and the degree of detail in the simulation. As such, it integrates both the bed zone and the freeboard zone on a same 3D grid and assumes the bed as a porous medium, where heterogeneous reactions are simulated by a modified laminar rate model. Liquid water, dry biomass and char are introduced as site species that react on the porous medium surfaces to produce and/or consume gas species (O2, CO, CO2, H2, H2O, light hydrocarbons and condensable gases). To validate the numerical tool, predictions have been compared to experimental data gathered in a 250kWth combustion test facility operated with a high quality woody pellet. Once validated, the tool has been applied to characterize the flow patterns as well as the temperature and the main gaseous emissions profiles within the combustion chamber. According to the analysis of the simulation results, significant improvements have been identified concerning not only operation but also design issues.

Three-Dimensional Simulation of the Effects of Demolition Waste Recycling into Rotary Cement Kilns

The performance of three-dimensional kiln chambers using recycled cement is studied using basic geometric and operational parameters. A comprehensive 3D-simulator is developed that includes separate yet coupled computational models for the bed and the freeboard regions. Complex swirling airflow is taken into account, considering solid-phase combustion, gas-phase combustion of volatile fuel, and radiative heat transfer in the freeboard zone. Clinkerization reactions are included in the bed zone, assuming that the solid matter behaves as pseudofluid. Appropriate terms are used to model CO2 transfer from the bed to the freeboard zone. Proper inclusion of an additional feed stream coming from mechanical separation of demolition waste matter is also considered. The predictions of the simulations are consistent with observations from literature data and industrial experience. The sensitivity of the clinker composition and temperature profiles to selected parameters is also studied, revealing significant effects...