Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier

Chemical looping combustion is a novel technology that can be used to meet the demand on energy production without CO₂ emission. To improve CO₂ capture efficiency in the process of chemical looping combustion of coal, a prototype configuration for chemical looping combustion of coal is made in this study. It comprises a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and a loop-seal. The loop-seal connects the spout-fluid bed with the fast fluidized bed and is fluidized by steam to prevent the contamination of the flue gas between the two reactors. The performance of chemical looping combustion of coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier in a 1 kW_th prototype. The experimental results show that the configuration can minimize the amount of residual char entering into the air reactor from the fuel reactor with the external circulation of oxygen carrier particles giving up to 95% of CO₂ capture efficiency at a fuel reactor temperature of 985 °C. The effect of the fuel reactor temperature on the release of gaseous products of sulfur species in the air and fuel reactors is carried out. The fraction of gaseous sulfur product released in the fuel reactor increases with the fuel reactor temperature, whereas the one in the air reactor decreases correspondingly. The high fuel reactor temperature results in more SO₂ formation, and H₂S abatement in the fuel reactor. The increase of SO₂ in the fuel reactor accelerates the reaction of SO₂ with CO to form COS, and COS concentration in the fuel reactor exit gas increases with the fuel reactor temperature. The SO₂ in the air reactor exit gas is composed of the product of sulfur in residual char burnt with air and that of nickel sulfide oxidization with air in the air reactor. Due to the evident decrease of residual char in the fuel reactor with increasing fuel reactor temperature, it results in the decrease of residual char entering the air reactor from the fuel reactor, and the decrease of SO₂ from sulfur in the residual char burnt with air in the air reactor.     

Simulation of a low temperature water gas shift reactor using the heterogeneous model/application to a pem fuel cell

In the last few years, a renewed interest in the water gas shift (WGS) reaction at low temperature has arisen due to its application to fuel cells.In this work, a simulation of a fixed bed reactor for this reaction, which forms part of a hydrogen production–purification train for a 10 kW PEM fuel cell using ethanol as the raw material, was carried out. A commercial Cu/Zn/Ba/Al₂O₃ catalyst was employed and a one-dimensional heterogeneous model was applied for the simulation. The catalyst deactivation due to thermal factors (sintering) was taken into account in the model. Isothermal and adiabatic regimes were analyzed as well.Results of the simulation indicate that the pellet can be considered isothermal but temperature gradients in the film cannot be disregarded. On the other hand, concentration gradients in the film can be ignored but CO profiles are established inside the pellet. Adiabatic operation can be recommended because of its simplicity of operation and construction. The reactor volume is strongly sensitive to the CO outlet concentration at CO levels lower than 6000 ppm. For a 10 kW PEM fuel cell, using adequate pellet size and taking into account the catalyst deactivation, a reactor volume of 0.64 l would be enough to obtain an outlet CO concentration of about 7160 ppm. This concentration value can be handled by the next purification stage, COPROX.     

Hydrogen production with integrated microchannel fuel processor for portable fuel cell systems

An integrated microchannel methanol processor was developed by assembling unit reactors, which were fabricated by stacking and bonding microchannel patterned stainless steel plates, including fuel vaporizer, heat exchanger, catalytic combustor and steam reformer. Commercially available Cu/ZnO/Al₂O₃ catalyst was coated inside the microchannel of the unit reactor for steam reforming. Pt/Al₂O₃ pellets prepared by ‘incipient wetness’ were filled in the cavity reactor for catalytic combustion. Those unit reactors were integrated to develop the fuel processor and operated at different reaction conditions to optimize the reactor performance, including methanol steam reformer and methanol catalytic combustor. The optimized fuel processor has the dimensions of 60 mm × 40 mm × 30 mm, and produced 450sccm reformed gas containing 73.3% H₂, 24.5% CO₂ and 2.2% CO at 230–260 °C which can produce power output of 59 Wt.     

Investigation of bubble effect in microfluidic fuel cells by a simplified microfluidic reactor

This study experimentally examines the influence of two-phase flow on the fluid flow in membraneless microfluidic fuel cells. The gas production rate from such fuel cell is firstly estimated via corresponding electrochemical equations and stoichiometry from the published measured current–voltage curves in the literature to identify the existence of gas bubble. It is observed that O₂ bubble is likely to be generated in Hasegawa’s experiment when the current density exceeds 30 mA cm^?2 and 3 mA cm^?2 for volumetric flow rates of 100 μL min^?1 and 10 μL min^?1, respectively. Besides, CO₂ bubble is also likely to be presented in the Jayashree’s experiment at a current density above 110 mA cm^?2 at their operating volumetric liquid flow rate, 0.3 mL min^?1. Secondly, a 1000-μm-width and 50-μm-depth platinum-deposited microfluidic reactor is fabricated and tested to estimate the gas bubble effect on the mixing in the similar microchannel at different volumetric flow rates. Analysis of the mixing along with the flow visualization confirm that the membraneless fuel cell should be free from any bubble, since the mixing index of the two inlet streams with bubble generation is almost five times higher than that without any bubble at the downstream.     

Packed-bed thermal storage for concentrated solar power – Pilot-scale demonstration and industrial-scale design

A thermal energy storage system, consisting of a packed bed of rocks as storing material and air as high-temperature heat transfer fluid, is analyzed for concentrated solar power (CSP) applications. A 6.5 MWh_th pilot-scale thermal storage unit immersed in the ground and of truncated conical shape is fabricated and experimentally demonstrated to generate thermoclines. A dynamic numerical heat transfer model is formulated for separate fluid and solid phases and variable thermo-physical properties in the range of 20–650 °C, and validated with experimental results. The validated model is further applied to design and simulate an array of two industrial-scale thermal storage units, each of 7.2 GWh_th capacity, for a 26 MW_el round-the-clock concentrated solar power plant during multiple 8 h-charging/16 h-discharging cycles, yielding 95% overall thermal efficiency.     

Development of 10-kWe preferential oxidation system for fuel cell vehicles

A preferential oxidation (PROX) reactor for a 10-kWe polymer electrolyte membrane fuel cell (PEMFC) system is developed. Pt-Ru/Al₂O₃ catalyst powder, with a size of 300–600 μm is applied for the PROX reaction. To minimize pressure drop and to avoid hot spots in the catalyst bed, the reactor is designed as a dual-staged, multi-tube system. The performance of the 10-kWe PROX unit is evaluated by feeding simulated gasoline reformate which contains 1.2 wt.% carbon monoxide (CO). The CO concentration of the treated reformate is lower than 20 ppm in the steady-state and is under 30 ppm at 65% load change. Hydrogen loss in the steady-state is about 1.5% and the pressure drop across the reactor is 4 psi. Start-up characteristics of the 10-kWe PROX system are also investigated. It takes 3 min to reduce the CO concentration to below 20 ppm. Several controllable factors are found to shorten the start-up time.     

A novel polygeneration system integrating the acetylene production process and fuel cell

This paper presents a novel polygeneration system that integrates the acetylene process and the use of fuel cells. The system produces acetylene and power by a process of the partial oxidation/combustion (POC) of natural gas process, a water–gas shift reactor, a fuel cell and a waste heat boiler auxiliary system to recover the exhaust heat and gas from the fuel cell. Based on 584.3 kg/h of natural gas feedstock, a POC reactor temperature of 1773 K, an absorber pressure of 1.013 MPa and a degasser pressure of 0.103 MPa, the simulation results show that the new system achieved acetylene production of 1.9 MW, net electricity production of 1.7 MW, power generation efficiency of 26.8% and exergy efficiency of 43.4%, which was 20.2% higher than the traditional acetylene production process. The new system's exergy analysis and the flow rate of the products were investigated, and the results revealed that the energy conversion and systematic integration mechanism demonstrated the improvement of natural gas energy conversion efficiency.     

Fuel effects on start-up energy and efficiency for automotive PEM fuel cell systems

This paper investigates the effects of various fuels on hydrogen production for automotive PEM fuel cell systems. Gasoline, methanol, ethanol, dimethyl ether and methane are compared for their effects on fuel processor size, start-up energy and overall efficiencies for 50 kW_e fuel processors. The start-up energy is the energy required to raise the temperature of the fuel processor from ambient temperature (20 °C) to that of the steady-state operating temperatures. The fuel processor modeled consisted of an equilibrium-ATR (autothermal), high-temperature water gas shift (HTS), low-temperature water gas shift (LTS) and preferential oxidation (PrOx) reactors. The individual reactor volumes with methane, dimethyl ether, methanol and ethanol were scaled relative to a gasoline-fueled fuel processor meeting the 2010 DOE technical targets. The modeled fuel processor volumes were, 25.9 L for methane, 30.8 L for dimethyl ether, 42.5 L for gasoline, 43.7 L for ethanol and 45.8 L for methane. The calculated fuel processor start-up energies for the modeled fuels were, 2712 kJ for methanol, 3423 kJ for dimethyl ether, 6632 kJ for ethanol, 7068 kJ for gasoline and 7592 kJ for methane. The modeled overall efficiencies, correcting for the fuel processor start-up energy using a drive cycle of 33 miles driven per day, were, 38.5% for dimethyl ether, 38.3% for methanol, 37% for gasoline, 34.5% for ethanol and 33.2% for methane assuming a steady-state efficiency of 44% for each fuel.     

Nitrogen transfer of fuel-N in chemical looping combustion

Chemical-looping combustion (CLC) is a novel technique used for CO₂ separation that has been investigated for gaseous fuel and solid fuel. The nitrogen transfer of fuel-N in the coal is experimentally investigated with a NiO/Al₂O₃ oxygen carrier under a continuous operation in a 1 kW_th interconnected fluidized bed prototype. The effects of the fuel reactor temperature, coal type and operation conditions on the release of gaseous products of nitrogen species in the air reactor and the fuel reactor are carried out. Results show that the nitrogen transfer direction of fuel-N is toward N₂ formation in the fuel reactor independent of fuel type. In the fuel reactor N₂ is the sole product of nitrogen transfer of fuel-N. The concentration of N₂ in the fuel reactor exit gas increases with the fuel reactor temperature. The NO_x precursor of HCN can be oxidized by the oxygen carrier to form NO or N₂ in the fuel reactor. However, in the fuel reactor NO from coal devolatilization and HCN oxidization by oxygen carrier is completely reduced to N₂. The other NO_x precursor of NH₃ is completely converted to N₂ due to oxidization by NiO and the catalytic effect of Ni on the decomposition of NH₃. After coal devolatilization, char-N conversion in the fuel reactor is toward N₂ formation according to the investigation of solid–solid reaction between char and oxygen carrier. The amount of residual char has a potential to cause formation of nitrogen contaminants in the air reactor. In the air reactor, NO is the only nitrogen contaminant, and there is no NO₂ formation. The high fuel reactor temperature results in little residual char coming into the air reactor. The proportion of char-N converted to NO in the air reactor increases from 16.98% to 18.85% when the fuel reactor temperature changes from 850 to 950 °C. For the fuels containing more volatile matter, the possibility of NO formation in the air reactor is smaller than the fuels containing less volatile matter. For the fuels containing less volatile matter, char gasification rate is still a significant factor both for the carbon capture efficiency and NO formation.     

Integration of solid oxide fuel cell and palladium membrane reactor: Technical and economic analysis

This paper presents a technical and economic analysis of a solid oxide fuel cell system equipped with a palladium membrane reactor (PMR–SOFC) with the aim of determining the benefits of such an integrated unit over the conventional reformer module (CON-SOFC). The performance of both SOFC systems under the conditions for energetically self-sustaining operation (Q_NET = 0) was achieved by varying the fuel utilization for each operating voltage. Two types of fuels, i.e., methane and desulphurized biogas, are considered. The simulation results show that the maximum power density of the CON-SOFC fuelled by methane (0.423 W/cm²) is higher than that of the CON-SOFC fuelled by biogas (0.399 W/cm²) due to the presence of CO₂ in biogas. For the PMR–SOFC, it is found that the operation at a higher permeation pressure offers higher power density because lower fuel utilization is required when operating the SOFC at the energy self-sustained condition. When the membrane reactor is operated at the permeation pressure of 1 bar, the methane-fuelled and biogas-fuelled PMR–SOFCs can achieve the maximum power density of 0.4398 and 0.4213 W/cm², respectively. Although the PMR–SOFC can offer higher power density, compared with the CON-SOFC, the capital costs of supporting units, i.e., palladium membrane reactor, high-pressure compressor, and vacuum pump, for PMR–SOFC need to be taken into account. The economic analysis shows that the PMR–SOFC is not a good choice from an economic viewpoint because of the requirement of a large high-pressure compressor for feeding gas to the membrane reactor.     

Modeling a counter-current moving bed for fuel and steam reactors in the TRCL process

A mathematical model for the moving bed is developed to simulate the fuel and steam reactor in the TRCL (Three-Reactor Chemical-Looping) process. An ideal plug flow of the solid and gas is assumed in modeling the fuel and steam reactor in the TRCL process. The model considered the mass, heat balances, equilibrium, physical properties, such as the heat capacity and viscosity, and kinetics. From this model, the temperature, gas conversion and solid conversion profiles can be predicted for fuel and steam reactors. The oxygen carrier inventory (the mass of the oxygen carrier) in the fuel and steam reactor was calculated with variation of the solid inlet temperature, solid conversion, Fe₂O₃ content and steam feed rate. The temperature of the oxygen carrier to the reactor was the most sensitive parameter for determining the required inventory of the oxygen carrier. An increase in the solid inlet temperature was predicted to decrease the required inventory of the oxygen carrier. In the steam reactor, a solid inlet temperature increase over 1150 K will cause an increase in the inventory of the oxygen carrier due to the equilibrium conversion. An excessively low or high active material content will require a larger inventory of the oxygen carrier in the fuel reactor. In this study, approximately 20 wt.% of the Fe₂O₃ content was suitable for reducing the inventory of the oxygen carrier while achieving a solid conversion of 0.9 in the fuel reactor.     

Parametric study of recuperative VOC oxidation reactor with porous media

Numerical study of volatile organic compounds (VOC) oxidation reactor consisting of two coaxial tubes, filled with inert porous media is performed. Influence of incoming gas flux, adiabatic temperature of gas combustion, reaction rate constant, diameter of porous body particles, reactor size and heat losses on maximal temperature of reactor, recuperation efficiency, combustion front position is investigated. It is shown that maximum temperature and recuperation efficiency of reactor has extremum in the field of incoming gas flow rate and porous body particle size parameters (for simulated configuration of reactor maximum corresponds to U_G ∼ 2 m/s and d₀ ∼ 6 mm). Numerical simulation shows non-monotonous character of maximal temperature and recuperation efficiency dependence from side heat losses of reactor. The obtained results can be used for construction optimization of practical VOC oxidation reactors.     

High-temperature carbonate/MgO composite materials as thermal storage media for double-walled solar reformer tubes

The composite materials of molten alkali-carbonate/MgO-ceramics are examined as thermal storage media in a tubular reformer using a double-walled reactor tube of a laboratory scale. The concept of a double-walled reformer tube is proposed as a solar tubular reformer and involves packing a molten salt/ceramic composite material in the annular region between the internal catalyst tube and the exterior solar absorber wall. The composite materials of Na₂CO₃, K₂CO₃, and Li₂CO₃ with magnesia are tested as thermal storage media. The reforming performances of the composite materials are tested in the cooling mode of the double-walled reactor tube. The experimental result obtained under feed gas mixture of CH₄/CO₂ = 1:3 at 1 atm shows that the use of 80 wt%Na₂CO₃/20 wt%MgO composite material successfully delayed the cooling time of the catalyst bed by 5–19 min in comparison to the case without a composite material. In addition, the Li₂CO₃/MgO and Na₂CO₃/MgO composite materials relatively revealed good performances: they prolonged the cooling time by over 10 min in the gas hourly space velocity (GHSV) range of 5000–12,500 h^?1. The application of the reactor tubes to solar tubular reformers is expected to realize stable operation of the solar reforming process under fluctuating insolation during cloud passage.     

Modified APEX reactor as a fusion breeder

An advanced fusion reactor project, called APEX, with improved effectiveness has been developed using a protective flowing liquid wall for tritium breeding and energy transfer. In the modified APEX concept, the flowing molten salt wall is composed of Flibe as the main constituent with increased mole fractions of heavy metal salt (ThF₄ or UF₄) for both fissile and fusile breeding purposes and to increase the energy multiplication. Neutron transport calculations are conducted with the help of the SCALE4.3 SYSTEM by solving the Boltzmann transport equation with the code XSDRNPM. By preserving a self sufficient tritium breeding ratio (TBR > 1.05) for a mole fraction up to 6% of ThF₄ or 12% of UF₄, the modified APEX reactor can produce up to ∼2800 kg of ²³³U/year or ∼4950 kg of ²³⁹Pu/year, assuming the same baseline fusion power production of 4000 MW_th, as in the original APEX concept. With 6% ThF₄ or 12% UF₄ in the coolant, the total energy output will increase to 5560 MW_th or 8440 MW_th, respectively. For a plant operation period of 30 full power years, the atomic displacement and helium production rates remain well below the presumable limits. The additional benefits of fissionable metal salt in the flowing liquid in a fusion reactor can be summarized as breeding of high quality fissile fuel for external reactors and increase of total plant power output.     

Application of a detailed mathematical model to the gasifier unit of the dual fluidized bed gasification plant

A one-dimensional steady state model for biomass-steam gasification has been developed. The reactor is a bubbling fluidized bed. With respect to hydrodynamics the model distinguishes two zones namely: dense zone and freeboard zone. The gasification process is modelled in three steps: drying, devolatilization and gasification of biomass char. The model assumes that solids are well mixed while the gases are in plug flow regime. Mass and energy balance is solved globally across the entire gasifier. The gas composition and temperatures predicted by the model for wood chips as fuel agree well with values measured at an 8 MW (fuel power) commercial plant.     

Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal

Chemical looping combustion (CLC) is an innovative combustion technology with inherent separation of CO₂ without energy penalty. When solid fuel is applied in CLC, the gasification of solid fuel is the rate-limiting process for in situ gasification of coal and reduction of oxygen carrier. The K₂CO₃-decorated iron ore after calcinations was used as oxygen carrier in CLC of anthracite coal, and potassium ferrites were formed during the calcinations process. The experiments were performed in a laboratory fluidized bed reactor with steam as a gasification medium. Effects of reaction temperature, K₂CO₃ loading in iron ore and cycle on the gas concentration, carbon conversion, gasification rate and yields of carbonaceous gases were investigated. The carbon gasification was accelerated during the fast reaction stage between 860 °C and 920 °C, and the water–gas shift reaction was significantly enhanced in a wider temperature range of 800 °C to 920 °C. With the K₂CO₃ loading in iron ore increasing from 0% to 20% at 920 °C, the carbon conversion was accelerated in the fast reaction stage, and the fast reaction stage became shorter. The yield of CO₂ reached a maximum of 94.4% and the yield of CO reached a minimum of 3.4% when use the iron ore loaded with 6% K₂CO₃. SEM analysis showed that the K₂CO₃-decorating in iron ore would cause a sintering on the particle surface of oxygen carrier, and the K₂CO₃ loading in iron ore should not be too high. Cycle experiments indicate that the K₂CO₃-decorated iron ore has a relative stable catalytic effect in the CLC process.     

Combustion characteristics of cotton stalk in FBC

The present work reports studies on the mixing and combustion characteristics of cotton stalk with 10–100 mm in length in FBC. Experiments on a cold model show that cotton stalk cannot fluidize, and adding bed material can improve the fluidization condition. Cotton stalk can mix well with 0.6–1 mm alumina at fluidization number N = 3–7. However, when the fluidization number is higher more than 7, the mixing bed will exist a little segregation comparing with N = 3–7. Thermogravimetric experiments show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick. Fluidized-bed combustion of cotton stalk was tested in a 0.2 MW_th test facility. According to the temperature distribution along the bed height, when the primary and secondary air is adapted cotton stalk can be burned stably in the fluidized bed. During pure cotton stalk combustion tests, silica sand and alumina are used as bed material to compare their agglomeration characteristics. SEM/EDX analysis on agglomerate samples after combustion about 38 h suggests that the high alkali metals content causes the formation of the coating around silica sand particles. The coating consists of compounds with low-melting temperature results in agglomeration of silica sand particles. By contrast, alumina is difficult to react with alkali metals from biomass ash, and the agglomeration of alumina was not found at 910 °C. It is found that alumina is more favorable than silica sand particle for use in a fluidized bed in cotton stalk combustion.     

Fast starting fuel processor for automotive fuel cell systems

A fuel processor was constructed which incorporated two burners with direct steam generation by water injection into the burner exhaust. These burners with direct water vaporization enabled rapid fuel processor start-up for automotive fuel cell systems. The fuel processor consisted of a conventional chain of reactors: auto-thermal reformer (ATR), water gas shift (WGS) reactor and preferential oxidation (PrOx) reactor. The criticality of steam to the fuel reforming process was illustrated. By utilizing direct vaporization of water, and hydrogen for catalyst light-off, excellent start performance was obtained with a start time of 20 s to 30% power and 140 s to full power.     

Magnesium oxide/water chemical heat pump to enhance energy utilization of a cogeneration system

A chemical heat pump using a magnesium oxide/water reaction system is expected to be applicable to cogeneration systems using gas engine, diesel engine, and fuel cells. The operability of the heat pump was examined experimentally under hydration operation pressures between 30 and 203 kPa. In the experiment, a reactant having high durability for repetitive operation was packed in a cylindrical reactor. The cycle of operation was repeated under various thermally driven operation conditions. The forward and reverse reactions were studied by measuring the reactor bed temperature distribution and the reacted fraction changes. The reactor bed stored heat at around 300–400 °C by the dehydration reaction and released heat at around 100–200 °C by the hydration reaction under the heat amplification mode operation. The practical possibility of the reactor bed was discussed based on the experimental results. The heat pump is expected to be applicable for load leveling in a cogeneration system by chemically storing surplus heat during low heat demand and supplying heat during peak demand. It was shown that the chemical heat pump would be able to improve the efficiency of energy utilization in cogeneration systems while also helping to reduce energy consumption and global carbon dioxide emissions.     

Implementation of an UASB anaerobic digester at bagasse-based pulp and paper industry

Upflow anaerobic sludge blanket (UASB) reactor was installed to replace the conventional anaerobic lagoon treating bagasse wash wastewater from agro-based pulp and paper mill, to generate bio-energy and to reduce greenhouse gas emissions. The plant was designed to treat 12 ML d⁻¹ of wastewater having two 5 ML capacity reactors, 5.75 kg COD m⁻³ d⁻¹ organic loading rate and 20 h hydraulic retention time. In the plant 80–85% COD reduction was achieved with biogas production factor of 520 L kg⁻¹ COD reduced. In 11 months 4.4 million m³ of biogas was generated from bagasse wash wastewater utilizing UASB process. Utilization of the biogas in the Lime Kiln saved 2.14 ML of furnace oil in 9 months. Besides significant economic benefits, furnace oil saving reduced 6.4 Gg CO₂ emission from fossil fuel and conversion of the anaerobic lagoon into anaerobic reactor reduced 2.1 Gg methane emission which is equal to 43.8 Gg of CO₂.