Chemical-looping combustion using syngas as fuel

Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, avoiding direct contact between air and fuel. Thus, CO₂ and H₂O are inherently separated from the rest of the flue gases and the carbon dioxide can be obtained in a pure form without the use of an energy intensive air separation unit. The paper presents results from a 3-year project devoted to developing the CLC technology for use with syngas from coal gasification. The project has focused on: (i) the development of oxygen carrier particles, (ii) establishing a reactor design and feasible operating conditions and (iii) construction and operation of a continuously working hot reactor. Approximately, 300 different oxygen carriers based on oxides of the metals Ni, Fe, Mn and Cu were investigated with respect to parameters, which are important in a CLC system, and from these investigations, several particles were found to possess suitable qualities as oxygen carriers. Several cold-model prototypes of CLC based on interconnected fluidized bed reactors were tested, and from these tests a hot prototype CLC reactor system was constructed and operated successfully using three carriers based on Ni, Fe and Mn developed within the project. The particles were used for 30–70 h with combustion, but were circulated under hot conditions for 60–150 h.     

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of sulphur

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of sulphur compounds, such as H₂S and COS. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH₄ and H₂S as fuel. The influence of H₂S concentration on the gas product distribution and combustion efficiency, sulphur splitting between the fuel reactor (FR) and the air reactor (AR), oxygen carrier deactivation and material agglomeration was investigated in a continuous CLC plant (500 W_th). The oxygen carrier to fuel ratio, ?, was the main operating parameter affecting the CLC system. Complete fuel combustion were reached at 1073 K working at ? values ≥1.5. The presence of H₂S did not produce a decrease in the combustion efficiency even when working with a fuel containing 1300 vppm H₂S. At these conditions, the great majority of the sulphur fed into the system was released in the gas outlet of the FR as SO₂, affecting to the quality of the CO₂ produced. Formation of copper sulphide, Cu₂S, and the subsequent reactivity loss was only detected working at low values of ? ≤ 1.5, although this fact did not produce any agglomeration problem in the fluidized beds. In addition, the oxygen carrier was fully regenerated in a H₂S-free environment. It can be concluded that Cu-based oxygen carriers are adequate materials to be used in a CLC process using fuels containing H₂S although quality of the CO₂ produced is affected.     

Numerical simulation of chemical looping combustion process with CaSO4 oxygen carrier

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. The technique involves the use of an oxygen carrier which transfers oxygen from combustion air to the fuel, and hence a direct contact between air and fuel is avoided. A chemical-looping combustion system consists of a fuel reactor and an air reactor. A metal oxide is used as oxygen carrier that circulates between the two reactors. The air reactor is a high velocity fluidized bed where the oxygen carrier particles are transported together with the air stream to the top of the air reactor, where they are then transferred to the fuel reactor using a cyclone. The fuel reactor is a bubbling fluidized bed reactor where oxygen carrier particles react with hydrocarbon fuel and get reduced. The reduced oxygen carrier particles are transported back to the air reactor where they react with oxygen in the air and are oxidized back to metal oxide. The exhaust from the fuel reactor mainly consists of CO₂ and water vapor. After condensation of the water in the exit gas from the fuel reactor, the remaining CO₂ gas is compressed and cooled to yield liquid CO₂, which can be disposed of in various ways.With the improvement of numerical methods and more advanced hardware technology, the time needed to run CFD (Computational fluid dynamics) codes is decreasing. Hence multiphase CFD-based models for dealing with complex gas-solid hydrodynamics and chemical reactions are becoming more accessible. Until now there were a few literatures about mathematical modeling of chemical-looping combustion using CFD approach. In this work, the reaction kinetics model of the fuel reactor (CaSO₄ + H₂) was developed by means of the commercial code FLUENT. The bubble formation and the relation between bubble formation and molar fraction of products in gas phase were well captured by CFD simulation. Computational results from the simulation also showed low fuel conversion rate. The conversion of H₂ was about 34% partially due to fast, large bubbles rising through the reactor, low bed temperature and large particles diameter.     

Effect of gas composition in Chemical-Looping Combustion with copper-based oxygen carriers: Fate of light hydrocarbons

Chemical-Looping Combustion (CLC) is an emerging technology for CO₂ capture because separation of this gas from the other flue gas components is inherent to the process and thus no energy is expended for the separation. Natural or refinery gas can be used as gaseous fuels and they may contain different amounts of light hydrocarbons. This paper presents the combustion results obtained with a Cu-based oxygen carrier using mixtures of CH₄ and light hydrocarbons (LHC) (C₂H₆ and C₃H₈) as fuel. The effect on combustion efficiency of the fuel reactor temperature, solid circulation flow rate and gas composition was studied in a continuous CLC plant (500 W_th). Full combustions were reached at 1073 and 1153 K working at oxygen to fuel ratios, ? higher than 1.5 and 1.2 respectively. Unburnt hydrocarbons were never detected at any experimental conditions at the fuel reactor outlet. Carbon formation can be avoided working at 1153 K or at ? values higher than 1.5 at 1073 K. After 30 h of continuous operation, the oxygen carrier exhibited an adequate behavior regarding attrition and agglomeration. It can be concluded that no special measures should be taken in a CLC process with Cu-based OC with respect to the presence of LHC in the fuel gas.     

Operating experience with chemical looping combustion in a 120 kW dual circulating fluidized bed (DCFB) unit

This study presents first operating experience with a 120 kW chemical looping pilot rig. The dual circulating fluidized bed reactor system and its auxiliary units are discussed. Two different oxygen carriers, i.e. ilmenite, which is a natural iron titanium ore, and a designed Ni-based particle, are tested in the CLC unit. The pilot rig is fueled with H₂, CO and CH₄ respectively at a fuel power of 65–145 kW. High solids circulation, very low solids residence time and low solids inventory are observed during operation. Owing to the scalability of the design concept, these characteristics should be quite similar to those of commercial CLC power plants. Ilmenite shows a high potential for the combustion of H₂-rich gases (e.g. from coal gasification with steam). The H₂ conversion is quite high but there is still a high potential for further improvement. The Ni-based oxygen carrier achieves the thermodynamic maximum H₂ and CO conversion and also very high CH₄ conversion. A variation of the air/fuel ratio and the reaction temperature indicates that the Ni/NiO ratio of the particle has an influence on the performance of the chemical looping combustor. Generally, low solids conversion in air and fuel reactors is observed in almost any conditions. Despite a very low H₂O/CH₄ molar ratio, no carbon formation is observed.     

Chemical-looping with oxygen uncoupling for combustion of solid fuels

Chemical-looping with oxygen uncoupling (CLOU) is a novel method to burn solid fuels in gas-phase oxygen without the need for an energy intensive air separation unit. The carbon dioxide from the combustion is inherently separated from the rest of the flue gases. CLOU is based on chemical-looping combustion (CLC) and involves three steps in two reactors, one air reactor where a metal oxide captures oxygen from the combustion air (step 1), and a fuel reactor where the metal oxide releases oxygen in the gas-phase (step 2) and where this gas-phase oxygen reacts with a fuel (step 3). In other proposed schemes for using chemical-looping combustion of solid fuels there is a need for an intermediate gasification step of the char with steam or carbon dioxide to form reactive gaseous compounds which then react with the oxygen carrier particles. The gasification of char with H₂O and CO₂ is inherently slow, resulting in slow overall rates of reaction. This slow gasification is avoided in the proposed process, since there is no intermediate gasification step needed and the char reacts directly with gas-phase oxygen. The process demands an oxygen carrier which has the ability to react with the oxygen in the combustion air in the air reactor but which decomposes to a reduced metal oxide and gas-phase oxygen in the fuel reactor. Three metal oxide systems with suitable thermodynamic properties have been identified, and a thermal analysis has shown that Mn₂O₃/Mn₃O₄ and CuO/Cu₂O have suitable thermodynamic properties, although Co₃O₄/CoO may also be a possibility. However, the latter system has the disadvantage of an overall endothermic reaction in the fuel reactor. Results from batch laboratory fluidized bed tests with CuO and a gaseous and solid fuel are presented. The reaction rate of petroleum coke is approximately a factor 50 higher using CLOU in comparison to the reaction rate of the same fuel with an iron-based oxygen carrier in normal CLC.     

Multi-stage chemical looping combustion (CLC) for combined cycles with CO2 capture

This paper presents application of the chemical looping combustion (CLC) method in natural gas-fired combined cycles for power generation with CO₂ capture. A CLC combined cycle consisting of single CLC-reactor system, an air turbine, a CO₂-turbine and a steam cycle has been designated as the base-case cycle. The base-case cycle can achieve net plant efficiency of about 52% at an oxidation temperature of 1200 °C. In order to achieve a reasonable efficiency at lower oxidation temperatures, reheat is introduced into the air turbine by employing multi CLC-reactors. The results show that the single reheat CLC-combined cycle can achieve net plant efficiency of above 51% at oxidation temperature of 1000 °C and above 53% at the oxidation temperature of 1200 °C including CO₂ compression to 110 bar. The double reheat cycle results in marginal efficiency improvement as compared to the single reheat cycle. The CLC-cycles are also compared with a conventional combined cycle with and without post-combustion capture in amine solution. All the CLC-cycles show higher net plant efficiencies with close to 100% CO₂ capture as compared to a conventional combined cycle with post-combustion capture, which is very promising.     

Ash and deposit formation from oxy-coal combustion in a 100 kW test furnace

Ash deposition is still an unresolved problem when retrofitting existing air-fired coal power plants to oxy-fuel combustion. Experimental data are quite necessary for mechanism validation and model development. This work was designed to obtain laboratory combustor data on ash and deposits from oxy-coal combustion, and to explore the effects of oxy-firing on their formation. Two bituminous coals (Utah coal and Illinois coal) and one sub-bituminous coal (PRB coal) were burned on a down-fired combustor under both oxy- and air-firing. Two oxy-fired cases, i.e., 27 vol% O₂/73 vol% CO₂ and 32 vol% O₂/68 vol% CO₂, were selected to match the radiation flux and the adiabatic flame temperature of air combustion, respectively. Once-through CO₂ was used to simulate fully cleaned recycled flue gas. The flue gas excess oxygen was fixed at 3 vol%. For each case, both size-segregated fly ash and bulk fly ash samples were obtained. Simultaneously, ash deposits were collected on an especially designed un-cooled deposition probe. Ash particle size distributions and chemical composition of all samples were characterized. Data showed that oxy-firing had insignificant impacts on the tri-modal ash particle size distributions and composition size distributions in the size range studied. Bulk ash compositions also showed no significant differences between oxy- and air-firing, except for slightly higher sulfur contents in some oxy-fired ashes. The oxy-fired deposits were thicker than those from air-firing, suggesting enhanced ash deposition rates in oxy-firing. Oxy-firing also had apparent impacts on the deposit composition, especially for those components (e.g., CaO, Fe₂O₃, SO₃, etc.) that could contribute significantly to ash deposition. Based on these results, aerodynamic changes in gas flow and changes in combustion temperature seemed more important than chemical changes of ash particles in determining deposit behavior during oxy-coal combustion.     

Waste products from the steel industry with NiO as additive as oxygen carrier for chemical-looping combustion

Fe₂O₃-containing waste materials from the steel industry are proposed as oxygen carrier for chemical-looping combustion. Three such materials, red iron oxide, brown iron oxide and iron oxide scales, have been examined by oxidation and reduction experiments in a batch fluidized-bed reactor at temperatures between 800 and 950 °C. NiO-based particles have been used as additive, in order to examine if it is possible to utilize the catalytic properties of metallic Ni to facilitate decomposition of hydrocarbons into more reactive combustion intermediates such as CO and H₂. The experiments indicated modest reactivity between the waste materials and CH₄, which was used as reducing gas. Adding small amounts of NiO-based particles to the sample increased the yield of CO₂ in a standard experiment, typically by a factor of 1.5–3.5. The fraction of unconverted fuel typically was reduced by 70–90%. The conversion of CH₄ to CO₂ was 94% at best, corresponding to a combustion efficiency of 96%. This was achieved using a bed mass corresponding to 57 kg oxygen carrier per MW fuel, of which only 5 wt% was NiO-based synthetic particles. The different materials fared differently well during the experiments. Red iron oxide was fairly stable, while brown iron oxide was soft and subject to considerable erosion. Iron oxide scales experienced increased reactivity and porosity as function of the numbers of reduction cycles.     

Assessment of the different parameters affecting the CO2 purity from coal fired oxyfuel process

The oxyfuel process is one of the most promising options to capture CO₂ from coal fired power plants. The combustion takes place in an atmosphere of almost pure oxygen, delivered from an air separation unit (ASU), and recirculated flue gas. This provides a flue gas containing 80–90 vol% CO₂ on a dry basis. Impurities are caused by the purity of the oxygen from the ASU, the combustion process and air ingress. Via liquefaction a CO₂ stream with purity in the range from 85 to 99.5 vol% can be separated and stored geologically. Impurities like O₂, NO_X, SO_X, and CO may negatively influence the transport infrastructure or the geological storage site by causing geochemical reactions. Therefore the maximum acceptable concentrations of the impurities in the separated CO₂ stream must be defined regarding the requirements from transportation and storage. The main objective of the research project COORAL therefore is to define the required CO₂ purity for capture and storage.     

Study of Hg and SO3 behavior in flue gas of oxy-fuel combustion system

Oxy-fuel combustion systems have been under development to reduce CO₂ emissions from coal-fired power plants. In oxy-fuel combustion system, Hg in the flue gas causes corrosion in CO₂ purification and compression units. Also, SO₃ in the flue gas corrodes the equipment and ducts of oxy-fuel combustion system. Therefore, Hg and SO₃ need to be removed.Babcock-Hitachi conducted tests using a 1.5 MWth Combustion & Air Quality Control System (AQCS) test facility which consists of oxygen supply unit, furnace, Selective Catalytic Reduction (SCR) catalyst, Clean Energy Recuperator (CER), Dry Electrostatic Precipitator (DESP), flue gas recirculation system, Wet Flue Gas Desulfurization (WFGD), and CO₂ Compression and Purification Unit (CPU). In both cases of air and oxy-fuel combustion, the Hg removal across the DESP could be improved, and SO₃ concentration at the DESP outlet could be reduced to less than 1 ppm by installing a CER upstream of the DESP and reducing the gas temperature at the DESP inlet. Hg was not dissolved in the drain recovered from CO₂ compressor, and may be adsorbed at an inner part of CO₂ compressor. This indicated that Hg needs to be removed at a location upstream of the CO₂ compressor to prevent corrosion of the compressor.     

Kinetics of sulfur dioxide- and oxygen-induced degradation of aqueous monoethanolamine solution during CO2 absorption from power plant flue gas streams

Studies of the kinetics of sulfur dioxide (SO₂)- and oxygen (O₂)-induced degradation of aqueous monoethanolamine (MEA) during the absorption of carbon dioxide (CO₂) from flue gases derived from coal- or natural gas-fired power plants were conducted as a function of temperature and the liquid phase concentrations of MEA, O₂, SO₂ and CO₂. The kinetic data were based on the initial rate which shows the propensity for amine degradation and obtained under a range of conditions typical of the CO₂ absorption process (3–7 kmol/m³ MEA, 6% O₂, 0–196 ppm SO₂, 0–0.55 CO₂ loading, and 328–393 K temperature). The results showed that an increase in temperature and the concentrations of MEA, O₂ and SO₂ resulted in a higher MEA degradation rate. An increase in CO₂ concentration gave the opposite effect. A semi-empirical model based on the initial rate, ?r_MEA = {6.74 × 10⁹ e^?(29,403/RT)[MEA]^0.02([O]^2.91 + [SO₂]^3.52)}/{1 + 1.18[CO₂]^0.18} was developed to fit the experimental data. With the higher order of reaction, SO₂ has a higher propensity to cause MEA to degrade than O₂. Unlike previous models, this model shows an improvement in that any of the parameters (i.e. O₂, SO₂, and CO₂) can be removed without affecting the usability of the model.     

Experimental validation of in situ CO2 capture with CaO during the low temperature combustion of biomass in a fluidized bed reactor

A novel concept for capturing CO₂ from biomass combustion using CaO as an active solid sorbent of CO₂ is discussed and experimentally tested. According to the CaO/CaCO₃ equilibrium, if a fuel could be burned at a sufficiently low temperature (below 700 °C) it would be possible to capture CO₂ “in situ” with the CaO particles at atmospheric pressure. A subsequent step involving the regeneration of CaCO₃ in a calciner operating at typical conditions of oxyfired-circulating fluidized combustion would deliver the CO₂ ready for purification, compression and permanent geological storage. Several series of experiments to prove this concept have been conducted in a 30 kW interconnected fluidized bed test facility at INCAR-CSIC, made up of two interconnected circulating fluidized bed reactors, one acting as biomass combustor-carbonator and the other as air-fired calciner (which is considered to yield similar sorbent properties than those of an oxyfired calciner). CO₂ capture efficiencies in dynamic tests in the combustor-carbonator reactor were measured over a wide range of operating conditions, including different superficial gas velocities, solids circulation rates, excess air above stoichiometric, and biomass type (olive pits, saw dust and pellets). Biomass combustion in air is effective at temperatures even below the 700 °C, necessary for the effective capture of CO₂ by carbonation of CaO. Overall CO₂ capture efficiencies in the combustor-carbonator higher than 70% can be achieved with sufficiently high solids circulation rates of CaO and solids inventories. The application of a simple reactor model for the combined combustion and CO₂ capture reactions allows an efficiency factor to be obtained from the dynamic experimental test that could be valuable for scaling up purposes.     

Pulverized coal stream ignition delay under conventional and oxy-fuel combustion conditions

The coal stream ignition process is critical to the performance of modern pulverized coal burners, particularly when operating under novel conditions such as experienced in oxy-fuel combustion. However, experimental studies of coal stream ignition are lacking, and recent modeling efforts have had to rely on comparisons with a single set of experiments in vitiated air. To begin to address this shortfall, we have conducted experiments on the ignition properties of two U.S. and two Chinese coals in a laminar entrained flow reactor. Most of the measurements focused on varying the coal feed rate for furnace temperatures of 1230–1320 K and for 12–20 vol.% O₂ in nitrogen. The influence of coal feed rate on ignition with a carbon dioxide diluent was also measured for 20 vol.% O₂ at 1280 K. A second set of measurements was performed for ignition of a fixed coal feed rate in N₂ and CO₂ environments at identical furnace temperatures of 1200 K, 1340 K, and 1670 K. A scientific CCD camera equipped with a 431 nm imaging filter was used to interrogate the ignition process. Under most conditions, the ignition delay decreased with increasing coal feed rate until a minimum was reached at a feed rate corresponding to a particle number density of approximately 4 × 10⁹ m^?3 in the coal feed pipe. This ignition minimum corresponds to a cold flow group number, G, of ～0.3. At higher coal feed rates the ignition delay increased. The ignition delay time was shown to be very sensitive to (a) the temperature of the hot coflow into which the coal stream is introduced, and (b) the coal particle size. The three high volatile bituminous coals showed nearly identical ignition delay as a function of coal feed rate, whereas the subbituminous coal showed slightly greater apparent ignition delay. Bath gas CO₂ content was found to have a minor impact on ignition delay.     

Conceptual design of a three fluidised beds combustion system capturing CO2 with CaO

In this work, the Aspen Hysys conceptual design of a new process for energy generation at large scale with implicit CO₂ capture is presented. This process makes use of the CaO capability for CO₂ capture at high temperature and the possibility of regenerating this sorbent working in interconnected fluidised bed reactors operating at different temperatures. The proposed process has the advantage of producing power with minimum CO₂ emissions and very low energy penalties compared with similar air-based combustion power plants. In this system, five main parts can be distinguished: the combustor where coal is burnt with air, the calciner where the fresh and the recycled CaCO₃ is calcined, the carbonator where the CO₂ produced in the combustor is captured, the supercritical steam cycle and the CO₂ compression system. In this arrangement, the three fluidised bed reactors are interconnected in such a way that it is possible to perform the CaCO₃ calcination at a temperature of 950 °C with the energy transported by a hot solid stream produced in the circulating fluidised bed combustor operating at 1030 °C. The stream rich in CaO produced in the calciner is split into three parts. One of them is transported to the carbonator operating at 650 °C where most of the CO₂ in the flue gas produced in the combustor is captured. The second one is sent to the combustor, where it is heated up and used as energy carrier. The third solid stream that leaves the calciner is a purge in order to maintain the capture system activity and to avoid inert material accumulation. Because of the high temperatures involved in all the system, it is possible to recover most of the energy in the fuel and to produce power in a supercritical steam cycle. A case study is presented and it is demonstrated that under these operating conditions, 90% CO₂ capture efficiency can be achieved with no energy penalty further than the one originated in the CO₂ compression system.     

Parametric investigation of the calcium looping process for CO2 capture in a 10 kWth dual fluidized bed

Calcium looping (CaL) is a promising post-combustion CO₂ capture technology which is carried out in a dual fluidized bed (DFB) system with continuous looping of CaO, the CO₂ carrier, between two beds. The system consists of a carbonator, where flue gas CO₂ is adsorbed by CaO and a regenerator, where captured CO₂ is released. The CO₂-rich regenerator flue gas can be sequestered after gas processing and compression. A parametric study was conducted on the 10 kW_th DFB facility at the University of Stuttgart, which consists of a bubbling fluidized bed carbonator and a riser regenerator. The effect of the following parameters on CO₂ capture efficiency was investigated: carbonator space time, carbonator temperature and calcium looping ratio. The active space time in the carbonator, which is a function of the space time and the calcium looping ratio, was found to strongly correlate with the CO₂ capture efficiency. BET and BJH techniques provided surface area and pore volume distribution data, respectively, for collected sorbent samples. The rate of sorbent attrition was found to be 2 wt.%/h which is below the expected sorbent make-up rate required to maintain sufficient sorbent activity. Steady-state CO₂ capture efficiencies greater than 90% were achieved for different combinations of operational parameters. Moreover, the experimental results obtained were briefly compared with results derived from reactor modeling studies. Finally, the implications of the experimental results with respect to commercialization of the CaL process have been assessed.     

Solid fuels in chemical-looping combustion

The feasibility of using a number of different solid fuels in chemical-looping combustion (CLC) has been investigated. A laboratory fluidized bed reactor system for solid fuel, simulating a chemical-looping combustion system by exposing the sample to alternating reducing and oxidizing conditions, was used. In each reducing phase 0.2 g of fuel in the size range 180–250 μm was added to the reactor containing 40 g oxygen carrier of size 125–180 μm. Two different oxygen carriers were tested, a synthetic particle of 60% active material of Fe₂O₃ and 40% MgAl₂O₄ and a particle consisting of the natural mineral ilmenite. Effect of steam content in the fluidizing gas of the reactor was investigated as well as effect of temperature. A number of experiments were also made to investigate the rate of conversion of the different fuels in a CLC system. A high dependency on steam content in the fluidizing gas as well as temperature was shown. The fraction of volatiles in the fuel was also found to be important. Furthermore the presence of an oxygen carrier was shown to enhance the conversion rate of the intermediate gasification reaction. At 950 °C and with 50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of 0.125–0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion was reached within 2–10 min. In almost all cases the synthetic Fe₂O₃ particle with 40% MgAl₂O₄ and the mineral ilmenite showed similar results with the different fuels.     

160 h of chemical-looping combustion in a 10 kW reactor system with a NiO-based oxygen carrier

Chemical-looping combustion, CLC, is a technology with inherent separation of the greenhouse gas CO₂. The technique uses an oxygen carrier made up of particulate metal oxide to transfer oxygen from combustion air to fuel. In this work, an oxygen carrier consisting of 60% NiO and 40% NiAl₂O₄ was used in a 10 kW CLC reactor system for 160 h of operation with fuel. The first 3 h of fuel operation excepted, the test series was accomplished with the same batch of oxygen carrier particles. The fuel used in the experiments was natural gas, and a fuel conversion to CO₂ of approximately 99% was accomplished. Combustion conditions were very stable during the test period, except for the operation at sub-stoichiometric conditions. It was shown that the methane fraction in the fuel reactor exit gas was dependent upon the rate of solids circulation, with higher circulation leading to more unconverted methane. The carbon monoxide fraction was found to follow the thermodynamical equilibrium for all investigated fuel reactor temperatures, 660–950 °C. Thermal analysis of the fuel reactor at stable conditions enabled calculation of the particle circulation which was found to be approximately 4 kg/s, MW. The loss of fines, i.e. the amount of elutriated oxygen carrier particles with diameter <45 μm, decreased during the entire test period. After 160 h of operation the fractional loss of fines was 0.00022 h⁻¹, corresponding to a particle life time of 4500 h.     

Carbon dioxide storage capacity in uneconomic coal beds in Alberta,Canada: Methodology,potential and site identification

Methodology is presented for a first-order regional-scale estimation of CO₂ storage capacity in coals under sub-critical conditions, which is subsequently applied to Cretaceous-Tertiary coal beds in Alberta, Canada. Regions suitable for CO₂ storage have been defined on the basis of groundwater depth and CO₂ phase at in situ conditions. The theoretical CO₂ storage capacity was estimated on the basis of CO₂ adsorption isotherms measured on coal samples, and it varies between ∼20 kt CO₂/km² and 1260 kt CO₂/km², for a total of approximately 20 Gt CO₂. This represents the theoretical storage capacity limit that would be attained if there would be no other gases present in the coals or they would be 100% replaced by CO₂, and if all the coals will be accessed by CO₂. A recovery factor of less than 100% and a completion factor less than 50% reduce the theoretical storage capacity to an effective storage capacity of only 6.4 Gt CO₂. Not all the effective CO₂ storage capacity will be utilized because it is uneconomic to build the necessary infrastructure for areas with low storage capacity per unit surface. Assuming that the economic threshold to develop the necessary infrastructure is 200 kt CO₂/km², then the CO₂ storage capacity in coal beds in Alberta is greatly reduced further to a practical capacity of only ∼800 Mt CO₂.     

CO2 recycling by reaction with renewably-generated hydrogen

A laboratory-scale reactor system was built and operated to demonstrate the feasibility of catalytically reacting carbon dioxide (CO₂) with renewably-generated hydrogen (H₂) to produce methane (CH₄) according to the Sabatier reaction: CO₂ + 4H₂ → CH₄ + 2H₂O. A cylindrical reaction vessel packed with a commercial methanation catalyst (Haldor Topsøe PK-7R) was used. Renewable H₂ produced by electrolysis of water (from solar- and wind-generated electricity) was fed into the reactor along with a custom blend of 2% CO₂ in N₂, meant to represent a synthetic exhaust mixture. Reaction conditions of temperature, flow rates, and gas mixing ratios were varied to determine optimum performance. The extent of reaction was monitored by real-time measurement of CO₂ and CH₄. Maximum conversion of CO₂ occurred at 300–350 °C. Approximately 60% conversion of CO₂ was realized at a space velocity of about 10,000 h^?1 with a molar ratio of H₂/CO₂ of 4/1. Somewhat higher total CO₂ conversion was possible by increasing the H₂/CO₂ ratio, but the most efficient use of available H₂ occurs at a lower H₂/CO₂ ratio.