Tobias Mattisson

A fluidized bed combustion process with inherent CO2 separation; application of chemical looping combustion

For combustion with CO2 capture, chemical-looping combustion has the advantage that no energy is lost for the separation of CO2. In chemical-looping combustion oxygen is transferred from the combustion air to the gaseous fuel by means of an oxygen carrier. The fuel and the combustion air are never mixed, and the gases from the oxidation of the fuel, CO2 and H2O, leave the system as a separate stream. The H2O can easily be removed by condensation and pure CO2 is obtained without any loss of energy for separation. This makes chemical-looping combustion a most interesting alternative to other CO2 separation schemes, which have the drawback of a large energy consumption. A design of a boiler with chemical-looping combustion is proposed. The system involves two interconnected fluidized beds, a high-velocity riser and a low-velocity bed. Metal oxide particles are used as oxygen carrier. The reactivities needed for oxygen carriers to be suitable for such a process are estimated and compared to available experimental data for particles of Fe2O3 and NiO. The data available on oxygen carriers, although limited, indicate that the process outlined should be feasible.

The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2

Abstract Chemical-looping combustion (CLC) has been suggested as an energy efficient method for capture of carbon dioxide from combustion. The technique involves the use of a metal oxide as an oxygen carrier which transfers oxygen from the combustion air to the fuel, and the direct contact between fuel and combustion air is avoided. Thus, the products of combustion, i.e. carbon dioxide and water, are kept separate from the rest of the flue gas. After condensation of the water almost pure CO 2 is obtained, without any energy lost for the separation. In this paper, the feasibility of using Fe 2 O 3 as an oxygen carrier has been investigated in a fixed bed quartz reactor. Iron oxide was exposed to repeated cycles of air and methane at 950 ° C, with the outlet gas concentrations measured. The time under reducing conditions and the amount of bed material were varied in a wide range. The reduction rate of Fe 2 O 3 to Fe (d X /d t ) with 100% methane was between 1–8%/min and was a function of both the conversion range of the solid material, Δ X , as well as the yield of methane to carbon dioxide, γ red . The rate of oxidation was also a function of Δ X and the gas conversion, γ ox , and was considerably faster than the reduction, with rates up to 90%/min. The parameters d X /d t , Δ X , and γ are closely related and can be used to establish design criteria of a CLC system based on interconnected fluidized beds. The rates of both reduction and oxidation found should be sufficient to be employed in a CLC system based on two interconnected fluidized beds.

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.

Materials for Chemical-Looping with Oxygen Uncoupling

Chemical-looping with oxygen uncoupling (CLOU) is a novel combustion technology with inherent separation of carbon dioxide. The process is a three-step process which utilizes a circulating oxygen carrier to transfer oxygen from the combustion air to the fuel. The process utilizes two interconnected fluidized bed reactors, an air reactor and a fuel reactor. In the fuel reactor, the metal oxide decomposes with the release of gas phase oxygen (step 1), which reacts directly with the fuel through normal combustion (step 2). The reduced oxygen carrier is then transported to the air reactor where it reacts with the oxygen in the air (step 3). The outlet from the fuel reactor consists of only CO2 and H2O, and pure carbon dioxide can be obtained by simple condensation of the steam. This paper gives an overview of the research conducted around the CLOU process, including (i) a thermodynamic evaluation, (ii) a complete review of tested oxygen carriers, (iii) review of kinetic data of reduction and oxidation, and (iv) evaluation of design criteria. From the tests of various fuels in continuous chemical-looping units utilizing CLOU materials, it can be established that almost full conversion of the fuel can be obtained for gaseous, liquid, and solid fuels.

Gas leakage measurements in a cold model of an interconnected fluidized bed for chemical-looping combustion

Abstract In chemical-looping combustion (CLC) a gaseous fuel is burnt with inherent separation of the greenhouse gas carbon dioxide. The oxygen is transported from the combustion air to the fuel by means of metal oxide particles acting as oxygen carriers. A CLC system can be designed similar to a circulating fluidized bed, but with the addition of a bubbling fluidized bed on the return side. Thus, the system consists of a riser (fast fluidized bed) acting as the air reactor. This is connected to a cyclone, where the particles and the gas from the air reactor are separated. The particles fall down into a second fluidized bed, the fuel reactor, and are via a fluidized pot-seal transported back into the riser. The gas leaving the air reactor consists of nitrogen and unreacted oxygen, while the reaction products, carbon dioxide and water, come out from the fuel reactor. The water can easily be condensed and removed, and the remaining carbon dioxide can be liquefied for subsequent sequestration. The gas leakage between the reactors must be minimized to prevent the carbon dioxide from being diluted with nitrogen, or to prevent carbon dioxide from leaking to the air reactor decreasing the efficiency of carbon dioxide capture. In this system, the possible gas leakages are: (i) from the fuel reactor to the cyclone and to the pot-seal, (ii) from the cyclone down to the fuel reactor, (iii) from the pot-seal to the fuel reactor. These gas leakages were investigated in a scaled cold model. A typical leakage from the fuel reactor was 2%, i.e. a CO 2 capture efficiency of 98%. No leakage was detected from the cyclone to the fuel reactor. Thus, all product gas from the air reactor leaves the system from the cyclone. A typical leakage from the pot-seal into the fuel reactor was 6%, which corresponds to 0.3% of the total air added to the system, and would give a dilution of the CO 2 produced by approximately 6% air. However, this gas leakage can be avoided by using steam, instead of air, to fluidize the whole, or part of, the pot-seal. The disadvantages of diluting the CO 2 are likely to motivate the use of steam.

Sulfur Tolerance of CaxMn1–yMyO3−δ (M = Mg, Ti) Perovskite-Type Oxygen Carriers in Chemical-Looping with Oxygen Uncoupling (CLOU)

Perovskite-structured oxygen carriers of the type CaxMn1–yMyO3−δ (M = Mg, Ti) have been investigated for the CLOU process. The oxygen carrier particles were produced by spray-drying and were calcined at 1300 °C for 4 h. A batch fluidized-bed reactor was used to investigate the chemical-looping characteristics of the materials. The effect of calcium content, dopants (Mg and Ti), and operating temperature (900, 950, 1000, and 1050 °C) on the oxygen uncoupling property and the reactivity with CH4 in the presence and absence of SO2 was evaluated. In addition, the attrition resistance and mechanical integrity of the oxygen carriers were examined in a jet-cup attrition rig. All of the investigated perovskite-type materials were able to release gas phase oxygen in inert atmosphere. Their reactivity with methane was high and increased with temperature and calcium content, approaching complete gas yield at 1000 °C. The reactivity decreased in the presence of SO2 for all of the investigated oxygen carriers. Decreasing the calcium content resulted in a less severe decrease in reactivity in the presence of SO2, with the exception of materials doped with both Mg and Ti, for which a higher resistance to sulfur deactivation could be maintained even at higher calcium contents. The drop in reactivity in the presence of SO2 also decreased at higher temperatures, and at 1050 °C, the decrease in the reactivity of the Mg- and Ti-doped material was minimal. Sulfur balance over the reactor system indicated that the fraction of the introduced SO2 that passed through the reactor increased with temperature. It was shown that it is possible to regenerate the oxygen carriers during reduction in the absence of SO2. Most of the materials also showed relatively low attrition rates. The results indicate that it is possible to modify the operating conditions and properties of perovskite-type oxygen carriers to decrease or avoid their deactivation by sulfur.

Examination of Perovskite Structure CaMnO 3-δ with MgO Addition as Oxygen Carrier for Chemical Looping with Oxygen Uncoupling Using Methane and Syngas

Perovskite structure oxygen carriers with the general formula CaMnxMg1-xO3-δ were spray-dried and examined in a batch fluidized bed reactor. The CLOU behavior, reactivity towards methane, and syngas were investigated at temperature 900°C to 1050°C. All particles showed CLOU behavior at these temperatures. For experiments with methane, a bed mass corresponding to 57 kg/MW was used in the reactor, and the average CH4 to CO2 conversion was above 97% for most materials. Full syngas conversion was achieved for all materials utilizing a bed mass corresponding to 178 kg/MW. SEM/EDX and XRD confirmed the presence of MgO in the fresh and used samples, indicating that the Mg cation is not incorporated into the perovskite structure and the active compound is likely pure CaMnO3-δ. The very high reactivity with fuel gases, comparable to that of baseline oxygen carriers of NiO, makes these perovskite particles highly interesting for commercial CLC application. Contrary to NiO, oxygen carriers based on CaMnO3-δ have no thermodynamic limitations for methane oxidation to CO2 and H2O, not to mention that the materials are environmentally friendly and can utilize much cheaper raw materials for production. The physical properties, crystalline phases, and morphology information were also determined in this work.

Oxygen Carriers for Chemcial-Looping Combustion

© 2015 Elsevier Ltd. All rights reserved. Experiences from actual operation with oxygen carriers have been reported from more than 20 pilot plants in the size range 0.3kW-3MW using gaseous, solid and liquid fuels. Total operational experience is 6700h and includes both manufactured materials and low-cost materials. The manufactured materials include oxides of nickel, copper, manganese, iron and cobalt, as well as combined oxides. The low-cost materials include iron ores, ilmenite ores, manganese ores, waste materials and calcium sulphate. Several materials studied show good performance with respect to both conversion and expected lifetime. Several materials can be expected to give low costs, and an oxygen carrier cost as low as 1/tonne CO2 captured may not be unrealistic.

Oxidation behaviour of desulphurization residues from gasification and fuel-rich combustion

Abstract One method of stabilizing CaS in desulphurization residues is to oxidize it to more stable CaSO 4 . In the present work the effects of operating variables on the CaS oxidation were investigated using a thermogravimetric analyser and mass spectrometer. It was found that the level of conversion of CaS to CaSO 4 reached a maximum at approximately 920°C. At temperatures below 920°C, there remained a significant amount of unreacted CaS in the sample particles. At 1000°C, there hardly remained any CaS but the conversion of CaS to the undesirable CaO reached 55.4%. Increasing the concentration of SO 2 in the gas phase improved the sulphation. The conversion to CaSO 4 increased as the degree of sulphidation of the residues decreased. An efficiency parameter of the CaS oxidation was proposed and was also used to evaluate the oxidation process. A correlation has been suggested which can make the estimation of the CaS conversion and the reaction rate possible. From the results, recommendations for the oxidation process have been made.

Thermogravimetric combined with mass spectrometric studies on the oxidation of calcium sulfide

Abstract The oxidation of CaS particles has been investigated by means of a thermogravimetric analyzer combined with a mass spectrometer, especially under fluidized bed combustion conditions. The products are CaSO 4 or both CaSO 4 and CaO, depending on the temperature and O 2 concentration. The oxidation reaction results in the evolution of SO 2 when CaO is formed in the products. The conditions under which the undesirable SO 2 will be released, or not, have been obtained. There is always an overall sample weight gain, even when SO 2 is evolved during the reaction. The investigation of the kinetics of the CaS oxidation in the no SO 2 releasing conditions shows that the oxidation reaction is of first order with respect to O 2 , and a higher conversion can be achieved at higher temperature and by reducing the particle size. The maximum conversion of CaS to CaSO 4 is 48.67% in 60 min at 865°C and 4 vol% O 2 in N 2 for 17 μm CaS particles. The rate constant and the effective product layer diffusivity are 4.1 × 10 −4 –2.88 × 10 −3 m s −1 and 3.15 × 10 −11 –4.39 × 10 −10 m 2 s −1 at 500–865°C in the light of the shrinking unreacted core model for gas-solid reaction, the former yielding an activation energy of 39.5 kJ mol −1 for the CaS oxidation reaction.