Henrik Leion

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.

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.

Effects of steam and CO2 in the fluidizing gas when using bituminous coal in chemical-looping combustion

Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel in order to avoid direct contact between air and fuel. Thus, the CO2 is inherently separated from the flue gases with a potential for considerably lower energy penalty and cost compared to other techniques for CO2 separation. The oxygen carrier is circulated between two reactors, a fuel and an air reactor, where the flue gas from the air reactor contains oxygen depleted air and the flue gas from the fuel reactor contains mainly CO2 and H2O. The water can easily be condensed and the remaining CO2 can be transported for underground storage. Most of the prior work with CLC has focused on using natural gas and syngas as fuel and oxygen carrying material normally produced from pure chemicals. However, recent work on adapting the CLC process for solid fuels with ores and natural minerals as oxygen carrier shows promising results. This paper will present results from reactivity investigations in a laboratory fluidized-bed reactor system using previously investigated natural mineral ilmenite as oxygen carrier and a bituminous Colombian coal as fuel. Experiments were conducted at a temperature of 970°C with N2, steam, and/or CO2 in the fluidizing gas. Synergy effects between steam and CO2 on fuel conversion was noted. The results show that the fuel conversion was a roughly a factor 5 faster with steam as compared to CO2 in the fluidizing gas.

Standard Enthalpy of Formation of CuAl2O4 Revisited

Due to discrepancy in the standard enthalpy of formation, , of copper(II) aluminate (CuAl2O4) reported in thermodynamic databases, the calculated reaction enthalpy, ΔHr, of CuAl2O4 with reducing gases varies considerably. In this work, the standard enthalpy of formation, , of CuAl2O4 is reassessed using the reaction enthalpy, ΔHr, of CuAl2O4 with CO measured by differential scanning calorimetry. A value of −1824.4 ± 4.1 kJ/mol was found for the standard enthalpy of formation, , of CuAl2O4.

In situ neutron powder diffraction study of the reaction M2O3 ↔ M3O4 ↔ MO, M = (Fe0.2Mn0.8): implications for chemical looping with oxygen uncoupling

The structural properties of (Fe0.2Mn0.8)xOy as a function of temperature under differing oxidizing and reducing atmospheres have been investigated using Thermal Gravimetric Analysis and neutron powder diffraction techniques. The principal observation is a reversible transition between the (low temperature) bixbyite and (high temperature) cubic spinel structured phases, which occurs over a significant temperature range and is dependent on the surrounding gas atmosphere. In addition to detailed information concerning the temperature dependence of the positional parameters, analysis of the powder neutron diffraction data shows that the two metal species are not completely randomly distributed over the two symmetry independent cation sites within the bixbyite and cubic spinel phases. More significantly, the two phases appear to be completely stoichiometric, with compositions (Fe0.2Mn0.8)2O3 and (Fe0.2Mn0.8)3O4, respectively. Under severely reducing conditions, the sample transforms to a rocksalt structured phase of stoichiometry (Fe0.2Mn0.8)O, but reverts to the spinel and bixbyite phases on re-oxidation. The implications of these findings for the use of (Fe1−xMnx)Oy as oxygen carriers in chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) techniques to capture CO2 during combustion of hydrocarbon fuels is briefly discussed.

Use of natural ores and waste materials as oxygen carriers for chemical-looping combustion

The increasing CO2 levels in the atmosphere require an immediate action in order to avoid irreversible climate changes. Chemical-looping combustion (CLC) is an innovative technology that provides an energy and cost-effective separation of CO2 for further capture and storage and thereby helps to mitigate the anthropogenic CO2 emissions from thermochemical fuel conversion. The solid oxygen carrier is a core component of every CLC system and the choice of the oxygen carrier depends on the fuel and operation conditions. Low-cost oxygen carriers tend to be more suitable for the process as the lifetime of the oxygen carrier material is often limited by side reactions with fuel ash, or by carryover losses in the ash separation. A series of natural ores and waste products have been investigated as potential oxygen cares, such as Mn-ore, Fe-ore, ilmenite. The materials are chosen based on their content of oxides that have proven to have promising oxygen transporting capabilities. The results lead to summary of criterion for further screening of the existing materials for more reactive and better suited candidates for the process.