Combined Oxides of Iron, Manganese and Silica as Oxygen Carriers for Chemical-Looping Combustion

Spray-dried particles with the chemical compositions of Fe0.66Mn1.33SiO3 and FeMnSiO3 have been examined as oxygen carrier materials for chemical-looping combustion. The performance of the materials was examined in oxygen release experiments and during fuel operation with natural gas and syngas. The experiments were carried out in a fluidized-bed chemical-looping reactor system designed for a thermal power of 300 W. The reactor system includes an air reactor and a fuel reactor, as well as loop seals and means for circulation of the oxygen carrier particles. Both materials were able to release gas phase oxygen in inert atmosphere at temperatures between 800-950°C, and with approximately equal oxygen concentrations. Fe0.66Mn1.33SiO3 provided higher conversion of natural gas as compared to FeMnSiO3 and the fuel conversion increased with temperature for both materials. During natural gas operation with Fe0.66Mn1.33SiO3 the conversion reached 100% at around 950°C with a fuel reactor inventory of 235 kg/MW. The fuel conversion was improved when the solids inventory was increased; this improvement could especially be observed for FeMnSiO3 as the fuel conversion was lower for this material. Fe0.66Mn1.33SiO3 provided higher fuel conversion than FeMnSiO3 also when syngas was used as fuel. The fuel conversion increased with temperature for both materials and full conversion was reached above 800°C with a fuel reactor inventory of 225 kg/MW for Fe0.66Mn1.33SiO3, while FeMnSiO3 was incapable of providing full conversion. A rather large elutriation of fines and a significant change in particle size distribution could be observed during operation for both materials. Both materials could work as oxygen carrier for chemical-looping with oxygen uncoupling. Fe0.66Mn1.33SiO3 would be preferred as it has higher conversion of both syngas and natural gas, but the attrition behavior of the material would need to be further investigated

Energy Efficiency Opportunities within the Heat Treatment Industry

Energy efficiency measures have become a top priority for large energy consuming companies because of the increasing energy prices and implemented energy policies. Many companies also receive demands from their customers to reduce their climate impact. Heat treatment processes are performed at high temperature, sometimes up to 1000°C, and the holding time can be up to several hours. A large amount of energy is needed for these processes and this reflects in a large energy cost for these companies. The purpose of this project was to identify advantageous, both economically and environmentally, energy efficiency improvements in a specific steel heat treatment plant. The first part of the project was to perform an energy audit and map the energy consumption in the plant. When the distribution of the energy consumption had been determined, the largest energy consumers could be identified. The search for energy efficiency opportunities was then focused on the largest energy consumers in the plant. The profitability of the identified energy saving possibilities was evaluated as well as the environmental benefits of the suggestions. The energy audit showed that the major part of the energy was consumed in the process itself and that the largest energy consumer among the support processes is the ventilation system. The hardening and nitrokarburizing furnaces, or the main furnaces, are the largest energy consumers of the process equipment. It was found that 753 MWh/year (7.7%) of electricity can be saved by housekeeping measures. The suggested measures were to remove unnecessary lighting, turn off the manual equipment during weekends, plan the production more energy efficiently, search the compressed air system for leaks and close a damper in a preheating furnace. The proposed energy saving investment measures will all together save 418 MWh/year (4.3%) of electricity and remove the district heating demand. The suggested investments were to switch the lighting to low energy lamps, insulate the door hoods of the main furnaces, move the intake to the compressor outdoors, heat exchange exhaust furnace gases with washing water and heat exchange the waste heat from the compressor with the heating of the offices. All suggested investment were shown to be profitable, i.e. having a positive net present value. However, the payback periods for the heat exchanging between the compressor and the offices and the low energy lighting may be regarded to be too long as they were more than five years. If all housekeeping measures and all investment measures were implemented, the energy cost for the plant would decrease with almost 1 MSEK/year

Chemical-looping combustion using combined iron/manganese/silicon oxygen carriers

Combined oxides of iron, manganese and silicon have been used as oxygen carriers for chemical-looping combustion. Three materials with varying composition of iron, manganese and silicon have been evaluated in oxygen release experiments and during continuous operation with syngas and natural gas as fuels. The concentration of oxygen released increased as a function of temperature and the highest concentrations of oxygen were measured with the material with the highest fraction of manganese. It was also this material which gave the best conversion of both syngas and natural gas; essentially full conversion of syngas and above 95% conversion of natural gas above 900° C. The other two materials showed similar performance, albeit with higher syngas conversion for the material with the lowest manganese fraction and the lowest conversion of natural gas for the same material. The materials lasted for 10–14 h of operation with fuel addition before circulation disruption occurred, which was likely caused by particle attrition in all three cases. A phase diagram of the iron–manganese–silicon–oxide system was constructed and the possible relevant phase transitions were identified. This analysis showed that more phase transitions could be expected for the materials with higher manganese content which could explain the superior performance during fuel operation of the material with the highest manganese content. It should however be noted that this material was operated with the highest fuel reactor inventory per thermal power which could also be a contributing factor to the better performance of this material. The study shows that it is possible to achieve very high fuel conversion with combined oxides of iron, manganese and silicon as oxygen carrier. The mechanical stability of the particles was rather poor though and would need to be improved. On the other hand the findings relating to material stability is not necessary valid for natural materials containing a number of additional elements. The results are also of interest as an indication of how natural materials with similar composition, i.e. manganese ores, would perform as oxygen carriers

CuO-Based Oxygen-Carrier Particles for Chemical-Looping with Oxygen Uncoupling - Experiments in Batch Reactor and in Continuous Operation

Chemical-looping with oxygen uncoupling (CLOU) is an innovative method to oxidize fuels with inherent CO2 sequestration, which utilizes a solid oxygen-carrier material to provide O-2 for fuel combustion. In this study, a range of CuO-based oxygen-carrier particles have been manufactured and examined. Out of 24 samples prepared, 10 were examined in a batch fluidized-bed reactor, of which three were selected for further examination by continuous operation in a small circulating fluidized-bed reactor system. Composite particles consisting of CuO as active phase and support material such as ZrO2, YSZ, CeO2, and MgAl2O4 were capable of providing full conversion of CH4 at 900 and 925 degrees C, and were also found to release gas phase O-2 into inert atmosphere when fluidized with N-2. Particles using semiactive support such as Fe2O3, Mn2O3, and Al2O3 formed combined spinel phases with CuO. Such materials were still capable of releasing gas phase O-2 but at different concentrations as compared to particles with inert support. Materials with semiactive support had less good reactivity with CH4. No formation of unexpected phases could be detected by X-ray diffractometry, and all chemical reactions were completely reversible. The three materials that were examined in continuous operation were readily capable of providing more or less full conversion of natural gas under the chosen conditions. However, they also suffered from quick attrition and turned into a flour-like substance after a few hours of continuous operation with fuel. Crushing strength analysis showed that particles used in continuous operation were physically much weaker than fresh. In total, 23 h of continuous operation with fuel addition was recorded

Using Low-Cost Iron-Based Materials as Oxygen Carriers for Chemical Looping Combustion

In chemical looping combustion with solid fuels, the oxygen-carrier lifetime is expected to be shorter than with gaseous fuels. Therefore, it is particularly important to use low-cost oxygen carriers in solid fuel applications. Apart from being cheap, these oxygen carriers should be able to convert the CO and H2 produced from the solid fuel gasification and be sufficiently hard to withstand fragmentation. Several low-cost iron-based materials displayed high conversion of syngas and high mechanical strength and can be used for further development of the technology. These materials include oxide scales from Sandvik and Scana and an iron ore from LKAB. All tested oxygen carriers showed higher gas conversion than a reference sample, the mineral ilmenite. Generally, softer oxygen carriers were more porous and appeared to have a higher reactivity towards syngas. When compared with ilmenite, the conversion of CO was higher for all oxygen carriers and the conversion of H2 was higher when tested for longer reduction times. The oxygen carrier Sandvik 2 displayed the highest conversion of syngas and was therefore selected for solid fuel experiments. The conversion rate of solid fuels was higher with Sandvik 2 than with the reference sample, ilmenite