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Chemical Looping Combustion CO2-Ready Gas Power

Final Report Summary - CLC GAS POWER (Chemical Looping Combustion CO2-Ready Gas Power)

The 'Chemical-looping combustion CO2-ready gas power' (CLC) project was a 30-month project devoted to taking the chemical-looping combustion (CLC) technology to the next level of development. The project was part of the EU's Sixth Framework programme with support from the Carbon Capture Project (CCP) and has mainly focused on the critical issues for an up-scaling of the technology.

Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, thus avoiding direct contact between air and fuel. The technique involves the use of metal oxide particles with the purpose of transferring oxygen from an air reactor to a fuel reactor.

The advantage with this system compared to normal combustion is that the CO2 is inherently obtained in a highly concentrated stream ready for sequestration and there is no cost or energy penalties for gas separation. CLC uses well-established boiler technology very similar to circulating fluidised bed boilers, which also means that costs can be assessed with great accuracy. Thus, CLC could play a significant role in achieving large reductions in emissions of CO2 and hence combating global warming.

It was the objective of the CLC gas power project to establish and validate solutions to these topics, thereby enabling a future demonstration phase:
1) Identify process suitability of raw materials commercially available at competitive prices.
2) Establish commercial particle production techniques for up-scaling from laboratory freeze-granulation.
3) Adapt alternate particle production paths with potentially lower production costs, such as impregnation.
4) Investigate possible effects of gas impurities on particles, primarily sulphur.
5) Perform long term testing to confirm mechanical and chemical integrity of particles.
6) Modify an existing CFB rig to intermediate CLC demonstration unit at 100-200 kWth scale.
7) Extend and verify modelling capability for process performance optimisation and scaleup.
8) Perform process and technology scale-up to prepare for industrial 20-50 MWe demonstration unit.

In the first phase of the project approximately 40 different oxygen carriers based on NiO with NiAl2O4 were prepared by freeze granulation. The oxygen carrier materials were investigated with respect to parameters important for CLC, such as reactivity under alternating oxidising and reducing conditions, mechanical strength and defluidisation phenomena. Oxygen carrier particles with high sphericity, good free-flowing properties and homogeneity on the micro-scale, were prepared by the industrial spray-drying method, using commercial raw materials.

A specific hot integrated pilot has been designed and built by ALSTOM to study the attrition behaviour of oxygen carriers. A preliminary test rig in ambient operating conditions was also constructed in order to optimise the final design. The 15 kWt rig is composed of two interconnected circulating fluidised bed and it is operated with natural gas. Four different oxygen carriers have been tested with the main part of the tests performed with the N-VITO particle. Carbon conversion up to 97 % has been obtained during experimental investigation. A limited attrition of particles has been observed.

The oxygen carriers prepared by freeze granulation and spray drying were tested with respect to physical properties, chemical composition, gas conversion under reducing and oxidising conditions as well as rate of reduction. The reactivity investigation was carried out in a batch fluidised bed reactor where the oxygen carriers were exposed to alternating CH4 and 5 % O2. It was also found that the addition of MgO or the use of MgAl2O4 as the 'inert' improved the methane conversion, where a series of different Mg-based materials were investigated. Hence a larger batch of material was produced, which was similar to the N-VITO oxygen carrier, but with 5 wt% MgO added during the preparation stage, hence the nomenclature N-VITOMg.

Two small continuous CLC reactors at Chalmers and CSIC were used to investigate reactivity of particles under continuous conditions. As was seen in the batch reactor investigation, it was found that the N-VITO particle had good oxygen transport characteristics, but had relatively poor methane conversion. The particle with 5 wt% MgO addition, on the other hand, showed excellent methane conversion, but with poor oxygen transport capability, especially at the lower temperature. The mixture of the two particles resulted in a potent oxygen carrier mixture with the desired qualities.

From the screening phase of the project the N-VITO particle sintered at 1450 degrees Celsius was selected to be investigated for long-term operation (> 1000 h) with natural gas and study changes with respect to reactivity and physical characteristics of the particle. No decrease in reactivity was seen during the test period. No gas leakage between the reactors was detected. Agglomeration of particles occurred on some occasions. The agglomerates were soft, and could be crushed and re-introduced into the reactor system.

At Vienna University of Technology, a dual circulating fluidised bed (DCFB) reactor system has been designed and built for CLC operation with natural gas at a fuel power of 120 kW. About 100 hours of CLC and CLR operation experience with Ni-based oxygen carriers at different bed inventories have been reached at the test rig between May and August 2008. The unit has been fuelled with CH4, CO, H2, C3H8 or mixtures of these gases and can be operated with different oxygen carriers. Based on solid samples taken directly out of the loop seals in the global loop, the solids circulation rate can be determined with great accuracy. The results show air reactor entrainment rates between 10 and 90 kg/(m2s). No problems with carbon deposition on oxygen carrier particles were found.

A mathematical model describing the behaviour of the 120 kWth CLC pilot plant located at Vienna University of Technology has been developed. The global model includes the submodels describing the FR and the AR. The FR model considers all the processes affecting the reaction of the fuel gas and the oxygen carrier such as the reactor fluid dynamics, the reactivity of the oxygen carrier and the reaction pathway. The main outputs of the model are the axial profiles of gas composition and flows (CH4, CO, H2, CO2 and H2O), gas composition at the outlet, solid conversion, profiles of solid concentration and flows in the dilute zone.

The critical issues for a future demonstration step of the CLC process have been addressed. These include, among others, the up-scaling of the technology to an industrial demonstration unit. As part of this an environmental assessment has been done to ensure the process meets (future) high standards of environmental performance and workplace safety. The issue of permitting was addressed; this novel technology might not fit into the existing permitting regime and finally scale-up of the technology from pilot to full size plant may reveal issues, which were not apparent at a smaller scale. These and other risks / recommendations need to be addressed in the next development phase of this technology.

The project has demonstrated that production of oxygen carriers which exhibit excellent properties with respect to important parameters for CLC can be produced with commercial materials and production methods. The effect of higher hydrocarbons and sulfur on the oxygen carriers does not present any major complications. Integrity tests for one Ni-based carriers for >1000 h of combustion showed good reactivity and limited attrition. Further, within the project the world largest CLC reactor has been constructed and operated successfully with some of the developed oxygen carriers.

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