Combined fixed bed processes for improved energy efficiency and with low penalty for CO₂ capture

-    Goal of the Project: This project aims at merging two different CO₂ capture technologies into one, in order to utilize and strengthen their advantages and at the same time moderate their disadvantages. In this case use Chemical Looping Oxygen Production (CLOP) for Enhanced Pressurised Coal Gasification (EPCG) to produce syngas to be utilized in Chemical Looping Combustion (CLC). The aim of the COMPOSITE project is to study the potential of such a new concept through, collecting kinetics data from experiments and further use them in modelling of the CLOP, EPCG and CLC parts to establish boundary condition that further can be used for the overall energy- and mass balances for calculation of energy efficiency and the penalty of CO₂ capture. The outcome will then be further used to benchmark this technology against state-of-the-art CO₂ capture technology. Milestone: Come up with design criteria based on simplified process simulation that has at least 41% efficiency with potential for further increase and compare it with the state of the art supercritical Bituminous coal fired plant without carbon capture (46%).
-    Technical content: WP1 Fabricate and characterise materials for the fixed bed (porous extruded granulates) with needed properties, like kinetics, capacities, and enthalpies, to the best operational window suggested by modelling for the process. WP2 Determination of produced materials kinetics, capacity and suitable operation window. WP3 Testing of oxygen carrier materials (OCM) under fixed bed condition using input parameters for optimal performance from the modelling work. WP4 Modelling and optimization of CLOP, EPCG, and CLC. WP5 Evaluation of hot gas cleaning processes. WP6 Overall system performance.
-    Technical advantages: In the combined process the oxygen carrier materials (OCM) will not be in direct contact with coal. Only the cleaned syngas produced after the hot gas cleaning step will be in direct contact thereby prolonging the materials life time. Several other advantages are seen in this process. E.g. an ASU for the oxygen production can be avoided, hot cleaning will avoid energy loss for this step, and the semi hot lean pressurised air can also be further heated and utilized in this concept with higher heating value. The OCM used in this concept will be exposed for lower temperature and gas gradients during the redox processes giving a better prospective for the life time of the OCM. All together this can pave the way for a more efficient and sustainable power production based on fossil fuel.
-    R&D challenges: Three processes are connected meaning a good scheme for gas flows and transient temperature effect must be considered. Evaluate how much of the oxygen needs to be consumed by the CLOP in order to be efficient and what should the O2 concentration in the recycled gasification gas be after the CLOP unit. One of the main challenges is the CLOP unit since materials working at to high temperatures will have problems with efficient heat transfer while the one working at to low temperature often lack kinetics. In between there is a window of operation which should be suitable for oxygen production. The compressed air at 350 °C is used to cool down the oxidation process which is exothermic, and at the same time preheat the lean air transferred to the CLC unit for further heating.

Results to date:
WP1: Material characterization, production and optimization. 3 CLOP material systems were identified, prepared and fabricated. These systems cover sole transition metal oxide system, mixed oxide system oxide, and a complex oxide system. The two first systems were synthesized at SINTEF, while powder of the third material was produced by an external supplier. Complex oxide system was identified as one of the most promising CLOP materials, and was further fabricated as porous extruded granulates at SINTEF for thorough thermogravimetric and packed bed reactor testing (WP2 and WP3). The effect of processing and annealing parameters on extrudability, porosity and strength of the extrudates were studied. Further SEM, XRD characterizations combined to measurements of sintering shrinkage and thermal expansion were performed to define the optimum fabrication conditions. Sintering at 1250 °C to 1300 °C resulted in sufficiently strong materials with 30% porosity.

WP2: Determination of kinetics for OCM's from thermogravimetric (TG) analyses. All the three CLOP material systems are studied in the atmospheric and pressurized TGAs in SINTEF. In case of the sole transition metal oxide, the material is expected to possess 10 wt% change during the first reduction stage at 550 °C to 600 °C. The pressurized thermogravimetry confirms that although the capacity of this redox couple is sufficient, and the reduction reaction is relatively fast, the extremely slow oxidation kinetics hinders the capacity of the material to be recovered, and after a few cycles the material simply loses its redox capabilities. The second system of study, mixed oxide shows promising redox results at 750 °C to 900 °C. As this redox reaction happens at higher temperatures, this system may result in higher overall efficiencies (being studied in WP4). Further investigation of this system under realistic pressurized condition is planned for the coming months. Applying the thermogravimetric analysis on the novel complex oxide system, reveals outstanding redox performance for this system. The redox temperature is in the range of 500 °C to 580 °C which fits well to both the pressurized air (350 °C) and sweep gas from CLC process (1000 °C). It was shown that the redox performance of this material is hysteresic with temperature, thus a temperature swing approach is beneficial to extract the required redox capacity and obtain fast kinetics. Preliminary results from the modelling WP shows that the redox kinetics for this material is fast enough for the large reactor, thus not to be the bottleneck in the oxygen exchange CLOP reactor. From literature it was found that the redox capacity and kinetics of this material can even be better and to a larger extent can be improved with better control on the elemental stoichiometry. Optimum synthesis and fabrication routes have result in much faster kinetics and twice capacity at the same temperature, which is outstanding. Further formulation of synthesis and fabrication route to take advantage of this improved operation window from composition stoichiometry at large scale is planned for phase 2.

WP3: Testing of OCM's under fixed-bed conditions. Scheduled for phase 2.
Preliminary tests under fixed-bed condition using a short reactor (3cm) shows that the material produce oxygen easily in the range 3-8% depending on temperature. Test performed at 10 bar between air (5 min, higher flow than inert flow) and inert (5min) shows that 4-5% oxygen into inert can be achieved at 520 °C. A temperature drop in the bed due to the endothermic release of oxygen is observed. See Figure 1.
In order to compensate for this and ease the release of oxygen some syngas return to the CLOP unit will be further investigated, based on promising modelling work performed in WP4 showing enhanced release of oxygen by mentioned approach. Reactor tests is scheduled for phase 2, and the new test conditions will also be investigated as well as a longer fixed bed reactor to verify the modelling.

WP4: Modeling and optimization of reactors: CLOP, EPCG, CLC.
Work has been done both on overall systems modeling as well as more detailed simulations of the CLOP unit. When it comes to the CLC unit, this has been studied in detail in previous projects; in particular in the DemoClock project.
For the overall systems simulations, we have simulated the overall process in order to fix the process conditions in the plant (temperature, pressure, composition etc.). The unit models at this level are quite simplistic: Gasification as an equilibrium reactor; the CLOP as a mass exchanger for O2; specified efficiencies for compressor/turbines etc. Based on this the preliminary conclusions are: 1) The most significant parameter for overall plant efficiency is the CLC temperature. Ideally, this should be as high as possible, but in our case limited to around 1150 °C due to the CLC material. This is the temperature available for the turbine. 2) At a given temperature the gasification is favoured by low pressure due to le Châtelier's principle. Operating pressure of around 20 bars is probably suitable. 3) Using the CLOP in the COMPOSITE process avoids the use of an air separation unit (ASU), as used in competing technologies. An ASU consumes electrical power corresponding to about 4% points decrease in the overall plant efficiency (LHV).
For the unit simulations, initial work focussed on the CLOP unit since this is the least known component in the COMPOSITE process.
The third material tested a promising complex oxide material with good kinetics was received from WP2 and incorporated into the CLOP simulation. This material also releases oxygen at relatively low temperatures, but displays substantially faster oxidation kinetics than that of the sole transition metal oxides. Even though the material tested was not optimized, resulting in fairly slow kinetics and reduced oxygen transport capacity, simulations showed that the kinetics and oxygen transport capacity achieved at this early stage is already sufficient for successful pressurized operation at industrial scale. Since the integration of the CLOP process is the key novelty in the COMPOSITE concept, this is a very encouraging result at this stage of the project.
The modelling data was incorporated into an IGCC scheme developed by Polymi, and together with a different gasifier and hot gas cleaning system based on RTI technology. The calculation shows that an efficiency of 45% can be achieved with CO₂ capture. See table below.

Table 1. COMPOSITE Phase 1 Simulation results with CO₂ capture: (ASU = air separation unit, PBCLC = packed bed chemical looping combustion, CLOP = chemical looping oxygen production. AB= air blown, ASC = Advanced supercritical PC, EBPG = European best practice guidelines for assessment of CO₂ capture technologies, Report D1.4.3 DECARBit, 2011).

WP5: Evaluation and testing of hot gas cleaning processes. Scheduled for phase 2.
    

WP6: Overall system performance and benchmarking.
System level model development based on the constituent sub-models and subsequent implementation into general-purpose software (for benchmarking) is in this case, dependent on input from other WPs. This software will both check the Polmi calculation but is also very flexible a will be used for further optimization of the process. Since considerable input is now available, work to make use of these results/data at system level has just started; targeting to establish mass and energy balances as well as interfacing the boundaries of constituent units. This is necessary in order to eventually make an overall evaluation. Deliberations to define the reference process as well as the basis (system boundaries, appropriate simulation tool for evaluation, the case-study choice, etc.) for comparison are also underway.