Chemical-looping combustion (CLC) for coal is a zero-emission technology, combining very low efficiency penalty (2-3%) and low CO2 capture cost, potentially as low as 10 €/ton CO2. Key issues for coal-CLC are:
The proposal builds on the great advances made in the ÉCLAIR project and the unique CLC solid fuel pilots developed and built. ÉCLAIR has clearly demonstrated the feasibility of the technology, but also that it would be an important advantage if process performance could be further improved. Such improvement primarily concerns the incomplete gas conversion and the corresponding need for oxy-polishing of CO2 product gas.
The proposal focuses on options to improve gas conversion with either new oxygen carriers, or with more advanced fuel reactor design. A number of oxygen carrier materials expected to give radical improvements in performance are known, but have not been sufficiently tested. The programme involves prequalification tests of such materials under sustained continuous operation in smaller chemical-looping combustors, 1-10 kW. Best candidates will be further tested in 100 kW and 1 MW. Advanced fuel reactor designs will be investigated in cold flow model experiments combined with use of validated models in order to assess options to improve gas conversion. Proposal also involves update and review of downstream gas treatment and full-scale power plant design, as well as studies of the fate and influence of sulphur and nitrogen in the fuel, depending on oxygen carrier. The project is expected to have very great impact because it aims at demonstrating significant advances of the chemical-looping technology for solid fuel. Thus, it is expected to demonstrate how the potential for very fundamental reductions of energy penalty and CO2 capture costs can be realized.
Chemical looping combustion (CLC) has emerged as one of the most promising long-term technologies for low-cost CO2 capture for coal. The idea of CLC is to use a metal oxide to transport oxygen from air to the fuel, thus avoiding direct contact between fuel and air. The process consists of an air reactor, where the metal is oxidized by the O2 content in the air, and a fuel reactor, where the metal oxide is reduced by the fuel that is converted to mainly CO2 and H2O. After condensing the steam, a stream of almost pure CO2 is obtained. The separation of CO2 is inherent in the process, resulting from the fact that combustion air and fuel are never mixed. Thus, no active separation of gases is needed, and there is no energy penalty for gas separation as in post-, pre- or oxy-combustion CO2 capture. Hence, the CLC process requires a uniquely low energy penalty of 2-3 percentage points, and CO2 capture costs are potentially as low as 10 €/ton of CO2 captured. The most promising approach for the two reactors is the fluidized bed technology that is widely applied to the conversion of coal.
Early testing with solid fuels in small CLC pilots has clearly shown three aspects where performance could be improved:
Here, solid fuel conversion is highly dependent on scale of operation and efficiency of cyclones. The height of the riser will determine the residence time of fine char particles per cycle of circulation, and the cyclones will determine the extent to which char fines of a given size are recirculated. It can be assumed that full-scale operation with high efficiency cyclones will give a completely different performance as compared to small bubbling beds with no return of elutriated material.
CO2 capture can be controlled by the addition of a so-called carbon stripper, which combines separation and conversion of char from the particle flow going from fuel reactor to air reactor.
Lastly, incomplete gas conversion means that measures are needed downstream of the fuel reactor. One option is to oxidize these gases with a flow of oxygen, so-called oxygen polishing, another is to separate these in connection with CO2 liquefaction and recycle them to the fuel reactor. Both options would involve additional costs. For this reason, the ACCLAIM project will have a focus on improving the gas conversion of the fuel reactor gas.
As explained above the key objective for this study should be to investigate ways to improve process performance, with a focus on improving gas conversion in fuel reactor. Two main routes will be assessed:
To reach this key objective the project involves the following main key steps for the study of oxygen carriers
In parallel with oxygen-carrier operation, activities related to reactor design are pursued.
Finally downstream gas treatment is updated and the full scale-design is reviewed.
Published July 4, 2013