Chemical Looping

 From a general point of view, "chemical looping" refers to a process where a chemical reaction takes place within two different reactors, and a reactive solid material circulates (loops) between both reactors to drive this chemical reaction. Depending on the respective species, transported by the looping solids and on the obtained process product, different chemical looping applications can be identified. The most important technologies, currently under investigation within research activities around the world, are shown in Figure 1.

Figure 1- Chemical looping technologies

 The research focus at Vienna University of Technology lies on chemical looping combustion and reforming with main activities in the sectors of reactor hydrodynamics, detailed reactor modelling, process modelling and process demonstration. Detailed introduction into the theroretical background of CLC and CLR, as well as more information about research activities can be found on the next pages and under "Research", respectively.

Chemical looping combustion for CO2 capture

  Today the international society has broadly accepted that an anthropogenic influence on the greenhouse effect and thus on the continued global warming exists. This influence is primarily caused by extensive utilization of fossil fuels and the related emission of so called greenhouse gases, with CO2 being their main representative. Even so, the worldwide energy demand is expected to increase and since carbon free technologies are still away from being mature and economical competitive, fossil fuel applications will still cover the major part of the energy demand, at least within the next decades.

To meet this contradicting situation, the European Union proposed a strategy with the overall goal of limitation of greenhouse gas emissions while, at the same time, increasing energy demands are covered. Among others like, expansion of renewable energy sources or increase of end-use efficiencies, one part of this strategy is fossil fuel conversion combined with capture and storage of formed CO2, hence the implementation of the so called carbon capture and storage (CCS) technology. The idea is to deposit concentrated CO2 in save geological storages like gas fields instead of emitting it uncontrolled into the atmosphere. Several projects are currently running to demonstrate the feasibility of carbon sequestration and long-term stability of the storages. On the other hand, research goes on to find cost-effective measures to obtain pure CO2 from power stations.

One of the most promising capture technologies is chemical looping combustion (CLC). While other capture technologies exhibit the need of cost and energy extensive gas-gas separation steps, CLC separates CO2 inherent to the process via unmixed combustion. Oxygen is transported in terms of an oxygen carrier (generally a metal oxide) from the combustion air stream to the fuel stream and thus air nitrogen and fuel are never mixed (Figure 1). The chemical reactions with oxygen carrier take place in two separated reactors. Oxidation of the oxygen carrier is performed within the so called air reactor, which is supplied with combustion air and reduction takes place inside the fuel reactor, where fuel is fed into the system. The total amount of heat released from the two reactors equals the heat released from ordinary combustion of the fuel fed. Most of the proposed CLC applications are using well-established boiler technology very similar to (dual) fluidized bed boilers, which also means that costs can be assessed with great accuracy.

Figure 1- CLC concept

In conclusion, CLC features up to 100% CO2 capture efficiency, a highly concentrated stream of CO2 ready for sequestration, no NOx emissions, and no costs or energy penalties for gas separation. Furthermore CLC uses well-established boiler technology, which means that costs can be assessed with great accuracy. Therefore, CLC is estimated to achieve CO2 capture cost reductions of 40 to 50% compared to today’s best available carbon capture technologies, namely post combustion amine and oxyfuel combustion.

Chemical looping reforming for production of H₂ rich synthesis gas

Nowadays, hydrogen and methanol is predominantly produced out of synthesis gas obtained from catalytic reforming of methane and lower hydrocarbons. While state of the art technologies like methane steam reforming (MSR) and autothermal reforming (ATR) have been established as an optimized standard, chemical looping reforming (CLR) offers a great potential for further optimization in terms of performance and economics of the synthesis gas production. Figure 1 shows the basic concept of a CLR process and as one can clearly see, it is quite similar to the chemical looping combustion concept. Thus, a CLR system consists of two separate reactors and a circulating material is exchanged between both reactors to drive the considered reactions and again a system of two interconnected fluidized beds is proposed as a proper realisation of such a system. The main difference to a CLC processes is that the oxygen transport into the reformer (fuel reactor) is sub-stoichiometric. Furthermore, the circulating bed material acts not only as an thermal energy and oxygen carrier, but also as a reforming catalyst, to reach higher H₂ yields. The obtainable H₂ yield for a considered CLR plant generally depends on the reformer operating temperature, the global air excess ratio and on the catalytic activity of the bed material.

Figure 1- CLR Concept

Compared to state-of-the-art synthesis gas production processes, CLR shows many advantages. First of all, for CLR operation neither an air separation unit (ATR) nor any (partial) internal or external combustion (ATR/MSR) is necessary. Thus, all carbon involved is available within the synthesis gas and a better performance in terms of CO₂ emission control is achieved. Another advantage of CLR is that less steam is required for the process. In fact, steam is just needed to prevent carbon formation on the bed material surfaces, if the global air ratio is reduced below a critical value. Due to the opportunity of compact design of the dual fluidized bed system, smaller reactor volumes and thus less catalyst per unit fuel feed are required. Since the reactors can be refractory lined, the operating temperatures can be increased to enhance the conversion of CH₄ and the thermal flywheel of the bed material leads to an uniform reactor temperature profile. As a general advantage of chemical looping processes, no formation of thermal NO_x occurs, whereas in ATR and MSR internal/external combustion might lead to some NO_x formation.

The main challenges that CLR faces are effective dust removal and fluidized bed operation at elevated pressure. However, comparing this outlook with state-of-the-art reforming technologies, the problems arising with CLR still have a better chance to be solved, because of the:
→ availability of suitable particles from commercial raw materials
→ availability of suitable dual circulating bed technology
→ improvements in dust removal technology (medium and high temperature filters, etc.).