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 NOx occurs, whereas in ATR and MSR internal/external combustion might lead to some NOx 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:
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