## 2 State of the Art
### Reforming Technologies  
Several reforming technologies are applied for syngas generation from natural gas: Steam methane reforming, autothermal reforming and dry methane reforming. Regarding the syngas production, steam methane reforming (SMR) is a proven and the most widely applied technology. In SMR, methane reacts in a highly endothermic reaction with steam over a catalyst, typically based on nickel, at temperatures between 800-1000 \({}^{\circ}\)C and pressures in the range of 20-30 bar [4; 25; 27]. The strongly endothermic reaction mechanism is associated with a high heat duty, which is supplied through the combustion of fuel or unreacted syngas from the synthesis unit in a reformer furnace. The large steam requirement at steam to carbon ratios above 3.5 results in large mass flows downstream the reforming unit. In consequence, the size and capital expenditures (CAPEX) for the components in the synthesis and distillation unit are high compared to process routes with alternative reforming technologies. Moreover, the supply of additional fuel to the reformer furnace and the high electricity consumption for compression units in the synthesis loop cause high operation costs (OPEX). The upscaling of SMR-based methanol production is only economically justifiable in a single train capacity of up to 2500 MTPD. Above this capacity, other reforming technologies become beneficial, since the economies of scale effect has a stronger impact on their cost reduction [8]. For a capacity range of 2500 to 5000 MTPD, a two-step reforming including a primary steam reformer followed by an oxygen-blown secondary reformer is the preferred concept. The use of oxygen reduces the steam to carbon ratio in the primary reformer, but additionally requires an air separation unit (ASU). Above a capacity of 5000 MTPD, ATR is the only appropriate technology, as higher cost reductions associated with the upscaling are achieved with an ASU than with tubular reforming. An ATR reactor has a compact design, accommodating a combustion chamber and a catalytic fixed bed, located in a refractory lined vessel. Oxygen or oxygen enriched air is deployed together with steam to partially oxidize the methane intake according to reactions in Table 1. The partial oxidation of methane is highly exothermic and results in high operation temperatures between 950-1400 \({}^{\circ}\)C (Table 1). To keep the temperature in a moderate range and to allow for adequate hydrogen content, steam is fed to the inlet to initiate the endothermic SMR in reaction given in Table 1. Asteam-to-carbon ratio (S/C-ratio) of 0.5-1.5 and an oxygen-to-carbon ration (O/C-ratio) in the range 0.6-1.0 have been reported by different manufacturers. The composition of the syngas produced by ATR has a typical S module in the range of 1-2 and a CO/CO\({}_{2}\)-ratio of 2.5-5.5 [5; 28; 29]. Furthermore, high conversion of methane is favored in a high-pressure range of 30-50 bar, which is advantageous in terms of syngas compression. In comparison to SMR, the use of an ATR reduces the steam requirement and therefore results in lower mass flow rates and smaller equipment size of the downstream components. Additionally, the heat of reaction can be integrated to cover the heat requirement of the pre-heating unit and other heat consumers. With respect to the synthesis of methanol, ATR as a stand-alone reforming technology cannot fulfill the stoichiometric requirement. Typically, the syngas composition is characterized by a hydrogen deficiency, which results in a lower per pass conversion in the methanol reactor. The dry methane reforming technology (DMR) is the third option of syngas production, which is analyzed in the present work. DMR is a relatively new technology that is considered as a promising carbon utilization concept [18; 19]. According to the reaction in Table 1, methane reacts with carbon dioxide over a nitrogen based catalyst to give hydrogen and carbon monoxide at a ratio of one.