## 3 Results  
### ADDmeth1 Concept  
Based on the requirements presented in Section 2.1 and the experimental experience with the structured predecessor reactor [11; 20], a scalable reactor concept named 'ADDmeth'  
Figure 3: Mounted tensile sample incl. thermocouples. Figure 2: Geometry of the tensile samples (dimensions in mm). was developed, which explicitly uses the advantages of AM. Figure 4 highlights the main features of the concept based on the bench-scale prototype ADDmeth1. The key innovation is the centrally located, conically widening reaction channel. This fulfils the need for kinetics improvement in the second half of the reactor, as discussed in Section 1. Based on the positive experience with a diameter of 8 mm for the reaction channels by [PERSON] et al. (2018) and [PERSON] et al. (2018), the minimum diameter of the reaction channel in the expected main reaction zone is also set to 8 mm for ADDmeth1. The widening of the reaction channel, which is 170 mm long in total, starts at about halfway and widens to a diameter of 32 mm. In this way, the cross-sectional area expands 16 times. As a result, the gas velocity drops and the thermal resistance of the catalyst bed increases, which in turn increases the mean temperature level in the second half of the reactor. These effects are expected to increase the once-through conversion. Three heat pipes serve as a heat dissipation system. They are evenly arranged around the reaction channel and twist around it to ensure an even better uniformity of the tempera  
Figure 4: CAD cutaway scheme of ADDmeth1 highlighting the innovations and an exemplary gas pathway. ture. To account for the increasing steam content (from bottom to top), their inner diameter increases from 8 mm at the fluid reservoir in the evaporation zone of the heat pipes to 12 mm in the condenser zones. Since the reactor is installed vertically, a capillary structure is dispensed with and only a transport structure (spiral grooves) is provided for an even backflow of condensate. A common fluid reservoir for all heat pipes homogenizes pressure and temperature. The heat pipe system works with water as a heat transfer medium. Internal gas preheating takes place in the space between the reaction channel, the heat pipes and the reactor's outer wall, which is filled with a lattice structure. It is based on a diamond unit cell reported by [PERSON] et al. [17], has a strut thickness of 0.6 mm and a porosity of 89.4%. It conducts the heat from the reaction channel's hot surface and increases the heat transfer area to ensure feed gas preheating. It also contributes significantly to the mechanical stability of the outer wall (see also Section 3.2.1). The reactor body is very compact, with a footprint of 2771 mm\({}^{2}\) (equilateral triangle with 80 mm edge length), a height of 230 mm and a total weight of \(\sim\)1.8 kg. Together with its design power of 5 kW\({}_{\text{Syn}}\), this results in a very low power-specific mass of 0.36 kg/kW considering the reactor body only (comparison: 5 kg/kW for predecessor reactor reported by [PERSON] et al. [11]). The share of functional volume is 52%. The short reaction channel (170 mm) also keeps the pressure drop low. In the design case (10 bar, 5 kW\({}_{\text{Syn}}\), H\({}_{2}\)/CO\({}_{2}\) = 4), a pressure loss of about 265 mbar can be expected according to the Ergun equation. In terms of scalability, the ADDmeth concept with its triangular unit cell ensures that the main innovation (the conic reaction channel) can be easily numbered-up by only moderate adaptations. The choice of the material 1.4404 assures the weldability of the reactor and standard fittings can be used for the test rig integration (see also Section 3.3). In addition, this facilitates the connection to a cooling unit at the reactor top. It consists of standard parts and can be removed by means of a flange construction. In order to be able to characterize the reactor in detail, both compressed air and evaporative cooling are planned.