Table of Contents:
In this section, a comparison is made between SRM. POX and the DRM processes in terms of syngas ratio, energy requirements and carbon formation tendency. Hie comparison is made at a pressure of 1 bar and temperature is varied between 200 and 1200 °C. The feed conditions for all these processes are kept based on stoicliiometiy as per equation (1)—(6) (SRM: H,0/CH4 = 1, POX: 0,/CH4 = 0.5, DRM: COyCH, = 1). Following this comparison, the proposed process of CARGEN and DRM+COSORB are described. Finally, an economic comparison between various combinations of the aforementioned processes in terms of their operational cost and carbon footprint is presented.
Comparison of syngas ratio
In this analysis, the effect of reforming technology on the H,/CO ratio of syngas is studied at different temperatures. The relative atomistic ratio of the feed components in terms of 0:C:H in each reforming technology is key to determining the H,/CO ratio of the syngas produced. The 0:C:H ratio in the feed is 1:1:6 for SRM. 1:1:4 for POX, and 1:1:2 for DRM. This shows that the hydrogen content in SRM is the highest, followed by POX and the lowest in DRM. This low quantity of hydrogen in DRM results in significantly low FT:CO of 1:1, which is not suitable for downstream operations that require at least 2:1. A comparison of H,:CO of the three reforming technologies as a function of temperature is provided in Table 2.
The ratio of hydrogen to carbon monoxide at different temperature conditions provided in Table 2 is seen to be highest for SRM, followed by POX and lowest for DRM. Notably, this is in the same order as that of the 0:C:H ratio stated earlier. If a hydrogen rich gas, steam, for example, is added, it will change the relative ratio of hydrogen to carbon monoxide and consequently other products from the reaction system.
Table 2. Syngas ratio of the three reforming technologies as a function of temperature.
Comparison of energy’ requirements
The energy requirement to drive the reforming reactions forms the major portion of energy requirements of reforming processes. Here, the energy requirements of the three reforming processes (SRM, POX, DRM) are shown as a function of temperature at a reaction pressure of 1 bar and depicted in Figure 3. The feed composition is kept at stoichiometric condition, as indicated earlier. It can be observed that POX technology produces energy as its energy requirements are in negative. However, with an increase in temperature, the energy requirements tend to increase; at ~ 750 °C they tend to cross the zero-energy line, indicating that it becomes net positive after this temperature. On the other hand, both SRM and DRM processes are observed to require energy for the reaction to happen under all temperature conditions. Another point to note is that energy requirements of the DRM process are more than that of SRM beyond 650 °C temperature, while below 650 °C, the energy requirements from the SRM are slightly higher than that of DRM.
Figure 3 shows the energy requirements for SRM, POX and DRM. However, this cannot be used to make comparisons since each of these processes produce syngas of a different H,/CO ratio and their
carbon footprint of oxidant production varies with the process. Section 3.6 includes comparison results that compare the overall carbon footprint. Nevertheless, Figure 3 helps in understanding the energy requirements at different temperatures of the reformer reactor for different processes.
Comparison of carbon formation tendency
This section compares the carbon formation tendency of each reforming technology as a function of temperature. The operational conditions of feed composition, reaction pressure and temperature are identical to the previous sections.
Carbon formation is a result of a combination of two major side reactions: The Boudouard reaction and the methane decomposition reaction, which happen in all reforming processes. However, their extent varies depending upon reaction conditions, oxidant type and its concentration. Equation (7) and equation (8) illustrate the aforementioned side reactions.
From several experimental and thermodynamic modelling studies, it is proven that the extent of methane decomposition reaction at low reforming temperature of 500 °C is very high compared to Boudouard reaction. Many industrial reports also claim that methane decomposition reactions are extremely prone at the reactor entrance, wherein the concentration of methane is very high, compared to any other reaction products. Near the reactor inlets, heating is generally done in order to increase the temperature of the reaction gases to desirable reforming temperatures of 900 °C and beyond. During this transition, since the gases pass through coking temperatures, a huge quantity of carbon is deposited near the inlet. On the other hand, on the exits of the reactor, since the carbon monoxide concentrations are high, the carbon disproportionation tendency is much higher when temperatures are quenched to the target outlet temperatures of 400 °C. Formation of solid carbon at the entrance and the exit of the reactor bed is not the only concern that the industry faces, the other and the most severe challenge is related to catalyst deactivation. Since reforming reactions take place in the presence of catalyst, the formation of carbon deactivates the catalyst by clogging its active sites. In addition to this, sintering is another problem that is widely reported in the scientific literature. Due to these challenges, carbon formation in the reformer is undesirable. Much of the scientific attention in the global community at present is focused on the reduction or elimination of solid carbon from the reforming process. Extensive literature studies that report novel methods of reduction of carbon formation tendency of the reforming reaction are available; some of these studies are either process related while others are purely catalytic. Figure 4 illustrates the carbon formation tendency of the three reforming reactions at stoichiometric feed condition of each reforming process and 1 bar pressure.
Figure 4. Carbon fonnation comparison between SRM, POX and DRM process at stoichiometnc feed conditions and
pressure of 1 bar.
From Figure 4, it can be clearly observed that the carbon formation tendency from the DRM reaction is higher than the other two reforming techniques (DRM > POX > SRM). The lowest carbon formation is observed in SRM, as it contains the highest amount of hydrogen among the reforming techniques. In conjunction to the discussion provided in the previous section of syngas ratio, it could be observed that the ratio of 0:C:H also carries huge implication on carbon formation tendency. Since the DRM process has the highest ratio of C/H compared to the other reforming technologies, the carbon formation tendency of this reaction is the highest. Therefore, in order to reduce coke formation tendency, it would be advisable to either remove excessive carbon and or increase hydrogen concentration in order to reduce the C/H ratio. In addition to this, it can also be observed that low temperatures favor more carbon formation than higher temperatures for DRM and POX, while for SRM, a peculiar trend is observed, in which carbon formation is only in the 400 °C to 800 °C temperature range.