Adiabatic pre-reforming

FCE tested a lab-scale carbonate fuel cell stack on a model diesel-like fuel (Exxsol) using an adiabatic pre-reformer to convert the liquid fuel to methane in 1991 to 1993. In 1995 and 1996, FCE verified a 32 kW MCFC stack operation on jet fuel (JP-8) and diesel (DF-2) in system inte-... 

Adiabatic pre-reforming may be used for steam reforming of hydrocarbon feedstock ranging from natural gas to heavy naphtha with a final boiling point above 200°C and an aromatics content up to 30%. The process is carried out in a fixed-bed adiabatic reactor loaded with a highly active reforming catalyst located upstream from the primary reformer (Figure 6.9). 

Adiabatic pre-reforming of desulfurised diesel and jet fuels was demonstrated for commercial nickel catalysts at 480 °C and S/C ratios around 2.5 for 1000- and 500-h duration, respectively [81]. The feed was not completely sulfur-free, but still contained about 1 ppm sulfur and thus catalyst deactivation originated from sulfur poisoning and rather than from coke formation. 

Another application for diesel fuel processors is the propulsion of naval systems. Krummrich et al. [626] reported from a conceptual study of a 2.5-MW fuel processor/fuel cell system, which was dedicated to submarine applications for the German ship manufacturer HDW. The system consisted of a desulfurisation step, an adiabatic pre-reformer operated between 400 and 550 °C, steam reforming at 800 °C and catalytic carbon monoxide clean-up. The critical step turned out to be the desulfurisation of F76 diesel fuel, which in Europe contains as much as 0.2 wt.% sulfur, world-wide as much as 1 wt.%. These workers then set up and operated a 25-kW demonstration model of the fuel processor, which achieved an efficiency of 82%. 

An adiabatic pre-reformer was modeled with the same kinetics used in the previous models. A reactor bed was configured to represent an industrial reactor that was newly commissioned. Consequently the catalyst activity was assumed to be uniform, that is, no poisoned front had time to be established. The catalyst activity was reconciled so that one temperature in the nonequilibrium zone near the front of the reactor was matched. All other simulated temperatures then also matched the remaining measured temperatures, confirming that the kinetics were yielding appropriate heats of reaction, and composition all along the reactor bed. Effluent compositions were directly measured, and are plotted versus simulated compositions in the parity plot, showing excellent agreement. 

In adiabatic pre-reformers, it is possible to process naphtha with a final boiling point up to 220 C and an aromatic content up to 30%. It may be possible to process even heavier feedstocks depending on the operating conditions. During the operation of an adiabatic pre-reformer, progressive deactivation takes place, mainly by sulphur poisoning. This phenomenon causes the temperature profile to move in the flow direction (Davies et al., 1967). 

It was possible to operate at heat fluxes close to 80.000 kcal/m /h in spite of the sulphur passivation. The catalyst temperature increases quickly to above 750-800 C, at which the reforming rates are sufficient for conversion of methane even at the high load. The high reaction temperature means that higher hydrocarbons must be converted in an adiabatic pre-reformer to eliminate the risk for carbon formation by thermal cracking (reaction (10)). 

Steam export, which is often undesirable in hydrogen plants, can be minimized by adiabatic pre-reforming which allows high preheat temperatures of the reformer feed, and by preheating of the combustion air to the reformer burners. 

An adiabatic pre-reformer is an attractive revamp option, especially when the aim is to minimize steam export and/or to increase plant capacity in cases where the primary reformer is the bottleneck. 

Natural gas is preheated to 400 °C and de-sulfurised in a zinc oxide bed. A very important criterion for design of FPS systems for fuel cells is the need to avoid carbon formation. The adiabatic pre-reformer quantitatively converts higher hydrocarbons to form a mixture of methane, hydrogen, carbon monoxide and carbon dioxide, and thus ehminates the risk of carbon formation. A typical composition of Danish natural gas is given in Table 1. A final in/out heat... 

Pre-Keformer A pre-reformer is based on the concept of shifting reforming duty away from the direct-fired reformer, thereby reducing the duty of the latter. The pre-reformer usually occurs at about 500°C inlet over an adiabatic fixed bed of special reforming catalyst, such as sulfated nickel, and uses heat recovered from the convection section of the reformer. The process may be attractive in case of plant retrofits to increase reforming capacity or in cases where the feedsock contains heavier components. 

Typically, the prereforming process is performed in an adiabatic fixed-bed reactor upstream of the main reformer. In the pre-reformer, higher... 

In some cases a plant may have a pre-reformer. A pre-former is an adiabatic, fixed-bed reactor upstream of the primary reformer. It provides an operation with increased flexibility in the choice of feed stock it increases the life of the steam reforming catalyst and tubes it provides the option to increase the overall plant capacity and it allows the reformer to operate at lower steam-to-carbon ratios166. The hot flue gas from the reformer convection section provides the heat required for this endothermic reaction. 

Description Syngas preparation section. The feedstock is first preheated and sulfur compounds are removed in a desulfurizer (1). Steam is added, and the feedstock-steam mixture is preheated again. A part of the feed is reformed adiabatically in pre-reformer (2). The half of feedstock-steam mixture is distributed into catalyst tubes of the steam reformer (3) and the rest is sent to TEC s proprietary heat exchanger reformer, "TAF-X" (4), installed in parallel with (3) as the primary reforming. The heat required for TAF-X is supplied by the effluent stream of secondary reformer (5). Depending on plant capacity, the TAF-X (4) and/or the secondary reformer (5) can be eliminated. 

Based on a co-flow configuration, the effect of various parameters on cell performance has been studied systematically. The study covers the effect of (a) air flow rate, (b) anode thickness, (c) steam to carbon ratio, (d) specific area available for surface reactions, and (e) extend of pre-reforming on cell efficiency and power density. Though the model predicts many variables such as conversion, selectivity, temperature and species distribution, overpotential losses and polarization resistances, they are not discussed in detail here. In all cases calculations are carried for adiabatic as well as isothermal operation, fii calculations modeling adiabatic operation the outer interconnect walls are assumed to be adiabatic. All calculations modeling isothermal operation are carried out for a constant temperature of 800°C. Furthermore, in all cases the cell is assumed to operate at a constant voltage of 0.7 V. 

Though the focus of this thesis is on direct internal reforming, the existing applications use some extend of pre-reformed fuel. Therefore, a systematic study to understand the influence of non-reformed and pre-reformed fuels on cell efficiency is carried out. It is well known that direct internal reforming can result in reduced cost and increased overall efficiency of the system. However, it is quite convincing from Fig. 7.19 that, the efficiency of the fuel cell is higher for pre-reformed fuel. Both efficiency and power density increases with extent of pre-reforming for both adiabatic and isothermal case (Fig. 7.20). 

Figure 7.19 Effect of pre-reforming on efficiency and power density under adiabatic condition. In call cases the pre-reformed fuel is assumed to enter at 800°C and air at 600°C. The non-reformed fuel is assumed to consist of 60% vol. CH4 and 40 % vol. H2O.

Pre-reforming and adiabatic reforming process optimization, especially if results obtained by different licensers could be combined and optimized this would open the possibility to further energy savings. 

---